the development and applications of infrared thermal imaging

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Beyond human vision: the development and applications of infrared thermal imaging E F J Ring* Medical Imaging Research Unit, Faculty of Advanced Technology, University of Glamorgan, Pontypridd CF37 1DL, UK

Abstract: From the identification of thermal energy beyond the visible spectrum by Herschel in 1800 to Robert Wood’s pioneering work in infrared photography, little progress to harness this energy for imaging had been made. In this overview of developments in thermal imaging, significant progress has been made especially in the mid- and far-infrared wavelengths. In the last 50 years, electronic detectors have developed to a stage where small hand-held cameras, often similar in appearance to video camcorders, have become a useful tool in a wide range of industries, and medical and veterinary applications where the study of temperature is of special importance. The impact of computer technology in this field is of special benefit. Keywords:

infrared, thermogram, infrared imaging, infrared detector

This paper is part of a special issue on infrared 1

INTRODUCTION

The ability to create visible images from energy outside the range of human perception is a relatively modern achievement. Infrared radiation was unknown before the astronomer Sir William Herschel conducted an experiment in 1800. In this simple attempt to test the component colours of the visible spectrum, emitted from a prism in a darkened room, he identified a source of heat just beyond the visible red. This became known as the infrared, and his son John Herschel in 1840 used this phenomenon with solar energy to create a thermal image by evaporography that he named a thermogram.1 However, in 1910, Professor Robert Wood (Fig. 1) brought a new dimension to the use of infrared energy with his images created on infrared sensitive film, known as infrared photography.2 Today we recognize that wavelengths between 750 nm and 300 mm as the infrared portion of the electromagnetic spectrum. The photographs of Robert The MS was accepted for publication on 3 March 2010. * Corresponding author, E. F. J. Ring, Medical Imaging Research Unit, Faculty of Advanced Technology, University of Glamorgan, Pontypridd CF37 1DL, UK; email: [email protected]

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Wood’s experiments are termed the near infrared, and use that part of the infrared spectrum nearest to visible red. Further energy is now termed the mid and far infrared (Table 1). Electronic sensor technology now gives us the ability to target specific bands 1 Professor Robert Wood in within the infrared, 1910 and thermography or thermal imaging, to be more precise, is widely used in engineering, astronomy and medicine to study temperature distribution.

2 THE DEVELOPMENT OF THERMAL CAMERA TECHNOLOGY The use of this technology gained ground during the Second World War where the ability to see in the dark IMAG IR5 # RPS 2010

DOI: 10.1179/174313110X12771950995671

APPLICATIONS OF INFRARED THERMAL IMAGING

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2 Optical mechanical scanning system used in the early infrared thermographs

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a Prototype thermal imaging scanner Pyroscan 1940; b Pyroscan developed for medical thermography 1960

has clear military advantages. There are now many other applications in military technology, such as heatseeking missiles where infrared sensors are used. Indium antimonide (2–5 mm) was one of the first successful detectors to be used that has increased sensitivity when cooled below ambient temperatures. Initially, liquid nitrogen was used to cool the detector, but more sustainable cooling systems using thermoelectric (Peltier) or gas expansion devices such as Sterling cooling systems were subsequently developed. These cooling methods also improve the flexibility of use since the camera can be positioned at any angle.3 Today, a wide range of electronic sensor technology exists, and thermal cameras are now available that use uncooled bolometers with great effect. Furthermore, the early scanning cameras used reflecting optics, creating an image by line raster, similar to the early Baird Television system (Fig. 2, 3). Multiple element detectors provided the advantages of both Table 1

high speed and high-resolution imaging. Before this, the earlier single element detectors could give either high resolution at lower speeds or low resolution at higher speed. This problem was overcome by the ‘SPRITE’ detector (8–12 mm) developed by Professor C. T. Elliott. SPRITE provided an efficient means of noise rejection. Multiple arrays of SPRITE units in the 1980s opened up many applications previously hindered by the speed or thermal resolution compromise. An eight-element SPRITE was equivalent to 64 discrete single elements, but with only one-third of the number of connections. For the first time, the output became easily compatible with a standard television or video display rate (Fig. 4, 5).4 Among the current successful infrared detectors for thermal imaging are the focal plane arrays that have become the general standard.5 Designers can now tailor the performance to suit the applications. For example, a military system may require a robust high specification capable of withstanding mechanical shock and vibration, compared to a more mass market with less demanding features. The image format can be 2406480 or 6406480 presently available at 25 mm pitch but higher specifications are achievable such as 10246768 with a finer element pitch. Table 2 illustrates some of the wide range of modern infrared detectors used for thermal imaging cameras. Detector improvements are also supported by the wider use of infrared transmitting lenses. Early scanning systems used mainly reflecting optics and focusing was made by mechanical adjustment of the optical pathway. Germanium and selenium lenses have played a predominant role in advanced camera technology, and calcium fluoride (CaF2) lenses can also be used for applications between deep ultraviolet

