Distorted helix ferroelectric liquid crystal cell

A distorted helix ferroelectric liquid crystal cell (DHF) is driven with a capacitance connected in series to the liquid crystal layer. Capacity is effected by either an external device (capacitor) or by incorporation of a insulator (dielectric) in the DHF cell. The value of the series capacity is typically smaller than about twice the capacity of the cell without the series capacitance. The capacitive drive circuit, DHF cell and method for driving the circuit and cell, function to shorten the switching times required by conventional cells.

FIELD OF THE INVENTION 
The invention relates to liquid crystal cells of the type known as 
distorted helix ferroelectric (DHF), and to a circuit and method suitable 
for this purpose. 
BACKGROUND OF THE INVENTION 
It is known to reduce switching times in DHF cells through the use of 
special driving pulse shapes (see for example, EP-A356 730, which 
corresponds to U.S. patent application Ser. No. 08/915,043, filed Jul. 16, 
1992, the contents of which are herein incorporated by reference). 
Operation via special driving pulse shapes permits not only gray scale 
values typical of DHF cells, but also allows multiplexing in combination 
with an active matrix. The DHF cell's relatively short switching time is 
advantageous in the industrial application of liquid crystal cells, e.g. 
in switchable filters, light valves (so-called spatial light modulators 
"SLMs") or 3D-spectacles. However, there exists a great need for even 
shorter switching times in liquid crystal cells. 
Unexpectedly, it has now been found that switching times can be reduced by 
a factor of 10 through the use of a novel driving circuit which drives a 
cell via a capacitance connected thereto in series. As a result, high 
switching speed with simultaneous availability of gray scale values is 
obtained. 
In the liquid crystal cell of the present invention a novel circuit is 
employed. This circuit is characterized in that its capacitance is smaller 
than the capacitance of the liquid crystal helix in series with the liquid 
crystal cell. 
In a particularly preferred embodiment of the invention, the additional 
capacitance takes the form of a transparent insulating layer located 
between the driving electrodes and adjacent to the liquid crystal. Times 
thus achievable are shorter than times for comparable DHF cells which are 
directly driven by square-wave pulses. The driving voltage is preferably 
higher than previous conventional driving voltages. In this mode of 
operation higher voltage can be used without damaging (i.e. unwinding) the 
helical structure of the liquid crystal. 
In contrast to a Lagerwall cell (see U.S. Pat. No. 4,904,064, issued Feb. 
27, 1990, the contents of which are herein incorporated by reference), the 
inventive cell allows for the display of gray scales. Consequently, any 
desired deformation (i.e. alteration of transmission) of the cell can be 
achieved by a suitable choice of the amplitude. Moreover, 
electrochemically induced degradation of the liquid crystal is avoided, 
since the insulating layer prevents direct current from flowing in the 
cell. 
The inventive cell can be used in display cells, including television sets 
and so-called terminals, in printers (a replacement for laser printers), 
and generally in all cases where high-speed light modulators or modulators 
with gray scales can be used. Thus, the inventive cell represents a major 
improvement over the prior art. 
SUMMARY OF THE INVENTION 
The subject distorted helix ferroelectric liquid crystal cell comprises two 
transparent substrates (each having an associated electrode and 
orientation means), capacitance means, and a liquid crystal layer disposed 
between the two substrates. The substrates, capacitance means and liquid 
crystal layer are coupled so that the capacity of the cell is electrically 
in series with the liquid crystal layer and smaller than the effective 
capacity of the cell without the series capacity. As used herein, the term 
"effective capacity" refers to capacity which includes the contribution of 
the ferroelectric helix. The term "smaller" is used as a comparative term. 
Quantitatively, it is preferred that the capacity of the cell is smaller 
than F times the effective capacity of the cell without the series 
capacity, and F ranges from about 3 to about 0.3. More preferably, F is 
from about 1 to about 0.3, and particularly about 0.3. 
Capacitance means preferably comprise one or more insulating layers located 
between an electrode and the liquid crystal layer. However, insulating 
layers may also be located between each electrode and the liquid crystal 
layer. Alternatively, capacitance means may comprise a capacitor connected 
to one of the electrodes. Generally, the two substrates are parallel to 
each other and are spaced about 2 .mu.m apart. 
The orientation means generally comprise a polymer layer that is treated so 
as to orient the liquid crystal molecules in a predetermined orientation. 
Typical polymer layers include polyvinyl alcohol and polyimide, and a 
typical treatment involves rubbing. 
Orientation means may comprise a polarizer which is parallel to or at an 
angle to the helical axis of the liquid crystal. For example, an angle of 
about 22.5.degree. to the helical axis of the liquid crystal may be 
utilized.

