U.S. patent application number 11/828583 was filed with the patent office on 2008-02-28 for liquid crystal display device.
Invention is credited to Shigesumi Araki, Emi Kisara, Kazuhiro Nishiyama, Mitsutaka Okita.
Application Number | 20080049178 11/828583 |
Document ID | / |
Family ID | 39113049 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080049178 |
Kind Code |
A1 |
Kisara; Emi ; et
al. |
February 28, 2008 |
LIQUID CRYSTAL DISPLAY DEVICE
Abstract
A liquid crystal display device includes an OCB mode liquid
crystal display panel, and an optical compensation element which is
disposed outside of the liquid crystal display layer. The optical
compensation element includes a polarizer plate, a first
retardation plate which is disposed between the polarizer plate and
the liquid crystal layer, and a second retardation plate which is
disposed between the polarizer plate and the first retardation
plate and has a biaxial refractive index anisotropy. The optical
compensation element compensates a difference of a polarization
state that differs between azimuth directions of light passing
through the liquid crystal layer and compensates a shift of the
polarization state of light, which passes through the retardation
plate, from an azimuth direction of an absorption axis of the
polarizer plate.
Inventors: |
Kisara; Emi; (Ishikawa-gun,
JP) ; Nishiyama; Kazuhiro; (Kanazawa-shi, JP)
; Okita; Mitsutaka; (Hakusan-shi, JP) ; Araki;
Shigesumi; (Kanazawa-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
39113049 |
Appl. No.: |
11/828583 |
Filed: |
July 26, 2007 |
Current U.S.
Class: |
349/118 ;
349/119 |
Current CPC
Class: |
G02F 1/133634 20130101;
G02F 1/1395 20130101; G02F 1/133631 20210101 |
Class at
Publication: |
349/118 ;
349/119 |
International
Class: |
G02F 1/13363 20060101
G02F001/13363 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 26, 2006 |
JP |
2006-203860 |
Jul 13, 2007 |
JP |
2007-184426 |
Claims
1. A liquid crystal display device comprising: an OCB mode liquid
crystal display panel which is configured such that a liquid
crystal layer is held between a first substrate and a second
substrate; and an optical compensation element which is disposed
outside of the liquid crystal layer and optically compensates a
retardation of the liquid crystal layer in a predetermined display
state in which a voltage is applied to the liquid crystal layer,
wherein the optical compensation element includes: a polarizer
plate; a first retardation plate which is disposed between the
polarizer plate and the liquid crystal layer and imparts a phase
difference of a 1/4 wavelength; and a second retardation plate
which is disposed between the polarizer plate and the first
retardation plate and has a biaxial refractive index anisotropy,
and the second retardation plate has a refractive index anisotropy
which is set in such a manner as to compensate (a) a difference of
a polarization state due to an influence of optical rotatory power
that differs between azimuth directions of light passing through
the liquid crystal layer and to compensate (b) a shift of the
polarization state of light, which passes through the first
retardation plate, from an azimuth direction of an absorption axis
of the polarizer plate.
2. The liquid crystal display device according to claim 1, wherein
the liquid crystal display panel includes a reflective electrode on
the first substrate, and the optical compensation element is
disposed on an outer surface of the second substrate.
3. The liquid crystal display device according to claim 2, wherein
an Nz coefficient in the second retardation plate is set in a range
of between 0.15 and 0.3, the Nz coefficient being given by
Nz=(nx-nz)/(nx-ny), where nx and ny are refractive indices in
mutually perpendicular directions in a plane of the second
retardation plate, and nz is a refractive index in a normal
direction to the second retardation plate.
4. The liquid crystal display device according to claim 3, wherein
the absorption axis of the polarizer plate is substantially
perpendicular to an optical axis of the second retardation
plate.
5. The liquid crystal display device according to claim 2, wherein
an Nz coefficient in the second retardation plate is set in a range
of between 0.7 and 0.9, the Nz coefficient being given by
Nz=(nx-nz)/(nx-ny), where nx and ny are refractive indices in
mutually perpendicular directions in a plane of the second
retardation plate, and nz is a refractive index in a normal
direction to the second retardation plate.
6. The liquid crystal display device according to claim 5, wherein
the absorption axis of the polarizer plate substantially agrees
with an optical axis of the second retardation plate.
7. The liquid crystal display device according to claim 1, wherein
the liquid crystal display panel includes a transmissive electrode
on the first substrate, and the optical compensation element is
disposed on one of an outer surface of the first substrate and an
outer surface of the second substrate.
8. The liquid crystal display device according to claim 7, wherein
an Nz coefficient in the second retardation plate is set in a range
of between 0.4 and 0.6, the Nz coefficient being given by
Nz=(nx-nz)/(nx-ny), where nx and ny are refractive indices in
mutually perpendicular directions in a plane of the second
retardation plate, and nz is a refractive index in a normal
direction to the second retardation plate.
9. The liquid crystal display device according to claim 8, wherein
the absorption axis of the polarizer plate substantially agrees
with, or is substantially perpendicular to, an optical axis of the
second retardation plate.
10. The liquid crystal display device according to claim 1, further
comprising a retardation plate RA with a refractive index
anisotropy corresponding to an A-plate and a retardation plate RC
with a refractive index anisotropy corresponding to a C-plate,
which are disposed between the liquid crystal display panel and the
first retardation plate.
11. The liquid crystal display device according to claim 1, further
comprising a retardation plate RB with a biaxial refractive index
anisotropy corresponding to an A-plate and a C-plate, which is
disposed between the liquid crystal display panel and the first
retardation plate.
12. The liquid crystal display device according to claim 1, further
comprising a retardation plate Rwv with such a refractive index
anisotropy that a major axis thereof is inclined to a normal
direction, which is disposed between the liquid crystal display
panel and the first retardation plate.
13. The liquid crystal display device according to claim 1, further
comprising a retardation plate RA with a refractive index
anisotropy corresponding to an A-plate, which is disposed between
the liquid crystal display panel and the first retardation plate,
wherein the first retardation plate imparts a phase difference of a
1/4 wavelength between light components of a predetermined
wavelength, which pass through a fast axis and a slow axis thereof,
and has a biaxial refractive index anisotropy corresponding to a
C-plate.
14. A liquid crystal display device comprising: an OCB mode liquid
crystal display panel which is configured such that a liquid
crystal layer is held between a first substrate and a second
substrate; and a first optical compensation element and a second
optical compensation element which are disposed, respectively,
outside of the liquid crystal layer, and optically compensate a
retardation of the liquid crystal layer in a predetermined display
state in which a voltage is applied to the liquid crystal layer,
wherein the first optical compensation element includes: a first
polarizer plate; a first retardation plate which is disposed
between the first polarizer plate and the liquid crystal layer and
imparts a phase difference of a 1/4 wavelength; and a second
retardation plate which is disposed between the first polarizer
plate and the first retardation plate and has a biaxial refractive
index anisotropy, the second optical compensation element includes:
a second polarizer plate; a third retardation plate which is
disposed between the second polarizer plate and the liquid crystal
layer and imparts a phase difference of a 1/4 wavelength; and a
fourth retardation plate which is disposed between the second
polarizer plate and the third retardation plate and has a biaxial
refractive index anisotropy, and the second retardation plate and
the fourth retardation plate have a refractive index anisotropy
which is set in such a manner as to compensate (a) a difference of
a polarization state due to an influence of optical rotatory power
that differs between azimuth directions of light passing through
the liquid crystal layer and to compensate (b) a shift of the
polarization state of light, which passes through the first
retardation plate, from an azimuth direction of an absorption axis
of the polarizer plate.
15. The liquid crystal display device according to claim 14,
wherein the absorption axis of the first polarizer plate
substantially agrees with an optical axis of the second retardation
plate, and the absorption axis of the second polarizer plate is
substantially perpendicular to an optical axis of the fourth
retardation plate, an Nz coefficient in the second retardation
plate is set in a range of between 0.7 and 0.9, the Nz coefficient
being given by Nz=(nx-nz)/(nx-ny), where nx and ny are refractive
indices in mutually perpendicular directions in a plane of the
retardation plate, and nz is a refractive index in a normal
direction to the retardation plate, and the Nz coefficient in the
fourth retardation plate is set in a range of between 0.15 and
0.3.
16. The liquid crystal display device according to claim 14,
wherein the absorption axis of the first polarizer plate is
substantially perpendicular to an optical axis of the second
retardation plate, and the absorption axis of the second polarizer
plate substantially agrees with an optical axis of the fourth
retardation plate, an Nz coefficient in the second retardation
plate is set in a range of between 0.15 and 0.3, the Nz coefficient
being given by Nz=(nx-nz)/(nx-ny), where nx and ny are refractive
indices in mutually perpendicular directions in a plane of the
retardation plate, and nz is a refractive index in a normal
direction to the retardation plate, and the Nz coefficient in the
fourth retardation plate is set in a range of between 0.7 and
0.9.
17. The liquid crystal display device according to claim 14,
wherein the absorption axis of the first polarizer plate is
substantially perpendicular to an optical axis of the second
retardation plate, and the absorption axis of the second polarizer
plate is substantially perpendicular to an optical axis of the
fourth retardation plate, and an Nz coefficient in each of the
second retardation plate and the fourth retardation plate is set in
a range of between 0.15 and 0.3, the Nz coefficient being given by
Nz=(nx-nz)/(nx-ny), where nx and ny are refractive indices in
mutually perpendicular directions in a plane of each of the second
retardation plate and the fourth retardation plate, and nz is a
refractive index in a normal direction to each of the second
retardation plate and the fourth retardation plate.
