Editing Latent image (section)

{{Short description|Invisible image produced by the exposure of a photosensitive material to light}}
{{other uses}}
{{more citations needed|date=November 2015}}
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 [[Photographic processing|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 [[Photographic emulsion|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]] [[atom]]s formed in or on a silver halide crystal due to [[Redox|reduction]] of interstitial silver ions by [[photoelectron]]s (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.

[[File:Latent Image.png|thumb|"Printed out" image on a 35mm B&W film, overexposed by approximately 24 stops (about two days of exposure at f/2), without any chemical processing, showing that the silver clusters can grow up to visible sizes without developing.]]

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.