Dual frequency, optically active liquid crystal cell

A high-speed responsive liquid crystal optical device with improved light transmissivity is provided. The device includes a liquid crystal panel including an optically active liquid crystal material having dielectric anisotropy which becomes zero at a crossing frequency ("fc") of 100 KHz or below. The dielectric anisotropy of the liquid crystal is positive at frequencies lower than fc ("fl") and negative at frequencies higher than fc ("fh"). Crossed polarizers are disposed on each side of the liquid crystal panel. A liquid crystal driving circuit selectively applies a signal of a frequency fh to open the device and a signal of a frequency fl to close the device. The optical device is particularly well suited for use as a light valve in a printer device. Response time within 1 m sec. permits a printing speed of 10 prints of A4 size to be produced per minute with a resolving power of about 10 dots per 1 m/m. This amounts to a printing speed of approximately 500 lines in one second with each line being written into m sec. or less. The optically active nematic liquid crystal is a composition comprising an optically inactive nematic liquid crystal and an optically active liquid crystal material added thereto.

BACKGROUND OF THE INVENTION 
The present invention relates to a liquid crystal optical device, and more 
particularly to a liquid crystal optical device having an optically active 
nematic liquid crystal composition capable of dielectric relaxation at low 
frequencies for high-speed responsiveness. The optical device is 
particularly well suited for use in a printing device including a liquid 
crystal light valve. It is known to use a liquid crystal device as a light 
modulation element in a printing device, a signal reading device, a signal 
converting device, a light signal switch, a device for adjusting a 
quantity of light, or simply as a light valve. (See, for example, Japanese 
Laid-Open Patent Publications Nos. 50-74340, 50-102343, 51-26053, 49-66149 
and 51-80242.) The disclosed proposals have merely been indicative of 
ideas in principle, and have not been feasible as a practical matter 
because conventional liquid crystal materials and systems for driving the 
liquid crystals have only attained a speed of responsiveness (from several 
tens msec. to several hundreds msec.) and a frequency of repetition (from 
10 to 1 Hz). Hence, they have not been completely satisfactory for the 
foregoing intended applications. 
As semiconductor technology has advanced to render CPU's, memories and the 
like less costly, more and more people other than specialists handling 
EDP's are having a chance to use microcomputers and office computers and 
to deal with the printouts of their computers. Stated otherwise, there has 
been a great need for output processing at the same level as ordinary 
documents. This includes outputs expressed in Chinese characters and Kana 
(Japanese syllabary), word processors for the Japanese language being an 
example. 
Although circuits and memories now do much to improve such a system in view 
of their ever reducing cost, printers as system output terminals require a 
resolving power as high as 32.times.32 dots suitable for printing Chinese 
characters which add to the cost of the systems. The printers should 
operate at speeds high enough to meet the high resolution requirement and 
compensate for low speed operation needed for high resolution printing. 
Devices available at the present time designed to meet the foregoing 
requirements are electro-photographic printers using lasers or OFT and 
multi-stylus electrostatic printers. Both of these are very expensive and 
the greatest item which increases, the cost of the systems. Thus, 
unavailability of desirable printers constitutes a bottleneck in 
popularizing the foregoing various systems in the market even though a 
need for such systems exists in the market. This situation basically holds 
true for high-speed facsimile, CRT hard-copiers and various other 
terminals. 
With these points in view, the present invention is aimed at increasing 
high-speed responsiveness and provides a liquid crystal optical device 
which is quite effective for meeting all of the above-mentioned 
applications. The present invention also provides a printing device of the 
liquid crystal light valve type. Such devices have been considered to be 
difficult to produce as a practical matter due to a variety of 
technological problems. 
High speed light valves for the foregoing and other applications are 
required to have the following characteristics: (1) They should be closed 
at high speeds; (2) They should be opened at high speeds; (3) They should 
be able to be opened and closed in short periods of time; (4) They should 
not allow much leakage of light when closed; and (5) They should have a 
high transmissivity of light when opened. With these characteristics in 
mind, conventional liquid crystal devices as light valves will be 
described to highlight their disadvantages. In order to achieve 
characteristics (4) and (5) above, twisted nematic liquid crystals are 
most effective for minimizing leakage of light. With the twisted nematic 
liquid crystals, leakage of light when the light valve is closed can be 
reduced to almost nothing by arranging polarizing planes of polarizers 
perpendicularly to each other. It is easy for some polarizers used to have 
100/1 as a ratio of light transmission when the light valve is opened to 
light transmission when the light valve is closed. With this ratio, the 
light transmissivity while the light valve is opened is from 20 to 40%, 
which meets the condition of (5). Other liquid crystals than twisted 
nematic liquid crystals cannot satisfy the conditions (4) and (5). 
For example, a dynamic scattering system, a guest-host system with a 
two-colored dye added to a nematic liquid crystal for a display, or a 
nematic-cholesteric phase-transition system allow much leakage of light, 
thus being unable to meet characteristic (4). Conditions (4) and (5) 
cannot be met by a system utilizing birefringence of liquid crystal 
molecules, since such a system only has an opening and closing function 
with respect to light of a particular wavelength, but not an opening and 
closing function with respect to all visible light. 
Accordingly, it is desirable to provide a liquid crystal optical device 
which is capable of operating at high speed and will not allow much 
leakage of light when closed and has a high transmissivity of light when 
opened. Such a device will permit construction of an electro-photographic 
picture of high resolving power and high quality which will operate at 
high speed, is simple in construction, reliable in operation, of a small 
size and inexpensive to construct. 
SUMMARY OF THE INVENTION 
Generally speaking, a liquid crystal optical device including a first 
transparent plate having at least one common electrode and an opposed 
transparent plate disposed in confronting relation to the first-mentioned 
transparent plate and having a plurality of signal electrodes disposed 
thereon, a liquid crystal composition sealed between the transparent 
plates, and polarizers disposed on each side of the liquid crystal panel 
is provided. The liquid crystal composition includes a nematic liquid 
crystal material having dielectric electric anisotropy which becomes zero 
at a crossing frequency (hereinafter referred to as "fc") of 100 kHz, or 
below at ordinary temperatures, and an optically active material added to 
the nematic liquid crystal material. The optical device further includes a 
drive circuit for selectively applying a signal having a frequency higher 
than fc (hereinafter referred to as "fh") and a signal having a frequency 
lower than fc (hereinafter referred to as "fl") across the common 
electrode and the signal electrodes for opening and closing the liquid 
crystal optical device. The liquid crystal layer has a thickness which is 
three times or less as large as the helical pitch of the liquid crystal 
composition. In a preferred embodiment, the two polarizers are disposed 
with their polarizing axes perpendicular to each other and the optical 
device is closable by application of a signal of frequency fl across 
opposed electrodes and the optical device is openable by application of 
the signal of frequency fh across the opposed electrodes. 
A printing device in accordance with the invention includes a light signal 
generator having a source of light, a liquid crystal light valve including 
the liquid crystal optical device in accordance with the invention and 
liquid crystal driving circuit, a photosensitive member, a developing 
section and a fixing section. Accordingly, it is an object of the 
invention to provide an improved liquid crystal optical device. 
It is another object of the invention to provide an improved liquid crystal 
optical device including an optically active nematic liquid capable of 
dielectric relaxation at low frequencies for high-speed responsiveness. 
It is a further object of the invention to provide high-speed light valves 
which may be closed and opened at high speed within small periods of time, 
do not allow much leakage of light when closed and have a high 
transmissivity of light when opened. 
Still another object of the invention is to provide an improved liquid 
crystal composition including a nematic liquid crystal composition 
including an optically active liquid crystal material. 
Still a further object of the invention is to provide an improved printing 
device including a liquid crystal light valve. 
Yet another object of the invention is to provide an improved 
electro-photographic printer. 
Yet a further object of the invention is to provide an improved printing 
device including a high-speed liquid crystal light valve. Another object 
of the invention is to provide an improved printing device including a 
high-speed liquid crystal light valve wherein the liquid crystal 
composition includes an optically active material added to a nematic 
liquid crystal material. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specifications. 
The invention accordingly comprises compositions possessing the 
characteristics, properties and relation of components which will be 
exemplified in the compositions described and the features of 
construction, combinations of elements, and arrangement of parts which 
will be exemplified in the constructions hereinafter set forth, and the 
scope of the invention will be indicated in the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A conventional twisted nematic liquid crystal system will be described 
first as an illustrative example Although the description is sufficient 
for applications which do not need to meet the characteristics (4) and (5) 
noted above, the principles of the present invention permit use for 
applications which do need to meet conditions (4) and (5). 
FIG. 1 shows the arrangement of a twisted nematic liquid crystal panel. A 
liquid crystal composition 8 is sandwiched between transparent base plates 
1 of glass sealed by a seal 3 and having transparent electrodes disposed 
thereon. Base plates 1 are rubbed in directions 4 and 5 such that liquid 
crystal molecules in composition 8 are oriented parallel to planes of the 
base plates 1. Direction 4 extends from the left to the right, and 
direction 5 extends away from the viewer into the paper. The major axes of 
the liquid crystal molecules are arranged in the directions of rubbing at 
the glass wall surfaces for twisted nematic molecular orientation. A pair 
of polarizers 6 and 7 are disposed outside plates 1 and are preferably 
arranged so that their polarizing directions will extend perpendicularly 
to each other. This minimizes the amount of leakage of light therethrough 
when the light valve is closed. The directions of rubbing preferably 
coincide with or extend normally to the directions of polarization for 
increased light transmission when the light valve is opened. Designated at 
9 is the thickness of the liquid crystal layer which will hereinafter be 
indicated as "d." 