Divisions of the infrared spectrum

Terminology

Near-infrared

Mid-infrared

Far-infrared

Wavelength

750 nm–2.5 mm

2.5–10 mm

10–300 mm

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Finally, the impact of computerisation on thermal imaging must be recognized as a significant and now vital addition to this modern technology. Image capture, image processing and storage, the use of false colour palettes and systems for quantification of temperature have facilitated almost all current applications from medicine, industry and engineering. Software for infrared thermal imaging is widely available, and with the increased reliability of small computers and inbuilt processors, former cameras that were heavy and bulky are now replaced with portable digital systems, which can be used both onand offline with their computer hardware6 (Fig. 6).

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a Infrared scanning system using a sprite multi element detector for real time thermal imaging and a pdp11 image processing computer 1980. b High speed macrothermogram of a finger showing the cooled areas by the evaporation of sweat from the skin.

to mid-infrared wavelengths (180–8000 nm). There is now a wider choice of modern infrared transmitting lenses, but in general, their cost remains high compared to the optical systems of conventional cameras for visible light photography. A number of infrared cameras now offer a parfocal digital camera for simultaneous visible recording, often useful when the thermal image alone can be difficult to interpret.

THE THERMAL IMAGE

An infrared thermogram is an image of temperature distribution of the target. The first imaging systems did not provide a direct display, but printed the image during the slow scanning process on electrosensitive paper, and an example is shown in Fig. 7. By the mid1960s, manufacturers were using techniques that allowed an image to be displayed on a long persistent cathode ray screen (CRT), usually a modified oscilloscope. The image could then be either shown as white for hot areas and black for cold with limited grey shades between or the reverse. A further step was to create isotherms, bright line points linked to a given temperature level. This was useful as a simple means of determining the temperature of a given feature in the image. Electronically generated isotherms also led to the simple means of creating the first colour thermograms by multiple exposure photography. An oscilloscope camera was fitted with a turret of colour filters, and linked to the isotherm level control. The shutter remained open, while each isotherm level in the image was exposed though a different colour filter, resulting in a composite colour

5 The SPRITE Mercury Cadmium Telluride infrared detector element developed by C.T.Elliott


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Modern examples of infrared cameras: a fixed installation (left) and two portable cameras (right)

thermogram. For immediate use, Polaroid film was used to avoid the delays of film processing, although the images were less stable than those recorded on silver halide media (Fig. 8). Subsequently, when colour monitors became available, the multiple filter technique became redundant. A new generation of professional CRT units for photography was introduced, providing an expensive but very efficient means of creating a photographic record of the thermogram in colour and monochrome. By this time, computers were available, and although the images were coarsely pixilated, measurement from digitally created regions of interest added to the speed and reliability of image analysis. Today, digital images and image processing have become the standard, with many different colour palettes applied by different users. For example, in engineering applications, a colour palette called ‘iron’ is used where yellow is hotter than red, and white is hotter than yellow, as found when iron is heated in a furnace. In medicine, where the temperature range is more limited (typically 10uC), a ‘rainbow’ palette is preferred, with red as hot and blue/black as cold. These colour scales may also be linear or logarithmic in distribution, and in some software packages, the

user can generate his own colour scale for a specific application if required. In general, the temperature colour scale used at the time of image capture, should be displayed alongside the final image. Without this, the image is poorly defined, since the range and level of temperatures are essential to the full information provided in the thermogram. These issues are frequently defined in any standardisation protocol, since they are essential elements for the comparison of thermograms to indicate change. They are also essential in evidential material for forensic and legal issues.7

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APPLICATIONS OF THERMAL IMAGING

The study of temperature has widespread applications in science and industry. Thermal imaging offers the great advantage of real-time two-dimensional temperature measurement. With modern technology, a single image may contain several thousands of temperature points, recorded in a fraction of a second.

