Infrared Thermography

Microwave Technology Series The Microwave Technology Series publishes authoritative works for professional engineers, researchers and advanced students across the entire range of microwave devices, sub-systems, systems and applications. The series aims to meet the reader's needs for relevant information useful in practical applications. Engineers involved in microwave devices and circuits, antennas, broadcasting communications, radar, infra-red and avionics will find the series an invaluable source of design and reference information.

Series editors: Michel-Henri Carpentier Professor in 'Grandes Ecoles', France, Fellow of the IEEE, and President of the French SEE

Bradford L. Smith International Patents Consultant and Engineer with the Alcatel group in Paris, France, and a Senior Member of the IEEE and French SEE

Titles available

1. The Microwave Engineering Handbook Volume 1 Microwave components Edited by Bradford L. Smith and Michel-Henri Carpentier

2. The Microwave Engineering Handbook Volume 2 Microwave circuits, antennas and propagation Edited by Bradford L. Smith and Michel-Henri Carpentier

3. The Microwave Engineering Handbook Volume 3 Microwave systems and applications Edited by Bradford L. Smith and Michel-Henri Carpentier

4. Solid-state Microwave Generation J. Anastassiades, D. Kaminsky, E. Perea and A. Poezevara

5. Infrared Thermography G. Gaussorgues

6. Phase Locked Loops J.B. Encinas

7. Frequency Measurement and Control Chronos Group

8. Microwave Integrated Circuits Edited by I. Kneppo

Infrared Thermography

G. Gaussorgues Technical Director, HGH lnfrared System Massy, and Director, Electro-oprics Laboratory of the French Navy, Fruna

Translated by

s. Chomet Department of Physics King's College Vniversity of London VK

!~1 ~--~ SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

English language edition 1994

© 1994 Springer Science+Business Media Dordrecht Originally published by Chapman & Hali in 1994 Softcover reprint ofthe hardcover lst edition 1994

Original French language edition - La Therrrwgraphie lnfrarouge­Principes, Technologies, Applications (3rd edition, revised) - © 1989 Technique et Documentation - Lavoisier

ISBN 978-94-010-4306-9 ISBN 978-94-011-0711-2 (eBook) DOI 10.1007/978-94-011-0711-2

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries conceming reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page.

The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made.

A catalogue record for this book is available from the British Library

Contents

Colour and black and white plates showing thermograms and images recorded in false colours appear at the end of the book

Foreword

Historical Background

1 Revision of Radiometry 1.1 The radiometric chain 1.2 Radiant flux 1.3 Geometrical spreading of a beam 1.4 Radiance 1.5 Irradiance 1.6 Radiant exitance 1. 7 Radiant intensity of a source in a given direction 1.8 Quantity of radiation and exposure 1.9 Bouguer's law 1.10 Radiation scattering 1.11 Note on units

2 Origins of Infrared Radiation

3 Thermal Emission by Matter 3.1 Black-body radiation

3.1.1 Planck's law 3.1.2 Wien's law 3.1.3 Stefan-Boltzmann law 3.1.4 Exitance of black-body in a given spectral band 3.1.5 Evaluation of exitance of a body by the method of

reduced coordinates 3.1.6 Thermal derivation of Planck's law 3.1.7 Thermal contrast

3.2 Different types of radiator 3.3 Problems with the emissivity of a material 3.4 Thermodynamic equilibrium

xiii

xv

1 1 2 2 3 4 5 5 5 6 6 7

8

11 11 12 13 15 16

17 22 23 24 25 26

vi Infrared Thermography

3.5 Problems with the reflectance of a material 26 3.6 Example of an application 30

3.6.1 Calculation of Te and Ee 32 3.6.2 Calculation of EO and To 34

3.7 Emissivity of materials 36 3.7.1 Spectral emissivity 36 3.7.2 Emissivity of dielectrics - the effect of temperature 37 3.7.3 Emissivity of metals - the effect of temperature 39 3.7.4 The effect of the angle of incidence on emissivity 40 3.7.5 Measurement of emissivity 42 3.7.6 The effect of emissivity in thermography 43 3.7.7 Emissivity of a rough surface 43 3.7.8 The emissivity of dihedrons and trihedrons 45

