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For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.Library of Congress Cataloging-in-Publication DataRogalski, Antoni.Infrared detectors / Antoni Rogalski. -- 2nd ed.p. cm.Includes bibliographical references and index.ISBN 978-1-4200-7671-4 (hardcover : alk. paper)1. Infrared detectors. I. Title. TA1570.R63 2011621.362--dc22 2010029532Visit the Taylor & Francis Web site athttp://www.taylorandfrancis.comand the CRC Press Web site athttp://www.crcpress.com In memory of my daughter MartaviiTable of ContentsPreface xviiAcknowledgments xixAbout the Author xxiI Part : Fundaments of Infrared Detection 1 1 Radiometry 211 Radiometric and Photometric Quantities and Units 212 Defnitions of Radiometric Quantities 413 Radiance 714 Blackbody Radiation 1015 Emissivity 1316 Infrared Optics 1517 Some Radiometric Aspects of Infrared Systems 17171 Night-Vision System Concepts 17172 Atmospheric Transmission and Infrared Bands 18173 Scene Radiation and Contrast 20References 21 2 Infrared Detector Characterization 2321 Historical Aspects of Modern Infrared Technology 2422 Classifcation of Infrared Detectors 2823 Cooling of IR Detectors 31231 Cryogenic Dewars 32232 JouleThompson Coolers 32233 Stirling Cycle Coolers 32234 Peltier Coolers 3324 Detector Figures of Merit 33241 Responsivity 34242 Noise Equivalent Power 34243 Detectivity 3425 Fundamental Detectivity Limits 35References 40 3 Fundamental Performance Limitations of Infrared Detectors 4531 Thermal Detectors 45311 Principle of Operation 45312 Noise Mechanisms 48313 Detectivity and Fundamental Limits 4932 Photon Detectors 53321 Photon Detection Process 53322 Theoretical Model of Photon Detectors 56viii3221 Optical Generation Noise 583222 Thermal Generation and Recombination Noise 59323 Optimum Thickness of Photodetector 60324 Detector Material Figure of Merit 60325 Reducing Device Volume to Enhance Performance 6233 Comparison of Fundamental Limits of Photon and Thermal Detectors 6534 Modeling of Photodetectors 69References 71 4 Heterodyne Detection 75References 83I Part I: Infrared Thermal Detectors 87 5 Thermopiles 8851 Basic Principle and Operation of Thermopiles 8852 Figures of Merit 9153 Thermoelectric Materials 9354 Micromachined Thermopiles 96541 Design Optimization 97542 Thermopile Confgurations 98543 Micromachined Thermopile Technology 98References 101 6 Bolometers 10461 Basic Principle and Operation of Bolometers 10462 Types of Bolometers 107621 Metal Bolometers 107622 Thermistors 107623 Semiconductor Bolometers 108624 Micromachined Room Temperature Silicon Bolometers 1116241 Bolometer Sensing Materials 1146242 Vanadium Oxide 1146243 Amorphous Silicon 1156244 Silicon Diodes 1166245 Other Materials 116625 Superconducting Bolometers 117626 High-Temperature Superconducting Bolometers 12163 Hot Electron Bolometers 126References 130 7 Pyroelectric Detectors 13871 Basic Principle and Operation of Pyroelectric Detectors 138711 Responsivity 139712 Noise and Detectivity 14272 Pyroelectric Material Selection 144ix721 Single Crystals 145722 Pyroelectric Polymers 147723 Pyroelectric Ceramics 147724 Dielectric Bolometers 148725 Choice of Material 15273 Pyroelectric Vidicon 152References 153 8 Novel Thermal Detectors 15781 Golay Cell 15782 Novel Uncooled Detectors 157821 Electrically Coupled Cantilevers 160822 Optically Coupled Cantilevers 163823 Pyro-Optical Transducers 166824 Antenna-Coupled Microbolometers 16883 Comparison of Thermal Detectors 169References 171I Part II: Infrared Photon Detectors 175 9 Theory of Photon Detectors 17691 Photoconductive Detectors 176911 Intrinsic Photoconductivity Theory 1769111 Sweep-Out Effects 1789112 Noise Mechanisms in Photoconductors 1809113 Quantum Effciency 1829114 Ultimate Performance of Photoconductors 1839115 Infuence of Background 1849116 Infuence of Surface Recombination 184912 Extrinsic Photoconductivity Theory 185913 Operating Temperature of Intrinsic and Extrinsic Infrared Detectors 19492 p-n Junction Photodiodes 197921 Ideal Diffusion-Limited p-n Junctions 1989211 Diffusion Current 1989212 Quantum Effciency 2019213 Noise 2029214 Detectivity 203922 Real p-n Junctions 2059221 GenerationRecombination Current 2069222 Tunneling Current 2089223 Surface Leakage Current 2109223 Space-Charge Limited Current 211923 Response Time 213x93 p-i-n Photodiodes 21494 Avalanche Photodiodes 21695 Schottky-Barrier Photodiodes 222951 SchottkyMott Theory and Its Modifcations 222952 Current Transport Processes 223953 Silicides 22596 Metal-SemiconductorMetal Photodiodes 22697 MIS Photodiodes 22798 Nonequilibrium Photodiodes 23299 nBn Detector 233910 Photoelectromagnetic, Magnetoconcentration, and Dember Detectors 2349101 Photoelectromagnetic Detectors 23591011 PEM Effect 23591012 Lile Solution 23691013 Fabrication and Performance 2389102 Magnetoconcentration Detectors 2399103 Dember Detectors 240911 Photon-Drag Detectors 242References 245 10 Intrinsic Silicon and Germanium Detectors 256101 Silicon Photodiodes 256102 Germanium Photodiodes 264103 SiGe Photodiodes 266References 269 11 Extrinsic Silicon and Germanium Detectors 272111 Technology 273112 Peculiarities of the Operation of Extrinsic Photodetectors 274113 Performance of Extrinsic Photoconductors 2761131 Silicon-Doped Photoconductors 2761132 Germanium-Doped Photoconductors 278114 Blocked Impurity Band Devices 280115 Solid-State Photomultipliers 284References 285 12 Photoemissive Detectors 290121 Internal Photoemission Process 2901211 Scattering Effects 2941212 Dark Current 2961213 Metal Electrodes 297122 Control of Schottky-Barrier Detector Cutoff Wavelength 298123 Optimized Structure and Fabrication of Schottky-Barrier Detectors 299124 Novel Internal Photoemissive Detectors 300xi1241 Heterojunction Internal Photoemissive Detectors 3001242 Homojunction Internal Photoemissive Detectors 301References 303 13 III-V Detectors 309131 Some Physical Properties of III-V Narrow Gap Semiconductors 309132 InGaAs Photodiodes 3151321 p-i-n InGaAs Photodiodes 3161322 InGaAs Avalanche Photodiodes 318133 Binary III-V Detectors 3211331 InSb Photoconductive Detectors 3211332 InSb Photoelectromagnetic Detectors 3221333 InSb Photodiodes 3241334 InAs Photodiodes 3311335 InSb Nonequilibrium Photodiodes 335134 Ternary and Quaternary III-V Detectors 3371341 InAs Sb Detectors 33913411 InAsSb Photoconductors 33913412 InAsSb Photodiodes 3411342 Photodiodes Based on GaSb-Related Ternary and Quaternary Alloys 348135 Novel Sb-Based III-V Narrow Gap Photodetectors 3501351 InTlSb and InTlP 3501352 InSbBi 3511353 InSbN 352References 352 14 HgCdTe Detectors 366141 HgCdTe Historical Perspective 366142 HgCdTe: Technology and Properties 3691421 Phase Diagrams 3691422 Outlook on Crystal Growth 3701423 Defects and Impurities 37614231 Native Defects 37614232 Dopants 378143 Fundamental HgCdTe Properties 3791431 Energy Bandgap 3791432 Mobilities 3801433 Optical Properties 3831434 Thermal GenerationRecombination Processes 38714341 ShockleyRead Processes 38714342 Radiative Processes 38914343 Auger Processes 389144 Auger-Dominated Photodetector Performance 391xii1441 Equilibrium Devices 3911442 Nonequilibrium Devices 392145 Photoconductive Detectors 3941451 Technology 3941452 Performance of Photoconductive Detectors 39614521 Devices for Operation at 77 K 39614522 Devices for Operation above 77 K 4001453 Trapping-Mode Photoconductors 4021454 Excluded Photoconductors 4021455 SPRITE Detectors 406146 Photovoltaic Detectors 4101461 Junction Formation 41114611 Hg In-Diffusion 41114612 Ion Milling 41214613 Ion Implantation 41214614 Reactive Ion Etching 41514615 Doping during Growth 41614616 Passivation 41714617 Contact Metallization 4191462 Fundamental Limitation to HgCdTe Photodiode Performance 4201463 Nonfundamental Limitation to HgCdTe Photodiode Performance 4321464 Avalanche Photodiodes 4371465 Auger-Suppressed Photodiodes 4421466 MIS Photodiodes 4461467 Schottky-Barrier Photodiodes 448147 Hg-Based Alternative Detectors 4491471 Crystal Growth 4501472 Physical Properties 4511473 HgZnTe Photodetectors 4531474 HgMnTe Photodetectors 454References 456 15 IV-VI Detectors 485151 Material Preparation and Properties 4851511 Crystal Growth 4851512 Defects and Impurities 4881513 Some Physical Properties 4891514 GenerationRecombination Processes 494152 Polycrystalline Photoconductive Detectors 4981521 Deposition of Polycrystalline Lead Salts 4981522 Fabrication 4991523 Performance 501xiii153 p-n Junction Photodiodes 5011531 Performance Limit 5021532 Technology and Properties 50715321 Diffused Photodiodes 51015322 Ion Implantation 51215323 Heterojunctions 512154 Schottky-Barrier Photodiodes 5141541 Schottky-Barrier Controversial Issue 5141542 Technology and Properties 517155 Unconventional Thin Film Photodiodes 522155 Tunable Resonant Cavity Enhanced Detectors 525156 Lead Salts Versus HgCdTe 527References 529 16 Quantum Well Infrared Photodetectors 542161 Low Dimensional Solids: Background 542162 Multiple Quantum Wells and Superlattices 5481621 Compositional Superlattices 5481622 Doping Superlattices 5491623 Intersubband Optical Transitions 5511624 Intersubband Relaxation Time 555163 Photoconductive QWIP 5561631 Fabrication 5571632 Dark Current 5581633 Photocurrent 5641634 Detector Performance 5661635 QWIP versus HgCdTe 570164 Photovoltaic QWIP 573165 Superlattice Miniband QWIPs 575166 Light Coupling 577167 Related Devices 5801671 p-Doped GaAs/AlGaAs QWIPs 5801672 Hot-Electron Transistor Detectors 5811673 SiGe/Si QWIPs 5821674 QWIPs with Other Material Systems 5841675 Multicolor Detectors 5851676 Integrated QWIP-LED 588References 589 17 Superlattice Detectors 601171 HgTe/HgCdTe Superlattice 6011711 Material Properties 6011712 Superlattice Photodiodes 604xiv172 Strained Layer Superlattices 608173 InAsSb/InSb Strained Layer Superlattice Photodiodes 609174 InAs/GaInSb Type II Strained Layer Superlattices 6111741 Material Properties 6111742 Superlattice