U.S. patent application number 13/244171 was filed with the patent office on 2012-01-26 for method for calibrating color image forming apparatus.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to HIROKI TEZUKA.
Application Number | 20120019850 13/244171 |
Document ID | / |
Family ID | 35731811 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120019850 |
Kind Code |
A1 |
TEZUKA; HIROKI |
January 26, 2012 |
METHOD FOR CALIBRATING COLOR IMAGE FORMING APPARATUS
Abstract
A calibration method for calibrating a color image forming
apparatus includes detecting the color of a patch formed on a
recording medium by the color image forming apparatus using a color
sensor, converting a detected color signal in a first color
specification system into a color signal in a second color
specification system, and adjusting at least one of the image
forming conditions for the color image forming apparatus, based on
the color signal converted in the conversion step, where the
conversion conditions in the color signal conversion are different
depending on the attributes of the patch.
Inventors: |
TEZUKA; HIROKI;
(Shinagawa-ku, JP) |
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
35731811 |
Appl. No.: |
13/244171 |
Filed: |
September 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11187808 |
Jul 25, 2005 |
8040581 |
|
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13244171 |
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Current U.S.
Class: |
358/1.9 |
Current CPC
Class: |
H04N 1/6033
20130101 |
Class at
Publication: |
358/1.9 |
International
Class: |
H04N 1/60 20060101
H04N001/60 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 28, 2004 |
JP |
2004-219971 |
Claims
1.-9. (canceled)
10. A color image forming apparatus comprising: a sheet discharging
portion to which a recording medium is discharged after an image
formation; a forming portion that forms a plurality of fixed
patches in a row on the recording medium; a detecting portion
disposed on a recording medium conveying path to detect, before the
recording medium is discharged to the sheet discharging portion,
colors of the plurality of fixed patches formed on the recording
medium; a converting portion that converts a color signal, obtained
by the detecting portion and corresponding to a detection signal in
a first color specification system, into a color signal in a second
color specification system as color signal conversion processes;
and an adjusting portion that adjusts an image forming condition
for the color image forming apparatus, based on the color signal in
the second color specification system, wherein the plurality of
fixed patches include plural kinds of patches, an order of
formation and an order of detection of the plural kinds of patches
being predetermined, and the converting portion uses a parameter of
each of the plurality of fixed patches to be subject to color
measurement to perform the color signal conversion processes for
the respective patches based on the order of detection or detection
positions of the plurality of fixed patches.
11. A color image forming apparatus according to claim 10, wherein
the color signal conversion processes include selecting a matrix
used in converting the color signal in the first color
specification system into the color signal in the second color
specification system, or changing a look-up table used in
converting the color signal in the first color specification system
into the color signal in the second color specification system.
12. A color image forming apparatus according to claim 10, wherein
the plurality of fixed patches include a monochromatic patch formed
with only black and a color mixture patch formed by yellow,
magenta, and cyan.
13. A method of adjusting a color image forming apparatus,
comprising: forming a plurality of patches in a row on a recording
medium; fixing the plurality of patches to the recording medium;
detecting colors of the plurality of patches fixed to the recording
medium in a medium conveying path before the recording medium is
discharged to a sheet discharging portion; converting a color
signal, obtained in the detecting and corresponding to a detection
signal in a first color specification system, into a color signal
in a second color specification system as color signal conversion
processes; and adjusting an image forming condition for the color
image forming apparatus, based on the color signal in the second
color specification system, wherein the plurality of patches fixed
to the recording medium include plural kinds of patches, an order
of formation and an order of detection of the plural kinds of
patches being predetermined, and the converting uses a parameter of
each of the plurality of patches fixed to the recording medium to
be subject to color measurement to perform the color signal
conversion processes for the respective patches based on the order
of detection or detection positions of the plurality of patches
fixed to the recording medium.
14. A method according to claim 13, wherein the color signal
conversion processes include selecting a matrix used in converting
the color signal in the first color specification system into the
color signal in the second color specification system, or changing
a look-up table used in converting the color signal in the first
color specification system into the color signal in the second
color specification system.
15. A method according to claim 13, wherein the plurality of
patches fixed to the recording medium include a monochromatic patch
formed with only black and a color mixture patch formed by yellow,
magenta, and cyan.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for calibrating a
color image forming apparatus such as a color printer or a color
copier, and more particularly to a color signal conversion method
for measuring a test chart outputted to improve the color stability
of a color image forming apparatus using a chromaticity detecting
means, and converting a color signal in a first color specification
system that is the detection result of the chromaticity detecting
means into a color signal in a second color specification
system.
