Color Compensation of STN Displays

Black and white operation of STN displays, necessary to produce a color display with the typical color filter technology, requires a compensation of the interference colors of a standard STN device. In this subsection several compensation techniques will be described. 

The transmission spectra of the nonselect state for the yellow mode (and the complementary blue mode) are characterized by a broad interference peak with a maxi- 

Consequently the overall retardation is zero (Soleil-Babinet compensator) resulting in a black and white operating LCD configuration. 

In this technique one standard polarizer is replaced by a dichroic color sensitive polarizer. For the yellow mode this results in a compensation of the off state and only slight influence on the on state. This mode of operation is called the neutral STN-mode (NSTN). For the complementary blue mode a compensation of the on state is achieved. A disadvantage of this technique is the low polarization efficiency of only (50-75)%. This limits its use to low-multiplexed displays with moderate requirements of contrast and viewing angle. 

Light propagating through a STN-cell will undergo a phase retardation A given by 

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