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Spie Press Book

Uncooled Thermal Imaging Arrays, Systems, and Applications
Author(s): Paul W. Kruse
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Book Description

This introduction to uncooled infrared focal plane arrays and their applications is aimed at professionals, students, and end users. Topics include principal uncooled thermal detection mechanisms; fundamental performance limits and theoretical performance; the state of the art; and applications, technical trends, and systems employing uncooled arrays.

Book Details

Date Published: 15 July 2001
Pages: 110
ISBN: 9780819441225
Volume: TT51
Errata

Table of Contents
SHOW Table of Contents | HIDE Table of Contents
List of Figures xiii
List of Tables xv
Preface xvii
Chapter 1 An Overview of Uncooled Thermal Imaging Detection Mechanisms and Their Figures of Merit / 1
1.1 Terminology / 1
1.2 Detection Mechanisms / 2
1.2.1 Photon detection mechanisms / 2
1.2.2 Thermal detection mechanisms / 4
1.2.2.1 Resistive bolometer / 4
1.2.2.2 Pyroelectric effect / 5
1.2.2.3 Field-enhanced pyroelectric effect/ferroelectric bolometer / 5
1.2.2.4 Thermoelectric effect/radiation thermocouple / 5
1.2.3 Wave interaction effects / 6
1.3 Figures of Merit / 6
1.3.1 Responsivity / 7
1.3.2 Noise equivalent power and D* / 7
1.3.3 Noise equivalent temperature difference / 8
1.3.4 Minimum resolvable temperature difference / 9
1.3.5 Thermal response time / 9
References / 10
Chapter 2 Fundamental Limits / 11
2.1 Introduction / 11
2.2 Photon Noise Limitations of Thermal Detectors / 12
2.3 Temperature Fluctuation Noise in Thermal Detectors / 14
2.4 Temperature Fluctuation Noise Limit to Focal Plane Array Performance / 20
2.5 Background Fluctuation Noise Limit to Focal Plane Array Performance / 20
2.6 Discussion / 22
References / 23
Chapter 3 Thermoelectric Arrays / 25
3.1 Introduction / 25
3.2 The Heat Flow Equation / 27
3.3 Responsivity / 29
3.4 Noise / 30
3.5 D* / 30
3.6 Noise Equivalent Temperature Difference / 30
3.7 Pixel Design Optimization / 31
References / 32
Chapter 4 Resistive Bolometers / 33
4.1 Introduction / 33
4.2 Responsivity / 34
4.2.1 Case I: No Joulean heating; constant bias / 34
4.2.2 Case II: Joulean heating; constant bias / 35
4.2.3 Case III: Joulean heating; pulsed bias / 40
4.3 Noise / 42
4.4 Noise Equivalent Temperature Difference / 44
4.5 Choice of Resistive Materials / 45
4.5.1 Vanadium oxide / 45
4.5.2 Amorphous silicon / 46
4.5.3 Thermistor materials / 47
4.5.4 Titanium / 47
4.5.5 P-N junction diodes / 47
References / 48
Chapter 5 Pyroelectric Arrays / 49
5.1 Introduction / 49
5.2 The Heat Flow Equation / 51
5.3 Responsivity / 52
5.4 Johnson Noise / 54
5.5 Temperature Fluctuation Noise / 55
5.6 Noise Equivalent Temperature Difference / 55
References / 56
Chapter 6 State of the Art and Technical Trends / 57
6.1 Introduction / 57
6.2 Resistive Bolometer Arrays and Their Applications in Thermal Imagers and Imaging Radiometers / 57
6.2.1 The Honeywell silicon microstructure resistive bolometer array and thermal imager / 57
6.2.2 Improvements on the Honeywell VOx 240 ( 332 pixel bolometer array / 63
6.2.2.1 Increase in fill factor / 63
6.2.2.2 CMOS ROIC / 63
6.2.2.3 Smaller pixels / 63
6.2.2.4 640 ( 480 pixel arrays / 64
6.2.2.5 160 ( 120 pixel arrays / 64
6.2.2.6 Removal of temperature stabilizer / 64
6.2.3 Use of amorphous silicon rather than vanadium oxide as the resistive material / 64
6.2.4 Use of diodes rather than resistive materials / 65
6.2.5 Thermal imagers employing uncooled VOx bolometer arrays / 65
6.2.6 Imaging radiometers based on 320 ( 240 pixel uncooled VOx bolometers / 65
6.2.7 Summary / 67
6.3 Pyroelectric and Ferroelectric Bolometer Uncooled Arrays and Thermal Imagers that Employ Them / 67
6.3.1 Introduction / 67
6.3.2 The Texas Instruments (now Raytheon) hybrid ferroelectric bolometer array and imagers / 68
6.3.3 Monolithic pyroelectric array development / 72
6.4 Uncooled Thermoelectric Arrays and Thermal Imagers and Imaging Radiometers that Employ Them / 72
6.4.1 Introduction / 72
6.4.2 Monolithic linear arrays / 73
6.4.3 Imaging radiometer employing linear thermoelectric arrays / 75
6.5 Status and Trends of Uncooled Arrays / 77
6.5.1 Status and trends of uncooled arrays for military systems / 77
6.5.2 Status and trends of commercial uncooled arrays and systems / 78
References / 79
Chapter 7 Choosing the Proper Technical Approach for a Given Application / 83
7.1 Introduction / 83
7.2 Thermal Imaging Applications / 83
7.3 Comparison of the Principal Types of Uncooled Thermal Detectors / 86
Index / 89

Preface

This SPIE Tutorial Text provides an introduction to the subject of uncooled infrared focal plane arrays and their applications. Like all Tutorial Texts, it is intended as an introduction to the subject for both professionals and students; it is not a comprehensive reference or academic textbook. It is a stand-alone version of the short course "Uncooled IR Focal Plane Arrays" which I have taught at SPIE meetings since 1995.

