Bias voltage adjusting means for electrographic apparatus

An electrographic apparatus comprising a photosensitive screen composed of an insulating layer, two electrically conductive layers coated on opposite sides of the insulating layer and a photosensitive layer coated on one of said electrically conductive layers, and a bias electric source connected between the two electrically conductive layers and producing, on a dielectric coated record sheet, an electrostatic latent image corresponding to an image to be reproduced is disclosed. The apparatus comprises means for adjusting a bias voltage supplied from the bias electric source and applied between the two electrically conductive layers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electrographic apparatus comprising a 
photosensitive screen composed of an insulating layer, two electrically 
conductive layers coated on opposite sides of the insulating layer and a 
photosensitive layer coated on one of the electrically conductive layers, 
a corona discharge device arranged at one side of the photosensitive 
screen and emitting a flow of corona ions, and a dielectric coated record 
sheet arranged at the other side of the photosensitive screen, whereby an 
electrostatic latent image produced on the photosensitive layer of the 
photosensitive screen causes the flow of corona ions directed from the 
corona discharge device through the photosensitive screen toward the 
dielectric coated record sheet to modulate so as to produce, on the 
dielectric coated record sheet, an electrostatic charge image 
corresponding to the electrostatic latent image on the photosensitive 
layer of the photosensitive screen. 
2. Description of the Prior Art 
Various kinds of electrographic apparatus have been proposed which can 
modulate a flow of corona ions by an electrostatic latent image produced 
on a photosensitive screen so as to produce on a dielectric coated record 
sheet an electrostatic charge image. 
One of these prior art apparatus makes use of a photosensitive screen of 
four layer construction composed of an insulating layer, two electrically 
conductive layers coated on opposite sides of the insulating layer and a 
photosensitive layer coated on one of the electrically conductive layers. 
Such apparatus can perform a first step of uniformly charging the 
photosensitive layer, a second step of illuminating the photosensitive 
screen with a light image and producing thereon an electrostatic latent 
image, and a third step of applying a high voltage between a field 
electrode arranged at the rear of a dielectric coated record sheet which 
is opposed to and spaced apart from the photosensitive layer of the 
photosensitive screen on the one hand and the photosensitive layer of the 
photosensitive screen on the other hand and directing a flow of corona 
ions from a corona discharge device arranged at the side of the 
electrically conductive layer of the photosensitive screen through the 
photosensitive screen toward the dielectric coated record sheet while 
applying a bias voltage between the two electrically conductive layers and 
producing on the dielectric coated record sheet an electrostatic charge 
image corresponding to the electrostatic latent image produced on the 
photosensitive layer of the photosensitive screen. 
This apparatus has the advantage that the picture image can be controlled 
by adjusting the bias voltage applied between the two electrically 
conductive layers in dependence with the number of copies set beforehand 
and that a plurality of copies can be reproduced by repeating the third 
step after the electrostatic latent image has been produced on the 
photosensitive screen. 
In practice, however, a concentration of the picture image becomes changed 
in dependence with the number of copies set beforehand owing to a dark 
decay characteristic of the photosensitive layer and to an undesirous 
detour of the flow of corona ions emitted from the corona discharge 
device. As a result, copies each having a picture image having a good 
quality are limited in number and it is impossible to obtain a large 
number of copies each of good quality. 
In order to obviate such drawback, another prior art electrographic 
apparatus has been proposed. In this apparatus, in the above mentioned 
third step, the bias voltage applied between the electrically conductive 
layers of the photosensitive screen can be changed in response to the 
number of copies set beforehand and hence can compensate for the decay of 
the electrostatic latent image on the photosensitive layer. In such 
electrographic apparatus constructed as above described, in the case of 
obtaining a plurality of copies from the electrostatic latent image 
produced on the photosensitive screen by repeating the third step, if the 
concentration of the first picture image becomes incorrect, the 
concentration of subsequent picture images becomes also incorrect. 
SUMMARY OF THE INVENTION 
An object of the invention, therefore, is to provide an electrographic 
apparatus which can change the reproduction property of subsequent picture 
image in dependence with the concentration of the first reproduced picture 
image. 
