Photography today relies on the silver halides. Silver halides are compounds formed by the combination of metallic silver and one of a group of elements known as halogens. These are bromide, chlorine, iodine, and fluoride. The last is not used in photographic materials as it dissolves very easily in water. The principle one used is silver bromide. Every manufacturer makes a range of different emulsions for a variety of purposes. The slowest of these are designed to have inherently fine grain and produce the finest photographic quality. Medium speed emulsions have moderately fine grain and can be relied on to produce very satisfactory results in all but the most extreme conditions. The fastest films are designed with the aim of providing sufficient sensitivity to secure an image however poor the lighting conditions. The price paid for the highest sensitivity is the inherently large silver halide grain size with resulting loss of fine quality.
The silver salt’s spectral sensitivity can be made to vary by the addition of special dyes:
All normal emulsions are sensitive to ultra violet and blue. These are called 'blue sensitive’ or 'colour blind’ emulsions.
Orthochromatic emulsions are sensitive to all the visible spectrum with the exception of the deep orange and red rays.
Panchromatic emulsions are sensitive to all the visible spectrum without exception.
Infrachromatic emulsions are sensitive to infra-red radiation as well as the visible spectrum, so one has to filter out the visible spectrum to obtain a truly infra-red image.