Cell design of gray-scale thin-film-transistor-driven liquid crystal displays

The desirable liquid crystal (LC) panel design (here called “cell design”) for a gray-scale thin-film-transistor (TFT)-driven twisted nematic (TN) liquid crystal display (LCD) is discussed in terms of display legibility and ease of fabrication. To optimize cell design for gray-scale application, some key display factors such as contrast ratio, color change, and viewing cone are evaluated for various cell geometries and cell thicknesses. The cell geometries discussed are combinations of two display modes (a normally white mode and a normally black mode, in which the optical axes of the exit polarizers are placed perpendicular and parallel to those of the entrance polarizers, respectively) and two optical eigenmodes (an extraordinary-ray mode and an ordinary-ray mode, in which the transmission axes of the entrance polarizers are parallel and perpendicular to the entrance rubbing directions, respectively). A new driving scheme of threshold-voltage bias application to the LC cell is proposed to overcome the TN LCD shortcoming of a narrow viewing cone. We have adopted a cell design for a 512-color TFT LCD: 1) a first minimum normally white (NW) mode as polarizer arrangement for ease of fabrication, 2) an extraordinary-ray mode (e-mode) as optical eigenmode with a novel driving scheme (threshold-voltage biased) for gray-scale improvement in eliminating brightness reversals, and 3) a retardation (dΔn) value of 0.47 µm for further optimization of proper gray-scale order and color change. We have called this mode “threshold-voltage-biased e-mode NW,” or “biased e-mode NW.”