In more physical terms, 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 silver cluster in the latent image can be as small as a few silver atoms. On the other hand, a developed silver grain can have billions of silver atoms. Therefore, photographic developer is a chemical amplifier acting on the latent image, 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.
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 latent image. In such a case, much of the photoelectrons will be wasted by recombination mechanism. 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 image. Gold sensitization of Koslowski creates a metallic gold specks on crystal surface, which by itself does not render the crystal developable. When 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 photohole away from photoelectrons and sensitivity sites. This should reduce the probability of recombination. Reduction sensitization is one possible implementation of this concept. Recent 2-electron sensitization technique is built on this concept. However, the scientific knowledge on the behavior of photoholes is less well understood 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 is a phenomenon that 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.
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 defect (edge dislocation).
In recent years, many photographic prints are made by scanning laser exposure. Each location on a photographic paper is exposed by 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 HIRF. Paper manufacturers use dopants and precise control of the dislocation sites to improve (to virtually eliminate) HIRF for this new application.
LIRF is due to inefficiency of forming a latent image, and this reduces photographic speed but increases contrast. Due to low level of exposure irradiance (intensity), a single crystal may have to wait for a significant amount of time between absorbing sufficient number of photons. In the process of making a stable latent image center, a smaller and less stable silver speck is made. Further generation of photoelectrons is necessary to grow this small speck to a larger, stable, latent image. There is a positive probability that this intermediate unstable speck will decompose before next available photoelectrons can stabilize it. This probability increases with decreasing irradiance level.
LIRF can be improved by optimizing the stability of latent subimage, optimizing sulfur sensitization, and introduction of crystalline defect (edge dislocation).
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 in in 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.
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, 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 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 determining the developability of the crystal.
One way to improve this problem is the use of gold sensitization 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.
A famous instance of latent-image stability is the picture taken of the ill-fated balloon expedition of Salomon Andree and his party to the North Pole in 1897. The pictures of the expedition and of the balloon stranded on the ice were not discovered and developed until some 33 years later (see Coe, ch 10 for picture).