Table 2 Some of the detectors used for infrared imaging Detector Indium gallium arsenide (InGaAs) photodiodes Germanium photodiodes Lead sulphide (PbS) photoconductive Lead selenide (PbSe) photoconductive Indium arsenide (InAs) photovoltaic Platinum silicide (PtSi) photovoltaic Indium antimonide (InSb) photoconductive Mercury cadmium telluride (MCT, HgCdTe) photoconductive Mercury zinc telluride (MZT, HgZnTe) photoconductive Amorphous silicon (a-Si) (microbolometer)

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Spectral range (mm) 0.7–2.6 0.8–1.7 1–3.2 1.5–5.2 1–3.8 1–5 2–5 0.8–25

7 2.5–20 8–13

Medical thermogram of knees, one hot (white) with arthritis, discs at the side are thermal reference temperatures for densitometric calibration of the image. Image recorded directly to thermal paper in 1960


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35mm camera with oscilloscope mount and rotating colour filter turret used to generate colour thermograms from a monochrome oscilloscope thermogram by multiple exposure on a single frame of film. 1970

Thermal imaging in medicine

The human body is homeothermic, i.e. self-generating and regulating the essential levels of temperature for survival. The core is relatively stable, but the shell of the body, the surface tissues, mainly the skin, form part of the regulatory process. Human skin behaves as an almost blackbody with an emissivity of 0.96– 0.98. An American Physiologist JD Hardy showed in 1934, that the emission of human skin peaks at 9– 12 mm. However, detectors operating at 2–5 mm and bolometer systems operating up to 15 mm have all proved to be equally successful in medical applications. As humans we increase our ‘comfort’ by added clothing for insulation in winter, or decreasing clothing levels in the summer. The association between disease and temperature is almost as old as medicine itself. For generations, the clinical thermometer, a simple maximum thermometer for a narrow range of body temperature close to 37uC, was the

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Thermogram of a human hand, rewarming after cooling, showing warm veins and fingers


A modern infrared thermogram of the human face, as required for fever screening

only means of recording the level of temperature measured in a cavity such as the mouth, for the detection of fever. Thermal imaging has been used mainly in research over the last 50 years to study a number of diseases where skin temperature can reflect the presence of inflammation in underlying tissues, or where blood flow is increased or decreased due to a clinical abnormality. In arthritis, hot joints can be imaged and changes due to treatment can be objectively measured. In some circulatory disturbances, such as Raynaud’s phenomenon, or handarm vibration syndrome, damage to small blood vessels from exposure to vibrating machinery, the effect of local blood circulation on skin temperature can be assessed with thermal imaging, especially after exposure of the hands to a stimulus of temperature or a vibrating surface at a known frequency (Fig. 9). In all medical applications, the technique can only provide an image of skin temperature distribution, and does not provide data at a specific depth inside the body, as is common in

11 Horses hooves, the left ankle is inflamed

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12 A sheep’s head with a warm face and horns

other imaging methods. However, there are advantages in that thermal imaging is non-invasive and objective, and is therefore safe and harmless. It is convenient for regular follow-up or monitoring of a treatment or surgical intervention where skin temperature can be related to the underlying condition.8 The small size and weight of the modern cameras that are very similar to a domestic camcorder mean that they can now be employed in the operating theatre, and have been used successfully to monitor the surgical procedures in open-heart surgery. Currently, there is interest in the use of thermal imaging for fever screening. Following the sever acute respiratory syndrome (SARS) outbreak in Southeast Asia, increasing use of thermal imaging has been made to screen travelling passengers at a time of pandemic fever. For this reason, the International Standards Organization has published two new documents defining the use of a thermal imaging camera for fever screening. The first in September 1998, describes the essential design and performance characteristics of a radiometric infrared camera for screening, where differences on the face can be little more than 1uC, and the second in March 2009, defines the recommended mode of deployment

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Thermogram of a slag pile with hot ash being added. (White is hot)

including the testing of the system, and the training of its users. An essential part of this application is that only a close-up image of the upper face (Fig. 10), where a minimum of 9 pixels can be located in each corner of the eye (inner canthus), will provide a true indication of the presence or absence of fever. The widespread idea that a camera can be used to survey a group of moving passengers at a distance is entirely wrong, since it is possible to have a proven fever, yet hot have a generalized increase in facial temperature, as may have been found in the SARS outbreak.9 4.2

Thermal imaging in veterinary

In the horse, where injuries to limbs (Fig. 11) and spinal problems occur, thermal imaging is an effective investigative tool.10,11 Other animals have been studied in specialist centres, much of the published work being of a research nature. The presence of hair is a limitation in many animals, which blocks the infrared emission from the skin surface (Fig. 12). If thermal imaging is needed for veterinary purposes in the clinic, the hair or coat overlying the area to be imaged is shaved, preferably a few hours before investigation. In veterinary applications, the modern hand-held cameras are particularly convenient, and

