U.S. patent application number 10/054940 was filed with the patent office on 2002-05-30 for liquid crystal display device with compensation for viewing angle dependency and optical anisotropic element used therein.
This patent application is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Hatoh, Hitoshi, Ishikawa, Masahito, Manabe, Atsuyuki, Okamoto, Masumi, Tanaka, Yasuharu.
Application Number | 20020063829 10/054940 |
Document ID | / |
Family ID | 26517532 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020063829 |
Kind Code |
A1 |
Manabe, Atsuyuki ; et
al. |
May 30, 2002 |
Liquid crystal display device with compensation for viewing angle
dependency and optical anisotropic element used therein
Abstract
In a liquid crystal display device having a driving liquid
crystal cell interposed between two polarizers 1 and 4, the cell
having a liquid crystal layer 3e held between two substrates 3a and
3b, the layer having a twisted molecular alignment when no voltage
is applied, and the liquid crystal cell performing optical control,
using the optical anisotropy of liquid crystal, there is provided
with an optical anisotropic element 2 between the polarizer and the
driving liquid crystal cell, the optical anisotropic element 2
comprising an optical anisotropic substance layer 2c in which the
optical rotatory power is minimal in the direction of layer
thickness and the optical anisotropy is negative. The angle of the
optical axis of the optical anisotropic element 2 varies
continuously or in stages in the direction of layer thickness of
the optical anisotropic element as against the surface of the
optical anisotropic element. Furthermore, optical anisotropic
element with negative optical anisotropy can be combine to the
anisotropically negative element.
Inventors: |
Manabe, Atsuyuki;
(Kanagawa-ken, JP) ; Ishikawa, Masahito;
(Kanagawa-ken, JP) ; Tanaka, Yasuharu;
(Kanagawa-ken, JP) ; Hatoh, Hitoshi;
(Kanagawa-ken, JP) ; Okamoto, Masumi;
(Kanagawa-ken, JP) |
Correspondence
Address: |
Intellectual Property Group
Pillsbury Winthrop LLP
1600 Tysons Boulevard
McLean
VA
22102
US
|
Assignee: |
Kabushiki Kaisha Toshiba
|
Family ID: |
26517532 |
Appl. No.: |
10/054940 |
Filed: |
January 25, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10054940 |
Jan 25, 2002 |
|
|
|
08698997 |
Aug 16, 1996 |
|
|
|
Current U.S.
Class: |
349/117 |
Current CPC
Class: |
G02F 2413/105 20130101;
G02F 1/133632 20130101 |
Class at
Publication: |
349/117 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 17, 1995 |
JP |
P07-209562 |
Nov 24, 1995 |
JP |
P07-305879 |
Claims
What is claimed is:
1. A liquid crystal display device comprising at least one
polarizer, a driving liquid crystal cell having two substrates and
a liquid crystal layer held between at least the two substrates, at
least one optical anisotropic element in which plural optical
anisotropic units arrange in the direction of the layer thickness,
wherein the optical anisotropic element is arranged for the optical
anisotropy of the optical anisotropic unit to be negative to the
direction of thickness, the angles of respective optical axes of
the optical anisotropic units being not constant against the
direction of the thickness and that the optical anisotropy having
the minimum optical rotatory power in the thickness direction.
2. The liquid crystal display device as claimed in claim 1, wherein
the angle of the optical axis of the optical anisotropic units
constituting the optical anisotropic element with the surface of
the substrate of the driving liquid crystal cell varies
continuously or stepwise in the direction of the layer thickness of
the optical anisotropic element.
3. The liquid crystal display device as claimed in claim 1, wherein
the optical axis of the optical anisotropic units is substantially
parallel to the surface of the optical anisotropic element adjacent
to the driving liquid crystal cell and varies in the layer of the
optical anisotropic element to be substantially normal on the other
surface of the optical anisotropic element.
4. The liquid crystal display device as claimed in claim 1, wherein
the optical axis of the anisotropic units is substantially normal
to the substrate of the driving liquid crystal cell on the surface
of the optical anisotropic element adjacent to the driving liquid
crystal cell and varies in the layer of the optical anisotropic
element to be substantially parallel to the substrate of the
driving liquid crystal cell on the other surface of the optical
anisotropic element.
5. The liquid crystal display device as claimed in claim 1, wherein
the orientation, as viewed from the normal of the substrate of the
liquid crystal cell, of the respective optical axes of the optical
anisotropic units is aligned in a single axis.
6. The liquid crystal display device as claimed in claim 1, wherein
the orientation of the optical axes of the optical anisotropic
units as viewed from the normal of the substrate of the driving
liquid crystal cell is twisted toward the surface of the optical
anisotropic element.
7. The liquid crystal display device as claimed in claim 1, wherein
at least one first optical anisotropic element and at least one
second optical anisotropic element are arranged respectively, and
the first optical anisotropic element has an optical axis whose the
angle varies within the layers of the first optical anisotropic
element to be parallel to the surface of the first optical
anisotropic element adjacent to the driving liquid crystal cell and
to be substantially normal to the other surface of the first
optical anisotropic element, and the second optical anisotropic
element has an optical axis whose the angle is substantially normal
to the surface of the second optical anisotropic element adjacent
to the driving liquid crystal cell and varies within the layers of
the second optical anisotropic element to be substantially parallel
to the other surface of the second optical anisotropic element.
8. The liquid crystal display device as claimed in claim 1, wherein
the optical axes of the optical anisotropic element vary in the
thickness direction of the layer of the optical anisotropic element
to be substantially parallel in one surface of the element and to
be slanted by 10 to 80 degrees from the normal of the surface to
the other surface of the optical anisotropic element.
9. The liquid crystal display device as claimed in claim 1, wherein
the optical axis of the optical anisotropic element has a first
angle of 10 to 90 degrees on the one surface of the optical
anisotropic element and varies in the layer of the element to have
a second angle of 0 to 80 degree on the other surface of the
optical anisotropic element, the second angle being smaller than
that of the first angle.
10. The liquid crystal display device as claimed in claim 3 or 4,
wherein the orientation of the optical axis of the optical
anisotropic element on the side nearly parallel to the substrate
surface of the driving liquid crystal cell is substantially
parallel to or substantially perpendicular with the absorption axis
of the polarizers when viewed from the normal direction of the
substrate.
11. A liquid crystal display comprising at least one polarizer, a
driving liquid crystal cell with two substrates and a liquid
crystal layer held between the two substrates, and at least one
optical anisotropic element with one or more optical anisotropic
units arranged between the polarizer and the cell, wherein the
angles of the optical axes of the optical anisotropic units with
the substrate of the element unit are substantially coincident to
each other on the both surfaces of the optical anisotropic element
and the angles of the optical axis varies in the intermediate
layer, and the optical anisotropy of the optical anisotropic
element is negative to the direction of the thickness.
12. A liquid crystal display device as claimed in claim 11, wherein
the angle of the optical axis of the optical anisotropic element
with the substrate surface of the driving liquid crystal cell is
substantially same on the both surfaces of the optical anisotropic
element, and the angle varies continuously or stepwise in its
intermediate portion in the layer of the optical anisotropic
element.
13. A liquid crystal display device as claimed in claim 11, wherein
the direction of the optical axis of the optical anisotropic units
resides on a single axis when viewed from the direction of the
normal to the substrate of the driving liquid crystal cell.
14. A liquid crystal display device as claimed in claim 11, wherein
the directions of the optical axis of the optical anisotropic units
are at least two when viewed from the direction of the normal to
the substrate of the driving liquid crystal cell.
15. The liquid crystal display device as claimed in claim 11,
wherein the optical axis of the optical anisotropic units is
twisted continuously or stepwise.
16. The liquid crystal display device as claimed in claims 1 or 11,
wherein a biaxial retardation film is disposed between a polarizer
and an optical anisotropic element.
17. The liquid crystal display device as claimed in claims 1 or 11,
wherein the layer of the optical anisotropic substance of the
optical anisotropic element is one selected from organic, inorganic
material and high molecular liquid crystal.
18. The liquid crystal display device as claimed in claims 1 or 9,
wherein the optical anisotropic element is disposed within the
driving liquid crystal cell.
19. An optical anisotropic element comprising a plurality of
optical anisotropic units arranged in the direction of the layer
thickness of the element, wherein angles between optical axes of
the optical anisotropic units and surfaces of the optical
anisotropic element differs in the vicinity of the upper and lower
surfaces of the optical anisotropic element, and the optical
anisotropy of the optical anisotropic element is negative to the
thickness direction.
20. The optical anisotropic element as claimed in claim 19, wherein
the angle of the optical axis of optical anisotropic units varies
continuously or stepwise against the surface of the optical
anisotropic element in the direction of the layer thickness of the
optical anisotropic element.
21. The optical anisotropic element in as claimed claim 19 or 20,
wherein the orientation of the optical axis of the optical
anisotropic units is substantially parallel to one surface of the
optical anisotropic element and substantially normal to the other
surface the optical anisotropic element, and the optical axis in
between the layers varies continuously within the layer of the
optical anisotropic element.
22. The optical anisotropic element as claimed in claim 19 or 20,
wherein the orientation of the optical axis of the optical
anisotropic units is substantially normal to one surface of the
optical anisotropic element and substantially parallel to the other
surface of the optical anisotropic element, and the optical axis
varies continuously within the layer of the optical anisotropic
element.
23. The optical anisotropic element as claimed in claim 19, wherein
the respective optical axes of the optical anisotropic units align
on a single axis when viewed from the direction of the normal to
the surface of the optical anisotropic element.
24. The optical anisotropic element as claimed in claim 19, wherein
the respective optical axes of the optical anisotropic units is
twisted in the direction of the element plane when viewed from the
direction of the normal to the surface of the optical anisotropic
element.
25. An optical anisotropic element comprising a plurality of
optical anisotropic units, wherein the optical anisotropic units
have optical axes of which the angles are substantially coincident
on the both units of the surfaces of the element and vary in the
intermediate units, and the optical anisotropy of the optical
anisotropic element is negative in the direction of the
thickness.
26. The optical anisotropic element as claimed in claim 25, wherein
the angles varies continuously or stepwise in the intermediate
units.
27. The optical anisotropic element as claimed in claim 25, wherein
the respective optical axes of the optical anisotropic units are on
a single axis when viewed from the direction of the normal to the
surface of the optical anisotropic element.
28. The optical anisotropic element as claimed in claim 25, wherein
the optical axes of the optical anisotropic units are directed
toward two or more orientations when viewed from the direction of
the normal to the surface of the optical anisotropic element.
29. The optical anisotropic element as claimed in claim 25, wherein
the respective optical axes of the optical anisotropic units are
twisted continuously or stepwise in the direction of the element
when viewed from the direction of the normal to the surface of the
optical anisotropic element.
30. The optical anisotropic element as claimed in claims 19 or 25,
wherein the optical anisotropic element is of any one selected from
organic, inorganic material and high molecular liquid crystal.
31. A liquid crystal display element comprising: at least two
polarizers; a driving liquid crystal cell sandwiched between the
polarizers, comprising two substrates with electrodes and a liquid
crystal layer interposed between the two substrates; and at least
one optical anisotropic layer with positive optical anisotropy, and
at least one optical anisotropic layer with negative optical
anisotropy, the optical anisotropic layers disposed between the
polarizer and the driving liquid crystal cell and of which an
optical rotatory power in a direction slanted from normal of the
optical anisotropic layers is greater than that of normal to the
optical anisotropic layers.
32. The liquid crystal display element as claimed in claim 31,
wherein the optical axis of the optical anisotropic layer with
positive optical anisotropy is uniformly slanted to the direction
of the layer thickness of the optical anisotropic layer or varies
continuously in the direction of the layer thickness of the optical
anisotropic layer.
33. The liquid crystal display device as claimed in claim 32,
wherein the optical axes of the optical anisotropic layer are in
the same orientation.
