A latent image is an invisible image produced by the exposure to light of a photosensitive material such as photographic film. When photographic film is developed, the area that was exposed darkens and forms a visible image. In the early days of photography, the nature of the invisible change in the silver halide crystals of the film's emulsion coating was unknown, so the image was said to be "latent" until the film was treated with photographic developer. In more physical terms, a latent image is a small cluster of metallic silver atoms formed in or on a silver halide crystal due to reduction of interstitial silver ions by photoelectrons (a photolytic silver cluster). If intense exposure continues, such photolytic silver clusters grow to visible sizes. This is called printing out the image. On the other hand, the formation of a visible image by the action of photographic developer is called developing out the image.
The size of a silver cluster in the latent image can be as small as a few silver atoms. However, in order to act as an effective latent image center, at least four silver atoms are necessary. On the other hand, a developed silver grain can have billions of silver atoms. Therefore, photographic developer acting on the latent image is a chemical amplifier with a gain factor up to several billion. The development system was the most important technology that increased the photographic sensitivity in the history of photography.
Mechanism of formation The action of the light on the silver halide grains within the emulsion forms sites of metallic silver in the grains. The basic mechanism by which this happens was first proposed by R. W. Gurney and N. F. Mott in 1938. The incoming photon liberates an electron, called a photoelectron, from a silver halide crystal. Photoelectrons migrate to a shallow electron trap site (a sensitivity site), where the electrons reduce silver ions to form a metallic silver speck. A positive hole must also be generated, but it is largely ignored. Subsequent work has slightly modified this picture, so that "hole" trapping is also considered (Mitchell, 1957). Since then, understanding of the mechanism of sensitivity and latent image formation has been greatly improved. A latent image is formed when light changes the charge atoms in the molecule. Taking bromine as a halide for this example, when light hits a silver halide molecule, the halide is 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 sensitivity with long exposure using old emulsion technology. HIRF is due to creation of many latent subimages that are not developable due to small size. Because of brief and intense exposure, many photoelectrons are created simultaneously. They make many latent subimages (that cannot render the crystal developable), rather than one or a few latent images (that can). HIRF can be improved by incorporating dopants that create temporary deep electron traps, optimizing the degree of sulfur sensitization, introducing crystalline defects (edge dislocation). In recent years, many photographic prints are made by scanning laser exposure. Each location on a photographic paper is exposed by a very brief but intense laser. Problems due to HIRF were the major technical challenge in development of such products. Color photographic papers are usually made with very high percentage of silver chloride (about 99%) and the rest is bromide and/or iodide. Chloride emulsions have particularly poor HIRF and usually suffer from LIRF. Paper manufacturers use dopants and precise control of the dislocation sites to improve (to virtually eliminate) HIRF for this new application.
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