U.S. patent application number 08/827798 was filed with the patent office on 2001-06-28 for liquid crystal display device.
Invention is credited to KANEMOTO, AKIHIKO, TAKAHASHI, HIROYUKI, TAKIGUCHI, YASUYUKI.
Application Number | 20010005246 08/827798 |
Document ID | / |
Family ID | 14699879 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005246 |
Kind Code |
A1 |
TAKIGUCHI, YASUYUKI ; et
al. |
June 28, 2001 |
LIQUID CRYSTAL DISPLAY DEVICE
Abstract
A liquid crystal display device is disclosed, comprising a
cholesteric liquid crystal layer which has an intrinsic helical
pitch of approximately twice as large as a thickness of the liquid
crystal layer and also has a bistable character capable of
switching by an application of voltage, and further provided with
at least one birefringent medium layer between a polarizer and a
substrate. This construction of the liquid crystal display device
results in a reduction of birefringent effects, whereby achieving a
high contrast, satisfactory color purity, and other display
characteristics such as a higher duty ratio operation and faster
time response, suitable for displaying a large volume of
information data.
Inventors: |
TAKIGUCHI, YASUYUKI;
(SAGAMIHARA-SHI, JP) ; TAKAHASHI, HIROYUKI;
(YOKOHAMA-SHI, JP) ; KANEMOTO, AKIHIKO;
(YOKOHAMA-SHI, JP) |
Correspondence
Address: |
GERARD M WISSING
COOPER & DUNHAM
1185 AVENUE OF THE AMERICAS
NEW YORK
NY
10036
|
Family ID: |
14699879 |
Appl. No.: |
08/827798 |
Filed: |
April 11, 1997 |
Current U.S.
Class: |
349/117 ;
349/123; 349/175; 349/181 |
Current CPC
Class: |
G02F 1/13718 20130101;
G02F 1/13363 20130101 |
Class at
Publication: |
349/117 ;
349/123; 349/175; 349/181 |
International
Class: |
G02F 001/1335; G02F
001/1337; C09K 019/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 15, 1996 |
JP |
08-116955 |
Claims
1. A liquid crystal display device comprising: a pair of spaced
apart substrates; each of said substrates being alignment treated
such that a direction of said alignment having a slight angle of
inclination to said substrate and being substantially parallel to
each other; a cholesteric liquid crystal layer disposed between
said pair of substrate, having an intrinsic helical pitch of
approximately twice as large as a spacing between said substrates,
to constitute a liquid crystal cell; said cholesteric liquid
crystal layer having a bistable character capable of switching by
an application of voltage such that a twist angle of said liquid
crystal layer along the direction of a thickness thereof being
either approximately 360.degree. or an angle subtracted 360.degree.
from the just above-mentioned angle value; a pair of polarizers
each provided on an upper face and lower face of said liquid
crystal cell; and at least one birefringent medium layer further
provided between said polarizer and said substrate at least for
either one of an upper or lower set of said polarizer and said
substrate, to reduce a birefringence effect thereof at one of said
bistable states.
2. The liquid crystal display device in accordance with claim 1,
wherein a .DELTA.nd value of said liquid crystal layer is from 0.15
to 0.4 .mu.m.
3. The liquid crystal display device in accordance with claim 1,
wherein the wavelength dependence of .DELTA.n for said liquid
crystal layer and said birefringent medium layer are equal or
approximately equal to each other.
4. The liquid crystal display device in accordance with claim 1,
wherein said birefringent medium layer is selected from the group
consisting of a stretched or extruded polymer film, a liquid
crystal cell having a homogeneous alignment, an aligned polymer
liquid crystal, and an alignment-immobilized liquid crystalline
polymer which is obtained by freezing molecules of the polymer
liquid crystal aligned homogeneously.
5. The liquid crystal display device in accordance with claim 1,
wherein said substrates themselves comprises said birefringent
medium layer.
6. The liquid crystal display device in accordance with claim 1,
wherein said birefringent medium layer comprises a plurality of
nearly uniaxial birefringent layers overlaid with its axis shifted
consecutively by a predetermined angle.
7. A liquid crystal display device having a pair of spaced apart
substrates, each of said substrates being alignment treated such
that a direction of said alignment having a slight angle of
inclination to said substrate and being substantially parallel to
each other, a cholesteric liquid crystal layer disposed between
said pair of substrate, having an intrinsic helical pitch of
approximately twice as large as a spacing between said substrates,
to constitute a liquid crystal cell; said cholesteric liquid
crystal layer having a bistable character capable of switching by
an application of voltage such that a twist angle of said liquid
crystal layer along a cell thickness being either approximately
360.degree. or an angle subtracted 360.degree. from the just
above-mentioned angle value, and a pair of polarizers each provided
on an upper and lower faces of said liquid crystal cell, the
improvement comprising: at least one birefringent medium layer
further provided between said polarizer and said substrate for at
least either one of an upper or lower set of said polarizer and
said substrate, to reduce a birefringence effect thereof at one of
said bistable states.
8. The liquid crystal display device in accordance with claim 7,
wherein a .DELTA.nd value of said liquid crystal layer is from 0.15
to 0.4 .mu.m.
9. The liquid crystal display device in accordance with claim 7,
wherein the wavelength dependence of .DELTA.n for said liquid
crystal layer and said birefringent medium layer are equal or
approximately equal to each other.
10. The liquid crystal display device in accordance with claim 7,
wherein said birefringent medium layer is selected from the group
consisting of a stretched or extruded polymer film, a liquid
crystal cell having a homogeneous alignment, an aligned polymer
liquid crystal, and an alignment-immobilized liquid crystalline
polymer which is obtained by freezing molecules of the polymer
liquid crystal aligned homogeneously.
11. The liquid crystal display device in accordance with claim 7,
wherein said substrates themselves comprises said birefringent
medium layer.
12. The liquid crystal display device in accordance with claim 7,
wherein said birefringent medium layer comprises a plurality of
nearly uniaxial birefringent layers overlaid with its axis shifted
consecutively by a predetermined angle.
