Infrared Photography

infrared photography [¦in·frə¦red fə′täg·rə·fē]

(graphic arts)

Photography in which an infrared optical system projects an image directly on infrared film, to provide a record of point-to-point variations in temperature of a scene.

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Infrared Photography 

(also IR photography), the production of photographs using infrared radiation. Photographs can be taken in IR radiation in several ways. The simplest method is that of direct photography, using photographic plates or film that are sensitive to IR radiation. In this case a light filter that passes IR radiation but is opaque to visible light is mounted on the camera lens. Modern materials for infrared photography have a sensitivity threshold in the long-wave region of λ = 1.2 microns (μ).

The sensitivity of infrared film and plates is relatively low; therefore, at low illumination intensities apparatus consisting of an image converter and a standard photographic camera is used for infrared photography. The image converter, which is placed in front of the camera lens, converts the invisible infrared image into a visible image, simultaneously increasing its brightness. Such apparatus makes possible the production of photographs on standard film in complete darkness and with a low-powered IR radiation source. The sensitivity threshold in the long-wave region is dependent on the photocathode of the image converter and does not exceed λ = 1.2 λ.

Special apparatus makes possible the production of infrared photographs in the region where λ > 1.2 η. One such device, the infrared vidicon, is a television system in which the screen of the camera tube is made of photoconductive semiconductor materials, which change their electrical conductivity under the influence of infrared radiation. A visible television image appearing on the screen of the picture tube is then photographed by a standard camera. The long-wave limitations of the vidicon depend on the nature of the material of the photoconductive screen and on its temperature; at T = 79°K (with liquid nitrogen cooling) the limit is λ ≈ 5 μ; at T = 21°K (with liquid hydrogen cooling), λ ≈ 20 μ.

Infrared photography makes it possible to obtain additional data about an object (as compared to photography in visible light or visual observation). When passing through haze or fog, infrared radiation is dispersed to a lesser degree than visible radiation, making possible the production of sharp photographs of remote objects over distances of hundreds of kilometers. Because of the difference in reflection and transmission coefficients between the visible range and the infrared range, infrared photographs show details invisible to the eye on standard photographs. These particular features of infrared photography are widely used in botany for studies of plant diseases, in medicine during diagnosis of skin and vascular diseases, in criminology for detection of forgeries, in infrared aerial surveying, and in astronomy for photographing stars and nebulas. Infrared photographs can be taken in complete darkness.

Devices are also made that are capable of recording thermal IR radiation by an object whose temperature is different at different points. The intensity of IR radiation at each point is recorded by the receiver and is converted into a light signal, which is in turn recorded on photographic film. The image produced in this case is not an infrared photograph in the usual sense, since it depicts only the temperature distribution on the surface of the object. Such devices are used in detecting overheated areas of machines and in infrared aerial surveying to produce a thermal map of a locality.

REFERENCE

Clark, W. Photography by Infrared, 2nd ed. New York, 1946.

V. I. MALYSHEV