U.S. patent application number 09/105175 was filed with the patent office on 2002-01-24 for liquid crystal display device.
Invention is credited to MURAYAMA, AKIO, TAKASE, TAKESHI.
Application Number | 20020008823 09/105175 |
Document ID | / |
Family ID | 15922039 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008823 |
Kind Code |
A1 |
MURAYAMA, AKIO ; et
al. |
January 24, 2002 |
LIQUID CRYSTAL DISPLAY DEVICE
Abstract
A liquid crystal display device includes an array substrate and
a counter substrate, a liquid crystal cell held between the
substrates 300 and containing liquid crystal molecules arranged to
have an alignment corresponding to alignment properties of inner
surfaces of the substrates, first and second polarizing plates
having individual polarization axes and mounted on outer surfaces
of the first and second substrates, respectively, a pixel electrode
and counter electrode formed on the array substrate to apply a
lateral electric field substantially parallel to the first and
second substrate into the liquid crystal cell. The liquid crystal
display device controls the alignment of the liquid crystal
molecules by the lateral electric field between the pixel and
counter electrodes. Particularly, in the liquid crystal display
device, an optical retardation plate is interposed at least between
the first polarizing plate and the array substrate, and an optical
axis and retardation value of the optical retardation plate are
determined to compensate for twisting of the alignment of the
liquid crystal molecules caused upon application of the lateral
electric field.
Inventors: |
MURAYAMA, AKIO; (IBO-GUN,
JP) ; TAKASE, TAKESHI; (IBO-GUN, JP) |
Correspondence
Address: |
CUSHMAN DARBY & CUSHMAN
INTELLECTUAL PROPERTY GROUP OF
PILLSBURY MADISON & SUTRO
1100 NEW YORK AVE NW 9TH FLOOR E TOWER
WASHINGTON
DC
200053918
|
Family ID: |
15922039 |
Appl. No.: |
09/105175 |
Filed: |
June 26, 1998 |
Current U.S.
Class: |
349/141 |
Current CPC
Class: |
G02F 2413/08 20130101;
G02F 1/133749 20210101; G02F 1/13363 20130101; G02F 1/134363
20130101; G02F 2202/40 20130101; G02F 1/1337 20130101; G02F 2413/02
20130101 |
Class at
Publication: |
349/141 |
International
Class: |
G02F 001/1337 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 27, 1997 |
JP |
9-171376 |
Claims
1. A liquid crystal display device comprising: first and second
substrates; a liquid crystal cell held between said first and
second substrates and containing liquid crystal molecules arranged
to have an alignment corresponding to alignment properties of inner
surfaces of said first and second substrates; first and second
electrodes formed on said first substrate to apply a lateral
electric field substantially parallel to said first and second
substrate into said liquid crystal cell; first and second
polarizing plates having individual polarization axes and mounted
on outer surfaces of said first and second substrates,
respectively; and an optical retardation plate interposed at least
between said first polarizing plate and said first substrate;
wherein an optical axis and retardation value of said optical
retardation plate are determined to compensate for twisting of the
alignment of said liquid crystal molecules caused upon application
of the lateral electric field.
2. A liquid crystal display device according to claim 1, wherein a
bright state and a dark state are designated by selectively
applying one of first and second voltages between said first and
second electrodes, said first and second voltages being
substantially not zero and differ from each other.
3. A liquid crystal display device according to claim 2, wherein
the first voltage is set lower than the second voltage to designate
the bright state by applying the first voltage and to designate the
dark state by applying the second voltage.
4. A liquid crystal display device according to claim 3, wherein
said liquid crystal molecules are aligned in a predetermined
alignment direction R on the inner surfaces of said first and
second substrates, and a direction E of the lateral electric field
and the predetermined alignment direction R form an angle less than
90.degree. and not less than 45.degree.
5. A liquid crystal display device according to claim 4, wherein
the direction E of the lateral electric field and the alignment
direction R form an angle of 60.degree. to 88.degree..
6. A liquid crystal display device according to claim 2, wherein
the first voltage is set lower than the second voltage to designate
the dark state by applying the first voltage and to designate the
bright state by applying the second voltage.
7. A liquid crystal display device according to claim 6, wherein
said liquid crystal molecules are aligned in a predetermined
alignment direction R on the inner surfaces of said first and
second substrates, and a direction E of the lateral electric field
and the predetermined alignment direction R form an angle less than
90.degree. and not less than 45.degree.
8. A liquid crystal display device according to claim 7, wherein
the direction E of the lateral electric field and the predetermined
alignment direction R form an angle of 60.degree. to
88.degree..
9. A liquid crystal display device according to claim 1, wherein
said liquid crystal molecules are aligned in a predetermined
alignment direction R on the inner surfaces of said first and
second substrates, and an optical axis W of said optical
retardation plate and the predetermined alignment direction R form
an angle of 45.degree. to 135 .
10. A liquid crystal display device according to claim 9, wherein
the optical axis W of said optical retardation plate and the
alignment direction R form an angle of 60.degree. to
90.degree..
11. A liquid crystal display device according to claim 1, wherein
the retardation value of said optical retardation plate is
{fraction (1/3 )}to {fraction (2/3 )}of a retardation value RLc of
said liquid crystal cell.
12. A liquid crystal display device according to claim 1, wherein
another optical retardation plate is interposed between said second
polarizing plate and said second substrate, wherein a retardation
value of each optical retardation plate is {fraction (1/7 )}to
{fraction (4/9 )} of a retardation value RLc of said liquid crystal
cell.
