Image forming apparatus

An image forming apparatus includes a light source; an optical shutter unit comprising a pair of parallel substrates respectively having a plurality of scanning electrodes and a plurality of data electrodes disposed to intersect with each other to form a matrix electrode structure on their opposite surfaces, and a ferroelectric liquid crystal disposed between the pair of substrates so as to form a microshutter at each intersection of the scanning electrodes and data electrodes; an optical system for forming an image at a desired position with light transmitted through the optical shutter unit; a photosensitive member as a medium for recording the thus formed image; a scanning side drive circuit for applying a scanning voltage signal to the plurality of scanning electrodes so as to sequentially select at least one scanning electrode in a prescribed cycle; and a data side drive unit for applying data voltage signals to the plurality of data electrodes in synchronism with said scanning voltage signal. Image data are transferred on the optical shutter unit corresponding to the movement of the photosensitive member so as to move an image formed thereby on the photosensitive member while keeping the relative position of the image with respect to the photosensitive member.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to an image forming apparatus, particularly 
an image forming apparatus provided with a liquid crystal-optical shutter 
array using a ferroelectric liquid crystal. 
Hitherto, there have been several proposals on a so-called "liquid 
crystal-optical shutter array" wherein the electro-optical modulation 
function of a liquid crystal (hereinafter sometimes abbreviated as "LC") 
is utilized, and light is irradiated to LC-modulation cells as 
microshutters arranged in the form of an array, so that selectively 
transmitted light is provided to a photosensitive member as light image 
signals. Such LC-optical shutter arrays are disclosed in, e.g., Japanese 
Laid-Open Patent Application Nos. (JP-A) Sho56-98073, 56-98967 and 
57-120466. 
Further, LC-optical shutter arrays utilizing the high speed responsiveness 
and memory characteristic of a ferroelectric liquid crystal have been 
disclosed in, e.g., U.S. Pat. No. 4,548,476 and JP-A No. Sho60-107023. 
FIG. 10 shows a schematic arrangement of an example of an image forming 
apparatus using an LC-optical shutter array. Referring to the figure, the 
image forming apparatus is roughly composed of a linear light source 1 
such as a fluorescent lamp, a pair of polarizers 2 and 3, an LC-optical 
shutter panel 4 comprising an LC-microshutter array interposed between the 
polarizers 2 and 3, a lens array 5, and a photosensitive drum 6. 
Accessories such as a charger are omitted from showing. Light emitted from 
the source 1 passes through a modulation system comprising the polarizers 
2 and 3 and the shutter panel 4, collected by the lens array 5, and then 
irradiated onto the photosensitive drum 6. An electrostatic recording 
apparatus like this has several advantages such as the ease of realizing a 
small apparatus and absence of mechanically moving parts to provide less 
noise. 
In image forming apparatuses of the electrostatic recording system using 
such an LC-optical shutter array, however, as light emitted from a light 
source is generally passed through polarizers, a liquid crystal cell and 
lenses to reach a photosensitive member, a considerable portion of the 
initial light leaks or is attenuated. Consequently the light reaching the 
photosensitive member can be weak resulting in the production of inferior 
quality images. In order to obviate this defect, a high luminescence light 
source or a photosensitive member with a high sensitivity is required, so 
that there arises another problem of high production cost. 
As another problem, it is difficult to obtain a half-tone image by an 
LC-optical shutter array. Particularly, in the case of an LC-optical 
shutter array using a ferroelectric liquid crystal (sometimes abbreviated 
as "FLC"), it is difficult to form an intermediate optical transmission 
state because the FLC assumes either one of two optically stable states, 
as shown in U.S. Pat. No. 4,367,924. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve the above mentioned problems 
and provide an image forming apparatus of a good image quality using a 
liquid crystal-optical shutter at a low production cost without requiring 
a high luminescent light source or a high sensitivity photosensitive 
member. 
Another object of the present invention is to provide an image forming 
apparatus suitable for preparing a half-tone image. 
