U.S. patent number 3,912,510 [Application Number 05/408,367] was granted by the patent office on 1975-10-14 for electrophotographic process employing a compound document screen.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Lawrence M. Marks.
United States Patent |
3,912,510 |
Marks |
October 14, 1975 |
Electrophotographic process employing a compound document
screen
Abstract
The present invention is directed toward an electrophotographic
imaging process and a method for extending the range capabilities
of said process. The process includes providing a compound document
screen adapted to be used at the exposure station proximate to the
image face of a document to be copied, such that light reflected
from the screened document is passed through a lens system and
imaged onto a photosensitive member. The document screen consists
of a clear transparent base member having a mixed dot pattern of
substantially light absorbing dots and substantially light
reflecting dots. The frequency of the like dots is such that the
lens system employed in the electrophotographic process passes the
fundamental spatial frequencies reflected from the screened
original but attenuates the harmonic spatial frequencies. The
developed image is found to consist of a plurality of halftone dots
of varying sizes, the dot sizes varying in accordance with the
screened output density reflected from the original document.
Inventors: |
Marks; Lawrence M. (Webster,
NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23615998 |
Appl.
No.: |
05/408,367 |
Filed: |
October 23, 1973 |
Current U.S.
Class: |
430/31; 430/6;
430/396 |
Current CPC
Class: |
G02B
27/46 (20130101); G03G 15/04027 (20130101) |
Current International
Class: |
G02B
27/46 (20060101); G03G 15/04 (20060101); G03G
013/04 () |
Field of
Search: |
;96/1R,1.2,45,116,1M,1PE,1PS |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Ikeda et al., "Tone Reproduction in Electrophotography," Deushi
Shashin (Electrophotography), 4, No. 2, pp. 5-12, (1962), (English
translation supplied). .
Crooks et al., "Obtaining Reproduction of Continuous Tone," IBM
Technical Disclosure Bulletin, Vol. 12, No. 2, July 1969, p. 262.
.
Anfilov, "The Nature of the Edge Effect in Electrophotography,"
Photographic Abstracts, Part 7, 1963, p. 319..
|
Primary Examiner: Torchin; Norman G.
Assistant Examiner: Miller; John R.
Claims
What is claimed is:
1. In an electrophotographic imaging process comprising the steps
wherein an original document is provided at an exposure station,
illuminated, and light reflected from said illuminated original
document is passed through a lens system and directed onto an
electrically photosensitive member, the improvement comprising
conducting said imaging process with a compound document screen
positioned proximate to the image face of said original document
between said document and said lens system, said compound document
screen comprising:
a clear transparent substrate material having clear areas and
bearing opaque areas;
said opaque areas comprising a repetitive pattern of substantially
opaque mixed dots comprising substantially light absorbing like
dots and substantially light reflecting like dots;
said like dots arranged with respect to other like dots at an
average like dot inch frequency such that the lens system employed
in the electrophotographic process passes the fundamental spatial
frequencies and attenuates the harmonic spatial frequencies.
2. The process of claim 1 wherein the substrate material comprises
a single sheet of clear transparent material having the
substantially light absorbing like dots affixed to one side of said
sheet and the substantially light reflecting like dots affixed to
the same or opposite side of said sheet.
3. The process of claim 1 wherein the substrate material comprises
two superimposed sheets of clear transparent material having the
substantially light absorbing like dots affixed to one of said
superimposed sheets and the substantially light reflecting like
dots affixed to the other of said sheets.
4. The process of claim 1 wherein said compound document screen is
positioned in contact with the image face of said original
document.
5. The process of claim 1 wherein each of said like dot patterns on
said substrate material is of substantially uniform frequency, like
dots being arrayed along generally rectilinearly directed lines
with respect to other like dots.
6. The process of claim 5 wherein said mixed dots on said substrate
material are arranged in a body centered pattern.
7. The process of claim 5 wherein the rectilinear arrays of
substantially light absorbing like dots are disposed at an angle
with respect to the rectilinear arrays of substantially light
reflecting like dots, said angle being appropriate to minimize
moire and provide optimum randomization of the mixed dot
pattern.
8. The process of claim 5 wherein the uniform like dot inch
frequency is within the range of about 50 to 400.