13. A liquid crystal display device according to claim 12, wherein
the retardation value of each optical retardation plate is
{fraction (1/4 )}to {fraction (4/9 )} of the retardation value RLc
of said liquid crystal cell.
14. A liquid crystal display device according to claim 1, wherein
one of the polarization axes of said first and second polarizing
plates is offset with reference to a twisted alignment of said
liquid crystal molecules.
15. A liquid crystal display device according to claim 14, wherein
the polarization axes of said first and second polarizing plates
intersect each other at an angle larger than 90.degree..
16. A liquid crystal display device according to claim 1, wherein
an optical axis of said optical retardation plate is
two-dimensionally twisted.
17. A liquid crystal display device comprising: first and second
substrates; a liquid crystal cell held between said first and
second substrates and containing liquid crystal molecules arranged
to have an alignment corresponding to alignment properties of inner
surfaces of said first and second substrates; first and second
electrodes formed on said first substrate to apply a lateral
electric field substantially parallel to said first and second
substrate into said liquid crystal cell; first and second
polarizing plates having individual polarization axes and mounted
on outer surfaces of said first and second substrates,
respectively; and first and second optical retardation plates
interposed between said first polarizing plate and said first
substrate and said second polarizing plate and said second
substrate, respectively; wherein when a first lateral electric
field is produced to obtain a dark state and a second lateral
electric field is produced to obtain a bright state, optical axes
and retardation values of said first and second optical retardation
plates are determined to compensate for twisting of the alignment
of said liquid crystal molecules caused upon application of the
first lateral electric field.
18. A liquid crystal display device according to claim 17, wherein
the first lateral electric field is smaller than the second
electric field.
19. A liquid crystal display device according to claim 18, wherein
said first polarizing plate and said first and second optical
retardation plates are arranged to obtain substantially linear
polarized light output toward said second polarizing plate, and the
optical axis of said second polarizing plate is arranged to be
substantially orthogonal to the output light when the twisted
alignment of said liquid crystal molecules occurs upon application
of the first lateral electric field.
20. A liquid crystal display device according to claim 17, wherein
the first lateral electric field is larger than the second electric
field.
21. A liquid crystal display device according to claim 20, wherein
said first polarizing plate and said first optical retardation
plate are arranged to obtain substantially linear polarized light
output toward said second polarizing plate, and the optical axis of
said second polarizing plate is arranged to be substantially
orthogonal to the output light when the twisted alignment of said
liquid crystal molecules occurs upon application of the first
lateral electric field.
22. A liquid crystal display device according to claim 17, wherein
said first optical retardation plate is arranged to obtain
substantially linear polarized light output toward said second
polarizing plate, and the optical axis of said second polarizing
plate is arranged to be substantially orthogonal to the output
light.
23. A liquid crystal display device according to claim 17, wherein
said first and second optical retardation plates have a same
structure.
24. A liquid crystal display device according to claim 17, wherein
an optical axis of one of said first and second optical retardation
plates is twodimensionally twisted.
25. A liquid crystal display device according to claim 1, wherein
said first and second substrates respectively include first and
second alignment films which serve as the inner surfaces to align
the liquid crystal molecules in a same direction.
26. A liquid crystal display device according to claim 17, wherein
said first and second substrates respectively include first and
second alignment films which serve as the inner surfaces to align
the liquid crystal molecules in a same direction.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a liquid crystal display device,
in particular, to a liquid crystal display device mainly using an
electric field substantially in parallel to a display screen.
[0002] In recent years, liquid crystal display devices have been
used in various fields by virtue of their merits: lightweight,
thinness, and low power consumption. Particularly, a widespread
liquid crystal display device has a structure in which twisted
nematic (TN) liquid crystal is held between electrode
substrates.
[0003] In such a conventional liquid crystal display device, the
brightness and color considerably vary with the viewing angle. This
is a factor which makes it difficult to comply with a demand for a
large display screen.
[0004] Under the circumstances, development of a liquid crystal
display device mainly using an electric field substantially in
parallel to a display screen has been continuing so as to solve the
problem. Such a liquid crystal display device is disclosed, for
example, in Jpn. Pat. Appln. KOKAI Publication No. 63-21907.
[0005] As shown in FIG. 9, the liquid crystal display device
comprises an array substrate 10 having a pixel electrode 1 and a
counter electrode 3 both formed thereon, a counter substrate 20
facing the array substrate 10, and a liquid crystal cell 30 which
contains TN liquid crystal molecules of a positive anisotropic
dielectric constant and is held between the substrates 10 and 20.
The liquid crystal cell 30 is held between the substrate 10 and 20
via aligning films 13 and 23 which are treated to align the liquid
crystal molecules in the same direction R. The alignment
(treatment) direction R forms a predetermined angle .oval-hollow.
1, for example, 80.degree. with respect to a direction of an
electric field E created between the pixel electrode 1 and the
counter electrode 3.
[0006] Polarizing plates 40 and 50 are respectively mounted on
outer surfaces of the substrates 10 and 20 to have a cross-Nicol
system in which the polarization axis P1 of the polarizing plate 40
is set in the alignment direction R, and the polarization axis P2
of the polarizing plate 50 is set in a direction orthogonal to the
alignment direction R.
[0007] With this system, the light transmittance is set at a
minimum value when no voltage is applied between the pixel
electrode 1 and the counter electrode 3, and at a maximum value
mainly by the birefringence effect of the liquid crystal molecules
aligned in the electric field direction E shown in FIG. 10 when a
voltage of a sufficient level is applied between the pixel
electrode 1 and the counter electrode 3.
