Editing Latent image (section)

==Development of silver halide crystals==
A [[photographic developer|developer solution]] converts silver halide crystals to metallic silver grains, but it acts only on those having latent image centers. (A solution that converts ''all'' silver halide crystals to metallic silver grains is called ''[[Fogging (photography)|fogging]] developer'' and such a solution is used in the second developer of reversal processing.) This conversion is due to electrochemical reduction, wherein the latent image centers act as a catalyst.

===Reduction potential of the developer===
A developer solution must have a reduction potential that is strong enough to develop sufficiently exposed silver halide crystals having a latent image center. At the same time, developer must have reduction potential that is weak enough not to reduce unexposed silver halide crystals.

In a suitably formulated developer, electrons are injected to the silver halide crystals only through silver speck (latent image). 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.

Generally, weakly exposed crystals have smaller silver clusters. Silver clusters of smaller sizes have a higher Fermi level, and therefore more crystals are developed as the developer's reduction potential is increased. However, again, the developer potential must be well below the conduction band of silver halide crystal. Thus there is a limit in increasing the photographic speed of the system by boosting the developer potential; if the solution's reduction potential is set high enough to exploit smaller silver cluster, at some point the solution begins to reduce silver halide crystals regardless of exposure. This is called [[fog]], which is metallic silver made from non-imagewise (exposure-nonspecific) reduction of silver halide crystals. It was also found that, when developer solution is optimally formulated, the maximum photographic speed is rather insensitive to the choice of developing agent (James 1945), and there exists a limit for the size of silver cluster that can be developed.

One way to improve this problem is the use of the gold sensitization technique of Koslowski. A small metallic gold cluster whose Fermi level is high enough to prevent development of the crystal is used to decrease the threshold size of metallic silver cluster that can render the crystal developable.

For further discussion, refer to Tani 1995 and Hamilton 1988.