Fundamentals of Lighting [1 ed.] 9781783320561, 9781842658796

Citation preview

Fundamentals of

Lighting

Fundamentals of

Lighting

O. N. Awasthi

α Alpha Science International Ltd. Oxford, U.K.

Fundamentals of Lighting 450 pgs.  |  224 figs.  |  22 tbls.

O.N. Awasthi Former Professor of Physics National Council of Educational Research and Training Visiting Professor of Lighting Technology and Energy Management Maharashtra Institute of Technology Pune Copyright © 2014 ALPHA SCIENCE INTERNATIONAL LTD. 7200 The Quorum, Oxford Business Park North Garsington Road, Oxford OX4 2JZ, U.K. www.alphasci.com ISBN 978-1-84265-879-6 E-ISBN 978-1-78332-056-1 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the publisher.

Dedicated to

My beloved parents and grand-parents, especially my mother late Smt. Vidyavati and maternal grandmother late Smt. Ramshri, who shaped my life with their selfless sacrifice, untiring hardwork and constant inspiration

Preface It gives me immense pleasure to present my book ‘Fundamentals of Lighting’ to the students, teachers and all others who may like to read it. Actually this book is the first volume of my two book series on the topic: Lighting Technology and Energy Management. The other volume ‘Applications of Light and Energy Management’ will follow shortly. This book has its origin in a course of lectures delivered by me for a number of years to the first and second year postgraduate students of MIT (Maharashtra Institute of Technology), Pune for their two years post graduate program in Lighting Technology and Energy Management. I could hardly find suitable books to refer to cover the syllabi. The material normally available was in the form of lighting handbooks or lighting manuals, etc. I had to collect suitable material from different sources after spending considerable time to teach any particular topic. The students were unable to find the relevant reference material at reasonable price in the market. I was continuously requested by the students to help them by writing a book suitable for their course. This is how I started writing this book. In order to understand the contents of this book an elementary knowledge of physics and mathematics including the vector analysis is assumed. In order to make the contents simple to understand I have given sufficient solved examples in the body of the chapters to clarify the relevant concepts wherever required. At the end of each chapter exercises have been given with relevant questions related to the chapter and in some cases objective type questions have been included with answers. A number of figures and tables have also been included in the book wherever needed for completeness. This book is expected to cover the syllabi of most of the Institutions of Engineering, Architecture and Universities offering the courses in the area of Lighting Technology, Illumination Engineering, Applied Physics, or the courses in Light and Lighting as well as in Architectural Lighting, etc. at the undergraduate and the postgraduate levels. It will also be useful to the students, teachers and researchers interested in lighting design at all levels.

viii    PREFACE

The book will be useful for lighting industries, lighting professionals and lighting designers including lighting research centres, interior designers, etc. The book will also be useful for all the organizations and individuals engaged in green lighting and energy saving programs in lighting globally. As is well known, in the present day world it is very important that the users understand the importance of energy efficiency and the implications of choosing a product in the overall scope of energy consumption including the current concepts of green lighting. There is a strong need to assist users with the selection process when specifying, recommending, designing and installing various lighting systems. In residential environment, lighting energy consumption has increased dramatically with the popularity of halogen downlight systems, which are grossly inefficient when compared to the increasing range of residential fluorescent tubular as well as compact fluorescent lamps (CFLs). This is an example where improved lighting design knowledge may lead to more efficient installations. Qualification as a lighting designer requires a great depth of knowledge in the lighting design process. This knowledge is acquired through extensive and detailed training covering a large variety of lighting design applications such as interior lighting, (differentiated into commercial, retail, industrial and residential), floodlighting, emergency and exit lighting, exterior lighting, road lighting, public lighting, facade lighting, and theatre lighting, etc. We have tried to do justice to our readers by giving them the basic knowledge of the required concepts; however, the understanding of the detailed design methodology requires sustained efforts and practical experience. In recent years, as we all know, there has been a great deal of interest in the development of lighting technology and lighting research and teaching of lighting related issues all over the world. We all know that light is very crucial for our survival. Without light, life on the earth would not be possible. All our senses are, of course, important, but our sense of vision is recognized as being our most important link with the surroundings. More than 80 per cent of all the information around us reaches our brain through our eyes and it strongly influences our way of life. Our vision is only possible in the presence of light, the natural light due to the sun or artificial light from various man made light sources. The sunlight reflected from the moon and the man made artificial light are now as important to us as food. The lighting is said to be of good quality when our eyes can clearly and pleasantly perceive the things around us. Today, the importance of the link between the functional, physiological and psychological basics of light is becoming increasingly more recognized. The ever more stringent requirements being placed on lighting together with the increasing significance being attached to the subjective appreciation of light as an environment-determining factor. The need to employ economic lighting techniques is seen as positive stimulants in the human as well as national development process. Good lighting contributes to the quality of life and increases the comfort level and efficiency



PREFACE    ix

with which various functions are performed. A good lighting system provides light for quality which helps people to see well enough to carry out their visual tasks effectively, to enhance their environment and to stimulate their performance and well-being. This may be related to productivity in factories and offices and to sales promotion through the lighting of merchandise. Good lighting also helps to the creation of a suitable ambience in hotels and restaurants; to road traffic safety or to national pride and heritage when historical buildings are provided with a flood lighting installations. Energy Management in lighting has gained importance since the early 1970s, stimulated by the escalation of energy costs, the gradual depletion of conventional energy resources and the concern for the protection of our environment. Lighting is the most conspicuous example of energy consumption. When inefficiently used, it leads not only to energy crisis but also to a reduction in human efficiency. Lighting is a large and rapidly growing source of energy demand and greenhouse gas emissions. At the same time the savings potential of lighting energy is high, even with the current technology, and there are new energy efficient lighting technologies coming into the market. Currently, more than 33 billion lamps operate worldwide, consuming more than 2650 TWh of energy annually, which is 19% of the global electricity consumption. The main goal of the energy use controlling agencies is to identify and to accelerate the widespread use of appropriate energy efficient high-quality lighting technologies and their integration with other building systems, making them the preferred choice of lighting designers, owners and users. There is significant potential to the improved energy efficiency of old and new lighting installations even with the existing technology. The introduction of more energy efficient lighting products and procedures can at the same time provide better living and working environments and also contribute in a cost-effective manner to the global reduction of energy consumption and greenhouse gas emissions. In India, where the power supply has consistently fallen short of the demand, it accounts for as much as 20 to 22% of the total energy consumed in lighting. This book has 6 Chapters in all which begin by introducing the basic concepts of light and lighting, and then explore the key requirements of a lighting system and what standards need to be met. It also explains what we mean by sustainability and energy efficiency and how good lighting design can contribute to these. Further, this book is supposed to be a comprehensive guide to the theory and practice of lighting design. It contains physics principles dealing with the light production, light measurement techniques and systems, light sources including the recent developments in lamp technology, luminaires, daylighting including the architectural lighting, lighting design methods (lumen as well as point by point method) with illustrations. It also contains discussions on glare and its effects on vision. There is a detailed discussion on unified glare rating (UGR) and procedure for its evaluation. Chapter 6

x    PREFACE

of this book outlines the guiding principles of lighting design methods and procedures adopted for interior lighting. This chapter also includes the lighting design applications for interior spaces, namely, residential lighting, office lighting, educational facility lighting, hospitality and entertainment lighting, theatre, television and photographic lighting, industrial lighting, retail lighting, jewellery store lighting, and museum lighting, etc. It also contains material on lighting design using computers. It has been identified that electric lighting has tremendous hidden potential for energy conservation. Demand-side management programmes (DSM) offer both the cost-energy and environmental benefits, while modern developments in artificial lighting offer alternatives that contribute to DSM strategies. How to exploit fully this hidden potential towards energy savings? This is possible by effective lighting education imparted to engineers, technical experts and consumers. In the present competitive world, successful industries are those which seize the opportunity to optimize production, keep product quality high and minimize operating costs. Lighting is a productivity tool that can help to accomplish these goals and gain a competitive edge. Through investigations, it has been established that good quality lighting contributes to better productivity and profitability through better visual performance, less spoilage, lesser unit cost, lesser eyestrain, lesser accidents, less absenteeism. This also provides increased accuracy and better worker morale etc. It is observed that normally, the consumers visit the retailers, get baffled by the range of lighting fixtures displayed and invariably make a wrong choice by either leaning on the aesthetics or the ever important cost factor. But later, get disappointed when they cannot get the desired effect and satisfaction. They may blame the lamp/luminaire/their manufacturers without realizing that their choice was wrong. As far as lighting effects are concerned, “seeing is believing”. Even the lighting professionals like electrical contractors, architects or interior decorators may be experts in artistic visualization but usually lack technical knowledge. They also need to be educated on the technical aspects/application guidelines of novel lighting products. In a nut-shell, the lighting manufacturers (industries) may market their most priced and technically advanced lighting products. But if the people i.e. the end-users, retailers and professionals are not educated on the technical aspects, the users may not appreciate these products and the lighting companies may get a negative feedback/disappointing market response. What is the remedy? Continuous educational programmes on lighting through effective ‘Industry-Institute-Lighting Society’ interactions. It is sincerely believed that this book will help to promote quality lighting and better life to the individuals including the national efforts in energy saving programs. The lighting industry in India has been growing at nearly 17 to 18% per annum over the last few years to an annual turnover of `. 80,000 million and more. One of the emerging lighting trends is eco-friendly and energy saving solutions. This has brought forward an immediate need for



PREFACE    xi

more energy efficient products and also has pushed the market towards LEDs and Solar Lighting. Light-emitting diodes (LEDs) are the future of lighting. Just as the incandescent gave way to the compact fluorescent lamp (CFL) in our homes and offices, the CFL is yielding to the light-emitting diodes. This will, however, take some time, since the LEDs are bit expensive as compared to fluorescent light sources. In few years time, the household incandescent lamps will vanish, and CFLs will ultimately be replaced by LEDs. The LED technology has been around since the 1950s, and for years we’ve seen LEDs in almost all our electronic equipments, regardless of the device, its function, or its maker. For decades they have been affordable to purchase and cheap to operate, but they’ve largely been relegated to the red, blue, and green status indicators on our computers, radios, and routers. Powered by just a few milliamps and usually outlasting any device they operated within, LEDs served their purpose but were far from fulfilling their potential. Recently, high-power, high-quality LEDs have started lighting our homes and offices, and the big lighting companies—General Electric, Osram Sylvania, and Philips—as well as a number of other companies have entered this field and we expect positive results very soon. It is sincerely hoped that this book will be well received by all concerned.

O.N. Awasthi

Acknowledgements A number of individuals and organizations have helped me during the preparation of this book. I wish to thank all of them for their help and cooperation. I am particularly grateful to Dr. Vishwanath D. Karad, Executive President and founder Director General, MAEER’s MIT, Pune for starting the lighting course and inviting me to teach the Lighting Technology and Energy Management course to the MIT students. Likewise, I am grateful to my colleagues Dr. P.C. Barjatia, Governing Body Member of ISLE (Indian Society of Lighting engineers) and the Director, Lighting Research Academy at MIT and Dr. Milind Pande, Executive Director, MITSOT, MIT, Pune for their whole hearted support for my work. My special thanks are due to Prof. Shilpa Pant, Prof. S.R. Yeolekar and Prof. D.S. Gandhe, Consultant in Energy Management at MIT, Pune and Prof. R.S. Aithal, M.I.T., Manipal, Prof.Anil Valia, Lighting Consultant, Mumbai, Prof. S. Rajagopalan, Dr. S.J. Dhoble and Dr. B.M. Suryavanshi of Nagpur, Prof. P.R. Lalitha, RIE, Mysore, Dr. S.P. Sharma, Dr. Sant Prakash, Prof. V.G. Jadhao and Dr.Praveen Kulshreshtha, RIE, Bhopal, Dr. D.S. Pandurang and Dr. Vijay H. Raybagkar, Pune, Dr. Vivek Pundhir, Bhopal, Dr. D.P. Singh, Aligarh, Dr. S.N. Awasthi, Agra and Mr. P.N. Awasthi, Kanpur for their support for this work. I also wish to offer my special thanks for technical support offered to me by Mr. Vikas (Wellington, NZ), Mr. Abhishek (London, UK), Dr Anurag and Dr. A.P. Agrawal, Pune. I also wish to thank all my colleagues at MIT, Pune and NCERT for their direct or indirect help in my work. I am also grateful to all my students of the Lighting Technology and Energy Management course at MIT, Pune for their whole hearted support for this work. I also wish to thank Prof. S. C. Srivastava of the Department of Electrical Engineering, IIT, Kanpur for his support for this book. Further, my sincere thanks are also due to Dr. Mark Rea, Director, Lighting Research Center, New York, USA who is also the Editor of the IESNA Lighting Hand Book.

xiv    ACKNOWLEDGEMENTS

I also wish to express my thanks to Mr. Gulshan Aghi, President, ISLE, Mr. C. G. S. Mani, President, ELCOMA (Electric Lamp and Component Manufacturers Association of India) and Dr. Avinash D. Kulkarni, former President, ISLE and Chairman & MD, Litex Electricals Pvt. Ltd., Pune for their immense contributions in the field of lighting in India. I also wish to express my sincere thanks and appreciation to Mandavi, my wife, for her extreme patience and continuous support for my work. Finally I extend my warm regards and gratefulness to Shri N. K. Mehra, Publisher and Managing Director and his colleagues of Narosa Publishing House Pvt. Ltd., New Delhi for doing the excellent job in bringing out this book. O.N. Awasthi

Contents Preface vii Acknowledgements xiii 1.

The Principles of Light and Optics 1.1 Introduction Why Lighting Research? Quantum Jump in Lighting Technology 1.2 The Science and the Nature of Light What is Light? Electromagnetic Theory of Light Applications of Ultraviolet Light Applications of IR Light 1.3 Black body Radiation Introduction The Black body Spectrum Identifying Individual Types of Atoms 1.4 Light Generation Natural Phenomena - Solar Radiation How Light is Produced in the Sun? Artificial Light Generation How does laser light Differ from other Light? What is Stimulated Emission? Components of Laser 1.5 Optical Control of Light Reflection and Reflectance of Light Types of Scattering Polarization of Light Polarization by Reflection 1.6 Exercises

1.1 1.1 1.3 1.4 1.5 1.6 1.7 1.13 1.15 1.16 1.16 1.17 1.23 1.23 1.23 1.23 1.25 1.29 1.30 1.34 1.53 1.53 1.63 1.64 1.66 1.70

xvi    CONTENT

1.7 Bibliography

1.71

2.

Light and its Measurement 2.1 2.1 Introduction 2.1 Radiometry 2.1 Photometry 2.2 2.2 Principles of Photometry and Radiometry 2.2 Photometry and the Eye 2.4 2.3 Photometric and Radiometric Quantities 2.4 Luminous Flux 2.5 Lumen 2.5 The Radiant Flux 2.7 2.4 Brightness, Luminance and Illuminance 2.12 Luminance 2.12 Illuminance 2.15 Unit of Illuminance, Lux 2.16 2.5 Efficacy, Efficiency and Laws of Illumination 2.18 Luminous Efficacy of a Lamp 2.18 Lambert’s Law or Cosine Law 2.21 2.6 Numerical Problems In Photometry 2.22 2.7 Techniques of Light Measurement and Measuring Systems 2.24 Light Measuring Systems 2.25 Luminous Flux Measurement and the Integrating Sphere 2.25 Substitution Method 2.29 Operating Principle of Photometers 2.32 Structure and Operating Principle 2.36 The Component Features 2.42 IES and CIE Formats 2.45 Sensors, Thermal Detectors and their Applications 2.46 Measurement of Color of a Light Source 2.50 2.8 Exercises 2.52 2.9 Bibliography 2.53

3.

Light Sources 3.1 An Overview of Light Sources 3.2 Classification of Light Sources 3.3 Materials Used in Light Generation Glass-Metal Seals 3.4 Incandescent Light Sources-Gls and T-H Lamps Pressure Tungsten Halogen Lamps

3.1 3.1 3.2 3.4 3.8 3.13 3.20 3.25



CONTENT    xvii

3.5 Electric Gas Discharge Lamps Most Common Gas-Discharge Lamps Compact Fluorescent Lamps (CFLs) Comparison of CFLs with Incandescent Lamps Sodium Vapour Lamps High-Intensity Discharge (HID) Lamps High Intensity Discharge (HID) Lamp Technology Variants Special Purpose HID Lamps such as Xenon and HMI Metal Halide HID Lamps 3.6 Recent Development in Lamp Technology Recent Advances in LEDs Technology LEDs and Heat Organic LED Electrode-less High Frequency Induction Lamps Magnetic Induction Lamps Fibre Optic Light Sources and Pulsed Light Sources 3.7 Comparison of all Types of Light Sources Color Temperature of Light Sources Color Rendering Index (CRI) of Light Sources CRI or Ra ratings of lamps CRI of HID and Low Pressure Sodium Lamps 3.8 Exercises 3.9 Bibliography

3.31 3.31 3.37 3.39 3.41 3.48 3.49 3.56 3.58 3.60 3.60 3.62 3.63 3.64 3.66 3.68 3.74 3.77 3.78 3.79 3.83 3.87 3.89

4. Luminaires 4.1 Introduction 4.2 General Description of Luminaires Functions of Luminaires 4.3 Light Output Ratio of Luminaire and its Use CIE Luminaire Types and their Light Distributions 4.4 Luminaires: Types and Classification The CIE Classification System Principal Types of Luminaires Outdoor Luminaires Light and Luminaire Design Approvals and Standards 4.5 Safety and Protection of Luminaires 4.6 Light Control Components and Optical Design of Luminaires Lenses Fresnel lens

4.1 4.1 4.1 4.2 4.3 4.4 4.5 4.7 4.8 4.10 4.16 4.17 4.18 4.21 4.37 4.38

xviii    CONTENT

4.7 Testing and Performance of Luminaires Components of Photometric Performance Goniometer Set Up Goniophotometer for Luminare Photometry Luminaire Efficiency Climatic Testing of the Luminaires 4.8 Materials used in Luminaires Luminaire with Transmitting Material 4.9 Exercises 4.10 Bibliography

4.41 4.43 4.52 4.52 4.54 4.55 4.56 4.56 4.56 4.57

5. Daylighting 5.1 Introduction 5.2 Daylight Sources and Daylighting Techniques CIE Overcast Sky Model Daylighting Techniques 5.3 Daylighting and Human Factors 5.4 Daylight Effects on Building Contents 5.5 Daylight Factor and its Evaluation Derivation of Formula for Daylight Factor (DF) The Sky Illuminance Numerical Examples for DF 5.6 Design Methods and Evaluation Techniques Design Recommendations 5.7 Energy Conservation and Daylighting 5.8 Exercises 5.9 Bibliography

5.1 5.1 5.4 5.7 5.8 5.13 5.14 5.15 5.16 5.18 5.21 5.22 5.26 5.26 5.28 5.29

6.

6.1 6.1 6.2 6.2 6.3 6.10 6.18 6.20 6.25 6.29 6.35 6.36

Lighting Design Criteria and Interior Lighting 6.1 Introduction 6.2 Fundamentals of Interior and Exterior Lighting Basic Concepts Visibility Unified Glare Rating (UGR) Formula Ballast and Power Supplies 6.3 Methods of Lighting Design Uniformity of Illumination Point-by-Point or Point Method of Lighting Design Lighting Levels for Interior Lighting 6.4 Applications of Interior Lighting



CONTENT    xix

Residential Lighting Lighting Methods Office Lighting Lamps and Ballasts Specifications for Office Lighting The Effects of Video Display Terminal Use Lighting Levels for Office Lighting Educational Facility Lighting Selection of Lamps and Lumnaires Lighting Levels for Educational Facility Lighting Hospitality Facilities and Entertainmet Lighting Lighting Management Theatre, Television and Photographic Lighting Lamp Position Lighting Levels for Theatres, Concert Halls and Cinemas Photography Lighting Techniques Industrial Lighting Industrial Quality Issues Energy Efficient Lighting Electric Power Limits for Industrial and other Utility Spaces Lighting Levels for Industrial Lighting Retail Lighting Types of Retails and Their Lighting Retail Lighting Elements Basic Retail Lighting Intermediate Retail Lighting Higher-End Retail Lighting Lamp Spot Light Lighting Levels for Retail Lighting Jewelry Store Lighting Museum Lighting Lighting Design Using Computers 6.5 Exercises 6.6 Bibliography

6.36 6.38 6.50 6.56 6.56 6.60 6.61 6.63 6.67 6.68 6.81 6.83 6.86 6.91 6.91 6.94 6.97 6.99 6.101 6.102 6.106 6.106 6.106 6.108 6.109 6.110 6.111 6.112 6.112 6.116 6.123 6.124 6.125

Index I.1

Chapter

1 The Principles of Light and Optics

1.1 INTRODUCTION The purpose of this introductory book on the fundamentals of lighting and energy management is to provide necessary reading material needed for engineering and physics students and lighting professionals who are interested in the field of lighting. This will also be useful as a necessary input for a large lighting and allied industry workforce with an overview of the key principles of light and lighting which includes an understanding of basic design concepts and lighting technologies currently available, in the context of sustainability. This book will help users to understand the importance of energy efficiency and the implications of choosing a product in the overall scope of energy consumption. It also aims to assist users with the selection process when specifying, recommending, designing and installing various lighting systems. In residential environment, lighting energy consumption has increased dramatically with the popularity of halogen downlight systems, which are grossly inefficient when compared to the increasing range of residential fluorescent tubular as well as compact fluorescent lamps (CFLs). Halogen lamps are incandescent lamps and they are quite inefficient. This is an example of where improved lighting design knowledge may lead to more efficient installations. Qualification as a lighting designer requires a great depth of knowledge in the lighting design process. This knowledge is acquired through extensive, detailed training, covering a large variety of lighting design applications such as interior lighting, (differentiated into commercial, retail, industrial and residential), floodlighting, emergency and exit lighting, exterior lighting, road lighting, public lighting, facade lighting, and theatre lighting. We shall try to do justice to our readers by giving them the basic knowledge of the above mentioned concepts, however,

1.2    FUNDAMENTALS OF LIGHTING

the understanding of the detailed design methodology requires sustained efforts and practical experience. This book has few chapters which introduce the basic concepts of light and lighting and explores the key requirements of a lighting system and what standards need to be met. It also explains what we mean by sustainability and energy efficiency and how good lighting design can contribute to these. In recent years, as we all know, there has been a great deal of interest in the development of lighting technology and lighting research and teaching of lighting related issues all over the world. We all know that light is very crucial for our survival. Without light, life on the earth would not be possible. All our senses are, of course, important, but our sense of vision is recognized as being our most important link with the surroundings. More than 80 per cent of all the information around us reaches our brain through our eyes and it strongly influences our way of life. Our vision is only possible in the presence of light, the natural light due to the sun. The sunlight reflected from the moon and the man-made artificial light are now as important to us as food. The lighting is said to be of good quality when our eyes can clearly and pleasantly perceive the things around us. Today, the importance of the link between the functional, physiological and psychological basics of light is becoming increasingly more recognized. The ever more stringent requirements being placed on lighting together with the increasing significance being attached to the subjective appreciation of light as an environment-determining factor. The needs to employ economic lighting techniques are seen as positive stimulants in the human as well as national development process. Good lighting contributes to the quality of life and increases the comfort level and efficiency with which work functions are performed. A good lighting system provides light for quality which helps people to see well enough to carry out their visual tasks effectively, to enhance their environment and to stimulate their performance and well-being. This may be related to productivity in factories and offices; to sales promotion through the lighting of merchandise; to the creation of a suitable ambience in hotels and restaurants; to road traffic safety or to national pride and heritage when historical buildings are provided with a flood lighting installation. Energy management in lighting has gained importance since the early 1970s, stimulated by the escalation of energy costs, the gradual depletion of conventional energy resources and the concern for the protection of our environment. Lighting is the most conspicuous example of energy consumption. When inefficiently used, it leads not only to energy crisis but also to a reduction in human efficiency. In India, where the power supply has consistently fallen short of the demand, it accounts for as much as 20 to 22% of the total energy consumed in lighting. The end-use energy efficiency in lighting is observed to be poor in our country. It has been identified that electric lighting has tremendous hidden potential for energy conservation. Demand-side management programmes (DSM) offer both the cost-energy and environmental benefits, while modern developments in artificial lighting offer alternatives that contribute to DSM strategies.



THE PRINCIPLES OF LIGHT AND OPTICS    1.3

How to exploit fully this hidden potential towards energy savings? This is possible by effective lighting education imparted to engineers, technical experts and consumers. In the present competitive world, successful industries are those which seize the opportunity to optimize production, keep product quality high and minimize operating costs. Lighting is a productivity tool that can help to accomplish these goals and gain a competitive edge. Through investigations, it has been established that good quality lighting contributes to better productivity and profitability through better visual performance, less spoilage, lesser unit cost, lesser eye-strain, lesser accidents, less absenteeism. This also provides increased accuracy and better worker morale etc. It is observed that normally, the consumers visit the retailers, get baffled by the range of lighting fixtures displayed and invariably make a wrong choice by either leaning on the aesthetics or the ever important cost factor. But later, get disappointed when they cannot get the desired effect and satisfaction. They may blame the lamp/luminaire/their manufacturers without realizing that their choice was wrong. As far as lighting effects are concerned, “seeing is believing”. Even the lighting professionals like electrical contractors, architects or interior decorators may be experts in artistic visualization but usually lack technical knowledge. They also need to be educated on the technical aspects/application guidelines of novel lighting products. In a nut-shell, the lighting manufacturers (industries) may market their most priced and technically advanced lighting products. But if the people i.e., the end-users, retailers and professionals are not educated on the technical aspects, the users may not appreciate these products and the lighting companies may get a negative feedback/disappointing market response. What is the remedy? Continuous educational programmes on lighting through effective ‘Industry-Institute-Lighting Society’ interactions.

Why Lighting Research? The most important reason for introducing lighting research programmes in our engineering and technology institutions is that the lighting conditions of our surroundings have a very significant impact on the fundamentals of human life: health, wealth and safety. Another reason why lighting education and research should be undertaken is to conserve energy and wealth of our nation. So, the very purpose of lighting education and research is to provide a precise understanding of the role of lighting conditions in human health. It also provides safety and wealth through three routes: visual system, circadian system and perceptual system, so that appropriate lighting can be designed at minimum overall costs. Internationally it has been realized that further investment in lighting education and research is worthwhile as lighting is under attack from a number of causes: One that is likely to grow in magnitude, is the desire to reduce global warming effect (environmental protection).

1.4    FUNDAMENTALS OF LIGHTING

Further, lighting is a convenient means to reduce electricity demand throughout a country’s building stock. In this context, the lighting industries have been called upon to contribute to world-wide effort to combat energy crisis. Another area in which lighting is under pressure is exterior lighting where there is a growing trend for light at night to be seen as a form of pollution. The development that is most likely to change how we use and value lighting is the growing interest in the effects of light exposure on human health, both positive and negative. Hence, the future of lighting research lies in a move beyond visibility, visual performance and visual comfort to the areas where lighting affects the mood and behaviour through the ‘message’ it sends and on health and task performance through the circadian system. For the successful explorations in the above direction, what is desired is strong and healthy Industry-institute interaction. The Illuminating Engineering Society of North America (IESNA) seeks to improve the lighted environment by bringing together those with lighting knowledge and by translating that knowledge into actions that benefit the public. International Commission on Illumination (CIE) is an organization “devoted to international cooperation and exchange of information among its member countries on all matters relating to the science and art of lighting”. CIE works globally to develop and publish lighting design standardization and best-practice documents.

Quantum Jump in Lighting Technology Although India has made tremendous progress during the last 66 years of its independence, still it is relying on imported technology. This is true in almost all the areas but the situation is alarming in the field of lighting. There were only about half a dozen big lighting industries in the world until few years ago which were the major players in the international market including India. However, this situation is changing very rapidly now-a-days. Still the problem of advanced technology transfer in the field of lighting is a big hurdle. The socalled multinationals are not willing to give advanced technology to the Indian lighting industries or to the other developing countries. It is, therefore, very much desirable to develop our own technology by giving due importance to the research, innovation and development in the field of lighting. In the present context the notion of light or lighting, in general, is taken as the visible light only through which our eyes are able to see the objects around us. However, the light is not just the visible light but it constitutes a vast spectrum of electromagnetic radiation. This spectrum starts from high-energy radiation called Gamma rays. After this region is X-ray region, then is the Ultraviolet region which is also of high energy range. Then comes the visible region with which all of us are familiar. This region is only a small fraction of the wide electromagnetic spectrum of light. Ultraviolet (UV) and Infrared (IR) light play a very significant role in the present day world.



THE PRINCIPLES OF LIGHT AND OPTICS    1.5

UV light is used for analyzing minerals, pest control, spectro-photometry, biotechnology, biochemistry, photolithography, sterilization, disinfecting drinking water, food processing, fire detection, telecom applications, and a variety of industrial applications. On the other hand IR region is used for night vision for military applications and during warfare, thermography, industrial heating, spectroscopy, communications and so many other applications. Although there is a vast scope of Lighting Technology in India and the world at large but not much efforts have been made in India to impart this vital knowledge to the students by our existing universities/institutes of technology/engineering colleges. United States of America, however, realized the importance of lighting research and, therefore, an internationally renowned Lighting Research Center (LRC) is established in New York, USA, which is providing a number of courses pertaining to lighting science and related areas. There is a strong need for such institutions in India and other countries of the world as well.

1.2 THE SCIENCE AND THE NATURE OF LIGHT The science of light is optics which is a branch of physics. In optics we study the behavior and properties of light including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet and infrared light. Practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces. Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the ray-based model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that light waves were in fact electromagnetic radiation. Some phenomena depend on the fact that light has both wave-like and particle-like properties. Explanation of these effects requires quantum mechanics. When considering light’s particle-like properties, the light is modeled as a collection of particles called photons. Quantum optics deals with the application of quantum mechanics to optical systems. Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, and medicine particularly in ophthalmology and optometry. Practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fiber optics. We shall discuss more on these issues, but let us now discuss about the nature of light.

1.6    FUNDAMENTALS OF LIGHTING

What is Light? Light is an electromagnetic radiation which is most essential for the survival of all living beings. It is necessary for our vision, heating of the atmosphere around us as well as for the food production due to photosynthesis etc. The light is produced due to excitation of atoms on receiving energy in the form of heat or by any other method of excitation to higher energy levels and then deexcitation to lower energy level. In this process a photon of energy is emitted which is the difference of excited and de-excited energy levels. Reference of light could be found in almost all the ancient scriptures like Vedas, Bible etc. However, the scientific studies of light and the sky started much later. Leonardo Da Vinci (1452-1519) investigated the nature of light and studied reflection, refraction and mirrors. He studied the structure and anatomy of the human eye and compared it to the camera. Galileo Galilei (1564-1642) studied light and observed the sky with a telescope and in 1609, discovered that Jupiter had satellites and that Venus displayed phases like the moon. Although Galileo did not invent the telescope he did invent modern astronomy. Sir Isaac Newton (1642-1727), an English scientist and mathematician, greatly contributed to many fields of science including motion, gravity and optics. He was the first to formulate the corpuscular theory of light. Newton said that luminous bodies radiate energy in the form of particles or corpuscles, and that these particles travel in straight lines. The particles then act on the retina of the eye and stimulate the optic nerve and produce the sensation of vision in the brain. Newton was born the same year that Galileo died. In 1666 Newton at the age of 23, performed his famous prism experiment. He noticed that sunlight is white light that contains all the colors of the spectrum. In 1704 he published the first edition of his famous book ‘Opticks’. Earlier in 1687 he had published ‘Principia Mathematica’. Newton correctly identified the principle of refraction associated with his experiment in that light is bent as it travels from one medium to another at an angle, dependent on its wavelength. He didn’t know that he was repeating what Leonardo Da Vinci had noted down, in mirror writing, approximately 200 years earlier. Newton also tried to discover a link between light and color. He was able to show that visible light consists of seven colors - Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR). Color is an electromagnetic wave phenomenon. It is a sensation produced when light stimulates the retina of the eye, and the brain interprets this sensation as ‘color’. Early scientists always considered the primary colors to be relatively large areas of the spectrum: red, orange, yellow, green, blue and violet. However in 1666, Newton, named a 7th color located between blue and violet, as indigo. Newton's corpuscular theory was not able to explain interference and diffraction phenomena of light. This resulted in the abandonment of the Newton’s corpuscular theory of light. In 1678, Dutch physicist, Christiaan Huygens (1629-1695), proposed his wave theory of light. According to this theory light is made up of waves. Huygens further suggested that in a vacuum, or other uniform mediums, the light waves are spherical, and these wave surfaces advance or spread out as they travel at the speed of light. This became



THE PRINCIPLES OF LIGHT AND OPTICS    1.7

known as ‘Huygens’ principle’. This theory explains why light shining through a pin hole or slit will spread out rather than going in a straight line. Huygens theory was the successful theory of light wave motion in the three dimensions. Wave theory could explain interference and diffraction experiments very well. Huygens’ principle allows us to predict where a given wavefront will be in the future, if we have the knowledge of where the given wavefront is in the present. In 1803, Thomas Young studied the interference of light waves by shining light through a screen with two slits equally separated. The light emerging from the two slits, spread out according to Huygen’s principle. Eventually the two wave fronts overlapped with each other. The screen was placed at the point of overlapping of the waves. On the screen a series of dark and bright interference fringes were observed. Later in 1815, Augustin Fresnel supported Young’s experiments with mathematical calculations. Later on the extensive work in the field of interference and diffraction of light by Augustin Jean Fresnel (1788-1827) and Joseph von Fraunhofer (1787-1826) firmly established the wave nature of light. However; at a later stage even Huygens’ wave theory ran into trouble as it could not explain the phenomenon of polarization which suggests that the light wave must be transverse in nature and not longitudinal as suggested by Huygens.

Electromagnetic Theory of Light The electromagnetic wave theory of light was developed in the middle of 19th century. In 1865, James Clark Maxwell (1831-1879) provided a mathematical theory that showed a close relationship between electric and magnetic phenomena. His theory is based upon the following four previously known laws and principles of physics: 1.

Electric fields are generated by the electric charges. Further, the electric fields originate on the positive charges and terminate on negative charges (Coulomb’s law and Gauss’s law for Electrostatics).

2.

Magnetic field lines always form closed loops Gauss’s law for Magnetostatics).

3.

A varying magnetic field induces an emf and hence an electric field (Faraday’s law of electromagnetic induction).

4.

Magnetic fields are generated by moving charges (Ampere’s law).

Using symmetry principles of electric and magnetic fields, Maxwell had to modify Ampere’s law for stationary current by adding a term known as displacement current so that the modified Ampere’s law is applicable for both stationary and varying currents. Maxwell, thus, established four differential equations connecting electric and magnetic fields which are known as Maxwell’s equations. In this way Maxwell unified electric and magnetic fields. Maxwell’s equations are the backbone of Electrodynamics. By using vector algebra and manipulating his four equations Maxwell obtained electromagnetic waves. He, thus, laid the foundation of electromagnetic theory of light. Maxwell’s theory also predicted

1.8    FUNDAMENTALS OF LIGHTING

that electric and magnetic fields can move through the space as waves. He calculated their speed to be equal to the speed of light which is c = 3 × 108 m/s and concluded that light waves are electromagnetic in nature. According to Maxwell, light consists of fluctuating electric and magnetic fields which are perpendicular to each other as well as to the direction of propagation. The EM wave that travel through the space is a transverse wave as shown in Fig. 1.1. It was very unfortunate that Maxwell could not see the experimental verification of his EM theory of light during his life time as he died in 1879, while the experimental proof came in 1887. It was in 1887 that Heinrich Hertz (1857 –1894), an intelligent German physicist who died at a very young age of 36 years, clarified and expanded the electromagnetic theory of light that had been put forth by Maxwell. He was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect radio waves. In fact he was the first to generate and detect electromagnetic waves in a laboratory experiment. He also measured the speed of the electromagnetic waves, verifying Maxwell’s prediction that they travel at the speed of light. In order to honour Heinrich Hertz for his monumental work, the international scientific community decided to name the unit of frequency after his name as Hertz. Maxwell’s derivation of the electromagnetic wave equation has been replaced in modern physics by a much less cumbersome method involving combining the corrected version of ampere's circuital law with Faraday’s law of induction. To obtain the electromagnetic wave equation in a vacuum using the modern method, we begin with the modern form of Maxwell’s equations. In a vacuum and charge free space, these equations are: ∇ . E = 0

∇ × E = −



∇ . B = 0



∂B ∂t

∇ × B = µ0 € 0

...1.1 ...1.2 ...1.3

∂E ∂t

...1.4

In these equations E is electric field, B is magnetic field, ρ is charge density, µ0 is permeability and €0 is the permittivity of the free space. In vacuum ρ = 0, because there’s no charge density in free space. The symbol ∇ is known as the del operator. In the above Maxwell’s equations, the first equation represents Gauss’s law of electrostatics which is a generalized form of well known Coulomb’s law. The second equation represents Faraday’s law of electromagnetic induction. The third equation represents Gauss’s law of magnetostatics. This equation clearly shows that the lines of force in a magnet are closed loops and a monopole does not exist. The fourth and the last Maxwell’s equation represents modified Ampere’s law. This equation, as is well known, was modified by



THE PRINCIPLES OF LIGHT AND OPTICS    1.9

Maxwell to include the time varying fields using symmetry considerations of electric and magnetic fields. Taking the curl of the curl equations (second and fourth) we get 2      ∇ × ∇ × E = − ∂ ∇ × B = µ0 ε0 ∂ E ∂t ∂t 2

     ∇ × ∇ × B = µ0 ε0

∂ ∂2 B ∇ × E = – µ0 ε0 2 ∂t ∂t

…1.5 …1.6

By using the vector identity:     ∇ × (∇ × V) = ∇(∇ ⋅ V) – ∇2V

...1.7

where V is any vector function of space. Using the above identity and the other two previous equations we get the following wave equations:       

∂2 E 0 − c02 ∇ 2 E = 2 ∂t

...1.8

      

∂2 B 0 − c02 ∇ 2 B = ∂t 2

...1.9

µ0 ∈0 = 3 × 108 m/s is the speed of light

From above equations c0 = in free space.

Solutions to the homogeneous electromagnetic wave equations: The general solution to the electromagnetic wave equation is a linear superposition of waves. An electromagnetic wave of real angular frequency ω can be written as:

E(r, t) = E0ei(k.r – wt)

ei(k.r – wt)

B(r, t) = B0

…1.10 …1.11

The wave-vector k, indicates the direction of propagation of the wave, and also its phase-velocity n, via

ω …1.12 k Since the wave is transverse in nature, so we must have E0 ⋅ k = B0 ⋅ k = 0 This means that electric field amplitude E0, magnetic field amplitude B0 and the propagation vector k are mutually perpendicular to each other. Finally, the familiar Maxwell equation

n =

∂B …1.13 ∂t leads us to the following relation between the constant vectors E0 and B0 kˆ × E0 B0 = …1.14 v

∇ × E =

1.10    FUNDAMENTALS OF LIGHTING

A unit vector pointing in the direction of wave propagation is given by: kˆ = k/k

…1.15

In addition, for a valid solution, the wave vector and the angular frequency are not independent; they must adhere to the dispersion relation: |k| = k = w/c = 2p/l



…1.16

where k is the wave number and l is the wavelength. The Fig. 1.1 is a 3D diagram which shows a plane linearly polarized wave propagating from left to right. y E Direction of wave travel x z

B

Fig. 1.1  Electromagnetic Waves

There has been tremendous progress in the field of since and technology during the end of 19th century and middle of 20th century. We mention here some of these discoveries. Important discoveries during 19th and 20th century year-wise: l

1861- Black body radiation: Kirchhoff

l

1884- Black body radiation law by Stefan and Boltzmann

l

1887- Heinrich Hertz discovers the photoelectric effect

l

1888- Heinrich Hertz discovers radio waves

l

1893- Wien’s radiation law

l

1895-X-rays by Röntgen

l

1896- Becquerel discovers the radioactivity of uranium

l

1897- Joseph Thomson discovers the electron

l

1899- Rutherford discovers that uranium radiation is composed of positively charged alpha particles and negatively charged beta particles

l

1900 - Planck states his radiation law

l

1905- (i) Special theory of relativity –Albert Einstein and (ii) Photoelectric effect –Albert Einstein: he gave the theory of photoelectric effect using Planck’s theory of quantization of energy and introducing the concept of photon for light energy quanta.



THE PRINCIPLES OF LIGHT AND OPTICS    1.11 l

1911-Discovery of Atomic nucleus by Rutherford

l

1913- Niels Bohr developed the model of the atom

l

1913- Millikan measures the fundamental unit of charge

l

1924- Wave-particle duality by Louis de Broglie

l

1925- Pauli’s exclusion principle by Wolfgang Pauli

l

1925-27- Development of Quantum Mechanics by Erwin Schrödinger and Heisenberg, etc.

l

1927- Uncertainty principle by Werner Heisenberg

l

1932- James Chadwick discovers the neutron

l

1932- Carl Anderson discovers the positron

l

1933- Wolfgang Pauli proposes the existence of neutrino

l

1948- Quantum Electrodynamics was developed

l

1958- Charles Townes invents the laser

l

1963- Gell-Mann and George Zweig proposed the quark model

Quantum Theory of Light In the year 1900 Max Planck (1858–1947), a German physicist, developed the quantum theory of black body radiation. He assumed that the energy gained orreleased by a particle is given by:

En = nhn

where n is a number which can have values as 0, 1, 2, 3, etc., (an integer). ν is the frequency of vibration and h is the Planck’s constant. Above equation is known as the quantization law of energy. Later in 1905 Einstein used Planck’s theory of quantization of energy to explain Photoelectric Effect. Einstein named the quantum of energy as photon. We can, therefore, consider the light beam as a bundle of photons. Each photon may have energy as hn, 2 hn, 3 hn, etc. Max Planck is regarded as the founder of quantum theory for which he received the Nobel Prize in Physics in 1918. Planck’s theory is based on the following two important assumptions: (i) The energy is emitted and absorbed in discrete quanta (photons). (ii) The magnitude of each quantum, E, is determined by the product of h and n, where h is Planck’s constant (h = 6.626 × 10–34 J.s), n is the frequency of photon vibration in Hz, and E is the energy in Joules. This theory provides a means of determing the amount of energy in each quantum. It also follows that from this theory that energy increases with frequency.

1.12    FUNDAMENTALS OF LIGHTING

Light and the Energy Spectrum The electric field E and the magnetic field B in the travelling electromagnetic wave usually follow the sine wave form as shown in Fig. 1.2. The sine wave is the fundamental waveform in nature. When dealing with light waves, we generally refer to the sine wave. The period (T) of the waveform is one full 0 to 360 degree sweep.

Amplitude

Wavelength

Fig. 1.2  The Sine Wave Representing the Variation of E Field and B Field with Time

The electric and magnetic fields at a fixed point in space can be represented as: E = E0 sin ωt and B = B0 sin ωt



…1.17

where, w = 2pn = angular frequency of the wave. The relationship of frequency and the period is given below. Frequency

n = 1/T and T = 1/n;

The waveforms are always in the time domain and go on for infinity. Wavelength is the distance between two consecutive crests. Frequency is the number of oscillations per second. The speed of a wave can be found by multiplying the two units together. The wave’s speed is measured in units of length (distance) per second Wavelength × Frequency = Speed or c = n. λ. Taking the distance of the sun from Earth as approximately 150,000,000 km, and the speed of light as 300,000 km/s, we see that light reaches us in 8 minutes and 20 sec. With the use of the SI units for wavelength (λ), frequency (n) and speed of light (c), we can derive some simple equation relating to wavelength, frequency and speed of light:

λ = c/ν.

The entire spectrum of electromagnetic radiation is presented in Fig. 1.3.



THE PRINCIPLES OF LIGHT AND OPTICS    1.13 Visible light Radio waves Microwaves Infrared Ultraviolet X-rays Gamma Name of wave Wavelength 2 1 –1 –2 –3 –4 –5 –6 –7 –8 –9 –10 –11 –12 (meters)10 1 m 1m 10 10 10 10 10 10 10 10 10 10 10 10

Fig. 1.3  Electromagnetic Spectrum from Low Energy to Higher Energy Waves

We know that the light is an electromagnetic radiation, however, what we see as visible light is only a tiny fraction of the entire electromagnetic spectrum. This extends from very low frequency radio waves through microwaves, infrared, visible and ultraviolet light to X-rays and ultra energetic gamma rays. Our eyes respond to only visible light and that is termed as illumination. The wavelengths in the visible region are detected by the human eye and then interpreted by the brain as colors, ranging from red at the longest wavelengths of about 780 nm and violet at the shortest wavelengths of about 380 nm. The frequencies can be recognized as orange, yellow, green, and blue. Outside the visible range that the human eye is able to perceive are called ultraviolet (UV) at the short wavelength (high frequency) end and infrared (IR) at the long wavelength (low frequency) end. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see infrared light. As such for a common human being what is visible is referred to as Light, whereas in fact it is visible region of light. The study of light and its design dealing with the visible region is known as Illumination Engineering. However, now-a-days lot of progress has been made in the utilization of light in the non-visible region like ultraviolet (UV) and infrared (IR) region. Hence, a generalized form of study of light and lighting is known as Lighting Technology.

Applications of Ultraviolet Light The ultraviolet radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. The name UV means “beyond violet”, violet being the color of the shortest wavelengths of visible light. In some cases the UV wavelengths are colloquially called black light, as it is invisible to the human eye. Some animals, including birds, reptiles, and insects such as bees, can see into the near ultraviolet. Many fruits, flowers, and seeds stand out more strongly from the background in ultraviolet wavelengths as compared to human color vision. It is observed that the Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the atmosphere’s ozone layer, 99% of the ultraviolet radiation that reaches the Earth’s surface is UVA.

1.14    FUNDAMENTALS OF LIGHTING

UVA………. . 315 to 400 nm UVB ………. . 280 to 315 nm UVC …………100 to 280 nm Some of the important applications of the UV light are discussed below: (i) Pest Control

The UV fly traps are used for the elimination of various small flying insects. They are attracted to the UV light and are killed using an electrical shock or trapped once they come into contact with the device.

(ii) Spectrophotometry

The spectroscopy using UV light is widely used as a technique in chemistry, for analysis of chemical structure, most notably conjugated systems.

(iii) Analyzing Minerals

The UV lamps are also used in analyzing minerals, gems, and in other detective work including authentication of various collectibles.

(iv) Biochemistry The green fluorescent protein is often used in genetics as a marker. Several substances, proteins for instance, have significant light absorption bands in the ultraviolet. This technique are of use and interest in biochemistry and related fields. Now days the UV spectrophotometers are common in various optical laboratories. (v) Photolithography

In the electronics industry UV radiation is used extensively because photolithography is used in the manufacture of semiconductors, integrated circuit components and printed circuit boards.

(vi) Sterilization

The UV lamps lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. However, these lamps are used only as a supplement to other sterilization techniques.

(vii) Drinking Water Purification

UV radiation can be an effective viricide and bactericide. The disinfection using UV radiation is finding increased usage in drinking water treatment and is also used in wastewater treatment application.

(viii) Food Processing

Ultraviolet radiation is used in several food processes to remove unwanted microorganisms. UV light can be used to pasteurize fruit juices by pumping the juice over a high intensity ultraviolet light source

(ix) Fire Detection

The ultraviolet detectors generally use either a solid-state device, such as one based on silicon carbide or aluminum nitride, or a gas-filled tube as the sensing element.



THE PRINCIPLES OF LIGHT AND OPTICS    1.15

(x) Telecom Applications

This has wide practical significance now that semiconductor manufacturing processes are using wavelengths shorter than 200 nm.

(xi) Industrial Applications of UV

Some of the adhesives and coatings are formulated with photoinitiators. When exposed to the correct wavelengths of UV light, polymerisation occurs, and so the adhesives harden or cure. Some of the important applications include glass and plastic bonding, optical fiber coatings, the coating of flooring, and dental fillings, etc.

Range of Infrared (IR) Light The infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of microwave radiation. The name means “below red”, red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750 nm and 1 mm. The infrared region is divided as follows: Near (short wavelength) infrared ……………… 780 nm to 1400 nm Mid (medium wavelength) infrared……………. . 1400 nm to 3000 nm Far (long wavelength) infrared …………………3000 nm to 1 mm

Applications of IR Light (i) Night Vision

For this purpose infrared light is used when there is insufficient visible light to see an object. The radiation is detected and turned into an image on a screen, enabling the police and military to acquire thermally significant targets, such as human beings and automobiles.

(ii) Digital Cameras

Infared light from the LED of a remote control as seen by a digital camera. Digital cameras often use infrared blockers.

(iii) Thermography

Infrared radiation can be used to remotely determine the temperature of objects. This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry.



Thermography (thermal imaging) is mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to the massively reduced production costs.

1.16    FUNDAMENTALS OF LIGHTING

(iv) Heating

Infrared radiation is used for heating in various industrial applications.

(v) Communications

The IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants. The Infrared communications are useful for indoor use in areas of high population density.



In the free space optical communication infrared lasers can be a relatively inexpensive way to install a Gigabit/s communications link in urban areas, compared to the cost of burying fibre optic cable. Infrared lasers are used to provide the light for optical fibre communications systems. Infrared is the most common way for remote controls to command appliances. Today’s communication system is possible only through the use of light energy.



The distance learning using the broad band internet is possible only with the use of light energy.

(vi) Spectroscopy

For studies of the composition of (usually) organic compounds IR spectroscopy is utilized.



Besides the above mentioned applications of UV and IR, visible light has a vast area of applications, the most important, of course, for our vision.



Light is life, because without light we cannot survive. It gives us energy in the form of heat It helps us and also all other living beings to see around us. It helps plants to produce food through photosynthesis, etc.



In the present book we shall be studying various applications of light including large verities of artificial light sources, light control technology and applications, etc.

1.3 BLACK BODY RADIATION Introduction A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Black body radiation is distributed in energy in a manner set by the temperature T. A black body in thermal equilibrium emits electromagnetic radiation called black body radiation with a spectrum and intensity that is independent of the nature of the body and characterized only by its temperature (see Fig. 1.4). 1860.

The concept of a black body originally was introduced by Kirchhoff in



THE PRINCIPLES OF LIGHT AND OPTICS    1.17

Generally a black body in thermal equilibrium has two notable properties. A black body allows all incident radiation to pass into it and internally absorbs all the incident radiation. This is true of radiation for all wavelengths and for all angles of incidence. Hence the black body is a perfect absorber for all incident radiation. The black body radiation infact is a type of e.m. radiation for a body in thermodynamic equilibrium with its environment, or emitted by a black body held at constant temperature. Black body Radiation T = 300 K

Intensity

T = 250 K T = 200 K

Wavelength (cm)

Fig. 1.4  The Black Body Radiation

The radiation has a specific spectrum and intensity that depends only on the temperature of the body. An insulated enclosure that is in thermal equilibrium internally contains black body radiation and will emit it through a hole made in its wall, provided the hole is small enough to have negligible effect upon the equilibrium. A black body at room temperature appears black, as most of the energy it radiates is infrared and cannot be perceived by the human eye. It has been observed that at higher temperatures, black bodies glow with increasing intensity and colors that range from dull red to brilliant blue-white as the temperature increases. In view of the fact that the stars and the planets are neither in thermal equilibrium with their surroundings nor perfect black bodies, hence, black body radiation is used as a first approximation for the energy they emit.