[0008] In such a liquid crystal display device, upon application of
the voltage, the alignment of liquid crystal molecules is twisted
in a range from the main surface of each substrate to the middle of
the liquid crystal cell since a binding force is applied to the
liquid crystal molecules from the main surface of each substrate
due to the alignment treatment.
[0009] Since a considerable period of time is required for resuming
the twisted alignment obtained by application of a voltage to an
initial alignment of the molecules, the display device has a
drawback that the response speed is slow. This drawback is also
raised in the case where the liquid crystal cell has a negative
anisotropic dielectric constant.
[0010] On the other hand, Jpn. Pat. Appln. KOKOKU Publication No.
7-261152 discloses a technique of controlling the light
transmittance by selection between application of a high level
voltage and application of a low level voltage, instead of
selection between application of a voltage and nonapplication of
the voltage, so as to use ICs of a low withstand voltage in a
liquid crystal display device. Since this technique enables
reduction in the amplitudes of voltages applied to the electrodes,
ICs of a low withstand voltage can be used in the liquid crystal
display device.
[0011] The inventors of the present invention have studied this
technique and found that the technique enhances the response speed
as a result of the control of liquid crystal molecules during which
voltage application is retained.
[0012] However, it is also recognized that the contrast ratio is
deteriorated due to the above-mentioned control of liquid crystal
molecules during which voltage application is retained.
BRIEF SUMMARY OF THE INVENTION
[0013] The present invention has been achieved in view of the
above-mentioned problem, and an object of the present invention is
to provide a liquid crystal display device capable of attaining
good viewing angle characteristics without deteriorating
characteristics of the device, such as contrast ratio and response
speed.
[0014] According to the invention, there is provided a liquid
crystal display device which comprises first and second substrates;
a liquid crystal cell held between the first and second substrates
and containing liquid crystal molecules arranged to have an
alignment corresponding to alignment properties of inner surfaces
of the first and second substrates; first and second electrodes
formed on the first substrate to apply a lateral electric field
substantially parallel to the first and second substrate into the
liquid crystal cell; first and second polarizing plates having
individual polarization axes and mounted on outer surfaces of the
first and second substrates, respectively; and an optical
retardation plate interposed at least between the first polarizing
plate and the first substrate; wherein an optical axis and
retardation value of the optical retardation plate are determined
to compensate for twisting of the alignment of the liquid crystal
molecules caused upon application of the lateral electric
field.
[0015] According to another aspect of the present invention, there
is provided a liquid crystal display device which comprises first
and second substrates; a liquid crystal cell held between the first
and second substrates and containing liquid crystal molecules
arranged to have an alignment corresponding to alignment properties
of inner surfaces of the first and second substrates; first and
second electrodes formed on the first substrate to apply a lateral
electric field substantially parallel to the first and second
substrate into the liquid crystal cell; first and second polarizing
plates having individual polarization axes and mounted on outer
surfaces of the first and second substrates, respectively; and
first and second optical retardation plates interposed between the
first polarizing plate and the first substrate and the second
polarizing plate and the second substrate, respectively; wherein
when a first lateral electric field is produced to obtain a dark
state and a second lateral electric field is produced to obtain a
bright state, optical axes and retardation values of the first and
second optical retardation plates are determined to compensate for
twisting of the alignment of the liquid crystal molecules caused
upon application of the first lateral electric field.
[0016] In the liquid crystal display device of the present
invention, a lateral electric field is used to switch the alignment
of liquid crystal molecules, and thus can attain good viewing angle
characteristics. Further, the optical axis and the retardation
value of each optical retardation plate is determined to compensate
for twisting of the liquid crystal molecules, and thus the contrast
ratio can be improved while maintaining a high response speed.
[0017] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0018] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments give below, serve to explain the principles
of the invention.
[0019] FIG. 1 is a perspective view schematically showing a liquid
crystal display device according to an embodiment of the present
invention;
[0020] FIG. 2 is a plain view schematically showing a part of an
array substrate shown in FIG. 1;
[0021] FIG. 3 is a sectional view schematically showing a section
of a part of the liquid crystal panel, taken along a line III-III
shown in FIG. 2;
[0022] FIGS. 4A and 4B are graphs showing a twisted alignment of
liquid crystal molecules in a liquid crystal cell shown in FIG.
3;
[0023] FIG. 5 is a diagram schematically showing a circuit
configuration of the liquid crystal display device shown in FIG.
1;
[0024] FIG. 6 is a timing chart showing waveforms for driving the
liquid crystal display device shown in FIG. 1;
[0025] FIG. 7 schematically shows an example of an alignment
structure of a liquid crystal panel shown in FIG. 3;
[0026] FIG. 8 schematically shows another example of the alignment
structure of the liquid crystal panel shown in FIG. 3; and
[0027] FIGS. 9 and 10 are views for explaining the conventional
liquid crystal display device using a lateral electric field.
DETAILED DESCRIPTION OF THE INVENTION
[0028] A liquid crystal display device according to an embodiment
of the present invention will be described with reference to the
accompanying drawings.
[0029] As shown in FIG. 1, the liquid crystal display device 100
includes a liquid crystal panel 500 in which a liquid crystal cell
400 (see FIG. 3) is held between an array substrate 200 and a
counter substrate 300, and a driving circuit section 600 for
driving the liquid crystal panel 500. The liquid crystal panel 500
includes an effective display area 111 having a diagonal size of 15
inches and capable of displaying a color image. In the liquid
crystal panel 500, (1024 .times.3) .times.786 display pixels are
arranged in matrix.
