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Spie Press Book

Common Sense Approach to Thermal Imaging
Author(s): Gerald C. Holst
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Book Description

Thorough explanation of heat transfer, with concepts supported by thermograms. Intended for all who work with thermal imaging systems: researchers, system designers, test engineers, sales staff, and military and civilian end users. Copublished with JCD Publishing.

Book Details

Date Published: 30 September 2000
Pages: 370
ISBN: 9780819437228
Volume: PM86

Table of Contents
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PART 1

1. Introduction 2
1.1. Heat 4
1.2. Temperature measuring devices 6
1.2.1. Contact devices 6
1.2.1. Non-contact devices 9
1.3. Temperature scales 10
1.4. The electromagnetic spectrum 12
1.4. Brief history of thermography 15
1.5. System development 16
1.6. Applications overview 18
1.7. Units 20
1.8. References 21
2. Heat 21
2.1. Heat capacity and specific heat capacity 23
2.2. Phase change 24
2.3. Heat transfer 26
2.3.1. Conduction 26
2.3.2. Convection 31
2.3.3. Radiation 34
2.4. Heat sources 35
2.4.1. Sun 35
2.4.2. Combustion 35
2.4.4. Chemical reaction 37
2.4.5. Nuclear power plant 37
2.4.6. Energy conversion 38
2.4.7. Mass transport 43
2.4.8. Man-made 43
2.5. Useful conversions and constants 44
2.6. References 45
3. Blackbody radiation 46
3.1. Radiation theory 46
3.1.1. Stefan-Boltzmann law 46
3.1.2. Planck's blackbody law 47
3.2. The background 50
3.2.1. Total power 51
3.2.2. Limited spectral response 52
3.3. Detection of radiation 54
3.3.1. General response 54
3.3.2. Limited spectral response 55
4. Emissivity 57
4.1. Conservation of energy 57
4.2. Target emissivity 58
4.3. Surface conditions 60
4.4. Environmental effects 60
4.5. Geometric factors 62
4.6. Spectral dependence 64
4.7. References 68
5. Atmospheric transmittance 69
5.1. Extinction 70
5.2. Path length dependence 72
5.3. Atmospheric constituents 73
5.3.1. Water vapor 74
5.3.2. Aerosols 75
5.4. Path radiance 76
5.5. Modeling the atmosphere 78
5.6. Back-of-the-envelope modeling 79
5.6.1. "Average" weather conditions 79
5.6.2. Range predictions 80
5.6. References 82
6. Camera design 83
6.1. Camera output 84
6.2. System designs 85
6.2.1. Military systems 85
6.2.2. Civilian systems 86
6.3. Optics 87
6.4. Scanners 87
6.5. Detectors and coolers 89
6.5.1. Detector operation 89
6.5.2. Specific detectors 91
6.5.3. Detector responsivity 92
6.5.4. Fixed pattern noise 94
6.5.5. AC coupling 94
6.5.6. Fill factor 95
6.6. Digitization 96
6.7. Image processing 97
6.8. Reconstruction 99
6.9. Monitors 99
6.10. MWIR versus LWIR 99
6.11. References 101
7. Performance parameters 102
7.1. Spatial frequency 104
7.2. Sensitivity (NEDT) 105
7.2.1. NEDT measurement 105
7.2.2. NEDT theory 106
7.3. Spatial resolution 108
7.3.1. Airy disk 109
7.3.2. Instantaneous-field-of-view 109
7.3.3. Pixel-angular-subtense 110
7.3.4. Slit response function 111
7.3.5. Measuring IFOV 112
7.3.6. Nyquist frequency 113
7.3.7. Optical zoom/electronic magnification 114
7.4. Observer response 115
7.5. MRT and MDT 116
7.6. References 118
8. Camera selection 119
8.1. Environment 120
8.1.1. Harsh environment 121
8.1.2. Ambient temperature 122
8.1.3. Viewing geometry 123
8.2. Camera 127
8.2.1. Physical attributes 127
8.2.2. Dynamic range 128
8.2.3. Frame rate 131
8.2.4. Image processing 132
8.2.5. Calibration 133
8.3. Storage and output 133
8.4. Other issues 134
8.5. References 135
9. Observer training 136
9.1. Certification 137
9.2. Military users 137
9.3. MRT observers 138
9.4. References 139