The Black body Spectrum Black body radiation has a continuous wavelength or frequency spectrum that depends only on the body’s temperature, called the Planck spectrum or Planck’s law (see Fig. 1.5). It has been observed experimentally that the spectrum is peaks at a characteristic frequency that shifts to higher frequencies with increasing temperature, and at room temperature most of the emission is in the infrared region of the electromagnetic spectrum. As the temperature

1.18    FUNDAMENTALS OF LIGHTING

Fig. 1.5  Experimentally Observed Black Body Radiation Curves

increases above 500 degree centigrade, the black bodies start to emit significant amounts of visible light. Viewed in the dark, the first faint glow appears as a ghostly grey. It is found that with rising temperature, the glow becomes visible even when there is some background surrounding light-first as a dull red, then yellow, and eventually a dazzling bluish-white as the temperature rises. Further, when the body appears white, it is emitting a substantial fraction of its energy as ultraviolet radiation. The sun, with an effective temperature of approximately 5800°C, is an approximately black body with an emission spectrum peaked in the central, yellow-green part of the visible spectrum, but with significant power in the ultraviolet as well. It is absoluteley clear that the black body radiation provides insight into the thermodynamic equilibrium state of radiation from a cavity. The study of the laws of black bodies and the failure of classical physics to describe them helped establish the foundations of quantum mechanics. Likewise, calculating the black body curve was a major challenge in theoretical physics during the late nineteenth century. The problem was solved in 1900 by Max Planck in the formalism now known as Planck’s law of black body radiation. By making changes to Wien’s radiation law (not to be confused with Wien’s displacement law) consistent with thermodynamics and electromagnetism, he found a mathematical expression fitting the experimental data satisfactorily. Planck had to assume that the energy of the oscillators in the cavity was quantized, i.e., it existed in integer multiples of some quantity. Einstein built on this idea and proposed the quantization of electromagnetic radiation itself in 1905 to explain the photoelectric effect. All these theoretical advances eventually resulted in the superseding of classical electromagnetism by electrodynamics. These quanta were called photons and the black body cavity was thought of as containing a gas of photons. All this led to the development of quantum probability distributions, called Fermi–Dirac statistics and Bose–Einstein statistics, each applicable to a different class of particles, fermions and bosons, as we all know. As the temperature decreases, the peak of the black body radiation curve



THE PRINCIPLES OF LIGHT AND OPTICS    1.19

moves to lower intensities and longer wavelength regions. The black body radiation graph is also compared with the classical model of Rayleigh and Jeans. The wavelength at which the radiation is strongest is given by Wien’s displacement law, and the overall power emitted per unit area is given by the Stefan–Boltzmann law. So, as temperature increases, the glow color changes from red to yellow to white to blue. As the peak wavelength moves into the ultraviolet, enough radiation continues to be emitted in the blue wavelengths that the body will continue to appear blue. It will never become invisible—indeed, the radiation of visible light increases with temperature.

Stefan–Boltzmann Law The Stefan–Boltzmann law states that the power emitted per unit area of the surface of a black body is directly proportional to the fourth power of its absolute temperature. This law can be written as: J = sT4

…1.18

where J is the total power radiated per unit area. T is the absolute temperature and

s = 5.67 × 10−8

W m−2 K−4 is the Stefan–Boltzmann constant.

Planck’s Law of Black body Radiation Planck’s law states that I (n, T), the energy radiated per unit time per unit area of a black body is given by the relation:

I(n, T) = (2 hn3/c2) {1/(ehn/kT – 1)}

…1.19

where I(ν, T) is the energy per unit time (or the power) radiated per unit area of emitting surface in the normal direction per unit solid angle per unit frequency by a black body at temperature T; h is the Planck constant; c is the speed of light in a vacuum; k is the Boltzmann constant; n is the frequency of the electromagnetic radiation; and T is the temperature of the body in Kelvin.

Displacement Law of Wien The utility of this law is that it shows how the spectrum of black body radiation at any temperature is related to the spectrum at any other temperature. In case we know the shape of the spectrum at one temperature, at any other temperature

1.20    FUNDAMENTALS OF LIGHTING

we can calculate its shape. Utility of the Wien’s displacement law is that the wavelength at which the intensity of the radiation produced by a black body is at a maximum, λmax, it is a function only of the temperature: λmax = b/T

…1.20

Here, the constant, b, known as Wien’s displacement constant, is equal to 2. 8977721(26) × 10–3 km. Please note that the peak intensity can be expressed in terms of intensity per unit wavelength or in terms of intensity per unit frequency. The equation for the peak wavelength given above refers to the intensity per unit wavelength; meanwhile the Planck’s Law as expressed earlier was in terms of intensity per unit frequency. The frequency at which the power per unit frequency is maximized is given by:

nmax = T × 103.5 GHzK–1.

Atomic Structure and Energy Radiation The atomic model proposed by Niels Bohr in 1913 to explain the hydrogen spectrum is the backbone of the atomic theory. There have been several modifications and refinement particularly after the birth of quantum mechanics in 1925-27, even then the main ideas of the atomic radiation remain intact, though modified. The electrons in atoms have certain values of energy, and are not all the same. When an atom gains energy, for instance being heated such as flames, the electrons of the atom increase in energy levels. They will then drop back down and emit energy as a photon - a single packet of energy. The energy of the photon is equal to the difference in energy between the electron’s original energy level and its new level. The higher the drop down the ‘ladder’ the more energy the electron loses and so the more energy in the emitted photon. This changes it’s wavelength in the electromagnetic spectrum. That’s why metal first glows red, as it emits the lower red part of the spectrum, and increases to white hot, as it emits all different wavelengths. The combination of colors builds up to form the color white. Very hot stars can even glow blue, only emitting the top end of the spectrum. Each element in the periodic table emits and absorbs different parts of the spectrum, giving us the ranges of colors we see. In 1868, by looking at the parts of the spectrum emitted from the corona of the sun during a total eclipse, Helium was discovered, as it did not match any other element known to the man. Atoms emit radiations when they are excited. An atom can be excited in a different ways. Exothermic chemical reactions, collision of energetic particles with atoms, collision between atoms, by energetic incident photons etc. The electrons in an atom’s electron shell have specific energy levels. If we add energy to an atom, the atom will absorb a specific amount of energy, and the electron will jump up to a higher energy level. Each different



THE PRINCIPLES OF LIGHT AND OPTICS    1.21

element has its own energy levels, and it can only absorb energy in specific amounts. When we add a lot of energy to the atom, the atom becomes ionized, as one or more electrons absorb enough energy to break free of the atom completely, leaving the atom with an unbalanced positive electrical charge. When the excited electron releases energy, the electron drops back to its previous level, and the atom (or more specifically, the electron) emits a photon, which is a particle of light. Each photon has a frequency or energy that is distinctive to the element and the energy level. Electrons cannot have intermediate energies; they absorb and release exact “packets” or “quanta” of energy (see Fig. 1.6). This is how a mass spectrometer works; the operator ionizes a sample of the material that he wants to analyze, and watches the resulting spectrum. Each wavelength of light emitted by the sample corresponds to one specific element. The main assumptions of the Bohr’s theory for hydrogen atom are: l

The atom has energy that arises from the motion of electron about the nucleus, and from the interactions between the electron and the nucleus.

l

Only certain values of electronic energy are allowed for an atom, meaning thereby that the electronic energy is quantized.

l

A change of electronic energy level (state) of an atom involves the absorption or emission of a definite amount, quantum, of energy.

l

As a convention the lowest electronic energy state is called the ground state.

l

The state with energy greater than the ground state is an excited state.

l

When atoms absorb or emit light in moving from one energy state to another, the energy of the photon absorbed or emitted is related to the energies of the two states by the following equation:

ΔE = Efinal – Einitial = hn

…1.21

where h = Planck’s constant and ν = frequency of light. The allowed energy states of atoms and molecules can be described by sets of numbers called quantum numbers. The quantum numbers are associated with individual electrons in an atom.

Relation between Energy Difference, ΔE, and Wavelength The wavelength is inversely related to the energy difference, ΔE. A large ΔE will produce short wavelengths . . . . . . . . 700 nm—infrared. We shall now discuss little more about the atomic energy radiation. We all know that the atoms are the building blocks of matter.

1.22    FUNDAMENTALS OF LIGHTING

Matter is anything that has mass and takes up space (volume). All matter is made up of atoms. The atom has a nucleus, which contains particles of positive charge (protons) and particles of neutral charge (neutrons). Surrounding the nucleus of an atom are shells of electrons - small negatively charged particles. These shells are actually different energy levels and within the energy levels, the electrons orbit the nucleus of the atom. The emited photon

The absorbed photon

Fig. 1.6  Bohr’s Model of the Hydrogen Atom; Electron Orbits Around a Central Nucleus

The electron emits or absorbs the energy on changing the orbits. The ground state of an electron is the state of lowest energy for that electron. There is also a maximum energy that each electron can have and still be part of its atom. Beyond that energy, the electron is no longer bound to the nucleus of the atom and it is considered to be ionized. It is true that when an electron temporarily occupies an energy state greater than its ground state, it is in an excited state of the atom. An electron can become excited if it is given external energy such as if it absorbs a photon, or packet of light, or collides with another nearby atom or particle. The orbit described by a particular electron revolving about the nucleus is determined by the energy of that electron. In other words, there is a particular energy associated with each orbit. The system of orbits or energy levels is characteristic of each element and remains stable unless disturbed by external forces. The electrons of an atom can be divided into two classes. The first class includes the inner shell electrons, which are not readily excitable. The other class includes the valence electrons of an atom in a solid. When valence electron is removed from its normal orbit due to excitation, then the so called conduction electron enters the conduction band and confers on the solid the conduction property of electrical conductivity. Upon the absorption of sufficient energy by an atom in the gaseous state, the valence electro is pushed to a higher enrgy state. Eventually, the electron returns to the normal orbit, the ground or an intermediate state, and in so doing, the difference of the energy that the atom loses is emitted as a quantum of radiation. Each orbital has a specific energy associated with it. with its quantum number. The wavelength of the radiation emitted by an electron jump is determined by the Planck’s formula.



THE PRINCIPLES OF LIGHT AND OPTICS    1.23

According to Planck’s formula the photon energy is given by the relation:

E2 – E1 = hν

…1.22

Here, E2 is the energy associated with the excited orbit, E1 is energy associated with the normal orbit, h is the Planck’s constant, and ν is the frequency of the emitted radiation as the electron moves from level 2 to level 1.

Identifying Individual Types of Atoms It is observed that the transitions among the various orbitals are unique for each element because the energy levels are uniquely determined by the protons and neutrons in the nucleus. It is well known that different elements have different numbers of protons and neutrons in their nuclei. When the electrons of a certain atom return to lower orbitals from excited states, the photons they emit have energies that are characteristic of that kind of atom. This gives each element a unique fingerprint, making it possible to identify the elements present in a container of gas, or even a star. We can use tools like the periodic table of elements to figure out exactly how many protons, and thus electrons, an atom has. First of all, we know that for an atom to have a neutral charge, it must have the same number of protons and electrons. If an atom loses or gains electrons, it becomes ionized, or charged. The periodic table will give us the atomic number of an element. The atomic number tells us how many protons an atom has. For example, hydrogen has an atomic number of one - which means it has one proton, and thus one electron - and actually has no neutrons. Different forms of the same chemical element that differ only by the number of neutrons in their nucleus are called isotopes. Most elements have more than one naturally occurring isotope.

1.4 LIGHT GENERATION Natural Phenomena – Solar Radiation Sun is a biggest source of light on earth. The generation of electromagnetic radiation in the Sun is completely different from the processes utilized for producing artificial light on earth.

How Light is Produced in the Sun? Nuclear fusion is the process that produces light in the Sun. Nuclear energy can be produced in two ways namely nuclear fission and nuclear fusion. In atomic power plants nuclear fission is taking place in which U35 atom splits into fast moving lighter elements. Neutron particle is a key agent in this process. But in nuclear fusion energy is produced by fusing nuclei like hydrogen to more massive helium. This activity is happening in our Sun. The Sun is made of plasma comprising hydrogen and helium in the ionized form.

1.24    FUNDAMENTALS OF LIGHTING

Around 3/4th of the total mass of the Sun is made up of hydrogen, while most of the remaining 1/4th is helium. Other than these two gases, elements like iron, carbon, neon, oxygen, nickel, chromium, sulfur, magnesium, silicon and calcium are also found in trace amounts in the Sun. Hydrogen, which accounts for approximately, 74% of the total star, makes up 92% of its total volume. On the other hand, helium, which accounts for 24%, makes up only 7% of its volume. This hydrogen and helium in the Sun were produced as a result of the Big Bang. Among the two, hydrogen was the first element to be formed. Obviously it is very hard to produce energy through nuclear fusion on Earth as it requires very high temperature and pressure. Also the process has to overcome the repulsion between the same positively charged protons or other nuclei like deuterons or alpha particles. Fusion process involves low mass nuclei whose combined mass is more than the resulting fused massive nucleus. The loss of mass in the process is converted to energy according to Einstein’s law of conservation of mass, E = mc2. where m = loss of mass in the process and c = velocity of the light. Sun is a star and all stars are big balls of gas, primarily made up of hydrogen and helium. We can simply say that the nuclear fusion is happening in the Sun as burning of mostly found hydrogen atoms to form helium atoms. The core part of the Sun which extends up to 25% of the radius from its centre is existing with very hot temperature with about 1.5 crore degree centigrade and extreme pressure. At this temperature at the core of the Sun all the atoms of hydrogen and helium are in the ionized form. That is the place where nuclear fusion is happening. But fusion won’t immediately happen even at such high temperature and pressure as similar charge particles are repelling each other. Every fusion of hydrogen atoms in the Sun may require many number of collisions which makes the possibility of lasting it for more than 10 billion years. The nuclear fusion in the Sun can be explained by the following two processes. 1.

The Proton-Proton Chain Reaction



This is the dominant fusion process in the Sun. In a ‘proton-proton chain reaction’, hydrogen fuel is progressively converted into helium, along with release of energy in the process. In the first stage, p-p chain reaction is the fusing of two hydrogen nuclei into a deuterium (an isotope of hydrogen) nucleus. There are four alternative pathways by which the rest of the fusion process occurs to give us He4 . The difference between the ‘fusing masses’ (the four protons) and ‘fused mass’ (He4) is 0.7% of the total mass of 4 protons, which is converted into energy. The total energy produced by the fusion of 4 protons is 26.8 MeV.

2.

Carbon-Nitrogen-Oxygen (CNO) Cycle



This nuclear fusion process occurs very marginally in the Sun, but is the dominant fusion pathway in stars 1.5 times more massive, than our Sun.



THE PRINCIPLES OF LIGHT AND OPTICS    1.25



This process also fuses four protons into a helium nucleus, by using carbon, nitrogen and oxygen nuclei as catalysts. The end products of both the processes are the same. However, CNO cycle is dominant in stars with stellar cores much hotter than that of Sun (in the range of 13 million Kelvin). However, despite occurring at higher temperature, overall energy released through the whole reaction is again 26.8 MeV, which is more or less the same as for p-p cycle. Through both these processes, about 3.6 × 1038 protons are fused and converted into helium nuclei releasing 3.8 × 1026 Watts of energy every second by the Sun. Only a very small fraction of the solar energy is received by the earth. The earth's outer atmosphere receives at an average rate of about 1350 W/m2. About 75% of this energy reaches the earth’s surface at sea level on a clear day. There are some other natural sources of light like moon light and star light, etc.

Artificial Light Generation Generally light sources are divided into two types: 1.

Incandescent light sources

2.

Luminescent light sources

In actual fact the cause of light emission in both types of light sources is the same: ’Electronic transitions from higher level to lower energy states in the atoms’. However, the mode of electron excitation and the resultant spectral distribution of the radiation are different. We shall discuss this in what follows. (a) The Phenon of Incandescence

Incandescence is the phenomenon of the emission of light in the visible region from a hot body as a result of its temperature. The term is derived from the Latin verb incandesce, to glow white. Incandescence is a special case of thermal radiation. This generally refers specifically to visible light, while thermal radiation refers also to infrared or any other electromagnetic radiation. In practice, virtually all solid or liquid substances start to glow around 798 K (525°C), with a very dull red color, when no chemical reaction takes place that produce light as a result of an exothermic process.



At higher temperatures, the substance becomes brighter and its color changes from red towards white. Incandescence phenomenon is exploited in incandescent light bulbs, in which a filament is heated to a temperature at which a fraction of the radiation falls in the visible spectrum. The majority of the radiation, however, is emitted in the infrared part of the spectrum, rendering incandescent lights relatively inefficient as a light source.



If the filament could be made hotter, efficiency would increase; however, there are currently no materials able to withstand such temperatures

1.26    FUNDAMENTALS OF LIGHTING

which would be appropriate for use in lamps. More efficient light sources, such as fluorescent lamps and LEDs, do not function by incandescence. We all know that the sunlight is the incandescence of the its white hot surface. Incandescence phenomenon is also utilized in the production of halogen-tungsten lamps. We shall discuss in detail the application of the phenomenon of incandescence for the preparation of incandescent lamps in Chapter 3. (b) Phenomenon of Luminescence

The phenomenon of luminescence describes any process in which energy is emitted from a material at a different wavelength from that at which it is absorbed in the material. Electromagnetic radiation from luminescent sources results from the excitation of single valence electrons of an atom. In this case line spectra are obtained, such as those of mercury or sodium arcs. It is an umbrella term covering following important processes:

1. Photoluminescence

This process can be further subdivided into 4 following processes: (i)

Gaseous discharge

(ii) Fluorescence (iii) Phosphorescence (iv) Lasers 2. Electroluminescence

This process can further be subdivided into 3 following processes: (i)

Electroluminescent lamps (ac capacitive)

(ii) Light emitting diodes (LEDs) (iii) Cathodoluminescence (electron excitation)

Photoluminescence he phenomenon of photoluminescence is a process in which a substance T absorbs photons (electromagnetic radiation) and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. This is one of many forms of luminescence (light emission). The period between absorption and emission is typically extremely short, of the order of 10 nanoseconds. However, this period can be extended into minutes or hours using a different mechanism for excitation of the material.



THE PRINCIPLES OF LIGHT AND OPTICS    1.27

Gaseous Discharge The discharge of gases and its utility for developing gas discharge lamps started in 1675 when French astronomer Jean-Felix Picard observed that the empty space in his mercury barometer glowed as the mercury jiggled while he was carrying the barometer. Investigators, including Francis Hauksbee, tried to determine the cause of the phenomenon. It was Hauksbee who first demonstrated a gas-discharge lamp in 1705. With his demonstrations he showed that an evacuated or partially evacuated glass globe, in which he placed a small amount of mercury, while charged by static electricity could produce a light bright enough to read a book . The phenomenon of electric arc was first described by Petrov, a Russian scientist, in 1802; Sir Humphry Davy demonstrated in the same year the electric arc at the Royal Institution of Great Britain. Since then, discharge light sources have been researched because they create light from electricity considerably more efficiently than incandescent light bulbs. Later it was discovered that the arc discharge could be optimized by using an inert gas instead of air as a medium. Therefore, noble gases neon, argon, krypton or xenon were used, as well as carbon dioxide historically. The introduction of the metal vapor lamp, including various metals within the discharge tube, was a later advancement in the lamp industry. The heat of the gas discharge vaporized some of the metal and the discharge is then produced almost exclusively by the metal vapor. The usual metals are sodium and mercury owing to their visible spectrum emission. Hundred years of research later led to lamps without electrodes which are instead energized by microwave or radio frequency sources. In addition, light sources of much lower output have been created, extending the applications of discharge lighting to home or indoor use. We shall discuss gas discharge lamps in Chapter 3.

Fluorescence The fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence phenomena. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons. This two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. The emitted radiation may also be of the same wavelength as the absorbed radiation, termed as resonance fluorescence. The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye and the emitted light is in the visible region.

1.28    FUNDAMENTALS OF LIGHTING

Fluorescence phenomenon has many practical applications including mineralogy, gemology, chemical sensors (fluorescence spectroscopy), fluorescent labeling, dyes, biological detectors, and, most commonly in fluorescent lamps. Fluorescent lamps will be discussed in Chapter 3.

The Phenomenon of Phosphorescence This is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with forbidden energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation may be reemitted at a lower intensity for up to several hours after the original excitation. Commonly seen examples of phosphorescent materials are the glow-in-the-dark toys, paint, and clock dials that glow for sometime after being charged with a bright light such as in any normal reading or room light. Typically the glowing then slowly fades out within minutes or up to a few hours in a dark room. The study of phosphorescent materials led to the discovery of radioactivity in 1896. In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for glow-in-the-dark materials which are charged by exposure to light. Unlike the relatively swift reactions in a common fluorescent tube, phosphorescent materials used for these materials absorb the energy and store it for a longer time as the processes required to re-emit the light occur less often. It is possible that after an electron absorbs a photon of high energy, it may undergo vibrational relaxations and intersystem crossing to another spin state. Again the system relaxes vibrationally in the new spin state and eventually emits light by phosphorescence. Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, on the order of 10 nanoseconds. Light is absorbed and emitted at these fast time scales in cases where the energy of the photons involved matches the available energy states and allowed transitions of the substrate. In the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity usually a triplet state. As a result, the energy can become trapped in the triplet state with only classically forbidden transitions available to return to the lower energy state. These transitions, although forbidden, will still occur in quantum mechanics but are kinetically unflavored and thus progress at significantly slower time scales. Most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, allowing these substances to effectively store light energy in the form of very slowly degrading excited electron states.



THE PRINCIPLES OF LIGHT AND OPTICS    1.29

Lasers The term laser means light amplification by stimulated emission of radiation. The emitted laser light is notable for its high degree of spatial and temporal coherence, unattainable using other technologies. A laser light source is a device that emits light through a process of optical amplification based on the stimulated emission of photons. Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a pencil beam. Laser beams can be focused to very tiny spots, achieving a very high brightness. Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase which vary randomly with respect to time and position, and thus a very short coherence length. Infact the single wavelength lasers actually produce radiation in several modes having slightly different frequencies (wavelengths), often not in a single polarization. Although temporal coherence implies monochromaticity, there are even lasers that emit a broad spectrum of light, or emit different wavelengths of light simultaneously. There are some lasers which are not single spatial mode and consequently their light beams diverge more than required by the diffraction limit. However all such devices are classified as lasers based on their method of producing that light: stimulated emission. Lasers are employed in applications where light of the required spatial or temporal coherence could not be produced using simpler technologies. In 1958, Charles Townes and Arthur Schawlow theorized about a visible laser, an invention that used infrared and/or visible spectrum light.

How does Laser Light Differ from Other Light? Light, as we all know is an electromagnetic wave. Each wave has brightness and color, and vibrates at a certain angle, so-called polarization. This is also true for laser light but it is more parallel than any other light source. Every part of the beam has almost the exact same direction and the beam will therefore diverge very little. With a good laser an object at a distance of 1 km can be illuminated with a dot about 60 mm in radius. As it is so parallel it can also be focused to very small diameters where the concentration of light energy becomes so great that you can cut, drill or turn with the beam. It also makes it possible to illuminate and examine very tiny details. It is this property that is used in surgical appliances and in CD players. It can also be made very monochromic, so that just one light wavelength is present. This is not the case with ordinary light sources. White light contains

1.30    FUNDAMENTALS OF LIGHTING

all the colors in the spectrum, but even a colored light, such as a red LED (light emitting diode) contains a continuous interval of red wavelengths. On the other hand, laser emissions are not usually very strong when it comes to energy content. A very powerful laser of the kind that is used in a laser show does not give off more light than an ordinary streetlight; the difference is in how parallel it is.

What is Stimulated Emission? Normally atoms emit light more or less randomly and in random directions and phases. All light created in normal light sources, such as bulbs, candles, neon tubes and even the Sun is generated in this way. If energy is stored in the atom and light of the correct wavelength passes close by something else can happen. The atom emits light that is totally synchronous with the passing light. This means that the passing light has been amplified which is necessary for the oscillation taking place between the mirrors in a laser. Light emitted in this way goes in random directions, with random phases and at random times. Albert Einstein predicted in 1917 that there is also another way for light to be emitted. It can amplify a passing beam, provided three conditions are met: 1.

Energy is stored in the atom (same as above)

2.

Light passes close enough to the atom before the time has expired and the light is emitted in the random fashion described above

3.

The passing light has a wavelength suitable for the atom

The process taking place in this case is called Stimulated Emission, which, together with feedback in a resonant cavity between mirrors, forms the conditions for laser. The emitted laser light is notable for its high degree of spatial and temporal coherence, unattainable using other technologies. A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. Technically the spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a pencil beam. Laser beams can be focused to very tiny spots, achieving a very high degree of brightness. Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase which vary randomly with respect to time and position, and thus a very short coherence length. The single wavelength lasers actually produce radiation in several modes having slightly different frequencies or wavelengths, often not in a single polarization. Although temporal coherence implies monochromaticity, there are even lasers that emit a broad spectrum of light, or emit different wavelengths



THE PRINCIPLES OF LIGHT AND OPTICS    1.31

of light simultaneously. There are some lasers which are not single spatial mode and consequently their light beams diverge more than required by the diffraction limit. However all such devices are classified as lasers based on their method of producing that light through stimulated emission. Lasers are employed in applications where light of the required spatial or temporal coherence could not be produced using simpler technologies. Emission of photons from excited atoms occurs in certain manners which we shall discuss in what follows.

Spontaneous Emission If the excited atoms emit photons spontaneously, then this type of emission is known as spontaneous emission. When an atom in an excited state falls to a lower energy level, it emits a photon of light. The atoms typically remain excited for no longer than a few nanoseconds. The atoms can also absorb photons, making a transition from a lower level to a more excited one. Atoms can also absorb photons, making a transition from a lower level to a more excited one. This is, of course, absorption. Consider an atom in higher state (E2). It can decay to lower energy level by emitting a photon. Emitted photon have energy hv = E2 – E1. Life time of excited state is 10-9 sec. This process is known as spontaneous emission.

Spontaneous Absorption Let us consider two energy level having energy E1 and E2 respectively. The atom will remain in ground state unless some external stimulant is applied to it. When an EM wave, i.e., a photon of a particular frequency falls on it, there is finite probability that atom will jump form energy state E1 to E2. If this happens instaneously, then we call this process as spontaneous aborption.

Stimulated Emission There are metastable states i.e., transition from these states is not allowed according to selection rules. The life time of metastable states is ~10-3 sec. Atom in this state can’t jump to lower state at their own. When a photon of suitable freqency arrives, it sends the atom in the metastable unstable state. The emitted photon is in coherence with incident photon. Physical processes are shown in Fig. 1.7. The stimulated photons have unique properties. They are in phase with the incident photon of same wavelength and travel in same direction as incident photon.

1.32    FUNDAMENTALS OF LIGHTING

We show in the following diagram the three main processes for laser action: 1. Photon absorption 2. Spontaneous emission 3. Stimulated emission E2

E2

E2

h12

h12 h12

E1

E1

(a) Absorption

E1

(b) Spontaneous emission

h12 (in phase)

(c) Stimulated emission

Fig. 1.7  Stimulated Emission

Stimulated emission is further elaborated in the following Fig. 1.8. Stimulated Emission Electron excited state D Incident photon

E2 Stimulated Emission

(E = h) E1

On stimulation the electron form E2 state jumps to E1 state by releasing a photon of energy h = E2 – E1

Fig. 1.8  Stimulated Emission

Stimulated vs Spontaneous Emission Stimulated emission requires the presence of a photon. An incoming photon stimulates an atom in an excited state to decay to the ground state by emitting a photon. It has been observed experimentally that the stimulated photons travel in the same direction as the incoming photon. Spontaneous emission does not require the presence of a photon. Instead an atom in the excited state can relax to the ground state by spontaneously emitting a photon. Spontaneously emitted photons are emitted in all directions. In 1917, Einstein showed that another process, stimulated emission, can occur. These processes account for absorption and emission of radiation and the attainment of thermal equilibrium. The excited state can return to the lower state spontaneously as well as by a process stimulated by radiation already present at the transition frequency.



THE PRINCIPLES OF LIGHT AND OPTICS    1.33

Properties of Laser Light Lasers emit light that is highly directional. Laser light is emitted as a relatively narrow beam in a specific direction. Ordinary light, such as coming from the Sun, a light bulb, or a candle, is emitted in many directions away from the source. The light from a laser is said to be coherent, which means the wavelengths of the laser light are in phase in space and time.

Population Inversion The process of producing a population inversion is called pumping. Examples: by lamps of appropriate intensity →by electrical discharge. A state in which a substance has been energized or excited to specific energy level is known as the excited state. In population inversion more atoms or molecules are in a higher excited state. Initially the atoms are pumped from state E1 to E3 and then the atoms from excited energy level E3 make a very fast non-radiative transition to arrive at the metastable state E2. The life time of the energy states E3 and E2 are shown in Fig. 1.9. –8

E3

10 sec E2

–3

10 sec

E1

Fig. 1.9  Population Inversion

Achieving Inversion by Pumping the Laser Medium Pumping the laser medium: Let I be the intensity of (flash lamp) light used to pump energy into the laser medium. Will this intensity be sufficient to achieve inversion, N2 > N1? It’ll depend on the laser medium’s energy level system. In what energy levels do atoms reside? It all depends on the Boltzmann population factor. Boltzmann population factor Ni is the number density of molecules in state i (i.e. the number of molecules per cm3). Let T is the temperature, and kB is Boltzmann’s constant. Boltzmann population factor can be used as follows: In equilibrium, the ratio of the populations of the two states is:

1.34    FUNDAMENTALS OF LIGHTING

N2/N1 = exp (–E/kBT )

…1.23

E = E2 – E1 = hn.

where

As a result, higher-energy states are always less populated than the ground state, and absorption is stronger than stimulated emission. In the absence of collisions, atoms tend to remain in the lowest energy state available. Collisions can knock an atom into a higher-energy state. The higher the temperature, the more this happens.

Components of Laser 1.

Pumping Source



A pump is the basic energy source for a laser. It gives energy to various atoms of laser medium and excites them so that population inversion can take place and it is maintained with time. The excitation of atoms occur directly or through atom to atom collision.



There are various types of pumps depending upon the nature of medium. Examples are- electric discharges, flashlamps, arc lamps and chemical reactions.



The type of pump source used depends on the gain medium. →

A helium-neon (He-Ne) laser uses an electrical discharge in the helium-neon gas mixture.

→

Excimer lasers use a chemical reaction.

2.

Gain Medium



When energy is given to laser medium, a small fraction of medium shows lasing action. This part of laser medium is called Active Centers.



For example, in ruby laser Cr+++ is active center, in He-Ne laser Ne are active centers. It is the major determining factor of the wavelength of operation of the laser. Excitation by the pump source produces a population inversion, where spontaneous and stimulated emission of photons takes place.

Example: solid, liquid, gas and semiconductor lasers.

Optical Resonator It is a set up used to obtain amplification of stimulated photons, by oscillating them back and forth between two extreme limits. This consists of two plane or concave mirrors placed co-axially. One mirror is reflecting and other is partially reflecting. Light is reflected by the mirrors back into the medium and is amplified. The design and alignment of the mirrors with respect to the medium is crucial. Spinning mirrors, modulators, filters and absorbers may be added to produce a variety of effects on the laser output.



THE PRINCIPLES OF LIGHT AND OPTICS    1.35

Stimulated emission can lead to a chain reaction and laser emission. If a medium has many excited atoms, one photon can become many. This is the essence of the laser. The factor by which an input beam is amplified by a medium is called the gain and is represented by G.

Requirements for Laser Action 1.

Four-level laser system



In this case laser transition takes place between the third and second excited states. Rapid depopulation of the lower laser level takes place. Step-1: Pumping: atoms are excited to higher energy level by providing energy from external source. Step-2: Population Inversion: atom via radiation less decay, decays to metastable state and hence population inversion takes place Step-3: Laser Action: atom from metastable state decays to lower state by stimulated emission and hence laser action take place. Step-4: Back to Ground State: atom from excited state decays to lower state by spontaneous emission.

2.

Three-level laser system



Initially excited to a short-lived high-energy state. Then quickly decay to the intermediate metastable level. Population inversion is created between lower ground state and a higher-energy metastable state.

3.

Two-level laser system



Unimaginable as absorption and stimulated processes neutralize one another. The material becomes transparent. Even with very a intense pump source, the best one can achieve with a two-level system is excited state population = ground state population.

Types of Lasers We can classify lasers according to the active material: examples are solidstate, liquid, gas, excimer or semiconductor lasers. We can also classify lasers according to the wavelength: examples are -infrared, visible, ultraviolet (UV) or X-ray lasers. We can also classify lasers according to the nature of pumping: examples are- flash type, chemical pumping and electric discharge lasers. We can also classify lasers according to the nature of output: examples are- pulsed and continuous lasers.

Continuous vs. Pulsed Lasers Excitation of the lasing atoms is normally done by an external source of light (such as a lamp) or by another laser. The output of the laser light can be a

1.36    FUNDAMENTALS OF LIGHTING

continuous wave if the pumping is continuous or pulsed if the pumping is pulsed. Pulsed lasers have very high peak intensities because the laser intensity is concentrated in very short time duration.

Solid-State Laser - Ruby Laser A ruby laser generally consists of a ruby rod that must be pumped with very high energy, usually from a flashtube, to achieve a population inversion. The rod is often placed between two mirrors, forming an optical cavity, which oscillate the light produced by the ruby’s fluorescence, causing stimulated emission. Ruby is one of the few solid state lasers that produce light in the visible range of the spectrum, lasing at 694. 3 nanometers, in a deep red color, with a very narrow linewidth of 0. 53 nm. The ruby laser is a three level solid state laser. The active laser medium is a synthetic ruby rod that is energized through optical pumping, typically by a xenon flashtube. Ruby has very broad and powerful absorption bands in the visual spectrum, at 400 and 550 nm, and a very long fluorescence lifetime of 3 ms. This allows for very high energy pumping, since the pulse duration can be much longer than with other materials. While ruby has a very wide absorption profile, its conversion efficiency is much lower than other mediums. In early examples, the rod’s ends had to be polished with great precision, such that the ends of the rod were flat to within a quarter of a wavelength of the output light and parallel to each other within a few seconds of arc. The finely polished ends of the rod were silvered; one end completely, the other only partially. The rod, with its reflective ends, then acts as a Fabry–Pérot etalon. Modern lasers often use rods with antireflection coatings, or with the ends cut and polished at Brewster’s angle instead. This eliminates the reflections from the ends of the rod. External dielectric mirrors then are used to form the optical cavity. Curved mirrors are typically used to relax the alignment tolerances and to form a stable resonator, often compensating for thermal lensing of the rod. In short we can summarize ruby laser as follows: Ruby Laser: operating wavelength: 694.3 nm (IR); 3 level system: absorbs green/blue light; gain medium: crystal of aluminum oxide (Al2O3) with small part of atoms of aluminium is replaced with Cr3+ ions; Pump source: flash lamp. The ends of ruby rod serve as laser mirrors (Fig. 1.10).

Working Process The high-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of Cr3+ ions in the ruby crystal to higher energy levels. At a specific energy level, some Cr3+ emit photons see Fig. 1.11. At first the photons are emitted in all directions.



THE PRINCIPLES OF LIGHT AND OPTICS    1.37 Flashtube

Ruby Beam

Trigger electrode

Fig. 1.10  Ruby Laser System

Fig. 1.11  Three Level Energy Diagram for Ruby Laser

Photons from one Cr3+ stimulate emission of photons from other Cr3+ and the light intensity is rapidly amplified. The presence of mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification. The photons leave through the partially silvered mirror at one end. This is the laser light. As the flash lamp stops operating, the population of the upper level decreases very rapidly and lasing action stops till the further operation of next flash. As the production of laser beam depends upon the operation of flash lamp the ruby laser is pulsed type laser.

He-Ne Laser The Helium-Neon laser was the first continuous laser. It was invented by Javan et al. in 1961. He-Ne lasers will have to compete in the times to come with laser diodes. He-Ne lasers are still unequalled as far as beam geometry and the purity of the modes are concerned. Laser diodes will have to be improved to a great extent before they pose a serious threat to helium-neon laser. A helium-neon laser is a type of small power gas laser.

1.38    FUNDAMENTALS OF LIGHTING

He-Ne lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics. Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum. He-Ne lasers are normally small, with cavity lengths of around 15 cm up to 50 cm. The optical cavity of the laser typically consists of a plane, highly reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end. Electric discharge pumping is used. Optical output powers range from 1 mW to 100 mW.

Construction of He-Ne Laser The set up of He-Ne laser consists of a discharge tube of length 80 cm and bore diameter of 1. 5 cm. The gain medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 Pa per cm of cavity length) in a glass envelope. The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for CW operation. As already indicated above the optical cavity of the laser typically consists of a plane, highly reflecting mirror at one end of the laser tube. There is a concave output coupler mirror of approximately 1% transmission at the other end. He-Ne lasers are normally small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW (see Fig. 1.12). Partially silvered mirror

Fully silvered mirror

He + Ne

Laser output

Discharge electrodes

Fig. 1.12  He-Ne Laser Set Up

He-Ne laser system involves four state energy level transitions which are depicted in Fig. 1.13. The energy exchange between He and Ne atoms and the subsequent transitions from the excited states of Ne atoms are self explanatory in the diagram.



THE PRINCIPLES OF LIGHT AND OPTICS    1.39

E3

Energy exchange through atomic collions E

3.39 m

6

E2

Excitation due to collision with electrons

E4

E5

6328Å 1.15 m 0.6 m Spontaneous emission

E3 E2 E1 He

De-excitation by collision Ne

Fig. 1.13  Energy Level Diagram with Electronic Transitions in He-Ne Laser

Commercial He-Ne lasers are relatively small devices, among gas lasers, having cavity lengths usually ranging from 15 cm to 50 cm but sometimes up to about 1 meter to achieve the highest powers. The optical output power levels for them ranges from 0. 5 to 50 mW. The red He-Ne laser wavelength of 633 nm has an actual vacuum wavelength of 632.991 nm, or about 632.816 nm in air. The wavelength of the lasing modes lie within about 0. 001 nm above or below this value, and the wavelengths of those modes shift within this range due to thermal expansion and contraction of the cavity. Frequency-stabilized versions enable the wavelength of a single mode to be specified to within 1 part in 108 by the technique of comparing the powers of two longitudinal modes in opposite polarizations. Absolute stabilization of the laser’s frequency or wavelength as fine as 2.5 parts in 1011 can be obtained through use of an iodine absorption cell.

Applications of He-Ne Laser Red He-Ne lasers have many industrial and scientific uses. They are widely used in laboratory demonstrations in the field of optics in view of their relatively low cost and ease of operation compared to other visible lasers producing beams of similar quality in terms of spatial coherence and long coherence length. However since about 1990 semiconductor lasers have offered a lower cost alternative for many such applications. A consumer application of the red He-Ne laser is the Laser Disc player, made by Pioneer. The laser is used in the device to read the optical disk.

Carbon Dioxide Laser The carbon dioxide gas laser is capable of continuous output powers above 10 kilowatts. It is also capable of extremely high power pulse operating.

1.40    FUNDAMENTALS OF LIGHTING

It exhibits laser action at several infrared frequencies but none in the visible region. The helium-neon laser employs an electric discharge for pumping, using a percentage of nitrogen gas as a pumping gas. The CO2 laser is the most efficient laser, capable of operating at more than 30% efficiency. That’s a lot more efficient than an ordinary incandescent light bulb at producing visible light (about 90% of the output of a light bulb filament is invisible). The carbon dioxide laser finds many applications in industry, particularly for welding and cutting. The CO2 laser is a laser based on a gas mixture as the gain medium, which contains carbon dioxide (CO2), helium (He), nitrogen (N2), and possibly some hydrogen (H2), water vapour and/or xenon (Xe). The CO2 laser is electrically pumped via a gas discharge, which can be operated with DC current, with AC current (e.g. 20–50 kHz) or min the radio frequency (RF) domain. Nitrogen molecules are excited by the discharge into a metastable vibrational level and transfer their excitation energy to the CO2 molecules when colliding with them. Helium serves to depopulate the lower laser level and to remove the heat. Other constituents such as hydrogen or water vapor can help (particularly in sealed-tube lasers) to reoxidize carbon monoxide (formed in the discharge) to carbon dioxide. CO2 lasers typically emit at a wavelength of 10.6 μm, but there are other lines in the region of 9–11 μm (particularly at 9. 6 μm). In most cases, average powers are between some tens of watts and many kilowatts. The power conversion efficiency can be well above 10%, i.e., it is higher than for most lamp-pumped solid-state lasers, but lower than for many diode-pumped lasers. The CO2 laser was one of the earliest gas lasers to be developed by Kumar Patel of Bell Labs in 1964, and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 micrometers.

Procedure for the Construction of CO2 Laser Since CO2 lasers operate in the infrared, special materials are necessary for their construction. Typically, the mirrors are silvered, while windows and lenses are made of either germanium or zinc selenide. For high power applications, gold mirrors and zinc selenide windows and lenses are preferred. There are also diamond windows and even lenses in use. It is seen that the diamond windows are expensive, however their high thermal conductivity and hardness make them useful in high-power applications and in dirty environments. Further, the optical elements made of diamond can even be sand blasted without losing their optical properties. The lenses and windows were made out of salt of either sodium chloride or potassium chloride. While the material is inexpensive, the lenses and windows degrade slowly with exposure to atmospheric moisture. The set up for the CO2 laser consists of a gas discharge (with a mix close to that specified above) with a total reflector at one end, and an output coupler usually a semi-reflective coated zinc selenide mirror at the output end. The



THE PRINCIPLES OF LIGHT AND OPTICS    1.41

reflectivity of the output coupler is typically around 5–15%. The laser output may also be edge-coupled in higher power systems to reduce optical heating problems. The CO2 laser can be constructed to have CW powers between mW and hundreds of kW. It is also very easy to actively Q-switch a CO2 laser by means of a rotating mirror or an electro-optic switch. This give rise to Q-switched peak powers up to gigawatts (GW) of peak power. Because the laser transitions are actually on vibration-rotation bands of a linear triatomic molecule, the rotational structure of the P and R bands can be selected by a tuning element in the laser cavity. Because transmissive materials in the infrared are rather lossy, the frequency tuning element is almost always a diffraction grating. By rotating the diffraction grating, a particular rotational line of the vibrational transition can be selected. The finest frequency selection may also be obtained through the use of an etalon. In practice, together with isotopic substitution, this means that a continuous comb of frequencies separated by around 1 cm−1 (30 GHz) can be used that extend from 880 to 1090 cm−1. Such line-tuneable carbon dioxide lasers are principally of interest in research applications.

Important Applications of CO2 Laser Due to the high power levels available combined with reasonable cost for the laser, CO2 lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving. They are also very useful in surgical procedures because water which makes up most biological tissue absorbs this frequency of light very well. Some examples of medical uses are laser surgery, skin resurfacing, laser facelifts which essentially consist of burning the skin to promote collagen formation. Also, it could be used to treat certain skin diseases. Because the atmosphere is quite transparent to infrared light, CO2 lasers are also used for military range finding techniques. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available. They are also quite efficient: the ratio of output power to pump power can be as large as 20%. The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 micrometers.

Semiconductor Laser Semiconductor lasers use semiconductor as active medium. The majority of semiconductor materials are based on a combination of elements in the third group of the Periodic Table (such as Al, Ga, In) and the fifth group (such as N, P, As, Sb) hence referred to as the III-V compounds. Under forward bias (the p-type side is made positive) the majority carriers, electrons in the n-side, holes in the p-side, are injected across the depletion region in both directions to create a population inversion in a narrow active region. The light produced by radioactive recombination across the band gap

1.42    FUNDAMENTALS OF LIGHTING

is confined in this active region. Laser diode is similar in principle to an LED. The principle of semiconductor laser is very different from CO2 and He-Ne lasers. It is based on “Recombination Radiation”. We shall now explain this principle. Semiconductor lasers are based on semiconductor gain media, where optical gain is usually achieved by stimulated emission at an interband transition under conditions of a high carrier density in the conduction band.

Electron energy E

The physical origin of gain in a semiconductor (for the usual case of an interband transition) is illustrated in Fig. 1.14.

Conduction band Pumping

Light emission Valence band

Electron wave number k

Fig. 1.14  Principles of Semiconductor Lasers

Without pumping, most of the electrons are in the valence band. A pump beam with a photon energy slightly above the bandgap energy can excite electrons into a higher state in the conduction band. The electrons from here quickly decay to states near the bottom of the conduction band. At the same time, the holes generated in the valence band move to the top of the valence band. Electrons in the conduction band can then recombine with these holes, emitting photons with an energy near the bandgap energy. This process can also be stimulated by incoming photons with suitable energy. A quantitative description can be based on the Fermi–Dirac distributions for electrons in both bands. Most semiconductor lasers are laser diodes, which are pumped with an electrical current in a region where an n-doped and a p-doped semiconductor material meet. However, there are also optically pumped semiconductor lasers, where carriers are generated by absorbed pump light, and quantum cascade lasers, where intraband transitions are utilized.

What Function can a Photodiode Provide in the Process? It is attached to the inactive side to serve as a sensor for the power supply in order to provide an element of control of the laser output. Under forward bias (the p-type side is made positive) the majority carriers, electrons in the n-side, holes in the p-side, are injected across the depletion



THE PRINCIPLES OF LIGHT AND OPTICS    1.43

region in both directions to create a population inversion in a narrow active region. It is found that the light produced by radioactive recombination across the band gap is confined in this active region. In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band.

Basic Structure of p-n Junction Laser Diode The structure of a p-n junction diode laser is shown in Fig. 1.15.

Contact layer

Laser intensity profile

p+ – GaAs p – Ga1–yAlyAs

d ~ 0.2 m Output

Active layer GaAs n – Ga1–xAlxAs

Electrons Holes

Fig. 1.15  Semiconductor Laser

Epitaxial growth of a p-type layer on an n-type substrate GaAs, Ga(1-x) AlxAs, GaxIn(1-x)As(1-y) Py are often used for different wavelengths. Two parallel end faces are used as partially reflected mirrors. Achievement of population inversion and production of laser When p-n junction diode is forward biased, then there will be injection of electrons into the conduction band along n-side and production of more holes in valence band along p-side of the junction. Thus, there will be more number of electrons in conduction band comparable to valence band, so population inversion is achieved. Therefore, when the electrons and holes are injected into the junction region from opposite sides with forward biasing, then population inversion is achieved between levels near the bottom of the conduction band and empty levels near the top of the valence band. When electrons recombine with the holes in junction region, then there will be release of energy in the form of photons. This release of energy in the form of photons happen only in special types of semiconductors like GaliumArsenide (GaAs). Otherwise in semiconductors like silicon and germanium, whenever holes and electrons recombine, energy is released in the form of heat, thus Si and Ge cannot be used for the production of laser. The spontaneously emitted photon during recombination in the junction region of GaAs will trigger laser action near the junction diode. The photons emitted have a wavelength from 8200 Å to 9000 Å in the infrared region.

1.44    FUNDAMENTALS OF LIGHTING

Applications of Semiconductor Lasers Semiconductor lasers are numerically the most common laser type. They find wide use in telecommunication as easily modulated and easily coupled light sources for fiber optics communication processes. They are used in various measuring instruments, such as range finders. Another common use is in barcode readers. Visible lasers, typically red but later also green, are common as laser pointers. Both low and high-power diodes are used extensively in the printing industry both as light sources for scanning (input) of images and for very highspeed and high-resolution printing plate (output) manufacturing. Infrared and red laser diodes are common in CD players, CD-ROMs and DVD technology. Violet lasers are used in HD DVD and Blue-ray technology. The diode lasers have also found many applications in laser absorption spectrometry (LAS) for high-speed, low-cost assessment or monitoring of the concentration of various species in gas phase. High-power laser diodes are used in industrial applications such as heat treating, cladding, seam welding and for pumping other lasers, such as diode-pumped solid-state lasers. Uses of laser diodes can be categorized in various ways. Most applications could be served by larger solid-state lasers or optical parametric oscillators, but the low cost of massproduced diode lasers makes them essential for mass-market applications. Diode lasers can be used in a great many fields; since light has many different properties (power, wavelength, spectral and beam quality, polarization, etc. It is useful to classify applications by these basic properties. Many applications of diode lasers primarily make use of the directed energy property of an optical beam. In this category one might include the laser printers, barcode readers, image scanning, illuminators, designators, optical data recording, combustion ignition, laser surgery, industrial sorting, industrial machining, and directed energy weaponry. Some of these applications are well-established while others are emerging. Medicine and especially dentistry have found many new uses for diode lasers. The shrinking size of the units and their increasing user friendliness makes them very attractive to clinicians for minor soft tissue procedures. The 800 nm – 980 nm units have a high absorption rate for hemoglobin and thus make them ideal for soft tissue applications. Uses which may make use of the coherence of diode-laser-generated light include interferometric distance measurement, holography, coherent communications, and coherent control of chemical reactions. Uses which may make use of “narrow spectral” properties of diode lasers include range-finding, telecommunications, infrared countermeasures, spectroscopic sensing, generation of radio-frequency or terahertz waves, atomic clock state preparation, quantum key cryptography, frequency doubling and conversion. Some other applications include water purification (in the UV), and photodynamic therapy where a particular wavelength of light would cause a substance such as porphyrin to become chemically active as an anti-cancer agent only where the tissue is illuminated by lig.



THE PRINCIPLES OF LIGHT AND OPTICS    1.45

Uses where the desired quality of laser diodes is their ability to generate ultra-short pulses of light by the technique known as “mode-locking” include clock distribution for high-performance integrated circuits, high-peak-power sources for laser-induced breakdown spectroscopy sensing. Also used for arbitrary waveform generation for radio-frequency waves, photonic sampling for analog-to-digital conversion, and optical code-divisionmultiple-access systems for secure communication.

The Phenomenon of Electroluminescence Electroluminescence is an electro-optical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. This is distinct from black body light emission resulting from heat (incandescence), from a chemical reaction (chemiluminescence), or from a mechanical action (mechanoluminescence). Electroluminescence is the result of radiative recombination of electrons and holes in a material, usually a semiconductor. The excited electrons release their energy as photons - light. Prior to recombination, electrons and holes may be separated either by doping the material to form a p-n junction in semiconductor electroluminescent devices such as light-emitting diodes. Electroluminescence can also be produced through excitation by impact of high energy electrons accelerated by a strong electric field as with the phosphors in electroluminescent displays. This phenomenon is also used to produce displays. The Electroluminescent displays are a type of flat panel display created by sandwiching a layer of electroluminescent material such as GaAs between two layers of conductors. When current flows, the layer of material emits radiation in the form of visible light. Electroluminescence phenomenon works by exciting atoms by passing an electric current through them, causing them to emit photons. By varying the material being excited, the color of the light emitted can be changed. The actual electroluminescence display is constructed using flat, opaque electrode strips running parallel to each other, covered by a layer of electroluminescent material, followed by another layer of electrodes, running perpendicular to the bottom layer. This top layer must be transparent in order to let light escape. At each intersection, the material lights, creating a pixel.

LEDs (Light Emitting Diodes) The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Chargecarriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The wavelength of the light emitted, and thus its color depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials.

1.46    FUNDAMENTALS OF LIGHTING

The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light. LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors. LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. p-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate. Most materials used for LED production have very high refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. Thus, light extraction in LEDs is an important aspect of LED production, subject to much research and development. The work for LED began in 1907 when a British scientist, H. J. Round, for the first time, reported of a light-emitting solid-state diode. Unfortunately, this discovery produced no practical use for many decades. It was only in the 1970s that LED indicator lights started to become commercially available for use in appliances such as TVs, radios, and telephones. Although these LEDs were bright enough to use as indicators, they were not able to illuminate an area. As the technology started to advance, the light output of LEDs increased, and have now become bright enough to be used for illumination. Today, LED technology has provided us with LEDs which can be used in many lighting applications. We have LED flashlights, headlights, boat lights, and you can find many types of LED bulbs to replace the bulbs you have in your home. LEDs are bulbs without a filament that are low in power consumption and have a long life span. LEDs are just starting to rival conventional lighting, but unfortunately they just don’t have the output (lumen) needed to completely replace incandescent, and other types of light sources just yet. Never the less, technology is advancing every day, and it will not be long until the LED bulb will be the bulb of choice for most applications in the home and work place (see Figs. 1.16(a and b)).