The responsiveness characteristic of the twisted nematic liquid crystal 
panel of FIG. 1 thus constructed is shown in FIG. 2. The nematic liquid 
crystal composition E24LV was used which is primarily a biphenyl liquid 
crystal manufactured by BDH, Co. The characteristics of the liquid crystal 
panel were measured at a temperature of 40.degree. C., the thickness of 
the liquid crystal layer being about 5 .mu.. The graph of FIG. 2 has a 
horizontal axis indicating time in terms of milliseconds, and a vertical 
axis indicating light transmissivity in terms of percentage. Light 
transmissivity is 100% with only two polarizers having parallel polarizing 
axes. FIG. 3 shows signals applied between the electrodes at voltages +a 
and -a (V) with no voltage applied in an interval of time between points S 
and E. Curves 10 and 11 of FIG. 2 indicate responsivenesses of the liquid 
crystal when the voltages of a=3.5 (V) and a=20 (V) are applied, 
respectively. With respect to responsiveness characteristics of twisted 
nematic liquid crystal panels, the time before a point I is frequently 
called a "rise time" and the time after a point E or N is frequent called 
a "fall time." Such designations will be used in the present description. 
Curve 10 shows responsiveness when a normal voltage is applied, the rise 
and fall times being several tens of milliseconds. Curve 11 was obtained 
when a higher voltage was applied and shows a much faster rise time, but a 
slower fall time past a peak indicated at 12. Peak 12 is derived from an 
intrinsic property of the twisted nematic liquid crystal. For the higher 
the applied voltage is, the longer time is needed to rearrange the twisted 
nematic molecular orientation when the applied voltage is cut off. The 
responsiveness shown in FIG. 2 represents the general characteristic of 
twisted nematic liquid crystal panels, including E24LV manufactured by 
BDH, Co. The conventional twisted nematic liquid crystal panels are 
therefore disadvantageous in that the response time, particularly the fall 
time cannot be reduced. One way to solve the problem is to utilize the 
phenomenon of dielectric relaxation in the direction of major axes of 
liquid crystal molecules for switching with signals of two different 
frequencies. 
FIG. 4 illustrates generally the frequency characteristic of dielectric 
anisotropy of a liquid crystal composition in which dielectric anisotropy 
is reversible. Assuming that a component of the dielectric constant 
parallel to the axes of liquid crystal molecules is indicated by 
.epsilon..sub.11 and a component thereof normal to the axes of liquid 
crystal molecules is indicated by .epsilon..sub.1, dielectric anisotropy 
can be expressed by .epsilon..sub.11 -.epsilon..sub.1. In FIG. 4, a 
frequency at which the dielectric anisotropy becomes zero is called a 
"crossing frequency," indicated as "fc". The dielectric anisotropy is 
reversed at frequencies above and below crossing frequency fc. It is 
assumed that a frequency below crossing frequency fc is indicated as "fl," 
a frequency above the crossing frequency fc is indicated as "fh," the 
dielectric anisotropy at frequency fl is indicated as 
".DELTA..epsilon..sub.L," and the dielectric anisotropy at frequency fl is 
indicated as ".DELTA..epsilon..sub.H." .DELTA..epsilon..sub.L is positive 
and .DELTA..epsilon..sub.H is negative. When the signal of fl is applied 
to a twisted nematic liquid crystal panel employing a nematic liquid 
crystal having such a property, the major axes of, liquid crystal 
molecules extend parallel to the electric field produced. When the 
electric field is removed, the liquid crystal molecules are rearranged 
into a twisted nematic orientation. Stated otherwise, the panel has 
responsivenesses as shown in FIG. 2 in response to the applied signals as 
shown in FIG. 3, as with the general twisted nematic liquid crystal 
panels. (Since the example given in FIG. 2 is indicative of a 
characteristic of E24LV, actual values of voltages and response times may 
differ dependent on liquid crystal compositions used.) The applied signal 
13 has a frequency fl, and the two polarizers have polarizing axes which 
extend perpendicular to each other. Then, signals as illustrated in FIG. 5 
are applied, the signals being different from those of FIG. 3 in that 
signal fh is included in a region indicated at 14. When signal fl is 
applied, the major axes of liquid crystal molecules are forced to extend 
parallel to the electric field. Application of signal fh thus produces a 
force tending to bring the major axes of the liquid crystal molecules 
parallel with surfaces of the electrodes. Nematic liquid crystal 1085 
manufactured by Merck & Co., Inc. produces dielectric relaxation at 
ordinary temperatures. FIG. 6 illustrates the characteristics of this 
liquid crystal in response to signals applied. The liquid crystal was 
measured at a temperature of 40.degree. C. with the thickness of the 
liquid crystal layer being about 5 .mu., fl:1 KHz, fh:100 KHz, and the 
voltage a=20 (V). The curve of FIG. 6 shows that fall time is faster than 
that of the ordinary twisted nematic liquid crystal panel as shown in FIG. 
2. This is because application of a signal of fh forcibly reduces the fall 
time. With the phenomenon of dielectric relaxation of the liquid crystal 
molecules being thus utilized, the fall time can be reduced from the 
disadvantageously long interval with the normal twisted nematic liquid 
crystals. This arrangement however cannot meet the foregoing 
characteristics required of high-speed light valves. FIG. 7 shows the 
responsiveness characteristic of Merck's nematic liquid crystal 1085 
obtained when a signal, of 100 KHz was applied during one millisecond at 
the voltage a=30 (V) with the same conditions as those for the 
characteristic curve in FIG. 6. The responsiveness characteristic of FIG. 
7 indicates insufficient response times and light transmissivity. 
The foregoing conventional arrangements show that the characteristics, such 
as response times, which are required of high-speed light valves cannot be 
realized. Hence, various devices utilizing liquid crystal light valves 
have not been prepared as a practical matter. 
The general structure of a conventional printing device using a liquid 
crystal light valve and the problems encountered with such a printing 
device will be described. Referring to FIGS. 8 and 9, a printing device 
including a liquid crystal light valve 22 is shown schematically. A source 
of light 21 is energized at all times to illuminate valve 22 continuously. 
Liquid crystal light valve 22 has a plurality of minute shutters 28 which 
are independently optically openable and closable by a liquid crystal 
driving circuit 29 to allow and prevent light transmission from light 
source 21. These parts jointly constitute a light signal generator 30. 
Light signals thus generated will reach a photosensitive member 23 of 
photosensitive material which has been charged by a charging station 26, 
whereupon the area of member 23 hit by the light signals is discharged. 
Therefore, electrostatic latent image is formed on photosensitive member 
23 in accordance with the external writing signals. The electrostatic 
latent image thus created is developed at a developing section 24 with 
coloring toner. The toner image is transferred at a transferring section 
25 onto a recording material, such as, for example, paper and fixed with 
heat at a fixing section 27, thereby producing a completely fixed printed 
image. 
Since such a system does not require what corresponds to a precision and 
high-speed optical scanning system as in a laser printer, the described 
system has various merits, such as a simpler structure and a reduced cost 
of manufacture. However, known printing devices have not been practical 
due to a fatal problem, that is, the slow speed of writing. 
Printers for the above-mentioned various systems proposed require a 
printing speed which allows ten prints of the A4 size to be produced per 
minute and a resolving power of about 10 dots per 1 mm. Stated otherwise, 
such performance requires a printing speed of approximately 500 lines in 
one second, that is, one line which can be written in 2 m sec. or less. 
The time interval of 2 m sec. for opening shutters cannot be achieved by a 
conventional twisted nematic liquid crystal driven by a normal alternating 
voltage. 
While there is demand for printing devices which will operate at such high 
speeds, is of high quality, less costly and small in size, there are no 
available proposals which sufficiently meet such a requirement. The 
present invention satisfies such demand and provides a printing device 
which will operate at a sufficiently high speed, is of high quality, and 
is small in size and less costly to construct. 
FIG. 10 illustrates a liquid crystal panel including construction of a 
liquid crystal light valve particularly well suited for use as a light 
valve for a printing device. Designated at 31 is a glass plate having 
signal electrodes 36, 36-1 and 36-2 thereof. Electrode 36 is a transparent 
body of indium oxide or stannous oxide, and electrodes 36-1, 36-2 are made 
of chromium and gold, respectively. A glass plate 32 supports common 
electrodes 37, 37-1 and 37-2 thereon. Electrode 37 is also transparent, 
and electrodes 37-1 and 37-2 are opaque bodies of a metal material. A 
transparent portion 38 corresponds to shutters 28 as shown in FIG. 9. 
Glass plates 31 and 32 are fixed together by sealant 33 to provide a space 
34 in which a liquid crystal compound 34 is sealed and polarizers 35 are 
positioned on both sides of the device. 
The advantageous features of liquid crystal light valve of the present 
invention having a responsiveness characteristic of light as transmitted 
through the liquid crystal light valve will first be described prior to 
the description of the printing characteristic. FIG. 11 is a block diagram 
of an optical measurement device which was used to measure light 
transmission responsiveness of a liquid crystal light valve according to 
the present invention. Light from a light source 39 is converted into 
parallel rays of light 41 by a lens 40 which illuminates a liquid crystal 
light valve 42. Liquid crystal light valve 42 is driven by a driver unit 
44 to allow light to be transmitted through light valve 42 and fall upon a 
light detector 43. Signals 49 are amplified by an amplifier 45 are stored 
in a digital memory 46 by synchronous signals 48 from driver unit 44. 
Signals from memory 46 are recorded by a recorder 47. The light 
transmission responsiveness of a liquid crystal valve was measured in the 
above process. Values with respect to the liquid crystal compound, such as 
dielectric anisotropy, fc and the like referred to in the present 
description were measured by multi-frequency LCR meters 4274A and 4275A 
manufactured by YHP, Co. 
Characteristics required of a liquid crystal light valve are as follows: 
(1) The liquid crystal light valve should have a high frequency of 
repetition and should not exhibit hysteresis; and 
(2) The liquid crystal light valve should have a large light transmissivity 
while it is open. 
The foregoing two are important characteristics to be met. The reciprocal 
of the frequency of repetition is an interval of time for writing a single 
line. 
The requirement that the liquid crystal light valve have no hysteresis is a 
very important consideration. Conventionally, the dynamic drive system of 
a liquid crystal device relies on the cumulative response effect which is 
a hysteresis effect of the liquid crystal. Such an effect is utilized with 
ordinary twisted nematic liquid crystals and a two-frequency dynamic drive 
system. However, such an effect is disadvantageous for the high-speed 
liquid crystal light valve according to the present invention, and hence 
should be avoided as much as possible. The hysteresis effect can be 
reduced by applying a signal of fl during an interval of time for each 
writing period which makes the potential level in the liquid crystal 
uniform at all times, and then driving the liquid crystal for entering the 
next interval of time for writing. 
