U.S. patent number 3,799,668 [Application Number 05/340,413] was granted by the patent office on 1974-03-26 for color standard and method of calibrating a multi-color electrophotographic printing machine.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to James H. McVeigh.
United States Patent |
3,799,668 |
McVeigh |
March 26, 1974 |
COLOR STANDARD AND METHOD OF CALIBRATING A MULTI-COLOR
ELECTROPHOTOGRAPHIC PRINTING MACHINE
Abstract
A color standard and method of use therefore in which a
multi-color electrophotographic machine is calibrated for
controlling the color balance and image density of a copy being
reproduced therein.
Inventors: |
McVeigh; James H. (Rochester,
NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23333254 |
Appl.
No.: |
05/340,413 |
Filed: |
March 12, 1973 |
Current U.S.
Class: |
399/28;
356/243.5; 355/88; 356/422; 250/252.1; 356/421 |
Current CPC
Class: |
G03G
15/01 (20130101) |
Current International
Class: |
G03G
15/01 (20060101); G03g 015/00 () |
Field of
Search: |
;355/4,17,88 ;95/1R
;96/1.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Moses; Richard L.
Attorney, Agent or Firm: Ralabate; James J. Fleischer; Henry
Green; Clarence A.
Claims
What is claimed is:
1. A method of calibrating a multi-color electrophotographic
printing machine of the type having a developability regulating
mechanism arranged to control the color balance and density of a
copy being reproduced thereby with a color standard including the
steps of:
setting the printing machine in the color calibration mode of
operation for producing a first color test copy;
actuating the printing machine to produce a first color test
copy;
comparing the first color test copy with the color standard;
and
adjusting the developability regulating mechanism until the density
of the first color test copy is intermediate the maximum and
minimum density ranges of the color standard.
2. A method as recited in claim 1, wherein said step of setting the
printing machine in the calibration mode includes the steps of:
inserting a toner dispenser housing toner particles of a color
corresponding to the first color test copy being reproduced into
the development system of the printing machine;
determining the number of copies required to actuate the
developability regulating mechanism to energize the toner dispenser
and increase the concentration of toner particles within the
developer mix;
placing the color standard in the printing machine as the original
document to be reproduced when the toner dispenser is energized
prior to the formation of the predetermined number of test copies;
and
forming a first color test copy without an original document in the
printing machine when the toner dispenser is not energized prior to
the formation of the predetermined number of test copies.
3. A method as recited in claim 2, further including the steps
of:
printing a plurality of first color test copies when the color
standard is disposed in the printing machine until a predetermined
number of first color test copies are produced without the toner
dispenser being energized by the developability regulating
apparatus; and
comparing the first color test copy immediately prior to the toner
dispenser being energized, and the first color copy immediately
following the printing of the predetermined number of first color
test copies with the color standard to determine if the first color
test copy prior to toner dispensing and the first color test copy
subsequent to the printing of the predetermined number of first
color test copies are intermediate the maximum and minimum density
ranges of the color standard.
4. A method as recited in claim 1, further including the steps
of:
setting the printing machine in the color calibration mode of
operation for producing a second color test copy;
actuating the printing machine to produce a second color test
copy;
comparing the second color test copy with the color standard;
and
adjusting the developability regulating mechanism until the density
of the second color test copy is intermediate the maximum and
minimum density ranges of the color standard.
5. A method as recited in claim 4, further including the steps
of:
setting the printing machine in the color calibration mode of
operation for producing a third color test copy;
actuating the printing machine to produce the third color test
copy;
comparing the third color test copy with the color standard;
and
adjusting the developability regulating mechanism until the density
of the third color test copy is intermediate the maximum and
minimum density range of the color standard.
6. A method as recited in claim 1, further including the steps
of:
setting the printing machine in the exposure calibration mode of
operation after said step of adjusting the developability
regulating mechanism;
actuating the printing machine to produce a plurality of first
color test copies with the color standard disposed therein as the
original document;
comparing each first color test copy with the color standard;
adjusting the exposure system of the printing machine until each
first color test copy reproduced therein is of a predetermined
exposure condition; and
determining the neutral density filter to be utilized in the
exposure system of the printing machine in the normal operating
mode thereof.
7. A method as recited in claim 1, further including the steps
of:
actuating the printing machine to produce a multi-color test
copy;
comparing the multi-color test copy with the color standard;
and
adjusting the electrical bias of each developer unit of the
development system in the printing machine until the multi-color
test copy is reproduced within the specified density ranges of the
color standard.
8. A color standard for calibrating a multi-color
electrophotographic printing machine, including:
a generally planar support member;
a plurality of color samples disposed on said support member, each
of said color samples being of a predetermined hue, value and
chroma so as to be substantially optimum for reproduction as a test
copy on the printing machine; and
a plurality of pairs of limit color samples disposed on said
support member, each of said pair of limit color samples
corresponding in hue, value and chroma to one color being
reproduced by the printing machine, one of said limit color samples
being of a maximum acceptable density and the other of said limit
color samples being of a minimum acceptable density to define the
permissible density range for each of said colors being reproduced
by the multi-color electrophotographic printing machine.