13. The liquid crystal display device in accordance with claim 7,
wherein said birefringent medium layer has a twist angle which has
approximately the same magnitude as, and an opposite direction to
that of said liquid crystal layer and has an approximately right
angle between a slow axis of said birefringent medium layer and an
alignment direction of the face of said liquid crystal cell
opposing to said birefringent medium layer.
14. The liquid crystal display device in accordance with claim 13,
wherein a .DELTA.nd value of said liquid crystal layer is from 0.15
to 0.4 .mu.m.
15. The liquid crystal display device in accordance with claim 13,
wherein the wavelength dependence of .DELTA.n for said liquid
crystal layer and said birefringent medium layer are equal or
approximately equal to each other.
16. The liquid crystal display device in accordance with claim 13,
wherein said birefringent medium layer is selected from the group
consisting of a stretched or extruded polymer film, a liquid
crystal cell having a homogeneous alignment, an aligned polymer
liquid crystal, and an alignment-immobilized liquid crystalline
polymer which is obtained by freezing molecules of the polymer
liquid crystal aligned homogeneously.
17. The liquid crystal display device in accordance with claim 13,
wherein said substrates themselves comprises said birefringent
medium layer.
18. The liquid crystal display device in accordance with claim 13,
wherein said birefringent medium layer comprises a plurality of
nearly uniaxial birefringent layers overlaid with its axis shifted
consecutively by a predetermined angle.
19. The liquid crystal display device in accordance with claim 7,
wherein said birefringent medium layer has a twist angle which has
approximately the same magnitude as, and an opposite direction to,
that of said liquid crystal layer and a .DELTA.nd value
approximately equal to that of said liquid crystal layer.
20. The liquid crystal display device in accordance with claim 19,
wherein a .DELTA.nd value of said liquid crystal layer is from 0.15
to 0.4 .mu.m.
21. The liquid crystal display device in accordance with claim 19,
wherein the wavelength dependence of .DELTA.n for said liquid
crystal layer and said birefringent medium layer are equal or
approximately equal to each other.
22. The liquid crystal display device in accordance with claim 19,
wherein said birefringent medium layer is selected from the group
consisting of a stretched or extruded polymer film, a liquid
crystal cell having a homogeneous alignment, an aligned polymer
liquid crystal, and an alignment-immobilized liquid crystalline
polymer which is obtained by freezing molecules of the polymer
liquid crystal aligned homogeneously.
23. The liquid crystal display device in accordance with claim 19,
wherein said substrates themselves comprises said birefringent
medium layer.
24. The liquid crystal display device in accordance with claim 19,
wherein said birefringent medium layer comprises a plurality of
nearly uniaxial birefringent layers overlaid with its axis shifted
consecutively by a predetermined angle.
25. The liquid crystal display device in accordance with claim 7,
wherein said birefringent medium layer is nearly uniaxial, having a
slow axis in the plane of the birefringent medium layer and a
.DELTA.nd value of said birefringent medium layer is approximately
equal to that of liquid crystal cell, and an approximately right
angle between a slow axis of said birefringent medium layer and an
alignment direction of the face of the liquid crystal opposing to
the birefringent medium layer.
26. The liquid crystal display device in accordance with claim 25,
wherein a .DELTA.nd value of said liquid crystal layer is from 0.15
to 0.4 .mu.m.
27. The liquid crystal display device in accordance with claim 25,
wherein the wavelength dependence of .DELTA.n for said liquid
crystal layer and said birefringent medium layer are equal or
approximately equal to each other.
28. The liquid crystal display device in accordance with claim 25,
wherein said birefringent medium layer is selected from the group
consisting of a stretched or extruded polymer film, a liquid
crystal cell having a homogeneous alignment, an aligned polymer
liquid crystal, or an alignment-immobilized liquid crystalline
polymer which is obtained by freezing molecules of the polymer
liquid crystal aligned homogeneously.
29. The liquid crystal display device in accordance with claim 25,
wherein said substrates themselves comprises said birefringent
medium layer.
30. The liquid crystal display device in accordance with claim 25,
wherein said birefringent medium layer comprises a plurality of
nearly uniaxial birefringent layers overlaid with its axis shifted
consecutively by a predetermined angle.
31. The liquid crystal display device in accordance with claim 7,
wherein said birefringent medium layer is nearly uniaxial, having a
slow axis in the plane of said birefringent medium layer and a
.DELTA.nd value of said birefringent medium layer is approximately
from 0.1 to 0.2 time of that of liquid crystal cell.
32. The liquid crystal display device in accordance with claim 31,
wherein the wavelength dependence of .DELTA.n for said liquid
crystal layer and said birefringent medium layer are equal or
approximately equal to each other.
33. The liquid crystal display device in accordance with claim 31,
wherein a .DELTA.nd value of said liquid crystal layer is from 0.17
to 0.47 .mu.m.
34. The liquid crystal display device in accordance with claim 31,
wherein the wavelength dependence or dispersion of .DELTA.n of said
birefringent medium layer is smaller than that of said liquid
crystal layer.
35. The liquid crystal display device in accordance with claim 31,
wherein said birefringent medium layer is selected from the group
consisting of a stretched or extruded polymer film, a liquid
crystal cell having a homogeneous alignment, an aligned polymer
liquid crystal, and an alignment-immobilized liquid crystalline
polymer which is obtained by freezing molecules of the polymer
liquid crystal aligned homogeneously.
36. The liquid crystal display device in accordance with claim 31,
wherein said substrates themselves comprises said birefringent
medium layer.
37. The liquid crystal display device in accordance with claim 31,
wherein said birefringent medium layer comprises a plurality of
nearly uniaxial birefringent layers overlaid with its axis shifted
consecutively by a predetermined angle.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] This invention relates in general to liquid crystal display
devices and more particularly, to the display devices comprising
cholesteric liquid crystals having bistable characteristics and
provided with birefringence medium layers.
[0003] 2. Description of the Related Art
[0004] Liquid crystal display devices typically comprise a layer of
liquid crystals placed between a pair of light transparent
substrates provided with alignment films and transparent
electrodes, a pair of polarizers disposed on outward surfaces of
the substrates, whereby constituting liquid crystal cells.
