Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-08-04
2002-12-10
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S118000, C349S119000
Reexamination Certificate
active
06493053
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and, more particularly, to a liquid crystal display apparatus which employs in combination a liquid crystal display device and a phase plate, thereby improving the response characteristics and the viewing angle characteristics of the display screen.
2. Description of the Related Art
A liquid crystal display device using a nematic liquid crystal display device has conventionally been used widely in number segment type display devices such as clocks and desktop calculators. In recent years, such liquid crystal display devices have also been used in word processors, notebook type personal computers, car liquid crystal TVs, and the like.
A liquid crystal display apparatus typically includes a light transmissive substrate on which electrode lines, and the like, are provided for turning pixels ON/OFF. For example, in an active matrix type liquid crystal display apparatus, active elements such as thin film transistors are provided on the substrate, along with the electrode lines, as switching means for selectively driving the pixel electrodes by which voltages are applied through the liquid crystal layer. In a color liquid crystal display apparatus, a color filter layer for providing colors, e.g., RGB, is provided on the substrate.
An appropriate liquid crystal display mode can be selected for use with each of such liquid crystal display devices according to the twist angle of the liquid crystal molecules. For example, an active drive type twisted nematic liquid crystal display mode (hereinafter, “TN mode”) and a multiplex drive type super twisted nematic liquid crystal display mode (hereinafter, “STN mode”) are well known in the art.
In the TN mode, nematic liquid crystal molecules are oriented in a 90° twist so as to guide light along the twist, thereby producing a display. The STN mode effectively utilizes the phenomenon that when the twist angle of the nematic liquid crystal molecules is increased to be greater than 90°, the transmission therethrough changes rapidly for voltages in the vicinity of the threshold voltage applied through the liquid crystal layer.
In the STN mode, the background of the display screen is colored in a unique color due to interference of colors because the STN mode utilizes the birefringence effect of a liquid crystal material. In order to solve such problems so as to produce a black and white display with the STN mode, it is believed to be effective to employ an optical compensator. Display modes employing an optical compensator can be generally classified into the double super twisted nematic phase compensation mode (hereinafter, “DSTN mode”) and the film type phase compensation mode in which an optically anisotropic film is employed (hereinafter, “film added mode”).
The DSTN mode employs a two-layer structure including a display liquid crystal cell in which the liquid crystal molecules are twisted in a certain direction and another liquid crystal cell in which the liquid crystal molecules are twisted in the opposite direction. The film added mode employs a structure in which an optically anisotropic film is provided. It is believed that the film added mode is advantageous in that it is light in weight and low in cost. Since the black and white display characteristics with the STN modes have been improved by the employment of such phase compensation methods, color STN liquid crystal display apparatuses have been realized in the art in which a color filter layer is provided in an STN mode display apparatus.
On the other hand, the TN modes can be generally classified into the normally black mode and the normally white mode. In the normally black mode, a pair of polarizers are arranged so that their polarization directions are parallel to each other, whereby a black display is produced in the absence of an ON voltage through the liquid crystal layer (i.e., in the OFF state). In the normally white mode, a pair of polarizers are arranged so that their polarization directions are orthogonal to each other, whereby a white display is produced in the OFF state. The normally white mode is advantageous in the terms of the display contrast, the color reproducibility, the viewing angle dependency of the display, etc.
However, a liquid crystal display apparatus of the above-described TN mode has a problem as follows. In a TN mode liquid crystal display apparatus, the liquid crystal molecules have a refractive index anisotropy &Dgr;n and the liquid crystal molecules are in an inclined orientation with respect to the upper and lower substrates. As a result, the viewing angle dependency may be substantial such that the contrast of the displayed image substantially varies depending upon the direction and the angle from which the display is viewed by the observer.
FIG. 34
schematically illustrates in a cross-sectional view the structure of a TN liquid crystal display device. In the state as shown, a gray-level voltage is being applied through the liquid crystal layer, whereby the liquid crystal molecules are slightly raised. With the TN liquid crystal display device in this state, linearly-polarized light travelling in the direction normal to the surfaces of the pair of substrates (the “normal direction”) passes through the liquid crystal molecules at an angle which is different from the angle at which linearly-polarized light travelling in a direction inclined with respect to the normal direction passes through the liquid crystal molecules. Since the liquid crystal molecules have the refractive index anisotropy &Dgr;n, passage of the linearly-polarized light from the different directions produces normal light and abnormal light with a phase difference therebetween. Due to such a phase difference, the incident light is converted into elliptically-polarized light, thereby causing the viewing angle dependency.
Within a liquid crystal layer in an actual liquid crystal display device, liquid crystal molecules in the vicinity of the midpoint between the substrates have a tilt angle which is different from that of other liquid crystal molecules on or near the substrate, and the liquid crystal molecules have a 90° twist between the substrates about the normal direction.
Thus, the linearly-polarized light passing through the liquid crystal layer is subject to various degrees of birefringent effect depending upon its direction and angle, thereby exhibiting a complicated viewing angle dependency.
This viewing angle dependency causes phenomena such as the following: the display screen is colored when the viewing angle is inclined from the direction normal to the screen toward the normal viewing direction, i.e., toward the lower side of the screen by a certain angle or more (hereinafter, the “coloring phenomenon”); and the black and the white are reversed (hereinafter, the “black and white reversal phenomenon”). When the viewing angle is inclined toward the counter-normal viewing direction, i.e., toward the upper side of the screen, the contrast is reduced rapidly.
The above-described liquid crystal display apparatus has another problem follows. The larger the display screen, the narrower is the viewing angle. When a large liquid crystal display screen is viewed by the observer from the normal direction within a short distance from the screen, the display color in an upper portion of the screen may be different from the display color in a lower portion of the screen due to an influence of the viewing angle dependency. This is because the observer being so close to the screen, although in the normal direction makes the situation substantially the same as that where a small display screen is viewed from an inclined direction.
In order to address the problems associated with the viewing angle dependency, it has been proposed in the art to insert a phase plate (a phase film) as an optionally anisotropic optical element between a liquid crystal display device and one of a pair of polarizers (see, for example, Japanese Laid-Open Publication No. 5-313159).
According
Miyachi Koichi
Nakata Morio
Shiomi Makoto
Yamahara Motohiro
Conlin David G.
Dike Bronstein Roberts & Cushman
Ngo Julie
Sharp Kabushiki Kaisha
Sikes William L.
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