The expression, "It is open," in the requirement (2) above means that a 
signal for opening the shutter is applied, or the shutters are actually 
open (both are substantially the same in meaning). The expression "an 
opening interval of time" as described below means an interval of time 
during which a signal for opening the shutters is applied. 
The light transmissivity of a liquid crystal light valve will now be 
described with reference to FIG. 12. FIG. 12 shows an ideal light 
transmissivity at (a) obtained when signals at (b) are applied to a liquid 
crystal valve. Indicated at V1 is the voltage, at T1 is the interval of 
time for writing, and T2 is an opening interval of time. A signal 51 
applied during time interval T2 has a frequency higher than fc, such 
frequency being referred to as fh. The shutter allows 100% of the light to 
be transmitted during a time interval t.sub.1 after signal 51 is first 
applied. (It is assumed that light transmission is 100% when the 
polarizing planes of the two polarizers are parallel to each other, and 
that light transmission is 0% when the polarizing planes are perpendicular 
to each other.) A signal 52 of frequency fl applied during interval of 
time T3 is below fc, and is referred to as fl. The shutter is closed time 
interval t2 after application of signal 51 ceases. Although interval of 
time T2' is equal to time interval T2, the frequency of signal 53 applied 
during T2' is different from fh in that signal 53 does not open the 
shutter. (While the frequency of the signal 53 may be of any value and may 
even be higher than fc so long as it does not open the shutter, it is 
preferably the same as fl for the ease with which signals are generated.) 
Signals for driving the liquid crystal light valve according to the present 
invention will now be described. As illustrated in FIG. 12 at (b), T1 
represents a period of time for writing (or an interval of time for 
writing), T2 an opening interval of time, T2' a non-opening interval of 
time, and T3 a closing interval of time. The interval of time T1 for 
writing includes the opening interval of time T2 and the closing interval 
of time T3, or of the non-opening interval of time T2' and the closing 
interval of time T3. The signal of fh is basically applied during the 
opening interval of time T2. However, a signal of other frequencies for 
opening the liquid crystal light valve will suffice (for example, a 
combination of fh and fl may be suitable with fh certain to open the 
liquid crystal light valve). 
The signal of fl is applied during the closing interval of time T3. 
However, a signal of other frequencies for closing the liquid crystal 
light valve will suffice (for example, a combination of fl and fh may be 
suitable with fl certain to close the liquid crystal light valve). During 
the interval of time T2', a signal which does not render the liquid 
crystal light valve open is applied. More specifically, a signal which is 
supposed to open the liquid crystal light valve, but does not during T2' 
because of slow responsiveness will suffice. For example, such a signal 
may be of a zero voltage applied to the liquid crystal light valve, may be 
of a frequency higher than fc but slow in responsiveness, and may be of a 
frequency of fl. The foregoing signals may be of a rectangular waveform, a 
sine waveform, or other waveforms. Light transmission responsiveness will 
again be described. An ideal light transmission property, as shown in FIG. 
12 at (a), has short t.sub.1, t.sub.2, allows 100% light transmission, 
exhibits no hysteresis (responsiveness is governed only by a signal 
applied irrespective of previous history, r-1, r-2, r-3, r-4 having the 
same responsiveness), and has a short interval of time T1 for writing. 
While 100% light transmission is not reached with no hysteresis exhibited 
for an example (c), a liquid crystal light valve having the characteristic 
(c) will suffice since irregularities in light transmission responsiveness 
do not exist, and an amount of light transmitted can be increased by using 
a more powerful source of light. At curve (d), r-1, r-2, r-3, r-4 are all 
different and light transmission again does not reach 100%. At curve (e), 
some signals allow 100% light transmission, but not all at the same 
responsiveness. Prior twisted nematic liquid crystal displays and 
two-frequencies driving displays have relied on the cumulative response 
effect for display. Although liquid crystals exhibit substantial 
hysteresis, such an effect should be reduced as much as possible to obtain 
characteristics suitable for liquid crystal light valves. 
Actual measurements in a liquid crystal light valve as shown in FIG. 10 
will now be described. FIG. 13 shows at (a) the frequency characteristic 
of dielectric anisotropy of nematic liquid crystal No. 1085 manufactured 
by Merck & Co. Inc., which is known as a liquid crystal which can produce 
dielectric relaxation. A layer of this liquid crystal having a thickness 
of 5.mu. was measured for light transmissivity at a temperature of 
40.degree. C., fl of 1 KHz, T1 of 2 m sec., T2 of 1 m sec., V1 of 30 V and 
fh of 100 KHz and 130 KHz. The measurements are shown in FIG. 14 at (a) 
and (b). A signal of fh was applied in time interval T2, and signals of fl 
were applied in time intervals T2' and T3. At (a), r-1 and r-2 show the 
same responsiveness irrespective of the previous conditions and have a 
maximum light transmissivity of 25%, but are characteristics which can be 
suitable for a liquid crystal light valve. Measurements of (b) have 
different r-1 and r-2, and are not suitable for characteristics of a 
liquid crystal light valve. The example of (a), though acceptable, has a 
25% light transmissivity which is disadvantageous in that light 
transmissivity varies substantially as fh varies slightly. Additionally, 
responsiveness changes as temperature varies slightly. The present 
invention eliminates all of these problems and provides a liquid crystal 
light valve which will operate at quite a high speed and is of increased 
light transmissivity. 
Accordingly, two important aspects of the present invention, first an 
optically active material is added to a nematic liquid crystal which can 
cause dielectric relaxation to produce a cholesteric liquid crystal 
capable of causing dielectric relaxation. The cholesteric liquid crystal 
is drivable by signals of fl and fh (the cholesteric liquid crystal herein 
used includes not only derivatives of cholesterol, but also liquid crystal 
compounds having cholesteric liquid crystal ordering by adding an 
optically active material such as a chiral nematic liquid crystal to a 
nematic liquid crystal). Secondly, the helical pitch of the cholesteric 
liquid crystal of the present invention is substantially the same as the 
distance, between the two glass plates. Thirdly, since the display device 
(for switching between light and dark indication) relies on the 
characteristic of polarizers, the liquid crystal light valve can be closed 
completely. Fourthly, as shown at (b) in FIG. 12, the shutter is closed 
and then opened for successive opening thereof. In other words, time 
interval T2 for the application of a signal of fh is constant, and a 
signal of fl is applied during time interval T3 in time interval T1 for a 
single writing. 
The characteristic of light transmission responsiveness of a liquid crystal 
light valve according to the present invention, which has the foregoing 
feature will be described. FIG. 15 illustrates the light transmission 
responsiveness characteristics of a liquid crystal light valve according 
to the present invention. The measurement conditions were as follows; 
thickness of a liquid crystal layer: 5.mu.; temperature: 35.degree. C.; 
fl: 2 KHz; fh: 130 KHz; T1: 2 m sec; T2: 1 m sec and V1: 30 V. The maximum 
light transmissivity was 100% compared to the results example of (a) in 
FIG. 14 which had a maximum light transmissivity of only 25%. 
The difference between the transmissivity of the examples of FIG. 14 and 
the example of FIG. 15 is based on the difference between the liquid 
crystal compounds used. The examples of FIG. 14 used nematic liquid 
crystal No. 1085 manufactured by Merck & Co., Inc. and the example of FIG. 
15 in accordance with the present invention used a cholesteric liquid 
crystal having a long helical pitch. This latter crystal composition was 
prepared by adding to the foregoing nematic liquid crystal (Merck No. 
1085) 3 percent by weight of an optically active material, such as, 
4-(2-methylbutyl)-4'-cyanobiphenyls. The frequency characteristic of 
dielectric anisotropy of this liquid crystal compound is shown in FIG. 13 
at (b). It indicates higher fc than that for curve (a) which was obtained 
without addition of an optically active material, and a greater dielectric 
anisotropy at 0 Hz than that for curve (a). Thus, addition of an optically 
active material advantageously resulted in an increase in dielectric 
anisotropy and an increase in light transmissivity of the liquid crystal 
light valve. Another advantage is that hysteresis has been eliminated. 
The present invention will now be described for its usefulness with 
reference to examples using other liquid crystal compositions. Nematic 
liquid crystal compositions such as the composition listed in Table 1 
exhibit dielectric relaxation at room temperature. These compositions are 
substantially the same as those in Example 3 described in Japanese Patent 
Application No. 55-81426 (hereinafter referred to as "Liquid Crystal-I"). 
Liquid Crystal-I is no optically active. 
TABLE I 
__________________________________________________________________________ 
Percentage 
of mixture 
Compounds (wt %) 
__________________________________________________________________________ 
##STR1## 8 
##STR2## 8 
##STR3## 6 
##STR4## 6 
##STR5## 6 
##STR6## 6 
##STR7## 6 
##STR8## 12 
##STR9## 12 
##STR10## 12 
##STR11## 12 
##STR12## 6 
__________________________________________________________________________ 
##STR13## 
FIG. 16 shows at (a) the frequency characteristic of dielectric anisotropy 
of the nematic liquid crystal compounds indicated in Table 1 and at (b) 
the frequency characteristic of dielectric anisotropy of a cholesteric 
liquid crystal compound having a long helical pitch obtained by adding to 
of Table I having the characteristic curve (a) 3.05 by weight of the 
foregoing optically active material, 4-(2-methylbutyl)-4'-cyanobiphenyls. 
FIG. 17 shows the deviation in fc when optically active material is added 
in varying amounts to the liquid crystal composition of Table 1. 
FIG. 19 shows the responsiveness characteristics of light transmission 
obtained when the amount of the optically active material is changed. FIG. 
19 illustrates at (i), (ii), (iii) and (iv) the characteristics of 
including 0,0.6,2.45 and 3.05 percent by weight respectively, the 
optically active material added. Curves (a), (b) and (c represents the 
light transmissivity based on the different signals applied as shown in 
FIG. 18. 