Photodiodes 6151743 nBn Superlattice Detectors 620References 622 18 Quantum Dot Infrared Photodetectors 629181 QDIP Preparation and Principle of Operation 629182 Anticipated Advantages of QDIPs 631183 QDIP Model 632184 Performance of QDIPs 6381841 RoA Product 6381842 Detectivity at 78 K 6381843 Performance at Higher Temperature 639References 641I Part V: Focal Plane Arrays 645 19 Overview of Focal Plane Array Architectures 646191 Overview 646192 Monolithic FPA Architectures 6501921 CCD Devices 6531922 CMOS Devices 657193 Hybrid Focal Plane Arrays 6601931 Interconnect Techniques 6601932 Readout Integrated Circuits 662194 Performance of Focal Plane Arrays 6651941 Noise Equivalent Difference Temperature 6671942 NEDT Limited by Readout Circuit 67019421 Readout Limited NEDT for HgCdTe Photodiode and QWIP 671195 Minimum Resolvable Difference Temperature 673196 Adaptive Focal Plane Arrays 673References 676 20 Thermal Detector Focal Plane Arrays 680201 Thermopile Focal Plane Arrays 681202 Bolometer Focal Plane Arrays 6862021 Manufacturing Techniques 6892022 FPA Performance 6922023 Packaging 696203 Pyroelectric Focal Plane Arrays 6972031 Linear Arrays 6972032 Hybrid Architecture 6982033 Monolithic Architecture 701xv2034 Outlook on Commercial Market of Uncooled Focal Plane Arrays 703204 Novel Uncooled Focal Plane Arrays 705References 707 21 Photon Detector Focal Plane Arrays 715211 Intrinsic Silicon and Germanium Arrays 715212 Extrinsic Silicon and Germanium Arrays 719213 Photoemissive Arrays 725214 III-V Focal Plane Arrays 7312141 InGaAs Arrays 7312142 InSb Arrays 73521421 Hybrid InSb Focal Plane Arrays 73521422 Monolithic InSb Arrays 738215 HgCdTe Focal Plane Arrays 7422151 Monolithic FPAs 7442152 Hybrid FPAs 745216 Lead Salt Arrays 751217 QWIP Arrays 755218 InAs/GaInSb SLS Arrays 759References 762 22 Terahertz Detectors and Focal Plane Arrays 776221 Direct and Heterodyne Terahertz Detection: General Considerations 778222 Schottky-Barrier Structures 780223 Pair-Braking Photon Detectors 784224 Thermal Detectors 7862241 Semiconductor Bolometers 7872242 Superconducting Hot-Electron Bolometers 7902243 Transition Edge Sensor Bolometers 792225 Field Effect Transistor Detectors 795226 Conclusions 798References 799 23 Third-Generation Infrared Detectors 808231 Benefts of Multicolor Detection 808232 Requirements of Third-Generation Detectors 810233 HgCdTe Multicolor Detectors 8122331 Dual-Band HgCdTe Detectors 8132332 Three-Color HgCdTe Detectors 821234 Multiband QWIPs 822235 Type-II InAs/GaInSb Dual-Band Detectors 832236 Multiband QDIPs 836References 839Final Remarks 846Index 849xviiPrefaceProgress in infrared (IR) detector technology has been mainly connected to semiconductor IR detectors, which are included in the class of photon detectors They exhibit both perfect signal-to-noise performance and a very fast response But to achieve this, the photon detectors require cryogenic cooling Cooling requirements are the main obstacle to the more widespread use of IR systems based on semiconductor photodetectors making them bulky, heavy, expensive, and inconvenient to useUntil the 1990s, despite numerous research initiatives and the appeal of room temperature operation and low cost potential, thermal detectors have enjoyed limited success compared with cooled photon detectors for thermal imaging applications Only the pyroelectric vidicon received much attention with the hope that it could be made practical for some applications Throughout the 1980s and early 1990s, many companies in the United States (especially Texas Instruments and Honeywells Research Laboratory) developed devices based on various thermal detection principles In the mid-1990s, this success caused DARPA (Defense Advanced Research Projects Agency) to reduce support for HgCdTe and attempt a major leap with uncooled technology The desire was to have producible arrays with useful performance, without the burden of fast (f/1) long-wavelength infrared opticsIn order to access these new changes in infrared detector technology, there was need for a com-prehensive introductory account of IR detector physics and operational principles, together with important references In 2000, the frst edition of Infrared Detectors was published with the inten-tion of meeting this need The last decade has seen considerable changes with numerous break-throughs in detector concepts and performance It became clear that the book needed substantial revision to continue to serve its purposeIn this second edition of Infrared Detectors, about 70% of the contents have been revised and updated, and much of the materials have been reorganized The book is divided into four parts: fundaments of infrared detection, infrared thermal detectors, infrared photon detectors, and focal plane arrays The frst part provides a tutorial introduction to the technical topics that are fundamental to a thorough understanding of different types of IR detectors and systems The second part presents theory and technology of different types of thermal detectors The third part covers theory and technology of photon detectors The last part concerns IR focal plane arrays (FPAs) where relations between the performance of detector arrays and infrared system quality are consideredThe short description below mainly concerns differences between the original edition and this revision I have added a discussion of radiometry and fux-transfer issues needed for IR detec-tor and system analysis in the frst part In the next two parts, in addition to updating traditional issues described in the previous book, I have included new achievements and trends in the devel-opment of IR detectors, most notably:novel uncooled detectors (eg, cantilever detectors, antenna and optically coupled detectors); type II superlattice detectors; and quantum dot infrared detectors In addition, I have highlighted new approaches to terahertz (THz) arrays and a new generation of infrared detectorsso-called third-generation detectors THz technologies are now receiving increasing attention, and devices exploiting this wavelength band are set to become increasingly important in a diverse range of human activity applications (eg, security, biological, drugs and explosion detection, gases fngerprints, imaging, etc) Today, researchers are developing third-generation systems that provide enhanced capabilities such as a larger number of pixels, higher frame rates, better thermal resolution, multicolor functionality, and other on-chip functionsThis book is written for those who desire a comprehensive analysis of the latest developments in infrared detector technology and basic insight into fundamental processes important to evolving detection techniques Special attention has been given to the physical limits of detector perfor-mance and comparisons of performance in different types of detectors The reader should gain a good understanding of the similarities and contrasts, the strengths and weaknesses of a mul-titude of approaches that have been developed over a century to improve our ability to sense IR radiationThe level of presentation is suitable for graduate students in physics and engineering who have received standard preparation in modern solid-state physics and electronic circuits This book xviiiis also of interest to individuals working with aerospace sensors and systems, remote sensing, thermal imaging, military imaging, optical telecommunications, infrared spectroscopy, and light detection and ranging (LIDAR) To satisfy the needs of the frst group, many chapters discuss the principles underlying each topic and some historical background before bringing the reader the most recent information available For those currently in the feld, the book can be used as a collec-tion of useful data, as a guide to the literature, and as an overview of topics covering a wide range of applications The book could also be used as a reference for participants of relevant workshops and short coursesThis new edition of Infrared Detectors gives a comprehensive analysis of the latest developments in IR detector technology and basic insight into the fundamental processes important to evolving detection techniques The book covers a broad spectrum of IR detectors, including theory, types of materials and their physical properties, and detector fabricationAntoni RogalskixixAcknowledgmentsIn the course of this writing, many people have assisted me and offered their support I would like, frst, to express my appreciation to the management of the Institute of Applied Physics, Military University of Technology, Warsaw, for providing the environment in which I worked on the book The writing of the book has been partially done under fnancial support of the Polish Ministry of Sciences and Higher Education, Key Project POIG010301-14-016/08 New Photonic Materials and Their Advanced ApplicationThe author has benefted from the kind cooperation of many scientists who are actively work-ing in infrared detector technologies The preparation of this book was aided by many informa-tive and stimulating discussions with the authors colleagues at the Institute of Applied Physics, Military University of Technology in Warsaw The author thanks the following individuals for providing preprints, unpublished information, and in some cases original fgures, which were used in preparing the book: Drs L Faraone and J Antoszewski (University of Western Australia, Perth), Dr J L Tissot (Ulis, Voroize, France), Dr S D Gunapala (California Institute of Technology, Pasadena), Dr M Kimata (Ritsumeikan University, Shiga, Japan), Dr M Razeghi (Northwestern University, Evanston, Illinois), Drs M Z Tidrow and P Norton (US Army RDECOM CERDEC NVESD, Fort Belvoir, Virginia), Dr S Krishna (University of New Mexico, Albuquerque), Dr H C Liu (National Research Council, Ottawa, Canada), G U Perera (Georgia State University, Atlanta), Professor J Piotrowski (Vigo System Ltd, Oz