The term "infrared focal plane array" refers to an assemblage of individual infrared detector picture elements ("pixels") located at the focal plane of an infrared imaging system. Although the definition includes one-dimensional ("linear") arrays as well as two-dimensional arrays, it is frequently applied only to two-dimensional arrays. In this text, the term refers to both types.

The term "uncooled" refers to arrays that are operated without any type of cryogenic system. Thus they operate at the imaging system ambient temperature. If the ambient is unstated, it is assumed to be "room temperature," which refers to 295 K and is frequently rounded off to 300 K.

The development of uncooled infrared focal plane arrays can be traced to the military need to see the battlefield at night without the need for any form of illumination, whether from natural sources (moonlight, starlight, airglow) or from artificial sources. In the United States, it was the Vietnam War that caused the military services to initiate the development of infrared systems that could provide imagery arising from the thermal emission of terrain, vehicles, buildings, and people. The result was the Common Modular FLIR (Forward Looking InfraRed), the development of which was led by the U.S. Army Night Vision Laboratory (now the U.S. Army CECOM Night Vision and Electronic Sensors Directorate). The "Common Mod" FLIR employed a (Hg,Cd)Te (mercury cadmium telluride) linear array that required cooling to 77 K; the temperature of liquid nitrogen at 1 atmosphere pressure. Excellent imagery is obtainable with this FLIR, but it requires a cryogenic system (i.e., a refrigerator), and a mechanical system to scan the scene across the linear array to form a two-dimensional image.

The need for the scanner has been obviated by the development of two-dimensional focal plane arrays. In addition to (Hg,Cd)Te focal plane arrays, today there are many other types, such as InSb (indium antimonide), PtSi (platinum silicide), and GaAs/AlGaAs (gallium arsenide/aluminum gallium arsenide) QWIPs (quantum well imaging photodetectors). All require cryogenic operation, although some do not require cooling to as low as 77 K.

These cryogenic arrays employ photon detection, in which the absorption of an incident photon of sufficient energy frees an electron or hole or both to increase the electrical conductivity or to generate a voltage at a potential barrier internal to the material. However, there is another class of detection mechanism, known as thermal, in which the action of the incident radiation is to slightly increase the temperature of a material, which can be detected, for example, by a change in electrical conductivity (resistance bolometer), generation of a voltage (thermoelectric effect, thermocouple), or change in the electrical polarization (pyroelectric effect). These effects do not require cryogenic operation.

When compared on a pixel basis, uncooled thermal detector arrays in general have a much poorer signal-to-noise ratio than the cryogenic photon detector arrays. On the other hand, a staring detector array can have a much smaller system noise bandwidth than a scanning array. Because it is easier to prepare staring arrays employing uncooled thermal detectors than those employing cryogenic photon detectors, the disparity in performance between the two is reduced. This is especially true if both types are operated at the TV frame rate (30 Hz in the U.S.). Thermal detection mechanisms are generally slower than photon; both are designed to have little degradation in performance at 30 Hz.

The most intensive effort in the development of uncooled infrared imaging arrays and systems has been in the U.S. The initial efforts at Honeywell Technology Center on thin film resistive bolometer arrays and at that part of Texas Instruments that later was acquired by Raytheon on pyroelectric arrays and ferroelectric bolometer arrays began in the late 1970s and early 1980s. These efforts, which were funded by U.S. Army Communication Electronics Command (CECOM) Night Vision and Electronic Sensors Directorate (NVESD) and by Defense Advanced Research Projects Agency (DARPA), were initially under military security restrictions. The work became unclassified in 1992, revealing outstandingly successful efforts by both contractors. Since then the field has flourished; many organizations worldwide have entered the field.

The outline of the text is as follows: the first chapter describes in broad terms the principal uncooled thermal detection mechanisms and the figures of merit used to describe their performance. The second chapter describes the fundamental limits to their performance. It is so placed in order that the theoretical performance of the three most important detection mechanisms, namely, thermoelectric, resistive bolometric and pyroelectric-the subjects of Chapters 3, 4, and 5-can be compared against these fundamental limits.

The above chapters constitute the first of three parts of this text. All except chapter one are highly mathematical, directed toward the reader who is participating in the technical development of uncooled arrays. Chapter 6, the second part, is a state-of-the-art description of uncooled arrays and systems employing them. This information comes from journal articles and SPIE Proceedings; manufacturers' literature is not employed. It is of interest not only to the array developers both also to the user community. The final part, Chapter 7, describes applications and technical trends of uncooled arrays and systems employing them. It is of general interest.

I am greatly indebted to Mrs. Jan Jacobs for typing the manuscript, and to Dr. Marc C. Foote, Prof. William L. Wolfe, and Dr. R. Andrew Wood for their very helpful comments.

Paul W. Kruse
December, 2000


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