VIEWING ANGLE ENHANCEMENT FOR VERTICALLY ALIGNED CHOLESTERIC LIQUID CRYSTAL DISPLAYS

摘要

A vertically aligned cholesteric liquid crystal display system (500) that provides a high contrast ratio and gray scale transmission that is largely invariant with respect to viewing angle is disclosed. The inventive display consists of an optical compensator and a novel cell design that improves the display's gray scale stability. The display's compensator comprises a negative C-plate (510) and either one or two pairs of crossed A-plates (550). The A-plates are oriented along the display's polarizer transmission axes. The achievable viewing angle using this compensator arrangement is wider than that achievable with crossed polarizers alone. The display's liquid crystal cell design incorporates two tilt domains with a relatively low phase thickness. The liquid crystal's chiral dopant concentration is adjusted to give a cell gap-to-pitch ratio of 0.2 to 0.32. The display's polarizers (505, 555) are oriented at 45 DEG and 135 DEG , whereas the average liquid crystal director is oriented at 90 DEG . Each pixel electrode (525, 540) contains horizontal stripes which produce lateral electric fields within the active pixel region. The lateral electric fields cause the liquid crystal to break up into two tilt domains that are oriented in opposite directions, approximately 90 DEG and 270 DEG . The resulting gray scale response is reproducible from pixel to pixel and relatively uniform with viewing angle because the response from different tilt domains are averaged over the entire pixel. The lateral electric fields also eliminate the instability that exists when an electric potential is first applied across a cell and, thereby, reduces the display's turn-on delay time.