18. The liquid crystal display device according to claim 14,
wherein the absorption axis of the first polarizer plate
substantially agrees with an optical axis of the second retardation
plate, and the absorption axis of the second polarizer plate
substantially agrees with an optical axis of the fourth retardation
plate, and an Nz coefficient in each of the second retardation
plate and the fourth retardation plate is set in a range of between
0.7 and 0.9, the Nz coefficient being given by Nz=(nx-nz)/(nx-ny),
where nx and ny are refractive indices in mutually perpendicular
directions in a plane of each of the second retardation plate and
the fourth retardation plate, and nz is a refractive index in a
normal direction to each of the second retardation plate and the
fourth retardation plate.
19. The liquid crystal display device according to claim 14,
further comprising a retardation plate RA with a refractive index
anisotropy corresponding to an A-plate and a retardation plate RC
with a refractive index anisotropy corresponding to a C-plate,
which are disposed between the liquid crystal display panel and the
first retardation plate and between the liquid crystal display
panel and the third retardation plate.
20. The liquid crystal display device according to claim 14,
further comprising a retardation plate RB with a biaxial refractive
index anisotropy corresponding to an A-plate and a C-plate, which
is disposed between the liquid crystal display panel and the first
retardation plate and between the liquid crystal display panel and
the third retardation plate.
21. The liquid crystal display device according to claim 14,
further comprising a retardation plate Rwv with such a refractive
index anisotropy that a major axis thereof is inclined to a normal
direction, which is disposed between the liquid crystal display
panel and the first retardation plate and between the liquid
crystal display panel and the third retardation plate.
22. The liquid crystal display device according to claim 14,
further comprising a retardation plate RA with a refractive index
anisotropy corresponding to an A-plate, which is disposed between
the liquid crystal display panel and the first retardation plate
and between the liquid crystal display panel and the third
retardation plate, wherein each of the first retardation plate and
the third retardation plate imparts a phase difference of a 1/4
wavelength between light components of a predetermined wavelength,
which pass through a fast axis and a slow axis thereof, and has a
biaxial refractive index anisotropy corresponding to a C-plate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Applications No. 2006-203860,
filed Jul. 26, 2006; and No. 2007-184426, filed Jul. 13, 2007, the
entire contents of both of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to a liquid crystal
display device, and more particularly to a liquid crystal display
device which uses an optically compensated bend (OCB) alignment
technique that is capable of realizing a wide viewing angle and a
high speed response.
[0004] 2. Description of the Related Art
[0005] Liquid crystal display devices have been applied to various
technical fields by virtue of their features of light weight, small
thickness and low power consumption.
[0006] In recent years, attention has been paid to a liquid crystal
display device to which an OCB mode is applied, as a liquid crystal
display device that is capable of improving a viewing angle and a
response speed. The OCB mode liquid crystal display device is
configured such that a liquid crystal layer including bend-aligned
liquid crystal molecules is held between a pair of substrates in
the state in which a predetermined voltage is applied between the
pair of substrates. Compared to a twisted nematic (TN) mode, the
OCB mode can realize a higher response speed and can optically
self-compensate the influence of birefringence of light that passes
through the liquid crystal layer by the alignment state of liquid
crystal molecules. Thus, the viewing angle can advantageously be
increased.
[0007] There is disclosed a circular polarization plate that is
applicable to the OCB mode liquid crystal display device (see, e.g.
Jpn. Pat. Appln. KOKAI Publication No. 2005-164957). This circular
polarization plate includes a liquid crystal film in which a
nematic hybrid alignment structure is fixed.
[0008] As regards the above-described OCB mode liquid crystal
display device, there has been a demand for a further increase in
viewing angle at which a high contrast can be obtained.
BRIEF SUMMARY OF THE INVENTION
[0009] The object of the present invention is to provide a liquid
crystal display device to which an OCB mode is applicable, and
which is capable of increasing a viewing angle.
[0010] According to an aspect of the present invention, there is
provided a liquid crystal display device comprising: an OCB mode
liquid crystal display panel which is configured such that a liquid
crystal layer is held between a first substrate and a second
substrate; and an optical compensation element which is disposed
outside of the liquid crystal layer and optically compensates a
retardation of the liquid crystal layer in a predetermined display
state in which a voltage is applied to the liquid crystal layer,
wherein the optical compensation element includes; a polarizer
plate; a first retardation plate which is disposed between the
polarizer plate and the liquid crystal layer and imparts a phase
difference of a 1/4 wavelength; and a second retardation plate
which is disposed between the polarizer plate and the first
retardation plate and has a biaxial refractive index anisotropy,
and the second retardation plate has a refractive index anisotropy
which is set in such a manner as to compensate (a) a difference of
a polarization state due to an influence of optical rotatory power
that differs between azimuth directions of light passing through
the liquid crystal layer and to compensate (b) a shift of the
polarization state of light, which passes through the first
retardation plate, from an azimuth direction of an absorption axis
of the polarizer plate.
[0011] According to another aspect of the present invention, there
may be provided a liquid crystal display device comprising: a
liquid crystal display panel which is configured such that a liquid
crystal layer is held between a first substrate and a second
substrate, and to which an OCB mode is applied; and a first optical
compensation element and a second optical compensation element
which are disposed, respectively, on outer surfaces of the first
substrate and the second substrate, and optically compensate a
retardation of the liquid crystal layer in a predetermined display
state in which a voltage is applied to the liquid crystal layer,
wherein the first optical compensation element includes: a first
polarizer plate; a first retardation plate which is disposed
between the first polarizer plate and the liquid crystal display
panel and imparts a phase difference of a 1/4 wavelength between
light components of a predetermined wavelength, which pass through
a fast axis and a slow axis thereof; and a second retardation plate
which is disposed between the first polarizer plate and the first
retardation plate and has a biaxial refractive index anisotropy,
and the second optical compensation element includes: a second
polarizer plate; a third retardation plate which is disposed
between the second polarizer plate and the liquid crystal display
panel and imparts a phase difference of a 1/4 wavelength between
light components of a predetermined wavelength, which pass through
a fast axis and a slow axis thereof; and a fourth retardation plate
which is disposed between the second polarizer plate and the third
retardation plate and has a biaxial refractive index anisotropy,
and wherein the absorption axis of the first polarizer plate
substantially agrees with an optical axis of the second retardation
plate, the absorption axis of the second polarizer plate is
substantially perpendicular to an optical axis of the fourth
retardation plate, an Nz coefficient in the second retardation
plate is set in a range of between 0.7 and 0.9, the Nz coefficient
being given by Nz=(nx-nz)/(nx-ny), where nx and ny are refractive
indices in mutually perpendicular directions in a plane of the
second retardation plate, and nz is a refractive index in a normal
direction to the second retardation plate, and the Nz coefficient
in the fourth retardation plate is set in a range of between 0.15
and 0.3.
[0012] According to still another aspect of the present invention,
there may be provided a liquid crystal display device comprising: a
liquid crystal display panel which is configured such that a liquid
crystal layer is held between a first substrate and a second
substrate, and to which an OCB mode is applied; and a first optical
compensation element and a second optical compensation element
which are disposed, respectively, on outer surfaces of the first
substrate and the second substrate, and optically compensate a
retardation of the liquid crystal layer in a predetermined display
state in which a voltage is applied to the liquid crystal layer,
wherein the first optical compensation element includes: a first
polarizer plate; a first retardation plate which is disposed
between the first polarizer plate and the liquid crystal display
panel and imparts a phase difference of a 1/4 wavelength between
light components of a predetermined wavelength, which pass through
a fast axis and a slow axis thereof; and a second retardation plate
which is disposed between the first polarizer plate and the first
retardation plate and has a biaxial refractive index anisotropy,
and the second optical compensation element includes: a second
polarizer plate; a third retardation plate which is disposed
between the second polarizer plate and the liquid crystal display
panel and imparts a phase difference of a 1/4 wavelength between
light components of a predetermined wavelength, which pass through
a fast axis and a slow axis thereof; and a fourth retardation plate
which is disposed between the second polarizer plate and the third
retardation plate and has a biaxial refractive index anisotropy,
and wherein the absorption axis of the first polarizer plate is
substantially perpendicular to an optical axis of the second
retardation plate, the absorption axis of the second polarizer
plate substantially agrees with an optical axis of the fourth
retardation plate, an Nz coefficient in the second retardation
plate is set in a range of between 0.15 and 0.3, the Nz coefficient
being given by Nz=(nx-nz)/(nx-ny), where nx and ny are refractive
indices in mutually perpendicular directions in a plane of the
second retardation plate, and nz is a refractive index in a normal
direction to the second retardation plate, and the Nz coefficient
in the fourth retardation plate is set in a range of between 0.7
and 0.9.