9. A color standard as recited in claim 8, wherein said support
member includes a plurality of apertures therein, each of said
apertures passing through one of said limit color samples
permitting the color copy corresponding thereto to be disposed
therebeneath so as to be visible through the aperture for
comparison therewith to ascertain whether the color is between the
maximum and minimum acceptable density range of said limit color
samples.
10. A color standard as recited in claim 9, wherein said plurality
of limit color samples include:
a high density cyan color sample and a low density cyan color
sample;
a high density magenta color sample and a low density magenta color
sample; and
a high density yellow color sample and a low density yellow color
sample.
11. A color standard as recited in claim 9, wherein said plurality
of limit color samples include:
a high density red color sample and a low density red color
sample;
a high density cyan color sample and a low density cyan color
sample; and
a high density black color sample and a low density black color
sample.
12. A color standard as recited in claim 8 for a multi-color
electrophotographic printing machine having a development system
with a controllable developer bias voltage arranged to adjust copy
image density, wherein said plurality of color samples include an
array of developer color samples disposed on said support member,
said array of developer color samples comprising a first set of
developer bias color samples adapted to be reproduced only when the
electrical bias of the development system is less than the optimum
value, a second set of developer bias color samples corresponding
to the optimum electrical bias of the development system, and a
third set of developer bias color samples adapted to indicate when
the electrical bias of the development system is greater than the
optimum value.
13. A color standard as recited in claim 12, wherein:
said first set of developer bias color samples include a light blue
color sample, a light pink color sample, and a light yellow color
sample;
said second set of developer bias color samples include a
substantially pure red color sample, a substantially pure green
color sample, and a substantially pure blue color sample; and
said third set of developer bias color samples include a
contaminated red color sample, a contaminated green color sample,
and a contaminated blue color sample.
14. A color standard as recited in claim 12, wherein:
said first set of developer bias color samples include a light red
color sample and a light blue color sample;
said second set of developer bias color samples include a
substantially pure red color sample and a substantially pure blue
color sample; and
said third set of developer bias color samples include a
contaminated red color sample and a contaminated blue color
sample.
15. A color standard as recited in claim 8, wherein said plurality
of color samples include a plurality of pairs of exposure color
samples disposed on said support member, one of said exposure color
samples of each of said pairs of exposure samples being arranged to
indicate an underexposed condition for the printing machine, and
the other of said exposure color samples of each of said pairs of
color samples being arranged to indicate an overexposed condition
for the printing machine.
16. A color standard as recited in claim 15, wherein said pairs of
exposure color samples include a first pair of exposure samples
having one exposure color sample arranged to indicate an
underexposed condition for the development of a cyan image in the
printing machine, and the other exposure color sample arranged to
indicate an overexposure condition for the development of the cyan
image in the printing machine.
17. A color standard as recited in claim 16, wherein said pairs of
exposure color samples include a second pair of exposure color
samples having one exposure color sample arranged to indicate an
underexposed condition for the development of a yellow and magenta
image in the printing machine, and the other exposure color sample
arranged to indicate an overexposed condition for the development
of a yellow and magenta image in the printing machine.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to a multi-color
electrophotographic printing machine, and more particularly
concerns a color standard and method of use therefore in which the
multi-color electrophotographic printing machine is calibrated for
controlling the color balance and density of copies being
reproduced thereon.
In a multi-color electrophotographic printing machine,
developability is defined as the ability of the developer mix used
therein to form an image having a specified density and color
balance. A developability control system adjusts the concentration
of developer mix to produce images on a copy which have a suitable
density and color balance. Developability is related to the
concentration of toner particles in the developer mix. By this, it
is meant that the percentage of toner particles relative to carrier
granules in the developer mix is a major factor in defining the
developability capability of the printing machine. Thus, the
function of the developability control system is to regulate the
toner particle concentration within the developer mix to aid in
maintaining the appropriate image density and color balance of the
copy being reproduced.
Generally, an electrophotographic printing machine, particularly a
multi-color printing machine, utilizes a plurality of discretely
colored toner particles. The density of toner particles relative to
one another on the copy defines the color balance of the system.
The density of toner particles on the copy is a function of the
toner particle concentration in the developer mix. The
developability control system has included therein appropriate
circuitry arranged to provide a variable reference which represents
the desired toner particle level within the respective developer
mix. Hence, as the level of the reference for the respective toner
particles within the developability control system is varied, the
color balance within the electrophotographic printing machine
changes. In order to obtain multi-color copies having the desired
color balance and density, the printing machine must be calibrated
in the field prior to the installation thereof.
Calibration of a multi-color printing machine requires adjustment
of the development system, exposure system and developability
regulating system to substantially optimize color copies being
produced thereon. Heretofore, no simple procedure has been
developed which facilitates the ready calibration of a multi-color
printing machine. Moreover, any such calibration procedure should
insure that the color balance and density of copies being
reproduced on the printing machine are within the desired standards
for the requisite period of operation.
Accordingly, it is the primary object of the present invention to
improve the method of calibrating a multi-color electrophotographic
printing machine.
SUMMARY OF THE INVENTION
Briefly stated, and in accordance with the present invention, there
is provided a method of calibrating a multi-color
electrophotographic printing machine for controlling the color
balance and density thereof.
