Liquid crystal device having birefringent plate next to polarizer

A liquid crystal device comprising a liquid crystal cell in which a ferroelectric liquid crystal are put between two sheets transparent substrates each formed with a transparent electrode to the inside thereof with the peripheral edges of the transparent substrates being sealed, a polymer film having a birefringent property appended on one side of the liquid crystal cell and two polarizing plates disposed on both sides of the liquid crystal cell and such that the polarizing directions thereof are in perpendicular to or in parallel with each other. Coloring for the transmission light is suppressed, by which the display quality can be improved.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns a liquid crystal device for use in liquid crystal 
display devices, liquid crystal light valves, etc. 
2. Description of the Prior Art 
Technical attention has been attracted in recent years to liquid crystal 
device using ferroelectric liquid crystals in the field where high speed 
responsivity is required. 
Heretofore, two types of systems have mainly been studied for the display 
by the use of ferroelectric liquid crystals, that is, a system of 
birefringent mode display in which a liquid crystal cell is placed between 
two polarizing plates and optical axes for the liquid crystal molecules 
are varied by the voltage application for the display and a guest-host 
mode in which a dichromic pigment is added to the liquid crystal and 
color-switching is conducted by using a sheet of polarizing plate. 
Although the birefringent mode display system has a feature that a high 
contrast can be obtained by the polarizing plates, the display quality is 
poor, since there occurs a remarkable coloration due to interferring 
colors unless the cell gap is restricted to less than 2 .mu.m and, in 
addition, the hue of the interferring color varies by the scattering of 
cell gaps making it difficult to obtain uniform display. If the cell gap 
is reduced to less than 2 .mu.m, although the coloration is less 
prominent, there are a lot of technical problems in the upon manufacturing 
a large area panel with such a thin cell gap. 
While on the other hand, in the guest-host mode display system, scattering 
of the cell gap provides no remarkable direct effects on the optical 
properties as in the birefringent mode cell and the liquid display device 
can be manufactured with cells having the same extent of gap as in the 
commercially available TN mode display devices. However, it is difficult 
to obtain a high contrast and, particularly, a great amount of pigment has 
to be added for black and white display to bring about problems in the 
brightness of the display and the life and reliability of the device. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to overcome the foregoing 
problems in the prior art and provide a liquid crystal device capable of 
suppressing the coloration caused by the interferring color and capable of 
attaining a preferred display quality even on a large screen in a 
birefringent mode display system using ferroelectric liquid crystals. 
The foregoing object can be attained in accordance with this invention by a 
liquid crystal device comprising a liquid crystal cell having 
ferroelectric liquid crystals put between two transparent substrates each 
formed with a transparent electrode at the inside thereof with the 
peripheral portion of the transparent substrates being sealed, a polymer 
film having a birefringent property appended on one side of the liquid 
crystal cell and two polarizing plates disposed on both sides of the 
liquid crystal cell and such that the polarizing directions are in 
perpendicular with each other. 
In accordance with this invention, since the polymer film having the 
birefringent property is appended on one side of the liquid crystal, the 
coloration can be suppressed to obtain a preferred display quality even if 
the size of the cell gap is increased to some extent. 
The polymer film of the birefringent property usable in this invention can 
include, for example, monoaxially stretched polyethylene terephthalate 
film. The film thickness of the polymer film is preferably from 1 to 100 
.mu.m and, more preferably, from 10 to 50 .mu.m. Further, the polymer film 
is desirably disposed with the optical axis thereof being displaced from 
the direction in parallel with or in perpendicular to the polarizing axis 
of one of the polarizing plates by, preferably, from 1.degree. to 
20.degree. and, more preferably, from 2.degree. to 7.degree.. This can 
effectively suppress the coloration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows one embodiment for the liquid crystal device according to this 
invention. 
In the liquid crystal device, a monoaxially stretched polymer film 31 is 
appended on one side of a liquid crystal cell 31 and, further, two 
polarizing plates 11, 12 are disposed on both sides of the liquid crystal 
cell 21 with the polarizing directions thereof being in perpendicular to 
each other. The liquid crystal cell 21 is constituted as described below. 
