Optical writing device with controlled driving voltages

Disclosed is an optical writing device having an array of a plurality of writing light shutter elements for an image forming purpose. The optical writing device also has at least one first monitoring light shutter element and at least one second monitoring light shutter element. During recording period, the first second monitoring light shutter element is frequently driven, and the second monitoring light shutter element is infrequently driven. After the recording period, the first and second monitoring light shutter elements are driven with varying voltage, and light amounts from the first and second monitoring light shutter elements are sensed by a sensor. Based on the output from the sensor, optimal drive voltage for the next recording period is set.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to an optical writing device that writes or 
displays images on an image receiving surface, e.g., a photosensitive 
material, a screen, or a naked eye, and more particularly, to an optical 
writing device in which an electro-optical material such as PLZT is used 
for the light shutter elements. 
2. Description of Related Art 
An image forming apparatus that forms images (latent images) by exposing 
photographic paper or film using a silver photosensitive material or a 
photosensitive drum for electronic photography by means of an optical 
writing device is conventionally known. One solid-state scanning type form 
of this optical writing device, in which a PLZT optical shutter element 
array is used, is known. In an optical writing device of this solid-state 
scanning type, a polarizer and an analyzer are located in the upstream 
side and the downstream side of the light path relative to the light 
shutter element array, respectively. The polarizer and the analyzer are 
arranged in a Cross-Nicoled fashion relative to the light shutter 
elements. Because PLZT is a material that has an electro-optical effect, 
as is publicly known, light may be allowed to pass through or prevented 
from passing through each light shutter element by controlling the applied 
voltage. FIG. 6 shows the relationship between the drive voltage for each 
light shutter element and the amount of pass-through light. In the 
drawing, the characteristic A represents the characteristic in the initial 
stage, and the light shutter element is driven at all times using a 
half-wavelength voltage V.sub.H that achieves the maximum amount of 
pass-through light. 
However, when an electrical field running in a certain direction is applied 
to the light shutter element at all times, the initial characteristic A 
changes into the characteristic B. If the application of the electric 
field is continued, the characteristic changes into the characteristic C. 
In other words, even if the same voltage H.sub.V is applied, the amount of 
pass-through light decreases by .DELTA.I.sub.B for the characteristic B 
and by .DELTA.I.sub.C for the characteristic C. This change in the amount 
of pass-through light is larger for elements that are driven more 
frequently per unit of time, which leads to the problem of degraded image 
quality. 
Therefore, the applicant has proposed, as disclosed in Laid-Open Japanese 
Patent Application HEI 4-115219, to incorporate a means to monitor the 
characteristic of the light shutter elements and control the recovery of 
the characteristic in accordance with the monitored amount of light. 
However, using this invention, because the attempt to recover the 
characteristic is made during the application of an alternating electrical 
field, a relatively long period of time is required, and due to the need 
to monitor the amount of light corresponding to the alternating electrical 
field, the circuit becomes complex in construction. 
SUMMARY OF THE INVENTION 
The object of the present invention, therefore, is to provide an improved 
optical writing device. Another object of the present invention is to 
provide a solid-state scanning optical writing device that can prevent, by 
means of a relatively simple construction, large variations in the amount 
of pass-through light among the light shutter elements, which occur due to 
variations in the frequency with which they are driven. 
In order to achieve at least one of the objects described above, the 
optical writing device of the present invention uses at least one first 
monitoring light shutter element and at least one second monitoring light 
shutter elements that may be made of an electro-optical material and are 
separate from writing light shutter elements used for image writing 
purposes. The first monitoring light shutter element is driven under a 
first condition, while the second monitoring shutter element is driven 
under a second condition, wherein the first condition is to drive the 
first monitoring light shutter element frequently, and the second 
condition is to drive the second monitoring light shutter element 
infrequently. It is preferred that the driving of the first and second 
monitoring light shutter elements occur while the image writing is being 
performed by means of the writing light shutter elements, i.e., during the 
recording period. When writing or recording is completed, the first and 
second monitoring light shutter elements are driven with at varying 
voltages. When the amount of light exiting from the first and second 
monitoring shutter elements is detected at this time, the average 
characteristic of the amount of pass-through light from the first 
monitoring light shutter element that is driven frequently and the second 
monitoring light shutter element that is driven infrequently may be 
obtained. The driving voltage at which the amount of exiting light was the 
largest is then determined and is fed back to the driving of the writing 
light shutter elements during the next recording session. 