According to the present invention, there is provided an image forming 
apparatus, comprising: a light source; an optical shutter unit comprising 
a pair of parallel substrates respectively having a plurality of scanning 
electrodes and a plurality of data electrodes disposed to intersect with 
each other to form a matrix electrode structure on their opposite 
surfaces, and a ferroelectric liquid crystal disposed between the pair of 
substrates so as to form a microshutter at each intersection of the 
scanning electrodes and data electrodes; an optical system for forming an 
image at a desired position with light transmitted through the optical 
shutter unit; a photosensitive member as a medium for recording the thus 
formed image; a scanning side drive circuit for applying a scanning 
voltage signal to the plurality of scanning electrodes so as to 
sequentially select at least one scanning electrode in a prescribed cycle; 
and a data side drive unit for applying data voltage signals to the 
plurality of data electrodes in synchronism with said scanning voltage 
signal, so that image data are transferred on the optical shutter unit 
corresponding to the movement of the photosensitive member so as to move 
an image formed thereby on the photosensitive member while keeping the 
relative position of the image with respect to the photosensitive member. 
According to another aspect of the present invention, there is provided an 
image forming apparatus, comprising: a light source; an optical shutter 
unit comprising liquid crystal-microshutters arranged in a plurality of 
rows and in a plurality of columns, each microshutter being capable of 
controlling the transmittance therethrough of light from the light source 
depending on given gradation data; an optical system for forming an image 
at a desired position with light transmitted through the optical shutter 
unit; and a photosensitive member as a medium for recording the thus 
formed image disposed in such a manner that a particular position on the 
photosensitive member is exposed to light a plurality of times 
respectively through a plurality of microshutters each belonging to one of 
the plurality of rows; the opening period of a microshutter being 
different for the respective rows of the microshutters. 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a diagram showing an LC-optical shutter unit and its peripheral 
circuit according to an embodiment of the image forming apparatus of the 
present invention. 
Referring to FIG. 1, an LC-optical shutter unit 11 comprises a pair of 
substrates (not shown) having 20 scanning electrodes 11a and 2000 data 
electrodes 11b, respectively, facing each other on their opposite faces so 
as to form a matrix electrode, and a ferroelectric liquid crystal (FLC, 
not shown) disposed between the substrates. In this embodiment, the 
intersections of the scanning electrodes 11a and the data electrodes 11b 
each constitute a microshutter or shutter window and are arranged in the 
form of a matrix as shown by a schematically enlarged view at the lower 
right corner in FIG. 1. 
The 20 scanning electrodes 11a of the LC-optical shutter unit 11 are 
connected via scanning lines 14 to a scanning side drive circuit 12 
including a 20-bit shift register 18 and a scanning line drive unit 16 
which successively supplies a scanning voltage based on scanning signals 
from the shift register. On the other hand, the 2000 data electrodes 11b 
are connected via data lines 15 to a data side drive circuit 13 including 
a 2000-bit shift register 19 for storing image data from an external 
circuit in parallel at specified points of time, a latch circuit 20 for 
once memorizing image signals from the shift register 19, and a data line 
drive unit 17 for supplying data voltages according to the data signals 
from the shift register 19. 
The system shown in FIG. 1 further includes a control circuit 21 for 
supplying control pulses .phi..sub.1, .phi..sub.2 and .phi..sub.3 to the 
shift registers 18, 19 and the latch circuit 20, respectively, and a RAM 
(random access memory) 22. The other members of the image forming 
apparatus are similar to those shown in FIG. 10. Thus, a fluorescent light 
source is disposed as a linear light source above the LC-optical shutter 
unit, and below the unit are disposed an optical system and a 
photosensitive drum. 
In the above-described system, when a control pulse .phi..sub.1 is supplied 
to the shift register 18 from the control circuit 21, the shift register 
18 supplies a scanning signal to the scanning line drive unit 16. In 
response to the scanning signal, the scanning line drive unit 16 supplies 
a scanning voltage whereby a scanning line 14 is selected or not selected. 
