Liquid crystal display device with compensation and LC twist angle varying in a nonlinear fashion in the thickness direction

In a liquid crystal display device having a voltage-driven liquid crystal cell sandwiched between two spaced polarizers, the cell comprises a liquid crystal layer having a twisted molecular alignment with no applied voltage and also having an optical rotary power with respect to visible rays. In the vicinity of the cell or in close contact therewith, there is disposed an optically anisotropic layer having an optical rotary power smaller than that of the cell.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention relates to a liquid crystal display device, and more 
particularly to a liquid crystal display device with improved dependence 
characteristics of contrast ratios and display colors upon viewing angles. 
2. Description of the Prior Art: 
Liquid crystal display devices have significant advantages in terms of thin 
size, light weight and low power consumption. They have been widely used 
in various products such as a watch, an electronic calculator, a word 
processor, a personal computer and the like. Most liquid crystal display 
devices employ twisted nematic liquid crystal. Further, simple matrix type 
liquid crystal display devices, which have been currently used in personal 
computers generally have a large display plane (about 10 inch diagonally) 
and large display capacity (e.g., 640.times.480 picture elements). Such a 
liquid crystal display device has a simple configuration that liquid 
crystal having a twisted (180.degree. or more) molecular alignment is 
sandwiched by two spaced glass substrates having transparent striped 
electrodes formed thereon (ST (super twist) mode). In order to realize a 
multiplex drive with a relatively large number of scanning lines by use of 
such a simple configuration, a steepness of electrooptical characteristic 
must be improved. The steepness represents an electrooptical 
characteristic of a liquid crystal cell when an applied voltage value of 
the cell is changed above a threshold voltage. The steepness of 
electrooptical characteristics can be improved by increasing a total twist 
angle (referred to as a twist angle) of a molecular alignment in liquid 
crystal. In practice, a twist angle of a liquid crystal display device in 
the ST mode must be 180.degree. at a minimum. At this twist angle is much 
larger compared to the twist angle of about 90.degree. for the TN-LCD, 
LCDs having such twist angle are referred to as "supertwisted" LCD. 
However, when a twist angle increases to 180.degree. or more, a display 
shows undesirable colors because of a birefringence phenomenon. To prevent 
the undesirable colors, there has been disclosed Japanese Patent 
Publication No. 63-53528 in which an achromatic display image can be 
realized by inserting a second liquid crystal cell, which serves to 
perform optical compensation, between one of polarizers and a first liquid 
crystal cell (which serves to display), the second cell having a molecular 
alignment twisted in a direction reverse to that of the first cell. This 
is based on the principle that light including ordinary ray components and 
extraordinary ray components is changed into elliptically polarized light 
by the first liquid crystal cell in which a liquid crystal molecular 
alignment is twisted. Further, the elliptically polarized light is 
converted into linearly polarized light by the second liquid crystal cell 
in such a manner that the ordinary ray components and the extraordinary 
ray components are replaced with each other. Thus, undesirable coloring, 
which will be caused by a birefringence phenomenon, can be avoided. As a 
result, an achromatic display image can be realized. In order to 
accurately convert elliptically polarized light into linearly polarized 
light, there must be provided the following conditions: 
First, the second liquid crystal cell for optical compensation has a 
retardation value substantially equal to that of the first liquid crystal 
cell for display. Second, twist directions of the molecular alignments of 
both the first and second cells are reversed to each other. Third, their 
molecular alignments, which are in close contact with each other, must 
intersect orthogonally. 
Besides the above-described technique, there have been disclosed various 
techniques to prevent a display image from being undesirably colored. For 
example, retardation films are used in place of a second liquid crystal 
cell. Specifically, several sheets of retardation films are deposited on a 
first liquid crystal cell so that the deposited films have substantially 
the same function as a second liquid crystal cell. 
As described above, even in the supertwisted device, a satisfactory a 
chromatic display image can be obtained when appropriate optical 
compensation has been provided. Further, with a prescribed combination of 
color filters, a satisfactory color display image, which is more 
attractive as a product, can also be obtained. However, in a simple-matrix 
system, display operations are performed by a multiplexed matrix 
addressing. Thus, as the number of scanning lines increases in order to 
increase a display capacity, a difference between a voltage value at which 
light is cut off and a voltage value at which light is transmitted 
decreases significantly. As a result, a contrast ratio and a response 
speed of a liquid crystal display device inevitably deteriorates. Further, 
in a conventional technique, a display image is reversed or completely 
disappears, or is undesirably colored depending on viewing directions and 
angles. These phenomena are essentially disadvantageous to realization of 
a liquid crystal display device with good quality. 
In the case of a liquid crystal display device in an active matrix system, 
switching elements composed of thin-film transistors and diodes are 
provided at respective picture elements. In this system, a voltage value 
at which light is cut off and a voltage value at which light is 
transmitted can be arbitrarily controlled independently of the number of 
scanning lines. Therefore, steepness of electrooptical characteristics of 
liquid crystal need not be significantly high, i.e., a twist angle need 
not be as large as in the case of a liquid crystal display device in the 
ST mode. 
A liquid crystal cell in a TN (twist nematic) mode, whose molecules are in 
an orientation of a twist angle of 90.degree., is inferior to a liquid 
crystal cell in the ST mode in terms of rapidity in electrooptic 
characteristics. However, a liquid crystal cell in the TN mode utilizes 
its optical rotatory power as a display principle. Thus, a high-contrast 
display image can be relatively easily obtained without undesirable 
coloring. Further, response to voltage in the TN mode is quicker than that 
in the ST mode. A combination of the active matrix system and the TN mode 
can realize a liquid crystal display device having a large display 
capacity, a high-contrast ratio, and quick response to voltage. Moreover, 
when prescribed color filters are added to the above-described 
combination, a full-color display image, which is more attractive as a 
product, can be realized. 
However, in this conventional mode, a display image is reversed or 
completely disappears, or is undesirably colored depending on viewing 
directions and angles. These phenomena are significantly disadvantageous 
to realization of a liquid crystal display device of good quality. 
To improve such dependence characteristics of a display image upon viewing 
angles, there has been disclosed Japanese Patent Disclosure No. S62-21423. 
In this application, a liquid crystal cell and a retardation film, which 
is a polymer film having optical anisotropy negative in its thickness, are 
disposed between two spaced polarizers. Further, these has been disclosed 
Japanese Patent Disclosure No. H3-67219, in which a retardation film is 
disposed on a liquid crystal cell. This double refraction layer consists 
of liquid crystal compound (or polymeric liquid crystal) which exhibits a 
cholesteric liquid crystal phase such that the product of a helical pitch 
and a refractive index is 400 nm at a maximum. In these references, the 
consideration has been made only to the case when liquid crystal molecules 
are aligned perpendicularly to substrates of a liquid crystal cell, but 
not to the case when liquid crystal molecules are in a twisted alignment, 
i.e., the case of the TN mode or the ST mode. 
The fundamental principle of display operation for the above-described 
liquid crystal display devices is such that when a voltage is applied to a 
liquid crystal cell, orientations of liquid crystal molecules therein are 
changed so as to cause the liquid crystal cell to be optically changed. 
Thus, if the liquid crystal display device is observed while being 
inclined to the display surface, the orientations of liquid crystal 
molecules are observed inaccurately. As a result, a display image is 
reversed or completely disappears. Particularly in the case of full-color 
display using prescribed color filters, a display image is significantly 
deteriorated. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide a liquid 
crystal display device in which dependence characteristics of contrast 
ratios and display colors upon viewing angles have been improved. 