9. The process of claim 1 wherein said repetitive pattern of
substantially opaque mixed dots occupies from about 2% to about 65%
of the image area of the compound screen, said substantially light
absorbing like dots constituting from about 1% to about 64% of said
image area and said substantially light reflecting like dots
correspondingly constituting from about 64% to about 1% of said
image area.
10. The process of claim 9 wherein the like dot inch frequency is
within the range of about 70 to about 150.
Description
BACKGROUND OF THE INVENTION
The present invention relates to electrophotographic processes.
More specifically, the present invention relates to halftone
screening techniques for extending the range of relatively high
contrast electrophotographic processes such as xerography.
In xerography, a special xerographic photoreceptor comprising a
layer of photoconductive insulating material placed upon a
conductive backing is used to support xerographic images. The
photoreceptor may be formed in any shape. An image is formed by
uniformly electrostatically charging the photoreceptive surface and
then exposing it to a radiation pattern in the form of the image to
be reproduced. This radiation selectively discharges areas of the
photoreceptor forming an electrostatic charge pattern conforming to
the radiation image. This radiation image is generally derived from
an original document or other object which is illuminated and
imaged on the photoreceptor through a lens.
The latent image on the photoconductive layer is then developed by
contacting it with a finely divided electrostatically attractable
material such as a resinous colored powder called a toner. The
toner is held to the image areas by electrostatic charge fields on
the layer. The toner is held proportionately to the charge field so
that the greatest amount of material is deposited where the
greatest charge field is located. Where there is a minimum charge
there is little or no material deposited. Therefore, a toner image
is produced to conform with the latent image previously placed on
the photoreceptor. In reusable xerographic systems the toner is
transferred to a sheet of paper or other support surface and
suitably fixed thereto to form a permanent print. This fixing may
take place by heat or vapor which fuses the toner to the support
material to which it has been transferred.
The xerographic process produces excellent results for the
reproduction of line copy, e.g., printed characters such as letters
or numerals, but presents inherent difficulties where the copy to
be reproduced comprises large solid dark areas of high density or a
continuous tone image of varying density such as a photograph. At
this point, a clear distinction is to be made between the problem
of xerographic reproduction of dense solid areas of an original and
accurate xerographic reproduction of density gradients in the
highlight and shadow regions of continuous tone originals having
areas of varying densities.
The former is a development problem associated primarily with an
open cascade development system which problem has been largely
overcome by employing specific development techniques or by
altering the charge pattern present on large areas of contiguous
charge on the photoreceptor, as hereinafter discussed. The latter
is partially a development problem and partially a problem inherent
in a high contrast and moderate range process such as xerography
caused by the inability of a given photoreceptor to sense or
appreciate, and consequently reproduce, small density gradients in
the highlight and shadow areas of a continuous tone original such
as a photograph. It is the solution of this latter problem by
extending the range and improving the tone reproduction response of
the xerographic process toward which the present invention is
directed.
Various techniques have been proposed in the prior art to improve
solid area cascade development in the xerographic process. Briefly,
the problem of solid area development is due to electric field
conditions in the regions of large contiguous areas of charge
present on the photoreceptor. Xerographic development in these
areas delineates only their outline, developing only in the areas
where there is a differential in charge on the xerographic surface.
Consequently, the centers of these areas of uniform high charge,
being large solid areas of dark input, do not attract and hold
xerographic toner, and thus appear white or very lightly toned on
the transfer copy sheet.
Since the problem of solid area development is primarily associated
with open cascade development systems, one solution to the problem
has been the adoption of development techniques other than cascade
such as the well known magnetic brush, powder cloud, or liquid
development systems, or by the use of development electrodes as for
example disclosed in U.S. Pat. No. 2,777,418 to Gundlach or U.S.
Pat. No. 2,952,241 to Clark et al.
Another approach towards the solution of the problem of solid area
development has been to break up the continuous charge pattern on
the photoreceptor using mechanical, optical, or electrical
techniques. For example, Carlson suggests in U.S. Pat. No.
2,599,542 that improved solid area coverage is obtained using an
electrophotographic plate which has been etched to resemble a
waffle-grid design, the depressions on the surface of which plate
are filled with a photoconductive substance. Weigl in U.S. Pat. No.