34. The liquid crystal display device as claimed in claim 31,
wherein a set of the optical anisotropic layer with negative
optical anisotropy and the optical anisotropic layer with positive
optical anisotropy are arranged on both sides of the driving liquid
crystal cell.
35. The liquid crystal display device as claimed in claim 34,
wherein the optical anisotropic layer with negative optical
anisotropy is arranged adjacent to the driving liquid crystal
cell.
36. The liquid crystal display device as claimed in claim 35,
wherein a plane with smaller oblique angle of the optical axis of
the optical anisotropic layer with negative optical anisotropy, the
oblique angle of the optical axis varying continuously in the
thickness direction of the optical anisotropic layer, is arranged
adjacent to the driving liquid crystal cell.
37. The liquid crystal display device as claimed in claim 35,
wherein the respective optical axes of the optical anisotropic
layer with positive optical anisotropy and the optical anisotropic
layer with negative optical anisotropy are cross at right angle to
each other.
38. The optical anisotropic element as claimed in claim 1 or 3,
wherein the optical anisotropy of optical anisotropic units is
negative, and the optical axis is substantially perpendicular to
one surface of the optical anisotropic element and is aligned
obliquely at 10 to 60 degrees without twist on the other surface of
the optical anisotropic element.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a liquid crystal display device
and the optical anisotropic element used therein.
[0003] 2. Description of the Related Art
[0004] The liquid crystal display device is not only used as
display unit for wrist watch, electronic calculator, word processor
and personal computer thanks to its crucial advantages of being
thin and light-weighted and of low power consumption, but also
widely used in many newly designed products.
[0005] The liquid crystal display device used in personal computer,
among others, is employed in larger and larger size of display
units with higher capacity and greater size of display surface 10
inches diagonally and 640.times.480 pixels or more. The display
system used in this class of liquid crystal display device may
roughly be divided into two: one is simple-matrix system and
another, active matrix system.
[0006] The simple-matrix system features a simple structure in
which the liquid crystal is held between two sheets of glass
substrates provided with stripe-shaped transparent electrodes. The
simple matrix system demands the liquid crystal a high performance
all the more.
[0007] Before describing this performance, we briefly explain the
display principle of the liquid crystal display device. The liquid
crystal display device achieves the display changing the
orientation of the liquid crystal molecules by varying the voltage
applied on the liquid crystal.
[0008] Generally a large contrast requires a large differential
voltage. The display with as many as 640.times.480 pixels has
however only about 1 V of voltage difference between dark state and
bright state. Only 1 V of difference requires a large state
alteration of molecular liquid crystal. Many of the researches have
thus far been conducted to realize such a feat. In 1985, the
research group of Shafer et al. found out the fact that the change
in the alignment of the liquid crystal molecules sensitively
responds to the change in voltage if the twist angle of the liquid
crystal display device is enlarged and that the liquid crystal
molecules have a certain tilt to get a stable arrangement with a
large twist angle. Since this research report, the alignment
technology to realize this has been briskly developed and invested
into its commercialization successfully.
[0009] In general, 180.degree. or more twist angle is necessary to
materialize a display with as many as 640.times.480 pixels. The
liquid crystal display device with such a large twist angle has
been called "Super Twist Nematic" (STN). Note however that the STN
display at early stage was not achromatic, but colored; for
example, with green characters in yellow background. This is due to
too large a twist angle. Japanese Patent Publication 63-53528
(1985) discloses a technique to resolve such problems as colored
display. This method technique realizes an achromatic display by
arranging a second liquid crystal cell with its alignment of liquid
crystal layer twisted in opposite direction between a polarizer and
a first liquid crystal cell.
[0010] The principle of this achromatization consists in resolving
the optically rotatory dispersion, that is, a wavelength dependence
of optical rotatory power, by transmitting, the light produced this
dispersion after passing through the first liquid crystal cell
caused to have a large twisted molecular structure in the cell,
through the second liquid crystal cell having a symmetrical
structure to that of the first liquid crystal cell. As a result,
the color caused by the optically rotatory dispersion was dissolved
to materialize the achromatic display. In order to perform such a
conversion exactly, it is necessary that the second liquid crystal
cell, which is an optical compensation plate, has a retardation
value substantially the same with that of the first liquid crystal
cell with their twist directions being opposed to each other and
their arrays are so configured that the directors of the liquid
crystal display device cell molecules coming most close to each
other should intersect each other.
[0011] A variety of other techniques have so far been proposed. For
example, optically anisotropic film may be used in place of the
second liquid crystal cell. Lamination of the optically anisotropic
film on the liquid crystal cell affords a performance substantially
equivalent to that of the second liquid crystal cell.
[0012] The optical compensation as above makes it possible to
display achromatically even on the STN display unit. Furthermore,
this achromatic display combined with color filter enables to have
a high value added colored display. Since however the simple
multiplex system is based on the principle of multiplex drive,
which in its turn is based on the average voltage method, if the
number of scanning lines is increased to augment the display
capacity, the difference reduces remarkably between the voltage
when the light is intercepted and that when the light is left to
transmit, which may result in lower contrast or slower response of
the liquid crystal. This is a critical weak point. Such
conventional techniques are much problematical if one tries to
realize a liquid crystal display device with higher display
quality, because they may cause such negative phenomena as display
screen seen reversed (that is, obverse and reverse) depending on
the orientation and angle when viewing it, disappearance of the
display image or display catching colors.
[0013] On the other hand, the active matrix system, which is
provided with switching element comprising, for each display pixel,
thin-film transistor or diode, allows us to set a given voltage
ratio on the liquid crystal layer of each pixel irrespectively of
the number of scanning lines. No special performance such as that
for the simple matrix system is required in the active matrix
system. There is therefore no need to increase the twist angle as
in the case of STN. It has been considered that angle of 90.degree.
suffice for the active matrix system.
[0014] In the liquid crystal cell (TN) with a small 90.degree.
twist angle, the optical rotatory dispersion is small since the
light rotates following faithfully the twist, which ensures a
colorless, high contrast display. The response to voltage is more
rapid than in the STN too. A favorable combination of the active
matrix system with the TN will realize a liquid crystal display
device featuring a large display capacity, higher contrast and
higher response speed. Since further there is a switching element
for each pixel, an intermediate voltage can be applied, which
enables to make a gray scale (half tone) image. Moreover, the TN as
combined with color filter will facilitate the materialization of
full colored display.
[0015] Even in the active matrix system, however, such phenomena
are observed as obverse-reverse display screen depending on the
orientation of view, total disappearance of display image and
colored display when a gray scale image (half tone) is displayed,
though not so with binary display. These phenomena are much
problematical when one wants to realize a high quality liquid
crystal display device.
[0016] Japanese Patent Laid-Open 62-21423 (1987) discloses a liquid
crystal cell and a birefringence layer, which is a polymer film
whose optical anisotropy is negative in the direction of its
thickness, are between two polarizers, as means to reduce the
visual angle dependency. On the other hand, Japanese Patent
Laid-Open 3-67219 (1991) discloses an arrangement, on liquid
crystal cell, of a birefringence layer composed of the liquid
crystal compound (or high molecular liquid crystal) presenting
cholesteric liquid crystal phase with 400 nm or less product of
helical pitch length and refractive index. These two propositions
have been contrived only for the cases of liquid crystal cells
homeotropically aligned liquid crystal cells (molecular liquid
crystal arranged perpendicularly to the aligned substrate), not for
such liquid crystal cell with twisted orientation as TN and STN
systems. Japanese Patent Laid-Open 4-349429 (1992) proposes to
control the viewing angle of liquid crystal display device by
optional compensation element with arrangement of 360.degree. or
more tilt angle, but the effect of enlarged viewing angle cannot
yet be considered sufficient for gradation display (gray scale
image).
[0017] Though we have some technical reports on the improved
viewing angle of TN-LCD by obliquely arranging the optical axis of
negative optical anisotropic substance (Lecture Manuscripts for the
21st Liquid Crystal Conference), the compensation can not cover all
the orientations of view.
[0018] The basic principle of the display by the liquid crystal
display device thus far described consists in performing an optical
control by changing the orientation of the liquid crystal molecules
through the voltage to be applied to the liquid crystal.
[0019] Thus, the liquid crystal display device has such a visual
angle dependency that this device, when viewed as tilted, changes
the orientation of the molecular liquid crystal thus changing the
way it is seen. When displaying a subtle gray scale image is
displayed, in particular, the viewing angle dependency is more
conspicuous since the inclination of the liquid crystal molecules
is changed minutely.
[0020] Such visual angle dependency of the way the alignment of the
liquid crystal molecules is seen gives rise to such phenomena as
reversed image of display and total lack of recognition. When, in
particular, colored display is made by combination with color
filter, the reproducibility of the display reduces remarkably,
which is one of the critical problems.
SUMMARY OF THE INVENTION
[0021] Accordingly, one of objects of the invention is to provide
the liquid crystal display device with enhanced contrast and
improved viewing angle dependency of the display colors, and the
optical anisotropic element.
[0022] Briefly, in accordance with one aspect of the invention,
there is provided a liquid crystal display device comprising at
least one polarizer, a driving liquid crystal cell having two
substrates and a liquid crystal layer held between at least the two
substrates, at least one optical anisotropic element in which
plural optical anisotropic units arrange in the direction of the
layer thickness, wherein the optical anisotropic element is
arranged so that the optical anisotropy of the optical anisotropic
unit is negative to the direction of thickness, the angles of
respective optical axes of the optical anisotropic units are not
constant against the direction of the thickness and that the
optical anisotropy has the minimum optical rotatory power in the
thickness direction.
[0023] A liquid crystal display device having at least one
polarizers, a driving liquid crystal cell having a liquid crystal
held between two substrates and at least one optical anisotropic
element in which plural optical anisotropic units run in a row in
the thickness direction characterized in that the optical
anisotropy of the optical anisotropic units of the optical
anisotropic element is negative to the thickness direction, that
the angle of the respective optical axes is not constant to the
thickness direction, and that the optical anisotropic elements are
so arranged as having minimum optical rotatory power in the
thickness direction.
[0024] The angle between the optical axis of the optical
anisotropic element and the substrate surface of the driving liquid
crystal cell preferably varies continuously or stepwise (in stages)
in the direction of the layer thickness of the optical anisotropic
element.
[0025] In another aspect of this invention, there is provided a
liquid crystal display comprising at least one polarizer, a driving
liquid crystal cell with two substrates and a liquid crystal layer
held between the two substrates, and at least one optical
anisotropic element with one or more optical anisotropic units
arranged between the polarizer and the cell, wherein the angles of
the optical axes of the optical anisotropic units with the
substrate of the element unit are substantially coincident to each
other on the both surfaces of the optical anisotropic element and
the angles of the optical axis varies in the intermediate layer,
and the optical anisotropy of the optical anisotropic element is
negative to the direction of the thickness.
[0026] Furthermore, in another aspect of this invention, there is
provided an optical anisotropic element comprising a plurality of
optical anisotropic units arranged in the direction of the layer
thickness of the element, wherein angles between optical axes of
the optical anisotropic units and surfaces of the optical
anisotropic element differs in the vicinity of the upper and lower
surfaces of the optical anisotropic element, and the optical
anisotropy of the optical anisotropic element is negative to the
thickness direction.
[0027] In another aspect of this invention, there is provided an
optical anisotropic element comprising a plurality of optical
anisotropic units, wherein the optical anisotropic units have
optical axes of which the angles are substantially coincident on
the both units of the surfaces of the element and vary in the
intermediate units, and the optical anisotropy of the optical
anisotropic element is negative in the direction of the
thickness.
[0028] In another aspect of this invention, there is provided a
liquid crystal display element comprising:
[0029] at least two polarizers;
[0030] a driving liquid crystal cell sandwiched between the
polarizers, comprising two substrates with electrodes and a liquid
crystal layer interposed between the two substrates; and
[0031] at least one optical anisotropic layer with positive optical
anisotropy, and at least one optical anisotropic layer with
negative optical anisotropy,
[0032] the optical anisotropic layers disposed between the
polarizer and the driving liquid crystal cell and of which an
optical rotatory power in a direction slanted from normal of the
optical anisotropic layers is greater than that of normal to the
optical anisotropic layers.
