Optimization of contrast ratio in reversed-polarizer, transmissive-type twisted nematic displays

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Abstract
Simple mathematical expressions are presented which describe the transmission of normally-incident light through absorbing, twisted nematic layers. These expressions are used to optimize the contrast ratio in white light of a reversed-polarizer, twisted nematic display with respect to the variables of twist angle, optical path length difference, dye concentration, and polarizer strength. The results are presented in the form of curves showing the optimum contrast and the optimal values of these variables as a function of the on-state brightness of the display. The maximum contrast ratios are compared to optimized twisted layers without dye and conventional 90° twisted layers with and without dye. Also, an explanation is given for the experimental observation reported in the literature that the contrast ratio of twisted layers may assume a maximum value for a certain concentration of dichroic dyes.