Abstract: Apparatus designed to measure, within a selectively variable defined range, the distance of an object's surface from opposite sides of a predetermined datum plane. The apparatus includes two pairs of light sources which are disposed on one side of the datum plane, and on opposite sides of a reflection viewing plane which is normal to the datum plane. In each source pair, the two sources are energized alternately to create, on opposite sides of the datum plane, and with regard to the viewing plane, alternate upper and lower preferential illumination zones. A photodetector, which "looks" for reflections along the viewing plane, has its output signal coordinately gated in such a manner that the level of such signal is directly interpretable to indicate the distance of a reflecting surface from the datum plane, as well as the side of such plane on which the surface is located.