3.8 Emission from the interior of a medium 46 3.9 Other sources of infrared radiation 50

3.9.1 The Nernst filament (Nernst glower) 50 3.9.2 The globar 51 3.9.3 Electroluminescent junctions 51 3.9.4 Sources employing stimulated emission (lasers) 52

4 Transmission by the Atmosphere 61 4.1 Self-absorption by gases 62 4.2 Scattering by particles 67 4.3 Atmospheric turbulence 68

4.3.1 Diffraction by inhomogeneities 70 4.3.2 The structure function 71 4.3.3 Measurements of turbulence 74

4.4 Methods for calculating atmospheric transmission 75 4.4.1 The 'line-by-line' method 76 4.4.2 The band model method 76 4.4.3 Empirical methods employing band models 77 4.4.4 The multiparametric model 78

4.5 A practical method for calculating atmospheric transmission 78 4.5.1 Molecular absorption 78 4.5.2 Scattering by particles 81 4.5.3 Example of application 94

5 Optical Materials for the Infrared 103 5.1 Propagation of an electromagnetic wave in matter 103 5.2 Optical properties of a medium 109

5.2.1 Refraction 110 5.2.2 Dispersion 110 5.2.3 Absorption, transmission and reflection 111

5.3 Physical properties of optical materials 114 5.3.1 Hardness 115

5.3.2 Thermal properties 5.3.3 Cost of materials

5.4 Types of material 5.4.1 Glasses 5.4.2 Crystals 5.4.3 Plastics 5.4.4 Metals

5.5 Properties of some optical materials 5.5.1 Glasses 5.5.2 Crystals

Contents vii

115 116 117 117 118 118 118 119 119 125

6 Optical Image Formation 135 6.1 Geometrical optics 135 6.2 Aberrations of optical systems 136

6.2.1 Chromatic aberrations 136 6.2.2 Geometrical aberrations 139

6.3. Calculation of geometrical aberrations 159 6.3.1 Path of a marginal ray - imaging by an objective 160 6.3.2 The path of a principal - stop imaging 162 6.3.3 Paraxial rays - the Gaussian approximation 164 6.3.4 The third-order approximation 167 6.3.5 Spherical aberration 167 6.3.6 The case of aplanatic optics - the Abbe's sine condition 168 6.3.7 Calculation of coma 169 6.3.8 Astigmatism and field curvature 171 6.3.9 Distortion 172

6.4 Diffraction 173 6.4.1 Diffraction by an aperture 173 6.4.2 Image formation - linear filter theory 178 6.4.3 The optical transfer function 181 6.4.4 Optics for the infrared 188 6.4.5 Reflecting telescopes 188 6.4.6 Catadioptric telescopes 196 6.4.7 Evaluation of image-spot abberration for different simple

optical systems 196 6.4.8 Refractive optics 200 6.4.9 Simple germanium lens for A. = 10,um 201

7 Scanning and Imaging 213 7.1 Radiometers 213 7.2 Radiometers for spatial analysis 214 7.3 Thermography 220 7.4 Scanning methods 220

7.4.1 Line scanners 221 7.4.2 Image scanning 231

YIll Infrared Thermography

7.5 Imaging 232 7.6 Imaging with multi-element detectors 234

7.6.1 Two-dimensional scanning with a single detector 234 7.6.2 Scanning by a parallel array of n elements 235 7.6.3 Scanning using an array of p elements in series 239 7.6.4 Serial-parallel scanning with a two-dimensional array 240

7.7 Electronic imaging 240 7.7.1 The pyroelectric image tube 242 7.7.2 Pyroelectric arrays 243 7.7.3 Solid state arrays 243

8 Spectral Filtering 244 8.1 Spectral transmittance of materials 244 8.2 The properties of thin layers 246 8.3 Antirefiective thin films 249

