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BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal device and an image
forming apparatus utilizing a liquid crystal device and more particularly
to an image forming apparatus with a printer head of the type in which a
light path is opened or closed by means of an optical modulator utilizing
a ferroelectric liquid crystal so as to produce light signals.
With recent remarkable advances in information processing technologies,
there have been increasing demands for high information or packaging
density and high speed of image forming apparatus. Furthermore, there has
been a strong demand for high printing quality. To satisfy these and other
demands, electrophotographic devices, laser beam printers and optical
fiber tube printers have been developed and put into commercial practice.
However, these image forming apparatus are very expensive and complicated
in construction and it is difficult to make them compact in size and light
in weight. Therefore, recently there have been proposed less expensive and
smaller image forming apparatus utilizing PLZT or optical shutters of
liquid crystal or image forming apparatus such as LED printers utilizing
light-emitting diodes. Of these image forming apparatus, liquid crystal
shutter printers utilizing electrooptical effects of liquid crystal are
considered promising as image forming apparatus capable of obtaining
images with less expense and high information density.
As for a liquid crystal used in a head of a liquid crystal shutter printer,
a method for driving a twisted nematic crystal by a two frequency system
is disclosed in, for instance, Japanese Laid-Open patent application No.
94377/1981. According to a printer head of this system, a liquid crystal
composition which exhibits a positive or a negative dielectric anisotropy
in response to different frequencies of applied voltage is used, and the
operation principle is based on the fact that when applied frequencies are
selectively varied, the liquid crystal is optically distinguished between
the state in which liquid crystal molecules are oriented in the direction
of electric field and the state in which liquid crystal molecules are
oriented in the direction perpendicular to the electric field. In general,
the higher the applied voltage is, the faster the response speed becomes.
Accordingly, as liquid crystals oriented in one direction produce a bright
state while those oriented in the other direction produce a dark state,
switching between the bright and the dark state can be effected by the
forced application of a voltage so that high-speed response becomes
possible if as high a voltage as possible is forcibly applied. However,
the response time is of the order of one microsecond at the shortest and
is considerably longer than the response time of the order of tens of
nanoseconds of LED printer heads so that the head of a liquid crystal
shutter printer has not been employed as a printer head requiring a high
response speed. As for an LED printer head, it is difficult to form an LED
array luminance. Therefore, when an electrostatic latent image formed by
receiving this light-emitting luminance is developed with a developer
comprising a toner having a charge of opposite polarity to that of the
electrostatic image, there arises a defect that the optical density
becomes nonuniform from dot to dot.
Meanwhile, ferroelectric liquid crystals with spontaneous polarization have
been discovered and it is well known that they have considerably fast
response as compared with the conventional liquid crystals because
electric dipoles of liquid crystal molecules can respond in about one
microsecond to the external electric field. When such a ferroelectric
liquid crystal is used to form a cell of a thickness of one to two microns
and used as an optical shutter, the brightness-to-darkness contrast of
1:20 can be obtained. Accordingly, research and development of high speed
liquid crystal shutter printers has been carried out so as to replace the
printer heads utilizing the conventional liquid crystal modes.
It is known however that when these ferroelectric liquid crystals operate
as liquid crystal shutters, they are in a chiral smectic C phase (SmC*) or
in a chiral smectic H phase (SmH*) which appears at relatively high
temperatures (for instance, about 60.degree.-90.degree. C.) as compared
with room temperature. Accordingly, there arises a problem that it is
difficult to apply these ferroelectric liquid crystals to an image forming
apparatus in which light signals are produced by a printer head utilizing
such a ferroelectric liquid crystal and illuminated over a photosensitive
drum of, for instance, an electrophotographic copying machine. More
particularly, for the sake of the normal operation of an image forming
apparatus, the liquid crystal in an optical modulator of a printer head
must be always maintained at a temperature between 60.degree.-90.degree.
