Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-10-11
2004-05-25
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S095000, C349S120000
Reexamination Certificate
active
06741306
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a liquid crystal display unit and, more particularly, to a wide viewing angle liquid crystal display having an optical compensator and an optical diffuser.
DESCRIPTION OF THE RELATED ART
The liquid crystal display has various attractive features such as light, thin and low in power consumption, and find a wide variety of application. A flat display panel is a typical consumer product of the liquid crystal display, and forms an essential part of an office machine, portable television set for vehicles or a navigation system for vehicles.
The liquid crystal display used in the vehicles is expected to have a wide viewing angle, because the liquid crystal display is usually placed to be close to the driver. An image produced on the liquid crystal display has a viewing angle dependency. The image is assumed to be recognized as a black configuration by a person siting in front of the liquid crystal display. However, when the person is laterally moved over a critical angle, the image whitens, and the person feels it unclear. For this reason, the liquid crystal display for vehicles is expected to have the wide viewing angle characteristics.
FIG. 1
illustrates a standard liquid crystal display. The prior art liquid crystal display largely comprises a surface light source
1
and a liquid crystal panel
2
. The surface light source
1
is implemented by a cold-cathode tube fluorescent lamp associated with an optical guide plate, and illuminates the liquid crystal panel
2
. The cold-cathode tube fluorescent lamp is assumed to be a line light source, and the light is incident into an input end surface of the optical guide plate. The light repeats the total reflection in the optical guide plate, and diffusing elements in the optical guide plate cause the light
1
a
to uniformly illuminate the liquid crystal panel
2
. If the light
1
a
is less uniform, a diffusing sheet is further attached to the output end surface of the optical guide plate.
The liquid crystal panel
2
includes a pair of transparent substrate structures
2
a
and
2
b
spaced apart from each other by a sealing layer
2
c
and twisted nematic liquid crystal
2
d
filling the gap between the transparent substrate structures
2
a
and
2
b
. The twisted nematic liquid crystal
2
d
has a twisting angle of about 90 degrees.
The transparent substrate structure
2
a
has a plurality of pixel electrodes arranged in matrix, a plurality of thin film transistors selectively connected between the pixel electrodes and image signal lines and gate control lines selectively connected to the gate electrodes of the thin film transistors, and these components are formed on and over a transparent glass substrate. On the other hand, the transparent substrate structure
2
b
has a transparent counter electrode and color filters formed on and over a transparent glass substrate.
The liquid crystal panel
2
further includes polarizing plates
2
e
/
2
f
attached to the outer surfaces of the transparent substrate structures
2
a
/
2
b
, respectively. When the standard liquid crystal display is designed to be normally white mode, the polarizing plates
2
e
/
2
f
have transmission axes angularly spaced at 90 degrees. The pixel electrodes, the twisted nematic liquid crystal and the counter electrode form a matrix of liquid crystal capacitors, and the liquid crystal capacitors serve as a matrix of pixels where an image is produced
When an image
3
a
is produced on a screen
3
b
, the counter electrode is biased to a certain constant potential level, and image-carrying signals sequentially bias the pixel electrodes to potential levels representing the gradations by controlling the thin film transistors. The droplets of liquid crystal droplet
2
d
rise at different angles depending upon the potential difference between the pixel electrodes and the counter electrode, and the pixels have a certain dispersion of transmittance so as to produce the image
3
a
on the screen
3
b.
When a person sees the image
3
a
in front of the screen
3
b
, the person clearly recognizes the image
3
a
on the screen
3
b
. However, the liquid crystal
2
d
has an anisotropic refractive index. If the image is obliquely seen, an inversion of gradation or deterioration of contrast ratio take place, and the person hardly discriminates the image
3
a
on the screen
3
b
. The inversion of gradation is the phenomenon where an image recognized by a person is produced in a gradation inverted from a gradation of an image intended to be produced on the screen. The contrast ratio is a ratio between white luminance and black luminance. In general, when the person is downwardly moved to a position at 5 degrees with respect to a vertical line to the screen
3
b
, the inversion of gradation takes place. If the angle exceeds 30 degrees toward an upper portion of the screen and 50 degrees toward a lower position of the screen, the contrast ratio is decreased to 10 or less.
Thus, the anisotropy of the refractive index makes the viewing angle of the liquid crystal display narrow, and various technologies have been proposed for wide viewing angle characteristics. One of the techniques is disclosed in Japanese Patent Publication of Unexamined Application No. 7-120619, and
FIGS. 2 and 3
illustrates an index ellipsoid of an optically anisotropic element and a liquid crystal panel using the optically anisotropic film both disclosed in the Japanese Patent Publication of Unexamined Application. The liquid crystal panel shown in
FIGS. 2 and 3
is hereinbelow referred to as “first prior art”.
The optically anisotropic film
10
has the principal indices of refraction nx, ny and nz where nx>ny>nz. As shown in
FIG. 2
, nx is on the optically anisotropic film
10
, and ny and nz decline with respect to a rotating axis aligned with the direction of nx. The principal indices of refraction are represented by an index ellipsoid. The optically anisotropic film
10
is effective against reduction of contrast due to a double refraction inherent in liquid crystal.
Using a pair of optically anisotropic films
10
a
/
10
b
, the first prior art liquid crystal panel comprises a liquid crystal cell
11
provided between the optically anisotropic films
10
a
/
10
b
and polarizing layers
12
b
/
12
b
, and the polarizing layers
12
a
/
12
b
are spaced from each other by a gap where the liquid crystal cell
11
and the optically anisotropic films
10
a
/
10
b
are inserted. The liquid crystal cell
11
is analogous to the prior art standard liquid crystal panel, and includes a bottom transparent substrate
11
a
, transparent electrodes
11
b
formed on the bottom transparent substrate
11
a
, a top transparent substrate
11
c
spaced from the bottom transparent substrate
11
a
by a spacer
11
d
, a transparent electrode
11
e
formed on the lower surface of the top transparent substrate
11
c
and liquid crystal
11
f
filling the gap between the bottom transparent substrate
11
a
and the top transparent substrate
11
c.
Although the optically anisotropic films
10
a
/
10
b
suppress the reduction of contrast due to the double refraction of the liquid crystal, the optically anisotropic films
10
a
/
10
b
can not sufficiently suppress the inversion of gradation. When twisted nematic liquid crystal is used for the first prior art, an image is unintentionally colored.
Another improvement is disclosed in Japanese Patent Publication of Unexamined Application No. 7-159614, and is hereinbelow referred to as “second prior art”.
FIG. 4
illustrates a polarization of light L
0
obliquely incident onto the second prior art. The second prior art comprises a liquid crystal cell
20
, an optically anisotropic element
21
and a pair of polarizing plates
22
a
/
22
b
, and liquid crystal LC fills a gap in the liquid crystal cell
20
.
When the light is obliquely incident onto the polarizing plate
22
a
, lineally polarized light Li takes place, and proceeds toward the liquid crystal cell
20
. The liquid crystal LC converts the linearly polarized light Li t
Maruyama Muneo
Ohi Susumu
Duong Tai
NEC LCD Technologies Ltd.
Sughrue & Mion, PLLC
Ton Toan
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