Briefly, in accordance with one aspect of the present invention, there is 
provided a liquid crystal display device which comprises two polarizers 
spaced apart from each other; a driving liquid crystal cell disposed 
between the two polarizers, the cell having two substrates with electrodes 
and a liquid crystal layer sandwiched therebetween, the liquid crystal 
forming a molecular twisted alignment with no voltage applied to the 
electrodes; and 
at least one optically anisotropic layer having at least one opticaly 
anisotropic media whose optical axis is continuously-twisted alignment and 
in substantially perpendicular to a substrate surface of the driving 
liquid crystal cell, the optically anisotropic layer having optical 
rotatory power smaller than that of the liquid crystal layer of the 
driving liquid crystal cell with respect to visible rays. 
In accordance with another aspect of the present invention, there is 
provided a liquid crystal display device in which a retardation value 
R.sub.1 [nm] and a twist angle T.sub.1 [deg] of the driving liquid crystal 
and a retardation value R.sub.2 [nm] and a twist angle T.sub.2 [deg] of 
the optically anisotropic layer have the following relationship: 
EQU (R.sub.1 /T.sub.1)&gt;(R.sub.2 /T.sub.2). 
In accordance with another aspect of the present invention, there is 
provided a liquid crystal display device in which a value obtained by 
multiplying .DELTA.n by P is smaller than a value in a range of visible 
ray wavelengths, where .DELTA.n represents refractive-index anisotropy and 
P represents a helical pitch, both of the optically anisotropic material 
of the optically anisotropic layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, embodiments according to the present invention will be 
described which are capable of achieving desirable contrast ratios and 
which can be arbitrarily determined to be in particular orientations and 
viewing angles. 
In a liquid crystal display device of a TN mode or a ST mode, a 
polarization state of light transmitted through the liquid crystal display 
device differs depending on incidences (i.e., whether or not an incident 
angle is a right angle) with respect to a display surface of the liquid 
crystal display device. The difference of polarization states has direct 
effects upon occurrence of undesirable phenomena such as inversion and 
coloring of a display image. As a viewing angle with respect to the normal 
line on the display surface of the liquid crystal display device 
increases, particular regions having such undesirable phenomena increase. 
Particularly, these phenomena can be significantly observed at respective 
picture elements when a voltage is applied to a driving liquid crystal 
cell. 
FIG. 2 shows dependence characteristics of contrast ratios upon viewing 
angles in the case of a conventional TN-type liquid crystal display 
device. In the figure, the viewing angles are in a range of 60.degree. 
(left) to 60.degree. (right) through 0.degree. from a normal on the 
display surface of the liquid crystal display device. 
Further, the curve (10.0) indicates the case of a normally-open mode, and 
the curve (11.0) indicates the case of a normally-close mode. The 
normally-open mode is such that a bright state is obtained when no voltage 
is applied to the driving liquid crystal cell, and a dark state is 
obtained when a voltage is applied to the driving liquid crystal cell. To 
the contrary, the normally-close mode is such that a dark state is 
obtained with no applied voltage, and a bright state is obtained with an 
applied voltage. As can be seen from FIG. 2, contrast ratios in the case 
of the normally-close mode are less dependent upon viewing angles than 
those in the case of the normally-open mode. A contrast ratio is a value 
obtained by dividing an intensity of a bright state by an intensity of a 
dark state. Therefore, a contrast ratio is changed depending on an 
intensity of a dark state. 
FIG. 3 shows dependence characteristics of intensities of a dark state upon 
viewing angles. In FIG. 3, the curve (10.1) indicates the case of a 
normally-open mode, and the curve (11.1) indicates the case of a 
normally-close mode. As can be seen from FIG. 3, intensities of a dark 
state in the case of the normally-close mode are less dependent upon 
viewing angles than those in the case of the normally-open mode. In other 
words, contrast ratios in the case of the normally-close mode are less 
dependent upon viewing angles than those in the case of the normally-open 
mode. 
When a difference between dark states in the normally-open mode and in the 
normally-close mode is studied, the following phenomena can be considered. 
Specifically, in the case of the normally-open mode, a voltage is applied 
to the liquid crystal cell to obtain a dark state. In this case, it can be 
roughly assumed that liquid crystal molecules are in a perpendicular 
alignment with respect to the substrate surface of the cell. In the case 
of the normally-close mode, a dark state can be obtained with no applied 
voltage. Thus, liquid crystal molecules are in a horizontally twisted 
alignment with respect to the cell substrate. Therefore, it is understood 
that the difference of viewing-angle characteristics in these modes is 
derived from a difference in molecular alignment states of the liquid 
crystal cells, i.e., viewing-angle characteristics in the perpendicular 
alignment state are inferior to those in the horizontally twisted 
alignment state. 
FIG. 4 shows an index surface which schematically represents a liquid 
crystal molecule in the perpendicular alignment state. 
A z-axis corresponds to a thickness-direction of the liquid crystal cell, 
and an x-y plane corresponds to a substrate surface of the liquid crystal 
cell. A birefringence phenomenon can be represented by a shape of a 
cross-sectional plane formed when an index surface 6 is cut by a plane 
including a normal which is on the center point of the index surface 6. 
The formed cross-sectional plane (which is an ellipse in most cases except 
when a viewing axis (6.1) is accurately on the z-axis) is called a 
refractive-index shape in a two-dimensional plane. A difference between 
the lengths of a longitudinal axis and a traverse axis of the 
refractive-index shape corresponds to a phase difference between an 
ordinary ray and an extraordinary ray. 
Assume that absorption axes of two spaced polarizers (which sandwich a 
liquid crystal cell) orthogonally intersect each other. In this case, if a 
phase difference between an ordinary ray and an extraordinary ray is zero, 
light to be transmitted through the liquid crystal cell is cut off. 
Further, if a phase difference between an ordinary ray and an 
extraordinary ray is not zero, light can be transmitted through the liquid 
crystal cell in accordance with this phase difference and a wavelength of 
an incident light. In the case of light incident on the liquid crystal 
cell perpendicularly to the substrate surface thereof (i.e., when the 
liquid crystal cell is observed from a right front), a refractive-index 
shape (6.5) in a two-dimensional plane becomes a circle (6.4). 
Consequently, a phase difference between an ordinary ray and an 
extraordinary ray becomes zero. In the case of light incident on the 
liquid crystal cell with an inclined viewing axis (6.1), a 
refractive-index shape (6.5) becomes an ellipse so that a phase difference 
between an ordinary ray and an extraordinary ray occurs. A polarization 
state of light which penetrates through the liquid crystal cell differs 
depending on a viewing axis (6.1). In order to improve viewing-angle 
characteristics of the liquid crystal cell, it is important to improve an 
index surface of the liquid crystal cell with applied voltage. 
More specifically, in FIG. 4, as a viewing angle (6.3) increases, a 
refractive-index shape (6.5) increases in a direction of n.sub.61 as does 
the amount of transmitted light. It is ideal that a shape of a 
refractive-index area in a two-dimensional plane stay the same even when a 
viewing angle was different. This can be substantially realized by 
performing the optical compensation as follows: A disc-shaped index 
surface shown in FIG. 7 is disposed on the z-axis of an index surface 6 
(i.e., to be disposed at a position adjacent to the upper or lower 
substrate of the liquid crystal cell). In this configuration, as a viewing 
angle (6.3) increases, a refractive-index shape (6.5) in a two-demensional 
plane of the index surface 6 increases. However, at the same time, a 
refractive-index shape in a longitudinal direction of n.sub.62 can also be 
increased. As a result, a synthesized refractive-index shape in a 
two-dimensional plane becomes a circle. This means that the index surface 
6 can be optically compensated. Thus, viewing-angle characteristics can be 
improved. 
