A Positive Contrast Guest-Host Display Using a Liquid Crystal of Negative Dielectric Anisotropy

Recently we described a novel guest-host cell which exhibits a positive contrast (dark digits on a clear background) employing a mixture of a nematic liquid crystal with negative dielectric anisotropy, a chiral component, and a dichroic dye. In this cell, the cell thickness and the alignment layers are chosen such that in the absence of a field, the mixture shows a uniform homeotropic structure. When a voltage greater than the threshold value is applied, a helicoïdal structure is adopted, which absorbs the incident illumination.

In this work the expression for the threshold voltage is derived and compared with experimental results. Experimental results are also presented which indicate, for a given cell thickness, the influence of the pitch of the mixture on the contrast and the response time.