A type of printing plate used in offset lithography which utilizes a film- or metal-based plate coated with a silver halide emulsion (similar to that used in photographic film) to allow transfer of the image to be printed to the surface of the plate. Silver halide plates can be etched using photographic negatives, which are brought into contact with the surface of the plate and then exposed to light, which only exposes the plate surface in the image areas. After exposure, the plate needs to be developed. Unlike some other types of plates which can be developed in regular, ambient light, silver halide plates are sensitive to blue light, so developing of the plates needs to be performed in a darkroom equipped with yellow-filtered light. Newer varieties of silver halide plates allow etching to be performed by lasers, allowing direct computer-to-plate platemaking. A drawback to silver halide plates involves their processing chemicals which contain heavy metals (in particular, silver) which, due to pollution regulations, need to be specially treated or sent to special treatment plants. Silver halide plates used for monocolored forms and other documents are film-based, while those used for color printing (either spot color or process color) are aluminum-based, and are commonly etched using digital information. (See Plate: Offset Lithography.)
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