3,248,216 teaches selective discharge of a charged electrostatic
plate by contacting the plate with a conductive element such as a
metallic gravure roller having a dot pattern provided by ridges or
projections, followed by exposure of the semidischarged plate to
the image. Optical techniques for improving solid area coverage by
breaking up the charge area on an electrophotographic plate involve
exposing the plate after charging and prior to or subsequent to
imaging to a screened light source. The screen may take the form of
a line or comb screen or a grid or dot pattern. The plate is
selectively discharged in those areas where the light passes
through the screen but retains its charge in those areas blocked by
the opaque areas in the screen. Examples of optical techniques for
improving solid area coverage may be found in U.S. Pat. Nos.
2,598,732, 3,121,010, 3,212,888, 3,335,003, and 3,535,036.
The use of screens consisting of alternating opaque and transparent
areas positioned between the object to be imaged and the
photoreceptor has also been suggested in the prior art as a means
for breaking up solid area images to allow uniform development. For
example, Pendry in U.S. Pat. No. 3,152,528 teaches a document
screen adapted to be superimposed over the document to be copied
between the document and the lens system of a xerographic copy
machine. The screen comprises a transparent base material having
printed thereon a plurality of opaque dots or lines which serve to
break up any dark or continuous tone areas present on the document
to be copied. Typical of such screens, which have been in
commercial use for the past several years, are those consisting of
a pattern of reflecting dots on a transparent substrate. These dots
cover about 30% of the area of the screen and are arranged in a
square array with a frequency of about 60-65 dots per inch.
Because of the improved solid area coverage in xerographic copies
achieved by the above techniques in shadow and middle tone areas of
an original such as a continuous tone photograph, the casual
observer is impressed that the process has been sensitized to the
point where it can "see" and consequently reproduce not only solid
areas but also density gradients in the middle tone areas of the
original. However, the use of such mechanical, electrical or
optical discharge techniques, or of reflecting document screens
wherein the opaque patterns of the screen appear faithfully
reproduced on the solid areas of output copy, does not serve to
extend the range of the process; that is, small density gradients
in the highlight and shadow areas of the original are not shown as
concomitant changes in density in the copy. Furthermore, the
density of the toned areas of the copy is necessarily less than the
maximum density achievable in the process because of the
intermittent areas of discharge of the xerograhpic plate evidenced
by small intermittent white areas in the copy.
The range of an electrophotographic system is usually defined in
terms of the input exposures over which changes in output density
can be observed. Range can be shown graphically using a tone
reproduction curve (TRC) wherein input density expressed in terms
of log.sub.10 (100/Ro) is plotted against output density expressed
in terms of log.sub.10 (100/Rc), where Ro is the percent
reflectivity of the original and Rc is the percent reflectivity of
the copy. Thus, where the reflectivity approaches 100% (white
areas), the density approaches 0 (log.sub.10 100/100=0); where the
reflectivity decreases, (black areas), the density increases. For
example, at 10% reflectivity, the density is 1; at 1% reflectivity,
the density is 2. A typical TRC of solid area xerography embodying
a selenium photoreceptor plotted over a plurality of input
densities is shown as the solid curve in FIG. 1. For the purposes
of the present invention, the range is defined as the density
differential on the abscissa axis between points where the slope of
the "S" shaped TRC is 0.5. The range of the system shown in FIG. 1
is about 0.6.
The TRC in FIG. 1 illustrates clearly why normal xerographic
systems have a limited capability in reproducing pictorial
originals. Opaque photographs typically have a density range in the
order of about 1.5 (D max - 1.6: D min = 0.1) and simply can not be
accurately reproduced by a system with a range of 0.6. Varying the
exposure above or below the point where the minimum output density
occurs for an input density of zero serves merely to shift the TRC
with no range extension and at the cost of sacrificing shadow or
highlight information. In fact, range extension can be achieved
only by "flattening" the TRC curve to approach as nearly as
possible the dotted straight line of FIG. 1 which represents the
optimum faithful reproduction of all densities.
Accordingly, it is an object of the present invention to provide a
simple and economical means for improving the range capabilities of
high contrast and moderate or low range electrophotographic
processes.
A more specific object is to extend the range of input densities
transmitted or reflected from an original document over which there
is a change of output density in a copy made using a high contrast
electrophotographic process such as xerography.
