The light-sensitive recording materials of photography, that is, photographic films, plates, and papers. They consist primarily of a support of plastic sheeting, glass, or paper, respectively, and a thin, light-sensitive layer, commonly called the emulsion, in which the image will be formed and stored. The material will usually embody additional layers to enhance its photographic or physical properties.
Film support, for many years made mostly of flammable cellulose nitrate, is now exclusively made of slow-burning “safety” materials, usually cellulose triacetate or polyester terephthalate, which are manufactured to provide thin, flexible, transparent, colorless, optically uniform, tear-resistant sheeting. Film supports usually range in thickness from 0.0025 to 0.009 in. (0.06 to 0.23 mm) and are made in rolls up to 60 in. (1.5 m) wide and 6000 ft (1800 m) long. See also Ester; Polyester resins.
Glass is the predominant substrate for photographic plates, though methacrylate sheet, fused quartz, and other rigid materials are sometimes used. Plate supports are selected for optical clarity and flatness. Thickness, ranging usually from 0.04 to 0.25 in. (1 to 6 mm), is increased with plate size as needed to resist breakage and retain flatness. See also Glass.
Photographic paper is made from bleached wood pulp of high α-cellulose content, free from ground wood and chemical impurities. It is often coated with a suspension of baryta (barium sulfate) in gelatin for improved reflectance and may be calendered for high smoothness. Fluorescent brighteners may be added to increase the appearance of whiteness. See also Paper.
Most emulsions are basically a suspension of silver halide crystals in gelatin. The crystals, ranging in size from 2.0 to less than 0.05 micrometers, are formed by precipitation by mixing a solution of silver nitrate with a solution containing one or more soluble halides in the presence of a protective colloid. During manufacture, the emulsion is ripened to control crystal size and structure. Chemicals are added in small but significant amounts to control speed, image tone, contrast, spectral sensitivity, keeping qualities, fog, and hardness; to facilitate uniform coating; and, in the case of color films and papers, to participate in the eventual formation of dye instead of metallic silver images upon development. The gelatin, sometimes modified by the addition of synthetic polymers, is more than a simple vehicle for the silver halide crystals. It interacts with the silver halide crystals during manufacture, exposure, and processing and contributes to the stability of the latent image. See also Emulsion; Gelatin; Silver.
The silver halides (and silver behenates) are normally sensitive only to x-radiation and to ultraviolet, violet, and blue wavelengths, but they can be made sensitive to longer wavelengths by adding special dyes, predominantly polymethines, to the emulsion. The process is known as spectral sensitizing to distinguish it from the chemical sensitizing used to raise the overall or inherent sensitivity of the grains. See also Photography.
