Definition: (1) Organic compound used in colour film or print processing that reacts or combines with development by-products to form a coloured dye. * Couplers are colourless until they react with the developer (usually para-phenylenediamine or the like) oxidised from turning exposed silver halide to silver. * They are made to produce dyes with the subtractive primary colours of cyan, magenta and yellow in separate layers of colour film. (2) Component in optical rigs used to distribute or divide optical power. (3) Device used to join a optical component with a light source or detector. * Also known as connector.

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