In FIG. 18, T2, T3 and T2' have the same meanings as those shown in FIG. 12 
at (b). At (a) a waveform of a signal of fh applied for 1 m sec. and a 
signal of for 1 m sec. is shown, such combined signals being applied 
repetitively. At (b) a waveform of a signal of fh applied for 1 m sec. a 
signal of fl applied for 3 m sec is shown, the combined signal being 
applied repetitively. At (c) a waveform of a signal of fh for 1 m sec. and 
a signal of fl applied for 15 m sec is shown, the combined signals being 
applied repetitively. 
The signal (a) is applied for successive openings, the signal (b) is 
applied for one opening during two intervals for writing, and the signal 
(c) is applied for one opening during four time intervals for writing. An 
ideal liquid crystal light valve must have the same responsiveness with 
respect to the opening signals during T2 at (a), (b) and (c). 
FIG. 19 will now be described again wherein the signals were applied under 
the following conditions: temperature: 40.degree. C.; thickness of the 
liquid crystal layer: 5 .mu.m; fh: 100 KHz and fl: 1 KHz. FIG. 19 
illustrates at (i) characteristics obtained without optically active 
material added. The liquid crystal light valve does not close when the 
signal of curve (a) of FIG. 18 is applied. When signals of curves (b) and 
(c) are applied, responsiveness of light transmission differs. At (ii) the 
characteristics with 0.6 percent by weight of the optically active 
material added are shown. The characteristics of (ii) are better than 
those of (i) in that the response to the signal of curve (a) exhibits 
better responsiveness. At (iii) the characteristics with 2.45 percent by 
weight of the optically active material are illustrated. While the 
response to signals of curves (b) and (c) are ideal, the response to the 
signal of curve (a) indicates a low light transmissivity. However, the 
response to the signal of curve (a) will be sufficient by reducing the 
repetitive frequency by half with T2 of 1 m sec. and T3 of 3 m sec. FIG. 
19 shows at (iv) the characteristics obtained when 3.05 percent by weight 
of the optically active material is added. The characteristic curve 
coincides completely with the responsiveness characteristic of the ideal 
liquid crystal light valve as shown in FIG. 12 at (a). The period of time 
for writing is 2 m sec. which enables high-speed operation of a light 
valve. 
A liquid crystal light valve having the responsiveness characteristic of 
light transmission as shown in FIG. 19 at (iv) is fully satisfactory for 
use as a light signal generator in a high-speed printer. Thus, as 
described above, a liquid crystal light valve which previously was merely 
an idea, and has not been practiced is now rendered feasible as a 
practical matter in accordance with the present invention. 
The construction of a liquid crystal light valve of the present invention 
suitable for use in a printing device will be described once again. The 
structure of such a liquid crystal panel is as shown in FIG. 10. Two glass 
plates 31 and 32 are treated for horizontal molecular orientation. Glass 
plates 31 and 32 need not necessarily be oriented perpendicular to each 
other. However, parallel orientation preferably should be avoided. If they 
are oriented parallel to each other, the ratio of the intensity of light 
transmitted when the liquid crystal light valve is open to that when the 
light valve is closed would be small. Two polarizers 35 are disposed on 
the outer sides of glass plates 31 and 32. With polarizing planes of 
polarizers 35 perpendicular to each other, when the liquid crystal light 
valve is closed, the intensity of light transmitted is reduced to a 
minimum (reduced leakage of light), such leakage of light being 
controllable by adjusting the polarizing plane. 
Orientation of liquid crystal molecules sealed within the device differs 
with the thickness of the liquid crystal layer and the amount of the 
optically active material. In the example of FIG. 19 at (iv) (treated for 
horizontal orientation with directions of orientation extending 
perpendicularly to each other) the liquid crystal has a helical structure 
twisted through one revolution and 90.degree. (or 450.degree.) without 
application of an electric field. At this time, the liquid crystal takes 
on a light blue color, as the helical pitch is short. When a signal of fl 
is applied, the liquid crystal molecules arrange parallel to the applied 
electric field and lose their optical activity. This results in a dark 
color due to the effect of the polarizers. With the electric field 
removed, only portions which have been subjected to the electric field are 
discolored. At this time, the molecules are ordered by a helical 
configuration twisted 270.degree.. Upon elapse of a certain interval of 
time after the electric field has been eliminated, the liquid crystal 
molecules are reoriented to their initial state. When a signal of fh is 
applied, it is presumed that the molecules are also oriented in a helical 
configuration of 270.degree., as will be described with reference to FIG. 
20. 
When there is no electric field applied, the light transmissivity is C1 
which is short of 100%. When a signal of fl is applied at E1, the light 
transmissivity becomes 0%. When the electric field is cut off at E2, the 
light transmissivity reaches C2 which is 100%. After a while, the light 
transmissivity decreases to C3 which is at the same level as C1. As signal 
of fl is applied at E3, and a signal of fh is applied at E4, whereupon the 
light transmissivity becomes C4 which is the same as C2, or 100%. 
Application of a signal of fl at E5 reduces the light transmissivity down 
to 0%. This constitutes a responsiveness characteristic of light 
transmission which is the same as at (iv) in FIG. 19. 
As described above, the liquid crystal molecules are stable when helically 
configured through 450.degree., but are ordered as a quasi-stable state by 
a helical configuration of 270.degree. when responding to applied signals 
at a high speed. When a signal of fl is applied to a layer of the liquid 
crystal compound having a thickness of 8 .mu.m, the liquid crystal layer 
is in a turbid state. Continued application of the signal of fl eliminates 
the turbidity, and application of a drive signal allows the liquid crystal 
layer to be operated normally. When the drive signal is applied while the 
liquid crystal layer is turbid, however, the light transmissivity is 
reduced. To cope with this problem, it is necessary to continue to apply 
the signal of fl across such a thick liquid crystal layer until the liquid 
crystal layer is no longer turbid. With a layer of the foregoing liquid 
crystal compound having a thickness of 4 .mu.m, the above-mentioned 
phenomenon does not take place, and the molecules are ordered at all times 
by a helical configuration twisted through 270.degree.. This liquid 
crystal layer has a light transmissivity which is 100% at all of C1, C2, 
C3 and C4 as indicated in FIG. 20. 
This demonstrates that even with irregularities in thickness in the same 
cell and irregularities in light transmission when not in operation, 
application of a signal of fl for a predetermined interval of time 
eliminates irregularities in light transmitted and results in uniform 
light transmission. (Application of an actual drive signal takes more 
time, but achieves the same result as when only a signal of fl is 
applied). Therefore, a large-sized panel which is 20 cm long for use as a 
liquid crystal light valve does not suffer from irregularities in 
thickness and can be constructed with utmost ease. Such an advantage is 
very important for an optical writing device prepared in accordance with 
the present invention. 
The liquid crystal optical devices in accordance with the present invention 
include: (i) a liquid crystal composition; (ii) a liquid crystal panel; 
and (iii) a system for driving the liquid crystal. Details are as follows: 
(i) The liquid crystal composition comprises (a) an optically active 
nematic liquid crystal, or an optically inactive nematic liquid crystal 
rendered optically active by the addition of an optically active material, 
and (b) such a nematic liquid crystal has a characteristic such that 
dielectric relaxation is produced in the direction of the major axes of 
liquid crystal molecules and dielectric anisotropy is reversible, the 
dielectric anisotropy having a frequency characteristic which is the same 
as that shown in FIG. 4; 
(ii) The liquid crystal panel includes (a) the liquid crystal composition 
sandwiched between two transparent base plates and sealed by a sealant, 
(b) the two base plates each having at least one electrode disposed 
thereon, the electrodes extending in crossing and confronting relation to 
each other and the confronting parts being transparent for use as a 
display or light valve, (c) the two base plates having their surfaces 
treated for parallel molecular orientation at the surfaces, and (e) 
polarizers are disposed, one on each side of the two base plates so that 
the polarizing plates extend perpendicularly to each other; and 
(iii) The liquid crystal driving system selectively applies signals of fl 
and fh between the electrodes to change liquid crystal molecular 
orientations, the arrangement being that the two molecular orientations 
and polarizers are combined to block and transmit light. 
The following are the advantages gained by combining crossed polarizers and 
a nematic liquid crystal composition which is optically active and capable 
of exhibiting dielectric relaxation in the direction of the major axes of 
the liquid crystal molecules for realizing a highly advantageous 
high-speed responsiveness characteristic according to the present 
invention. The following refers to Liquid Crystal-I of Table I above. 
To this liquid crystal composition is added 3 percent by weight of 
4-(2-methylbutyl)-4'-cyanobiphenyls (liquid crystal CB-15 manufactured by 
Merck & Co., Inc.) which is an optically active material known as a chiral 
nematic liquid crystal to produce a liquid crystal composition 
(hereinafter referred to as "Liquid Crystal-II"), and is added 3 percent 
by weight of 4-n-amyl-4'-cyanobiphenyls which is an optically inactive 
nematic liquid crystal (hereinafter referred to as "Liquid Crystal-III"). 
The frequency characteristics of the dielectric anisotropy of these liquid 
crystal compositions are shown in FIG. 16 at (b). The difference between 
the dielectric anisotropies and frequency characteristics of Liquid 
Crystal -II and Liquid Crystal III is within measurement errors, and is 
negligibly small. 
While both of the cyanobiphenyl liquid crystals have an alkyl group 
comprising five carbons, the CB-15 has an asymmetric carbon and hence is 
optically inactive. Accordingly, Liquid Crystal-II with the 
4-(2-methylbutyl)-4'-cyanobiphenyls liquid crystal added is optically 
active, and Liquid Crystal-III with the 4-n-amyl-4'-cyanobiphenyls liquid 
crystal added is optically inactive. Liquid Crystal II with the optically 
active material added is in the helical cholesteric liquid crystal phase 
with an intrinsic helical pitch being about 4 .mu.m at room temperatures. 
The intrinsic pitch is defined in the description such that one pitch is 
given by a 360.degree. rotation. 