C. so that the liquid crystal is kept in the SmC* or SmH* phase, whereby
additional power is consumed. Moreover, there arises a problem that when
the liquid crystal in the SmC* or SmH* phase is heated in excess of the
above-described temperature range, smectic A phase (SmA) appears so that
high speed response cannot be obtained.
SUMMARY OF THE INVENTION
In view of the above, an object of the present invention is to provide a
liquid crystal device and an image forming apparatus utilizing the liquid
crystal device which can substantially overcome the above and other
problems of the prior art.
Another object of the present invention is to provide an image forming
apparatus capable of forming an image at high speed.
The above and other objects of the present invention can be attained by a
liquid crystal device of the type comprising: a light signal generator
which comprises an exposure light source and a light-path switching means
capable of interrupting the transmission of light rays at arbitrary
portions of the cross-sectional area of the path of the light rays emitted
from the light source, and an image-bearing member so disposed as to be
illuminated with light signals from the light signal generator, wherein
the light signal generator comprises: the light-path switching means which
in turn comprises scanning lines, data lines and a ferroelectric liquid
crystal material having an operational temperature range in which the
ferroelectric liquid crystal material behaves as a ferroelectric liquid
crystal and is driven by selective application of signals to the scanning
lines and the data lines so as to assume either one of light-transmitting
state and light-interrupting state, and temperature control means for
controlling the temperature of the liquid crystal material within the
operational temperature range by means of at least one of heating means
and cooling means.
These and other objects, features and advantages of the present invention
will become more apparent upon a consideration of the following
description of the preferred embodiments of the present invention taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are perspective views illustrating an optical modulation
element and its operation principle, respectively, used in the present
invention;
FIG. 3 is a view illustrating schematically a light-path opening and
closing means used in the present invention;
FIG. 4 is a view used to explain the change in liquid crystal phase in
response to temperature rise and drop;
FIG. 5 is a view used to explain the temperature dependency of an optical
modulation element used in the present invention;
FIG. 6 shows a temperature control flowchart for controlling the light-path
switching means;
FIG. 7 is a perspective view of another light-path switching means in
accordance with the present invention;
FIG. 8 is a plan view illustrating an electrode matrix arrangement used in
the light-path opening and closing means of the present invention;
FIGS. 9A-9D show waveforms of electrical signals applied to the electrode
matrix;
FIGS. 10A-10D show waveforms applied to SmC* or SmH*; and
FIGS. 11 and 12 are schematic sectional views of an image forming apparatus
in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the present invention, ferroelectric liquid crystals which are in a
chiral smectic C phase (SmC*) or in a chiral smectic H phase (SmH*), may
preferably be used. Such crystal liquids have bistability of giving a
first optically stable state and a second optically stable state.
Therefore, unlike an optical modulation device utilizing the
above-mentioned TN type liquid crystal, liquid crystals are oriented in
the first optically stable state in response to a first or one electric
field vector and in the second optically stable state in response to a
second or the other electric field vector.
These ferroelectric liquid crystals are described in, e.g., "LE JOURNAL DE
PHYSIQUE LETTERS" 36 (L-69), 1975 "Ferroelectric Liquid Crystals";
"Applied Physics Letters" 36 (11) 1980, "Submicro Second Bistable
Electrooptic Switching in Liquid Crystals"; "Solid State Physics" 16
(141), 1981 "Liquid Crystal", etc. Ferroelectric liquid crystals disclosed
in these publications may be used in the present invention.
Examples of ferroelectric liquid crystal compounds are as follows:
__________________________________________________________________________
##STR1## (1)
n = 6: T.sub.1 .apprxeq. 75.degree. C., T.sub.2 .apprxeq. 85.degree. C.
n = 7: T.sub.1 .apprxeq. 68.degree. C., T.sub.2 .apprxeq. 90.degree. C.
n = 8: T.sub.1 .apprxeq. 65.degree. C., T.sub.2 .apprxeq. 90.degree. C.
n = 9: T.sub.1 .apprxeq. 65.degree. C., T.sub.2 .apprxeq. 90.degree. C.
n = 10: T.sub.1 .apprxeq. 60.degree. C., T.sub.2 .apprxeq. 90.degree. C.
