Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
2000-09-22
2001-12-11
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S490020, C349S096000, C349S099000
Reexamination Certificate
active
06330108
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a composite retarder plate and an optically compensatory polarizing plate which can highly compensate for birefringence owing to TN liquid crystal in order to form a liquid-crystal display device excellent in viewing angle and contrast.
The present application is based on Japanese Patent Application No. Hei. 11-269059, which is incorporated herein by reference.
2. Description of the Related Art
TFT-LCD (liquid-crystal display) devices using TN liquid crystal have come into wide use for television sets, personal computer monitors, etc. because quick response characteristic and frontal-view high contrast characteristic have attracted a great deal of attention. In the meantime, the viewing angle range permitting good visibility has been narrowed owing to remarkable lowering of oblique-view contrast, reversal of gradation display (tone reversal), etc. The viewing angle range has needed to be mended. Improvement of contrast, widening of the viewing angle range, suppression of display color change owing to viewing angle and uniformity of screen display have become particularly important subjects with the advance of increase in screen size.
A proposal to use a retarder for compensating for phase difference owing to birefringence of TN liquid crystal in order to widen the viewing angle has been made heretofore as a measure against the aforementioned narrowed viewing angle range. A Wide View Film (tradename: made by Fuji Photo Film Co., Ltd.), an NH Film (tradename: made by Nippon Petrochemicals Co., Ltd.) and superposed compensating plates (Unexamined Japanese Patent Publication No. Hei. 7-306406, Unexamined Japanese Patent Publication No. Hei. 7-35924 and Unexamined Japanese Patent Publication No. Hei. 10-123506) are known as compensating plates for widening the viewing angle. The. Wide View Film is made of discotic liquid crystal with an inclined optical axis and exhibits negative refractive index anisotropy. The NH Film is made of nematic liquid crystal with an inclined optical axis and exhibits positive refractive in ex anisotropy. Each of the superposed compensating plates is made of a laminate of retarders. Each of the retarders is made of a uniaxial oriented film of a polymer exhibiting birefringence characteristic. In each of the superposed compensating plates, a retarder having an optical axis in a plane and a retarder having an optical axis inclined with respect to the plane are used in combination so that refractive index directions of the retarders cross at right angles.
The Wide View Film, however, had a problem that a coloring phenomenon occurred in a white display state without application of any voltage because of remarkable lowering of contrast at a viewing angle inclined by 60 degrees or more with respect to the frontal direction. The NH Film had a problem that a black display state with application of a voltage was not kept when the viewing angle was changed in the black display state. The superposed compensating plates had a problem that a remarkable coloring phenomenon occurred owing to the change of the viewing angle. Hence, the background-art compensating plates could not be adapted to phase difference characteristic of TN liquid crystal satisfactorily, and there was a problem that improvement in visibility characteristic of the background-art compensating plates was not sufficient.
SUMMARY OF THE INVENTION
An object of the present invention is to develop a retarder by which it is possible to constitute a liquid-crystal display device which is able to highly compensate for phase difference owing to birefringence of TN liquid crystal even in the case of a large screen size, and which is excellent in viewing angle while being contrast free from tone reversal, in suppression of display color change owing to the viewing angle and in uniformity of screen display.
According to an aspect of the present invention, there is provided a composite retarder plate comprising a laminate including (A) at least one retarder layer made of a high-molecular film satisfying the relation nx>ny>nz, (B) at least one retarder layer satisfying tie relation nx≧ny>nz and having an optical axis inclined relative to a direction of a normal line with respect to a plane of the layer, and (C) at least one retarder layer satisfying the relation nx>nz≧ny or nx>ny>nz when nx and ny are in-plane refractive indices and nz is a refractive index in a direction of thickness, wherein &Dgr;Nxy and Rth given by Nx≧Ny, (Nx−Ny)·D=&Dgr;Nxy and {(Nx+Ny)/2−Nz}·D=Rth and based on monochromatic light with a wavelength of 590 nm are in a range of from 25 to 100 nm and in a range of from 100 to 300 nm respectively when Nx and Ny are in-plane refractive indices in the laminate, Nz is a refractive index in a direction of thickness of the laminate and D is the thickness of the laminate.
The present invention further provides: an optically compensatory polarizing plate which comprises a laminate including the aforementioned composite retarder plate and a polarizing plate; and a liquid-crystal display device which comprises a polarizing plate, a liquid-crystal cell, and the aforementioned composite retarder plate disposed between the polarizing plate and the liquid-crystal cell.
According to the present invention, the phase difference characteristic of &Dgr;Nxy and Rth can be achieved by composite phase difference based on the aforementioned combination of the retarder layers (A), (B) and (C). Accordingly, it is possible to obtain a retarder that can highly compensate for phase difference owing to birefringence of TN liquid crystal at all azimuth angles. Hence, it is possible to form a liquid-crystal display device which is free from tone reversal even in the case where the liquid-crystal display device is applied to a large-size screen, which is so wide in viewing angle and the display color hardly changes owing to the viewing angle, and which is excellent in uniformity of contrast and screen display.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.
REFERENCES:
patent: 5504603 (1996-04-01), Winker et al.
patent: 5793455 (1998-08-01), Nakamura
patent: 5986732 (1999-11-01), Ozeki et al.
patent: 6034754 (2000-03-01), Sato et al.
patent: 7-35924 (1995-02-01), None
patent: 7-306406 (1995-11-01), None
patent: 10-123506 (1998-05-01), None
Patent Abstracts of Japan 10123506 May 15, 1998.
Patent Abstracts of Japan 07035924 Feb. 7, 1995.
Nishikouji Yuuichi
Yoshimi Hiroyuki
Nitto Denko Corporation
Schuberg Darren
Sughrue Mion Zinn Macpeak & Seas, PLLC
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