Light and Light Sources

Light and Light Sources

P. Flesch Light and Light Sources High-Intensity Discharge Lamps With 226 Figures, 5 in Color, and 4 Tables 123

PD Dr.-Ing. Peter Flesch OSRAM GmbH Nonnendammallee 44-61 13629 Berlin Germany E-mail: peter@flesch.de Cover: Kleiner Leuchtturm Borkum, Germany, North Sea (Courtesy of Frank Toussaint, Der Leuchtturm-Atlas", http://leuchtturm-atlas.de/) Library of Congress Control Number: 2006923689 ISBN-10 3-540-32684-7 Springer Berlin Heidelberg New York ISBN-13 978-3-540-32684-7 Springer Berlin Heidelberg New York This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. Springer is a part of Springer Science+Business Media. springer.com Springer-Verlag Berlin Heidelberg 2006 Printed in the Netherlands The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting by SPI Publisher Services Cover design: design & production GmbH, Heidelberg Printed on acid-free paper SPIN 11534952 57/3100/SPI 543210

Dedicated to my family

Preface High-intensity discharge lamps (HID lamps, also high-pressure discharge lamps) are very important light sources for visible, UV, as well as IR radiation. They have captured a major share of the markets for automotive headlight lamps (D2 lamp), video projection (UHP lamp), general lighting, street/industrial lighting, commercial lighting, floodlighting, sun tanning, microscopy, endoscopy, photochemistry, lithography, etc. This renders HID lamps an interesting and seminal field of research. High-pressure discharge lamps are normally very small (typically 0.7 mm to some centimeters electrode gap), have short time scales (typical ac frequency: 50 500 Hz or higher), and the electrical power input lies between some tens and several thousand watts. This book is concerned with the understanding of these lamps. Beginning with the human eye and an explanation of light and color, the working principle of different light sources is explained and the light sources are compared with each other. Starting point is the incandescent lamp, then low- and high-pressure (high-intensity) discharge lamps are discussed in detail. Furthermore, a large part of this book deals with important subjects concerning HID lamps like electrode and plasma physics as well as the state of the art in HID lamp diagnostics and modeling. The aim of this book is to give an introduction to the working principle of HID lamps and to point out challenges and problems associated with the development and operation of high-pressure discharge lamps. This book is directed at students interested in high-pressure discharge lamps as well as persons already involved in the research and development or the usage of HID lamps. Karlsruhe, January 2006 Peter Flesch

Acknowledgments Many thanks to Prof. Dr. Neiger, Prof. Dr. Dr. h.c. Thumm, and Prof. Dr. Zissis, as well as Prof. Dr. Lemmer, the Lichttechnische Institut, and the Fakultät für Elektrotechnik und Informationstechnik of the Universität Karlsruhe (TH) for all the support and the stimulating atmosphere. Furthermore I would like to thank all my colleagues of the Lichttechnische Institut and all colleagues from other universities and industry with whom I worked in different BMBF and EU projects or who I met at conferences and workshops. Special thanks to the BMBF for supporting the projects FKZ 13N7107/0, FKZ 13N7765, and FKZ 13N8073 as well as to the EU for supporting the cost action 529 Efficient Lighting for the 21st Century. Last but not least, many thanks to all the people supporting the work on this book by discussions, proofreading, and providing many of the images and figures shown in this book. This book would not have been possible without this support.