SUMMARY OF THE INVENTION
The foregoing and other objects of the invention are realized by
providing a half tone compound document screen to be used proximate
to an original document to be copied at the exposure station in an
electrophotographic process. The halftone screen is constructed of
a clear transparent substrate material having on at least one
surface thereof a plurality of substantially opaque dots of uniform
density, and is adapted to be positioned proximate to, preferably
in contact with, the face of the document to be copied between the
document face and lens system employed in the electrophotographic
system. The dots present on the screen comprise a mixed or compound
dot pattern of a plurality of substantially light-absorbing dots
and a plurality of substantially light-reflecting dots. The
frequency and array of these dots is such that light reflected by
the screened original is modulated by the lens in accordance with
the Modulation Transfer Function of the particular lens system
employed such that the lens passes the fundamental spatial
frequencies in the pattern and attenuates the harmonic spatial
frequencies in the pattern. Spatial modulation of a continuous tone
image on an original document by screening according to the present
invention gives rise to an area modulated pattern of halftone dots
in the copy. The copy image of a continuous tone black and white
original is found to consist of a plurality of black halftone dots
of varying sizes, the sizes of these dots varying in accordance
with the screened output density in various areas of the original.
Accordingly, minute changes in density in all areas of the original
document, including highlight and shadow areas, are accurately
recorded as minute changes in halftone dot size, thereby conveying
the impression of accurate electrophotographic reproduction of
density gradients and effectively extending the range of the
electrophotographic process, as well as providing for solid area
coverage.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a tone reproduction curve for a typical xerographic
system embodying a selenium photoreceptor.
FIG. 2 is a tone reproduction curve for a xerographic system
employing a selenium photoreceptor and embodying a document screen
according to the present invention.
FIG. 3 is an enlarged view of a small area of suitable compound
screen pattern of absorbing and reflecting opaque dots on a
transparent substrate arranged in a body centered pattern.
FIG. 4 is an enlarged view of a small area of a compound
transparent screen comprising a first screen containing absorbing
opaque dots superimposed over a second screen containing reflecting
dots arranged at a suitable angle to achieve randomization of the
dot pattern.
DETAILED DESCRIPTION OF THE INVENTION
As indicated above, the present invention involves specific
halftone screening techniques to extend the range of high contrast
electrophotographic processes. The invention is specifically
described as applied to the xerographic process but it should be
understood that it is equally applicable to any electrophotographic
process involving projection through a lens of an image reflected
from a colored or black and white original document onto a
photosensitive member, such as the photoelectrophoretic process
exemplified in U.S. Pat. No. 3,384,556, the manifold imaging
process exemplified in U.S. Pat. No. 3,707,368 and like
processes.
The halftone screen used in the present invention comprises clear
transparent support material having on at least one surface thereof
a mixed dot pattern of appropriate frequency comprising a plurality
of substantially opaque dots of uniform density, some of which dots
are substantially light-absorbing and others of which are
substantially light-reflecting. The term "dots" are used herein is
intended not only to emcompass dots in the classical sense such as
the circular shapes depicted in FIGS. 3 and 4, but also is intended
to encompass areas of uniform density forming other geometrical
shapes such as elipses, squares, triangles or polygons in general,
inasmuch as any of these shapes proves operable in the present
invention. The opacity of the dots should be sufficient to
optically block out from the photosensitive member white or denser
image information, or colored image information, contained on those
areas of an original over which the dots are superimposed. The
substantially light-absorbing dots, hereafter referred to as black
dots, should be of such a density as to absorb more light of all
wavelengths than is reflected. Conversely, the substantially
light-reflecting dots, hereafter referred to as white dots, should
be of such a density as to reflect more light of all wavelengths
than is absorbed. Best results, in terms of range extension, are
obtained where the black dots are at least 80% absorbing and the
white dots at least 80% reflecting, with optimum results achieved
as both values approach 100%. The base material supporting the dot
patterns may comprise any clear transparent material such as glass
or plastic. Clear films made from plastics, such as polyesters,
methacrylate polymers or vinyl halide polymers and having a
thickness of less than about 100 mils, are especially preferred
because such screens can be used with both flat and curved platen
electrophotographic machinery.
The frequency of the screen dot pattern is defined for the purposes
of the present invention in terms of the average period of like
dots present on a given linear or area measurement of screen
surface. By the term "like dots" is meant dots of similar
reflectivity or absorbancy, i.e., white dots or black dots.
Frequency is the reciprocal of the average period of like dots and
can be defined by the equation: f = 1/P, where P equals the average
distance between the geometrical center of one like dot and its
closest like dot neighbor of the total like dot population per
linear or area measurement of screen surface. Thus, a screen having
a like dot inch frequency of about 100, or the equivalent like dot
millimeter frequency of about 4, would be a screen where the
average distance between like dots present in 1 linear inch or
linear millimeter, or 1 square inch or square millimeter where the
dots are not in rectilinear array, would be about .01 inch or about
0.25 millimeter respectively.