FIG. 21 shows the responsiveness characteristic of light transmissivity of 
the foregoing liquid crystal compositions sealed in the liquid crystal 
panel of FIG. 1. FIG. 23 illustrates the driving signals applied to the 
liquid crystal materials. A signal 34 which is defined as A and a signal 
35 which is defined at A are applied to the opposite electrodes, A and A 
denoting rectangular waves of opposite phases. Designated at T2 (32) and 
T3 (33) are intervals of time in which opening and closing signals are 
applied, respectively, and such intervals of time are contained in a 
period of time T1 (31) and are repeated. T1 being referred to as writing 
synchronization. In T2 are applied signals fh indicated at 36, 38 in 
opposite phases, and in T3 are applied signals fl indicated at 37 and 39 
in opposite phases. When the signals A and A are applied between the 
confronting electrodes, signals +V1 and -V1 are applied across the liquid 
crystal layer. 
In FIG. 21, designated at 40, 41 and 42 are characteristic curves of 
responsiveness of Liquid Crystal-I, Liquid Crystal-II and Liquid 
Crystal-III. The measurement was made of the liquid crystals having a 
thickness of 5 to 5.5 82 m at a temperature of 40.degree. C. with T1=2 m 
sec., T2=1 m sec., T3=1 m sec., fh=130 KHz, fl=1 KHz and V1=30 (v). 
Addition of an optically active material to Liquid Crystal-I changes curve 
40 into the curve 41 of Liquid Crystal-II. It is apparent from comparison 
with the curve 42 of Liquid Crystal-III that addition of the optical 
material makes such a difference in responsiveness. The responsiveness 
curve 42 of Liquid Crystal-III is slower than the responsiveness curve 40 
due to a reduced .DELTA..epsilon.H as shown by the dielectric anisotropy 
as shown in FIG. 16. 
The above measurement data provide the following important result. Addition 
of an optically active material greatly improves the responsiveness 
characteristic, and this results from the fact that the liquid crystal 
composition itself is optically active. More specifically, the difference 
in responsiveness between Liquid Crystal-II and Liquid Crystal-III which 
have the same dielectric anisotropy results from the fact that the former 
is optically active and the latter is optically inactive. Incidentally, 
Liquid Crystal-III has a liquid crystal layer twisted 90.degree. due to 
orientation treatment of the liquid crystal panel, and Liquid Crystal-II 
has a 450.degree.-twisted structure due to the orientation treatment, 
intrinsic helical construction and thickness of the liquid crystal layer. 
The responsiveness characteristics shown in FIG. 22 will be utilized to 
explain that being optically active is the governing factor, irrespective 
of the kinds of optically active materials used. To Liquid Crystal-I were 
added 2.2 percent by weight of optically active materials, that is, a 
chiral nematic liquid crystal, 4-(4-hexyloxybenzoloxy)-benzoic 
acid-d-2-octyl ester (liquid crystal compound S811 manufactured by Merck & 
Co., Inc.) and a cholesteric liquid crystal cholesteryl nonanoate 
(CH.sub.3 (CH.sub.2).sub.7 COOC.sub.27 H.sub.45), to produce liquid 
crystal compositions, referred to as Liquid Crystal-IV and Liquid 
Crystal-V, respectively. The frequency characteristics of dielectric 
anisotropy of these liquid crystals coincide with that of Liquid Crystal-I 
as shown in FIG. 16 at (a). Both Liquid Crystal-IV and Liquid Crystal-V 
have an intrinsic pitch which is about 4 .mu.m at room temperatures. In 
FIG. 22 Liquid Crystal-I has a responsiveness characteristic indicated by 
curve 40, Liquid Crystal-IV has a responsiveness characteristic indicated 
by the curve 43 and Liquid Crystal-V has a responsiveness characteristic 
indicated by the curve 44. The measurements were made for a liquid crystal 
layer having a thickness of 5 to 5.5 .mu.m at a temperature of 40.degree. 
C. with T1=3 m sec., T2=2 m sec., T3=1 m sec., fh=130 KHz, fl=1 KHz and 
V1=30 (v). FIG. 22 indicates that Liquid Crystal-IV and Liquid Crystal-V 
have greatly improved responsiveness characteristics. The results show 
that the improved responsiveness characteristics do not depend on the 
particular optically active materials added. 
The present invention is not dependent on the amount of an optically active 
material added, but on the relationship between the helical pitch of a 
liquid crystal layer generated by the added optically active material and 
the thickness of the liquid crystal layer. How the amount of the optically 
active material added affects the responsiveness characteristic will now 
be described. FIG. 26 shows the relationship between the helical pitch and 
the amount of the chiral nematic liquid crystal, that is, 
4-(4-hexyloxybenzoloxy)-benzoic acid-d-2-octyl ester added to a liquid 
crystal composition (hereinafter referred to as "Liquid Crystal-VI" given 
in Table 2. 
TABLE II 
__________________________________________________________________________ 
Percentage 
of mixture 
Compounds (wt %) 
__________________________________________________________________________ 
##STR14## 5 
##STR15## 17 
##STR16## 14 
##STR17## 17 
##STR18## 17 
##STR19## 6 
##STR20## 6 
##STR21## 2 
##STR22## 2 
##STR23## 4 
##STR24## 6 
##STR25## 4 
__________________________________________________________________________ 
Referring to FIG. 26, 52 to 58, inclusive, are points of measurement at 
room temperature. Addition of the chiral nematic liquid crystal to Liquid 
Crystal-VI does not substantially change dielectric anisotropy and its 
frequency characteristic. Thus, comparison of the responsiveness 
characteristic with respect to the amount of the added liquid crystal is 
equivalent to the responsiveness characteristic with respect to one 
variable, namely the "pitch". Data points 52 to 58 in FIG. 26 represent 
pitches given when 1.3, 2.0, 2.5, 3.1, 3.4, 4.5 and 5.0 percent by weight 
of the liquid crystal are added, respectively. Responsiveness 
characteristics at this time are shown in FIGS. 24 and 25 by curves 60 to 
68. Curves 60 and 61 represent responsiveness characteristics obtained 
when 0 and 0.5 percent by weight, respectively, of the liquid crystal are 
added. The curves 62 and 65 correspond respectively to data points 52 to 
58 of FIG. 26. These measurements were made of the liquid crystals sealed 
in a liquid crystal panel as shown in FIG. 1. The panel has a thickness 
ranging from 5 to 5.5 .mu.m at a temperature of 40.degree. C. with T1=2 m 
sec., T2=0.5 m sec., T3=1.5 m sec., fh=130 KHz, fl=1 KHz, V1=30 (V). The 
direction of rubbing corresponded to that indicated by arrow 5 in FIG. 1, 
however, the rubbing is in a direction opposite to the direction of arrow 
5 of FIG. 1. 
A review of FIGS. 24 and 25 indicates that there is an optimum amount of 
the optically active material which may be added. Additionally, an 
excessive amount of optically active material added causes responsiveness 
to become slower than the responsiveness without optically active material 
added. With the thickness of the liquid crystal layers being between 5 to 
5.5 .mu.m and at a temperature of 40.degree. C., angles of twist of the 
liquid crystal layers are 90.degree. for curves 60 and 61 in FIGS. 24 and 
25, 270.degree. for curve 62, 450.degree. for curves 63 and 64, 
630.degree. for curve 65, 810.degree. for curves 66 and 67 and 990.degree. 
for curve 68. When the liquid crystal layer has a structure twisted 
through substantially three rotations, no further advantage can be gained 
by adding more optically active material. Although the liquid crystal 
layer about 5 .mu.m thick has been described, substantially the same 
result about twisting can be obtained irrespective of the thickness of 
liquid crystal layers. 
As described above, it is possible to provide a high-speed light valve by 
using a liquid crystal composition comprising a nematic liquid crystal 
capable of producing dielectric relaxation in the direction of major axes 
of liquid crystal molecules with an optically active material added to the 
nematic liquid crystal and by applying signals of fh and fl. 
Driving signals which render the present invention more effective will now 
be described. The following measurements were made with a liquid crystal 
composition (hereinafter referred to as "Liquid Crystal-VII". Liquid 
Crystal-VII was prepared by adding 2.2 percent by weight of 
4-(4-hexyloxybenzoloxy)-benzoic acid-d-2-octyl ester to a nematic liquid 
crystal composition hereinafter referred to as "Liquid Crystal-VIII" as 
shown in Table III. The liquid crystal composition was measured at a 
temperature of 40.degree. C. for liquid crystal layer thicknesses of 5 to 
5.5 .mu.m, with fh=130 KHz, fl=500 Hz or 1 KHz. 
FIG. 27 shows by way of example, signals applied for opening and closing a 
liquid crystal light valve once every 2 m sec. Designated as 70 is a 
signal of fh, and as 71 is a signal of fl. At B1, the signal of fh is 
applied for 2 m sec., and the signal of fl is applied for 4 m sec. At B2, 
the signal of fh is applied for 4 m sec., and the signal of fl is applied 
for 2 m sec. At B3, the signal of fh is applied for 6 m sec. 
TABLE III 
__________________________________________________________________________ 
Percentage 
of mixture 
Compounds (wt %) 
__________________________________________________________________________ 
##STR26## 5 
##STR27## 15 
##STR28## 13 
##STR29## 13 
##STR30## 14 
##STR31## 4 
##STR32## 10 
##STR33## 7 
##STR34## 3 
##STR35## 2 
##STR36## 1 
##STR37## 1 
##STR38## 2 
##STR39## 7 
##STR40## 3 
__________________________________________________________________________ 
The signals B1, B2 and B3 are repeatedly applied to the liquid crystal. 
These signals are applied to one of the electrodes, and a signal of 0 (v) 
is applied to the other electrodes, V1 being 30 (v). The responsiveness 
characteristics are illustrated in FIG. 30. The responsiveness 
characteristics represented by curves 83, 85 and 87 correspond 
respectively to the light transmissivity response to application of 
signals B1, B2 and B3 of FIG. 27. The responsiveness characteristics 
represented by curves 82, 84 and 86 correspond to transmissivity in 
response to repeated signals in which signals of fh are applied for 1 m 
sec., 3 m sec., and 5 m sec., respectively, and signals of fl are applied 
for 5 m sec., 3 m sec., and 1 m sec., respectively. As is apparent from 
FIG. 30, responsiveness characteristics are different with respect to all 
of the applied signals. Thus, the light transmissivity varies with the 
applied signals. 
