Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2002-05-29
2004-11-23
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C345S094000, C345S095000, C345S096000
Reexamination Certificate
active
06822700
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device applying Twist OCB mode, and, more particularly, to a liquid crystal display device having stairs type driving signal which can eliminate over-transmittance due to voltage application and have rapid response time by successively applying data signals having different size when a gate pulse is applied.
Generally, liquid crystal display devices have a structure that liquid crystal molecules are aligned between a pair of transparent substrates. In the structure, screen display is performed by transmitting or cutting off light for electrically controlling the alignment state of liquid crystal molecules. The liquid crystal display device has been used as an indicator of electronic calculators and digital watches and at the same time, rapidly extended its application to a screen of laptop computer, television receiver and word processor.
The conventional liquid crystal display has generally employed TN mode which has improved optical properties, clear B/W display and rapid response time. As shown in
FIG. 1
, the TN mode liquid crystal display device comprises an upper and a lower substrates
1
,
5
, a pixel electrode
2
arranged on the inner side of lower substrate
1
, a common electrode
6
arranged on the inner side of upper substrate
5
, alignment layers (not shown) respectively arranged on the opposite surfaces of upper and lower substrates, polarizing plates
4
,
7
on the outer sides of upper and lower substrates
1
,
5
and a liquid crystal layer
8
interposed between upper and lower substrates
1
,
5
, including a plurality of liquid crystal molecules.
The alignment layer (not shown) is a horizontal alignment layer wherein a rubbing axis
3
b
thereof is crossed by 90° and upper and lower polarizing plates and polarizing axes
4
a
,
7
a
are attached in a crossed direction. That is, the rubbing axis
2
a
of lower alignment layer and the polarizing axis
4
a
of lower polarizing plate are attached in the same direction, and the rubbing axis
3
b
of upper alignment layer and the polarizing axis
7
a
of upper polarizing plate are attached in the same direction. And, the pixel electrode
2
and the common electrode
6
are formed in a plate shape.
As shown in
FIG. 1
, liquid crystal molecules
8
are levorotatorily twisted by 90° under the influence of upper and lower alignment layers and chiral dopant before the electric field is applied to the region between the pixel electrode
2
and the common electrode
6
. Therefore, after passing through the lower polarizing plate
4
, light can pass through levorotatorily twisted liquid crystal molecules
8
a
, and then the upper polarizing plate
7
. As a result, the screen becomes white.
Although it is not shown in the drawings, when the electric filed is applied to the region between the pixel electrode
2
and the common electrode
6
, the liquid crystal molecules
8
a
are arranged to be parallel with the electric field (perpendicular to the substrate) formed between driving electrodes. Therefore, after passing through the lower polarizing plate
4
, light cannot pass through the crossed upper polarizing plate since the major axis of liquid crystal molecules
8
a
is perpendicular to the surface of substrate. As a result, the screen becomes dark.
However, the TN liquid crystal display device has different refractive anisotropies according to the direction since the interposed liquid crystal molecules have a shape of bar. Thus, transmittance is drastically diversified according to the viewing angle and therefore, the liquid crystal display has disadvantages that it is difficult to apply to large scale display and the response time is too slow to realize moving pictures. As a result, it has difficulty in being applied to large scale TV due to its narrow viewing angle and slow response time.
In order to solve the problems, various liquid crystal modes have been proposed. However, they have not completely satisfied wide viewing angle and at the same time, high speed response time. Recently, OCB (Optically Compensated Bend) mode has been proposed to have a wide viewing angle by a phase compensation film and at the same time, a high speed response time below 10 ms. The OCB is a mode using bend of liquid crystals generated when the upper and lower substrates are rubbed to be parallel with each other and a predetermined voltage is applied.
The OCB mode liquid crystal display (Reference: SID 93 Digest P277, “Wide-Viewing-Angle Display Mode for the Active-Matrix LCD Using Bend-Alignment Liquid Crystal Cell, Y. Yamaguchi, T. Miyashita, T. Uchida) can compensate refractive anisotropy of liquid crystal molecules without several times of rubbing processes, thereby maintaining regular viewing angle in any direction of screen.
FIGS. 2A
to
2
C are drawings showing a conventional OCB mode liquid crystal display device.
Referring to
FIG. 2A
, a lower substrate
10
and an upper substrate
15
are arranged opposite to each other with a predetermined distance. A liquid crystal layer
18
is interposed between the lower substrate
10
and the upper substrate
15
. The liquid crystal layer includes a plurality of liquid crystal molecules
18
a
, made of materials having positive dielectric anisotropy. And, driving electrodes
11
,
16
are arranged on the inner sides of lower and the upper substrates
10
,
15
to drive liquid crystal molecules, wherein a first alignment layer
12
is disposed on the inner side of lower substrate
10
, that is, between the lower substrate
10
and the liquid crystal layer
19
and a second alignment layer
17
is disposed on the inner side of upper substrate
15
, that is, between the upper substrate
15
and the liquid crystal layer
19
. The first and the second alignment layers
12
,
17
are horizontal alignment layers having a pretilt angle of below 10°, rubbed to the direction parallel with each other. Furthermore, polarizing plates
19
a
,
19
b
are attached on the outer sides of lower and upper substrates
10
,
15
, having a predetermined of polarizing axes. It is desirable that the polarizing axes of the polarizing plates
19
a
,
19
b
are cross-arranged with each other.
As shown in
FIG. 2A
, when the voltage is not applied to the OCB mode liquid crystal display device, liquid crystal molecules
18
a
are arranged in a shape of splay under the influence of first and second alignment layers
12
,
17
.
As shown in
FIG. 2B
, when the voltage is applied between the driving electrodes to the critical voltage Vs, that is, as much as liquid crystal molecules
18
a
in the middle layer of the liquid crystal layer
18
are affected by electric field, the liquid crystal molecules
19
a
in the middle layer are twisted by the effect of electric field E
1
, so that the electric field and the major axis thereof are parallel with each other. However, the liquid crystal molecules
19
a
arranged on upper and lower parts are affected by alignment layers
12
,
17
than by electric field, thereby maintaining the initial alignment. Here, it is possible to control d&Dgr;n of liquid crystals to make white state.
Thereafter, as shown in
FIG. 2C
, when the voltage greater than the critical voltage Vs is applied between driving electrodes, liquid crystal molecules are affected by electric field E2 in the middle layer and the vicinities thereof. Therefore, they are twisted so that the electric field and the major axis thereof are parallel with each other, thereby screen becomes dark. The liquid crystal molecules
19
a
adjacent to the surface of substrates
10
,
15
are affected by alignment layers
12
,
17
than by electric field, thereby maintaining the initial alignment.
In the OCB mode liquid crystal display device, liquid crystal molecules
19
a
are arranged symmetrically with respect to the middle layer when the electric field is formed. Therefore, it is possible to accomplish phase compensation when light passes through the upper substrate
15
from the lower substrate
10
. And, when the electric field is not formed, back
Hong Seung Ho
Lee Seung Hee
Boe-Hydis Technology Co., Ltd.
Caley Michael H.
Chowdhury Tarifur R.
Ladas & Parry LLP
LandOfFree
Liquid crystal display device having stairs type driving signal 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 device having stairs type driving signal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal display device having stairs type driving signal will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3280230