[0005] Several liquid crystal display devices using bistable
cholesteric liquid crystals have been developed (for example,
Japanese Patent H1-51818). Liquid crystal cells of the display
device include a layer of cholesteric liquid crystals which are
constituted to have an intrinsic helical pitch of approximately
twice as large as a thickness of the liquid crystal layer.
[0006] In addition, the liquid crystal cells have a bistable
character and can be switched between two states by an application
of voltage, such that a twist angle of the liquid crystal layer
along the layer thickness is either approximately 360.degree. or
0.degree., corresponding to a twisted state or a uniform state
(non-twisted state), respectively. It may be noted that the latter
value (approximately 0.degree.) is not necessarily 0.degree. but a
value subtracted 360.degree. from the above-mentioned
"approximately 360.degree.".
[0007] The pair of polarizing plates are each provided on an upper
and a lower faces of the cell, whereby constituting the liquid
display device, as aforementioned.
[0008] When the polarizing plates are each arranged such that a
transparency axis of one plate makes a right angle to the other,
and that a direction of the liquid crystal alignment at the uniform
state makes a 45.degree. angle to the transparent axis of
polarizing plate, birefringent colors are generally observed.
[0009] These colors are not preferable for black and white
displays, and a white display color is obtained by adjusting a
.DELTA.nd value of the device to about 270 nm, where .DELTA.n and d
represent an optical anisotropy of the liquid crystal and a
thickness of the liquid crystal layer, respectively. Although some
birefringence may also arise in the twisted state, this results in
a nearly black display color due to relatively small values of the
birefringence in that state, without significantly affecting
display qualities.
[0010] As above-mentioned, display colors of nearly black and white
quality can be obtained by conventional bistable liquid crystal
display devices using cholesteric liquid crystals. However, due to
the birefringence in the twisted state of the liquid crystal, in
practice, there exist a certain amount of the light which breaks
through display devices in black display state, resulting in an
insufficient contrast. In addition, when these liquid crystal cells
are used as color displays equipped with color filters, the
above-mentioned coloration gives rise to serious problems, and
satisfactory color purity has not been achieved.
SUMMARY OF THE INVENTION
[0011] It is therefore an object of the present invention to
provide liquid crystal display which overcomes the above-noted
difficulties.
[0012] A further object of the present invention is to provide a
display device with a high contrast and satisfactory color quality
when used as color displays equipped with color filters, and also
to achieve display characteristics such as a higher duty ratio
operation and faster time response, suitable for displaying a large
volume of information data.
[0013] These and other objects of the present invention are
accomplished by the provision of a liquid crystal display device
including a cholesteric liquid crystal layer which has an intrinsic
helical pitch of approximately twice as large as a thickness of the
liquid crystal layer, which is disposed between a pair of
substrates and has a bistable character capable of switching by an
application of voltage. The display device also includes a pair of
polarizers each provided on a upper and lower faces of a liquid
crystal cell.
[0014] In the present invention, at least one birefringent medium
layer is further provided between a polarizer and a substrate for
either one of a upper or lower set of the polarizer and the
substrate. This results in a reduction of birefringent effect of
the display device at either one of the bistable states, whereby
solving the afore-mentioned problems of the conventional display
devices.
[0015] According to another aspect of the present invention, the
birefringent medium layer has a twist angle which has approximately
the same magnitude as, and an opposite direction to, that of the
liquid crystal layer, a .DELTA.nd value approximately equal to that
of the liquid crystal layer, and an approximately right angle
between a slow axis of the birefringent medium layer and an
alignment direction of a face opposing to the birefringent medium
layer, of the liquid crystal cell.
[0016] According to yet another aspect of the invention, the
birefringent medium layer is nearly uniaxial, has a slow axis in
the plane of the birefringent medium layer and a .DELTA.nd value
approximately equal to that of a liquid crystal cell. In addition,
the birefringent medium layer is provided so as to have an
approximately right angle between a slow axis of the birefringent
medium layer and an alignment direction of the face of the liquid
crystal, opposing to the birefringent medium layer.
[0017] According to another aspect of the invention, the
birefringent medium layer is selected from the group consisting of
a stretched or extruded polymer film, a liquid crystal cell having
a homogeneous alignment, an aligned polymer liquid crystal, or an
alignment-immobilized liquid crystalline polymer which is obtained
by freezing molecules of the polymer liquid crystal aligned
homogeneously.
[0018] By this construction of the liquid crystal display, the
birefringent effect can be reduced, whereby achieving a high
contrast, satisfactory color purity, and other display
characteristics such as a higher duty ratio operation and faster
time response, suitable for displaying a large volume of
information data.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Preferred embodiments of the invention are described
hereinbelow with reference to the drawings wherein:
[0020] FIG. 1 is a cross section of a conventional liquid crystal
display having a bistable character, comprising a cholesteric
liquid crystal layer;
[0021] FIG. 2 is a cross section of a liquid crystal display in
accordance with the invention, wherein a birefringent medium layer
is further provided between an upper substrate and a polarizer in
addition to the display structure of FIG. 1;
[0022] FIG. 3 is a schematic showing alignments of liquid crystal
molecules in liquid crystal cells;
[0023] FIG. 4 shows transmittance spectra of a conventional liquid
crystal display of FIG. 1, without the provision of a birefringent
medium layer;
[0024] FIG. 5 is a schematic showing an arrangement of a plurality
of axes of a liquid crystal display in accordance with a first
embodiment of the invention wherein a birefringent medium layer is
provided, having a twist angle which has approximately the same
magnitude as, and an opposite direction to, that of a liquid
crystal layer;
[0025] FIG. 6 shows transmittance spectra of a liquid crystal
display of FIG. 5;
[0026] FIG. 7 a schematic showing an arrangement of a plurality of
axes of a liquid crystal display in accordance with a second
embodiment of the invention wherein a birefringent medium layer is
selected such that a birefringent medium layer to be nearly
uniaxial with its slow axis in the plane of the layer its .DELTA.nd
value approximately from 0.1 to 0.2 time of that of liquid crystal
cell;
[0027] FIG. 8 shows transmittance spectra of a liquid crystal
display of FIG. 7; and
[0028] FIG. 9 shows other transmittance spectra of a liquid crystal
display in accordance with a first embodiment of the invention
wherein a liquid crystal cell was used as a birefringent medium
layer, which had a parallel preferred orientation.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0029] In the description which follows, specific embodiments of
the invention particularly useful in liquid crystal display
devices, comprising cholesteric liquid crystal layers having a
bistable character.