In practice, the index surface shown in FIG. 7 can be realized by an 
optically anisotropic layer, e.g., consisting of an optically anisotropic 
material, whose optical axis is in a continuously twisted alignment. 
In general, a driving liquid crystal cell performs displaying operation by 
means of, in accordance with an applied voltage, positively changing a 
polarization direction of light in a range of visible wavelengths (i.e., a 
range of 380 nm through 750 nm). On the other hand, in the case of an 
optically anisotropic layer for optical compensation according to the 
present invention, optical rotatory power occurs depending on optical 
conditions of the optically anisotropic layer. This is because the optical 
axis of the optically anisotropic material layer is continuously twisted. 
Optical rotatory power represents properties of a medium, the properties 
being such that as light progresses, in the medium a vibration direction 
of the light rotates clockwise or counter clockwise about an axis which is 
a light-progressing direction. Assume that a retardation value of an 
optically anisotropic layer, whose optical axis is continuously twisted, 
is constant. In this case, if the optical axis has a long twist pitch, 
light rotates its polarizing plane in accordance with the twist of the 
optical axis. However, if the optical axis has a short twist pitch, light 
cannot follow the twist of the optical axis, and consequently an optical 
rotatory phenomenon does not occur. When optical rotatory power of an 
optically anisotropic layer is large, a polarizing plane of light which 
transmits through the layer is inevitably changed. As a result, a contrast 
ratio is inevitably decreased. In some cases, a polarizing plane is 
variously changed by wavelengths of light which penetrate through an 
optically anisotropic layer, and transmitted light is undesirably colored. 
Therefore, optical rotatory power with respect to visible rays of an 
optically anisotropic layer must be smaller than that of a driving liquid 
crystal cell. 
Optical rotatory power significantly depends on wavelengths of light which 
penetrates through a medium and also on inherent properties of this 
medium. The amount of optical rotatory power can be expressed by the 
degree of change in a retardation value of a medium with respect to the 
change of an optical axis. Therefore, the amount of optical rotatory power 
of a driving liquid crystal cell can be expressed by the following 
equation: 
EQU .DELTA.n.sub.1 .multidot.d.sub.1 /T.sub.1 =R.sub.1 /T.sub.1(1.1) 
where R.sub.1 (=.DELTA.n.sub.1 .multidot.d.sub.1) represents a retardation 
value, .DELTA.n.sub.1 (=n.sub.e -n.sub.o) represents refractive-index 
anisotropy, which is a difference between a refractive index n.sub.o with 
respect to an ordinary ray and a refractive index n.sub.e with respect to 
an extraordinary ray both of liquid crystal of the driving liquid crystal 
cell, d.sub.1 represents thickness of a liquid crystal layer of the 
driving liquid crystal cell, and T.sub.1 represents a twist angle of a 
twisted molecular alignment in the liquid crystal layer. Similarly, the 
amount of optical rotatory power of a viewing-angle compensating optically 
anisotropic layer can be expressed by the following equation: 
EQU .DELTA.n.sub.2 .multidot.d.sub.2 /T.sub.2 =R.sub.2 /T.sub.2(1.2) 
where R.sub.2 (=.DELTA.n.sub.2 .multidot.d.sub.2) represents a retardation 
value, .DELTA.n.sub.2 represents refractive-index anisotropy of an 
optically anisotropic material of the viewing-angle compensating optically 
anisotropic layer, d.sub.2 represents thickness of a deposited optically 
anisotropic material layer, and T.sub.2 represents a total twist angle of 
the optically anisotropic material layer. 
Therefore, the quantitative relationship of optical rotatory power between 
the viewing-angle comensating optically anisotropic layer and the driving 
liquid crystal cell can be expressed by the following equation: 
EQU (R.sub.1 /T.sub.1)&gt;(R.sub.2 /T.sub.2) (1.3) 
Transmission of light through an optically anisotropic layer, i.e., an 
optically anisotropic material, whose optical axis is continuously twisted 
can be expressed by parameters indicated by the following equation (C. Z. 
Van Doorn, Physics Letters 42A, 7 (1973)): 
EQU f=.lambda./(P.times..DELTA.n) (1.4) 
where .lambda. represents a wavelength (in a range of visible wavelengths) 
of light in a vacuum, and P represents a helical pitch (P=d/T) of an 
optical axis. 
When f&lt;&lt;1, light in an optically anisotropic layer changes its polarizing 
plane in accordance with a twist angle of an optical axis of the layer, 
i.e., there exists significant optical rotatory power in the layer. As 
described above, it is preferable for an optically anisotropic layer to 
have optical rotatory power less than that of a driving liquid crystal 
cell. Further, the optically anisotropic layer must satisfy a condition of 
f&gt;&gt;1. Therefore, the following equation must be held for an optically 
anisotropic layer: 
EQU P.times..DELTA.n&gt;.lambda. (1.5) 
EQU (see equation (1.4)) 
Liquid crystal, which has a very large total twist angle, i.e., a very 
short helical pitch, is generally called cholesteric liquid crystal. 
Assume that a value of n.times.p (the product of an averaged refractive 
index n and a helical pitch p) of cholesteric liquid crystal exists in a 
range of visible wavelengths (shortest wavelength=360 nm through 400 nm, 
and longest wavelength=760 nm through 830 nm, which differ depending on 
conditions). In this case, selective reflection occurs (J. L. Fergason; 
Molecular Crystal. 1. 293. (1966).) This selective reflection is not a 
phenomenon observed only in cholesteric liquid crystal, but can also be 
observed in an optically anisotropic layer consisting of an optically 
anisotropic material whose optical axis is continuously twisted. When 
selective reflection occurs in an optically anisotropic layer, a coloring 
phenomenon occurs therein, and display colors are undesirably changed. 
Therefore, a value of n.times.p (the product of an averaged refractive 
index n and an optical-axis twist pitch P, both of an anisotropic material 
layer constituting an optically anisotropic layer) is determined to be out 
of a range of visible wavelengths so as to prevent an undesirable coloring 
phenomenon. 
Hereinbefore, the principle of viewing-angle expansion or viewing-angle 
control according to the present invention have been conceptually 
described. However, in practice, the above-described index surface of the 
driving liquid crystal cell (when a voltage more than a threshold voltage 
is applied thereto) is not a simple ellipsoid as shown in FIG. 4. Actual 
calculations have been made as to a molecular alignment state in a driving 
liquid crystal cell (with applied voltage to be in a dark state) in the TN 
mode. The results the calculations are shown in FIG. 5. In FIG. 5, the 
curve 7 indicates a tilt angle, and the curve 8 indicates a twist angle. 
In FIG. 6, a tilt angle 7 is an angle of the direction axis (8.1 L) of a 
liquid crystal molecule (8.1) tilting with respect to an X-Y plane which 
is a display plane of a liquid crystal cell. Further, in FIG. 6, a twist 
angle 8 is an angle constituted by an X-axis and an axis obtained by 
projecting the liquid crystal molecule (8.1) from a Z-axis onto the X-Y 
plane. 
