infrared imaging

the formation of a visual image of an object by means of the object’s own thermal (infrared) radiation or the thermal radiation reflected from the object. Infrared imaging is used to determine the location and shape of objects in darkness or in optically opaque media and to study the degree of heating of individual sections of complex surfaces and the internal structure of bodies that are opaque in visible light. Every heated body emits thermal radiation whose intensity and spectrum depend on the body’s properties and temperature. Radiation in the infrared region of the electromagnetic spectrum is characteristic of bodies with temperatures of several tens of degrees Celsius. Infrared radiation is invisible to the human eye, but it can be detected by various thermal radiation detectors (seeRADIATION DETECTOR) and converted to a visual image by several methods.

The first infrared imaging systems were built in the late 1930’s and found some applications during World War II in detecting military and industrial objects; these systems used heat sensors—bolometers and thermocouples—that converted infrared radiation into electrical signals. By means of an optomechanical scanning system, discrete points on the object were successively projected to the sensor, and the electrical signals obtained from the sensor were fed to the input of a cathode-ray tube that was similar to a television picture tube; a visual image of the object was formed on the luminescent screen of the tube. Development of these systems continued in the 1970’s with the use of both heat sensors and cooled photoelectric sensors (for example, sensors based on InSb or HgCdTe₂), which are capable of sensing radiation with wavelengths down to 5–6 micrometers (the maximum thermal radiation at room temperature falls at wavelengths of approximately 10 micrometers); pyroelectrical detectors have also been used. These detectors have high sensitivity, commensurate with fluctuations in thermal radiation, which makes it possible to use them to obtain visual images of objects at distances up to 10–15 km and surface temperatures differing from the temperature of the surrounding medium by less than 1°C. Such infrared systems make it possible to detect temperature differences (to 0.1°C) in individual sections of the human body, which is of considerable interest in the early diagnosis of tumors and circulatory disorders.

In the late 1960’s and early 1970’s, fundamentally new and simpler infrared devices were developed, whose use is preferable whenever their sensitivity is adequate. In these devices, the thermal image of the object is projected directly (without intermediate conversion of the infrared radiation into electrical signals) onto a screen coated with a thin layer of a special substance. As the result of a certain physicochemical process taking place when it is heated, the substance changes its own optical characteristics—its coefficient of reflection or transmission of visible light, the intensity or color of its own luminescence, or the like. Visual images of the object may be observed and photographed on the device’s screen. Liquid crystals, crystal phosphors, thin films of semiconductors, magnetic thin films, and heat-sensitive lacquers and paints can be used as temperaturesensitive substances.

As liquid crystals are heated, they gradually change their color and hues from red to violet; multicomponent mixtures of choles-teric liquid crystals have a color-indication temperature interval of less than 0.1°C. Heat-sensitive paints change their color once or twice (usually irreversibly) when heated, thus recording one or two temperature values. This is convenient in cases where it is sufficient to know whether the object being investigated, such as a machine part, has been heated to a certain critical temperature. In certain semiconductor films (particularly films of Se and its derivatives), as the temperature rises the spectral region of transparency shifts in the direction of long waves; this makes it possible, by using an additional source of visible light, to detect changes of 1°–5°C in the temperature of the film.

The use of phosphors in infrared imaging is based on the phenomenon of the quenching of luminescence: the brightness of the luminescence of certain phosphors (for example, compounds of ZnS CdS·Ag Ni) excited by ultraviolet radiation diminishes sharply as the phosphors are heated. Such phosphors make it possible to observe temperature changes of 0.2°–0.3°C visually, and the quenching effect is completely reversible. Instruments that make use of phosphors permit imaging from radio waves as well as infrared waves (seeRADIO IMAGING).

When magnetic thin films are heated, there is a change in the orientation of the axes of magnetization of the magnetic domains; the domains, in turn, orient the ferromagnetic particles of the colloid solution applied to the film surface. This “magnetic relief,” which emerges under the effect of the heat rays on the magnetized film, becomes visible in ordinary reflected light where the film is magnetized. The infrared imaging techniques considered above have been applied in various devices.

Films of the substances mentioned above may also be applied directly to an object in order to study the temperature distribution over the object’s surface; this field of science is called thermography. In this case, however, it is the temperature of the object, not the object’s thermal radiation, that is detected. Another form of infrared imaging is the use of infrared lasers, for example, CO₂ gas lasers, with a wavelength of 10.6 micrometers, corresponding to the maximum thermal radiation at a temperature of 23°C; such lasers can be used to transilluminate objects that are opaque to visible light. This technique was developed in the 1970’s.

Infrared imaging is increasingly used in medicine and technical diagnosis, navigation, geological exploration, meteorology, flaw detection, research on thermal processes, and military affairs.