PART 2

10. Introduction to applications 141
10.1. Condition monitoring 143
10.2 Process control/quality control 147
11. Target signatures 149
11.1. Thermal anomaly identification 149
11.2. The environment 151
11.3. Passive targets 154
11.3.1. Solar heating 154
11.3.2. Clouds 157
11.3.3. Wind 157
11.3.4. Rain and snow 158
11.3.5. The scanning window 159
11.3.6. Outdoor considerations 160
11.4. Emissivity variations 162
11.5. Active targets 165
11.5.1. Vehicles 165
11.5.2. Small targets 166
11.5.3. Estimate of the scanning window 167
11.6. Limited Access Components 169
11.7. References 170
12. Temperature measurements 171
12.1. Apparent temperature 171
12.2. Temperature calibration 173
12.3. Emissivity 174
12.3.1. Emissivity correction 175
12.3.2. Angle of incidance 176
12.3.3. Emissivity uncertainty 177
12.3.4. Increasing the emissivity 180
12.4. Measurement of Ts 183
12.5. Atmospheric corrections 184
12.6. Dual band measurements 184
12.7. Signal averaging 185
12.8. References 186
13. Building envelope inspections 187
13.1. Building Science 188
13.1.1. Historic Buildings 188
13.1.2. 1880 to 1940 Construction 190
13.1.3. 1940 to 1960 Construction 190
13.1.4. 1960 to 1990 Construction 190
13.1.5. Modern Construction 191
13.1.6. Windows 191
13.2. Heat transfer 192
13.2.1. Conductivity 193
13.2.2. Evaporation 195
13.2.3. Mass Transport 196
13.3. Building Inspection 198
13.4. References 198
14. Roof inspections 200
14.1. Roof construction 202
14.2. Temperature differential 203
14.2.1. Solar loading 203
14.2.2. Clouds 205
14.2.3. Wind 206
14.2.4. Surface moisture 206
14.2.5. High relative humidity 206
14.2.6. Phase change 207
14.2.7. Reflections 207
14.2.8. Interior effects 207
14.3. Inspection 208
14.4. References 210
15. Power distribution 211
15.1. Resistance 212
15.2. Temperature 216
15.2.1. Radiation 217
15.2.2. Conduction 218
15.2.3. Convection 219
15.3. Power distribution inspection 221
15.4. REFERENCES 223
16. Electrical/Mechanical Inspection 224
16.1. Baseline thermogram 224
16.2. Electrical connections 225
16.3. Motors: electrical components 228
16.4. Motors: mechanical components 229
16.5. Machinery 231
16.6. Emissivity 234
16.7. Temperature guidelines 235
16.7.1. Electrical circuits 235
16.7.2. Mechanical systems 238
16.8. References 239
17. Buried objects 241
17.1. Heat transfer 242
17.2. Environment 245
17.3. Underground objects 246
17.3.1. Hot fluids 246
17.3.2. Voids 248
17.4. References 248
18. Surveillance 250
18.1. Perceived signal-to-noise ratio 251
18.2. Three-dimensional noise model 252
18.3. Two-dimensional MRT 256
18.4. Range predictions 257
18.4.1. Target ?T 257
18.4.2. Johnson criteria 258
18.4.3. Discrimination 258
18.4.4. Target transfer probability function 260
18.4.5. Range prediction methodology 261
18.5. Surveillance applications 263
18.5.1. Military 264
18.5.2. Law enforcement 267
18.5.3. Search and rescue 268
18.5.4. Animal monitoring 269
18.5.5. Other airborne applications 269
18.6. References 270
19. Nondestructive testing 272
19.1. Applications 274
19.2. Heat transfer 275
19.2.1. Flash pulse (impulse or Dirac pulse) 277
19.2.2. Step and square pulse irradiation 279
19.2.3. Modulated Irradiation 280
19.3. Test methodology 281
19.3.1. Heat injection methods 282
19.3.2. Data analysis 285
19.3.3. Temperature/time guidelines 287
19.4. References 288
20. Process/quality control 289
20.1 Automotive 290
20.2. Petrochemical 290
20.3. Die castings and molding 292
20.4. Printed circuit boards 293
20.5. Metals 294
20.5.1. Aluminum 294
20.5.2. Steel 295
20.6. Paper industry 296
20.7. References 296
21. Inspection procedures 298
21.1. Preliminary work 299
21.1.1. Who 299
21.1.2. What 300
21.1.3. When 300
21.1.4. Where 301
21.1.5. Why 301
21.1.6. How 301
21.2. Safety 301
21.3. Standards and guidelines 302
21.4. Test procedure 302
21.4.1. Test plan 302
21.4.2. Good practices 306
21.5. Exit conference 307
21.6. Final report 307
21.7. References 309
Appendix A:Temperature conversion 310
Appendix B: EMISSIVITY 316
Appendix C: Thermal Sensing and Imaging 323
Index 350

Preface

Objects are characterized by a variety of physical parameters such as size, shape, and weight. However, the most frequently measured physical property is temperature. Heat is a byproduct of all work whether it is from electrical, mechanical, or chemical activity. We generate, contain, and transfer heat to run our industries and make our every day lives comfortable. Unexpected temperature variations may indicate design flaws, poor workmanship, or damaged components. A temperature variation can also be used to recognize an intruder, locate a buried object, or to identify geological events.

Thermal imaging systems are used by the military to detect, recognize, and identify enemy personnel, equipment, and buildings. Police patrol border crossings and use thermal imaging systems for search and rescue. The systems are particularly useful for evaluating the condition of power lines, transformers, circuit breakers, and motors. Simply put, they can be used to evaluate the "health" of any electrical or mechanical component.

