Molecular Orientation in Twisted Liquid Crystal cells

Abstract

A useful model of the molecular orientation within twisted liquid crystal cells has been developed by Berreman (1). This model predicts a surprisingly large homogeneous components of orientation at fields at which a typical display is “on”. It also predicts the retention of twist at voltagaes considerably above that normal “turn-on” voltage. At normal operating voltage the molecular configuration of partially homogeneous molecules with twist cofined to the center of the cell can produce considerable retardation of off-axis polarized light. This degrades readability of the display.

A method of verifying this predicted molecular structure has been developed utilizing the interference fringes formed by reflections at the internal electrode-liquid crystal interfaces. Using HeNe laser light, these interference fringes are of considerable amplitude and permit an accurate measure of refractive index and birefringence. In 90[ddot] twisted cells, these interference fringes exhibit a 90[ddot] asymmetry. They easily show changes in molecular configuration at voltages well above the normal turn-on voltge.