Location of Latent Image
Depending on the silver halide crystal, the latent image may be formed inside or outside of the crystal. Depending on where the LI is formed, the photographic properties and the response to developer vary. Current emulsion technology allows very precise manipulation of this factor by a number of ways.
Each emulsion has a place within each crystal where LI's are formed preferentially. They are called "sensitivity centers." Emulsions that form LI 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 dislocation is 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 center built into the core of the emulsion, which is bleached by photoholes generated upon exposure. This type of emulsion produces positive image upon development in a conventional developer, without reversal processing.
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