Fig. 1.16 (a)  Light Emitting Diodes with Bulbs (b)  Single LED



THE PRINCIPLES OF LIGHT AND OPTICS    1.47

The LEDs do not have a filament that will burn out, and hence, they don’t get especially hot while in operation. They are special diodes that emit light when connected in a circuit. They are illuminated solely by the movement of electrons in a semiconductor material, and they last as long as a standard transistor. LED light bulbs give off directional light, which means that the light goes where the bulb is pointing. LEDs are closer to the color of daylight than incandescent bulbs, with an excellent Color Rendering Index of around 85. They are about 5 times more energy-efficient than florescent lamps, and can last up to 10 times as long. The LEDs are just starting to rival conventional lighting, but unfortunately they just don’t have the output (lumen) needed to completely replace incandescent, and other types of lamps just yet. Since they are new, hence, the price of LED lighting is still high enough to discourage many home owners. LEDs do dozens of different jobs and are found in all kinds of devices. They form numbers on digital clocks, transmit information from remote controls, light up watches and tell you whether you have turned on your appliances. They are used in television or illuminate a traffic light. As already indicated; LED is a p-n junction diode with a forward bias as shown in Fig. 1.17(a) along with its energy diagram.

p

n

IF +

V

–

Potential energy

Conductive band hc  =  Eg

Eg

–

–

+

+

–

–

–

Non radiative deep trap recombination

Radiative recombination +

–

+ +

+

Valence band

Distance

Fig. 1.17(a)  Diagram of an LED p-n junction

We are showing in Fig. 1.17(b) the internal circuit details of LED.

1.48    FUNDAMENTALS OF LIGHTING

Fig. 1.17(b)  LED's Circuit Symbol and Internal Details

Polarity of LED is indicated by size of its leads. The lead with longer length is positive and lead with short length is negative. Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don’t have a filament that will burn out. The lifespan of an LED surpasses the short life of an incandescent bulb by thousands of hours. Tiny LEDs are already replacing the tubes that light up LCD HD TVs to make dramatically thinner televisions. Light emitting diode (LED) is a special type of diode that emits light. It gives lights when current is passed through it. Since it does not require heating of filament or gas, it does not have the problem of burning out. The LED details and its internal components are shown in the above figure. When a 1.5 V battery is connected to the single load with LED then the current will flow from positive terminal through load to the negative terminal of the battery. We show a LED flasher circuits below which operates on a single 1.5 volt battery. The circuit uses the popular LM3909 LED flasher IC and requires only a timing capacitor and LED. As a consequence of current LED starts glowing (Fig. 1.18).

Fig. 1.18  LED Circuit



THE PRINCIPLES OF LIGHT AND OPTICS    1.49

LED Lamps The light emitted from LEDs varies from visible to infrared and ultraviolet regions. They operate on low voltage and power. LEDs are one of the most common electronic components and are mostly used as indicators in circuits.

LED Lamp with AC Supply In this case the lamp uses two circuits, one converts AC voltage into DC voltage and the second circuit supplies power to the LEDs. One disadvantage of this circuit is 6 LEDs are in series and if one gets burnt then the whole circuit stops functioning. It can be used as a night lamp or as a light source in place of any other lamp. This lamp can be used directly into 230V AC sockets (see Fig. 1.19).

Working of the LED Lamp This is the simple version of a white LED lamp that can be directly powered from mains. It can give ample light even for reading purpose. Capacitor CX along with diodes D1 through D4 forms the AC step down circuit. CX reduces high voltage AC from mains to a low voltage AC which is rectified by the diodes D1 to D4. Capacitor C1 removes ripples from AC so that low voltage DC is available to power the LEDs. CX is the X rated AC capacitor that reduces AC voltage through capacitive reactance property. Resistor R1 is very important to remove the stored voltage from CX when power is switched off. This prevents lethal shock. Resistor R2 limits the inrush current. More LEDs can be added by reducing the value of R2. Since the circuit is directly connected to the mains, take utmost care to avoid shock. No components should be touched when it is connected to mains. The circuit consumes low power up to 1 watt. R1 470 K 1 W

CX 472 K 400 V D1 230 V AC

D5

P

N

D3

D2 D6 D4

R2 470 R 1 W D1-D4 IN 4007

D7 C1 100 F 25 V

D8

D5-D8 White LED

Fig. 1.19  Low power LED Lamp Operating on 230 V

1.50    FUNDAMENTALS OF LIGHTING

Applications: 1.

Used as night lamp.

2.

It can also be used where less light is needed like in toilets, etc.

3.

More useful at the time of load sheding in rural areas.

LED lamp with DC Supply These lamps run on 12 V, 9 V, 6 V or 3 V battery and can be used in many household applications. A modern ‘A rated’ compact LED lamp with 3 watt power and operating with 12 V DC and having E 27 base fitting ideal for a huge range of low voltage lighting applications is shown in Fig. 1.20 below. The 3 W LED bulb provides a very useful light source which is a near equivalent to a regular 25 W domestic incandescent lamp with a massive 220 lumens. This is an ideal 12 volt lamp for low energy lighting in workshops, large garden sheds, outbuildings and remote applications as well as household, boating and caravans to name but a few.

Fig. 1.20  LED Lamp for 12 V DC Power Supply

Solid state electronics resists damage from high vibration environments. It's colour temperature is 4200°K and it gives out bright white light with average 220 lumens. The average lamp life is 50, 000 hours. Operating temperature: –10°C to +50°C. Lamp has built in Polarity Protection. Approximate equivalent light output (compared to regular incandescent lamps): 25W LED devices are becoming popular because they consume very less power than other light devices. LEDs have been used in electronics circuit for long time. They are available in red, yellow, green and multicolor and mainly used as indicators in electronic devices. But the new technological advancement makes it possible to have white LEDs. Super bright LEDs made it possible to get more light with very low power consumption.



THE PRINCIPLES OF LIGHT AND OPTICS    1.51

Therefore now LEDs find its use as a light source.

Fig. 1.21(a)  LED-tube light (b) A typical LED-bulb

LEDs are so far used in digital display, indicator on electronic instruments like TV, Computer. But now they started finding application in making bulb, torch, and emergency lamps, traffic signal, street lights and so on. A LED-tube light and also a LED-bulb are shown in Figs. 1.21(a) and (b)).

LEDs and their Advantages 1.

A Range of colors: LEDs are available in variety of colors like a violet, blue, yellow, green, orange, red and white.

2.

Efficiency: LED consumes very less energy they are very efficient than incandescent bulb.

3. Low maintenance: LED does not necessarily need maintenance. Their rated life is 10000 hrs. 4.

Durability: LEDs are extremely resistance to shock, vibration.

5.

The low operation voltage of LEDs eliminates sparks.

Disadvantage 1.

The viewing angle is less.

2.

Direct viewing into LED may damage your eyes.

LED Light Bulbs Many LED lamps have become available as replacements for screw-in incandescent or compact fluorescent light bulbs, ranging from low-power 5–60 watt incandescent bulbs. Conventional replacement bulbs for 60 watt incandescent bulbs typically require about 7 watts of power. The manufacturers now claim that the 16 watt LED bulb is as bright as a 150 W halogen lamp. According to the European Union Standard, an energy-efficient bulb that claims to be the equivalent of a 60 W tungsten bulb must have a minimum light output of 806 lumens. Only some models of LED bulbs are designed to work with dimmers as used for incandescent lamps. LED lamps often have directional light characteristics. These lamps have declined in cost in recent years. LED

1.52    FUNDAMENTALS OF LIGHTING

lamps are more power-efficient than compact fluorescent bulbs and offer lifespan of 30,000 or more hours, reduced if operated at a higher temperature than specified. Incandescent bulbs have a typical life of 1,000 hours, compact fluorescents about 8,000 hours. One year is taken on the average as 8,760 hours. The bulbs maintain output light intensity very well over their life-times. Energy Star Specifications require the bulbs to typically drop less than 10% after 6000 or more hours of operation, and in the worst case not more than 15%. They are also mercury-free, unlike fluorescent lamps. LED lamps are available with a variety of color properties. The higher purchase cost than other types may be more than offset by savings in energy and maintenance LED Tubes. Now LED tubes are available in various lengths. Several companies offer LED lamps and tubes for general lighting purposes. The technology is improving rapidly and new energy-efficient consumer LED lamps are available.

The Phenomen of Cathodoluminescence This is also an electro-optical phenomenon whereby a beam of electrons is generated by an electron gun as in a cathode ray tube. It then impacts on a luminescent material such as a phosphor, causing the material to emit visible light. The most common example is the screen of a television. In geology, mineralogy and materials science a scanning electron microscope with specialized optical detectors, or an optical cathodoluminescence microscope, is used to examine internal structures of semiconductors, rocks, ceramics, glass, etc., in order to get information on the composition, growth and quality of the material. Cathodoluminescence occurs because the impingement of a high energy electron beam onto a semiconductor will result in the promotion of electrons from the valence band into the conduction band, leaving behind a hole. When an electron and a hole recombine, it is possible for a photon to be emitted. The energy (color) of the photon, and the probability that a photon and not a phonon will be emitted, depends on the material, its purity, and its defect state. In this case, the semiconductor examined can, in fact, be almost any non-metallic material. In terms of band structure, classical semiconductors, insulators, ceramics, gemstones, minerals, and glasses can be treated the same way. In materials science and semiconductor engineering, cathodoluminescence will mostly be performed in either a scanning electron microscope or a scanning transmission electron microscope. In these cases, the highly focused beam of electrons impinges on a sample and induces it to emit light from a localized area. This light will be collected by an optical system, such as an elliptical mirror. From there, a fiber optic will transfer the light out of the microscope where it will be separated by a monochromator and then detected with a photomultiplier tube. By scanning the microscope’s beam in an X-Y pattern and measuring the light emitted with the beam at each point, a map of the optical activity of the specimen can be obtained. The advanced techniques are useful for examining low-dimensional semiconductor structures, such a quantum wells or quantum dots. In short, cathodoluminescence is a technique that can be implemented in an optical



THE PRINCIPLES OF LIGHT AND OPTICS    1.53

or electron microscope with the proper accessories, and allows the optical properties of non-metallic materials to be examined.

1.5 OPTICAL CONTROL OF LIGHT Optical control of light is mostly provided in a number of ways. However, all the methods used in the light control process use one or more of the following physical phenomena: Reflection, refraction, diffraction, interference, polarization, scattering and absorption. We shall discuss these phenomena.

Reflection and Reflectance of Light Reflection means the change in the direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection. Reflection is observed with many types of electromagnetic waves, besides visible light. Reflection of VHF and higher frequencies is important for radio transmission and for radar. Even hard X-rays and gamma rays can be reflected at shallow angles with special grazing mirrors. Reflection of light is either specular-mirror-like or diffuse (retaining the energy, but losing the image) depending on the nature of the interface (Fig. 1.22. Furthermore, if the interface is between a dielectric and a conductor, the phase of the reflected wave is retained; otherwise if the interface is between two dielectrics, the phase may be retained or inverted, depending on the indices of refraction. A mirror provides the most common model for specular light reflection, and typically consists of a glass sheet with a metallic coating where the reflection actually occurs. i=r

i

r

(a) Specular reflection

(b) Diffuse reflection

Fig. 1.22  Specular and Diffuse Reflection

1.54    FUNDAMENTALS OF LIGHTING

In the diagram above, a light ray strikes a mirror at a point, and the reflected ray is either going in a predetermined direction (specular reflection) or in any arbitrary direction (diffuse reflection). Specular reflection follows the law of reflection which states that angles i = r; or in other words, the angle of incidence equals the angle of reflection. In fact, reflection of light may occur whenever light travels from a medium of a given refractive index into a medium with a different refractive index. In the most general case, a certain fraction of the light is reflected from the interface, and the remainder is refracted. Solving Maxwell’s equations for a light ray striking a boundary allows the derivation of the Fresnel equations, which can be used to predict how much of the light is reflected, and how much is refracted. Total internal reflection of light from a denser medium occurs if the angle of incidence is above the critical angle. Total internal reflection is used as a means of focusing waves that cannot effectively be reflected by common means. X-ray telescopes are constructed by creating a converging tunnel for the waves. When light reflects off a material denser (with higher refractive index) than the external medium, it undergoes a polarity inversion. In contrast, a less dense, lower refractive index material will reflect light in phase. This is an important principle in the field of optics. Specular reflection helps to form images. Reflection from a flat surface forms a mirror image, which appears to be reversed from left to right because we compare the image we see to what we would see if we were rotated into the position of the image. Specular reflection at a curved surface forms an image which may be magnified or demagnified; curved mirrors have optical power. Such mirrors may have surfaces that are spherical or parabolic. If the reflecting surface is very smooth, the reflection of light that occurs is called specular or regular reflection. The laws of reflection are as follows: 1.

The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane.

2.

The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.

3.

The reflected ray and the incident ray are on both sides of the normal.

What is Reflectance? Most practical surfaces give a mixture of free and diffuse reflection properties. The amount of reflection at a surface is measured by the reflection factor or reflectance. Reflectance is the ratio of the luminous flux reflected from a surface to the flux incident upon the surface. Maximum reflectance has a value near 1, for light, shiny surfaces. Minimum reflectance has a value near 0, for dark, dull surfaces.



THE PRINCIPLES OF LIGHT AND OPTICS    1.55

Reflected components of light are an important factor in illumination. In optics and photometry, reflectance or reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength. However it is also commonly averaged over the reflected hemisphere to give the hemispherical spectral reflectivity (see Fig. 1.23. The reflectivity ρ(λ) can be expressed as:

r(l) = Grefl (λ)/Gincid (λ)

…1.24

where Grefl(λ) and Gincid(λ) are the reflected and incident spectral (per wavelength) intensity, respectively.

Fig. 1.23  Experimental Spectral Reflectance Curves for Aluminium (Al), Silver (Ag), and Gold (Au) Metal Mirrors at Normal Incidence [Source: Max Born and E. Wolf, Principles of Optics, 7th Edition, Elmsford, N. Y., Pergamon Press (1999)]

CIE (the International Commission on Illumination) has explained that the reflectivity is distinguished from reflectance by the fact that reflectivity is a value that applies to thick reflecting objects. It is seen that when reflection occurs from thin layers of material, internal reflection effects can cause the reflectance to vary with surface thickness. Reflectivity is the limiting value of reflectance as the surface becomes thick. It is the intrinsic reflectance of the surface, hence it is independent of other parameters such as the reflectance of the rear surface. Another way to interpret this is that the reflectance is the fraction of electromagnetic power reflected from a specific sample, while reflectivity is a property of the material itself, which would be measured on a perfect machine if the material filled half of all space. The reflectance spectrum or spectral reflectance curve is the plot of the reflectance as a function of wavelength.

1.56    FUNDAMENTALS OF LIGHTING

Refraction of Light When light travels through a medium, such as glass, diamond, or plastic, it travels at a different speed than in air or vacuum. The speed of light in a given material is related to a quantity called the index of refraction, n, which is defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v): Index of refraction: n = c/v

…1.25

Although as the speed changes, the wavelength changes, however the frequency of the light remains constant in a non-dispersive medium. The change in speed that occurs when light passes from one medium to another is responsible for the bending of light, or refraction, that takes place at an interface. If light is travelling from medium 1 into medium 2, and angles are measured from the normal to the interface, the angle of transmission of the light into the second medium is related to the angle of incidence by Snell’s law. In Fig. 1.24 the angles θ1, θ2 and θ1r represent angle of incidence, angle of refraction and the angle of reflection respectively.

Fig. 1.24  Refraction of Light

Thus, refraction of lightis described by Snell’s law, which states that

sin θ1/sin θ2 = v1/v2 = n2/n1

...1.26

In general, the incident wave is partially refracted and partially reflected; the details of this behavior are described by the Fresnel equations. Figure 1.24 shows the refraction of light at the interface between two media of different refractive indices, with n2 > n1 on the left side and reverse of it on the right side. Since the phase velocity is lower in the second medium (v2 < v1), the angle of refraction θ2 is less than the angle of incidence θ1; that is, the ray in the higher-index medium is closer to the normal. In optics, refraction is a phenomenon that often occurs when waves travel from a medium with a given refractive index to a medium with another at an oblique angle. At the boundary between the media, the wave’s phase velocity is altered, usually causing a change in direction. Its wavelength increases or decreases but its frequency remains constant. For example, a light ray will refract as it enters and leaves glass, assuming there is a change in refractive index. A ray travelling along the normal (perpendicular to the boundary) will change speed, but



THE PRINCIPLES OF LIGHT AND OPTICS    1.57

not the direction. Refraction still occurs in this case. Understanding of this concept led to the invention of lenses and the refracting telescope. Recently some metamaterials have been created which have a negative refractive index. With metamaterials, we can also obtain total refraction phenomena when the wave impedances of the two media are matched. There is then no reflected wave. Also, since refraction can make objects appear closer than they are, it is responsible for allowing water to magnify objects. First, as light is entering a drop of water, it slows down. If the water’s surface is not flat, then the light will be bent into a new path. This round shape will bend the light outwards and as it spreads out, the image you see gets larger.

Clinical Significance In optometry, ophthalmology and orthoptics, refraction (also known as refractometry) is a clinical test in which the appropriate eye care professional determines the eye’s refractive error for vision. The doctor uses this data for the best corrective lenses to be prescribed. A series of test lenses in graded optical powers or focal lengths are presented to determine which provides the sharpest, clearest vision.

Interference of Light Two or more light waves travelling in the same medium travel independently and can pass through each other. In regions where they overlap, we only observe a single disturbance. We call this phenomenon as interference. When two or more waves interfere, the resulting displacement is equal to the vector sum of the individual displacements. If two waves with equal amplitudes overlap in phase i.e., if crest meets crest and trough meets trough, then we observe a resultant wave with twice the amplitude. We call this constructive interference. If the two overlapping waves, however, are completely out of phase i.e., if crest meets trough, then the two waves cancel each other out completely. We call this destructive interference.

The Double Slit Experiment It was British scientist Thomas Young who in 1803 performed an interference experiment using double slits as shown in Figs. 1.25(a) and (b).

y d

 

L

Fig. 1.25(a)  Two Slits Acting as two Sources Produce Interference Pattern on the Screen

1.58    FUNDAMENTALS OF LIGHTING

P

r1

y

A d E

r2





C

D B

S L

Fig. 1.25(b) Young's Double Slit Experiment with Details of Path Difference Evaluation

Path difference = S = r2 – r1 = d sin θ If light is incident on to an obstacle which contains two very small slits a distance d apart, then the wavelets emanating from each slit will constructively interfere behind the obstacle. If we let the light fall onto a screen behind the obstacle, we will observe a pattern of bright and dark stripes on the screen. This pattern of bright and dark lines is known as a fringe pattern. The bright lines indicate constructive interference and the dark lines indicate destructive interference. The bright fringe in the middle of the diagram above is caused by constructive interference of the light from the two slits traveling the same distance to the screen. It is known as the zero-order fringe. Crest meets crest and trough meets trough. The dark fringes on either side of the zero-order fringe are caused by destructive interference. Light from one slit travels a distance that is 1/2 wavelength longer than the distance traveled by light from the other slit. Crests meet troughs at these locations. The dark fringes are followed by the first-order fringes, one on each side of the zero-order fringe. Light from one slit travels a distance that is one wavelength longer than the distance travelled by light from the other slit to reach these positions. Crest again meets crest. The diagram shows the geometry for the fringe pattern. If light with wavelength λ passes through two slits separated by a distance d, we will observe constructively interference at certain angles. These angles are found by applying the condition for constructive interference:

d sin θ = m λ, where m = 0, 1, 2.

…1.27

The angles at which dark fringes occur can be found be applying the condition for destructive interference, which is d sin θ = (m + 1/2)λ, m = 0, 1, 2.

…1.28

If the interference pattern is viewed on a screen a distance L from the slits, then the wavelength can be found from the spacing of the fringes. We have: sin θ = y/L, and hence, λ = y d/m L, (L >> y)

...1.29



THE PRINCIPLES OF LIGHT AND OPTICS    1.59

where y is the distance from the center of the interference pattern to the math bright line in the pattern. Using this experiment and by measuring the fringe width, we can measure the wavelength of light.

Applications of Optical Interferometry Interferometry has played an important role in the advancement of physics, and also has a wide range of applications in physical and engineering measurement. Interferometry has been used in defining and calibrating length standards. When the meter was defined as the distance between two marks on a platinum-iridium bar, Michelson and Benoît used interferometry to measure the wavelength of the red cadmium line in the new standard, and also showed that it could be used as a length standard. Sixty years later, in 1960, the metre in the new SI system was defined to be equal to 1, 650, 763. 73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. This definition was replaced in 1983 by defining the metre as the distance travelled by light in vacuum during a specific time interval. Interferometry is still fundamental in establishing the calibration chain in length measurement. Interferometry is used in the calibration of slip gauges (called gauge blocks in the US) and in coordinate-measuring machines. It is also used in the testing of optical components.

Diffraction of Light Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Diffraction was first observed by Francesco Grimaldi in 1665. He noticed that light waves spread out when made to pass through a slit. Later it was observed that diffraction not only occurs in small slits or holes but in every case where light waves bend round a corner. Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, x-rays and radio waves. As physical objects have wave-like properties (at the atomic level), diffraction also occurs with matter and can be studied according While diffraction occurs whenever propagating waves encounter an obstacle. Its effects are generally most pronounced for waves where the wavelength is roughly similar to the dimensions of the diffracting objects. If the obstructing object provides multiple, closely spaced openings, a complex pattern of varying intensity can result. This is due to the superposition, or interference, of different parts of a wave that travelled to the observer by different paths. The formalism of diffraction can also describe the way in which waves of finite extent propagate in free space. For example, the expanding profile of a laser beam, the beam shape of a radar antenna and the field of view of an ultrasonic transducer can all be analysed using diffraction equations. One of the most common examples of difraction in nature is the tiny specks or hair-like transparent structures, known as “floaters” that we can see when we look up at the sky. This illusion is produced within the eye-ball,

1.60    FUNDAMENTALS OF LIGHTING

when light passes through tiny bits in the vitreous humour. They are more prominently observed when one half-closes his eyes and peeps through them. The phenomenon of diffraction can be readily explained using Huygens’ principle. When the wavefront of a light ray is partially obstructed, only those wavelets which belong to the exposed parts superpose, in such a way that the resulting wavefront has a different shape. This permits bending of light around the edges. Colourful fringe patterns are observed on a screen due to diffraction. In the early 1800s, most of the people who wrote and submitted papers on diffraction of light were believers of the wave-theory of light. However, their views contradicted those of Newton’s supporters’ and there would be regular discussions between these two sides. One such person, who believed in the wave theory, was Augustin Fresnel, who in 1819, handed a paper to the French Academy of Sciences, about the phenomenon of diffraction. However, the Academy mainly consisting of Newton’s supporters, tried to challenge Fresnel’s point of view by saying that if light was indeed a wave, these waves, which were diffracted from the edges of a sphere, would cause a bright area to occur within the shadow of the sphere. This was indeed oberved later, and the area is today known as the Fresnel Bright Spot.

Diffraction by a Single Slit Let us assume a slit of width 'a' at which a parallel beam of light consisting of light rays of wavelength λ, is incident (refer to Fig. 1.26). According to Huygens ‘Principle each particle that is reached by the wavefronts of these waves becomes a source of secondary wavelets. By geometry of the figure we see that a distructive interference occurs when a sin θ = mλ and a dark fringe is observed on the screen.

Diffraction Grating In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions.

Fig. 1.26  A Single Slit Experimental Set Up



THE PRINCIPLES OF LIGHT AND OPTICS    1.61

The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element. Because of this, gratings are commonly used in monochromators and spectrometers. The principles of diffraction gratings were discovered by James Gregory, about a year after Newton’s prism experiments, initially with artifacts such as bird feathers. The first man-made diffraction grating was made around 1785 by Philadelphia inventor David Rittenhouse, who strung hairs between two finely threaded screws. This was similar to notable German physicist Joseph von Fraunhofer’s wire diffraction grating in 1821.

Screen (Wall)

y2 Diffraction grating

m=2

y1

m=1

Target holder

m=0

Laser pointer

m=1 m=2

D

Optical bench

(About 1m)

Fig. 1.27  Experimental Set Up of Grating with Laser Pointer

Instead of a single slit, or two narrow slits side by side, let us assume that we have a very large number of parallel slits all of the same width, and spaced at regular intervals. Such an arrangement is known as a diffraction grating which can be used for wavelength measurement (see Fig. 1.27). We can find the intensity of the light transmitted by the grating using the combined principles of interference and diffraction. If many slits are used the conditions for intense maxima (waves from all slits in phase) become more critical, and the maxima sharpen. The spacing between maxima, however, is still determined by the distance between slits of the grating. Thus, the distance between maxima depends on the distance between slits and the resolution, the relative sharpness of the maxima, depends on the total number of slits. Often a grating is characterized by the number of slits per unit length. From this information one can, of course, deduce the distance between the slits. When the condition [d sin θ = mλ] is satisfied for a grating used at normal incidence, the wavelength λ will give a maximum in intensity at the angle θ as measured from the normal to the grating.

1.62    FUNDAMENTALS OF LIGHTING

In above equation, d is the spacing between the grating slits and m is an integer called the order number. The number m = 0 is called the central maximum or zeroth order and corresponds to θ = 0 for all values of λ. For measuring the wavelengths of light, we need to know the spacing d. This is done by observing several emission lines of known wavelength from the element Mercury. In particular we should observe (1) a purple line at 4358.3 Angstroms, (2) a green line at 5460.7 Angstroms, (3) two yellow lines, one at 5769.6 Angstroms and the other at 5790.7 Angstroms. The Angstrom is a common unit of wavelength used in spectroscopy, 1 Angstrom = 0.1 nm. For measuring the wavelength of any light source including laser light, we need to put the light source as close as possible to the slit of the collimator of the spectrometer. Observe the central maximum and adjust the collimator slit to a moderate width. Centre the vertical cross hair on the slit image and focus the eyepiece. If we do not get a bright image, we need to move our source back and forth sideways until we do. The zeroth order angle should be close to 180. Test the ability to read the angle scale by reading this angle. By simply measuring the diffraction angle and knowing d, we can estimate the value of wavelength, λ.

Scattering of Light Scattering is a form of reflection in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media. The deviations from the law of reflection due to irregularities on a surface are also usually considered to be a form of scattering process. In case the irregularities are random and dense enough that their individual effects average out, then this kind of scattered reflection is commonly referred to as diffuse reflection. Most objects that one sees are visible due to light scattering from their surfaces. Indeed, this is our primary mechanism of physical observation. Scattering of light depends on the wavelength or frequency of the light being scattered. Since visible light has wavelength of the order of a micron, objects much smaller than this cannot be seen, even with the aid of a microscope. The transmission of various frequencies of light is essential for applications ranging from window glass to fiber optic transmission cables and infrared (IR) heat-seeking missile detection systems. Light propagating through an optical system can be attenuated by absorption, reflection and scattering. The interaction of light with matter can shed light on important information about the structure and dynamics of the material being examined. If the scattering centers are in motion, then the scattered radiation is Doppler shifted. An analysis of the spectrum of scattered light can thus yield information regarding the motion of the scattering center. The periodicity or structural repetition in the scattering medium will cause interference in the spectrum of scattered light. Thus, a study of the



THE PRINCIPLES OF LIGHT AND OPTICS    1.63

scattered light intensity as a function of scattering angle gives information about the structure, spatial configuration, or morphology of the scattering medium.

Types of Scattering 1. Rayleigh Scattering This is the elastic scattering of light by molecules and particulate matter much smaller than the wavelength of the incident light. It occurs when light penetrates gaseous, liquid, or solid phases of matter. Rayleigh scattering intensity has a very strong dependence on the size of the particles. It is proportional the sixth power of their diameter. It is inversely proportional to the fourth power of the wavelength of light. This means that the shorter wavelength in visible white light (violet and blue) are scattered stronger than the longer wavelengths toward the red end of the visible spectrum. This type of scattering is therefore responsible for the blue color of the sky during the day and the orange colors during sunrise and sunset. Rayleigh scattering is the main cause of signal loss in optical fibers.

2. Mie Scattering This is a broad class of scattering of light by spherical particles of any diameter. The scattering intensity is generally not strongly dependent on the wavelength, but is sensitive to the particle size. Mie scattering coincides with Rayleigh scattering in the special case where the diameter of the particles is much smaller than the wavelength of the light; in this limit, however, the shape of the particles no longer matters. Mie scattering intensity for large particles is proportional to the square of the particle diameter.

3. Tyndall Scattering This is similar to Mie scattering without the restriction to spherical geometry of the particles. It is particularly applicable to colloidal mixtures and suspensions.

4. Brillion Scattering This occurs from the interaction of photons with acoustic phonons in solids, which are vibrational quanta of lattice vibrations, or with elastic waves in liquids. The scattering is inelastic, meaning it is shifted in energy from the Rayleigh line frequency by an amount that corresponds to the energy of the elastic wave or phonon, and it occurs on the higher and lower energy side of the Rayleigh line, which may be associated with the creation and annihilation of a phonon. The light wave is considered to be scattered by the density maximum or amplitude of the acoustic phonon, in the same manner that X-rays are scattered by the crystal planes in a solid. In solids, the role of the crystal planes in this process is analogous to the planes of the sound waves or density fluctuations. Brillouin scattering measurements require the use of a high-contrast Fabry–Pérot interferometer to resolve the Brillouin lines from the elastic scattering, because the energy shifts are very small (< 100 cm−1) and very weak in intensity.

1.64    FUNDAMENTALS OF LIGHTING

Brillouin scattering measurements yield the sound velocities in a material, which may be used to calculate the elastic constants of the sample.

5. Raman Scattering This is another form of inelastic light scattering, but instead of scattering from acoustic phonons, as in Brillouin scattering, the light interacts with optical phonons, which are predominantly intra-molecular vibrations and rotations with energies larger than acoustic phonons. Raman scattering may therefore be used to determine chemical composition and molecular structure. Since most Raman lines are stronger than Brillouin lines, and have higher energies, standard spectrometers using scanning monochromators may be used to measure them. Raman spectrometers are standard equipment in many chemical laboratories. Static and dynamic scattering A common dichotomy in light scattering terminology is static light scattering versus dynamic light scattering. The chief distinction is whether the scattering is observed to be changing over time (dynamic) or constant over the observation (static). This terminology is especially commonly encountered in the field of polymer chemistry, though λ it can obviously be applied to a broad range of situations.

Polarization of Light As we all know, light is an electromagnetic wave. The electric field of a light wave propagating in the z-direction is given by E = E0 exp(kz – ωt). Light wave is a transverse wave. Therefore, E is a vector lying in the plane perpendicular to z, E = (Ex, Ey). Polarized light is produced when the direction of E in the plane perpendicular to the direction of propagation is constrained in some fashion. The electric field vector E can always be resolved into two perpendicular components. The light is called an elliptically polarized, when the two components have a constant phase difference and the tip of the electric field vector traces out an ellipse in the plane perpendicular to the direction of propagation. Ex = E0xexp(i(kz – ωt)), Ey = E0yexp(i(kz – ωt + φ))

…1.30

Linearly polarized light is a special case of elliptically polarized light. If the light is linearly polarized, then the two components oscillate in phase, Ex = E0xexp(i(kz – ωt)), Ey = E0yexp(i(kz – ωt)), j = 0. The direction of E and the direction of propagation define a plane. The electric vector traces out a straight line. For example, E = Ei = E0x exp(i(kz – ωt)). Circularly polarized light is also a special case of elliptically polarized light in which E0x = E0y and the two components have a 90° phase difference. In this case the electric field vector traces out a circle in the plane perpendicular to the direction of propagation. When viewed looking towards the source, a right circularly polarized beam has a field vector that describes



THE PRINCIPLES OF LIGHT AND OPTICS    1.65

a clockwise circle, while left circularly polarized light has a field vector that describes counter clockwise circle.

Unpolarized Light Most sources of electromagnetic radiation contain a large number of atoms or molecules that emit light. The orientation of the electric fields produced by these emitters may not be correlated, in which case the light is said to be unpolarized. If there is partial correlation between the emitters, the light is partially polarized. If the polarization is consistent across the spectrum of the source, partially polarized light can be described as a superposition of a completely unpolarized component, and a completely polarized one. One may then describe the light in terms of the degree of polarization, and the parameters of the polarization ellipse. In unpolarized light j, Ey, Ex are randomly varying on a timescale that is much shorter than that needed for observation (Fig. 1.28). Natural light is, in general, unpolarized

Fig. 1.28  Unpolarized Light

Polarization Mechanisms Dichroism Certain naturally occurring crystalline materials have transmittance properties which depend on the polarization state of light. Dichroism is the selective absorption of one plane of polarization in preference to the other, orthogonal polarization during transmission through the material. The most common method of producing polarized light is to use polaroid material, made from chains of organic molecules, which are anisotropic in shape. Light transmitted is linearly polarized perpendicular to the direction of the chains. If linearly polarized light passes through polaroid material, then the transmitted intensity is given by the following Law of Malus:          It = I0cos2 θ

…1.31

Here θ is the angle between E and transmission direction. If θ = 90° the transmitted intensity is zero. A polarizer produces linearly polarized light. It is often convenient to orient the transmission axis of a polarizer vertically or horizontally to produce light with vertical or horizontal linear polarization (Fig. 1.29).

1.66    FUNDAMENTALS OF LIGHTING

Fig. 1.29  Vertical and Horizontal Polarization

Polarization by Reflection When unpolarized light is incident on a boundary between two dielectric surfaces, for example, on an air-glass boundary, then the reflected and transmitted components are partially plane polarized. The reflected wave is 100% linearly polarized when the incident angle is equal to the Brewster angle.

Brewster’s Angle Brewster’s angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized. This special angle of incidence is named after the Scottish physicist, Sir David Brewster (1781–1868). When light encounters a boundary between two media with different refractive indices, some of it is usually reflected as shown in the figure above. The fraction that is reflected is described by the Fresnel equations, and is dependent upon the incoming light’s polarization and angle of incidence (Fig. 1.30).

Fig. 1.30  Brewster's Angle (An Illustration of the Polarization of Light which is Incident on an Interface at Brewster’s Angle)

The Fresnel equations predict that light with the p polarization(electric field polarized in the same plane as the incident ray and the surface normal) will not be reflected if the angle of incidence is

θB = arc tan (n2/n1)

…1.32



THE PRINCIPLES OF LIGHT AND OPTICS    1.67

In this equation n1 and n2 are the refractive indices of the two media. This equation is known as Brewster’s law, and the angle defined by it is Brewster’s angle. The physical mechanism for this can be qualitatively understood from the manner in which electric dipoles in the media respond to p-polarized light. One can imagine that light incident on the surface is absorbed, and then reradiated by oscillating electric dipoles at the interface between the two media. The polarization of freely propagating light is always perpendicular to the direction in which the light is travelling. The dipoles that produce the transmitted (refracted) light oscillate in the polarization direction of that light. These same oscillating dipoles also generate the reflected light. However, dipoles do not radiate any energy in the direction of the dipole moment. Consequently, if the direction of the refracted light is perpendicular to the direction in which the light is predicted to be specularly reflected, the dipoles cannot create any reflected light. With simple geometry this condition can be expressed as: θ1 + θ2 = 90° where θ1 is the angle of incidence and θ2 is the angle of refraction. Using Snell’s law: n1 sin θ1 = n2 sin θ2 One can calculate the incident angle θ1 = θB at which no light is reflected. n1 sin θB = n2 sin (90 – θB) = n2 cos θB Solving for θB gives:

θB = arc tan (n2/n1)

For a glass medium (n2 ≈ 1.5) in air (n1 ≈ 1), Brewster’s angle for visible light is approximately 56°, while for an air-water interface (n2 ≈ 1.33), it is approximately 53°. Since the refractive index for a given medium changes depending on the wavelength of light, Brewster’s angle will also vary with wavelength. The phenomenon of light being polarized by reflection from a surface at a particular angle was first observed by Etienne-Louis Malus in 1808. He attempted to relate the polarizing angle to the refractive index of the material, but was frustrated by the inconsistent quality of glasses available at that time. In 1815, Brewster experimented with higher-quality materials and showed that this angle was a function of the refractive index, defining Brewster’s law. Brewster’s angle is often referred to as the polarizing angle, because light that reflects from a surface at this angle is entirely polarized perpendicular to the incident plane (s-polarized). A glass plate or a stack of plates placed at Brewster’s angle in a light beam can thus be used as a polarizer. When the sun is at a low angle in the sky during the sunrise and sunset, the sunlight reflecting off the surface of water is nearly 100% horizontally polarized because the angle of incidence is close to the Brewster angle.

1.68    FUNDAMENTALS OF LIGHTING

In a practical application for glare-reduction, the sunglasses are coated with a polarizer with a vertical transmission axis and therefore block the reflected light.

Use of Polarized Light The polaroids are used in sun glasses. They reduce the intensity and the glare by cutting down the horizontally polarized light. The polarized light is used in photo elastic stress analysis. For 3-D view, special type of glasses, which have polaroids with perpendicular axes, are used. Polarized light is useful to determine size and shape of viruses. This polarized light finds many practical applications in industry and engineering science.

Uses of Polaroids The polaroids polarize light. A number of needle shaped crystals quinine iodosulphate with their axes parallel to one another are packed between two sheets of plastic. This arrangement serves as the polaroids. Some of the important uses are discussed below. These reduce excess glare and hence sun glasses are fitted with Polaroid sheets. These are also used to reduce headlight glare of cars. They are used to improve color contrast in old oil paintings. These are useful in 3-D motion pictures i.e., holography. The wind shields of automobiles are also made of Polaroid sheets for glare protection. Now let us define the terms s and p of polarization. Generally a coordinate system relates to the plane made by the propagation direction and a vector perpendicular to the plane of a reflecting surface. This is known as the plane of incidence. The component of the electric field parallel to this plane is termed p-like (parallel) and the component perpendicular to this plane is termed s-like (from senkrecht, a German word for perpendicular). Light with a p-like electric field is said to be p-polarized, pi-polarized, tangential plane polarized, or is said to be a transverse-magnetic (TM) wave. Light with an s-like electric field is s-polarized, also known as sigma-polarized or sagittal plane polarized, or it can be called a transverse-electric (TE) wave. However, there is no universal convention in this TE and TM naming scheme, and certain authors do refer to light with p-like electric field as TE and light with s-like electric field as TM. The polarized sunglasses use the principle of Brewster’s angle to reduce glare from the sun reflecting off horizontal surfaces such as water or road, etc. In a large range of angles around Brewster’s angle the reflection of p-polarized light is lower than the s-polarized light, as is generally observed. Thus, if the sun is low in the sky reflected light is mostly s-polarized. Some sunglasses



THE PRINCIPLES OF LIGHT AND OPTICS    1.69

known as polarized glasses use a polarizing material such as polaroid film to block horizontally-polarized light, preferentially blocking reflections from horizontal surfaces. The effect is strongest with smooth surfaces such as water, but reflections from road and the ground are also reduced. The photographers use the same principle to remove reflections from water so that they can photograph objects beneath the surface. In this case, the polarizing filter camera attachment can be rotated to be at the correct angle s-surface polarized; p-plane polarized.

Absorption of Light The absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom in the substance. Here the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is often called attenuation in the substance. Usually, the absorption of waves does not depend on their intensity (linear absorption), although in certain conditions (usually, in optics), the medium changes its transparency dependently on the intensity of waves going through, and the saturable absorption (or non-linear absorption) occurs. In the absorption process , the frequency of the incoming light wave is at or near the energy levels of the electrons in the matter. The electrons will absorb the energy of the light wave and change their energy state. There are several options as what can happen next, either the electron returns to the ground state emitting the photon of light or the energy is retained by the matter and the light is absorbed. If the photon is immediately re-emitted the photon is effectively reflected or scattered. If the photon energy is absorbed the energy from the photon typically manifests itself as heating the matter up. The absorption of light makes an object dark or opaque to the wavelengths or colors of the incoming wave. Wood is opaque to visible light. Some materials are opaque to some wavelengths of light, but transparent to others. The glass and water are opaque to ultraviolet light, but transparent to visible light. By which wavelengths of light are absorbed by a material the material composition and properties can be understood. Another manner that the absorption of light is apparent is by their color. If a matter absorbs light of certain wavelengths or colors of the spectrum, an observer will not see these colors in the reflected light. On the other hand if certain wavelengths of colors are reflected from the material, an observer will see them and see the material in those colours. For example, the leaves of green plants contain a pigment called chlorophyll, which absorbs the blue and red colors of the spectrum and reflects the green. Leaves therefore appear green. Let the initial intensity of light is I0.

1.70    FUNDAMENTALS OF LIGHTING

The intensity of light after passing through an optical system for distance x can be attenuated by absorption and let the intensity now is Ix. The exponential law of absorption is the basic working relationship and we have: Ix = I0. e–αx.



...1.33

where α is called attenuation or absorption coefficient.

1.6 EXERCISES Q. 1 What is light? Explain how it is produced? Discuss any of its two phenomena which are useful for lighting. Q. 2 What do you mean by electromagnetic spectrum? Give the approximate range of light wavelengths corresponding to different colours. Mention some of the applications of UV and IR radiations for the lighting industry. Q. 3 Explain the following Phenomena of light with their applications for lighting:

(i)

Reflection and Reflectance



(ii) Refraction



(iii) Diffraction



(iv) Absorption



(v) Polarization



(vi) Scattering of light.

Q. 4 Write short notes on:

(i)

Raleigh and Mie Scattering



(ii) Black body Radiation.

Q. 5 What do you mean by light? Explain how it is generated in an incandescent lamp. Q. 6 Write short notes on any five from the following:

(a) Electromagnetic spectrum



(b) Black body radiation



(c)



(d) Scattering of light.

Reflection of light

Q. 7 Answer the following:

(a) What do you mean by electromagnetic spectrum? Give the range of light wavelengths corresponding to visible region. Mention some of the applications of UV radiations for the lighting industry.



(b) Reflectance of a surface and its use in lighting.



THE PRINCIPLES OF LIGHT AND OPTICS    1.71

Q.8 Choose the correct answers for the following objective type questions:

(i)



The light has velocity c in air. What will be its velocity in a glass having refractive Index as 1.5? Ans. (a) c/3 (b) 2c/3 (c) 3c/2.

[Ans: (b)]

(ii) Incandescent lamps don’t need ballast because:



(a) The current is regulated by increase in resistance of tungsten wire



(b) The lamp has inert gas which controls the current



(c) The glass bulb regulates the current.

[Ans: (a)]

1.7 BIBLIOGRAPHY 1.

C.S. Williams and O.A. Buklund, Optics: A short course for Engineers and Scientists, John Wiley, N.Y.(1983).

2.

Max Born and E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, 7th Edition, Elmsford, N.Y., Pergamon Press (1999).

3.

A. Nussbaum and R.A. Phillips, Contemporary Optics for Scientists and Engineers, Englewood Cliff, N.J., Prentice –Hall, Inc (1976).

4.

T. Tamir, Integrated Optics, 2nd Edition, New York, Springer-Verlag (1971).

5.

B.A. Lingyel, Lasers, 2nd Edition, N.Y., John Wiley and Sons, Inc.(1966).

6.

H.G. Kuhn, Atomic Spectra, 2nd Edition N.Y. Academic Press, Inc.(1969).

7.

Beiser, Concept of Modern Physics, 6th Edition, Tata McGraw-Hill, New Delhi (2003).

8.

A. Stimson, Photometry and Radiometry for Engineers, New York, WileyInterscience (1974).

9.

Mark S. Rea, Editor-in-chief, IESNA Lighting Handbook: Reference and Application, 9th ed., New York (2000).

10. M.A. Cayless and A.M. Marsden, “Lamps and Lighting², 3rd edition, Oxford and IBH Publishing Co., New Delhi (1983).

Chapter

2 Light and its Measurement

2.1 INTRODUCTION Accurate measurement of needed quantities is the most important factor in the progress and development of science and technology. Lord Kelvin (1824-1907), a famous British scientist, has expressed this idea in the following words: “When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be”. The human visual system responds only in the range of wavelengths from 380 to 780 nm, depending on the individual observer. It should be kept in mind that for any source of illumination, the radiant energy produced is rarely limited to wavelengths within these boundaries. Although the primary concern is the measurement of radiation that results in visual sensation. The measurements of radiant quantities outside the visible spectrum are also important because of the use of nonvisual radiation in various scientific and technological applications. Optical radiation generally refers to all radiation that can be measured using certain techniques and equipment (mirrors, lenses, filters, diffraction gratings, prisms, etc.). Thus visible, ultraviolet (UV), and infrared (IR) radiation are collectively considered as optical radiation. The measurement of optical radiation is called radiometry.

Radiometry It is the science of measuring radiant quantities without regard for the visual effects of the radiation.

2.2    FUNDAMENTALS OF LIGHTING

Light almost always refers to wavelengths visible to humans, although sometimes invisible radiation is also called light when describing radiation on plants or on skin.

Photometry It is a special branch of radiometry, the measurement of radiation in terms of human visual response. The Commission Internationale de l’Éclairage (CIE) has established a standard observer response curve (also known as the photopic luminous efficiency function), denoted by V(λ) (to be discussed in detail in next section).This standard observer response curve, with its peak at approximately 555 nm, is used as a standard weighting function that, when applied to a spectral power distribution (SPD) of the light being measured, is an approximation of the perceived brightness of that light. The standardization of the eye spectral sensitivity function is the key to photometry, removing the influence of the observer from the measurements. However, despite the industry-wide acceptance of this function, one should recognize that it represents a compromise in assuming a predictable correlation of physical measurements with visual response, and that there are some circumstances where the system works poorly.

2.2 PRINCIPLES OF PHOTOMETRY AND RADIOMETRY Radiometry, as explained above, is the subject concerned with the measurement of the energy content in optical radiation and how this energy flows through optical systems. Photometry is the subject of the measurement of the energy content in optical radiation that can induce a visual response and the quantification of that response through the human visual system. The measures and units of the various quantities used in radiometry and photometry will be described below. Photometry, as explained above is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy (including light) in terms of absolute power whereas in photometry, the radiant power at each wavelength is weighted by a luminosity function (i.e., visual sensitivity function) that represents the human brightness sensitivity. Although the scotopic function may also be applied in the same way when the light is dim. Photometry is a very important segment of lighting industries. Usually every lighting industry has a photometric laboratory where scientists measure various useful data of lamps and luminaires. The main aim of the photometry is to measure visible optical radiation (light) in such away that the results correlate with what the visual sensation is to a normal human observer exposed to that radiation. This is done in view of the fact that the artificial light by and large is to be used by the human beings.



LIGHT AND ITS MEASUREMENT    2.3

Until about 1940, visual comparison techniques of measurements were predominant in photometry, whereby an observer was required to match the brightness of two visual fields viewed either simultaneously or sequentially. This method of photometry is called visual photometry, and is seldom used today. In modern photometric practice, measurements are made with photodetectors. This is referred to as physical photometry. In order to achieve the aim of photometry, one must take into account the characteristics of human vision. The relative spectral responsivity of the human eye was first defined by CIE (Commission Internationale de l’Éclairage) in 1924 and redefined as part of colorimetric standard observers in 1931. The purpose of photometry in lighting is to accurately describe the performance of a luminaire ( light source + fixture including all the accessories) to enable the designer to select the lighting equipment and to design a fixture layout which best meets the needs of the job. The photometric distribution curve is one of the lighting designers most valuable tools. It is a cross sectional map of intensity (candelas) measured at many different angles. It is a two dimensional representation and therefore shows data for one plane only. If the distribution of the unit is symmetric, the curve in one plane is sufficient for all calculations. If asymmetric, such as with street lighting and fluorescent units, three or more planes are required. In general, incandescent and HID reflector units are described by a single vertical plane of photometry. Fluorescent luminaires require a minimum of one plane along the lamp axis, one across the lamp axis and one at a 45° angle. The greater the departure from symmetry, the more planes are needed for accurate calculations. The coefficient of utilization (CU) or the utilization factor(UF) refers to the ratio of lumens which ultimately reach the working plane to the total lumens generated by the lamp. CU values are necessary for calculating the average illuminance levels for designing, and are very important part of the photometry of luminaires. The manufacturers of the luminaires are obliged to follow the CIE guidelines and provide the photometric data of their luminaires such as CU table, etc., to the consumers. A utilization curve is usually provided for luminaire units intended for outdoor use or units with a distribution radically asymmetric. A CU table is provided for units which are used primarily indoors, where the lumen or zonal cavity method of calculation applies. Use of CU data will be discussed further in Chapter 6. Isofootcandle charts are often used to describe the light pattern when a fixture produces a distribution other than symmetric. These charts are derived from the candlepower data and show exact plots or lines of equal footcandle levels on the working plane when the fixture is at a designated mounting height. Use of isofootcandle charts in determining illuminance at designated points will be discussed later. Spacing criteria provides the designer with information regarding how far apart to space luminaires and maintain acceptable illumination uniformity on the working plane. Criteria for spacing is generally conservative i.e., it takes into account the direct component of illumination only. We shall discuss this issue further in Chapter 6.

2.4    FUNDAMENTALS OF LIGHTING

Photometry and the Eye Human eye is very significant for photometric measurements. However; the sensitivity of human eye varies with light wavelengths. This is precisely the region that the photometric instruments are designed in such a way that they take into account the human eye sensitivity curve. A curve of the human eye’s response to light as a function of wavelength is known as the luminosity function, V(λ) (Fig. 2.1). In most cases, the region 380 nm to 780 nm is used for calculation with negligible errors because the V(λ) function falls below 10-4 outside this region. Thus, a photodetector having a spectral responsively matched to the V(λ) function replaced the role of human eyes in photometry. Likewise, V(λ) function replaced the role of human eyes in photometry. Note that the eye has different responses as a function of wavelength when it is adapted to light conditions (photopic vision) and dark conditions (scotopic vision). In photopic vision eye is most sensitive at 555 nm wavelength, whereas in scotopic vision it is most sensitive at 507 nm wavelength.

Fig. 2.1  Eye Luminosity Function (V(λ) Curve) - Photopic and Scotopic Visibility Curves

The graph of intensity plotted against wavelength for the dark-adapted eye is called the scotopic curve and the light-adapted eye is called the photopic curve.

2.3 PHOTOMETRIC AND RADIOMETRIC QUANTITIES There are a number of photometric and radiometric quantities used for different purposes. We wish to discuss them along with their applications. We are familiar with the idea that the adjective heavy can refer to weight or density, which are fundamentally different things.



LIGHT AND ITS MEASUREMENT    2.5

Similarly, the adjective bright can refer to a light source which delivers a high luminous flux (measured in lumens), or to a light source which concentrates the luminous flux it has into a very narrow beam (candelas), or to a light source that is seen against a dark background. The light propagates through three-dimensional space–spreading out, becoming concentrated, reflecting off shiny or matte surfaces. Since, light consists of many different wavelengths, the number of fundamentally different kinds of light measurement that can be made is large, and so are the numbers of quantities and units that represent them.

Luminous Flux The radiant power is the total radiated power in watts, also called radiant flux. This power must be factored by the sensitivity of the human eye to determine luminous flux in lumens. In the visible wavelength range, namely 380 nm to 780 nm, radiant flux is considered to have associated with it a luminous flux, Φ, which is a measure of the visual response. The unit of luminous flux is the lumen (lm) and is defined in terms of the candela. A point source emitting a uniform intensity of 1cd in all directions emits a total flux of 4p lm. Alternatively, the lumen can be defined as the luminous flux associated with a radiant flux of (1/683) Watt at a wavelength of 555 nm in air; at any other wavelength the associated luminous flux is V(λ) lm. To find the luminous flux dΦ associated with a spectral radiant flux ψ over a wavelength range dλ, the procedure is given below. We have by definition

dΦ = 683 V(λ) ψ dλ …2.1

and the total luminous flux F is found by integrating equation (2.1) over the whole of visible range 780



Φ = 683

∫ V(λ) ψ dλ …2.2

380

The sensitivity of the eye peaks at 555 nm and falls off to approximately 10-4 at 380 and 750 nm. This constitutes the range of daylight sensitivity, or photopic vision. The eye’s night time sensitivity, called scotopic vision, shifts toward the blue end of the visible, peaking at 507 nm and falling to 10–4 at 340 and 670 nm. This weighting factor, or luminosity function V(λ), allows for conversion of radiant flux to luminous flux at any wavelength. In the photopic region, the peak at 555 nm is assigned a conversion value of 683 lumens per Watt.