[0013] According to still another aspect of the present invention,
there may be provided a liquid crystal display device comprising: a
liquid crystal display panel which is configured such that a liquid
crystal layer is held between a first substrate and a second
substrate, and to which an OCB mode is applied; and a first optical
compensation element and a second optical compensation element
which are disposed, respectively, on outer surfaces of the first
substrate and the second substrate, and optically compensate a
retardation of the liquid crystal layer in a predetermined display
state in which a voltage is applied to the liquid crystal layer,
wherein the first optical compensation element includes: a first
polarizer plate; a first retardation plate which is disposed
between the first polarizer plate and the liquid crystal display
panel and imparts a phase difference of a 1/4 wavelength between
light components of a predetermined wavelength, which pass through
a fast axis and a slow axis thereof; and a second retardation plate
which is disposed between the first polarizer plate and the first
retardation plate and has a biaxial refractive index anisotropy,
and the second optical compensation element includes: a second
polarizer plate; a third retardation plate which is disposed
between the second polarizer plate and the liquid crystal display
panel and imparts a phase difference of a 1/4 wavelength between
light components of a predetermined wavelength, which pass through
a fast axis and a slow axis thereof; and a fourth retardation plate
which is disposed between the second polarizer plate and the third
retardation plate and has a biaxial refractive index anisotropy,
and wherein the absorption axis of the first polarizer plate is
substantially perpendicular to an optical axis of the second
retardation plate, the absorption axis of the second polarizer
plate is substantially perpendicular to an optical axis of the
fourth retardation plate, and an Nz coefficient in each of the
second retardation plate and the fourth retardation plate is set in
a range of between 0.15 and 0.3, the Nz coefficient being given by
Nz=(nx-nz)/(nx-ny), where nx and ny are refractive indices in
mutually perpendicular directions in a plane of each of the second
retardation plate and the fourth retardation plate, and nz is a
refractive index in a normal direction to each of the second
retardation plate and the fourth retardation plate.
[0014] According to still another aspect of the present invention,
there may be provided a liquid crystal display device comprising: a
liquid crystal display panel which is configured such that a liquid
crystal layer is held between a first substrate and a second
substrate, and to which an OCB mode is applied; and a first optical
compensation element and a second optical compensation element
which are disposed, respectively, on outer surfaces of the first
substrate and the second substrate, and optically compensate a
retardation of the liquid crystal layer in a predetermined display
state in which a voltage is applied to the liquid crystal layer,
wherein the first optical compensation element includes: a first
polarizer plate; a first retardation plate which is disposed
between the first polarizer plate and the liquid crystal display
panel and imparts a phase difference of a 1/4 wavelength between
light components of a predetermined wavelength, which pass through
a fast axis and a slow axis thereof; and a second retardation plate
which is disposed between the first polarizer plate and the first
retardation plate and has a biaxial refractive index anisotropy,
and the second optical compensation element includes: a second
polarizer plate; a third retardation plate which is disposed
between the second polarizer plate and the liquid crystal display
panel and imparts a phase difference of a 1/4 wavelength between
light components of a predetermined wavelength, which pass through
a fast axis and a slow axis thereof; and a fourth retardation plate
which is disposed between the second polarizer plate and the third
retardation plate and has a biaxial refractive index anisotropy,
and wherein the absorption axis of the first polarizer plate
substantially agrees with an optical axis of the second retardation
plate, the absorption axis of the second polarizer plate
substantially agrees with an optical axis of the fourth retardation
plate, and an Nz coefficient in each of the second retardation
plate and the fourth retardation plate is set in a range of between
0.7 and 0.9, the Nz coefficient being given by Nz=(nx-nz)/(nx-ny),
where nx and ny are refractive indices in mutually perpendicular
directions in a plane of each of the second retardation plate and
the fourth retardation plate, and nz is a refractive index in a
normal direction to each of the second retardation plate and the
fourth retardation plate.
[0015] The present invention can provide a liquid crystal display
device to which an OCB mode is applicable, and which is capable of
increasing a viewing angle.
[0016] 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
[0017] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate embodiments of
the invention, and together with the general description given
above and the detailed description of the embodiments given below,
serve to explain the principles of the invention.
[0018] FIG. 1 schematically shows the structure of a transmissive
liquid crystal display device or a transreflective liquid crystal
display device according to an embodiment of the present
invention;
[0019] FIG. 2 schematically shows the structure of a liquid crystal
display panel which is applicable to the liquid crystal display
device shown in FIG. 1;
[0020] FIG. 3 schematically shows the structure of an OCB mode
transmissive liquid crystal display panel which is applicable to
the liquid crystal display device shown in FIG. 1;
[0021] FIG. 4A schematically shows an example of the structure of
an optical compensation element which is applicable to the liquid
crystal display device shown in FIG. 1;
[0022] FIG. 4B schematically shows another example of the structure
of the optical compensation element which is applicable to the
liquid crystal display device shown in FIG. 1;
[0023] FIG. 4C schematically shows still another example of the
structure of the optical compensation element which is applicable
to the liquid crystal display device shown in FIG. 1;
[0024] FIG. 4D schematically shows still another example of the
structure of the optical compensation element which is applicable
to the liquid crystal display device shown in FIG. 1;
[0025] FIG. 5 is a view for explaining a relationship of
compensation with liquid crystal molecules in a case where the
optical compensation element shown in FIG. 4C is applied;
[0026] FIG. 6 is a view for explaining the definitions of axial
angles relative to a rubbing direction of an alignment film in the
liquid crystal display device shown in FIG. 1;
[0027] FIG. 7 is a view for explaining the structure of a
transmissive liquid crystal display device or a transreflective
liquid crystal display device to which a first optical compensation
element and a second optical compensation element according to a
first example of structure are applied;
[0028] FIG. 8A shows a result of simulation of a viewing angle
dependency of a contrast ratio in a transmissive liquid crystal
display device according to a comparative example;
[0029] FIG. 8B shows a result of simulation of a viewing angle
dependency of a contrast ratio in the transmissive liquid crystal
display device according to the present embodiment;
[0030] FIG. 9A schematically shows another structure of a
transmissive liquid crystal display device or a transreflective
liquid crystal display device according to an embodiment of the
present invention;
[0031] FIG. 9B schematically shows still another structure of the
transmissive liquid crystal display device or a transreflective
liquid crystal display device according to the embodiment of the
present invention;
[0032] FIG. 10A is a view for explaining the structure of a
transmissive liquid crystal display device or a transreflective
liquid crystal display device to which a first optical compensation
element and a second optical compensation element according to a
second example of structure are applied;
[0033] FIG. 10B shows a result of simulation of a viewing angle
dependency of a contrast ratio in the transmissive liquid crystal
display device according to the present embodiment;
[0034] FIG. 10C shows a result of simulation of a viewing angle
dependency of a contrast ratio in another transmissive liquid
crystal display device according to the present embodiment;
[0035] FIG. 11 schematically shows the structure of an OCB mode
transreflective liquid crystal display panel which is applicable to
the liquid crystal display device shown in FIG. 1;
[0036] FIG. 12A shows a result of simulation of a viewing angle
dependency of a contrast ratio in a transmissive part of a
transreflective liquid crystal display device according to a
comparative example;
[0037] FIG. 12B shows a result of simulation of a viewing angle
dependency of a contrast ratio in a transmissive part of the
transreflective liquid crystal display device according to the
present embodiment;
[0038] FIG. 13A shows a result of simulation of a viewing angle
dependency of a contrast ratio in a reflective part of the
transreflective liquid crystal display device according to the
comparative example;
[0039] FIG. 13B shows a result of simulation of a viewing angle
dependency of a contrast ratio in a reflective part of the
transreflective liquid crystal display device according to the
present embodiment;
[0040] FIG. 14 schematically shows the structure of an OCB mode
reflective liquid crystal display device according to an embodiment
of the invention;
[0041] FIG. 15 is a view for explaining an example of structure and
an example of modification of a reflective liquid crystal display
device;
[0042] FIG. 16A shows a result of simulation of a viewing angle
dependency of a contrast ratio in a reflective liquid crystal
display device according to a comparative example; and
[0043] FIG. 16B shows a result of simulation of a viewing angle
dependency of a contrast ratio in the reflective liquid crystal
display device according to the present embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0044] Liquid crystal display devices according to an embodiment of
the present invention will now be described with reference to the
accompanying drawings. Liquid crystal display devices, to which an
OCB mode is applied, will be described as examples of the liquid
crystal display devices. Such OCB mode liquid crystal display
devices include a transmissive liquid crystal display device in
which each pixel is composed of only a transmissive part that
displays an image by selectively passing backlight; a reflective
liquid crystal display device in which each pixel is composed of
only a reflective part that displays an image by selectively
reflecting ambient light; and a transreflective liquid crystal
display device in which each pixel is composed of both the
reflective part and the transmissive part.
<<Transmissive Liquid Crystal Display Devices>>
[0045] As shown in FIG. 1, a transmissive liquid crystal display
device includes a liquid crystal display panel 1 to which an OCB
mode is applied, a backlight 60 which illuminates the liquid
crystal display panel 1, a first optical compensation element 40
which is disposed between the liquid crystal display panel 1 and
the backlight 60, and a second optical compensation element 50
which is disposed on an observation surface side of the liquid
crystal display panel 1.
[0046] As shown in FIG. 2 and FIG. 3, the liquid crystal display
panel 1 is configured such that a liquid crystal layer 30 is held
between a pair of substrates, namely, an array substrate (first
substrate) 10 and a counter-substrate (second substrate) 20. The
liquid crystal display panel 1 includes an active area ACT which
displays an image. This active area ACT is composed of a plurality
of pixels PX which are arrayed in a matrix.
[0047] The array substrate 10 is formed by using a
light-transmissive insulating substrate 11 such as a glass
substrate. The array substrate 10 includes, on one major surface of
the insulating substrate 11, a plurality of scanning lines Sc which
are disposed along rows of pixels PX; a plurality of signal lines
Sg which are disposed along columns of pixels PX; switch elements
12 which are disposed near intersections between the scanning lines
Sc and signal lines Sg in association with the individual pixels
PX; pixel electrodes 13 which are disposed in association with the
individual pixels PX and are connected to the associated switch
elements 12; and an alignment film 16 which is disposed so as to
cover the entire major surface of the insulating substrate 13.