As pointed out above, the frequency and array of the dot pattern
present on the screen is determined by the frequency response
function, specifically, the Modulation Transfer Function (MTF), of
the particular lens system employed in the electrophotographic
process. The relationship between spatial frequency and optical
response function is discussed, inter alia, in "Optics: A short
Course for Engineers and Scientists", Charles S. Williams and
Orville A. Becklund, John Wiley and Sons, N.Y., N.Y., 1972, at
pages 215 through 228. For a given lens system MTF, the frequency
of the dot pattern is too low if the dot pattern is accurately
imaged by a properly focused lens, for in this case the aerial
image of the dot pattern would be a square wave which according to
conventional Fourier analysis comprises sine waves at the
fundamental dot pattern frequency and many higher harmonics. Such a
square wave aerial image produces only a single dot size on the
photosensitive member rather than a variety of dot sizes for
different input densities. Conversely, the frequency of the dot
pattern is too high if the dot pattern is completely smeared by the
lens, since in this case resolution of the dot pattern would be
completely lost giving an unmodulated image and producing no dot
pattern whatever on the photosensitive member. Lens systems
commonly employed in most electrophotographic processes and in
commercially available xerographic equipment begin to exhibit the
desired modulation at a spatial like dot millimeter frequency of
about 2, or a like dot inch frequency of about 50, and modulation
may be lost completely at like dot millimeter frequencies ranging
anywhere from about 6 to about 16, or like dot inch frequencies of
approximately 150 to 400, depending on the quality of the lens.
Thus, for the purposes of the present invention, halftone compound
screens having a like dot inch frequency within the range of about
50 to 400 are generally suitable. Specifically, the MTF of lens
systems commonly used in the xerographic process or in xerographic
equipment is such that compound screens having a uniform like dot
inch frequency within the range of about 70 to 150 are sufficient
for appropriate image modulation such that the lens will pass the
fundamental spatial frequencies and attenuate the harmonic spatial
frequencies.
The fundamental and harmonic frequencies of the screen dot pattern
mentioned above refer to the frequencies of sine waves required to
synthesize the reflectivity patterns or like dots within the screen
according to conventional Fourier analysis. Within the scope of
this invention it should be appreciated that like dots may be
positioned in any regular array or may occupy random positions with
respect to other like dots. Examples of the regular array would be
square, triangular, or hexagonal lattices, with the fundamental
screen frequency defined by the basic periodicity of the array of
like dots. The frequency is given by f = 1/p where p is the average
distance between like dots per rectilinear measurement of screen
surface. In the random case, the fundamental frequency is
substantially that defined where p is the average distance between
one like dot and its closest like dot neighbor in the random array
per area of screen surface. Although the like dots may occupy
completely random positions in the random array, it has been found
to be advantageous for like dots not to overlap. It should also be
pointed out that it is not necessary that the frequency of the
white dot pattern be identical to the frequency of the black dot
pattern, nor is it necessary for the frequency to be uniform on all
areas of screen surface, so long as the frequency of each like dot
pattern is sufficient to achieve appropriate modulation within the
modulation or frequency parameters specified above.
One embodiment of dot array is the body centered regular pattern
shown in FIG. 3 which consists of a plurality of square arrays of
like dots surrounding a centrally positioned different dot. The
square array in FIG. 3 is depicted in the area encompassed by the
dotted line which shows four black dots in square array with a
white dot positioned at the intersection of black dot diagonals. Of
course, the array may be equally described at another area as four
white dots in a square array surrounding a centrally positioned
black dot. Assuming the like dot inch frequency of the black and
white dots of the compound screen of FIG. 3 to be 100, this means
for the purposes of the present invention that there is a
repetitive two dimensional pattern of 100 black dots along each of
two mutually perpendicular rectilinearly directed imaginary lines 1
inch long encompassing a common end dot and 100 white dots along
each of two mutually perpendicular different rectilinearly directed
imaginary lines also one inch long and encompassing a common end
dot. Thus, 1 square inch of compound screen surface with a body
centered like dot inch frequency of 100 would contain approximately
10,000 black dots and 10,000 white dots.