Contents Introduction.................................................... 1 1 Light and Light Sources....................................... 3 1.1 The Human Eye.......................................... 3 1.1.1 Rods and Cones and More............................ 4 1.1.2 Color Space....................................... 8 1.1.3 Mixing Colors..................................... 10 1.2 Photometric Characteristics of Light Sources................... 11 1.2.1 Luminous Flux..................................... 12 1.2.2 Luminous Efficacy.................................. 13 1.2.3 Luminous Intensity................................. 13 1.2.4 Illuminance........................................ 14 1.2.5 Color Rendering Index............................... 14 1.2.6 Correlated Color Temperature......................... 15 1.3 Light Sources............................................ 16 1.3.1 Sun.............................................. 17 1.3.2 Incandescent Lamp................................. 18 1.3.3 Gas Discharge Lamps............................... 22 1.3.4 Fluorescent Lamp................................... 29 1.3.5 Low-Pressure Sodium Lamp.......................... 33 1.3.6 High-Pressure Mercury Lamp......................... 37 1.3.7 High-Pressure Sodium Lamp......................... 41 1.3.8 Metal Halide Lamp................................. 45 1.3.9 Other Discharge Lamps.............................. 50 2 Plasma and Electrode Physics.................................. 51 2.1 Gas Laws............................................... 51 2.1.1 Boltzmann Distribution.............................. 51 2.1.2 Population Density of Excited Atoms................... 52 2.1.3 Maxwell Velocity Distribution......................... 53 2.1.4 Temperature....................................... 56 2.1.5 Pressure or Ideal Gas Law............................ 57 2.1.6 Collision Frequency and Mean Free Path................ 58

xii Contents 2.1.7 Thermal Equilibrium................................ 59 2.1.8 Local Thermal Equilibrium........................... 60 2.2 Plasma Properties......................................... 62 2.2.1 Evaporation and Partial Pressure....................... 62 2.2.2 Saha Equation..................................... 62 2.2.3 Plasma Radiation................................... 65 2.3 Transport Coefficients..................................... 76 2.3.1 Diffusion......................................... 77 2.3.2 Electrical Conductivity............................... 79 2.3.3 Thermal Conductivity............................... 82 2.3.4 Radiative Energy Transport........................... 84 2.4 Electrode Properties....................................... 89 2.4.1 Thermal Conductivity of Tungsten..................... 89 2.4.2 Mass Density and Specific Heat of Tungsten............. 91 2.4.3 Black Body Radiation............................... 91 2.4.4 Electron Emission.................................. 96 3 Experimental Investigations.................................... 105 3.1 Electrode Diagnostics..................................... 106 3.1.1 Electrode Pyrometry................................ 108 3.1.2 Bolometer......................................... 109 3.1.3 1-λ Pyrometry..................................... 109 3.1.4 Examples of 1-λ Pyrometry........................... 114 3.1.5 2-λ Pyrometry..................................... 118 3.1.6 Pyrometry with Plasma Correction..................... 119 3.1.7 Example of (2+1)-λ Pyrometry........................ 126 3.1.8 More Pyrometry.................................... 138 3.1.9 Calorimetric Measurements on Electrodes............... 138 3.1.10 External Laser Heating of Electrodes................... 143 3.1.11 In Situ Laser Diagnostics of Work Function.............. 145 3.1.12 Monolayer of Sodium on Cathode...................... 147 3.1.13 Deformation of Electrodes............................ 148 3.2 Plasma Diagnostics....................................... 152 3.2.1 Emission Spectroscopy.............................. 152 3.2.2 Electrode Fall Voltage and Electric Field of Column........ 156 3.2.3 Plasma Potential.................................... 161 3.2.4 Determination of Lamp Pressure....................... 166 4 Numerical Simulations........................................ 169 4.1 Modeling of Electrodes.................................... 170 4.1.1 Cathode Sheath Model............................... 171 4.1.2 Examples of Electrode Models........................ 175 4.1.3 Cathode and Anode................................. 179 4.2 Plasma Models........................................... 184 4.2.1 Description of the Plasma............................ 184 4.2.2 Example of a Plasma Column Model................... 189 4.2.3 Example of a Plasma Model Including Convection......... 193

Contents xiii 4.3 Extended models......................................... 198 4.4 A Self-Consistent Electrode Plasma Model.................... 200 4.4.1 Model Equations and Boundary Conditions.............. 202 4.4.2 Numerical Procedure................................ 220 4.4.3 Comparing Different Cathode Models................... 226 4.4.4 Different Electrode Shapes........................... 238 4.4.5 External Laser Heating of Electrodes................... 256 4.4.6 D2 Automotive Headlight Lamp....................... 278 4.4.7 Mass, Pressure, and Electrode Gap..................... 298 4.4.8 Spot Diffuse Transition and Time-Dependent Behavior..... 311 4.4.9 Summary: Self Consistent Electrode Plasma Model....... 320 Summary...................................................... 325 Bibliography................................................... 329 Index.......................................................... 339