Various conditions may be used for the first and second conditions. For 
example, for the first condition, the same condition as for the writing 
light shutter element which is most frequently driven may be used, and for 
the second condition, the same condition as for the writing light shutter 
element which is driven most infrequently may be used. This enables 
control that accurately reflects the actual hysterisis based on the 
driving of the writing light shutter elements. 
As another example, the first condition can be that the first monitoring 
light shutter element is continuously activated and the second condition 
can be that the second monitoring light shutter element is not activated 
at all. Using this method, while the accuracy decreases slightly in 
comparison with the previous example, control that is problem-free as a 
practical matter may be obtained. Further, because it is not necessary to 
refer to the driving condition of the writing light shutter elements, the 
construction of the optical writing device may be made simpler. 
In addition, when the first and second monitoring light shutter elements 
are driven at varying voltages, it is preferred that the voltage be 
continuously varied within a prescribed range. It is preferred that the 
prescribed range include zero volts up to a voltage that exceeds the 
high-wavelength voltage. 
Using the present invention, a proxy of an average characteristic of the 
amount of pass-through light of all of the writing light shutter elements 
may be obtained by detecting the amount of exiting light from the first 
and second monitoring light shutter elements, and the average optimal 
driving voltage may be fed back, allowing the variation in the amount of 
pass-through light among the light shutter elements that occurs during the 
recording period due to differences in the frequency of driving to be 
reduced, such that stable high-quality images may be obtained. In 
addition, in comparison with the method in which the characteristic of the 
light shutter elements is sought to be recovered based on the monitoring 
of the light amount while applying an alternating electric field, the 
present invention may be realized using a simpler construction and the 
processing takes a shorter period of time. 
These and other objects, advantages and features of the invention will 
become apparent from the following description thereof taken in 
conjunction with the accompanying drawings which illustrates a specific 
embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiments of the optical writing device pertaining to the present 
invention are explained below with reference to the accompanying drawings. 
First Embodiment 
An image forming apparatus in which the optical writing device, which 
comprises the first embodiment of the present invention, is applied is 
shown in FIG. 1. This image forming apparatus forms electrostatic latent 
images on the surface of the photoreceptor drum 7 by exposing the drum 7 
by the optical writing device. The optical writing device includes a light 
source (halogen lamp) 1, an optical fiber array 2, a polarizer 3, a light 
shutter module 4, an analyzer 5 and an image forming lens array 6. 
Further, as a characteristic element of this embodiment, a driving power 
supply 11 for the corrective process and a photosensor 13, which are 
described below, are included. The optical fiber array 2 comprises 
numerous individual optical fibers bound together. The light emitted from 
the light source 1 irradiates the light incidence end 2a and exits the 
optical fiber array 2 from the other end 2b in a linear fashion. 
The light shutter module 4 comprises a ceramic or glass substrate 21 having 
a slit opening and an array 22 comprising multiple light shutter chips 
made of PLZT and located on the substrate 21, as shown in FIG. 2. Each of 
the light shutter chips has multiple light shutter elements, each of which 
comprises one pixel. The light shutter elements are arranged in two rows 
such that the elements are arranged in a zigzag fashion and the two rows 
of light shutter elements together form images corresponding to one line 
in the main scanning directions. PLZT comprises a light-permeable ceramic 
substance having an electro-optical effect with a large Kerr constant, as 
is commonly known in the field. The light that undergoes linear 
polarization by the polarizer 3 experiences rotation of the plane of 
polarization when a voltage is applied to the light shutter element, and 
exits through the analyzer 5. When no voltage is applied, the plane of 
polarization does not rotate and the pass-through light, which did not 
undergo rotation of the plane of polarization, is cut off by the analyzer 
5. 