Another object of the invention is to provide an electrographic apparatus 
which can select an intermediate concentration tone reproduction property 
of successive picture images to be reproduced. 
A feature of the invention is the provision, in an electrographic apparatus 
comprising a photosensitive screen composed of an insulating layer, two 
electrically conductive layers coated on opposite sides of the insulating 
layer and a photosensitive layer coated on one of the electrically 
conductive layers, a corona discharge device arranged at one side of the 
photosensitive screen and emitting a flow of corona ions, and a dielectric 
coated record sheet arranged at the other side of the photosensitive 
screen, whereby an electrostatic latent image produced on the 
photosensitive layer of the photosensitive screen causes the flow of 
corona ions directed from the corona discharge devices through the 
photosensitive screen toward the dielectric coated record sheet to 
modulate so as to produce, on the dielectric coated record sheet, an 
electrostatic charge image corresponding to the electrostatic latent image 
on the photosensitive layer of the photosensitive screen, of the 
improvement comprising means for adjusting a bias voltage applied between 
the two electrically conductive layers of the photosensitive screen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIGS. 1A and 1B are shown, in section, two examples of a photosensitive 
screen applicable to an electrographic apparatus according to the 
invention. Each of these photosensitive screens is of four layer 
construction composed of a mesh-shaped electrically conductive core 1 
which is coated on one side with an insulating layer 2 and an electrically 
conductive layer 3 in the order as mentioned and coated on the other side 
with a photosensitive layer 4. 
In FIGS. 2A to 2C are illustrated successive steps of modulating a flow of 
corona ions by means of an electrostatic latent image produced on the 
photosensitive screen shown in FIG. 1A and then producing an electrostatic 
charge image on a dielectric coated record sheet. As shown in FIG. 2A, at 
first, between the electrically conductive core 1 and the electrically 
conductive layer 3 is connected a bias electric source 5 for supplying a 
voltage of E.sub.B0. The photosensitive layer 4 is uniformly charged with 
a desired potential by a corona discharge device 7 connected to a corona 
electric source 6. In the example shown in FIG. 2A, the photosensitive 
layer 4 is uniformly charged by the corona discharge device 7 arranged at 
the side of the electrically conductive layer 3. But, the photosensitive 
layer 4 may also be uniformly charged by a corona discharge device 
arranged at the side of the photosensitive layer 4. In the above mentioned 
first charging step, the bias electric source 5 also functions to 
uniformly charge the photosensitive layer 4 with the desired potential. 
But, the first charging step may be carried out without using the bias 
electric source 5. 
In FIG. 2B is illustrated a second step of illuminating the photosensitive 
layer 4 with a light image as shown by arrows. In the imagewise exposed 
area of the photosensitive layer 4, its resistance value becomes decreased 
to decrease or eliminate the charge produced thereon. In the dark area of 
the photosensitive layer 4, its high resistance is kept as it is, thus 
retaining its charge produced thereon. In this way, an electrostatic 
latent image corresponding to the light image is produced on the 
photosensitive layer 4. 
In FIG. 2C is illustrated a third step which makes use of a dielectric 
coated record sheet 8 and a field electrode 9 opposed to and separated 
from the photosensitive layer 4 of the photosensitive screen. Between the 
photosensitive screen and the field electrode 9 is connected an 
accelerating electric source 10 so as to apply a high voltage 
therebetween. A flow of corona ions is directed from a second corona 
discharge device 11 through the photosensitive screen toward the 
dielectric coated record sheet 8. In this case, a polarity of a corona 
electric source 12 connected to the second corona discharge device 11 is 
made opposite to that of the corona electric source 6 connected to the 
first corona discharge device 7. As a result, the force produced on the 
photosensitive screen mesh 1 acts on the flow of corona ions in the 
directions shown by the arrows. That is, in the dark area of the 
photosensitive layer 4 where the charge remains, the flow of corona ions 
are directed in a direction for promoting its passage therethrough as 
shown by the dotted line arrows. In the imagewise exposed area of the 
photosensitive layer 4 where no is present, the flow of corona ions are 
prevented from passing therethrough as shown by full line arrows. In this 
case, the force for preventing the flow of corona ions from passing 
through the photosensitive screen is determined by a bias voltage E.sub.B1 
applied from the bias electric source 13 between the two electrically 
conductive layers 1, 3 of the photosensitive screen. The action of 
promoting the passage of the flow of corona ions through the 
photosensitive screen is determined by the value obtained by subtracting 
the bias voltage E.sub.B1 applied between the two electrically conductive 
layers 1, 3 from the surface potential built up on the photosensitive 
layer 4. 
