Abstract: Graphic data entry in an interactive graphic display system is achieved by means of an apparatus that provides automatic centering of a pointing laser beam on a hand held cursor. The graphic display system provides a means for displaying and comparing through superposition full size overlaid graphical data and projected graphical data on a common display screen. It includes a steerable pointing laser the beam of which scans the display screen in response to a laser beam deflection circuit. The system also includes laser beam position sensing and indicating circuits which accurately measure the laser beam position. Centering of the laser beam on the cursor at the display screen is accomplished by dithering the beam uniformly about its average position in a discrete dither pattern and utilizing the dither pattern to determine misalignment of the beam's average position with respect to the center of the cursor.

Abstract: A system for sensing the position of a laser writing beam is provided wherein at least two laser beams of unlike frequency are superimposed on the laser writing beam. The combined laser beams are simultaneously deflected in a common optical deflection system. Once deflected, the unlike frequency laser beams are separated from the writing laser beam and focused onto a diffraction grating. The position of the focused beams of unlike frequency on the diffraction grating corresponds to the position of the writing laser beam on the writing media. The focused laser beams of unlike frequency produce a traveling interference pattern on the grating and are diffracted by the grating into a plurality of beams. By sensing the frequency of at least one of the beams diffracted by the grating, the position and movement of the writing laser beam can be determined.