An improved positive contrast guest-host type display using nematic-cholesteric phase change

A positive contrast guest-host type liquid crystal display which is legible when viewed in reflection is described. The display is based on nematic to cholesteric phase change by electric field. The condition showing nematic phase in the voltage-off state is d/P<0.9 in the liquid crystal mixture studied (d: cell thickness, P: helical pitch). A d/P ratio nearly equal to 0.75 has been found a preferable condition in relation to display characteristics. The effect of homeo-tropic alignments with or without pretilt angle on display characteristics is reported. Recently developed nematic liquid crystals with large negative dielectric anisotropy and low viscosity have resulted in reducing driving voltage and response times. Contrast ratio and reliability have also been improved by using stable anthraquinone dyes with higher dichroic ratio. The display obtained is operated with static or 1/2 duty cycle drive at 3 to 4.5V, and has wide viewing angle as well as bright images.