[0048] Each of the switch elements 12 is composed of, e.g. a
thin-film transistor (TFT). The switch element 12 has a gate
connected to the associated scanning line Sc. The switch element 12
has a source connected to the associated signal line Sg. The pixel
electrode 13 is disposed on an insulation film 14, and is
electrically connected to the drain of the switch element 12. The
pixel electrode 13 functions as a transmissive electrode and is
formed of a light-transmissive electrically conductive material
such as indium tin oxide (ITO). In short, each pixel PX corresponds
to a transmissive part.
[0049] The counter-substrate 20 is formed by using a
light-transmissive insulating substrate 21 such as a glass
substrate. The counter-substrate 20 includes, on one major surface
of the insulating substrate 21, a counter-electrode 22 which is
disposed commonly for the plural pixels PX, and an alignment film
23 which is disposed so as to cover the entire major surface of the
insulating substrate 21. The counter-electrode 22 is formed of a
light-transmissive electrically conductive material such as
ITO.
[0050] The array substrate 10 and counter-substrate 20 having the
above-described structures are disposed with a predetermined gap
provided therebetween by spacers (not shown), and are attached to
each other by a seal material. The liquid crystal layer 30 is
sealed in the gap between the array substrate 10 and
counter-substrate 20.
[0051] In this embodiment, the OCB mode is applied to the liquid
crystal display panel 1 not only in the example of the transmissive
liquid crystal display device, but also in examples of the
transreflective liquid crystal display device and reflective liquid
crystal display device, which will be described later. The liquid
crystal layer 30 is formed of a material including liquid crystal
molecules 31 which have positive dielectric constant anisotropy and
optically positive uniaxiality. In this liquid crystal layer 30, as
shown in FIG. 3, the liquid crystal molecules 31 are bend-aligned
between the array substrate 10 and counter-substrate 20 in a
predetermined display state in which a voltage is applied to the
liquid crystal layer 30.
<<First Example of Structure of the Optical Compensation
Element>>
[0052] In a first example of structure, the first optical
compensation element 40 and second optical compensation element 50
have functions of optically compensating retardation of the liquid
crystal layer 30 in a predetermined display state in which a
voltage is applied to the liquid crystal layer 30 in the
above-described liquid crystal display panel 1. Specifically, as
shown in FIG. 1, the first optical compensation element 40 is
disposed on the outside surface of the array substrate 10, and the
second optical compensation element 50 is disposed on the outside
surface of the counter-substrate 20. The first optical compensation
element 40 and second optical compensation element 50 have
substantially the same structure and are configured to be symmetric
with respect to the liquid crystal display panel 1.
[0053] Specifically, the first optical compensation element 40
includes a first polarizer plate PL1, a first retardation plate R1
and a second retardation plate R2. The second optical compensation
element 50 includes a second polarizer plate PL1, a third
retardation plate R3 and a fourth retardation plate R4.
[0054] Each of the first polarizer plate PL1 and second polarizer
plate PL2 is configured such that a polarization layer of, e.g.
polyvinyl alcohol (PVA) is held between a pair of support layers
of, e.g. triacetate cellulose (TAC). Each of the first polarizer
plate PL1 and second polarizer plate PL2 has a transmission axis
and an absorption axis which are substantially perpendicular to
each other.
[0055] The first retardation plate R1 is disposed between the first
polarizer plate PL1 and the liquid crystal display panel 1. The
third retardation plate R3 is disposed between the second polarizer
plate PL2 and the liquid crystal display panel 1. Each of the first
retardation plate R1 and the third retardation plate R3 has, in a
plane thereof, a fast axis and a slow axis which are substantially
perpendicular to each other. Each of the first retardation plate R1
and the third retardation plate R3 is a so-called "1/4 wavelength
plate", which imparts a phase difference of a 1/4 wavelength
between light components of a predetermined wavelength, which pass
through the fast axis and the slow axis.
[0056] Each of the combination of the first polarizer plate PL1 and
first retardation plate (1/4 wavelength plate) R1 and the
combination of the second polarizer plate PL2 and third retardation
plate (1/4 wavelength plate) R3 ideally functions as a circular
polarization element that converts linearly polarized light of a
predetermined wavelength, which has passed through the transmission
axis of the polarizer plate, to circularly polarized light.
[0057] The second retardation plate R2 is disposed between the
first polarizer plate PL1 and the first retardation plate R1. The
fourth retardation plate R4 is disposed between the second
polarizer plate PL2 and the third retardation plate R3. Each of the
second retardation plate R2 and the fourth retardation plate R4 is
a retardation plate having biaxial refractive index anisotropy, and
has, in a plane thereof, a fast axis and a slow axis which are
substantially perpendicular to each other. Each of the second
retardation plate R2 and the fourth retardation plate R4 is a
so-called "1/2 wavelength plate", which imparts a phase difference
of a 1/2 wavelength between light components of a predetermined
wavelength, which pass through the fast axis and the slow axis. The
details of the second retardation plate R2 and the fourth
retardation plate R4 will be described later.
[0058] Each of the first optical compensation element 40 and second
optical compensation element 50 includes a compensation layer CL.
Specifically, the first optical compensation element 40 includes a
first compensation layer CL1 which is disposed between the liquid
crystal display panel 1 and first retardation plate R1. The second
optical compensation element 50 includes a second compensation
layer CL2 which is disposed between the liquid crystal display
panel 1 and third retardation plate R3.
[0059] The structures of the first optical compensation element 40
and second optical compensation element 50, which include the first
compensation layer CL1 and second compensation layer CL2, will now
be described specifically. Examples of the structure of the first
optical compensation element 40 are described with reference to
FIG. 4A to FIG. 4D. The same examples of the structure are also
applicable to the second optical compensation element 50. The first
compensation layer CL1 and second compensation layer CL2 may not
necessarily have the same structure.
[0060] In an example of structure shown in FIG. 4A, the first
compensation layer CL1 includes a retardation plate RA which is
disposed between the liquid crystal display panel 1 and the first
retardation plate R1 and has a refractive index anisotropy which
substantially corresponds to an A-plate, and a retardation plate RC
which is disposed between the first retardation plate R1 and the
retardation plate RA and has a refractive index anisotropy which
substantially corresponds to a C-plate.
[0061] The retardation plate RA has such an in-plane phase
difference as to cancel a residual retardation in the plane thereof
in a specified voltage application state (e.g. a state in which
black is displayed by applying a high voltage). Specifically, the
retardation plate RA has a refractive index anisotropy of
nx>ny.apprxeq.nz or nz.apprxeq.nx>ny, where nx and ny are
refractive indices in mutually perpendicular directions in the
plane of the retardation plate RA, and nz is a refractive index in
a normal direction to the retardation plate RA. By the function of
the retardation plate RA, the in-plane phase difference of the
liquid crystal layer 30 can be canceled, and a display quality at a
time of observation in a frontal direction of the screen (a normal
direction to the screen) can be improved (in particular, contrast
can be improved).
[0062] The retardation plate RC has such a normal-directional phase
difference as to cancel a residual retardation in the normal
direction thereof in a specified voltage application state (e.g. a
state in which black is displayed by applying a high voltage).
Specifically, the retardation plate RC has a refractive index
anisotropy of nx.apprxeq.ny.noteq.nz. By the function of the
retardation plate RC, the normal-directional phase difference of
the liquid crystal layer 30 can be canceled, and a display quality
at a time of observation in an oblique direction of the screen can
be improved (in particular, a viewing angle can be improved).
[0063] In an example of structure shown in FIG. 4B, the first
compensation layer CL1 includes a retardation plate RB which has a
biaxial refractive index anisotropy and is disposed between the
liquid crystal display panel 1 and the first retardation plate R1.
This retardation plate RB has both a refractive index anisotropy
which substantially corresponds to an A-plate and a refractive
index anisotropy which substantially corresponds to a C-plate. To
be more specific, the retardation plate RB has a refractive index
anisotropy of nx>ny>nz. In this example of structure, the
same advantageous effects as in the example of structure shown in
FIG. 4A can be obtained. Moreover, since the number of retardation
plates is less than in the example of structure shown in FIG. 4A,
the reduction in thickness can be achieved.
[0064] In an example of structure shown in FIG. 4C, the first
compensation layer CL1 includes a retardation plate Rwv which is
disposed between the liquid crystal display panel 1 and the first
retardation plate R1. This retardation plate Rwv is an anisotropic
film which compensates retardation of the liquid crystal layer 30.
The retardation plate Rwv is an anisotropic film having such a
refractive index anisotropy that a substantial major axis is
inclined to the normal line, when consideration is given to the
total refractive index anisotropy of the retardation plate Rwv
itself. As such a retardation plate Rwv, a WV (Wide View) film
(manufactured by FUJIFILM Corporation) is applicable. The WV film
is a liquid crystal film in which discotic liquid crystal molecules
having an optically negative uniaxial refractive index anisotropy
are fixed in the state in which an optical axis is hybrid-aligned
along the normal direction in a liquid crystal state (i.e. in the
state in which the major axis is hybrid-aligned).
[0065] In particular, in the case where both the first compensation
layer CL1 and second compensation layer CL2 are composed of
retardation plates Rwv in relation to the OCB mode liquid crystal
display panel 1, the discotic liquid crystal molecules which
constitute the retardation plates Rwv, as shown in FIG. 5,
optically compensate the bend-aligned liquid crystal molecules 31,
respectively. Therefore, the combination of the OCB mode liquid
crystal display panel 1 and the retardation plates Rwv is effective
in terms of optical compensation.
