A silver halide is one of the compounds formed between silver and one of the halogens — silver bromide (AgBr), chloride (AgCl), iodide (AgI), and three forms of silver fluorides.
As a group, they are often referred to as the silver halides, and are
often given the pseudo-chemical notation AgX. Although most silver
halides involve silver atoms with oxidation states of +1 (Ag+), silver halides in which the silver atoms have oxidation states of +2 (Ag2+) are known, of which silver(II) fluoride is the only known stable one.
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.[1]
When a silver halide crystal is exposed to light, a sensitivity speck on the surface of the crystal is turned into a small speck of metallic silver (these comprise the invisible or latent image). If the speck of silver contains approximately four or more atoms, it is rendered developable - meaning that it can undergo development
which turns the entire crystal into metallic silver. Areas of the
emulsion receiving larger amounts of light (reflected from a subject
being photographed, for example) undergo the greatest development and
therefore results in the highest optical density.
F.E.Sullivan has been shooting people, places and things since 2001. He lives in Bakersfield, Ca.