8.3.1 Antirefiective coating using a single layer 249 8.3.2 Two-layer antirefiective coating 251 8.3.3 Multilayer antirefiective coating 251 8.3.4 Examples of surface treatments for improving the

transmission of materials 252 8.4 Filters 254

8.4.1 Different types of filter 255 8.4.2 Filter fabrication technologies 256

9 Radiation Detectors 261 9.1 Generalities 261 9.2 Characteristics of detectors 262

9.2.1 Current-voltage characteristic 262 9.2.2 Shape of signal 264

9.3 Noise 264 9.3.1 The spectral distribution and technological causes of noise 264 9.3.2 Signal-to-noise ratio 265 9.3.3 The noise equivalent power (NEP) 267 9.3.4 Detectivity 267 9.3.5 Detectivity limit of a perfect detector 268

9.4 Detector sensitivity 268 9.4.1 Local variation of sensitivity 268 9.4.2 Spectral sensitivity 268 9.4.3 Global sensitivity 269 9.4.4 Sensitivity as a function of frequency 270

9.5 Thermal detectors 271 9.5.1 Fluctuations 271 9.5.2 General principle of operation 271 9.5.3 Signal-to-noise ratio 273

9.5.4 Detectivity of heat detectors 9.6 Different types of thermal detector

9.6.1 Bolometers 9.6.2 Pyroelectric detectors 9.6.3 Thermopiles 9.6.5 Pneumatic detectors

9.7 Quantum detectors 9.7.1 Fluctuations 9.7.2 Detectivity of quantum detectors

9.8 Different types of quantum detector 9.8.1 Photoemissive detectors 9.8.2. Summary of solid state physics 9.8.3. Photoconductive detectors 9.8.4 Photovoltaic detectors

9.9 Applications of detectors 9.9.1 Spectral sensitivity range 9.9.2 Sensitivity 9.9.3 Noise and detectivity 9.9.4 Frequency response of detectors 9.9.5 Detector bias arrangements 9.9.6 Effect of detector field angle 9.9.7 Passivation of detectors

9.10 Multielement detectors 9.11 Detectors used in thermography 9.12 Charge coupled devices

9.12.1 Three-phase CCD 9.12.2 Two-phase CCD 9.12.3 Transfer efficiency 9.12.4 Reading of a detector array with a CCD 9.12.5 Imaging with a CCD matrix 9.12.6 Charge injection devices (Cms) 9.12.7 Spectral response and characteristics of CCD and

cm imaging devices 9.13 Infrared charge coupled devices (IRCCD)

9.13.1 HgTeCd detectors 9.13.2 Indium antimonide 9.13.3 Silicon-platinum Schottky diode 9.13.4 Performance of IRCCDs

9.14 Sprite detectors 9.15 Detector cooling

9.15.1 Cooling by liquified gas 9.15.2 Cooling by Joule-Thomson expansion 9.15.3 Cooling by cryogenic cycles 9.15.4 Thermoelectric cooling

Contents ix

273 274 274 275 276 276 276 277 278 280 280 282 284 287 289 289 291 292 293 294 295 295 296 297 298 298 299 300 301 301 302

304 304 305 306 306 307 308 311 311 311 313 316

x Infrared Thermography

10 Signal Processing 319 10.1 The analogue signal 319 10.2 Processing of analogue signals 323 10.3 Processing of digital signals 323 10.4 Example of application 324

10.4.1 Analogue acquisition 325 10.4.2 Digitisation of the signal 325 10.4.3 Visualisation 329 10.4.4 Architecture of image reconstruction 331 10.4.5 Image processing 333 10.4.6 Temperature calibration of images 334 10.4.7 Description of program 337

11 Characterisation of infrared systems 340 11.1 Generalities 340

11.1.1 Noise equivalent irradiance (NEI) 341 11.1.2 Thermal resolution 341 11.1.3 Spatial resolution 342 11.1.4 Spectral response 342 11.1.5 The signal- temperature relation 342 11.1.6 Temporal stability and drift 343