FIG. 28 illustrates the waveforms of examples of signals applied in 
accordance with the present invention. Signals Cl, C2 and C3 correspond 
respectively to the signals B1, B2 and B3 of FIG. 27. The examples of 
signals of FIG. 27 differ from those of FIG. 28 in that the repetitive 
period T1 is divided into T2 and T3 in FIG. 28, with the signal of fh 
applied in T2 and the signal of fl applied in T3. In the examples of FIG. 
27, the opening signal is fh only, whereas the present invention utilizes 
a unit of combined signals of fh and fl. The closing signal is of fl in 
both T2 and T3. 
FIG. 31 shows responsiveness characteristics obtained when the signals C1 
to C3 are applied to one of the electrodes of the optical device while a 
signal of 0 (V) is applied to the other electrode with V1=30 (V). 
Designated as 90 is the responsiveness characteristics for C1, 90, 91 and 
92 for C3. These responsiveness characteristics are the same shape as the 
responsiveness characteristic 82 of FIG. 30. The characteristics shown in 
FIG. 31 are highly advantageous in that they permit high speed operation, 
and uniformly provide the same light transmissivity. Obtaining the 
foregoing characteristics and reducing the voltage in half can be achieved 
by a system for applying a signal shown by waveform CO in FIG. 28 to the 
electrode to which the voltage of 0 (V) is applied. Curve 95 and 96 
represent signals of fh and fl, respectively, the signals being in 
opposite phase with the signals 70 and 71. The signal CO is in opposite 
phase with the signal C3. Opening and closing signals are the same as the 
signals C1 to C3. In order to obtain the characteristics of FIG. 31, V1 
may be 15 (V) or the amplitude between +V1 and -V1 as shown in FIG. 28 may 
be changed into an amplitude between 0 and +V1. 
The effectiveness of the driving system employing a liquid crystal 
composition capable of producing dielectric distribution, and of opening 
and closing by switching between the frequencies fl and fh will be 
described again, and the features of the present invention will be 
described. Conventional signals for driving a liquid crystal as shown in 
FIG. 29 are applied to the foregoing liquid crystal light valve. 
Designated at D1, D2 and D3 are opening and closing signals corresponding 
respectively to C1, C2 and C3 of FIG. 28. The signals D1, D2 and D3 are 
applied to one of the electrodes, and the signal D0 is applied to the 
opposite electrode. The opening signals at D1, D2 and D3 are in phase with 
the signal D0 and the closing signals are in opposite phase with the 
signal D0. Thus, the signals D0-D1, D0-D2 and D0-D3 are applied across the 
liquid crystal layer. FIG. 32 shows the responsiveness characteristics 
corresponding to application of these signals. The characteristic curves 
103, 105 and 107 correspond respectively to the signals D1, D2 and D3. The 
curves 102, 104 and 106 are indicative of signals in which intervals of 
time during which they are in phase are 1 m sec., 3 m sec. and 5 m sec., 
respectively. 
These measurements were obtained under the same conditions as those for the 
measurements of FIGS. 30 and 31. The same liquid crystal material was 
used, and V1 was 30 (V). The examples shown in FIG. 32 are different from 
the present invention shown in FIG. 31 in that an applied signal of fh was 
not used. The examples of FIG. 32 are characterized in that they have a 
fast responsiveness characteristic, while when the conventional drive 
signals as shown in FIG. 29 are applied to ordinary twisted nematic liquid 
crystal elements, they respond with transmitted light under the mean 
effective voltage as with the examples described in connection with FIG. 
2. This is due to the optically active material added. High-speed response 
could likewise be obtained by E24LV used for the measurement of FIG. 2 to 
which an optically active material was added. However, the light 
transmissivity is low as shown in FIG. 32. 
In accordance with the present invention, a liquid crystal light valve can 
be provided which will operate at a higher speed and which will provide 
higher light transmissivity. It should also be noted that although 
addition of an optically active material reduces the pitch of the liquid 
crystal, resulting in optical rotatory dispersion and a reduction in the 
light transmissivity when no voltage is applied, light transmissivity of 
100% can be obtained by applying a signal of fh. FIGS. 30, 31 and 32 show 
responsiveness characteristics of the same liquid crystal panel. While the 
light transmissivity when no voltage is applied is 81% as shown at 107, 
the light transmissivity according to the present invention reaches 100%. 
The reason why this happens is still under study. 
An important thing about the driving signals according to the present 
invention is that the signals of fl are required to be applied in a given 
period (T1). Application of the signals of fl causes the major axes of the 
liquid crystal molecules to be oriented in a quasi-stable state 
perpendicular to the surfaces of the panel base plates, resulting in a 
high-speed response capability. Measurements indicative of advantages of 
applying the signals of fl and applications will be described. The 
responsiveness characteristic shown in FIG. 31 are available when the 
driving signals is applied continuously. The responsiveness characteristic 
obtained when the applied voltage is raised from 0 (V) to V1 for operation 
until a stable response is reached will be described for an application 
(Example 1) to be described below. 
FIGS. 33, 34, 35 and 36 illustrate light transmissivity P indicated by the 
arrow 100 in FIG. 31 as it varies with time. For the example of FIG. 33, a 
liquid crystal panel having a liquid crystal layer 5.3 .mu.m thick was 
used. For the examples of FIGS. 34, 35 and 36, a liquid crystal panel 
having a liquid crystal layer 6.5 .mu.m thick was a liquid crystal panel 
with a liquid crystal layer 5.3 .mu.m thick of FIG. 33 has liquid crystal 
molecules twisted through an angle of 450.degree.. With a liquid crystal 
layer 6.5 .mu.m thick of FIGS. 34, 35 and 36 the liquid crystal molecules 
are twisted through 630.degree.. 
In FIG. 33 which indicates data on the liquid crystal panel with molecules 
twisted through 450.degree. will be described. Designated as 120 is an 
interval of time between turning on of a power supply switch and 
generation of a voltage, and as 121 is an interval of time required until 
the responsiveness characteristic becomes stabilized. The interval of time 
121 is about 200 m sec. Once the responsiveness characteristic is 
stabilized, such a stable condition continues. During an interval of time 
indicated at 123, a light transmission characteristic shown by the broken 
lines 140 results when the signals of FIG. 28 are applied after only the 
signal of fl has been applied for one second. Such a characteristic 
becomes stable in a time interval 124 which ranges from 10 to 20 m sec. 
Application of the signal of fl can render the operation stable more 
quickly. 
The liquid crystal panel with liquid crystal molecules twisted 630.degree. 
will be described with reference to FIG. 34. The measurements were made 
under the same conditions as those for FIG. 33. An interval of time 
designated as 122 is required until a responsiveness characteristic 
becomes substantially constant, the interval of time being about 300 m 
sec. The light transmissivity is low. Designated at 153 is an interval of 
time, which is about 5 seconds, required until the transmissivity becomes 
continuously constant. The continuously constant transmissivity does not 
reach 100%, but is 90% in the example of FIG. 34. However, when the 
operating signals are applied after the signal of fl has been applied for 
an interval of time (one second) indicated at 123, the transmissivity 
changes as shown by the broken line and reaches 100% when it becomes 
stable at 128. The transmissivity as shown by the broken line 140 changes 
such that it rapidly rises in a period of time 125 and becomes stable in a 
period of time 126 (on the order of one second). The phenomenon is 
considered to be a quasi-stable condition gained by application of the 
signal of fl. 
According to the example shown in FIG. 35, a signal of fl is applied for an 
interval of time 131 (0.75 sec.) after the transmissivity of 90% has been 
stabilized at 127, and then the operating signals are applied for an 
interval of time 132 (0.75 sec.), the signals being applied repeatedly. 
The transmissivity gradually increased as indicated at 127, 129, 130, 128 
until it reached 100%. In another example of FIG. 36, a signal of 0 (V) is 
applied for an interval of time 133 (one second) and then the signal of fl 
is applied for an interval of time 134 (one second) after the 
transmissivity of 90% has been stabilized at 127, and thereafter the 
operating signals are applied. The transmissivity is restored in a period 
of time 135 up to 100%. As described above, the stable operating condition 
can be reached in a short period of time by applying the signal of fl, a 
feature which is important in practice because application of the signal 
of fl can prevent a drop in the transmissivity as described above which 
would otherwise be caused by irregularities in the thickness of the liquid 
crystal panel occurring in the manufacturing process. As described in the 
foregoing examples, the present invention provides a display device and 
light valve which will operate at high speeds and respond rapidly by 
employing a liquid crystal composition panel and a system for driving the 
panel. Embodiments in which the liquid crystal optical device is 
incorporated will be described. 
EXAMPLE 1 
An embodiment will first be described which embodies the present invention 
as a microshutter array for photowriting. FIGS. 37 and 38 show the 
arrangement of a liquid crystal panel. The liquid crystal panel comprises 
a base plate 217 of glass supporting common signal electrodes 219, 220, a 
base plate 218 of glass supporting signal electrodes 221, 222, a liquid 
crystal composition 225 sealed between base plates 217, 218 and spacers 
226, and polarizers 223, 224 disposed one on each side of base plates 217 
and 218. Common signal electrodes 219 are transparent, common signal 
electrode 220 is opaque, and signal electrodes 221, 222 are transparent. 
Polarizers 223, 224 have their polarizing planes extending perpendicularly 
to each other. Light is modulated by a microshutter composed of 
transparent common electrodes 219 and signal electrodes 221 and 222. 2,000 
in number of such microshutters are arranged in a straight interval of 20 
cm, the microshutters being spaced at an interval or pitch of 100 .mu.m. A 
integrated circuit driver has been fabricated on a trial basis which is 
capable of opening and closing the microshutters in response to 
time-series picture-element data, and packaged fifty output drivers, a 
total of forty such driver ICs have been mounted, twenty on each side of 
the 2,000 microshutters. 