[0030] It is understood, however, that the invention is not limited
to these embodiments. For example, it is appreciated that the
construction and the fabrication methods of the liquid crystal
display in the present invention are adaptable to any form of
liquid crystal display devices. Other embodiments will be apparent
to those skilled in the art upon reading the following
description.
[0031] The invention provides a liquid crystal display device
including a cholesteric liquid crystal layer which has an intrinsic
helical pitch of approximately twice as large as a thickness of a
liquid crystal layer, which is disposed between a pair of
substrates and has a bistable character capable of switching by an
application of voltage such that a twist angle of said liquid
crystal layer along the direction of a layer thickness is either
approximately 360.degree. or an angle subtracted 360.degree. from
the angle value above-mentioned, which is not necessarily
0.degree..
[0032] The substrates are treated for liquid crystal alignment
(hereinafter referred to as "alignment treated") such that a
direction of the alignment has a slight inclination angle relative
to the substrate and substantially parallel to each other. The
display device also includes a pair of polarizers each provided on
a upper and lower faces of the liquid crystal cell.
[0033] In the present invention, at least one birefringent medium
layer is further provided between the polarizer and the substrate
for either one of an upper or lower set of the polarizer and the
substrate. This results in a reduction of the birefringent effect
of the display device, whereby solving the afore-mentioned problems
of the conventional display devices.
[0034] By "substantially parallel" in the above description, it is
meant that, each having a slight inclination angle relative to each
of the substrate as shown in FIGS. 5 and 7, rL and rU are
approximately parallel each other or the angle between these two is
within 30.degree.. Similarly, by "approximately 360.degree.", it is
meant that an angle of from 330.degree. to 390.degree., and more
preferably from 340.degree. to 380.degree..
[0035] FIG. 1 is a cross section of a conventional liquid crystal
display, having a bistable character, comprising a layer 30 of
liquid crystals placed between a pair of light transparent
substrates, a lower substrate 11 and upper substrate 12, which are
provided with transparent electrodes 21 and 22 for applying
voltage, and alignment films 31 and 32 for aligning liquid
crystals, and a pair of polarizers 41 and 42.
[0036] FIG. 2 is a cross section of a liquid crystal display of the
present invention, wherein a birefringent medium layer 51 is
further provided to reduce a birefringent effect between an upper
substrate 12 and a polarizer 41, in addition to the structure of
FIG. 1.
[0037] FIG. 3 is a schematic showing alignment of liquid crystal
molecules in liquid crystal cells, wherein the characters U, P and
C represent a uniform (or non-twisted) state, 180.degree. twisted
state and 360.degree. twisted state, respectively.
[0038] There preferably used is a liquid crystal layer comprised of
nematic phase liquid crystals mixed with liquid crystals of a
cholesteric phase so as to have such a composition that a resultant
liquid crystal layer exhibits a cholesteric phase.
[0039] By the alignment films, liquid crystal molecules in the cell
are aligned so as to have a slight angle of inclination relative to
the face of the substrate. Moreover, the angles of inclination
relative to each of the upper and lower substrates are opposite in
its sign, shown as the state P in FIG. 3.
[0040] As aforementioned, the liquid crystal layer is constructed
such that the layer has an intrinsic helical pitch of approximately
twice as large as a thickness of the liquid crystal layer.
Therefore, without any restraint by the alignment layer, the
approximately 180.degree. twisted state is the stable state for the
layer of this construction. By the above-mentioned "approximately
twice", it is meant that the d/P value is preferably from 0.3 to
0.8, where d and P represent a thickness of the liquid crystal
layer and the intrinsic pitch of the liquid crystal, respectively.
In addition, the above-mentioned twisted state may have an angle
from 150.degree. to 210.degree..
[0041] By contrast, when the liquid crystal cell is constructed so
as to have inclination angles under the restraint by the alignment
films as above-mentioned P state in FIG. 3, two metastable states
arise due to the splay deformation with an increased elastic
energy, which are represented as U and C in FIG. 3 and referred to
as a uniform state and a 360.degree. twisted state, respectively.
These two matastable states can be reversibly switched by the
application of the voltage, as aforementioned.
[0042] FIG. 4 shows transmittance spectra of a conventional liquid
crystal display of FIG. 1, without the provision of a birefringent
medium layer, wherein TS and US represent, respectively, the
spectra for a twisted state and for a uniform (or non-twisted)
state. In FIG. 4, a maximum transmissivity value of the TS spectrum
is normalized to 50% and the values in succeeding figures are
tabulated accordingly.
[0043] Although there is observed a certain wavelength dependence
in transmissivity, a nearly white display quality is obtained for
the US spectrum. Similarly, a nearly black color is obtained for
the TS spectra. However, the transmissivity is generally high
throughout the spectrum region and there exists some light leakage
especially in the short wavelength region, due to birefringent
nature in the TS state.
[0044] In the present invention, by the provision of a birefringent
medium layer, the birefringent effect of the display device can be
reduced and liquid display devices having a high display contrast
can be provided.
[0045] A first embodiment of the invention will now be described.
In order to compensate a birefringent effect of the liquid crystal
display in the twisted state of the present embodiment, a
birefringent medium layer is further provided, having a twist angle
which has approximately the same magnitude as, and an opposite
direction to, that of a liquid crystal layer of the present
embodiment.