When a voltage is applied to the liquid crystal cell, liquid crystal 
molecules in the vicinity of the center of the cell tilt by about 
90.degree.. However, liquid crystal molecules near the upper and lower 
surfaces of the cell substrates do not tilt so significantly. This is 
because the liquid crystal molecules near the surfaces have been affected 
by an anchoring effect of the substrate surfaces of the cell. Further, the 
twist angle 8 becomes an S-shaped distribution, as indicated by the curve 
8a shown in FIG. 5. As can be seen from FIG. 5, a molecular alignment in 
the liquid crystal cell with an applied voltage does not become a perfect 
perpendicular alignment state. This molecular alignment state in the 
liquid crystal cell has significant effects on viewing-angle 
characteristics. FIG. 8a is a graph illustrating viewing-angle 
characteristics of a dark state in a normally-open mode of a TN-LCD (when 
a voltage more than a threshold voltage is applied thereto). In FIG. 8a, 
four different curves respectively indicate measurements of transmission 
of a liquid crystal cell measured at four different direction angles 
.phi., i.e., 0.degree. (right direction), 90.degree. (upper direction), 
180.degree. (left direction), and 270.degree. (lower direction) with 
variation of viewing angles in a range of 0.degree. through 60.degree. 
(see a coordinate system shown in FIG. 10). As can be seen from FIG. 8a, 
viewing angle versus transmission curves of upper and lower directions do 
not coincide with those of left and right directions. Further, 
transmission of upper and lower directions at the same viewing angles 
differ significantly. On the other hand, in the case of a liquid crystal 
cell having perpendicularly aligned molecules, viewing angle versus 
transmissivity curves of upper and lower directions substantially coincide 
with those of left and right directions, as shown in FIG. 8b. Thus, in the 
case of the liquid crystal cell having a twisted molecular alignment with 
an applied voltage, an index surface of the cell does not become a simple 
shape as shown in FIG. 4, but becomes a shape obtained by deforming the 
shape of FIG. 4. This deformation is caused by the inclination of liquid 
crystal molecules in the vicinity of the center of the liquid crystal 
cell, and the twisted molecular alignment near the substrate surface of 
the liquid crystal cell. Therefore, an optically compensating index 
surface, which is used for a driving liquid crystal cell in a TN mode or 
in an ST mode, is intentionally deformed into a slightly complicated shape 
so as to accord with an index surface of the driving liquid crystal cell, 
as shown in FIG. 7. 
FIGS. 8c through 8e are graphs illustrating viewing-angle characteristics 
of three different optically anisotropic layers, each being disposed 
between two spaced polarizers having transmission axes that orthogonally 
intersect each other. The measurements have been made in the same manner 
as in the cases shown in FIGS. 8a and 8b. All of the measured optically 
anisotropic layers are made of chiral nematic liquid crystal, and have 
liquid crystal layers of 12 .mu.m thick in common. However, as for a 
helical pitch and a twist angle are different as follows. Specifically, 
optical-axis twist pitches are: 
0.248 .mu.m in the case of FIG. 8c, 
0.738 .mu.m in the case of FIG. 8d, and 
5.3 .mu.m in the case of FIG. 8e. 
Further, twist angles, when expressed by the number of rotations are: 
48,25 rotations in the case of FIG. 8c, 
16,25 rotations in the case of FIG. 8d, and 
2,25 rotations in the case of FIG. 8e. 
In FIGS. 8c through 8e, the broken line with x mark curves indicate the 
results of measurement in upper and lower directions, and the line with o 
mark curves indicate the results of measurement in left and right 
directions, respectively. As can be seen from FIGS. 8c through 8e, when 
the twist angles are small, transmission of the respective directions 
differ significantly. Further, when the twist angles are large, 
transmission of the respective directions coincide with each other. This 
means that a refractive-index ellipsoid of an optically anisotropic layer 
becomes a perfect disc-shape when a twist angle is large, but becomes a 
deformed disc-shape when a twist angle is small. 
The advantages of improving viewing-angle characteristics according to the 
present invention can be obtained in all the cases of FIGS. 8c through 8e. 
However, as described above, an index surface of a driving liquid crystal 
cell is deformed when a voltage is applied to the cell. Thus, the 
advantages of this invention can be obtained more effectively when an 
index surface of an optically anisotropic layer, which is deposited on a 
driving liquid crystal cell, is also deformed correspondingly to 
deformation of a refractive-index ellipsoid of the driving liquid crystal 
cell. Specifically, it is more preferable to employ optically anisotropic 
layers having characteristics of FIGS. 8d and 8e, in which the numbers of 
rotations (representing the twist angles) are relatively smaller. As 
described above, when the number of rotation in an optically anisotropic 
layer becomes smaller, a helical pitch P becomes longer. On the other 
hand, a refractive index n of optically anisotropic layer can be regarded 
as n=1.5. Thus, it is more preferable for a value of n.times.P to be 
greater than a value of the longest wavelength in a range of visible 
wavelengths. Further, the advantages of this invention are changed 
depending on optical conditions of an optically anisotropic layer such as 
a retardation value (.DELTA.n.multidot.d: the product of refractive-index 
anisotropy by thickness), the number of rotations, a tilt angle and a 
rotational direction both of a twisted optical axis, and a direction of an 
optical axis of optically anisotropic material constituting an optically 
anisotropic layer. Particularly, as for a rotational direction of an 
optical axis, the advantages of this invention are changed depending on 
whether a rotational direction of an optical axis of an optically 
anisotropic layer and a twisting direction of liquid crystal molecules in 
a driving liquid crystal cell are equal to each other or reverse to each 
other. These changes in advantages of this invention will be later 
described in detail referring to various embodiments. 
Hereinbefore, a driving liquid crystal cell in the TN mode has been 
described as an example. However, the optical comensation technique of 
this invention is based on the principle of optical anisotropy. Thus, as 
for a driving liquid crystal cell, the same advantages of this invention 
as described above can be naturally obtained not only in the case of the 
TN mode (with a twist angle of about 90.degree.) but also in the case of 
the ST mode (with a twist angle of 180.degree. or more). Specifically, the 
advantages of this invention can be obtained in either case as long as a 
molecular alignment state of a driving liquid crystal cell (when a voltage 
more than a threshold voltage is applied to the cell) is such that tilt 
angles of liquid crystal molecules near the substrate surface of the cell 
and in the vicinity of the center of the cell differ from each other, and 
twist angles of liquid crystal molecules are aligned in a nonlinear 
fashion with respect to the thickness of the liquid crystal layer. 
Further, the optically anisotropic layer can be realized by depositing 
retardation films which have been manufactured by extending polymer films 
so as to produce opto-anisotropy therein. Moreover, the optically 
anisotropic layer can also be realized by use of a liquid crystal cell 
having a twisted molecular alignment, or by use of a thin film having a 
twisted alignment of polymeric liquid crystal. In this case, polymeric 
liquid crystal is applied to a surface of at least one of upper and lower 
substrates of a driving liquid crystal cell, so that a satisfactory liquid 
crystal display device can be easily manufactured. For example, 
polysiloxane can be used as a main chain, and polymer copolymer liquid 
crystal having biphenylbenzoate and cholesteril-radical in an appropriate 
proportion can be used as a side chain. 
FIG. 9 is a cross-sectional view illustrating a liquid crystal display 
device of this embodiment according to the present invention. In FIG. 9, a 
liquid crystal display device 10 comprises two spaced polarizers 1 and 4 
(LLC 2-92-18: manufactured by SANRITZ Co., Ltd.), and a viewing-angle 
compensating liquid crystal 2 and a driving liquid crystal 3, the cells 2 
and 3 being sandwiched between the polarizer 1 and 4. The polarizers 1 
comprises a polarizing film 1a adhered to inner sides of two transparent 
substrates 1b. Similarly, the polarizer 4 comprises a polarizing film 4a 
adhered to inner sides of two transparent substrates 4b. 