[0033] In the context of this specification, the optical
anisotropic unit means the respective layers of an optical
anisotropic element with predetermined thickness that has a
multi-layered structure. Each layer is a unit having an optical
axis oriented toward a particular direction and comprises, when
layered, a configuration that changes the inclination of the
optical axis gradually in continuous or staged fashion. This
invention defines here that the configuration in which the optical
axes change in the direction of thickness means that the optical
axes of optical anisotropic units in optical anisotropic element
change in series in the direction of the thickness. This invention
contains an optical anisotropic element without multi-layered
construction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0035] FIG. 1 is a cross-sectional view showing the constitution of
embodiment 1 by the invention;
[0036] FIG. 2a is an exploded perspective view illustrating the
embodiment 1 by the invention and FIG. 2b shows an illustration
showing the coordinate system for measuring the electrooptic
characteristics;
[0037] FIG. 3a and FIG. 3b are diagrams depicting the principle of
operation of TN-LCD;
[0038] FIG. 4a and FIG. 4b are diagrams illustrating how the
viewing angle characteristics of TN-LCD are generated;
[0039] FIG. 5 is a diagram that illustrates how the viewing angle
characteristics of TN-LCD are generated.
[0040] FIG. 6a through FIG. 6d are schematic diagrams that
illustrates the alignment of the optical anisotropic element by
this invention;
[0041] FIG. 7a through FIG. 7f are schematic diagrams that
illustrates the principle of the optical compensation when the
optical anisotropic element by this invention is used.
[0042] FIG. 8 is another schematic diagram that illustrates the
optical indicatrix for driving liquid crystal cell when voltage is
applied;
[0043] FIG. 9 is another schematic diagram showing the optical
indicatrix of the optical anisotropic element which is negative in
the refractive index anisotropy in the direction of the
thickness;
[0044] FIG. 10 is graphs for showing the electrooptic
characteristic of the liquid crystal display device in embodiment
1;
[0045] FIG. 11 is graphs for showing the electrooptic
characteristics of the liquid crystal display device by comparison
example 1;
[0046] FIG. 12 is an exploded perspective view illustrating the
constitution of embodiment 2 according to this invention;
[0047] FIG. 13 is graphs showing the electrooptic characteristics
in comparison example;
[0048] FIG. 14 is graphs showing the effects of embodiment 3 of
this invention;
[0049] FIG. 15 is an exploded perspective view showing the
configuration of embodiment 4 of this invention;
[0050] FIG. 16 is graphs showing the effects of embodiment 4 of
this invention;
[0051] FIG. 17 is an exploded perspective view showing the
configuration of embodiment 5 of this invention;
[0052] FIG. 18 is graphs showing the effects of embodiment 5 of
this invention;
[0053] FIG. 19 is graphs showing the effects of embodiment 6 of
this invention;
[0054] FIG. 20 is a schematic cross sectional view showing the
configuration of liquid crystal cell for compensation of viewing
angle of embodiment 6 of this invention;
[0055] FIG. 21 is an exploded perspective view showing the
configuration of embodiment 7 of this invention;
[0056] FIG. 22 is a cross sectional view illustrating the
constitution of embodiment 9;
[0057] FIG. 23 is a schematic diagram illustrating the function of
embodiment 9;
[0058] FIG. 24a through FIG. 24d are schematic diagrams
illustrating the alignment of the optical axes of the optical
anisotropic element relating to this invention;
[0059] FIG. 25a through FIG. 25f are schematic diagrams showing the
optical compensation principle using the optical axes of the
driving liquid crystal cell relating to this invention;
[0060] FIG. 26a through FIG. 26c are schematic diagrams
illustrating the configuration of the optical anisotropic element
by embodiment 9;
[0061] FIG. 27 is graphs showing the electrooptic characteristic of
the optical anisotropic element by embodiment 9;
[0062] FIG. 28 is graphs showing the electrooptic characteristic of
the optical anisotropic element by prior art;
[0063] FIG. 29 is graphs showing the electrooptic characteristic of
the optical anisotropic element by comparative example 2;
[0064] FIG. 30 is graphs showing the electrooptic characteristic of
the optical anisotropic element by comparative example 3;
[0065] FIG. 31 is a cross sectional view showing the cross
sectional view of the liquid crystal display device by embodiment
11;
[0066] FIG. 32 is an exploded perspective view showing the
configuration of the liquid crystal display device by embodiment
11;
[0067] FIG. 33 is a schematic diagram showing the scheme of the
liquid crystal display device by embodiment 11;
[0068] FIG. 34a and FIG. 34b are graphs showing the electrooptic
characteristic of the liquid crystal display device by embodiment
11;
[0069] FIG. 35a and FIG. 35b are graphs showing the visual angle
dependency of the luminance of conventional TN type liquid crystal
display device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0070] Hereinafter, embodiments according to the invention will be
described which are capable of achieving desirable viewing angle
and bright display color when displaying gray scale is alleviated
at the same time.
[0071] The polarized state of the visible light propagating in such
a liquid crystal element as TN and STN depends upon whether the
light comes in vertical or slanted to the display plane of the
liquid crystal display device. This difference in polarized state
is directly reflected on reversed display or colored display image.
Such a phenomenon is supposed to go on slanting largely the viewing
angle of the display plane of the liquid crystal element from the
normal (frontal face) of the display plane. This tendency is
elicited particularly in the pixels where voltage is applied on the
liquid crystal layers of a liquid crystal cell having the means to
apply voltage on the liquid crystal layers (hereafter referred to
as "driving liquid crystal cell").
[0072] FIG. 35a and FIG. 35b show diagrammatically the angle
dependency of display luminance when the horizontal and vertical
inclination from the normal of display face (substrate face) of the
conventional TN liquid crystal display device is from 0.degree. to
60.degree.. Levels 1 to 8 represent respective gradation numbers in
the tonal display (assigning intensity levels), which manifests
that the voltage applied to the liquid crystal cell differs
sequentially. Applied to the liquid crystal cell is 0 V for level
1, and 5 V for level 8. In the case of upper orientation, for
instance, the greater the angle (viewing angle) slanted from the
normal of the display plane of the display unit (from 0.degree.
[front] to 60.degree.), the greater the luminance becomes
gradually. In the real display, the display color this luminance
intensity is recognized as whitish (excessively bright image).
[0073] Referring to the upper orientation shown in FIG. 35b, the
luminance reduces the more (contrary to the upper orientation),
when the viewing angle is slanted gradually from front (0.degree.)
to 60.degree.. In the actual display screen, this phenomenon is
recognized as darkening (excessively dark) image. The brightest
display level 1 and lower gradation level 2 on the front face
reverse themselves in their largeness relationship at 35.degree. of
viewing angle in the upper orientation, which are observed as
reverse image such as the negative of the photo film in the actual
display image. It is idealistic that the transmittance of light
does not change however the viewing angle may vary at any gradation
level whatsoever. However, the viewing angle characteristic of
actual TN is relatively good in the right and left orientation, but
bad in vertical one.
[0074] The reason why such a phenomenon occurs is that the visual
or viewing angle characteristic of the liquid crystal display
device results from the polarized state which differs depending on
the angle of incident light. We will argue this point later
referring to an exemplary TN type device.
[0075] FIG. 3 illustrates the principle of the operation of TN-LCD
(TN type liquid crystal device). FIG. 3a represents the alignment
of the liquid crystal molecules in the TN cell when no voltage is
applied on the electrodes 3c and 3d. When voltage V is not applied,
the liquid crystal molecules manifest a continuous twisted
alignment of liquid crystal molecules, parallel to each other, in
the direction of the thickness of the liquid crystal layers
(direction of Z axis in the figure) nearly in parallel with the
substrate. The liquid crystal molecules having an optical axis in
the direction of their longer axis, the parallel alignment of
liquid crystal molecules form an optical axial plane.
[0076] When the light beam Li as polarized by the polarizer Pi
among the incident light beams LA impinges this alignment, the
polarized plane rotates according to the twisted alignment of the
liquid crystal molecules LM, and when the light quits the liquid
crystal layer, it turns, by the twist angle of the liquid crystal,
about the polarized plane before the light enters the liquid
crystal layer. The transmitting light Lo is obtained when the
transmission axis Pot of the analyzer Po is matched with this
rotational direction.
[0077] FIG. 3a depicts the array of the liquid crystal cells in the
TN cell when voltage is applied. Applying voltage V will raise the
liquid crystal molecule LM, and the liquid crystal molecule LMc
near the center of the cell is more tilted than the liquid crystal
molecule LMs in the vicinity of the electrodes. The liquid crystal
molecules LMs tilts little in the vicinity of the electrodes 3c and
3d because of the anchoring power at the electrode/liquid crystal
layer interface, which is necessary to array the liquid crystal. As
the voltage V grows higher, the liquid crystal molecules are tilted
more, and at the same time the twist array distorts. Finally, the
twisting is released when the voltage arrives at a certain level.
If under these conditions the polarized light Li incides, the
polarized plane Lp does not rotate, and the liquid crystal layers
are made to progress due to the inexistence of the twisted array,
namely because the optical axial plane is on a single axis. Where
the light quits the liquid crystal layer, the polarized plane
remains the same as before it entered the liquid crystal layer.
Since the transmission axis Pot of the analyzer Po gets orthogonal
with the polarized plane Lp, the polarized light cannot transmit.
To display the half tone (gray scale image), the voltage to be
applied on the liquid crystal layer is set lower and the twisted
array is somewhat left and the polarized plane on which the light
quits the liquid crystal layer is rotated to a certain extent to
get an intermediate transmitting light.
[0078] This is the principle on which the TN device controls the
system making use of the twisted array of the transmitting light.
We now explain what phenomena occur to the slanted light.
[0079] FIG. 4 illustrates how the light comes in slant on the
molecular array when the half tone (gray scale image) is displayed.
FIG. 4a is a perspective diagram showing the relationship of the
molecular arrangement LMint when the half tone is displayed with
the directions L and U of two incident light. For this diagram to
be more comprehensible, FIG. 4b and FIG. 4c depicts the views from
the direction of Y axis, where Z axis represents the direction of
the norm of the substrate of the driving liquid crystal cell while
the X and Y axes indicate the substrate faces. This slant or tilt
is called "pretilt." In general, the pretilt signifies the tilting
of liquid crystal molecules at the substrate/liquid crystal
interface, and the tilting angle is called "pretilt angle
.alpha.0."
[0080] When no voltage is applied the liquid crystal molecules are
held inclined at a same angle over the upper and lower substrates
3a and 3b. If there exists a certain slant (pretilt) over the range
with voltage to be applied, the directions of the inclination are
justified into the pretilt direction, which will cause
discrimination lines on the boundary of the region with different
tilting directions to degrade remarkably the display quality.
Consequently the pretilting is indispensable to get a uniform
display, the angle of which is 1.degree. through 6.degree. in
general. Note that any liquid crystal display device is endowed
with this pretilt.
[0081] As has been illustrated in FIG. 4b and FIG. 4c, therefore,
the array of he liquid crystal molecules becomes asymmetric to the
Z axis particularly when the half tone is displayed. As for the
polarized light obliquely impinging from +X axis to +Z axis in FIG.
4b, the array of the liquid crystal molecules LM loses the tilting
characteristic as if in the array with no voltage applied as shown
by LM-L in FIG. 5, thus enabling to rotate largely the polarized
plane. As a result, the intensity of the transmitting light becomes
greater than that of the outgoing light as against the incident
light parallel to the Z axis. To the polarized light U inciding
from the symmetric orientation (obliquely from--X axis to +Z axis)
with the opposed normal of substrate as reference, the array of the
liquid crystal molecules LM becomes largely tilted as shown by LM-U
in FIG. 5 as if further higher voltage were applied, disabling thus
the polarized plane to be rotated. As a result, the intensity of
the transmitting light becomes smaller than intensity of the
outgoing light as against the incident light parallel to Z axis. In
the corresponding relationship to FIG. 35, the orientation of L in
FIG. 35b and that of U in FIG. 4, to the lower orientation in FIG.