[0046] An arrangement of a plurality of axes for the case mentioned
just above is schematically represented in FIG. 5. In the figure,
rL and rU represent the alignment directions of a lower and upper
substrates, respectively, and pL and pU represent transmittance
axes of a lower and upper polarizer, respectively. In a cell,
liquid crystal molecules are arranged between the substrates so as
to have a uniform (or non-twisted) state or 360.degree. twisted
state which is represented by .omega..sub.Lc in the figure.
[0047] A birefringent medium layer is provided between the liquid
crystal cell and the upper substrate. The direction rL in the
figure is the alignment direction of a lower face of, or a face
opposing to the liquid crystal cell of, the twisted birefringent
medium layer. Similarly, the direction rU in the figure is the
alignment direction of an upper face of, or a face opposing to the
polarizer of, the twisted birefringent medium layer. The
birefringent medium layer can be arranged between the above two
alignment directions so as to have an twist angle .omega.R. The
angle .theta.pL is between an alignment direction rL of the lower
face of the liquid crystal and a transparency axis of the lower
polarizer, .theta. pU is an angle between an alignment direction rU
of the upper face of the birefringent medium layer and a direction
of a transparency axis pU of the upper polarizer, and .theta.r is
an angle between an alignment direction of the upper face of the
liquid crystal cell and a alignment direction rL of the lower face
of the birefringent medium layer.
[0048] The magnitude of an angle .omega.R and .DELTA.nd value of a
birefringent medium layer preferred for the present embodiment have
a relationship with a .DELTA.nd value of the liquid crystal layer,
represented as follows,
.DELTA.nd(LC)= 20.times. .DELTA.nd(Birefringent medium
layer)/.vertline..omega.R.vertline..sup.0.5.
[0049] A twist angle of the birefringent medium layer may range
from between about .omega.LC-270.degree. and about
.omega.LC+1800.degree. and a compensation is feasible as long as
the above-mentioned relationship is satisfied. Since a decrease of
the twist angle results in a decrease of the contrast, and a twist
angle that is to large, by contrast, makes uniform alignment
difficult, the twist angle is more preferably from between about
.omega.LC-180.degree. and about .omega.LC+360.degree.. For the
above range of the twist angle, a .DELTA.nd value of the
birefringent medium layer is obtained by the above-mentioned
equation. The difference between calculated and practically usable
.DELTA.nd values is preferably within 40%, and more preferably
within 20%.
[0050] Moreover, it is especially preferred to have the magnitude
of the angle .omega.R is approximately same to the twist angle in
the twisted state, and also to have approximately same values of
.DELTA.nd between the liquid crystal and the birefringent medium
layer. By "approximately equal" above-mentioned, it is meant that
the difference in the angles is within 90.degree.. The difference
in .DELTA.nd values is preferably within 40%, and more preferably
within 20%.
[0051] In addition, the angle .theta.r is preferably from
70.degree. to 110.degree., more preferably from 80.degree. to
100.degree.. Beyond the above-mentioned range, a satisfactorily
compensation can not achieved, resulting in a reduced contrast. The
angles .theta.pL and .theta.pU are preferably from 30.degree. to
60.degree., and more preferably from 35.degree. to 55.degree..
Beyond this range, a reduction of the contrast also results.
[0052] With the present construction of the display, the
compensation of the birefringent effect in the twisted state can be
achieved almost completely. As seen from TS spectra in FIG. 6, an
complete black display state can be obtained. In addition, the
spectra US at the uniform state remains almost unchanged from the
spectra before the compensation and has a relatively broad range of
the transmittance, resulting in a fewer wavelength dependence of
the transmissivity and therefore approaching to an improved white
display quality. As a result, liquid crystal displays having
excellent display qualities can be realized.
[0053] Results shown in FIG. 6 is obtained for the displays with
.DELTA.nd= 0.27. If the value becomes greater, a coloration in the
uniform state results. By contrast, if the .DELTA.nd value becomes
smaller, the display becomes bluish colored, and a reduction in
transmissivity appears for much smaller values. A satisfactory
black and white display quality is achieved for the liquid crystal
.DELTA.nd value of preferably from 0.15 to 0.4 .mu.m, and more
preferably from 0.2 to 0.35 .mu.m.
[0054] In addition, in the present construction of the displays,
the wavelength dependencies of .DELTA.n of both of the liquid
crystal cell and the birefringent medium layer also may affect the
display contrast. If the above .DELTA.n values are different each
other, a complete compensation can be achieved in some range of the
wavelength but a certain degree of light leak results in other
range. Accordingly, the wavelength dependence of .DELTA.n for both
of the liquid crystal cell and the birefringent medium layer have
to be equal or approximately equal to each other to achieve a high
display contrast. For example, if the magnitude of the .DELTA.n
dispersion is defined as
.nu.= (.DELTA.n.sub.F-.DELTA.n.sub.C)/.DELTA.n.sub.D
[0055] the difference in .nu. for the both of the liquid crystal
cell and birefringent medium layer is preferably within 50%, and
more preferably within 30%, wherein .DELTA.n.sub.D, .DELTA.n.sub.F,
and .DELTA.n.sub.C, are .DELTA.n values at 589 nm, 486 nm, and 658
nm, respectively.
[0056] In the present invention, examples of birefringent medium
layer having twisted structures include a liquid crystal cell
having a twisted structure, a polymer liquid crystal with a twisted
alignment, and an alignment-immobilized liquid crystalline polymer
which is obtained by freezing molecules of the polymer liquid
crystal aligned homogeneously. The liquid crystalline polymer is
especially preferred, since it has a self-sustaining nature and its
thickness can be considerably reduced.
[0057] The birefringent medium layer may be constructed with a
single layer, which has a continuous twisted structure throughout
the entire layer. In addition, the birefringent medium layer may
also be constructed with a plurality of optically uniaxial
birefringent layers overlaid with its axis shifted consecutively by
a predetermined angle and adjusted to have optical characteristics
which are substantially equal to that of single twisted layer
structure.
[0058] In the present case, although a contrast is obtained
somewhat reduced compared with that of the afore-mentioned
continuous structure, the fabrication cost can be reduced, since
relatively less expensive materials such as, for example, stretched
polymer films can be used. Although the birefringent medium layer
may be constructed with a single layer, as above-mentioned, having
an entire layer has a uniform birefringent properties, some of the
medium layer may also provided separately onto a transparent
substrate or between the substrates.
