Liquid crystal display device

A liquid crystal display device of this invention includes a pair of opposing substrates, a plurality of electrodes arranged on the inner surfaces of the pair of opposing substrates so as to oppose each other, aligning films formed on the inner surfaces of the pair of substrates on which the plurality of electrodes are formed, a nematic liquid crystal sealed between the pair of opposing substrates, and a pair of polarizing plates arranged to sandwich the nematic liquid crystal. The nematic liquid crystal has a value of dielectric ratio .DELTA..epsilon./.epsilon..sub..perp. of 0.5 or less, a value of elastic constant ratio K.sub.33 /K.sub.11 of 0.8 or less, and values of liquid crystal layer thickness d and optical anisotropy .DELTA.n with which a value of product .DELTA.n.multidot.d of the liquid crystal layer thickness d and optical anisotropy .DELTA.n becomes 0.5 (.mu.m) or more and 0.7 (.mu.m) or less.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a multiplex drive liquid crystal display 
device using a field effect type twisted nematic effect. 
2. Description of the Related Art 
A liquid crystal display apparatus having a plurality of pixels arranged in 
a matrix manner is applied to a display unit of a computer terminal, an 
image display unit of a television receiver, and the like. Recently, a 
demand has arisen for a large size and high image quality of the image 
display unit. Therefore, an increase in number of pixels and an 
improvement in contrast are desired. A liquid crystal display apparatus 
applied to the image display unit has a simple matrix type twisted nematic 
liquid crystal display device (to be referred to as a matrix TN. LC. 
device) arranged such that a plurality of electrodes are aligned on inner 
surfaces of a pair of opposing substrates and opposing portions of the 
electrodes form a plurality of pixels aligned in a matrix manner. The 
matrix TN. LC. device is driven in a multiplexed manner. 
In the matrix TN. LC. device, if the number of pixels is increased in order 
to improve the resolution and increase a display area, the number of 
scanning lines is naturally increased. Therefore, high multiplex drive 
must be performed. However, if a multiplexing degree is increased, a 
difference in effective voltages between an on electric field to be 
applied to a liquid crystal to turn on pixels and an off electric field to 
be applied to the liquid crystal to turn off the pixels is reduced. As a 
result, an operating margin of a drive voltage is reduced, the contrast is 
lowered, and a viewing angle characteristic is degraded. 
The operating margin and the contrast of a liquid crystal display device 
depend on a voltage-luminance characteristic. That is, when a change in 
transmittivity with respect to a change in intensity of the electric field 
to be applied to the liquid crystal is steep, the operating margin can be 
increased, and the contrast can be increased. As shown in FIG. 1, the 
steepness of the voltage-luminance characteristic is represented by a 
ratio (to be referred to as .gamma. value hereinafter) between voltage 
V.sub.50 at which the transmittivity is 50% and threshold voltage V.sub.C. 
When the .gamma. value becomes closer to 1, the change in transmittivity 
becomes steeper. Therefore, the operating margin can be increased, and the 
contrast can be increased. 
In addition, in the matrix TN. LC. device which is of high multiplex drive 
type, a multiplexing degree is increased, and one selection period is 
shortened. Therefore, the matrix TN. LC. device must respond at high 
speed. 
As described above, the matrix TN. LC. device of high multiplex degree must 
have: 
(1) a .gamma. characteristic close to 1; 
(2) a wide viewing angle; and 
(3) a high response speed. 
The .gamma. characteristic is studied by M. Schadt et al. According to 
their studies, the .gamma. value representing the steepness of the 
voltage-luminance characteristic is given by the following equation (I) 
and coincides well with the characteristic of an actual device: 
##EQU1## 
where V.sub.50 : the applied voltage when a transmittivity of 50% is 
obtained 
V.sub.C : the threshold voltage 
K.sub.11 : the splay elastic constant of the liquid crystal 
K.sub.33 : the bending elastic constant of the liquid crystal 
.DELTA..epsilon. : the dielectric anisotropy of the liquid crystal 
.epsilon..sub..perp. : the dielectric constant in a direction 
perpendicular to a liquid crystal molecular axis 
.DELTA.n : the optical anisotropy of the liquid crystal 
d : the liquid crystal layer thickness 
.lambda. : the wavelength of light 
According to equation (I), it is apparent that when the first, second, and 
third terms of equation (I) are close to 1 the .gamma. value is close to 
1. Therefore, in order lo improve the .gamma. value characteristic, the 
following conditions must be simultaneously satisfied: 
(a) a ratio (to be referred to as elastic constant ratio K.sub.33 /K.sub.11 
hereinafter) of bending elastic constant K.sub.33 to splay elastic 
constant K.sub.11 is small; 
(b) a ratio (to be referred to as dielectric ratio 
.DELTA..epsilon./.epsilon..perp. hereinafter) of dielectric anisotropy 
.DELTA..epsilon. to the dielectric constant in a direction perpendicular 
to the liquid crystal molecular axis, is small; and 
(c) a value of product .DELTA.n.multidot.d of liquid crystal optical 
anisotropy .DELTA.n and liquid crystal layer thickness d is 1.1 (.mu.m) 
when a wavelength of incident light is 550 nm. 
Dependency (to be referred to as a viewing angle characteristic 
hereinafter) of the contrast to an observing direction is studied by Mr. 
