Abstract: An optical encoder comprises a housing capable of being clamped in an end wall of a motor or rotating component about its shaft, a shaft opening in the housing, a fixture for mounting an optical encoding disc firmly in an end portion of the shaft when projecting through the housing opening, a peripheral aperture in the housing provided with a removable closure supporting a caliper assembly inside the housing at a position at which its opposite arms overlie opposite sides of the disc, the arms carrying light generating and light-sensitive elements respectively so that an electrical signal provided by the light-sensitive elements is a function of the rotational movement of the disc. An adjustment facility is also provided which allows the caliper assembly limited arcuate movement with respect to the shaft axis during setting up of the encoder.