A technique for obtaining positional information using a Ronchi grating is described. By employing the grating as a diffraction element, modulation of the phase of a coherent light beam is accomplished. The phase contains information as to the position of the beam on the grating surface. The use of an interference technique allows one to monitor the light phase, and therefore the beam’s position. Using a single grating, directional as well as positional information is obtained in two dimensions. Advantages and other system characteristics are described.