Contrast compensation system for optical display - uses liquid crystal enabling operation in high ambient light levels

摘要

A filter placed in front of the screen comprises a film of liquid crystal (11) placed between two polarisers (20, 21) with different polarisation directions. A voltage is applied between the film's conducting faces (18, 19) using a voltage source (15) and a switch (14) mounted in series at the terminals of the two faces. The voltage is applied for strong illumination of the screen by ambient light and is not applied when the screen is only feebly illuminated. The filter's transmission factor is low when the voltage is applied and higher when the voltage is not applied. Thus the system provides compensation for variations in contrast between the information on the screen and the bottom of the screen following ambient light variations.