centers." Emulsions that form LIs in the interior are called internal(ly) sensitive emulsions, and those that form LI on the surface are called surface sensitive emulsions. 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. Because multiple sensitivity centers are present, the emulsion had both internal and surface sensitivity. That is, photoelectrons may migrate to one of many sensitivity centers. 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. Many modern negative emulsions introduce a layer just under the crystal surface where a sufficient number of edge dislocations are intentionally created, while maintaining the bulk of the crystal interior defect-free. Chemical sensitization (e.g., sulfur plus gold sensitization) is applied on the surface. 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. Emulsions with different structures were made for other applications, such as direct positive emulsions. Direct positive emulsion has fog centers built into the core of the emulsion, which is bleached by photoholes generated upon exposure. This type of emulsion produces a positive image upon development in a conventional developer, without reversal processing. Development of silver halide crystals A 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 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