Although the body centered pattern of FIG. 3 is very desirable in
terms of dot pattern spatial array, it is often a tedious and
relatively expensive matter to prepare screens where the body
centered pattern can be accurately reproduced throughout a large
screen area, particularly at higher screen frequencies. Improper
registration of the body centered pattern at various areas of the
screen can give rise to an undesirable moire pattern which
adversely affects the modulation of the dot pattern. Accordingly, a
simpler realization of the compound screen is a random mixed dot
pattern which may be achieved by orientating a black dot and white
dot linear array at a suitable angle to achieve randomization and
minimize moire. This is best accomplished by orientating a regular
linear array of white dots at a suitable angle, such as about
30.degree. or about 60.degree., with respect to a regular linear
array of black dots. In this type of array, the relative spacing of
black and white dots is not uniform as in the body centered pattern
and, in fact, at various areas of screen surface some of the black
and white dots will overlap. An example of a dot pattern formed by
superimposing a linear black dot screen over a linear white dot
screen orientated at an angle of 30.degree. is shown in FIG. 4. As
in the case of compound screens having a body centered pattern, the
inch frequency of like dots in the orientated array should be
within the range of about 50 to 400 for best results.
The mixed dot pattern forming the compound screen serves to extend
the range of the electrophotographic process in both the highlight
and shadow areas of a continuous tone original document, with the
black dots modulating in the highlight areas of the original and
the white dots modulating in the shadow areas of the original.
Thus, the degree of range extension achieved in the highlight or
shadow areas is controlled within certain limits as a function of
the relative surface area of the compound screen containing black
dots and white dots respectively. For example, a half tone document
screen of regular array and appropriate frequency, e.g., 100 dots
per inch, consisting solely of black opaque dots covering about 30%
of the screen surface was evaluated in the xerographic process
using a black and white continuous tone photograph as the original
document. After adjusting exposure to compensate for additional
light absorption caused by the screen, it was found that range
extension in the copy has been achieved only in the highlight areas
of the original document, i.e., the low density end of the tone
reproduction curve. Similarly, a half tone document screen
consisting solely of white substantially opaque dots with a
frequency of 100 dots per inch and coverage of about 30% gave range
extension in the shadow areas of the original, i.e., the high
density end of the TRC. It is thus evident, that with the mixed
black and white dot patterns of the present invention, the dots of
each gray scale color operate independently to achieve range
extension at both ends of the TRC, thereby flattening the curve to
more nearly approximate the ideal TRC represented by the dotted
lines in FIGS. 1 and 2. FIG. 2 depicts such a flattened curve. Note
that the range has been extended to about 1.1 as opposed to the
range of about 0.6 shown in FIG. 1.
The relative proportion of the area of the compound screen covered
by black or white dots may vary as a factor of the type of
electrophotographic process in which the screen is to be used, the
nature of the particular continuous tone document to be copied, and
exposure limitations in the electrophotographic process. In
general, it has been found that desirable results in terms of range
extension in the xerographic process have been achieved using
compound screens having from about 2% up to about 65% opaque area
coverage, 1 to 64% of which opaque area coverage is provided by
either black or white dots. As the black dot area increases above
1%, additional exposure in the form of increased document
illumination or longer exposure time of the screened document is
necessary to compensate for the absorbance of the screen. As the
white dot area is increased above 1%, there is a corresponding
lowering of the maximum output density in solid or dense areas of
the copy. Thus, the composition of a screen to suit a particular
process, apparatus or category of document may require some trial
and error work within the parameters specified above on the part of
the technician to achieve optimum results in terms of range
extension.
For pictorial reproduction via the xerographic mode, screens having
about 40% total opaque dot coverage, composed of about 30% black
dots and 10% white dots have been found to be most satisfactory.
Use of such a document screen requires approximately double the
unscreened exposure to achieve accurate xerographic reproduction of
the original. Where such a screen is to be used as a document
screen with commercially available xerographic equipment, it may be
necessary in some cases to modidfy the equipment to increase the
exposure twofold either by providing additional exposure lamps, by
using exposure lamps of higher lumen values, by slowing down the
equipment to provide a longer exposure time of the document to the
photosensitive member, or by combinations of these.
The halftone screen is designed for use proximate the original
document at the exposure station in an electrophotographic process.
By the term "proximate" is meant that the screen is used positioned
either in direct contact with the image face of the original
document or at a distance away from the image face within the focal
capabilities of the lens, usually not greater than about 1/4
inch.
