Note: Descriptions are shown in the official language in which they were submitted.
~375i4S
BACKGROUND OE' THE INV_ENTION
This invention relates generally to an electrophoto-
graphic printing machine, and ~ore particularly concerns a
transfer apparatus employed therein which produces a color
corrected copy from an original document.
In the process of electrophotographic printing, a
photoconductive surface is uniformly charged and exposed to a
light image of the original document. Exposure of the photo-
conductive surface creates an electrostatic latent image corres-
ponding to the original document. Toner particles are then
electrostatically attracted to the latent image to render it
viewable. Subsequently, the toner powder image is transferred
to a sheet of support material and permanently affixed thereto
to produce a copy o~ the original document. The foregoing
process is described in detail in U.S. Patent ~o. 2,297,691
issued to Carlson in 1942.
Multi-color electrophotographic printing is substan-
tially identical to the heretofore~discussed process of black
and white printing with the following distinctions. Rather
than forming a total light image of the original, the light
image is filtered producing a single color light image which is
a partial light image of the original document. The foregoing
single color light image exposes the photoconductive surface to
create a single color electrostatic latent image. The single
color electrostatic latent image is developed with toner
particles of a color complementary to the single color light
image. The single color toner powder image is then transferred
from the electrostatic latent image to a sheet of support
material. This proce~ss is repeated a plurality of cycles with
differently colored light images and the respective complementary
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~V3'~5~S
colored toner particles. Each single color toner powder image
is transferred to the sheet of support material in superimposed
registration with the prior toner powder image. This creates
a composite multi-layered toner powder image on the sheet of
support material~ Thereafter, this composite multi-layered
toner powder image is permanently affixed to the sheet of --
support material to create a color copy corresponding to the
; colored original document.
The fidelity of the color is limited by the imperfect
nature of the spectral transmittance of the toner particles.
Ideal toner particles perfectly absorb over a preselected
spectral region and perfectly transmit over the remaining
spectral region. For example, ideal cyan will perfectly absorb
red light and perfectly transmit blue and green light. Similarly,
, 15 ideal magenta will perfectly absorb green light and transmit
both blue and red light. Finally, ideal yellow will absorb
perfectly in the blue region while transmitting both red and
` green light. However, real materials differ from these ideal
`l colorants by exhiblting unwanted absorption in regions where
20~ they should be perfectly transmitting. Typical cyan toner
; particles absorb not only red but also some green thus, cyan
toner particles contain some~magenta impurities therein.
Similarly, typical magenta toner particles absorb some blue and
:~ .
~ therefore contain some yellow impuritie~s therein. It should ~ -
;~ 25 be noted that the yellow toner particles are substantiallypure. It is therefore apparent that a comblnation of the ~;
foregoing toner particles will produce not only the desired
resultant color but a color produced from the impurities which
is an undesired effect. This will result in the colors of the
3G copy differing from that of the original. Compensation for the
_ 3 _
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~6~375~a~
impure spectral characteristics of the toner particles is
termed color correction~ A set of ideal toner particles would
require no color correction.
Thus, i-t is a primary object of the present invention
to improve the method and apparatus employed in reproducing
color copies so as to correct for the impurities of colors
employed therein.
SUMM~RY OF THE INVENTION
'~ Briefly stated and in accordance with the present
invention there is providèd an electrophotographic printing
` machine for creating color corrected copies from a color
original document.
.:., .
This is achieved, in the present instance, by an
electrophotographic printing machine employing means ~or
c}larging a photoconductive member to a substantially uniform
;~' potential. Means are provided ~or exposing the charged photo-
conductive member to successive single color light images.
This records successive single color electrostatic latent
images on the photoconductive member. A plurality of developing
units are arranged to act on the photoconductive member. Each
developer unit brings into oparative communication with the
photoconductive member toner particles containing a predetermined
colorant. The colorant of each o~ the toner particles corresponds
, .
to the single color light image employed to record the single
color electrostatic latent image on the photoconductive member.
In this manner, successive single color electrostatic latent
` images are rendered visible with toner particles having the. ~
corresponding colorant therein. In addition, means are provided
for transferring toner particles Erom successive single color
electrostatic latent images to a sheet of support material.
, ~ .
:
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~37~
Each successive layer of toner particles transferred to the
sheet of support material contains a colorant corresponding in
color to the color of the impurity contained in the previously
transferred layer of toner particles. Successive layers of
; 5 toner particles are transferred in superimposed registration
with one another. Hence, each successive transferred layer of
toner particles corrects for the impurity contained in the
colorant of the previously transferred layer of toner particles.