A vapor deposition film of indium oxide is formed on a glass substrate 22 
on the segment side and patterned into a desired configuration through a 
photoetching process to form a transparent electrode 23. Further, after 
coating a 10% solution of a polyimide resin ("PI.times.1400": trade name 
of products manufactured by Hitachi Kasei) in N-methylpyrrolidone by using 
a spinner and removing those portions of the film other than the display 
area, they are polymerized by crosslinking at 370.degree. C. for one hour 
to form an oriented film 24. The oriented film 24 is further oriented by 
rubbing in one direction with absorbent cotton. While on the other hand, a 
transparent electrode 26 made of a vapor deposition film of indium oxide 
is formed on a glass substrate 29 on the common side to form a transparent 
electrode 26, which is then applied with a surface treatment by using a 
silane coupling agent "SH6020" (manufactured by Toray Silicone Co.). Then, 
while holding the glass substrate 22 on the segment side and the glass 
substrate 25 on the common side so as to keep a predetermined gap, the 
peripheral portions therebetween is secured with a sealant 27 made of an 
epoxy resin. Then, liquid crystals 28 are sealed to the inside of the 
liquid crystal of 21. The liquid crystals 28 comprise a mixture of 43.8% 
by weight of p'-(act-amyloxy)-phenoxy p-n-octyloxy benzoate), 14.2% by 
weight of p'-(act-amyloxy)-phenoxy p-n-hexyloxy benzoate, 14.2% by weight 
of p'-(n-octyloxy)phenoxy p-n-octyloxy benzoate, 12.8% by weight of 
p'-octyloxy phenyl 4-(2-methyl butyl) biphenyl carboxylate, 10% by weight 
of p'-hexyloxy phenyl 4-(2-methyl butoxy) biphenyl carboxylate and 5% by 
weight of (-)-4'-octyloxy biphenyl carboxylic acid P-1-methyl heptyl 
alcohol ester. The liquid crystals exhibit a chiral smectic C phase within 
a temperature range from 10.degree. to 50.degree. C. and exhibit the value 
of polarization of the permanent dipole moment of 12.8 nC/cm.sup.2 at 
15.degree. C. 
The liquid crystal cell 21 is disposed between the two polarizing plates 11 
and 12 with the polarizing axes thereof being in perpendicular to each 
other, such that the optical axes of the liquid crystal molecules and the 
polarizing axis of one of the polarizing plates coincide with each other 
under the state of applying an electrical field with negative (-) polarity 
onto the substrate on the segment side to provide a dark state. Then, a 
polyethylene terephthalate film 31 on about 35 .mu.m thickness stretched 
monoaxially is inserted with the optical axis thereof being deviated by 
about 5.degree. relative to the polarizing axis of one of the polarizing 
plates and the colored state of the interferring transmission light is 
measured under the bright state applied with an electrical field of 
positive (+) plarity. Upon measurement, x, y values for the transmission 
light are measured by using a BM-5 color luminance meter (manufactured by 
Tokyo Kogaku Co.) As the liquid crystal cell 21, those having two kinds of 
cell gaps are used. Table 1 shows the results of the comparison between 
the embodiment according to this invention using the birefringent film 31 
and the conventional embodiment without using such film 31. 
TABLE 1 
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Cell gap Transmission light 
(um) x value y value 
______________________________________ 
Example 1 3.0 0.445 0.438 
Example 2 2.1 0.395 0.423 
Comparative 
3.0 0.491 0.448 
Example 1 
Comparative 
2.1 0.419 0.435 
Example 2 
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FIG. 2 shows the results of the measurement plotted on the chromaticity 
diagram. In the figure, a curve with the wavelength graduation shows a 
spectrum trace, while a straight line connecting both ends of the spectrum 
trace is a pure-purple trace points A, B, C and D.sub.65 represent the 
chromaticity coordinate for the standard light A, B, C and D.sub.65. Plots 
with the mark "x" represent the values for the conventional embodiments 1, 
2 and plots with the mark "o" represent the values for the embodiments 1 
and 2 according to this invention. It can be seen that the values on the 
chromaticity diagram are displaced toward the center by the insertion of 
the birefringent polymer film 31 and the colored state of the transmission 
light is reduced remarkably toward white light. 
As has been described above according to this invention, since a pair of 
polarizing plates are disposed on both sides of the liquid crystal cell 
and a polymer film with the birefringent property is disposed on one side 
of the liquid crystal cell, the coloration for the transmission light is 
suppressed, by which the display quality can further be improved.