On the other hand, image signals supplied to the RAM 22 are stored therein 
in an amount required for 20.times.2000 pixels, and new image signals for 
one line (2000 bits) are supplied from the exterior and the oldest image 
signals for one line are discarded therefrom for each field scanning 
operation. Image data sent from the RAM 22 are converted by the shift 
register 19 into parallel signals for one line, which are then sent to the 
latch circuit 20 by a control pulse .phi..sub.3. The data signals stored 
in the latch circuit 20 are sent to the data line drive unit 17 by a 
control pulse .phi..sub.2 at completion of each line. By the data line 
drive unit 17, data voltages are applied corresponding to the data signals 
whereby the data lines 15 are respectively selected or not selected. The 
application of the data voltages is synchronized with the application of 
the scanning voltages as the scanning lines are described above, whereby 
an image pattern is formed on the shutter unit (array) 11. 
The control circuit 21 generates the control pulses .phi..sub.1 
-.phi..sub.3, and in synchronism therewith, supplies a readout address 
signal to the RAM 22, wherein the address signal is supplied so that an 
addressed row is shifted by one row for each one field scanning. After the 
20 scanning lines are scanned once in this manner, an image pattern is 
formed on the shutter unit, and in the subsequent scanning, an image 
pattern is formed by shifting one bit in the direction of a data line. If 
the above-operation is repeated thereafter, an image pattern on the 
shutter unit is moved, as if it flows in the direction of the data lines, 
to effect so-called "scrolling". At this time, an image formed on the 
photosensitive member moves at an equal speed in the reuse direction as 
the optical system provides an inverted image of an equal size. Therefore, 
if the photosensitive member is rotated corresponding to the above 
movement, a light image is caused to illuminate the same part of the 
photosensitive member without changing a relative position with respect to 
the photosensitive member. As a result, the exposure quantity (illuminance 
X time) is increased to twenty times that given by a conventional 
single-row shutter array if it is assumed that the exposure illuminance 
during the movement is constant. In an actual case using an optical 
system, however, the light transmitted through the LC-optical shutter unit 
11 reaches the photosensitive drum at different percentages depending on 
the position of an image in question. As a result, in the particular 
actual embodiment as described above, the total exposure quantity was 
about 12 times that obtained by a single-row shutter array. It was further 
observed that undesirable phenomena such as after image and tailing 
occurred along with the movement of an image on the LC-optical shutter 
unit 11 when a TN-type device was used in the shutter unit, whereas a 
clear image having a high contrast and free of after image or tailing was 
obtained when a ferroelectric liquid crystal device was used. 
FIG. 2 illustrates another embodiment of the present invention which is 
similar to the one shown FIG. 1 except that microshutters formed at 
intersections of 20 scanning lines and 500 data lines are arranged in a 
staggered fashion as shown at the lower right corner in FIG. 2. In FIG. 2, 
the same reference numerals as used in FIG. 1 denote equivalent parts. In 
the previous embodiment, a point on the photosensitive member is 
repeatedly exposed 20 times during the movement, whereas it is exposed 5 
times in this embodiment. Accordingly, it is necessary to reduce the image 
movement or transfer velocity and the peripheral speed of the 
photosensitive member to about 1/4 times those in the previous embodiment 
in order to obtain the same exposure quantity. In this embodiment, 
however, the number of data lines is reduced to 1/4 so that the data line 
drive unit is simplified to result a remarkable reduction in apparatus 
cost. 
In the above embodiments, image data on the shutter unit are moved by one 
line for one-frame scanning. It is however possible to adopt a scheme 
wherein image data are moved by one line for scanning of two or more 
frames or a scheme wherein image data are moved by two or more lines for 
one-frame scanning (interlaced scanning). In any scheme, image data formed 
on a photosensitive member are moved or transferred accompanying the 
movement of a photosensitive member without changing the relative 
position. 
