Note: Descriptions are shown in the official language in which they were submitted.
1095S'~
BACKGROUND OF THE INVENTION
This invention relates generally to an electrostato-
graphic printing machine, and more particularly concerns a printing
machine adapted to operate either in a pictorial or functional
copying mode.
In a typical electrostatographic printing machine, a
latent image is recorded on a surface and developed with charged
~particles. A sheet of support material is positioned closely ad-
jacent to the latent image and arranged to have the particles
io transferred thereto. After the particles are transferred to the
sheet of support material, they are permanently affixed thereto
forming a copy of the original document. Electrophotographic
printing and electrographic printing are different versions of
electrostatographic printing. In the process of electrophoto-
graphic printing, a photoconductive member is charged to a sub-
stantially uniform level. The light image of an original document
irradiates the charged photoconductive member dissipating the
charge in accordance with the intensity thereof. This records an
electrostatic latent image on the photoconductive member corres-
ponding to the original document being reproduced. Electrographic
printing differs from electrophotographic printing in that neither
a photoconductive member nor a light image is required to create
a latent image of the original document. Generally, both of the
foregoing processes employ heat settable particles to develop the
latent image. Heat is applied to these particles permanently
affixing them to the sheet of support material.
A multi-color e]ectrophotographic printing machine is
essentially the same as a black and white printing machine. How-
ever, in multi-color printing, the foregoing process is repeated
a plurality of cycles, each cycle being for a discrete color. In
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this process, the light image is filtered to record an electro-
static latent image on the photoconductive member corresponding
to a single color. A plurality of different single color light
images are fQrmed. Each single color electrostatic latent image
is developed with toner particles complementary in color to the
color of the filtered light image. The toner powder images are
then transferred tc the sheet of support material in superimposed
~registration with one another. The multi-layered toner powder
image is then permanently affixed by the application of heat there-
to to the sheet of support material producing a permanently colored
copy of the original document.
Hereinbefore, an electrophotographic printing machine
was merely adapted to produce either a functional copy or a pic-
torial copy. A functional copy is a copy of a document wherein
subtle variations of tone or color are not present, such as in a
graph, chart, lines, etc. Functional copying machines have great
difficulty in forming tone gradations. Contrawise, a pictorial
copying machine employs a hal~-tone screen to overcome this defect.
The screen produces tonal gradations by forming half-tone dots or
; 20 lines of varyin~ size~ In the highlight regions, the half-tone
; pattern may comprise narrow lines or small dots. The lines increase
in width or the dots in size throughout the intermediate shades
until they merge together at the shadow end. In this manner, there
will be complete whiteness at the highlight end and nearly solid
black at the shadow end of the tone scale.
Numerous patents teach the concept of screening. Exem-
plary of these patents are U. S. Patent No~. 2,598,732; 3~535,036;
3,121,010; 3,493,381; 3,776,633; and 3,809,555. A recent Japanese
application shows the utilization of an auxiliary light source in
conjunction with a screen in the optical light path. This is
.
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Japanese application No. 47-124,202 dated December 11, 1972
having a laid open No. 49-82345, the inventor being Masayasu
Anzai and the applicant being Hitachi, Ltd. Other U.S.
patents relevant thereto are Nos. 3,936,173 and 3,981,577.
However, none of the prior art references appear to
disclose an electrophotographic printing machine having the
capability of operating in either the functional or pictorial
copying mod~. Nor do the prior art references appear to
teach electrophotographic printing machines having the
ability to vary the reproduction contrast of copies by
adjusting the half-tone screen. Thus, no printing machine
hereinbefore developed had the capability of de-energizing
the screen selectively so as to change from the pictorial
mode to the functional mode.
In accordance with one aspect of this invention
there is provided an electrophotographic printing machine of
the type having an arcuate photoconductive member, including:
a screen member mounted movably in the printing machine
closely spaced to the photoconductive member, said screen
member comprises an arcuate member having a curvature equal
to the curvature of said arcuate photoconductive member
with the centers of curvature being in coincidence with one
another and the radius of curvature of said screen member
being greater than the radius of curvature of said photo-
conductive member; means for adjusting the spacing between
the photoconductive member and said screen member; and means
for translating said screen member relative to said photo-
conductive member while main~aining the spacing between said
screen member and photoconductive member substantially constant.
BRIEF DESCRIPTION OF THE DRAWINGS
. . _ . . .