G. BAUR and reported in "The Influence of Material and Device Parameters 
on the Optical Characteristics of Liquid Crystal Displays", Molecular 
Crystals and Liquid Crystals, Volume 63, Nos. 1 to 4, 1981. According to 
this report, the viewing angle characteristic of a liquid crystal display 
device depends on liquid crystal layer thickness d and liquid crystal 
optical anisotropy .DELTA.n of a liquid crystal. That is, in a liquid 
crystal display device having large product .DELTA.n.multidot.d (to be 
referred to as .DELTA.n.multidot.d hereinafter) of layer thickness d and 
optical anisotropy .DELTA.n, an apparent change rate of 
.DELTA.n.multidot.d obtained when the liquid crystal display device is 
viewed from its front and in an oblique direction is large, resulting in a 
poor viewing angle characteristic. To the contrary, a liquid crystal 
display device having small .DELTA.n.multidot.d has a good viewing angle 
characteristic. In addition, when liquid crystal display devices having 
equal .DELTA.n.multidot.d are compared, a better viewing angle 
characteristic is obtained with smaller optical anisotropy .DELTA.n of the 
liquid crystal. That is, a better viewing angle characteristic is obtained 
when a change in contrast with respect to a change in observing direction 
is small. Therefore, in order to improve the viewing angle characteristic: 
(d) .DELTA.n.multidot.d must be reduced; and 
(e) .DELTA.n must be reduced. 
As for the response characteristic, response time t.sub.ON required for 
turning on the liquid crystal display device and response time t.sub.OFF 
required for turning off the liquid crystal display device are represented 
by the following logic equations (II) and (III), respectively, and 
coincide well with measurement values: 
EQU t.sub.ON =.eta./(.epsilon..sub.0 .DELTA..epsilon.E.sup.2 -Kq.sup.2)(II) 
EQU t.sub.OFF =n/Kq.sup.2 (III) 
where q=.pi./d, K=K.sub.11 +[(K.sub.33 -2K.sub.22)/4] 
.eta. : viscosity 
.epsilon..sub.0 : dielectric constant in vacuum 
E : electric field intensity 
K.sub.22 : a twist elastic constant 
According to equations (II) and (III), the response speed depends on 
viscosity .eta. and electric field intensity E. That is, in order to 
increase the response speed: 
(f) viscosity .eta. must be reduced; and 
(g) the electric field intensity must be increased. 
Of the above conditions (a) to (c) for obtaining a steep .gamma. 
characteristic, the condition of .DELTA.n.multidot.d has a largest 
influence on the .gamma. characteristic. Therefore, in consideration of 
the above technical background, a value of .DELTA.n.multidot.d of a 
conventional matrix TN. LC. device is set to be about 1.1 (.mu.m) because 
the center of a wavelength range of a visual light beam is about 550 nm. 
In this case, since optical anisotropy .DELTA.n of a liquid crystal 
generally falls within the range of 0.13 to 0.16, liquid crystal layer 
thickness (interelectrode gap) d is set to fall within the range of 7.0 to 
8.5 (.mu.m). 
The above conventional liquid crystal display device has a 
.DELTA.n.multidot.d value of 1.1 and therefore has a relatively high 
contrast. However, the contrast is not sufficient yet. In addition, since 
the value of .DELTA.n.multidot.d is large, the viewing angle 
characteristic is poor, and the response speed is low because liquid 
crystal layer thickness d is increased. If thickness d is reduced in order 
to increase the response speed, the value of .DELTA.n must be increased to 
be 0.147 or more to satisfy the condition of .DELTA.n.multidot.d for 
obtaining a high contrast. In this case, however, the viewing angle 
characteristic is further degraded because the value of .DELTA.n is large. 
If the values of thickness d and .DELTA.n are reduced in order to improve 
the viewing angle characteristic and the response speed, the value of 
.DELTA.n.multidot.d largely becomes different from an optimal value of 
1.1, and the contrast is significantly lowered. Therefore, a high 
contrast, a wide viewing angle, and a high-speed response cannot be 
satisfied. 
As described above, according to the conventional liquid crystal display 
device, it is difficult to obtain an image display with sufficiently high 
contrast and to satisfy above conditions (1) to (3) required for the 
matrix TN. LC. device of high multiplex drive type. As a result, an image 
display cannot be obtained with sufficiently high display quality. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above situation 
and has as its object to provide a liquid crystal display device which can 
display an image with high contrast. 
It is another object of the present invention to provide a liquid crystal 
display device which satisfies all of a high contrast, a wide viewing 
angle, and a high response speed and is suitable for high multiplex drive. 
In order to achieve the above objects, a liquid crystal display device 
according to the present invention comprises: 
a pair of opposing substrates; 
a plurality of electrodes arranged on opposing inner surfaces of the pair 
of substrates so as to oppose each other; 
aligning films formed on the inner surfaces of the pair of substrates on 
which the plurality of electrodes are formed, and oriented in a 
predetermined direction; 
a nematic liquid crystal, interposed between the opposing electrodes formed 
on the pair of opposing substrates, and having a value of dielectric ratio 
.DELTA..epsilon./.epsilon..perp. of 0.5 or less which is a ratio of 
dielectric anisotropy .DELTA..epsilon. to dielectric constant 
.epsilon..perp.in a direction perpendicular to a liquid crystal molecular 
axis direction and a value of elastic constant ratio K.sub.33 /K.sub.11 of 
0.8 or less which is a ratio of bending elastic constant K.sub.33 to splay 
elastic constant K.sub.11 ; and 
a pair of polarizing plates arranged to sandwich the nematic liquid 
crystal. 
