Editing Latent image (section)

==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.

===Low-intensity reciprocity failure (LIRF)===
Low-intensity reciprocity failure (LIRF) occurs when the crystal is exposed with weak light of long duration, such as in astronomical photography.

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 speck to a larger, stable, latent image. There is a finite 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 defects (edge dislocation).