Abstract: A method and apparatus for defining coordinates of an object intersecting a predefined plane. More particularly, the invention provides a means for generating first and second light beams and a means for sweeping the first and second light beams across the predefined plane. A continuous optical sensor, which could be a plurality of solar cells, is provided around a portion of the periphery of the predefined plane so that each light beam when swept across the plane intersects a portion of the optical sensor. Each light beam is swept across the plane by a rotating reflective surface. The reflective surfaces are rotated by respective motors which have control systems configured so that both light beams cannot simultaneously irradiate the optical sensor. A means for generating timing pulses when each light beam defines a predetermined reference angle is provided.