The rotation of a shaft is sometimes measured by detecting the position of a narrow beam of light, reflected by a mirror rigidly mounted to the shaft. For small angles of rotation, a centering detector can be used to determine the position or the change in position of the beam. One type of centering detector is comprised of two photo-detectors each having the shape of a hemi-disc. These hemi-discs are placed close to each other so that their straight edges are aligned parallel. If a radially symmetric beam of light is incident on two such detectors with equal sensitivity, their electrical output signals will be of equal magnitude when the beam center coincides with the geometric center of symmetry of the pair. If the beam is displaced, the detector outputs are no longer equal, and the unbalance is related to the magnitude of the displacement.