[0030] The array substrate 200 for the liquid crystal panel 500
includes 1024 .times.3 signal lines 211 having a multi-layered
structure of molybdenum/ aluminum/ molybdenum, 786 scanning lines
221 of molybdenumtungsten alloy (Mo-W alloy), and thin film
transistors (TFTs) 231. The signal lines 211 and the scanning lines
221 are arranged to intersect at right angles and the TFTs 231 are
arranged near intersections of the signal lines 211 and the
scanning lines 221, on a transparent grass substrate 201. The glass
substrate 201 has a polished surface and a thickness of 0.7 mm.
More specifically, each TFT 231 has a gate electrode is formed of
one of the scanning lines 221, a gate insulating film 241 made of
silicon nitride (SiNx), an amorphous silicon hydride semiconductor
layer (a-Si:H) 233 formed over the gate electrode via the gate
insulating film 241. A channel protection film 235 made of a
silicon nitride film (SiNx) is positioned on the amorphous silicon
hydride semiconductor layer (a-Si:H) 233, and source and drain
electrodes 251 and 261 are electrically connected to the a-Si:H
film 233 via low-resistance amorphous silicon hydride semiconductor
layers (n+a-Si:H) 237 and 239 in which phosphorus is doped. The
drain electrode 261 is integrally formed with one of the signal
lines 211. The source electrode 251 has a multi-layered structure
of molybdenum/aluminum/molybdenum, similarly to the signal lines,
and extends along one of the signal lines 211 to form a stripe
serving as a pixel electrode 271. Further, a first storage
capacitance electrode 273 is formed in contact with an end of the
stripe and serves as a storage capacitance Cs. A counter electrode
281 is made of Mo-W alloy similarly to the scanning line 221, and
arranged substantially in parallel with the scanning line 221. The
counter electrode 281 has first and second electrodes 283 and 285
arranged substantially in parallel with the pixel electrode 271,
and a second storage capacitance electrode 287 which overlaps the
first storage capacitance electrode 273 via the gate insulating
film 241 interposed therebetween. With this structure, the liquid
crystal molecules are controlled by a lateral electric field
between the pixel electrode 271 and the first electrode 283 and a
lateral electric field between the pixel electrode 271 and the
second electrode 285. The array substrate 200 further has an
aligning film 291 arranged to cover these electrodes.
[0031] The counter substrate 300 for the liquid crystal panel 500
includes a transparent glass substrate 301 which has a polished
surface and a thickness of 0.7 mm, and a light shielding film 311
which is made of resin and arranged on the glass substrate 301 for
the pixel matrix. The light shielding film 311 shields lights
leaked from gaps between the signal lines 211 and the counter
electrodes 281 and between the scanning lines 221 and the counter
electrodes 281 in the array substrate 200, and lights undesirably
irradiating the TFTs 231. In openings of the shielding film 311,
color filters 321 of red (R), blue (B), and green (G) are arranged
to enable displaying of a color image. Further, an aligning film
341 is formed on a smoothing layer 331 which is made of transparent
resin and covers the color filters 321.
[0032] In a gap between the array substrate 200 and the counter
substrate 300, fine polymers (not shown) are dispersed so that a
distance d between the substrates 200 and 300 is maintained at 3.5
.mu.m, for example. The substrate distance d is preferable to be
set within a range of 1.5 to 5.5 .mu.m, more desirably, within a
range of 3.0 to 4.0 .mu.m,in order to attain an adequate response
speed of liquid crystal with a low voltage, and to secure the
uniformity of display performance.
[0033] The liquid crystal cell 400 is made of a nematic liquid
crystal material which is held in the gap between the substrates
200 and 300 and has positive anisotropic dielectric constant
.epsilon. of 10.7, anisotropic refractive index .delta.n of 0.10,
and viscosity of 21 cps.
[0034] On the array substrate 200, the liquid crystal molecules in
the liquid crystal cell 400 are aligned by the aligning film 291
which is alignment-treated such that the liquid crystal molecules
form a pretilt angle .theta.of 5.degree. with respect to the array
substrate 200 and form an acute angle .oval-hollow. R1 with respect
to the electric field direction E of the lateral electric field
between the pixel electrode 271 and the counter electrode 281. On
the counter substrate 300, the liquid crystal molecules in the
liquid crystal cell 400 are aligned by aligning film 341 which is
alignment-treated such that the liquid crystal molecules form a
pretilt angle .theta. of 5.degree. with respect to the array
substrate 300 and form an acute angle .oval-hollow. R2 with respect
to the electric field direction E of the lateral electric field
between the pixel electrode 271 and the counter electrode 281.
Alignment directions R1 and R2 of the liquid crystal molecules are
opposite to each other.
[0035] The angles .oval-hollow. R1 and .oval-hollow. R2 are
normally set at the same value less than 90.degree. and not less
than 45.degree., preferably within a range of 60.degree. to
88.degree..
[0036] optical retardation plates 411 and 421 are respectively
mounted on the outer surface of the array substrate 200 and the
outer surface of the counter substrate 300 such that the optical
axis W1 of the plate 411 forms a predetermined angle .oval-hollow.
W1 with respect to the alignment direction R1, and the optical axis
W2 of the plate 421 forms a predetermined angle .oval-hollow. W2
with respect to the alignment direction R2. The angles
.oval-hollow.W1 and .theta. W2 are preferably set within a range of
45.degree. to 135.degree., or 60.degree. to 90.degree., or more
preferably, 80.degree. to 90.degree..