arw Mazowiecki, Poland), Dr M Reine (Lockheed Martin IR Imaging Systems, Lexington, Massachusetts), Dr F F Sizov (Institute of Semiconductor Physics, Kiev, Ukraine), and Dr H Zogg (AFIF at Swiss Federal Institute of Technology, Zrich) Thanks also to CRC Press, especially Luna Han, who encouraged me to undertake this new edition and for her cooperation and care in publishing this second editionUltimately, it is the encouragement, understanding, and support of my family that provided me the courage to embark on this project and see it to its conclusionxxiAbout the AuthorAntoni Rogalski is a professor at the Institute of Applied Physics, Military University of Technology in Warsaw, Poland He is a lead-ing researcher in the feld of IR optoelectronics During the course of his scientifc career, he has made pioneering contributions in the areas of theory, design, and technology of different types of IR detec-tors In 1997, he received an award from the Foundation for Polish Science (the most prestigious scientifc award in Poland) for achieve-ments in the study of ternary alloy systems for infrared detectorsmainly an alternative to HgCdTe new ternary alloy detectors such as lead salts, InAsSb, HgZnTe, and HgMnTe In 2004, he was elected as a corresponding member of the Polish Academy of SciencesProfessor Rogalskis scientifc achievements include determining the fundamental physical parameters of InAsSb, HgZnTe, HgMnTe, and lead salts; estimating the ultimate performance of ternary alloy detectors; elaborating on studies of high-quality PbSnTe, HgZnTe, and HgCdTe photodiodes oper-ated in 35 m and 812 m spectral ranges; and conducting comparative studies of the perfor-mance limitation of HgCdTe photodiodes versus other types of photon detectors (especially QWIP and QDIP IR detectors)Professor Rogalski has given about 50 invited plenary talks at international conferences He is author and co-author of over 200 scientifc papers, 11 books, and 13 book chapters He is a fellow of the International Society for Optical Engineering (SPIE), vice president of the Polish Optoelectronic Committee, vice president of the Electronic and Telecommunication Division at the Polish Academy of Science, editor-in-chief of the journal Opto-Electronics Review, deputy editor-in-chief of the Bulletin of the Polish Academy of Sciences: Technical Sciences, and a member of the editorial boards of Journal of Infrared and Millimeter Waves and International Review of Physics.Professor Rogalski is an active member of the international technical community He is a chair and co-chair, organizer and member of scientifc committees of many national and international conferences on optoelectronic devices and material sciences1I PAR T FunDAmenTs oF InFRAReD DeTeCTIonPart I: Fundaments oF InFrared detectIon21 RadiometryTraditionally, infrared (IR) technologies are connected with controlling functions and night-vision problems with earlier applications connected simply with detection of IR radiation, and later by forming IR images from temperature and emissive differences (systems for recogni-tion and surveillance, tank sight systems, anti-tank missiles, airair missiles, etc) Most of the funding has been provided to fulfll military needs, but peaceful applications have increased continuously, especially since the last decade of the twentieth century (see Figure 11) It is pre-dicted currently that the commercial market is about 70% in volume and 40% in value, largely connected with volume production of uncooled imagers [1] These include medical, industry, earth resources, and energy conservation applications Medical applications include thermogra-phy in which IR scans of the body detect cancers or other trauma, which raise the body surface temperature Earth resource determinations are done by using IR images from satellites in conjunction with feld observation for calibration (in this manner, eg, the area and content of felds and forests can be determined) In some cases even the health state of a crop is determined from space Energy conservation in homes and industry has been aided by the use of IR scans to determine the points of maximum heat loss Demands for these technologies are quickly growing due to their effective applications, for example, in global monitoring of environmental pollution and climate changes, longtime prognoses of agriculture crop yield, chemical process monitoring, Fourier transform IR spectroscopy, IR astronomy, car driving, IR imaging in medi-cal diagnostics, and othersThe infrared range covers all electromagnetic radiation longer than the visible, but shorter than millimeter waves (Figure 12) The divisions between these categories are based on the different source and detector technologies used in each region Many proposals in the division of IR range have been published and these shown in Table 11 are based on limits of spectral bands of com-monly used IR detectors Wavelength 1 m is a sensitivity limit of popular Si detectors Similarly, wavelength 3 m is a long wavelength sensitivity of PbS and InGaAs detectors; wavelength 6 m is a sensitivity limit of InSb, PbSe, PtSi detectors, and HgCdTe detectors optimized for a 35 m atmospheric window; and fnally wavelength 15 m is a long wavelength sensitivity limit of HgCdTe detectors optimized for an 814 m atmospheric windowThe IR devices cannot be designed without an understanding of the amount of radiation power that impinges on the detector from the target, and the radiation of the target cannot be under-stood without a radiometric measurement This issue is critical to the overall signal-to-noise ratio achieved by the IR systemOur discussion in this chapter is simplifed due to certain provisions and approximations We specifcally consider the radiometry of incoherent sources and ignore the effects of diffraction In general, we make small-angle assumptions similar to those made for paraxial optics The sine of an angle is approximated by the angle itself in radiansThis chapter provides some guidance in radiometry For further details, see [27]1.1 RADIOMETRIC AND PHOTOMETRIC QUANTITIES AND UNITSRadiometry is the branch of optical physics that deals with the measurement of electromagnetic radiation in the frequency range between 31013 and 31016 Hz This range corresponds to wave-lengths between 10 nm and 10 m and includes the regions commonly called the ultraviolet, the visible, and the infrared Radiometry deals with the actual energy content of the light rather than its perception through a human visual system Typical radiometric units include watt (radiant fux), watt per steradian (radiant intensity), watt per square meter (irradiance), and watt per square meter per steradian (radiance)Historically, the power of a light source was obtained by observing brightness of the source It turns out that brightness perceived by the human eye depends upon wavelength; that is, color of the light and differs from the actual energy contained in the light The eye is most sensitive to the yellowgreen light and less sensitive to red and blue lights of the spectrum To take the difference into account, a new set of physical measures of light is defned for the visible light that parallel the quantities of radiometry, where the power is weighted according to the human response by multiplying the corresponding quantity by a spectral function, called the V() func-tion or the spectral luminous effciency for photopic vision, defned in the domain from 360 to 830 nm, and is normalized to one at its leak, 555 nm (Figure 13) The V() function tells us the appropriate response of the human eye to various wavelengths This function was frst defned by the Commission Internationale de lclairage (CIE) in 1924 [8] and is an average response Part I: Fundaments oF InFrared detectIon4Photometry is the measurement of light, which is defned as radiation detectable by the human eye It is restricted to the visible region and all quantities are weighted by the spectral response of the eye Typical photometric units include lumen (luminous fux), candela (luminous intensity), lux (illuminance), and candela per square meter (luminance)The units as well as the names of similar properties in photometry differ from those in radiom-etry For instance, power is simply called power in radiometry or radiant fux, but it is called the luminous fux in photometry While the unit of power in radiometry is watt, in photometry it is lumen A lumen is defned in terms of a superfuous fundamental unit, called candela, which is one of the seven independent quantities of the SI system of units (meter, kilogram, second, ampere, Kelvin, mole, and candela) Candela is the SI unit of the photometric quantity called luminous intensity or luminositry that corresponds to the radiant intensity in radiometry Table 12 lists the radiometric and photometric quantities and units along with translation between both groups of unitsRadiometry is plagued by a confusion of terminology, symbols, defnitions, and units The origin of this confusion is largely because of the parallel or duplicate development of the funda-mental radiometric practices by researchers in different disciplines Consequently, considerable care should be exercised when reading publications The terminology used in this chapter follows international standards and recommendations [7,9]1.2 DEFINITIONS OF RADIOMETRIC QUANTITIESRadiant fux, also called radiant power, is the energy Q (in joules) radiated by a source per unit of time and is defned by dQdt (11)The unit of radiant fux is the Watt (W=J/s)3501.21.00.80.60.40.20.0Wavelength (nm)Value750 700 650 600 550 500 450 400Figure 1.3 CIE V() functionTable 1 2 Radiometric and Photometric Quantities and unitsPhotometric Quantity UnitRadiometric Quantity Symbol UnitUnit ConversionLuminous fux lm (lumen) Radiant fux W (Watt) 1 W=683 lmLuminous intensity cd (candela)=lm/sr Radiant intensity I W/sr 1 W/sr=683 cdIlluminance lx (lux)=lm/m2Irradiance E W/m21 W/m2=683 lxLuminance cd/m2=lm/(sr m2) Radiance L W/(sr m2) 1 W/(sr m2)=683 cd/m2Luminous exitance lm/m2Radiant exitance M W/m2Luminous exposure lx s Radiant exposure W/(m2 s)Luminous energy lm s Radiant energy Q J (Joule) 1 J=683 lm sPart I: Fundaments oF InFrared detectIon8refector, but typically exhibit semispecular refection characteristics at oblique viewing angles An ideal thermal source (blackbody) is perfectly Lambertian, while certain special diffusers also closely approximate the condition An actual source is typically approximately Lambertian within a range of view angles s that is less than 20Even for a Lambertian source, the intensity depends on s Making the assumption of L indepen-dent of source position, from Equation 111 follows I L dA LA IdsAs s s n ss

