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 elucidated by R W Gurney and N F Mott in 1938. The incoming photon liberates an electron from a halide ion. This electron migrates to a trap site (a latent image site). This in turn attracts a silver ion, which gains the electron to become a metallic atom.

Subsequent work has slightly modified this picture, so that 'hole' trapping rather than electron trapping is now seen as important (Mitchell, 1957)

Under normal conditions the latent image, which typically comprises only 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 optimised to work most quickly where silver atoms are already present on the grain).

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)

References

Coe, Brian, 'The Birth of Photography' Ash & Grant, 1976

Mitchell, J.W 'Photographic Sensitivity' Rep. Prog. Phys., v20, pp433-515, 1957