Page "Silver halide" Paragraph 2

Silver halides are used in photographic film and photographic paper, including graphic art film and paper, where silver halide crystals in gelatin are coated on to a film base, glass or paper substrate.

The gelatin is a vital part of the emulsion as the protective colloid of appropriate physical and chemical properties.

Gelatin may also contain trace elements ( such as sulfur ) which increase the light sensitivity of the emulsion, although modern practice uses gelatin without such components.

When absorbed by an AgX crystal, photons cause electrons to be promoted to a conduction band ( de-localized electron orbital with higher energy than a valence band ) which can be attracted by a sensitivity speck, which is a shallow electron trap, which may be a crystalline defect or a cluster of silver sulfide, gold, other trace elements ( dopant ), or combination thereof, and then combined with an interstitial silver ion to form silver metal speck.