Several texts that describe the applications of thermal imaging systems are
1. Nondestructive Evaluation of Materials by Infrared Thermography, X. P. V. Maldague, Springer-Verlag, New York (1992).
2. Applications of Thermal Imaging, S. G. Burney, T. L. Williams, and C. H. N. Jones, Adam Hilger, Philadelphia, PA (1988).
3. Practical Applications on Infrared Thermal Sensing and Imaging Equipment, second edition, H. Kaplan, SPIE Press Vol. TT34, Bellingham WA (1999).

SPIE has assembled two compendiums:
1. Selected SPIE papers on Thermal Sensing and Imaging 1980-1999, SPIE CD-ROM, Volume 7, J. Snell and D. Burleigh, eds. (1999).
2. Selected Papers on Temperature Sensing: Optical Methods, R. D. Lucier, ed., SPIE Milestone Series Vol. MS 116, Bellingham WA (1995).

The first is a collection of papers from the proceedings of the SPIE conference Thermosense. Since this is an important contribution to the literature, Appendix C of this book contains the Table of Contents of the CD-ROM. The second compendium contains reprints of articles that appeared in professional journals.

This book differs from these texts by clearly describing the phenomenology of heat transfer and providing numerous thermograms to support the concepts. It also covers a diverse set of applications. This book is divided into two parts. The first part (Chapters 1-9) provides the physics background that is necessary to interpret thermograms. The second part (Chapters 10-20) discusses various applications. Except for Chapter 19, Nondestructive Testing, minimal math is used in the second part. Heat transfer during nondestructive testing is a complex phenomenon and therefore requires more math. The researcher and scientist will read the first part in detail whereas the thermographer performing inspections will concentrate on the second part.

Heat transfer (Chapter 2), radiation theory (Chapter 3), and emissivity (Chapter 4) form the backbone of all thermal imaging system applications. The atmosphere (Chapter 5) may attenuate the received signal. This becomes an issue over long path lengths - typical of military applications. Camera design, performance parameters, and camera selection are contained in Chapters 6 through 8. It is can not be understated that each camera has specific design features and unique performance parameters. As a result, it is somewhat of a challenge to find a system that is best for a specific application. Interpretation of thermograms requires training (Chapter 9). This training must include the material provided in Chapters 2 through 5.

Part 2 begins with an introduction to applications (Chapter 10). Chapter 11 discusses the influence of the environment of target signatures. With this knowledge, it is possible to perform quantitative temperature measurements. For some tests, qualitative results are adequate. These include building envelope inspections (Chapter 13), roof inspections (Chapter 14), and the location of buried objects (Chapter 17). Electrical and mechanical inspections often require quantitative results (Chapters 15 and 16). Here, the temperature of a component is compared to a standard or guideline. If the temperature is too high, then the component must be repaired or replaced. Although surveillance refers to the observation of a person, this definition is extended to include the detection, recognition, and identification of both people and objects (Chapter 18). It includes search and rescue, endangered species monitoring, border patrol, law enforcement, and military applications. Nondestructive testing (Chapter 19) can locate disbonding, delamination, and corrosion. These are of prime concern of our aging commercial aircraft. Chapter 20 briefly discusses applications in six different industries.

The physics is described by three equations. The first is the "easy" approach to describe the phenomenology. For example, the output of a detector is given by:

where RD is the detector's responsivity, M is the radiation from the target, and k is a constant. The effect of RD and M on VDETECTOR is described and supported by numerous examples. When the concept of wavelength is introduced, the voltage at a specific wavelength becomes

The radiant intensity also depends upon the target's temperature, T. The sum of all the outputs due to the various wavelengths must be added together. This is represented by an integral:

Long-wave infrared (LWIR) and mid-wave infrared (MWIR) systems have different wavelength intervals, [ 1 , 2]. The reader who is less interested in the math complexities will use the first equation. But he will fully understand the phenomenology. If detailed calculations are necessary, the reader will use the third equation.

By using both simple and complex math, this book is intended for all that work with thermal imaging systems. This includes the researcher, system designer, test engineer, salesman, and end user. Since civilian and military applications are discussed, this book is useful to both communities.

I extend my deepest gratitude to all my coworkers and students who have contributed to the ideas in this book. They are too many to mention by name. I especially thank all those who read draft copies of the manuscript: Mary Lee Cassetta, consultant; Arnold Daniels, Optics-1; Dennis Hewins, Academy of Infrared Thermography; A. J. Holst, American Credit; Chris Johnston, IRcameras.com; Ron Newport, Academy of Infrared Thermography; Harold Orlando, Northrup Grumman; Jim Porter, Raytheon Systems; Austin Richards, Indigo; Elliot Rittenberg, EFR Associates; John Snell, Snell Infrared; and Gary Weil, EnTech Engineering. Although these reviewers provided valuable comments, the accuracy of the text is solely my responsibility. Douglas F. Marks provided the graphics and manuscript layout.

The thermograms were obtained from a number of sources. However, some images have been so widely distributed that the original owner is not known to many. If I missed a credit or gave credit to the wrong person, I apologize. Every attempt has been made to authenticate the owners.

I hope that you find the title accurate: That this book IS the common sense approach to thermal imaging.

Gerald C. Holst
Winter Park, FL


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