Lumen The lumen is the standard unit for the luminous flux of a light source. It is an SI derived unit based on the candela. It can be defined as the luminous flux emitted into unit solid angle (1 sr) by an isotropic point source having a

2.6    FUNDAMENTALS OF LIGHTING

luminous intensity of 1 candela. The unit (lumen) is then equal to cd × sr. The abbreviation is lm and the symbol is Φv. The electromagnetic radiation contains all types of radiations- like gamarays, X-rays, UV, visible, IR, microwaves and radio waves. Thus, the visible light is a small fraction of total e.m. radiation. The luminous flux is the fraction of the total e.m. power expressed in lumens where eye luminosity function has been multiplied to the e.m. power expressed in watt. Thus,luminous flux is the light power as perceived by the human eye. At light wavelength of 555 nm, the human eye has maximum sensitivity (100%). It is found through experiments that the visible light of wavelength 555 nm emitting 683 lumens equals 1 watt of power. A typical 100 watt incandescent bulb has a luminous flux of about 1700 lumens.As explained earlier the lumen is the unit of luminous flux, and is defined in terms of the candela, an SI base unit like the meter or second. 1 lumen is defined to be 1/4 p candela, the SI base unit of luminous intensity. Since the eye does not see all wavelengths equally well, the luminosity function is a very important way to determine the luminous flux from a source. The luminous flux from a monochromatic source producing light at a single wavelength is easiest to determine, as follows:

Φv = Φ × V(λ) × (683 lm/W) ...2.3

For instance, a 5 mW laser pointer using at a wavelength of 680 nm produces 0.005 W × 0.017 × 683 lm/W = 0.058 lm, while a 5 mW laser pointer at 630 nm produces 0.005 W × 0.265 × 683 lm/W = 0.905 lm, a significantly greater luminous flux. To find the luminous flux from a source radiating over a spectrum is more difficult. It is necessary to determine the spectral power distribution (SPD) for the particular source. Once that is done, it is necessary to calculate the luminous flux at each wavelength, or at regular intervals for continuous spectra. Adding up the flux at each wavelength gives a total flux produced by a source in the visible spectrum. Some sources are easier to do this than others. An incandescent lamp produces a continuous spectrum in the visible region and various intervals of wavelength must be used to determine the total luminous flux. For sources like a mercury vapor lamp, however, it is slightly easier. Mercury emits light primarily in a line spectrum. It emits radiant flux at 6 primary wavelengths. This makes it easier to determine the luminous flux of this lamp versus the incandescent lamp. Generally, it is not necessary to determine the luminous flux for yourself. It is commonly given for a lamp based on laboratory testing during manufacturing process. As an example, the luminous flux for a 40 W incandescent lamp is approximately 680 lm. We can use this information to extrapolate to similar lamps. Thus the average luminous efficacy for an incandescent lamp is about 17 lm/W.



LIGHT AND ITS MEASUREMENT    2.7

The Radiant Flux The radiant flux (Φ) is energy per unit time (dQ/dt) that is radiated from a source over optical wavelengths, which are defined to be from 3 × 1011 and 3 × 1016 Hz. This range is approximately equivalent to wavelengths from 0.01 µm to 1000 μm and includes the regions of the electromagnetic spectrum commonly referred to as Ultraviolet (UV), Visible, and Infrared (IR). The flux is measured in units of Joules per second (J/s), or Watts (W). A radiant flux of 1 W means that a source produces 1 Joule every second. If we integrate radiant flux over time we obtain the total Energy (Q) output by the source. Most of the time when we deal with light sources, we do not really need to qualify optical radiation. For instance, if we look at the standard incandescent lamp, we know that the flux output below .01 μm is negligible, and that there is also very little energy output in the microwave and radio regions of the e.m. spectrum. So we can reasonably assume that the flux output for a standard lamp is restricted to be in the optical range, and in doing so we may take the nominal wattage as our radiant flux. This means that a 40 W incandescent lamp would have a radiant flux of 40 W, and a 250 W Mercury vapor lamp would have a radiant flux of 250 W. Thus, the radiant flux must be factored by the sensitivity of the human eye to determine luminous flux in lumens for a light source. The radiant flux ψ is equal to the total power in watts of electromagnetic radiation emitted or received. It may include both visible and non-visible components. The spectral radiant flux ψλ is the radiant flux per unit wavelength interval usually taken to be 1nm,so that ψλ is expressed as watts per nanometer (W nm-1). The total radiant flux ψ is found by integrating ψλ over the whole spectrum:      ψ =

∞

∫ ψ λ dλ ,

(where limits of integration are 0 to ∞) ...2.4

0

Definition of Luminous Exitance For a radiator, the luminous exitance or luminous emittance of a radiator is the total flux emitted in all directions from a unit area of the radiator. Luminous exitance is symbolized by M and has units of lumens per square meter, just like illuminance. In symbols

M = F/S ...2.5

where a total flux F comes from a source of area S. Both luminance and luminous exitance pertain to radiating sources of finite area, but luminance quantifies the flux radiated in a particular direction while luminous exitance pertains to the total flux emitted. Exitance is a much easier concept to grasp but, unfortunately, not as useful as luminance. Luminous Intensity We can define the luminous intensity, I of a light source as the luminous flux emitted by the source per unit solid angle.

2.8    FUNDAMENTALS OF LIGHTING

The SI unit of luminous intensity is candela (cd). Luminous intensity is related to luminous flux as stated above. Luminous flux is the total perceived power emitted in all directions by a source. Luminous intensity is the perceived power per unit solid angle. Luminous intensity is also not the same as the radiant intensity, the corresponding objective physical quantity used in the measurement science of radiometry. We can thus define the luminous intensity as an expression of the amount of light power emanating from a point source within a solid angle of one steradian. For reference, a frequency of 540 THz (or 5.40 × 1014 Hz ) is specified. Frequency of 540 THz corresponds to a wavelength of about 555 nanometer (nm), and is generally accepted as the frequency and wavelength at which the average human eye is most sensitive; this wavelength is in the middle of the visible-light spectrum. A steradian is the standard unit solid angle. A sphere encloses 4 p (approximately 12.57) steradians. Relationship between Luminous Flux and Luminous Intensity Let us suppose, we have an isotrpic point source which is enclosed in a sphere of radius r. The light emitted by this source will fill the entire sphere uniformly. We can, therefore, say that the illuminance, E, due to the point source on the surface of the sphere of area A can be expressed as:

E = F/A …2.6

where F is the luminous flux emitted by the point source. For a sphere of radius r, the area A = 4p r2. We, therefore, have

E = F/4p r2 = (F/4p)/r2 = I/r2

…2.7

We, thus have, luminous intensity as I = (F/4p) = Flux emitted per unit solid angle. The luminous intensity is a measure of how much flux (lumens) is emitted within a small conical angle in a particular direction from a light source. This small solid angle is taken to be one steradian. The total solid angle for spherical distribution is 4p. The symbol for lumiminous intensity is I. We can, thus, define the luminous intensity as the total luminous flux Φ emitted per unit solid angle. If the total solid angle is Ω, then we can write: I = Φ/Ω …2.8 The standard SI unit of luminous intensity is the candela (cd): one candela is equal to one lumen per steradian. Candela The candela is the SI unit of light intensity as has already been stated above. This unit was adopted by the international scientific community during 1948. Till then the standard unit for luminous intensity was candle power. In fact a unit of light intensity, candela, refers to a given direction. A light source radiating a light intensity of one candela in all directions produces a well-defined quantity of light per second, which has been given the name



LIGHT AND ITS MEASUREMENT    2.9

‘lumen’. This has become the principal lighting standard in use today. For example, for radiation with a wavelength of 490 nm the eye sensitivity is only 20 per cent of that for radiation with a wavelength of 555 nm. One watt of power radiated at 490 nm therefore equals 0.2 light-watts. Basically a candela is approximately equal to an older unit, the candlepower, which was the light intensity emitted by the flame of a candle. So a candle flame has a luminous intensity of about 1 candela; by comparison, a 60-watt incandescent light bulb has a luminous intensity of about 65 candelas, while a typical searchlight has a luminous intensity of about 800 million candelas. If a light source is isotropic (so it emits light equally in all directions), then there is a simple relationship between luminous flux Φ and luminous intensity I. I = Φ/(4π sr) …2.9



If a source emits the same luminous flux in all directions, then the luminous intensity is the same in all directions. For most sources, however, the flux emitted in each direction is not the same. Since light intensity is direction dependent for most of the light sources, hence lamp manufactures use their photometric laboratories for plotting a graph of luminous intensity of the light source at various angles in horizontal and vertical planes. These graphs are known as polar graphs. A polar diagram or graph for a typical light source is shown in Fig. 2.2. Usually the reflector lamps will have a light output stated in candela. This value is the peak intensity, usually quoted at 0 degrees or directly below the lamp in the vertical position. 120°

150°

180°

150°

120°

90°

90° 20 40

60°

60°

60 30°

0°

30°

Fig. 2.2  A Polar Graph of a Typical Light Source

Important SI Photometric and Radiometric units Quantity

SI unit

Abbreviation

lumen second lumen (= cd . sr)

lm . s

Photometric quantity Luminous energy Luminous flux

lm

2.10    FUNDAMENTALS OF LIGHTING

Luminous intensity Luminance Luminous efficacy Illuminance

candela (= lm/sr) cd candela per square metre cd/m2 lumen per watt lm/W lux (= lm/m2) lx

Radiometric quantity Radiant energy Radiant flux Radiant intensity Radiance Irradiance

joule watt watt per steradian watt per steradian per square metre watt per square metre

J W W· sr−1 W· sr −1· m − 2 W· m − 2 (intensity)

Comparison of Watt with Lumen Unit of radiant flux is watt, while lumen is the unit of luminous flux. The comparison of the watt and the lumen illustrates the distinction between radiometric and photometric units. The watt is a unit of power. We are accustomed to thinking of light bulbs in terms of power in watts. This power is not a measure of the amount of light output, but rather indicates how much energy the bulb will use. Since the incandescent bulbs have fairly similar characteristics (same spectral power distribution), power consumption provides a rough guide to the light output of incandescent bulbs. The incandescent bulbs are now considered to be inefficient since they produce a lot of heat energy which falls in the infrared region of electromagnetic spectrum and is not visible to the humans. These bulbs are, in fact, sometimes used as heat sources (as in a chick incubator), but usually they are used for the purpose of providing light. As such, they are very inefficient, because most of the radiant energy they emit is invisible infrared. A CFL can provide light comparable to a sixty watt incandescent lamp while consuming as little as fifteen watts of electricity. Many people still think of light in terms of power consumed by the bulb. It has been a business requirement for several decades that light bulb packs give the output in lumens. The package of a 60 watt incandescent bulb indicates that it provides about 900 lumens, as does the pack of the 15 watt compact fluorescent lamp. Combining these definitions, we see that 1/683 watt of 555 nm green light provides one lumen. The relation between watts and lumens is not just a simple scaling factor. We know this already, because the 60 watt incandescent bulb and the 15 watt compact fluorescent can both provide 900 lumens. Lumen definition tells us that 1 watt of pure green 555 nm light is worth 683 lumens. The lumen is defined as the amount of light given into one



LIGHT AND ITS MEASUREMENT    2.11

steradian by a point source of one candela strength; while the candela, a base SI unit, is defined as the luminous intensity of a source of monochromatic radiation, of frequency 540 terahertz, and a radiant intensity of 1/683 watts per steradian. 540 THz corresponds to about 555 nanometres, the wavelength, in the green, to which the human eye is most sensitive. The number 1/683 was chosen to make the candela about equal to the standard candle, the unit which it superseded. It does not say anything about other wavelengths. As we can see from the eye luminosity function, the sensitivity of the eye varies with wavelength of light. For photopic vision, the human eye has the maximum sensitivity at 555 nm. Now one could choose to define one watt of light power radiated at a wavelength of 555 nm as being equal to one ‘light-watt’. One watt of power radiated at a different wavelength within the visible range would then have to be multiplied by the relative eye-sensitivity factors as defined by the spectral eye sensitivity curve for photopic vision, that is to say the V(λ) curve. In this way, one arrives at the light-watt value corresponding to that wavelength value. Since the lumen is photometric units, its relationship to watt depends on the wavelength according to how visible the wavelength is to us. Infrared and ultraviolet radiation, for example, are invisible and do not count. One watt of infrared radiation (which is where most of the radiation from an incandescent bulb falls) is worth zero lumens. In the visible spectrum, wavelengths of light are weighted according to a function called the “photopic spectral luminous efficiency. According to this function, 700 nm red light is only about 4% as efficient as 555 nm green light. Thus, one watt of 700 nm red light is worth only 27 lumens. Because of the summation over the visual portion of the e.m. spectrum that is part of this weighting, the unit of “lumen” is color-blind: there is no way to tell what color a lumen will appear. This is equivalent to evaluating groceries by number of bags: there is no information about the specific content, just a number that refers to the total weighted quantity. Comparison of Photometric with Radiometric Quantities We know that there are two parallel systems of quantities known as photometric and radiometric quantities in lighting. Every quantity in one system has an analogous quantity in the other system. Some examples of parallel quantities include: Luminance (photometric) and radiance (radiometric), Luminous flux (photometric) and radiant flux (radiometric), Luminous intensity (photometric) and radiant intensity (radiometric). In photometric quantities every wavelength is weighted according to how sensitive the human eye is to it, while radiometric quantities use unweighted absolute power. For example, the eye responds much more strongly to green light than to red, so a green source will have greater luminous flux than a red source with the same radiant flux would. Radiant energy outside the visible spectrum does not contribute to photometric quantities at all.

2.12    FUNDAMENTALS OF LIGHTING

For example, a 1000 watt space heater may put out a great deal of radiant flux (1000 watts, in fact), but as a light source it puts out very few lumens (because most of the energy is in the infrared, leaving only a dim red glow in the visible).

2.4 BRIGHTNESS, LUMINANCE AND ILLUMINANCE The luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction on a surface. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square meter (cd/ m2). The luminance is often used to characterize emission or reflection from flat, diffuse surfaces. The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye’s pupil. Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between 50 and 300 cd/m2. The sun has luminance of about 1.6 × 109 cd/m2 at noon. Luminance is invariant in geometric optics. This means that for an ideal optical system, the luminance at the output is the same as the input luminance. For real, passive, optical systems, the output luminance is at most equal to the input.

Luminance The luminance of an element of a surface is defined in terms of the luminous intensity it produces divided by the area apparently producing the intensity. Therefore, if the luminous element is viewed at an angle φ then it is the projected area of the surface in that direction that is included in the formula. At an angle φ to the normal of the luminous element of surface area A, the luminance L is given by L = I/A cos φ cd/m2



…2.10

This definition of luminance gives us a means of measuring the physical brightness of an element of a surface or of the whole surface. However, the background luminance is very essential to see the actual brightness of a surface. Typical luminance values of certain objects are listed below: Luminance value 1.6 × 109 cd/m2 600,000 cd/m2 120,000 cd/m2 11,000 cd/m2 8,000 cd/m2 2,500 cd/m2

Object Solar disk at noon Solar disk at horizon Frosted bulb 60 W T8 cool white fluorescent tubular lamp Average clear sky Moon surface



LIGHT AND ITS MEASUREMENT    2.13

2,000 cd/m2 30 cd/m2 0.0004 cd/m2

Average cloudy sky Green electroluminescent source Darkest sky

Luminance Contrast The concept of luminance contrast has been developed to see an object distinctly in the luminous background. Several statistics have been proposed for the measurement of luminance contrast. None can be said at this point to have been accepted by the community as universally most useful. Luminance contrast statistics generally have two components: (i) The first is the difference between the two luminances in question (e.g. of a symbol and its background). If the state of adaptation of the visual system stays constant, larger luminance differences produce larger brightness differences (higher brightness contrasts). (ii) The second component of any luminance contrast statistics is some measure describing the adaptation state of the eye. A luminance difference that produces a large brightness difference on a dim background will produce a smaller brightness difference on a brighter background due to visual adaptation. To capture this behavior, designers of luminance contrast statistics generally divide a numerator that describes the luminance change by a denominator that describes the average luminance to which the eye is adapted. Weber Contrast We can, thus, define luminance contrast in two ways, namely Weber contrast and Michelson contrast. One of the oldest luminance contrast statistics, Weber Contrast, is also often used for certain patterns (small, sharpedged graphic objects like symbols and text characters on larger uniform backgrounds):

Cw = (Ls – Lb)/Lb …2.11

where Ls is the luminance of the symbol and Lb is the luminance of the immediately adjacent background. When the background is lighter than the symbol Cw is negative and ranges from zero to 1. When the background is darker than the symbol Cw is positive and ranges from zero to potentially very large numbers. Michelson Contrast For simple periodic patterns (e.g. textures) there is no large area of uniform luminance that dominates the users brightness adaptation. There is no clear choice for the denominator of the above statistics.

2.14    FUNDAMENTALS OF LIGHTING

Given that a target is above the minimum size, it will be visible only if it differs from its immediate background in luminance or color. If it differs in luminance from the immediate background, the target has a luminance contrast. Michelson contrast is defined as:

Cm = (Lmax – Lmin)/(Lmax + Lmin) …2.12

where,

Lmax = maximum luminance,



Lmin = minimum luminance.

The quantity defined by the above equation is often called contrast, or Michelson contrast, but usually and more properly called modulation. It gives a value between 0 and 1 for all objects. It applies to periodic patterns, such as gratings, which have one maximum and one minimum in each cycle. Because there are several different definitions of luminance contrast and different definitions have different ranges of possible values, it is important to know which definition is being used when the contrast of a target is specified. When a target and its background are both diffuse reflectors, the luminance contrast is not affected by changing the illuminance, so the luminance contrast can be calculated from the reflectance. However, if either the object or the background are directional reflectors, luminance must be used to calculate contrast. It should be noted that for calculating luminance contrast, it does not matter how the luminance is achieved. It makes no difference whether the luminance is produced by reflection from a surface, such as print; from a self-luminous source, such as a VDT screen; or by some combination, such as a display on a VDT screen with a reflected image superimposed. The Brightness and its Perception Generally the brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. This is a subjective attribute/property of an object being observed. For objects in the illuminant or aperture modes, the perception of brightness is a function of luminance. Specifically, brightness is related to luminance by a power law of the form:

B = A × L0.33 …2.13

where, B = brightness,  A = constant and  L = luminance in cd/m2. Despite the logical inconsistency of describing a perception of brightness to an object in the surface mode, studies of the perception of brightness of surfaces in an interior have been made. The room contained both self-luminous objects, which appeared in the illuminant mode, and reflecting surfaces, which appeared in the surface mode. The luminance range in the interior was two log



LIGHT AND ITS MEASUREMENT    2.15

units. These studies have shown that the perceived brightness of any single surface increases with luminance according to a power law with an exponent of 0.35, but that the brightness of a number of surfaces seen simultaneously follows a power law with an exponent of approximately 0.6. These relationships can be used to estimate the relative brightness of surfaces in an interior by assuming that the brightest surface in the room has a brightness given by:

Bmax = L0.35 max

…2.14

Then, other surfaces will have a brightness given by:

B = A × L0.6,

where

A = Bmax/L0.6 max.

…2.15

This simple system under estimates the brightness of highly saturated colored surfaces and overestimates the brightness of translucent surfaces. These relationships are given for guidance only. The data from which these relationships were derived represented just over half the data collected. The data from other subjects were eliminated to reduce the “noise” in the data, “noise” that may reflect the lack of meaning some people find in attempting to describe the brightness of an object in the surface mode. The above discussion of perceptual constancy and modes of appearance is concerned mainly with the perception of individual objects in an interior.

Illuminance Illuminance is the quantity of light, or luminous flux, falling on unit area of a surface. It is designated by the symbol E. The unit is the lux (lx). One lux equals one lumen per square metre (lm/m2). In the CGS system, the unit of illuminance is the phot, which is equal to 10,000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography. In the past illuminance was often called brightness, but this leads to confusion with other uses of the word. “Brightness” should never be used for quantitative description, but only for non quantitative references to physiological sensations and perceptions of light. The human eye is capable of seeing somewhat more than a 2 trillion-fold range: The presence of white objects is somewhat discernible under starlight, at 5 × 10−5 lux, while at the bright end, it is possible to read large text at 108 lux, or about 1,000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting after images. A typical office has an illuminance of between 300 to 500 lux or 30 to 50 foot candles on the desktops. We all are aware that the horizontal illuminance describes the amount of light landing on a horizontal surface, such as a desk, and vertical illuminance describes the illuminance landing on a vertical surface, such as a wall or a face.

2.16    FUNDAMENTALS OF LIGHTING

Typical examples of illuminance: Typical Range Lux

Situation

100,000

Bright sunny day

10,000

Cloudy day

1000 to 2000

Watch repairman’s bench

100 to 1000

Typical office

200 to 1000

Night sports field

1 to 10

Residential street lighting

0.25

Cloudy moonlight

Illumination follows a square law. For example, for any given reading, if the light meter is held twice as far away from the light, the meter will read only one fourth as much; if the light meter is held half as far away from the light, the meter will read four times as much. Luminance describes the amount of light leaving a surface in a particular direction, and can be thought of as the measured brightness of a surface as seen by the eye. Luminance is expressed in candelas per square foot, or more commonly candelas per square meter (cd/m²). A typical computer monitor has a luminance of about 100 cd/m². Luminance is a measure of light coming from a surface (in contrast to illuminance, or light falling on a surface). If the surface is similar to a sheet of bond typewriter paper, a good mental image is that in an ordinary office, the luminance of the sheet is about 100 cd/m2. Luminance does not follow a square law, but the measurement area must be defined. The luminance of a wall, for example, is the same whether measured two meters away or four. Similarly, moving closer to or further away from a source document does not change its apparent brightness. The particular luminance of a bright surface is usually referred to as its brightness because it is the quality of brightness that we perceive.

Unit of Illuminance, Lux The lux (symbol: lx) is the SI derived unit of illuminance or illumination. It is equal to one lumen per square metre. Sunlight on an average day ranges from 32 000 to 100000 lux. TV studios are lit at about 1000 lux [i.e., 1000 lumens per square metre]. A bright office has about 400 lux of illumination. At sunset and sunrise, ambient outdoor light is also about 400 lux (if the sky is clear). Moonlight represents about 1 lux and starlight measures a mere 0.00005 lux. Lumens measure “luminous flux”. This is a measure of the total number of packets (or quanta) of light produced by a light source (e.g. a globe or fluorescent tube). This is the “quantity” of light emitted by the light source. The purpose of lux is intended to tell you how many lumens you need given the area you are trying to illuminate. Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 1200 lumens.



LIGHT AND ITS MEASUREMENT    2.17

To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area with the same number of lux requires a larger number of lumens. The differences between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres produce a dimmer illuminance of only 100 lux. Relationship between Various Lighting Units (1) Relation between luminous flux (F) and luminous intensity (I),(2) Relation between luminous intensity (I) and illuminance (E) and (3) Cosine Law and Vertical illuminance are important. Light measurement process generally uses two dissimilar parameters. We know that the light is the visible electromagnetic radiation, thus we are concerned on the one hand with energy and on the other hand with a sensation obtained through the eye. These two are principally dissimilar things which makes it difficult to talk about light in quantitative terms. To solve the problem, a convention has been adopted that fits in well with illuminating engineering practice: viz., the product of radiant energy and eye sensitivity. Thus, light is regarded as radiation measured in terms of human eye sensitivity, as already explained. Illuminance Vs Luminance Illuminance is the amount of light coming from a light fixture that lands on a surface. It is measured in footcandles (lux in the metric system). A typical office has an illuminance of between 30 to 50 foot-candles (300 to 500 lux) on desktops. The horizontal illuminance describes the amount of light landing on a horizontal surface, such a desk, and vertical illuminance describes the illuminance landing on a vertical surface, such as a wall or a face. Luminance describes the amount of light leaving a surface in a particular direction, and can be thought of as the measured brightness of a surface as seen by the eye. Luminance is expressed in candelas per square foot, or more commonly, Candelas per square meter (cd/ m²). A typical computer monitor has a Luminance of about 100 cd/m². Relationship between Luminance and Illuminance Luminance and Illuminance are related as follows: or

Luminance = (R × Illuminance)/p L = R × E/p …2.16

where, L is luminance in cd/m2, R is reflectance of the surface and E is the illuminance of the surface in lux.

2.18    FUNDAMENTALS OF LIGHTING

2.5 EFFICACY, EFFICIENCY AND LAWS OF ILLUMINATION Luminous Efficacy of a Lamp Luminous efficacy is a figure of merit for light sources. It is the ratio of luminous flux (in lumens) to power (usually measured in watts). As most commonly used, it is the ratio of luminous flux emitted from a light source to the electric power consumed by the source, and thus describes how well the source does at providing visible light from a given amount of electricity. This is also referred to as luminous efficacy of a source. Lighting, like any commodity, needs some means of indicating its quantity. We normally buy an ordinary incandescent lamp in terms of wattage, e.g., 40 Watt, 60 W or 100 W, etc. We know that our 100 W lamp will give more light than a 40 W lamp of the same type. However, when it comes to a technical specification for, say, a large office, then such a simple approach is inadequate. Offices are lit often with tubular fluorescent lamps. A typical white 100 W tubular fluorescent lamp gives almost seven times as much light as a 100 W incandescent lamp. Therefore, watts are an inadequate measure of the amount of light provided. Differences in light output for a given wattage of lamp can be caused by several factors. The main reason for a fluorescent lamp being more efficient than incandescent lamp lies in the fact that the atomic processes for producing light is more efficient in the fluorescent lamp than the incandescent lamp. Another reason is that when the electric power in watts is converted into light the amount of light produced depends upon its wavelength (i.e. colour). This is because the eye is more sensitive to some colours than to others. For example, under daytime or photopic conditions the eye is more sensitive to the yellow/green wavelengths than it is to red or blue wavelengths. The relative magnitude of the difference is indicated in the curve shown in Fig. 2.1. To determine the output of a lamp the fundamental method is to measure the power emitted in watts over a series of narrow wavebands covering the whole spectrum of the lamp and to multiply each value by the relative response of the eye (called the spectral luminous efficiency, V(λ). These values are then added together and multiplied by a constant (683) used to relate all light output measurements to an internationally agreed base. This quantity of light, which depends upon the wavelength as well as electromagnetic power, is expressed in lumens. We can thus say that the luminous efficacy is a figure of merit for light sources. It is the ratio of luminous flux (in lumens) to power (usually measured in watts). As most commonly used, it is the ratio of luminous flux emitted from a light source to the electric power consumed by the source, and thus describes how well the source does at providing visible light from a given amount of electricity. An example of this calculation is shown here in a simplified form. In particular, the wavelength intervals are 25 nm, whereas the usual interval is 1 nm. Example 2.1: A 100 W incandescent lamp has a power distribution as shown in Table 2.1. Calculate the lamp output in lumens and also find the luminous efficacy of the lamp.



LIGHT AND ITS MEASUREMENT    2.19 Table 2.1 Wavelength (lnm)

Power radiated W per wavelength Interval

Spectral luminosity function V(l)

683 × W × V(l) = light output per unit wavelength interval (lumen)

380 400

0.034 0.062

3.9 × 10–4 4 × 10–4

0.0009 0.017

425

0.096

7.26 × 10–4

0.476

450

0.133

3.8 × 10–4

3.452

475 500 525 550

0.176 0.234 0.298 0.365

0.1226 0.3230 0.7832 1.0002

14.75 51.62 161.44 249.34

575

0.433

0.9154

270.72

600 625 650 675 700 725 750 780

0.505 0.757 0.644 0.712 0.790   

0.6310 0.3210 0.1070 0.0323 4 × 10–3 7.4 × 10–4 1.2 × 10–4 1.499 × 10–5

217.64 126.06 47.06 11.28 2.21   

Total = 5.023 watts

Total = 1156.10 lumens

From such a process it is found that a 100 watt incandescent lamp produces 1156.10 lumens. Thus its efficacy is 11.561 lumens/watt i.e., approximately 12 lm/watt. Likewise a 2400 mm 100 watt tubular fluorescent lamp produces about 8000 lumens and a 100 watt high pressure sodium lamp about 10,000 lumens. Dividing the lumen output by the wattage gives luminous efficacy for fluorescent tube as 80 lm/W and for high pressure sodium lamp as 100 lm/W. A comparison of efficacies of various lamps is shown in the Fig. 2.3.

Luminous Efficiency Artificial light sources are usually evaluated in terms luminous efficacy of a source, also sometimes called overall luminous efficacy. As has been explained above, this is the ratio between the total luminous flux emitted by a device and the total amount of input power (electrical, etc.) it consumes. The overall luminous efficacy is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve of the human eye (the “luminosity function”). When expressed in dimensionless form (for example, as a fraction of the maximum possible luminous efficacy), this value may be called overall luminous efficiency, wall-plug luminous efficiency, or simply the lighting efficiency.

2.20    FUNDAMENTALS OF LIGHTING

The main difference between the luminous efficacy of radiation and the luminous efficacy of a source is that the latter accounts for input energy that is lost as heat or otherwise exits the source as something other than electromagnetic radiation. Luminous efficacy of radiation is a property of the radiation emitted by a source. Luminous efficacy of a source is a property of the source as a whole. Comparison of luminous efficacy of various lamps

Comparison of luminous efficacy of various lamps

Fig. 2.3  Luminous Efficacy of Various Lamps

Difference between Luminous Efficacy and Luminous Efficiency Many a times confusion arises as to how luminous efficiency and luminous efficacy are calculated. Both of these calculations are a ratios of measured output (either lumens or watts) to input power.  More specifically, Efficiency, sometimes called “Power Efficiency” is calculated by taking actual measured optical “watts” from your lamp and dividing this by the electrical power driving your lamp. This result is reported as a percentage. On the other hand, if you were looking to report the luminous efficacy of your lamp, you would calculate this by taking the ratio of the measured luminous flux and again dividing this by the electrical power driving your lamp. Reported units, in this case, would be lumens/watt. Depending on what instrument manufacturer you are working with, the software may or may not report the actual input power (Watts). If the input power is not reported directly, you can obtain the power if you know the current and voltage, usually read directly from your power supply, by simply multiplying the supplied current in Amps by the measured voltage. This will give you the electrical power supplied to the lamp. As a quick example, consider a high powered LED driven at a typical current of 350 mA  (0.350 Amps).The measured voltage as reported by your meter is 3.582 V. Multiplying the current and voltage, 0.350 A × 3.582 V = 1.2537 W. The “Measured” optical output is reported to be (0.15378 W, 43 Lumens). Dividing 0.15387 W by 1.2537 Watts gives us a ratio



LIGHT AND ITS MEASUREMENT    2.21

of 0.1226 or 12.26%.To calculate Luminous Efficacy, simply take 43 Lumens and divide this by 1.2537 Watts to get 34.3 lm/W.

Laws of Illumination

There are two important laws of illumination. These are: 1.

Inverse Square Law

2.

Lambert's Law or Cosine Law

Inverse Square Law This law assumes that the illuminance received on a surface from a light source is inversely proportional to the square of its distance from the source. In other words, the farther away we move, the less is the illuminance which is expressed by the eqn. (2.17) and shown in Fig. 2.4.

9A 4A A

P 1d

2d 3d

Fig. 2.4  Inverse Square Law

The inverse square law can be expressed mathematically as follows: E = I/d2 lux

…2.17

where E is illuminance, I is the luminous intensity and d is distance of the plane from the light source.

Lambert’s Law or Cosine Law This law allows us to calculate the illuminance with one or more lamps or with reflection from the surroundings. This law can be explained as follows: I(0)

I()



Fig. 2.5 Lambert’S Law

2.22    FUNDAMENTALS OF LIGHTING

In case light is falling on a surface at an angle θ from the normal, then instantaneity of the light in the direction of θ is I(θ), which is I(0) cos θ (see Fig. 2.5). Hence the illuminance in that direction is:

E(θ) = E(0) cosθ = I(0) cos(θ)/d2

…2.18

This is known as Lambert’s law. Using this law it is easy to obtain the horizontal and vertical illuminance. (see Fig. 2.6). Horizontal and Vertical Illuminance: EH = I cosθ/d2 and EV = I sinθ/d2 where I is the luminous intensity in the direction of point P. Since d = h/cosθ, hence EH and EV can be written as:

EH = I cos3 θ/h2 and EV = I cos2 θ sinθ/h2

Fig. 2.6  Geometry for Horizontal and Vertical Illuminance

2.6 NUMERICAL PROBLEMS IN PHOTOMETRY In order to clarify various concepts related to photometry we shall like to present here some solved examples. Example 1 The illuminance of a surface is 1000 lux. If the area of the surface is 30 cm2, find the luminous flux incident on the surface. Solution: In this case illuminance, E = 1000 lux and



A = 30 cm2 = 30 × 10-4 m2

Since,

E = Φ/A,

hence,

Φ = E × A = 1000 × 30 × 10–4 lm = 3 lm.

Example 2 The luminous efficacy of a lamp is 5 lm/watt and its power is 88 watts. Calculate the illuminance on a surface placed at a distance of 10 m, when the light is falling normally.



LIGHT AND ITS MEASUREMENT    2.23

Solution:

Φ = luminous efficacy × wattage of the lamp



Φ = 5 × 88 = 440 lm



E = I/r2 = Φ/4 π r2;

Hence,

E = 440 × 7/(4 × 22 × 102) lux = 0.35 lux.

Example 3 A small source of luminous intensity of 100 cd is suspended 4 m above a point P on a horizontal surface. Calulate the value of illuminance at the point P and a point Q 3 m away from P. Solution: Using the relation for illuminance,

E = I/d2,

As shown in Fig. 2.7 we have, EP = 100/42 = 100/16 = 6.25 lux. Using Lambert’s law E Q = I(θ)/d2 = 100 cosθ/52 = 100 × 0.8/52 = 3.2 lux

Fig. 2.7  Geometry for the Problem

Example 4 A circular patch of a source has a luminance of 500 cd/m2 when viewed at an angle of 45 degrees. If the patch has a diameter of 0.1 m, what is the luminous intensity of that patch in the direction of view. Solution:

Fig. 2.8  Geometry of the Problem

We know that the luminance, L is given as follows: L = I/A cos j cd/m2

2.24    FUNDAMENTALS OF LIGHTING

where, I is luminous intensity, A is area of the circular patch and j is angle between normal to the area and the direction of view. I = LA cosj,

φ = 45° in this case (Fig. 2.8).

Hence,

I = L × π (0.1/2)2 cos 45°



I = (500 × 3.14 × 0.01)/4√2 ;

Hence Luminous intensity, I = 2.88 cd.



Example 5 The luminous efficacy (L.E.) of a lamp is 5 lm/W and its power is 88 watt. Calculate the illuminance on a surface placed at a distance of 10 m, when the light is falling normally. Solution: Here,

L.E. = 5 lm/watt Power = 88 watt, hence,

Luminous flux = j = 5 × 88 = 440 lm Illuminance = j/4 π r2 = 440 × 7/(4 × 22 × 102 = 7/20 = 0.35 lux Intensity of light = φ/4 π = 440 × 7/(4 × 22)

= 35 lm/sr, or candela (cd). Example 6 The illuminance of a surface is 1000 lux. If the total area of the surface is 30 cm2, find the luminous flux incident on the surface. Solution: Here, As

E = 1000 lux; A = 30 cm2 = 30 × 10-4 m2 E = φ/A, hence, φ = 3 lm.

2.7 TECHNIQUES OF LIGHT MEASUREMENT AND MEASURING SYSTEMS Light Measurement Techniques Let us now discuss the approximation methods for the following cases: 1.

Illumination from point sources

2.

Illumination from linear and surface sources

Light from Point Sources An ideal point source is a source that radiates light equally in all directions. The radially diverging lines represent energy flux. Of course, there is no such thing as a perfect real point source. However, an illumination source may be considered a point source if its dimensions are small relative to the target distance.



LIGHT AND ITS MEASUREMENT    2.25

The most important thing to note regarding a point source is that the flux density for a unit area of a target drops off as that area moves farther away from the point source, assuming angular orientation of the area stays the same. In other words, there are fewer Flux lines impinging on the area the farther away it is from the point source. As long as a point source is set at a constant distance and angle to the target, the flux density remains the same. Not a problem for flat targets that can be fixed in position. But what about targets with large textures, or targets that move? The flux density at surface A is greater than at surface B when the object is close to the point source.

How to Calculate Illumination? For calculating the illumination we usually pose following questions: 1. 2. 3. 4. 5.

How much illumination is required? How much effective light flux is required to give this much illumination? What is the total number of lumens output of the lamps? What total power rating will be needed? In what manner should this total power supply be divided up among numerous units and how these units should be distributed? We need to seek answers for the above questions with the following inputs: (i) The amount of illumination required depends upon the type of work for which light illumination is needed. Usually it is given in standard tables. (ii) The average illumination in the known area in lux can be used in calculating the lumens required. (iii) With the effective lumens required given and the lumens output of the source also known, the number of light sources required may be calculated.

Light Measuring Systems Special terminology and the use of both the metric and English systems tend to somewhat complicate discussions about light measurements. The English system is still in use in U.K. and some of its protectorates. However, by and large the metric or SI system is used universally. The intent of this content is to give a brief explanation of certain terms used in light measurements and the procedure to obtain them.

Luminous Flux Measurement and the Integrating Sphere The accurate measurement of total luminous flux from various lamps including the long tubular fluorescent lamps is usually achieved by using a large diameter Integrating sphere.

2.26    FUNDAMENTALS OF LIGHTING

Integrating sphere is a hollow sphere where the inner surface provides a high and diffuse reflectance over a wide spectral range. They are ideal optical diffusers. Integrating spheres operate as light collectors. The collected light becomes either a source of illumination – uniform source, or a source of measurement – photometer, radiometer and reflectance/transmittance sphere. In either case the operation is the same. The input light is diffusely reflected inside the sphere so the output port is uniformly illuminated (see Fig. 2.9). The measurement of the total radiant power or luminous flux of light sources using integrating spheres is one of the most important tests in light measurement. As mentioned above the inside surface of an integrating sphere is coated with a diffusely reflecting material which guarantees complete integration and homogenization of the emitted radiation. The integrated light is then measured at the detector port. There is a series of integrating spheres, available in the market in a wide range of sizes and providing functionality for a broad spectrum of applications. A deciding factor in choosing the appropriate integrating sphere is the maximum dimension of the light source to be measured. When the measurements of luminous flux is required from larger sources, the use of larger integrating spheres is common in order to keep measurement errors to a minimum. Generally all the integrating spheres have a side entrance port for determining luminous flux in the 2π configuration (i.e. for determining emission into the forward hemisphere only). For the larger integrating spheres, the test object can also be mounted in the centre, thus permitting measurements in the 4π configuration. The research studies have suggested that the use of an integrating sphere with long tubular fluorescent lamps can result in errors due to the increased screen size necessary to shield the sensor from direct light. The use of shield then removes dissimilar contributions of flux along the lamp length from reaching the sphere wall. These are the basic reasons that is why the dimensions of the integrating sphere should be at least six times the overall length of the lamp. Even when strict substitution is applied the diameter of the sphere should be at least 1.5 times the length of the lamp. The minimum sphere diameter required to measure a 1.5 m long tubular fluorescent lamp would be 2.25 m for direct substitution and 9 m for absolute measurements. The diffused light in the integrating sphere ensures that the output is insensitive to spatial, angular and polarization changes in the output. The sphere output provides a true cosine response and a very repeatable uniform illumination plane. Integrating sphere’s interior is covered with a diffuse white reflective coating, with small holes for entrance and exit ports. The important property of the integrating sphere is a uniform scattering or diffusing effect. Light rays incident on any point on the inner surface are, by multiple scattering reflections, distributed equally to all other points. Effects of the original direction of light are made minimum.



LIGHT AND ITS MEASUREMENT    2.27

We may consider the integrating sphere as a diffuser which preserves power but destroys spatial information. It is typically used with some light source and a detector for optical power measurement. It is well known that the practical implementation of the integrating sphere was due to work by R. Ulbricht (1849-1923) published in 1900.

Fig. 2.9  A Typical Integrating Sphere (Open View)

It has become a standard instrument in photometry and radiometry. It has the advantage over a goniophotometer for measuring the light produced by a source that total power can be obtained in a single measurement. Light scattered by the interior of the integrating sphere is evenly distributed over all the angles. The integrating sphere is used in optical measurements. The total power (flux) of a light source can be measured without inaccuracy caused by the directional characteristics of the source. Reflection and absorption of samples can be studied. The sphere creates a reference radiation source that can be used to provide a photometric standard. The commercial model of integrating sphere employs four separate lamps that can be specified to achieve the required spectral output from ultraviolet through infrared. Since all the light incident on the input port is collected, a detector connected to an integrating sphere can accurately measure the sum of the entire ambient light incident on a small circular aperture. The total power of a laser beam can also be measured, free from the effects of beam shape, incident direction, and incident position. Integrating spheres are ideal optical diffusers; they are used for radiometric measurements where uniform illumination or angular collection is essential, for reflectance and transmittance measurements, or even to mix two light sources. The light is collected by an integrating sphere and uniformly reflected and scattered around the sphere’s interior so the output is a uniform, spatially integrated field (Lambertian) of radiation which is insensitive to the spatial, angular and polarization changes of the input. Integrating spheres are equipped with internal baffles to prevent direct ray path from input, to output

2.28    FUNDAMENTALS OF LIGHTING

ports. These baffles and small wall imperfections make these real diffusers less perfect. That is, they are the closest to Lambertian diffusers of any simple optical devices, but exhibit some departure from the ideal. For critical radiometric measurements, always use an integrating sphere; they are close to ideal diffusers. Integrating spheres are considerably more expensive, and suffer similar or higher throughput losses than disk diffusers, but they provide a true cosine response and very repeatable uniform illumination of a spectrometer grating or detector. Disk diffusers are suitable for applications requiring relatively uniform illumination, such as detectors which are not uniformly sensitive across the photosensitive area. Integrating spheres are also recommended for diffuse reflectance and transmittance measurements. They are standard for UV or IR analytical surface spectroscopy. Most backscatter and turbidity measurements are improved by use of an integrating sphere due to higher angle collection (a full 180° hemisphere). The flux measurements made with integrating spheres are more reliable than other devices. The sphere reduces or removes sensitivity to beam shape and angle, and detector spatial response variations. Most of the integrating spheres are available with a choice of different interior materials. For example in certain applications one can choose barium sulphate-based white coated spheres. The coating is highly reflective, >97% in the visible region.

Theoretical Principle of Integrating Sphere The principle of working of an integrating sphere is depicted in Fig. 2.10.



L

A

B

Photometer V()

Fig. 2.10  Geometry of Integrating Sphere

Flux distribution and Principle of measurement F = Luminous flux in lumens; L = Light source B = Baffle A = Lambertian highly reflective coating (80-98%) Consider an integrating sphere with highly diffused reflecting surface inside as shown in Fig. 2.9. If the sphere is coated with a paint of reflectance



LIGHT AND ITS MEASUREMENT    2.29

R, it is possible to obtain by simple calculation an estimate of the reflected illuminance when a light source is suspended within the sphere. If we use the principle of the conservation of energy, then we can set an energy balance equation for the sphere. All the light emitted by the lamp is eventually absorbed by the sphere, but at any moment the total flux F received on the inside of the sphere equals the direct flux emitted by the lamp Fd plus reflected flux Fr. We, thus, have:

F = Fd + Fr = Fd + R × F,

thus

F = Fd/(1 – R) …2.19

The reflected flux Fr can be written as Fr = F – Fd = Fd/(1 – R) – Fd = Fd {(1/(1 – R) – 1} = Fd. R/(1 – R) Dividing both sides by the area of the sphere, A gives the reflected illuminance Er, in terms of the direct illuminance Ed and the reflectance of the sphere, R.

Er = Ed R/(1 – R) …2.20

The reflected illuminance, Er is measured at a convenient point inside the sphere. A baffle is fixed within the sphere between the light source and the photocell to ensure that only the reflected illuminance is measured. There are errors in the measurements because of the presence of source itself and the integrating sphere may not be fulfilling ideal conditions.

Substitution Method Some of these problems may be overcome or minimized by using the substitution method. In this method, a lamp of known luminous flux is placed inside the integrating sphere and the illuminance of the wall is measured. The test lamp is then put inside the sphere in place of the standard lamp and the illuminance is measured. If the two lamps are physically identical, except for the their luminous outputs, then from the ratio of the illuminance readings the flux of the test lamp can be calculated. In this arrangement both the standard and the test lamps absorb the same amount of radiation. If the test and standard lamps are physically different, it is necessary to measure the ratio of self-absorption. This is achieved by placing an auxiliary lamp close to the sphere wall and screening it to prevent light falling directly onto the photocell or the test lamp. A reading is taken with the auxiliary lamp, after which the standard lamp is placed in its normal position in the centre of the sphere, but unlit, and a further reading is taken. Let the ratio of these readings be Ks. This procedure is then repeated with the test lamp in place of the standard lamp. Let this ratio be Kt. The ratio Ks/Kt is used to correct readings taken in the normal way.

2.30    FUNDAMENTALS OF LIGHTING

Spectrophotometer A spectrophotometer (Fig. 2.11) is a photometer (a device for measuring light intensity) that can measure intensity as a function of the light source wavelength. Important features of spectrophotometers are spectral bandwidth and linear range of absorption or reflectance measurement.

Fig. 2.11  A Typical Spectrophotometer

A spectrophotometer is commonly used for the measurement of transmittance or reflectance of solutions, transparent or opaque solids, such as polished glass, or gases. However they can also be designed to measure the diffusivity on any of the listed light ranges that usually cover around 200 nm - 2500 nm using different controls and calibrations. Within these ranges of light, calibrations are needed on the machine using standards that vary in type depending on the wavelength of the photometric determination. The use of spectrophotometers spans various scientific fields, such as physics, materials science, chemistry, biochemistry, and molecular biology. They are widely used in many industries including semiconductors, laser and optical manufacturing, printing and forensic examination, as well in laboratories. Ultimately, a spectrophotometer is able to determine, depending on the control or calibration, what substances are present in a target and exactly how much through calculations of observed wavelengths. An experimental set up of spectrophotometer is shown in Fig. 2.11(a).

Fig. 2.11(a)  A Typical Spectrophotometer Set Up



LIGHT AND ITS MEASUREMENT    2.31

The light source (far right) sends light via an input fiber into a cuvette in a cuvette holder (bottom center). The light interacts with the sample. The output fiber carries light from the sample to the spectrometer (top center), which is connected to the PC (far left). Using this set up measurement of  absorbance, transmission, reflection, relative irradiance, absolute irradiance, color measurements for light and light signaling devices can be made.

Photometer A photometer (Fig. 2.12) is an instrument that measures the luminous intensity of a light source by comparing it with a standard source whose intensity is known. The distances between the instrument and the two light sources are adjusted until they each provide the same illuminance at the photometer. The human eye is a very good judge of this equal illuminance, and photometers generally use some system which allows two screens to be compared. A photocell light meter is an instrument that directly measures the illuminance on a surface.

Fig. 2.12  A Typical Photometer

The electrical resistance of some semiconductors, such as selenium, changes with exposure to light and this property is used in an electrical circuit connected to a galvanometer. This meter may be calibrated in lux or footcandles. Photometers are also used to measure: (i) Illuminance, (ii) Irradiance (iii) Light absorption, (iv) Scattering of light by media (v) Reflection of light, (vi) Fluorescence, (vii) Phosphorescence and (viii) Luminescence. The photometer is usually placed on an optical bench such that the illuminance from the source being investigated is equal to that of the standard source as equal illuminance could be judged by the eye.

2.32    FUNDAMENTALS OF LIGHTING

The values of the relative luminous fluxes can then be calculated as the illuminance decreases proportionally to the inverse square of the distance. A well known such photometer used to have a paper with an oil spot, that could make the paper slightly more transparent – when the spot was not visible from either side the illuminance from the two sides is equal.

Photoresistor A photoresistor or light dependent resistor or cadmium sulphide (CdS) cell is a resister whose resistance decreases with increasing incident light intensity. It can also be referenced as a photoconductor. A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The free electrons thus obtained and the holes conduct electricity, thereby lowering resistance. A photoelectric device can be either intrinsic or extrinsic. In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire band gap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

Operating Principle of Photometers Photometers detect the light with photoresistors, photodiodes or photomultiplier tubes. To analyze the light, the photometer may measure the light after it has passed through a filter or through a monochromator for determination at defined wavelengths for analysis of the spectral distribution of the light. There are some photometers which measure light by counting individual photons rather than incoming flux. The operating principles are the same but the results are given in units such as photons/cm2 or photons· cm–2· s r –1 rather than W/cm2 or W· cm–2· s r–1. Due to their individual photon counting nature, these instruments are limited to observations where the irradiance is low. The irradiance is limited by the time resolution of its associated detector readout electronics. With current technology this is in the megahertz range. The maximum irradiance is also limited by the throughput and gain parameters of the detector itself. The light sensing element in photon counting devices in near infrared (NIR), visible and ultraviolet wavelengths is a



LIGHT AND ITS MEASUREMENT    2.33

photomultiplier to achieve sufficient sensitivity. In airborne and space-based remote sensing such photon counters are used at the upper reaches of the electromagnetic spectrum such as the X-ray to far ultraviolet. This is usually due to the lower radiant intensity of the objects being measured as well as the difficulty of measuring light at higher energies using its particle-like nature as compared to the wavelike nature of light at lower frequencies. Conversely, radiometers are typically used for remote sensing from the visible, infrared though radio frequency range. The measuring of luminous flux, luminous intensity and luminance is based on the measuring of illuminance by means of the photometric fundamental law. That is why this quantity has got a special significance. A state-of-the-art luminance-meter is shown in Fig. 2.13.

Fig. 2.13 Luminance Meter with Data Processor

Luminance meter measures (in candelas per square metre) the brightness of a point on a surface that is radiating or reflecting light. In this instrument the luminous intensity in a given direction of a small element of surface area divided by the orthogonal projection of this area onto a plane at right angles to the direction which is measured.

Illuminance Meter or Lux Meter Technical requirements for an illuminance or lux meter are as follows: There should be fine approximation to the spectral luminous efficiency curve V(λ) of the human eye and the photometer-head should be having the cosine-correction. There should be linear relation between illuminance and  photocurrent with low temperature influence, ageing and fatigue of the light sensor. There should be correct light-measurement of AC-powered discharged lamps with wide measure range, which should make possible the following measurements with just one instrument. These measurements are: l

measuring of daylight up to 200,000 lx,

l

measuring of indoor lighting up to 2,000 lx,

l

measuring of low illuminances, e.g., street lighting up to 20 lx.

l

emergency-lighting up to 2 lx.

2.34    FUNDAMENTALS OF LIGHTING

Simple operation, that means no application of weakening filters and correction-factors,correct reading of the measuring results by a digital display. Accurate photometry of high-pressure discharge lamps with pulsed current supply calls for a special photocurrent amplifier, reliably estimate the linear mean value of luminous flux. Ordinary luxmeters are generally not designed to measure high pulsating light sources . Therefore, to avoid errors, we should heed this recommendation before measuring such light sources. It is always essential to critically evaluate and investigate the suitability of the photometer for measurements of pulsating light when it is required. All these conditions are generally met by a good digital lux meter (see Fig 2.14).