FIG. 39 is a timing chart of various signals necessary for driving the 
panel, and FIG. 40 is a block diagram of a circuit for producing the 
driving signals. Designated as 301 in FIG. 39 is a reset signal for 
starting operation, as 302 is a line starting signal indicative of 
starting of data in one line, and as 303 is a request clock signal for 
requesting the data. The clock signal comprises 2,000 pulses for one line 
supplied in synchronism with the line starting signal. The data is 
received in synchronism with the clock pulses. Designated as 304 is a 
shift clock signal for transferring data from a shift register in the 
integrated driving circuits as 305 are latch pulses for latching the data 
immediately after the data has been transferred, and 306, 307, 308 are 
driving signals to be applied to the liquid crystal. Signal 308 is to be 
applied to the common electrodes. When the ON signal 306 is applied, the 
liquid crystal microshutters are opened, and when the OFF signal 307 is 
applied, the microshutters are closed. The ON signal and the common 
electrode signal are composed of combined signals of a high frequency fh 
and a low frequency fl, and are in opposite phase with each other. The OFF 
signal is of a low frequency in phase with the ON signal of fl. The 
interval of time in which the signal of fh is applied in one period of the 
ON signal is called an opening time. The circuit arrangement for 
generating these signals comprises a clock generator 310 for producing a 
base clock signal of 4.2 MHz which is frequency-divided by a divider 311 
for generating various signal waveforms. 
The line starting signal having a period of 2 m sec. is generated at 315 
for synchronization. 2,000 pulses are counted at counter 312, and a 
request signal is generated at request clock 320 and applied with together 
the line staring signal to a time-series picture-element signal generator 
324 which is outside equipment. Data which is supplied from the 
picture-element signal generator 324 in synchronism with request clock 320 
is distributed at 319 for supply to the driver ICs 322 packaged in an 
inter-digital structure. 
The opening time is determined at 316, and the ON signal, OFF signal and 
common electrode signal are generated at 317. The latch pulses are 
generated at 313 and the shift clock is generated at 314, these signals 
and the ON and OFF signal data being supplied to the integrated driver 
circuits 322. The common electrode signals are converted by an output 
buffer 318 so as to be as a voltage of 30 (V), and are applied to the 
common electrode 323. A control 321 receives a reset signal from the 
outside equipment 324 for starting and stopping operation of various 
circuit components. 
FIG. 41 is a block diagram of the integrated driver circuit. The data 
supplied as an input from the foregoing circuit of FIG. 40 is transferred 
by a shift register 330 of 50 bits in synchronism with a shift clock 
signal 335. The shift register in the twenty ICs on one side of the panel 
are connected in cascade. Data from a data output 340 is transferred to 
the shift register in an adjacent IC driver. When data of 1,000 bits on 
one side, or of 2,000 bits on both sides of the panel has been 
transferred, the data is latched by a latch 331 of 50 bits at a timing of 
the latch pulses 337. ON and OFF signals 338 and 339 are selected one at a 
time at 332 dependent on the latched data, and the logic level of the 
selected signal is converted by a lever converter 332 into a signal at a 
driving voltage of 30 (V), which is supplied via a driver buffer 334 to 
the signal electrodes. The microshutter array is thus driven. With such an 
arrangement, a high-speed light valve array is actuatable in 2 m sec. for 
one line. 
EXAMPLE 2 
The microshutter array of Example 1 with Liquid Crystal-VIII sealed in is 
maintained at 40.degree. C. with T1 being 2 m sec., and T2 being 0.6 m 
sec. A photosensitive body of zinc oxide sensitized with rose bengal is 
illuminated by light transmitted through the microshutter array from a 
halogen lamp with a brightness of 2,000,000 cd/m.sup.2 disposed behind the 
microshutter array, and was developed. An image is formed on the 
photosensitive array in accordance with signals applied. The 
photosensitive body is caused to move at a speed of 5 cm/sec. Thus, a 
high-speed photo-writing printer can be provided. 
EXAMPLE 3 
As shown in FIG. 42, a photo sensor 503 such as CdS is disposed closely to 
a liquid crystal optical device 502 of the invention which comprises 500 
shutters 501 with 8 dots/mm arranged rectilinearly. Sources of light 505 
are disposed adjacent to the surface of an original 504 to be read for 
illuminating the surface of the original 504, there being a convergent 
optical fiber element 506. The original 504 is optically read out at a 
speed of 500 Hz for 8 dots/mm. Since the shutters 501 in the liquid 
crystal optical device 502 are opened one at a time, the single 
photosensor 503 may suffice, and hence the device is quite inexpensive to 
construct. 
EXAMPLE 4 
As shown in FIG. 43, a liquid crystal optical device 509 of the present 
invention has apertures 508 formed correspondingly to a plurality of 
optical fibers 507. A single optical fiber 510 is disposed in opposite 
relation to the optical fibers 507 across the liquid crystal optical 
device 509, the single optical fiber 510 having an end covering all of the 
apertures. With this arrangement, a photoswitch can be constructed which 
can switch light signals as desired at a speed of 500 Hz. 
EXAMPLE 5 
As illustrated in FIG. 44, a liquid crystal optical device 514 according to 
the present invention is placed in an optical path in front of a 
single-panel television camera sensor 511, device 514 comprising 37 
shutters 513 each having a tri-colored filter 512. With such an 
arrangement, television signals can be generated which includes color 
signals with 10 m sec. for each filter and hence 30 m sec. for all 
filters. Designated as 515 is a lens, as 516 is a mirror, and as 517 is a 
dichroic mirror. 
EXAMPLE 6 
FIG. 45 shows a liquid crystal optical device 521 according to the present 
invention which has two openings 520. Device 521 and a group of mirrors 
519 are located in front of a television camera sensor 518. By switching 
mirrors 519 once every 40 m sec., three-dimensional television signals can 
be generated. Each opening 520 includes a lens 522. 
EXAMPLE 7 
A liquid crystal optical device 524 of the present invention as shown in 
FIG. 46 has apertures 525 positioned to correspond to human eyes 523. By 
alternately opening and closing of apertures 525 in every 30 m sec. in 
synchronism with the switching signal in Example 5, a television image on 
a television receiver 526 receiving television signal in Example 5 can be 
seen three-dimensionally. Designated as 527 is a synchronous signal 
generator. 
EXAMPLE 8 
As illustrated in FIG. 47, a signal lens 528 has thereon a plurality of 
lens portions 529 having different focal lengths. A liquid crystal optical 
device 531 in accordance with the present invention has openings 530 
corresponding to each of group of lens portions 529. There is thus 
provided an automatic focal length adjustment device which can select an 
appropriate focal length with a delay of 2 m sec. in response to a signal 
from an automatic focal point detector 532. 
EXAMPLE 9 
A pair of sunglasses having a photosensor 533 is constructed with a liquid 
crystal optical device 534 in accordance with the present invention as 
shown in FIG. 48. The sunglasses can protect the wearer's eyes with a 
shutter responsiveness of 1 m sec. Designated as 535 is a hood. 
EXAMPLE 10 
A liquid crystal optical device 537 in accordance with the invention 
includes a plurality of picture elements 536 arranged in a row is 
incorporated in a level meter as shown in FIG. 49. Such a level meter can 
perform switching in about 2 m sec. and give a sharply defined indication. 
EXAMPLE 11 
As shown in FIG. 50, a liquid crystal optical device 540 in accordance with 
the present invention has a plurality of concentric circular openings 539 
disposed adjacent to a camera lens 538 and used as both a shutter and an 
aperture. The aperture thus constructed has an ON-OFF ratio of 1/2,000 and 
a minimum opening interval of time of 1 m sec. An auxiliary hood 542 is 
necessary since light tends to leak and fall on a film 541 even with the 
ON-OFF ratio of 1/2,000, since a hood may be opened and closed slowly in 
about one second. 
As specifically described in the various foregoing embodiments, the present 
invention provides a liquid crystal optical device having a responsiveness 
characteristic which is several tens times faster than that of 
conventional liquid crystal devices, and hence the liquid crystal optical 
device of the present invention can find a variety of useful applications. 
The liquid crystal optical device according to the present invention is 
highly advantageous in that it is inexpensive to construct, has a large 
area, is characterized by low power consumption, is flexible in design to 
vary the shape of the opening, and features inherent in liquid crystal 
devices. 
Advantages of a liquid crystal light valve according to the present 
invention have been described in terms of its responsive characteristic of 
light transmission. A light signal generator including a liquid crystal 
light valve will now be described with reference to a preferred embodiment 
of the present invention. 
A printing device will be described in detail with reference to FIG. 51 of 
the drawings. FIG. 51 shows in (a) a front elevational view, in (b) a side 
elevational view with portions in cross-section, and in (c) a front 
elevational view. A casing 1101 houses the unit of the present invention 
which includes a halogen lamp 1102, electrical contact terminals 1103, a 
filter 1104 for absorbing infrared radiation, a rod lens 1105, a mounting 
plate 1106 for a liquid crystal light valve, a spring 1107, screws 1108 
for attaching a focusing lens, screws 1109 for attaching mounting plate 
1106 for a liquid crystal light valve, a liquid crystal light valve unit 
1110, screws 1111 for mounting unit casing 1101, a photosensitive drum 
1113, a housing 1114 for a copying system, terminal plates 1115 for 
supplying halogen lamp 1102 with electrical power, a filament of the 
halogen lamp 1116, a gradient index fiber array lens 1117 and screws 1118 
for attaching gradient index fiber array lens 1117. Light generated by 
halogen lamp filament 1116 illuminates liquid crystal light valve unit 
1110 through rod lens 1105. Gradient index fiber array lens 1117 is 
arranged such that liquid crystal light valve 1110 and photosensitive drum 
1113 are located at the conjugate lengths of gradient index fiber array 
lens 1117. The rod lens may be replaced with a gradient index fiber array 
lens, and both lenses may be replaced with rod lenses. 
Although the source of light has been described to be a halogen lamp, it 
may be a fluorescent lamp, flash lamp, or the like. The source of light 
and lenses may be arranged in a variety of combinations and should be 
positioned taking into account an overall design balance of the unit. A 
fluorescent lamp should be used if heat generated is likely to cause 
problems. 
