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photographic emulsionA light-sensitive layer of silver halide crystals suspended in gelatin and coated on a flexible or rigid base, from which a permanent record of an optical or near-ultraviolet image, focused on the surface, may be produced. Both black-and-white and color films or plates are used in astronomy. Emulsions highly sensitive to infrared and ultraviolet radiation and to X-rays are also now available.
The sensitivity of a particular type of emulsion to light or other radiation is known as its speed. This is usually expressed numerically as its ISO number; very high speed color and black-and-white emulsions are commercially available and are suitable for astronomical work. The sensitivity of an emulsion can be increased by hypersensitization; this reduces the required exposure time or improves the image quality, sometimes both.
Photographic emulsions have high resolution, wide spectral response, and good dimensional stability and permanence, and they produce images with a reasonably high signal/noise ratio. The light from a star, well below the visual threshold, can be recorded during a long exposure: exposures of over an hour may be used. Emulsions are, however, inefficient detectors compared to electronic recording devices – they can record only a very small number of the incident photons of radiation. In addition, a linear relationship between intensity of incident radiation and resulting image density exists for only a narrow range of exposures (‘exposure’ here means intensity of illumination times exposure time); at shorter or longer exposures the relationship is nonlinear and the emulsion suffers from reciprocity failure. See also imaging; photography.
a suspension of photosensitive silver halide microcrystals, called grains, uniformly distributed in gelatin or another protective colloid, such as cellulose derivatives, albumin, or polyvinyl alcohol. The term is also applied to a dry photosensitive layer consisting of a film of dry gelatin gel containing silver halide microcrystals. The microcrystals present in photographic emulsions are well-formed cubic or cuboctahedral crystals of the following sizes: 0.01–0.02 micrometer (especially fine-grained nuclear photographic emulsion), 0.2–0.3 micrometer (high-speed photographic emulsions), and more than 0.5 micrometer (X-ray emulsions). The photographic sensitivity of an emulsion increases with an increase in the dimensions of the microcrystals, although grain size also increases.
Emulsion additives that provide various desired properties include tanning agents (chromium acetate, chromium potassium sulfate, and others), plasticizers (glycerin and ethylene glycol), spectral sensitizing dyes (usually polymethine dyes), stabilizers (trisazoindolizine derivatives and others), antioxidants (pyrocatechol), antiseptics (phenol and chloromethyl phenol), antifogging agents (potassium bromide and others), and surfactants. The use of such additives allows the preparation of photographic emulsions for the production of a wide range of photographic materials that differ in overall and spectral sensitivity, gradation, and structurometric characteristics.
The manufacture of photographic emulsions consists of the preparation of a silver halide suspension in a protective colloid medium with subsequent physical and chemical ripening. The silver halide is formed upon interaction of the halides of alkali metals or ammonium halides with silver nitrate (in the ammonia method, from silver ammoniate) in an aqueous gelatin solution. The crystallization process, whereby silver halide microcrystals of varying sizes are formed, occurs during the physical ripening stage. At the same time, as a result of the differences in solubility of small and large microcrystals, there occurs a gradual disappearance of small crystals with a simultaneous increase in the size of large crystals to some given value.
Chemical ripening is characterized by the adsorption of active microscopic impurities contained in the gelatin onto the surface of the silver halide microcrystals and by the formation of complex compounds between the impurities and the silver ions. The resulting unstable complexes decompose, which leads to the formation of dislocations in the crystal lattice structure. The sites of dislocations form centers of photosensitivity, which determine the fundamental photographic properties of the emulsion. (Upon exposure to light, the centers of photosensitivity change into centers of development, which constitute a latent photographic image.) After chemical ripening, additives are introduced into the emulsion, which is then prepared to be poured onto a backing.
REFERENCESKilinskii, I. M., and S. M. Levi. Tekhnologiia proizvodstva kinofotoplenok. Leningrad, 1973.
Chibisov, K. V. Khimiia fotograficheskikh emul’sii. Moscow, 1975.
Mees, C, and T. James. Teoriia fotograficheskogo protsessa. Leningrad, 1973. (Translated from English.)
V. S. CHELTSOV