In other words, as the application of a voltage to the light shutter 
element is turned ON and OFF, the light permeation property is also turned 
ON and OFF. The light that exits the analyzer 5 forms an image on the 
photoreceptor drum 7 via the image forming lens array 6, resulting in the 
formation of an electrostatic latent image on the drum 7. The light 
shutter elements are turned ON and OFF one line at a time in accordance 
with the image data (main scanning). By synchronizing the main scanning 
and the speed of rotation of the photoreceptor drum 7 in one direction 
(secondary scanning), a two-dimensional image (or a latent image) is 
formed on the drum 7. 
The light shutter elements are individually driven by the drive ICs 30 that 
are located on both sides and along the array 22. Each drive IC 30 
comprises a shift register 31, a latch circuit 32, AND gates 33 (33.sub.1 
.about.33.sub.n), and high-voltage drivers 34 (34.sub.1 .about.34.sub.n). 
Image data DATA is sent to the shift register 31 in synchronization with a 
clock signal CLK and is latched to the latch circuit 32 when a latch 
strobe signal LS is issued. A voltage VD is applied to the light shutter 
elements 23.sub.1 through 23.sub.n from the high-voltage drivers 34.sub.1 
through 34.sub.n, respectively, in a pulse fashion based on the image data 
DATA and the turning ON and OFF of a signal BLK via the AND gates 33.sub.1 
through 33.sub.n. When this takes place, bi-refraction occurs in the PLZT 
and the incident light passes through while being polarized. The 
relationship between the incident light and the exiting light is expressed 
by the following equation. 
EQU I.sub.o /I.sub.i =sin.sup.2 (-.pi..multidot.n.sup.2 
.multidot.R.multidot.L.multidot.E.sup.2 /2.lambda.) 
I.sub.o : Amount of exiting light 
I.sub.i : Amount of incident light 
n: PLZT refractive index 
R: Kerr constant 
L: Length of light path 
E: Strength of electrical field 
.lambda.: Light wavelength 
The light shutter element made of PLZT is electrically equivalent to a 
condenser and is charged when a voltage is applied to its electrodes. 
While a regular condenser releases all electrical charge by 
short-circuiting the electrodes, with PLZT, even if the electrodes are 
short-circuited, some electrical charge remains. As the residual 
electrical charge accumulates, as shown in FIG. 6, the pass-through light 
amount characteristic changes from that of the beginning stage, i.e., the 
initial characteristic A, to the characteristic B and then to the 
characteristic C, where the amount of pass-through light decreases by 
.DELTA.I.sub.B and .DELTA.I.sub.C, respectively, depending on the 
frequency with which the light shutter element is driven. 
FIG. 7 shows the amount of pass-through light after the light shutter 
element is driven for a certain recording period. In the drawing, the 
characteristic D represents the amount of pass-through light relative to 
the driving voltage of the light shutter element which is driven most 
infrequently and the characteristic E represents the amount of 
pass-through light relative to the driving voltage of the light shutter 
element which is driven most frequently. It is presumed that the 
characteristic of other light shutter elements falls somewhere between the 
characteristics D and E. Here, the driving voltage that can minimize the 
variation in the output light amount among the light shutter elements is 
the voltage at the point at which the characteristic D and the 
characteristic E intersect. This intersection point voltage is a voltage 
VD by which the maximum light amount for the characteristic F, which is 
obtained by adding the characteristics D and E, is obtained. 
In consideration of the facts described above, in this embodiment, the same 
number of multiple light amount-monitoring multiple light shutter elements 
23A and 23B were located next to the recording light shutter elements 23 
on one end such that they would comprise a single array 22, as shown in 
FIG. 4. Further, a driving voltage source or power supply 11 (see FIG. 1) 
to drive all the light shutter elements including the monitoring light 
shutter elements 23A and 23B, and a photosensor 13 to detect the amount of 
light exiting from the monitoring light shutter elements 23A and 23B were 
also used. The light shutter elements 23A are driven using the same 
condition as the recording light shutter element that is driven most 
frequently during a recording period. On the other hand, the light shutter 
elements 23B are driven using the same condition as the recording light 
shutter element that is driven most infrequently during a recording 
period. 