11.2 Characteristics of infrared detectors 343 11.2.1 Sensitivity 343 11.2.2 Time constant 344 11.2.3 Noise equivalent power (NEP) 344 11.2.4 Noise equivalent irradiance (NEI) 345 11.2.5 Detectivity 345

11.3 Calculation of the characteristics of infrared systems 347 11.3.1 Calculation of noise equivalent irradiance (NEI) 347 11.3.2 Calculation of noise equivalent temperature difference

(NETD) 351 11.4 Measurement of the characteristics of an infrared system 353

11.4.1 Measurement of NEI 354 11.4.2 Measurement of NETD 357 11.4.3 Measurement of MRTD 357 11.4.4 Measurement ofMDTD 359 11.4.5 Measurement of relative spectral response 359 11.4.6 Measurement of spatial resolution - the modulation

transfer function 360 11.4.7 Determination of the signal-temperature relation 363 11.4.8 Measurement of drift 368

11.5 Example: Characterisation of a system 369 11.5.1 Evaluation of NEI 370 11.5.2 Evaluation of NETD 372 11.5.3 Measurement of NEI 374 11.5.4 Measurement ofNETD 373

11.5.5 Measurement of spatial resolution 11.5.6 Determination of the signal-temperature relation

12 Imaging and Measurement 12.1 Spatial resolution 12.2 Thermal resolution 12.3 Imaging and measurement

12.3.1 Thermal imaging 12.3.2 Thermal measurements 12.3.3 Conclusion

12.4 Examples of applications 12.4.1 Spatial resolution 12.4.2 Thermal resolution 12.4.3 Temporal stability 12.4.4 Other characteristics 12.4.5 Recording of signals

13 Choosing the Spectral Band 13.1 Spectral emissivity 13.2 Radiated power 13.3 Thermal contrast 13.4 Atmospheric transmission 13.5 Radiation detectors 13.6 Stray radiation due to the measuring system itself 13.7 Conclusions 13.8 Two-band thermal imaging

14 Industrial and Military Applications 14.1 Infrared thermography in nondestructive testing

14.1.1 Recapitulation 14.2 Thermography in industrial processes

14.2.1 Why use thermography for process control? 14.2.2 Thermographic methods in industrial processes 14.2.3 Thermographic systems for process control 14.2.4 Examples of applications 14.2.5 Conclusions

Contents

14.3 Acquisition, digitisation and processing in two-band image

xi

375 376

379 379 383 383 384 384 386 387 387 392 393 394 395

397 397 398 400 402 402 403 405 407

414 414 414 418 418 419 421 424 425

processing 426 14.3.1 The two-band (bispectral) infrared camera 426 14.3.2 Measurement of the characteristics of a bispectral system 432 14.3.3 Data processing chain 437 14.3.4 Operation of the system 439

14.4 Infrared signatures - new acquisition and processing techniques 443

14.5 Integrated systems for the nuclear industry 449 14.6 Conclusions 449

xii Infrared Thermography

15 Infrared Spectroradiometry 453 15.1 Spectroradiometry 453

15.1.1 Spectroradiometry by spectral dispersion 454 15.1.2 Fourier transform spectroradiometry 456 15.1.3 Spectroradiometry with interference filters 457 15.1.4 Limitations of spectroradiometry 458

15.2 An infrared spectroradiometer 459 15.2.1 Optics 460 15.2.2 Elimination of stray flux 462

16 Line Scanners 471 16.1 First generation thermography 471 16.2 Second generation thermography 472 16.3 The infrared line scanner 473 16.4 Description of the ATL 100 system 473 16.5 ATL 100 used for welding control 476 16.6 Example of application: control ofroll welding 478