Other advantages of the present invention will
become apparent upon reading the following detailed descrip-
tion and upon reference to the drawings, in which~
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1~95~'77
Figure 1 is a schematic, perspective view of an electro-
photographic printing machine incorporating the features of the
present invention therein;
Figure 2 is an elevational view of one embodiment of the
exposure system employed in the Figure 1 electrophotographic print-
ing machine;
- Figure 3 is an elevational view of another embodiment of
the exposure system employed in the Figure 1 printing machine; and
Figure 4 is a schematic perspective view depicting the
relationship of the screen member and photoconductive member em-
ployed in the Figure 1 printing machine.
While the present invention will be described in connec-
tion with the preferred embodiments thereof, it will be understood
that it is not intended to limit the invention to that embodiment.
On the contrary, it is intended to cover all alternatives, modifi-
cations and e~uivalents as may be included within the spirit and
scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
A general understanding of an electrophotographic print-
; 20 ing machine having the features of the present invention incorpo-
rated therein may be had by referring to Figure 1. In all of the
drawings, like reference numerals have been used throughout to
designate like elements. The electrophotographic printing machine
shown in Figure 1 is arranged to produce functional or pictorial
;~ 25 copies from a colored original document. The original document
may be in the form of single sheets, books, three-dimensional
objects, or colored slides. Preferably~ the type of copy that is
desired would depend upon the original document being reproduced.
For example, bar charts, graphs, etc. would be reproduced as func-
tional copies whereas photographs would be reproduced in the pic-
torial copy mode.
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As shown in Figure l, the electrophotographic printing
machine includes a photoconductive member having a rotatable drum
10 with a photoconductive surface 12 entrained thereabout and
secured thereto. Drum 10 is mounted on a shaft (not shown) and
rotates in the direction of arrow 14. This moves photoconductive
sur~ace 12 sequentially through a series of processing stations.
Preferably, photoconductive surface 12 is made from a suitable
selenium alloy such as is described in U.S. Patent No. 3,655,377
issued to Sechak in 1972. A timing disc (not shown) is mounted on
one end o~ the shaft of drum 10. This timing disc cooperates with
a light source and photosensor to produce an electrical signal
which is coupled to the machine logic. In this way, as drum lO
rotates, the appropriate processing station is activated.
For purposes of the present disclosure, the various pro-
cessing stations in the printing machine will be briefly described
hereinafter.
As drum 10 rotates in the direction of arrow 14, it passes
through charging station A. Charging station A includes a corona
generating device, indicated generally by the reference numeral 16.
Corona generating device ]6 charges photoconductive surface 12 to
a reLatively high substantially uniform level. Preferably, corona
generating device 16 extends in a generally transverse direction
across photoconductive surface 12 to produce a spray of ions for
the charging thereof. One type of suitable corona generating device
is described in U.S. Patent No. 2,778,946 issued to Mayo in 1957.
Thereafter, drum lO rotates the charged portion of photo~
conductive surface 12 to exposure station B. At exposure station B,
the charged area of photoconductive surface 12 is exposed to a color
filtered light image of the original document. A moving lens system,
generally designated by the reference numeral 18, and a color filter
1~9~;~5'~
mechanism, shown generally at 20, are positioned at exposure sta-
tion B. U.S. Patent No. 3,062,108 issued to Mayo in 1952 describes
a moving lens system suitable for use in electrophotographic print-
ing. A color filter mechanism suitable for use in the Figure 1
electrophotographic printing machine is described in U.S. Patent
No. 3,775,006 issued to Hartman et al. in 1973. Original document
22 is disposed upon transparent vie~ing platen 24. Lamp assembly
-26 is positioned beneath transparent viewing platen 24, and, in
conjunction with lens system 1~ and filter 20, moves in a timed
io relationship with drum 10 to scan successive incremental areas of
original document 22. Preferably, lens 18 is a six element split
dagor type of lens having front and back compo~nd components with
a diaphragm located centrally therebetween. The front lens com-
ponent has three lens elements including, in the following order;
a first lens element of positive power, a second lens element of
negative power cemented to the first lens element, and a third lens
element of positive power disposed between the second lens element
and diaphragm. Preferably, the first lens element has a double
convex lens as the front component, a concave lens as the second
component and a convex-concave lens as the third component. Lens
18 has a preferred speed ranging from about F/4.5 to about F/8Ø
A suitable type of lens is described in U.S. Patent No. 3,592,531
