Taking a Picture: Exposure Chemistry

­So, either manually or automatically, you now have an image that is focused on the film surface, and the proper exposure has been set through a combination of film speed, aperture settings (f-stop) and exposure time (usually fractions of a second, from one thirtieth to one one-thousandth of a second). Say cheese and push the button. What happened? While outwardly unexciting, the moment of exposure is when a lot of photochemistry happens.

By opening the camera's shutter for a fraction of a second, you formed a latent image of the visible energy reflected off the objects in your viewfinder. The brightest portion of your picture exposed the majority of the silver-halide grains in that particular part of the film. In other parts of the image, less light energy reached the film, and fewer grains were exposed.

When a photon of light is absorbed by the spectral sensitizer sitting on the surface of a silver-halide grain, the energy of an electron is raised into the conduction band from the valence band, where it can be transferred to the conduction band of the silver-halide-grain electronic structure. A conduction-band electron can then go on to combine with a positive hole in the silver-halide lattice and form a single atom of silver. This single atom of silver is unstable. However, if enough photoelectrons are present at the same time in the crystal lattice, they may combine with enough positive holes to form a stable latent-image site. It is generally felt that a stable latent-image site is at least two to four silver atoms per grain. A silver-halide grain contains billions of silver-halide molecules, and it only takes two to four atoms of uncombined silver to form the latent-image site.

In color film, this process happens separately for exposure to the red, green and blue portions of the reflected light. There is a separate layer in the film for each color: Red light forms a latent image in the red-sensitive layer of the film; green light forms a latent image in the green-sensitive layer; blue light forms a latent image in the blue-sensitive layer. The image is called "latent" because you can't detect its presence until the film is processed. The true photoefficiency of a film is measured by its performance as a photon detector. Any photon that reaches the film but does not form a latent image is lost information. Modern color films generally take from 20 to 60 photons per grain to produce a developable latent image.