Viewing Angle Compensators for Liquid Crystal Displays based on Layers with a Positive Birefringence

New compensators to enhance the viewing angle dependence of active matrix twisted nematic displays are designed. In contrast to the well-known discotic compensation films, these compensators are based on layers with a positive birefringence. It is shown that by combining layers with a positive birefringence, the optical properties of layers with a negative birefringence can be mimicked. Expressions are derived that relate the optical properties of the stacks of positively birefringent layers to those of negatively birefringent layers. Calculated conoscopic plots confirmed the correspondence. Two compensator configurations are experimentally tested: the combination of two positively birefringent layers with a tilted optical axis, referred to as T x T configuration, and the combination of a positively birefringent layer with a planar optical axis and a layer with a lilted optical axis, referred to as the P x T configuration. In both cases the projections of the optical axes onto the plane of the layers are crossed with respect to each other. It is demonstrated that the compensator based on the T x T configuration already results in a substantial improvement of the viewing angle of the active matrix display. The contrast values, the linearity of the grey scales;:and the color purity of the compensated display for oblique viewing angles are all improved with respect to the noncompensated:display. The contrast as a function of viewing angle is further improved by using compensators based on two P x T layer stacks.