Abstract: A method and apparatus for determining the location of a target surface by projecting a periodically scanning pattern of light onto the target surface, and comparing the time of projection of a predetermined point of the pattern along a predetermined path with the time of observing an image of the predetermined point of the pattern reflected along an intersecting receiver path. An apertured disc is rotated between a light source and a collimating projection lens assembly to produce scanning pattern of light. Photoelectric sensors generate reference and receiver electrical signals in response to projection and reception of the pattern of light, and the distance of the target surface from a reference location may be electrically determined as a geometrically defined function of the phase angle between the electrical signals.