[0066] In an example of structure shown in FIG. 4D, the first
compensation layer CL1 includes a retardation plate RA which is
disposed between the liquid crystal display panel 1 and the first
retardation plate R1 and has a refractive index anisotropy which
substantially corresponds to an A-plate. In this case, a
retardation plate having biaxial refractive index anisotropy is
applied to the first retardation plate R1. Specifically, the
refractive index anisotropy of the first retardation plate R1 is
set to have a refractive index anisotropy substantially
corresponding to a C-plate, in addition to its own function as a
1/4 wavelength plate. To be more specific, the first retardation
plate R1 has a refractive index anisotropy of nx>ny>nz. In
this example of structure, the same advantageous effects as in the
example of structure shown in FIG. 4A can be obtained. Moreover,
since the number of retardation plates is less than in the example
of structure shown in FIG. 4A, the reduction in thickness can be
achieved.
[0067] In the above-described liquid crystal display device, the
respective structural elements are arranged with axial angles
described below, in relation to a reference direction that is the
rubbing direction of the alignment film 16 on the array substrate
10 side and the alignment film 23 on the counter-substrate 20 side.
The axial angle, in this context, refers to a counterclockwise
angle, relative to the reference azimuth direction (X axis), of the
absorption axis of the polarizer plate and the slow axis (or
optical axis) of the retardation plate, as defined in FIG. 6.
Specifically, when the liquid crystal display device is observed
from the counter-substrate 20 side, an X axis and a Y axis, which
are perpendicular to each other, are defined, for the purpose of
convenience, in a plane that is parallel to the major surface of
the array substrate 10 (or counter-substrate 20), and a normal
direction to this plane is defined as a Z axis. The term "in-plane"
means "within a plane" that is defined by the X axis and Y
axis.
[0068] A description is given of the case in which the first
compensation layer CL1 and second compensation layer CL2 in the
first optical compensation element 40 and second optical
compensation element 50 according to the first example of structure
are composed of the retardation plate RA and retardation plate RC
as shown in FIG. 4A.
[0069] Specifically, in the liquid crystal display panel 1, the
rubbing direction is set at 0.degree. azimuth. In the first optical
compensation element 40, the absorption axis of the first polarizer
plate PL1 is set at 45.degree. azimuth. The slow axis of the first
retardation plate R1 is set at 0.degree. azimuth (i.e. crossing the
absorption axis of the first polarizer plate PL1 at about
45.degree.). The optical axis of the second retardation plate R2 in
the X-Y plane is set at 45.degree. azimuth (i.e. substantially
parallel to the absorption axis of the first polarizer plate PL1).
The slow axis of the retardation plate RA is set at 90.degree.
azimuth.
[0070] In the second optical compensation element 50, the
absorption axis of the second polarizer plate PL2 is set at
135.degree. azimuth. The slow axis of the third retardation plate
R3 is set at 90.degree. azimuth (i.e. crossing the absorption axis
of the second polarizer plate PL2 at about 45.degree.). The optical
axis of the fourth retardation plate R4 in the X-Y plane is set at
45.degree. azimuth (i.e. substantially perpendicular to the
absorption axis of the polarizer plate PL). The slow axis of the
retardation plate RA is set at 90.degree. azimuth.
[0071] The above-described axial angles of the respective
structural elements are summarized in FIG. 7.
[0072] In the liquid crystal display device that adopts the OCB
mode, the liquid crystal molecules 31 are bend-aligned, as shown in
FIG. 6, in a predetermined voltage application state (e.g. black
display state) in an X-Z plane. When the screen is observed from a
direction at 90.degree. azimuth in the X-Y plane, the liquid
crystal molecules 31 are aligned counterclockwise from the lower
side (array substrate side) to the upper side (counter-substrate
side). On the other hand, when the screen is observed from a
direction at 270.degree. azimuth in the X-Y plane, the liquid
crystal molecules 31 are aligned clockwise from the lower side to
the upper side.
[0073] Thus, the optical rotatory power of the light that passes
through the liquid crystal layer 30 is affected by the alignment of
liquid crystal molecules 31 in opposite rotational directions at
90.degree. azimuth and 270.degree. azimuth. Owing to the influence
of asymmetry of optical rotatory power, the polarization states of
light, which passes through the liquid crystal layer 30 toward the
respective azimuth directions, are different. In other words, a
difference occurs in the display quality of the screen due to the
difference in polarization state of light passing through the
liquid crystal layer 30, between the case where the viewing angle
is increased from the normal direction (i.e. Z-axis direction) of
the screen toward the 90.degree. azimuth in the X-Y plane and the
case where the viewing angle is increased from the normal direction
of the screen toward the 270.degree. azimuth in the X-Y plane.
Hence, the viewing angle at which a high contrast is obtained is
limited.
[0074] In the present embodiment, the difference of the
polarization state, which varies depending on the influence of
optical rotatory power that differs between azimuth directions of
light passing through the liquid crystal layer 30, is optically
compensated.
[0075] Moreover, in the predetermined voltage application state
(black display state), the polarization state (ideally a linear
polarization state) of light, which emerges from the third
retardation plate R3 of the second optical compensation element 50,
shifts from the azimuth direction of the absorption axis of the
second polarizer plate PL2, due to the influence of retardation of
not only the liquid crystal layer 30 but also other structural
elements. Consequently, the transmittance of the screen at the time
of black display cannot sufficiently be lowered, and the contrast
may deteriorate. To solve this problem, use is made of the optical
compensation elements having the function of optically compensating
the shift of the polarization state of light, which passes through
the third retardation plate R3, from the azimuth direction of the
absorption axis of the second polarizer plate PL2. Thereby, the
contrast can be improved, and the viewing angle at which a high
contrast is obtained, can be increased.
[0076] A more detailed description is given below.
[0077] In the case where the first optical compensation element 40
and second optical compensation element 50 according to the first
example of structure are applied to the transmissive liquid crystal
display, the first optical compensation element 40 includes, as
shown in FIG. 1, the second retardation plate R2 with the biaxial
refractive index anisotropy between the first retardation plate R1
and the first polarizer plate PL1. In addition, the second optical
compensation element 50 includes the fourth retardation plate R4
with the biaxial refractive index anisotropy between the third
retardation plate R3 and the second polarizer plate PL2. The
inventor has found that the above-described optical compensation
can be achieved by optimizing an Nz coefficient, which is given by
Nz=(nx-nz)/(nx-ny), in the second retardation plate R2 and fourth
retardation plate R4 each having the biaxial refractive index
anisotropy.
[0078] In particular, in specifying the optimal range of the Nz
coefficient, it was verified that the influence of different
optical rotatory power and the influence of the shift of the
polarization state from the azimuth direction of the absorption
axis can be improved by varying the Nz coefficient.
[0079] In the structure shown in FIG. 1, the second retardation
plate R2 of the first optical compensation element 40 mainly
compensates the difference of the polarization state which varies
mainly due to the influence of optical rotatory power, depending on
the azimuth direction of light passing through the liquid crystal
layer 30. The fourth retardation plate R4 of the second optical
compensation element 50 mainly compensates the shift of the
polarization state of the light, which has passed through the third
retardation plate R3, from the azimuth direction of the absorption
axis of the second polarizer plate PL2.
[0080] In the above-described structure, the Nz coefficient that is
necessary for each compensation differs, and the Nz coefficient of
the second retardation plate R2 of the first optical compensation
element 40 differs from the Nz coefficient of the fourth
retardation plate R4 of the second optical compensation element 50.
Specifically, in order to compensate the difference of the
polarization state which varies depending on the azimuth direction
of light passing through the liquid crystal layer 30, it is
desirable to use the second retardation plate R2 having the Nz
coefficient that is set in the range of between 0.7 and 0.9. If the
Nz coefficient is less than 0.7 or greater than 0.9, it is
difficult to secure a viewing angle contrast at, e.g. 90.degree.
azimuth.
[0081] On the other hand, in order to compensate the shift of the
polarization state of the light, which has passed through the third
retardation plate R3, from the azimuth direction of the absorption
axis of the second polarizer plate PL2, it is desirable to use the
fourth retardation plate R4 having the Nz coefficient that is set
in the range of between 0.15 and 0.3. If the Nz coefficient is less
than 0.15 or greater than 0.3, the compensation of the polarization
state would become deficient and it is difficult to secure the
viewing angle contrast.
[0082] Thereby, a sufficiently wide viewing angle can be obtained,
and a good display quality can be obtained.
[0083] Next, verification was conducted on the advantageous effects
that are obtained in the case where the first optical compensation
element 40 and second optical compensation element 50 according to
the first example of structure are applied to the transmissive
liquid crystal display. The structure of the transmissive liquid
crystal display device according to the present embodiment is as
shown in FIG. 1. The Nz coefficient of the second retardation plate
R2 of the first optical compensation element 40 was set at 0.9
(nx=1.580951, ny=1.578995, nz=1.579191), and the Nz coefficient of
the fourth retardation plate R4 of the second optical compensation
element 50 was set at 0.2 (nx=1.580951, ny=1.578995, nz=1.580560).
In a comparative example, a first optical compensation element 40
including no second retardation plate and a second optical
compensation element 50 including no fourth retardation plate were
applied to a liquid crystal display device, with the other
structural aspects being the same as in the present embodiment.
[0084] FIG. 8A shows a result of simulation of the viewing angle
dependency of a contrast ratio in the liquid crystal display device
according to the comparative example. In FIG. 8A, the center
corresponds to the normal direction (Z axis) of the liquid crystal
display panel. Concentric circles defined about the normal
direction indicate tilt angles (viewing angles) to the normal
direction, and correspond to 20.degree., 40.degree., 60.degree. and
80.degree., respectively. The characteristic diagram of FIG. 8A was
obtained by connecting regions corresponding to contrast ratios
(CR) of 100:1 to 10:1 in all azimuth directions.