In the preferred method, the printing machine is placed in the
calibration mode of operation for producing first color test
copies. The printing machine is actuated to produce the first color
test copies which are then compared to the color standard.
Thereafter, the developability regulating mechanism of the printing
machine is adjusted until successive first color test copies are
intermediate a maximum and minimum density color samples of the
color standard corresponding to the first color test copies.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will become
apparent upon reading the following detailed description and upon
reference to the drawings, in which:
FIG. 1 is a schematic perspective view of a multi-color
electrophotographic printing machine having a developability
regulating mechanism for controlling the color balance and density
of copies being reproduced thereby;
FIG. 2 is a front elevational view of the color standard utilized
for calibrating the FIG. 1 printing machine; and
FIG. 3 is a back elevational view of the FIG. 2 color standard.
While the present invention will be described in connection with
the preferred embodiment and method of use therefore, it will be
understood that it is not intended to limit the invention to that
embodiment and method of use. 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.
DETAILED DESCRIPTION OF THE INVENTION
With continued reference to the drawings wherein like reference
numerals have been used throughout to designate like elements, FIG.
1 schematically depicts an electrophotographic printing machine in
which the present invention may be utilized for the calibration
thereof. The electrophotographic printing machine illustrated
schematically in FIG. 1 shows the various components utilized
therein for producing multi-color copies from a colored
original.
Electrophotographic printing machines employ a drum 10 mounted
rotatably within the machine frame (not shown) and having a
photoconductive surface 12 mounted on the exterior circumferential
surface thereof. One type of suitable photoconductive material is
disclosed in U.S. Pat. No. 3,655,377 issued to Sechak in 1972. A
series of processing stations are disposed such that as drum 10
rotates in the direction of arrow 14, it passes sequentially
therethrough. Drum 10 is driven at a predetermined speed relative
to the other machine operating mechanisms from a common drive motor
(not shown). The various machine operations are coordinated with
one another to produce the proper sequence of events at the
appropriate processing stations.
Initially, drum 10 rotates so that photoconductive surface 12 moves
through charging station A. Charging station A has positioned
thereat a corona generating device, indicated generally at 16. As
shown in FIG. 1, corona generating device 16 extends in a generally
transverse direction across photoconductive surface 12. This
readily enables corona generating device 16 to charge
photoconductive surface 12 to a relatively high substantially
uniform potential. Preferably, corona generating device 16 is of a
type described in U.S. Pat. No. 2,778,946 issued to Mayo in
1957.
Drum 10, thereafter, is rotated to exposure station B where the
charged photoconductive surface 12 is exposed to a color filtered
light image of the original document. Exposure station B includes
thereat a moving lens system, generally designated by the reference
numeral 18, and a color filter mechanism shown generally at 20. A
suitable moving lens system is disclosed in U.S. Pat. No. 3,062,108
issued to Mayo in 1962, and a suitable color filter mechanism is
described in copending application Ser. No. 830,282 filed in 1969.
Filter mechanism 20 includes a neutral density filter adapted to
regulate the minimum discharge level of photoconductive surface 12.
In association with the neutral density filters are blue, red and
green filters. These filters are utilized therein in order to form
color separated light images of the original document. Each colored
filter may have a corresponding neutral density filter associated
therewith. As part of the calibration procedure for the multi-color
electrophotographic printing machine, the appropriate neutral
density filter, if required, is selected to correspond with its
respective colored filter.
With continued reference to FIG. 1, an original document 22, such
as a sheet of paper, book, or the like is placed face down upon
transparent viewing platen 24. Lamp assembly 26 and lens system 18
are moved in a timed relation with drum 10 to scan successive
incremental areas of original document 22 disposed upon platen 24.
In this manner, a flowing light image of original document 22 is
projected onto photoconductive surface 12. Filter mechanism 20 is
adapted to interpose selected color filters and their respective
neutral density filters into the optical light path. The
appropriate color filter and its associate neutral density filter
operate on the light rays passing through lens 18 to record an
electrostatic latent image on photoconductive surface 12
corresponding to a preselected region of the electromagnetic wave
spectrum hereafter referred to as a single color electrostatic
latent image. An exposure slit (not shown) is operatively
associated with filter mechanism 20 and lens 18. The exposure slit
slot is used to control the length of time the flowing light image
is projected onto photoconductive surface 12. Since the rotation of
drum 10 is substantially constant, increasing the width of the slit
increases the amount of time light strikes any area on
photoconductive surface 12. Thus, the exposure slit acts in a
similar manner as the aperture of lens 18 which may be changed to
alter the amount of light passing therethrough. The exposure slit
may be disposed beneath platen 24 or above drum 10 in the optical
light path.