.j . .
- This produces a combination of toner particles substantially
approximating the ideal color.
BRIEF DESCRIPTION OF THE DRAWI~GS
Other objects and advantages of the present inven-
tion will bacome apparent upon reading the following detailed
description and upon reference to the drawings, in which:
Figure 1 is a schematic perspective view of a muIti-
color electrophotographic printing machine incorporating the
~,~ features of the present invention thereinî
Figure 2 i5 a schematic perspective view of the
~ transfer apparatus employed in the Figure 1 printing machine,
;~ 20 Figure 3 is a graphic representation diagramatically
:
; depicting the characteristics typifying the transfer of two
layers of toner particles by the Figure 2 transfer apparatus; and
Figure 4 is a graphic representation diagramatically
~ .
illustrating the characteristics typifying the transfer of three
layers of toner particles by the Figure 2 transfer apparatus. -
While the present invention will be described in
connection with a preferred embodiment, 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,
modifications and equivalents as may be included within the
spirit and scope of the invention as defined by the appended claims.
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7~4~
DET~ILED DESCRIPTION OF_INVENTION
- For a general understanding of the disclosed multi-
; color electrophotographic printing machi~e in which the present
invention may be incorporated, continued reference is had to
the drawings wherein like reference numerals have been used
throughout to designate like elements. Figure 1 schematically
illustrates the various components of a printing machine for
- producing color corrected copies from a colored origlnal docu-
ment. Although the transfer apparatus of the present invention
is parbicularly well adapted for use in an electrophotographic
; printing machine, i-t should become evident from the following
~; discussion that it is equally well suited for use in a wide
.. ..~:
`~ variety of electrostatographic printing machines and is not -
necessarily limited in its application to the particular embodi-
ment shown herein.
~`~ The process employed in the multi-color electrophoto-
graphic printing machine depicted in Figure 1 is a subtractlve
;color-to-color reproducing process wherein toner~particles
hav~ing colorants containing the subtractive prlmaries cyan,
,,~ 20 ~magenta and~yellow are employed to provide a wide range of
colors found in the original document on the ~olor copy. The
first step in producing a color copy is to ascertain the color
`~ composition of the original subject matter and to record thisinformation on an image bearing member. The color original
~ocument is optically scanned a number of times~to formulate
successive electrostatic latent images on the i~age bearing
member. Each light image is passed through a color filter to
I form a color separated electrostatic latent image. The electro-
static latent image created by passing the light image through
a filter is developed by toner particles containing colorants ~`
complementary theretoO Areas of relatively high charge density
.:
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~375~5
on the image bearing member indicate the absence of the filtered
light, while areas of relatively low charge density on the i~age
bearing member indicate the presence oE the filtered light in
the colored original. For example, the electrostatic latent
image formed by passing the light image through a green filter
will record magentas as areas of relatively high charge density
on the image bearing member while the green light rays will
cause the charge density on the image beaxing member to be
reduced to an ineffective development level. The magentas
are then made visible by applying toner particles containing
a green absorbing, i.e. magenta, colorant to the electrostatic
latent image recorded on the image bearing member. Similarly,
a blue separation is developed with toner particles containing
a yellow pigment while a red separation is developed with toner
particles containing a cyan colorant. The three developer
color separated toner powder images are then brought together,
in registration, on a sheet of final support material to produce
a multi-color copy.
Turning now to Figure l, the detailed structural
2~ configuration of the electrophotographic printing machine
employing the process hereinbefore described will now be dis-
cussed. The electrophotographic printing machine utilizes an
image bearing member having a drum 10 with a photoconductive
surface 12 secured to and entrained about the exterior circum-
ferential surface thereof. Preferably, photoconductive surface
12 is made from a material having a relatively panchromatic
. ~
response to white light. One type of suitable photoconductive
material is disclosed in U.S. Patent No. 3,655,377 issued to
Sechak in 1972, Drum 10 is mounted rotatably within the printin~
machine on the frame thereof (not shown). A series of processing
." '' ~'
~)3'7545
stations are disposed such that as drum 10 rotatas in the
direction of arrow 14, photoconductive surface 1~ passes
sequentially therethrough. Drum 10 is driven at a predetermined
speed relative to the other machine operating mechanisms by a
drive motor (not shown). A timing disc is mounted in the region
of one end of drum 10 and is adapted to trigger the logic cir-
cuitry of the printing machine. This coordinates the various
machine operations with one another to produce the proper
sequence of events at the various processing stations.
