Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1997-02-28
2001-05-15
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S180000, C349S099000
Reexamination Certificate
active
06233032
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display apparatus. More specifically, the present invention relates to a liquid crystal display apparatus having improved viewing angle characteristics.
2. Description of the Related Art
Conventionally, liquid crystal display apparatuses using nematic liquid crystal display devices have been widely used as numerical value segment type liquid crystal display apparatuses such as clocks and calculators. In recent years, such apparatuses have been used for displays of word processors, computers, and navigation systems, and TVs.
In the above-mentioned liquid crystal display apparatus, an active element such as a thin film transistor (hereinafter, referred to as a “TFT”) is generally formed for selectively driving a pixel electrode which applies a voltage to a liquid crystal layer. Furthermore, a color filter layer of red, green, blue, etc. is provided for the purpose of conducting a color display. As a display system, the following two liquid crystal display systems have been used, depending upon the twist angle of nematic liquid crystal to be used:
(a) An active drive type twisted nematic (TN) liquid crystal display system in which nematic liquid crystal molecules are aligned so as to be twisted by 90°.
(b) A multiplex drive type super twisted nemetic (STN) liquid crystal display system in which nematic liquid crystal molecules are aligned so as to be twisted by more than 90°.
The liquid crystal display devices driven by the above-mentioned two systems have problems such as viewing angle dependence and a small viewing angle. Therefore, various methods for solving these problems have been suggested.
For example, it is suggested in Japanese Laid-Open Patent Publication No. 57-186735 that each pixel be divided into two regions and the viewing angle characteristics of these regions made different, whereby each pixel is provided with a plurality of viewing angle characteristics. Furthermore, it is suggested in Japanese Laid-Open Patent Publication No. 7-248497 that liquid crystal molecules be aligned so as to be twisted by a plurality of twist angles in each pixel.
According to the method disclosed in Japanese Laid-Open Patent Publication No. 57-186735, each pixel has two viewing angle directions: a 12 o'clock direction and a 6 o'clock direction. In a liquid crystal panel having such viewing angle characteristics, viewing angle dependence exists in the vertical direction (i.e., 6-12 o'clock direction) and horizontal direction (i.e., 3-9 o'clock direction). In particular, the viewing angle dependence in the case where the display panel is seen in the horizontal direction (i.e., 3-9 o'clock direction) is almost the same as that in a TN liquid crystal panel to which a conventional alignment division method is not applied. More specifically, the problem of the viewing angle dependence is not solved even by applying the alignment division method. When a viewing angle is increased from the vertical direction (viewing angle of 0°) to the horizontal direction to the display panel, black end white (contrast) of an image is inverted in the vicinity of a viewing angle of 30° (gray-scale inversion phenomenon).
In the present specification, a “viewing angle” refers to an angle formed by a normal line to a display surface of a liquid crystal display device (liquid crystal panel) and a viewing angle direction (i.e., a direction in which a user sees the display surface: a line of sight). An “azimuth angle” refers to an angle formed by a component of a display surface in a viewing angle direction and a reference direction of the display surface. The reference direction is typically set in an upper direction (i.e., a 12 o'clock direction, for example, in the face of a clock) in the case where a user sees the display surface.
The wording “viewing angle characteristics” of a liquid crystal display apparatus refers to the dependence on the direction of a line of sight of a user in a wide sense. Specifically, it refers to the dependence of display characteristics on the above-mentioned “viewing angle” and “azimuth angle”.
When the viewing angle is increased from 0° in the vertical direction, averaged characteristics of the viewing angle characteristics in the 6 o'clock direction and in the 12 o'clock direction in the conventional TN liquid crystal panel can be obtained. Thus, the viewing angle characteristics obtained by increasing the viewing angle in the 12 o'clock direction become symmetric with respect to those obtained by increasing the viewing angle in the 6 o'clock direction. However, there is a limit to the extent contrast can be increased when the viewing angle is tilted down.
According to the methods described in the abovementioned publications, there is a limit to improvement of the viewing angle characteristics in the azimuth angle of 45° from an absorption axis or a transmission axis of a polarizing plate.
SUMMARY OF THE INVENTION
The liquid crystal display apparatus of the present invention, having a plurality of display pixels arranged in a matrix, includes: a pair of substrates opposing each other; a liquid crystal layer placed between the pair of substrates; an alignment film formed on a surface of at least one of the pair of substrates opposing the liquid crystal layer and subjected to a treatment for aligning liquid crystal molecules in the liquid crystal layer; and a pair of polarizing plates placed so as to interpose the pair of substrates, wherein the liquid crystal layer is divided into at least two regions in each of the display pixels, alignment states of the liquid crystal molecules are different in the at least two regions, and one absorption axis of one of the pair of polarizing plates is substantially parallel to a 6-12 o'clock direction or a 3-9 o'clock direction.
Alternatively, the liquid crystal display apparatus of the present invention, having a plurality of display pixels arranged in a matrix, includes: a pair of substrates opposing each other; a liquid crystal layer placed between the pair of substrates; an alignment film formed on a surface of at least one of the pair of substrates opposing the liquid crystal layer and subjected to a treatment for aligning liquid crystal molecules in the liquid crystal layer; and a pair of polarizing plates placed so as to interpose the pair of substrates, wherein the liquid crystal layer is divided into first and second liquid crystal regions in each of the display pixels, the first and second liquid crystal regions have a different viewing angle dependence, a relationship &psgr;1+&psgr;2=180° is satisfied, where &psgr;1 is a first twist angle of the liquid crystal molecules in the first liquid crystal region and &psgr;2 is a second twist angle of the liquid crystal molecules in the second liquid crystal region, one absorption axis of the pair of polarizing plates is placed in a direction so as to substantially bisect the first twist angle, and the other absorption axis of the pair of polarizing plates is placed in a direction so as to substantially bisect the second twist angle.
In one embodiment of the present invention, the one absorption axis of one of the pair of polarizing plates is substantially parallel to the 6-12 o'clock direction, and the other absorption axis is substantially parallel to the 3-9 o'clock direction.
In another embodiment of the present invention, an angle formed by the one absorption axis of the pair of polarizing plates and the other absorption axis is in a range of 90°±5°.
In another embodiment of the present invention, retardation of the liquid crystal layer is in a range of 340 nm to 440 nm.
In another embodiment of the present invention, the liquid crystal molecules in the liquid crystal layer are aligned so as to be twisted by 90°.
Hereinafter, the function of the present invention will be described.
According to the present invention, a viewing angle can be increased both in the vertical direction and in the hori
Inoue Iichiro
Mizushima Shigeaki
Yamahara Motohiro
Conliu David G.
Daley William J.
Dike Bronstein Roberts & Cushman IP Group
Sharp Kabushiki Kaisha
Ton Toan
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