The above described electrographic apparatus is capable of obtaining a 
plurality of copies from the electrostatic latent image which has been 
produced on the photosensitive screen by repeating the third step shown in 
FIG. 1C. 
The electrographic apparatus according to the invention makes use of means 
for adjusting the bias voltage. This means for adjusting the bias voltage 
may be mounted on an operation panel of the electrographic apparatus and 
may be manually operated so as to change the bias voltage as will be 
described with reference to one embodiment of the invention. 
Alternatively, the means for adjusting the bias voltage may be 
automatically operated in response to the amount of light in an optical 
path or in response to an electric potential built up on the electrostatic 
latent image produced on the photosensitive screen. In this way, the 
invention is capable of changing the bias voltage in response to 
occurrence of a picture image having an incorrect concentration and hence 
of reproducing a picture image having a correct concentration. 
In FIG. 3 is shown one embodiment of a bias electric source circuit for a 
photosensitive screen of an electrographic apparatus according to the 
invention. 
In the present embodiment, provision is made for a transformer 20 which 
functions to supply a suitable bias voltage to a photosensitive screen. 
The transformer has a secondary winding across which are connected, in 
series, a diode 21, a variable resistor 22, a voltage dividing resistor 
23, and a variable resistor 24 for correcting the bias voltage. In 
parallel with the secondary winding of the transformer 20 and the diode 21 
is connected a smoothing condenser 32. The voltage dividing resistor 23 is 
provided with a number of taps which are connected to contacts 26-1, 26-2, 
26-3, . . . 26-m . . . 26-(n-1), respectively, of a rotary switch 25. Both 
terminals of the voltage dividing resistor 23 are connected to contacts 
26-0 and 26-n, respectively, of the rotary switch 23. A change-over arm 27 
of the rotary switch 25 is connected to an electrically conductive layer 3 
of a photosensitive screen whose electrically conductive mesh 1 is 
connected to that terminal of the secondary winding of the transformer 20 
which is connected to the bias correcting variable resistor 24. The diode 
21 is connected in such polarity that a positive voltage is applied to the 
electrically conductive layer 3 and that a negative voltage is applied to 
the electrically conductive mesh 1. The variable resistor 22 operates to 
adjust the charge characteristic of the photosensitive screen, electric 
source circuit, etc. after the photosensitive screen has been exposed. The 
voltage dividing resistor 23 operates to change the bias voltage in 
response to the number of copies set beforehand when a number of copies is 
to be reproduced from one manuscript. 
In the case of reproducing a first copy, the change-over arm 27 is 
connected to the contact 26-1. In the case of reproducing a second, a 
third . . . copy, the change-over arm 27 is connected to the contact 26-2, 
26-3 . . . in succession. The change-over arm 27 is interlocked with a 
switch for selecting the number of copies set beforehand such that the 
change-over arm 27 is automatically connected to successive contacts in 
response to the number of copies set beforehand. In this way, it is 
possible to change the bias voltage in response to the number of copies 
set beforehand and to obtain a substantially constant picture image 
concentration irrespective of changes of the surface potential built up on 
the electrostatic latent image owing to the reproduction of copies. In 
this case, however, if the concentration of the first picture image 
reproduced is not correct, all of the concentrations of the second, third 
. . . picture images also become incorrect. In order to eliminate such 
drawback, in accordance with the invention, provision is made for the bias 
correction variable resistor 24. The bias correction variable resistor 24 
may be mounted on an operation panel exposed to the outside of the 
electrographic apparatus and can manually be operated by an operator so as 
to adjust the bias voltage. 
If the picture image concentration of the first copy is not correct, the 
operator can adjust the variable resistor 24 in a direction and range 
which are determined in dependence with the first picture image 
concentration and hence obtain a second copy having a correct picture 
image. Then, a desired number of copies set beforehand each having a 
correct picture quality can be obtained. The variable resistor 24 may be 
replaced by a change-over switch 31 which can change-over resistors 28, 29 
and 30 as shown in FIG. 3. 
As seen from the above, the electrographic apparatus according to the 
invention is also capable of selectively changing an concentration of the 
picture image of the intermediate copy by adjusting the variable resistor 
24 shown in FIG. 3. 
In FIG. 4A is shown a graph showing a curve of a potential difference 
E.sub.p -E.sub.R as a function of the proportion of a flow of corona ions 
passing through the photosensitive meshes (a relative concentration of a 
picture image). In the potential difference E.sub.p -E.sub.R, E.sub.p is 
an electrostatic latent image potential produced on the photosensitive 
layer 4 and building up an electric field which can promote the passage of 
the flow of corona ions through the photosensitive screen and E.sub.R is a 
bias voltage applied between the two electrically conductive layers 1, 3 
and preventing the flow of corona ions from passing through the 
photosensitive screen. In practice, in order to completely intercept the 
flow of corona ions through 200 screen mesh, for example, the potential 
difference E.sub.p -E.sub.R must be of the order of +50 V as shown in FIG. 
4A owing to the presence of an electric field built up by the corona 
discharge device and an electric field (0.5 to 1 kv/mm) applied between 
the photosensitive screen and the field electrode. 
In FIG. 4A, the value of the potential difference E.sub.p -E.sub.R which 
can completely intercept the flow of corona ions is given by E.sub.0. This 
shows that, even when the electrostatic latent image potential E.sub.p =0 
V, a bias voltage of the order of 50 V is required in order to completely 
intercept the flow of corona ions. In addition, if the electrostatic 
latent image potential is added to the residual potential, use must be 
made of a bias voltage composed of at least the residual potential 
+E.sub.0. 
In FIG. 4B is shown a graph showing a characteristic curve of the amount of 
exposed light to photosensitive layer formed of Se as a function of 
electrostatic latent image potential E.sub.p and showing the range of 
utilization of the bias voltage in the characteristic curve. If the 
residual potential is not present in the imagewise exposed area of the 
photosensitive layer, it is possible to obtain a picture image which is 
not over developed by making the bias voltage E.sub.R equal to E.sub.0. In 
practice, however, the residual potential is present in the imagewise 
exposed area of the photosensitive layer, so that the lowest bias voltage 
E.sub.B1 required for obtaining the correct picture image must be equal to 
E.sub.0 plus the residual potential. In this case, a reproduced picture 
image has a concentration tone within a range shown by 1 in FIG. 4B. If 
the bias voltage is made equal to E.sub.B2 which is higher than E.sub.B1, 
the lowest voltage within which the flow of corona ions can pass through 
the photosensitive screen becomes higher than the residual voltage. As a 
result, a reproduced picture image has a concentration tone within a range 
shown by 2 in FIG. 4B. In the range 1, the portion reproducing the 
concentration tone increases slowly and hence it is possible to reproduce 
copies which are rich in harmony. In the range 2, the portion producing 
the concentration tone increases sharply and hence can reproduce copies 
each having a good contrast. 
As a result, the range 1 is suitable for reproducing photographs or faint 
letters, etc. each having a poor contrast and the range 2 is suitable for 
reproducing clear letters, etc. each having a good contrast. 
As stated hereinbefore, the use of the bias adjusting variable resistor 24 
or the bias adjusting change-over switch 31 mounted on the operation panel 
of the electrographic apparatus ensures a change of the bias voltage and 
provides the important advantage that the concentration tone reproduction 
characteristic of the picture image to be reproduced can be selected. 
The invention is not limited to the above described embodiments, but many 
changes and alternations may be made. For example, as shown in FIG. 5, the 
bias voltage may be changed at the primary winding side of the bias 
electric source transformer 20.