In black-and-white photographic film, there is usually one layer of silver halide crystals. By similar techniques, special-purpose films can be made sensitive to the infrared (IR) region of the spectrum. Panchromatic film renders all colors in shades of gray approximately matching their subjective brightness. First orthochromatic (sensitive to blue and green) and finally panchromatic (sensitive to all visible colors) films were developed. This problem was resolved with the discovery that certain dyes, called sensitizing dyes, when adsorbed onto the silver halide crystals made them respond to other colors as well. Unmodified silver halide crystals are sensitive only to the blue part of the visible spectrum, producing unnatural-looking renditions of some colored subjects.

In addition to visible light, all films are sensitive to ultraviolet light, X-rays, gamma rays, and high-energy particles.

This creates an invisible latent image in the emulsion, which can be chemically developed into a visible photograph. Instead, a very short exposure to the image formed by a camera lens is used to produce only a very slight chemical change, proportional to the amount of light absorbed by each crystal. The emulsion will gradually darken if left exposed to light, but the process is too slow and incomplete to be of any practical use. The sizes and other characteristics of the crystals determine the sensitivity, contrast, and resolution of the film.

Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals.

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