After exposure, drum 10 rotates the single color electrostatic
latent image recorded on photoconductive surface 12 to development
station C. Development station C includes thereat three individual
developer units, generally designated by the reference numerals 28,
30 and 32, respectively. A suitable development station employing a
plurality of developer units is disclosed in copending application
Ser. No. 255,259, filed in 1972. Preferably, the developer units
are all of a type referred to generally as magnetic brush developer
units. In a magnetic brush development system, a developer mix
typically comprising magnetic iron carrier granules together with
colored resin toner particles is supplied to an electrostatic
latent image recorded on photoconductive surface 12. In such an
apparatus, the iron particles are held by a magnetic roller in a
bristle-like formation resembling a brush with the toner particles
adhering to the iron by electrostatic attraction. The bristles of
iron particles are electrically conductive and contribute to the
transfer therefrom of toner particles to the charged
photoconductive surface. A magnetic roller of this type is
generally electrically biased at some fixed potential above ground.
The 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 contain
discretely colored toner particles corresponding to the complement
of the spectral region of the wavelength of light transmitted
through filter mechanism 20, e.g. a green filtered electrostatic
latent image is rendered visible by depositing green absorbing
magenta toner particles thereon, blue and red latent images are
developed with yellow and cyan toner particles, respectively.
Preferably, each developer unit, 28, 30 and 32, is electrically
biased to a potential of about 500 volts.
Additional toner particles may be added to their respective
developer mix when developability is reduced deleteriously. The
developability regulating systems, indicated generally at 34,
includes a transparent electrode 36 mounted on photoconductive
surface 12 of drum 10. A light source 38 cooperates with fiber
optic light pipe 40 to transmit light rays through transparent
electrode 36. During development, transparent electrode 36 is
biased electrically to attract toner particles thereto. The toner
particles are deposited on transparent electrode 36 during
development and the intensity of the light rays passing
therethrough is indicative of the density thereof. A photosensor
42, located in oven 43, is in a light receiving relation with light
rays transmitted through transparent electrode 36 via fiber optic
light pipe 45 and produces an electrical output signal
corresponding to the intensity of rays passing therethrough.
Suitable analog reference circuitry compares the electrical output
signal from photosensor 42 with an adjustable reference to generate
a logic control signal for dispensing selected toner particles into
the corresponding developer unit. The logic elements, preferably,
include a suitable discriminator circuit for comparing the
reference with the electrical output signal from photosensor 42.
The discriminator circuit may utilize a silicone control switch
which turns on and effectively locks in after an electrical output
signal has been obtained having a magnitude greater than the
reference (i.e. set point). The signal from the discriminator
circuit changes the state of a flip-flop to generate an output
signal therefrom. The output signal from the flip-flop, in
conjunction with an output signal from the appropriate developer
unit actuates an AND gate. The AND gate transmits a control signal
to the toner dispenser housing the toner particles for the
developer unit generating the signal output to the AND gate. The
control signal also resets the flip-flop. The type of logic circuit
heretofore disclosed is on-off. However, in the alternative, it is
possible to utilize portional circuitry which varies the quantity
of toner particles dispensed to the respective developer units as a
function of the magnitude of the control signal. This may be
achieved by a suitable integrated circuit module arranged to
produce a stepped proportional dispensing rate. Duplicate logic
elements are utilized for each developer unit, i.e. yellow
developer unit, cyan developer unit and magenta developer unit.
Hence, there are three separate independent logic channels, each
channel being associated with its respective developer unit. The
density of toner particles deposited on photoconductive surface 12
is a function of the concentration of toner particles within the
developer mix. The concentration of toner particles is, in turn, a
function of the magnitude of the reference in the developability
regulating mechanism. Thus, by adjusting the respective references,
image density as well as color balance is regulated. The foregoing
is described in greater detail in copending application Ser. No.
213,056, filed in 1971.
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 final support material 44. Final
support material 44 may be, amongst others, plain paper or a
thermoplastic sheet. A transfer roll, shown generally at 46,
recirculates support material 44 and is biased electrically to a
potential of sufficient magnitude and polarity to attract
electrostatically toner particles from the latent image recorded on
photoconductive surface 12 to support material 44. A suitably
electrically biased transfer roll is described in U.S. Pat. No.
3,612,677 issued to Langdon et al. in 1971. Transfer roll 46
rotates in the direction of arrow 47 in synchronism with drum 10,
i.e. in this case at substantially the same angular velocity.
Inasmuch as support material 44 is secured releasably thereon for
movement in a recirculating path therewith, successive toner powder
images may be transferred thereto in superimposed registration with
one another.
After the toner powder images have been transferred to support
material 44, support material 44 is separated from bias transfer
roll 46 and advanced to a suitable fuser (not shown) which
coalesces the transferred powder image thereto. One type of
suitable fuser is described in U.S. Pat. No. 3,498,592 issued to
Moser et al. in 1970. After the fusing process, support material 44
is advanced by a plurality of endless belt conveyors (not shown) to
a catch tray (not shown) for subsequent removal therefrom by the
machine operator.
Although a preponderance of toner particles are transferred to
support material 44, invariably some residual toner particles
remain on photoconductive surface 12 after the transfer of the
toner powder image to support material 44. These residual toner
particles are removed from photoconductive surface 12 as it moves
through cleaning station E. Here the residual toner particles are
first brought under the influence of a cleaning corona generating
device (not shown) adapted to neutralize the electrostatic charge
remaining on the toner particles. The neutralized toner particles
are then mechanically cleaned from photoconductive surface 12 by a
rotatably mounted fibrous brush 48. A suitable brush cleaning
device is described in U.S. Pat. No. 3,590,412 issued to Gerbasi in
1971. Rotatably mounted brush 48 is positioned at cleaning station
E and maintained in contact with photoconductive surface 12. In
this manner, neutralized residual toner particles remaining on
photoconductive surface 12 after each transfer operation are
readily removed therefrom.