This control is executed by the CPU 17 selecting the light shutter element 
that is driven most frequently and the light shutter element that is 
driven most infrequently based on the image data visualized on a data map 
which is located in memory 18 in FIG. 1 and inputting the same driving 
conditions as used for these elements to the drive power supply 11 via the 
data bus 16 and the D/A converter 12 to drive the monitoring light shutter 
elements 23A and 23B using their respective conditions. Naturally, the 
light shutter elements 23A and 23B are located in an area that is outside 
the imaging or recording area of the photoreceptor drum 7. 
During a non-imaging or non-recording (break) period, the monitoring light 
shutter elements 23A and 23B are driven using a voltage that continuously 
changes from 0V to a level exceeding the half-wavelength voltage and the 
amount of light exiting from them is detected by means of the photosensor 
13. The driving voltage and the output from the photosensor 13 at this 
time appear as shown under the `break period` in the timing chart of FIG. 
5. The output from the photosensor 13 corresponds to the sum 
characteristic F shown in FIG. 7. The driving voltage VD at which the 
maximum light amount is obtained is determined from the peak value of the 
characteristic F. In the next recording period, the recording light 
shutter elements 23 are driven using the voltage VD thus obtained. Through 
such feedback control, the variation in light amount caused by the 
differences in driving frequency among the recording light shutter 
elements 23 may be reduced substantially. 
The control circuit for monitoring the light amount is shown in FIG. 1. The 
detection output (analog electric current value) is converted into an 
analog voltage value by the I/V converter 14, and then converted into a 
digital value by the A/D converter 15. It is then stored in a 
predetermined area of the memory 18 via the data bus 16. This process is 
mainly controlled by the CPU 17, and is repeated during the non-imaging 
period with varying the voltage VD. When the next recording period begins, 
the CPU 17 performs control the power supply via the D/A converter 12 so 
that the voltage VD that corresponds to the maximum digital value stored 
in the memory 18 will be supplied to the drive ICs 30. 
It is preferred that the same numbers of monitoring light shutter elements 
23A and 23B be used. They should be placed in an area at which the light 
amount from both types of monitoring light shutter elements may be 
detected at the same time by the photosensor 13. Naturally, the added 
characteristic F may be detected by separately detecting the light amount 
from the light shutter elements 23A and 23B and calculating the total 
amount. In addition, it is preferred that all of the writing light shutter 
elements 23 be turned OFF during the light amount monitoring period, in 
order to prevent the light source 1 that is turned ON for the purpose of 
monitoring from irradiating and causing fatigue to the photoreceptor drum 
7. 
Variation 
The optical writing device pertaining to the present invention is not 
limited to the embodiment described above, and may be modified in various 
ways within the scope of the invention. For example, the optical writing 
device is not limited to a device that exposes a photoreceptor, but may be 
a device that projects images onto a screen, for example. 
In the first embodiment, for the conditions to drive the light amount 
monitoring light shutter elements 23A and 23B, the same condition as for 
the writing light shutter element driven most frequently and the same 
condition as for the writing light shutter element driven most 
infrequently were used, respectively, but the present invention is not 
limited to this implementation. For example, it is also acceptable if the 
conditions for driving the light amount monitoring light shutter elements 
23A and 23B are that the light shutter elements 23A are driven throughout 
a recording period and the light shutter elements 23B are not driven 
throughout a recording period. While this method entails slightly less 
accuracy compared with the first embodiment, control may be carried out 
without any problems for practical purposes. Further, because it no longer 
becomes necessary to refer to the driving condition of the writing light 
shutter elements, the construction of the optical writing device may be 
made simpler. 
Moreover, the detection circuit in which a monitoring photosensor is used 
and the optimal driving voltage determination circuit in the first 
embodiment may have various other constructions. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be constructed as 
being included therein.