Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1996-08-16
2002-06-25
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S119000
Reexamination Certificate
active
06411355
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a liquid crystal display device and the optical anisotropic element used therein.
2. Description of the Related Art
The liquid crystal display device is not only used as a display unit for wrist watches, electronic calculators, word processors and personal computers thanks to its crucial advantages of being thin and light-weight and of low power consumption, but also widely used in many newly designed products.
The liquid crystal display device used in personal computers, among others, uses larger and larger display units with higher capacity and greater display surface size, for example, 10 inches diagonally with 640×480 pixels or more. The display systems used in this class of liquid crystal display device may roughly be divided into two: one is a simple-matrix system and the other an active matrix system.
The simple-matrix system features a simple structure in which the liquid crystal is held between two sheets of glass substrates provided with stripe-shaped transparent electrodes. The simple matrix system demands a high performance all the more from the liquid crystal.
Before describing this performance, we briefly explain the display principle of the liquid crystal display device. The liquid crystal display device achieves the display, changing the orientation of the liquid crystal molecules by varying the voltage applied on the liquid crystal.
Generally a large contrast requires a large differential voltage. A display with as many as 640×480 pixels has however only about 1 V voltage difference between the dark state and the bright state. Only 1 V of difference requires a large state alteration of the molecular liquid crystal. Much research has thus far been conducted to realize such a feat. In 1985, the research group of Shafer et al. found that the change in the alignment of the liquid crystal molecules responds sensitively to the change in voltage if the twist angle of the liquid crystal display device is enlarged and that the liquid crystal molecules have a certain tilt to get a stable arrangement with a large twist angle. Since this research report, the alignment technology to realize this has been briskly developed and successfully commercialized.
In general, 180° or more twist angle is necessary to materialize a display with as many as 640×480 pixels. The liquid crystal display device with such a large twist angle has been called “Super Twist Nematic” (STN). Note however that the STN display at early stage was not achromatic, but colored; for example, with green characters in yellow background. This is due to a twist angle that is too large. Japanese Patent Publication 63-53528 (1985) discloses a technique to resolve such problems as colored display. This method technique realizes an achromatic display by arranging a second liquid crystal cell with its alignment of the liquid crystal layer twisted in the opposite direction between a polarizer and a first liquid crystal cell.
The principle of this achromatization consists in resolving the optical rotatory dispersion, that is, a wavelength dependence of optical rotatory power, by transmitting the light produced this dispersion after passing through the first liquid crystal cell caused to have a large twisted molecular structure in the cell, through the second liquid crystal cell having a symmetrical structure to that of the first liquid crystal cell. As a result, the color caused by the optical rotatory dispersion was dissolved to materialize the achromatic display. In order to perform such a conversion exactly, it is necessary that the second liquid crystal cell, which is an optical compensation plate, has a retardation value substantially the same as that of the first liquid crystal cell with their twist directions being opposed to each other and their arrays being so configured that the directors of the liquid crystal display device cell molecules coming most closely to each other should intersect each other.
A variety of other techniques have so far been proposed. For example, optically anisotropic film may be used in place of the second liquid crystal cell. Lamination of the optically anisotropic film on the liquid crystal cell affords a performance substantially equivalent to that of the second liquid crystal cell.
The optical compensation as above makes it possible to display achromatically even on the STN display unit. Furthermore, this achromatic display combined with a color filter enables one to have a high value added colored display. Since, however, the simple multiplex system is based on the principle of multiplex drive, which in turn is based on the average voltage method, if the number of scanning lines is increased to augment the display capacity, the difference reduces remarkably between the voltage when the light is intercepted and that when the light is left to transmit, which may result in lower contrast or slower response of the liquid crystal. This is a critical weak point. Such conventional techniques are much problematical if one tries to realize a liquid crystal display device with higher display quality, because they may cause such negative phenomena as the display screen seen as reversed (that is, obverse and reverse) depending on the orientation and angle when viewing it, disappearance of the display image or the display catching colors.
On the other hand, the active matrix system, which is provided with a switching element comprising, for each display pixel, a thin-film transistor or diode, allows us to set a given voltage ratio on the liquid crystal layer of each pixel irrespectively of the number of scanning lines. No special performance such as that for the simple matrix system is required in the active matrix system. There is therefore no need to increase the twist angle as in the case of STN. It has been considered that an angle of 90° suffices for the active matrix system.
In the liquid crystal cell (TN) with a small 90° twist angle, the optical rotatory dispersion is small since the light rotates following faithfully the twist, which ensures a colorless, high contrast display. The response to voltage is more rapid than in the STN too. A favorable combination of the active matrix system with the TN will realize a liquid crystal display device featuring a large display capacity, higher contrast and higher response speed. Since further there is a switching element for each pixel, an intermediate voltage can be applied, which enables one to make a gray scale (half tone) image. Moreover, the TN as combined with a color filter will facilitate the materialization of a full colored display.
Even in the active matrix system, however, such phenomena are observed as an obverse-reverse display screen depending on the orientation of view, total disappearance of the display image and colored display when a gray-scale image (half tone) is displayed, though not so with a binary display. These phenomena are much problematical when one wants to realize a high quality liquid crystal display device.
Japanese Patent Laid-Open 62-21423 (1987) discloses a liquid crystal cell and a birefringence layer (which is a polymer film whose optical anisotropy is negative in the direction of its thickness) that are between two polarizers as a means to reduce the visual angle dependency. On the other hand, Japanese Patent Laid-Open 3-67219 (1991) discloses an arrangement on a liquid crystal cell of a birefringence layer composed of the liquid crystal compound (or high molecular liquid crystal) presenting cholesteric liquid crystal phase with 400 nm or less product of helical pitch length and refractive index. These two propositions have been contrived only for the cases of liquid crystal cells with homeotropically aligned liquid crystal cells (molecular liquid crystal arranged perpendicularly to the aligned substrate), not for such liquid crystal cell with twisted orientation as TN and STN systems. Japanese Patent Laid-Open 4-349429 (1992) proposes to control the viewing angle of liquid crystal display device by optio
Hatoh Hitoshi
Ishikawa Masahito
Manabe Atsuyuki
Okamoto Masumi
Tanaka Yasuharu
Pillsbury & Winthrop LLP
Ton Toan
Toshiba Techno Center Inc.
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