Latent image

In photography a latent image is formed when light (or in radiography, X-rays) acts on a photographic emulsion. This image is invisible until the emulsion is developed using photographic developers, such as Metol.

The action of the light with the silver halide grains within the emulsion forms sites of metallic silver on 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 from a silver halide molecule. This electron migrates to a shallow electron trap site (a sensitivity site), where the electron reduces a silver ion to form a metallic silver speck. In the above picture, positive hole must also be generated but it is largely ignored. Subsequent work has slightly modified this picture, so that 'hole' trapping is also called into question (Mitchell, 1957). Since then, the mechanism of sensitivity and latent image formation has been greatly improved.

Under normal conditions the latent image, which may be as small as a few atoms of metallic silver on each halide grain, is stable for many months. Subsequent development can then reveal a visible metallic image. (Photographic developers reduce the silver halide grains to silver, but are designed to work preferentially on silver halide crystals with a latent image centers present.

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, also at [1]).

References

Coe, Brian, The Birth of Photography, Ash & Grant, 1976
Mitchell, J.W., Photographic Sensitivity, Rep. Prog. Phys., v20, pp433-515, 1957

References

See also