Photographic emulsion is a light-sensitive colloid used in film-based photography. Most commonly, in silver-gelatin photography, it consists of silver halide crystals dispersed in gelatin. The emulsion is usually coated onto a substrate of glass, films (of cellulose nitrate, cellulose acetate or polyester), paper, or fabric. The substrate is often flexible and known as a film base.
Photographic emulsion is not a true emulsion, but a suspension of solid particles (silver halide) in a fluid (gelatin in solution). However, the word emulsion is customarily used in a photographic context. Gelatin or gum arabic layers sensitized with dichromate used in the dichromated colloid processes carbon and gum bichromate are sometimes called emulsions. Some processes do not have emulsions, such as platinum, cyanotype, salted paper, or kallitype.