Initially, drum 10 moves photoconductive sur~ace 12
through charging station A. A corona generating device,
indicated generally at 16, is disposed at charging station A~
Corona generating device 16 extends in a generally longitudinal
direction transversely across photoconductive surface 12. This
; 15 readily enables corona generating device 16 to charge photocon-
` ductive surface 12 to a relativaly high substantially uniformpotential, Preferably, corona generating device 16 is of the
type described in U.S. Patent No~ 2,778,946 issued to Mayo in
1957.
Thereafter, dru~ 10 is rotated to exposure station B.
Exposure station ~ includes thereat an optical system generally
designated by the reference numeral 18. Optical system 18
includes a moving lens system, generally designated by the
reference numeral 20, and a color filter shown generally at 220
An original document 24 is disposed upon transparent viewing
,
platen 26. Scan lamps 28 are disposed beneath transparent platen
26 to illuminate original document 24 positioned thereon. Lamps
: 28, lens 20 and filter 22 move in a timed relation with drum 10
to scan successive incremental areas of original document 24
disposed upon platen 26. Mirror 30 reflects light rays reflected
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~37~4~
from original document 2~ through lens 20. After passing
through lens 20, the light rays are transmitted through filter
22, i.e. a selected color separation filter inserted into the
path o~ the light rays. Thereafter, the light rays are re~lected
from a second mirror 32 onto photoconductive surface 12 of drum
10 to selecti~ely dissipate the charge thereon in the irradiated
areas forming a single color electrostatic latent image thereon.
- As previously indicated, the appropriate color filter operates
on the light rays passing through lens 20 to record an electro-
static latent image on photoconductive surface 12 corresponding
to a preselected spectral region of the electromagnetic wave
spectrum, hereinafter referred to as a single color electrostatic
~i latent image. Preferably, filter mechanism 22 includes three
., .
filters, a blue filter, a red filter and a green filter. Each
. .
" 15 of the filters is associated with its respective toner particles
and the associate colorant, i.e. the complement of the color
.,
thereof to produce a subtractive system. By way of example,
a green filtered light image is cleveloped with toner particles
containing a magenta colorant, a blue filtered light image is
developed with toner particles containing a yellow colorant, and~
a red filtered light image is developed with toner particles
.: .
containing a cyan colorant.
-; With continued reference to Figure l, after exposure,
drum lO rotates the single color latent electrostatic latent
image recorded on photoconductive surface 12 to development
` station C. Development station C includes three developer
~- units, generally indicated by the reference numerals 34, 3~
and 38, respectively. Preferably, the developer units are all
of a type generally referred to as magnetic brush developer
~ 30 units. A typical magnetic brush developer unit employs a
:,,
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,
~(~37~i45
magnetiæable developer mix of carrier granules and toner par-
ticles. The developer mix is continually brought through a
directional flux field to form a brush thereof. Each developer
unit includes a developer roll electrically biased to the
appropriate potential such that the toner particles are
attracted from the carrier granules to the areas of photocon-
ductive surface 12 having a greater charge thereon, i.e. the
single color electrostatic latent image. The single color
electrostatic latent image recorded on photoconductive surface
12 is developed by bringing the brush of developer mix into
contact therewith. Each of the respective developer units
contains toner particles having discrete colorants therein
corresponding to the complement oE the spectral region of the
wave length of light transmitted through filter 22. As herein-
before indicated, a green filtered electrostatic latent image
is rendered visible by depositing toner particles having a
m~genta colorant therein adapted to absorb green. Similarly,
blue and red electrostatic latent images are developed with
toner particles having a yellow colorant and toner particles
having a cyan colorant therein, respectively.
Drum 10 is, next, rotated to transfer station D
where the toner powder image adhering electrostatically to
photoconductive surface 12 is transferred to a sheet of support
material 40. Support material 40 may be plain paper or a sheet
;~ 25 of thermoplastic material, amongst others. Transfer station D
. : ~
includes a transfer member, designated generally by the
reference numeral 42. Transfer member 42 is a roll adapted to
recirculate support material 40 and is electrically biased to
the appropriate voltage by a variable power supply 44. This
potential is of suf~icient magnitude and polarity to attract
'
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,~,1 , . . . . . ..
