Latent Image - Photographic Sensitivity

Photographic Sensitivity

One very important way to increase photographic sensitivity is to manipulate the electron traps in each crystal. A pure, defect-free crystal exhibits poor photographic sensitivity, since it lacks a shallow electron trap that facilitates the formation of latent image. In such a case, much of the photoelectrons will be wasted by recombination mechanism. Shallow electron traps are created by sulfur sensitization, introduction of a crystalline defect (edge dislocation), and incorporating a trace amount of non-silver salt as a dopant. The location, kind and number of shallow traps have a huge influence on the efficiency by which the photoelectrons create latent image centers, and consequently, on photographic sensitivity.

Another important way to increase photographic sensitivity is to reduce the threshold size of developable latent image. Gold sensitization of Koslowski creates a metallic gold specks on crystal surface, which by itself does not render the crystal developable. When latent image is formed around the gold speck, the presence of gold is known to reduce the number of metallic silver atoms necessary to render the crystal developable.

Another important concept in increasing photographic sensitivity is to separate photohole away from photoelectrons and sensitivity sites. This should reduce the probability of recombination. Reduction sensitization is one possible implementation of this concept. Recent 2-electron sensitization technique is built on this concept. However, the scientific knowledge on the behavior of photoholes is less well understood than that of photoelectrons.

On the other hand, a deep electron trap or a site that facilitates recombination will compete for photoelectrons and therefore reduces the sensitivity. However, these manipulations are used, for example, to enhance contrast of the emulsion.