Optical properties of a new bistable twisted nematic liquid crystal boundary layer display

Abstract

In a new display cell, a holding voltage compresses the regions where the director is not essentially vertical into thin boundary layers adjacent to the surfaces. With opposite surfaces tilt biased in opposite senses, and with suitable isolation regions, the director field contains a horizontal director line. Above a certain threshold voltage the stable states are asymmetric, with the horizontal director line nearer one surface than the other. In a twist cell, these bistable states can be discriminated optically with either a dichroic dye and a single polarizer or with two polarizers using birefringence effects. A dye gives poor results because the horizontal absorbing layers are thin. However, the birefringence effects enable good contrast to be obtained. We examine the brightness and contrast by studying the transmission of monochromatic light passing normally through the cell in the two states. We include an analysis of polarized light propagating through two birefringent layers of arbitrary phase difference, and whose principal planes are at arbitrary angles to each other and to the polarizers. A separate analysis relates the phase difference of the layers to the voltage and cell boundary conditions. In order to determine suitable operating conditions with two polarizers, we carry out several different calculations. These include: (1) calculating, with one state totally extinguished, the transmission of the other state when the cell twist is 45° and when the cell twist is adjusted to maximize that transmission; (2) calculating the transmission and contrast ratio under conditions that maximize the transmission difference of the two states; and (3) calculating the contrast ratio when the polarizer and cell twist are set to make the brighter state give circular polarization (in order to minimize chromatic effects). A qualitative experimental confirmation is included.