In a liquid-crystal display, each pixel is associated with a sensor which detects incoming light projected by a light pen. In response to the detected light, the sensor produces a signal which causes its associated pixel to darken. Thus, as a user moves the light pen along the display, the...http://www.google.de/patents/US5751453?utm_source=gb-gplus-sharePatent US5751453 - Liquid crystal display with pen-input capability