Image data formed on one line of shutters arranged in a matrix are 
transferred line by line so that the image formed on the photosensitive 
member does not change its relative position with respect to the 
photosensitive member. As the transfer operation is repeated, an image 
pattern on the shutter unit is moved as if it flows in the direction of 
the data lines to effect so-called "scrolling". As a result, a particular 
point of the photosensitive member is continually exposed to the same bit 
of light image, so that even a low-luminance light source or a 
low-sensitivity photosensitive member can provide an image of a 
sufficiently high quality. 
As an ordinary TN-type liquid crystal has a slow response time on the order 
of 100 msec, the response of the liquid crystal at the time of scrolling 
cannot follow the change in applied voltage to cause a phenomenon of after 
image or tailing so that it is difficult to effect normal transfer of 
image data. However, it is possible to effect scrolling without such after 
image or tailing by using a ferroelectric liquid crystal showing a 
response time on the order of 100 .mu.sec. 
As described above, according to the image forming apparatus of the present 
invention, shutter windows or microshutters of an LC-optical shutter unit 
are arranged in the form of a matrix using preferably a ferroelectric 
liquid crystal, and image data on the shutter unit are moved or 
transferred along the movement of a photosensitive drum so that the 
resultant image formed on the photosensitive member does not change its 
relative position with respect to the photosensitive member, whereby an 
image of a sufficiently high quality can be obtained even by using a light 
source of low luminance or a photosensitive member of a low sensitivity. 
Particularly, by using a ferroelectric liquid crystal device in the 
LC-optical shutter unit, a clear and high-contrast image can be formed on 
the photosensitive drum, the image quality is further improved. Further, 
if a staggered arrangement of unit shutters is adopted, the drive circuit 
can be simplified to reduce a production cost. 
FIGS. 3(a)-3(d) are time charts showing a set of driving waveforms used in 
the present invention and optical responses obtained as a result 
respectively expressed in time series, and FIG. 4 is a schematic partial 
plan view of a matrix electrode structure used in an apparatus of the 
present invention. 
The matrix electrode structure shown in FIG. 4 allows a gradational display 
with 16 steps of 0/15, 1/15, . . . 15/15 by 4 shutter rows. The scanning 
lines S.sub.1.sup.0, S.sub.2.sup.0, S.sub.3.sup.0 and S.sub.4.sup.0 are 
supplied with scanning signals S.sub.1, S.sub.2, S.sub.3 and S.sub.4, 
respectively, shown at FIG. 3(a), and the data lines I.sub.1.sup.0, 
I.sub.2.sup.0, . . . are supplied with image signals I.sub.1, I.sub.2, . . 
. in the form of binary signals of open and close in 4 times as shown at 
FIG. 3(b). 
At this time, 4 pixels A, B, C and D on a particular data line 
(I.sub.1.sup.0 in FIG. 4 in this embodiment) are supplied with voltages in 
time-serial waveforms shown at FIG. 3(c) A, B, C and D, whereby the pixels 
are provided with light transmission states as shown at FIG. 3(d) A, B, C 
and D, respectively, as the open or closed state of a pixel is determined 
depending on the electric field direction of a voltage applied to the 
pixel exceeding the threshold voltage of the ferroelectric liquid crystal. 
In this embodiment, the voltage value V.sub.0 is set to satisfied the 
relationship of .vertline.V.sub.0 
.vertline.&lt;.vertline.Vth.vertline.&lt;.vertline.3V.sub.0 /2.vertline., 
wherein Vth denotes the threshold value of the ferroelctric liquid 
crystal. As a result, referring to FIG. 5 which shows a positional 
relationship between a shutter array 51 and a photosensitive member 52 in 
an image forming apparatus, image forming positions A.sup.0, B.sup.0, 
C.sup.0 and D.sup.0 on the photosensitive member are exposed in this 
scanning cycle to quantities of light in ratios of 8:4:2:1 as the scanning 
lines S.sub.1, S.sub.2, S.sub.3 and S.sub.4 are supplied with scanning 
selection signals having durations set at ratios of 8:4:2:1 in this 
embodiment. Referring further to FIG. 5, light images transmitted through 
microshutters (pixels) A, B, C and D on the shutter array 51 are focused 
at the image-forming positions A.sup.0, B.sup.0, C.sup.0 and D.sup.0 on 
the photosensitive member 52 by the action of a lens array 53 providing an 
inverted image of an equal size. 