16.6.1 Description of process and objective of measurement 478 16.6.2 Installation of the ATL 100 479

16.7 The HGH ATL infrared scanner 481 16.8 Applications of the ATL 020 482

16.8.1 Monitoring of the rolling process in steel industry 482 16.8.2 Glass industry 482

16.9 The Agema THP 5 and 6 infrared scanner 487 16.10 The HGH ATL 080 infrared scanner 488

17 Advances in Thermographic Systems 492 17.1 Agema (Sweden) 492 17.2 Avio-Nippon Avionics (Japan) 495 17.3 Inframetrics (USA) 496 17.4 Comparison of infrared sensitivities of different

thermographic systems 499 17.4.1 AGA 780SW 499 17.4.2 AGA 900SW 499 17.4.3 AVIO TVS 2000 499

17.5 Inframetrics 700 499 17.6 Cooling of detectors 501

Appendix 502

Bibliography 503

Index 505

Colour and Black and White Plates 509

Foreword

Spectral filter Detector

Object

a Display unit

The infrared radiation emitted by a body carries specific information about its material, its recent history and its configuration.

This information can be captured and processed by a suitable acquisition system, and the results can then be employed to monitor and control certain parameters that can only be measured indirectly or with great difficulty.

It is clear that each particular case of remote detection or measurement of such parameters requires a system capable of producing the desired end result .

This book describes the techniques that can be used to access the in­formation contained by the material of a body by capturing the infrared radiation emitted by it.

The radiometric measurement chain consists of the source of radiation under investigation, the environment in which it is found, the medium through which the radiation propagates and, finally, a properly specified measurement system.

It is desirable to examine point by point the different links in this chain, and to evaluate their performance, before we can understand the functional possibilities and limitations of the system in relation to the required goal.

It would be hazardous to take measurements with equipment that is

poorly suited to the phenomena under investigation, and the results thus obtained would not reflect the true possibilities of a judiciously chosen method.

The acquisition chain will first be followed element by element in a va­riety of possible configurations, and the properties of such systems will be explored. We will then examine the characterization of the performance that can be achieved by experimental methods.

Particular attention will be devoted to the qualitative and quantitative aspects of infrared thermography, on the one hand, and to the analysis of the different spectral aspects of operating conditions, on the other.

Finally, these concepts will be illustrated by examples of the use of thermography in industrial and military applications.

Display of processed data in real time

Memorised result

Data acquisition (evidence of satifaJionl) and processing centre

)) J) I

o V Detector

Front optics

(;9ifIc~ ~)

/ l'

/ /' Object (optical signal)

Historical Background

In 1800, Sir William Herschel discovered the presence of thermal radiation outside the spectrum of visible light.

With the help of a thermometer placed beyond the red part of the spec­trum of visible light, produced by a prism, Herschel demonstrated the pres­ence of invisible radiation whose energy could be detected by its heating effect.

He proved subsequently that this radiation, christened infrared, obeyed the same laws as visible light. However, it was not until 1830 that the first detectors were developed for this type of radiation. They were based on the principle of the thermocouple, and were called thermopiles.

The bolometer, which relies on a material whose electrical resistance varies with temperature, made its appearance in 1880 and introduced a significant improvement in infrared detection sensitivity.

Between 1870 and 1920, technological advances led to the development of the first quantum detectors based on the interaction between radiation and matter. The detection process thus ceased to depend on the creation of an electrical signal due to the heating effect of radiation and, instead, relied on the direct conversion of radiation into an electrical signal.

Photoconducting or photovoltaic detectors are found to have much shorter response times and higher sensitivities. The subsequent chronology was as follows.

1930 - 1944. Development of lead sulfide (PbS) detectors, specifically for military needs. These detectors are sensitive in the 1.3-3 J.Lm band.

1940 - 1950. Extension of the spectral range to middle infrared, i.e., 3-5 J.Lm by the use of indium antimonide (InSb).

1960. Exploration of the far infrared, 8-14 J.Lm, by mercury-tellurium­cadmium detectors (HgTeCd) .

The last type of detector requires cooling. Because of their higher sen­sitivity and short response times, these quantum detectors have led to the development of thermal imaging systems that rely on the detection of in­frared radiation emitted by matter in the range 2-15 J.Lm.