Optical structure to improve the γ-curve distortion of a four-domain (4-D) twisted nematic liquid crystal cell

The γ -curve represents the viewing angle characteristics of a liquid crystal (LC) cell in all gray levels, including the dark state. In this paper, we propose a optical configuration of a four-domain (4-D) twisted nematic (TN) LC cell for improving the γ -curve distortion in the oblique direction, without any deterioration in the optical property of a conventional 4-D TN cell. A positive A -plate and a negative A -plate are applied at the top and the bottom of the 4-D TN LC layer, respectively. The two A -films applied can be optimized at all viewing angles by calculating the polarization difference (Δ p ) as functions of the retardation and the optical axis of the films, so that the transmittance distortion in the middle gray scale can be compensated for in all viewing directions. As a result of a calculation of the gamma distortion index (GDI), the γ -curve can be improved by more than 70% in the off-axis at 0° and 180°, and by more than 50% in the off-axis at 90° and 270°.