Specifically, the viewing-angle compensating liquid crystal cell 2 is 
disposed between the polarizer 1 and the driving liquid crystal cell 3, as 
shown in FIG. 1. The viewing-angle compensating liquid crystal cell 2 
comprises two spaced transparent substrates 2a and 2b, and liquid crystal 
2c introduced into a space therebetween. The liquid crystal 2c is a 
mixture of twist nematic liquid crystal (ZLI-2806; manufactured by E. 
Merck Co., Ltd.) and chiral dopant (S811; manufactured by E. Merck Co., 
Ltd.). More specifically, the liquid crystal 2 is introduced into the 
space between upper and lower substrates 2a and 2b with a total twist 
angle of 990.degree.. The liquid crystal molecules of the liquid crystal 2 
are in an alignment twisted counterclockwise from the lower substrate 2b 
to the upper substrate 2a (counterclockwise-twist). The liquid crystal 
material used for the viewing-angle compensating liquid crystal cell 2 has 
refractive-index anisotropy .DELTA.n of 0.039, and a twist pitch P of 3.27 
.mu.m, and constitutes a liquid crystal layer of 9 .mu.m thick. 
The driving liquid crystal cell 3 is disposed between the viewing-angle 
compensating liquid crystal cell 2 and polarizer 4. The driving liquid 
crystal cell 3 comprises an upper substrate 3a having transparent 
electrode 3c formed thereon, and a lower substrate 3b having a transparent 
electrode 3d formed thereon. Further, the driving liquid crystal cell 3 
comprises liquid crystal 3e introduced into a space between the upper and 
lower substrates 3a and 3b. The electrodes 3c and 3d are connected to a 
drive power source 3f. The liquid crystal 3e is such that a mixture of 
twist nematic liquid crystal (ZLI-4287; manufactured by E. Merck Co., 
Ltd.) and chiral dopant (S811; manufactured by E. Merck Co., Ltd.) is 
introduced into the space between the upper and lower substrates 3a and 3b 
with a twist angle of 90.degree.. A state of the liquid crystal 3e is 
changed in accordance with a voltage supplied from the drive power source 
3f. The liquid crystal 3e has refractive-index anisotropy .DELTA.n of 
0.093 and constitutes a liquid crystal layer of 5.5 .mu.m thick. Further, 
liquid crystal molecules are in an alignment twisted counterclockwise from 
the lower substrate 3b to the upper substrate 3a (counterclockwise twist). 
FIG. 1 is an exploded perspective view illustrating a configuration of a 
liquid crystal display device of this embodiment according to the present 
invention. In FIG. 1, transmission axes (1.1) and (4.1) of the polarizers 
1 and 4 are perpendicular to each other. Further, the transmission axis 
(1.1) is disposed on an imaginary line which deviates counterclockwise 
from a y-axis by an angle of about 135.degree. when observed from a +z 
direction. In FIG. 1, rubbing axes (3.1) and (3.2) of the upper and lower 
substrates 3a and 3b of the driving liquid crystal cell 3 are in 
perpendicular to each other. Further, the rubbing axis (3.1) is disposed 
on an imaginary line which deviates counterclockwise from the y-axis by an 
angle of about 45.degree. when observed from the +Z direction. 
In FIG. 1, rubbing axes (2.1) and (2.2) of the upper and lower substrates 
2a and 2b of the viewing-angle compensating liquid crystal cell 2 are 
perpendicular to each other. Further, the viewing-angle compensating 
liquid crystal cell 2 is disposed in such a manner that the rubbing axis 
(2.1) is parallel to the rubbing axis (3.1). The polarizer 1 is disposed 
in such a manner that the transmission axis (1.1) is parallel to the 
rubbing axis (2.1) of the viewing-angle compensating liquid crystal cell 
2. 
The optical rotatory power of the driving liquid crystal cell 3 is 
calculated by use of the equation (1.1) as follows: 
EQU R.sub.1 /T.sub.1 =0.5115 .mu.m/90.degree.=5.6833 [nm/deg] 
Similarly, the optical rotatory power of the viewing-angle compensating 
liquid crystal cell 2 is calculated by use of the equation (1.2) as 
follows: 
EQU R.sub.2 /T.sub.2 =0.351 .mu.m/990.degree.=0.3545 [nm/deg] 
When both the values are compared, the following relationship is held: 
EQU (R.sub.1 /T.sub.1)&gt;(R.sub.2 /T.sub.2) 
Specifically, the optical rotatory power of the viewing-angle compensating 
liquid crystal cell 2 is less than 1/10 of the optical rotatory power of 
the driving liquid crystal cell 3. 
The viewing-angle characteristics of the above-described liquid crystal 
display device of this embodiment are measured by use of the coordinate 
system shown in FIG. 10. In measurement, the following prescribed voltages 
are supplied from the drive power source 3f so as to apply the same 
between the electrodes 3c and 3d of the driving liquid crystal cell 3. 
Specifically, the applied voltages are 1 V in the case of a bright state, 
and 5 V in the case of a dark (where a threshold voltage of the liquid 
crystal is 2 V). The results of measurement are expressed by use of polor 
coordinates shown in FIG. 11. In FIG. 11, Iso-contrast characteristics are 
expressed in the following manner. Specifically, the abscissa represents 
the values of .theta. (e.g., 20.degree., 40.degree. and 60.degree.), and 
an angle .phi. is represented by 90.degree., 180.degree. and 270.degree., 
for example. Further, numerals (such as 5, 10, 30 and 50) adjacent to the 
respective curves represent the contrast ratios. For example, the curve 
with the numeral 10 indicates an Iso-contrast curve which represents that 
a ratio of brightness and darkness is 10:1. For instance, a point A 
represents that a contrast ratio is 10:1 when observed at 
.theta.=50.degree. and .phi.=60.degree.. As can be seen from FIG. 11, 
viewing angles with respect to directions of 0.degree. through 
180.degree., which are mainly practical range, are larger than viewing 
angles in Iso-contrast characteristics of a conventional liquid crystal 
cell shown in FIG. 12 (which will be later described). Thus, in this 
embodiment, dependence characteristics of contrast ratios upon viewing 
angles have been improved. As for display color, in the case of a 
conventional liquid crystal display device, a display color in a dark 
state was changed depending on viewing angles. However, in this 
embodiment, a satisfactory black display color in a dark state was 
invariably obtained even when viewing angles were changed. Experimentally, 
a 10-inch (diagonal size) TFT-LCD (thin-film transistor-liquid crystal 
display) was manufactured by use of a liquid crystal display device of 
this embodiment with a color filter provided therein. As a result, there 
was obtained a satisfactory full-color display device capable of 
discriminating its display contents independently of change of direction 
and viewing angles. 
COMISON EXAMPLE 
For the sake of comparison, Iso-contrast characteristics in the case of a 
liquid crystal display device without a viewing-angle compensating liquid 
crystal cell 2 were measured (other elements were the same as those in the 
embodiment 1). In this case, viewing angles were small in directions of 
0.degree. through 50.degree. and 130.degree. through 180.degree.. Further, 
a display color in this case was undesirably colored when viewing angles 
were changed. 
EMBODIMENT 2 
This embodiment differs from the embodiment 1 in that a viewing-angle 
compensating liquid crystal cell 2 is modified in the following manner. 
