Optical performance of the π cell compensated with a negative-birefringence film and an a-plate

The pi 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 pi cell, we formulated the idea of using a negative-birefringence structure which has a mirror image of the director configuration of the pi 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 pi cell. An extended Poincare 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 pi cell is much less than that of the twisted-nematic liquid-crystal display compensated with a similar negative-birefringence film.