[0085] As shown in FIG. 8A, it is understood that in the liquid
crystal display device of the comparative example, the contrast
ratio becomes 10:1 or less in the range of viewing angles of
60.degree. or more, in particular, at 0.degree. azimuth and
180.degree. azimuth. For the purpose of convenience, assume that
the azimuth direction parallel to the rubbing direction
(0.degree.-180.degree. azimuth) is the vertical direction of the
screen, the 90.degree. azimuth direction is the right direction of
the screen, and the 270.degree. azimuth direction is the left
direction of the screen. In this case, it was confirmed that the
contrast ratio considerably lowers as the viewing angle increases
from the normal direction toward the upward and downward directions
of the screen.
[0086] FIG. 8B shows a result of simulation of the viewing angle
dependency of a contrast ratio in the liquid crystal display device
according to the present embodiment. As is clear from FIG. 8B, it
was confirmed that the contrast ratio of 10:1 or more was obtained
in the range of viewing angles of 80.degree. or more in all azimuth
directions, and sufficient viewing angles were obtained.
[0087] The same advantageous effects were confirmed as regards
another structure of the transmissive liquid crystal display device
according to the present embodiment, wherein the Nz coefficient of
the second retardation plate R2 of the first optical compensation
element 40 was set at 0.8 (nx=1.580951, ny=1.578995, nz=1.579386),
and the Nz coefficient of the fourth retardation plate R4 of the
second optical compensation element 50 was set at 0.15
(nx=1.580951, ny=1.578995, nz=1.580658).
[0088] Moreover, the same advantageous effects were confirmed as
regards still another structure of the transmissive liquid crystal
display device according to the present embodiment, wherein the Nz
coefficient of the second retardation plate R2 of the first optical
compensation element 40 was set at 0.7 (nx=1.580951, ny=1.578995,
nz=1.579582), and the Nz coefficient of the fourth retardation
plate R4 of the second optical compensation element 50 was set at
0.3 (nx=1.580951, ny=1.578995, nz=1.580364).
[0089] However, in the case where the Nz coefficient of the second
retardation plate R2 of the first optical compensation element 40
was set at 0.5 (nx=1.580951, ny=1.578995, nz=1.579973), the
influence of undesired optical rotatory power could not be
compensated and a high viewing angle contrast failed to be secured.
Similarly, in the case where the Nz coefficient of the second
retardation plate R2 of the first optical compensation element 40
was set at 1.1 (nx=1.580951, ny=1.578995, nz=1.58799), the
influence of undesired optical rotatory power could not be
compensated and a high viewing angle contrast failed to be
secured.
[0090] As shown in FIG. 7, in the first example of structure, the
absorption axis of the first polarizer plate PL1 and the optical
axis of the second retardation plate R2 substantially agree in the
first optical compensation element 40. In addition, in the second
optical compensation element 50, the absorption axis of the second
polarizer plate PL2 is substantially perpendicular to the optical
axis of the fourth retardation plate R4. The invention, however, is
not limited to this example. Specifically, since the first optical
compensation element 40 and second optical compensation element 50
are symmetric in the liquid crystal display panel 1, the first
optical compensation element 40 and second optical compensation
element 50 may substantially be transposed in an alternative
structure.
[0091] To be more specific, in Modification 1, the optical axis of
the second retardation plate R2 in its X-Y plane is set at
135.degree. in the first optical compensation element 40. In short,
the absorption axis of the first polarizer plate PL1 is
substantially perpendicular to the optical axis of the second
retardation plate R2. In addition, in the second optical
compensation element 50, the optical axis of the fourth retardation
plate R4 in its X-Y plane is set at 135.degree.. In short, the
absorption axis of the second polarizer plate PL2 substantially
agrees with the optical axis of the fourth retardation plate R4. In
this Modification 1, the same advantageous effects as in the
above-described first example of structure can be obtained by
making use of the fourth retardation plate R4 in which the Nz
coefficient is set in the range of between 0.7 and 0.9, and the
second retardation plate R2 in which the Nz coefficient is set in
the range of between 0.15 and 0.3.
[0092] For example, it was confirmed that a sufficient viewing
angle was obtained in the case where the Nz coefficient in the
second retardation plate R2 of the first optical compensation
element 40 was set at 0.2 and the Nz coefficient in the fourth
retardation plate R4 of the second optical compensation element 50
was set at 0.8.
[0093] In the first example of structure, in the case where the
absorption axis of the first polarizer plate PL1 substantially
agrees with the optical axis of the second retardation plate R2 in
the first optical compensation element 40 and the absorption axis
of the second polarizer plate PL2 substantially agrees with the
optical axis of the fourth retardation plate R4 in the second
optical compensation element 50, it should suffice to use the
second retardation plate R2 and fourth retardation plate R4, in
each of which the Nz coefficient is set in the range of between 0.7
and 0.9. Similarly, in the case where the absorption axis of the
first polarizer plate PL1 is substantially perpendicular to the
optical axis of the second retardation plate R2 in the first
optical compensation element 40 and the absorption axis of the
second polarizer plate PL2 is substantially perpendicular to the
optical axis of the fourth retardation plate R4 in the second
optical compensation element 50, it should suffice to use the
second retardation plate R2 and fourth retardation plate R4, in
each of which the NZ coefficient is set in the range of between
0.15 and 0.3. In each of these cases, the same advantageous effects
as in the above-described first example of structure can be
obtained, and the common structural elements are usable and the
reduction in cost is realized.
<<Second Example of the Structure of the Optical Compensation
Element>>
[0094] In a second example of structure, like the above-described
first example of structure, the first optical compensation element
40 and second optical compensation element 50 have functions of
optically compensating retardation of the liquid crystal layer 30
in a predetermined display state in which a voltage is applied to
the liquid crystal layer 30 of the liquid crystal display panel
1.
[0095] Specifically, as shown in FIG. 9A, the first optical
compensation element 40, which is disposed on the outer surface of
the array substrate 10, is configured to include a first polarizer
plate PL1, a first retardation plate R1 and a second retardation
plate R2. The second optical compensation element 50, which is
disposed on the outer surface of the counter-substrate 20, is
configured to include a second polarizer plate PL2 and a third
retardation plate R3. The first optical compensation element 40 and
the second optical compensation element 50 are configured to be
asymmetric with respect to the liquid crystal display panel 1.
According to the liquid crystal display device to which the optical
compensation elements according to the second example of structure
are applied, the number of retardation plates is less than in the
liquid crystal display device to which the optical compensation
elements according to the first example of structure are applied.
Therefore, the reduction in cost and thickness can be achieved.
[0096] The first polarizer plate PL1, second polarizer plate PL2,
the first retardation plate R1 and the third retardation plate R3
are the same as those in the first example of structure.
Specifically, each of the combination of the first polarizer plate
PL1 and the first retardation plate (1/4 wavelength plate) R1 and
the combination of the second polarizer plate PL2 and the third
retardation plate (1/4 wavelength plate) R3 functions ideally as a
circular polarization element that converts linearly polarized
light of a predetermined wavelength, which has passed through the
transmission axis of the polarizer plate, to circularly polarized
light.
[0097] The second retardation plate R2 is disposed between the
first polarizer plate PL1 and the first retardation plate R1. The
second retardation plate R2 is a retardation plate with a biaxial
refractive index anisotropy.
[0098] The first optical compensation element 40 includes a first
compensation layer CL1 which is disposed between the liquid crystal
display panel 1 and the first retardation plate R1. The second
optical compensation element 50 includes a second compensation
layer CL2 which is disposed between the liquid crystal display
panel 1 and the third retardation plate R3. Like the first example
of structure, the structures as shown in FIG. 4A to FIG. 4D are
applicable to the first optical compensation element 40 and the
second optical compensation element 50, which include the first
compensation layer CL1 and the second compensation layer CL2,
respectively.
[0099] A description is given of the case in which the first
compensation layer CL1 and second compensation layer CL2 in the
first optical compensation element 40 and second optical
compensation element 50 according to the second example of
structure are composed of the retardation plate RA and retardation
plate RC as shown in FIG. 4A.
[0100] Specifically, in the liquid crystal display panel 1, the
rubbing direction is set at 0.degree. azimuth. In the first optical
compensation element 40, the absorption axis of the first polarizer
plate PL1 is set at 45.degree. azimuth. The slow axis of the first
retardation plate R1 is set at 0.degree. azimuth (i.e. crossing the
absorption axis of the first polarizer plate PL1 at about
45.degree.). The optical axis of the second retardation plate R2 in
the X-Y plane is set at 45.degree. azimuth (i.e. substantially
parallel to the absorption axis of the first polarizer plate PL1).
The slow axis of the retardation plate RA is set at 90.degree.
azimuth.
[0101] In the second optical compensation element 50, the
absorption axis of the second polarizer plate PL2 is set at
135.degree. azimuth. The slow axis of the third retardation plate
R3 is set at 90.degree. azimuth (i.e. crossing the absorption axis
of the second polarizer plate PL2 at about 45.degree.) The slow
axis of the retardation plate RA is set at 90.degree. azimuth.
[0102] The above-described axial angles of the respective
structural elements are summarized in FIG. 10A.