cos cos cos (113)It is Lamberts cosine law, where In is the intensity of the ray leaving in a direction perpendicular to the surface For non-Lambertian surfaces, the radiance L is a function of angle itself, and the falloff of I with s is faster than coss

To receive relationship between radiation exitance and radiance for a planar Lambertian source, we return to Equation 112 and integrate MAL d d L dss d sd

cos cos sin 0202 212 L L, (114)where the Lambertian-source assumption has been used to pull L outside of the angular integrals For a non-Lambertian source, the integration yields a proportionality constant different from Let us simplify further considerations assuming s=0 Then, for the geometrical confguration shown in Figure 111, the radiant power on the detector can be obtained by multiplying the detec-tors solid angle by the area of the source and the radiance of the source [10] d s ds ds dLALA ArL A 2 (115)From this equation results that the fux on the detector is expressed as the radiance of the source multiplied by an areasolid angle (A) product To fulfll Equation 115, two provisions are required: a small-angle assumption for the approximation of the solid angle of a fat surface by A/r2 and the fux transfer is unaffected by absorption losses in the systemAnother situation occurs for a tilted receiver shown in Figure 112 The source normal is along the line of centers, so s=0 in this case The angle d is the angle between the line of centers and the normal to the detector surface In this situation d s dLA (116)SourceAs AdDetectordsrFigure 1.11 Radiant power transfer from source to detectorSources = 0 AdAs rDetectordFigure 1.12 Radiant power transfer from source to a titled detectorPart I: Fundaments oF InFrared detectIon10From Equation 121 results, the image irradiance is equal EALAqimgimaglens 2 (122)1.4 BLACKBODY RADIATIONAll objects are composed of continually vibrating atoms, with higher energy atoms vibrating more frequently The vibration of all charged particles, including these atoms, generates electromagnetic waves The higher the temperature of an object, the faster the vibration, and thus the higher the spectral radiant energy As a result, all objects are continually emitting radiation at a rate with a wavelength distribution that depends upon the temperature of the object and its spectral emissivity, ()Radiant emission is usually treated in terms of the concept of a blackbody [5] A blackbody is an object that absorbs all incident radiation and, conversely according to the Kirchhoffs law, is a per-fect radiator The energy emitted by a blackbody is the maximum theoretically possible for a given temperature A device of this type is a very useful standard source for the calibration and testing of radiometric instruments Further, most sources of thermal radiation radiate energy in a manner that can be readily described in terms of a blackbody emitting through a flter, making it possible to use the blackbody radiation laws as a starting point for many radiometric calculationsThe blackbody or Planck equation was one of the milestones of physics The Plancks law describes the spectral radiance (spectral radiant exitance) of a perfect blackbody as a function of its temperature and the wavelength of the emitted radiation, in the forms L Thc hckT , exp ( )