35b.
[0082] As has so far been explained, the orientation dependency of
the transmitting light at half (intermediate) tone results from the
asynmmetric array of the liquid crystal molecules. Due to this
asymmetry of array, the rotational angle (optical rotatory power)
of the polarized face depends on the direction in which the light
comes, which will result in change in the transmittance. In the
case of TN-LCD, it may be that there is a tendency for the optical
rotatory power to be generated in the upper orientation and the
same power to be reduced in the lower one. In consequence, an
addition of such optical anisotropic element that decreases the
optical rotatory power in the upper orientation and generates the
same in the lower one will contribute to the improvement of the
viewing angle dependency of the liquid crystal display device.
[0083] To summarize, the first characteristic required for the
optical anisotropic element is that the rotational direction of
polarized light in the upper orientation is reversed to that in the
lower one for any driving liquid crystal cell whose viewing
characteristic is not good either in upper or lower
orientation.
[0084] The second characteristic required is enhancement of the
viewing characteristic in other orientations.
[0085] This invention provides the optical anisotropic element
having such characteristics as above and the liquid crystal display
device equipped with this optical anisotropic element.
[0086] Now we describe the configuration of the optical anisotropic
element according to this invention.
[0087] The optical anisotropic element by this invention is an
optically anisotropic membrane, plate or sheet-shaped planar body
of a certain thickness. As shown in FIG. 9, the element with
negative optical anisotropy has small refractive index of the
optical axis, that is, the Z axis which is perpendicular to the
direction of the X-Y axes that constitute the plane.
[0088] We define any optically anisotropic thin layer with its
optical axis oriented to a certain direction as optical anisotropic
units assuming that the foregoing optical anisotropic element
consists of these units stacked into multilayer. The element
includes also any configuration of these units without clear
division of layers.
[0089] In consequence, in the representative embodiment of this
invention is a liquid crystal display device, the optical
anisotropic element has the optical anisotropy of the optical
anisotropic units which is negative in the direction of thickness,
that the angle of the respective optical axes is not constant to
the thickness direction and that the minimal optical rotatory power
is obtained in the direction.
[0090] An embodiment of this liquid crystal display device of this
invention is an optical anisotropic element of a hybrid
construction wherein the optical axis runs substantially parallel
to one plane, from the one plane of the element to the other, it
changes its slant gradually toward other plane and becomes
substantially perpendicular on the plane of other orientation.
[0091] This invention further has an embodiment with improved
viewing angle dependency that consists in a combined body of
optical anisotropic elements wherein at least a layer of optical
anisotropic units with an optical anisotropy is combined with at
least another layer of other optical anisotropic units with an
optical anisotropy different therefrom. The set of combination uses
a pair of optical anisotropic structures, for example positive and
negative in anisotropy, in which the optical rotatory power in the
direction slanted from the direction of the normal is greater than
that in the direction of the normal. This set is disposed, for
example, on both sides of the driving liquid crystal cell.
[0092] The characteristic required for the optical anisotropic
element is the "rotational direction of polarized light in the
upper orientation is reversed to that in the lower one." FIG. 6
illustrates the alignment of the optical axes in the optical
anisotropic element according to this invention, while FIG. 6a
depicts the cross sectional view of optical anisotropic element in
an embodiment of this invention where the ellipse represents the
optical anisotropic bodies that constitute the optical anisotropic
element, the longer axis of this ellipse corresponding to the
optical axis OL.
[0093] The normal to the shorter axis of the ellipse is equivalent
to the optical axis OL. The unit may be a molecule or plural
molecules running in rows into laminated layer.
[0094] From the lower substrate 2b over to the upper one 2a, the
inclination of the longer axis changes continuously, being nearly
parallel to the substrate face in the vicinity of the lower
substrate 2b and substantially vertical to the same near the upper
substrate 2a (hybrid alignment). FIG. 6b is a top view of this
array. The arrows in the ellipses in this figure represent the
orientation of optical axes., Note that the orientations of
respective optical axes in the layer converge into a same plane,
that is, are justified in a row on a single axis. FIG. 6c
illustrates the array as viewed obliquely from the Z axis. The
directions of the slant are indicated by X, Y and Z axes in the
figure. FIG. 6d depicts the same, but viewed from reversed oblique
direction. As is clear from FIGS. 6c and FIG. 6d, if the array of
FIG. 6a viewed obliquely from Z axis, the array twists leftward in
the direction of progress increasingly as it progresses from lower
to upper area in FIG. 6c, while it twists rightward in FIG. 6d. The
optical anisotropic element with such oblique array contributes to
the realization of the characteristic: "the rotational direction of
the polarized light is reversed in upper orientation from that in
the lower one."
[0095] The optical anisotropic elements by this invention may be
regarded as a structure in which the layer units of optical
anisotropic substance are optically laminated into multilayer
construction in the direction of the thickness. In this
construction the respective layer units have their own optical axes
and the slants of these axes vary continuously or stepwise.
Furthermore, such optical axis array has been adopted where the
minimal optical rotatory power is had in the direction of
thickness.
[0096] Now we will describe how these optical anisotropic elements
should be combined with the drive cell to get an excellent
compensation effect.
[0097] FIG. 7a illustrates the liquid crystal cell as shown in
FIGS. 3, 4, and 5, adding arrows similar to those in FIG. 6, where
the symbol Lip represents the polarized axis of incident light and
Lop, that of outgoing light. FIG. 7a shows an optical anisotropic
element and FIG. 7b shows the driving liquid crystal cell (TN) to
which a voltage equivalent to half (intermediate) tone is applied,
both as viewed from the Z axis.
[0098] FIG. 7c illustrates the array of the molecules of respective
optical anisotropic substance layers that constitute the optical
anisotropic element viewed as projected on +X axis side from Z axis
side. This figure depicts the optical rotational status when the
straight polarized light incides on.
[0099] FIG. 7c depicts the array of the optical axes of an optical
anisotropic element as viewed throwing it from on Z axis to +X axis
where is shown the status of polarized light when a straight
polarized light comes in. In this direction, the optical
anisotropic element is apt to rotate the polarized face of the
incident light into the left direction (left rotatory power). FIG.
7d shows an array of drive cell viewed in the same was as in FIG.
7c.
[0100] The liquid crystal molecules are slanted due to the
application of a voltage a little higher than the voltage
equivalent to the half (intermediate) tone (critical or threshold
voltage at which the liquid crystal can function). If viewed from
this direction, there arises an alignment portion where the length
of the liquid crystal molecules in the longer axis of ellipse is
substantially equal to the length in the shorter axis direction.
The polarized incident light, therefore, transmits without much
rotation, and the direction of the polarized axis Lop of the
outgoing light is nearly the same as the polarized axis Lip of the
incident light. This causes the abnormal display called excessively
dark image, which may be improved if the polarized light is rotated
counterclockwise to intensify the optical rotatory power. The
optical anisotropic element as shown above in FIG. 7c is suited to
this improvement. The optical anisotropic element as shown in FIG.
7c has the left rotatory power, which makes up for the polarized
light lacking in the driving liquid crystal cell.
[0101] The same will be explained, but in reversed orientation,
referring to FIG. 7e and FIG. 7f. The FIG. 7e and FIG. 7f represent
the array of optical axes when the optical anisotropic element as
shown in FIG. 7a is observed from the direction of Z axis toward -X
axis. This array has a characteristic to rotate clockwise the
incident light shown (right rotatory power). FIG. 7f schematically
shows a state with voltage of half tone applied as was the case
with FIG. 7d. From this direction the liquid crystal molecules look
like slanted though not in reality, and a large optical rotatory
power is given for that reason. This may cause too bright a
display, which is an abnormal display called "excessive bright
image." If we apply, however, the right polarized light that
controls the counterclockwise polarized light, excessive polarized
light can be dissolved to improve the "excessive bright image." The
optical anisotropic element as shown in FIG. 7e has the right
rotatory power, which if, combined with the drive cell, will
acquire an improvement of characteristic.
[0102] Thus far we have illustrated the principle of enlarged
viewing angle by example of an optical anisotropic element with
hybrid alignment whose rotatory power in a direction slanted from
the normal to the optical anisotropic element is greater than that
in the direction of the normal. Further the optical anisotropic
element with twisted hybrid alignment and one with uniformly tilted
alignment between upper and lower substrates may present
characteristics similar to those of the optical anisotropic element
with hybrid alignment, which can be selected in terms of the design
specification of the liquid crystal display device.
[0103] Thus far we have explained with TN taken as an example.
Since the same principle is applicable also to STN, it can be used
as a means to improve the viewing angle of STN.
[0104] The optical anisotropic body with negative optical
anisotropy is endowed with such hybrid array as gives an optical
rotatory power oblique to the element greater than that in frontal
direction (Z axis). This characteristic is much effective in
eliminating such abnormal display as "excessively darkened image"
and "excessively bright image". The explanation above showed an
example where the optical anisotropy of the optical anisotropic
units constituting the optical anisotropic element is negative.
[0105] Needless to say, similar effect is displayed even when the
anisotropy is positive if the oblique rotatory power is greater
than that in frontal direction.
[0106] The foregoing description involves the hybrid array, but it
is not only in this array that the oblique optical rotatory power
can be greater than that in frontal direction. The same effect may
be had also with the array where the optical axis of the optical
anisotropic units constituting the optical anisotropic element is
twisted in the plane of the element as viewed from the direction of
the normal to the element, with the array where the orientation of
the optical axis on both end faces of the optical anisotropic
element is the same and the internal arrays varies continuously or
stepwise (or in stages), and with the bend and spray arrays where
the hybrid arrangements are superposed.
[0107] The retardation value of the optical anisotropic element is
preferably smaller than that of the driving liquid crystal cell,
and more preferably be near to the retardation value of the driving
liquid crystal cell when the voltage for displaying the half tone
(gray scale image) is applied. But this value, which depends on the
product specification and mass production, is not necessarily
limited to those values. The array and alignment of the optical
anisotropic element vary largely depending upon the product
specification, mass production and cost in terms of the optical
anisotropic elements combined and their number.
[0108] It goes also without saying that similar effects may be
given when combining the viewing characteristic with the optical
anisotropic element where the optical anisotropic bodies with
optically positive anisotropic units are diversely arranged or with
the optical anisotropic elements where the optical anisotropic
bodies with optically biaxial optical anisotropic substance are
diversely arranged.
[0109] As an exemplary substance presenting negatively optical
anisotropy, we may enumerate C.sub.18H.sub.6(OCOC.sub.7H.sub.15)
having a triphenylene core with alkyl chain by ester bond and
[C.sub.6(OCOC.sub.mH.sub.2m+1).sub.6] having a benzene core, which
are called discotic liquid crystal. These discotic liquid crystals
may be used in crystal phase so that their array should not alter
by forming desired array in a temperature zone presenting liquid
crystal phase. Furthermore, it is possible to control, by voltage,
the viewing characteristic if the temperature range presenting
liquid crystal phase is employed as the working temperature range
for liquid crystal module and the optical anisotropic elements are
so produced as can control the array by electric field.
[0110] Because the present invention uses the optical anisotropic
substance constituting the optical anisotropic element whose
optical axis slants negative differently, the element displays
still enhance the visual angle improving effect. Now we will
describe the principle by which the angle of field characteristic
is enhanced when using any liquid crystal of optically negative
anisotropy.
[0111] FIG. 8 represents the conditions of driving liquid crystal
cell under which the voltage equal to or higher than the threshold
voltage is applied to the cell, as expressed by a three-dimensional
optical index shape (index ellipsoid). In this figure, the Z axis
exhibits the direction of the thickness of liquid crystal cell,
whole X and Y planes correspond to the substrate face of the liquid
crystal cell. The phenomenon of birefringence is represented by the
geometrical shape of elliptical cut face (called "index ellipsoid
with two-dimensional plane") as formed when the normal plane on the
central point of the optical indicatrix (index ellipsoid) RA of a
line connecting an observation point when the central point of the
liquid crystal cell is viewed from a direction on the one hand, and
the central point of this optical indicatrix, on the other.