The liquid crystal display device of the present invention having the above 
arrangement employs a nematic liquid crystal having very small dielectric 
ratio .DELTA..epsilon./.epsilon..sub..perp. and elastic constant ratio 
K.sub.33 /K.sub.11 as a liquid crystal. The values of dielectric ratio 
.DELTA..epsilon./.epsilon..sub..perp. and elastic constant ratio K.sub.33 
/K.sub.11 are much smaller than .DELTA..epsilon./.epsilon..sub..perp. =1.0 
or more and K.sub.33 /K.sub.11 =1.2 to 1.3 of a conventional liquid 
crystal composition. The liquid crystal display device of the present 
invention obtained by interposing the liquid crystal composition having 
very small dielectric ratio .DELTA..epsilon./.epsilon..sub..perp. and 
elastic constant ratio K.sub.33 /K.sub.11 between the electrodes has an 
improved .gamma. characteristic and a high contrast. 
In addition, equation (I) representing the .gamma. characteristic is not 
applicable to the liquid crystal display device of the present invention. 
That is, an optimal .gamma. characteristic is obtained under conditions 
different from conditions (a), (b), and (c) for optimizing the .gamma. 
characteristic. 
More specifically, when a liquid crystal display device employs a liquid 
crystal composition having dielectric ratio 
.DELTA..epsilon./.epsilon..perp. of 0.5 or less and elastic constant ratio 
K.sub.33 /K.sub.11 of 0.8 or less, a value of .DELTA.n.multidot.d for 
optimizing the contrast obtained when the device is viewed from its front 
falls within the range of 0.50 (.mu.m) to 0.70 (.mu.m), preferably, 0.54 
to 0.70, and more preferably, 0.54 to 0.65. As described above, according 
to the liquid crystal display device of the present invention, the values 
of dielectric ratio .DELTA..epsilon./.epsilon..sub..perp. and elastic 
constant ratio K.sub.33 /K.sub.11 are very small. Therefore, a high 
contrast can be obtained with small .DELTA.n.multidot.d. In addition, 
since liquid crystal layer thickness d can be reduced, a high response 
speed can be obtained. Furthermore, since .DELTA.n and .DELTA.n.multidot.d 
are small, the viewing angle characteristic can be improved. 
As a result, electrooptical characteristics of the liquid crystal display 
device which is driven by high multiplex drive can be improved. 
In the present invention, it is preferable that the values of elastic 
constant ratio K.sub.33 /K.sub.11 and dielectric ratio 
.DELTA..epsilon./.epsilon..sub..perp. are minimized. More specifically, 
the value of dielectric ratio .DELTA..epsilon./.epsilon..sub..perp. falls 
within the range of, preferably, 0.1 to 0.5 and, more preferably, 0.2 to 
0.4. The value of elastic constant ratio K.sub.33 /K.sub.11 is larger than 
a value which can be realized as a liquid crystal composition and smaller 
than 0.8, e.g., falls within the range of 0.2 to 0.8 or 0.4 to 0.8. Liquid 
crystal layer thickness d is set within the range of, preferably, 4.0 
.mu.m to 8.0 .mu.m and, more preferably, 4.0 .mu.m to 7.0 .mu.m. 
When .DELTA.n.multidot.d is set within the range of 0.54 .mu.m to 0.70 
.mu.m, layer thickness d is set within the range of, preferably, 4.0 .mu.m 
to 7.0 .mu.m, and more preferably, 6 .mu.m or less. Liquid crystal optical 
anisotropy .DELTA.n, which meets the mentioned condition of 
.DELTA.n.multidot.d, is set within the range of, preferably, 0.08 to 0.14 
and, more preferably, 0.12 or less.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described in detail below by way of its 
examples. A matrix TN. LC. device of the present invention will be 
described with reference to the accompanying drawings. 
In FIGS. 2 and 3, a plurality of first electrodes 2 which vertically extend 
in FIG. 3 are arranged on lower substrate 1 consisting of a transparent 
glass plate or an optically isotropic plastic plate. Aligning film 3 which 
is subjected to an aligning treatment is formed to cover the substrate 
surface on which electrodes 2 are arranged. A plurality of second 
electrodes 5 which extend in a transverse direction in FIG. 3 are arranged 
on upper substrate 4 consisting of a material similar to that of lower 
substrate 1. Aligning film 6 which is subjected to an aligning treatment 
is formed to cover the substrate surface on which second electrodes 5 are 
arranged. Lower and upper substrates 1 and 4 oppose each other with a 
predetermined gap therebetween such that the surfaces on which first and 
second electrodes 2 and 5 are formed face inward and are adhered by 
sealing member 7. Nematic liquid crystal 8 to be described below is sealed 
between substrates 1 and 4. Nematic liquid crystal 8 forms a liquid 
crystal layer having thickness d between first and second electrodes 2 and 
5. A pair of polarizing plates 9 and 10 are arranged on the outer surfaces 
of substrates 1 and 4, respectively. 
