Abstract: A device for detecting alternating light and dark areas of a Vernier or Moire interference pattern. The device comprises a plurality of paired light sources and light detectors. The detector of each such pair is positioned to receive light from the corresponding source of each of such pairs. An opaque scale having transparent graduations and a grating having transparent graduations are included. The scale and grating are movable relative to each other and light from each light source is directed toward its corresponding detector through the scale and grating so that alternating light and dark areas in the form of an interference pattern are formed. The device also includes a light directing arrangement for independently allowing adjustment of the light emanating from each light source in a direction along a line passing through the light and dark areas and for allowing change of the amount of light received by the light detectors. The adjustments for phase and gain are substantially independent of each other.