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Optical Performance of the π Cell Compensated with a Negative-Birefringence Film and an A-plate

The π cell is known to be a fast and wide-viewing-angle liquid-crystal-display mode due to its unique director configuration. To compensate the viewing angle dependence of the positive birefringence of the π cell, we formulated the idea of using a negative-birefringence structure which has a mirror image of the director configuration of the π cell. Our calculation showed that this combination provides an excellent viewing angle, which was experimentally confirmed by applying a fabricated optical compensation film composed of a discotic layer and an aromatic polyimide layer, both of which have negative birefringence, to a test π cell. An extended Poincaré sphere analysis explained the optical effect of the compensation film and revealed that polarizers are responsible for the low contrast ratio at oblique angles. A complete viewing angle was achieved by adding a positive a-plate which suppresses light leakage at oblique incidence. Color shift upon viewing from oblique angles was also examined. Our experiment showed that the color shift of the π cell is much less than that of the twisted-nematic liquid-crystal display compensated with a similar negative-birefringence film.