Introduction This book is concerned with high-intensity discharge (HID) lamps. High-intensity or high-pressure discharge lamps have captured a major share of the markets for automotive headlight lamps (D2 lamp), video projection (UHP lamp), general lighting, street/industrial lighting, commercial lighting, floodlighting, sun tanning, microscopy, endoscopy, photochemistry, lithography, etc. This renders HID lamps an interesting and seminal field of research. High-pressure discharge lamps are normally very small (typically 0.7 mm to some centimeters electrode gap), have short time scales (typical ac frequency: 50 500 Hz or higher), and the electrical power input lies between some tens and several thousand watts. The aim of this book is to give an introduction to the working principle of HID lamps and to point out challenges and problems associated with the development and operation of high-pressure discharge lamps. The fundamentals of plasma and electrode physics as well as the current research on HID lamps will be treated. This book is directed at students interested in high-pressure discharge lamps as well as persons already involved in the research and development or the usage of HID lamps. An overview of light and light sources is given in Chap. 1. This includes a short introduction to the human eye and to photometric characteristics of light sources as well as the discussion of different light sources, covering the sun, the incandescent lamp, and low-pressure and high-pressure discharge lamps. The laws of physics needed for the description and understanding of plasma and electrodes in HID lamps are the subject of Chap. 2. The plasma generates the light and must be heated to sufficiently high temperatures to achieve high efficiencies. The electrodes must supply the electric current to the plasma and must therefore have a high enough temperature, e.g., to emit electrons. On the other hand, the electrodes may cause an early end of life of the lamp, if they get too hot, or the melting of an electrode tip might change the electrode shape and thus alter the operating conditions of the lamp. The current research on HID lamps is the focus of Chaps. 3 and 4. Chapter 3 is concerned with the experimental investigations of high-pressure discharge lamps, namely electrode and plasma diagnostics, whereas numerical simulations of highpressure discharge lamps are discussed in Chap. 4. The starting point of the

2 Introduction numerical simulations is the modeling of the electrodes, followed by the theoretical treatment of the plasma. However, plasma and electrodes are coupled in real lamps, and so the numerical simulation of electrodes and plasma including the interactions between them is a special focus of Chap. 4 and the research work of the author. Chapters 3 and 4 not only present the state of the art of HID lamp research, but also aim at giving a deeper insight into the working principle of high-intensity discharge lamps and the problems and challenges associated with HID lamps. Part of this work has already been published in [53 65]. The generation of light matching our demands in home, office, factory, or shop lighting, for the illumination of roads or sports stadiums, or for the usage in video projection, or for automotive headlight lamps is not the only goal of the development of HID lamps. Environmental issues are also important guidelines for the usage and improvement of high-pressure discharge lamps: Today, the generation of light by approximately 9 billion incandescent lamps, 4 billion fluorescent lamps, and 500 million high-pressure discharge lamps consumes 25% of worldwide electric energy production [111]. Therefore, there is a huge potential to save energy and thus reduce environmental pollution and save energy resources. Moreover, the replacement or reduction of mercury in high-pressure discharge lamps is also an important current issue. The generation of light is not the only purpose of HID lamps, but it is the most visible application area of all lamps and has an obvious effect on our quality of life (cf. Figure 1). Nevertheless, high-pressure discharge lamps are also used in many other fields of application where electromagnetic radiation in the UV and IR part is important. Photolithography, disinfection, curing, or laser excitation are only some of the manifold possibilities offered by HID lamps. Fig. 1. Earth at night. The Eastern USA, Europe, and Japan are brightly lit by their cities, while the interiors of Africa, Asia, Australia, and South America remain (for now) dark [124]