##STR2## (2)
T.sub.1 .apprxeq. 25.degree. C., T.sub.2 .apprxeq. 35.degree. C.
##STR3## (3)
n = 10, T.sub.1 .apprxeq. 70.degree. C., T.sub.2 .apprxeq. 75.degree. C.
n = 14, T.sub.1 .apprxeq. 45.degree. C., T.sub.2 .apprxeq. 70.degree. C.
##STR4## (4)
T.sub.1 .apprxeq. 60.degree. C., T.sub.2 .apprxeq. 75.degree. C.
##STR5## (5)
T.sub.1 .apprxeq. 45.degree. C., T.sub.2 .apprxeq. 55.degree. C.
##STR6## (6)
T.sub.1 .apprxeq. 120.degree. C., T.sub.2 .apprxeq. 130.degree. C.
##STR7## (7)
T.sub.1 .apprxeq. 25.degree. C., T.sub.2 .apprxeq. 65.degree. C.
##STR8## (8)
n = 4; T.sub.1 .apprxeq. 85.degree. C., T.sub.2 .apprxeq. 110.degree. C.
n = 8; T.sub.1 .apprxeq. 5.degree. C., T.sub.2 .apprxeq. 140.degree. C.
n = 12; T.sub.1 .apprxeq. 55.degree. C., T.sub.2 .apprxeq. 145.degree. C.
##STR9## (9)
n = 8; T.sub.1 .apprxeq. 45.degree. C., T.sub.2 .apprxeq. 110.degree. C.
n = 10; T.sub.1 .apprxeq. 75.degree. C., T.sub.2 .apprxeq. 110.degree. C.
##STR10## (10)
T.sub.1 .apprxeq. 35.degree. C., T.sub.2 .apprxeq. 90.degree. C.
##STR11## (11)
T.sub.1 .apprxeq. 140.degree. C., T.sub.2 .apprxeq. 155.degree. C.
__________________________________________________________________________
In the above description including the formulae, C* represents an
asymmetric carbon atom; T.sub.1 represents the lowest temperature of SmC*;
and T.sub.2 represents the highest temperature of SmC*.
When these compounds are subjected to temperature change, the following
phase change occurs:
##STR12##
The present invention will be described in detail with reference to the
accompanying drawings.
FIG. 1 schematically shows a ferroelectric liquid crystal cell and is used
to explain the mode of operation of a ferroelectric liquid crystal.
Reference numerals 11 and 11a denote base plates (glass plates) on each of
which is disposed a transparent electrode of, e.g. In.sub.2 O.sub.3,
SnO.sub.2, ITO (Indium-Tin Oxide), etc. A liquid crystal in an SmC* or
SmH* phase in which liquid crystal molecular layers 12 are oriented
perpendicular to surfaces of the glass plates is hermetically disposed
therebetween. A full line 13 shows liquid crystal molecules. Each crystal
molecule 13 has a dipole moment (P.vertline.) 14 in a direction
perpendicular to the axis thereof. When a voltage higher than a certain
threshold level is applied between electrodes formed on the base plates 11
and 11a a helical structure of the liquid crystal 13 is loosened or
unwound to change the alignment direction of respective liquid crystal
molecules 13 so that the dipole moments (P.vertline.) are all directed in
the direction of the electric field. The liquid crystal molecules 13 have
an elongated shape and show refractative anisotropy between the long axis
and the short axis thereof. Accordingly, it is easily understood that
when, for instance, polarizers arranged in a cross nicol relationship i.e.
with their polarizing directions being crossing each other are disposed on
the upper and the lower surfaces of the glass plates, the liquid crystal
cell thus arranged functions as a liquid crystal modulation device of
which optical characteristics vary depending upon the polarity of an
applied voltage.