[0037] Further, polarizing plates 431 and 441 such as G1220DU
(manufactured by NITTO DENKO CO., LTD.) are respectively mounted on
the outer surfaces of the optical retardation plates 411 and 421
such that the polarizing axis of the plate 431 forms an angle
.oval-hollow. P1 with respect to the lateral electric field E
between the pixel electrode 271 and the counter electrode 281, and
the polarizing axis of the plate 441 forms an angle .theta. P2 with
respect to the lateral electric field E between the pixel electrode
271 and the counter electrode 281.
[0038] With the above-mentioned structure, the geometrical aperture
ratio of the liquid crystal panel 500 is set at 30% when the
transmittance of the color filters 321 and the like are not taken
into consideration.
[0039] The function of the optical retardation plates 411 and 421
of the liquid crystal panel 500 will be described below. FIG. 4A
shows an alignment of the liquid crystal molecules obtained when a
voltage of 4.0 V is applied between the pixel electrode 271 and the
counter electrode 281, and FIG. 4B shows an alignment of the liquid
crystal molecules obtained when a voltage of 10.0 V is applied
between the pixel electrode 271 and the counter electrode 281.
[0040] As should be clear from these graphs, the alignment of the
liquid crystal molecules is twisted both in the cases where a low
voltage is applied thereto and where a high voltage is applied
thereto. Twisting of the alignment of the liquid crystal molecules
decreases the contrast ratio of the liquid crystal display device
since the light transmittance is reduced when a bright state is
designated, and light leakage occurs when a dark state is
designated.
[0041] In the present embodiment, the optical retardation plates
411 and 421 are provided to compensate for undesired twisting of
the alignment of the liquid crystal molecules which may decrease
the constant ratio. With this compensation, only liquid crystal
molecules located in the middle of the liquid crystal cell and
contributing to switching of the display state are aligned into a
substantially uniform direction, and this alignment direction is
determined as a reference for improving the light transmittance in
the bright state and suppressing light leakage in the dark
state.
[0042] The retardation values Rr1 and Rr2 of the optical
retardation plates 411 and 421 can be determined on the basis of
the retardation value RLc of the liquid crystal cell 400, in the
following manner, for example:
[0043] When the retardation value RLc of the liquid crystal cell
400 can be obtained from the formula
.delta.n.multidot.d.multidot.cos.sup.2.theta- . where the pretilt
angle of the liquid crystal molecules is denoted as 0 , the
substrate distance is denoted as d, and anisotropic refractive
index is denoted as An. In this embodiment, the retardation value
RLc is 347 nm. As described above, the alignment of the liquid
crystal molecules near each of the aligning films 291 and 341
cannot be changed according to the electric field direction since
this alignment is fixed due to a binding force applied from each of
the aligning films 291 and 341.
[0044] For example, when the dark state (where a high voltage is
applied) is designated in a normally-white mode, compensation needs
to be performed for the alignment of liquid crystal molecules
separated from the aligning films 291 and 341 by a distance of
about 0.05.multidot.d to 0.20d and not accurately aligned according
to the electric field. Assuming that the optical retardation plates
411 and 421 are formed a uniaxial oriented films, the retardation
values Rr1 and Rr2 thereof are preferable to be set at 0.05 RLc to
0.20.multidot.RLc, more desirably, 0.08 .multidot.RLc to 0.12
.multidot.RLc where RLc is the retardation value of the liquid
crystal cell 400. If one of the optical retardation plates 411 and
421 is eliminated, the retardation value of the remaining optical
retardation plate is preferably set within a range of
0.10.multidot.RLc to 0.40.multidot.RLc, more desirably,
0.15.multidot.RLc to 0.25.multidot.RLc.
[0045] On the other hand, when the dark state (where a low voltage
is applied) is designated in a normallyblack mode, compensation
needs to be performed for the alignment of liquid crystal molecules
separated from the aligning films 291 and 341 by a distance about
d.multidot.{fraction (1/7)}to d .multidot.{fraction (4/9 )} and not
accurately aligned according to the electric field. Assuming that
the optical retardation plates 411 and 421 are formed as uniaxial
oriented films, the retardation values Rr1 and Rr2 thereof are
preferable to be set at {fraction (1/7 )} RLc to {fraction
(4/9)}RLc, more desirably, {fraction (1/4 )} RLc to {fraction
(4/9)} RLc where RLc is the retardation value of the liquid crystal
cell 400. If one of the optical retardation plates 411 and 421 is
eliminated, the retardation value of the remaining optical
retardation plate is preferably set within a range of {fraction
(+B1/3 )}RLc to {fraction (2/3 )} RLc.
[0046] Instead of the uniaxial oriented films, the optical
retardation plates 411 and 421 may be formed as films for a twisted
alignment, for example. In this case, the retardation values can be
slightly larger than the above-mentioned values, and preferably
increased from the above-mentioned values by about 20%.
[0047] The optical retardation plates 411 and 421 may be formed of
materials having the same retardation value, and also formed of
materials having retardation values different on the side of the
array substrate 200 and on the side of the counter substrate
300.
[0048] Since the electric field is more effective on the side of
the array substrate 200 than on the side of the counter substrate
300, the amount of compensation on the side of the array substrate
200 can be more reduced than that on the side of the counter
substrate. Accordingly, In the case where the optical retardation
plates 411 and 421 are set to different retardation values, it is
preferable that the retardation value of the plate 411 on the side
of the array substrate 200 is set smaller than that of the plate
421 on the counter substrate 300.
