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image converterSee image tube.
a phototube for the conversion of an infrared, ultraviolet, or X-ray image of an object, which is invisible to the eye, into a visible image; it may also serve to increase (intensify) the brightness of a visible image (image intensifier). Its operation is based on the conversion of an optical or X-ray image into an electron image by means of a photocathode, with the subsequent conversion of the electron image into a visible image, which is produced on a fluorescent screen (seeCATHODE LUMINESCENCE and PHOSPHORS).
In an image converter (Figure 1) an image of an object is projected by an objective onto a photocathode (with X rays the shadow image is projected directly onto the photocathode). Radiation from the object causes photoemission on the surface of the photocathode; the photoemission from different parts of the photocathode varies according to the brightness distribution of the projected image. The photoelectrons are accelerated by an electric field in the region between the photocathode and the screen and are then focused with an electric and/or magnetic field (constituting an electron lens), after which they bombard the screen, thus exciting luminescence. The luminous intensity of individual points on the screen depends on the flux density of the photoelectrons so that a visible image of the object appears on the screen. Image converters are divided into single-stage and multistage (cascade) types; the latter have a tandem arrangement of two or more single-stage converters, such that the luminous flux from the screen of the first stage is directed onto the photocathode of the second and so on.
The principal parameters of an image converter are the total sensitivity, resolution, and conversion efficiency. The total sensitivity is the ratio of the photoelectric current to the intensity of the radiation incident on the photocathode; it is mainly determined by the properties of the photocathode used. For example, in an image converter having a cesium oxide-silver photocathode, which is used to convert an infrared image (at wavelengths between 0.78 and 1.5 micrometers), the total sensitivity may reach 70 microamperes per lumen; multialkali photocathodes made of compounds of Sb with Cs and of Sb with K and Na are used to intensify the brightness of a visible image and provide a total sensitivity up to 103 microamperes per lumen. The resolution is determined by the maximum number of resolvable lines in the image on a portion of the screen 1 mm long; it ranges from 25 to 60 or more lines per mm. The conversion efficiency is the ratio of the luminous flux radiated by the screen to the radiant flux from the object incident on the photocathode; the conversion efficiency of single-stage image converters is several thousand, and multistage converters may exhibit an efficiency of 106 or higher.
The principal disadvantages of multistage image converters are poor resolution and relatively high dark noise, which degrade the image quality. The latter drawback is eliminated in converters that have a microchannel amplifier, as proposed in 1940 by the Soviet engineer 1. F. Pes’iatskii. Such converters have a glass plate, perforated by a multitude of minute channels 15–25 micrometers in diameter, which is placed in the path of the photoelectrons. The inner walls of the microchannels are coated with a material having a high secondary-emission ratio. With a voltage of several kilovolts applied to the plates, the photoelectrons that enter the microchannels are accelerated to energies sufficient to produce secondary emission from the microchannel walls, thus permitting the original electron stream to be intensified by a factor of 105–106. The electrons from each microchannel strike a corresponding point on the screen to create a visible image. No electron focusing is needed in this type of image converter.
The Soviet scientists P. V. Timofeev, V. V. Sorokina, M. M. Butslov, and others have made important contributions to the development of various kinds of image converters.
I. F. USOL’TSEV
Image converters are used in infrared technology, spectrosco-py, medicine, microbiology, motion-picture technology, nuclear physics, and other branches of science and technology. In the late 1940’s infrared image converters with a wavelength sensitivity extending to 1.1 micrometers were used to photograph the spectrum of the night sky and the invisible region in the central part of the Galaxy; the research led to the general use of image converters in astronomy.
Modern multistage image converters can register on a photoe-mulsion the light flashes (scintillations) from a single electron emitted by the input photocathode. In observations of faint (weakly radiating or illuminated) celestial objects, they are also capable of accumulating the signals from the flashes in a computer memory. Some spectroscopic instruments operate on this principle to record simultaneously approximately 1,000 elements of the spectrum of a celestial body and an equal number of elements of comparison spectra. The ability to accumulate information is limited in practice by the memory capacity of the computer. Such instruments have a substantial advantage for observing faint objects against the background glow of the night sky. The advantage is proportional to , where η is the ratio of the number of photoelectrons emitted by the receiver to the number of incident quanta and t is the accumulation period. With such instruments the images obtained by several telescopes can be added together.
In some types of image converters the image is registered on a matrix of electron-sensitive elements (from ten to 100 in number) that is used in place of a luminescent screen.
P. V. SHCHEGLOV
REFERENCESZaidel’, I. N., and G. I. Kurenkov. Elektronno-opticheskie preobrazovateli. Moscow, 1970.
Kozelkin, V. V., and I. F. Usol’tsev. Osnovy infrakrasnoi tekhniki, 2nd ed. Moscow, 1974.
Kurs astrofiziki i zvezdnoi astronomii, 3rd ed., vol. 1. Edited by A. A. Mikhailov. Moscow, 1973.
Shcheglov, P. V. Elektronnaia leleskopiia. Moscow, 1963.