_,

1]121251W/(cm2sr m), (123) M Thc hckT , exp ( )

_,

1]121251W/(cm22m), (124)where is the wavelength, T is the temperature, h is the Plancks constant, c is the velocity of light, and k is the Boltzmanns constant The corresponding equations for spectral radiant exitance, M(,T), and spectral radiance, L(,T), are related by M=LThe units listed in Table 12 are based on joule as the fundamental quantity An analogous set of quantities can be based on number of photons A conversion between two sets of units is easily accomplished using the relationship for the amount of energy carried per photon: =hc/ For example joule photon joule photon s s ( ) ( ) ( ) (125)In a similar way, the Equations 123 and 124 may be transformed to the forms L Tc hckT , exp ( )

_,

1]12141photon/(s cm sr m),2 (126) M Tc hckT , exp ( )

_,

1]12141photon/(s cm m)2 (127)Figure 115 shows a plot of these curves for a number of blackbody temperatures As the temperature increases, the amount of energy emitted at any wavelength increases too, and the wavelength of peak emission decreases The latter is given by the Wiens displacement law [11]: mwT 2898 mK for maximum watts, (128) mpT 3670 mK for maximum photons, (129)1 radIometry11which is derived from the condition for the peak of the exitance function by setting the derivative equal to zero dM Td, ( ) 0 (130)and solving for wavelength at maximum exitanceThe loci of these maxima are shown in Figure 115 Note that for an object at an ambient temper-ature of 259 K, mw and mp occur at 100 m and 127 m, respectively We need detectors operat-ing near 10 m if we expect to see room temperature objects such as people, trees, and vehicles without the aid of refected light For hotter objects such as engines, maximum emission occurs at shorter wavelengths Thus, the waveband 215 m in infrared or thermal region of the electro-magnetic spectrum contains the maximum radiative emission for thermal imaging purposes It is interesting to note that the mw for sun is near 05 m, very close to the peak of sensitivity of the human eyeTotal radiant exitance from a blackbody at temperature T is the integral of spectral exitance over all wavelengths M T M T dhchckT( ) ( )

_,

,exp02521 11]1 05 42 34 4215dkc hT T , (131)where =25k4/15c2h3 is called the StefanBoltzmann constant and has an approximate value of 5671012 W/cm2K4

The relation determined by Equation 131 between the total radiant exitance of a blackbody and its temperature is called the StefanBoltzmann law The total exitance can be interpreted as the area under the spectral exitance curve for a given temperature, as is shown in Figure 116The radiant exitance of blackbody between a and b is obtained by integrating the Plancks law over the integral [a,b] as shown in Figure 116: M T M T dhchckTab ( ) ( )

_,,exp2125

1]1abd (132)0.11020101810161014101210101081061041021001.0 10Wavelength (m)Spectral radiant exitance (photons s1 cm2 m1)WattsPhotons500 K 290 K 1000 K1000 KLocus of peaks3670 mK500 KLocus ofpeaks2898 mK290 K100104103102101100101102Spectral radiant exitance (W cm2 m1)Figure 1.15 Plancks law for spectral radiant exitance (From Burnay, S G, Williams, T L, and Jones, C H, Applications of Thermal Imaging, Adam Hilger, Bristol, England, 1988)1 radIometry13 ( ) ( )M TThckTM T ,, 2 (134)For a system operating within a fnite passband (), it is important at what wavelength the source (target) exitance changes the most with temperature This question is fundamental for sen-sitivity of an infrared system Comparing the second partial derivative to zero ( )