[0049] The angles .theta. W1 and .oval-hollow. W2 respectively
formed by the optical axes W1 and W2 of the optical retardation
plates 411 and 421 and the alignment direction R are suitable to be
set within a range of 450 to 1350 , more preferably, 60.degree. to
90.degree., in particular, within a range of 80.degree. to
90.degree.. The angles .oval-hollow. W1 and .theta. W2 are
respectively obtained with respect to the optical axes W1 and W2 in
the twisted direction of liquid crystal molecules upon application
of a voltage.
[0050] Next, the driving circuit section 600 will be described with
reference to FIGS. 5 and 6. The driving circuit section 600
includes a controller 641 which receives digital data DATA and a
synchronization signal Sync supplied from the outside to output a
vertical scanning start signal VST and a vertical scanning clock
signal VCK to a vertical scanning circuit 611, to output a
horizontal scanning start signal HST, a horizontal scanning clock
signal HCK, a polarity inversion signal POL, and to output the
digital data DATA in synchronism with these signals to a horizontal
scanning circuit 621, and the polarity inversion signal POL to a
counter electrode driving circuit 631.
[0051] The vertical scanning circuit 611 has a shift register for
serially shifting the vertical scanning start signal VST in
response to the vertical scanning clock signal VCK to output
scanning signals VY1 to VY786 to the scanning lines.
[0052] The horizontal scanning circuit 621 includes a shift
register for serially shifting the horizontal scanning start signal
HST in response to the horizontal scanning clock signal HCK, a
sampling circuit for sequentially sampling the digital data DATA
according to outputs of the shift register, and a DAC circuit for
D/A converting the digital data DATA into video signal voltages
Vsig1 - Vsig3072 to be supplied to the scanning lines, on the basis
of the polarity inversion signal POL.
[0053] The counter electrode driving circuit 631 is arranged to
output a counter electrode voltage VCOM determined according to the
polarity inversion signal POL.
[0054] [First Example]
[0055] The first example of the alignment structure of the liquid
crystal panel 500 will be described with reference to FIG. 7.
[0056] In FIG. 7, E denotes an electric field direction of the
lateral electric field applied between the pixel electrode 271 and
the counter electrode 281. The aligning films 291 and 341 of the
array and counter substrates 200 and 300 form angles .oval-hollow.
R1 and .theta. R2 of 70.degree. with respect to the direction of
the lateral electric field E, and the liquid crystal molecules on
the array and counter substrates 200 and 300 are respectively
aligned in the alignment directions R1 and R2 opposite to each
other.
[0057] The optical retardation plates 411 and 421 are formed as
uniaxial oriented films made of triacetylcellulose and having a
retardation value of 30 nm. The optical axes W1 and W2 of the
optical retardation plates 411 and 421 are set to respectively form
angles .theta. W1 and .theta. W2 of substantially 90.degree. with
respect to the alignment directions R1 and R2. Polarization axes P1
and P2 of the polarizing plates 431 and 441 are arranged to be
orthogonal to each other and slanted toward the alignment direction
R1 from the lateral electric field direction E by 3 and by
93.degree., respectively. In this manner, the liquid crystal panel
500 of the normally-white mode is formed.
[0058] In the liquid crystal display device 100 having the
above-mentioned structure, when a voltage of 3.6 V is applied to
the liquid crystal, the light transmittance is set at a maximum
level (bright state), i.e., 3.3% (33% in a dummy cell from which
influence by a color filter and the like is excluded). When a
voltage of 10.0 V is applied to the liquid crystal, the light
transmittance is set at a minimum level (dark state), i.e., 0.03%
(0.3% in the dummy cell). As should be clear from this, the device
attains a remarkably high contrast ratio. Regarding to the response
speed, the switching from the bright state to the dark state takes
9 ms, and the switching from the dark state to the bright state
takes 17 ms: a sufficiently high response speed is also
recognized.
[0059] [Example for Comparison]
[0060] In contrast, in a liquid crystal display device having the
same structure as that of the first example, from which the optical
retardation plates 411 and 421 are removed, the bright state is
attained when a voltage of 3.8 V is applied to the liquid crystal,
and the light transmittance of 3.1% (31%, in the dummy cell) is
attained in the time. The dark state is attained when the liquid
crystal is applied with a voltage of 10.2 V, and light
transmittance of 0.09% (0.9% in the dummy cell) is recognized. As
is clear from this, a sufficient contrast ratio as attained by the
structure of the first example could not be attained with use of
the structure of this example.
[0061] According to the first example, the twisted alignment of the
liquid crystal molecules, which may increase the light
transmittance, in the dark state, is compensated by the optical
retardation plates 411 and 421. By virtue of the optical
retardation plates, the alignment direction of the liquid crystal
molecules are set to be a substantially uniform direction
determined by only the liquid crystal molecules constituting the
middle part of the liquid crystal cell 400, which substantially
contributes to the switching of display state. In this manner, the
light transmittance in the dark state could be effectively
suppressed. In the bright state, the twisted alignment of the
liquid crystal molecules is compensated by the optical retardation
plates 411 and 421 so that the alignment direction of the liquid
crystal molecules forms an angle of substantially 450 with respect
to the polarization axes P1 and P2 of the polarizing plates 431 and
441 in order to form an ideal homogeneous alignment. In this
manner, the light transmittance in the bright state could be
sufficiently enhanced.