issued to McCrobie in 1971. In this manner, a flowing light image
of original document 22 is produced. This is a single color light
image. The single color flowing light image is transmitted through
screen member 28. In one embodiment, screen member 28 is inter-
posed into the optical light path. In an alternate embodiment,
screen member 28 is located out of the optical light path. Both of
these embodiments will be discussed hereinafter in greater detail
with reference to Fiyures 2 and 3. Screen member 28 is an arcuate
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5~'77
sheet having a plurality of opaque Lines the~eon. One skilled in
the art will appreciate that while a curved screen member will be
described, a flat screen may be employed in lieu thereof. The
detailed structural con~iguration of screen member 28 will be dis-
cussed hereinafter with reference to Figure 4. In operation, the
flowing light image passing through screen member 28 is modulated
so as to form a modulated single color light image which irradiates
~the charged portions of photoconductive surface 12. As hereinbefore
noted, filter mechanism 20 interposes selected color filters into
the optical light path. Successive color filters operate on the
; light rays passing through lens 18 to create a modulated single
color light image which records a modulated sirlgle colored electro-
static latent image on photoconductîve surface 12. The modulated
single color latent image is that which is created by discharging
the photoconductive member with a single color light image. Screen
member 28 is operational only in the pictorial mode~ Contrawise,
if the printing machine is in the functional mode, screen member 28
is no longer operational. More particularly, when screen member 28
is in the optical path and the machine is switched from the pic-
torial mode to the functional mode, screen member 28 is removed
from the optical path or spaced a sufficient distance from photo-
conductive surface 12 so as to be de-focused and ineffective.
After the electrostatic latent image is recorded on
photoconductive surface 12, drum 10 rotates to development station
C. Three developer units, generally indicated by the reference
numerals 30, 32 and 34, are positioned at development station C.
A suitable development station employing a plurality of developer
units (in this case three) is described in U.S.Patent No~ 3,854,449
issued to Davidson in 1974. The developer units described therein
are magnetic brush developer units. A typical magnetic brush
55~77'
development unit employs a magnetizable developer mix of carrier
granules and toner particles. The developer unit forms a direc-
tional flux field to continually create a brush of developer mix.
This developer mix brush is brought into contact with the modulated
single color electrostatic latent image recorded on photoconductive
surface 12. The toner particles adhering electrostatically to the
carrier granules of the developer mix are attracted by the greater
-electrostatic force to the latent image and render it visible.
Developer units 30, 32 and 34, respectively, contain dis-
cretely colored toner particles. Each of the toner particles con-
tained in the respective developer units correspond to the comple-
ment of the single color light images transmitted through three
different color filters 20. For example, a modulated electrostatic
latent image formed from a green filtered light image is rendered
visible by depositing green absorbing magenta toner particles there-
on using one of the developer units. Similarly, electrostatic
latent images formed from blue and red light images are developed
with yellow and cyan toner particles using the other developer
units.
After the modulated electrostatic latent image recorded
on photoconductive surface 12 is developed, drum 10 rotates to
transfer station D. At transfer station D, the toner powder image
adhering electrostatically to photoconductive surface 12 is trans-
ferred to a copy sheet or a sheet of support material 36. A biased
transfer roll, shown generally at 38, recirculates support material
36. Transfer roll 38 is electrically biased to a potential of
sufficient magnitude and polarity to electrostatically attract toner
particles from photoconductive surface 12 thereto~ In this manner,
transfer roll 38 rotates in the direction of arrow ~0, at substan-
tiall~ the same tangential velocity as drum 10. Thus, as transfer
_ g _
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,
1~5S'7t~
roll 38 rotates in synchronism with drum 10, successive toner pow-
der images may be transferred from photoconductive surface 12 to
sheet 38. A suitable electrically biased transfer roll is described
in U.S. Patent No. 3,612,677 issued to Langdon et al. in 1971.
Briefly describing the sheet feeding pathJ support
material 36 is advanced from a stack 42 thereof disposed upon tray
44. Feed roll 46, in operative communication with retard roll 48,
~separates and advances the uppermost sheet from stack 42. The
advancing sheet moves into chute 50 and is directed thereby into
io the nip of register rolls 52. Register rolls 52 align and forward
the advancing sheet, in synchronism with the movement of transfer
roll 38. Transfer roll 38 has gripper fingers 54 mounted therein.