Fig. 2.14 A typical Digital Lux Meter

Fields of Applications of Lux Meter The instrument is suitable for measuring of illuminance,luminance and reflection coefficient.It can also be used for measuring of luminous intensity by means of the photometric laws. The luminous flux  measurements  are done in accordance with the relation:

Flux = E × Area in m2 ...2.21

Further applications are possible, e.g., (i) Control of the turning on and off of street luminaires (ii) Brightness control for roadway tunnels (iii) Measurements of building materials used in lighting technology (e.g. for measurement of reflection, transmission, absorption, extinction etc.) (iv) Measurements for solar facilities (v) Light measurements for applications in physics, optoelectronics, meteorology, botany, biology and medicine, etc. 

The Photodiode This is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. The photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with



LIGHT AND ITS MEASUREMENT    2.35

a window or optical fibre connection to allow light to reach the sensitive part of the device.

Operational Principle Photodiode is a P-N junction device. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a mobile electron and a hole which is positively charged. If the absorption occurs in the junction’s depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is calibrated in the form of amount of light falling at the junction. Thus photodiode can measure the amount of light.

Photovoltaic Mode If used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up in the process. The diode becomes forward biased and “dark current” begins to flow across the junction in the direction opposite to the photocurrent. This mode is responsible for the photovoltaic effect, which is the basis for solar cells— in fact, a solar cell is just an array of large group of photodiodes.

Applications of Photodiodes P-N photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes. Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for remote controls in VCRs and televisions, etc. In other consumer items such as camera light meters, clock radios (the ones that dim the display when it’s dark) and street light photoconductors are often used rather than photodiodes, although in principle either could be used. Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors. They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators) or instruments to analyze samples.

Comparison of Photodiode with Photomultiplier Tube Advantages of photodiode as compared to photomultiplier tube: 1.

There is excellent linearity of output current as a function of incident light.

2.

There is good spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with other semiconductor materials.

2.36    FUNDAMENTALS OF LIGHTING

3.

Low noise.

4.

Low cost.

5.

Compact and light weight.

6.

Long lifetime.

7.

High quantum efficiency, typically 80%.

8.

No high voltage required.

Disadvantages Compared to Photomultipliers 1.

Small area

2.

No internal gain (except avalanche photodiodes, but their gain is typically 10² – 10³ compared to up to 108 for the photomultiplier)

3.

Much lower overall sensitivity

4. The photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits; However, response time for many designs is slower.

The Photomultiplier Tubes The photomultiplier tube (Fig. 2.15) is a member of a class of vacuum tubes. This is an extremely sensitive detector of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the current produced by incident light by as much as 100 million times , in multiple stages, enabling individual photons to be detected when the incident flux of light is very low. The combination of high gain, low noise, high frequency response, and large area of collection has earned photomultipliers an essential place in nuclear and particle physics, astronomy, medical diagnostics including blood tests, medical imaging, motion picture film scanning (telecine), and high-end image scanners known as drum scanners. Semiconductor devices, particularly avalanche photodiodes, are alternatives to photomultipliers; however, photomultipliers are uniquely well-suited for applications requiring low-noise, high-sensitivity detection of light that is imperfectly collimated. While photomultipliers are extraordinarily sensitive and moderately efficient, research is still underway to create a photon-counting light detection device that is much more than 99% efficient. Such a detector is of interest for applications related to quantum information and quantum cryptography. The elements of photomultiplier technology, when integrated differently, are the basis of night vision devices which have wide applications.

Structure and Operating Principle Photomultipliers are constructed from a glass envelope with a high vacuum inside, which houses a photocathode, several dynodes, and an anode. Incident photons strike the photocathode material, which is present as a thin deposit on



LIGHT AND ITS MEASUREMENT    2.37

the entry window of the device, with electrons being produced as a consequence of the photoelectric effect. These electrons are directed by the focusing electrode toward the electron multiplier, where electrons are multiplied by the process of secondary emission. Photocathode Incident photon

Electrons

Anode Electrical connectors

Scintillator

Light photon

Focusing Dynode electrode

Photomultiplier tube (PMT)

Fig. 2.15 Photomultiplier Tube

In this apparatus the electron multiplier consists of a number of electrodes usually called dynodes. Each dynode is held at a more positive voltage than the previous one. The electrons leave the photocathode, having the energy of the incoming photon (minus the work function of the photocathode). As the electrons move toward the first dynode, they are accelerated by the electric field and arrive with much greater energy. Upon striking the first dynode, more low energy electrons are emitted, and these electrons in turn are accelerated toward the second dynode. The geometry of the dynode chain is such that a cascade occurs with an ever-increasing number of electrons being produced at each stage. Finally, the electrons reach the anode, where the accumulation of charge results in a sharp current pulse indicating the arrival of a photon at the photocathode. There are two common photomultiplier orientations, the head-on or end-on design, as shown above, where light enters the flat, circular top of the tube, and the side-on design, where light enters at a particular spot on the side of the tube. The use of different photocathode materials affect its performance which is also affected by the transmission of the window material that the light passes through, and by the arrangement of the dynodes. A large number of photomultiplier models are available having various combinations of these, and other, design variables.

Applications The photomultiplier tubes generally utilize 1000 to 2000 volts to accelerate electrons within the chain of dynodes as shown above in the diagram. The most negative voltage is connected to the cathode, and the most positive voltage is connected to the anode. Negative high-voltage supplies (with the positive terminal grounded) are preferred, because this configuration enables the photocurrent to be measured at the low voltage side of the circuit for amplification by subsequent electronic circuits operating at low voltage. The voltages are distributed to the dynodes by a resistive voltage divider, although variations such as active designs with transistors or diodes are possible.

2.38    FUNDAMENTALS OF LIGHTING

The divider design, which influences frequency response or rise time, can be selected to suit varying applications. Some instruments that use photomultipliers have provisions to vary the anode voltage to control the gain of the system. Being powered, the photomultipliers must be shielded from ambient light to prevent their destruction through over excitation. If used in a location with strong magnetic fields, which can curve electron paths, photomultipliers are usually shielded by a layer of mu-metal. This magnetic shield is often maintained at cathode potential. When this is the case, the external shield must also be electrically insulated because of the high voltage on it.

Goniophotometer System The word gonio is derived from the Greek word which means angle. Thus, a goniometer is an instrument that measures angles most accurately. Likewise, the word photometer is derived from the Greek word photon which means light and thus photometer is an instrument that measures light. Therefore, Goniophotometer = Goniometer + Photometer. A goniophotometer therefore performs the measurement of the spatial distribution of a radiation source and displays the photometric properties of the light visible to the human eye in relation to a defined angular position. The automotive and general lighting industries use goniophotometers for lighting research and as a control measure in their manufacturing process. Goniophotometer is a device used for measurement of the light emitted from an object at different angles in the space. The use of goniophotometers has been increasing in recent years with the introduction of LED-light sources, which are mostly directed light sources, where the spatial distribution of light is not homogeneous. Due to strict regulations, the spatial distribution of light is of high importance to automotive lighting and its design. Goniophotometer performs angular analysis of light sources and displays (see Fig. 2.16).

Fig. 2.16 Goniophotometer System

Goniophotometers are generally used to test flood light sources, vehicle lighting and road signs, etc. Near-field goniophotometers are used for testing flat panel displays and luminaires; a source imaging goniophotometer is used for generating radiant source.



LIGHT AND ITS MEASUREMENT    2.39

Goniophotometer system which comprises a goniophotometer and all its accessories is a very important lighting measurement tool for any standard photometric laboratory. Floodlights, projectors, automobile heallights, and other concentrating beams have to be tested at much greater distances than do indoor luminaires. The detector should be far enough away so that it sees the whole of the reflector flashed with light. For normal floodlighting luminaires as used in stadium lighting, 33 meters is sufficient. However by international agreement automobile headlights are tested at 25 meters. Far field goniophotometer and detector with stray light tube positioned at a distance from which the light source can be regarded as point source (25 m for automotive headlamps, 3.162 m for signal lamps). One consequence of the long path length is that the photocell has to be fixed and the luminaire rotated. The goniophotometer required for measuring the angles of elevation and azimuth needs to be very strongly made to take the weight of the luminaires without sagging. The luminaire holder turns in elevation and also moves in azimuth. The detector can be a photovoltaic cell or, if this is not sensitive enough, a photomultiplier tube. It is convenient to arrange for the readout of the photocell to be at the same position as the goniophotometer so that only one person is needed to take the readings and work with the goniophotometer. The equipment can be computer-controlled so that the readings are taken at predetermined intervals and goniophotometer turned by electric motors controlled by the computer. The measurements are carried out in darkened room with luminaire mounted on the goniophotometer. In order to obtain the necessary path length it is sometimes convenient to locate the detector at the end of a light-tight tunnel which can pass through adjoining rooms. There should be baffles (a mechanical device to limit or regulate the flow of light) fixed to the wall of the tunnel to reduce the amount of stray light.

Principles and Theory of Goniophotometer As mentioned above, a positioning system and photometer as a whole are termed ‘goniophotometer’, which is defined as a specialized instrument for measuring the angular variation of a given photometric level. In recent years, procedures applied to quality control have spread to practically all production sectors. The lighting industry in particular now sees the application of new legislations in various countries governing aspects of power efficiency in lights and ballasts. The new legislations will make it compulsory for manufacturers to supply luminaires with photometric characteristics in accordance with currently approved quality and safety standards. The levels of interest, from the viewpoint of processing certificates for luminaires, are light flux, the distributions of illuminance and the light intensity. In order to achieve these levels, a positioning system is required that is capable of shifting a photometer, with precision, on the surface of a sphere in whose centre is the source of light to be evaluated.

2.40    FUNDAMENTALS OF LIGHTING 25 meter optical axis

Photoreceiver

Fig. 2.17  Goniophotometer’s Operation Scheme

Such a positioning system along with a photometer is shown in Fig. 2.17. The spherical run of the photometer around the luminaire while pointing at the same divides the surrounding sphere into sectors where luminous intensity and illuminance values are calculated from data supplied by the sensor.

Moving Mirror Goniophotometer Recently there has been some very significant advancement in the field of photometry. One such advancement has been in the development of moving mirror goniophotometer. The Illuminating Engineering Society, USA has announced in its annual progress report of the year 2010 that Lighting Sciences Inc. has developed ‘Moving Mirror Goniophotometer Series 6400 T’ which has been selected for 2010 as a significant engineering advancement for the lighting industry. This recognition is primarily because the Series 6400 T incorporates a triple amplifier system that automatically optimizes amplifier sensitivity for all different intensity ranges during a test. The multiple amplifier and electronic channels developed for the Series 6400 T enable lighting laboratories to run an entire indoor photometric test in three minutes, without manual intervention or concern for the accuracy of low-end intensity measurements. Data collection occurs at a rate of 18,000 readings per second, allowing the mirror to rotate at 4 rotations per minute (RPM) or faster throughout the entire test. Moving mirror goniophotometer set up is illustrated in Fig. 2.18.

Fig. 2.18  The Moving Mirror Goniophotometer System



LIGHT AND ITS MEASUREMENT    2.41

Important characteristics of the above set up are listed below: 1.

The luminaire remains at a fixed height during the test; mirror rotates around luminaire. This eliminates errors due to thermal effects in laboratory for temperature sensitive lamps such as fluorescent and LEDs.

2.

The high-quality construction delivers trouble-free performance for years.

3.

The steel framework allows the safe attachment of heavy luminaires in the system.

4.

Precision digital drives provide unrivalled accuracy.

5.

Signal Maximizer circuitry with three amplifiers enhances system sensitivity, dynamic range and accuracy.

6.

16-bit analog-to-digital converters.

7.

The programmed computer system options avoid error-prone operator Inputs in the system.

8. Luminaire mounting system lowers to average chest height. Avoids hazardous step-ladder operations. 9.

Attention to detail in design and appearance.

10. The automatic dark current compensation reduces errors and boosts accuracy of measurement. 11. The rotating aperture mask in front of photodetector allows only light from the mirror to enter. 12. Room reflections are almost entirely eliminated. 13. The unmatched software selection for any test situation with the convenience of using the test data with nearly any design and analysis software concerned. 14. Luminaire hard-wire power connections eliminate wear and electrical resistance errors for measurements. 15. High-speed data collection - needs only a fraction of the time required by competitive systems.

The Goniophotometer Set up Components The High Speed Moving Mirror Goniophotometer consists of four basic components: 1. The Mirror Swing-Arm and Support Structure The height of the test luminaire or lamp is fixed. The mirror rotates in circles around the luminaire, eliminating constant repositioning of the test item to different heights and possible different air temperatures.

This component holds the test luminaire or lamp during testing and includes the support tower, mirror, swing-arm and luminaire mounting fixtures. The structure can be manufactured in several sizes, depending on the maximum size of item to be tested.

2.42    FUNDAMENTALS OF LIGHTING

2. The Photodetector System The specially shielded unit houses the high-sensitivity photodetector used to obtain the photometric data for the luminaire concerned. The rotating aperture mask that is sequenced with the mirror removes stray light. The photodetector is connected to a signal maximizer amplifier circuit, which automatically adjusts the system sensitivity or gain settings to ensure the highest possible accuracy throughout the test. Three-amplifier system is provided for exceptional dynamic range. The data are relayed to the computer interface through three analog-to-digital converters of the system.

The Goniophotometer Integrated Console This unit contains all of the system’s electric and electronic equipment including main power switches, test lamp voltage adjustment, motor controllers and computer interfaces.

The Computer System This system includes a Windows 7 computer and the photometric software to control the operation and data collection functions of the Moving Mirror Goniophotometer during testing. All information displays are on the computer monitor. The additional automated software produces all forms of required reports and graphs.

The Component Features The Mirror Swing-Arm and Support Structure The main support tower, swing-arm and mounting assembly are all engineered and manufactured to produce the most stable test platform. Precision assembly ensures that the mirror swing-arm and counterweight are perfectly balanced to provide for continuously accurate alignment throughout all rotational positions of the set up. In order to avoid the common problems associated with similar devices, the mirror of the high speed moving mirror goniophotometer is bonded to its rigid steel base. This is then bolted to the steel framework of the swing-arm, which guarantees a perfectly flat mirror surface, even at its uppermost zenith position. The test luminaire or lamp is at a fixed height throughout the test, while the mirror rotates continuously around it. This allows use of an extremely strong steel framework for attachment of the luminaire system. The raising and lowering of the entire lamp/luminaire mounting assembly is motorized. It may be lowered to about 1.5 m. (5 feet) above ground level so that the test technician can easily attach and connect the test lamp or luminaire. It then rises to the proper test height which is indicated by a wall mounted laser that is aimed at the goniophotometer center. This design eliminates the need for inconvenient mounting platforms or ladders, which can be a dangerous liability, particularly when testing large or heavy luminaires.



LIGHT AND ITS MEASUREMENT    2.43

The separate mounting turntables are provided for uplight and downlight testing. In both instances the luminaire is positioned at the goniophotometer’s centerline axis, which allows the goniophotometer to accommodate very tall fixtures either above or below this centreline. Because the width of the luminaire support mechanism is only about 0.2 m (8 inches.), light loss due to shadowing is kept to an absolute minimum. Precision digital motors are coupled to the mirror shaft and the luminaire rotation shaft, and are constantly monitored by the system’s computer, ensuring that both the mirror instantaneous position and the luminaire angles are precise throughout the test sequence. Resolution of each motor of the High Speed Moving Mirror Goniophotometer is an extremely fine 0.01 degrees.

Photodetector System The photodetector is a high-sensitivity silicon cell with spectral sensitivity correction filtering. It offers high-speed response, excellent linearity and low drift, and is housed in a shielded chamber to exclude stray light and to prevent light from being received directly from the luminaire itself. Inside the photodetector housing is a digitally-controlled rotating aperture or pivoting collimating tube, which is synchronized electronically with the mirror rotation. This serves to block stray light from room surfaces, by allowing the photodetector to see only the mirror. The photodetector is interfaced to the system computer. This consists of a triple amplifier system. The primary amplifier sensitivity is set by the software to the highest gain setting that will not cause saturation or over-ranging. This is performed during a preliminary mirror scan. If higher light intensity is encountered during the test, auto-ranging occurs to reset the gain to the optimum level. Also, two amplifiers set with different sensitivities also constantly deliver readings; they record the lower light intensities encountered during the mirror scans. All three amplifiers are read simultaneously. This Signal Maximizer three-channel system ensures that there is a separate amplifier channel available for optimum readings of high, medium and low light intensities at every instant during the mirror scans. This dramatically increases the goniophotometer dynamic range. The Signal Maximizer circuitry also automatically compensates for “dark current” – a signal generated by the electronics even when there is no light on the photodetector. This signal varies from one gain setting to the next, and if not properly accounted for, can create test result errors. Dark current values are automatically measured and applied to each gain stage throughout the test sequence without the need for operator input. The amplifiers and electronics are located immediately adjacent to the photodetector, minimizing noise and maximizing signal stability. Interfacing between the photodetector amplifier and the computer is through use of three 16 bit analog-to-digital converters, one for each channel.

2.44    FUNDAMENTALS OF LIGHTING

Goniophotometer Console This component contains all of the system’s electrical and electronic controls. Main power switches and a test lamp voltage adjustment are included in the goniophotometer console. A hand-held control box is also attached to the unit on a long cable so the test operator can walk around the lab and manually position the luminaire during test set up. A remote power analyzer meter is installed which incorporates a large digital display of volts, amps and watts. The meter measures the exact lamp characteristics by use of remote sensing leads to the test luminaire. An option for measurement of Power Factor and Total Harmonic Distortion is available. An additional benefit of this system is that the power measurements on the ballast secondary of a high intensity discharge lamp are also facilitated. Readings are transmitted to the computer and displayed so that all electrical monitoring can be done from the computer station. Automated voltage or power adjustment controlled throughout the test by the computer is also available as an option.

Computer Softwares The heart of the high speed moving mirror goniophotometer is the computer hardware and software which control the system’s entire operation. Actually all functions of the system are computer automated including the mirror and lamp/luminaire rotation, selection of data points and the recording of data. Although the goniophotometer’s operation is computer-controlled, manual over-rides additionally allow for unusual testing situations. The computer system provided with the High Speed Moving Mirror Goniophotometer consists of MS Windows based equipment totally configured and integrated to provide full system operation, data collection, processing, and report output.

Control Software Comprehensive software developed in the field controls the operation of the High Speed Moving Mirror Goniophotometer and gathers the appropriate test data for the particular type of luminaire being tested. Standard test formats are provided for indoor luminaires, area and roadway luminaires, floodlights, spotlights and bare lamps, and may be selected by the test operator from easyto-use menus. The software developed is usually MS Windows 7 based. It allows automatic or manual operation of the system, with pre-stored horizontal and vertical angle formats for commonly-used test procedures. Test data are automatically collected and stored for all types of tests. A custom software package is also provided that allows the user to specify unique sets of data collection angles, with increments as small as 0.01 degrees. Specialized test formats therefore can be developed and applied.



LIGHT AND ITS MEASUREMENT    2.45

Data Processing Software Data output files are produced. This provides a wide range of test data processing software, which has been developed to produce attractive and easy-to-understand reports and graphical presentations. These data files may be processed on the photometer’s computer or another compatible system for processing, including a network computer. The data processing software is equipped with a simple-to-use Windows interface that generates complete reports, including building of test report headings, application of calibration factors and insertion of a luminaire sketch. Using a single command, data processing can be switched between relative and absolute test reports. The system is IES LM-79-08 compliant for LED luminaire testing. With the custom software package, data can be exported to external spreadsheet software.

Plotting Programs Software is also available to provide graphical output. Example plot packages that are provided include: 1.

Polar intensity (candlepower).

2.

Axially symmetric, bisymmetric and quadrilaterally symmetric distributions.

3.

Roadway plane and cone intensities.

4.

CIE C-0 and C-90 graphs.

5. Isofootcandle/Isolux. 6.

IESNA and CIE roadway classification.

7.

Floodlight intensity (rectilinear).

8.

Floodlight isocandela (Type B/Beta).

IES and CIE Formats The supplied software supports both IES and CIE testing formats and reports. The floodlight reports are generated in Type B output (CIE B-Beta) while all others are in IESNA normal polar format or CIE type C-Gamma format. The American or International terminology is selectable, e.g., candlepower or intensity, efficiency or light output ratio, etc., as per requirement. Reports can be based upon absolute or relative lamp lumens, or 1000 lumens.

The Output Data Files The provision is also made within the software for the generation of data files in accordance with various industry-standard file formats (IES, CIBSE, EULUM or CIE, as per need. This flexibility ensures that the test data produced by the

2.46    FUNDAMENTALS OF LIGHTING

High Speed Moving Mirror Goniophotometer Series 6400T are compatible with virtually all lighting software packages available.

Size of Luminaires Different goniophotometer models are available depending upon the maximum size of luminaire to be tested; all other features are identical.

The Models for Luminaire Sizes Model 6220 T 0.6 × 0.6 m (2 × 2 ft.) 0.85 m (2.8 ft.) Model 6240 T 0.6 × 1.2 m (2 × 4 ft.) 1.3 m (4.5 ft.) Model 6440 T 1.2 × 1.2 m (4 × 4 ft.) 1.7 m (5.7 ft.) We can now summarize some of the important applications of the goniophotometer system: 1.

Testing of automobile headlamps, tail-lamps and direction indicators,

2.

Testing of luminance of license plates,

3.

Testing of road traffic signs,

4.

Testing of reflector lights;

5.

Testing of retro-reflecting materials,

6.

Testing of LEDs, LED modules and LED luminaires,

7.

Testing of airport taxiway lighting,

8.

Testing of bicycle and motorbike lights,

9.

Testing of aerospace and marine lamps,

10. Testing of endoscopic illuminators, 11. Testing of general lighting (directional radiation sources), etc.

Sensors, Thermal Detectors and their Applications A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards.

Uses of Sensors Sensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) and lamps which dim or brighten by touching the base. There



LIGHT AND ITS MEASUREMENT    2.47

are also innumerable applications for sensors of which most people are never aware. Applications include cars, machines, aerospace, medicine, manufacturing and robotics. A sensor is a device which receives and responds to a signal or stimulus. Here, the term “stimulus” means a property or a quantity that needs to be converted into electrical form. Hence, sensor can be defined as a device which receives a signal and converts it into electrical form which can be further used for electronic devices. A sensor differs from a transducer in the way that a transducer converts one form of energy into other form whereas a sensor converts the received signal into electrical form only. A sensor’s sensitivity indicates how much the sensor’s output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1°C, the sensitivity is 1 cm/°C. Sensors that measure very small changes must have very high sensitivities. Sensors also have an impact on what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer. Sensors need to be designed to have a small effect on what is measured, making the sensor smaller often improves this and may introduce other advantages. Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS (Micro-Electro-Mechanical Systems) technology. In most cases, a micro sensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. Good sensor obeys the following rules: l

Is sensitive to the measured property

l

Is insensitive to any other property

l

Does not influence the measured property

Ideal sensors are designed to be linear. The output signal of such a sensor is linearly proportional to the value of the measured property. The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement.

Sensors in Nature All living organisms contain biological sensors with functions similar to those of the mechanical devices. Most of these are specialized cells that are sensitive to: Light, motion, temperature, magnetic fields, gravity, humidity, vibration, pressure, electrical fields, sound, and other physical aspects of the external environment; physical aspects of the internal environment, such as stretch,

2.48    FUNDAMENTALS OF LIGHTING

motion of the organism, and position of appendages (proprioception); environmental molecules, including toxins, nutrients, and pheromones; estimation of bio molecules interaction and some kinetics parameters; internal metabolic milieu, such as glucose level, oxygen level, or osmolality, internal signal molecules, such as hormones, neurotransmitters, and cytokines, differences between proteins of the organism itself and of the environment or alien creatures.

Biosensors in Nature The sensors which detect analytes thanks to a biological component, such as cells, protein, nucleic acid or biomimetic polymers, are called biosensors, where as a non-biological sensor, even organic (=carbon chemistry), for biological analytes is referred to as sensor or nanosensor (such a microcantilevers). The optical sensors are a class of sensors that use light waves as an input. These light waves can be used directly to determine the proximity of an object, or indirectly to measure other properties. There are several different types of optical sensors: Photodetectors as Sensors – The photodetectors, also known as proximity sensors, are used to determine if a moving object enters the range of a sensor. The most commonly found photodetector is the “electric eye”. This type of sensor works by projecting a beam of light from a transmitter to a receiver across a specific distance. As long as the beam of light maintains a connection with the receiver, the circuit remains closed. If an object passes through the beam of light, the continuity of the circuit is lost, and the circuit opens. An example of this type of sensor is a garage door opener safety sensor that will halt the closing of the door if an object breaks the beam. Infrared Sensors – Active infrared sensors project a beam of light in the infrared spectrum and receive the returning reflection from objects in the sensor’s range. Infrared sensors can be used as proximity sensors, such as in automatic doors. The passive infrared sensors are used to measure the radiation of heat within its range. Examples of passive infrared sensors include “heat-seeking” missile guidance systems and infrared thermography systems. Fiberoptic sensors – Fiberoptic sensors can be used to measure a wide range of physical phenomena, depending on the configuration of the sensor. Optical fibers can be coated with materials that respond to changes in strain, temperature, or humidity. Optical gratings can be etched into the fiber at specific intervals to reflect specific frequencies of light. As the fiber is strained, the distances between the gratings change, allowing the physical strain to be measured. The Interferometers – The interferometer is a device that adds waves from two light sources, generating an interference pattern. The pattern can be used to determine properties of the waves. Interferometry is used in many industries, but its most visible use is in astronomy.



LIGHT AND ITS MEASUREMENT    2.49

Initial interferometers were limited to large wavelength radio telescopes, but are now applied to shorter wavelengths of light. This method allows astronomers to measure the diameters of stars, and future projects using interferometers will help astronomers detect and perhaps measure extra solar planets. Two small telescopes mounted a fixed distance apart can achieve the resolution of a single telescope with a diameter equal to the distance between the two telescopes. The optical sensors in many cases can provide non-contact measurement in environments where direct contact of electrical circuitry is not possible. The optical sensors are used for both very low-tech and very-high tech applications.

The Thermal Detectors The thermal detectors (see Fig. 2.19) include heat sensitive coatings, thermoelectric devices and pyroelectric devices, etc. Heat sensitive coatings use substances that are blended to melt at certain temperatures to specially formulated paint and greases that change color as there is change in temperatures. Heat sensitive coatings are relatively inexpensive but do not provide good quantitative data. Thermoelectric devices include thermocouples, thermopiles,thermistors and bolometers. These devices produce an electrical response based on a change in temperature of the sensor. They are often used for point or localized measurement in a contact or near contact mode. However, thermal sensors can be miniaturized. For example, microbolometers are the active elements in some high-tech portable imaging systems, such as those used by fire departments. Benefits of thermal detectors are that the element does not need to be cooled and they are comparatively low in price. Thermal detectors are used to measure the temperature in everything from home appliances to fire and intruder detection systems to industrial furnaces to rockets.

Fig. 2.19 Thermal Detectors

Occupancy Sensors for Lighting Control Motion sensors are often used in indoor spaces to control electric lighting. If no motion is detected, it is assumed that the space is empty, and thus does not need to be lit. Turning off the lights in such circumstances can save substantial amounts of energy. In lighting practice occupancy sensors are sometime also called “presence sensors” or “vacancy sensors”.

2.50    FUNDAMENTALS OF LIGHTING

Motion detection is a process of confirming a change in position of an object relative to its surroundings or the change in the surroundings relative to an object. This detection can be achieved by both mechanical and electronic methods. In addition to discrete, on or off motion detection, it can also consist of magnitude detection that can measure and quantify the strength or speed of this motion or the object that created it. When motion detection is accomplished by natural organisms, it is called perception. Motion can be detected by: sound (acoustic sensors), opacity (optical and infrared sensors and video image processors), geomagnetism (magnetic sensors, magnetometers), reflection of transmitted energy (infrared laser radar, ultrasonic sensors, and microwave radar sensors), electromagnetic induction (inductive-loop detectors), and vibration (triboelectric, seismic, and inertiaswitch sensors). Acoustic sensors are based on: electret effect, inductive coupling, capacitive coupling, triboelectric effect, piezoelectric effect, and fiber optic transmission. Radar intrusion sensors have the lowest rate of false alarms.

System Design and Components Occupancy sensors for lighting control use infrared (IR) or acoustic technology, or a combination of the two. The field of view of the sensor must be carefully selected/adjusted so that it responds only to motion in the space served by the controlled lighting. For example, an occupancy sensor controlling lights in an office should not detect motion in the corridor outside the office. Sensors and their placement are never perfect, therefore most systems incorporate a delay time before switching. This delay time is often user-selectable, but a typical default value is 15 minutes. This means that the sensor must detect no motion for the entire delay time before the lights are switched. Most systems switch lights off at the end of the delay time, but more sophisticated systems with dimming technology reduce lighting slowly to a minimum level (or zero) over several minutes, to minimize the potential disruption in adjacent spaces. If lights are off and an occupant re-enters a space, most current systems switch lights back on when motion is detected. However, systems designed to switch lights off automatically with no occupancy, and that require the occupant to switch lights on when they re-enter are gaining in popularity due to their potential for increased energy savings. These savings accrue because in spaces with access to daylight the occupant may decide on their return that they no longer require supplemental electric light.

Measurement of Color of a Light Source In order to understand the basic principles of color and its complexities for light sources the reader is advised to consult Chapter 1 of our book entitled: Applications of Light and Energy Measurement. Likewise, in order to understand the concept of color temperature or correlated color temperature and color rendering index for a light source, the reader is advised to consult Chapter 3 (light sources) of this book.



LIGHT AND ITS MEASUREMENT    2.51

The color of an object depends on the colors of light it reflects. A particular material absorbs the light frequency that matches the frequency at which electrons in the atoms of that material vibrate. A color can be specified by its colorimetric values. A colorimeter is an instrument that measures color using numbers derived from CIE values. A spectrophotometer is another instrument for measuring color. It samples wavelengths across the color spectrum. Measuring color allows us to compare the color gamut, or range of colors produced by different methods. We find that color transparency film produces a wide range of colors including some a monitor cannot display. The lighting experts are generally not able to predict the color appearance of a light source precisely, except under fairly restricted conditions. However, the International Lighting Commission (Commission Internationale de l’Eclariage, referred to as CIE) established a colorimetry system for color matching that has, with minor changes, remained in use for nearly a century. Human color vision begins with absorption of light by the eyes’ three cone photoreceptors. In the 19th century, scientists discovered that any light can be exactly matched in appearance with the proper combination of three different colored lights, known as primaries. They also discovered that color matching followed all the rules of linear algebra; addition, subtraction, multiplication, and division. The CIE system has been the foundation for all color calculations used by the lighting industry, in large part because color matching follows these algebraic rules. This three-primary principle is utilized today with color television and other electronic displays. Just by incorporating different amounts of just three highly saturated, red, green and blue primaries, a wide array of color perceptions can be created on the display. Generally lighting manufacturers publish spectral power distribution (SPD) data for their light sources, these data are cumbersome and more detailed than necessary for accurate, unambiguous color representation. Instead, the industry most commonly describes a light source’s color appearance using chromaticity, which is derived from the SPD of the light source using the CIE system. In that system the absolute amounts of the three primaries needed to match a given light are normalized so the sum of the amounts of the three primary lights equals one. In this way, any two of the normalized numbers give a complete description of a light source color. The two numbers used to describe a light source color mathematically are known as its chromaticity coordinates, or simply its chromaticity. The true color appearance of a light source is too complex to be represented precisely by chromaticity. However, the chromaticity of a light source is useful as an approximate representation of its color appearance. Lights with chromaticity coordinates at the bottom left of the diagram will generally appear blue, while those in the far right will appear red. Those near the black body locus will appear “white”. Chromaticity diagrams are often produced in color so that they resemble an artist’s palette, but this approach is technically inappropriate despite its visual appeal. The color temperature is a characteristic of visible light

2.52    FUNDAMENTALS OF LIGHTING

that has important applications in lighting. The color temperature of a light source is the temperature of an ideal black body radiator that radiates light of comparable hue to that of the light source. In practice, color temperature is only meaningful for light sources that do in fact correspond somewhat closely to the radiation of some black body, i.e., those on a line from reddish/orange via yellow and more or less white to blueish white; it does not make sense to speak of the color temperature of e.g., a green or a purple light. Color temperature is conventionally stated in the unit of absolute temperature, the Kelvin, having the unit symbol K. Color temperatures over 5,000K are called cool colors (blueish white), while lower color temperatures (2,700–3,000K) are called warm colors -yellowish white through red. This relation, however, is a psychological one in contrast to the physical relation implied by Wien’s displacement law, according to which the spectral peak is shifted towards shorter wavelengths resulting in a more blueish white for higher temperatures. The color rendering of light sources is often interpreted as indicating object color quality. However, color rendering is actually defined as the “effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant”. The color rendering only refers to color fidelity, the accurate representation of object colors compared to those same objects under a reference source, and does not include other aspects of color quality, such as chromatic discrimination and color preference. The CIE color rendering index (CRI) is the only internationally-accepted metric for assessing the color rendering performance of light sources.

2.8 EXERCISES Q.1. Explain the principle of photometry and the radiometry and the difference between them. Q.2. Define the luminous flux of a light source. Explain how it is measured using the integrating sphere. Q.3. Define the luminous intensity of a light source. Explain the relationship between luminous intensity (I), Luminance (L) and Illuminance (E) of a light source. Q.4. Give a list of various light measuring instruments. Explain in detail the working of any one instrument for measurement of light. Q.5. Explain the differentiate between:

(i)

Luminous efficacy and Luminous efficiency



(ii) Photometry and Radiometry



(iii) Luminous flux and Radiant flux



LIGHT AND ITS MEASUREMENT    2.53

Q.6. Write short notes on

(i)

Thermal detectors



(ii)

Luminous efficacy



(iii) Photomultiplier tube



(iv) Photodiodes



(v) Laws of illumination



(vi) Occupancy sensors

Q.7. Explain the structure and working of the following instruments:

(i)

Illuminance or Lux meter



(ii) Spectrophotometer



(iii) Goniophotometer

Q.8. What do you mean by luminous efficacy of a light source? Explain the procedure to obtain the same for an incandescent lamp. Q.9. Compare the luminous efficacy of various lamps. Q.10. Discuss the structure, working principle and applications of a goniophotometer. Q.11. At which light wavelength the human visual system is most sensitive?

(a)  555 nm

(b)  507 nm

(c)  400 nm



[Ans:(a)]

Q.12. At which wavelength of light the rods in the retina are most sensitive?

(a) 507 nm   (b)  600 nm   (c) 570 nm



[Ans:(a)]

Q.13. A blue light source and a green light source have the same luminous intensity. Which of the two sources radiates less power?

(a) Blue light source       (b)  Green light source [Ans:(b)]

2.9 BIBLIOGRAPHY 1.

I.E.S.N.A, New York, “Lighting Hand Book” 9th Edition, 2000.

2.

Ronald N. Helms, “Illumination Engg. for Energy Efficient Luminous Environments”, P.H. Inc, N.J., 1980. 3. J. Trost, “Efficient Building Design Series-Electrical and Lighting”, Vol. 1, P.H., Inc., N.J., 1999.

4.

H.A.E. Keitz, “Light Calculations and Measurements”, 2nd Edition, Philips Technical Library, The Netherlands, 1971.

2.54    FUNDAMENTALS OF LIGHTING

5. Leslie Cromwell, Fred J. Weibell and Erich A. Pfeiffer, “Biomedical Instrumentation and Measurement”, Prentice Hall of India, New Delhi, 1996. 6.

Vicki Bruce and Patric R. Green, “Visual Perception Physiology, Psychology and Ecology”, Lawrence Erlbaum Associates Ltd., London, 1989.

7.

H. Cotton, “Principles of Illumination”, Chapman and Hall Ltd., London, 1960.

8.

Decusatis, C. “Hand Book of Applied Photometry”, AIP Press, 1997.

Chapter

3 Light Sources

3.1 AN OVERVIEW OF LIGHT SOURCES Without light, there would be no life. We all need light to see around us. However, in the initial stage of life sun was the only source of light for human beings. Our sun is an atomic furnace that turns mass into energy due to thermonuclear fusion. Every second, it converts over 657 million tons of hydrogen into 653 tons of helium. The missing 4 million tons of mass is discharged into space as energy. The earth receives only about one two-billionths of this. Scientists calculate that the sun should keep burning for another 10 to 15 billion years. It has been estimated that in 15 minutes our sun radiates as much energy as mankind consumes in all forms, during an entire year. The sun is approximately 150 millions km from the earth. Energy, with a color temperature of approximately 6500 degrees Kelvin, is received on earth, from the Sun. It takes light from the sun approximately 8 minutes and 20 seconds to reach the earth. The illumination on the earth’s surface by the sun may exceed 100,000 lux in midsummer. We shall like to menetion below some of the landmark periods in history when significant contributions were made in the process of light source preparation or in the related areas of lighting technology. Lamp History

- Journey through Times

From the initial stage of life - Sun 65 million years BC

- Fire

450 BC

- Oil Lamp (Egypt)

1808 AD

- Carbon Arc Lamp (Davy)

1879 AD

- Incandescent Lamp (Edison)

3.2    FUNDAMENTALS OF LIGHTING

1906 AD

- High Pressure Mercury Vapour (HPMV) Lamp

1910 AD

- Drawn Tungsten Filament Lamp

1923 AD

- Low Pressure Sodium Vapour (LPSV) Lamp

1924 AD

- Gas Filled Incandescent Lamp

1933 AD

- Fluorescent Lamp

1958 AD

- Laser Beam Light Source

1968 AD

- High Pressure Sodium Vapour (HPSV) Lamp

1969 AD

- Metal Halide Lamp

1990 AD

- Induction Lamp

2000 AD

- Light Emitting Diodes (LEDs)

3.2 CLASSIFICATION OF LIGHT SOURCES Anything that gives out light is called a light source. The Sun is a light source, stars are light sources. Fire is a light source. A candle is a light source. An electric light bulb is also a light source.

Visibility of Objects We see objects when they reflect light and the reflected light enters into our eyes. We see the moon when it reflects light from the Sun. When we look in a mirror we see an image of ourselves reflected back. Lamps are typically classified by the process used to produce the light from them. We can initially classify lamps into 3 main categories (see Fig. 3.1:

Family Tree of Lamps 1.

Incandescent lamps

2.

Discharge lamps

3.

Solid state lamps (LEDs) Lamp Family Tree

Incandescent Lamps

Discharge Lamps (Gas/Vapour)

Solid State Lighting (LEDs)

Fig. 3.1  Family Tree of Lamps



LIGHT SOURCES    3.3

1.

Incandescent Lamps



The lamps using the phenomenon of incandescence are known as incandescent lamps. These lamps can be grouped as follows (see Fig. 3.2) : (a) Vacuum lamps (b) Gas filled lamps Gas filled lamps can further be subdivided as: (a) Halogen lamps (b) Non-halogen lamps Incandescent Lamps

Vacuum Lamps

Gas filled Lamps

Halogen

Non-Halogen

Fig. 3.2  Types of Incandescent Lamps

2.

Discharge Lamps



Lamps using electric discharge process are known as discharge lamps. These lamps can be grouped as follows: (a) Low pressure discharge lamps (b) High pressure discharge lamps Low pressure discharge lamps can further be subdivided as: (a) Fluorescent tubular lamps and Compact fluorescent lamps (b) Low pressure sodium vapour lamps



High Intensity Discharge (HID) can be divided into 3 following categories: (i)

High pressure mercury vapour lamps

(ii) High pressure sodium vapour lamps (iii) High pressure metal halide lamps Figure 3.3 illustrates this classification.

3.4    FUNDAMENTALS OF LIGHTING

Fig. 3.3  Types of Discharge Lamps

3.

Solid State Lamps (LEDs)



Light emitting diodes (LEDs) can be classified as follows: Red, Green, Blue and White LEDs. LEDs have a very long life > 50,000 to 100,000 hours but their luminous efficacy is generally low. However recent research and development in the field has been able to increase luminous efficacy of LEDs considerably.

Till some time back the LEDs were mostly used in traffic signals, electronic goods, watches and automobiles, etc. However, due to recent advancement in the LED technology there is spurt in the demand of LED lighting in a big way. Some of these advances are discussed in section 3.6 of this chapter.

3.3 MATERIALS USED IN LIGHT GENERATION The materials which are needed for light generation should have certain important properties. For example, if we require a filament material then we usually look for a material which should have high electrical and thermal conductivity, high melting point, should have good elastic properties and also should be less reactive. Likewise, if we are looking for envelop of the lamp, then we require good reflecting and transmitting properties. The main materials used in lighting are the following: (i) Glass

(iv) Metals and Alloys

(ii) Ceramics

(v) Electrodes

(iii) Glass-Metal Seals

(vi) Gases-Inert Gases and Halogens

(vii) Getter Materials

(viii) Capping Cements

(ix) Semiconductors

(x) Metal Halides, etc.

Glasses Used in Light Generation There are three main groups of glasses: (i) Soda lime silicate



LIGHT SOURCES    3.5

(ii) Lead-alkali silicate (iii) Borosilicate glasses

Soda Lime glass or Soda Lime Silicate This type of glass is the mostly used in windowpanes, glass containers like bottles and jars for beverages and food items etc. Soda Lime glass is prepared by melting the raw materials, such as soda, lime, silica, alumina, and small quantities of fining agents (e.g. sodium sulphate, sodium chloride) in a glass furnace at temperatures locally up to 1675°C. The temperature is only limited by the quality of the furnace superstructure material and by the glass composition. Green and brown bottles are obtained from raw materials containing iron oxide. For lowering the price of the raw materials, pure chemicals are not used, but relatively inexpensive minerals such as trona, sand, and feldspar. The mix of raw materials is termed batch. Soda Lime glass is divided technically into glass used for windows, called float glass or flat glass, and glass for containers, called container glass. Both types differ in the application, production method (float process for windows, blowing and pressing for containers), and chemical composition. Soda Lime silicate glass is the most commonly used glass in the lamp industry because with only minor variations in batch composition it is used as the envelope material for general incandescent, fluorescent and low wattage discharge lamps. The internal glass components of fluorescent and general incandescent lamps and also many types of small incandescent bulbs are made from lead-alkali glass.

Lead-Alkali Silicate or Lead Alkali Glass Lead-alkali glass is preferred to Soda Lime glass because of its higher electrical resistivity which prevents electrolysis occurring in the pinch seal. If lime is replaced by lead oxide (PbO) in soda lime glasses and if potash (K2O) is used as a partial replacement for soda, lead-alkali-silicate glasses result that have lower softening points than lime glasses. Refractive indices, dispersive powers, and electrical resistance of these glasses are generally much greater than those of Soda Lime-silica glass. Float glass has a higher magnesium oxide and sodium oxide content as compared to container glass, and lower silica, calcium oxide, and aluminium oxide content. From this follows a slightly higher quality of container glass concerning the chemical durability against water, which is required especially for storage of beverages and food. The standard lead glass traditionally used in lamp making contains about 30% by weight of lead oxide. Since the early 1980s, however, due to the relatively high cost and the introduction of increasingly stringent health and safety regulations concerning lead compounds, glass containg 20-22% of lead oxide have almost entirely replaced the old standard lead glasses.

3.6    FUNDAMENTALS OF LIGHTING

The Borosilicate Glass This glass is made from silica by mixing with it boron oxide. Further, these glasses are having very low thermal coefficient and hence they are resistant to thermal shocks. Typically, the glass composition for this type of glass is about 70% silica, 10% boron oxide, 8% sodium oxide, 8% potassium oxide, and 1% calcium oxide (lime). The refractory properties and physical strength of this glass make it ideal for use in laboratories, where it is used to make high-durability glass lab equipment, such as beakers and test tubes, etc. In addition, borosilicate glass warps minimally when exposed to heat allowing a borosilicate container to provide accurate measurements of volume over time. During 1950s borosilicate glass tubing was used to pipe coolants (often distilled water) through high power vacuum tube–based electronic equipment, such as commercial broadcast transmitters. The heaters for aquarium are sometimes made out of borosilicate glass. Because of its high heat resistance, it can tolerate the great temperature differences between water and the nichrome heating element. Many high quality flashlights use borosilicate glass for the lens, since this allows a higher percentage of light transmittance through the lens than compared to plastics and lower-quality glass materials. Innovative lamp working techniques led to artistic applications such as contemporary glass marbles, etc. The modern glass art movement, spurred largely by the rapid development of a borosilicate colour palette at Northstar Glass in the 1980s and 1990s, has provided vast economic growth for borosilicate glass suppliers. This glass is commonly used in the glassblowing form of lamp working and the artists create a range of products ranging from jewellery, kitchenware, to sculpture as well as for artistic glass tobacco pipes. Generally the astronomical reflecting telescope glass mirror components are made of borosilicate glass because of its low coefficient of expansion with heat. This makes very precise optical surfaces possible that change very little with temperature, and matched glass mirror components that track across temperature changes and retain the optical system’s characteristics. Generally it is somewhat more difficult to make than traditional glass, it is economical to produce because of its superior durability, chemical and heat resistance which finds excellent use in chemical laboratory equipment, cookware, lighting, and in certain cases, windows, etc. The optical glass most often used for making instrument lenses is Schott BK-7 (or the equivalent from other makers), a very finely made borosilicate crown glass. This is designated as 517642 glass after its 1.517 refractive index and 64.2 Abbe number. Some less costly borosilicate glasses, such as Schott B270 or the equivalent, are used to make crown glass eyeglasses lenses. Low cost borosilicate glass, like that used to make kitchenware and even reflecting telescope mirrors, cannot be used for high quality lenses because of the striations and inclusions common to lower grades of this type of glasses. The borosilicate is also a material of choice for evacuated tube solar thermal technology, because of its high strength and heat resistance. These glasses also find application in the semiconductor



LIGHT SOURCES    3.7

industry in the development of micromechanical devices, known as MEMS, as part of stacks of etched silica wafers bonded to the etched borosilicate glass.

Characteristics of Reflecting Materials It is not that only glasses reflect the light. Even polished materials are found to reflect light. During reflection the light beam returns to the same medium where from the incident beam originated. The reflection of light may be specular (that is, mirror-like) or diffuse (that is, not retaining the image, only the energy) depending on the nature of the interface. Furthermore, if the interface is between dielectric-conductor or dielectric-dielectric media, the phase of the reflected wave may or may not be inverted, respectively. Generally a polished mirror provides the most common model for specular light reflection and consists of a glass sheet in front of a metallic coating where the reflection actually occurs. The reflection phenomenon is enhanced in metals by suppression of wave propagation beyond their skin depths. The reflection usually occurs from the surface of transparent media, such as water or glass. Total internal reflection of light from a denser medium occurs if the angle of incidence is above the critical angle. Total internal reflection is used as a means of focussing waves that cannot effectively be reflected by common means. The X-ray telescopes are constructed by creating a converging tunnel for the waves. As the waves interact focus point (or toward another interaction with the tunnel surface, eventually being at low angle with the surface of this tunnel they are reflected toward the directed to the a detector at the focus). A conventional reflector would be useless as the X-rays would simply pass through the intended reflector. When light reflects off a material denser (with higher refractive index) than the external medium, it undergoes a 180° phase reversal. In contrast, a less dense, lower refractive index material will reflect light in phase. This is an important principle in the field of thin-film optics. Specular reflection at a curved surface forms an image which may be magnified or demagnified; curved mirrors have optical power. Such mirrors may have surfaces that are spherical or parabolic.

Laws of Regular Reflection If the reflecting surface is very smooth, the reflection of light that occurs is called specular or regular reflection. The laws of reflection are as follows: The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane. The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.

3.8    FUNDAMENTALS OF LIGHTING

Glass-Metal Seals One of the major factor affecting the choice of vitreous (vitreous refers to a material in an amorphous, glassy state) material for a particular application is its ability to seal hermetically to other materials particularly metals. A hermetic seal is a seal which, for practical purposes, is considered airtight. For example, tin cans are hermetically sealed. The properties required of a glass in order for it to produce an ideal stress-free seal to a metal are as follows: 1.

Its thermal expansion coefficient should match that of the metal over a wide range of temperatures;

2.

It must be readily workable in the sealing region;

3.

It must exhibit satisfactory chemical resistance to atmospheric attack;

4.

Its electrical resistivity, dielectric constant must be satisfactory;

5.

It must be homogeneous.

Generally Dumet alloy is used almost exclusively throughout the lighting industry for sealing to lead-alkali glass. Dumet is a composite material consisting of 42-58% nickel-iron alloy sheathed with copper which constitutes about 25% by weight of the complete wire. The surface of the wire is coated with sodium borate.

Capping Cements Capping cements are required to provide a reliable mechanical joint between materials with widely different thermal expansion characteristics over a wide temperature range after many thousands of hours. The material used for fixing metal caps onto glass bulbs consists of about (0% marble dust filler plus phenolic, natural and silicone resins. For fixing ceramaic caps onto fused silica-bodied lamps, much more refractory cement is used and this consists essentially of silica powder mixed with an inorganic binder, such as sodium silicate.

Lamp Phosphers Whenever a substance absorbs energy in any form, a fraction of the absorbed energy may be re-emitted in the form of electromagnetic radiation in the visible or near visible region of the electromagnetic spectrum. This phenomenon is called luminescence. Luminescent solids are usually referred to as phosphors. The luminescence process involves at least two steps: (i) The excitation of the electronic system of the solid and (ii) The subsequent emission of photons. These two steps may or may not be separated by intermediate processes. The excitation may be achieved by bombardment with photons-photoluminescence, with electrons-cathodoluminescence, or with other particles.



LIGHT SOURCES    3.9

Luminescence can also be induced as a result of a chemical reactionchemiluminescence or by the application of electric field-electroluminescence. When one speaks of fluorescence, one usually has in mind the emission of light during excitation. The emission of light after the excitation has ceased is then referred to as phosphorescence or after glow. It is now established that the ability of a material to exhibit luminescence is associated with the presence of ‘Activators’. Generally the activators are impurity atoms occurring in relatively small concentrations in the host material. Some of the important groups of luminescent crystalline solids are as follows: The alkali halides activated with thallium or other heavy metals, ZnS and CdS activated with Cu, Ag, Au, Mn etc., and the silicate phosphors, such as zinc orthosilicate.

Phosphor Materials Phosphor materials play a very important role in lighting. The best known phosphor material is a Cu activated ZnS or Ag activated ZnS. The host materials are typically oxides, sulphides, selenides, halides or silicates of zinc, cadmium, manganese, aluminium, silicon, or various rare earth metals. The activators prolong the emission time (afterglow). In turn, other materials (e.g. nickel) can be used to quench the afterglow and shorten the decay part of the phosphor emission characteristics.

White LEDs LEDs with white light are usually blue In GaN LEDs with a coating of a suitable material. Cerium(III)-doped YAG (YAG: Ce3+, or Y3 Al5) O12: Ce3+ is often used; it absorbs the light from the blue LED and emits in a broad range from greenish to reddish, with most of output in yellow. The emission of pale yellow light from the Ce3+: YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium. These can even be further adjusted by substituting some or all of the aluminium in the YAG with gallium. However, this process is not one of phosphorescence. The yellow light is produced by a process known as scintillation, the complete absence of an afterglow being one of the characteristics of the process. White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulphide (ZnS: Cu, Al). This is a method analogous to the way fluorescent lamps work.

Halophosphate Lamp Phosphor This an old type of phosphor which has been used in fluorescent lamps to convert ultraviolet light into visible light. Generally calcium halophosphate

3.10    FUNDAMENTALS OF LIGHTING

phosphors doped with antimony and manganese are widely used in fluorescent lamp even now, however these phosphors are now being replaced by more efficient triband phosphors.

Triband Phosphors These phosphors are based upon inorganic material containing ‘rare earth’ elements. A combination of red, green and blue results in a 50% increase in efficacy with regard to standard phosphors and excellent color rendering properties. Philips lighting was among the first to introduce triband phosphors on a worldwide scale. Triband phosphors are available in ready-for-use premixed blends or in single components. In partnering with Philips, Elmet Technologies is able to offer customer product-driven innovations as well as technical support, including assistance with the suspension making process. With the industry’s continuous demand for coating weight reduction, Philips has taken a leadership role in striving to meet this requirement.