[0062] In addition to the above, according to the first example,
the switching of the liquid crystal molecules in the middle part of
the liquid crystal cell 400 other than those near the main surfaces
of the substrates 101 and 201 is mainly used to enhance the
response speed since the alignment of the liquid crystal molecules
is twisted both in the bright and dark states. Further, in the
switching to the bright state, the voltage applied to the liquid
crystal cell 400 increases as compared with the switching from the
initial molecular alignment.
[0063] [Second Example]
[0064] Next, the second example of the liquid crystal panel 500
will be described below. In this example, the optical retardation
plates 411 and 421 which have retardation twisted in a direction of
thickness are interposed, instead of the optical retardation plates
of the first example. The optical retardation plates 411 and 421
are arranged such that optical axes W1 and W2 on sides on which the
plates 411 and 421 contact glass substrates 201 and 301 form angles
.oval-hollow. W1 and .oval-hollow. W2 of substantially 90.degree.
with respect to alignment directions R1 and R2, respectively. The
twisting angles of the optical axes are set at substantially 70
.degree. along a twisting direction of the liquid crystal
molecules. The optical retardation plates 411 and 421 are set to
have the retardation value of 40 nm in the direction of the optical
axes W1 and W2 contacting the glass substrates 201 and 301. The
liquid crystal display device of the second example is formed in
the similar manner to that of the first example, except that the
anisotropic refractive index An of the liquid crystal cell 400 is
set at 0.103 in order to compensate the reduction of the
retardation effective to the switching of the liquid crystal
display device.
[0065] In the second example, when the voltage of 3.4 V is applied
to the liquid crystal, the light transmittance is set at the
maximum level (the bright state), i.e., the light transmittance of
3.5% (35% in the dummy cell), and when the voltage of 9.8 V is
applied to the liquid crystal, the light transmittance is set at
the minimum level (the dark state), i.e., the light transmittance
of 0.02% (0.2% in the dummy cell). As is clear from this, the
device of the second example also attained remarkably high level of
contrast ratio.
[0066] The response speed is also recognized to be sufficiently
high: the switching from the bright state to the dark state takes 8
ms, and the switching from the dark state to the bright state takes
17 ms.
[0067] [Third example]
[0068] The third example of the alignment structure of the liquid
crystal panel 500 will be described with reference to FIG. 8. The
liquid crystal panel 500 of the third example is constituted as the
normally-black mode device.
[0069] Unlike the first example, the liquid crystal cell 400 of the
third example is formed of nematic liquid crystal material having
the anisotropic dielectric constant .epsilon. of +9.9 , the
anisotropic refractive index An of 0.09, and the viscosity of 21
cps. In the third example, a uniaxial oriented film of
polycarbonate having a retardation value of 50 nm is used to form
the optical retardation plates 411 and 421, which are arranged to
have the optical axes W1 and W2 substantially orthogonal to the
alignment directions R1 and R2. The polarizing plates 431 and 441
are arranged such that the polarization axis P1 of the polarizing
plate 431 is slanted toward the alignment direction R1 from the
lateral electric field direction E by an angle of .oval-hollow. P1
of 70.degree. (such that the polarization axis P1 corresponds to
the alignment direction R1), the polarization axis P2 of the
polarizing plate 441 is slanted toward the alignment direction R1
from the lateral electric field direction E by an angle .theta. P2
of 160.degree. , and the axes P1 and P2 are orthogonal to each
other.
[0070] In the third example, when a voltage of 1.0 V is applied to
the liquid crystal, the light transmittance is set at a minimum
level (the dark state), i.e., 0.02% (0.2% in the dummy cell). When
a voltage of 6.3 V is applied to the liquid crystal, the light
transmittance is set at a maximum level (the bright state), i.e.,
3.1% (31% in the dummy cell). As should be clear from this, the
device of the third example attains a remarkably high contrast
ratio.
[0071] Since the liquid crystal in the third example is applied
with the voltage between 1.0 V and 6.3 V, the response speed is
lower than those attained in the other examples.
[0072] [Fourth Example]
[0073] The fourth example of the alignment structure of the liquid
crystal panel 500 will be described below. The liquid crystal panel
500 of the fourth example is also constituted as the normally-black
mode device.
[0074] Unlike the third example, the polarizing plates 431 and 441
of the fourth example are arranged such that the polarization axis
P1 of the polarizing plate 431 is slanted toward the alignment
direction R1 from the lateral electric field direction E by an
angle of .theta. P1 of 60.degree. , the polarization axis P2 of the
polarizing plate 441 is slanted toward the alignment direction R1
from the lateral electric field direction E by an angle .theta. P2
of 150.degree., the axes P1 and P2 are orthogonal to each other. In
short, the polarizing plates are arranged not by making the
polarization axis P1 of the polarizing plate 431 correspond to the
alignment direction R1, but by reducing the angles set in the third
example by 100 in the twisting direction of the liquid crystal
molecules to offset with reference to the twisted alignment of the
liquid crystal molecules.
[0075] In the fourth example, when a voltage of 2.4 V is applied to
the liquid crystal, the light transmittance is set at a minimum
level (the dark state), i.e., 0.04% (0.4% in the dummy cell). When
a voltage of 8.4 V is applied to the liquid crystal, the light
transmittance is set at a maximum level (the bright state), i.e.,
3.1% (31% in the dummy cell). As should be clear from this, the
device attains remarkably high contrast ratio. The response speed
is also recognized to be sufficiently high: the switching from the
bright state to the dark state takes 30 ms, and the switching from
the dark state to the bright state takes 10 ms.
[0076] In this example, the dark state is attained by applying a
low voltage. In accordance therewith, the polarizing plate 431 is
arranged to have an orientation offset with respect to the
alignment direction, thereby leakage light is more decreased to
attain high contrast in comparing the examples described above.