DETAILED DESCRIPTION OF THE INVENTION 
The subject invention will now be described in terms of its preferred 
embodiments. These embodiments are set forth to aid in understanding the 
invention, but are not to be construed as limiting. 
The liquid crystal cell shown schematically in section in FIG. 1 is a 
distorted helix ferroelectric (DHF) cell as described in Swiss Patent 
Application No. 1555/88, dated 26 Apr. 1988, corresponding to U.S. patent 
application Ser. No. 08/200,939, filed Feb. 23, 1994, the contents of 
which are herein incorporated by reference. The liquid crystal cell has 
two parallel substrates (e.g. glass plates 1, 2) spaced apart by about 2 
.mu.m, and is provided with electrode layers 3, 4 on its facing surfaces. 
Plates 1, 2 also have on their facing surfaces polyvinyl alcohol (PVA) 
coatings 5, 6 which are treated (e.g. by rubbing) so as to orient the 
liquid crystal molecules in a preferred direction. Such treatment is known 
to the skilled artisan. Liquid crystal layer 7 is disposed between plates 
1, 2. 
In the one preferred embodiment, the liquid crystal comprises a mixture of: 
13.1 wt. % of 5-nonyl-2-p-(nonyloxy)phenyl!pyrimidine, 
21.9 wt. % of 2-p-(hexyloxy)phenyl!-5-nonylpyrimidine, 
4.4 wt. % of 2-p-(E)-2-octenyloxy!phenyl!-5-octylpyridine, 
8.8 wt. % of 2-(p-heptylphenyl)-5-(E)-2-octenyloxy!pyrimidine, 
8.8 wt. % of 5-(E)-2-decenyloxy!-2-(p-heptylphenyl)pyrimidine, 
4.4 wt. % of (E)-5-heptyl-2-(4-oct-2-enyloxy-phenyl)pyridine, 
9.3 wt. % of 
2,2'-(4,4'-biphenylene)bis(2S,4S,5S)-4-methyl-5-octyl-m-dioxan! and 
29.3 wt. % of bis(R)-1-methylheptyl! p-terphenyl-4,4"dicarboxylate. 
Insulating layer 8 is disposed on one of the two glass plates 1, 2 between 
electrode layer 3 and oriented (PVA) layer 5, and constitutes a dielectric 
of a capacitance which lies in series with the capacitance of liquid 
crystal layer 7. Alternatively, the cell can be driven using a 
commercially obtainable capacitor connected in series with the cell. Such 
an alternative drive can be used in place of the capacitance integrated in 
the liquid crystal layer. 
In the alternative inventive cell circuit diagram shown in FIG. 2, the 
subscript hx denotes a parameter of the liquid crystal layer. The 
electro-optical effect of the DHF cell is brought about by deformation of 
a ferroelectric helix. This deformation is coupled with a charge 
(polarization charge) on the helix. The charge Q.sub.hx is therefore a 
measure of the deformation of the helix and consequently of the 
electro-optical effect. The fact that an applied voltage deforms the helix 
can be described by introducing a capacitance C.sub.hx which is charged 
when a voltage is applied. The viscosity of the liquid crystal is thus 
described by a series resistance (R.sub.hx). The series resistance of the 
ITO layer is also present in R.sub.hx. 
The electro-optical effect f.sub.eo (Q.sub.hx) is therefore only a function 
of Q.sub.hx. This model leads to a frequency dependence for Q.sub.hx of 
the following form: 
##EQU1## 
where p=j .omega., .omega.=frequency, j=imaginary unit. The frequency 
behavior of f.sub.eo therefore corresponds to that of an RC element with a 
characteristic time .tau..sub.hx =R.sub.hx C.sub.hx. The electro-optical 
effect follows the applied voltage up to the cut-off frequency of the RC 
element. 
If an additional capacitance C.sub.ext is connected in series with the 
cell, then C.sub.hx in formula (1) must be replaced by C, the capacitance 
of the series connection of C.sub.hx and C.sub.ext : 
EQU C=C.sub.hx C.sub.ext /(C.sub.hx +C.sub.ext) (2) 
The new characteristic time is therefore .tau.=R.sub.hx C. Since C is 
always smaller than C.sub.ext, this time can be made very short for small 
values of C.sub.ext. Since the extent of the electro-optical effect is 
also proportional to C (equation 1), the lower time is limited ultimately 
by the highest voltage which can be applied without damaging the cell. 
No direct current voltage component can be applied to capacitively-coupled 
liquid crystal cells. This is very desirable since no space charges are 
separated in the liquid crystal cell, thus averting any possible adverse 
effect on orientation, stability or working point (ghost images). Two 
modes of operating a DHF cell can be distinguished, depending upon the 
chosen position of the crossed polarizers relative to the helix. In both 
modes, dc-free driving is possible within limitations: 
If a polarizer is parallel to the helical axis, then light transmission is 
at a minimum when a zero voltage (0 V) is applied to the liquid crystal 
cell. For both positive and negative voltages, light transmission is 
greater. Accordingly, if the liquid crystal cell is driven alternately 
positively and negatively, the average voltage at the liquid crystal cell 
will be zero. Thus, in the case of a television image, a positive and a 
negative image can be recorded in alternation. Unfortunately, a 
disadvantage of this method is that the useful electro-optical effect is 
halved. 
If a polarizer is rotated through a fixed angle (preferably 22.5.degree.) 
to the helical axis, light transmission is at a minimum, e.g. at a given 
negative voltage U.sub.- and at a maximum for a positive voltage U.sub.+. 
To guarantee dc-free driving, therefore, U.sub.+ t.sub.+ must equal 
U.sub.- t.sub.-, t.sub.+, .sub.- denoting the times during which the 
positive and, respectively, negative voltage is applied. This mode of 
operation is therefore particularly suitable for applications having a 
constant scanning ratio, such as occurring with serially connected filters 
or spectacles for watching stereo images on a screen where the left and 
right images are presented in alternation. 
The advantages of the inventive driving method include short switching 
time, absence of a dc current across the cell, substantially simpler 
driving (since no decomposition of the driving pulses in switching pulses 
and holding pulses is necessary), and inherent protection by isolation 
from short circuits. These advantages are in addition to the known 
advantages of DHF cells regarding the ability to generate gray scales, 
thin cells with little dependence on the angle of viewing, and combination 
with active addressing. 
Upon reading the present specification, various alternative embodiments 
will become obvious to those skilled in the art. These embodiments are to 
be considered within the scope and spirit of the subject invention which 
is only to be limited by the claims which follow and their equivalents.