Latent image
A pure, defect-free crystal exhibits poor photographic sensitivity, since it lacks a shallow electron trap that facilitates the formation of a latent image.
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 images.
On the other hand, a deep electron trap or a site that facilitates recombination will compete for photoelectrons and therefore reduces the sensitivity.
High-intensity reciprocity failure (HIRF) is common when the crystal is exposed by intense but brief light, such as flash tube.
HIRF can be improved by incorporating dopants that create temporary deep electron traps, optimizing the degree of sulfur sensitization, introducing crystalline defects (edge dislocation).
Color photographic papers are usually made with very high percentage of silver chloride (about 99%) and the rest is bromide and/or iodide.
Paper manufacturers use dopants and precise control of the dislocation sites to improve (to virtually eliminate) HIRF for this new application.
Low-intensity reciprocity failure (LIRF) occurs when the crystal is exposed with weak light of long duration, such as in astronomical photography.
LIRF is due to inefficiency of forming a latent image, and this reduces photographic speed but increases contrast.
Due to low level of exposure irradiance (intensity), a single crystal may have to wait for a significant amount of time between absorbing sufficient number of photons.
The sensitivity type largely reflects the site of very shallow electron traps that form latent images effectively.
Most, if not all, old technology negative film emulsions had many unintentionally created edge dislocation sites (and other crystalline defects) internally and sulfur sensitization was performed on the surface of the crystal.
In order to exploit the maximum sensitivity of such emulsions, it is generally considered that the developer must have some silver halide solvent action to make the internal latent image sites accessible.
As a result, the photoelectrons are concentrated to a few sensitivity sites on or very near the crystal surface, thereby greatly enhancing the efficiency with which the latent image is produced.
Therefore, it is very important for the chemical reduction potential of the developer solution (not the standard reduction potential of the developing agent) to be somewhere higher than the Fermi energy level of small metallic silver clusters (that is, the latent image) but well below the conduction band of unexposed silver halide crystals.
Under normal conditions the latent image, which may be as small as a few atoms of metallic silver on each halide grain, is stable for many months.
A famous instance of latent-image stability are the pictures taken by Nils Strindberg, the photographer in S. A. Andrée's ill-fated arctic balloon expedition of 1897.
