any member of a class of highly coloured organic compounds used for increasing the range of wavelengths of light to which photographic emulsions are sensitive. A few members of the class are used in textile dyeing, but most are too easily destroyed by acids or by light to be satisfactory for this purpose.

The silver salts in photographic films or plates are affected only by light of wavelengths in the range of 350 to 450 nanometres, which includes only a small part of the visible spectrum (about 400 to 700 nanometres). In 1873 it was found that cyanine, a blue dye discovered in 1856, rendered the emulsion sensitive to a wider range of visibly coloured light. Since then, related dyes have been developed that sensitize the emulsions to all visible wavelengths and to some in the infrared region. Such sensitizers are especially useful in long-distance photography.