Abstract: Apparatus for measuring angular displacement of an object in which an alternating potential source is applied to the electrodes of a light source, such as a gaseous discharge light, resulting in alternating illuminations in the medium, e.g., gas, about the respective electrodes, which are detected by photodetector means. The light source and the photodetector means are mounted such that the distances between the photodetector and the respective electrodes vary proportionately with any rotation of the object whose angular displacement is being monitored. The output signals from the photodetector means are separated and compared and, in response thereto, a signal is generated indicative of the angular displacement of the object.