It is believed that the foregoing description is sufficient for
purposes of the present application to illustrate the general
operation of a multi-color electrophotographic printing machine
adapted to be calibrated by the color standard of the present
invention.
Referring now to specific subject matter of the present invention,
FIGS. 2 and 3 depict the front and back elevational views of the
color standard of the present invention. As shown in FIG. 2, color
standard 50 includes a generally planar support member 52. Color
standard 50 has disposed upon support member 52 a plurality of
color samples of the primary input colors adapted to be reproduced
within a specified toner particle density and made from spectrally
photometrically calibrated inks. The ink colors are selected so as
to achieve satisfactory copies from the printing machine of FIG. 1.
Preferably, color sample 54 is black, color sample 56 is cyan,
color sample 58 is magenta, and color sample 60 is yellow. The
foregoing color samples have a predetermined hue, value and chroma
so as to be spectrally photometrically compatable with the
sensitometric response of the printing machine depicted in FIG. 1.
Color standard 50 also includes a matrix of color samples which are
adapted to be reproduced in full color. The color samples are
selected such that when the printing machine has the proper color
balance, color samples 74, 76 and 78 reproduce blank on a
multi-color copy. In addition, color samples 62, 64 and 66 are
reproduced as pure colors when the printing machine has the proper
color balance. Finally, color samples 68, 70 and 72 contain
unwanted or contaminated colors therein. For example, red is
contaminated by cyan being produced in the red color sample. Hence,
if pure colors are reproduced when color samples 68, 70 and 72 are
being copied, i.e. the output colors of color samples 68, 70 and 72
in the copy are comparable to the colors of color samples 62, 64
and 66, this indicates that the developer bias in the development
system should be decreased. Similarly, if development occurs in the
copy of any of the color samples 74, 76 and 78, this indicates that
the developer bias of the development system is too low and should
be raised. Preferably, color samples 74, 76 and 78 are light blue,
light pink and light yellow, while color samples 62, 64 and 66 are
pure red, pure green, and pure blue, respectively. Finally, color
samples 68, 70 and 72 are low chroma red, green and blue,
respectively, i.e. a red, green and blue having a high unwanted
absorption therein.
With continued reference to FIG. 2, exposure color sample 80 is
adapted to indicate an over-exposed condition, while exposure color
sample 82 is adapted to indicate an underexposed condition for a
red filtered copy, i.e. cyan development. For example, if in the
copy, no development occurs in exposure color sample 80 an
over-exposed condition exists, while if light development occurs in
exposure color sample 82 an underexposed condition exists. In a
similar manner, exposure color sample 84 is adapted to indicate an
over-exposed condition for blue and green filtered copies or
magenta and yellow development, while exposure color sample 86 is
adapted to indicate an under-exposed condition therefore. Thus, if,
in the copy, no development occurs in exposure color sample 84
there exists an over-exposed condition, while, if light development
occurs in exposure color sample 86 there exists an under-exposed
condition. Preferably, exposure color samples 80 and 84 are a
darker gray, while exposure color samples 82 and 86 are lighter
gray. All gray samples are spectrophotometrically defined for the
FIG. 1 printing machine.
Referring now to FIG. 3, there is shown a back elevational view of
color standard 50. As shown therein a plurality of limit color
samples are disposed on support member 52. Each limit color sample
has an aperture therein to permit ready comparison with the
reproduction of any single color image produced on a copy. Limit
color sample 88 is adapted to indicate the maximum acceptable
density yellow image reproduction on a copy, and includes an
aperture or hole 90 therein for facilitating a comparison
therewith. Similarly, limit color sample 92 is adapted to indicate
the minimum acceptable image density for yellow, and also includes
a hole 94 therein for comparing with the copy thereof. Limit color
sample 96 is adapted to indicate the maximum acceptable magenta
density of a copy, and also includes a hole 98 therein for
facilitating a comparison with the copy. In association therewith,
limit color sample 100 indicates the minimum acceptable magenta
density of a copy, and also includes a hole 102 therein for readily
enabling the copy to be compared therewith. Limit color sample 104
is adapted to indicate the maximum cyan density for a copy and also
includes a hole 106 therein for facilitating a comparison with the
copy. Finally, limit color sample 108 is associated with limit
color sample 104 to indicate the minimum acceptable cyan density of
a copy, and also includes a hole 110 therein for readily enabling
the copy to be compared therewith. Thus, color standard 50 may be
disposed upon platen 24 (FIG. 1) and utilized as original 22 to
have copies reproduced therefrom. In this way, single color copies
and multi-color copies may be compared with color standard 50 to
insure that the density of toner particles being reproduced thereon
are of a satisfactory degree. Moreover, the exposure
characteristics of the printing machine may also be checked. Should
the printing machine not be producing copies having the requisite
color balance and density, the machine is placed in a calibration
mode and suitably adjusted with the color standard. Utilization of
color standard 50 to calibrate the printing machine of FIG. 1
insures that multi-color copies produced therefrom have uniform
high quality.
