Note: Descriptions are shown in the official language in which they were submitted.
~ 36653
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 therefor in which ~
the multi-color electrophotographic printing machine is cali- ~ -
brated 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 speci-
fied 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
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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 cali-
bration 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 opera-
tion. -
SUMMARY OF THE INVENTI ON
In accordance with one aspect of this invention i '
there is provided a method of calibrating a multi-color electro-
photographic printing machine of the type having a develop-
ability 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
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copy is intermediate the maximum and minimum density ranges of
the color standard.
In accordance with another aspect of this inven-
tion there is provided a color standard for calibrating a multi-
color electrophotographic printing machine, including: a general-
ly 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 corres-
ponding 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
permisslble density range for each of said colors being reprodu-
ced by the multi-color electrophotographic printing machine.
36653
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present inven-
tion will become apparent upon reading the following detailed
description and upon reference to the drawings, in which:
Figure 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;
Figure 2 is a front elevational view of the color -
standard utilized for calibrating the Figure 1 printing machine;
and
Figure 3 is a back elevational view of the
Figure 2 color standard.
While the present invention will be described in ~-
connection with the preferred embodiment and method of ~
use therefor, 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 desig~
nate like elements, Figure 1 schematically depicts an electro-
photographic printing machine in which the present invention
may be utilized for the calibration thereof. The electro-
photographic printing machine illustrated schematically
in Figure 1 shows the various components utilized therein
for producing multi-color copies from a colored original.
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103~653
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.
Patent 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 Figure 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. Prefer-
ably, corona generating device 16 is of a type described
in U.S. Patent 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
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moving lens system is disclosed in U.S. Patent No. 3,062,108
issued to Mayo in 1962, and a suitable color filter mechanism
is described in U.S. Patent No. 3,775,006 issued November
27, 1973. 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 appro-
priate neutral density filter, if required, is selected to
correspond with its respective colored filter.
With continued reference to Figure 1, an original
document 22, such as a sheet of paptr, 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 respec-
tive 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
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20 and lens 18. The exposure slit 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 sur-
face 12 to development station C. Development station C
includes thereat three individual developer units, generally
designated by the reference numerals 28, 30 and 32, respect-
ively. A suitable development station employing a plurality
of developer units is disclosed in U.S. Patent No. 3,854,449
issued December 17, 1974. 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 photo-
conductive 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 adher-
ing 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 ~
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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, indi-
cated 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 develop-
ment, 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 refer-
ence to generate a logic control signal for dispensing
selected toner particles into the corresponding developer
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unit. The logic elements, preferably, include a suitable
discriminator circuit for comparing the reference with the
electrical output signal from photosensor 42. The discrimin-
ator 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 tùe magnitude of the reference in the develop- ~
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1~36653
ability 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 U.S. Patent No. 3,754,821 issued August 22, 1973.
Drum 10 is next rotated to transfer station D where
the toner powder image adhering electrostatically to photo-
conductive 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. Patent 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
registratio~ 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. Patent 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
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1C)366S3
tray (not shown) for subseque~1t 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 mechan-
ically cleaned from photoconductive surface 12 by a rotatably
mounted fibrous brush 48. A suitable brush cleaning device
is described in U.S. Patent 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 sur-
face 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 illu-
strate 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, Figures 2 and 3 depict the front and back
elevational views of the color standard of the present inven-
tion. As shown in Figure 2, color standard 50 includes a
generally planar support member 52. Color standard 50 has
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1~36~3
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 Figure 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 compatible with the sensitometric response
of the printing machine depicted in Figure 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 compar-
able to the colors of color samples 62, 64 and 66, this indi-
cates 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
0 and 78 are light blue, light pink and light yellow, while
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color samples 62, 64 and 66 are pure red, pure green, and pureblue, 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 Figure 2, exposure color
sample 80 is adapted to indicate an over-exposed condition,
while exposure color sample 82 is adapted to indicate an
under-exposed 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 under-exposed 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 therefor.
Thus, if, in the copy, no development occurs in exposure l~
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. l printing
machine.
Referring now to Figure 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
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to indicate the maximum acceptable density yellow image repro-
duction 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 m~ximum cyan density
for a copy and also lncludes a hole 106 therein for facilitat-
ing 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 llO therein for readily enabling the copy to be compared
therewith. Thus, color standard 50 may be disposed upon platen
24 (Figure l) and utilized as original 22 to have copies repro-
duced 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 character-
istics 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 calibra-
tion mode and suitably adjusted with the color standard.