The compound screens of the present invention may be fabricated by
printing, etching, dye transfer, photographic processes or other
well-known techniques which are employed to prepare analogous
screens used in the graphic arts. The simplest and most effective
procedure is to print directly onto the clear transparent base
member by offset printing techniques using opaque black or white
inks or pigments to provide the desired black and white dot
patterns. The total percentage of opaque area coverage at a given
frequency for a given area of screen may be established by
controlling the size of the dots printed on the screens, i.e. the
larger the fixed frequency dot size, the greater the area of dot
coverage. The relative proportion of black and white dot area
coverage can be controlled in the same manner. For example, to
print a compound screen having a like dot inch frequency of about
100, or a like dot millimeter frequency of about 4, with a total
opaque dot area coverage of 40% consisting of 20% black dots and
20% white dots, simple calculations indicate that each of the
approximately 16 black and 16 white dots per square millimeter
should be printed to occupy an area of about 0.0125 square
millimeters per dot. To print a similar screen where the black dots
account for about 30% screen opacity and the white dots account for
about 10% screen opacity, each of the 16 black dots should be
printed to occupy an area of about 0.019 square millimeters and
each of the 16 white dots should be printed to occupy an area of
about 0.006 square millimeters.
Compound screens having the body centered dot pattern similar to
that shown in FIG. 3 may be printed on a clear transparent
substrate by first applying dots of ink of one color to one side of
the substrate, and subsequently of the substrate and subsequently
printing dots of the color on the same ink of the other color in
proper spatial array to the same or opposite side of the substrate.
Alternatively, the body centered compound screen pattern may be
provided by two separate sheets or layers of substrate with white
dots printed on one sheet and black dots printed on the other sheet
such that when the two sheets are superimposed and fixed in place,
the body centered pattern of FIG. 3 is evident. The orientated
compound screen pattern of FIG. 4 may be printed in a similar
fashion by first printing dots of one color on one side of the
substrate or opposite side of the substrate, care being taken to
insure that the latter dots are printed orientated at suitable
linear angles to minimize moire, e.g., angles of 30.degree. or
60.degree., with respect to the former dots. With this technique,
no specific care need be taken with regard to the relative spatial
array between black and white dots. Alternatively, the black and
white dots may be printed on separate sheets, and a compound screen
formed by superimposing and orientating these sheets at appropriate
linear dot angles, e.g., 30.degree. or 60.degree.. The laminated
sheets may then be fixed in place such that relative movement of
the sheets is prevented, followed by trimming to the desired screen
dimensions.
As previously indicated, the compound half tone screen of the
present invention is suitable for use in any electrophotographic
imaging process, both color and black and white, and designed to be
positioned proximate to, preferably adjacent and in substantial
contact with, the image face of the original to be copied, and
between the original and lens system employed in the
electrophotographic process. The compound screens are particularly
adapted for the xerographic process as half tone document screens
used in contact with the image face of an opaque, colored or black
and white original document such as a continuous tone photograph.
Light illuminating the original passes through the transparent
areas of the screen and is selectively reflected or absorbed by the
opaque dot areas of the screen. The pattern of light reflected by
the screened original is passed through a lens system and focused
on a charged photoconductive plate. This spatial modulation of a
continuous tone image on an original document gives rise, after
xerographic development of the latent image formed on the plate, to
an area modulated pattern of half tone dots in the copy, said dots
varying in size as a function of the screened output density in
various areas of the original. In a black and white process, these
dots are black; in a color process, these dots would be of
appropriate color.
The dimensions of the compound screen should be sufficient to cover
either the entire image area of the document or selective pictorial
areas of the document. Thus, an 81/2 inch .times. 11 inch opaque
original photograph requires an 81/2 inch by 11 inch compound
screen. Other originals containing both pictorial and line copy
require screens of dimensions sufficient to cover the pictorial
copy only. When used with commercial xerographic equipment, the
compound screen is simply positioned at the platen or exposure
station and the original document placed over it. If desired, the
glass platen of a xerographic apparatus may itself constitute the
screen, having the appropriate dot pattern directly affixed
thereto.
While the invention has been described with reference to the
structure disclosed herein, it is not confined to the specific
embodiment set forth, and this application is intended to cover
such operative modifications or changes as may come within the
scope of the following claims.
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