Inert Gases Inert gases are also called the noble gases. An inert gas is any gas that is not reactive with elements. Like the noble gases an inert gas is not necessarily elemental and are often compound gases. Like the noble gases the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases. This is a tendency, not a rule, as noble gases and other “inert” gases can react to form compounds. Helium and neon are the only true elemental inert gases, because they do not form any (known) true chemical compounds, unlike the heavier noble gases (argon, krypton, xenon and radon). In marine applications, inert gas refers to gases with a low content of oxygen that are used to fill void spaces in and around tanks for explosion protection. There are two types of inert gas which are either based on nitrogen or on flue gas.

Production of Inert Gases The elemental inert gases are usually obtained by evapourating them off from condensed air at their respective vapour pressures. Nitrogen based inert gas is produced on board of chemical tankers and product carriers (smaller vessels) with compressors and a Nitrogen specific membrane. Inert gas is produced on board crude oil carriers (above 20000 tonnes) by using either a Flue gas system or by burning kerosene in a dedicated inert gas generator. The flue gas system uses the boiler exhaust as its source, so it’s important that the fuel/air ratio in the boiler burners is properly regulated to ensure that high quality IG is produced. Too much air would result in an oxygen content exceeding 5%, too much fuel oil would result in carryover of dangerous



LIGHT SOURCES    3.11

hydrocarbon gas. The flue gas is cleaned and cooled by the scrubber tower. Various safety devices prevent overpressure, return of hydrocarbon gas to the engine room or supply of IG with too high oxygen content. Gas tankers and product carriers cannot rely on flue gas systems (because they require IG with O2 content of 1% or less) and so use IGGs instead. The Inert Gas Generator consists of a combustion chamber and scrubber unit supplied by fans and a refrigeration unit which cools the gas. A drier in series with the system removes moisture from the gas before it is supplied to the deck. Regular calibration and testing to equipment is required to ensure that it works correctly.

Applications of Inert Gases Because of the non-reactive properties of inert gases they are often useful to prevent undesirable chemical reactions from taking place. For example, molecular nitrogen, a molecular inert gas, is often used in food packaging to ensure that food does not spoil in transit since no bacteria or fungi can flourish without the reactive gases oxygen or carbon dioxide, which the molecular nitrogen displaces. The most extant cells on Earth require the reactions which these gases are involved in to function. Most importantly since molecular nitrogen is inert it will not cause any reactions to take place in the food, possibly changing the intrinsic taste or smell, nor will it cause any chemical reactions in the human body. Thus the inert gas is used as a passive preservative, preventing biological decay, while being undetectable to the consumer since taste and olfactory senses require a chemical reaction to take place in order to send a signal to the brain. This is in contrast to active preservatives which react with the biological material of bacteria, fungi, and possibly the food itself changing the food’s intrinsic taste or smell, or may even act directly on the consumer’s taste and olfactory mechanisms. As chemists sometimes need to perform experiments on air-sensitive compounds, air-free techniques have been developed to handle them under inert gas. Inert or noble gases have very low boiling and melting points, which makes them useful as cryogenic refrigerants. In particular, liquid helium, which boils at 4.2  K (−268.95°C; −452.11°F), is used for superconducting magnets, such as those needed in nuclear magnetic resonance imaging and nuclear magnetic resonance. Although liquid neon does not reach temperatures as low as liquid helium, it also finds use in cryogenics because it has over 40 times more refrigerating capacity than liquid helium and over three times more than liquid hydrogen. Helium is used as a component of breathing gases to replace nitrogen, due its low solubility in fluids, especially in lipids. Gases are absorbed by the blood and body tissues when under pressure like in scuba diving, which causes an anesthetic effect known as nitrogen narcosis.

3.12    FUNDAMENTALS OF LIGHTING

Due to its reduced solubility, little helium is taken into cell membranes, and when helium is used to replace part of the breathing mixtures, such as in trimix or heliox, a decrease in the narcotic effect of the gas at depth is obtained. Inert or noble gases have very low boiling and melting points, which makes them useful as cryogenic refrigerants. In particular, liquid helium, which boils at 4.2 K (−268.95°C; −452.11°F), is used for superconducting magnets, such as those needed in nuclear magnetic resonance imaging and nuclear magnetic resonance. Liquid neon, although it does not reach temperatures as low as liquid helium, also finds use in cryogenics because it has over 40 times more refrigerating capacity than liquid helium and over three times more than liquid hydrogen. Helium is used as a component of breathing gases to replace nitrogen, due its low solubility in fluids, especially in lipids. Gases are absorbed by the blood and body tissues when under pressure like in scuba diving, which causes an anesthetic effect known as nitrogen narcosis. Due to its reduced solubility, little helium is taken into cell membranes, and when helium is used to replace part of the breathing mixtures, such as in trimix or heliox, a decrease in the narcotic effect of the gas at depth is obtained. Helium is used as the carrier medium in gas chromatography, as a filler gas for thermometers, and in devices for measuring radiation, such as the Geiger counter and the bubble chamber. Helium and argon are both commonly used to shield welding arcs and the surrounding base metal from the atmosphere during welding and cutting, as well as in other metallurgical processes and in the production of silicon for the semiconductor industry.

Inert Gases for Lighting Inert gases are commonly used in lighting because of their lack of chemical reactivity. Argon, mixed with nitrogen, is used as a filler gas for incandescent light bulbs. Krypton is used in high-performance light bulbs, which have higher color temperatures and greater efficiency, because it reduces the rate of evapouration of the filament more than argon; halogen lamps, in particular, use krypton mixed with small amounts of compounds of iodine or bromine. The inert gases glow in distinctive colors when used inside gas-discharge lamps, such as neon lights, which produce an orange-red color. Xenon is commonly used in xenon arc lamps which, due to their nearly continuous spectrum that resembles daylight, find application in film projectors and as automobile headlamps. The inert gases are also used in excimer lasers, which are based on short-lived electronically excited molecules known as excimers. The excimers used for lasers may be noble gas dimers such as Ar2, Kr2 or Xe2, or more commonly, the inert gas is combined with a halogen in excimers such as ArF, KrF, XeF, or XeCl. These lasers produce ultraviolet light which, due to its short wavelength (193 nm for ArF and 248 nm for KrF), allows for high-precision imaging. The excimer lasers have many industrial, medical, and scientific applications.



LIGHT SOURCES    3.13

They are used for microlithography and microfabrication, which are essential for integrated circuit manufacture, and for laser surgery, including laser angioplasty and eye surgery.

3.4 INCANDESCENT LIGHT SOURCES-GLS AND T-H LAMPS Beginning of Artificial Lighting The incandescent lamps are the standard bulbs that most people are familiar with. These bulbs work by using electricity to heat a tungsten filament in the bulb until it glows. The filament is either in a vacuum or in a mixture of argon/ nitrogen gas. Most of the energy consumed by the bulb is given off as heat, causing its lumens per watt performance to be low. Due to the filament’s high temperature, the tungsten tends to evapourate and collect on the sides of the bulb. The impurities present in the tungsten filament causes it to become thinner unevenly. When a bulb is turned on, the sudden surge of energy can cause the thin areas to heat up much faster than the rest of the filament, which in turn causes the filament to break and the bulb to burn out. Incandescent bulbs produce a steady warm, light that is good for most household applications. A standard incandescent bulb can last for 700-1000 hours, and can be used with a dimmer. Soft white bulbs use a special coating inside the glass bulb to better diffuse the light; but the light color is not changed. Incandescent light sources are based on the phenomenon of Incandescence. The phenomenon of incandescence is the release of electromagnetic radiation, usually visible radiation, from a body due to its temperature. Planck’s law describes the distribution of energy emissions across the electromagnetic spectrum at temperatures occurring on Earth (say 100-2000K). The release of radiation is usually predominantly in the infrared and visible regions of the electromagnetic spectrum. The total power emitted by radiation is given by the Stefan-Boltzmann law. Light is produced by incandescence when light comes from a heated solid. We see an example of incandescence when an iron bar is heated to a very high temperature. Incandescence occurs in light bulbs, because the filament resists the flow of electrons. This resistance heats the filament to a temperature where part of the black body radiation falls in the visible spectrum. The majority of radiation, however, is emitted in the invisible infrared and lower frequency spectra, which is why incandescent light bulbs are very inefficient. Fluorescent lamps do not function by means of incandescence, rather by a combination of thermionic emission and atomic excitation due to collision with high energy electrons. In an incandescent lamp, only the electrons at the top of the band can participate. Higher temperature can increase efficiency but we do not have materials that can withstand much higher temperature.

3.14    FUNDAMENTALS OF LIGHTING

Light is produced by incandescence when light comes from a heated solid. We see an example of incandescence when an iron bar is heated to a very high temperature. As the temperature rises it glows--red at first and then white. The process we are seeing which turns heat energy into light energy is called incandescence. Incandescent lamps derive their name from this process. In these lamps the filament is made of tungsten --a special metal which can stay at a high temperature for a hundreds of hours without burning through. Incandescent Lamps were first invented by Edison in 1879. Initially carbonized paper were used as filament. It was fragile. Later it was improved by coating with a hydrocarbon. In 1893 cellulose filament was developed from absorbent cotton dissolved in ZnCl. Normally filament is mounted in a glass bulb and maintained in vacuum. It gets heated upon passage of current and typically radiating 3.3 lm/W. They are called Type ‘B’ lamps. In 1905, metalizing by heating carbon filament at high temperature in an electric furnace efficiency improved to 4.0 lm/W. In Europe Osmium a Rare and expensive – Fragile filaments were employed with 5 lm/W radiation. It was soon, replaced by Tantalum a ductile material (1906 - 1913) by crystallizing by application of ac leading to 5 lm/W radiation. In 1907 Tungsten filaments entered with 7 lm/W radiation. Finely divided Tungsten powder is mixed with a binder and squirted through a die. In 1911 Coolidge developed Tungsten in ductile form which could result in a continuous uniform filament. It was rugged and had very high efficiency. Langmuir introduced use of inert gases and improved the radiation efficiency – (1913). They were called type ‘C’. The figure of an incandescent lamp is shown in Fig. 3.4 with its various parts.

Fig. 3.4  Incandescent Lamp with its Parts



LIGHT SOURCES    3.15

Inert gases are introduced in the glass envelope to decrease the vapourizations of Tungsten. The gases Nitrogen and Argon are most suitable. Conduction losses in a gas are proportional to velocity of gas molecules. Velocity is inversely proportional to square root of atomic weight. Argon with atomic weight of 39.8 and Nitrogen with atomic weight 28.0 are most suitable. Ionization potential of Argon is low. Hence a mixture of Argon and Nitrogen in the ratio of 85% Argon – 15% Nitrogen are employed. Concentrate the filament over a small region. To adopt tightly wound helical coil. During operation Tungsten filament evapourates and Tungsten particles deposit on the interior of the bulb in a vacuum lamp. Tungsten filament cross section decides the current rating and varies as square of the diameter. Filament characteristics depend on filament length, diameter, coil spacing, lead wires, number of supports, method of mounting, properties of gas, gas pressure, bulb size and shape of the bulb. The lamp is said to be most economical for the intended service, if uniform radiation is there at stated wattage with guaranteed efficiency and life rating. Filament lamps are used mainly for domestic and display lighting. There are many types of filament lamp, the most common being General Lighting Service (GLS) and decorative. Their finish – clear, diffuse/pearl or colored – is a significant factor in their application. Reflector lamps are similar but have an envelope with an internal reflective coating. Advantages of filament lamps include low initial cost, simple operation (no control gear required) and good color rendering index. Disadvantages of filament lamps are low efficiacy (measure of the energy efficiency of a light source, i.e., lumens per watt) and a relatively short life. Certain extended life filament lamps have only about half the efficacy of standard lamps. The light output of filament lamps is particularly sensitive to voltage variations.

GLS Lamps Design and Performance Characteristics Early lamps were laboriously hand-assembled; cost of lamps fell after automatic machinery was developed. In manufacturing the glass bulb, a type of “ribbon machine” is used. A continuous ribbon of glass is passed along a conveyer belt, heated in a furnace, and then blown by precisely aligned air nozzles through holes in the conveyer belt into molds. Thus the glass bulbs are created. After the bulbs are blown, and cooled, they are cut off of the ribbon machine. A machine of this sort may be able to produce 50,000 bulbs per hour. To improve the efficiency of the lamp, the filament usually consists of coils of coiled fine wire, also known as a ‘coiled coil’. For a 60-watt 120-volt lamp, the uncoiled length of the tungsten filament is usually 22.8 inches or 580 mm, and the filament diameter is 0.0018 inches (0.045 mm). The advantage of the coiled coil is that evapouration of the tungsten filament is at the rate of a tungsten cylinder having a diameter equal to that of the coiled coil. A coiled-coil filament evapourates more slowly than

3.16    FUNDAMENTALS OF LIGHTING

a straight filament of the same surface area. If the filament is then run hotter to bring back evapouration to the same rate, the resulting filament is a more efficient light source.

Efficiency of Incandescent Lamps Approximately 90% of the power consumed by an incandescent light bulb is emitted as heat, rather than as visible light. The effectiveness of an electric lighting source is determined by two factors - the relative visibility of electromagnetic radiation, and the rate at which the source converts electric power into electromagnetic radiation. Luminous efficacy of a light source is a ratio of the visible light energy emitted (the luminous flux) to the total power input to the lamp. Visible light is measured in lumens, a unit which is defined in part by the differing sensitivity of the human eye to different wavelengths of light. Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy is a measure of how well the distribution of energy matches the perception of the eye. The unit of luminous efficacy is lumen/watt. The maximum efficacy possible is 683 lm/W for monochromatic green light at 555 nanometers wavelength, the peak sensitivity of the human eye. For white light, the maximum luminous efficacy is around 240 lumens per watt, but the exact value is not unique because the human eye can perceive many different mixtures of visible light as “white”. Generally a 40W tungsten incandescent lamp has 12.6 lm/W as its luminous efficacy.

The Tungsten Filament As we are aware that a metal must be heated to extreme temperatures before it will emit a useful amount of visible light. Generally metals melt before reaching such extreme temperatures -- the vibration will break apart the rigid structural bonds between the atoms so that the material becomes a liquid. Light bulbs are manufactured with tungsten filaments because tungsten has an abnormally high melting temperature. The tungsten metal will catch on fire at such high temperatures, if the conditions are right. Combustion is caused by a reaction between two chemicals, which is set off when one of the chemicals has reached its ignition temperature. On Earth, combustion is usually a reaction between oxygen in the atmosphere and some heated material, but other combinations of chemicals will combust as well. The filament in a light bulb is housed in a sealed, oxygen-free chamber to prevent combustion. In the first light bulbs, all the air was sucked out of the bulb to create a near vacuum -- an area with no matter in it. Since there wasn’t any gaseous matter present (or hardly any), the material could not combust. The problem with this approach was the evapouration of the tungsten atoms. At such extreme temperatures, the occasional tungsten atom vibrates enough to detach from the atoms around it and flies into the air. In a



LIGHT SOURCES    3.17

vacuum bulb, free tungsten atoms shoot out in a straight line and collect on the inside of the glass. As more and more atoms evapourate, the filament starts to disintegrate, and the glass starts to get darker. This reduces the life of the bulb considerably. In a modern light bulb, inert gases, typically argon, greatly reduce this loss of tungsten. When a tungsten atom evapourates, chances are it will collide with an argon atom and bounce right back toward the filament, where it will rejoin the solid structure. Since inert gases normally don’t react with other elements, there is no chance of the elements combining in a combustion reaction. Cheap, effective and easy-to-use, the light bulb has proved a monstrous success. It is still the most popular method of bringing light indoors and extending the day after sundown. But by all indications, it will eventually give way to more advanced technologies, because it isn’t very efficient. Incandescent light bulbs give off most of their energy in the form of heat- carrying infrared light photons -- only about 10% of the light produced is in the visible spectrum. This wastes a lot of electricity. Cool light sources, such as fluorescent lamps and LEDs, don’t waste a lot of energy generating heat -- they give off mostly visible light. For this reason, they are slowly edging out the old reliable light bulb.

Important Properties of Tungsten The main properties of interest for tungsten are listed below:   1.

Tungsten is the heaviest engineering material with a density of 19.25 gcm3.      

2.

It has the highest melting point of all metals at 3410°C· 

3.

It has the lowest vapour pressure of all metals·  

4.

The tungsten metal has the highest modulus of elasticity of the metals (E = 400 GPa). 

5.

It is the hardest pure metal. 

6.

Excellent high temperature strength characteristics

7.

It has the highest tensile strength at temperatures above 1650°C

8.

It has a low thermal expansion co-efficient (4.4 × 10-6 m/m/°C ) as to that of borosilicate glass, and therefore makes it useful for glass to metal seals.

9.

It does not oxidise in air and needs no protection from oxidation at elevated temperatures.

10. Its corrosion resistance is excellent, and it is not attacked by nitric, hydrofluoric, or sulphuric acid solutions. There are several different shapes of filament used in lamps, with differing characteristics. Manufacturers designate the types with codes such as C-6, CC-6, C-2V, CC-2V, C-8, CC-88, C-2F, CC-2F, C-Bar, C-Bar-6, C-8I, C-2R, CC2R, and Axial. Electrical filaments are also used in hot cathodes of fluorescent

3.18    FUNDAMENTALS OF LIGHTING

lamps and vacuum tubes as a source of electrons or in vacuum tubes to heat an electron-emitting electrode (see Fig. 3.5).

Fig. 3.5 Tungsten Filament of a 100 Watt Incandescent Light Bulb (Magnified)

Lead glass or flint glass is more ‘brilliant’ because the increased refractive index causes noticeably more “sparkles”, while boron may be added to change the thermal and electrical properties, as in Pyrex. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eye glasses. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium oxide can be used for glass that absorbs UV wavelengths. Another common glass ingredient is recycled glass. The recycled glass saves on raw materials and energy. However, impurities in the cullet can lead to product and equipment failure. Finally, fining agents such as sodium sulfate, sodium chloride, or antimony oxide are added to reduce the bubble content in the glass. The glass batch calculation is the method by which the correct raw material mixture is determined to achieve the desired glass composition.

Contemporary Glass Production Following the glass batch preparation and mixing, the raw materials are transported to the furnace. Soda Lime glass for mass production is melted in gas fired units. Smaller scale furnaces for specialty glasses include electric melters, pot furnaces, and day tanks. After melting, homogenization and refining (removal of bubbles), the glass is formed. Flat glass for windows and similar applications is formed by the float glass process, developed between 1953 and 1957 by Sir Alastair Pilkington and Kenneth Bickerstaff of the UK’s Pilkington Brothers, who created a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity. The top surface of the glass is subjected to nitrogen under pressure to obtain a polished finish. Container glass for common bottles and jars is formed by blowing and pressing methods. Once the desired form is obtained, glass is usually annealed for the removal of stresses. Surface treatments, coatings or lamination may follow to improve the chemical durability (glass container coatings, glass



LIGHT SOURCES    3.19

container internal treatment), strength (toughened glass, bulletproof glass, windshields), or optical properties (insulated glazing, anti-reflective coating).

Vacuum Technology Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes, and a wide array of vacuum technology has since become available. The recent development of human space flight has raised interest in the impact of vacuum on human health, and on life forms in general. The 19th century saw increasing research with evacuated tubes, such as the Geissler and Crookes tubes. Scientists who experimented with such tubes included Eugen Goldstein, Nikola Tesla, Johann Wilhelm Hittorf, Thomas Edison, and many others. These tubes were mostly for specialized scientific applications, or were novelties, with the exception of the light bulb. The groundwork laid by these scientists and inventors, however, was critical to the development of vacuum tube technology. Though the thermionic emission effect was originally reported in 1873 by Frederick Guthrie, it is Thomas Edison’s 1884 investigation of the “Edison Effect” that is more often mentioned. Edison promptly patented what he found, but he did not understand the underlying physics, or the potential value of the discovery. The English physicist John Ambrose Fleming worked as an engineering consultant for many technology firms of his day, including Edison Telephone; in 1904, as a result of experiments conducted on Edison Effect bulbs imported from the USA and while working as scientific adviser to the Marconi company, he developed a device he called an “oscillation valve” (because it passes current in only one direction) or kenotron, which can also be used as part of a radio wave detector. Later known as the Fleming valve and then the diode, it allowed electrical current to flow in only one direction, enabling the rectification of alternating current. It was during 1906 that Robert von Lieben filed patent for a three electrode amplifying vacuum tube. His invention included also a beam focusing electromagnet. Likewise in 1907 Lee DeForest placed a bent wire serving as a screen, later known as the grid electrode, between the filament and plate electrode. As the voltage applied to the grid was varied from negative to positive, the number of electrons flowing from the filament to the plate would vary accordingly. In this way the grid was said to electrostatically control the plate current. The resulting three-electrode device was therefore an excellent and very sensitive amplifier of voltages. DeForest called his invention the “Audion”. In 1907, DeForest filed patent for a three-electrode version of the Audion for use in radio communications. The device is now known as the triode. De Forest’s device was not strictly a vacuum tube, but clearly depended for its action on ionisation of the relatively high levels of gas remaining after evacuation. The DeForest company, in its Audion leaflets, warned against operation which might cause the vacuum to become too hard.

3.20    FUNDAMENTALS OF LIGHTING

Later the Finnish inventor Eric Tigerstedt significantly improved on the original triode design in 1914, while working on his sound-on-film process in Berlin, Germany. The first true vacuum triodes were the Pliotrons developed by Irving Langmuir at the General Electric research laboratory (Schenectady, New York) in 1915. Langmuir was one of the first scientists to realize that a harder vacuum would improve the amplifying behaviour of the triode. Pliotrons were closely followed by the French ‘R’ Type which was in widespread use by the allied military by 1916. These two types were the first true vacuum tubes. Earlier the vacuum levels in production vacuum tubes typically ranged between 10 µPa to 10 nPa. The batteries were designed to provide the various voltages required by tubes in early radio sets. These were often rechargeable—usually of the lead-acid type ranging from 2 to 12 volts (1-6 cells) with single, double and triple cells being most common. Because these batteries produced 2 V, 4 V or 6 V, tube heaters were designed to operate at those voltages—a scheme which continues to be followed even today. Earlier in portable radios, torch batteries were sometimes used. The special-purpose tubes are intentionally constructed with various gases in the envelope. For instance, voltage regulator tubes contain various inert gases such as argon, helium or neon, and take advantage of the fact that these gases will ionize at predictable voltages. Thyratron is a special-purpose tube filled with low-pressure gas or mercury, some of which vapourizes. A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Although the envelope was typically glass, power tubes often use ceramics, and military tubes often use glass-lined metal. The gas-filled tubes operate by ionizing the gas with applied voltage to start electrical conduction. Both hot cathode and cold cathode type devices are encountered. Depending on application, either the glow from the gas or the arc discharge may be the desired function.

Pressure Pressure is the force per unit area applied to an object in a direction perpendicular to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.

Units of Pressure The SI unit for pressure is the Pascal (Pa), equal to one Newton per square meter (N· m–2 or kg· m–1· s–2). This special name for the unit was added in 1971; before that, pressure in SI was expressed simply as N/m2. The standard atmosphere (atm) is an established constant. It is approximately equal to air pressure at earth mean sea level and is defined on next page.



LIGHT SOURCES    3.21

Standard atmosphere = 101325 P­a = 101.325 kPa = 1013.25 hPa. Presently or formerly popular pressure units include the following: Atmosphere (atm) Manometric units: –  centimeter, inch, and millimeter of mercury (torr) –  millimeter, centimeter, meter, inch, and foot of water 1 Pascal = 1N/m2 = 10–5 bar = 1.0197 × 10–5 atm 1 atm = 101,325 Pa = 0.980665 = 760 Torr Example reading: 1 Pa = 1 N/m2   = 10–5 bar = 9.8692 × 10–6 atm. Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges do not have universally agreed definitions, but a typical distribution is as follows: l

Atmospheric pressure 760 torr or 101.3 kPa

l

Low vacuum 760 to 25 torr or 100 to 3 kPa

l

Medium vacuum 25 to 1 × 10–3 torr or 3 kPa to 100mPa

l

High vacuum 1 × 10–3 to 1 × 10–9 torr or 100 mPa to 100 nPa

l

Ultra high vacuum 1 × 10–9 to 1 × 10–12 torr or100 nPa to 100 pPa

l

Extremely high vacuum 2.5 mm

spray water

IP 40

foreign bodies > 1 mm

unprotected

IP 44

foreign bodies > 1 mm

splash water

IP 50

dust protected

unprotected

IP 54

dust protected

splash water

IP 65

dust proof

jet water

IP 66

dust proof

floodwater

Examples and applications of some IP classified luminaires: 1.

IP20 Luminaires



IP 20 luminaires can be utilised indoors only if no specific pollution rates are expected. Offices, dry, heated industrial halls, shops, shopping malls and theatres are typical application segments.

2.

IP50 Luminaires



IP 50 luminaires are utilized in dusty environments to prevent rapid pollution of the luminaire. The exterior of IP 50 luminaires can be cleaned easily. In the food industry, closed luminaires are specified to prevent glass particles from accidentally broken lamps from entering the production area, preventing contamination of the products under preparation.



Although the ingress protection is specified to protect the luminaire function, it also means that particles cannot leave the luminaire housing, thereby meeting the specification of the food industry. In the ‘wet’ food industry, luminaires meeting the IP 50 classifications shall not be utilized.

3.

IP54 Luminaires



IP 54 is the traditional water protected classification. Luminaires can be cleaned with water without any harmful effect. Once again this classification is often specified in the food processing industry, for industries where dust and moisture is generated in the hall, and for use under canopies.

4.

IP65 Luminaires



IP 65 jet-proof luminaires are applicable where the surroundings are cleaned frequently by water jets, or where luminaires are utilized in a dusty environment. Although the luminaires are not fully watertight, the potential ingress of moisture will not have any harmful effect on the luminaire function. IP 65 luminaires are often available in impact protected versions.



LUMINAIRES    4.21

Electrical Safety of Luminaires If a proper earth connection is available, Class I luminaires are used. However, in situations where no earth connection, or only a poor-quality earth connection, is available, or where eddy currents are present, Class II luminaires shall be applied. Class II street lanterns and floodlights, and water protected luminaires, are often used in (semi-)outdoor locations. Local electricity boards can provide the appropriate advice.

4.6 LIGHT CONTROL COMPONENTS AND OPTICAL DESIGN OF LUMINAIRES Incandescent lamps have compact filaments and those with clear bulbs are generally regarded as point sources that are easy to control optically. In the residential sector, however, many consumers prefer frosted lamps for non-glaring, softer light. With frosted incandescent lamps, the glass envelope becomes the source size. Their shapes still make them easier to control optically than fluorescent lamps, which have tubes of various sizes and shapes that emit light along their entire length. The usual purpose of optical control is todeliver as much light as possible from the luminaire to the intended surface (task, floor, ceiling, or wall). For specification and commercial grade luminaires, the most common optical control device is a reflector specifically designed for the intended lamp(s) and coated with reflective materials. Commodity residential grade products include few, if any, sophisticated optical control devices in the luminaire housing. Often the interiors of the housings are painted white to provide large diffuse and reflective surfaces at relatively low costs.

Luminaire Accessories Luminaire accessories such as lenses, diffusers, and louvers provide optical control and glare reduction. The most common accessory is the diffuser. In general diffusers in commodity residential grade luminaires are made of plastic usually acrylic or polycarbonate. Other materials include glass, alabaster, and perforated metal. Lenses control, or refract the distribution of light and offer more precise lighting control than diffusers. Plastic diffusers and lenses are inexpensive, lightweight, and easy to form. Fluorescent lamps give off ultraviolet (UV) radiation that gradually yellows some types of plastic diffusers and lenses. Incandescent lamps emit much less UV radiation. UV degraded plastic diffusers and lenses affect luminaire appearance and reduce the amount of light that can be transmitted through them. As a general rule, acrylic plastics are much more resistant to UV radiation than polycarbonate plastics. For controlling the light in a specific direction as well as its quantity and quality usually we utilize the light controlling components which are listed below.

4.22    FUNDAMENTALS OF LIGHTING

Light Control Components 1.

Reflectors

2.

Refractors

3. Diffusers 4.

Shields, etc.

What is a Reflector? A device used to redirect the light by the process of reflection is known as a Reflector.

Optical Design of Reflector Systems Before we discuss the optical designs of various types of reflectors we need to know the various types of reflectors. There are generally following types of reflectors: 1.

Concave reflectors 2.  Convex reflectors

3.

Circular reflectors 4.  Parabolic reflectors

5.

Elliptical reflectors 6.  Hyperbolic reflectors

7.

Plane mirror reflectors

What is a Mirror? A mirror is an object with a surface that has good specular reflection; that is, it is smooth enough to form an image. The most familiar type of mirror is the plane mirror, which has a flat surface. The curved mirrors are also used to produce magnified or diminished images or focus light or simply distort the reflected image, as per requirement. The mirrors are most commonly used for personal grooming, decoration and architecture, etc. Mirrors are also used in scientific apparatus such as telescopes and lasers, cameras and industrial machinery. Most mirrors are designed for visible light; however, mirrors designed for other types of waves or other wavelengths of electromagnetic radiation are also used, especially in optical instruments. Most mirrors are made by applying a reflective coating to a suitable substrate. The most common such substrate is glass, due to its ease of fabrication, its rigidity, and its ability to take a smooth finish. The reflective coating is typically applied to the back surface of the glass, so that it is protected from corrosion and accidental damage. (Glass is much more scratch-resistant than most substrates.)



LUMINAIRES    4.23

Historically, mirrors in the classical antiquity were made of solid metal (bronze, later silver) and they were too expensive for widespread use as well as being prone to corrosion. Due to polished metal’s low reflectivity, antique mirrors also gave a darker picture compared to modern ones, making them unsuitable for indoor use with artifical lighting (candles or lanterns at the time). The method of making mirrors out of ordinary glass was discovered by 16th century venetian glassmakers on the island of Murano, who covered the backside of plate glass with mercury, obtaining near-perfect reflection and imaging qualities. For over one hundred years venetian mirrors installed in richly decorated frames served as luxury decoration for palaces throughout Europe, but the secret of mercury process eventually arrived to London and Paris during the 17th century, due to industrial espionage. French workshops succeded in large scale industrialization of the process, eventually making mirrors affordable to the masses, although mercury’s toxicity remained a problem. In modern times the mirror substrate is shaped, polished and cleaned and is then coated. Glass mirrors are most often coated with non-toxic silver or aluminium, implemented by a series of coatings: Tin, silver, chemical activator, copper, paint. The tin is applied because silver will not bond with the glass. The activator causes the tin/silver to harden. Copper is added for long-term durability. The paint protects the coating on the back of the mirror from scratches and other accidental damage. In some applications, generally those that are cost-sensitive or that require great durability, mirrors are instead made from a single, bulk material such as polished metal. For technical applications such as laser mirrors, the reflective coating is typically applied by vacuum deposition on the front surface of the substrate. This eliminates double reflections and reduces absorption of light in the mirror. Cheaper technical mirrors use a silver, aluminium, or gold coating (the latter typically for infrared mirrors), and achieve reflectivities of 90–95% when new. A protective overcoat may be applied to prevent oxidation of the reflective layer. Applications requiring higher reflectivity or greater durability use dielectric coatings, which can achieve reflectivities as high as 99.999% over a narrow range of wavelengths.

Plane Mirror Reflectors Image Formation in Plane Mirrors and Image Characteristics: An image location is the location in space where all the reflected light appears to diverge from. Since light from the object appears to diverge from this location, a person who sights along a line at this location will perceive a replica or likeness of the actual object.

4.24    FUNDAMENTALS OF LIGHTING

In the case of plane mirrors, the image is said to be a virtual image. Virtual images are images which are formed in locations where light does not actually reach. Light does not actually pass through the location on the other side of the mirror; it only appears to an observer as though the light is coming from this location. Whenever a mirror (whether a plane mirror or otherwise) creates an image which is virtual, it will be located behind the mirror where light does not really come from. We will study instances in which real images are formed by curved mirrors. Such images are formed on the same side of the mirror as the object and light passes through the actual image location. Besides the fact that plane mirror images are virtual, there are several other characteristics which are worth noting. The second characteristic has to do with the orientation of the image. If you view an image of yourself in a plane mirror (perhaps a bathroom mirror), you will quickly notice that there is an apparent left-right reversal of the image. That is, if you raise your left hand, you will notice that the image raises what would seem to be it’s right hand. If you raise your right hand, the image raises what would seem to be its left hand. This is often termed left-right reversal. This characteristic becomes even more obvious if you wear a shirt with lettering. For example, a shirt displaying the word “NIKE” will read “EKIN” when viewed in the mirror. Another characteristic (3rd) of plane mirror images pertains to the relationship between the object’s distance to the mirror and the image’s distance to the mirror in question. For plane mirrors, the object distance (often represented by the symbol do) is equal to the image distance (often represented by the symbol di).That is the image is the same distance behind the mirror as the object is in front of the mirror. If you stand a distance of 2 meters from a plane mirror, you must focus at a location 2 meters behind the mirror in order to view your image. A fourth and final characteristic of plane mirror images is that the dimensions of the image are the same as the dimensions of the object. If a 1.6-meter tall person stands in front of a mirror, he/she will see an image which is 1.6-meters tall. If a coin with a diameter of 18-mm is placed in front of a plane mirror, the image of the coin has a diameter of 18 mm. The ratio of the image dimensions to the object dimensions is termed the magnification. Plane mirrors produce images which have a magnification of 1. In conclusion, plane mirrors produce images with a number of distinguishable characteristics. Images formed by plane mirrors are virtual, upright, left-right reversed, the same distance from the mirror as the object’s distance, and the same size as the object (see Figs. 4.19(a & b).



LUMINAIRES    4.25

Eye

Eye

Diagram A

Diagram B

Fig. 4.19(a) Diagram A and B Showing Image Formation

No light travels along this path

Object

Virtual Image

Fig. 4.19(b) Ray Diagram for Plane Mirror

What are the uses of plane mirrors? Mirrors are most commonly used for personal grooming, decoration, and architecture. Mirrors are also used in scientific apparatus such as telescopes and lasers, cameras, and industrial machinery. Most mirrors are designed for visible light, however, mirrors designed for other wavelengths of electromagnetic radiation are also used, especially in optical instruments.

Curved Mirrors Curved mirrors could be a part of the spherical mirror. Spherical mirrors can be thought of as a portion of a sphere which was sliced away and then silvered on one of the sides to form a reflecting surface. Concave mirrors are silvered on the inside of the sphere and convex mirrors are silvered on the outside of the sphere. We will now focus on concave mirrors and then on convex mirrors. What is a Concave Mirror?

4.26    FUNDAMENTALS OF LIGHTING

Sometimes referred to as a converging mirror, the concave mirror is configured with a surface that bends inward. Because the center of the curvature in a concave mirror is directed away from any incident light, this creates a reflective image that is typically larger than the actual focal point. The identification of concave mirrors as a converging device has to do with the fact that a concave mirror collects the light that falls into the bowl created by the inward bulge of the surface. This collection creates a refocus of the collected light into a single focal length. The light is collected at different angles, since the concave nature of the bulge allows the light rays to make normal contact at differing depths at each point on the surface of the mirror. The unique reflection that is created by a concave mirror makes the device extremely helpful with a number of different devices. One of the most common applications is with the production of automobile headlights. By placing the source of light at the center of the concave mirror reflector, the result is a parallel beam of light that provides both plenty of visibility for the driver of the vehicle as well as points of light that are apparent to approaching vehicles. In a similar vein, the concave mirror provides a focused beam of light for searchlights. The concave construction of the lens allows the production of that parallel beam that can be easily directed when the searchlight is positioned. Featuring a relatively easy construction, the concave mirror for search lights helps to keep the cost of the units within reason. At the same time, this type of concave mirror is also one of the more durable components on the device. The dental profession also benefits from the use of the concave mirror. The device is used to focus light onto the area of the mouth that the dentist is working with, providing a clear reflection of the tooth or set of teeth. The concave mirror is used in both hand held equipment as well as mirrors that can be mounted over the dental chair. A concave mirror can be very helpful around the house as well. Men can enjoy the use of a concave mirror while shaving, since the device will provide a magnified image of the beard line. In a similar application, women can enjoy the services of a concave mirror while applying make up. The larger image can make the intricate work of applying make up much easier to accomplish. Even the field of alternative energy benefits from the present of a concave mirror. Solar heating devices often make use of concave mirror panels as a means of collecting solar energy for storage. The bowl-like design helps to collect rays and focus them into a battery system, proving to be much more efficient than some other methods of powering solar heating systems (see Fig. 4.20).



LUMINAIRES    4.27

Fig. 4.20 Ray Diagram for the Concave Mirror

We see an object because light from the object travels to our eyes as we sight along a line at the object. Similarly, we see an image of an object because light from the object reflects off a mirror and travel to our eyes as we sight at the image location of the object. From these two basic premises, we have defined the image location as the location in space where light appears to diverge from. Ray diagrams have been a valuable tool for determining the path of light from object to mirror to our eyes. We will investigate the method for drawing ray diagrams for objects placed at various locations in front of a concave mirror. To draw these diagrams, we will have to recall the two rules of reflection for concave mirrors: Any incident ray travelling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection. Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection. In Fig. 4.20, five incident rays are drawn along with their corresponding reflected rays. Each ray intersects at the image location and then diverges to the eye of an observer. Every observer would observe the same image location and every light ray would follow the law of reflection. Yet only two of these rays would be needed to determine the image location since it only requires two rays to find the intersection point. Of the five incident rays drawn, two of them correspond to the incident rays described by our two rules of reflection for concave mirrors. Because they are the easiest and most predictable pair of rays to draw, these will be the two rays which are generally used for calculations (see Fig. 4.21).

4.28    FUNDAMENTALS OF LIGHTING

Object

C

F Complete Image

Fig. 4.21 Ray Diagram for Concave Mirror

The Mirror Equation Ray diagrams can be used to determine the image location, size, orientation and type of image formed of objects when placed at a given location in front of a concave mirror (Fig. 4.22). Ray diagrams provide useful information about object-image relationships, yet fail to provide the information in a quantitative form. Object d0 C

f

F Image di

Fig. 4.22 Concave Mirror Distances used for Mirror Equation

While a ray diagram may help one determine the approximate location and size of the image, it will not provide numerical information about image distance and object size. To obtain this type of numerical information, it is necessary to use the Mirror Equation and the Magnification Equation. The mirror equation expresses the quantitative relationship between the object distance (do), the image distance (di), and the focal length (f). The equation is stated as follows:

1/f = 1/d0 + 1/di

…4.1

The magnification equation relates the ratio of the image distance and object distance to the ratio of the image height (hi) and object height (ho). The magnification equation is stated as follows:

M = hi/ho = –di/d0

…4.2

These two equations can be combined to yield information about the image distance and image height if the object distance, object height, and focal length are known. The negative values for image height indicate that the image is an inverted image. As is often the case in physics, a negative or positive sign in front of the numerical value for a physical quantity represents



LUMINAIRES    4.29

information about direction. In the case of the image height, a negative value always indicates an inverted image. Generally in optics the distance of the object from the pole of the concave mirror is represented as u and that of image as v, then we can write: and

1/u + 1/v = 1/f …4.3 v/u = M …4.4

Where symbols have there usual meanings.

The +/– Sign Conventions The sign conventions for the given quantities in the mirror equation and magnification equations are as follows: f is + if the mirror is a concave mirror f is – if the mirror is a convex mirror di is + if the image is a real image and located on the object’s side of the mirror. di is – if the image is a virtual image and located behind the mirror. hi is + if the image is an upright image (and therefore, also virtual) hi is – if the image an inverted image (and therefore, also real)

Convex Mirrors and Image Formation A convex mirror or diverging mirror, is a curved mirror in which the reflective surface bulges toward the light source. Such mirrors always form a virtual image, since the focus F and the centre of curvature 2F are both imaginary points “inside” the mirror, which cannot be reached. Therefore images formed by these mirror cannot be taken on screen. (As they are inside the mirror). A collimated (parallel) beam of light diverges (spreads out) after reflection from a convex mirror, since the normal to the surface differs with each spot on the mirror.

F

F

F

Fig. 4.23 Ray Diagram for Image Formation in Convex Mirrors

The image is always virtual (rays haven’t actually passed though the image), diminished (smaller), and upright.

4.30    FUNDAMENTALS OF LIGHTING

These features make convex mirrors very useful: Everything appears smaller in the mirror, so they cover a wider field of view than a normal plane mirror does as the image is “compressed” (see Fig. 4.23).

Rules for Image Formation in Convex Mirror Any incident ray traveling parallel to the principal axis on the way to a convex mirror will reflect in such a manner that its extension will pass through the focal point. Any incident ray traveling towards a convex mirror such that its extension passes through the focal point will reflect and travel parallel to the principal axis. The ray diagrams are constructed in order to determine the location, size, orientation, and type of image formed by concave mirrors. The ray diagram constructed for a convex mirror reveals that the image of the object is virtual, upright, reduced in size and located behind the mirror. But will these always be the characteristics of an image produced by a convex mirror? Can convex mirrors ever produce real images? Inverted images? Magnified Images? To answer these questions, we will look at three different ray diagrams for objects positioned at different locations along the principal axis. The diagrams are shown in the previous slide. The diagrams in Fig. 4.23, show that in each case, the image is located behind the convex mirror. The image is a virtual image, an upright image and is reduced in size (i.e. smaller than the object).

The Mirror Equation - Convex Mirrors Ray diagrams can be used to determine the image location, size, orientation and type of image formed of objects when placed at a given location in front of a mirror. Ray diagrams provide useful information about object-image relationships, yet fail to provide the information in a quantitative form. Generally a ray diagram helps us to determine the approximate location and size of the image, however it does not provide numerical information about image size and image distance. To obtain this type of numerical information, it is necessary to use the magnification equation and the mirror equation. The mirror equation expresses the quantitative relationship between the object distance (d0), focal length (f ) and the image distance (di). The equation is stated as follows:

1/f = 1/d0 + 1/di

…4.5



LUMINAIRES    4.31

The magnification equation relates the ratio of the image distance and object distance to the ratio of the image height (hi) and object height (h0) and this equation is stated as follows:

M = hi/h0 = –di /d0

Both these two equations can be combined to yield information about the image distance and image height if the object distance, object height, and focal length are known in a problem. Uses of concave mirrors: 1.

Concave mirrors are used by dentists to see enlarged image of tooth.

2.

Concave mirrors are used by doctors (ENT specialists) to focus light into ears, throat etc.

3.

Concave mirrors are used in car headlights, reflector lamps.

4.

Concave mirrors are used in reflector type telescopes.

5.

Concave mirrors are used as make up or as shaving mirrors because an enlarged image can be obtained using a concave mirror.

Uses of Convex Mirrors In a car the passenger-side mirror is typically a convex mirror and in some countries, these are labelled with the safety warning “Objects in mirror are closer than they appear”, to warn the driver of the convex mirror’s distorting effects on distance perception. These mirrors are also used in some automated teller machines as a simple and handy security feature, allowing the users to see what is happening behind them. Similar devices are sold to be attached to ordinary computer monitors as well. Camera phones use convex mirrors to allow the user correctly aim the camera while taking a self-portrait. Convex mirrors are used as streetlight reflectors as they spread light over greater area. Convex mirrors are used as rear view mirrors in vehicles because convex mirrors increase field of view.

Parabolic Reflector A parabolic reflector (or dish or mirror) is a Parabola-shaped reflective device, used to collect or distribute energy such as light, sound, or radio waves. The parabolic reflector functions due to the geometric properties of the paraboloid shape: if the angle of incidence to the inner surface of the collector equals the angle of reflection, then any incoming ray that is parallel to the axis of the dish will be reflected to a central point, or “focus”. Because many types of energy can be reflected in this way, parabolic reflectors can be used to collect and concentrate energy entering the reflector at a particular angle. Similarly, energy radiating from the “focus” to the dish can be transmitted outward in a beam that is parallel to the axis of the dish. Parabolic reflectors suffer from an

4.32    FUNDAMENTALS OF LIGHTING

aberration called coma. This is primarily of interest in telescopes because most other applications do not require sharp resolution off the axis of the parabola.

Applications of Parabolic Reflectors The most common modern applications of the parabolic reflectors are in satellite dishes, reflecting telescopes, radio telescopes, parabolic microphones, and many lighting devices such as spotlights, car headlights and LED housings. The Olympic Flame has been lit using a parabolic reflector concentrating sunlight. Parabolic mirrors are one of many shapes for a burning-glass. A parabolic reflector pointing upward can be formed by rotating a reflective liquid, like mercury, around a vertical axis. This makes the liquid mirror telescope. Parabolic geometry is well known, and it was probably the very first type of solar cooker. In this category, there are maximum variations. The parallel light rays converge at the focus as shown in Figs. 4.24 and 4.25). Axis

Fig. 4.24  Parabolic Reflector

The reason for its popularity was the focus which was much better and sharper than that of other types of reflectors. However, at the same time it was very sensitive to even a slight change in the position of the sun and hence the use of such reflectors means constant tracking.

Fig. 4.25  Focussing Action of a Parabolic Reflector



LUMINAIRES    4.33

Parabolic reflectors (or paraboloids) and mirrors are used in astronomical telescopes, car headlights and satellite dishes. The paraboloid has the unique property that an on-axis parallel beam of radiation will be reflected by the surface and concentrated at its focus (or conversely, a point source located at the focus will produce a parallel beam on reflection). This feature has been illustrated in the diagram above - parallel rays enter from the left and are brought to a focus at a single point.

The Principle of Parabola If you want to understand how the algorithm works, we’ll need to have a look at the maths. We start by considering the parabola; this is a one dimensional curve and is a section through a paraboloid - a paraboloid is formed by rotating a parabola about its axis. The equation of a parabola is:

y = a.x²

…4.6

where a is a constant. For a parabola with a focal length of f : a = 1/(4f ).  Figure 4.26 of a parabola has the axis which is coincident with the y-axis and the focus is located at (0, f ). y 2

y=x 4f

(0, f )

0

x

Fig. 4.26  The Parabola with its Axis Along y-axis

Luminaire with Parabolic Reflector

Fig. 4.27  Luminaire with Parabolic Reflector

4.34    FUNDAMENTALS OF LIGHTING

As we already know, the luminaire includes the light source. Figure 4.27 shows such a luminaire with a parabolic reflector. Actually the most widely used reflectors are parabolic reflectors. They allow light to be controlled in a variety of ways, e.g., narrow-beam, wide-beam or asymmetrical distribution, and provide for specific glare limitation characteristics. If the reflector contour is constructed by rotating a parabola or parabolic segment around its own axis, the result is a reflector with narrow-beam light distribution. In the case of linear light sources a similar effect is produced when rectangular reflectors with a parabolic cross section are used.

Design and Use of Elliptical Reflectors In the elliptical reflectors the light radiated by a lamp located at the first focal point of the ellipse is reflected to the second focal point of the reflector and this second focal point of the ellipse can be used as an imaginary, (secondary) light source as shown in Fig. 4.28. These reflectors are used in recessed ceiling washlights to produce a light effect from the ceiling downwards. Further, these reflectors are also ideal when the smallest possible ceiling opening is required for downlights for use. The second focal point will be an imaginary light source positioned at ceiling level; it is, however, also possible to control the light distribution and glare limitation by using an additional parabolic reflector. The diagram of double-focus downlight (elliptical reflector)

Fig. 4.28  A Typical Elliptical Reflector

The elliptical reflectors are widely used in lighting as a means to concentrate the light of a source in the secondary focal plane,as explained above. To collect as much light from the source as possible, the reflector must cover a wide angular range of the emitted radiation, hence forward and backward reflections on the reflector must be discussed and conditions for paraxial optics do not apply.

Ellipse and its Properties In mathematics, an ellipse is a conic section, the locus of points in a plane such that the sum of the distances to two fixed points is a constant.



LUMINAIRES    4.35

The two fixed points are called foci (singular- focus). Actually an ellipse is the path traced out by a point whose distance from a fixed point, called the focus, maintains a constant ratio less than one with its distance from a straight line not passing through the focus, called the directrix (see Fig. 4.29). Equation of an ellipse:

x2/a2 + y2/b2 = 1

…4.7 10 7.5 5 2.5

-10 -7.5 -5 -2.5 -2.5

-2.5 -5 -7.5 10

-5 -7.5 10

Fig. 4.29  A Typical Ellipse

Reflective Properties of Ellipses In the ellipse the interior is silvered to produce a mirror and thus the rays originated at ellipse’s focus are reflected to the other focus of the ellipse (see Fig. 4.30). P

B

A

Fig. 4.30  Elliptical Reflector

Hyperbolic Reflector Hyperbola is shown in Fig. 4.31 and its reflective action is shown in Fig. 4.32.

4.36    FUNDAMENTALS OF LIGHTING

Fig. 4.31  A Typical Hyperbola 3 2 1 –3 –2 –1 –1

1

2

3

–2 –3

Fig. 4.32  Hyperbolic Reflector

Refractors The materials which undergo the phenomenon of refraction of light are generally used as the refractors. The phenomenon of refraction, there is the change in direction of a wave due to a change in its speed in the matter. This is most commonly seen when a wave passes from one medium to another. Refraction of light is the most commonly seen example, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth. Refraction is described by Snell’s law, which states that the angle of incidence is related to the angle of refraction by

μ = sin i/sin r …4.8

where i is the angle of incidence and r is the angle of refraction. If a photon is emitted from one focus of a hyperbola and strikes a branch of that hyperbola, it is reflected so that it behaves as though it had come from the other focus. Here, a red pulse is emitted from the left focus and strikes the right branch of the hyperbola. A green pulse is emitted from the right focus, timed so that the two pulses hit the vertex of the right branch simultaneously. Note the striking effect: The wavefronts appear to exchange places.



LUMINAIRES    4.37

Lenses A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens is a lens consisting of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element. Manufactured lenses are typically made of glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called lenses: for instance, a microwave lens can be made from paraffin wax. There are various types of lenses-like biconvex lens, plano convex lens, biconcave lens etc.

Types of Lenses Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres, with the lens axis ideally perpendicular to both surfaces. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens. Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex (or double convex, or just convex) if both surfaces are convex, A lens with two concave surfaces is biconcave (or just concave).If one of the surfaces is flat, the lens is planoconvex or plano -concave depending on the curvature of the other surface. A lens with one convex and one concave side is convex-concave or meniscus. It is this type of lens that is most commonly used in corrective lenses.

Imaging Properties of Lens

f 0 Object distance 0 = 30 cm

f

Focal length f = 20 cm i

Image distance i = 60 cm

M = –2

Fig. 4.33  The Image Formation in a Convex Lens

Lens formula:

1/o + 1/i = 1/f or 1/u + 1/v = 1/f …4.9

4.38    FUNDAMENTALS OF LIGHTING

where o or u is the object distance, i or v is the image distance and f is the focal length of the lens. distance are measured from the pole of the lens (see Fig. 4.33).