Specifically, the viewing-angle compensating cell 2 was replaced by a 
polymeric liquid crystal consisting of polysiloxane as a main chain and 
both of biphenylbenzoate and cholesteril-radical as a side chain. The 
polymeric liquid crystal had refractive-index anisotropy .DELTA.n of 0.20, 
a helical pitch P of 3.273 .mu.m, and layer thickness d of 1.76 .mu.m. 
Further, the number of helical rotations was 2.75 with a twist angle of 
990.degree., and a twist direction was counterclockwise. The values of the 
chiral pitch P and the layer thickness d were determined by a condition in 
which the value of .DELTA.n.times.P and .DELTA.n.times.d consistent with 
one of the compensating liquid crystal cell 2 in embodiments each other. 
The viewing-angle characteristics of the above-described liquid crystal 
display device of this embodiment were measured in the same manner as in 
the embodiment 1. The results of measurement are shown in FIG. 13. As can 
be seen from FIG. 13, the Iso-contrast characteristics of the display 
device in this embodiment are substantially equal to those in the 
embodiment 1. This is because the values of .DELTA.n.times.P and 
.DELTA.n.times.d have been determined to the same as those in the 
embodiment 1. Thus, a contrast ratio of 32:1 or more was obtained in 
directions of 0.degree. through 180.degree. in a 30.degree.-cone, and the 
viewing angles were expanded. 
EMBODIMENT 3 
This embodiment differs from the embodiment 1 in that the positional 
relationship of rubbing axes are changed in the following manner. 
Specifically, there were provided four different liquid crystal display 
devices such that an angle .omega. constituted by a rubbing axis (2.2) of 
a lower substrate of a viewing-angle compensating liquid crystal cell 2 
and a rubbing axis (3.1) of an upper substrate of a driving liquid crystal 
cell 3 was changed into four different angles such as 90.degree., 
120.degree., 150.degree. and 180.degree.. The viewing-angle 
characteristics of the four different liquid crystal display devices were 
respectively measured. The results of measurement are shown in Table 1. 
TABLE 1 
______________________________________ 
Iso-contrast Characteristics of Respective Configurations 
Iso-contrast 
configu- angle .omega. between 
characteristics 
rations rubbing axes shown in 
______________________________________ 
1 90.degree. FIG. 1 
2 120.degree. FIG. 14a 
3 150.degree. FIG. 14b 
4 180.degree. FIG. 14c 
______________________________________ 
As the angle .omega. is changed from 90.degree. to 180.degree., the 
Iso-contrast characteristics are changed as seen from the changes shown in 
FIG. 11 and FIG. 14a through FIG. 14c. Specifically, as the positional 
relationship between two axes (2.2) and (3.1) is changed from orthogonal 
intersection toward parallel, the Iso-contrast curves move to distribute 
around the viewing angle .theta.0.degree.. As described above, in this 
embodiment, the Iso-contrast characteristics can be easily changed by 
changing the positional relationship between a viewing-angle compensating 
liquid crystal cell 2 and the driving liquid crystal cell 3. 
EMBODIMENT 4 
This embodiment differs from the embodiment 1 in that a viewing-angle 
compensating liquid crystal cell 2 is modified in the following manner. 
Specifically, the cell 2 was manufactured by use of a mixture of a liquid 
crystal material (ZLI-2806; manufactured by E. Merck Co., Ltd.) having 
refractive-index anisotropy .DELTA.n of 0.039 and a chiral dopant (R811; 
manufactured by E. Merck Co., Ltd.) causing a liquid crystal molecular 
alignment to have a clockwise twist with a helical pitch of 2.96 .mu.m. 
The clockwise-twist viewing-angle compensating liquid crystal cell 2 had a 
twist angle of 450.degree., and a liquid crystal layer of 3.7 .mu.m thick. 
Further, an angle .omega. constituted by rubbing axis (2.2) of a lower 
substrate of the viewing-angle compensating liquid crystal cell 2 and a 
rubbing axis (3.1) of an upper substrate of a driving liquid crystal cell 
3 was determined to be 90.degree.. Iso-contrast characteristics of a 
liquid crystal display device of this embodiment were measured in the same 
as in the embodiment 1. The results of measurement are shown in FIG. 15. 
As can be seen from FIG. 15, an original 90.degree. viewing direction was 
changed to a 180.degree. viewing direction. For the purpose of 
experimental observation a gray scale display operations were performed by 
use of the liquid crystal display device of this invention. As a result, 
satisfactory viewing-angle characteristics were obtained in directions 
from 180.degree. to 360.degree. through 270.degree.. 
EMBODIMENT 5 
This embodiment differs from the embodiment 1 in that a viewing-angle 
compensating liquid crystal cell 2 is modified in the following manner. 
Specifically, the cell 2 was manufactured by use of a mixture of a liquid 
crystal material and a chiral dopant (R811; manufactured by E. Merck Co., 
Ltd.) causing liquid crystal molecular alignment to have clockwise twist 
with a twist angle of 990.degree. (a twist pitch=3.27 .mu.m). The 
clockwise-twist viewing-angle compensating liquid crystal cell 2 had a 
liquid crystal layer of 9 .mu.m thick. Further, the positional 
relationship of rubbing axes are changed in the following manner. 
Specifically, there were provided four different liquid crystal display 
devices such that an angle .omega. constituted by a rubbing axis (2.2) of 
a lower substrate of a viewing-angle compensating liquid crystal cell 2 
and a rubbing axis (3.1) of an upper substrate of a driving liquid crystal 
cell 3 was changed into four different angles such as 90.degree., 
120.degree., 150.degree. and 180.degree.. The Iso-contrast characteristics 
of the four different liquid crystal display devices were respectively 
measured. The results of the measurement are shown in Table 2. 
TABLE 2 
______________________________________ 
viewing-Angle Characteristics of Respective Configurations 
Iso-contrast 
configu- angle .omega. between 
characteristics 
rations rubbing axes shown in 
______________________________________ 
1 90.degree. FIG. 16a 
2 120.degree. FIG. 16b 
3 150.degree. FIG. 16c 
4 180.degree. FIG. 16d 
______________________________________ 
In this embodiment, a twist direction of the liquid crystal molecular 
alignment in the liquid crystal layer of the viewing-angle compensating 
liquid crystal cell 2 is reverse to that in the liquid crystal layer of 
the driving liquid crystal cell 3. Thus, as can be seen from FIG. 16a 
through FIG. 16d, an original 90.degree. viewing direction was changed to 
a 180.degree. viewing direction. Further, in this embodiment, the angles 
.omega. were changed, so that the shapes of the curves indicating 
Iso-contrast characteristics were also changed. 
EMBODIMENT 6 
This embodiment differs from the embodiment 1 in that a viewing-angle 
compensating liquid crystal cell 2 is modified in the following manner. 
Specifically, a twist angle (or the number of rotations) of liquid crystal 
molecular alignment in a liquid crystal layer of the cell 2 was changed 
into seven different angles by adjusting the mixing concentration of a 
counterclockwise-chiral dopant (S811; manufactured by E. Merck Co., Ltd.). 
More specifically, there were provided seven different liquid crystal 
display devices having seven different twist angles of 810.degree. through 
1170.degree.. The viewing-angle characteristics of these display devices 
were respectively measured. The results of measurement are shown in Table 
3. 