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13 A visible image of electrical equipment, b infrared thermogram showing the hot cable to the left of the image

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often used in conjunction with the par-focal visible light camera. 4.3

Thermal imaging in industry

The applications in industry are so wide that it is impossible to name all here in this brief overview. In the electrical industry, infrared microscopic images are used to assess circuits and hand-held cameras play an important role in checking hot spots indicating faulty fuses or connections (Fig. 13). This is particular important in insurance and safety surveys of industrial and faming buildings where fire risk must be regularly checked. In the power industry, aerial surveillance of overhead power lines for faulty insulators or junctions is common, and in many industries where furnaces or boilers are used, thermal inspection of insulation is of great importance to the continuance of production. Energy conservation generally is a field where thermal imaging is of increasing value, and building surveys both interior and exterior provide useful information on thermal leakage or the need for insulation to conserve heat. Other uses of aerial surveillance with thermal imaging include heat effluence from power stations, and industrial waste into rivers and waterways (Fig. 14). A number of manufacturing industries use thermal imaging to monitor temperature from paper mills to motorcars. Military intelligence, which has provided valuable advances in research and development, continues to use imaging often as a means of night vision, but also in the search and rescue services. Media reports have shown how army and fire service services have saved lives from the rubble of earthquakes or bomb damage to inhabited buildings. Thermal imaging can also be used to identify human bodies in dense smoke that would otherwise be invisible to the human eye. Some of the widespread applications of thermal imaging can be found in specialist conference proceedings.12

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CONCLUSION

Modern-day thermal imaging has developed, largely over the last 50 years to a point where a false colour image representing heat distribution is now widely recognized, and in fact used in public advertising. The concept of being able to visualize heat patterns in this way is greatly enhanced by modern computer


technology and image processing. Infrared thermal imaging has brought a new dimension to scientific imaging in a number of fields, particularly where noncontact temperature mapping is required. The portability of modern cameras and the use of infrared transmitting lenses have greatly increased the use of this technique.

REFERENCES 1 Ring, E. F. J. The discovery of infrared radiation in 1800. Imag. Sci. J., 2000, 48, 1–8. 2 Wood, R. Photography by invisible rays. Photogr. J., 1910, 50, 329–338. 3 Norton, P., Horn, S., Pellegrino, J. G. and Perconti, P. In Medical Devices and Systems (Ed. J. D. Bronzino), 2006, pp. 37-1–37-26 (Taylor & Francis, Boca Raton, FL). 4 Elliott, C. T. New detector for thermal imaging systems. Electron. Lett., 1981, 17, 312. 5 Cuthbertson, G. M. In The Thermal Image in Medicine and Biology (Ed. K. Ammer and E. F. J. Ring), 1995, pp. 21–32 (Uhlen Verlag, Vienna). 6 Ring, E. F. J. The historical development of temperature measurement in medicine. Infrared Phys. Technol., 2007, 49, 297–301. 7 Ring, E. F. J. and Ammer, K. In The Biomedical Handbook, 3rd edition, Medical Devices and Systems (Ed. J. D. Bronzino), 2006, pp. 36.1–36.14 (CRC Press, New York). 8 Ring, E. F. J., Hartmann, J., Ammer, K., Thomas, R., Land, D. and Hand, J. W. In Experimental Methods in the Physical Sciences, Vol. 43, Radiometric Temperature Measurements (Ed. B. Tsai, Z. Zhang and G. Machin), 2010, pp. 393–448 (Elsevier, Amsterdam). 9 Mercer, J. B. and Ring, E. F. J. Fever screening and infrared thermal imaging: concerns and guidelines. Thermol. Int., 2009, 19, 67–69. 10 Colles, C., Holah, G. and Pusey, A. In The Thermal Image in Medicine and Biology (Ed. K. Ammer and E. F. J. Ring), 1995, pp. 164–l67 (Uhlen Verlag, Vienna). 11 Tunley, B. V. and Henson, F. M. D. Reliability and repeatability of thermographic examination and the normal thermographic image of the thoracolumbar region in the horse. Equine Vet. J., 2004, 36, 306–312. 12 Rogalski A. Multispectral Infrared Detector Arrays Procdeedings of V111th Conference on Thermography and Thermometry in Practice TP 2009. Ustron, Poland. 43–60. Politechnika Lodz. 2009 ISBN 978-83-7283332-7.

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