LIGHT-INDUCED TRANSMISSION CHANGES IN LIQUID-CRYSTAL DISPLAYS FOR ADAPTIVE PATTERN PROJECTION

We studied the changes in the transmission and contrast properties of a super-twisted nematic liquid crystal display panel, induced by the light impinging on the panel itself. Upon illumination by laser radiation with power density close to that used in practice, the transmittance of the panel increases or decreases according to the brightness voltage that sets the transmittance level. The dynamics of the transmittance change are typically bi-exponential, with a shorter decay time of the order of the fraction of a second and a longer decay of the order of the tens of seconds. The observed changes were interpreted and modeled by considering local temperature changes in the crystal because of the light impinging onto it. The temperature changes produce a shift of the transmittance curve and a change of slope in its central region. The presence of the dip after the Frederickzs region accounts for the anomalous behavior of the transmittance in that portion of brightness voltages. The study is significant for use of liquid crystal panels as spatial light modulators (phase and amplitude modulation) upon illumination with moderate light patterns of nonuniform power density.