[0059] Although the birefringent medium layer is exemplified as
being provided on an upper face of a liquid crystal cell, the
medium layer may also be provided on a lower face or both sides of
the liquid crystal cell. The arrangement of the optical axes for
the latter case is the same as that shown in FIG. 5, when viewed
from the rear side of the liquid crystal cell. Also in the above
example, although .theta.pL and .theta.pU are assumed to represent
transparent axes of the polarizer, a similar description can be
made if absorption axes are assumed in place of the transparent
axes.
[0060] A second embodiment of the invention will be described,
wherein a birefringent medium layer is selected such that a
birefringent medium layer is nearly uniaxial with its slow axis in
the plane of the layer and that its .DELTA.nd is approximately from
0.1 to 0.2 time of that of liquid crystal cell. In the present
case, it is the birefringent effect at the twisted state which is
to be compensated, similarly to embodiment 1.
[0061] FIG. 7 a schematic showing an arrangement of a plurality of
axes of a liquid crystal display in accordance with the present
embodiment.
[0062] A nearly uniaxial birefringent medium layer is provided
between a liquid crystal cell and an upper polarizer. In the
figure, rL and rU represent slow axes of the birefringent medium
layer, .theta.pU is an angle between an alignment direction of the
upper face of the birefringent medium layer and a transparency axis
of the upper polarizer, .theta.r is an angle between an alignment
direction of the upper face of the liquid crystal cell and a slow
axis of the birefringent medium layer, and .theta.pU represents an
angle between a slow axis of the birefringent medium layer and a
transparency axis of the upper polarizer. Other notations are the
same as those in FIG. 5.
[0063] The angle .theta.r is preferably from 60.degree. to
120.degree., and more preferably from 70.degree. to 110.degree.;
angle .theta.pL is preferably from 30.degree. to 60.degree., and
more preferably from 35.degree. to 45.degree.; and angle .theta.pU
is preferably from 25.degree. to 50.degree., and more preferably
from 30.degree. to 45.degree.. Beyond the above-mentioned ranges of
.theta.r, .theta.pL, and/or .theta.pU, a satisfactory compensation
can not achieved and the contrast is reduced.
[0064] In the present embodiment, coloration in the U state depends
on a .DELTA.nd value of the liquid crystal cell, as well as that of
the birefringent medium layer. The preferable range of the
.DELTA.nd of the liquid crystal cell to achieve a satisfactory
black and white display quality is slightly larger than that of the
embodiment 1 and is preferably from 0.17 to 0.47 .mu.m, and more
preferably from 0.02 to 0.42 .mu.m. A preferable .DELTA.nd value of
the birefringent medium layer is from 0.02 to 0.07 .mu.m.
[0065] FIG. 8 shows transmittance spectra of a liquid crystal
display in accordance with the present embodiment, indicating
improved spectra of TS in black state and US in white state. When
these spectra are compared to those of the previous embodiment, in
which a twisted birefringent medium layer is used, a certain amount
of light leak is noticeable in the black state. However, the
present construction is still an considerable improvement over
conventional methods for the compensation.
[0066] Also in the present construction of the displays, the
wavelength dependencies of .DELTA.n for both of the liquid crystal
cell and the birefringent medium layer affect display contrast.
[0067] When the wavelength dependence of .DELTA.n of the
birefringent medium layer is comparable to or larger than that of
the liquid crystal cell, the compensation in visible wavelength
range is relative small and results in a light leak at the black
state. In order to achieve a high contrast, therefore, it is
necessary to adjust the wavelength dependence of .DELTA.n of the
birefringent medium layer to be smaller than that of the liquid
crystal cell.
[0068] Examples of nearly uniaxial birefringent medium layer
suitable for the present invention include stretched or extruded
films of polymers such as, for example, polycarbonate,
polyvinylalcohol, cellulose triacetate, polyethylene, and
polypropylene; a liquid crystal cell having a homogeneous
alignment, an aligned polymer liquid crystal, or an
alignment-immobilized liquid crystalline polymer which is obtained
by freezing molecules of the polymer liquid crystal aligned
homogeneously.
[0069] The birefringent medium layer may be used individually as
mentioned above, and also a plurality of the above-mentioned
birefringent layers can be used by overlaying with its slow axis
shifted consecutively by a predetermined angle. The birefringent
medium layer may be constructed with a single layer, having a
uniform birefringent properties throughout the entire layer, and be
provided separately onto a transparent substrate or between the
substrates.
[0070] If substrates of the liquid crystal display are of polymer
films, the substrates themselves can be used as birefringent medium
layer by adjusting its .DELTA.nd value. By this construction, the
parts number for the display device can be reduced and a thickness
of the device can also be reduced. In addition, a birefringent
medium layer can be provided at the bottom of the display in the
present embodiment.
[0071] A third embodiment of the invention will be described,
wherein a birefringent medium layer is selected to be nearly
uniaxial with its slow axis in the layer plane and its .DELTA.nd
value approximately equal to that of liquid crystal cell. By
contrast with the previous two embodiments, a compensation in the
uniform state is intended in the present embodiment.
[0072] Although the arrangement of the optical axes is similar to
that shown in FIG. 7, the present construction is different from
that of embodiment 2 at the point a birefringent medium layer is
selected such that its .DELTA.nd is approximately same as that of
liquid crystal cell in the present embodiment.
[0073] The angle .theta.r is preferably from 70.degree. to
110.degree., and more preferably from 80.degree. to 100.degree.,
and the angles .theta.pL and .theta.pU are preferably from
30.degree. to 60.degree., and more preferably from 35.degree. to
55.degree.. Beyond these ranges, a satisfactory compensation can
not achieved and a display contrast is reduced.
