Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-12-20
2003-09-30
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06628359
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display for a computer monitor, and a liquid crystal display device for displaying video images etc., and more particularly to a liquid crystal display device having an excellent viewing angle characteristic.
2. Description of the Related Art
There are two known techniques for improving or widening the viewing angle of a liquid crystal display device: (1) changing orientation directions of liquid crystal molecules within a plane substantially parallel to a substrate surface; and (2) in a display mode where orientation directions of liquid crystal molecules are changed to a direction perpendicular to a substrate surface, dividing into domains having different orientation directions (i.e., azimuthal angle direction) of liquid crystal molecules in a plane parallel to the substrate surface. A typical example of the former technique is the In-Plane-Switching (IPS) mode. Examples of the latter technique include: (1) a wide viewing angle liquid crystal display mode in which each pixel has a liquid crystal region where Np-type liquid crystal (nematic liquid crystal having positive dielectric anisotropy), which is oriented horizontally with respect to a substrate surface in the absence of applied voltage, is oriented axially symmetrically (Japanese Laid-Open Publication No. 7-120728); (2) a wide viewing angle liquid crystal display mode in which Nn-type liquid crystal (nematic liquid crystal having negative dielectric anisotropy), which is oriented substantially perpendicular to a substrate surface in the absence of applied voltage, is divided into a plurality of domains in which the falling directions in the presence of applied voltage are different (Japanese Laid-Open Publication No. 7-64089); and (3) a wide viewing angle liquid crystal display mode in which Np-type liquid crystal molecules in each pixel are divided substantially equally into four and are horizontally oriented (AM-LCD '96, p. 185(1996)). In the latter technique, a phase difference compensation element is requited on a principle so as to compensate a viewing angle of a 45° direction with respect to the absorption axis of a polarizer element.
Herein, the term “phase difference compensation element” means an optical component having birefringence in the form of a plate, sheet, or film. The term “polarizer element” means an optical component which absorbs one of two orthogonal beams of polarized light and passes the other beam. The absorption axis and transmission (polarization) axis of the polarizer are orthogonally crossed.
A phase difference film is currently made of stretched film. As resin material of the stretched film, polyvinylalcohol (PVA) and polycarbonate (PC) are commonly used. The phase difference film is required to have the following optical and mechanical characteristics,
(1) optical characteristics: less nonuniformity of phase difference, ability to match a wavelength dispersion characteristic of birefringence of a liquid crystal layer, ahigh level of heat resistance and moisture resistance, no optical defects such as light axis disturbance and contaminants, ahigh level of transmission, a small photoelasticity coefficient, performance of resisting transmission deterioration due to ultraviolet light, and the like; and
(2) mechanical characteristics: a high level of elasticity, a high level of tensile strength, a high level of yield bending strength, and the like.
The phase difference film is also required to be easy to process (e.g., easy to stretch) in the course of its manufacture. Among the above-described required characteristics, practically important characteristics are less phase difference nonuniformity, ability to match a wavelength dispersion characteristic of birefringence of a liquid crystal layer, and a small photoelasticity coefficient.
Japanese Laid-Open Publication No. 6-3524 discloses a uniaxial optical phase difference film made of crystalline copolymer of monochlorotrif luoroethylene (80-98 wt %) and fluorovinylidene (2-20 wt %) which is aligned in one direction.
The inventors have found that liquid crystal display devices using the above-described conventional phase retardation member have the following problems.
A phase retardation member disclosed in Japanese Laid-Open Publication No. 6-3524 cannot satisfy a retardation condition necessary for improving the display quality of a liquid crystal display device with a large screen (e.g., 42 inch type) or a liquid crystal display device including a liquid crystal layer having two or more different kinds of liquid crystal regions taking different initial orientation states in each of a plurality of pixels; or a liquid crystal layer having a liquid crystal region in which orientation directions of liquid crystal molecules continuously vary. In other words, when the phase retardation member is applied to these liquid crystal display devices, an insufficient viewing angle compensation effect is obtained and thus a viewing angle is so narrow that an image on the display screen becomes yellowish when viewing from a slanted direction. It is also difficult to manufacture the phase retardation member without nonuniformity of color having such a size that the member may be applied to a liquid crystal display device with a large screen (e.g., 42 inch type). Herein below, problems that the inventors have found will be described in detail with reference to the accompanying drawings.
In a large-size display device, e.g., 42-inch liquid crystal display device, various stresses are put on a phase difference compensation element. The various stresses are, for example, (1) a stress generated by attachment of a polarizer to the phase difference compensation element, or attachment of a liquid crystal cell to a polarizer attached to the phase difference compensation element; and (2) a stress due to heat generated by backlighting. As a result, the phase difference compensation element has retardation caused by the locally occurring stresses. This state is fixed by an adhesive. As a result, the transmission of the device is locally increased in a crossed-Nicols arrangement of the polarizers, whereby the brightness of the device becomes irregular and thus its display quality is largely impaired.
FIG. 1
schematically illustrates a display surface
10
of a large-size liquid crystal display device.
FIG. 1
shows local areas having increased transmission (light leakage) caused by the above-described stresses in a black display state. Light leakage (type 1) caused by a stress generated by attachment of a polarizer to a phase difference compensation element often emerges around the center of each of four edges of the display surface
10
. Light leakage (type 2) caused by a stress due to heat generated by backlighting emerges at the four corners of the display surface
10
. The degree of light leakage caused by these stresses depends on the magnitudes of the stresses and the photoelasticity coefficient of the material of the phase difference compensation element.
In the above-described display mode in which a pixel is divided into domains having different orientation directions of liquid crystal molecules, there is a problem that a viewing angle characteristic in a direction of an axis bisecting an angle between the absorption axes of an upper polarizer and a lower polarizer both sandwiching a liquid crystal cell (a viewing angle of a 45° direction with respect to the absorption axis of a polarizer) becomes significantly worse compared with a viewing angle characteristic in a direction of the absorption axis (see
FIG. 14
that illustrates isocontrast contour curves of Comparative Example 1 which will be described below).
Positions of polarizers in a liquid crystal display device and a definition of a direction of a viewing angle will be described with reference to
FIGS. 2A and 2B
.
FIG. 2A
is a diagram schematically illustrating a crossed-Nicols arrangement of polarizers in a liquid crystal display device. A polarizer disposed at a side nearer a viewer of a liquid cryst
Kohzaki Shuichi
Terashita Shin-ichi
Nixon & Vanderhye PC
Parker Kenneth
Sharp Kabushiki Kaisha
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