1]1 M TT,, 0 (135)produces a constrain on the wavelength of exitance contrast [10], similar to Wien displacement law max contrastm 2410T (136)For example, at a source temperature of 300 K, the maximum contrast occurs at a wavelength of about 8 m, which is not the wavelength for maximum exitance1.5 EMISSIVITYAs was mentioned previously, the blackbody curve provides the upper limit of the overall spectral exitance of a source for any specifc temperature Most thermal sources are not perfect blackbod-ies Many are called graybodies A graybody is one that emits radiation in exactly the same spec-tral distribution as a blackbody at the same temperature, but with reduced intensityThe ratio between the exitance of the actual source and the exitance of a blackbody at the same temperature is defned as emissivity In general, emissivity depends on and T: ,,, TM TM T( ) ( )( )sourceblackbody (137)and is a dimensionless number1For a perfect blackbody =1 for all wavelengths The emissivity of graybody is independent of (see Figure 118) A selective source has an emissivity that depends on wavelengthThe total radiant exitance for graybody at all wavelengths is equal M T Tgb( ) 4 (138)When radiant energy is incident on a surface, fraction is absorbed, fraction r is refected, and fraction t is transmitted Since energy must be conserved, the following relationship can be written: + + r t 1 (139)4103300 KBlackbodySelective radiatorGraybody31032103110311040 5 10Wavelength (m)Spectral radiant exitance (W/cm2 m)15 20 25 30Figure 1.18 Spectral radiant exitance of three different radiatorsPart I: Fundaments oF InFrared detectIon14Kirchhoff observed that at a given temperature the ratio of the integrated emissivity to the integrated absorptance is a constant for all materials and that it is equal to the radiant exitance of a blackbody at that temperature Known as Kirchhoffs law, it can be stated as M TM T ,, ( ) ( )sourceblackbody (140)This law is often paraphrased as good absorbers are good emitters Combining Equations 131 and 137 give TT44 (141)From this follows that (142)Thus the emissivity of any materials at a given temperature is numerically equal to its absorptance at that temperature Since an opaque material does not transmit energy, +r=1 and 1 r (143)Table 13 lists the emissivity of a number of common materials that are frequently a single num-ber, and are seldom given as either a function of or T unless it is an essentially well- characterized material [13] The dependence of emissivity on wavelength results from the fact that many sub-stances (glass, for example) have a negligible absorption and consequent low emissivity at certain wavelengths, while they are almost totally absorbent at other wavelengths For many materials, emissivity decreases as wavelength increases For nonmetallic substances, typically >08 for room temperature and decreases with increasing temperature For a metallic substance, the emissivity is very low at room temperature and generally increases proportional to temperatureTable 1.3: emissivity of a number of materialsMaterial Temperature (K) EmissivityTungsten 500 0051000 0112000 0263000 0333500 035Polished silver 650 003Polished aluminium 300 0031000 007Polished cooper 002015Polished iron 02Polished brass 4600 003Oxidized iron 08Black oxidized cooper 500 078Aluminium oxide 80500 075Water 320 094Ice 273 0960985Paper 092Glass 293 094Lampblack 273373 095Laboratory blackbody cavity 098099Source: Smith, W J, Modern Optical Engineering, McGraw-Hill, New York, 20001 radIometry151.6 INFRARED OPTICSThe optical block in the IR system creates an image of observed objects in a plane of the detector (detectors) In the case of a scanning imager, the optical scanning system creates an image with the number of pixels much greater than the number of elements of the detector In addition, the optical elements like windows, domes, and flters can be used to protect a system from the environment or to modify detector spectral responseThere is no essential difference in design rules of optical objectives for visible and IR ranges A designer of IR optics is only more limited because there is signifcantly fewer materials suitable for IR optical elements, in comparison with those for visible range, particularly for wavelengths over 25 mThere are two types of IR optical elements: refective elements and refractive elements As the names suggest, the role of refective elements is to refect incident radiation and the role of refrac-tive elements is to refract and transmit incident radiationMirrors used extensively inside IR systems (especially in scanners) are most often met as refective elements that serve manifold functions in IR systems Elsewhere they need a protective coating to prevent them from tarnishing Spherical or aspherical mirrors are employed as imaging elements Flat mirrors are widely used to fold an optical path, and refective prisms are often used in scanning systemsFour materials are most often used for mirror fabrication: optical crown glass, low-expansion borosilicate glass (LEBG), synthetic fused silica, and Zerodur Less popular in use are metallic substrates (beryllium, copper) and silicon carbide Optical crown glass is typically applied in nonimaging systems It has a relatively high thermal expansion coeffcient and is employed when thermal stability is not a critical factor The LEBG, known by the Corning brand name Pyrex, is well suited for high quality, front-surface mirrors designed for low optical deformation under thermal shock Synthetic fused silica has a very low thermal expansion coeffcientMetallic coatings are typically used as refective coatings of IR mirrors There are four types of metallic coatings used most often: bare aluminium, protected aluminium, silver, and gold They offer high refectivity, over about 95%, in 315 m spectral range Bare aluminium has a very high refectance value but oxidizes over time Protected aluminium is a bare aluminium coating with a dielectric overcoat that arrests the oxidation process Silver offers better refectance in near IR than aluminium and high refectance across a broad spectrum Gold is a widely used material and consistently offers very high refectance (about 99%) in the 0850 m range However, gold is soft (it cannot be touched to remove dust) and is most often used in laboratoryThe most popular materials used in manufacturing refractive optics of IR systems are: germa-nium (Ge), silicon (Si), fused silica (SiO2), glass BK-7, zinc selenide (ZnSe), and zinc sulfde (ZnS) The IR-transmitting materials potentially available for use as windows and lenses are gathered in Table 14 and their IR transmission is shown in Figure 119 [14]Germanium is a silvery, metallic-appearing solid of very high refractive index (4) that enables designing of high resolution optical systems using a minimal number of germanium lenses Its useful transmission range is from 2 m to about 15 m It is quite brittle and diffcult to cut but accept a very good polish Additionally, due to its very high refractive index, antire-fection coatings are essential for any germanium transmitting optical system Germanium has a low dispersion and is unlikely to need color correcting except in the highest-resolution systems In spite of the high material price and cost of antirefection coatings, germanium lenses are par-ticularly useful for 812 m band A signifcant disadvantage of germanium is serious depen-dence of its refractive index on temperature, so germanium telescopes and lenses may need to be athermalizedPhysical and chemical properties of silicon are very similar to properties of germanium It has a high refractive index (345), is brittle, does not cleave, takes an excellent polish, and has large dn/dT Similarly to germanium, silicon optics must have antirefection coatings Silicon offers two transmission ranges: 17 m and 25300 m Only the frst one is used in typical IR systems The material is signifcantly cheaper than germanium It is used mostly for IR systems operating in 35 m bandSingle crystal material has generally higher transmission than polycrystalline one Optical-grade germanium used for the highest optical transmission is n-type doped to receive a conduc-tivity of 514 cm Silicon is used in its intrinsic state At elevated temperatures semiconducting materials become opaque As a result, germanium is of little use above 100C In the 814 m region, semi-insulating GaAs may be used at the temperatures up to 200CPart I: Fundaments oF InFrared detectIon16Ordinary glass does not transmit radiation beyond 25 m in IR region Fused silica is charac-terized by very low thermal expansion coeffcient that makes optical systems particularly useful in changing environmental conditions It offers transmission range from about 03 m to 3 m Because of low refection losses due to low refractive index (145), antirefection coatings are not needed However, an antirefection coating is recommended to avoid ghost images Fused silica is more expensive than BK-7, but still signifcantly cheaper than Ge, ZnS, and ZnSe and is a popular material for lenses of IR systems with bands located below 3 m The BK-7 glass characteristics are similar to fused silica; the difference is only a bit shorter transmission band up to 25 mZnSe is expensive material comparable to germanium; has a transmission range from 2 to about 20 m, and a refractive index about 24 It is partially translucent when visible and reddish in color Table 1.4: Principal Characteristics of some Infrared