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 t wisted nematic displays are designed. In contrast to the well-known discoti c compensation films, these compensators are based on layers with a positiv e birefringence. It is shown that by combining layers with a positive biref ringence, the optical properties of layers with a negative birefringence ca n be mimicked. Expressions are derived that relate the optical properties o f the stacks of positively birefringent layers to those of negatively biref ringent layers. Calculated conoscopic plots confirmed the correspondence. T wo 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 l ayer with a planar optical axis and a layer with a lilted optical axis, ref erred to as the P x T configuration. In both cases the projections of the o ptical axes onto the plane of the layers are crossed with respect to each o ther. It is demonstrated that the compensator based on the T x T configurat ion already results in a substantial improvement of the viewing angle of th e active matrix display. The contrast values, the linearity of the grey sca les;:and the color purity of the compensated display for oblique viewing an gles are all improved with respect to the noncompensated:display. The contr ast as a function of viewing angle is further improved by using compensator s based on two P x T layer stacks.