A touch screen input device and method for use with a rear-mounted video output projection system incorporates a transparent sheet preferably having substantially parallel top and bottom surfaces as the touch screen panel. A rear-mounted light source projects light onto the rear surface of the touch screen panel, on which a diffusing surface is imposed, with the result that some of the light from the light source is totally internally reflected within the touch screen panel and is imaged by a rear-mounted video camera. When the front surface of the touch screen panel is touched by a finger or rubbery object, total internal reflection is interrupted and the video camera images the touch as a dark spot. Because the touch screen panel is a sheet having substantially parallel surfaces, the video output system with which the input device is designed to be used is a projection system, and the technology used to detect touches is optical imaging, the touch screen input device can be readily scaled up to desktop or large work-surface sizes. The touch screen input device operates equally well in an environment with any level of ambient light or no ambient light and can simultaneously detect multiple touches. The touch screen input device remains operative while books or other common desktop objects are left resting on portions of the touch screen panel.