The optical modulation element or device preferably used in the liquid
crystal device and the image forming apparatus utilizing such liquid
crystal device may have a sufficiently thin (for instance, one micrometer)
thickness. That is, as shown in FIG. 2, the spiral structure of liquid
crystal molecules is loosened or unwound to result in a non-spiral
structure even when an electric field is applied so that the bipolar
moment P or Pa is directed upwards (24) or downwards (24a). When an
electric field E or Ea which is higher than a certain threshold value and
is different in polarity is applied by voltage application means 21 and
21a, the bipolar moment responds to the vector of the electric field E or
Ea and are re-oriented upwards (24) or downwards (24a). Therefore, the
liquid crystal molecules are oriented in a first stable state 23 or a
second stable state 23a.
Two advantages can be obtained when such a ferroelectric liquid crystal is
used as an optical modulation element. First is that response becomes very
fast and second is that the orientation of liquid crystal molecules
exhibits bistability. The second advantage will be further explained in
detail with reference to FIG. 2. When the electric field E is applied to
the liquid crystal molecules, they are oriented in the first stable state
23. This state is kept stable even if the electric field is removed. On
the other hand, when the electric field Ea of which direction is opposite
to that of the electric field E is applied thereto, the liquid crystal
molecules are oriented in the second stable state 23a, whereby the
directions of the molecules are changed. Likewise, the latter state is
kept stable even if the electric field is removed. Further, as long as the
magnitude of the electric field E being applied is not above a certain
threshold value, the liquid crystal molecules are placed in the respective
orientation states. In order to effectively realize high response speed
and bistability, it is preferable that the thickness of the cell is as
thin as possible.
An image forming apparatus in accordance with the present invention
utilizes ferroelectric liquid crystals of the type described above. One
preferred embodiment thereof will be described in detail with reference to
FIG. 3 and so on.
FIG. 3 shows light-path switching means or light-path opening and closing
means in the form of a liquid crystal cell and temperature control means
of a light signal generator incorporated in an image forming apparatus in
accordance with the present invention. The light-path switching means is
provided with temperature control means for controlling the temperature of
a ferroelectric liquid crystal within a temperature range wherein the
ferroelectric liquid crystal can remain in the SmC* or SmH* phase.
The light-path switching means shown in FIG. 3 is provided with temperature
raising means so that when it is used as a printer head of an image
forming apparatus, an electrooptic modulation material 301 in the cell can
remain in the SmC* or SmH* phase. Temperature raising means are in contact
with the electrooptic modulation material 301 in the SmC* or SmH* phase.
Alternatively, temperature raising means comprise heat-generating resistor
members 302 and 303 upon which are formed insulating layers or films (not
shown) and a power supply 304. The heat generating resistor members 302
and 303 may be formed in the form of a film of a transparent metal oxide
compound such as indium oxide, zinc oxide, titanium oxide or the like.
As shown in FIG. 4, many ferroelectric liquid crystals exhibit different
phase boundaries when they are raised or lowered in temperature. More
particularly, the stable temperature range in which a ferroelectric liquid
crystal remains in the SmC* phase upon heating is different from that upon
cooling. In general, the ferroelectric liquid crystal stably remains in
the state SmC* down to a lower temperature (T.sub.1a) when it is subjected
to temperature decrease, as compared with a case where the liquid crystal
is subjected to temperature increase and changes from SmH* phase to the
SmC* phase at a temperature T.sub.1 which is higher than T.sub.1a. There
exists no problem when the lowermost temperatures T.sub.1 and T.sub.1a are
substantially equal to each other. When T.sub.1 is higher than T.sub.1a,
however, as the power consumption is less when the temperature of liquid
crystal is maintained at T.sub.1a rather than T.sub.1, it is preferable
that in this embodiment, as shown in FIG. 5, the temperature of liquid
crystal is once raised above the temperature T.sub.2 and then gradually
lowered and finally maintained at the SmC* phase temperature range T which
is given by
T.sub.1a +.beta.<T<T.sub.2 -.alpha.
where .alpha. and .beta. are constants and
T.sub.1a <T.sub.1a +.beta.<T.sub.1 <T.sub.2 -.alpha.<T.sub.2.
