Abstract: An apparatus is disclosed for sensing rotational or angular position of a shaft. The apparatus incorporates two light transmission paths, each having a separate photodetector providing an electrical signal in response to the intensity of received light. The electrical signals from the photodetectors are differentially processed to obtain a signal representative of rotational position. Devices are provided in each of the light transmission paths to vary the intensity of light transmitted in the path, and these devices are mechanically coupled to the shaft so that the variations in the respective light paths are opposite. The differentially processed photodetector signals are therefore unaffected by the absolute position of the shaft but are responsive only to deviations from a reference position. Furthermore, this differentially processed signal exhibits a higher degree of linearity was heretofore obtainable. Various embodiments are disclosed for achieving this opposite variation in the respective light paths.