TABLE 3 
______________________________________ 
Number of Twist Rotations in Compensation 
Cells and Advantages thereof 
number of twist rotations 
Iso-contrast 
(twist angles) of characteristics 
compensation cells shown in 
______________________________________ 
2.25 (810.degree.) FIG. 17a 
2.50 (900.degree.) FIG. 17b 
2.583 (930.degree.) FIG. 17c 
2.75 (990.degree.) FIG. 17d 
2.916 (1050.degree.) 
FIG. 17e 
3.00 (1080.degree.) 
FIG. 17f 
3.25 (1170.degree.) 
FIG. 17g 
______________________________________ 
As can be seen from Table 3 and FIG. 17a through FIG. 17g, as a twist angle 
in the viewing-angle compensating liquid crystal cell 2 increases, an 
Iso-contrast curve moves toward the center of the polar coordinate. 
Specifically, a viewing-angle direction becomes a direction of a normal on 
the display surface of the liquid crystal display device. Further, when 
the cases of FIGS. 17a, 17b and 17g are sequentially compared, in which a 
twist angle increases as 450.degree.+a multiple of 180.degree. (i.e., the 
number of twist rotations increases as 2.25, 2.75 and 3.25), it can be 
understood that the shapes of Iso-contrast curves are substantially 
constant. Further, in all the cases, the shapes of the Iso-contrast 
curves, which indicate that a contrast ratio=10:1, are substantially 
semicircular in the directions of 0.degree. through 180.degree.. This 
suggest that a contrast ratio of a certain constant value can be obtained 
in all the directions when observed at a fixed viewing angle. 
The shapes of Iso-contrast curves in the cases of FIGS. 17b, 17c, 17e and 
17f (i.e., twist angle are other than 450.degree.+a multiple of 
180.degree. ) differ from the shapes of Iso-contrast curves in the cases 
of FIGS. 17a, 17d and 17g, but become shapes which are laterally long. As 
described above, Iso-contrast characteristics of a liquid crystal display 
device can be changed by changing the number of twist rotations in a 
viewing-angle compensating liquid crystal cell. 
EMBODIMENT 7 
This embodiment differs from the embodiment 1 in that a viewing-angle 
compensating liquid crystal cell 2 is modified in the following manner. 
Specifically, as the cell 2, there was provided a counterclockwise-twist 
liquid crystal cell having a liquid crystal layer of 3.5 .mu.m thick and a 
twist angle of 270.degree.. In this embodiment, a liquid crystal material 
was the same as in the embodiment 1. A liquid crystal display device 
employing the above-described cell 2 was manufactured, and Iso-contrast 
characteristics of the device were measured. The results of measurement 
are shown in FIG. 18. 
As can be seen from FIG. 18, viewing angles with respect to directions of 
0.degree. through 180.degree., which are mainly a practical range, are 
larger than viewing angles in Iso-contrast characteristics of a 
conventional liquid crystal display device (see FIG. 12). Thus, in this 
embodiment, dependence characteristics of contrast ratios upon viewing 
angles have been improved. As for display color, in the case of a 
conventional liquid crystal display device, a display color in a dark 
state was changed depending on viewing angles. However, in this embodiment 
a satisfactory black display color in a dark state was invariably obtained 
even when viewing angles were changed. Experimentally, a 10-inch (diagonal 
size) TFT-LCD was manufactured by use of a liquid crystal display device 
according to this embodiment with a color filter provided therein. As a 
result, there was obtained a satisfactory full-color display device 
capable of discriminating its display contents independently of change of 
directions and viewing angles. 
EMBODIMENT 8 
This embodiment differs from the embodiment 1 in that a driving liquid 
crystal cell 3 is modified in the following manner. Specifically, the cell 
3 was manufactured by use of a mixture of nematic liquid crystal 
(ZLI-2293; manufactured by E. Merck Co., Ltd.) and a chiral dopant (S811; 
manufactured by E. Merck Co., Ltd.), the cell 3 having refractive-index 
anisotropy .DELTA.n of 0.13, a liquid crystal layer of 6.5 .mu.m thick and 
a twist angle of 240.degree. (counterclockwise in the ST mode). The 
driving liquid crystal cell 3 was disposed in such a manner that a rubbing 
axis of a lower substrate of the cell 3 was disposed on a line deviated 
clockwise by 30.degree. from a y-axis as observed from a +z-axis. Further, 
a transmission axis (1.1) of an upper polarizer was disposed on a line 
deviated counterclockwise by 90.degree. from a y-axis as observed from a 
+z-axis. Further, a transmission axis (4.1) of a lower polarizer was 
disposed on a line deviated counterclockwise by 110.degree. from the 
y-axis as observed from the +Z-axis. A liquid crystal display device of 
this embodiment exhibits a blue display with no applied voltage, and a 
white display with applied voltage. Experimentally, a 640.times.400--pixel 
liquid crystal display device was manufactured in accordance with the 
configuration of this embodiment. This display device was operated under 
conditions of 1/200--duty and 1/15--bias, and viewing-angle 
characteristics of the display device were measured. The results of the 
measurement are shown in FIG. 19. 
In FIG. 19, particular viewing angles relative to respective directions are 
illustrated. The particular viewing angles are such that values, which are 
obtained by dividing intensities (in the case when viewing angles are 
changed in a bright state) by intensities (of a normal on a display 
surface), become 0.4 or more. In FIG. 19, the solid lines indicate 
normalized Iso-transmission characteristics in the case of this 
embodiment, and the dotted lines indicate those in the case when a 
viewing-angle compensating liquid crystal cell 2 is not used. As can be 
seen from FIG. 19, regions surrounded by the solid lines (this embodiment) 
are smaller than regions surrounded by the dotted lines (without the cell 
2). This indicates that normalized Iso-transmission characteristics in a 
bright state of this embodiment have been significantly improved. 
EMBODIMENT 9 
This embodiment differs from the embodiment 1 in that a viewing-angle 
compensating liquid crystal cell 2 is replaced with an optically 
anisotropic layer in the following manner. Specifically, a chiral dopant 
(S811; manufactured by E. Merck Co., Ltd.) was mixed with a liquid crystal 
material (ML-1007; manufactured by E. Merck Co., Ltd.) having a 
refractive-index anisotropy .DELTA.n of 0.2. There were provided three 
different optically anisotropic layers having all the same twist pitch P 
of 0.19 .mu.m but different in their liquid crystal layer thickness. These 
optically anisotropic layers respectively had values of n.times.P (where 
refractive indexes are 1.5 or more) are 285 nm being smaller than a value 
in a range of visible wavelengths. There were provided three different 
liquid crystal display devices respectively having the above-described 
three different optically anisotropic layers. The viewing-angle 
characteristics of these display devices were measured. The results of 
measurement are shown in Table 4. 
TABLE 4 
______________________________________ 
Iso-contrast Characteristics of Respective Configurations 
thickness of liquid 
Iso-contrast 
configu- crystal layer of characteristics 
rations optically anisotropic layer 
shown in 
______________________________________ 
1 0.95 .mu.m FIG. 20a 
2 1.9 .mu.m FIG. 20b 
3 2.85 .mu.m FIG. 20c 
______________________________________ 
As can be seen from Table 4 and FIGS. 20a and 20c, as the thickness of a 
liquid crystal layer of an optically anisotropic layer increases, a 
preferable viewing angle expands in directions of 0.degree. and 
180.degree. (left and right direction). Further, the Iso-contrast curves 
are changed into shapes symmetrical with respect to the 
0.degree.-180.degree.-direction line. As described above, viewing-angle 
characteristics can be easily changed by changing the layer thickness of 
an optically anisotropic layer. 