Then, another image is formed on the shutter array 51 by repeating the 
above-mentioned scanning cycle while the photosensitive member is shifted 
for a distance of one time. As a result, light images through the pixels 
B, C and D are focused at the image-forming positions A.sup.0, B.sup.0 and 
C.sup.0, respectively, and the image-forming position D.sup.0 is moved out 
of the image forming region while an image-forming position Z.sup.0 comes 
into a position for receiving transmitted light through the pixel A. 
By repeating the above scanning operation, a particular point on the 
photosensitive member receives light transmitted successively through the 
pixels A, B, C and D to complete a whole exposure stage by 4 scanning 
cycles. In order to provide the particular point with an exposure for 
providing a gradation of 11/15, the pixels A, B, C and D are supplied with 
signals of "open", "close", "open" and "open", respectively, as 11 is 
given by the sum of 8+0+2+1. FIG. 6 shows a set of scanning line voltages 
and data line voltages to be applied in that case. The designation of 
"open" or "close" signal to the pixels A, B, C and D at their respective 
exposure time is listed in the following Table 1 corresponding to a series 
of required gradation. 
TABLE 1 
__________________________________________________________________________ 
Gradation 
Pixel 
0 1 2 3 4 5 6 7 8 9 10 
11 
12 
13 
14 
15 
__________________________________________________________________________ 
A 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 
B 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 
C 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 
D 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 
__________________________________________________________________________ 
1: OPEN 
0: CLOSE 
In a preferred embodiment of the present invention, a driving scheme as 
disclosed by, e.g., U.S. Pat. No. 4,655,561 in addition to the one shown 
in FIGS. 3(a)-3(d) may be used. 
In the above embodiments, there has been utilized a property of a 
ferroelectric liquid crystal that it causes a transition between the 
orientation states only when it is supplied with a voltage exceeding a 
threshold and retains its state when supplied with a voltage below the 
threshold. In another preferred embodiment of the present invention, a 
ferroelectric liquid crystal pixel is always supplied with an electric 
field exceeding a threshold and the open or closed state of the pixel is 
governed by the direction of the electric field. FIGS. 7(a)-7(d) show an 
example set of driving waveforms used in such an embodiment. More 
specifically, FIG. 7 shows (a) scanning pulses S.sub.1 -S.sub.4 applied to 
4 rows of unit shutters in a matrix shutter array, (b) data pulses I.sub.1 
and I.sub.2, (c) resultant voltages applied to pixels A, B, C and D, and 
(d) resultant light-transmission states at the respective pixels. This 
driving scheme utilizes a property that a pixel assumes "open" when 
supplied with a positive direction voltage and "close" when supplied with 
a negative direction voltage, respectively exceeding a voltage V.sub.0, as 
described in detail in U.S. Pat. No. 4,548,476. The respective pixels show 
light transmission states as shown at FIG. 7(d). 
As the selection periods for the respective scanning lines are set to 
ratios of 8:4:2:1, the pixels A, B, C and D are respectively exposed to 
quantities of light in ratios of 8:4:2:1. By repeating the scanning 
operation, a gradational image formation can be effected in the same 
manner as in the previous embodiment. 
As the liquid crystal showing bistability used in the image forming 
apparatus of the present invention, a chiral smectic liquid crystal 
showing ferroelectricity is most preferred. Particularly, a liquid crystal 
in chiral smectic C phase (SmC*) or H phase (SmH*) is suited. 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", "Kotai Butsuri (Solid State 
Physics)" 16 (141), 1981 "Liquid Crystal"; U.S. Pat. Nos. 4,561,726, 
4,589,996, 4,596,667, 4,613,209, 4,614,609, and 4,639,089, etc. 
