Reflective Liquid-Crystal Displays
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Reflective LiquidCrystal Displays
Tatsuo Uchida and Takahiro Ishinabe Abstract Reflective full-color liquid-crystal displays (LCDs) are attracting a great deal of interest as portable information systems because of their extremely low power consumption and light weight; also, the color does not wash out in outdoor use. In this article, reflective LCDs are classified into three types. Among them, the diffusing-reflector type and the front-diffusing film type are suitable for high-quality active-matrix displays. Diffusingreflector LCDs have the advantage of uniform reflectance at the desired viewing angle due to the design of the surface microstructure of the reflector. Front-diffusing film LCDs using metallic mirrors and an optimally designed light-controlling film enable high contrast in a wide viewing-angle range and uniform reflectance with no blurring. Thus, both types have a high potential for achieving excellent color quality comparable to printed paper. In the near future, these reflective LCDs will likely be applied not only to portable systems, but also to high-performance wireless monitor displays and various other information systems.
strate, backed by an absorbing material. Typical examples of the liquid crystal used are polymer-stabilized cholesteric (PSCH) liquid crystal6,7 and polymer-dispersed liquid crystal (PDLC).8–10 PSCH uses the selective reflection of the cholesteric liquid crystal, and PDLC uses the scattering arising from the difference of refractive indices between the polymer wall and the liquid crystal. PSCH liquid crystal is superior in brightness and memory (retention of display characters when voltage is turned off); it also achieves high color capability by stacking three layers of different colors. Although present PSCH systems require a relatively high drive voltage and have insufficient switching speed for large-scale direct-matrix displays, they have the potential to be applied to electronic books. On the other hand, PDLC has the drawback of insufficient brightness when the thickness of the PDLC layer
Keywords: flat-panel displays, reflective liquid-crystal displays.
Introduction Liquid-crystal displays (LCDs) were originally developed as black-and-white, reflective-type displays for alphanumeric and matrix applications. In the 1980s, fullcolor displays were achieved by using color microfilters inside the cells.1,2 Color LCDs have been widely used for applications including notebook computers, monitor displays, and liquid-crystal televisions. However, LCDs lost the advantage of low power consumption because of the low transmittance of the display, thus backlighting is required. Power-consumption considerations have led to attempts to realize reflective color LCDs without backlighting.3 The reflective LCD has many advantages, such as extremely low power consumption, a very thin profile, light weight (owing to a small battery), and no fading in bright or outdoor environments. Even if equipped with a front light, the luminance of reflective LCDs does not need to be as high as the
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