[0103] In the second example of structure, like the first example
of structure, use is made of the optical compensation elements
having the function of optically compensating the difference of the
polarization state, which varies depending on the influence of
optical rotatory power that differs between azimuth directions of
light passing through the liquid crystal layer 30, and also
optically compensating the shift of the polarization state of
light, which passes through the third retardation plate R3, from
the azimuth direction of the absorption axis of the second
polarizer plate PL2. Thereby, the contrast can be improved, and the
viewing angle at which a high contrast is obtained can be
increased.
[0104] A more detailed description is given below.
[0105] In the case where the first optical compensation element 40
and second optical compensation element 50 according to the second
example of structure are applied to the transmissive liquid crystal
display, the first optical compensation element 40 includes, as
shown in FIG. 9A, the second retardation plate R2 with the biaxial
refractive index anisotropy between the first retardation plate R1
and the first polarizer plate PL1. In the structure shown in FIG.
9A, the second retardation plate R2 of the first optical
compensation element 40 has the function of compensating the
difference of the polarization state which varies due to the
influence of optical rotatory power, depending on the azimuth
direction of light passing through the liquid crystal layer 30, and
compensating the shift of the polarization state of the light,
which has passed through the third retardation plate R3, from the
azimuth direction of the absorption axis of the second polarizer
plate PL2.
[0106] In the above-described structure, it is desirable to use the
second retardation plate R2 having the Nz coefficient that is set
in the range of between 0.4 and 0.6. If the Nz coefficient is less
than 0.4 or greater than 0.6, the optical compensation becomes
deficient and it is difficult to secure the viewing angle
contrast.
[0107] Thereby, a sufficiently wide viewing angle can be obtained,
and a good display quality can be obtained.
[0108] Next, verification was conducted on the advantageous effects
that are obtained in the case where the first optical compensation
element 40 and second optical compensation element 50 according to
the second example of structure are applied to the transmissive
liquid crystal display. The structure of the transmissive liquid
crystal display device according to the present embodiment is as
shown in FIG. 9A. The Nz coefficient of the second retardation
plate R2 of the first optical compensation element 40 was set at
0.5 (nx=1.580951, ny=1.578995, nz=1.579973). The viewing angle
dependency of the contrast ratio in the liquid crystal display
device according to the present embodiment was simulated, and it
was confirmed, as shown in FIG. 10B, that the contrast ratio of
10:1 or more was obtained in the range of viewing angles of
60.degree. or more in all azimuth directions of the screen, and the
contrast ratio of 10:1 or more was obtained in the range of viewing
angles of 80.degree. or more in azimuth directions except
90.degree. azimuth, and thus the sufficient viewing angles were
obtained.
[0109] As shown in FIG. 10A, in the second example of structure,
the second retardation plate R2 is disposed between the first
polarizer plate PL1 and the first retardation plate R1 in the first
optical compensation element 40, and the absorption axis of the
first polarizer plate PL1 and the optical axis of the second
retardation plate R2 substantially agree with each other. The
invention, however, is not limited to this example. Specifically,
in the liquid crystal display panel 1, the first optical
compensation element 40 and second optical compensation element 50
may substantially be transposed.
[0110] To be more specific, in Modification 2, as shown in FIG. 9B
and FIG. 10A, the second retardation plate R2 is not disposed in
the first optical compensation element 40, and the second
retardation plate R2 is disposed between the second polarizer plate
PL2 and the third retardation plate R3 in the second optical
compensation element 50. The optical axis of the second retardation
plate R2 in its X-Y plane is set at 135.degree.. In short, the
absorption axis of the second polarizer plate PL2 substantially
agrees with the optical axis of the second retardation plate R2. In
this Modification 2, the same advantageous effects as in the
above-described second example of structure can be obtained by
making use of the second retardation plate R2 in which the Nz
coefficient is set in the range of between 0.4 and 0.6.
[0111] For example, it was confirmed that a sufficient viewing
angle was obtained in the case where the Nz coefficient in the
second retardation plate R2 of the second optical compensation
element 50 was set at 0.5.
[0112] In the second example of structure, in the case where the
second retardation plate R2 is disposed in the first optical
compensation element 40 and the absorption axis of the first
polarizer plate PL1 is substantially perpendicular to the optical
axis of the second retardation plate R2 (e.g. the absorption axis
of the first polarizer plate PL1 is set at 45.degree. azimuth and
the optical axis of the second retardation plate R2 in the X-Y
plane is set at 135.degree. azimuth) and in the case where the
second retardation plate R2 is disposed in the second optical
compensation element 50 and the absorption axis of the second
polarizer plate PL2 is substantially perpendicular to the optical
axis of the second retardation plate R2 (e.g. the absorption axis
of the second polarizer plate PL2 is set at 135.degree. azimuth and
the optical axis of the second retardation plate R2 in the X-Y
plane is set at 45.degree. azimuth), it should suffice to use the
second retardation plate R2 in which the Nz coefficient is set in
the range of between 0.4 and 0.6. In each of these cases, the same
advantageous effects as in the above-described second example of
structure can be obtained.
[0113] As regards the above-described first example of structure
and the second example of structure, in the case where the example
of structure as shown in FIG. 4B is applied to at least one of the
first compensation layer CL1 and second compensation layer CL2, the
same advantageous effects can be obtained by setting the optical
axis of the retardation plate RB in its X-Y plane at 90.degree.
azimuth.
[0114] For example, as regards the first example of structure, in
the case where the first compensation layer CL1 and second
compensation layer CL2 were composed of the retardation plates RB,
the Nz coefficient of each retardation plate RB was set at 4.8, the
retardation Re of each retardation plate RB was set at 90 nm, the
Nz coefficient of the second retardation plate R2 of the first
optical compensation element 40 was set at 0.2 and the Nz
coefficient of the fourth retardation plate R4 of the second
optical compensation element 50 was set at 0.8, it was confirmed,
as shown in FIG. 10C, that sufficient viewing angles were
obtained.
[0115] For example, as regards the second example of structure, in
the case where the first compensation layer CL1 and second
compensation layer CL2 were composed of the retardation plates RB
and the Nz coefficient of the second retardation plate R2 of the
second optical compensation element 50 was set at 0.5, it was
confirmed that sufficient viewing angles were obtained.
[0116] As regards the above-described first example of structure
and the second example of structure, in the case where the example
of structure as shown in FIG. 4C is applied to at least one of the
first compensation layer CL1 and second compensation layer CL2, the
same advantageous effects can be obtained by setting the optical
axis of the retardation plate Rwv in its X-Y plane at 0.degree.
azimuth.
[0117] Furthermore, as regards the above-described first example of
structure and the second example of structure, in the case where
the example of structure as shown in FIG. 4D is applied to at least
one of the first compensation element 40 and second compensation
element 50, the same advantageous effects can be obtained by
setting the optical axis of the first retardation plate R1 in its
X-Y plane at 0.degree. azimuth and by setting the optical axis of
the retardation plate RA in its X-Y plane at 90.degree.
azimuth.
<<Transreflective Liquid Crystal Display Device>>
[0118] Next, a transreflective liquid crystal display device
according to the embodiment of the invention is described. The
structure of the transreflective liquid crystal display device is
as shown in FIG. 11. The basic structure of this transreflective
liquid crystal display device is the same as that of the
transmissive liquid crystal display device shown in FIG. 3.
However, the structure of transreflective liquid crystal display
device differs from that of the transmissive liquid crystal display
device in that each of a plurality of display pixels PX, which are
arrayed in a matrix, includes a reflective part PR that displays an
image by selectively reflecting ambient light, and a transmissive
part PT that displays an image by selectively transmitting light
from a backlight 60.
[0119] In the array substrate 10, the insulation layer 14 forms a
gap difference of the liquid crystal layer 3, thereby to impart a
retardation difference between the reflective part PR and the
transmissive part PT. Each pixel electrode 13 includes a reflective
electrode 13R which is provided in association with the reflective
part PR, and a transmissive electrode 13T which is provided in
association with the transmissive part PT. These electrodes 13R and
13T are electrically connected to each other, and are controlled by
one switching element W. The reflective electrode 13R is formed of
a light-reflective electrically conductive material such as
aluminum. The transmissive electrode 13T is formed of a
light-transmissive electrically conductive material such as indium
tin oxide (ITO). The reflective electrode 13R and transmissive
electrode 13T are electrically connected to the switching element
13.
[0120] In the example shown in FIG. 11, in the transmissive part PT
and reflective part PR, liquid crystal molecules 31 are
bend-aligned between the array substrate 10 and counter-substrate
20 in a predetermined display state in which a voltage is applied
to the liquid crystal layer 30.
[0121] To this transreflective liquid crystal display device, too,
applicable are the first optical compensation element 40 and second
optical compensation element 50 according to any one of the first
example of structure shown in FIG. 1, its Modification 1, the
second example of structure shown in FIG. 9A and its Modification 2
shown in FIG. 9B. In addition, any one of the structures shown in
FIG. 4A to FIG. 40 is applicable to the first optical compensation
element 40 and second optical compensation element 50.
[0122] Next, verification was conducted on the advantageous effects
that are obtained in the case where the first optical compensation
element 40 and second optical compensation element 50 according to
the first example of structure are applied to the above-described
transreflective liquid crystal display. The structure of the
transreflective liquid crystal display device according to the
present embodiment is as shown in FIG. 1. The Nz coefficient of the
second retardation plate R2 of the first optical compensation
element 40, which is disposed on the outer surface of the array
substrate 10, was set at 0.9 (nx=1.580951, ny=1.578995,
nz=1.579191), and the Nz coefficient of the fourth retardation
plate R4 of the second optical compensation element 50, which is
disposed on the outer surface of the counter-substrate 20, was set
at 0.2 (nx=1.580951, ny=1.578995, nz=1.580560). In a comparative
example, a first optical compensation element 40 including no
second retardation plate and a second optical compensation element
50 including no fourth retardation plate were applied to a
transreflective liquid crystal display device, with the other
structural aspects being the same as in the present embodiment.