~ ~375~5
electrostatically the toner particles from the electrostatic
latent image recorded on photoconductive surface 12 to support
material 40. Transfer roll 42 rotates in synchronism with
photoconductive surface 12. Inasmuch as support material 40
is secured releasably thereon for movement in the recirculated
path therewith, succecsive toner pow~er images may be trans-
ferred thereto into superimposed registration with one another.
In this case, transfer roll 42 ~otates in the direction of
arrow 46 at substantially the same angular velocity as drum 10.
Transfer member 42 will be described hereinafter in greater
detail with reference to Figure 2.
Support material 40 is advanced from a stack 48
thereof. ~tack 48 is disposed upon tray 50~ Feed roll 52, `
in operative communication with retard roll 54, advances and
separates the uppermost sheet from stack 48 disposed upon tray ~`
50. The advancing sheet moves into a chute 56 whi~h directs
it into the nip between register rolls 58. Thereafter, gripper
fingers, indicated generally at 60, mounted on transfer roll 42 ~ -~
secure releasably thereon support material 40 for movement
j 20 therewith in a recirculated path. After a pluraIity of toner
powder images have been transferred to support ma~terial 40,
gripper fingers 60 release support material 40 and space it
from transfer roll 42. Stripper bar 62 is then interposed
therebetween to separate support material 40 from transfer roll
42. Thereafter, endless belt conveyor 64 advances support
material 40 to fixing station E. At fixing station E, a fuser,
indicated generally at 66, generates sufficient heat to per-
manently affix the multi-layered toner powder image to support
material 40. Moreover, the toner powder layers are rendered
substantially transparent to act as filters. In this manner,
1~37S~,5
the light rays are transmitted through the respective toner
powder layers to the support material and then reflected back
therefrom to the eye of the observer. The observer then sees
the copy in the colors substantially corresponding to that of
the original document. One type of suitable fuser is described
in U.S. Patent No. 3,498,592 issued to ~oser et al. in 1970.
After the fusing process, support material 40 is advanced by
endless belt conveyors 68 and 70 to catch tray 72 for subsequent
removal therefrom by the machine operator.
Thereafter, drum 10 is advanced to cleaning station F.
Although a preponderance of the toner particles are transferred
to support material 40, invariably some residual toner particles
remain on photoconductive surface 12 after the transfer of the
toner powder image therefrom. These residual toner particles
are removed from photoconductive surface 12 as it passes through
cleaning station F. Here, the residual toner particles are
initiall~ brought under the influence of a cleaning corona
generating device (not shown) adapted to neutralize the electro-
static charge remaining thereon. The neutralized toner particles
are then removed from photoconductive surface 12 by a rotatably
mounted fibrous brush 74 in contact therewith. A suitable
brush cleaniny device is described in U.S. Patent No. 3,590,412
issued to Gerbasi in 1971.
'~ It is believed that the foregoing description is
'25 sufficient for purposes of the present application to depict
` the general operation of a multi-color electrophotographic
printing machine embodying the teachings of the present inven-
tion therein.
Referring now to the specific subject matter of the
present invention, Figure 2 depicts the transfer apparatus
associated with photoconductive surface 12 of drum 10. Transfer
- 12 -
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~3~5~5
roll 42 includes an aluminum tube 76, preferably having about
a 1/4 inch thick layer of urethane 78 cast thereabout. A
polyurethane coating 80, preferably of about 1 mil thick, is
sprayed over the layer of cast urethane 78. Preferably,
transfer roll 42 has a durometer hardness ranging from about
10 units to about 30 units on the Shore A scale. The resistivity
of transfer roll 42, preferably, ranges from about 108 to about
1011 ohm-centimeters. Variable voltage source 44 applies a
direct current bias voltage to aluminum tube 76 by suitable
means such as a carbon brush and brass ring assembly (not shown).
