How does photographic film give such good images with so little exposure to light? The answer, in a word, is amplification. Light causes clusters of silver atoms to form on silver halide microcrystals in a photographic emulsion. These clusters, which may contain as few as three or four silver atoms, catalyze the reduction of all the silver ions in the microcrystals to which they are attached, giving a “gain” of over a billion. This is the basis for the high sensitivity and image quality of silver halide photographic film. As I will explain in this article, much is known about the physical mechanisms involved, and by using this knowledge one can improve the sensitivity and efficiency of film.
The high sensitivity and efficiency of silver halide film can be traced to mesoscopic metal clusters—small groups of silver atoms subject to quantum size effects and the influence of their immediate environment.