Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-07-24
2002-04-16
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S129000
Reexamination Certificate
active
06373542
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device in which viewing angle dependence of the display screen is improved by combining a liquid crystal display element with an optical retardation plate.
BACKGROUND OF THE INVENTION
In the past, liquid crystal display devices using a nematic liquid crystal display element were widely used in numeral segment display devices in watches, calculators, etc., but recently, they have come to be used in devices such as word processors, laptop personal computers, and in-car liquid crystal televisions.
Generally, liquid crystal display elements include a transparent substrate, on which are provided electrode lines, etc. for turning the pixels on and off. For example, in an active matrix type liquid crystal display device, in addition to the electrode lines, active elements such as thin film transistors are provided on the substrate, as switching means for selective driving of pixel electrodes which apply a voltage to the liquid crystal. Further, in liquid crystal display devices capable of color display, color filter layers of, for example, red, green, and blue, are also provided on the substrate.
With regard to liquid crystal display methods used in liquid crystal display elements of this type, different methods are selected, as needed, depending on the twist angle of the liquid crystal. Two well-known examples are the twisted nematic liquid crystal display method (hereinafter referred to as the “TN method”), which is an active driving method, and the super twisted nematic liquid crystal display method (hereinafter referred to as the “STN method”), which is a multiplexing driving method.
In the TN method, nematic liquid crystal molecules are aligned in an arrangement twisted 90°, and display is performed by guiding light along the direction of twist. The STN method, on the other hand, makes use of the fact that, by expanding the twist angle of the liquid crystal molecules to more than 90°, light transmittance changes abruptly around the liquid crystal's applied voltage threshold.
Since the STN method makes use of the birefringence effect of the liquid crystal, color interference causes the background of the display screen to have a distinctive color. To remedy this kind of shortcoming and perform black-and-white display using the STN method, it is considered effective to use an optical compensation plate. Display methods which use an optical compensation plate can be broadly divided into the double-layered super twisted nematic phase compensation method (hereinafter referred to as the “D-STN method”) and the film-type phase compensation method (hereinafter referred to as the “film provision method”), in which a film having optical anisotropy is provided.
The D-STN method uses a two-layer structure of a liquid crystal cell for display and another liquid crystal cell in a twisted alignment with a twist angle of a direction opposite that of the liquid crystal cell for display. The film provision method, on the other hand, uses a structure provided with a film having optical anisotropy. From the point of view of weight and cost, the film provision method is considered superior. Since black-and-white display characteristics have been improved by adopting phase compensation methods of this type, color STN liquid crystal display devices have also been realized, which enable color display by providing a color filter layer in a display device using the STN method.
The TN method, on the other hand, can be broadly divided into normally-black and normally-white methods. In the normally-black method, two polarization plates are arranged so that their directions of polarization are mutually parallel, and black is displayed when an ON voltage is not applied to the liquid crystal (OFF state). In the normally-white method, two polarization plates are arranged so that their directions of polarization are mutually perpendicular, and white is displayed in the OFF state. From the points of view of display contrast, color repeatability, dependence of display on viewing angle, etc., the normally-white method is superior.
However, with TN liquid crystal display devices, since the liquid crystal molecules have anisotropy of the refractive index &Dgr;n, and since the liquid crystal molecules are aligned so as to tilt with respect to the upper and lower substrates, the contrast of the display image varies according to the direction and angle from which it is viewed; thus viewing angle dependence is large.
FIG. 12
schematically shows the structure of a TN liquid crystal display element
31
in cross-section. The Figure shows a state in which a voltage for display of a gray shade is being applied, and the liquid crystal molecules
32
are tilted up slightly. In the TN liquid crystal display element
31
, a linearly polarized light ray
35
, traveling in the normal direction of the surfaces of substrates
33
and
34
, and linear polarized light rays
36
and
37
traveling at an incline with respect to the normal angle, cross the liquid crystal molecules
32
at different respective angles. Since the liquid crystal molecules
32
have anisotropy of the refractive index &Dgr;n, the linearly polarized light rays
35
,
36
, and
37
of each direction, in passing among the liquid crystal molecules
32
, produce ordinary light and extraordinary light, and due to a difference in phase thereof, the linearly polarized light rays
35
,
36
, and
37
are converted into elliptically polarized light. This is the cause of viewing angle dependence.
Further, in an actual liquid crystal layer, the angle of tilt of liquid crystal molecules
32
around the midpoint between the substrates
33
and
34
differs from that of the liquid crystal molecules in the vicinity of either substrate
33
or
34
, and the liquid crystal molecules
32
are also twisted 90° along the axis of the normal direction.
For these reasons, in passing among the liquid crystal molecules
32
, the linearly polarized light rays
35
,
36
, and
37
are subject to various birefringence effects depending on their direction and angle of travel, and show a complex viewing angle dependence.
Specifically, viewing angle dependence is evident as phenomena such as the following. As viewing angle is gradually inclined in the standard viewing angle direction (below the normal direction of the display screen), above a certain angle from the normal direction, phenomena occur such as coloring of the display image (hereinafter referred to as “coloring phenomenon”) and reversal of black and white (hereinafter referred to as “reversal phenomenon”). Again, as viewing angle is gradually inclined in the opposite viewing angle direction (above the normal direction of the display screen), contrast is drastically impaired.
Further, the foregoing liquid crystal display device has the problem that, as the size of the display screen is increased, the viewing angle field is decreased. If a large liquid crystal display screen is viewed at close range from directly in front of the screen, there are cases in which viewing angle dependence causes colors in the upper and lower parts of the display screen to appear to differ. This is because the actual angles of view needed to view peripheral portions of the screen are increased, which is equivalent to viewing the screen from a more inclined viewing angle.
In order to improve this kind of viewing angle dependence, Japanese Unexamined Patent Publication Nos. 55-600/1980 (Tokukaisho 55-600) and 56-97318/1981 (Tokukaisho 56-97318), for example, have proposed insertion of an optical retardation plate (optical retardation film), as an optical element having optical anisotropy, between a liquid crystal display element and one of the polarizing plates.
In this method, light which has been converted from linearly polarized to elliptically polarized light by passing among the liquid crystal molecules (which have anisotropy of the refractive index) passes through an optical retardation plate. Consequently, th
Mizushima Shigeaki
Yamahara Motohiro
Chowdhury Tarifur R.
Conlin David G.
Daley, Jr. Williams J.
Dike Bronstein Roberts & Cushman
Sharp Kabushiki Kaisha
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