[0074] As aforementioned, a birefringent medium layer is selected
such that its .DELTA.nd is approximately same as that of liquid
crystal cell. In order to achieve a satisfactory black and white
display quality the difference in .DELTA.nd for the both of the
liquid crystal cell and birefringent medium layer is preferably
within 20%, and more preferably within 10%. In addition, .DELTA.nd
of the liquid crystal cell is preferably from 0.15 to 0.4 .mu.m,
and more preferably from 0.2 to 0.35 .mu.m.
[0075] FIG. 9 shows transmittance spectra of a liquid crystal
display in accordance with the present embodiment. The spectra US
and TS corresponds to a uniform state or black state, and a twisted
state or white state, respectively. Moreover, as seen from the
spectra, the black display quality of this uniform state is quite
complete without any of light leak and quite high in contrast. In
addition, the spectra at the twisted state is broader in the
transmittance region with a reduced wavelength dependence in the
transmissivity, resulting in a white display quality considerably
improved.
[0076] The present construction has advantages such as a fewer
fabrication cost and others, since less expensive materials such
as, for example, stretched films can be used as the birefringent
medium layer.
[0077] In order to achieve a high contrast, a birefringent medium
layer may be selected to have nearly the same wavelength dependence
of .DELTA.n as that of the liquid crystal cell. For example, if the
magnitude of the .DELTA.n dispersion is defined as
.nu.= (.DELTA.n.sub.F-.DELTA.n.sub.C)/.DELTA.n.sub.D
[0078] the difference in .nu. for the both of the liquid crystal
cell and birefringent medium layer is preferably within 50%, and
more preferably within 30%, wherein .DELTA.nd, .DELTA.nf, and
.DELTA.nc are .DELTA.n values at 589 nm, 486 nm, and 658 nm,
respectively.
[0079] Examples of nearly uniaxial birefringent medium layers
suitable for the present invention include stretched or extruded
polymer films, a liquid crystal cell having a homogeneous
alignment, an aligned polymer liquid crystal, or an
alignment-immobilized liquid crystalline polymer which is obtained
by freezing molecules of the polymer liquid crystal aligned
homogeneously.
[0080] The liquid crystalline polymer is especially preferred,
since the thickness of the birefringent medium layer can be
considerably reduced and the dispersion of refractive indices can
be designed in variety of ways by the use of liquid crystals.
[0081] The birefringent medium layer may be used individually as
mentioned above, and a plurality of the present birefringent medium
layer can also be used constructed by overlaying a plurality of the
layers with its slow axis shifted consecutively by a predetermined
angle. The birefringent medium layer may be constructed with a
single layer which has a uniform birefringent properties throughout
the entire layer has a single, and some part of the medium layer
may also provided separately onto a transparent substrate or
between the substrates.
[0082] If substrates of the liquid crystal display are of polymer
films, the substrates themselves can be used as birefringent medium
layer, by adjusting its .DELTA.nd value.
[0083] As liquid crystals suitable for the present invention,
liquid crystal comprising a nematic liquid crystal having a
positive dielectric anisotropy, mixed with cholesteric liquid
crystal is preferably used. As the nematic liquid crystal, a
dual-frequency addressable liquid crystal, of which dielectric
anisotropy change its sign with frequency, may also be used. An
intrinsic pitch the liquid crystal is preferably about twice of a
thickness of the liquid crystal cell.
[0084] The alignment of liquid crystal molecules is preferably
tilt-aligned, wherein the aligned molecules are tilted by an angle
of from 1.5.degree. to 30.degree. to the substrate. This alignment
can be achieved by using conventional substrates such as, for
example, polymer films of polyimide, polyamide, or polyvinyl
alcohol, of which surface is formed by well known method such as,
for example, by rubbing with a cotton cloth, or by oblique vacuum
evaporation of metal oxides.
[0085] The above description was carried out for the transparent
type of liquid crystal displays and a backlight unit may be
provided in the rear side of the display. In addition, liquid
crystal displays are used also as the transparent type by providing
a reflecting plate in place of the backlight unit. In addition, by
further providing color filters, liquid crystal displays may
implement color displays.
[0086] The following examples are provided further to illustrate
preferred embodiments of the invention.
EXAMPLES
Example 1
[0087] A sheet of plate glass having transparent electrodes was
coated with polyimide (AL3046 from Japan Synthetic Rubber Co) and
subsequently alignment treated by rubbing, thereby forming a first
substrate.
[0088] A second substrate was prepared in a similar manner as
above. The first and second substrates were subsequently arranged
apart from, and opposed to each other with silica beads disposed
in-between as spacers.
[0089] A liquid crystal was then sealed between the substrates to
constitute a liquid crystal display cell. In this embodiment, a
liquid crystal prepared by adding 1.1% by weight of a chiral
nematic liquid crystal (S811 from Merck & Co), which induces a
left-handed helical structure, in a nematic liquid crystal
(.DELTA.n= 0.79; ZLI3412-000 from Merck & Co) so as to have a
helical pitch of 8.2 .mu.m.
[0090] A thickness of the liquid crystal layer was adjusted to 4.1
.mu.m by selecting a diameter of the spacer beads. In addition, the
alignment directions by rubbing treatment, of the top and bottom
substrates were arranged in an anti-parallel fashion.
[0091] On top of the display cell, a polycabonate film having a
.DELTA.nd value of 40 nm was provided such that a slow axis of the
film was arranged to have a right angle relative to the direction
of the rubbing alignment of the top substrate. Subsequently, by
disposing a pair of polarizers on the top and bottom faces of the
cell substrate, a liquid crystal display of the present invention
was fabricated. At this point, the direction of transparency axis
for the top and bottom polarizing plates were arranged to be
45.degree. and 35.degree., for .theta.pL and .theta.pU,
respectively.
[0092] Transmittance spectra of the display device were measured
for each of the states TS and US. FIG. 8 represents the spectra
observed, indicating a considerable improvement on the light
leakage in short wavelength region, achieved by the provision of
the birefringent medium layer of the present invention over the
spectra shown in FIG. 4, which were previously obtained for the
display device without birefringent medium layers.
Example 2
[0093] A first liquid crystal display device was fabricated in a
similar manner to Example 1, with the exception that the display
had a thickness of a layer of liquid crystal of 3.5 .mu.m and a
helical pitch of 1.8 .mu.m.