Gripper fingers 54 receive the advancing sheet 36 and secure it
releasably to transfer roll 38. After the requisite number of toner
lS powder images have been transferred to sheet 36, in superimposed
registration with one another, sheet 36 is removed from transfer
roll 38. The foregoing is achieved by having gripper fingers 54
space sheet 36 from transfer roll 38 as it rotates in the direction
of arrow 40. This permits stripper bar 56 to be interposed there-
between separating sheet 36 from transfer roll 38. Sheet 36 passes
over stripper bar 56 onto conveyor 58. Endless belt conveyor 58
moves support material 36 to fixing station E.
At fixing station E, a fuser, indicated generally by the
reference numeral 60, permanently affixes the transferred toner
powder images to support material 36. One type of suitable fuser
is described in U.S. Patent No. 3,826,892 issued to Draugelis in
1974. After the fixing process, sheet 36 is advanced by endless
belt conveyors 62 and 64 to catch tray 66 for subsequent removal
therefrom by the machine operator.
Invariably, following the transfer process, residual toner
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955';'~7
particles adhere to photoconductive surface 12. Cleaning station
F, the final processing station in the direction of rotation of
' drum 10, as indicated by arrow 14, removes these residual toner
particles. A pre-clean corona generating device ~not shown) neu-
tralizes the charge on photoconductive surface 12 and that of the
residual toner particles. This enables fibrous brush 68, in con-
tact with photoconductive surface 12, to remove the residual toner
- particles therefrom. A suitable brush cleaning system is described
, in U.S. Patent No. 3,590,412 issued to Gerbasi in 1971.
io It is bel ieved that the foregoing description is sufficient
for purposes of the present application to describe the features
of the electrophotographic printing machine in which the present
invention is incorporated.
Referring now to Figure 2, there is shown one embodiment
of exposure station B. As shown thereat, lamps 26 moves across
platen 24 with original document 22 disposed face down thereon.
The light rays reflected from original document 22 pass through
transparent platen 24 onto mirror 70. Mirror 70 reflects the light
rays through lens 18 which forms a flowing light image thereof.
The flowing light image is then transmitted through the appropriate
filter of filter mechanism 20 so as to produce a single color flow-
ing light image. This single color flowing light is reflected by
mirror 72 through screen member 28 forming a modulated single color
flowing light image. The modulated single color light image there-
upon irradiates the charged portion of photoconductive surface 12.
` This selectively discharges photoconductive surface 12 to record
thereon a modulated single color electrostatic latent image. Light
source or lamp 82 provides additional non-image illumination. The
- light rays from lamp 82 pass through screen member 28 in superim-
posed registration with the light image. This reduces the illumi-
nation power level rec~uired from lamps 26 and flattens contrast.
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1~95St;t7
The foregoing is the manner of operation when the print-
ing machine is in the pictorial mode. In this mode of operation,
contrast may be adjusted by moving the screen member 28 vertically
so as to adjust the spacing between photoconductive surface 12 and
screen member 28. Preferably, screen member 28 is an arcuate mem~
ber having a curvature equal to the curvature of drum 10. In
addition, the centers of curvature of screen member 28 and drum 10
-are coincident with one another. The radius of curvature of screen
member 28 is greater than the radius of curvature of drum 10, the
difference defining the spacing therebetween. The spacing between
photoconductive surface 12 and screen member 28 may be adjusted.
This regulates the contrast of the resultant copy. Thus, in the
pictorial mode of operation, contrast may be adjusted by regulating
the spacing between screen member 28 and photoconductive surface 12.
This may be achieved by slidably mounting screen member 28 on rails.
A rack and pinion assembly may be employed to move screen member 28
along the foregoing rails. This is achieved by energizing drive
system 74, which, in this mode of operation, rotates the pinion,
thereby translating the rack and moving screen member 28 along the
rails. This adjusts the spacing between screen member 28 and photo-
conductive surface 12~
In the functional mode of operation, drive system 74
moves screen member 28 out of the optical path so that the light
image is not modulated. Alternatively, drive system 74 may move
2S screen member 28 to a position spaced an optimum distance from
photoconductive surface 12 for high contrast copying. Thus, in the
functional mode, an unmodulated single color light image irradiates
charged photoconductive surface 12 producing a single color elec-
; trostatic latent image~ Successive single color electrostatic
latent images are recorded on photoconductive surface 12 and
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:.