FIGS. 4a and 4B show an oriented direction of aligning films 3 and 6 and a 
direction of polarizing axis of polarizing plates 9 and 10. Films 3 and 6 
formed on the electrode-formed surfaces of lower and upper substrates 1 
and 4 are rubbed as shown in FIG. 4A. That is, film 3 on substrate 1 is 
rubbed in oriented direction 11 indicated by a broken arrow, and film 6 on 
substrate 4 is rubbed in oriented direction 12 indicated by a solid arrow 
which crosses direction 11 at substantially 90.degree.. In this manner, 
nematic liquid crystal 8 sealed between substrates 1 and 4 rubbed in 
directions different by substantially 90.degree. is subjected to twisted 
orientation in which liquid crystal molecules are twisted through 
substantially 90.degree.. As shown in FIG. 4A, polarizing axis 13 
(indicated by the broken arrow) of lower polarizing plate 9 and polarizing 
axis 14 (indicated by the solid arrow) of upper polarizing plate 10 are 
substantially parallel to each other and to direction 12 of film 6 formed 
on substrate 4. Note that as shown in FIG. 4B, polarizing axis 13 of plate 
9 and polarizing axis 14 of plate 10 may be substantially parallel to each 
other and to direction 11 of film 3 formed on substrate 1. 
As shown in FIG. 3, in the above liquid crystal display device, terminals 
2a of first electrodes 2 extending from an end portion of lower substrate 
1 are connected to driver 16 through lead lines 15, and terminals 5a of 
second electrodes 5 extending from an end portion of upper substrate 4 are 
connected to driver 16 through lead lines 17. In the liquid crystal 
display device having the above arrangement, electrodes 2 formed on 
substrate 1 constitute column electrode 18, and electrodes 5 formed on 
substrate 4 constitute row electrode 19. Each portion at which column 
electrode 18 crosses row electrode 19 through the nematic liquid crystal 
forms a pixel. A scanning signal for applying a voltage sequentially to 
electrodes 5 is supplied from driver 16 to row electrode 19, and a data 
signal corresponding to image data is supplied to electrodes 2 of column 
electrode 18 in synchronism with the scanning signal. In this manner, an 
electric field is applied to the nematic liquid crystal at the portion at 
which row electrode 19 crosses column electrode 18 to activate liquid 
crystal molecules, thereby controlling an on/off state of each pixel. That 
is, this liquid crystal display device is driven in a multiplex manner. 
The liquid crystal display device of the present invention employs, as a 
nematic liquid crystal, a liquid crystal composition in which ratio 
.DELTA..epsilon./.epsilon..sub..perp. of dielectric anisotropy 
.DELTA..epsilon. to dielectric constant .DELTA..sub..perp. in a direction 
perpendicular to a liquid crystal molecular axis is 0.5 or less and ratio 
K.sub.33 /K.sub.11 of bending elastic constant K.sub.33 to splay elastic 
constant K.sub.11 is 0.8 or less. A value of product .DELTA.n.multidot.d 
of liquid crystal layer thickness d and optical anisotropy .DELTA.n is 
from 0.54 to 0.7. That is, in the liquid crystal display device having 
small dielectric ratio .DELTA..epsilon./.epsilon..sub..perp. of the 
nematic liquid crystal, a change in dielectric constant corresponding to a 
change in alignment of liquid crystal molecules is small, and a change in 
equivalent impedance of the liquid crystal is small. Therefore, since the 
linearity of the electric field to be applied to the liquid crystal with 
respect to the voltage to be applied between the electrodes is improved, 
the contrast is improved. The response speed of the liquid crystal display 
device is represented by equations (II) and (III) described above. In this 
case, assuming that: 
##EQU2## 
the following equation (V) is obtained: 
EQU t.sub.ON =.eta..sub.1 d.sup.2 /.vertline..sup.2 K(V.sup.2 /V.sub.C.sup.2 
-1)(V) 
As is apparent from equations (V) and (III), in the liquid crystal display 
device of the present invention using the liquid crystal composition in 
which splay elastic constant K.sub.11 of the nematic liquid crystal is 
increased to reduce elastic constant ratio K.sub.33 /K.sub.11, both of 
rise time t.sub.ON and decay time t.sub.OFF are reduced. Therefore, the 
liquid crystal display device can respond at high speed. In this manner, 
equation (I) described above is not applied to the liquid crystal display 
device using a nematic liquid crystal having small dielectric ratio 
.DELTA..epsilon./.epsilon..perp. and elastic constant ratio K.sub.33 
/K.sub.11, and a high contrast can be obtained. In addition, in this 
liquid crystal display device, optimal electrooptical characteristics can 
be obtained when the value of .DELTA.n.multidot.d falls within a 
predetermined range smaller than 1.1. Therefore, since the value of 
.DELTA.n.multidot.d is small, the viewing angle characteristic is 
improved. Furthermore, since the value of d can be reduced, the intensity 
of the electric field to be applied to the liquid crystal can be increased 
to increase the response speed. 