Ferroelectric liquid crystals disclosed in these publications may be used 
in the present invention. 
More particularly, examples of ferroelectric liquid crystal compound usable 
in the method according to the present invention include 
decyloxybenzylidene-p'-amino-2-methylbutyl cinnamate (DOBAMBC), 
hexyloxybenzylidene-p'-amino-2-chloropropyl cinnamate (HOBACPC), 
4-O-(2-methyl)-butylresorcylidene-4'-octylaniline (MBRA 8), etc. 
When a device is constituted using these materials, the device may be 
supported with a block of copper, etc., in which a heater is embedded in 
order to realize a temperature condition where the liquid crystal 
compounds assume an SmC* or SmH*. 
Further, in the present invention, it is also possible to use a 
ferroelectric liquid crystal in chiral smectic F phase, I phase, J phase, 
G phase or K phase in addition to one in SmC* or SmH*. 
Referring to FIG. 8, there is schematically shown an example of a 
ferroelectric liquid crystal cell for explanation of the operation 
thereof. Reference numerals 81a and 81b denote substrates (glass plates) 
on which a transparent electrode of, e.g., In.sub.2 O.sub.3, SnO.sub.2, 
ITO (Indium-Tin-Oxide), etc., is disposed, respectively. A liquid crystal 
of, e.g., an SmC*-phase in which liquid crystal molecular layers 82 are 
oriented perpendicular to surfaces of the substrates is hermetically 
disposed therebetween. Full lines 83 show liquid crystal molecules. Each 
liquid crystal molecule 83 has a dipole moment (P.perp.) 84 in a direction 
perpendicular to the axis thereof. When a voltage higher than a certain 
threshold level is applied between electrodes formed on the substrates 81a 
and 81b, a helical structure of the liquid crystal molecule 83 is unwound 
or released to change the alignment direction of respective liquid crystal 
molecules 83 so that the dipole moments (P.perp.) 84 are all directed in 
the direction of the electric field. The liquid crystal molecules 83 have 
an elongated shape and show refractive 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 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 optical 
modulation device, of which optical characteristics vary depending upon 
the polarity of an applied voltage. Further, when the thickness of the 
liquid crystal cell is sufficiently thin (e.g., 1 .mu.), the helical 
structure of the liquid crystal molecules is unwound even in the absence 
of an electric field whereby the dipole moment assumes either of the two 
states, i.e., Pa in an upper direction 93a or Pb in a lower direction 93b 
as shown in FIG. 9. When electric field Ea or Eb higher than a certain 
threshold level and different from each other in polarity as shown in FIG. 
9 is applied to a cell having the above-mentioned characteristics, the 
dipole moment is directed either in the upper direction 94a or in the 
lower direction 94b depending on the vector of the electric field Ea or 
Eb. In correspondence with this, the liquid crystal molecules are oriented 
in either of a first stable state 93a (bright state) and a second stable 
state 93b (dark state). 
When the above-mentioned ferroelectric liquid crystal is used as an optical 
modulation element, it is possible to obtain two advantages. First is that 
the response speed is quite fast. Second is that the orientation of the 
liquid crystal shows bistability. The second advantage will be further 
explained, e.g., with reference to FIG. 9. When the electric field Ea is 
applied to the liquid crystal molecules, they are oriented in the first 
stable state 93a. This state is stably retained even if the electric field 
is removed. On the other hand, when the electric field Eb of which 
direction is opposite to that of the electric field Ea is applied thereto, 
the liquid crystal molecules are oriented to the second stable state 93b, 
whereby the directions of molecules are changed. This state is also stably 
retained even if the electric field is removed. Further, as long as the 
magnitude of the electric field Ea or Eb 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 and generally 0.5 to 20 .mu., particularly 1 
to 5 .mu.. 
As described above, according to the present invention, a half tone image 
can be produced by a binary state-controlling drive scheme of controlling 
a state of open or close, so that the designing of an image forming 
apparatus, particularly of a drive circuit therefor, can be remarkably 
simplified.