There is a large variety of photochemical processes whereby the action of light serves to darken the material in which it is absorbed. Many of these processes have no photographic gain in the sense that one photon affects at most one molecule of the absorbing material. The old blueprint papers are of this character. The ordinary, but potentially photographic, process of acquiring a suntan is of the same kind. Other and more useful photochemical processes make use of a much smaller optical exposure that produces only a latent (invisible) image which can then be developed to produce a visible image. The most popular and versatile of these processes makes use of micron-sized silver halide crystals imbedded in a gelatin matrix. The silver bromide is sensitized to various parts of the visible spectrum by the adsorption of a wide variety of organic dyes. The gain of the development process is in the order of 109, that is, 109 silver atoms are generated per “usefully” absorbed photon.
KeywordsZinc Recombination Bromide Selenium Retina
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