developed in the manner heretofore described. The resultant toner
powder images are transferred to the sheet of support material
mounted on the transfer roll. These toner powder images are then
permanently affixed to the sheet of support material creating a
functional copy of the original document. It is, therefore, apparent
that the major distinction between the functional copying mode and
the pictorial copying mode is the location of the screen. In the
pictorial copy mode the screen member modulates the light image pro-
ducing a pictorial copy. Contrawise, in the functional copying
mode the screen member is removed from the optical light path or
spaced an optimum distance from photoconductive surface 12 to pro-
duce high contrast copies, and the light image is unmodulated re-
sulting in a functional copy. Mode selection is an operator func-
tion. By this, it is meant that the operator by depressing a button
marked functional or pictorial, selects the mode of operation. If
the operator selects the functional mode of operation drive system
74 is energized to translate screen member 28 away from the optical
; light path, or, in lieu thereof, to an optimum distance from photo-
conductive surface 12. The translation of screen member 28 is
achieved by a solenoid having an arm secured thereto. Energization
of the solenoid removes screen member 28 from the optical light
path. Contrawise, depression of the pictorial mode button de-ener-
gizes the solenoid of the drive system and moves screen member 28
into the optical ]ight path. In addition to the foregoing controls,
a contrast control is mounted in the printing machine. The con-
trast control permits the operator to actuate drive system 74 so as
to regulate the spacing between screen member 2~ and photoconductive
surface 12. This, in turn, ad~usts the contrast of the pictorial
copy reproduced thereby, or moves the screen to an optimum position
for functional copying.
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1Cil95S'7~
Turning now to Figure 3, there is shown still another
embodiment of the printing machine. As shown in Figure 3, a screen
member 28 is positioned at exposure station B prior to the path of
the light image. It should be noted that in this mode of operation,
the screen member may be located either prior ~o or subsequent to
the light image optical path. Once again, lamps 26 move across
platen 24 scanning original document 22. The light rays reflected
-from original document 22 are, in turn, reflected by mirror 70
through lens 18 forming a flowing light image. The flowing light
image passes through the corresponding filter of filter mechanism 20
producing a single color flowing light image. This single color
flowing light image is reflected in a downwardly direction by mirror
72 onto photoconductive surface 12. The flowing light image irra-
diates the portion of photoconductive surface 12 having the screen
pattern thereon. In the event that the screen is located after the
formation of the flowing light image, the screen light pattern will
be'projected into superimposed registration with the latent image of
the original document recorded on photoconductive surface 12. The
screen pattern is formed by screen member 28 having light rays from
light source or lamps 78 passing therethrough and irradiates photo-
conductive surface 12 prior to or subsequent to the formation of the
original document and latent image. The intensity of illumination
emitted from lamp 78 is adjustable by varying the power level of
the voltage source 76 exciting lamp 78.
The spacing, between the screen member 28 and photoconduc-
tive sur~lce 12, may be adjusted by drive system 74 as heretofore
described. Briefly, a screen member 28 is mounted on rails and drive
system 7~ in conjunction with rack and pin assembly regulates the
spacing between photoconductive surface 12 and screen member 28.
This, in turn, adjusts the contrast of the resultant copy. Light
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source 78 projects light rays through screen member 28 forming a
screen pattern on photoconductive surface 12. This screen pattern
moves in the direction of arrow 14 and has the flowing light image
of the original document projected thereon in superimposed registra-
tion therewith. Thus, the resultant composite electrostatic latent
image formed on photoconductive surface 12 is modulated. This is
a sequential optical exposure rather than the multiplicative optical
-exposure as is shown in Figure 3. In the pictorial mode of opera-
tion, screen member 28 and light source 78 are operational. Lamp
I0 78 is excited by voltage source 76. In order to place the printing
machine in the functional mode of operation, voltage source 76 is
de-energized, thereby de-energizing lamp 78. ~n this manner, light
rays are not projected through screen member 28 and a screen pattern
is not formed on photoconductive surface 12. Thus, the single color
flowing light image irradiates the charged portions of the photo-
conductive surface producing a single color electrostatic latent
image which is unmodulated. Hence, in the functional mode the
latent image is unmodulated, whereas in the pictorial mode the
latent image is modulated. Modulation is achieved by the formation
of a screen pattern on the photoconductive surface in superimposed
registration with a latent image of the original document. There-
after the modulated latent image is rendered visible by the de-
velopment process heretofore described.
Referring now to Figure 4, there is shown a detailed
description of screen member 28 and its association with drum 10.