Therefore, according to the present invention, a large current is first
caused to pass through the heat-generating resistor members 302 and 303 so
that the temperature rises fast (first stage heating) and after the
temperature of liquid crystal exceeds T.sub.2, the current passing through
the heat-generating resistor members 302 and 303 is reduced (second stage
heating) simultaneously with the actuation of a cooler so that the
temperature of liquid crystal is lowered down to and maintained at T. Such
temperature control can be attained by actuating or energizing the heat
generating resistor members 302 and 303 stepwise in response to the output
signal of a temperature sensor 305. Such temperature control sequence is
shown in a flow chart of FIG. 6. FIG. 6 shows a two-stage heating scheme
comprising first and second stages. The conditions imposed upon the
heating in each stage and the cooling are as follows:
First stage heating capacity>Cooling capacity, and
Second stage heating capacity<Cooling capacity.
The temperature control sequence as shown in FIG. 6 can be carried out in
the electric circuit as shown in FIG. 3. In Step 1, a main power supply
306 is turned on and the temperature of the electrooptic modulation
material 301 in the cell is detected by the temperature sensor 305. In
Step 2, when the temperature T of the modulation material 301 is lower
than T.sub.1 ; that is, when T.sub.1 >T is YES, the power supply 304 is
turned on so that the first stage heating is carried out. When the
temperature T of the modulation material T is higher than T.sub.1 ; that
is, when T.sub.1 >T is NO, it is further detected whether or not
T>T.sub.2. When T>T.sub.2 (YES), Step 6 is executed, but when T<T.sub.2
(NO), the first stage heating is continued.
In Step 3, the first stage heating is effected and in response to the
output signal from a microprocessor 319, a temperature control circuit 307
and a current regulator 308 are actuated to control the current passing
through the heat generating resistor members 302 and 303 so that the
modulation material 301 is heated until T>T.sub.2 (YES).
In Step 4, the first stage heating is stopped. Simultaneously with
termination of the first stage heating, Step 5 is executed. That is, upon
completion of Step 4, in response to the control signal from the
microprocessor 319, the current regulator 308 decreases current flowing
through the heat-generating resistor members 302 and 303 so that generated
heat is decreased.
In Step 6, a cooling fan 309 is turned on and, in Step 7, the temperature T
of the modulation material 301 which is higher than T.sub.2 is lowered
until T<T.sub.2 -.alpha. (YES). Step 7 is a step in which a temperature
below the upper temperature of the temperature range in which the
modulation material 301 exhibits the properties of ferroelectric liquid
crystal is secured. Next step 8 is a step in which a temperature above the
lower limit of the temperature range in which the modulation material 301
exhibits the properties of ferroelectric liquid crystal is secured. Thus,
when T>T.sub.2 -.alpha.; that is, NO in step 7, cooling is continued.
However, when T.sub.1a +.beta.>T, that is, when NO in Step 8, Step 2 is
executed again so that Steps 2-7 are executed again.
In Step 9, the temperature T of the modulation material 301 is such that
T.sub.1a +.beta.<T<T.sub.2 -.beta. (YES). As a result, an image forming
apparatus (for instance, an electrophotographic copying machine) is
brought to the state of "Copy Ready".
In accordance with the above-described temperature control sequence, the
temperature of the modulation material 301 in the cell is controlled as
indicated by the curve as shown in FIG. 5.
In actual operations, .alpha. and .beta. are preferably set to be 1.degree.
