Abstract: An electronic light pen used in conjunction with a video display permits the position of the light pen to be determined with respect to the video display by the particular color or grey scale value of light emitted by the video display and received by the electronic light pen. The light from the video display is separated into three primary colors for a color display, namely red, blue, and green, and separated into grey scale shades for a monochrome display. Positional information of the light pen is calculated based on the relation of the color measured to the color previously programmed and presently displayed on that area of the screen, at any instant in time. The light pen has a focusing system that focuses light from the display to its optical detectors. The optical detectors have red, green, and blue filters to determine the brightness of each color. A grey scale value may be determined if the display is monochrome.