The voltage applied to roll 42 may range from about 1500 volts
to about 6000 volts. This voltage may be adjusted for various
layers of toner particles being transferred to support material
40. Thus, when the first layer of toner particles is transferred
from transfer roll 42 to support material 40, the voltage applied
thereto may be about 5000 volts, while a bias volta~e applied
for the transfer of the next successive layer of toner particles
may be 4000 volts. Finally, when the third layer of toner
particles is transferred to support material 40, the bias --
voltage may be 3000 volts. However, the bias voltage may also
be maintained constant at a preferred value, i.e. 5000 volts. `-~
This depends upon the desired color correction being achieved
, by the system. The technique of color ~orrection will be dis-
i cussed further with reference to Figures 3 and 4. Transfer
i 25 roll 42 is substantially the same diameter as drum 10 and is
driven at substantially the same speed thereat. Contact between
, photoconductive surface 12 of drum 10 and transfer roll 42 with
s support material 40 interposed therebetween, is preferably,
limited to a ma~imum of about 1.0 pound linear force. A syn-
chronous speed main drive motor rotates transfer roll 42. This
- 13 -
~3~5~i
drive is coupled directly to transfer roll 42 by flexible metal
bellows 82 which permits the lowering and raising of transfer
roll 42. Synchronization of transfer roll 42 and drum 10 is
achieved by precision gears (not shown) coupling the main drive
motor to both transfer roll 42 and drum 10.
Turning now to Figure 3, there is shown support
material 40 with a multi-layered toner powder image transferred
thereto. When a layer of toner particles is deposited on the
support material the effective resistivity of the transfer
roll increases. Hence, if the voltage applied thereto remains
constant, the magnitude of the electrostatic field applied to
the toner particles adhering electrostatically to photocon-
ductive surface 12 will decrease. Thus, the thickness of the
toner particle layer transferred in superposition with the
; 15 previous toner paxticle layer will be less than that of the
previously -transferred toner particle layer. This principle
may be utilized to color correct copies produced on a multi- ;~
' color electrophotographic printing machine. As hereinbefore
J' indicated, toner particles having a cyan colorant contain a
magenta colorant impurity. Similarly, toner particles having a
~ magenta colorant contain a yellow colorant impurity. Thus, it
; is desirable to transfer less toner particles containing magenta
colorant therein over toner particles containing cyan colorant
therein. The foregoing is exemplified in Figure 3. As shown
therein, initially cyan toner particles 84 are transferred to
the sheet of support material 40. Cyan toner particles 84
differ from ideal cyan toner particles in that they contain a
magenta impurity. The thickness of the cyan toner particles
may be represented by the letter G. Voltage source 44 maintains
a constant potential on transfer roll 42. The next successive
- 14 -
~3'Y5~i
layer of toner particles contain a magenta colorant therein.
Thus, magenta toner particles 86 are next transferred to
support material 40 and in superposi-tion with cyan toner
particles 84. As shown in Figure 3, when magenta toner par- `
ticles 86 are transferred directly to support material 40 the
thickness of the layer is substantially the same as that of cyan
toner particles 84, i.e. a thickness of G. However, when
magenta toner particles 86 are transferred to support material
40 in superposition with cyan toner particles 84 the thickness
of the toner particle layer is le 9S than that transferred to
the bare sheet of support material 40. Thus, the magenta toner
particles S6 are transferred over cyan toner particles 84
and have a thickness o~ H. As shown in Figure 3, the thickness H
of magenta toner particles 86 superimposed over cyan toner
particles 84 is less than the thickness G of magenta toner -~
particles 86 transferred directly to support material 40. The
foregoing corrects for the impurities in the cyan colorant.
Hence, the total color produced by magenta toner particles 86
superimposed with cyan toner particles 84 will contain substan-
tially the correct amount of cyan colorant therein. This is
due to the fact that the magenta toner particle layer 86
transferred over the cyan toner particle layer 84 is not as
thick as the cyan toner parti¢le layer. The magenta impurity
`~ in cyan toner particle layer 84 in combination with the layerOf magenta toner particles 86 transferred thereto results in
the total amount of magenta being approximately the ideal
amount. The foregoing may also be achieved by adjusting the
voltage produced from voltage source 44. By this it is meant
that the voltage produced from voltage source 44 ~ill be `~
, 30 decreased for magenta toner particle transfer as compared to
15 -
~037~45
cyan toner particle transfer. However, one should note that
it would also decrease the thickness of the layer of the magenta
toner particles transferred directly to support material 40.
Turning now to Figure 4, there is shown the effect
of transferring three layers of toner particles in superposi-
tion with one another. Figure 4 clearly illustrates the color
correcting effect produced by maintaining voltage source 44
substantially constant. Once again, as shown in Figure 4,
cyan toner particle layer 84 is initially transferred to support
material 40. The thickness of cyan toner particle layer 84
is represented by the letter G. Thereafter, magenta toner
particles are transferred to support material 40 having cyan
toner particles 84 adhering thereto. Magenta toner particles
86 are transferred directly to support material 40 and have a
layer thickness of G and H. Thus, where the magenta toner
particles are transferred directly to support material 40 they
have the same thic~ness as the cyan toner particles 84, i.e. G.
