The light-sensitive layer on photographic film, plates and paper. It is a thin coating of gelatin that holds millions of tiny crystals of silver halide. When light hits the crystals, it forms an invisible image that the developer then makes visible. Color films have several emulsion layers. Gelatin emulsions replaced the wet collodion process in the 1870s and 1880s and made factory-made dry plates and later roll film possible.
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