Electrooptical characteristics of a plurality of liquid crystal display 
devices of the present invention having the above arrangement were 
measured. These liquid crystal display devices are examples having 
different liquid crystal layer thicknesses d and using different liquid 
crystal compositions. Constituent factors and electrooptical 
characteristics of the examples are summarized in Table 1. In Table 1, 
optical anisotropy .DELTA.n is a measured value obtained when .lambda.=589 
nm. The contrast is a value (Y.sub.ON /Y.sub.OFF) obtained by dividing 
value Y.sub.ON of transmittivity in an ON state in a direction of viewing 
angle .theta.=10.degree. when the device was driven by a drive signal of 
1/64 duty by value Y.sub.OFF of transmittivity in an OFF state. The 
temperature during measurement was 25.degree. C. The threshold voltage, 
viewing angle characteristic, and response speed are measured values 
obtained when the device was driven by a static drive signal of 1 kHz. 
Note that above threshold voltage V.sub.th is defined as an applied 
voltage when a transmittivity of 50% is obtained. The response speed is 
defined as (T.sub.r +T.sub.D)/2 assuming that a rise time required for the 
luminance to reach from 10% to 90% is T.sub.r and a decay time required 
for the luminance to reach from 90% to 10% is T.sub.D. The viewing angle 
characteristic is defined as V.sub.th (.theta.=-10.degree.)/V.sub.th 
(.theta.=10.degree.) at a temperature of 25.degree. C. assuming that a 
threshold voltage observed from direction P inclined through 10.degree. 
from the Z axis perpendicular to the substrate surface of liquid crystal 
display device A toward a viewing angle direction is V.sub.th 
(.theta.=10.degree.) and a threshold voltage observed from direction Q 
inclined from the Z axis toward a direction opposite to the viewing angle 
direction is V.sub.th (.theta.=-10.degree.). Note that the viewing angle 
characteristic is better when its value is closer to 1, i.e., the viewing 
angle is wider. 
TABLE 1 
______________________________________ 
Example No. 
Example Example Example 
1 2 3 
______________________________________ 
Device .DELTA.n 0.160 0.140 0.120 
Constituent 
d[m] 7.0 4.0 5.0 
Factors .DELTA.n.d 
1.12 0.56 0.60 
Viscosity 21 24 24 
[cp] 
.DELTA..epsilon./.epsilon..perp. 
0.35 0.50 0.40 
K.sub.33 /K.sub.11 
&lt;0.80 &lt;0.75 &lt;0.75 
Electro- Contrast 21 18 20 
optical Ratio 
Character- 
Viewing 1.15 1.07 1.07 
istics Angle 
Character- 
istic 
Response .apprxeq.30 
&lt;25 .apprxeq.25 
Speed 
[msec] 
Threshold 4.5 4.0 4.3 
Voltage 
[V] 
______________________________________ 
Example No. 
Example Example Example 
4 5 6 
______________________________________ 
Device .DELTA.n 0.112 0.112 0.080 
Constituent 
d [.mu.m] 5.8 5.8 7.0 
Factors .DELTA.n.d 
0.65 0.65 0.56 
Viscosity 24 26 22 
[cp] 
.DELTA..epsilon./.epsilon..perp. 
0.35 0.20 0.35 
K.sub.33 /K.sub.11 
&lt;0.75 &lt;0.70 &lt;0.80 
Electro- Contrast 23 26 21 
optical Ratio 
Character- 
Viewing 1.06 1.05 1.05 
istics Angle 
Character- 
optical istic 
Response &lt;30 .apprxeq.30 
.apprxeq.30 
Speed 
[msec] 
Threshold 4.5 5.5 4.5 
Voltage 
[V] 
______________________________________ 
Example No. 
Example Example Example 
7 8 9 
______________________________________ 
Device .DELTA. n 0.100 0.080 0.112 
Constituent 
d [m] 7.0 8.0 4.8 
Factors .DELTA.n.d 
0.70 0.64 0.54 
Viscosity 22 22 24 
[cp] 
.DELTA..epsilon./.DELTA..perp. 
0.25 0.35 0.35 
K.sub.33 /K.sub.11 
&lt;0.80 &lt;0.80 &lt;0.75 
Electro- Contrast 24 22 17 
optical Ratio 
Character- 
Viewing 1.07 1.06 1.05 
istics Angle 
Character- 
istic 
Response .apprxeq.30 
&gt;30 .apprxeq.25 
Speed 
[msec] 
Threshold 5.0 4.5 4.4 
Voltage 
[V] 
______________________________________ 
The contrast ratios in Examples 1 to 9 are as very high as 17 or more. Of 
the above examples, the value of .DELTA.n.multidot.n in Example 1 is 1.12 
which is substantially equal to that of the conventional liquid crystal 
display device. However, the values of dielectric ratio 
.DELTA..epsilon./.epsilon..sub..perp. and elastic constant ratio K.sub.33 
/K.sub.11 are as very small as 0.35 and 0.80 or less, respectively. For 
this reason, the device of Example 1 has a very high contrast. 
According to Examples 2 to 9, the viewing angle characteristics are 1.07 or 
less, i.e., the viewing angles are wide, and the response speeds are about 
30 msec or less to realize high-speed response. Therefore, these examples 
satisfy all the three characteristics which have been considered difficult 
to satisfy simultaneously. Especially, since liquid crystal layer 
thicknesses d of Examples 2 to 4 are 6 .mu.m or less, their response 
speeds are very high. Therefore, Examples 2 to 4 are optimal as a liquid 
crystal display device for displaying a motion picture such as a 
television image. In addition, the values of .DELTA.n.multidot.d and 
elastic constant ratio K.sub.33 /K.sub.11 of Examples 2 to 4 substantially 
equal each other. In this case, the contrast is improved in an example 
having smaller dielectric ratio .DELTA..epsilon./.epsilon..sub..perp.. 