While the present invention has been described in connection with
cyan, magenta and yellow limit color samples, one skilled in the
art will appreciate that the invention is not necessarily so
limited. For example, the printing machine of FIG. 1 may utilize,
in lieu of cyan, magenta and yellow toner particles, black toner
particles to form a single color black copy, as well as red and
cyan toner particles to form partial color copies. In the foregoing
arrangement red, cyan and black maximum and minimum limit color
samples are provided for evaluating the respective toner particle
density on the copy.
In calibrating the multi-color electrophotographic printing machine
of FIG. 1 with the color standard depicted in the FIGS. 2 and 3, a
three-part procedure is utilized. Prior thereto, however, the
printing machine is placed in the calibration mode. Calibration of
the printing machine comprises adjusting the developability
regulating mechanism to achieve copies having the requisite
density, determining the neutral density filters necessary to
obtain the requisite exposure with all three color filters,
determining the F/stop and the exposure slit to be used in the
imaging system during operation, and final color balance. The
foregoing is required in order to compensate for the different
sensitives of photoconductive surfaces 12, and the varying spectral
irradiance between lamps 26. Finally, the electrical bias levels of
developer units 28, 30 and 32, respectively, are adjusted to
provide optimum multi-color copy quality. Lens system 18 has
operating therewith an exposure slit associated with lamps 26 in
order to provide incremental width exposure of original document
22. The exposure slit is configured in the shape of a butterfly to
correct for the cos.sup.4 light attenuation effect through lens 18
and thereby provide a substantially uniform illumination intensity
across the width of photoconductive surface 12. During the
calibration mode, a specific set of exposure slits are utilized
therein. Preferably, the set-up exposure slit has a maximum width
at the end regions thereof of about 0.341 inches, and a minimum
width at the center thereof of about 0.271 inches.
The following procedure may be utilized to place the multi-color
printing machine in the calibration mode. The developability
regulating mechanism 34 is adjusted by setting the electrical bias
level of transparent electrode 36 at a voltage level of about 200
volts DC above the developer unit electrical bias. Thereafter, the
electrical bias of transfer roll 46 is set at about 2,000 volts DC.
The electrical bias of developer units 28, 30 and 32 are adjusted.
Preferably, the developer electrical bias for the cyan developer
unit is set at about 470 volts DC, the electrical bias for the
magenta developer unit is set at about 500 volts DC, and the
electrical bias for the yellow developer unit is set at about 550
volts DC. Lens 20 is set an an F/stop of about 4.5. Each of the
toner dispensers housing the toner particles for the respective
developer units 28, 30 and 32 are removed from the machine, and all
neutral density filters are removed from filter pack 20. The
multi-color electrophotographic printing machine is now ready to be
calibrated.
Initially, the developability regulating mechanism is adjusted. The
calibration procedure for the developability regulating mechanism
requires the adjustment of the reference levels for cyan, magenta
and yellow, preferably, in that sequence. Initially, a single color
cyan copy of the front side (FIG. 2) of color standard 50 is
reproduced. For example, color sample 54 is compared with high and
low limit color samples 104 and 108, respectively, to determine
whether or not the density of the copy of color sample 54 is
satisfactory. If the copy density of color sample 54 is too low,
the cyan toner cartridge is inserted into the printing machine and
the developability regulating mechanism is actuated so as to
increase the concentration of cyan toner particles within the
developer mix. The preceding procedure is repeated until the
density of the copy of color sample 54 is satisfactory, i.e. it
lies intermediate color samples 104 and 108, respectively. If the
copy density is too high, the cyan reference level of the
developability regulating system is set to prevent cyan toner
particles from being added to the developer mix. Thereafter, a
plurality of copies are reproduced until the copy of color sample
54 has a density which lies between limit color samples 104 and
108. Brush 48 is, thereafter, removed from the printing machine to
prevent the cleaning of transparent electrode 36. The printing
machine of FIG. 1 is then actuated for at least one cycle, and the
reference levels of the developability regulating system adjusted
such that toner particle dispensing is just initiated. Alternately,
the light rays passing through transparent electrode 36 may be
attenuated by a neutral density filter or aperture simulating the
appropriate amount of toner particles thereon. Brush 48 is replaced
and developability regulating mechanism 34 is monitored so as to
insure that as a plurality of copies are being made, cyan toner
particles are periodically dispensed to the developer mix.
Developability regulating system 34 is checked to determine if
toner particles are being dispensed into the developer mix at least
once during the formation of the initial three copies. After a
plurality of copies have been made, the copies from the color
standard are compared with the appropriate limit color samples 88,
92, 96, 100 and 104 and 108 on the back (FIG. 3) thereof. For
example, the cyan copies must have a density which lies between the
high density limit color sample 104 and the low density limit color
sample 108. If the density of the cyan copy is not within the
limits of color sample 104 and color sample 108, the reference on
the developability regulating mechanism is suitably adjusted.