materialsMaterialWaveband(m) n4m, n10mdn/dT(106 K1)Density(g/cm3) Other CharacteristicsGe 35, 812 4025, 4004 424 (4 m) 533 Brittle, semiconductor, can be diamond-turned, visibly opaque, hard404 (10 m)Si 35 3425 159 (5 m) 233 Brittle, semiconductor, diamond-turned with diffculty, visibly opaque, hardGaAs 35, 812 3304, 3274 150 532 Brittle, semiconductor, visibly opaque, hardZnS 35, 812 2252, 2200 43 (4 m) 409 Yellowish, moderate hardness and strength, can be diamond-turned, scatters short wavelengths41 (10 m)ZnSe 35, 812 2433, 2406 63 (4 m) 526 Yellow-orange, relatively soft and weak, can be diamond-turned, very low internal absorption and scatter60 (10 m)CaF235 1410 81 (339 m) 318 Visibly clear, can be diamond-turned, mildly hygroscopicSapphire 35 1677(no) 6 (o) 399 Very hard, diffcult to polish due to crystal boundaries1667(ne) 12 (e)AMTIR-1 35, 812 2513, 2497 72 (10 m) 441 Amorphous IR glass, can be slumped to near-net shapeBK7 (Glass) 03523 34 251 Typical optical glassSource: Couture, M E, Challenges in IR optics, Proceedings of SPIE 4369, 64961, 2001 With permissionVisibleSapphireZnSAMTR-1As2S3 SiGeMgF2KCIZnSe100806040200BeF20.1 0.3 0.5 1 3 5 10Wavelength (m)Transmittance (%)30 50 100Figure 1.19 Transmission range of infrared materials (From Couture, M E, Challenges in IR optics, Proceedings of SPIE 4369, 64961, 2001 With permission)Part I: Fundaments oF InFrared detectIon18detectors is addressed and displayed so that it is visible to the naked eye Usefulness of the system is due to the following aspects:It is a totally passive technique and allows day and night operation It is ideal for detection of hot or cold spots, or areas of different emissivities within a scene Thermal radiation can penetrate smoke and mist more readily than visible radiation It is a real-time, remote sensing technique The thermal image is a pictorial representation of temperature difference Displayed on a scanned raster, the image resembles a television picture of the scene and can be computer pro-cessed to color-code temperature ranges Originally developed (in the 1960s) to extend the scope of night-vision systems, thermal imagers at frst provided an alternative to image intensifers As the technology has matured, its range of application has expanded and now extends into the felds that have little or nothing to do with night vision (eg, stress analysis, medical diagnostics) In most current thermal imagers, an optically focused image is scanned (mechanically or electroni-cally) across detectors (many elements or two-dimensional array), the output of which is converted into a visual image The optics, mode of scanning, and signal processing electronics are closely interrelated The number of picture points in the scene is governed by the nature of the detector (its performance) or the size of the detector array The effective number of picture points or resolu-tion elements in the scene may be increased by an optomechanical scanning device that images different parts of the scene onto the detector sequentially in timeFigure 121 shows representative camera architecture with three distinct hardware pieces [16]: a camera head (which contains optics, including collecting, imaging, zoom, focusing, and spectral fltering assembles), electronics/control processing box, and the display Electronics and motors to control and drive moving parts must be included The control electronics usually consist of com-munication circuits, bias generators, and clocks Usually a cameras sensor, focal plane array (FPA), needs cooling and therefore some form of cooler is included, along with its closed-loop cooling control electronics A signal from the FPA is of low voltage and amperage and requires analog preprocessing (including amplifcation, control, and correction), which is located physically near the FPA and included in the camera head Often, the A/D is also included here For user conve-nience, the camera head often contains the minimum hardware needed to keep volume, weight, and power to a minimumTypical costs of cryogenically cooled imagers are around $50,000 and restrict their installation to critical military applications allowing conducting of operations in complete darkness Moving from a cooled to an uncooled operation (eg, using silicon microbolometer) reduces the cost of an imager to below $15,000 Less expensive infrared cameras present a major departure from camera architecture presented in Figure 1211.7.2 Atmospheric Transmission and Infrared BandsMost of the above mentioned applications require transmission through air, but the radiation is attenuated by the processes of scattering and absorption Scattering causes a change in the direction of a radiation beam; it is caused by absorption and subsequent reradiation of energy by suspended particles For larger particles, scattering is independent of wavelength However, for small particles, compared with the wavelength of the radiation, the process is known as Rayleigh scattering and exhibits a 4 dependence Therefore, scattering by gas molecules is negligibly small for wavelengths longer than 2 m Also smoke and light mist particles are usually small with respect to IR wavelengths, and IR radiation can therefore penetrate further through smoke and mists than visible radiation However, rain, fog particles, and aerosols are larger and consequently scatter IR and visible radiation to a similar degreeFigure 122 is a plot of the transmission through 6000 feet of air as a function of wavelength [17] Specifc absorption bands of water, carbon dioxide, and oxygen molecules are indicated, which restricts atmospheric transmission to two windows at 35 m and 814 m Ozone, nitrous oxide, carbon monoxide, and methane are less important IR absorbing constituents of the atmosphereIn general, the 814 m band is preferred for high performance thermal imaging because of its higher sensitivity to ambient temperature objects and its better transmission through mist and smoke However, the 35 m band may be more appropriate for hotter objects, or if sensitivity is less important than contrast Also additional differences occur; for example, the advantage of 1 radIometry19MWIR band is a smaller diameter of the optics required to obtain a certain resolution and that some detectors may operate at higher temperatures (thermoelectric cooling) than is usual in the LWIR band where cryogenic cooling is required (about 77 K)Summarizing, MWIR and LWIR m spectral bands differ substantially with respect to back-ground fux, scene characteristics, temperature contrast, and atmospheric transmission under diverse weather conditions Factors which favor MWIR applications are: higher contrast, superior clear-weather performance (favorable weather conditions, eg, in most countries of Asia and Africa), higher transmittivity in high humidity, and higher resolution due to ~3 times smaller optical diffraction Factors which favor LWIR applications are: better performance in fog and dust conditions, winter haze (typical weather conditions, eg, in West Europe, northern United States, Canada), higher immunity to atmospheric turbulence, and reduced sensitivity to solar glints and fre fares The possibility of achieving higher signal-to-noise (S/N) ratio due to greater radiance levels in LWIR spectral range is not persuasive because the background photon fuxes are higher Blackbodyreferences Cooler Cooler controlelectronicsInternalimagingoptics AnalogpreprocessingFPAdewarFPAcontrolelectronicsMechanismcontrolelectronicsAuxiliaryelectronics boxInter-changeableopticsCameraheadPowersupplyPower in Controlpanel interfaceelectronicsA/DconvertersFramegrabberHigherorder imageprocessingDigitalcorrectionsVideoreformatterDisplayDatarecorderPixelformatterand dead pixelreplacementFigure 1.21 Representative infrared camera system architecture (From Miller, J L, Principles of Infrared Technology, Van Nostrand Reinhold, New York, 1994 With permission)Part I: Fundaments oF InFrared detectIon20to the same extent, and also because of readout limitation possibilities Theoretically, in staring arrays charge can be integrated for full frame time, but because of restrictions in the charge-handling capacity of the readout cells, it is much less compared to the frame time, especially for LWIR detectors for which background photon fux exceeds the useful signals by orders of magnitude1.7.3 Scene Radiation and ContrastThe total radiation received from any object is the sum of the emitted, refected, and transmit-ted radiation Objects that are not blackbodies emit only the fraction () of blackbody radiation, and the remaining fraction, 1 (), is either transmitted or, for opaque objects, refected When the scene is composed of objects and backgrounds of similar temperatures, refected radiation tends to reduce the available contrast However, refections of hotter or colder objects have a signifcant effect on the appearance of a thermal scene The powers of 290 K blackbody emission and ground-level solar radiation in MWIR and LWIR bands are given in Table 15 [11] We can see that while refected sunlight have a negligible effect on 813 m imaging, it is important in the 35 m bandA thermal image arises from temperature variations or differences in emissivity within a scene When the temperatures of a target and its background are nearly the same, detection becomes very diffcult The thermal contrast is one of the important parameters for IR imag-ing devices It is the ratio of the derivative of spectral radiant exitance to the spectral radiant exitance CM T TM T

