Light-actuated liquid-crystal light valves

Certain nematic liquid crystals exhibit strong small-angle scattering of light in the presence of an electric field. This "dynamic scattering" has been exploited in both directly viewed and projected displays, in which different electric fields are applied to different picture elements. One way of applying these fields is through a photoconductor in series with each picture element; the amount of dynamic scattering at each picture element is then controlled by the amount of signal light falling on the corresponding photoconductor. The voltages (tens of volts) and currents (µA/cm²) involved are quite modest compared to those used in conventional photoconductor-electroluminescent panel light amplifiers. Furthermore, the scattered or "projection" light may be very bright since it is derived from an external source; its intensity is limited only by heating in the liquid crystal cell due to absorption. Consequently, the gain and output light level may be very large compared to those of conventional light amplifiers. The design of practical light valves centers on preventing projection light from activating the photoconductor. Cells have been made in which: 1) the photoconductor is insensitive to the wavelength of the projection light; and 2) an opaque barrier prevents projection light from entering the photoconductor. Resolutions in excess of 5 lines per mm have been achieved in very simple cells in which the active elements are in the form of continuous thin layers. Experimental results on the gain and sensitivity will be presented.