Therefore, smaller dielectric ratio .DELTA..epsilon./.epsilon..sub..perp. 
is preferable. 
The device of Example 5 has smallest dielectric ratio 
.DELTA..epsilon./.epsilon..sub..perp. and elastic constant ratio K.sub.33 
/K.sub.11 and very good contrast and viewing angle characteristic. 
Therefore, in order to improve the contrast and viewing angle 
characteristic, it is preferable to minimize dielectric ratio 
.DELTA..epsilon./.epsilon..sub..perp. and elastic constant ratio K.sub.33 
/K.sub.11. 
The liquid crystal display devices according to Examples 1 to 9 have 
excellent characteristics. However, as indicated by Example 9, although 
the response speed and viewing angle characteristic can be improved when 
.DELTA.n.multidot.d is reduced even if the liquid crystal composition 
similar to that of Example 4 was used, the contrast is reduced. In 
addition, as shown in Example 7, when .DELTA.n.multidot.d exceeds 0.7, the 
response speed and viewing angle characteristic are degraded. Therefore, 
in order to obtain a high response speed and a wide viewing angle, it is 
preferable to set .DELTA.n.multidot.d to be 0.54 to 0.70. 
In this case, when optical anisotropy .DELTA.n is increased, the viewing 
angle characteristic is degraded. Therefore, in order to obtain a liquid 
crystal display device having a wider viewing angle, optical anisotropy 
.DELTA.n is preferably 0.14 or less as shown in Example 2. As can be seen 
from Examples 6 and 8 in which the same liquid crystal composition was 
used and liquid crystal layer thicknesses d were set to be 7.0 .mu.m and 
8.0 .mu.m, respectively, the device of Example 8 has a large layer 
thickness of 8 .mu.m. Therefore, the response speed of the device of 
Example 8 is lower than 30 msec, and its viewing angle characteristic is 
degraded. Therefore, in order to obtain response speed of 30 msec required 
for displaying a motion picture such as a television image, the liquid 
crystal layer thickness is preferably 7.0 .mu.m or less. 
The devices of Examples 1 to 9 are superior in various electrooptical 
characteristics, but their threshold voltages are relatively high. This is 
because in order to reduce dielectric ratio 
.DELTA..epsilon./.epsilon..sub..perp. to be 0.5 or less, the value of 
.DELTA..epsilon. of the composition can be increased to at most 3 because 
the value of .epsilon..sub.195 of a liquid crystal compound which is 
normally used is 3 to 5. Therefore, as represented by equation (IV), 
threshold voltage V.sub.C is increased when .DELTA..epsilon. is reduced. 
High threshold voltage V.sub.C increases a voltage of a drive signal for 
driving the liquid crystal display device. However, the drive signal can 
be arbitrarily set by a driver. For example, assuming that the device is 
driven by an optimal bias using a drive signal of 1/64 duty, the voltage 
of the drive signal is set to be 24 V when the threshold voltage is 4.0 V 
as in Example 2, it is set to be 27 V when the threshold voltage is 4.5 V 
as in Examples 4, 6, and 8, it is set to be 33 V when the threshold 
voltage is 5 V as in Example 7, and it is set to be 33 V when the 
threshold voltage is 5.5 V as in Example 3. As described above, the value 
of permittivity anisotropy .DELTA..epsilon. is less than 3 and preferably 
falls within the range of 0.5 to 1.5, and more preferably, 0.8 to 1.2. 
As described above, the liquid crystal display device of the present 
invention employs a liquid crystal composition having small elastic 
constant ratio K.sub.33 /K.sub.11 and .DELTA..epsilon./.epsilon..perp.. 
Examples of the liquid crystal composition (in % by weight) having these 
characteristics are listed in Table 2. In Table 2, R and R' represent an 
alkyl group. Note that Examples 1 to 7 use liquid crystal compositions of 
corresponding numbers in Table 2, respectively. Examples 8 and 9 use 
liquid crystal compositions of No. 6 and No. 4, respectively. 
TABLE 2 
__________________________________________________________________________ 
Composition No. 
Composition 1 2 3 4 5 6 7 
__________________________________________________________________________ 
Np Liquid Crystal Compound 
##STR1## -- -- 2 2 -- -- 2 
##STR2## -- 2 -- -- 1 -- -- 
##STR3## 2 -- -- -- -- 4 -- 
##STR4## 2 -- -- -- -- -- -- 
Nn Liquid Crystal Compound 
##STR5## 30 16 37 27 24 -- 23 
##STR6## 34 
##STR7## 15 25.5 
-- -- 15 40 15 
##STR8## -- -- 10 5 8 -- -- 
##STR9## -- 4 -- -- -- -- -- 
##STR10## -- -- 18 -- 16 10 12 
##STR11## 29 4.5 -- -- -- -- -- 
##STR12## 10 -- -- -- -- -- -- 
##STR13## -- -- -- 27 -- -- 12 
##STR14## 12 48 33 39 36 12 36 
Characteristics 
m .multidot. p (.degree.C.) 
&lt;- 20.degree. 
&lt;-20.degree. 
&lt;-40.degree. 
&lt;-30.degree. 