Thereafter, additional copies are made and the foregoing procedure
repeated. Finally, after a copy is reproduced which has a toner
image thereon falling within the density range of color samples 104
and 108, a plurality of copies are made until at least three
consecutive copies are formed wherein no toner particles are
dispensed to the developer mix. The copy immediately before the
three copies and the three copies immediately following the last
dispense signal are compared with the high and low density limit
color samples 104 and 108 to insure that they are within
specification. If the foregoing copies are within specification,
the developability regulating mechanism is calibrated. However, if
the copies are not within specification, a developer unit problem
is identified, and the appropriate repair procedure indicated. This
calibration procedure is repeated for magenta and yellow toner
particles.
Next, the exposure balance of the system is calibrated. This is
achieved by setting lens 18, preferably to an F 5.6 stop and
replacing the set-up exposure slit preferably with a number 4
exposure slit. Table 1 presents a tabulation of the preferred
dimensions for various exposure slits.
TABLE 1
Exposure Maximum Minimum Slit Width Width No. Inches Inches 2 0.682
0.542 4 0.751 0.597 6 0.829 0.659 8 0.913 0.726 10 1.006 0.800
In addition, the electrical bias for the cyan developer unit is set
at about 370 volts DC, the electrical bias for the magenta
developer unit being set at about 450 volts DC, and the electrical
bias for the yellow developer unit being set at about 420 volts.
DC. Thereafter, single color copies of cyan, magenta and yellow are
made in which color standard 50 is used as an original document in
the printing machine of FIG. 1. These color copies are compared to
color standard 50 in order to ascertain whether or not the system
exposure criteria are being satisfied by the printing machine.
Color samples 80 and 82 are utilized for cyan development and color
samples 84 and 86 are utilized for yellow and magenta development.
The exposure criteria is met when copies of color samples 80 and 84
have a very light density toner powder image thereon and copies of
color samples 82 and 86 remain completely devoid of toner
particles. Contrawise, if no development occurs in color samples 80
or 84, an over-exposed condition exists. When an over-exposed
condition exists, the printing machine exposure is decreased until
the exposure criteria is met. The slit number and F/stop is
recorded on the copy that meets the exposure criteria. This slit is
termed the separation slit. Each succeeding F/stop and slit is
adapted to reduce exposure and raise image potential. Exposure is
continually reduced until development is within the standards
prescribed by color samples 80 and 84. If, however, light
development occurs in the no development color samples 82 and 86,
an under-exposed condition exists. For each copy that indicates an
under-exposed condition a plurality of copies are run wherein
exposure is increased between each copy. Each succeeding set of
F/stops and slits increases image exposure and reduces image
potential. Image exposure is continually increased until correct
development is indicated, as evidenced by no development color
samples 82 and 86. The slit number and F/stop on the copy that
meets the exposure criteria is noted once again, this slit is
termed the separation slit. After determining the F/stop and
separation slit numbers that satisfy the exposure criteria for each
of the colors, the condition of the F/stop and slit number
resulting in the largest exposure is selected. This slit is termed
the run slit. For example, the F/stop for cyan is 4.5 and the
separation slit is slit number 4, the F/stop number for magenta is
4.5 and the separation slit is slit number 8, and the F/stop number
for yellow is 6.3 and the separation slit is slit number 2. The
condition representing the largest amount of light F/4.5 slit
number 8, the cyan example, indicates the F/stop and run slit. The
run slit is one that will be left on the machine in the operating
mode. The F/stop associated with the run slit is the proper lens
setting for the printing machine. This is a combination that
provides the maximum allowable exposure energy. Having selected the
appropriate F/stop and slit, the associate neutral density filters
for each of the colored filters, i.e. red, blue and green, must be
chosen to balance the exposure for the remaining color separations
which do not require the same amount of exposure energy. The values
of the neutral density filters are determined by calculating the
slit difference from the following formula:
Slit Difference = (Run Slit Number + Correction factor) -
Separation Slit.
The correction factor may be determined from Table 2.
TABLE 2
Run Separation Slit Slit Correction At At Factor F/4.5 F/4.5 0
F/4.5 F/5.6 9 F/4.5 F/6.3 14 F/4.5 F/8 20 F/4.5 F/11 33 F/5.6 F/5.6
0 F/5.6 F/6.3 5 F/5.6 F/8 14 F/5.6 F/11 24 F/6.3 F/8 9 F/6.3 F/11
19 F/8 F/8 0 F/8 F/11 10 F/11 F/11 0
The correction factor is required due to an overlap in exposure for
some combination of F/stop and slit numbers. The correction factors
of Table 2 may be evaluated from the overlap of available exposure
energy for different combinations of F/stop and slit numbers. Table
3 only presents the combination of slit numbers with F/stops 4.5,
5.6 and 6.3. The correction factors for F/stops of 8 and 11, as
shown in Table 2, may be calculated in a similar fashion. An F/stop
of 4.5 with the largest slit, i.e. slit number 10, is treated as
furnishing the maximum available energy, i.e. 100 percent.