[0123] As is shown in FIG. 12A, as regards the transreflective
liquid crystal display device according to the comparative example,
it is understood that in the transmissive part a contrast ratio of
10:1 or less is obtained in the range of viewing angles of
60.degree. or more, in particular, at 0.degree. azimuth and
180.degree. azimuth. On the other hand, as is shown in FIG. 12B, as
regards the transreflective liquid crystal display device according
to the present embodiment, it is understood that in the
transmissive part a contrast ratio of 10:1 or more is obtained in
the range of viewing angles of 80.degree. or more at all azimuth
directions of the screen, and sufficient viewing angles are
obtained, compared to the comparative example.
[0124] As is shown in FIG. 13A, as regards the transreflective
liquid crystal display device according to the comparative example,
it is understood that in the reflective part a contrast ratio of
10:1 or less is obtained in the range of viewing angles of
40.degree. or more, in particular, at 0.degree. azimuth and
180.degree. azimuth. On the other hand, as is shown in FIG. 13B, as
regards the transreflective liquid crystal display device according
to the present embodiment, it is understood that in the reflective
part a contrast ratio of 10:1 or more is obtained in the range of
viewing angles of 80.degree. or more at 90.degree.-720.degree.
azimuth and a contrast ratio of 10:1 or more is obtained in the
range of viewing angles of 50.degree. or mote at
0.degree.-180.degree. azimuth. It was confirmed that the viewing
angle was increased, compared to the comparative example.
[0125] Next, verification was conducted on the advantageous effects
that are obtained in the case where the first optical compensation
element 40 and second optical compensation element 50 according to
the second example of structure are applied to the transreflective
liquid crystal display. The structure of the transreflective liquid
crystal display device according to the present embodiment is as
shown in FIG. 9A. The Nz coefficient of the second retardation
plate R2 of the first optical compensation element 40, which is
disposed on the outer surface of the array substrate 10, was set at
0.5 (nx=1.580951, ny=1.578995, nz=1.579973). The viewing angle
dependency of the contrast ratio in the transreflective liquid
crystal display device according to the present embodiment was
simulated, and it was confirmed that a contrast ratio of 10:1 or
more was obtained in both the transmissive part and reflective part
in the range of viewing angles of 60.degree. or more in all azimuth
directions of the screen, and a contrast ratio of 10:1 or more was
obtained in the transmissive part in the range of viewing angles of
80.degree. or more at azimuth directions except 90.degree. azimuth,
and thus the sufficient viewing angles were obtained.
<<Reflective Liquid Crystal Display Device>>
[0126] Next, a reflective liquid crystal display device according
to the embodiment of the invention is described. The structure of
the reflective liquid crystal display device is as shown in FIG.
14. The basic structure of this reflective liquid crystal display
device is the same as that of the transmissive liquid crystal
display device shown in FIG. 3. However, the structure of
reflective liquid crystal display device differs from that of the
transmissive liquid crystal display device in that the array
substrate 10 includes reflective electrodes as pixel electrodes 13.
Each pixel electrode 13 is formed of a light-reflective
electrically conductive material such as aluminum. In short, each
pixel PX corresponds to a reflective part.
[0127] In this reflective liquid crystal display device, an optical
compensation element 70 is disposed on only the outer surface of
the counter-substrate 20. The optical compensation element 70 is
configured to include a polarizer plate PL, a first retardation
plate (1/4 wavelength plate) R1 which is disposed between the
polarizer plate PL and the liquid crystal display panel 1, a second
biaxial retardation plate (1/4 wavelength plate) R2 which is
disposed between the polarizer plate PL and the liquid crystal
display panel 1, and a compensation layer CL which is disposed
between the first retardation plate R1 and the liquid crystal
display panel 1. Any one of the examples of structure shown in FIG.
4A to FIG. 4D is applicable to the optical compensation element
70.
[0128] A description is given of the case in which the compensation
layer CL in the optical compensation element 70 is composed of the
retardation plate RA and retardation plate RC as shown in FIG.
4A.
[0129] Specifically, in the liquid crystal display panel 1, the
rubbing direction is set at 0.degree. azimuth. In the optical
compensation element 70, the absorption axis of the polarizer plate
PL is set at 135.degree. azimuth. The optical axis of the second
retardation plate R2 in the X-Y plane is set at 45.degree. azimuth
(i.e. substantially perpendicular to the absorption axis of the
polarizer plate PL). The optical axis of the first retardation
plate R1 in the X-Y plane is set at 90.degree. azimuth, and the
optical axis of the retardation plate RA in the X-Y plane is set at
90.degree. azimuth.
[0130] The above-described axial angles of the respective
structural elements are summarized in FIG. 15.
[0131] In this reflective liquid crystal display device, too, use
is made of the optical compensation element having the function of
optically compensating the difference of the polarization state,
which varies depending on the influence of optical rotatory power
that differs between azimuth directions of light passing through
the liquid crystal layer 30, and also optically compensating the
shift of the polarization state of light, which passes through the
first retardation plate R1, from the azimuth direction of the
absorption axis of the polarizer plate PL. Thereby, the contrast
can be improved, and the viewing angle at which a high contrast is
obtained can be increased.
[0132] Specifically, in the optical compensation element 70 of the
reflective liquid crystal display device, the second retardation
plate R2 with biaxial refractive index anisotropy, which is
disposed between the first retardation plate R1 and the polarizer
plate PL, compensates the difference of the polarization state
which varies due to the influence of optical rotatory power,
depending on the azimuth direction of the light that is incident
from the observation side and passes through the liquid crystal
layer 30. In addition, the second retardation plate R2 compensates
the shift of the polarization state of the light, which is
reflected by each pixel electrode 13 and passes through the first
retardation plate R1, from the azimuth direction of the absorption
axis of the polarizer plate PL.
[0133] In the structure in which the optical axis of the second
retardation plate R2 in the X-Y plane is substantially
perpendicular to the absorption axis of the polarizer plate PL, it
is desirable to use the second retardation plate R2 in which the Nz
coefficient is set in the range of between 0.15 and 0.3. If the Nz
coefficient is less than 0.15 or greater than 0.3, the optical
compensation becomes deficient and it is difficult to secure the
viewing angle contrast.
[0134] Thereby, a sufficiently wide viewing angle can be obtained,
and a good display quality can be obtained.
[0135] Next, verification was conducted on the advantageous effects
that are obtained in the case where the optical compensation
element 70 is applied to the above-described reflective liquid
crystal display. The structure of the reflective liquid crystal
display device according to the present embodiment is as shown in
FIG. 14. The Nz coefficient of the second retardation plate R2 of
the optical compensation element 70 was set at 0.2 (nx=1.580951,
ny=1.578995, nz=1.580560). In a comparative example, an optical
compensation element 70 including no second retardation plate was
applied to a reflective liquid crystal display device, with the
other structural aspects being the same as in the present
embodiment.
[0136] As is shown in FIG. 16A, as regards the reflective liquid
crystal display device according to the comparative example, it is
understood that a contrast ratio of 10:1 or less is obtained in the
range of viewing angles of 50.degree. or more, in particular, at
0.degree. azimuth and 180.degree. azimuth. On the other hand, as is
shown in FIG. 16B, as regards the reflective liquid crystal display
device according to the present embodiment, it is understood that a
contrast ratio of 10:1 or more is obtained in the range of viewing
angles of 80.degree. or more at all azimuth directions of the
screen, and sufficient viewing angles are obtained, compared to the
comparative example.
[0137] The same advantageous effects were confirmed as regards
another structure of the reflective liquid crystal display device
according to the present embodiment, wherein the Nz coefficient of
the second retardation plate R2 of the optical compensation element
70 was set at 0.15 (nx=1.580951, ny=1.578995, nz=1.580658).
[0138] Further, the same advantageous effects were confirmed as
regards still another structure of the reflective liquid crystal
display device according to the present embodiment, wherein the Nz
coefficient of the second retardation plate R2 of the optical
compensation element 70 was set at 0.3 (nx=1.580951, ny=1.578995,
nz=1.580364).
[0139] In the case where the Nz coefficient in the comparative
example was set at 0.5 (nx=1.580951, ny=1.578995, nz=1.579973), a
high viewing angle contrast could not be secured, as in the
above-described comparative example.
[0140] As shown in FIG. 15, in the example of structure, the
absorption axis of the polarizer plate PL and the optical axis of
the second retardation plate R2 substantially agree in the optical
compensation element 70. The invention, however, is not limited to
this example. Specifically, in a modification of this structure, in
the optical compensation element 70, the optical axis of the second
retardation plate R2 in the X-Y plane is set at 135.degree.. Thus,
the absorption axis of the polarizer plate PL and the optical axis
of the second retardation plate R2 substantially agree with each
other. In this modification, the same advantageous effects as in
the above-described structure can be obtained by using the second
retardation plate R2 in which the Nz coefficient is set in the
range of between 0.7 and 0.9.
[0141] The present invention is not limited directly to the
above-described embodiments. In practice, the structural elements
can be modified without departing from the spirit of the invention.
Various inventions can be made by properly combining the structural
elements disclosed in the embodiments. For example, some structural
elements may be omitted from all the structural elements disclosed
in the embodiments. Furthermore, structural elements in different
embodiments may properly be combined.
* * * * *