changed from a negative charge to a neutral one, releasing an electron that then changes the charge of the silver from a positive one to a neutral one. 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 a latent image. In such a case, many of the photoelectrons will recombine with the silver halide crystal and be wasted. 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. Gold sensitization of Koslowski creates metallic gold specks on the crystal surface, which by itself does not render the crystal developable. When a 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 photoholes away from photoelectrons and sensitivity sites. This should reduce the probability of recombination. Reduction sensitization is one possible implementation of this concept. The recent 2-electron sensitization technique is built on this concept. However, the scientific understanding of the behavior of photoholes is more limited 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. Reciprocity Law Failure Reciprocity law failure is a phenomenon where the same amount of exposure (irradiance multiplied by duration of exposure) produces different image density when the irradiance (and thus duration) is varied. There are two kinds of reciprocity failure. They are both related to poor efficiency of utilizing photoelectrons to create latent image centers. High intensity reciprocity failure (HIRF) High intensity reciprocity failure (HIRF) is common when the crystal is exposed by intense but brief light, such as flash tube. This reduces photographic speed and contrast. This is common with emulsions optimized for highest