&lt;-20.degree. 
&lt;-20.degree. 
&lt;-40.degree. 
c .multidot. p (.degree.C.) 
&gt;55.degree. 
&gt;55.degree. 
&lt;61.degree. 
57.degree. 
&gt;55.degree. 
&gt;55.degree. 
55.degree. 
.DELTA.n 0.160 
0.140 
0.120 
0.112 
0.112 
0.080 
0.100 
.DELTA..epsilon. 1.05 
1.48 
1.18 
0.93 
0.60 
1.04 
0.80 
Viscosity (cp) 21 24 24 24 26 22 22 
.DELTA..epsilon./.epsilon..perp. 
0.35 
0.50 
0.40 
0.35 
0.20 
0.35 
0.25 
K.sub.33 K.sub.11 0.8 0.75 
0.75 
0.75 
0.7 0.8 0.80 
or less 
or less 
or less 
or less 
or less 
or less 
or less 
__________________________________________________________________________ 
In the liquid crystal compositions listed in Table 2, a mixing ratio of an 
Np liquid crystal compound having a positive dielectric anisotropy is 
reduced to reduce dielectric ratio .DELTA..epsilon./.epsilon..sub.195 , 
and that of a pyrimidine liquid crystal compound having small elastic 
constant ratio K.sub.33 /K.sub.11 is increased to reduce elastic constant 
ratio K.sub.33 /K.sub.11 of the liquid crystal compositions. That is, 
these liquid crystal compositions were prepared by mixing a small amount 
of a liquid crystal compound having positive and relatively large 
dielectric anisotropy .DELTA..epsilon. e.g., an Np liquid crystal compound 
having a cyano group at its terminal end in a mixture containing an Nn 
liquid crystal compound in which dielectric anisotropy .DELTA..epsilon. is 
negative or substantially 0. As the Nn liquid crystal, a relatively large 
amount of a pyrimidine liquid crystal compound having small elastic 
constant ratio K.sub.33 /K.sub.11 was used, and a low-viscosity liquid 
crystal, a high-temperature liquid crystal, and the like were mixed in 
addition to the compound. The values of dielectric ratio 
.DELTA..epsilon./.epsilon..sub..perp. and elastic constant ratio K.sub.33 
/K.sub.11 of the liquid crystal compositions prepared as described above 
were 0.5 or less and 0.8 or less, respectively. Note that since a large 
amount of the high-viscosity pyrimidine compound was mixed in the liquid 
crystal compositions shown in Table 2, the viscosity values are larger 
than 20 cp. However, in the liquid crystal display device of the present 
invention, the response speed is not much reduced by an increase in 
viscosity because liquid crystal layer thickness d is relatively small. 
Comparative examples for the examples of the present invention are listed 
in Table 3 below. In Table 3, definitions of measuring conditions and 
characteristics are the same as those in Table 1. 
TABLE 3 
______________________________________ 
Comparative Example No. 
a b c d e f 
______________________________________ 
Device Constituent 
Factors 
.DELTA.n 0.144 0.130 0.160 
0.187 
0.100 
0.120 
d [.mu.m] 7.0 8.5 7.5 7.5 6.0 5.0 
.DELTA.n.d 1.00 1.10 1.20 1.40 0.60 0.60 
Viscosity (cp) &lt;20 .rarw. .rarw. 
.rarw. 
.rarw. 
.rarw. 
Electroptical 
Characteristics 
Contrast Ratio 10 14 13 12 6 4 
Viewing Angle 1.17 1.16 1.20 1.23 1.09 1.10 
Characteristic 
Response Speed &gt;25 &gt;30 &gt;25 &gt;25 &gt;25 &gt;20 
[msec] 
Threshold Voltage 
.apprxeq.2.2 
.rarw. .rarw. 
.rarw. 
.rarw. 
.rarw. 
[V] 
______________________________________ 
FIG. 6 shows values of the contrast with respect to values of 
.DELTA.n.multidot.d in Examples 1 to 9 of the present invention and 
Comparative Examples a to f. In FIG. 6, the abscissa represents the values 
of .DELTA.n.multidot.d, and the ordinate represents the values of contrast 
(CR), thereby plotting the values of Examples 1 to 9 shown in Table 1 and 
those of Comparative Examples a to f. In FIG. 6, symbols o represent the 
examples of the present invention, and symbols .DELTA. represent the 
comparative examples. 
As is apparent from FIG. 6, the values of contrast in Examples 1 to 9 of 
the present invention having dielectric ratio 
.DELTA..epsilon./.epsilon..sub..perp. and elastic constant ratio K.sub.33 
/K.sub.11 of 0.5 or less and 0.8 or less, respectively, are much higher 
than those of the comparative examples. Especially when Example 1 in which 
the value of .DELTA.n.multidot.d is around 1.1 is compared with 
Comparative Example a and Example 3 in which the value of 
.DELTA.n.multidot.d is 0.6 is compared with Comparative Examples e and f, 
it is apparent that the contrast values in the examples according to the 
present invention are very high. 
Of matrix TN. LC. devices according to Comparative Examples a to f listed 
in Table 3, the device according to Comparative Example b which satisfies 
condition (c) for improving the .gamma. characteristic given by equation 
(I), i.e., .DELTA.n.multidot.d=1.10 has the highest contrast among the 
comparative examples. However, the response speed of this device is slow. 