[0112] The difference between the longer axis and shorter one of
this optical indicatrix within two-dimensional plane correspond to
the phase difference (retardation value) between the ordinary and
extraordinary lights, and if the transmission axes of the
polarizers holding the liquid crystal cells in-between are
orthogonal with each other, the transmitting light of the liquid
crystal cells is intercepted if the retardation value is zero, and
if the same value is not zero, the transmitting light corresponding
to the retardation value and wavelength of the incident light is
generated.
[0113] When the light incides vertically on the substrate face of
the liquid crystal cell (namely, when the cell is viewed from a
position directly opposite), the optical indicatrix RA4 in the
two-dimensional plane becomes a circle, and the retardation value
is zero between the ordinary and extraordinary lights. When the
light enters from the direction RA1 slanted from the substrate face
of the liquid crystal cell, the optical indicatrix RA5 becomes
elliptic, retardation values produces a difference between the
ordinary and extraordinary lights. Thus the polarized state of the
light transmitting through the liquid crystal cell differs in the
direction directly opposite and slanted direction.
[0114] As the viewing angle RA3, that is, the angle at which the
optical indicatrix RA shown in FIG. 8 is seen, is made to grow, the
optical indicatrix RA5 in the two-dimensional plane grows larger in
the longitudinal direction, thereby showing a transmitting light
larger than when viewed from the direction of the visual (viewing)
axis RA1. Idealistically it is desirable that the optical
indicatrix in the two-dimensional plane does not change in its
geometrical shape in any orientation whatsoever when the viewing
angle is changed.
[0115] The optical compensation as above can be realized by placing
the disk-like optical indicatrix RB as shown in FIG. 9 on the Z
axis of the optical indicatrix RA shown in FIG. 8 (namely, to be
arranged just on or under the liquid crystal cell). Thus, when the
viewing angle RA3 is made to grow gradually, the optical indicatrix
RA5 in the two-dimensional plane of the optical indicatrix RA grows
in the direction of length, while the refractive index of the
optical indicatrix B increases in the direction of the length of
nRA2. As a result, the optical indicatrix composed in the
two-dimensional plane becomes circular, thereby enabling to
compensate optically for the optical indicatrix A to improve the
visual angle characteristic.
[0116] In an actual liquid crystal display device, the longer axis
of the optical indicatrix of the driving liquid crystal cell is not
perpendicular to the display plane as shown in FIG. 8, but a little
slanted. In consequence, it is desirable that the shorter axis of
the disk-like optical indicatrix B of the optical anisotropic
element shown in FIG. 9 be slanted in response thereto to
compensate for the above inclination.
[0117] Actually, such optical indicatrix as shown in FIG. 9 can be
realized by constituting it with the optical anisotropic element
composed of a layer of optical anisotropic substance having an
array of continuously twisted optical axes or with some material
whose refractive index is smaller in the in-plane direction than in
thickness direction.
[0118] We now attempt to describe how to realize an optical
anisotropic element with negative optical anisotropy composed of
layers of optical anisotropic substance with optical axes in
continuously twisted array.
[0119] Generally the liquid crystal cell performs the display,
changing the polarization direction of the light with visible
wavelength range (from 380 nm to 750 nm under normal conditions) by
the voltage to be applied to the liquid crystal cell.
[0120] In the case of the optical anisotropic element for optical
compensation according to this invention, the optical rotatory
power may be produced depending on the optical conditions of the
optical anisotropic element, because the optical axes of the layer
of optical anisotropic substance are continuously twisted. The
rotatory power in this context means the nature of the light whose
vibrational direction rotates right or left about the advancing
direction as the light progresses through the medium. Suppose now
constant the retardation value of the optical anisotropic element
whose optical axes are continuously twisted. If the twist pitch of
optical axis is long, the light rotates its polarized face in
accord with the twist of optical axes, while the light cannot
follow the twist of optical axes if the twist pitch is too short,
thus the optical rotatory power being not produced. If the optical
rotatory power of the optical anisotropic element is great enough,
the polarized face of the light transmitting through this element
comes to be changed, resulting in reduced contrast, or in some
cases, variations in polarized face due to the wavelength of the
light. From this such problems may arise as coloring of the light
which has transmitted through the optical anisotropic element.
[0121] It is therefore necessary that the optical rotatory power of
the optical anisotropic element to the visible light be smaller
than that of the driving liquid crystal cell to the visible light.
The optical rotatory power largely depends on the wavelength of the
light passing through the medium and on this medium itself. The
largeness of the optical rotatory power is expressed by the degree
of the change in retardation value of the medium to the change in
optical axes.
[0122] Therefore, the largeness of the optical rotatory power of
the driving liquid crystal cell may be expressed by the following
formula:
.DELTA.n1.times.d1/T1=R1/T1 [1.1]
[0123] where R1=.DELTA.n1.times.d1 (retardation value)
[0124] .DELTA.n1: difference of the refractive index no of the
liquid crystal of the driving liquid crystal cell to the ordinary
light from the refractive index to the extraordinary light ne
(=ne-no: refractive index anisotropy)
[0125] d1: thickness of liquid crystal layer
[0126] T1: angle of the twisted array of liquid crystal layer
(twist angle)
[0127] Similarly, the largeness of the optical rotatory power of
optical anisotropic element for compensation can be expressed by
the following formula:
.DELTA.n2.times.d2/T2=R2/T2 [1.2]
[0128] where R2=.DELTA.n2.times.d2
[0129] .DELTA.n2: refractive index anisotropy of the optical
anisotropic substance layer of optical anisotropic element for
compensation
[0130] d2: thickness of the laminated optical anisotropic substance
layer
[0131] T2: total twist angle of the optical axes of the optical
anisotropic substance layers.
[0132] From the formulas [1.1] and [1.2], the largeness
relationship of the optical rotatory power of optical anisotropic
element for compensation with that of the driving liquid crystal
cell can be represented by the following formula:
(R1/T1)>(R2/T2) [1.3]
[0133] The propagation of the light through the optical anisotropic
element whose optical axes of optical anisotropic substance layer
is twisted continuously may be represented by the parameters shown
by the following formula (C. Z Van Doorn, Physics Letters 4, 2A, 7
(1973)).
f=.lambda./(p.times..DELTA.n) [1.4]
[0134] where .lambda.=wavelenth of the light in vacuum (visible
wavelength range)
[0135] p=twist pitch length of optical axes (p=d/T).
[0136] If f<<1, the polarized face of the light in the
optical anisotropic element changes pursuant to the twist angle
acquiring thus the optical rotatory power. As has been described
earlier, the optical anisotropic element is desired to have small
rotatory power and must satisfy the condition f>>1. From
formula [1.4],
p.times..DELTA.n<.lambda. [1.5]
[0137] should hold for the optical anisotropic element.
[0138] Any liquid crystal with extremely large twist angle, namely
with shorter helical pitch is called "cholesteric liquid crystal"
in general. If the product (n.times.p) of the length of the helical
pitch of this liquid crystal, p and the average refractive index of
the cholesteric liquid crystal, n falls into the visible wavelength
range (depending on conditions, 360 nm to 400 nm for the extreme of
short wavelength and 760 nm to 830 nm for long wavelength extreme),
there arises selective scattering (J. L. Fergason; Molecular
Crystals. 1. 293 (1966)). Such a phenomenon is observed not merely
in the cholesteric liquid crystal cell, but also in the optical
anisotropic element whose optical axes of optical anisotropic body
are continuously twisted. If the selective scattering occurs, the
coloring phenomenon of optical anisotropic element produces to
change the display color. This coloring phenomenon can therefore be
prevented if the visible wavelength range excludes the product
n.times.p of the average refractive index of the optical
anisotropic substance layer constituting the optical anisotropic
element and the twisted pitch of the optical index p.
[0139] Further the optical anisotropic element can be made from
laminated retardation film in which optical anisotropy has been
developed by drawing high molecular film, liquid crystal cell with
twisted array, and thin film in which the high molecular liquid
crystals are twist-arranged. In this case the optical anisotropic
element can be had by applying this high molecular layer on at
least one of the substances of the driving liquid crystal cell.
This process facilitates the production and allows to get more
desirable liquid crystal display device. For instance, one can use
such high molecular copolymer as having polysiloxane as principal
chain and suitable proportional ratio of biphenyl benzoate and
cholesteryl group as side chains.
[0140] The similar effects may be given by these optical
anisotropic elements that can be manufactured not merely between
the polarizers and substrates but also in the cells inside the
substrates. For example, the high molecular liquid crystal may be
applied on the inside of the substrate on which the film may be
aligned.
[0141] However, a sheet of optical anisotropic element or plural
sheets of same type is used to realize the required characteristic
that the rotational direction of polarized light in the upper
orientation is reversed in the lower one, the thickness becomes too
great, retardation value too large or the display color altered.
The reason why is that the light transmitting through the optical
anisotropic element produces the birefringence effect with the
rotation of light causing thus the straight polarized light to
become elliptical polarized light. The ellipticity of this
elliptical polarized light, which depends on the wavelength of the
light, causes the transmitting light to be dependent on wavelength,
thereby causing the coloration.
[0142] We then discovered the optical rotatory power and coloration
can be duly dissolved by a combination of an optical anisotropic
element consisting of optical anisotropic units of negative or
positive optical anisotropy with similar element, but of positive
or negative anisotropy which is hybrid-arrayed.
[0143] Now we will describe the positive optical anisotropic
element to be combined with the negative one.
[0144] As was explained earlier with regard to the negative optical
anisotropic element, we will describe a characteristic wherein the
rotational direction of the polarized light in upper orientation is
reversed in the lower one.
[0145] FIG. 24 illustrates how the optical axes of the optical
anisotropic element by this invention has been aligned. FIG. 24a
represents a cross-sectional view of the optical anisotropic
element in an embodiment by this invention. The ellipse depicts the
optical anisotropic bodies LD that constitute the optical
anisotropic element where the longer axis of the ellipse
corresponds to the optical axis OL. The inclination of the longer
axis changes continuously from the electrode of the lower substrate
2b to the electrode of the upper substrate 2a, which is nearly
parallel to the substrate plane in the vicinity of the lower
substrate electrode and substantially vertical in the neighborhood
of the upper substrate electrode (hybrid alignment). FIG. 24b
illustrates an example of the arrangement as viewed from the top.
The arrow in the ellipse indicates the orientation of the optical
axis. Note that the orientation of the respective optical axes in
the layer lies in a same plane, that is, lies justified in a row on
a single axis. FIG. 24c represents an array diagram as observed
obliquely from the Z axis direction. The slanting direction is
expressed by X-Y-Z axes in the figure. FIG. 24d is the same view as
observed from diagonally oblique direction. As is clear from FIGS.
24c and 24d, the alignment of FIG. 24a, if viewed obliquely from
the Z axis direction, twists counterclockwise in the direction of
progress as it goes from lower to upper portion in FIG. 24c, and it
twists counterclockwise in FIG. 24d. Thus, the optical anisotropic
element with optical axes obliquely aligned contributes to the
realization of the foregoing characteristic that the rotational
direction of the polarized light in the upper orientation is
reversed in the lower orientation.
[0146] We now explain how such optical anisotropic element should
be combined with the driving liquid crystal cell to get an
excellent compensation effect.
[0147] FIG. 25a is a schematic diagram of the liquid crystal cell
as shown in FIGS. 3, 4, and 5 with arrows similar to those in FIG.