( ) ( ),, (144)Table 1.5: Power Available in each mWIR and LWIR Imaging BandsIR region(m)Ground-level solar radiation (W/m2)Emission from 290 K blackbody (W/m2)35 24 41813 15 127Source: Burnay, S G, Williams, T L, and Jones, C H, Applications of Thermal Imaging, Adam Hilger, Bristol, England, 1988CO2 CO2H2O H2O CO2CO2CO2O201008060402001 2 3 4 5 6 7Wavelength (m)Absorbing moleculeTransmission (%)8 9 10 11 12 13 14 15O2H2O CO2 CO2Figure 1.22 Transmission of the atmosphere for a 6000 foot horizontal path at sea level contain-ing 17 mm of precipitate water (From Hudson, R, Infrared System Engineering, Wiley, New York, 1969 With permission)1 radIometry21Figure 123 is a plot of C for several MWIR subbands and the 812 LWIR spectral band [18] The contrast in a thermal image is small when compared with visible image contrast due to differences in refectivity We can notice the contrast in the MWIR bands at 300 K is 354% compared to 16% for the LWIR bandREFERENCES 1 P R Norton, Infrared Detectors in the Next Millennium, Proceedings of SPIE 3698, 65265, 1999 2 F Grum and R J Becherer, Optical Radiation Measurements, Vol 1 Academic Press, San Diego, CA, 1979 3 W L Wolfe and G J Zissis, The Infrared Handbook, SPIE Optical Engineering Press, Bellingham, WA, 1990 4 W L Wolfe, Radiation Theory, in The Infrared and Electro-Optical Systems Handbook, ed G J Zissis, SPIE Optical Engineering Press, Bellingham, WA, 1993 5 W R McCluney, Introduction to Radiometry and Photometry, Artech House, Boston, MA, 1994 6 W L Wolf, Introduction to Radiometry, SPIE Optical Engineering Press, Bellingham, WA, 1998 7 Y Ohno, Basic Concepts in Photometry, Radiometry and Colorimetry, in Handbook of Optoelectronics, Vol 1, eds J P Dakin and R G W Brown, 287305, Taylor & Francis, New York, 2006 8 CIE Compte Rendu, p 67, 1924 9 Quantities and Units, ISO Standards Handbook, 3rd ed, 1993 10 E L Dereniak and G D Boreman, Infrared Detectors and Systems, Wiley, New York, 1996 11 S G Burnay, T L Williams, and C H Jones, Applications of Thermal Imaging, Adam Hilger, Bristol, England, 19880.0600.0500.0400.0300.0200.010250 270 290 310 330 3503.54.1 m3.55.0 m4.55.0 m8.012.0 mScene temperature (K)Thermal contrastFigure 1.23 Spectral photon contrast in the MWIR and LWIR (From Kozlowski, L J, and Kosonocky, W F, Handbook of Optics, McGraw-Hill, New York, 1995)Part I: Fundaments oF InFrared detectIon22 12 G Gaussorgues, La Thermographie Infrarouge, Technique et Documentation, Lavoisier, Paris, 1984 13 W J Smith, Modern Optical Engineering, McGraw-Hill, New York, 2000 14 M E Couture, Challenges in IR optics, Proceedings of SPIE 4369, 64961, 2001 15 D C Harris, Materials for Infrared Windows and Domes, SPIE Optical Engineering Press, Bellingham, WA, 1999 16 J L Miller, Principles of Infrared Technology, Van Nostrand Reinhold, New York, 1994 17 R Hudson, Infrared System Engineering, Wiley, New York, 1969 18 L J Kozlowski and W F Kosonocky, Infrared Detector Arrays, in Handbook of Optics, eds M Bass, E W Van Stryland, D R Williams, and W L Wolfe, McGraw-Hill, New York, 19952 InFrared detector characterIzatIon232 Infrared Detector CharacterizationInfrared (IR) radiation itself was unknown until 2010 years ago when Herschels experiment with the thermometer was frst reported The frst detector consisted of a liquid in a glass thermometer with a specially blackened bulb to absorb radiation Herschel built a crude monochromator that used a thermometer as a detector so that he could measure the distribution of energy in sunlight In April 1800 he wrote [1]:Thermometer No. 1 rose 7 degrees in 10 minutes by an exposure to the full red colored rays. I drew back the stand thermometer No. 1 rose, in 16 minutes, 83/8 degrees when its centre was inch out of the visible rays.The early history of IR was reviewed about 50 years ago in two well-known monographs [2,3] Many historical information can be also found in a more recently published monograph [4]The most important steps in the development of IR detectors are the following [5,6]:In 1821 Seebeck discovered the thermoelectric effect and soon thereafter demonstrated the frst thermocoupleIn 1829 Nobili constructed the frst thermopile by connecting a number of thermocouples in seriesIn 1833 Melloni modifed the thermocouple and used bismuth and antimony for its design Langleys bolometer appeared in 1880 [7] Langley used two thin ribbons of platinum foil, con-nected to form two arms of a Wheatstone bridge Langley continued to develop his bolometer for the next 20 years (400 times more sensitive than his frst efforts) His latest bolometer could detect the heat from a cow at a distance of a quarter of a mile The beginning of the development of IR detectors was connected with thermal detectorsThe photoconductive effect was discovered by Smith in 1873 when he experimented with selenium as an insulator for submarine cables [8] This discovery provided a fertile feld of investigation for several decades, though most of the effort was of doubtful quality By 1927, over 1500 articles and 100 patents had been listed on photosensitive selenium [9] Work on the IR photovoltaic effect in naturally occurring lead sulfde or galena was announced by Bose in 1904 [10], however, this effect was not used in a radiation detector for the next several decadesThe photon detectors were developed in the twentieth century The frst IR photoconductor was developed by Case in 1917 [11] He discovered that a substance composed of thallium and sulfur exhibited photoconductivity Later he found that the addition of oxygen greatly enhanced the response [12] However, instability of resistance in the presence of light or polarizing voltage, loss of responsivity due to overexposure to light, high noise, sluggish response, and lack of reproduc-ibility seemed to be inherent weaknessesSince about 1930 the development of IR technology has been dominated by the photon detec-tors In about 1930, the appearance of the CsOAg phototube, with more stable characteristics, to a great extent discouraged further development of photoconductive cells until about 1940 At that time, interest in improved detectors began in Germany [13,14] In 1933, Kutzscher at the University of Berlin, discovered that lead sulphide (from natural galena found in Sardinia) was photocon-ductive and had response to about 3 m This work was, of course, done under great secrecy and the results were not generally known until after 1945 Lead sulfde was the frst practical IR detec-tor deployed in a variety of applications during the war In 1941, Cashman improved the technol-ogy of thallous sulfde detectors, which led to successful production [15] Cashman, after success with thallous sulfde detectors, concentrated his efforts on lead sulfde and after World War II found that other semiconductors of the lead salt family (PbSe and PbTe) showed promise as IR detectors [15] Lead sulfde photoconductors were brought to the manufacturing stage of develop-ment in Germany in about 1943 They were frst produced in the United States at Northwestern University, Evanston, Illinois in 1944 and, in 1945, at the Admiralty Research Laboratory in England [16]Many materials have been investigated in the IR feld Observing a history of the development of the IR detector technology, a simple theorem, after Norton [17], can be stated: All physical phenomena in the range of about 0.11 eV can be proposed for IR detectors Among these effects are: thermoelectric power (thermocouples), change in electrical conductivity (bolometers), gas expansion (Golay cell), pyroelectricity (pyroelectric detectors), photon drag, Josephson effect (Josephson junctions, SQUIDs), internal emission (PtSi Schottky barriers), fundamental absorption (intrinsic photodetectors), 2 InFrared detector characterIzatIon25In 1967 the frst comprehensive extrinsic Si detector-oriented paper was published by Soref [20] However, the state of extrinsic Si was not changed signifcantly Although Si has several advan-tages over Ge (namely, a lower dielectric constant giving shorter dielectric relaxation time and lower capacitance, higher dopant solubility and a larger photoionization cross section for higher quantum effciency, and lower refractive index for lower refectance), these were not suffcient to warrant the necessary development efforts needed to bring it to the level of the highly devel-oped Ge detectors by then After being dormant for about 10 years, extrinsic Si was reconsidered after the invention of charge-coupled devices (CCDs) by Boyle and Smith [21] In 1973, Shepherd and Yang [22] proposed the metal-silicide/silicon Schottky-barrier detectors For the frst time it became possible to have much more sophisticated readout schemesboth detection and readout could be implemented in one common silicon chipAt the same time, rapid advances were being made in narrow bandgap semiconductors that would later prove useful in extending wavelength capabilities and improving sensitivity The frst such material was InSb, a member of the newly discovered IIIV compound semiconduc-tor family The interest in InSb stemmed not only from its small energy gap, but also from the fact that it could be prepared in single crystal form using a conventional technique The end of the 1950s and the beginning of the 1960s saw the introduction of narrow-gap semiconduc-tor alloys in IIIV (InAs1xSbx), IVVI (Pb1xSnxTe), and IIVI (Hg1xCdxTe) material systems These alloys allowed the bandgap of the semiconductor and hence the spectral response of the detector to be custom tailored for specifc applications In 1959, research by Lawson and coworkers [23] triggered the development of variable bandgap Hg1xCdxTe (HgCdTe) alloys, providing an unprecedented degree of freedom in IR detector design This frst paper reported both photoconductive and photovoltaic response at the wavelength extending out to 12 m Soon thereafter, working under a US Air Force contract with the objective of devising an 812 m background-limited semiconductor IR detector that would operate at temperatures as high as 77 K, the group lead by Kruse at the Honeywell Corporate Research Center in Hopkins, Minnesota developed a modifed Bridgman crystal growth technique for HgCdTe They soon reported both photoconductive and photovoltaic detection in rudimentary HgCdTe devices [24]The fundamental properties of narrow-gap semiconductors (high optical absorption coeffcient, high electron mobility, and low thermal generation rate), together with the capability for bandgap engineering, make these alloy systems almost ideal for a wide range of IR detectors The diffcul-ties in growing HgCdTe material, signifcantly due to the high vapor pressure of Hg, encouraged the development of alternative detector technologies over the past 40 years One of these was PbSnTe, which was vigorously pursued in parallel with HgCdTe in the late 1960s and early 1970s [2527] PbSnTe was comparatively easy to grow and good quality LWIR photodiodes were readily demonstrated However, in the late 1970s two factors led to the abandonment of PbSnTe detector work: high dielectric constant and large temperature coeffcient of expansion (TCE) mismatch with Si Scanned IR imaging systems of the 1970s required relatively fast response times so that the scanned image is not smeared in the scan direction With the trend today toward staring arrays, this consideration might be less important than it was when frst-generation systems were being designed The second drawback, large TCE, can lead to failure of the indium bonds in hybrid structure (between silicon readout and the detector array) after repeated thermal cycling from room temperature to the cryogenic temperature of operationThe material technology development was and continues to be primarily for military applica-tions In the United States, the Vietnam War caused the military services to initiate the devel-opment of IR systems that could provide imagery arising from the thermal emission of terrain vehicles, buildings, and people As photolithography became available in the early 1960s, it was applied to make IR detector arrays Linear array technology was frst applied to PbS, PbSe, and InSb detectors The discovery in the early 1960s of extrinsic Hg-doped germanium [28] led to the frst FLIR systems operating in the LWIR spectral window using linear arrays Because the detec-tion mechanism was based on an extrinsic excitation, it required a two-stage cooler to operate at 25 K The cooling requirements of intrinsic narrow bandgap semiconductor detectors are much less stringent Typically, to obtain the background-limited performance (BLIP), detectors for the 35 m spectral region are operated at 200 K or less, while those for the 814 m are typically operated at liquid nitrogen temperature In the late 1960s and early 1970s, frst-generation linear arrays (in which an electrical