[0094] In addition, a second display device was fabricated by using
display cell fabricated in a similar manner to Example 1, with the
exception that a liquid crystal was prepared by adding a chiral
nematic liquid crystal (R811 from Merck Co), which induces a
right-handed helical structure, to the ZLI3412-000 nematic liquid
crystal so as to have a helical pitch of 3.5 .mu.m and twisted by
360.degree. in the opposite direction.
[0095] Subsequently, the second display device was overlaid onto
the first display device such that a direction of the rubbing
alignment of an upper substrate of the first display device had a
right angle to that of a lower substrate of the second display
device, followed by the provision of a pair of polarizing plates on
the upper and lower face of the cell substrates, whereby
constituting a liquid crystal display of the present invention. At
this point, the direction of transparency axis for both of the
upper and lower polarizer .theta.pL and .theta.pU was arranged to
be 45.degree..
[0096] Transmittance spectra of the display device were measured
for both of the states TS and US. FIG. 6 represent the spectra,
indicating a nearly complete contrast achieved in the black TS
state. Moreover, the spectra in the TS state have a broader range
in transmittance curve which is indicative of an improved white
display quality.
Example 3
[0097] A liquid crystal display was fabricated in a similar manner
to Example 1, with the exception that, in place of the carbonate
film of Example 1, a liquid crystal cell was used which had a
parallel preferred orientation, a .DELTA.n value of 0.27 .mu.m, and
45.degree. angle for both .theta. pL and .theta. pU.
[0098] Light transmittance spectra of the display device were
measured at each of states TS and US. FIG. 9 represents the
measured spectra, indicating a complete compensation in the black
state was achieved in the US state, resulting in a quite high
contrast of the display.
Example 4
[0099] A liquid crystal display was fabricated in a similar manner
to Example 3, with the exception that, in place of the cell of
Example 3, a polycarbonate film having a .DELTA.n value of 0.27
.mu.m, was used. At this point, it was found that the wavelength
dependence of the .DELTA.n value was about one third of that of the
above liquid crystal cell.
[0100] Although the obtained contrast was practically satisfactory,
a slight leak of light was observed in the black state, indicating
display characteristics not so satisfactory as of Example 3.
Example 5
[0101] A liquid crystal display was fabricated in a similar manner
to Example 1, with the exception that, in place of the carbonate
film of Example 1, a liquid crystal cell was used which had a
parallel preferred orientation and a .DELTA.n value of 0.27
.mu.m.
[0102] Although a practically satisfying contrast was obtained, a
slight leak of light was observed in the black state, indicating
the contrast previously obtained in Example 1 was superior to the
present display.
Example 6
[0103] A liquid crystal display was fabricated in a similar manner
to Example 1, with the exception that a carbonate film which had a
thickness of 100 .mu.m and a corresponding .DELTA.n value of 0.04
.mu.m, was used as an upper substrate of a liquid crystal cell and
no phase difference plate was provided.
[0104] The display was found to have as much excellent display
characteristics as that of Example 1, even with such a thickness of
the present substrate as one half of the glass plate (1.1 mm thick)
in the Example 1.
Example 7
[0105] A phase shift plate was fabricated as follows. Solution of a
thermotropic-cholesteric polymer liquid crystal, having a .DELTA.n
value of 0.12 and a left-handed twist pitch of 2.3 .mu.m, was
coated to a thickness of 2.3 .mu.m on a cellulose triacetate film
which had a thickness of 100 .mu.m and was alignment treated by
rubbing.
[0106] The coated film was then dried and heated to such a
temperature that a cholesteric phase of the polymer liquid crystal
was formed and resulted in a preferred orientation in a direction
twisted by 360.degree., followed by a quenching to room temperature
at which the twisted structure was sufficiently fixed.
[0107] The twisted phase shifting plate prepared as above was used
in place of the reverse-twisted compensation cell in Example 2, to
fabricate a display cell of the present invention.
[0108] The display device was found to have as much excellent
optical characteristics as that of Example 2, even with a half
thickness of the compensation liquid crystal cell.
Example 8
[0109] A plurality of first and second (or compensation) liquid
crystal cells were fabricated in a similar manner to Example 2,
such that the cells had various cell thickness under the condition
that .DELTA.nd values for the first and second cells were kept
equal to each other.
[0110] When .DELTA.nd values were smaller than about 0.15 or
greater than about 0.4, a significant coloration was observed. The
results are shown in Table 1.
1TABLE 1 .DELTA.nd color 0.1 blue 0.15 bright blue 0.2 bluish white
0.25 white 0.3 white 0.35 yellowish white 0.4 yellow 0.45
orange
Example 9
[0111] A plurality of first and second (or compensation) liquid
crystal cells were fabricated in a similar manner to Example 1,
such that the cells had various thickness under the condition that
the ratio of .DELTA.nd of the first cell to that of the second cell
remained constant.
[0112] When the .DELTA.nd value for the first cell was smaller than
about 0.17 or greater than about 0.42, a significant coloration was
observed.
Example 10
[0113] A liquid crystal cell was fabricated in a similar manner to
Example 2, with the exception that, in place of the second (or
compensation) cell, a phase shifting plate was provided, which was
constituted of four sheets of carbonate films, each having a
.DELTA.nd value of 0.07 .mu.m and overlaid in such a manner that
the slow phase axis of each film was consecutively shifted by
120.degree..
[0114] Although a resultant cell exhibited a light leak in TS state
larger than that of Example 2, the display device showed a contrast
twice as good as conventional devices.
Example 11
[0115] A second liquid crystal cell was fabricated in a similar
manner to Example 2, with the exception that the liquid crystal
cell had a twist angle of 720.degree. in the direction opposite to
that of the first liquid crystal cell with a thickness of 5.3
.mu.m.
[0116] Although the present display device has a transmittance
value at 550 nm in the TS state, 0.1% higher than that of
embodiment 2, the display device exhibited a satisfactory
contrast.
[0117] Additional modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *