Method and apparatus for controlling the gray scale response of a multilayer image forming screen

An electrostatic latent image is impressed on a multilayer apertured screen to establish within the apertures electric fields that modulate the passage of charged particles through the screens so that the charged particles are distributed in correspondence with the image. After imaging the screen is operated to control the gray scale response thereof in order to optimize copy quality. The gray scale screen response is expanded by applying a bias voltage of at least two different values to the screen during the duplication interval. The response is limited by altering the latent image charge to provide a high density cutoff and by applying a bias voltage to establish a low density cutoff during the duplication interval. Copy quality is further improved by adjusting the integrated charged particle current to establish the desired degree of density in regions of the copy image corresponding to high density regions of the original image.

This invention relates to a method and apparatus for controlling the tonal 
response of an apertured multilayer image-forming screen of the type 
interposed between a stream of charged particles and a medium (e.g., 
paper) toward which the charged particles are propelled to form an image 
on the medium. 
In operating a multi-layer apertured screen of the type disclosed in U.S. 
Pat. No. 3,713,734 for Method and Apparatus for Forming a Positive 
Electrostatic Image, a photoconductive layer of the screen is impressed 
with an electrostatic latent image of a picture or pattern to be 
reproduced so that the charge level on the screen, or the potential 
difference between opposite surfaces of the photoconductive layer, is 
proportional to the density of the image. As explained in more detail in 
the cited patent, the charge on the photoconductive layer determines the 
magnitude and/or polarity of the field within an aperture through the 
screen so that the number of charged particles passing through an aperture 
bears a relation to the density of the image or picture at a corresponding 
location. Thus a dielectric medium placed behind the screen receives a 
charge pattern corresponding to that of the picture or pattern to be 
reproduced. 
Faithful and linear reproduction is achieved so long as there is a 
monotonic relationship between the charge pattern that can be impressed 
and stored on the photo-conductive layer and the charged particle pattern 
impressed and stored on the dielectric medium. The latter pattern is 
determined by the total amount of charged particles incident to the 
apertured screen during the duplication interval when the latent image on 
the screen is being duplicated on the dielectric medium and the control of 
charged particle flow during the duplication interval produced by the 
electric field created within the screen apertures. 
It has been found that as the physical size of the screen apertures is 
reduced in order to achieve finer resolution of reproduction, the range of 
control provided by the field within the aperture between the maximum and 
minimum value is narrower than the range of charge variations on the 
photoconductive layer and the screen becomes saturated at one or both 
extremes. Now a typical pattern or picture has completely white or 
transparent portions, completely black or opaque portions, and a 
continuous gradation of gray tones between the two extremes. All 
gradations of tone are not reproduced, however, when the range of charged 
particle control within the apertures is not co-extensive with the range 
of charges on the photoconductive layer. In such a case, if a range of 
operation is selected that accurately reproduces the white or transparent 
portions of the pattern or picture, the portions of the picture that range 
in tone from some intermediate portion of the gray scale to black will all 
be reproduced as black or opaque portions since further control is not 
possible beyond the upper limit of the charged particle control range. 
Similarly, if a range of operation is selected that accurately reproduces 
the black portions of the picture, those portions of the picture that 
range in tone from some intermediate portion of the gray scale to white 
will all be reproduced as white or transparent portions since further 
control is not possible below the lower limit of the charged particle 
control range. Corresponding, if a middle range of operation is selected, 
those portions of the picture or pattern that range in tone from white to 
light gray will reproduce as white and portions of the picture that range 
in tone dark gray to black will reproduce as black. 
Although it is theoretically possible to avoid the above-described 
saturation condition within the screen apertures by confining the 
photoconductive layer charge within a range equal to the limited charged 
particle control range provided by the screen, such a reduction of the 
charge range in the photoconductive layer renders the system more 
sensitive to electrical noise, which is manifested by graininess or 
mottling on the prints. Thus, in applications requiring a faithful 
reproduction of the original image over the entire gray scale it has 
heretofore been necessary to sacrifice the fineness of image resolution 
obtainable with smaller apertured screens in order to provide the desired 
gray scale screen response. 
It has further been found that even with an apertured screen possessing a 
charged particle control range sufficiently broad to encompass the range 
of tones of an original image to be reproduced, the reproduced image may 
still exhibit a range of tonal resolution which is inferior to the 
original. Thus, black portions of an original may develop as medium or 
light grey portions, while the intermediate tonal portions of the original 
image may exhibit a shift toward the white end of the tonal resolution 
scale. Thus, even though fineness of resolution has been sacrificed, the 
reproduced image still does not possess the desired tonal range. 
On the other hand, not all original images can be optimally reproduced with 
the above-noted electrostatic printing technique by providing a screen 
possessing a charged particle control range corresponding to a broad gray 
scale. For example, if a document to be reproduced has a faded text on a 
dirty background, faithful reproduction produces a copy having the same 
poor quality. To optimally reproduce an original of poor quality, the 
contrast ratio should be improved and the gray scale shifted so that the 
text appears darker on a lighter background. Moreover, this must be done 
in such a way that the reproduction of a good quality original having 
dark, well-defined text on a white background will not be adversely 
affected. Efforts in the past to reconcile the above-noted conflicting 
objectives have not met with wide success. 
SUMMARY OF THE INVENTION 
The invention comprises a method and apparatus for controlling the tonal 
resolution range of an electrostatic printer with an apertured 
multilayered screen in order to produce optimum copies of originals of 
varying quality. In another aspect of the invention an electrostatic 
printer having an apertured multi-image forming screen is operated in such 
a manner as to provide a reproduced image having a tonal range which 
corresponds to that of the original image. This is achieved by varying the 
integrated charged particle current in such a manner that the darkest 
portions of the reproduced original image are reproduced with the desired 
density. As defined herein the term "integrated charged particle current" 
is the total quantity of charged particles incident to the apertured 
screen during the duplication interval. In one embodiment of the invention 
the rate of charged particle emission from the source is varied to 
establish the desired integrated current; in another embodiment the 
duration of the duplication interval is varied to achieve the same result. 
In still another aspect of the invention, an electrostatic printer with an 
apertured multilayered screen is operated in such a manner as to limit the 
substantially linear screen response to a range intermediate the extreme 
limits of the gray scale and to provide saturated response outside this 
intermediate range. This is accomplished by impressing a latent 
electrostatic image on the screen, reducing the image potential to a 
predetermined maximum value corresponding to a predetermined upper density 
cutoff, and biasing the screen to provide a predetermined lower density 
cutoff so that image regions lying below the lower density limit are 
reproduced with minimum intensity and image regions corresponding to 
regions of the original image lying above the upper density limit are 
reproduced with maximum intensity. 
For a fuller understanding of the nature and advantages of the invention, 
reference should be had to the ensuing detailed description taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring more particularly to the drawings, reference numeral 12 indicates 
an electrostatic printing system that incorporates the present invention. 
The system includes an electrode plate 14 for supporting a suitable 
dielectric medium 16 on which an image is to be reproduced. Medium 16 is 
typically a piece of paper. A source of charged particles 18 is provided 
for supplying charged particles to be propelled toward medium 16, the 
propelling force being provided by a power source 20 which biases 
electrode plate 14 with respect to charged particle source 18 so that the 
particles are propelled toward the electrode plate. Interposed in the path 
of charged particles from source 18 to medium 16 is a multilayer apertured 
screen 22, such as is described in more detail in the aforecited patent. 
Screen 22 has a plurality of apertures therein in which apertures are 
formed electric fields that pass or block charged particles in a pattern 
that corresponds to a picture pattern to be reproduced on medium 16. 
As described in more detail in the above cited copending U.S. patent, a 
charge pattern or electrostatic latent image is formed on screen 22, 
typically at a location remote from medium 16 as indicated by broken lines 
in FIG. 1, by first bombarding the screen with charged particles such as 
air ions from a source 24 thereby establishing on the reverse face of the 
screen a uniform double-layer charge across the photoconductive layer. 
Thereafter the image of the picture or pattern to be reproduced is 
projected from a projector 26 on to the photoconductive layer so that the 
photoconductive layer is locally discharged to a degree proportional to 
the light intensity of the image. An electrostatic double-layer charge 
latent image of the pattern is thereby established on the screen. The 
screen is then moved back to the solid line position shown in FIG. 1 after 
which charged particles from source 18 are directed toward the screen so 
that a corresponding image is formed on medium 16. Since the completion of 
the reproduction on medium 16 is not part of the invention, it suffices 
for the present to say that toner particles of suitable color are applied 
to the charge pattern on medium 16 and adhere thereto in correspondence 
with the intensity of the image formed on the medium. Thereafter the toner 
particles are fixed or fused in accordance with known technology. 
In the present specification and claims "charged particles" is intended to 
encompass ions as well as charged particles of toner material that can be 
projected through screen 22 so as to dispense with a subsequent toner 
particle application step. 
Referring to FIG. 2, screen 22 is a multi-layer screen that includes an 
outer conductive layer 28 one surface of which defines the obverse face 29 
of the screen. Abutting layer 28 is an insulative layer 30 followed by 
another conductive layer 32 on which is disposed a photoconductive layer 
33. The exposed surface of layer 33 defines the reverse face 32 of the 
screen. Apertures 35 are formed in the screen and each of the previously 
enumerated layers bounds each of the apertures to permit establishment and 
control of fields within respective apertures. A conductor 36 is connected 
to layer 28 and a conductor 38 is connected to layer 32; conductors 36 and 
38 are in turn connected to a bias voltage supply source 40 which biases 
the electrodes with respect to one another and with respect to the field 
between charged particle source 18 and medium 16 such that a field 
indicated by field lines 42 is formed in apertures 35. The effect of the 
field indicated by lines 42 is to block particles as they approach the 
aperture so as to prohibit passage of the particles through screen 22. The 
voltage imposed on conductive layer 28 through conductor 36 is referred to 
hereinafter as V.sub.B. For controlling the magnitude of V.sub.B according 
to this invention a conventional bias control circuit 43 is provided and 
has connections to bias supply 40 and power source 20. 
When photoconductive layer 33 is initially charged from source 24 and is 
maintained in a dark or unexposed state, a substantial charge is 
established across the photoconductive layer 33, i.e., a voltage exists 
between conductive layer 32 and the reverse face 34 of the screen. Such 
voltage will be referred to hereinafter as V.sub.C. Voltage V.sub.C 
creates in apertures 35 a field indicated by field lines 44; the field 
represented by lines 44 is polarized in a direction opposite from that 
represented by field lines 42 so that the blocking field is counteracted 
by the field identified by lines 44 when photoconductor 33 is in the dark 
state. Accordingly, within each aperture 35 there is formed a field that 
passes and in fact enhances the flow of charged particles through screen 
22 so long as photoconductive layer 33 is maintained in the dark or 
unexposed state. 
When reverse face 34 of screen 22 is exposed to the image from image source 
26, conductive layer 33 is locally discharged in accordance with the 
intensity and distribution of the image so that each aperture 35, 
depending on its spatial position, passes or blocks particles in 
correspondence with the image. Thus, during the duplication interval, when 
the charged particles are propelled by power source 20 from particle 
source 18 to medium 16, the particles become arranged before impingement 
on the medium into a pattern corresponding to that of the image to be 
reproduced. 
The size and spacing of apertures 35 in screen 22, i.e., the relative 
fineness of the screen, determines the fineness of resolution of the image 
produced. If the apertures 35 are relatively small and closely spaced, a 
high degree of resolution of reproduction is achieved; if on the other 
hand, the apertures are relatively large, the degree of resolution is 
somewhat lower. An example of a screen that has sufficiently large 
apertures to afford control of charged particles without the necessity of 
employing the variable screen bias feature of the present invention is a 
screen formed of 140-line/inch woven wire mesh, which has 40% open area. 
An example of a screen the operation of which can be materially improved 
by employing the variable screen bias feature of the present invention is 
a screen formed of 300-line/inch woven wire mesh and having a 20% open 
area. 
A characteristic that is related to screen aperture size is the range of 
electric field variation within a given aperture. More specifically, for a 
relatively large aperture, a relatively broad range of voltage 
differentials V.sub.C -V.sub.B are required to produce fields within the 
aperture whose magnitudes range between complete charged particle blocking 
and maximum charged particle passage. A screen having apertures of this 
type thus possesses a relatively broad charged particle control range. In 
contrast, a relatively small aperture requires only a comparatively narrow 
range of voltage differentials V.sub.C -V.sub.B in order to produce fields 
whose magnitudes range between full charged particle blocking and maximum 
charged particle passage. A screen having apertures of this type thus 
possesses a relatively narrow charged particle control range. 
The response or transmission characteristic of a screen having relatively 
large diameter apertures, and thus a broad charged particle control range, 
such as that specifically referred to in the aforecited U.S. patent, is 
shown in FIG. 3. The ordinate of FIG. 3 represents the ratio expressed as 
a percentage of the amount of emergent charged particles that pass through 
the apertures in the screen to the amount of incident charged particles 
that approach observe face 29 of the screen from source 18. For example, a 
value of 30 indicates that 30% of the incident particles are transmitted 
through the screen. The abscissa represents the voltage differential 
V.sub.C -V.sub.B, wherein V.sub.C and V.sub.B are as defined above. It 
will be seen from curve 50, FIG. 3, that when the quantity V.sub.C 
-V.sub.B is -20 volts (assuming that the particles from source 18 are 
negatively charged), no charged particles pass through apertures so 
biased. As the value of V.sub.C -V.sub.B increases, the percent current 
transmission increases monotonically at a gradual rate which is nearly 
linear in the range - 20 V .ltoreq.V.sub.C -V.sub.B .ltoreq.120 V. 
Although not depicted in FIG. 3, beyond 120 volts the percent current 
transmission asymptotically approaches a maximum value of 60% for the 
particular screen whose characteristic is depicted in the Fig. 
FIG. 4 illustrates the characteristic of a typical photoconductive material 
employed for layer 33 on an apertured screen such as that having the 
characteristics of FIG. 3. In the graph of FIG. 4, the ordinate represents 
the voltage charge, V.sub.C, across photoconductive layer 33 after 
exposing the layer. The abscissa of the graph of FIG. 4 represents the 
light exposure to which the photoconductive layer is subjected and is 
calibrated as the logarithm of the exposure or number of stops. Relatively 
large values of V.sub.C correspond to relatively dark areas of the image 
and relatively small values of V.sub.C correspond to relatively light 
areas of the image. As is evident from the curve of FIG. 4, the monotonic 
response characteristic of the photo-conductive material provides 
substantial variation of photo-conductor charge density with increasing 
exposure between the lower limit of stop 2 and the upper limit of stop 8, 
corresponding to photo-conductor voltages in the range 140 V 
.gtoreq.V.sub.C .gtoreq.33 V. Accordingly, by operating the 
photo-conductive material in this voltage range, an expsoure range having 
the excellent tonal resolution range of a 6-stop system can be provided. 
Comparing the photo-conductive material characteristic of FIG. 4 with the 
screen characteristic of FIG. 3, it is seen that the desired 6-stop 
exposure range can be obtained by initially charging the photo-conductive 
layer of the screen to approximately 140 volts and providing a screen bias 
voltage V.sub.B of approximately 53 volts during the duplication interval 
following exposure. From the characteristic of the photo-conductive 
material, it is seen that the highest possible screen voltage, 
corresponding to black portions of the original image, is 140 volts, while 
the lowest possible screen voltage is approximately 33 volts corresponding 
to white portions of the original image. The screen characteristic 
illustrates that the selected bias voltage V.sub.B of approximately 53 
volts completely blocks transmission of charged particles through screen 
apertures in white portions of the image (33-53=-20 Volts) and enables 
transmission of approximately 34 percent of the incident charged particles 
through screen apertures in black portions of the image (140-53=97 Volts). 
In addition, the screen response in the range of voltages over which it is 
operated (-20 V .ltoreq. V.sub.C -V.sub.B .ltoreq.97 V) is nearly linear, 
so that the charged particle density variation on the copy medium 16 can 
be a virtual duplicate of the latent image on the screen. 
In order to obtain a visible copy having the tonal resolution of the 
original, however, it is necessary to adjust the integrated charged 
particle current incident to the screen. This is necessary in order to 
insure that the charged particle density on a portion of the copy 
corresponding to a black portion of the original is great enough to 
develop as a black portion. This may be effected in accordance with a 
first aspect of the invention in two ways: first, by varying the charged 
particle emission rate of source 18; second, by varying the duration of 
the duplication interval. 
The integrated charged particle current can be adjusted according to the 
first method by varying the power supplied by the power source 20 to the 
charged particle source 18. Means for adjusting the power source 20 are 
schematically indicated in FIG. 1 by bias control 43. The actual 
configuration of control 43 depends on the type of charged particle source 
utilized in a given electrostatic printer. If a high voltage wire corona 
source is employed as a source 18, control 43 may simply comprise a 
circuit for varying the magnitude of the high voltage applied to the 
corona source. If source 18 is charged toner particle source employing an 
air stream, control 43 may comprise both a voltage varying circuit for 
accelerating the toner particles into the stream and a means for varying 
the air flow rate. Other arrangements will occur to those skilled in the 
art. 
In operation, with a latent image on screen 27 voltage V.sub.B is set at a 
value just sufficient to block the flow of charged particles through white 
area apertures. Next, the power source is adjusted by means of control 43 
until the charged particle density on a portion of the copy medium 
corresponding to a black portion of the original is sufficient to develop 
as a black portion. When a charged toner particle source is employed, the 
charged particle density can be viewed directly. When a non-visible 
charged particle source is employed, the charged particle density can be 
rendered visible by applying visible toner particles to the copy. 
The integrated charged particle current can be adjusted according to the 
second method by varying the duration of the duplication interval while 
keeping the charged particle supply substantially constant. This method 
proceeds in a similar manner to that discussed above, viz. the voltage 
V.sub.B is set at a value just sufficient to block the flow of charged 
particles through white area apertures, after which the duplication 
interval is adjusted until the charged particle density on a black portion 
of the copy is sufficient to develop as a purely black portion. Adjustment 
of duplication interval may be achieved by installing a timing device for 
controlling the length of the duplication interval. 
In some applications, it has been found desirable to provide for adjustment 
of both the charged particle emission rate and the duplication interval so 
that full tonal range can be initially achieved with a minimum duplication 
interval duration. As the emissive efficiency of source 18 deteriorates 
with prolonged use the duplication interval can then be lengthened to 
compensate for the reduced density of the changed particles incident to 
screen 22. 
Finer mesh screens than those having a transmission characteristic similar 
to that shown in FIG. 3 are desirable for reproducing images with a higher 
degree of resolution and detail. Allusion has been made hereinabove to the 
fact that a screen with smaller apertures has a narrower range over which 
the electrostatic fields within the apertures can be controlled. FIG. 5 
depicts in graphic form a typical transmission characteristic of a screen 
substantially finer, i.e., having apertures substantially smaller, than 
the screen having the characteristic of FIG. 3. The ordinate and abscissa 
of FIG. 5 are the same as FIG. 3; from curve 43 in FIG. 5 it will be noted 
that the voltage range over which the apertures can be controlled to 
influence passage of charged particles through the apertures extends only 
from about -10 volts to about 60 volts, which corresponds to a voltage 
range between total blocking of charged particles and maximum passage of 
charged particles of 70 volts. Until now, utilization of a screen having a 
response characteristic as shown in FIG. 5 has been limited by certain 
disadvantages which include sensitivity to noise, inadequate gray scale 
response (particularly near the light or white portion of the image), and 
inadequate image density range. 
Sensitivity to noise occurs because a small change in the voltage quantity 
V.sub.C -V.sub.B produces a relatively large change in the percentage of 
charged particles which are transmitted through a given aperture. Such 
sensitivity to noise is manifested in the copy or reproduction by 
graininess or mottling which represent spurious signals that are not found 
in the original from which the copy of reproduction is made. 
Gray scale response of the screen, particularly near the white or highlight 
end of the curve, is poor because of the steepness of the curve near that 
extremity, the left-hand extremity as viewed in FIG. 5. Thus tones that 
are light gray in the original are reproduced as white or substantially 
white, whereupon the copy or reproduction has a chalky appearance in the 
light gray portions are reproduced as white. 
Inadequate image density range can be appreciated by referring to FIG. 4, 
in conjunction with FIG. 5 and noting that a 70 volt range across 
photoconductive layer 33 e.g. 37 volts to 107 volts accomodates an input 
image density range equivalent to only 3 stops, a range inadequate to 
reproduce the full tonal variations in the original. 
In a second aspect, the present invention overcomes the disadvantages and 
shortcomings inherent in fine mesh screens as enumerated above in the 
following manner. During the duplication interval when charged particles 
are permitted to selectively pass through the apertures in screen 22, the 
voltage V.sub.B on conductive layer 28 is changed so that the quantity 
V.sub.C -V.sub.B varies over a range larger than that shown in FIG. 5. 
Stated otherwise, by employing the present invention with a screen having 
a transmission characteristic as shown in FIG. 5, the effective response 
or transmission characteristic of the screen is improved to be 
substantially that of FIG. 3. According to the invention, photoconductive 
layer 33 is charged so that the range of V.sub.C thereacross encompasses 
an adequate image density range (e.g. six stops). Such range of V.sub.C 
substantially exceeds the response range of the screen. In one system 
designed according to the present invention, V.sub.C is charged to 160 
volts and is exposed until the highlight areas are discharged to 20 volts, 
thereby affording a range, between the maximum and minimum magnitude of 
V.sub.C, of 140 volts. Such range of variation is twice the range of a 
screen having the transmission characteristic of FIG. 5.V.sub.B is then 
established at a first level of 30 volts and power source 20 is activated 
for a first time interval to propel charged particles from source 18 to 
medium 16. Thus, apertures within the screen that are associated with 
areas of the photoconductive layer that are charged in the range of 20 
volts to 90 volts control charged particle passage according to the 
magnitude of the charge, V.sub.C, on the photoconductor. Apertures 
associated with the areas of the photoconductive layer that are charged in 
the range of 90 volts to 160 volts pass the maximum number of charged 
particles during the first time interval. This level of operation is 
permitted to persist for a period less than that required for tonal 
saturation of the black areas of the pattern. The voltage on conductive 
layer 36 is then switched to a second level of 100 volts and power source 
20 is activated for a second time to propel charged particles from source 
18 to medium 16. During this second time interval, apertures associated 
with portions of the photoconductive layer 33 that are charged at 
magnitudes ranging from 90 volts to 160 volts will pass charged particles 
in proportion to the specific magnitude within that range; however, 
apertures associated with portions of the photoconductive layer that are 
charged in the range of 20 to 90 volts will totally block passage of 
charged particles. Thus, during this second interval, the upper portion of 
the range of charges on the photoconductive layer will be effective to 
reproduce the corresponding portion of the pattern or image. 
The mode of operation described in the above example is shown graphically 
in FIG. 6. In FIG. 6, for simplicity, the response curve is depicted as 
linear rather than curved as in FIG. 5. The ordinate of FIG. 6 is 
percentage transmission which corresponds to the amount of charged 
particles passed through the apertures in the screen. The abscissa is 
calibrated in volts and represents the magnitude of V.sub.C over the 
reverse face of the screen. During the first interval of operation of the 
screen, the interval during which V.sub.B is set at 30 volts, the screen 
passes charged particles in linear relationship to the charge on 
photoconductive layer 33 on those portions of the photoconductive layer 
that are charged between 20 volts and 90 volts. Line segment 60 of the 
curve of FIG. 6 represents this range of operation. During operation of 
the screen at this interval, all apertures corresponding to locations in 
which V.sub.C is in the range of 90 to 160 volts pass uniform quantities 
of charged particles as indicated in line segment 62 in the Figure. During 
the second time interval, the interval during which V.sub.B is set at 100 
volts, apertures associated with areas in which V.sub.C is 20 to 90 volts 
are biased so as to completely block the passage of charged particles; 
line segment 64 represents this range. Apertures associated with the 
screen areas in which V.sub.C ranges from 90 to 160 volts pass charged 
particles in proportion to the particular value of V.sub.C within the 
range. Operation in this range is depicted by line segment 66 in FIG. 6. 
The overall response of the screen when operated in accordance with the 
invention is represented by the combination of line segment 60 and line 
segment 66. The latter segment is derived by adding line segment 62 to 
line segment 66. The overall response of the screen is seen to be 
substantially linear and is increased insofar as the range of linear 
operation is concerned. 
Another example will be helpful in appreciating the operation of this 
aspect of the present invention. In such example, a screen having a 
relatively narrow charged particle control range of 50 volts, cutoff 
voltage of -5 volts, and a saturation voltage of 45 volts is, by 
employment of the present invention, expanded to respond to an 85 volts 
range on the photoconductive layer. In operating a screen of the type 
characterized in FIG. 7, the photoconductive layer 33 is initially charged 
up to about 110 volts, after which the photoconductive layer is exposed to 
the image to be reproduced. Black or dark portions of the image will not 
discharge the photoconductor so that the voltage V.sub.C at such dark 
areas corresponds to 110 volts whereas light portions will discharge the 
photoconductive layer to voltage V.sub.C of about 25 volts. Because, as 
stated above, the screen has a cutoff voltage of -5 volts, V.sub.B is 
initially set at 30 volts so that the screen operates to permit passage of 
particles in proportion to the magnitude of V.sub.C in the range of 25 
volts to 75 volts. Operation in this range is depicted in FIG. 7 by line 
segments 70a and 70b. Apertures associated with areas of the 
photoconductive layer that are charged at 75 volts and above pass charged 
particles uniformly irrespective of the particular voltage within such 
range; operation in this part of the system is depicted by line segment 72 
on the graph. 
After a first interval of operation as described above, V.sub.B is switched 
to a second value of 65 volts so that areas of the screen at which the 
photoconductive layer is charged in the V.sub.C range of 60 volts to 110 
volts will pass charged particles in accordance with the value of V.sub.C 
in such range. Such range is identified in FIG. 7 by line segment 74. 
Apertures associated with areas of the screen at which the photoconductive 
layer is charged to a level below 60 volts will block passage of charged 
particles. This range is designated in FIG. 7 by line segment 76. 
The overall response is represented in FIG. 7 by line segment 70A, a second 
line segment, 78 and a third line segment 79. Line segments 78 and 79 are 
derived by adding to line segment 74 the respective magnitudes of line 
segments of 70B and 72. 
The examples described hereinabove with respect to FIG. 6 and 7 employ two 
discreet DC levels at which V.sub.B is set for intervals during the total 
period of projection of charged particles toward screen 22. Although such 
mode of operation has been found to provide excellent results in some 
applications, it is preferred to vary V.sub.B continuously for the period 
during which charged particles are projected toward the screen. FIG. 8 is 
a graph or curve 80 of the variation of V.sub.B, plotted on the ordinate, 
with time plotted on the abscissa. Time t.sub.1 represents the total 
period during which charged particles are projected toward the screen; it 
will be noted that V.sub.B continually and linearly increases during such 
period. The overall response of a screen biased in accordance with FIG. 8 
is shown by curve 82 in FIG. 9 in which the ordinate represents the 
percentage of charged particle transmission of the screen and the abscissa 
represents the relative density of the image in stops. It will be noted in 
curve 82 in FIG. 9 that both extremes, i.e., total blocking of charged 
particles and maximum passage of charged particles, is approached 
gradually so that gray tones near the extremes are accurately reproduced. 
Another mode of operation of a multilayer image forming screen in 
accordance with this aspect of the present invention is depicted in FIG. 
10 which plots V.sub.B over a time period t.sub.1 equivalent to that 
during which the charged particles are projected toward the screen. 
Operation according to FIG. 10 is substantially identical to that in 
accordance with FIG. 8 in that V.sub.B resides at any particular magnitude 
for the same total time. An advantage to using the biasing arrangement of 
FIG. 10 is that the duration of the total period during which charged 
particles are directed toward the screen is less critical, because even 
though one sawtooth wave of FIG. 10 may be cut off by inaccurate timing, 
insignificant influence on overall response of the screen occurs. 
The bias voltage V.sub.B may be varied by utilizing known voltage switching 
devices. Bias supply 40, e.g., may comprise a source of two voltages of 
the required magnitudes, and a two-position switch having a common output 
terminal may be coupled between supply 40 and lead 36 in a known manner to 
provide the two level bias voltages for the FIG. 6 and 7 embodiments. To 
provide the linearly swept, single cycle voltage of the FIG. 8 embodiment, 
and the linearly swept periodic voltage of the FIG. 10 embodiment, known 
mechanical or electrical voltage sweeping systems may be utilized. Such 
control devices, being well known, are not shown in detail herein to avoid 
prolixity, and are schematically depicted in FIG. 1 as bias control 43. 
When a relatively fine screen of the type discussed above in conjunction 
with FIGS. 5-10 is provided with a variable bias V.sub.B to expand the 
charged particle control range to equal that of a wide aperture screen, 
the tonal resolution of the visible copy may be improved by adjusting the 
integrated charged particle current in the manner discussed above in 
conjunction with FIGS. 3 and 4. Thus, by combining variable screen bias 
with adjustable integrated charged particle current, visible copies having 
improved tonal resolution and the inherent fineness of image resolution 
obtainable with relatively fine screens can be produced. 
The gray scale response of an electrostatic printer utilizing a 
multi-layered apertured screen may be further controlled using the 
technique of adjusting the bias voltage V.sub.B and the integrated charged 
particle current to provide a reproduction of an original in which a pair 
of selected original densities may be reproduced as a pair of selected 
desired densities which may be the same as or different from the original 
density values. The ensuing discussion is drawn to the preferred manner of 
achieving control of the gray scale response in accordance with this 
technique. The original image to be reproduced is first measured with a 
scanning densitometer 19 (FIG. 1), or a suitable equivalent instrument, to 
determine two parameters: firstly, that image density which is to be 
reproduced as white (which may or may not correspond to a white level in 
the original image); and secondly, a density lying intermediate the white 
and black levels to be reproduced as a particular density which may be the 
same as, or different from, the actual density in the original. After the 
value of these two parameters are determined, a plot of the FIG. 4 type is 
consulted to determine the corresponding photoconductor voltages V.sub.C. 
Next, the value of V.sub.B required to completely block transmission of 
charged particles through screen apertures having photoconductive voltages 
V.sub.C lying below the preselected white level cutoff is determined from 
a screen characteristic plot of the type shown in FIGS. 3 and 5. The same 
plot will also provide an indication of the value of the current 
transmission of the second density point previously measured. Lastly, the 
integrated charged particle current is adjusted so that the desired 
reproduction density will be obtained in those image areas corresponding 
to image areas in the original having the preselected density. As will be 
apparent to those skilled in the art, this preselected reproduction image 
density may be the same, greater or less than the density of the 
corresponding areas in the original. 
This process is illustrated in FIG. 11 which shows a plot of image density 
versus net effective screen control voltage for three different values of 
integrated charged particle current drawn to an arbitrary scale. Curve 90 
represents a normal plot of image density versus effective screen bias 
voltage in which a given photoconductor voltage V.sub.C corresponding to a 
region on the original having a density of 0.5 is reproduced with the same 
density. Curve 91 shows the effect of increasing the integrated charged 
particle current on the same density point. As shown in the FIG., the 
latent image region corresponding to the region on the original having a 
density of 0.5 is now reproduced with a density of 1.0. Similarly, curve 
92 illustrates the effect on the density point of reducing the integrated 
charged particle current. As is evident from the FIG., the density of the 
corresponding region on the reproduced image is lowered to 0.3. As will 
now be apparent, the effect of adjusting or altering the integrated 
charged particle current in this manner is to alter the characteristic of 
the curve of image density versus net effective screen control voltage. 
As will be apparent to those skilled in the art, the above described 
technique may be implemented in a number of equivalent ways. For example, 
the density measuring instrument may be omitted, if desired, and voltage 
V.sub.B and the integrated charged particle current may be empirically 
adjusted until the selected pair of density points in the original are 
reproduced with the desired densities in the reproduced image. 
In summary, the gray scale response of an electrostatic printer utilizing a 
multilayered apertured screen may be controlled to provide optimum full 
scale duplicate copies of an original image by varying the integrated 
charged particle current during the duplication interval and by adjusting 
the bias voltage V.sub.B in the above described manner. As will now be 
apparent, even a screen having relatively fine apertures and a 
correspondingly limited charged particle control range may be operated in 
such a manner as to provide a broadened gray scale response in excess of 
that heretofore obtainable with such screens. These techniques are 
particularly useful when an original image having a broad gray scale range 
is to be duplicated. In some applications, however, faithful reproduction 
of the original image leads to relatively undesirable copy. For example, 
as noted above, an original document which embodies faded text on a dirty 
background will be reproduced with equally poor quality if the gray scale 
response of the reproducing device is substantially linear or monotonic 
over a broad range. By modifying the above described techniques in the 
following manner, copies can be produced which possess enhanced quality 
over the original document. 
In accordance with this aspect of the invention, screen 22 of FIG. 2 is 
operated in a special saturation mode, hereinafter termed line copy mode, 
so that portions of the latent electrostatic image having a voltage lying 
below a preselected value V.sub.P completely block transmission of charged 
particles therethrough while portions of the image area having a voltage 
originally lying above a second preselected threshold V.sub.S transmit 
charged particles through the screen apertures at a uniform rate. Screen 
22 is first charged to the maximum photoconductor voltage V.sub.C by 
source 24 and then is subsequently exposed to the image to be reproduced. 
After the latent electrostatic image has been impressed upon screen 22, a 
bias voltage V.sub.S is applied to elements 28, 32 by bias control 43 and 
bias supply 40. Source 18 or source 24 is next energized to provide an ion 
current of opposite sign to that of the original ion current used to 
initially charge screen 22. Due to the presence of bias voltage V.sub.S, 
those portions of photoconductor 33 having a voltage lying above the value 
of V.sub.S are reduced to this upper limit. FIGS. 12 and 13 illustrate the 
manner in which the magnitude of the photoconductor voltage V.sub.C is 
altered when an original image of a variable density discrete bar gray 
scale is impressed onto screen 22 and screen 22 is operated with bias 
voltage V.sub.S. In both FIGS. the ordinate represents the magnitude of 
the photoconductor voltage while the abscissa represents the density of 
the original image. V.sub.D and V.sub.L represent the voltage to which the 
photoconductor layer 33 is discharged by exposure to the opposite end 
points of the scale. Thus, in FIG. 12, representing the state of the 
photoconductor 33 after exposure to the original image, the voltage range 
on the photoconductor 33 varies between V.sub.L and V.sub.D. In FIG. 13, 
representing the voltages on photoconductor 33 after operation of screen 
22 with bias voltage V.sub.S, photoconductor 33 exhibits voltages ranging 
from V.sub.L to V.sub.S. Those portions of photoconductor 33 formerly 
exhibiting voltages above V.sub.S have been discharged to this saturation 
level. After the photoconductor voltage V.sub.C has been so altered, the 
biasing voltage is adjusted to a lower density cutoff valve V.sub.P and 
the duplication interval is commmenced. 
With reference to FIG. 14, during the ensuing duplication interval, the 
passage of charged particles through apertures 35 within photoconductor 33 
regions having a voltage V.sub.C lying below the bias cutoff voltage 
V.sub.P is completely blocked. On the other hand, apertures 35 within 
regions of photoconductor 33 having a voltage V.sub.C corresponding to the 
saturation voltage V.sub.S transmit charged particles therethrough at a 
uniform saturation rate. Regions having a voltage V.sub.C lying in the 
range between V.sub.P and V.sub.S transfer charged particles therethrough 
at a varying rate depending upon the screen characteristic. It is noted 
that by choosing a screen 22 having a steep characteristic the transition 
range from cutoff to saturation can be made extremely narrow. The 
resulting copy will exhibit well defined dark regions corresponding to the 
textual material on a white background. The optimum values for V.sub.S and 
V.sub.P can best be determined on an empirical basis for any particular 
application. The integrated charged particle current may be adjusted in 
accordance with the above discussion in order to obtain high density 
regions for developing the reproduced textual material with the desired 
degree of blackness. 
When operating screen 22 in the line copy mode, it is noted that good 
original copies, i.e., copies having dark textual material on a white 
background, are duplicated with the same quality as poor original copies 
since all image areas having a photoconductor voltage V.sub.C greater than 
V.sub.S are duplicated with the same intensity. Thus, once adjusted for 
line copy mode, screen 22 may be used to produce copies of originals of 
varying quality without regard to the quality of the original. Further, it 
is understood that documents such as line or block charts, graphs, and the 
like can also be reproduced during operation in line copy mode with 
equally successful results. 
As will now be apparent, the above described invention enables the 
production of copies having optimum quality from originals of widely 
varying quality and nature. When operating in the gray scale mode, the 
invention provides duplicate copies having a fineness of tonal resolution 
superior to that hitherto obtainable and a uniformity or faithfulness of 
tonal reproduction likewise. When operating in the line copy mode, the 
invention enables the production of duplicate copies of superior quality 
to the original document. 
It is to be understood that the specific embodiments described hereinabove 
are by way of example only and that various biasing arrangements can be 
employed. For example, waves shaped different from linear or sawtooth 
waves can be employed during full gray scale operation without departing 
from the teachings of the invention. Moreover, in systems employing two or 
more levels of DC bias the bias ranges can overlap as in FIG. 7, the ends 
of the bias ranges can be coincident as FIG. 6, or there can be a gap 
between the two or more bias ranges. Further, it is understood that the 
examples of voltage magnitudes described are by way of illustration only. 
The specific quality of reproduction desired and the specific screen 
characteristics will dictate which particular biasing system is most 
desirable. 
For purposes of simplicity, the disclosure of the invention has been 
restricted up to this point to a system and method for producing a 
positive tonal reproduction of the original image. However, the same 
principles extend to the reproduction of a negative image from a positive 
original and a positive image from a negative original. For example, to 
produce a negative image reproduction of a positive original, the polarity 
of the latent image on photoconductive layer 33 must be the same as the 
polarity of the charged particles projected through the apertures in 
screen 22. Thus, either the polarity of both V.sub.B and of V.sub.C or the 
polarity of the charged particles from source 18 may be changed in the 
above described system to effect a positive to negative mode of 
reproduction. Negative to positive reproduction may be effected likewise. 
It is noted that when the mode of reproduction is inverse, the roles of 
the range limits are reversed. For example, in line copy mode with a 
positively charged latent image on photoconductive layer 33 and positive 
charged particles being supplied by source 18, the passage of charged 
particles through apertures 35 within photoconductor 33 regions having a 
voltage V.sub.C .gtoreq.voltage V.sub.S is completely blocked. 
Correspondingly, apertures 35 within regions of photoconductor 33 having a 
voltage V.sub.C .ltoreq.voltage V.sub.P transmit charged particles 
therethrough at a uniform saturation rate. Thus, in the inverse line copy 
mode, V.sub.S becomes the bias cutoff voltage while V.sub.P becomes the 
bias saturation voltage. The same analysis applies to the reproduction of 
a positive image from a negative latent image. 
Although several embodiments of the invention have been shown and 
described, it will be obvious that other adaptations and modifications can 
be made without departing from the true spirit and scope of the invention.