Fresnel Lens A Fresnel lens is a type of lens invented by French physicist Augustin -Jean Fresnel. Originally developed for light houses, the design enables the construction of lenses of large aperture and short focal length without the weight and volume of material which would be required in conventional lens design. Compared to earlier lenses, the Fresnel lens is much thinner, thus passing more light and allowing lighthouses to be visible over much longer distances. The idea of creating a thinner, lighter lens by making it with separate sections mounted in a frame is often attributed to Georges-Louis Lecler, Comte de Buffon. However, it is difficult to find any other sources that link Buffon to work with optics. French physicist and engineer Augustin-Jean Fresnel is most often given credit for the development of this lens for use in lighthouses. According to Smithsonian, the first Fresnel lens was used in 1823 in the Cordouan lighthouse at the mouth of the Gironde estuary. Its light could be seen from more than 20  miles (32  km) out. Scottish physicist Sir David Brewster is credited with convincing The United Kingdom to use these lenses in their lighthouses. The Fresnel lens reduces the amount of material required compared to a conventional spherical lens by breaking the lens into a set of concentric annular sections known as Fresnel zones. In the first (and largest) variations of the lens, each of these zones was a different prism. Though a lens might look like a single piece of glass, closer examination reveals that it is many small pieces. It was not until modern computercontrolled milling equipment (CNC) could turn out large complex pieces that these lenses were single pieces of glass. For each of these zones, the overall thickness of the lens is decreased, effectively chopping the continuous surface of a standard lens into a set of surfaces of the same curvature, with discontinuities between them. This allows a substantial reduction in thickness (and thus weight and volume of material) of the lens, at the expense of reducing the imaging quality of the lens.

Graphic Examples The construction process of a Fresnel`s lens is shown in Fig. 4.34 and the used lens is shown in Fig. 4.35.



LUMINAIRES    4.39

1.

Cross section of a Fresnel lens

1

2

Fig. 4.34 (1)  Cross Sections of a Fresnel’s Lens (2) Cross Section of a Conventional Plano-Convex Lens of Equivalent Power

Fig. 4.35  Fresnel Lens Displayed in the Musée National De La Marine in Paris, France

Uses of Fresnel Lens The use of a Fresnel lens reduces image quality, so they tend to be used only where quality is not critical or where the bulk of a solid lens would be prohibitive. Cheap Fresnel lenses can be stamped or moulded out of transparent plastic and are used in overhead projectors, projection televisions, and handheld sheet magnifying glasses. Fresnel lenses have been used to increase the visual size of CRT displays in pocket televisions, notably the Sinclair TV80. They are also used in traffic lights. Fresnel lenses are also used to correct several visual disorders, include several ocular motility disorders such as strabismus. Since Fresnel lenses can be made larger than glass lenses, as well as being much cheaper and lighter, they are used to concentrate sunlight for heating in solar cookers, solar forges, and solar collectors to heat water for domestic

4.40    FUNDAMENTALS OF LIGHTING

use. Perhaps the most widespread use of Fresnel lenses was in automobile headlamps, where they allow the roughly-parallel beam from the parabolic reflector to be shaped to meet requirements for dipped and main beam patterns, often both in the same headlamp unit. For reasons of cost, weight and impact resistance, newer cars have dispensed with glass Fresnel lenses, using multi-faceted reflectors with plain polycarbonate lenses. However, Fresnel lenses continue to be widely used in automobile tail, marker and backup lights. Fresnel lenses with high-quality glass were used in lighthouses, where they were ‘state of the art’ in the late 19th and through the middle of the 20th centuries. Fresnel lens systems used in lighthouse, typically include extra annular prismatic elements, arrayed in faceted domes above and below the central planar Fresnel, in order to catch all light emitted from the light source. The light path through these elements can include an internal reflection, rather than the simple refraction in the planar Fresnel element. These lenses conferred many practical benefits upon the designers, builders, and users lighthouses and their illumination. Among other things, smaller lenses could fit into more compact spaces. Greater light transmission over longer distances, and varied patterns made it possible to triangulate a position. Glass Fresnel lenses also are used in lighting instruments for theatre and motion pictures; such instruments are often called simply Fresnels. The entire instrument consists of a metal housing, reflector, lamp assembly, and Fresnel lens. A holder in front of the lens can hold a colored plastic film (gel) to tint the light or wire screens or frosted plastic to diffuse it. Many Fresnel instruments allow the lamp to be moved relative to the lens focal point, to increase or decrease the size of the light beam. The Fresnel lens is useful in the making of motion pictures not only because of its ability to focus the beam brighter than a typical lens, but also because the light is a relatively consistent intensity across the entire width of the beam of light. The aircraft carriers typically use Fresnel lenses in their optical landing system all over the world. The “meatball” light aids the pilot in maintaining proper glideslope for the landing. In the center are amber and red lights composed of Fresnel lenses. Although the lights are always on, the angle of the lens from the pilot’s point of view determines the color and position of the visible light. If the lights appear above the green horizontal bar, the pilot is too high. If it is below, the pilot is too low, and if the lights are red, the pilot is very low. New applications have appeared in solar energy, where Fresnel lenses are used to concentrate sunlight (with a ratio of almost 500) onto solar cells. Thus the active solar cell surface can be reduced to a fraction compared to conventional solar modules.



LUMINAIRES    4.41

This offers a considerable cost-saving potential by low material consumption, and it is possible to use high-quality and expensive solar cells, which achieve a very high efficiency under concentration due to thermodynamic effects. Fresnel reflectors are also currently being incorporated into next-generation solar thermal energy systems. The Polaroid SX-70 camera used a Fresnel reflector as part of its viewing system. Multi-focal Fresnel lens are also used as a part of retina identification camera, where they provide multiple in- and out-of-focus images of a fixation target inside the camera. For virtually all users, at least one of the images will be in focus, thus allowing correct eye alignment. Fresnel lens has seen applications in to enhancing passenger reading lights on Airbus aircraft. In a dark cabin, the focused beam of light does not dazzle neighboring passengers. Fresnel lenses have also been used in the field of popular entertainment. The British rock artist Peter Gabriel made use of them in his early solo live performances to magnify the size of his head, in contrast to the rest of his body, for dramatic and comic effect.

Projection Uses Fresnel lenses of different focal lengths (one collimator, and one collector) are used in commercial and DIY projection. The collimator lens has the lower focal length, and is placed closer to the light source, and the collector lens, which focuses the light into the triplet lens, is placed after the projection image (an active matrix LCD panel in LCD projectors).

4.7 TESTING AND PERFORMANCE OF LUMINAIRES The testing and performance of luminaires can be considered a combination of photometric, electrical, mechanical and environmental testing as well as their performance. The Lighting laboratory is usually equipped to carry out performance measurements on all commercially available lamps and luminaires. Lighting experts undertake performance tests using advanced equipment. Additional ageing tests are carried out when necessary. Results are compiled in detailed test reports. The lighting experts use the equipments to measure any type of lamp- Incandescent, halogen, Compact fluorescent, Fluorescent lamps, discharge lamps and LEDs and luminaire- indoor, emergency lighting and street lighting. More specifically, experts can measure the following parameters: 1.

Luminous flux (lumen)

2.

Luminous intensity distribution (candela)

3.

Power (W)

4.42    FUNDAMENTALS OF LIGHTING

4.

Energy efficiency (lm/W)

5.

European Energy Label

6.

Power factor and harmonics

7.

Color Rendering Index (CRI, Ra)

8.

Color temperature

9.

Spectral distribution including UV

10. Start-up and warm-up time Photometric laboratory generally provides clients with a report that compiles all test results. This report could be either as a paper document or in an electronic format (PDF). When applicable, the report comes with a photometric file (in LDT, IES or CIB format) containing all test results. This file can be used in simulation tools (DIALux, Relux, etc., to simulate the lighting of a specific light source in a given environment. Lighting experts also performs laboratory ageing test of lamps and luminaires in compliance with ISO 9001 standard. It allows to assess the reliability of lighting devices (including LED) using different testing methods: 1.

Lamp Survival Factor (LSF) and Lamp Luminous Maintenance Factor (LLMF)

2.

Fast Switching test

3.

Endurance test at different temperature (–20°C to 80°C, Tc + 10K,…)

The lighting experts generally perform the tests on lamps and luminaires according to the latest following international standards: 1.

EC N° 244/2009 and 245/2009 - European Ecodesign directive

2.

EC N° 874/2012 – European Energy labelling of electrical lamps and luminaires

3.

1998/11/EC - European Energy Label of lamps

4.

2000/55/EC - Efficiency of ballasts

5.

EN 13032 - Measurement of lamps and luminaires

6.

IES-LM79 - Measurement of LED products

7.

IEC 62612 - Self-ballasted LED lamps

8.

IEC 62384 - Performance of LED control gear

9.

IEC 60064 - Tungsten Filament lamps

10. IEC 60357- Tungsten Halogen lamps 11. IEC 60969 - Fluocompact lamps 12. IEC 60081, IEC 60901 - Fluorescent lamps 13. IEC 60662 - High pressure sodium lamps 14. IEC 61167 - Metal Halide lamps 15. CIE 63, CIE 84, CIE 121 - Measurement methods 16. CIE S020, IEC 60598-2-22 - Emergency lighting



LUMINAIRES    4.43

17. IEC 62442 - Energy performance of HID and LED control gears 18. IEC/PAS 62722 - Luminaire performance (LED) 19. IEC/TR 61341 – reflector lamps 20. IEC 62717 – LED modules The photometric testing and performance of a luminaire describes the efficiency and effectiveness with which it delivers the light produced by the lamp to the intended target. This performance is determined by the photometric properties of the lamp, the design and quality of the light control components, and to some extent any auxiliary equipment required by the lamp. The electrical testing and performance of a luminaire describes the efficacy with which the luminaire generates light and the electrical behavior of any auxiliary equipment such as ballasts. Luminaire efficacy is determined by lamp efficacy and, if present, the ballast and its interaction with the lamp. The electrical parameters, such as power factor, waveform distortion, and various forms of electromagnetic interference, are properties of the lamp and ballast. The mechanical and environmental testing and performance of a luminaire describes its behavior under stress. This can include extremes of temperature, water spray or moisture, mechanical shock, and fire etc.

Components of Photometric Performance Luminaire Photometric Reports Lighting industries which are engaged in luminaires designing and manufacture usually have a photometric laboratory which is needed for photometric testing of all types of luminaires. These laboratories are usually equipped with high speed mirror Goniophotometers. The lighting experts conduct testing and performance photometry of various lighting units consisting of light sources and fixtures including: Lamps, Indoor Fixtures, Outdoor Fixtures, Street Lighting, Flood Lighting, Spotlights, Flashlights and LED Lighting Products etc. These laboratories produce IESNA (Illuminating Engineering Society of North America) and CIE (International Commission on Illumination) approved photometric reports for their customers who would like to evaluate the photometric performance of their lighting products. Typical photometric measurements include: (i) Candela Tabulation (ii) Polar Candela Plots (iii) Beam Spread (iv) Zonal Lumen Output (v) Coefficient of Utilization Curves (vi) Total Luminaire Efficiency

4.44    FUNDAMENTALS OF LIGHTING

Testing may be conducted on a maximum luminaire size of 4 feet × 4 feet. Luminaire photometric performance is summarized in a photometric report. This report ontains the luminous intensity data for a typical ceiling mounted luminaire in an interior space alongwith the luminous intensity or candle power (CP) distribution curve for the same luminaire. Luminous intensity values are determined from laboratory measurements and are reported as the luminaire’s luminous intensity distribution. Electrical and thermal measurements are made and often reported. These include input watts, input volts, and ambient air temperature. In addition, some calculated application quantities are usually reported. These include zonal lumens, efficiency, and coefficients of utilization; Polar curves and row-area illumination; Luminous Intensity Distribution Curve;

Luminaire Efficiency Luminaire efficiency is obtained from standard photometry testing, which specifies the use of a goniophotometer in a black room with a 25±1°C ambient temperature. To measure luminaire efficiency, the luminaire is first installed in the goniophotometer in the same position that it would occupy in practice. (For example, a ceiling-mounted luminaire is installed horizontally, and a wall sconce is installed vertically). The luminaire’s light distribution and total light output are measured. Next, the lamp or lamps are taken out of the luminaire and installed in the goniophotometer in the same position as inside the luminaire, using the same ballast. The light distribution and total light output of the bare lamps are measured. Luminaire efficiency is then calculated as the ratio of the total light output from the luminaire to the total light output from the bare lamp(s), expressed as a percentage. Luminaire efficiency is a good predictor of light output for luminaires using incandescent lamps, but for luminaires using fluorescent lamps, luminaire efficiency is not enough to allow an evaluator to predict the actual light output. Lamp position, ambient temperature, and ballast factor also affect the light output. The National Electrical Manufacturers Association (NEMA) of USA has developed a related metric for “energy efficiency” of luminaires that use linear fluorescent lamps (NEMA 1995).

Light Distribution Curves The photometric report of a luminare illustrates the vlues of the luminous intensity in the form of a table which shows the distribution of the light at various angles from the source. The shape of the light distribution plot indicates the directions (via the angles) the light travels, and the intensity of light going in a particular direction [measured in candelas (cd)]. The performance of the luminaires must be known in order to predict the lighting levels and uniformities.



LUMINAIRES    4.45

Photometric testing of the luminaires must be conducted to determine the performance so that the data can be used in lighting calculations. The procedure for testing and reporting the data are contained in the guides published by the Illuminating Engineering Society of North America (IESNA) for North America and the CIE in Europe. Test equipments consist of the goniometer, the photometer, electrical measuring equipments and the thermometer. The goniophotometer is a device for measuring angles in a spherical coordinate system. The photometer is a device for measuring intensity of the light. Luminous intensity distribution curves are typically represented in polar plots because this format allows us to visualize both the orientation and the light distribution of the light fixture. The luminous intensity distribution of a light fixture depends upon reflector design, shielding type, and lamp-ballast selection. In order to provide suitable illuminance at a particular row or in a specified area the light output of the luminare is very important. For this the Polar Luminous Intensity Graph are available in the manuals supplied by the luminaires manufactures. These polar curves are the photometric data studied by the lighting experts using goniometers, phocell or lux meters etc. The diagram illustrates the distribution of luminous intensity, in candelas, for the transverse (solid line) and axial (dashed line) planes of the luminaire as shown in Fig. 4.36. The curve shown provides a visual guide to the type of distribution expected from the luminaire e.g., wide, narrow, direct, indirect etc., in addition to intensity.

90°

90°

1000 60°

2000

60°

3000

4000 30°

30°

Fig. 4.36 Polar Luminous Intensity Graph

Cartesian Luminous Intensity Graph The diagram in Fig. 4.37 indicates the distribution of luminous intensity, in candelas of the luminaire. The curve shown provides a visual guide to the type of distribution expected from the luminaire e.g., narrow or wide beam etc., in addition to intensity. This diagram is useful when light intensity changes rapidly within a small angular area.

4.46    FUNDAMENTALS OF LIGHTING

Fig. 4.37  Cartesian Luminous Intensity Graph

Cone Diagram for Illuminance Usually used for spotlights or lamps with reflectors, the diagram in Fig. 4.38 shows the maximum illuminance at different distances from the lamp used.

Fig. 4.38  Cone Diagram for Illuminance

Polar Curve of a Luminaire The polar curve of a luminaire is shown in Fig. 4.39.



LUMINAIRES    4.47

Fig. 4.39  Polar Curve of Luminous Intensity of a Luminaire

Illumination due to Flat Linear Source In order to evaluate the light output from an extended light source. It is assumed that the light fixture position is at the crossing of two axes (horizontal and vertical), and that 0° is beneath the light fixture. Other angles, which represent the various placements of a photocell as it moves in a circular pattern around the light fixture, are marked on the polar graph as well (see Fig. 4.40). 180° Vertical Axis Horizontal Axis

90° 65° 55°

0° Vertical Axis

Fig. 4.40  Fluorescent Lamp for Luminous Intensity Measurement

When light distribution is not symmetrical in all directions around the vertical axis, such as for a 2ft. × 4ft. light fixture, luminous intensity values may be taken in a number of vertical planes through the light fixture, (see Fig. 4.41). The planes shown in photometric reports are 0°, 22.5°, 45°, 67.5°, and 90°. The planes most commonly used in lighting practice are 0° or parallel

4.48    FUNDAMENTALS OF LIGHTING

to the lamp axes, 90° or perpendicular to the lamp axes, and at an angle 45° to the lamp axes (see Fig. 4.42). A vertical luminous intensity distribution curve is obtained by taking measurements at various angles of elevation in a vertical plane through the light center. Unless the plane is specified, the vertical curve is assumed to represent an average such as would be obtained by rotating the lamp or light fixture about its vertical axis. Luminous intensity curves can then be plotted for the plane of choice (see Fig. 4.43). Each lamp and lamp-light fixture combination has a unique set of luminous intensity distributions. The luminous intensity distribution of a light fixture depends upon reflector design, shielding type and lamp-ballast selection. 0° 45°

90°

Fig. 4.41  Intensity Measurement for FTL

Fig. 4.42 Cross Section of Planes for Intensity Measurement of FTL



LUMINAIRES    4.49 90°

60°

30° 0°

Fig. 4.43  Luminous Intensity Curve Perpendicular to the Lamp Axis of a 2ft. × 4ft. Light Fixture

If the distribution of the light is symmetrical in all directions around a vertical line, as with most recessed incandescent downlights, then only one luminous intensity distribution curve is necessary. In fact, only one half of that curve is usually shown, with the missing half implied to be an exact match. This is done primarily to simplify the reporting. If, however, the distribution of light is not symmetrical in all directions around a vertical line, luminous intensity or candela values will be taken in more than one vertical plane through the light fixture but are shown in the same plot. Only half of the curve for each plane is shown if the candela distribution is symmetrical within the plane. From the candela distribution curve, we can roughly estimate the luminous intensity at certain angle. If for example on the 0° -plane (parallel to the lamp axis), the candela value at 55° falls between 550 and 1100 candelas (cd) and is about 700 cd. On the 45° -plane, the candela value at 55° also falls between 550 and 1100 cd and is about 800 cd. On the 90° -plane (perpendicular to the lamp axis), the candela value at 55° falls between 1100 and 1650 cd and is about 1300 cd (see Fig. 4.44).

90°

75°

55°

Candelas

550 60°

1100 45° 0° 45° 90°

1650 0°

15°

30°

Fig. 4.44 Luminous Intensity Distribution Curve of a Luminaire

4.50    FUNDAMENTALS OF LIGHTING

The illuminance on a plane is usually calculate using the Lambert’s law or cosine law. The illuminance at a point in a plane not perpendicular to the direction of the luminous intensity is equal to the luminous intensity in the direction of the point, divided by the square of the distance between the light source. It has to be multiplied by the cosine of the angle θ that the direction of light incidence makes with the perpendicular to the plane. Graphical Integration, zonal method and the use of Rousseau diagram, Graphical integration method and use of polar graphs are some of the standard techniques to estimate illumination at a place. It is usually realized that picking lighting equipment for the job is sometimes more difficult with an outdoor installation than with an indoor area, because fewer fixtures contribute their light to a given area. Generally, this means there’s little margin for error in an outdoor lighting design. Although you can install lighting equipment on any high structure, pole mounting offers the most versatility. It is observed that luminaires on poles can provide illumination in every direction at distances of two to two and half times the mounting height from the pole location. Thus, luminaires on a singe pole can serve an area of about four times the mounting height squared. For example, a 50-ft pole can cover about 40,000 sq. ft and a 150-ft pole about 369,000 sq. ft. You can use narrow beam floodlights to light a flat area extending to five times the mounting height from the pole. However, at distances greater than two times the mounting height, uniformity and system efficiency drop off considerably. For a horizontal distance from the pole of twice the mounting height, the length of the shadow will be twice the height of the object casting the shadow. Once you establish the luminaire locations and mounting heights, determine the quantity and type of luminaire. If you select tall poles, you can use higher wattage lamps, which are more efficient than lower wattage light sources. It is seen that generally, 1000W or 750W high-pressure sodium (HPS) or metal-halide (MH) lamps are the choice for applications for high mast luminaires. In addition to choosing the lamp type, wattage, and number and location of luminaires, a designer must consider the beam spread, or the candlepower distribution pattern provided by the luminaires. High mast luminaires and floodlights offer symmetrical and asymmetrical beam spreads. Naturally, a high mast luminaire delivers most of its light directly downward. But, you can vary the beam spread of a high mast luminaire by vertically adjusting the lamp in the reflector assembly. You can also select reflector-/refractor-type units, which cast light at a high angle. When you rotate the optical assembly of an asymmetrical-beam, a high mast luminaire allows you to shape the distribution pattern of a cluster of these luminaires. Typically installed on structures and low poles, floodlights have a circular reflector, with the lamp mounted in the center. Floodlight beam spreads and their effective projection distances are classified. Type classification assumes a symmetrical beam shape, meaning that the beam spread angle in the vertical and horizontal axes are identical. Generally narrow projection beams (Type



LUMINAIRES    4.51

1, 2, 3, and 4), which are useful for directing a long throw of light, have a symmetrical beam spread. However, outdoor floodlights with Type 5, 6 and 7 beam spread have different beam spread for the vertical and horizontal axes, since they’re generally used to project their light output at medium to close distances. If the lighting project must satisfy only general criteria, then simple calculation is sufficient. However, a better method of designing an outdoor lighting system is to use an isofootcandle plot. An isofootcandle plot graphically represents the light distribution pattern on a horizontal surface. The graph consists of a series of lines, or contours, that represent the same illuminance anywhere on the line, with each line representing a different footcandle. Each contour from the center out represents approximately 50% of the value of the previous contour. The plot is placed over a grid, which you can use to indicate mounting height divisions. An isofootcandle plot can vary in shape from a circle, oval, or triangle, and may be symmetrical or asymmetrical. Essentially, you can use an isofootcandle curve at the same scale as a plan view of the area to be lighted to determine the contribution of each luminaire to the entire area. Today, manufacturers have powerful and relatively inexpensive software programs to perform these calculations. Many of these programs perform lighting design calculations based on isofootcandle curves and foot-candle tables for each luminaire type.

Rousseau Diagram The Rousseau diagram is developed from a polar curve in such a way as to give an area proportional to the total flux emitted by a lamp. Thus we can say that it is a geometric construction used to determine the total luminous flux of a lamp from a number of polar diagrams which give the effective luminous intensity of the lamp in various directions.

Approximation Methods Illumination calculations for any lighting design are not exact due to various reasons. Therefore, we need to use approximate methods for lighting design. Why design calculations quite often differ from the actual ones? When using software, a contractor might find that after the installation, the measured illuminance differs from the computer-predicted illuminance. Why? A number of factors may cause this variation. First of all, site conditions frequently vary from the assumptions used in preparing the design. Then, consider that a lamp can vary 55% in light output and still be within the manufacturer’s tolerances. An HID ballast can vary 57% and still be within tolerance. Thus, it’s possible for a lamp/ballast combination to be 12.5% under the predicted output. You may find that the installer skewed the lamp’s arc tube or mounted the fixture slightly out of alignment - resulting in the distribution of light at angles other than those intended. Another factor could

4.52    FUNDAMENTALS OF LIGHTING

be a reflector or a refractor also mounted slightly off axis, producing similar results. It takes only a few degrees of tilt to produce significant change in the light distribution pattern. Low voltage at the ballast of the fixtures could also be a problem, resulting from excessive voltage drop in the feeder or branchcircuit conductors. A regulator-type ballast is available for such a situation. While making measurements of luminous intensity for a luminaire the electrical measurements are required to make sure that the lamps or ballasts are operated at their correct characteristics for the test. Generally volts, amperes and watts are recorded. For outdoor uminaires we generally need a goniophotometer setup as discussed in Chapter 2.

Goniometer Set Up A goniometer is an instrument that either measures angle or allows an object to be rotated to a precise angular position. The term goniometry is derived from two Greek words, gonia, meaning angle and metron, meaning measure. A photograph of a goniophotometer set up is shown below in Fig. 4.45.

Fig. 4.45  Goniophotometer Set Up

Goniophotometer for Luminare Photometry Positioning the Goniophotometer Since goniophotometer is an instrument which can measure photometric quantities of a luminaire in a three dimensional space, hence a preliminary knowledge of the three-dimensional coordinate system is necessary. We give below the relationship of three dimensional Cartesian coordinates x, y, z in terms of the spherical coordinate system which is very useful for luminare photometry.



LUMINAIRES    4.53

Spherical Coordinate System The spherical coordinate system in mathematics and physics represents geometric figures in three dimensions using three coordinates r, θ, j. The radial distance r is measured from a fixed origin, the polar angle θ is measured from the positive z-axis to the point, and the azimuth angle j is measured from the positive x-axis to the orthogonal projection of the point in the x-y plane as shown in (see Fig. 4.46).

Fig. 4.46  Spherical Coordinate System

The relationship between the Cartesian coordinates and the polar coordinates is the following: x = r sin θ cos φ y = r sin θ sin φ z = r cos θ Floodlights, projectors, automobile heallights, and other concentrating beems have to be tested at much greater distances than do indoor luminaires. The detector should be far enough away so that it sees the whole of the reflector flashed with light. For normal floodlighting luminaires as used in stadium lighting, 33 meters is sufficient. However, by international agreement automobile headlights are tested at 25 meters. One consequence of the long path length is that the photocell has to be fixed and the luminaire rotated. The goniophotometer required for measuring the angles of elevation and azimuth needs to be very strongly made to take the weight of the luminaires without sagging. The luminaire holder turns in elevation and also moves in azimuth. The detector can be a photovoltaic cell or, if this is not sensitive enough, a photo-multiplier. It is convenient to arrange for the readout of the photocell to be at the same position as the goniophotometer so that only one person is needed to take the readings and work the goniophotometer the equipment can be computer-controlled so that the readings are taken at predetermined intervals and goniophotometer turned by electric motors controlled by the computer.

4.54    FUNDAMENTALS OF LIGHTING

The measurements are carried out in darkened room with luminaire mounted on the goniophotometer. In order to obtain the necessary path length it is sometimes convenient to locate the detector at the end of a light-tight tunnel which can pass through adjoining rooms. There should be baffles (a mechanical device to limit or regulate the flow of light) fixed to the wall of the tunnel to reduce the amount of stray light.

Luminaire Efficiency Definition: It is the ratio of the luminous flux (measured in lumens) emitted by the luminaire to that emitted by the lamp or lamps used therein.

Electrical Test System for HID Luminaires Now these days we have automatic electrical test systems which offer fully automatic testing for quality control of high intensity discharge (HID) lighting equipments (see Fig. 4.47).

Fig. 4.47  A Typical Electrical Test System

When used on the factory production line, these high-speed totally digital test systems are able to perform 100% quality control testing by verifying electrical performance characteristics. These test units consist of a computer, power circuits, electronic logic and measuring circuits. Electrical specifications for as many as 2,000 ballast or luminaire types can be retained on disk storage for verification of pass/ fail comparisons.



LUMINAIRES    4.55

Electrical Test System Usually the specifications and test procedure protocol are stored on disk with the capability to add others for additional ballast/luminaire types l

No meters to read—all data acquisition is performed by the computer

l

Tests are performed automatically and compared against stored specifications

l

Opto-couplers isolate computer and all electronics from the power circuits.

l

Built-in protection against surges and transients

l

Line voltage is checked frequently during testing. (Automatically adjusted with computerized power supply option)

l

Modular electronic construction and rugged, factory-floor design

l

Testing luminaires could be done up to 1500 watts.

Safety of Lumnaires In automatic models generally all circuitry is designed for the primary goal of operator safety. l

Digital and ballast power circuitry are protected using ground fault interrupting (GFI) circuits. These circuits provide fast shutdown in the event of excessive current flow to earth ground, as the result of ballast malfunctions or luminaire wiring errors.

l

Green safety lights are mechanically interlocked to the ballast power output circuitry. This insures that the safety lights cannot operate if ballast power is active.

l

Dual coupled start buttons insure the operator must use both hands during testing, eliminating the possibility of touching the fixture under test. The test program is halted in the event the start buttons are deactivated. Upon re-activation of the start buttons, the test proceeds.

Climatic Testing of the Luminaires Environmental simulations in climate test chambers allow experts to measure the effects of temperature and humidity on the luminaires in a controlled manner. Climatic testing is required to ensure that the luminaires will perform to standard under the climatic conditions they will encounter in service use. Materials can deteriorate over time, ageing prematurely due to either high or low temperature extremes, while humid conditions may lead to condensation which can be damaging to electronic components. Generally the climate test chambers have a test space of up to 1,500 litres. They are used to undertake heat tests of up to +180°C, cold tests of down to –70°C and humidity examinations in a range from 15% to 95%. The lighting

4.56    FUNDAMENTALS OF LIGHTING

experts are able to reproduce the effect on the luminaires of a wide range of temperature and humidity levels, including extreme stress parameters such as rapid temperature change. The accurate simulation of authentic environmental conditions speeds up the research into new materials and improves the quality and reliability of the products.

4.8 MATERIALS USED IN LUMINAIRES The main materials used in luminaires are the following: (i) Glasses; (ii) Plastic and Ceramic materials; (iii) Colours and Paints; (iv) Metals-steel, copper, aluminium and alloys, etc.; (v) Lenses; (vi) Mirrors; (vii) Electric wires of various sizes; (viii) Screws, Nut-bolts; (ix) Wood, etc.

Luminaire with Transmitting Material There is a plate-type luminaire which comprise a prism plate of light transmitting material. The prism plate has a smooth front plate surface and also has an opposite second prism surface. The second prism surface comprising a plurality of parallel longitudinally extending ribs alternating with and defining grooves between adjacent ribs, and the ribs extending in a longitudinal direction along the plate. Acover plate comprised of light transmitting material and having a smooth third plate surface resting on the second prism surface of the prism plate, the cover plate having an opposite fourth surface away from the prism plate; an optical-waveguide plate of light transmitting material and having a fifth surface shaped to the smooth first surface of the prism plate and resting on the smooth first surface of the prism plate and having an opposite sixth surface away from the prism plate.

4.9 EXERCISES Q.1 what do you mean by a luminaire and its efficiency? Explain different the functions of the luminaire with examples. Q.2 Explain with examples how various types of reflectors and refractors are used in luminaires?



LUMINAIRES    4.57

Q.3 Discuss the procedure of photometric testing and performance of automotive lighting luminaires using photogoniometer. Q.4 What do you mean by a Fresnel’s lense. Explain some of its important applications. Q.5 Discuss various types of mirrors and lenses. Explain some of their applications in our daily life. Q.6 What do you mean by IP (Ingress Protection) classification? Explain the procedure of classifying luminaires with IP code based on the protection levels with examples. Q.7. Write short notes on: Q.8

(i) Parabolic reflectors (ii) Concave mirror and its applications (iii) Convex lens and its applications (iv) Indoor luminaires (v) Outdoor luminaires (vi) IP Classification (vii) Electrical test of luminaires (viii) Convex mirror and its applications (ix) Mirror equation and its uses (x) Lens formula and its uses Which are the light control components of a luminare? Discuss the optical design and functions of any such component.

4.10 BIBLIOGRAPHY 1.

A.R. Bean and R.H. Simons, “Lighting Fittings – Performane and Design”, Pergamon Press, Oxford, 1968

2.

I.E.S.N.A., New York, “Lighting Hand Book”, (Reference Volume), 2000.

3.

M.A. Cayless and A.M. Marsden, “Lamps and Lighting”, (3rd Edition), Oxford and IBH Publishing Co, New Delhi, 1983

4.

Philips Lighting Division, Eindhoven, “Lighting Application Course”, (Volume 15), 1989.

5.

Bureau of Indian standard, New Delhi, “Code of Practice- IS-10322 (Part4)”, 1984.

6.

Brain Fitt and Joe Thornley, “Lighting by Design – A technical guide”, Focal Press, London 1992

7.

Verne Carlson and Sylvia E. Carlson, “Professional Lighting Hand Book”, (2nd Edition), Focal Press, Boston, 1991.

8.

Philips Lighting Division, Eindhoven, “Lighting Manual”, 5th Edition, 1993.

Chapter

5 Daylighting

5.1 INTRODUCTION The use of natural light, or daylighting, has traditionally been a desirable building feature and a hallmark of good design. When skillfully introduced, daylight creates an ambience of quietcontemplation and visual comfort. Daylighting design has recently taken on a new importance, beyond the esthetic and psychological aspects, with the advent of energy shortages and sustainability concerns. The alternative to daylighting, the use of electric power for lighting, contributes to the strain on electric generation capacity as well as the inefficient use of non-renewable energy resources. Furthermore, the cost of lighting a household or any other facility like library or school, etc., has become a major burden to communities and will continue to increase in the future. Daylight, which is free, provides the opportunity to greatly reduce these negative impacts created by the over dependence on electric lighting sources. Effective use of daylight in facility design is both an art and a science. The purpose here is to present the technical principles of daylighting design involved in a building design project, with some illustrations that suggest the esthetic possibilities within that framework. Daylighting is the practice of placing windows, or other openings, and reflective surfaces so that, during the day, natural light provides effective internal illumination. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use. Energy savings can be achieved either from the reduced use of artificial lighting, or from passive solar heating or cooling. In reality daylighting is just a fancy name given to a centuries old, geography and culture independent common design basic by 20th century

5.2    FUNDAMENTALS OF LIGHTING

architects who had given the go-by to common sense until energy costs and global warming became serious issues. There is no direct sunlight on the polar-side wall of a building from the autumnal equinox to the spring equinox in parts of the globe north of the Tropic of Cancer and in parts south of the Tropic of Capricorn. Traditionally, in these parts, with largely overcast skies, houses are designed with minimal windows on the polar side but more and larger windows on the equatorialside. For daylighting people generally use the windows and skylights to bring sunlight into the homes. Today’s highly energy-efficient windows, as well as advances in lighting design, allow efficient use of windows to reduce the need for artificial lighting during daylight hours without causing heating or cooling problems. The best way to incorporate daylighting in your home depends on your climate and home’s design. The sizes and locations of windows should be based on the cardinal directions rather than their effect on the street-side appearance of the house. South-facing windows are most advantageous for daylighting and for moderating seasonal temperatures. They allow most winter sunlight into the home but little direct sun during the summer, especially when properly shaded. North-facing windows are also advantageous for daylighting. They admit relatively even, natural light, producing little glare and almost no unwanted summer heat gain. Although east- and west-facing windows provide good daylight penetration in the morning and evening, respectively, they should be limited. They may cause glare, admit a lot of heat during the summer when it is usually not wanted, and contribute little to solar heating during the winter. If you’re constructing a new house, you want to consider daylighting as part of your whole-house design—an approach for building an energy-efficient home. Equatorial-side windows receive at least some direct sunlight on any sunny day of the year, so they are effective at daylighting areas of the house adjacent to the windows. Even so during mid-winter, light incidence is highly directional and casts deep shadows. This may be partially ameliorated through light diffusion and somewhat reflective internal surfaces. Most simply, daylighting is the practice of using natural light to illuminate building spaces. Rather than relying solely on electric lighting during the day, daylighting brings indirect natural light into the building. Daylighting reduces the need for electric lighting and connects people to the outdoors. And it provides pleasing illumination at a fraction of the cost of the most efficient electric lights. The good daylighting creates beautiful, appropriately lit spaces while saving energy. A successfully daylit building is the result of a combination of art and science, of architecture and engineering. It is the result of an integrated design process, and is not simply a technology that is installed once the building is complete. The daylighting designers toolbox includes concepts of lighting power density (W/ft2), illuminance levels, contrast ratios, window to wall ratios, ceiling to skylight area percentages, and reduction in glare. However,



DAYLIGHTING    5.3

we don’t have prescribed values for these concepts that designers can use knowing they’ll result in good daylighting. While there are efforts underway to establish metrics for good daylighting, they aren’t available yet. Even with proven metrics, daylighting will always be a mix of art and science, of logical thinking and common sense. Climate and geographical region, building type and use and building orientation are big factors in designing a successfully daylit building. Designers must always apply basic lighting and building performance principles to successfully employ daylighting. The daylighting does not have to increase construction costs if it’s done using an integrated design approach. An integrated approach considers the effect of lighting on air conditioning. The electric lights in modern buildings produce a lot of heat, while properly directed natural lighting generates almost no heat at all. The decrease in internally generated heat allows designers to downsize the air conditioning system. The resulting cost reduction helps pay for daylighting improvements. The light-to-heat ratio for daylighting is far better than even the most efficient electric lights. Properly designed daylighting screens out 99% of the sun’s heat while providing 50 foot-candles of light, which is more than enough for most tasks. Glare happens when too much light enters a building and this happens all the time in conventionally lit buildings. A properly daylit building uses carefully placed windows, shading devices and low-transmittance glasstechniques that block direct sunlight and greatly reduce glare. It is better to reduce the need for electric lighting and cooling in the first place. Cool daylighting does both. Natural light reduces the amount of installed electric lighting (within the limits of what’s needed for nighttime use). Less electric lighting means less heat gain, which means less heat to remove with air conditioning, using less energy. What lighting and cooling is left can then be done by the most efficient equipment available. Being efficient is always a good idea, but needing less energy is even better. Clear glass windows let in too much light, far more than what’s needed for effective lighting. The sun provides 70000 to 100,000 lux of light, while indoor office spaces need only about 500 lux. Too much light causes glare and the cave effect, where the back of the room appears dark compared to other surfaces. This encourages people to close the blinds and turn on overhead lights to cut down the contrast in the room. Well-designed daylighting lets in natural light that balances overhead electric lighting while curtailing glare. Properly designed skylighting is a good technique in certain situations, such as enclosed hallways or very deep spaces. However, in many schools and offices, windows can provide most of the daylighting that’s needed. It’s the placement and size of the windows that matters for effective daylighting. Clerestory windows—a row of small windows near the top of the wall—bring light in high in the room, producing a natural glow on the ceiling that mimics our experience of the sky. Skylights aren’t usually needed to achieve good results

5.4    FUNDAMENTALS OF LIGHTING

until you get beyond 25 feet of the perimeter windows. Even a completely overcast sky provides 50,000 to 60,000 lux of illumination—a hundred times more light than needed for daylighting. In some ways, overcast skies typical of northern climates provide a better lighting source because the light is more diffuse and even. Daylighting is most challenging in the sunny climates of the south because of the immense amount of illumination from the sky and sun. This illumination must be reduced and carefully controlled. Specific daylighting techniques vary, depending on location, number of building stories, building orientation and computer use in the building. Daylighting techniques can be adapted to meet the needs of almost any building, whether it’s a warehouse, school, office, or government building. All-glass buildings don’t provide good daylighting because they get too hot and have massive problems with glare. Windows constitute about 25–40% of the wall area of effectively designed daylit buildings. On the average, window area in daylit buildings isn’t all that different than windowed area in non-daylit buildings. Good daylighting technique depends on the proper placement of windows and performance characteristics such as visible light transmittance and solar heat gain coefficient—not having large amounts of glass.

5.2 DAYLIGHT SOURCES AND DAYLIGHTING TECHNIQUES The first and the major source of daylight is the sun. However, the amount of sunlight at different locations of the earth varies due the motion of earth around the sun and also on its own axis. Superimposed on this predictable pattern is variation caused by changes in the weather, temperature, and air pollution. Of the solar energy received at the earth’s surface, 40% is visible radiation. The rest is ultraviolet (UV) and infrared (IR) wavelengths. When absorbed, virtually all the radiant energy from the sun is converted to heat. The amount of usable visible energy in the solar spectrum varies with the depth and condition of the atmosphere through which the light travels. Because the spectral distribution of daylight changes continuously with sun position and sky conditions, the Commission Internationale de l’Éclairage (CIE) has adopted three standard spectral radiant power distributions for Daylight (Fig. 5.1). The second daylight source is the sky. As sunlight passes through the atmosphere, a portion of it is scattered by dust, water vapor, and other suspended particles. This scattering, acting in concert with clouds, produces sky luminance. The Commission International de l’Eclairage (CIE) has developed a series of mathematical models of ideal luminous distributions under different sky conditions - of which the three most common are clear, uniform and overcast. However, there are many different types of sky and many different mathematical models used to describe them.

DAYLIGHTING    5.5

Relative radiant power

140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 300

500

700

900

Wavelength in nanometers

Fig. 5.1  Three Spectral Radiant Power Distributions of Daylight

Developed by the CIE, daylight at 5500 k (D55), 6500 K (D65), and 7500 K (D75). Thus, it is now a standard practice to divide the sky into three categories (Fig. 5.2(a), (b) and (c)): (a) overcast sky, (b) clear sky and (c) partly cloudy sky. (a) Overcast sky

Fig. 5.2(a)  An Overcast Sky

5.6    FUNDAMENTALS OF LIGHTING

(b) Clear Sky

Zenith

Horizon

Fig. 5.2(b) Clear Sky

(c) Partly Cloudy Sky

Fig. 5.2(c) Partly Cloudy Sky

When the sky is not completely overcast, the sky luminance distribution may change rapidly and by a large amount as the sun is alternately obscured, partly obscured, or fully revealed. As clouds form and move through the sky, the distribution of light can change almost minute by minute. This means that we cannot really design for any specific distribution, but must rely on ‘average’ conditions. Each of these models assume the entire sky dome has some level of luminance, varying with angle from the horizon to the zenith, and with the relative angle from the current Sun position. The sky gets this luminance from sunlight that is scattered by air molecules or suspended particulates and reflected around by moisture vapour and clouds.



DAYLIGHTING    5.7

As a worst-case, the overcast sky condition is usually used. However in some tropical regions the uniform sky is considered by some researchers to be more appropriate. For the UK the BRE recommend the use of an average sky based on their own mathematical model.

CIE Overcast Sky Model The CIE Overcast Sky distribution model is based on a completely clouded sky where the Sun and its position are not apparent. The passage of radiation through the clouds usually produces close to white light by mixing as moisture droplets are quite large and affect all frequencies of light. However, if the atmosphere is heavily polluted the overcast sky color appearance can be slightly yellow. The distribution of luminance in such a sky is symmetrical about the zenith and is lower at the horizon than overhead (see Fig. 5.3). Zenith LZ

The luminance of the sky varies. The luminance in this zone = L

Z

Horizon



Fig. 5.3  Luminance Distribution Due to CIE Overcast Sky

Given a zenith illuminance (Lz) and its laltitude (θ), the luminance at any point (Lθ) in the sky is given by the following luminance formula:

Lθ = Lz × (1 + 2 sin (θ))/3

…5.1

Looking at this formula, you can see that the illuminance at the zenith (θ = 90) is three times brighter than at the horizon (θ = 0). This is significant as it means that skylights will be much more effective for daylighting per unit area than side windows as they allow more light in. As is well known, the illuminance levels due to the sunlight differ during the daytime because of the local climatic condition. If there is bright sunlight, the luminance level may go as high as 1,20,000 lux. During the nighttime there is no sunlight, however there may be some illuminance due to moonlight which is about 0.25 lux in full moonlight on a claear night. Otherwise the illuminance level s very small during the night. We list out illuminance level during the day and nigh in Table 5.1.

5.8    FUNDAMENTALS OF LIGHTING

Table 5.1  Daylight Intensity in Different Conditions Daytime Illuminance Levels

Examples

120000 lux 110000 lux 20000 lux 10000 - 25000 lux 8 m, light sources need to be mounted higher. This facilitates avoiding obstruction to guide rails of cranes and tall machinery. Here, dispersive narrow beam reflector luminaries fitted with metal halide or high pressure sodium vapour lamps that are color corrected are used. Special Tasks in Industrial Environment Best way of assessing visual requirement is known by doing it one self. Lighting design should create necessary contrast between the details to be distinguished against the background. If general lighting does not meet these requirements then additional aids such as Illuminated magnifying glass, Stroboscopic lighting for viewing objects in motion or monochromatic light in glass and ceramic manufacture are used.

High Bay and Low Bay Lighting High Bay and Low Bay light fixtures are perfect for low mount applications such as warehouses, gyms, assembly areas, food processing plants and hangars. Standard high bay or low bay fixtures normally use HID (High Intensity Discharge) bulbs which is a special type of lighting that is much more intense than most other light sources. Metal Halide is most commonly used for indoor application since it emits a bluish white bright light that is much more pleasant to the eyes than HPS. Low bay fixtures should be used in areas where the bottom of the luminaire is less than 20 feet above the floor. Low bay fixtures are usually 22 - 28” in diameter in order to spread the light evenly. Low bay HID lighting fixtures have optical refractors that cover the lamp and reduce glare. Their widespread distribution improves vertical illumination and permits spacings as much as two or more times their mounting height. In addition, they can be mounted up to 25’ when high vertical illuminance is required. Fluorescent fixtures are also good for low bay lighting due to their excellent uniformity and relatively low lumen package compared to HID.



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.97

High bay lighting fixtures should be used in areas where the bottom of the light fixture is more than 20 feet above the floor. Typical high bay fixtures have a 15-18” open reflector allowing for a more concentrated beam spread with a prominent downward component. High wattage sources in fluorescent and HID are usually required in order to properly illuminate the space. In recent years, manufacturers have begun to offer T5 High Output fluorescent High Bay and Low Bay light fixtures designed to deliver energy efficiency and cost-effective illumination to retail and industrial applications.

T5 HO (high output) High Bay Light As already explained the industrial buildings, like all workplaces, require wellplanned lighting systems to support various activities. Appropriate quantity of light is essential, but quality issues are equally important in providing a comfortable and safe working atmosphere. When the lighting meets both quantity and quality needs it adds significantly to worker performance and productivity.

High Bay & Low Bay Lighting Fixtures High Bay and Low Bay lighting fixtures including T5HO (high output) are shown in Figs. 6.59 and 6.60). High Bay

Low Bay

Fig. 6.59 High Bay and Low Bay MH Fixtures

Fig. 6.60 T5 HO (High Output) High Bay Light

Industrial Quality Issues Quality lighting contributes to the comfort and productivity of warehouse and manufacturing personnel.

6.98    FUNDAMENTALS OF LIGHTING

It also contributes to their safety, especially around moving machinery. Glare control, balanced brightness ratios and reduced lamp flicker or strobe effect must be taken into account to ensure safety and security in the work space.

Brightness, Contrast and Reflectance The ability to see detail is dependent on good contrast. When the task blends with the background it is difficult to see. If contrast is too great people may experience adaptation discomfort when looking from bright surfaces to very dark surfaces. People are most comfortable when the visual environment is relatively uniform. In the next slide we present the light levels ratios as recommended by the IESNA for areas where reflectance of the work area can be controlled, and where control of the remote surroundings is limited.

Recommended Maximum Brightness Ratios Tasks to adjacent darker areas 3 to 1 Tasks to adjacent lighter areas 1 to 3 Tasks to more remote darker areas 20 to 1 Tasks to more remote lighter areas 1 to 20 Between light fixtures or windows and the surfaces next to them 20 to 1

Lamp Colour Temperature Matters Outdated fluorescent and HID lamps such as cool white and mercury are inferior at colour rendering and are associated with noisy ballasts that often produced flicker in fluorescent lamps. Therefore light sources recommended are high colour rendering sources. If used consistently, will allow people to see colour accurately and work in a comfortable and safe environment.

Recommended Surface Reflectance Values Dark colours absorb light and light colours reflect light. To achieve comfortable brightness ratios, we need to encourage the building owner to select reflectance values for equipment and room surfaces based on the values shown in the picture. Many paint and ceiling material manufacturers publish reflectance values for their products Fig. 6.61.

Glare Control When the brightness ratios exceed maximum recommended values, disturbing glare can result. To minimize glare-producing conditions, consider the following: l

Use more lower wattage fixtures to reduce individual lamp brightness while maintaining required light levels.



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.99

  Fig. 6.61  Recommended Surface Reflectance Values l

Locate control panels and computer screens facing away from windows or bright light fixtures. If these elements are fixed, adjust the lighting fixture locations and shade the windows.

l

Raise bright fixtures above normal field of view.

l

Direct some light toward the ceiling to balance space brightness ratios and reduce contrast between the ceiling and fixtures.

Seeing and Productivity Ability to see well is based on several conditions including the age of the worker. A 40-year-old person generally requires twice as much light to perform a task as a 20-year old. The size of the task and the amount of time available to see it, dramatically affects the need for light. For example, small font text, moving rapidly past a reader and out of sight, needs a significant amount of light for comprehension. Large, stationary objects are easier to see in lower light levels.

Energy Efficient Lighting Luminaire Efficiency Rating (LER) When choosing luminaires for small industrial projects, look for catalog information indicating that each selected fixture meets or exceeds minimum LER values. This standard was developed to provide uniform practical metrics for evaluating the energy efficiency of luminaires (fixtures). The following formula is used for calculating LER: LER =

Total Reted Lamp Lumens × FB × Luminaire Efficiency Input Watts

6.100    FUNDAMENTALS OF LIGHTING

Input Watts = Total published rated input wattage of the ballasts. BF = Ballast Factor BF for HID ballasts is assumed to be 1.0, fluorescent ballasts - .95.

Fig. 6.62 Industrial Unit with Proper Lighting

400 Watt metal halide is currently the standard practice in industrial buildings in several places. Newer, pulse-start metal halide is 20% more efficient than standard metal halide. Both MH lamps are initially more efficient than fluorescent. But close comparison to fluorescent sources show that because MH light output degrades rapidly (lumen depreciation) after a few monthsof use, fluorescent may be a better choice. Both twin-tube and linear fluorescent fixture equivalents to HID fixtures are available.A well lit industrial unit is shown in Fig. 6.62. The lamp comparison table shows the difference between light output at the beginning of a lamp’s life (initial lumens) and when 40% of the lamp’s rated life is over, (design lumens). Fluorescent lamps have better lumen maintenance as the lamps age, compared to metal halide.

Lamp Comparison Lamp Type

Rated Input (Watts)

Rated Life (Hours)

Initial Lumens

Lumens @40% of Rated Life

Metal Halide —

Standard 400

20,000

40,000

26,000

Metal Halide —

Standard 250

10,000

20,500

13,500

Metal Halide —

Pulse Start 400

20,000

44,000

31,000

Metal Halide —

Pulse Start 250

10,000

23,800

16,000

Fluorescent T-8

Standard 32

20,000

2,950

2,800

Fluorescent T-5

High Output 54

20,000

5,000

4,740

Fluorescent T-5

Twin-tube38

20,000

3,300

2,970

Note: Other HID sources such as high pressure sodium or mercury vapour have not been recommended because of inherently poor colour rendition (CRI).



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.101

General Industrial Workspace Lighting The light fixture options should be selected for providing the right amount of light, while meeting or exceeding the requirements of national and local energy codes. The layouts may apply to the general factory and warehouse lighting for typical highbay fixture mounting heights between 25 and 35 feet above the floor. Fluorescent lamps using magnetic ballasts, pulse at 50 cycles per second and cause a perceptible flicker. This effect may be irritating to some people, and can be eliminated using electronic high frequency ballasts. HID ballast cycles can create a stroboscopic effect, and can cause a potentially dangerous condition where rotating machinery appears to be not moving.

Task Lighting It is always better to use task lighting see Fig. 6.63. Relying on the general lighting system to provide adequate light for detailed tasks could result in high energy costs and less than comfortable lighting. Locating light close to the task can provide higher light levels and eliminate shadows created by machinery or the workers themselves. For example, if general factory lighting is planned for 300 lux, and 40% of the area requires 500 lux, to provide the higher level from ceiling mounted fixtures would require 2.4 Watts per square meter additional power and cost 58% more than using localized task lighting.

Fig. 6.63 Use of Task Light

Electric Power Limits for Industrial and other Utility Spaces Internationally recommended values for energy in terms of power density measured in Watt per squre meter for industrial and other utility a spaces are given in Table 6.12. These values may differe from place to place. It is therefore suggested that the lighting consultant must look for local energy code before taking up any lighting projet.

6.102    FUNDAMENTALS OF LIGHTING Table 6.12  Electric Power Limits for Industrial and other Utility Spaces Whole Building Method

Watts / Sq.m.