C. or larger, particularly 5.degree. C. or larger and so as to satisfy the
relationship:
0.1.degree. C..ltoreq.(T.sub.2 -.alpha.)-(T.sub.1
+.beta.).ltoreq.10.degree. C.
In the light-path switching means shown in FIG. 3, a liquid crystal drive
circuit 310 applies selectively signals to electrodes 311 and 312 disposed
within the cell. As a result, the orientation of the electrooptic
modulation material 301 is selectively controlled so that the light path
is opened or closed. The change in orientation is detected by polarizers
313 and 314 disposed on both sides of the electrodes 311 and 312.
In FIG. 3, reference numerals 315 and 316 denote glass or plastic base
plates or substrates; 317 and 318, insulating films of polyimide,
polycarbonate, polyamide or the like.
FIG. 7 shows another embodiment of the lightpath switching means in
accordance with the present invention. In this embodiment a heating unit
703 comprising a heat insulating body arranged with a resistance heating
element 701 is attached to the whole or a part of periphery or side
surfaces of a liquid crystal cell 702. The current flowing through this
resistance heating element 701 can be controlled in a manner substantially
similar to that described with reference to the flowchart shown in FIG. 6.
Next, referring to FIGS. 8-10, the mode of operation of light-path
switching means in accordance with the present invention will be
described.
FIG. 8 shows schematically a cell 81 having a matrix electrode arrangement
in which a ferroelectric liquid crystal compound is interposed between a
pair of groups of electrodes oppositely spaced from each other. Reference
numeral 82 denotes scanning lines (common electrode group) to which are
applied scanning signals; 83, data lines (signal electrode groups) to
which are applied data signals. FIGS. 9A and 9B show an electrical signal
applied to a selected scanning line 82(s) and an electrical signal applied
to non-selected scanning lines 82(n), respectively. FIGS. 9C and 9D show
an electrical signal applied to selected data lines 83(s) and an
electrical signal applied to non-selected data lines 83(n), respectively.
In FIGS. 9A-9D, time is plotted along the abscissa while a voltage, along
the ordinate. For instance, when a moving picture is to be displayed, the
scanning lines 82 are sequentially and periodically selected. It is now
assumed that a threshold voltage for maintaining a liquid crystal cell
with bistability in a first stable state is V.sub.th1 and a threshold
voltage for maintaining the liquid crystal cell in a second stable state
is V.sub.th2. Then, as shown in FIG. 9A, an electrical signal applied to
the selected scanning line 82(s) is an alternative voltage which is V at
time t.sub.1 and is -V at time t.sub.2. The remaining non-selected
scanning lines 82(n) are grounded as shown in FIG. 9B. That is, the
electrical signal applied to them is zero. As shown in FIG. 9C, the
electrical signals applied to the selected data lines 83(s) are V and the
electrical signals applied to the non-selected data lines 83(n) are -V. In
this instance, the voltage V is set to a desired value satisfying the
relationships of:
V<V.sub.th 1<2 V;
and
-V>V.sub.th 2>2 V.
FIGS. 10 show voltage waveforms applied to respective picture elements when
the above-described electrical signals are applied to the scanning and
data lines in the manner described above. FIGS. 10A-10D correspond to
picture elements A, B, C and D, respectively, shown in FIG. 8. That is, as
is clear from FIG. 10, during a time period t.sub.2, a voltage 2 V
exceeding the threshold value V.sub.th1 is applied to a picture element A.
A voltage -2 V exceeding the threshold value -V.sub.th2 in terms of the
absolute value is applied to the picture element B arranged in the same
scanning line during a time period t.sub.1. Therefore, depending upon
whether or not data lines intersecting a selected scanning line are
selected, the orientation of liquid crystal molecules changes. Namely,
when data lines intersecting a selected scanning line are selected, liquid
crystal molecules are oriented in the first stable state. On the other
hand, when data lines are not selected, liquid crysta | | |
