A sensitivity speck or sensitivity center is an imperfection or other specific point in a silver halide crystal which traps electrons, causing photosensitivity.
This can produce a latent image in the crystal, having applications in photography[1] and dosimetry.
[2] A sensitivity speck is very often the site of shallow electron traps, such as crystalline defect (particularly edge dislocation) and silver sulfide specks created by sulfur sensitization process.
When a photon is absorbed by a silver halide crystal, a free-carrier (electron in the conduction band) is generated.
Subsequent exposure can grow the size of silver cluster through the same mechanism.