When the matrix TN. LC. device is used for motion display of, e.g., a 
television receiver, its response speed preferably corresponds to a 
response time of 30 msec or less. In order to obtain this response speed, 
liquid crystal layer thickness d must be reduced to enhance the electric 
field intensity in accordance with condition (g) for obtaining the desired 
response speed. For example, as indicated by Comparative Examples a, c, 
and d in Table 4, high response speeds are obtained with layer thickness d 
of 7.5 .mu.m or less. In this case, optical anisotropy .DELTA.n of the 
liquid crystal must be increased to be 0.147 or more. 
Of comparative examples a to d, the device of comparative example a having 
smallest .DELTA.n has a good viewing angle characteristic. Therefore, it 
is apparent that smaller .DELTA.n.multidot.d and .DELTA.n are preferable 
as indicated by conditions (d) and (e) for improving the viewing angle 
characteristic. Therefore, a liquid crystal display device having large 
.DELTA.n of 0.147 has a poor viewing angle characteristic. 
When .DELTA.n and .DELTA.n.multidot.d are reduced to improve the viewing 
angle and response characteristics, although these characteristics are 
improved, the contrast is significantly lowered as indicated by 
Comparative Examples e and f in Table 3. 
As described above, of the matrix TN. LC. devices in Comparative Examples a 
to f, the device of Comparative Example a in which .DELTA.n.multidot.d=1.1 
(.mu.m) has the highest contrast. However, the response characteristic of 
this device is slow, and its .DELTA.n.multidot.d is relatively large, 
resulting in a poor viewing angle characteristic. When liquid crystal 
layer thickness d is reduced to improve the response characteristic, 
.DELTA.n must be increased to be 0.147 or more to obtain a good contrast, 
and the viewing angle characteristic is further degraded because .DELTA.n 
is large. When layer thickness d and .DELTA.n are reduced to improve the 
viewing angle and response characteristics as in Comparative Examples e 
and f, the value of .DELTA.n.multidot.d is largely deviated from optimal 
value 1.1, and the contrast is significantly degraded. Therefore, 
Comparative Examples a to f do not satisfy all of the high contrast, wide 
viewing angle, and high response speed. 
Physical characteristics of liquid crystals according to Comparative 
Examples a to f listed in Table 3 are obtained with liquid crystal 
compositions prepared by mixing the liquid crystal compounds shown in 
Table 4 with respective mixing ratios. Comparative Examples a to f use 
liquid crystal compositions represented by corresponding symbols. In Table 
4, R and R" represent an alkyl group, and R' represents an alkyl group and 
an alkoxy group. 
TABLE 4 
__________________________________________________________________________ 
Liquid Crystal Mixing Ratio (wt %) 
Compound No. a b c d e f 
__________________________________________________________________________ 
##STR15## 10 
14 
20 
17 
27 
10 
##STR16## 18 
14 
5 8 -- 
17 
##STR17## 36 
36 
39 
25 
22 
32 
##STR18## -- 
-- 
-- 
-- 
12 
-- 
##STR19## -- 
-- 
-- 
-- 
15 
15 
##STR20## -- 
-- 
20 
40 
-- 
-- 
##STR21## 36 
16 
-- 
-- 
-- 
-- 
##STR22## -- 
20 
-- 
-- 
24 
26 
##STR23## -- 
-- 
16 
10 
-- 
-- 
__________________________________________________________________________ 
Values of .DELTA..epsilon. and viscosity of these liquid crystal 
compositions are about 10 and 20 cp or less (at a measuring temperature of 
20.degree. C.), respectively. Since the value of dielectric constant 
.epsilon..sub..perp. of the liquid crystal compound is generally about 3 
to 5, the value of .DELTA..epsilon./.epsilon..sub..perp. is 1 or more. A 
liquid crystal compound (e.g., pyrimidine liquid crystal compound) having 
small elastic constant ratio K.sub.33 /K.sub.11 has high viscosity and 
poor phase miscibility with other components. As a result, the viscosity 
of a composition is increased, and a smectic phase is easily generated. 
Therefore, in a conventional liquid crystal composition, a mixing ratio of 
the above compound is small, and the value of elastic constant ratio 
K.sub.33 /K.sub.11 is 1.2 to 1.3 or more. 
As described above, the examples of the present invention provide better 
contrast, viewing angle characteristic, and response speed than those of 
the comparative examples. Of these examples, especially Examples 2 to 7 
provide high response speeds and therefore are optimal for displaying a 
motion image such as a television image. 
In the above examples, a liquid crystal display device having a twisting 
angle of about 90.degree. is used. However, the present invention is not 
limited to the above examples but can be applied to a liquid crystal 
display device having a twisting angle smaller or larger than 90.degree.. 
As has been described above, the liquid crystal display device of the 
present invention having the improved electrooptical characteristics can 
be used in a display apparatus for displaying characters, numerals, and 
figures of computer peripheral equipment. For example, color filters of 
red, blue, and green are provided in correspondence to the column 
electrodes shown in FIGS. 2 and 3 so that one pixel is formed by three 
intersections of the column and row electrodes corresponding to the three 
colors, and a color data signal is applied to the column electrode of each 
color. As a result, the present invention can be applied to a color liquid 
crystal display apparatus capable of displaying pixels in full color.