24. In this FIG. 25a the characters Lip and Lop represent the
polarized axis of incident light and that of outgoing light,
respectively. FIGS. 25a and 25b depict the optical anisotropic
element and the driving liquid crystal cell (TN) to which voltage
equivalent to the half tone (gray scale image) has been applied,
respectively, both viewed from the Z axis direction. FIG. 25c
represents the array of the optical axes of the optical anisotropic
element as projected on the +X axis side from the Z axis side, in
which the polarized light status is shown when the straight
polarized light comes in. In this direction, the optical
anisotropic element has the nature to rotate, right and left, the
polarized plane of the incident light (left rotatory power). FIG.
25d shows the array of the driving liquid crystal cell when viewed
from the direction same as in FIG. 25c. The liquid crystal
molecules are slanted obliquely due to the voltage applied which is
a little higher than the voltage equivalent to the half tone
(critical voltage or threshold voltage for the liquid crystal to
function). If viewed from this direction, there will arise an
alignment portion where the length of the liquid crystal molecule
in the direction of longer axis becomes the same as that in the
direction of shorter axis.
[0148] The incident light therefore transmits without much
rotation, and the direction of the polarized axis Lop of the
outgoing light is nearly same as that of the outgoing light Lip.
This causing the abnormal display called "excessive dark image,"
this darkened display will be improved if the polarized light is
rotated counterclockwise to increase the light rotatory power.
Suited to this is the optical anisotropic element shown in FIG.
25c. This optical anisotropic element having the left rotatory
power, the driving liquid crystal cell may compensate for the
rotational light.
[0149] It can be explained that the same principle in reverse
orientation referring to FIGS. 25e and 25f, which show the array of
the optical axes of optical anisotropic element as observed from -X
axis in Z axis direction. This array has the characteristic to
rotate the axes of the incident light shown clockwise (right
rotatory power). FIG. 25f illustrates the state of voltage
equivalent to the half tone (gray scale image) applied as was the
case with FIG. 25d. The liquid crystal molecules look like slanted,
though not so in reality, which contributes to the generation of
large rotatory power. This causes the abnormal display called
"excessive bright image." Thus, an application of the right
polarized light may control the counterclockwise polarized light to
dissolve the excessive polarized light thereby improving the
excessive brightness. The optical anisotropic element as shown in
FIG. 25e having the right rotatory power, the combination of this
power with the driving liquid crystal cell will enhance the
characteristic.
[0150] We have explained the principle for enlargement of the angle
of field taking an example of the optical anisotropic element with
hybrid array where the optical rotatory power in the direction
slanted to the normal to the surface of the optical anisotropic
element is greater than that in the direction of the normal.
However, the optical anisotropic element with twisted hybrid
alignment or that with the alignment uniformly tilted between the
upper and lower substrates will elicit the characteristic analogous
to that of the optical anisotropic element with hybrid alignment.
One of these optical anisotropic elements may therefore be selected
in terms of the design specification of the liquid crystal display
device. Though we described the principle with TN as an example,
the same principle can apply also to STN and hybrid-aligned nematic
liquid crystal. Thus the principle may contribute to the
enhancement of the viewing angle of the STN.
[0151] Thus, the hybrid array of the optical anisotropic units
showing negative optical anisotropy gives the characteristic that
the optical rotatory power of the element in oblique direction is
greater than that in frontal direction, which has much improving
effects mainly on such abnormal display as "excessive dark image"
and "excessive bright image." This same characteristic can display
similar effect even when the optical anisotropy of the optical
anisotropic substance is positive.
[0152] This invention may improve the visual angle dependency of
the driving liquid crystal cell in every orientation by combining
at least one, positive in optical anisotropy, and one negative
sheet of optical anisotropic layers or elements.
[0153] The foregoing explanation took up, as an example, the case
where the optical anisotropic element is hybrid-aligned in optical
anisotropic units, but the characteristic that the optical rotation
in oblique direction is greater than that in frontal direction is
not limited to such an alignment. The same characteristic, and
consequently the same effect can be had in such an array wherein
the optical axes of the optical anisotropic substance layer
constituting the optical anisotropic element is twisted when viewed
from the direction of the normal to the element or else the
orientation of the optical axes on both end faces of the optical
anisotropic element is one and same with the internal array
changing continuously or stepwise.
[0154] It is preferable to use the ultraviolet-hardening type
liquid crystal which is the liquid crystal provided with such
polymerisable functional group as acryoyloxy group for the
foregoing optical anisotropic element to be realized.
EMBODIMENT 1
[0155] FIGS. 1 and 2 represent the cross-sectional views of the
liquid crystal display device in this embodiment. The liquid
crystal display device consists of two polarizers 1 and 4
(LLC2-92-18: manufactured by SANRITZ) and the liquid crystal cell
2, which uses a liquid crystal element for viewing angle
compensation, and driving liquid crystal cell 3, both held between
the polarizers. The polarizer 1 is a transparent substrate 1b
inside of which is held a polarizer film 1a, and the polarizer 4 is
a similar substrate 4b to which is applied a polarizer film 4a.
[0156] The liquid crystal cell 2 for visual angle compensation as
an optical anisotropic element is provided between the polarizers 1
and 4, and an optical anisotropic layer 2c interposed between the
transparent substrates 2a and 2b.
[0157] On the surface of the substrates (2a and 2b), (SiO.sub.2 is
oblique-evaporated respectively) at different angles, and
introduced therebetween is an optical anisotropic substance layer
which is a discotic liquid crystal
[C.sub.18H.sub.6(OCOC.sub.7H.sub.15).sub.6 with triphenylene core
with alkyl chain by ester bond], that is introduces as an optical
anisotropic substance layer with pretilt angle 30 degrees if nearer
to the driving liquid crystal cell, and 60 degrees if far from it.
The retardation value .DELTA.nd of the optical anisotropic
substance layer used for the liquid crystal cell for compensation
of viewing angle is -570 nm.
[0158] The driving liquid crystal cell 3 which comprises an upper
substrate 3a and a lower substrate 3b having respective transparent
electrodes 3c and 3d connected to a power supply 3E is arranged
between the liquid crystal cell 2 as an optical anisotropic element
and the polarizer 4. Between the two substrates 3a and 3b, a
twisted nematic liquid crystal layer (ZLI-4287, E. Merck Co., Ltd.)
of positive dielectric anisotropy to which is mixed the chiral
agent S811 (of E. Merck Co., Ltd.) is filled. The twisted angle of
the layer is 90 degree which alters the state in response to the
voltage applied from the power supply 3E. The twisted alignment is
maintained when no voltage is applied.
[0159] The difference .DELTA.n of the liquid crystal used for the
driving liquid crystal cell is 0.093, the thickness of the liquid
crystal layer is 5.5 micron. The molecular liquid crystal of the
driving liquid crystal cell 3 is twisted counterclockwise (left
twist) from the lower substrate 3b toward the upper substrate 3a.
This cell 3 functions as TN cell with 90.degree. twist angle and
optically control by optical rotatory power.
[0160] FIG. 2a is an exploded perspective diagram showing the
composition of the liquid crystal display device in this
embodiment. (1.1) and (4.1) represent respectively the transmission
axes of the polarizers 1 and 4, which are orthogonal (1.1) with
each other and arranged at 135.degree. counterclockwise viewed from
the +Z direction which is the normal direction of the substrate to
the Y axis. (3.1) and (3.2) are the rubbing axes; namely, alignment
directions of the upper substrate 3a and lower substrate 3b of the
driving liquid crystal cell 3, which are orthogonal with each other
and arranged at an angle 45.degree. between Y axis and rubbing axis
(3.1) counterclockwise viewed from the +Z direction
[0161] The optical anisotropic elements (2.1) and (2.2) of the
liquid crystal cell 2 for viewing angle compensation are the
rubbing axes of the upper and lower substrates 2a and 2b
respectively, which are orthogonal with each other, and the liquid
crystal cell for visual angle compensation 2 is so arranged that
the rubbing axis (2.2) is parallel to the rubbing axis (3.1) of the
driving liquid crystal cell 3. That is, the optical axis OL (FIG.
6) of the molecular liquid crystal LM is placed along these rubbing
axes to be the optical axes of the liquid crystal layer on the side
on which the liquid crystal layer comes in contact with the rubbed
face of the substrate.
[0162] The polarizer 1 has been so arranged that the transmission
axis (1.1) is perpendicular with the rubbing axis (2.1) of the
liquid crystal cell 2 for viewing angle compensation as an optical
anisotropic element.
[0163] The electrooptic characteristics of the liquid crystal cell
display device of the present constitution were measured in the
coordinate system as shown in FIG. 2b. The voltage in the
measurement (voltage to be applied between the electrodes 3c and 3d
of the driving liquid crystal cell 3 from the drive power supply
3f) was varied from 1 v to 5 V. The results of this measuring are
shown in FIG. 11. This figure, which shows the applied
voltage-transmittance in four orientations (up and down, right and
left) indicating the transmittance when the viewing angle is varied
from front face to 60.degree. by 30.degree.. Idealistically the
transmittance should be identical with the transmittance curve at
the frontal face (visual angle .theta.=0.degree.) at any viewing
angle. In the frontal direction, the transmittance reduces with the
increase of voltage when a certain voltage is exceeded.
[0164] In this embodiment, it is understood referring to FIG. 10
that the characteristic in the lower orientation hardly changes,
that in right and upper orientation worsens and that only in the
left orientation is improved with "reversing" at 60 degrees of
visual angle reduced.
[0165] Such a characteristic is available when the viewing angle
only in particular direction is to be improved as in the cases of
car navigation system or private information terminal.
COMPARISON EXAMPLE 1
[0166] In the first embodiment we measured the
voltage-transmittance characteristic when there existed no liquid
crystal cell 2 for visual angle compensation. The results of this
measurement are shown in FIG. 11. This comparison example revealed
a phenomenon that the display brightens in the upper orientation
and it darkens or the gradation reverses in the lower orientation,
both depending on the angle.
EMBODIMENT 2
[0167] FIG. 12 is an exploded perspective diagram showing the
constitution of the liquid crystal display device in this
embodiment. In the embodiment 1, polyimide AL-1051 (manufactured by
Japan Synthetic Rubber Co., Ltd. ) has been applied on both sides
of the lower substrate 2b contacting with the liquid crystal cell 2
for visual angle compensation which is an optical anisotropic
element. Rubbing has been processed on the surface of this coating.
The pretilt angle is 1.degree. in this case. On the other hand,
vertical alignment is processed on the side contacting with liquid
crystal of the upper substrate 2a. The differential .DELTA.n is
0.039, and the thickness of the liquid crystal layer is 4.4 micron.
The optical axis of the liquid crystal molecule, namely that of the
optical anisotropic element runs parallel on the side of the
driving liquid crystal cell 3 and goes substantially along the
normal of the cell substrate on the side isolated from the liquid
crystal cell 3 continuously changing in the direction of the layer
thickness. The twist angle is 0.degree..
[0168] (1.1) and (4.1) are the transmission axes of polarizers 1
and 4 respectively, which run orthogonal to each other, and (1.1)
is arranged at 135.degree. to the Y axis counterclockwise viewed
from +Z direction. (3.1) and (3.2) are the rubbing axes of the
upper and lower substrates 3a and 3b for the liquid crystal cell 2
for visual angle compensation which run orthogonal to each other
and arranged at 45.degree. of angle with the rubbing axis (3.1) to
the Y axis counterclockwise viewed from +Z direction.
[0169] The optical axis (2.2) of the liquid crystal cell 2 for
visual angle compensation is the rubbing axis of the lower
substrate 2b, which runs orthogonal to the rubbing axis (3.1) of
the upper substrate for the driving liquid crystal cell 3 to be
parallel with the rubbing axis (3.2) of the lower substrate.
[0170] The transmission axis (1.1) of the polarizer 1 has been
provided parallel to the rubbing axis (3.1) of the upper substrate
for the driving liquid crystal cell 3.
[0171] The electrooptic characteristics of the liquid crystal cell
display device of the present constitution were measured in the
coordinate system as shown in FIG. 2b. The voltage in the
measurement (voltage to be applied between the electrodes 3c and 3d
of the driving liquid crystal cell 3 from the drive power supply
3f) was varied from 1 V to 5 V. The result of this measuring are
shown in FIG. 13.
