Scattering Mode Liquid Crystal Devices

Liquid crystal displays utilizing scattering effects operated in the reflective mode have a mirrored backing that throws the light scattered at the liquid crystal back through the device into the viewing field. Reflective back electrodes generally limit the readability appreciably because of unwanted reflections. Display devices of this type are therefore hardly usable in practice unless special measures are taken. 

For driving matrix liquid crystal display panels, the silicon metal-oxide semiconductor field effect transistor (MOSFET) fabricated on a silicon monolithic wafer has been investigated by several groups [134-150]. The MOS transistor circuit fabrication techniques are well developed and have been used to produce various LSI devices. A dynamic scattering mode, a planar type GH mode or a polymer dispersed (PD) mode are used in these displays because the silicon wafer is intrinsically opaque. The circuit configuration of the panel is essentially the same as that of the p-Si TFT switch matrix addressed liquid crystal display panel as shown its equivalent circuit in Fig. 18(a). 

An improvement of this device has been described in which the nematic-cholesteric mixture is replaced by a smectic material. Thermal writing induces the change of the smectic from the perpendicular to a scattering texture. Unlike the nematic-cholesteric materials, selective erasure is possible with the smectic device. The thermal writing is too slow for television-rate applications because of the thermal inertia of the glass-liquid-crystal system. With a laser-beam power of 20 mW, addressing speed is approximately 10 elements/sec for the smectic device. In the projection mode, the resolution is 50 lines/mm at a contrast ratio of approximately 10 1. 

Many liquid crystal display modes have been devised, but the versatility and balance of properties offered by the twisted nematic (TN) device have proved very difficult to beat. It superseded the nematic djmamic scattering display used in early displays. To improve its performance, the TN device has been developed into new displays, i.e., STN and active matrix-addressed TN. Displays using dichroic dyes find a niche market in large information displays (airport displays), and, recently, devices using liquid crystals in conjunction with polymeric materials have been discovered. 

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