However, where magenta toner particles 86 are transferred in
superposition with cyan toner particles 84, they have a thickness
~o The thickness of the magenta toner particles 86 superimposed
over the cyan toner particles 84 is less than that of the cyan
toner particles. Hence, the magenta toner particles in com-
bination with the magenta impurity contained in the cyan toner
particles produce substantially the correct amount of magenta
combination formed therebetween. If the thickness of the
magenta toner particle layer 86 transferred over the cyan toner
particle layer 84 were the same thickness as the cyan toner
particle layer, the combined color formed thereby would have
excessive magenta due to the magenta impurity contained in
cyan toner paxticles 84. Finally, yellow toner particles 88
are transferred over magenta toner particles 86 superimposed
over cyan toner particles 84. The thickness of the layer of
- 16 -
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~3~S4S
yellow toner particles 88 is represented by the letter ~. The
thickness of the ~ yer of yellow toner particles 88 is less
than that of the magenta toner particle layer 86 and the cyan
toner particle layer 84. Thus, it may be said that toner
particle layer having a thickness I is less than the toner
particle layer having a thickness H which in turn is less than
the toner particle layer having a thickness G. The magenta
toner particles 86 contain a yellow impurity. Thus, by decreasing
the thickness of the yellow toner particle layer transferred in
superposition with the magenta toner particle layer, the
resultant combined color formed therebetween is color corrected.
The foregoing process may be easily understood by the following.
A layer of cyan toner particles 84 is transferred to support
material 40. Thereafter, a thinner layer of magenta toner
particles 86 are superimposed over the layer of cyan toner
particles 84, However, the magenta colorant of the combined
image is substantially correct inasmuch as cyan toner particles
84 contain a magenta impurity7 Finally, a layer of yellow
toner particles having still a lesser thickness are trans-
ferred in superposition over the layer of magenta toner par-
ticles. This corrects for the yellow impurity contained in
the magenta toner particles. Thus, the resultant color formed
from a combination of yellow, magenta and cyan is closely
approximate to the ideal color, i.e. black. The foregoing
color correction will not occur if the sequence of transfer is
varied. Thus, in order to achieve substantial color correction
initially, cyan toner particles must be transferred to the
support material, thereafter, magenta toner particles, and
finally, yellow toner particles. If the transfer sequence is
varied, it may significantly increase the color errors rather ~;
than correcting therefore. Without color correction all colors
~ 17 -
~93754S
reproduced may be desaturated i.e. dulled and yrayish. For
example, dulled cyan or blue, and dulled magenta becomes red.
The extent of desaturation depends upon the transmission quality
of the foregoing toner particles. The hereinbefore described
transfer substantially minimizes color desaturation and optimizes
the color copies to substantially correct for imperfections
of the toner colorants. While in the preferred transfer
sequence voltage source 44 is constant, it is evident that
voltage source 44 may be adjusted so as to decrease the thickness
of the yellow toner particle layer transferred over the pre-
viously transferred toner particle layer. However, this would
produce a decrease in all of the yellow toner particles transferred
thereto rather than a selective decrease in the thickness of
the layer which is achieved by maintaining the voltage source 44
substantially constant.
In recapitulation, it is apparent that the transfer
roll cooperating with the electrical biasing voltage source and
the corresponding sequence of transfer operations minimizes the
reproduction of desaturated colors. The method and apparatus
2~ heretofore described is adapted to correct for the impurities
contained in the colorants employed in the toner particles
utilized in the electrophotographic printing machine. This
color correction automatically provides for high fidelity colors
substantially approximating that of the ~iginal document.
It is, therefore, evident that there has been pro-
vided in accordance with the present invention a transfer
apparatus and method of sequentially transferring successive
layers of toner particles that fully satisfies the objects,
aims and advantages set forth above. ~hile this invention has
been described in conjunction with specific embodiments thereof,
- 18 -
~37~4S
it is apparent that many alternatives, modifications and
variations will be evident to those skilled in the art.
Accordingly, it is intended to embrace all alternatives,
modifications and variations as fall within the spirit and
broad scope of the appended claims.
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