Utilization of color standard 50 to calibrate the printing
machine of Figure l insures that multi-color copies produced
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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 Figure 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 Figure 1 with the color standard depicted
in the Figures 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 there-
with an exposure slit associated with lamps 26 in order to
provide incremental width exposure of original document 22.
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The exposure slit is configured in the shape of a butterfly to
correct for the cos4 (see U.S. Patent No. 3,669,538 issued
June 13, 1972) light attenuation effect through lens 18
and thereby provide a substantially uniform illumination inten-
sity 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 electri-
cal bias of developer units 28, 30 and 32 are adjusted.
Preferably, the developer electrical bias for the cyan devel-
oper unit is set at about 470 volts DC, the electrical biasfor 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 in 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 electrophoto-
graphic printing machine is now ready to be calibrated.
Initially, the developability regulating mechanism
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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 (Figure 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 satis~
factory. 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 satis-
factory, 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 repro-
duced until the copy of color sample 54 has a density whichlies 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 Figure 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
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~(~3~653
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 (Figure 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 re-
peated. 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 devel-
opability regulating mechanism is calibrated. However, if
the copies are not within specification, a developer unit
problem is identified, and the appropriate repair proce-
dure indicated. This calibration procedure is repeated for
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magenta and yellow toner particles.
Next, the exposure balance of the system is cali- ;
brated. This is achieved by setting lens 18, preferably to
an F 5.6 stop and replacing the set-up exposure slit prefer-
ably 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
11.006 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
Figure 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 develop-
ment. The exposure criteria is met when copies of color ;~
samples 80 and 84 have a very light density toner powder
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lQ36653
image thereon and copies of color samples 82 and 86 remaincompletely 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 ex-
posure and reduces image potential. Image exposure is con-
tinually 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 ex-
posure 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
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1(~366~i3
.
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,
iOe. red, blue and green, must be chosen to balance the ex-
posure for the remaining color separations which do not re-
quire 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.
'~.: ''"' ''' '''''.
.: : .
~ ~
. : .
:,- .
: -
-21- -
.
~)3~6S3
Table 2
l ' - ''
Run Separation Correction
Slit At Slit 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/ll 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/ll 24
F/6.3 F/8 9
F/6.3 F/ll 19
F/8 F/8 0
F/8 F/ll 10
F/ll F/ll
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 pre-
sents 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%.
- :
1(~36653
Table 3
I - .
Percent of Slit No. Slit No. Slit No.
Available for F/4.5 for F/4.5 for F/6.3
Exposure
Energy
1000 10 _ _ ~. ~
7~ 1 5
67.4 2 _ _
64.1 1 10 _ ~ - ~
61.5 _ 9 _ . :
58 6 _ 8 _
53.1 _ 6
48.1 _ 54 10
45 7 _ 2 8
1 "~ ~ ~
' ~ - ' '
- . :.~ . . . .: - .:
- .. - . .. . . ... . .. ..
1~3~65;3
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 a~ailable
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 correc-
tion 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. g+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 preceding 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 near-
est 0.05 neutral density filter so as to be consistent with
the required sensitivity and to facilitate manufacture.
Table 4
Slit Neutral ;
Difference Density
Filter
,. . .
0 None
1 None
2 0.05
3 0.10
4 0.10
0.10
6 0.15
7 0.15
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1~366$3
Table 4 (cont)
, Slit Neutral
Difference Density :
Filter
.:
8 0.15 ~ -
9 0.20
0.20 ::
11 0.25 ~ :
12 0.25 ~ :
13 0.25 :
14 0.30 ~ :
0.30 ; :
16 0.35
17 0.35
18 0.40 : .
19 0.40 : . -.
. 0.40 i~
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.
-25-
The printing malc~l3ne 61s, 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 Figure 1. If development
occurs in color samples 74, 76 and 78 the appropriate devel-
oper 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 print-
ing 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 part-
icles 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 appro-
-26-
- 1~)366S3
priate calibration therefor. 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 util-
ized 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 therefor 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, modifi-
cations 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.
'' ~. '.
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.~ . - .. , . ~ ,
