Sensitivity speck
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A sensitivity speck is an imperfection (dislocation) in the crystal lattice of silver halide (usually silver bromide), where a positively charged cluster of silver ions is exposed to the crystal surface. When the silver halide grain is exposed to light, the photons interact with the halide anions and knock out their electrons. The electrons then travel through the lattice, until they are captured by the dislocation. There they recombine with silver ions, forming neutral silver atoms. Tiny clusters of these silver atoms form the latent image.
If such speck of silver contains approximately four or more atoms, corresponding to an absorption of four or more photons, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver.
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