Reduction of grey scale inversion in a film-compensated twisted nematic liquid crystal display using beam steering optical film
Chi-Hyuck Park, Panel design, LG Philips LCD, 642-3, Jinpyung-dong, Gumi, 730-726, South Korea, Seung Hee Lee, School of Advanced Materials Engineering, Chonbuk National University, Duckjindong 664-14, Chonju, 561-756, South Korea, and Hyun Chul Choi, Panel Design, LG.Philips LCD, 624-3, JinPyung-Dong, Gumi, 730-726, South Korea.
Liquid crystal displays (LCDs) play an important role in human to machine interfaces. At present, twisted nematic (TN) LCD is the most common and commercialized LCD mode since it was first appeared in 1970s. However, TN shows limited viewing angles due to asymmetric director orientation in the gray and dark state. This causes non-uniformity in luminance along viewing direction called excessive brightness and darkness along vertical direction, resulting in strong gray scale inversion, which is the worst problem of the TN mode. There were some approaches to solve the intrinsic problems such as two domain- or four domain- TN, but they have failed to commercialize due to difficulty in fabrication. We propose a new cell structure that overcomes the intrinsic problems of the WV-TN cell. By employing a beam steering film (BSF) above the cell, the grey scale inversion is totally suppressed and the TN cell exhibits a high image quality even in wide viewing directions, opening a possibility for the TN cell to be applied to even large size displays. Figure shows the structures of the conventional TN cell applied WV film (WV TN) and the new BSF cell applied WV film plus beam steering film (BSF TN). In both cells, the cell retardation value was 0.41micro meter with the WV film at both top and bottom substrate. Owing to the BSF film, the maximum luminance reduced 264 nit to 140 nit at normal direction. The uniformity in the white and mid-grey state was much improved, especially, the region in which the relative luminance was larger than 25 % was much extended compared to that of the WV TN cell. In addition, the dark was well controlled by the WV film and luminance uniformity in the white state was improved so that the CR large than 50 existed over 80 deg. of the polar angle in vertical direction and large than 20 existed over 60 deg. of the polar angle in horizontal direction, which was much better result than that of the normal. Finally, the viewing angle dependency of the 9-grey levels for the WV TN cell with the BSF film was measured, as shown in Figure. As expected, a high luminance larger than 50 nit at G8 was maintained even at large polar angle of +/-80 deg., owing to the BSF film, while it was less than 25 nit in the WV TN cell. In summary, we propose a new cell structure in which the long standing intrinsic problem of gray scale inversion that the WV TN has is solved with the help of beam steering film on the top of the WV TN LCDs.