FIG. 52 is an exploded perspective view of the liquid crystal light valve 
unit, and FIG. 53 is a cross-sectional view of the unit. Light from the 
focusing lens enters a polarizer 1201, is transmitted through a B glass 
1202 having common electrodes and a NESA film 1203 of In.sub.2 O.sub.3 
disposed thereon, and passes through slits 1216 in a film 1204 of Cr and a 
film 1205 of Au on NESA film 1203. Slits 1216 have a dimension of about 
40. Light which has passed through slits 1216 reaches NESA film 1211 on 
In.sub.2 O.sub.3 disposed on an A glass 1212. NESA films 1211 of In.sub.2 
O.sub.3 are patterned at intervals of 0.1 mm on A glass 1212. NESA films 
1211 have a width of 80.mu. and are spaced at 20.mu.. The patterned 
spacing is determined by the resolving power required of a printing 
system. It is possible to obtain a resolving power of 10 lines/mm for a 
printing system for producing high-quality prints. The higher the 
resolving power, the higher the cost. However, this may be accomplished 
without cost increase by using a liquid crystal-driven switching element. 
The opposite electrodes of the liquid crystal valve unit, namely, NESA 
film 1211 of In.sub.2 O.sub.3 and NESA film 1203 of In.sub.2 O.sub.3 on B 
glass 1202, cause the liquid crystal enclosed by a seal 1206 to change 
orientation during differing modes of operation. More specifically, the 
modes of operation of the liquid crystal are changed by the common 
electrodes, that is, NESA film 1203 of In.sub.2 O.sub.3 on B glass 1202 
and the electrodes, that is, NESA film 1211 of In.sub.2 O.sub.3 patterned 
on A glass 1212, to allow or prevent passage of light through slits 1216. 
Polyimide resin is coated as an orienting material on NESA films 1203 and 
1211 on A and B glasses 1212 and 1202, respectively, and is rubbed in 
different directions oriented at an angle of 90.degree.. The layer of 
liquid crystal is approximately 6 .mu.m. Light which has passed through 
NESA film 1211 passes through A glass 1212 and polarizer A 1213. The 
elements between polarizers 1201 and 1213 are fixed together by adhesive 
and vapor deposition and unitized as a whole on a plate of aluminum 1214. 
The electrodes are coupled to an IC block 1222 by a flexible substrate 
1223 on a ceramic plate 1221 mounted on aluminum plate 1214. The 
electrodes comprise a layer 1210 of Cr deposited by vapor deposition on 
NESA film 1211 of In.sub.2 O.sub.3, a film 1209 of Au deposited by vapor 
deposition on the film 1210, a film 208 of Ni-P mounted on film 1209, and 
an uppermost film 1207 of Au-P. The flexible substrate 1223 is attached to 
a film of Zn mounted on film 1207 of Au-P. 
A process for generating signals for opening and closing the liquid crystal 
light, valve as illustrated in FIGS. 52 and 53 will be described. FIG. 54 
shows in a block diagram a drive circuit for the liquid crystal light 
valve and in FIG. 55 the waveform of the signals. The liquid crystal drive 
circuit comprises an interface 1301 for converting print signals from an 
external source of signals into time-series picture element signals, a 
serial-in, parallel-out shift register 1302 corresponding to the number of 
picture elements per line, a latch signal generator 1303 for generating 
signals for latching the output from shift register 1302, a switch 1306 
for switching drive signals, a buffer 1307 for producing outputs at a high 
potential, a drive signal generator 1305 and a control 1304. 
A serial output for data per line from interface 1301 is read by shift 
register 1302 according to a train of clock pulses 1308 supplied from the 
control 1304. Then, the content in shift register 1302 is latched by a 
latch signal 1309 supplied also from the control 1304. 
The drive signal generator 1305 produces an opening signal 1311 and a 
closing signal 1312 simultaneously as they are held in synchronism with 
the latch signal 1309 by control 1304. Drive signal switch 1306 serves to 
apply either drive signal 1311 or 1312 to high-voltage output buffer 1307 
based on the output from latch signal generator 1303. A synchronous signal 
1310 generated by the control 1304 keeps the drive signals synchronized. 
When signal 1310 is at a high level, a high-frequency portion of opening 
signal 1311 is generated. High-voltage output buffer 1307 serves to drive 
the liquid crystal cell. 
The signal which is applied to the liquid crystal light valve may be in any 
shape as long as it has the characteristics described above with reference 
to light transmission, signals 1311 and 1312 being shown by way of 
example. The signal applied to the liquid crystal light valve is a 
combination of a signal applied to the common electrodes and a signal 
applied to the signal electrodes. For example, no voltage may be applied 
to the common electrodes while a signal may be applied only to the signal 
electrodes. Alternatively, the liquid crystal light valve may be driven by 
a combination of a signal applied to the common electrodes and another 
signal applied to the signal electrodes. Thus, a light signal generator in 
accordance with an embodiment of the present invention may be provided as 
described above. 
Various processes for forming images according to the present invention 
will now be described. A process for forming a positive latent image will 
first be described with reference to FIG. 56. Such a process comprises the 
steps of discharging an image remaining on a photosensitive drum 1400 
rotating at a peripheral speed of 5 cm/sec. with a discharger 1401 
comprising a lamp or an AC corona discharger, charging photosensitive drum 
1400 with a corona charger 1402 (charger with a guard or a grid electrode 
being effective for uniform charging of the photosensitive drum), forming 
a latent image by eliminating latent image charges other than the area for 
an image to be formed with a light signal generator 1403, developing the 
image with toner at a developer 1404 of the magnetic roller or brush type, 
transferring the toner image, with a transfer corona discharger 1406 onto 
a recording paper 1405 traveling at a speed in synchronization with the 
peripheral speed of photosensitive drum 1400 and fixing the toner image to 
recording paper 1405 with light, heat, or pressure. Toner residuals 
remaining on photosensitive drum 1400 after the transferring step are 
removed by a blade 1407. 
FIG. 57 illustrates a process for forming an image based on the same 
principles as those of the process described above. Here, a latent image 
is transferred into electrostatically charged recording paper 1405 after 
the step of forming the latent image. Electrostatically charged recording 
paper 1405 is pressed against photosensitive drum 1400 by a backing 
electrode 1410 to effect latent image transfer due to contact or peeling. 
Thereafter, the image is developed by developer 1404 and then is fixed. 
The above two embodiments are simpler than those which will be described 
below. Noises or image distortions tend to be generated depending on the 
diameter of a light spot while a latent image is formed. More 
specifically, when the light spot diameter is small, the ground gets 
dirty. When the ground dirt is completely removed, then a latent image 
spot becomes small, making the contour of the image thinner. Although the 
foregoing processes are sufficient for use in a plotter, they will produce 
images of poor quality and are not satisfactory for use in a printing 
device. To avoid such a problem, a process is preferable which causes an 
area illuminated by a light spot to form a charged latent image. 
The process is illustrated in FIG. 58. A photosensitive body 1420 including 
a light-transmissive electrode and a photosensitive layer mounted on a 
light-transmissive base plate, and electrostatically charged recording 
paper 1405 held in contact with a backing electrode 1421 are biased as 
shown. After information has been optically written by liquid crystal 
shutter writing unit 1403, recording paper 1405 is peeled off on which a 
positive charged latent image is formed. Thereafter, the latent image is 
developed and fixed as in the above-mentioned processes. 
FIG. 59 shows a process for positively developing a charged latent image on 
electrostatically charged recording paper 1405. Photosensitive drum 1400 
has formed thereon a negative latent image as in the foregoing latent 
image forming processes, and electrostatic recording paper 1405 is charged 
in a polarity opposite to that of the charges on photosensitive drum 1400. 
Electrostatically charged recording paper 1405 is pressed against 
photosensitive drum 1400 by a transfer paper, whereby charges of opposite 
polarities on photosensitive drum 1400 and electrostatically charged 
recording paper 1400 are neutralized to allow a positive latent image to 
remain on electrostatic recording paper 1405. The image is developed and 
fixed as in the foregoing processes. 
Finally, a process for reversably developing a negative latent image will 
be described with reference to FIG. 60. Here, a negative electrostatic 
latent image is formed on electrostatically charged recording paper 1405. 
To effect reverse development, magnetic roller developer 1404 is biased at 
the same potential and in the same polarity as the potential and polarity 
of the charged area on electrostatically charged recording paper 1405. As 
a result, toner is caused to adhere only to the non-charged area on 
electrostatically charged recording paper, to form a reverse image. The 
biased development may be effected by either a developer using toner of a 
single constitutent or a developer using toner of two constitutents. The 
image is then developed and fixed as in the above-mentioned processes. 
As described above, various processes for forming images can be carried out 
using the light signal generator according to the present invention. 
EXAMPLE 12 
Printing as described above was used to produce printed copies under the 
following conditions. A liquid crystal light valve was driven at a voltage 
V1 of 30 V at a temperature of 40.degree. C. with fl of 1 KHz, fh of 100 
KHz and T1 of 2 m sec. Minute shutter openings were in the form of a 
square dimensioned at 40 .mu.m. A halogen lamp with a brightness of about 
1,000,000 Cd/m.sup.2 was used as a source of light, and zinc oxide 
sensitized with rose bengal was used as a photosensitive body. After 
information was written, toner was developed, transferred and fixed. Dots 
having a diameter of about 80 m were printed in a pattern according to 
printing signals. No irregular dots due to different signals and hence 
uniform dots were observed. 
As described above, the present invention provides a printing device having 
a high-speed liquid crystal light valve, the printing device being small 
in size, highly reliable in operation and less costly to construct due to 
the features of the liquid crystal light valve. Thus, a high-speed 
printing device of a high quality which will be required by office 
automation in the future is available inexpensively. 
It will thus be seen that the objects set forth above, and those made 
apparent from the preceding description, are efficiently attained and, 
since certain changes may be made in the above construction without 
departing from the spirit and scope of the invention, it is intended that 
all matter contained in the above description or shown in the accompanying 
drawings shall be interpreted as illustrative and not in a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention herein 
described, and all statements of the scope of the invention which, as a 
matter of language, might be said to fall therebetween. 
Particularly it is to be understood that in said claims, ingredients or 
compounds recited in the singular are intended to include compatible 
mixtures of such ingredients wherever the sense permits.