Manufacturing Facility

22

Warehouse

12

General Lighting High-bay

30

Active Storage/Bulky

11

Transition/Corridors

7

Equipment Room

8

Workshop

25

Control Room

5

Hospital

16

Library

15

Museum

16

Offices

13

Parking Garage

3

Retail

19

School

15

Lighting Levels for Industrial Lighting We have compiled Internationally recommended lighting levels in terms of illuminance values in lux, limited glare rating (UGR) and the minimum color rendering (Ra) for some important industrial units as shown in Table 6.13. For detailed lighting levels one may look for the data available on the website of the concerned country. European and US energy usage guidelines are, by and large, acc epted all over the world. Table 6.13  Lighting Levels for Industral Lighting Area

Illuminance (lux)

Limited Glare Rating

Minimum Color Rendering (Ra)

(I) Electrical industry



Cable and wire manufacture

300

25

80



Assembly work for large Transformers

300

25

80



Assembly work for Switchboards

500

22

80



Assembly work for Telephones

750

19

80



Assembly work for Measuring Equipments

1000

16

80



Electronic workshops, testing, adjusting

1500

16

80

Working with precious stones

1500

16

90

Manufacture of jewellery

1000

16

90

(II) Jewellery manufacturing





LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.103



Watch making (manual)

1500

16

80

Watch making (automatic)

500

19

80

(III) Rolling mills, Iron and Steel works



Production plants with continuous manual operation

200

25

80



Furnaces

200

25

20



Test, Measurement and Inspection

500

22

80

500

22

80

Painting, spraying and polishing chambers

750

22

80

Final inspection

1000

19

80

Constantly manned processing installations

300

25

80

Precision measuring rooms, laboratories

500

19

80

Pharmaceutical and Tyre production

500

22

80

(IV) Vehicle construction



Body work and assembly

(V) Chemical, plastics and rubber industry



Lighting Fixtures for Industrial Lighting These fixtures specifications include fixtures that ensure a balance of performance, energy savings, comfort, lighting quality and ease of maintenance, at a costeffective price. Many standard products meet these generic specifications. For those fixtures for which a Luminaire Efficacy Rating (LER) has been established, those values are given in the description. Luminaires for special applications such as hazardous areas, or using automatically switched quartz standby circuits or any fixtures under 150 input Watts, do not have LER values. The types of fixtures used in industrial spaces are limited compared to the vast array of equipment available for other work places. Appropriately applied, however, they can help to create a comfortable and energy effective environment.

Linear Fluorescent Fixtures With slotted reflectors are designed to minimize accumulation of dirt by allowing upward air-flow. Lenses or diffusers are uncommon for this reason. Where airborne particles call for greater protection, dust-tight covers are used. In damp locations, diffusers with vapor -tight gaskets are necessary. Improvements in fluorescent lamp technology, with the introduction of high output T-5 lamps, have made fluorescent an attractive alternative to more commonly used HID fixtures. High Intensity Discharge (HID) Fixtures Designed for metal halide lamps is often categorized as high bay or low bay distribution. The light distribution of highbay fixtures is usually symmetrical, and is often adjustable to produce narrow to medium wide (44 - 60 degrees) with spacing criteria values of 1.0 or less. This light distribution

6.104    FUNDAMENTALS OF LIGHTING

is meant to concentrate light on horizontal work surfaces from lofty mounting heights of 25 feet or more.

Aisle-lighting Fixtures Designed with asymmetric light distribution to specifically solve the unique requirements of this kind of area. In two directions, perpendicular to the stacks, the light distribution is high and broad to light the stored material top to bottom. Parallel to the aisle, light distribution is narrow so that workers are not disturbed by high angle light as they travel down the aisles. Both twin-tube and linear fluorescent fixtures equivalent to HID are available; typically these have better colour rendering properties. A. Metal Halide, Open Metal Reflector (Standard Practice) Lamp: Standard 400W Metal Halide and Pulse-start 250W Metal Halide

Description: Pendant mounted open clear anodized metal housing with interior multi-faceted specular reflector, direct distribution. Field adjustable light pattern for concentrated to medium to wide light distribution for various ceiling height.LER: 40-50.

B. Metal Halide, Open Prismatic Glass Reflector Lamp: Standard 400W Metal Halide

Description: Pendant mounted open metal housing with interior faceted specular reflector for direct distribution. Approximately 25% uplight. Field adjustable socket position for medium to wide light distribution suitable for various mounting heights. LER : 50

C. T-8 Fluorescent, Pendant Industrial Reflector Lamps: (4) 32W T8

Description: Pendant mounted fluorescent fixture in 8 foot lengths, 2 lamps in cross section, 4 lamps total. White baked enamel finish.Reflector slotted for 20% uplight. Optional “V”shaped centre baffle provides 30 degree glare shielding. LER: 68

D. T-5 Fluorescent, Pendant Industrial Reflector Lamps: (2) 54W T5 HO

Description: Pendant mounted fluorescent fixture in 8 foot lengths, 1 lamp in cross section,2 lamps total. White baked enamel finish. Reflector slotted for 20% uplight.Not rated for LER

E. Metal Halide, Open Reflector, Aisle-lighting Lamps: Type E, 400W Metal Halide and Type E-1, 250W Metal Halide

Description: Pendant mounted open clear anodized metal housing with interior multi-faceted specular reflector, direct distribution. Field adjustable light pattern for concentrated to medium to wide light distribution for various ceiling height.LER Type E: 50; Type E-1: 40

F.

Metal Halide, Prismatic Glass Reflector, Aisle-lighting Lamp: Type F, (1) 400W Metal Halide Type F-1, (1) 250W Metal Halide



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.105



Description: Pendant mounted open clear anodized metal housing with interior multi-faceted specular reflector, direct distribution. Field adjustable light pattern for concentrated to medium to wide light distribution for various ceiling height. LER Type F: 50; Type F-1: 45

G. Pendant Fluorescent Reflector Lamps: (2) 32W T-8 4’ fluorescent

Description: Cantilevered shelf mounted linear fluorescent task light. Not rated for LER

H. Under shelf Fluorescent Task light Lamps: (2) 32W T8

Descrition: Cantilevered shelf mounted linear fluorescent task light. Not rated for LER.

Lighting Controls When an area is not in use, reduced light levels save energy and operating expense for the building owner. There are several ways of doing this including: l

Occupancy sensors

l

Manual or equipment activated interval timer switches

l

High/low switched ballasts

l

Building time-switch or automated control systems

Medium Activity Areas The amount and type of control depends on the type of source and how often an area is used. Some light should be on in all areas of medium to heavy use when the building is occupied, especially in areas that are visible from other workspaces. Reduced light levels in unoccupied areas, instead of complete darkness, help to maintain acceptable contrast ratios.

High /Low Switching Switching fixtures to a lower light level on an occupancy or operator activated system is a cost- effective energy saver in medium use areas. While switching may reduce lamp life hours, calendar life will be significantly longer. New “Programmed Rapid Start” ballast technology for fluorescent lamps significantly improve the life of lamps that are switched frequent Two level control also works well for metal halide systems because when the high level is switched on, the lamps ramp up quickly and return to optimum colour and light output. In areas where abundant daylight is available for at least 25% of the time, the electric lighting energy usage can be reduced. Photo-sensor controlled fluorescent dimming systems or high/low metal halide systems are effective and comfortable ways to optimize energy use in those areas.

High Use Areas In those areas where the space is in constant use or heavily trafficked, the lighting level usually should remain constant when the building is occupied.

6.106    FUNDAMENTALS OF LIGHTING

Automatic Shut-Off A time-switch control or computer system can be programmed to assure that all non-essential lighting including task lighting, is off during unoccupied hours. Automatic shut-off is a requirement of IESNA Standard 90.1-1999.

Control Caution Be sure that reduced light levels do not cause dark areas in the worker’s immediate field of view. Metal halide lamps change colour when wattage is reduced, to a Mercury type greenish blue. Occupants should be aware of and accept this change beforea dual level system is installed.

Retail Lighting Goals of Retail Lighting The selection of the right lighting can be a major contribution to retail sales. Lighting can establish a store’s image, lead customers inside, focus their attention, make the products attractive and visible, and in general encourage purchasing.” Energy Effective” lighting provides all these benefits for the lowest life cycle cost, while saving energy, operating costs and maintenance.

Types of Retails and Their Lighting 1.

Basic Retail Lighting

2.

Intermediate Retail Lighting

3.

Higher-end Retail Lighting

General Design Considerations Retail lighting must have good color, contrast and balance between lighted surfaces. There is no single formula for all retail lighting. A professional lighting designer or retail designer may be able to create successful designs while breaking all the rules suggested here.

Retail Lighting Elements Ambient Lighting General, uniform lighting using light fixtures that distribute the light widely, directly or indirectly. Ambient lighting enables the customer to see and examine the merchandise, and the sales staff to complete the sale and perform their other duties.



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.107

Accent Lighting Spotlighting used to provide higher levels of light in a focused pattern to accentuate selected objects in relation to their surroundings. Accent lighting establishes the importance of certain objects through the use of contrast, and highlights the form, structure, texture or colour of the merchandise.

Perimeter Lighting and Valance Lighting Lighting the vertical surfaces. Asymmetrical light fixtures can direct light on tall vertical shelving and displays, typically located at the perimeter of the merchandise area. Valance lighting allows the source to be quite close to the merchandise, providing a shield or “valance” to conceal the light sources from the view of the customer. Valances are often built into the wall, shelving unit. Although primarily intended to provide light down on the merchandise, they also can be designed to light up on signage or provide indirect ambient lighting for the space

Shelf Lighting and Case Lighting Small or miniature light sources located very close to the objects being displayed, shielded from the customer’s view. This lighting must be carefully selected for the particular application to avoid accidental contact with hot lamps and to prevent damaging the merchandise with too much ultra-violet radiation or heat.

What is Aisle? An aisle is, in general, a space for walking with rows of seats on either side or with rows of seats on one side and a wall on the other. Aisles can be seen in certain types of buildings such as churches, synagogues, meeting halls, parliaments and legislatures, courtrooms, theatres, and in certain types of passenger vehicles. Aisles can also be seen in shops, warehouses, and factories, where rather than seats they have shelving to either side. In warehouses and factories aisles may consist of storage pallettes and in factories aisles may separate work areas. In health clubs, exercise equipment normally is arranged in aisles. Aisles are distinguished from corridors, hallways, walkways, footpaths/pavements lighting for retail is all about contrast and focus. Too much accent lighting means no contrast and no focus. The greatest lighting value is achieved by balancing ambient and accent lighting.

Some Important Suggestions for Retail Lighting 1.

Put the light source close to the merchandise.

2.

For ambient lighting, use efficient, diffuse sources,such as fluorescent lamps.

6.108    FUNDAMENTALS OF LIGHTING

3.

For accent lighting, use narrow beam spotlights such as Halogen lamps.

4.

Use the fewest types of lamp to get the desired effect, reducing relamping mistakes and maintenance headache.

5.

Illuminate the aisles with spill light from the accented merchandising areas or displays.

6.

Lower levels of ambient lighting require fewer watts of accent lighting.

7.

Use the lightest colours on the interior surfaces of shelving.

8.

Use organized patterns of light fixtures. Chaotic patterns may confuse, agitate or fatigue the customers.

9.

Use high colour rendering lamps for both ambient and task lighting.

Basic Retail Lighting Types of Stores High activity retailing such as mass merchandising, discount stores, hardware, video, fast food, grocery, service establishments and sale of bulk or large objects such as appliances or furniture. A destination store that doesn’t require lighting to draw customers inside, and that offers inexpensive products and/or significant value for money.

Self-service Purpose of lighting: To light all objects uniformly, provide good visibility for reading labels and to create a bright, clean, stimulating environment.

Lighting Strategies Exposed sources are effective to project a “discount” or “speedy service” image. Fluorescent sources provide the best value, giving good colour rendering, high efficiency lighting with the longest life.Use light coloured finishes on all wall surfaces to increase overall brightness and reflected light.For a greater sense of brightness and consistency with daylight use “841” colour lamps. This stands for a Colour Rendering Index of 80+ and a Correlated Colour Temperature of 4100 K Consider neon or coloured lights for window displays or to identify departments.

Lighting Levels 50-80 foot candles ambient lighting.

Basic Retail Fixture Schedule (a) 3-lamp Parabolic 2’ × 4’ (b) 2-lamp Fluorescent pendant



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.109

Figure 6.64 displays the basic retail lighting.

Fig. 6.64  Basic Reail Lighting

Intermediate Retail Lighting Types of stores: Clothing, stationary, beauty shop,gourmet shops, accessories, house wares, furniture and small objects. Most common store type, with average level of retail activity. Purpose of lighting: Sufficiently uniform illumination necessary to see and examine product and read labels. Limited accent lighting is desirable to set products apart, to create highlights or enhance textureand to attract attention to window displays.

What is Merchandising? Merchandising refers to the methods, practices and operations conducted to promote and sustain certain categories of commercial activity. The term is understood to have different specific meanings depending on the context. Merchandise in general is the sale of goods at a store. Lighting Strategies for Intermediate Retail: Partially conceal ambient light sources with louvers or baffles to create more emphasis on product. Locate accent lights close to displays. Use exposed or decorative accent lights to create attention or establish image. Fig. 6.65 displays Intermediate Retail Lighting. Lighting levels: 30-50 fc ambient, 75-100 fc accents. Intermediate Retail Fixture Schedule (1) 3-lamp Parabolic 2’ × 4’ (2) 2-lamp Parabolic 2’ × 2’ (3) 1-lamp Fluorescent Valance (4) 50 Watt Accent Lights

6.110    FUNDAMENTALS OF LIGHTING

Fig. 6.65 Intermediate Retail Lighting

Higher-End Retail Lighting Types of stores: More expensive or exclusive merchandise, such as jewellery, gifts, antiques, fine clothing and accessories, fine house wares and beauty salons. Lower activity than other retail types: Most personalized attention and assistance from sales personnel. Purpose of lighting: To establish image and enhance product colour, sparkle or texture. Encourage lingering, examination and impulse buying. Lighting Strategies: Use lower illumination levels for ambient lighting to enhance contrast of accent lighting, but do not eliminate ambient system. Use fluorescent lighting for ambient lighting. White painted parabolic louvers may be preferable in small spaces or low ceilings. For highest end applications, consider smaller diameter (T-5 or T-2) fluorescent lamps for concealed applications such as coves, valances and shelf lighting. Use the best colour rendering lamps (CRI above 80) and a warmer colour temperature of 3000 to 3200 Kelvin.For example, select fluorescent lamps designated 830. Use accent lighting to establish a hierarchy of importance. Since the eye is attracted to the brightest object in the field of view and then to the next brightest object, provide the highest wattage or the tightest focus lamps on the most important items or areas of the store. Use exposed or decorative sources to attract attention to specific displays or areas of the shop. Lighting Levels: 15-40 fc ambient. 75-100 fc accent. Accent lighting is coupled with areas of lowest ambient lighting levels.



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.111

Higher–end Retail Fixture Schedule 1.

2-lamp Parabolic 2’ × 2’

2.

1-lamp Fluorescent Valance

3.

Decorative Pendant

4.

50 to 90 Watt Accent Lights

5.

50 Watt Recessed Accent Light

General Lighting Fixture Schedule Ambient Luminaires: B: 2’ × 2’ Parabolic Troffer, two-lamp C: Valance :Side-socket Fluorescent Channel D: Small Decorative Pendant Bowl E: 2-Lamp Fluorescent Industrial with Reflector Pendant or Surface Mounted Accent Luminaires Halogen PAR 20.

Lamp Spot Light New Fluorescent Lamp Colors The standard three-digit system for classifying fluorescent lamps includes information about the Color Rendering Index (CRI) and the Correlated Color Temperature. Thus “835” stands for a CRI of 80+ and a color temperature of 3500 Kelvin. The Color Rendering Index indicates how well a given lamp renders the colours of the objects it illuminates. For Basic retailing, a CRI of 70+ is generally adequate. For Intermediate and Higher-End shops, 80+ is preferable. Correlated color temperature refers to the appearance of any light source. Low wattage incandescent is very “warm” at about 2700 Kelvin. Halogen is somewhat warm at 3000 Kelvin, and daylight is quite “cool” at 5000 to 10,000 Kelvin. The new fluorescent lamps are available in a wide range of color temperatures, but those most appropriate for retailing are 3000 K for Higher-End shops, 3500 K for Intermediate shops, and 4100 K for basic retailing. (a) Ambient Light Sources: 1.

CFL with electronic ballast

2.

T-5 Twin Tube with electronic ballast

3.

T-8 Linear with electronic ballast

(b) Accent Light Sources: 1.

Quartz Halogen PAR

2.

Low Voltage MR

3.

Halogen PAR Infra-Red

6.112    FUNDAMENTALS OF LIGHTING

Figure 6.66 displays Higher-End Retail Lighting.

Fig. 6.66  Higher-End Retail Lighting

Lighting Levels for Retail Lighting Table 6.14 shows the recommended values of lighting levels for retail premises. Table 6.14  Lighting Levels for Retail Premises Area

Illuminance (lux)

Limiting Glare Rating

Minimum Color Rendering (Ra)

Sales area

300

22

80

Till area

500

19

80

Wrapper table

500

19

80

Jewellery Store Lighting Jewelery lighting can attract customers to any jewellery store, help sell more products and strengthen a store’s image. Ceramic metal halide overhead lighting provides the high intensity light projection that is necessary to properly display diamonds and jewellery, while in-case LED fixtures provide that extra sparkle that attracts customers. Jewellery lighting includes halogen, fluorescent, metal halide and LED lighting.A complete showcase illumination system that allows store owners to display jewellery in the best light possible, both inside and outside of the showcase are now available. Without the proper lighting, merchandise on display will not attract attention and will not sell.That’s why it pays to highlight your jewellery and window displays with high-impact lighting design.Efficient lighting creates a bright, cheery shopping environment that converts browsers into buyers. Efficient lighting adds sparkle and brilliance to gems, jewellery, stones, glassware, and silverware. It reduces your lighting power costs to the absolute minimum. Outlasts conventional lighting, and saves



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.113

money on replacement bulbs. Remember...good lighting “invites” customers into your store and entices them to buy. The lighting designer should focus on the proper way to illuminate a jewellery store. He should also know how different diamonds and jewellery react to the different sources of lighting, how the proper use of lighting can direct a customer’s attention to a desired areas of a jewellery store as well as the current advances in lighting technology. Most jewellers are experts about all facets of their business, from diamonds to gemstones and from settings to the latest industry trends. But even more important to the success of a jewellery store is having the proper lighting to show their merchandise to consumers in the best way possible. Sadly, this crucial element is the one that most jewellery store owners know the least about. Fig. 6.67 displays the Jewellery Store Lighting. Proper store lighting can attract more customers to any establishment, help sell more products, and strengthen a store’s image. Many designers argue that the most important element of a store’s design may be its lighting. Unfortunately, lighting is often the most neglected component of store design. When choosing the lighting for any store or display cases, here are a few ideas to keep in mind. Ceiling Height The height of a store’s ceiling is one of the most important factors that goes into a plan for lighting a jewelry store in such a way that the merchandise looks spectacular. As the source of light moves farther away from the merchandise the power and intensity of the light diminishes. Stores with higher ceilings (over 9’) face some challenges with lighting. Stores with ceiling that are taller either have to use more lights that are spaced closer together to illuminate a showcase or bring the light source closer by dropping down a track or using a pendant.

Fig. 6.67  A Typial Jewellery Store Lighting

Color of light The color of light is measured in Kelvin (temperature). The higher the temperature (Kelvin) reading the cooler the color of light and the lower the

6.114    FUNDAMENTALS OF LIGHTING

temperature the warmer the color. For example a light that is of 3000K would be a warmer color and a light that is of 4000K would be a cooler light. As the temperature of the light gets too high (over 4200K) the color of the light begins to take on a blue quality. Lights that are in the 5500K to 6500K will look “blue” to the eye. Bulb Life/Quality How long a bulb is expected to last is based upon its “rated life”. The better bulbs have a rated life in excess of 10,000 hours. The quality of a bulb can also be measured by its CRI (Color Rendition Index). The higher the CRI of a bulb the better the quality of the light that it projects will be. CRI numbers over 80 signify a very high quality of light. CRI numbers over 90 signify an exemplary quality of light. Power of Light The power of a bulb is measured in lumens. The amount of light that is generated is measured in lux or, more commonly, in foot-candles (“fc”). The higher the lumens the higher the amount of foot-candles that will be generated by the bulb. Different Light Sources There are four main sources of light that are used in Jewellry stores. They are (1) Ceramic Metal Halide (2) Halogen (3) Fluorescent and (4) LEDs. LED technology is being used more and more and is quite popular along with the other three light sources. The best source of lighting for jewelry is still Ceramic Metal Halide. This is due to its qualities. They are energy efficient, powerful (lumens over 6000), come in warm and cool colors (3000K to 4200K) has excellent CRI (over 80 and in most cases over 90 CRI) and can provide foot-candles that can exceed 400. Halogens are hot, have good CRI’s, but do not maintain their color over the life of the bulb and are about a quarter to a third of power of a ceramic metal halide. Fluorescents are energy efficient but do not project enough power to be useful in a jewellry store when it comes to illuminating merchandise.LED’s are quite good but they have limitations and problems. LED technology is constantly changing. An LED fixture you buy today is (as in the case of a personal computer) will be outdated within a year. Keeping a consistent color of the LED over time can be problematic due to the changes in LED technology. LED’s can make jewelry look great inside of the showcase but are not powerful enough to be placed over the case where the sale is actually made. Due to this limitation, a different source of light is required above the showcase such as ceramic metal halide or halogen. This is where the problems occur. What happens is that there are two different sources of light each making a piece of jewellry look different. As an example - a customer see’s a piece of jewellry inside of a showcase being lit by an LED strip. They ask to have the salesperson take it out for them to see it. It is taken out and is now over the glass being illuminated by a different light source which makes the piece look different. The customer begins to wonder if the lighting inside the case is there to “trick” them into thinking the jewellry looks good. As sales are finalized on



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.115

the “glass” having a light source that makes the jewelry look the same inside the case as it does above the case is crucial to closing a sale. Some Important Tips: 1.

Most lighting designers who understand the proper way to light a jewellry store will tell you that you light “for diamonds” and the rest of your merchandise will survive and if you light for gold your diamonds will die. Diamonds require a bright white light that provides enough power to make them sparkle. Diamonds should be shown under lights that are in the 4000 -4200K range. This provides a beautiful white color that stops short of taking on blue color qualities. Gold and warmer colored stones look the best under warmer colored lights 3000K to 3500K.



However, if a store is going to use a single color of light throughout its showcases then the adage of “lighting for diamonds” is the way to proceed.

2.

Make certain that lighting is powerful enough to produce a minimum of 2000 lx. Having light that is less than 2000 lx is universally agreed to be lower than the minimum amount of light to properly light jewellry, especially diamonds. Readings of between 2000 and 4000 lx is the optimum amount of light power over a showcase depending on the store owner’s personal preference as to the amount of light that they think looks best.

3.

Have ambient or general lighting be warmer and less powerful than the lighting over the showcases. The human eye is attracted to brighter light. If the general lighting in a store is the same color and power as that over the showcases then there will be nothing to attract the customers to the showcases and the store will have a cold non-inviting feel to it. By having the general (ambient) lighting less powerful (about 750 lx) and of a warmer color (between 2500 and 300K) the lighting over the merchandise will stand out and attract the customer to the cases.

4.

When lighting a window display that has natural sunlight shining into it use twice the amount of light that would be commonly used over a showcase in order to combat the power of the sun. When upgrading lighting the trend is commonly a change from Halogen technology. Customers who are using Halogen and who switch to ceramic metal halide will notice an immediate difference in the quality and power of the light, the decrease in heat and the remarkable change that it makes to their merchandise. Things to consider when choosing the type of lighting:

Track Lighting This provides the most versatility as fixtures can be moved and repositioned with ease. If merchandise is moved from case to case or showcases are moved track lighting provides the store owner with the most options.

6.116    FUNDAMENTALS OF LIGHTING

Recessed Lighting This provides the sleekest look. The limitations are that the store owner will be limited in their ability to add or decrease light as well as limitations with moving merchandise and showcases around inside of the store. Once a recessed light is installed the cost to add or take away lighting becomes a large expense as electricians are required to remove/move lighting as well as the issues involved with repairing the holes in the ceiling where the recessed fixture was installed. Pendants Lighting Pendants are a great way to bring a light source closer to the showcases where the ceilings are high and the store owners do not want to spend the money required to add additional lights to compensate for high ceilings. Having lighting inside of a showcase can really add sparkle and glitter to your jewellry. The best lighting for inside a case is LED. Be careful though when choosing the LED to make sure that the color of the light is not blue (5500K = color range). Having one color shining on your merchandise inside the case and a different color shining on your merchandise above the case has the real potential to lose a sale. Stores do not want their customers to be attracted to how jewellry and diamonds looks inside only to have it look different on top of the glass where the sale is actually closed. Customers will think that stores are trying to trick them through the use of fancy lighting.

Museum Lighting Functions of Museums The main functions of a museum are: 1.

Collect and exhibit art and historic artefacts for public education and enjoyment,

2.

Protect the collection from damage, and

3.

Do this all as efficiently as possible.

This means preserving and presenting our art and heritage. If the public (young or old) cannot see, understand and enjoy the exhibits, the building is not a museum, it is an archive. If items are not preserved, whatever it may be, the building is not a museum. Without both good presentation and effective preservation, a museum won’t be able to continue. Funding is directly proportional to both the quality of art or artifacts and the quality of presentation.The problem is that exhibition always increases the risk of damage. The more visible and more accessible an artefact, the higher the risk. For our purposes, exhibition increases the dangers of light damage, both photochemical damage (fading) and photomechanical damage (structural damage). The safest place for a collection is in a vault, stored away in the dark, but that defeats the museum mission.The light energy hitting an artefact either reflects off or is absorbed. Reflected energy hits our eyes where it causes chemical changes.



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.117

We call that vision. Absorbed energy causes chemical changes in the artefact. We call that photochemical damage. Eyes recover fairly rapidly from lightinduced chemical changes. Artifacts do not. Museums can minimize light damage by minimizing the energy absorbed by artifacts. First, we do this by using good lighting design to lower the overall light levels. Full visible spectrum lighting with all colours represented and none over or under-represented can double visibility without increasing intensity. Eliminating shadows, glare and reflections, lighting artifacts and not areas, and keeping backgrounds subdued also increase visibility while letting us lower overall light intensity. On top of that, it is easy to greatly reduce total absorbed energy without reducing any of the reflected energy that we see. We do this by choosing the proper light sources (following IESNA guidelines), eliminating nonvisible radiation (UV and IR), and filtering lighting to match colors. Every museum is engaged in a continuing war against the damaging effects of light. The war begins as light rays (photons) cut through atoms of fragile color molecules and organic materials. The casualties are rare historic documents, sensitive textiles, fragile watercolours, fragile printed materials and organically dyed native arts. Damaged artifacts are given a quiet burial in the archives, never to be displayed again. Even the merchandise in the museum gift shop; clothes, books, posters, videos, can end up light damaged, discounted and sold at a loss. Priceless (or at least, costly) artifacts are set up like targets in a shooting gallery. The attacking weapons are the recessed lights and track fixtures you bought and paid for. The ammunition is the electrical power on every months’ electric bill. Those lights magically convert electrical power (electrons) into photon bullets. Visible light along with invisible ultraviolet and infrared radiation shoot deep into materials, past and through the open spaces in and between many thousands of atoms. “Reflective” materials need to be 50,000 atoms thick to reflect just half of the photons of a beam of light. Even then, they only reflect photons of certain colors of light. The rest of the photons in the light beam bore into the material to seek out and cut the atomic bonds that hold the molecules together. The more light, the more photons. The more photons, the more damage. That’s quantum physics, pure and simple. If we see the complete spectral output of a quartz halogen track light at 3000°K. Then we find that most of the energy (95%) is outside the visible spectrum. About 1% is ultraviolet, below 380 nm. Roughly 94% is infrared, above 770 nm. None of that energy is visible to you or your museum light meter. Lighting manufacturers almost never give you full spectrum data. Now you know why.

Exhibit Lighting Elements The elements of exhibit lighting should cater to the three goals of any museum:

6.118    FUNDAMENTALS OF LIGHTING

1.

Presentation: Showing the true beauty of art and historic artifacts

2.

Preservation: Protecting exhibits from fading and damage, and

3.

Conservation: Conserving energy, resources, manpower and funds

These three elements of exhibit lighting (and the three museum goals) are dependent on the things we’ve discussed above, understanding what light is, how people see it, and what happens when it hits something. Once you understand these fundamentals, you can control the light, protect the collection, and save money in the process.

Presentation Presentation is an easy term to understand. It is simply the visitors’ view of an exhibit- good or bad.Turn off the lights, what do you see? Nothing. You can exhibit the most wonderful masterpieces or historical treasures, but without light, who will know? Who will come? When you realize that people only see what the lighting reveals, it turns exhibit design upside down. You can have the most beautiful objects in the world, but only light transmits beauty to a viewer. When you walk into a museum, you experience just the lighting. It is the only communication link between the objects and the people. If colors are not present in the illumination, or if they are too strong or out of balance, it distorts that communication link. Poor lighting always creates poor exhibits. Ignoring good lighting design, glare and reflection can make even wonderful artifacts difficult to see.Bad lighting will make artwork or exhibits dull, lifeless, or distorted. The bottom line is that any exhibit is only as good as what the visitor sees. And the visitor only sees the light you provide. No UV/IR fiber optic systems produce a full, balanced colour spectrum very close to that of sunlight, but with no UV or IR. Thus, there is nothing more dramatic than seeing the change when you switch off ordinary lights and turn on this remarkable fiber optics system. The result of doing that hasbeen very consistent. There is usually a moment of total silence, followed by a gasp. Then someone says, “I never knew that was so beautiful!” The human mind has a unique ability to correct color-distorted light coming through tinted glasses or from off-color lights. The mind corrects the colors to somewhat normal vision in a process called “chromatic adaptation”. As you adapt, you will still mis-identify colors, lose details and miss variations in hue, but you stop noticing how bad the lighting really is.Typical museum incandescent and halogen lamps (particularly when dimmed) have poor color balance. There is too much energy at the yellow/red end of the spectrum and too little energy in the blue/violet hues. This is functionally identical to seeing through yellow shooting glasses. Your mind will adapt to the world turned yellow. The blue hues are suppressed and the violets are missing from your perception. But, when everything in a gallery has the same distortion, your mind conceals it. You start to think washed out colors and no blues look normal. A quantity that helps judge color balance or distortion is Color Rendition Index



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.119

(CRI). To get this number, test subjects with colours under specific light sources. CRI is the percentage of correctly identified colours. A CRI of 80 sounds good, but what it really means is that 20% of all of the colours were misidentified. Anything less than 100 CRI shows distortion.As you can see, HID sources have very similar output to tri-stimulus fluorescent. As with fluorescent lighting, the missing reds make everything look blue-green. “White” LEDs also have strong blue tint. In LEDs it is because of the huge blue spike in their output. LEDs are naturally mono-chromatic (one color). Just as fluorescent lamps generate UV to excite a phosphor coating to emit “white” light, “white” LEDs generate blue light at 450 nm to excite a “white “phosphor. The spectral output makes this distortion obvious. Watercolours, oil paintings, textiles, ancient glass, minerals, documents, photographs, etc., are all sensitive to bad lighting. The more complex the colouring and the finer the details, the greater poor lighting interferes with true perception. On top of this, for every 10°C rise in temperature the rate of chemical reactions double. The question is not, will bad lighting damage your artefacts? When will your plates or ceramics thermally craze? When will the background fabrics fade and need replacement? When will your textile, watercolour, etc., be damaged by UV, IR or both? The true bottom line is that presentation, preservation, and conservation are all interdependent. Bad lighting will have severe and inevitable consequences, it’s just a matter of time.

Preservation Preservation is also an easy term to understand. It is simply the protection of the collection from the damage.By the way, protection can extend to case linings, carpets, graphics and even the merchandise in our museum stores. As we’ve seen above, light is either reflected by an object, helping you see, or it is absorbed by the object, causing photochemical and/or photomechanical damage. Ultraviolet and infrared don’t help you see. They can only cause damage, so the first task is to remove all light energy that doesn’t help you see. Getting information on energy outside the visible spectrum is sometimes difficult.It’s even harder when you understand that most UV meters only read UV above 300 nm. But UV emissions below 300 nm are found in almost all sources. Few manufacturers bother to filter sources in UV ranges where no one has measuring tools. Fewer still give you accurate data. It’s a matter of “what you don’t know won’t hurt me”. Full spectrum outputs (UV through IR) most people won’t show you. Removing all UV and IR dramatically retards fading and damage. Both laboratory and museum testing have proven that lighting systems with no UV or IR extend the exhibit life a minimum of 3 to 5 times, compared to typical museum lighting. Tests of various lights with both widelyused ISO blue wool and fugitive dye samples in assorted colors showed some surprising results:

6.120    FUNDAMENTALS OF LIGHTING

1.

Fluorescent lamps faded the ISO samples only 10% less than sunlight.

2.

Dimmed incandescent and halogen lamps caused fading within 4% of fluorescent lamps.

3.

UV filters on fluorescent lamps only reduced fading by 30%.

4.

Adding glass IR filters to halogen lamps only reduced fading by 10%.

5.

Fibre optics with no UV and IR reduced fading by 80% (five times the exhibit life) compared to fluorescent, incandescent or halogen lamps.

6.

Lightly tinting fibre optics to match artefact color reduced fading by 91% (twelve times the exhibit life) compared to fluorescent, incandescent or halogen lamps.

7.

Color filtering fibre optic lighting to exactly match the color of an artefact reduced fading by 99% (increased exhibit life by 100 times). Colormatched lighting stops fading. This testing led to the science of Reflected Energy Matching.

Remember, you only see reflected light and only absorbed light causes damage. Matching light color to artefact color does not change appearance. Again, you only see what is reflected. What is to be done now is to eliminate absorbed energy (and damage). Hundreds of fading tests show that No UVIR fibre optic lighting with color matching filters can virtually stop photochemical damage.

Conservation Conservation is simply stewardship of a museum’s materials, personnel, and financial resources. The whole idea is in the old saying, “Waste not, want not.” And in an era of tight budgets, it can be a matter of museum and personal survival. Bad lighting results in more than poor presentation and destructive preservation. It also means that the lighting is inefficient.It wastes power. It generates heat. And every watt of power put into lighting takes 3 to 4 watts of air-conditioning to pump out. Each NoUVIR projector powers up to 32 individual fibre optic luminaires.It is not unusual for a single projector (i.e., one light bulb) to illuminate two or more cases. NoUVIR installations require less energy and far fewer light bulbs. With the added saving in HVAC load, NoUVIR installations can provide 50% to 70% overall gallery energy savings. Energy savings are only one way NoUVIR reduces operating costs. Case linings, signs and graphics last longer. IR driven case breathing and associated dust problems are eliminated. Exhibit rotations and cleaning cycles are extended. Maintenance and labour costs drop, all the result of good lighting. Lighting experts suggested the need to eliminate UV and IR in the early 1990’s. By 2000 the Illuminating Engineering Society of North America (IESNA) established guidelines recommending filtering all UV and IR in museum (and commercial) environments. You’ll find these guidelines in the IESNA Lighting Handbook.



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.121

Museum Lighting Design must Follow Basic Lighting Principles A – Light artifacts, not areas People come to a museum to see the art and artefacts. Except for historic sites, people don’t visit museums to see walls and ceilings. Good lighting design will make each artefact a centre of interest. One of the most important features of the NoUVIR system is control, the ability to put an exact amount of light precisely where you want it. In lighting a painting, don’t splash light all over the wall around it. Instead, illuminate the painting evenly and uniformly from top to bottom, to show exactly what the artist painted. Conventional lighting can’t do that because the light on the top of the painting will always be several times brighter than the light on the bottom.That’s the 1/r2 law at work. NoUVIR’s adjustable beams let you control intensity on each part of the painting top to bottom. You’ll see color and details you never knew were there. Lighting a small artifact, such as an Egyptian faience or ancient jade sculpture, doesn’t require flooding an entire case with light. Just zoom the fibre optic luminaire’s beam to the size of the artefact, and let light scattered from the object add a little soft glow to the interior. Now the case lining is not visually competing with the object. Similar objects can be grouped and lit with a single beam, showing the viewer the items are related, something impossible if you are lighting areas. Figures 6.68. 6.69, and 6.70 show the paintings and a jug in a museum.

Fig. 6.68 A Painting in a Museum

Fig. 6.69 A Painting in the Musium

Lighting artefacts instead of areas lets you place artefacts made of different materials in the same case or on the same wall. Now each object can have a lighting intensity and effect that specifically fits the individual size, color, and importance of each item. Fragile papers can be displayed next to

6.122    FUNDAMENTALS OF LIGHTING

engraved silver; gemstones may be displayed on fugitive costumes. Water colors and oils may be exhibited on the same wall. Your imagination chooses the placement and emphasis for each artefact and for each light pattern. The moon behind the hawk on the right shown in next slide is simply a beam of light from a NoUVIR spotlight. Lighting artefacts not only gives you more effective presentation, it is more efficient as well. One NoUVIR gem and geology exhibit used 40 NoUVIR projectors (40 light bulbs) to replace 700 halogen light fixtures (and their 700 light bulbs).This took 60,000 watts off line; quadrupled the light levels on the individual gems; and improved the color, appearance, and depth of the specimens. The exhibit not only looked better, but the museum dramatically cut its operating costs. As a matter of fact, the savings are funding doing it all again in another gallery. B – Backgrounds must be subdued Exhibits should always be brighter than the background and graphics around them. Exhibits should be on stage, the centre of attention. Two-to-one is a good rule. Artefacts should be twice the intensity of the background. On top of that, nothing in view should be more than twice as bright as the dimmest lit exhibit in a gallery. This includes windows, banners, large lightly coloured graphics and video presentations. More museum vision problems are caused by glare than low light levels. Your eye sees the entire scene - artefact, background and any visible light sources. Then your eyes set the iris diameter like the f-stop of a camera, adjusting to average the total light present. A window, a bright background or an over lit graphic will cause your iris to contract. The result is a mild case of “snow blindness”. Direct glare from overhead lights does the same thing, only to a greater degree.

Fig. 6.70  A Jug in the Museum

You can read quite well at 2 to 3 foot-candles’ (20-30 lux) given a few minutes to adjust and no bright visual distractions. If the gallery or lighting design draws a visitor’s attention and gaze to a light source, they can be effectively blinded to the exhibit for up to fifteen



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.123

minutes. A unique advantage of fibre optic lighting is that the light sources can be kept completely away from view; often hidden entirely inside a display case behind door. This allows the iris of the eye to open up to a larger extent, admitting more light and information to create a quality image on the retina. C – Control glare and case reflections A museum must protect exhibits from things like dust, contamination, visitors’ hands, breath, sneezes, etc. Usually that means a case or “window” between the artefact and the visitor. But windows reflect. A museum gallery should have the lowest ambient light levels possible. Low ambient lighting minimizes damage to both artifacts and furnishings. Conceal lighting. Use draperies over “historic” windows. (Draperies were common and important insulation in winter.) Use reversed graphics to avoid large white areas. Light artifacts to conservation levels and then control ambient light to about half of that level. You’ll be amazed at the result.

Lighting Design Using Computers Computers play a very significant role in our life. Likewise; computers are very useful for lighting designers. The students are,therefore,advised to learn the basic functions of computers. They can undertake programming exercises using C++/MATLAB 6.5. They may also undertake the following activities with the help of some computer experts, namely(i) Familiarization with MATLAB, simple problems on lighting. (ii) To obtain polar curves and CU charts by inputting photometric data. (iii) Glare evaluation –using Luminance limiting curves/UGR schemes. (iv) Interior lighting design obtain grid illuminance, uniformity (3D mesh, surf, contour plots) and layouts. (v) Exterior lighting design – For a given scheme obtain the grid illuminance and # D plots-using vector algebra method. (vi) Daylight availability and interior daylight illuminance (vii) Road lighting calculations (viii) Develop Graphical user interface for the above programs. After the students have become familiar with the basics ides of computer applications for lighting design,they may attempt some virtual design problems to know various inputs required for perfect lighting design. We, therefore, suggest that the students may take up the following projects one by one using lighting softwares which can be downloaded from the net: (i) Interior lighting design, sports lighting and Road lighting using Calculux (ii) Day light artificial integrated schemes using ADELINE:

6.124    FUNDAMENTALS OF LIGHTING

(a) Study of different input options (Simple input/Scribe/CAD tools). (b) Simulation using SUPERLITE IEA and RADIANCE. (c)

Energy Calculations

(iii) Day Light artificial light integrated schemes and exterior lighting using RELUX. (iv) Day Light – artificial light schemes using ECOTECT (a) Simulating with different sky conditions. (b) Use of blinds/window screening devices. (v) Interior lighting design using Lumen designer (vi) Interior and exterior simulation using Desktop Radiance (vii) Reflector design using the software Photopia Note: Matlab is a commercial “Matrix Laboratory” package which operates as an interactive programming environment. It is a mainstay of the Mathematics Department software line up and is also available for PC’s and Macintoshes and may be found on the CIRCA VAXes. Generally the Matlab is well adapted to numerical experiments. The Matlab program and script files always have filenames ending with “.m” and the programming language is exceptionally straightforward since almost every data object is assumed to be an array. The graphical output is available to supplement numerical results and the Online- help is also available from the Matlab prompt (a double arrow), both generally (listing all available commands): >> help [a long list of help topics follows] and for specific commands: >> help fft [a help message on the fft function follows]. The paper documentation is on the document shelf in compact black books and locally generated tutorials are available and are used in courses and a quick tutorial on Matlab is also on the net which one can easily use. Note: We wish to mention here that mostof the lighting companies have developed their lighting softwares which can e used by the lighting designers. Most of these softwares can easily be downloaded.Their operation is alo simple.

6.5 EXERCICES Q.1 What do mean by Lumen method of lighting design? Explain this method for Interior lighting to calculate the number of luminaires needed for a given illuminance. Q.2 Discuss the factors which need to be considered for residential lighting. Explain the importance of color temperature and CRI for interior lighting.



LIGHTING DESIGN CRITERIA AND INTERIOR LIGHTING    6.125

Q.3 Justify the need for industrial lighting. Explain some of its salient features. Q.4 Discuss Retail lighting giving some of its important salient features. Q.5 Attempt the following to explain the terms involved for interior and exterior lighting: (a) Visual performance and visual comfort (b) Luminance distribution and Illuminance uniformity Part-II: Write short notes on the following Q.6 Point by point method for Interior and exterior lighting. Q.7 Office Lighting Q.8 Museum lighting Q.9 Maintenance factor for lighting Q.10 Significance of power factor in lighting Q.11 Lighting systems for retail lighting Q.12 Education facility lighting Q.13 Glare and light pollution Q.14 Visual performance and visual comfort Q.15 Jewellery store lighting Part IV: Case lets: Q.16 Answer the following: (a) What do you mean by energy conservation in lighting?

Explain some of the steps which will help energy conservation and light pollution.

(b) Write a note on Unified Glare Index (GRI) and its significance in lighting design (c)

Write a note on the importance of energy conservation in lighting.

(d) Explain the difference between Luminance and Illuminance

6.6 BIBLIOGRAPHY 1.

J.B.de Boer and D.Fischer, “Interior Lighting”, 2nd edition, Philips Technical Library, 1981.

2.

D.W.Durrant, “Interior Lighting Deisgn”, 5th edition, Lighting Industry Federation Ltd. London 1977

3.

Bureau of Indian Standards, “Code of Practice for Interior Illumincation” IS-3646 (Part-Ward Name:), New Delhi, 1992

4.

Kao Chen, “Energy Effective Industrial Illuminating systems – Deisgn & Engg. Considerations “ The Fairmont Press, GA, 1994.

6.126    FUNDAMENTALS OF LIGHTING

5.

Jack L. Burton, “Building Systems Deisgn Series- Vol1: Fundamentals of Interior Lighting Prentice Hall, NJ, 1999.

6.

Ronals N. Helms, ‘Illumination Engg. For Energy Efficient Luminous Environments”, Prentice Hall, NJ, 1999.

7.

I.E.S.N.A, New York, “Lighting Hand Book” 9th Edition, 2000.

8.

D.C.Prichard, “Lighting”, 5th Edition, Longman Group Ltd. England, 1995.

9.

Philips Lighting Division, Eindhoven, “Lighting Manual”, 5th Edition, 1983.

10. Stanley L.Lyons, “Hand Book of Industrial Lighting” Butterworth & Co.Ltd, London, 1981. 11. M.A.Cayless and A.M.Marsden, “Lamps and Lighting”, 3rd Edition, Oxford and IBH Publishing Co, New Delhi, 1983. 12. Anil Valia, Designing with Light, International Lighting Academy, 1st edn, 2002.

Index A Absorption 1.13, 1.14, 1.21, 1.31 Absorption Coefficient 1.70 Accent Lighting 4.8, 6.41, 6.107 Acceptance Half-Angle 3.73 Accommodation 6.60 Acrylic 3.71, 4.21, 5.11 Ageing 2.33, 4.41, 4.55 Altitude 5.7, 6.31 Aluminium 1.36, 1.55, 3.5, 3.24 Ambient Light 2.27, 2.38, 3.63 Arc Tube 3.25, 3.42, 3.42 Atmospheric Loss 6.33, 6.34 Autotransformer 3.35 Average Daylight Factor 5.20 Atmospheric Pressure 3.21, 3.29, 3.33 Azimuth Angle 4.53

B Back Light 6.86, 6.93 Baffles 2.27, 2.28, 4.54 Ballast 2.39, 2.44, 3.35 Bare (Exposed) Lamp 2.44 Beam Angle 6.86 Beam Spread 4.43, 4.50, 6.97 Blackbody 1.16, 1.17, 1.19, 2.51 Blackbody Radiator 2.52, 3.78, 3.79

Blackbody Temperature 1.17, 3.78 Blackbody Spectrum 1.17 Blackbody (Planckian) Locus 2.51 Black Light (or UV) 1.13, 3.86 Boltzmann Constant 1.19 Brewster's Angle 1.36, 1.66, 1.67 Brightness 1.29, 1.30, 2.12, 2.14 Bulbs 1.25, 1.46, 1.51, 3.53, 3.54

C Calculations 1.7, 2.3, 2.20, 2.30 Candela (cd) 2.8, 2.24 Candle Power (cp) 4.44 Capacitor 1.48, 1.49, 3.35, 3.39 Caps and Capping 3.4, Car Park 3.84, 6.71 Cathodes 3.17, 3.40 Ceramics 1.52, 3.4, 3.20, 6.119 Chroma 6.29 Chromatic Adaptation 6.118 Chromaticity 2.51 Chromaticity Diagram 2.51 CIE 1.4, 2.2, 5.15, 6.22 Clear Sky 2.12, 5.5, 5.6 Clubs 6.107 Coatings 1.15, 1.36, 2.49 Coefficient of Utilization (CU) 2.3, 4.43, 6.21, 6.22

I.2   INDEX

Color Contrast 1.68, 6.106 Color 1.6, 1.13, 1.15, 1.20, 2.52, 3.77 Color Matching 2.51, 6.120 Color Rendering 1.47, 2.52, 3.78 Color Temperature 2.50, 2.51, 3.77, 3.81, 6.87 Computers in Lighting 3.49, 6.123 Cones 2.45, 2.51, 4.46 Conservation of Energy 2.29, 5.17 Contrast 1.54, 1.68, 2.13, 2.16, 6.54 Correlated Color Temperature (CCT) 2.50, 3.47, 6.111 Cosine Law 2.17, 2.21, 4.50, 6.30 Current 1.7, 1.38, 1.42, 1.48, 2.32

D

F Fresnel Lens 4.38, 4.39, 4.40 Floodlighting 1.1, 2.39, 4.6 Flux Density 2.24, 2.25

G Gas-Filled Lamps 3.3 Gaseous Discharge 1.26, 1.27 Goniophotometer 2.38, 2.39, 2.40

H Headlamp 4.40 High-Intensity Discharge (HID) Lamp 3.48, 3.74

Daylighting 5.1, 5.2, 5.3, 5.4, Daylight Availability 5.8, 5.28, 5.26, 6.16 Daylight Factor 5.15, 5.20, 6.1 Depreciation 6.25, 6.100 Diffraction 1.5, 1.6, 1.59, 1.60 Diffusion 2.35, 4.3, 5.2 Diffuse Transmittance 2.26 Diffusers 2.26, 2.28, 4.22, 6.50, 6.103 Dimmers 1.51, 3.32, 6.2 Direct-Indirect Lighting 6.65 Disability Glare 6.9, 6.11 Discharge Lamps 1.27, 2.44, 3.31 Discomfort Glare 6.9, 6.11, 6.13 Display Lighting 3.15, 3.80, 6.64

I

E

Lambert’s Cosine Law 2.21 Luminous Flux 1.54, 2.5, 2.9 Laser 1.11, 1.29, 1.30 Light Loss Factor (LLF) 3.62 Low-Pressure Mercury Lamp a Discharge Lamp 3.53, 3.54 Low-Pressure Sodium (LPS) Lamp 3.41, 3.44, 3.48

Efficacy 2.6, 2.18, 2.19, 3.40 Efficiency 1.1, 1.2, 1.40, 2.18, 2.20 Electric Discharge 1.35, 1.38, 3.3 Electroluminescence 1.26, 1.45, 3.63 Electromagnetic Spectrum 1.4, 1.13, 1.20

Illuminance 2.3, 2.10, 2.12, 2.15 Incandescent Filament Lamp 3.80 Infrared Lamp 6.64 Infrared (IR) Radiation 1.15, 2.1 Illumination or Illuminance 1.4, 1.13, 2.16, 2.18, 2.24 Intensity (Candlepower) 2.45 Isolux (Isofootcandle) 2.45

K Kelvin 1.19, 2.1, 2.52

L

INDEX    I.3

Lumen 1.46, 2.5., 2.9 Luminous 1.6, 2.5, 2.8 Luminaire 1.3, 2.3, 2.41, 4.3 Luminance 2.7, 2.10, 2.11, 2.12, 1.13 Luminous Efficacy of a Source 1.18, 2.19, 2.20 Luminous Exitance 2.7 Luminous Flux 2.5 Luminous Intensity 2.7 Mounting Height 2.3, 4.10, 6.23

N Nanometer 2.7, 2.8

O Opaque 1.45, 1.69, 2.30

P Par Lamp 6.85 Pendant Luminaire 1.12, 6.40 Peripheral Vision 6.15 Peripheral Visual 6.15, 6.58 Photography 1.5, 6.91, 6.93 Photometer 2.26, 2.31, 2.38 Photometry 2.2, 2.4 Photopic Vision 2.4, 2.5, 2.11 Pupil 2.12

R Radiance 2.10, 2.11, 6.124 Radiant Energy 2.1, 2.2, 2.10 Recessed Luminaire 4.8, 4.15 Reflectivity 1.41, 1.55 Refraction 1.6, 1.56, 4.36 Rods 1.36, Room Surfaces 2.43, 2.17, 2.19, 2.21, 6.28

S Scotopic Vision 2.4, 2.5, Searchlight 2.9, 3.23, 3.56, 4.26 Self-Ballasted Lamps 3.48 Semi-Direct Lighting 4.7 Semi-Indirect Lighting 4.7 Sky Light 5.18, 6.95, 6.96 Spectral Luminous Efficacy 2.11, 2.18, 2.33 Spectral Radiant Flux 2.5, 2.7 Spectral Luminous Efficiency for Scotopic Vision, V(Λ) 2.18 Spectrophotometer 2.30, 2.51 Speed of Light 1.6, 1.8, 1.11, 3.73 Stefan-Boltzmann Law 1.19, 3.13 Scotopic Vision 2.4, 2.5 Street Lighting 2.3, 2.16, 1.33, 4.43 Surface-Mounted Luminaire 4.8

T Table Lamp 3.33, 6.40 Task Ambient Lighting 5.27 Transmittance 1.65, 2.26, 3.6, 5.8 The Color of the Light 1.45, 6.114, 6.116 Tungsten-Halogen Lamp 6.39, 6.83

V Vacuum Lamp 3.3, 3.15 Veiling Reflection 5.22, 5.25, 5.29, 6.9 Visibility 1.4, 3.2, 6.3 Photopic Vision 2.4, 2.5, 2.11 Visual Acuity 2.25 Visual Perception 2.14 Visual Performance 1.3, 5.25, 6.10

Z Zonal-Cavity Method 2.3, 6.20