Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
1998-05-28
2001-02-13
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C349S179000, C349S181000
Reexamination Certificate
active
06188455
ABSTRACT:
This application is based on Japanese Patent Application No. 9-163142 filed on Jun. 19, 1997, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to techniques of improving the display quality of a liquid crystal display device by using optical compensation components.
b) Description of the Related Art
A liquid crystal display device having a homeotropic orientation liquid crystal cell disposed between two polarizers shows sharp threshold characteristics in an ECB (
e
lectrically
c
ontrolled
b
irefringence) mode. A high duty is therefore possible by matrix time divisional drive. However, light incidence in an oblique direction relative to the display screen of a liquid crystal display device has parasitic birefringence. Therefore, there is light transmission even without voltage application, and the contrast lowers considerably as compared to light incidence in a perpendicular direction relative to the display screen.
In order to solve this problem, optical compensation has been proposed by combining an optical compensation plate with an ECB mode liquid crystal cell as shown in FIG.
4
. Reference numeral
10
represents an ECB mode liquid crystal cell which is made of nematic liquid crystal having a positive refractive index anisotropy and a negative dielectric anisotropy and being disposed generally vertical to a glass substrate surface having electrodes. Two orthogonal Nicol configuration linear polarizers
12
and
13
sandwich the ECB mode liquid crystal cell
10
. When a voltage is not applied, the cell appears black, and when a voltage is applied, the cell enters a light transmission state and appears white.
An optical compensation plate
11
is inserted between the liquid crystal cell
10
and the linear polarizer
13
, the optical compensation plate
11
having a negative refractive index ellipsoid indicated at
15
in
FIG. 4. A
combination of the positive anisotropic refractive index ellipsoid
14
of the liquid crystal cell
10
and the negative refractive index ellipsoid
15
of the optical compensation plate
11
produces optical isotropy so that the optical compensation plate
11
functions as a view angle compensator. In
FIG. 4
, n. represents an extraordinary ray refractive index, and no represents an ordinary ray refractive index.
Optical compensation plates having a property described above have been manufactured by the following methods.
(1) A mixture of inorganic layer compound and polymer or the like is coated on a film (JP-A-5-196819, JP-A-6-82777).
(2) Discotic liquid crystal is coated on a film.
(3) A polycarbonate film is pulled or drawn in two directions.
(4) A thermosetting film is held between two glass plates and applied with heat and pressure (JP-B-7-69536).
The films formed by the above methods are all negative uniaxial.
In an ideal case of the optical compensation plate shown in
FIG. 4
, a combination of the positive anisotropic refractive index ellipsoid
14
of the liquid crystal cell
10
and the negative refractive index ellipsoid
15
of the optical compensation plate
11
produces optical isotropy, and the optical compensation plate
11
functions as a view angle compensator. In this case, the negative refractive index ellipsoid
15
of the optical compensation plate
11
is required to be a negative uniaxial or biaxial refractive index ellipsoid. Optical compensation by a positive refractive index ellipsoid is impossible.
The structure of a liquid crystal display device using a conventional optical compensation plate made of films formed by either of the methods (1) and (2) will be described with reference to
FIG. 5
which is a cross sectional view of a lamination structure of a liquid crystal cell, a compensation film, and a polarizer. On a liquid crystal cell
10
, an optical compensation film
17
is adhered with a binder layer
16
. On the optical compensation film
17
, a polarizer
18
is adhered with a binder layer
19
.
As a base for the polarizer
18
and optical compensation film
17
, TAC (triacetate cellulose) is generally used because of a good balance between cost and performance. The polarizer
18
has a structure of a polarizer layer
18
a
sandwiched between a pair of TAC films
18
b
and
18
c
. The optical compensation film
17
is formed by coating a negative uniaxial film layer
17
b
on the surface of a base TAC film
17
a.
The material of the negative uniaxial film layer
17
b
is an inorganic layer compound and an organic binder such as polyvinyl alcohol (PVA). Therefore, a birefringence dependency of this film
17
b
upon wavelength is determined by the inorganic layer compound. Generally, the refractive index dependency of an inorganic compound upon wavelength is almost constant over wavelengths of visible light rays. However, it is known that since liquid crystal is an organic compound including an aromatic compound, the refractive index greatly depends upon wavelength. Generally, both the ordinary and extraordinary ray refractive indices tend to become larger on the shorter wavelength side.
Therefore, a combination of a liquid crystal cell and a compensation plate made of an inorganic layer compound cannot realize an optical compensation uniformly over the whole range of wavelengths of visible rays. In the example described above, if the birefringence near 550 nm is optimized, a yellowish image appears on the display screen when viewed obliquely. In particular in a color display, color tone of blue becomes near achromatic color.
If a TAC film is exposed for a long time in an atmosphere at 60° C. or higher or in high temperature and humidity environments, the film shrinks in arrow directions as shown in
FIGS. 6A and 6B
and optical uniaxes are induced in the shrinking directions by a shrinking stress, independently from the optical axis inherent to the TAC film.
FIG. 6A
is a plan view of the TAC film, and
FIG. 6B
is a cross sectional view along a dotted chain line of
FIG. 6A
, in a liquid crystal display device as shown in FIG.
4
. Optical uniaxes caused by shrinking have different directions at different areas because of different shrinking stresses in various in-plane areas, although they are influenced by the shape and size of the film. Therefore, even if the polarizer
18
is adhered in an optimum direction, there is always an in-plane area where the direction of a uniaxis is shifted from a predetermined angle to the transmission/absorption axis of the polarizer. The effects of the optical compensation plate are therefore lost, and the induced uniaxis generates birefringence and a white spot (leakage light) is formed on the display screen.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a liquid crystal display device having excellent view angle characteristics, by using an optical compensation unit having a negative refractive index ellipsoid.
It is another object of the present invention to provide a liquid crystal display device having less deterioration of a display performance even in an atmosphere at 60° C. or higher or in high temperature and humidity environments.
According to one aspect of the present invention, there is provided a liquid crystal display device comprising: a drive liquid crystal cell having a pair of substrates disposed to face each other through a predetermined gap, each substrate having an electrode on one surface thereof, and a liquid crystal layer disposed between the pair of substrates, and comprising liquid crystal molecules being controlled by a voltage applied between electrodes; a pair of polarizers disposed outside of the drive liquid crystal cell; and optical compensation means disposed between the drive liquid crystal cell and one of the pair of polarizers, wherein the optical compensation means comprises a lamination of a left-twist liquid crystal layer comprising liquid crystal molecules having a chirality in a left-twist direction and a right-twist liquid crystal layer
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Sikes William L.
Stanley Electric Co. Ltd.
Ton Toan
LandOfFree
Liquid crystal display with two optical compensation units does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Liquid crystal display with two optical compensation units, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal display with two optical compensation units will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2608750