[0077] [Fifth Example]
[0078] The fifth example of the alignment structure of the liquid
crystal panel 500 will be described below. The liquid crystal panel
500 of the fifth example is constituted as the normally-black mode
device.
[0079] The device of the fifth example differs from the fourth
example in that the polarizing plate 441 is arranged so as to
incline a polarization axis P2 of the polarizing plate 441 by an
angle .theta. P2 of 155.degree. in the alignment direction RI with
respect to the lateral electric field direction E, such that the
axis P1 of the polarizing plate 431 and the axis P2 cross each
other to form an angle of 95.degree. larger than a right angle
formed in the other examples.
[0080] In the fifth example, when a voltage of 2.2 V is applied to
the liquid crystal, the light transmittance is set at a minimum
level (the dark state), i.e., 0.03% (0.3% in the dummy cell). When
a voltage of 8.6 V is applied to the liquid crystal, the light
transmittance is set at a maximum level (the bright state), i.e.,
3.1% (31% in the dummy cell). As should be clear from this, the
device attains remarkably high contrast ratio.
[0081] The response speed is also recognized to be sufficiently
high: the switching from the bright state to the dark state takes
28 ms, and the switching from the dark state to the bright state
takes 8 ms.
[0082] According to the fifth example, by setting the crossing
angle of the polarizing plates 431 and 441 larger than 900 , the
deviation of polarized light components due to the alignment of the
liquid crystal molecules could be compensated, thereby higher
contrast in comparing with the fourth example is attained.
[0083] [Sixth Example]
[0084] The sixth example of the alignment structure of the liquid
crystal panel 500 will be described below. The liquid crystal panel
500 of the sixth example is also constituted as the normally-black
mode device.
[0085] The sixth example differs from the fifth example in that an
unaxial stretched film of polycarbonate having a retardation value
of 100 nm is used as the optical retardation plates 411 and
421.
[0086] According to the liquid crystal display device of the sixth
example, when a voltage of 2.1 V is applied to the liquid crystal,
the light transmittance is set at a minimum level (the dark state),
i.e., 0.02% (0.2% in the dummy cell). When a voltage of 8.5 V is
applied to the liquid crystal, the light transmittance is set at a
maximum level (the bright state), i.e., 3.1% (31% in the dummy
cell). As should be clear from this, the device attains remarkably
high contrast ratio.
[0087] The response speed is also recognized to be sufficiently
high: the switching from the bright state to the dark state takes
25 ms, and the switching from the dark state to the bright state
takes 7 ms.
[0088] [Seventh Example]
[0089] The seventh example of the alignment structure of the liquid
crystal panel 500 will be described below. The liquid crystal panel
500 of the seventh example is constituted in the normally- black
mode.
[0090] This example also differs from the fifth example in that the
optical axes W1 and W2 of the optical retardation plates 411 and
421 contacting the surfaces of the glass substrates 201 and 301
form the angles .oval-hollow. W1 and .oval-hollow. W2 of
substantially 90.degree. with respect to the alignment directions
R1 and R2 respectively, and the twisting angle thereof is
substantially 20.degree. along the twisting direction of the liquid
crystal molecules. The optical retardation plates 411 and 421 have
a retardation value of 150 nm in directions of the optical axes W1
and W2 contacting the surfaces of the glass substrates 201 and
301.
[0091] In the seventh example, when a voltage of 2.1 V is applied
to the liquid crystal, the light transmittance is set at a minimum
level (the dark state), i.e., 0.01% (0.1% in the dummy cell). When
a voltage of 8.0 V is applied to the liquid crystal, the light
transmittance is set at a maximum level (the bright state), i.e.,
3.2% (32% in the dummy cell). As should be clear from this, the
device attains remarkably high contrast ratio. The response speed
is also recognized to be sufficiently high: the switching from the
bright state to the dark state takes 25 ms, and the switching from
the dark state to the bright state takes 5 ms.
[0092] It is understood that the present invention is not limited
to the examples described above, and that various changes and
modifications may be effected therein by one skilled in the art
without departing from the scope or spirit of the invention.
[0093] In the above-mentioned embodiment, the material having
positive anisotropic dielectric constant is used for the liquid
crystal cell. It goes without saying that the material having
negative anisotropic dielectric constant is used for the liquid
crystal cell.
[0094] In this embodiment, the optical retardation plates are
provided to the devices in all the examples. The effect of the
present invention can be attained by providing the optical
retardation plate to at least one side of the liquid crystal cell
of the device. It is more preferable, of course, to provide the
optical retardation plate to both sides.
[0095] The substrates for the liquid crystal display device can be
formed of the optical retardation plates.
[0096] The pixel electrode 271 and the counter electrode 281 are
arranged to be parallel to each other and to extend in the same
direction in various portions in the above-mentioned embodiment.
These electrodes can be bent at a predetermined angle in each
pixel. In this case, the electric field direction E in each pixel
may have more than one direction, but the present invention can be
used by setting the electric field direction E as general one.
[0097] Further, the driving circuit is arranged outside of the
liquid crystal panel in the above-mentioned embodiment. The driving
circuit can be incorporated in the liquid crystal panel if TFTs of
polycrystalline silicon or the like are formed for the driving
circuit.
[0098] As described above, the liquid crystal display device of the
present invention can sufficiently enhance the response speed and
attain a high contrast ratio.
[0099] Additional advantages and modifications will readily occurs
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents. Claims
* * * * *