As shown in Figure 4, screen member 28 includes a plurality of
substantially equally spaced opaque lines. During the pictorial
mode of operation, screen member 28 mhy be shifted a distance equal
to 1/3 of the spacing between adjacent lines. This shift occurs
between the formation of successive single color images. By way of
~ 9 ~t~,.7
example, screen member 28 may be located in its normal position
when the green light image is projected therethrough. However,
when the next light image, such as the red light images projected
therethrough, screen member 28 is shifted 1/3 of a period, i.e. 1/3
of the distance between the spacing of adjacent opaque lines there-
on. This shift does not change the spacing between screen member
28 and photoconductive surface 12. This shift prevents the occur-
rence of Moire' patterns and improves color saturation. The shift-
ing between successive single color light images in the pictorial
mode is achieved automatically by a vernier system. This is a
precise control system which moves screen member 28 incrementally
independently from drive system 74. As heretofore indicated~ drive
system 74 includes a solenoid adapted to move screen member 28
away from the optical light path in the embodiment depicted in
Figure 2. However, independent thereof, screen member 28 is shifted
incrementally 1/3 of the spacing between adjacent lines. This
shift occurs between the formation of successive single color light
images preventing the formation of Moire' patterns.
Motor 80 in conjunction with a suitable linkage such as
~o a parallel bar mechanism or a precision rac~. and pinion system ~oves
screen mernber 28. Screen member 28 is moved a distance equal to
1/3 of the spacing between adjacent lines. Actuation of shifting
mechanism 80 is achieved by the timing system in the printing
machine and the logic associated therewith. Thus, when the timing
disc indicates that the first single color light images is no longer
being projected through screen member 28, motor 80 is energized
moving screen mem~er 28 a distance equal to 1/3 of the space be-
tween ad~acent lines and the next successive single color light
image is projected therethrough. In the event that screen member
-30 -2~ i5 not in-the optical-light path,--(Figure-3) the foregoing occurs
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in the same manner. The screen pattern formed on photoconductive
surface 12 is shifted 1/3 of the distance between adjacent lines.
The lateral shift of screen member 28 also improves color saturation.
Screen member 28 may be formed on a transluscent layer or
substrate which adheres to a transparent portion or substrate. The
transparent portion may be made preferably from a suitable flexible
transparent plastic sheet such as Mylar. Screen member 28 may in-
clude a plurality of lines printed on a substantially transparent
substrate by a suitable chemical etching technique, or by a photo-
graphic technique. The screen itself may be made from any number
of opaque metallic material suitable for chemical etching such as
copper or aluminum. As heretofore i}ldicated, the transparent portion
is made preferably from a suitable plastic material. A finer screen
size generally results in a more natural or higher quality copy.
Hence, while a coarse screen having 50 to 60 lines per linear inch
will be useful for some purposes, finer screens such as those having ;
anywhere from 100 to 400 or more lines per inch will give a more
nearly continuous toner appearance to the finished copy. With
finer screens, the screen pattern may be karely perceptible on the
finished copy and a copy will have the appearance of a continuous
tone photograph.
In recapitulation, it is evident that the electrophoto-
graphic printing machine heretofore described operates in one of
two mode, i.e. a pictorial mode or a functional mode. In the pic-
torial mode, a screen is utilized to modulate the electrostatic
latent image of the original document.
Contrawise, in the functional mode the electrostatic
latent image remains unmodulated. In operation, the screen may be
moved into or out of the optical light path as the mode of opera-
tion is changed from the pictorial mode to the functional mode.
* trade mark
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When an additive system is employed, the light source for generat-
ing light rays which pass through the screen member may be de-
energized so as to decouple the screen member from the printing
machine system. Contrast is readily adjustable by regulating the
spacing between the screen member and photoconductive surface.
Moire' patterns are minimized and color saturation improved by
shifting the screen member a distance equal to 1/3 of the spacing
between adjacent lines on the screen. This shift occurs between
successive single color light images. In this manner, high quality
pictorial copies and functional copies may be produced.
It is, therefore, apparent that there has been provided
in accordance with the present invention, an electrophotographic
printing machine that operates in the pictorial or functional mode.
This printing machine fully satisfies the objects, aims, and ad-
lS vantages hereinbefore set forth. While the present invention has
been describ~d in conjunction with specific embodiments thereof, it
; is evident that many alternatives, modifications and variations
will be apparent to those skilled in the art in light of the fore-
going description. Accordingly, it is intended to embrace all such
alternatives, modifications and variations as fall within the spirit
and broad scope of the appended claims.
,