EMBODIMENT 10 
This embodiment differs from the embodiment 1 in that a viewing-angle 
compensating liquid crystal cell 2 is modified in the following manner. 
Specifically, the viewing-angle compensating cell 2 was replaced by a 
liquid crystal polymer consisting of polysiloxane as a main chain and both 
of biphenylbenzoate and cholesteril-radical as a side chain. The polymeric 
liquid crystal had refractive-index anisotropy .DELTA.n of 0.20, and a 
chiral pitch P of 0.468 .mu.m. Thickness of the polymeric liquid crystal 
layer was 0.117 .mu.m, and a twist angle was 90.degree., and a twist 
direction was counterclockwise. There was provided a liquid crystal 
display device employing the above-described viewing-angle compensating 
layer. The viewing-angle characteristics of the display device were 
measured in the same manner as in the embodiment 1. The results of 
measurement are shown in FIG. 21. As can be seen from FIG. 21, contrast 
ratios of 30:1 or more are obtained in a 30.degree.-cone in directions of 
0.degree. through 180.degree.. 
EMBODIMENT 11 
This embodiment differs from the embodiment 1 in that a driving liquid 
crystal cell 3 and a viewing-angle compensating liquid crystal cell 2 are 
modified in the following manner. Specifically, the cell 3 was 
manufactured by use of liquid crystal in an ST mode (counterclockwise 
twist with a twist angle of 240.degree.). The employed liquid crystal was 
twist nematic liquid crystal being a mixture of nematic liquid crystal 
(ZLI-2293; manufactured by E. Merck Co., Ltd.) and a chiral dopant (S811; 
manufactured by E. Merck Co., Ltd.). Thickness of a liquid crystal layer 
of the cell 3 was 6.5 .mu.m, and refractive-index anisotropy .DELTA.n 
thereof was 0.131. Next, the cell 2 was manufactured by use of chiral 
nematic liquid crystal (clockwise twist with a twist angle of 360.degree.) 
being a mixture of nematic liquid crystal (ZLI-2293; manufactured by E. 
Merck Co., Ltd.). Thickness of a liquid crystal layer of the cell 2 was 7 
.mu.m. The value of .DELTA.n.times.P of the cell 2 was 0.917 .mu.m greater 
than a value in a range of visible wavelengths, so that display colors 
were changed depending on viewing angles. Experimentally, a 
640.times.400--pixel liquid crystal display device of an ST-type was 
manufactured by use of the cells 2 and 3 in the manner as shown in FIG. 1. 
Specifically, the cell 2 and the cell 3 were disposed in close contact 
with each other. Further, a rubbing axis (2.2) of the cell 2 and a rubbing 
axis (3.1) of the cell 3 orthogonally intersected each other. Further, the 
rubbing axis (3.1) was disposed on a line deviated counterclockwise by 
30.degree. from a y-axis as observed from a +Z-axis. Further, polarizers 1 
and 4 were disposed in such a manner that a transmission axis (1.1) was 
disposed on a line deviated counterclockwise by 95.degree. from the y-axis 
as observed from the +Z-axis, and a transmission axis (4.1) was disposed 
on a line deviated clockwise by 5.degree. from y-axis as observed from the 
+Z-axis. The thus obtained display device of this embodiment was operated 
in a multiplex drive under conditions of 1/200-duty and 1/13-bias. As a 
result, a satisfactory black-and-white display was realized without 
bifringence colors peculiar to an ST-type liquid crystal display device. 
Further, viewing-angle characteristics of the liquid crystal display 
device of this embodiment were measured. The results of the measurement 
are shown in FIG. 22. As can be seen from FIG. 22 preferable viewing 
angles are expanded (better understood as compared to FIG. 23 which will 
be later described). 
COMISON EXAMPLE 
For the sake of comparison, viewing-angle characteristics in the case of a 
liquid crystal display device without a viewing-angle compensating liquid 
crystal cell 2 (other elements being the same as those in the embodiment 
11) were measured in the same manner as in the embodiment 11. The results 
of the measurement are shown in FIG. 23. As can be seen from FIG. 23, a 
display image is inevitably inverted when a viewing angle is greater than 
20.degree. in a lower half region of the diagram, i.e., in a 
270.degree.-direction. Further, a display color is changed into blue. This 
is significantly disadvantageous to applications in which a multi-color 
display is performed in combination with a color filter and like. 
EMBODIMENT 12 
This embodiment differs from the embodiment 1 in that a viewing-angle 
compensating liquid crystal cell 2 is replaced with an optically 
anisotropic layer in the following manner. Specifically, an optically 
anisotropic layer was manufactured, in place of the cell 2, by use of 
plural sheets of films made of TAC (triacetylcellulose) having a 
retardation value of 0.002 .mu.m, being deposited one after another. More 
specifically, 14 sheets of TAC films were deposited in such a manner that 
respective optical axes of the films were disposed with continuous 
counterclockwise deviation by every 6.5.degree.. As a result, an optically 
anisotropic layer having a retardation value of 0.028 .mu.m was obtained. 
Iso-contrast characteristics of a liquid crystal display device of this 
embodiment were measured. The results of measurement are shown in FIG. 24. 
As can be seen from FIG. 24, a satisfactory display image can be observed 
even when a viewing angle is 60.degree. or more in a direction of 
135.degree.. 
EMBODIMENT 13 
A configuration of this embodiment is shown in FIG. 25. This embodiment 
differs from the embodiment 1 in that a direction of a rubbing axis (2.1) 
on an upper substrate 2a of a viewing-angle compensating liquid crystal 
cell 2 is reversed as compared to the direction of the rubbing axis (2.1) 
in the embodiment 1. A driving liquid crystal cell 3 and physical 
properties of the cell 2, such as a twist angle (=990.degree.) of a liquid 
crystal layer, a twist direction, layer's thickness (=9 .mu.m), a twist 
pitch (=3.27 .mu.m), and refractive-index anisotropy .DELTA.n (=0.039) are 
all the same as those in the embodiment 1. Experimentally, a liquid 
crystal display device was manufactured in accordance with the 
above-described configuration. The Iso-contrast characteristics of this 
display device were measured in the same manner as in the embodiment 1. 
The results of the measurement are shown in FIG. 26. 
As can be seen from FIG. 26, Iso-contrast curves substantially the same as 
in the embodiment 1 were obtained in directions of 
315.degree.-0.degree.-45.degree. and 135.degree.-180.degree.-225.degree.. 
Further, advantages better than those in the embodiment 1 were obtained in 
directions of 45.degree.-90.degree.-135.degree., so that a preferable 
viewing angle was expanded. On the other hand, a preferable viewing angle 
was decreased to some extent as compared to that in the embodiment 1 in 
directions of 225.degree.-270.degree.-315.degree.. However, this decrease 
of preferable viewing angle was insignificant in practical applications. 
As described above, according to this embodiment, viewing-angle 
characteristics can be changed by reversing a direction of a rubbing axis 
of one of substrates of a viewing-angle compensating liquid crystal cell. 
As described above, according to the present invention, there can be 
provided a liquid crystal display device in which viewing-angle 
characteristics have been improved. The display device of this invention 
can display an image of good quality with superior visibility. Further, 
this invention can also be applied to various active-matrix liquid crystal 
display devices using 3-terminal or 2-terminal elements of TFT or MIM and 
the like. Naturally, in these applications, the above-described 
significant advantages can also be obtained. 
Obviously, numerous additional modifications and variations of the present 
invention are possible in light of the above teachings. It is therefore to 
be understood that within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described herein.