[0172] As is clear from the comparison with the characteristic
diagram 11 in the conventional example, the viewing angle worsens
in the upper and right orientations, but the "reversing" nearly
vanishes in the lower orientation, and the contrast of 60 degrees
of visual angle is improved in the left orientation.
EMBODIMENT3
[0173] In Embodiment 2, the liquid crystal layer of the liquid
crystal cell for compensation of visual angle 2 was put into
twisted alignment with 10 degrees of twist angle. The twist
direction is counterclockwise (left twist) from the lower substrate
2b toward the upper substrate 2a. Any other conditions are exactly
the same as in the Embodiment 2. FIG. 14 shows the
voltage-transmittance characteristic. Though no great difference is
shown from Embodiment 2 due to the twist angle of 10 degrees, the
reversing in right and left orientation increases due to the twist
applied to the liquid crystal cell for compensation of viewing
angle, thereby improving the contrast in the lower orientation.
EMBODIMENT4
[0174] FIG. 15 is an exploded perspective view showing the
configuration of the liquid crystal display device in this
embodiment. The liquid crystal cell for compensation of visual
angle 2, which is the first optical anisotropic element, is same as
the liquid crystal cell for compensation of visual angle 2 in
Embodiment 2.
[0175] The liquid crystal cell for compensation of visual angle 5,
which is the first optical anisotropic element, is of the
construction of the liquid crystal cell for compensation of visual
angle 2, which is however vertically inverted. Polyimide AL-1051
(of Japan Synthetic Rubber Co., Ltd.) is applied on the side of the
plane where the lower substrate 2b of the liquid crystal cell for
compensation of viewing angle 2 and the upper substrate 5a of the
liquid crystal cell for compensation of viewing angle 5 come in
contact with the liquid crystal, and the rubbing is performed on
this polyimide. The pretilt angle is 1 degree. On the other hand, a
homeotropic alignment is processed on the side where the lower
substrate 2a of the liquid crystal cell for compensation of viewing
angle 2 and the upper substrate 5b of the liquid crystal cell for
compensation of viewing angle 5 come in contact with the liquid
crystal.
[0176] The differential .DELTA.nd of the cell for compensation of
viewing angle is -570 nm for both cells. The optical axis of the
liquid crystal molecule, that is, the optical axis of the optical
anisotropic element is parallel to the cell substrate surface on
the side near to the driving liquid crystal cell of the liquid
crystal for compensation of viewing angle 2 and varies continuously
in the direction of the layer thickness to go substantially along
the direction of the normal to the cell substrate on the side
isolated from the liquid crystal cell 4 and reverse to this for
cell 5. The twist angle is 0 degree for both cells.
[0177] (1.1) and (4.1) are the transmission axes of the polarizers
1 and 4, which run orthogonal to each other and are arranged at 135
degrees counterclockwise as viewed from +Z direction against Y
axis. (3.1) and (3.2) are the rubbing axes of the upper and lower
substrates 3a and 3b of the driving liquid crystal cell 3, which
run orthogonal to each other and the angle between the Y axis and
the rubbing axis (3.1) is arranged at 45 degrees counterclockwise
as viewed from +Z direction.
[0178] The optical axis (2.2) of the liquid crystal cell for
compensation of viewing angle 2 is the rubbing axis of the lower
substrate 2b, which is so arranged that it goes orthogonal with the
rubbing axis (3.1) of the upper substrate of the driving liquid
crystal cell 3 and parallel to the rubbing axis (3.2) of the lower
substrate.
[0179] The optical axis (5.1) of the liquid crystal cell for
compensation of viewing angle 5 is the rubbing axis of the upper
substrate 5a, which is so arranged that it goes orthogonal with the
rubbing axis (3.1) of the upper substrate of the driving liquid
crystal cell 3 and parallel to the rubbing axis (3.1) of the lower
substrate.
[0180] The transmission axis (1.1) of the polarizer 1 has been so
arranged that it runs parallel to the rubbing axis (3.1) of the
upper substrate of the driving liquid crystal cell 3.
[0181] The electrooptic characteristic of the liquid crystal
display device by this configuration was measured on the coordinate
system as shown in FIG. 2(b). The voltage at the measurement
(voltage to be applied between the electrodes 3c and 3d of the
driving liquid crystal cell 3 from the drive supply source 3E) was
changed from 1 V to 5 V. FIG. 16 shows the results of this
measurement. Compared with the characteristic diagram FIG. 11 in
the conventional example, the characteristic in left and lower
orientation has worsened, but the "reversing" has vanished in the
right orientation and the "excessive bright image" in the upper
orientation has been improved.
EMBODIMENT5
[0182] FIG. 17 is an exploded perspective view showing the
configuration of the liquid crystal display device in this
embodiment. The liquid crystal cell for compensation 2 which is the
first optical anisotropic element resembles in its structure to the
liquid crystal cell for compensation 2 in Embodiment 2, provided
however that SiO2 is oblique-vaporized on one side of substrate.
The pretilt angle of the lower substrate is 60 degrees. Homeotropic
alignment has been processed on the side of the upper substrate 2a
which comes in contact with the liquid crystal.
[0183] The liquid crystal cell for compensation of viewing angle 5
which is the second optical anisotropic element has the structure
of the liquid crystal cell for compensation of viewing angle 2,
which however is vertically inverted. The differential .DELTA.nd of
the cell for compensation of viewing angle is -180 nm for both
cells. The tilt angle is 0 degree for both cells.
[0184] (1.1) and (4.1) are the transmission axes of the polarizers
1 and 4, which run orthogonal to each other and (1.1) is arranged
at 135 degrees counterclockwise as viewed from +Z direction against
Y axis. (3.1) and (3.2) are the rubbing axes of the upper and lower
substrates 3a and 3b of the driving liquid crystal cell 3, which
run orthogonal to each other and the angle between the Y axis and
the rubbing axis (3.1) is arranged at 45 degrees counterclockwise
as viewed from +Z direction.
[0185] The optical axis (2.2) of the liquid crystal cell for
compensation of viewing angle 2 is so arranged that it runs
orthogonal with the rubbing axis (3.1) of the upper substrate of
the driving liquid crystal cell 3 and parallel to the rubbing axis
(3.2) of the lower substrate in the alignment direction of the
substrate 2b.
[0186] The optical axis (5.1) of the liquid crystal cell for
compensation of viewing angle 5 is so arranged that it goes
orthogonal with the rubbing axis (3.1) of the upper substrate of
the driving liquid crystal cell 3 and parallel to the rubbing axis
(3.2) of the lower substrate in the alignment direction of the
lower substrate 2b.
[0187] The transmission axis (1.1) of the polarizer 1 has been so
arranged that it runs parallel to the rubbing axis (3.1) of the
upper substrate of the driving liquid crystal cell 3.
[0188] The electrooptic characteristic of the liquid crystal
display device by this configuration was measured on the coordinate
system as shown in FIG. 2(b). The voltage at the measurement
(voltage to be applied between the electrodes 3c and 3d of the
driving liquid crystal cell 3 from the drive supply source 3E) was
changed from 1 V to 5 V. FIG. 18 shows the results of this
measurement. Compared with the characteristic diagram FIG. 11 in
the conventional example (Comparative Example 1), the "reversing"
in the lower orientation has worsened, but the "reversing" has
vanished in the right and left orientation and the "excessive
bright image" in the upper orientation has been much improved.
EMBODIMENT6
[0189] FIG. 19 is an exploded perspective view showing the
configuration of the liquid crystal display device in this
embodiment. The liquid crystal cell for compensation of viewing
angle, which is an optical anisotropic element in this embodiment,
consists of two optical anisotropic layers, that are no other than
superposed cells 2 and 5 for compensation of viewing angle in
Embodiment 4. The differential .DELTA.nd is -380 nm.
[0190] FIG. 20 is a cross sectional view of liquid crystal cell 2
for compensation of viewing angle as an optical anisotropic aliment
as viewed from the direction of +X axis. With the substrate 2d
vertically aligned (homeotropic) on both sides as boundaries, the
direction of optical axis as viewed from +Z axis is that of the
rubbing axis (2.1) in the cell 2 shown in FIG. 17 between upper
substrates 2a and 2e, and (2.2) between the lower substrates 2d and
2b. If these rubbing axes (2.1) and (2.2) are placed in a same
direction, the optical axis will lie on a single axis. If they are
placed in different directions, there will be two optical axes as
shown in this embodiment. The optical axes will change continuously
if the liquid crystal layer is twisted.
[0191] The electrooptic characteristic of the liquid crystal
display device by this configuration was measured on the coordinate
system as shown in FIG. 2(b). The voltage at the measurement
(voltage to be applied between the electrodes 3c and 3d of the
driving liquid crystal cell 3 from the drive supply source 3E) was
changed from 1 V to 5 V. Compared with the characteristic diagram
FIG. 11 in the conventional example, the "reversing" and "excessive
dark image" in the lower orientation has been improved, but
substantially no change in any other orientations.
[0192] Several sheets of the optical anisotropic elements by this
invention, as shown in Embodiment 4, give the same characteristics
as those of the optical anisotropic elements used in this
embodiment. Further, the bend and bend-twist alignments of the
liquid crystal with negative optical anisotropy make it possible to
manufacture an optical anisotropic element consisting of an optical
anisotropic layer.
EMBODIMENT7
[0193] FIG. 21 illustrates the configuration of this embodiment. A
biaxial phase difference film 50 is provided between the driving
liquid crystal cell 3 and the liquid crystal cell for compensation
of viewing angle 2 which is the first optical anisotropic element
in Embodiment 2 to compensate for the optical anisotropy with
unstable discotic alignment. Though no remarkable improvement was
given from characteristic viewpoint, irregular display due to poor
alignment could be dissolved.
EMBODIMENT8
[0194] Manufacturing, in Embodiment 2, of a high molecular
copolymer, in which the polysiloxane principal chain includes the
liquid crystal cells 2 and the side chains, are a suitable
proportion of biphenyl benzoate and cholesteryl group, revealed the
characteristics similar to those in Embodiment 2. Preparing an
optical anisotropic element from high molecular copolymer allows to
realize a thinner liquid crystal display device.
EMBODIMENT9
[0195] FIG. 22 represents a cross sectional view of a liquid
crystal display device by this embodiment. FIG. 23 illustrates a
diagram which explains the function of the device. The liquid
crystal display device comprises two polarizers (LLC2-92-18: of
Sanitize make) 1 and 4, between which are held the liquid crystal
cells 2, 5, 6, 7 that are the optical anisotropic elements for
compensation of viewing angle and the driving liquid crystal cell
3.
[0196] The liquid crystal cells for compensation of viewing angle
used as the optical anisotropic elements 2 and 5 are provided
between the polarizer 1 and the driving liquid crystal cell 3. The
optical anisotropy of the optical anisotropic element is
positive.
[0197] The liquid crystal cell structure has the liquid crystal 2c
between the transparent substrates 2a and 2b. The substrate 2a
undergoes vertical alignment (homeotropy) and substrate 2b,
horizontal one. In the horizontal alignment of 2b, rubbing is
performed on the aligned film to give about 2 degrees of tilted
alignment. Used as the liquid crystal 2c is a positive anisotropic
nematic liquid crystal (ZL1-4287, of E. Merck Co., Ltd. make) whose
thickness is 1.9 micrometers and retardation (product of the
optical anisotropy of liquid crystal and thickness of liquid
crystal layer), 180 nm. The optical anisotropic element 7
(substrates 7a and 7b) is anisotropically positive element
identical with the optical anisotropic element 2 and aligned
reversibly.
[0198] The optical anisotropic element 5 comprising the cell for
compensation of viewing angle of negative anisotropy is of liquid
crystal cell structure having the liquid crystal 5c between the
transparent substrates 5a and 5b.
