Touch-screen input device using monitor as light source operating at intermediate frequency

The present invention comprises a standard CRT that has a light source provided by two strips of light patterns in the unused area of the CRT screen, typically at the right and left margins. In a preferred embodiment, the light patterns are created by a simple electronic circuit that is used in conjunction with the raster scan of the screen. No external light sources are required. The light patterns are created in a novel way in that they have a unique pattern frequency which is determined by causing the raster scan to create a point of light at the margins during certain raster cycles but not during others. The light pattern frequency is selected to be easily discriminated, such that the environmental disturbance and the signal from the CRT itself can be completely rejected. The light patterns are reflected from the unused areas of the CRT screen by reflective surfaces. The reflective surfaces are positions in proximity to the strips of light patterns and reflect the light across the surface of the CRT. Four single photodetectors located at the four corners of the CRT screen are used as detectors to monitor the reflected light. A pointer positioned on or near the surface of the CRT will block the light in such a way that two or more of the sensors can provide enough information to determine the pointer position by using geometry.