Silver halide

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.

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 a silver metal speck.

[1] Silver halides are also used to make corrective lenses darken when exposed to ultraviolet light (see photochromism).

[3] Examples of compounds that increase the solubility include: cyanide, thiocyanate, thiosulfate, thiourea, amines, ammonia, sulfite, thioether, crown ether.

For example, the Arctic World Archive uses film developed with silver halides[4] to store data of historical and cultural interest, such as a snapshot of the Open Source code in all active GitHub repositories as of 2020[update].