[0199] SiO.sub.2 is oblique-vaporized at certain angle on the
surface of the transparent substrate 3b. Anisotropically negative
discotic liquid crystal [C.sub.18H.sub.6(OCOC.sub.7H.sub.15).sub.6]
with triphenylene core with alkyl chain by ester bond) is
introduced as an optical anisotropic substance layer. The pretilt
angle is 60 degrees if it is near to the driving liquid crystal
cell 4 and 90 degrees if far from it. The retardation is -220 nm.
The optical anisotropic element 6 (substrates 6a and 6b) consists
of the anisotropically negative cell for compensation of viewing
angle identical with that of the optical anisotropic element 5, and
aligned in reverse.
[0200] The optical anisotropic elements 2 and 5, and optical
anisotropic elements 6 and 7 make pairs respectively and so aligned
that anisotropically negative elements do face the driving liquid
crystal cell 3. Namely, the driving liquid crystal cell 3 is
disposed between the anisotropically negative elements 5 and 6.
[0201] The driving liquid crystal cell 3 has a liquid crystal 3c
which is held between the substrate 3a with a transparent electrode
3a1 formed on a color filter and the substrate 3c with a thin-film
transistor and pixel electrode 3b1 formed on each picture element
and forms 90-degree twisted alignment when no voltage is applied.
Introduced into the liquid crystal 3c at 90-degree twist angle is
the nematic liquid crystal (ZL1-4287, of E. Merck Co., Ltd. make)
into which chiral agent S811 (Commercial name, of E. Merck Co.,
Ltd. make) is mingled. This liquid crystal alters its state in
response to the voltage applied from the drive source 4E, which is
a means to apply voltage. It maintains the twisted alignment if no
voltage is applied.
[0202] The differential .DELTA.nd of the liquid crystal used for
the driving liquid crystal cell 3 is 0.093 and the thickness of the
liquid crystal layer is 5.0 micrometers. The liquid crystal
molecule of the driving liquid crystal cell 3 is twisted
counterclockwise (left twist) from the lower substrate 3b toward
the upper substrate 3a. This cell 3 acts as TN cell with 90 degrees
of twist angle and controls the light by optical rotatory
power.
[0203] FIG. 26 is an explicative exploded view showing the
configuration of the liquid crystal display device in this
embodiment, where FIG. 26a is a perspective view, FIG. 26b top
view, and FIG. 26c side view. (1.1) and (4.1) are the absorption
axes of the polarizers 1 and 7, which run orthogonal to each other
and (1.1) is arranged at 45 degrees counterclockwise as viewed,
from +Z direction which is the direction of the normal to the
substrate, against Y axis.
[0204] (3.1) and (3.2) are the rubbing axes of the upper substrate
3a and lower substrates 3b of the driving liquid crystal cell,
namely, the direction of the alignment, which run orthogonal to
each other, and the angle between the X axis and the rubbing axis
(3.1) is arranged at 135 degrees counterclockwise as viewed from +Z
direction.
[0205] The anisotropically positive element 2 has the optical axes
20U of optical anisotropic units which are justified in one and
same orientation and is tilt-aligned at about 2 degrees on the
lower substrate as shown in the side view of FIG. 26c and at about
90 degrees on the upper substrate, being continuously tilted in
between (hybrid alignment). The average of these optical
anisotropic units, that is the average optical axis 20A is disposed
at 225 degrees counterclockwise as viewed from +Z direction against
X axis.
[0206] The anisotropically positive optical anisotropic element 7
provided on the other side of the liquid crystal cell 3 has the
same configuration as the optical anisotropic element 2. The array
of the element 7 is vertically inverted (upside down) array of the
element 2 as viewed from +Z axial direction and its average optical
axis 60A is 135 degrees from +Z axis to X axis.
[0207] On the other hand, the optical axis 50U of the optical
anisotropic units of optical anisotropic element 5 (the optical
axis in this case being defined as the direction with the smallest
refractive index) slants 90 degrees on the upper substrate and 60
degrees on the lower substrate as shown in FIG. 26, and the slant
angle varies therebetween successively. Therefore, the average
optical axis 50A tilts against the element face and its orientation
is -45 degrees from X axis when viewed from +Z axis.
[0208] The anisotropically negative optical anisotropic element 5
has also the same configuration as the optical anisotropic element
3. The array of the element 5 is vertically inverted (upside down)
array of the element 2 as viewed from +Z axial direction and its
average optical axis 60A is the average optical axis 20A which is
45 degrees separated from +Z axis to X axis.
[0209] The electrooptic characteristic of the liquid crystal
display device by this configuration was measured on the coordinate
system as shown in FIG. 2b, but changing the angle of the
observation point, azimuth .phi. and viewing (visual) angle
.theta.. The voltage at the measurement (voltage to be applied on
the liquid crystal layer 4c of the driving liquid crystal cell 3
from the drive voltage 3f) was changed from 1 V to 5 V. FIG. 27
shows the results of this measurement. FIG. 27 indicates the
applied voltage-transmittance characteristics in four orientations
(upper/lower and right/left) except the reduction in transmittance
by the color filter, showing thus the transmittance when the visual
angle is changed by 30 degrees from 0 (front) to 60 degrees. The
idealistic characteristic is that the transmission curve is same at
any angle with that at the front (visual angle .theta.=0).
[0210] FIG. 28 shows the applied voltage-transmittance
characteristic when any optical anisotropic element by conventional
art is not used. The characteristic in the lower orientation is
that the transmittance reduces as the visual angle grows larger.
This means that the "excessive dark image" produces when the tonal
display is practiced. The re-increase of the transmittance at 60
degrees of visual angle is equivalent to the "reversing" in the
actual display. In the upper orientation, the transmittance
increases as the visual angle at 3 V of voltage grows from 0 to 60
degrees. This is equivalent to the "excessive bright image" in the
actual display.
[0211] In this embodiment, FIG. 27 manifests improved
characteristic, in particular, the transmittance at 0 V and at
slanted 60 degrees, exhibiting a higher value except in the upper
orientation, which signifies that a bright display is possible even
in the slanted direction.
[0212] An excellent display could be obtained even at larger angle
when this liquid crystal display element 64 was displayed with 64
gradations.
COMPARISON EXAMPLE2
[0213] The voltage-transmittance characteristic was measured in
Embodiment 9 where there are anisotropically negative elements 3
and 5 only, without positive optical anisotropic elements 2 and 6.
FIG. 29 shows the results of this measurement. In this comparative
example, the characteristic in the lower orientation is improved
more or less, but the transmittance is high and contrast low at 5 V
in the right and left orientations.
COMPARISON EXAMPLE3
[0214] The voltage-transmittance characteristic was measured in
Embodiment 9 shown in FIG. 22, where there are anisotropically
positive elements 2 and 7 only, without negative optical
anisotropic elements 5 and 6. FIG. 30 shows the results of this
measurement. In this comparative example, the transmission is
restricted to low rate in the upper, right and left orientations,
but the transmittance in the lower orientation is marked to be
suddenly lowered, producing thus the reversing and excessive dark
image.
EMBODIMENT10
[0215] A biaxial phase difference film is provided between the
driving liquid crystal cell 3 and the liquid crystal cell for
compensation of viewing angle 2 which is the first optical
anisotropic element in Embodiment 9 to compensate for the optical
anisotropy with unstable discotic alignment. Though no remarkable
improvement was given from characteristic viewpoint, irregular
display due to poor alignment could be alleviated.
EMBODIMENT11
[0216] FIGS. 31 and 32 show the liquid crystal display device by
this embodiment. The liquid crystal display device 10 comprises two
polarizers (LLC2-92-18: of Sanitize make) 1 and 4, between which
are held the liquid crystal cell which is the optical anisotropic
elements for compensation of viewing angle 2 and the driving liquid
crystal cell 3. The polarizers 1 and 4 have respectively a
polarizing film held between their transparent substrates.
[0217] The liquid crystal cells for compensation of viewing angle 2
used as the optical anisotropic elements are provided between the
polarizers 1 and 4.
[0218] The liquid crystal cell structure has the liquid crystal 2c
between the transparent substrates 2a and 2b. SiO.sub.2 is
oblique-vaporized at different angles on the surface of the
transparent substrate 2a and 2b. Anisotropically negative discotic
liquid crystal [C.sub.18H.sub.6(OCOC.su- b.7H.sub.15).sub.6 with
triphenylene core with alkyl chain by ester bond] is introduced
between these substrates as an optical anisotropic substance layer.
The pretilt angle (slant of the optical axis against the substrate
face) is 30 degrees if it is near to the driving liquid crystal
cell 3 and 90 degrees if is far from it. The effective .DELTA.nd of
the optical anisotropic substance layer used in the liquid crystal
cell for compensation of visual angle is -60 nm.
[0219] The driving liquid crystal cell 3 is provided between the
polarizer 4 and the liquid crystal cell for compensation of visual
angle 2. The two substrates, upper substrate 3a and lower substrate
3b, form respectively the transparent electrodes 3a1 and 3b1, which
are connected to the drive source 3E. Charged between the
substrates 3a and 3b is the nematic liquid crystal with
dielectrically positive anisotropy (ZL1-4287, of E. Merck Co. Ltd.
make), which changes its state depending on the voltage applied by
the drive source 3E. It maintains the hybrid alignment when no
voltage is applied.
[0220] The differential .DELTA.nd of the liquid crystal molecule of
the driving liquid crystal cell 3 is 0.093 and the thickness of the
liquid crystal layer is 5.0 micrometers. The liquid crystal
molecule of the driving liquid crystal cell 3 is arrayed changing
its tilt angle, from substantially vertically to horizontally, from
the lower substrate 3b toward the upper substrate 3a, that is
hybrid alignment.
[0221] FIG. 32 is an exploded perspective view showing the
configuration of the liquid crystal display device in this
embodiment.
[0222] (1.1) and (4.1) are the transmission axes of the polarizers
1 and 4, which run orthogonal to each other and (1.1) is arranged
at 45 degrees counterclockwise as viewed from +Z direction which is
the direction of the normal to the substrate, against Y axis. (3.1)
is the rubbing axis of the upper substrate 3a, namely, the
direction of the alignment, and the angle between the Y axis and
the rubbing axis (3.1) is arranged at 135 degrees counterclockwise
as viewed from +Z direction. (3.2) is the alignment direction of
the lower substrate 3b, which substantially coincides with the -Z
direction.
[0223] The (2.1) and (2.2) of the liquid crystal cell for
compensation of visual angle 2, which is an optical anisotropic
element, are the alignment direction of the upper and lower
substrates 2a and 2b respectively as shown in FIG. 1, and the
liquid crystal cell for compensation of visual angle 2 is so
arrayed that its alignment direction (2.2) runs parallel to the
rubbing axis (3.1) of the driving liquid crystal cell 3.
[0224] The polarizer is so arranged that the transmission axis
(1.1) makes an angle of 45 degrees with the rubbing axis (2.1) of
the liquid crystal cell for compensation of visual angle 2.
[0225] The electrooptic characteristic of the liquid crystal
display device by this configuration was measured on the coordinate
system as shown in FIG. 2(b). The voltage at the measurement
(voltage to be applied on the electrodes 3a1 and 3b1 of the driving
liquid crystal cell 3 from the drive voltage 3E) was changed from 0
V to 5 V. FIG. 34 shows the results of this measurement. FIG. 34
indicates the visual angle-luminance characteristic for upper/lower
and right/left gradations showing the respective gradations and
luminance when the visual angle is changed by 10 degrees from the
front (0) to 60 degrees. The idealistic characteristic is that the
transmission curve is same at any angle with that at the front
(visual angle .theta.=0).
[0226] As is understood from FIG. 34, in this embodiment the
reversing is further reduced than TN in any of upper/lower and
right/left orientations. Since the transmittance of black color
hardly increases, an excellent contrast with larger visual angle
can be obtained.
[0227] This invention is also useful when only such particular
angle is to be changed under special conditions as in the car
navigation system.
[0228] Though this specification mentioned only TN type liquid
crystal display device using TFT, it is needless to say that
excellent effects can be had if it is applied to such simple matrix
liquid crystal display device as STN.
* * * * *