TABLE 3
Percent of Slit No. Slit No. Slit No. Available for F/4.5 for F/4.5
for F/6.3 Exposure Energy 100.0 10 -- -- 95.4 9 -- -- 90.6 8 -- --
86.3 7 -- -- 82.1 6 -- -- 78.1 5 -- -- 74.4 4 -- -- 70.7 3 -- --
67.4 2 -- -- 64.1 1 10 -- 61.5 -- 9 -- 58.6 -- 8 -- 55.7 -- 7 --
53.1 -- 6 -- 50.4 -- 5 10 48.1 -- 4 9 45.7 -- 3 8 43.6 -- 2 7 41.4
-- 1 6 39.1 -- -- 5 37.2 -- -- 4 35.4 -- -- 3 33.7 -- -- 2 32.1 --
-- 1
Referring now to Table 3, it is shown therein that the overlap in
exposure between F/stop 4.5 and F/stop 5.6 occurs at 64.1 percent
of available exposure energy. Slit number 1 and F/stop 4.5 provide
64.1 percent of the available exposure energy, as does slit number
10 and F/stop 5.6. Thus, the correction factor for a run slit
number at an F/stop of 4.5 and a separation slit number at an
F/stop of 5.6 is 9, i.e. 10-1. Similarly, the overlap between
F/stop 5.6 and F/stop 6.3 occurs at 50.4 percent of the available
exposure energy and the correction factor is 5, i.e. 10-5. The
correction factor from an F/stop of 4.5 to an F/stop of 6.3 is
determined by summing the correction factor of F/4.5 to F/5.6 with
the correction factor of F/5.6 to F/6.3, i.e. 9+5, to obtain a
correction factor of 14 as indicated in Table 2. In this manner,
the various correction factors of Table 2 may be determined for
utilization in the preceeding formula for determining the slit
difference. Having determined the slit difference, the neutral
density filter may be determined from Table 4. The neutral density
filters of Table 4 are determined from the transformation of
percent transmission to density. The values are rounded to the
nearest 0.05 neutral density filter so as to be consistent with the
required sensitivity and to facilitate manufacture.
TABLE 4
Neutral Slit Density Difference Filter 0 None 1 None 2 0.05 3 0.10
4 0.10 5 0.10 6 0.15 7 0.15 8 0.15 9 0.20 10 0.20 11 0.25 12 0.25
13 0.25 14 0.30 15 0.30 16 0.35 17 0.35 18 0.40 19 0.40 20 0.40
For example, if the run slit is slit number 2, at an F/stop of 4.5
and the separation slit for magenta is slit number 4 at an F/stop
of 6.3, the correction factor from Table 2 will be 14 and the slit
difference will be 12. The required neutral density filter may be
determined from Table 4. For a slit difference of 12, an 0.25
neutral density filter is required. The appropriate neutral density
filters are inserted in the machine in filter mechanism 20. Lens 18
is set to the F/stop heretofore determined and the appropriate run
slit is also inserted in the machine. Thereafter, the developer
unit electrical bias for cyan is adjusted to 410 volts DC, for
magenta to 450 volts DC, and for yellow to 450 volts DC for final
calibration.
The printing machine is, then, finally calibrated for optimum color
balance. This is achieved by adjusting the electrical biases within
each of the developer units 28, 30 and 32, respectively. A
plurality of multi-color copies are produced with color standard 50
utilized as original document 22 in the printing machine of FIG. 1.
If development occurs in color samples 74, 76 and 78 the
appropriate developer bias is raised in increments of 10 volts and
additional copies run to verify that no development occurs in color
samples 74, 76 and 78, respectively. If the color of the copies
from color samples 68, 70 and 72 are as pure (not contaminated), as
the color in color samples 62, 64 and 66, the appropriate developer
bias is decreased in increments of 20 volts until the image is
acceptable. Once the copies of color samples 68, 70 and 72 and 74,
76 and 78, respectively, are satisfactory, the multi-color
electrophotographic printing machine is calibrated. However, if the
foregoing criteria cannot be achieved by adjusting the developer
bias of the respective developer units, the developability
regulating system or exposure system must be readjusted in
accordance with the procedure hereinbefore described.
It will be evident to one skilled in the art that a printing
machine utilizing black, red and cyan toner particles will require
an array of color samples including light blue, pure red, and
contaminated red as one set thereof, while light red, pure blue and
contaminated blue is the second set thereof.
In recapitulation, it is apparent that the color standard of the
present invention may be utilized in a multi-color
electrophotographic printing machine to provide appropriate
calibration therefore. In use, the color standard permits the
calibration of the developability regulating mechanism, the
selection of the appropriate lens F/stop number as well as the
appropriate exposure slit to be utilized in conjunction therewith
in the image exposure system. Moreover, the color standard permits
the electrical bias of the development system to be adjusted to the
proper levels so as to optimize copies being reproduced in the
printing machine.
Thus, it is apparent that there has been provided in accordance
with the present invention, a color standard and method of use
therefore that fully satisfies the objects, aims and advantages set
forth above. While the invention has been described in conjunction
with specific embodiments and methods of use, it is evident that
many alternatives, modifications and variations will be apparent to
those skilled in the art in light of the foregoing 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.
* * * * *