[0003] 2. Background of the Related Art
[0004] Recently, there has been increasing demand for a color image
forming apparatus of the electrophotography type or the inkjet
type, such as a color printer or a color copier, having high color
stability of its output image.
[0005] Thus, the color image forming apparatus having a sensor for
detecting the chromaticity of a patch on the recording medium after
the forming and fixing of a monochromatic gradation patch of cyan
(C), magenta (M), yellow (Y) or black (K) or a mixed color patch in
which CMY are mixed on the recording medium (hereafter referred to
as a color sensor) is well known (e.g., refer to U.S. Patent
Application Publication No. 2003/049040).
[0006] In this color image forming apparatus, the color stability
of a final output image formed on the recording medium is
controlled by feeding back the detected result to a calibration
table for correcting the exposure amount, the process conditions
and the color gradation characteristics of an image forming
portion. Also, the output image of the color image forming
apparatus may be detected by an external image reading apparatus or
a chromaticity meter to make the same control.
[0007] This color sensor uses a light emitting element having three
or more kinds of light sources with different emission spectra of
red (R), green (G) and blue (B), respectively, and a light
receiving element with sensitivity in the visible region, or a
light emitting element having a light source that emits light of
white color (W) and a light receiving element formed with three or
more filters of different spectral transmittances. Thereby, three
or more kinds of outputs such as the RGB outputs are obtained.
[0008] In the color image forming apparatus of the ink jet type,
since the color balance changes depending on a change in the ink
discharge amount with the lapse of time, an environmental
difference from one place or time of use to another, or the
individual differences among ink cartridges, the color gradation
characteristics cannot be kept constant. Thus, some color image
forming apparatuses effect color stabilization control by
substituting a color sensor for the ink head and detecting the
chromaticity of a patch on the recording medium.
[0009] In the above color stabilization control, there is a process
of converting the sensor outputs, which are RGB values, into XYZ
chromaticity values as defined by the International Commission on
Illumination (CIE). For this conversion, the prior art has used a
method employing a matrix, as well as a method using a look-up
table.
[0010] However, the above prior art had the following problems.
[0011] Generally, the color matching functions for the spectral
sensitivity of RGB outputs of the color sensor and the XYZ
chromaticity values as defined by the International Commission on
Illumination do not have completely linear relations. Therefore,
there is a problem that some differences may occur between the XYZ
chromaticity values obtained by converting the RGB outputs of the
sensor and the XYZ chromaticity values obtained from the spectral
reflectance as defined by the International Commission on
Illumination.
[0012] That is, the XYZ chromaticity values calculated from the RGB
output signals of the sensor for a certain patch and the XYZ
chromacity values calculated from the spectral reflectance for the
patch as defined by the International Commission on Illumination
may be different in some cases. And this difference between the
chromaticity values may vary in magnitude, depending on the color
material or substratum color of the patch used in forming the
patch.
SUMMARY OF THE INVENTION
[0013] This invention has been achieved in the light of the
above-mentioned problems, and it is an object of the invention to
provide a color signal conversion method for reducing the
differences occurring between the XYZ chromaticity values obtained
by converting the RGB outputs of the sensor and the XYZ
chromaticity values obtained from the spectral reflectance as
defined by the International Commission on Illumination in a simple
manner when measuring a test chart outputted by the color image
forming apparatus.
[0014] In order to accomplish the above object, the invention
provides a color image calibration method as defined in the
claims.
[0015] With this invention, it is possible to reduce the
differences occurring between the XYZ chromaticity values obtained
by converting the RGB outputs of the sensor and the XYZ
chromaticity values obtained from the spectral reflectance as
defined by the International Commission on Illumination by changing
various kinds of parameters used in converting the RGB outputs of
the sensor into the XYZ chromaticity values depending on the
attribute of patch. And the precision of color stability control
using the XYZ chromaticity values can be improved.
[0016] Other objects, constitutions and effects of the invention
will be apparent from the following detailed description of the
invention and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a flowchart for converting the RGB outputs into
XYZ chromaticity values according to an embodiment 1;
[0018] FIG. 2 is a diagram showing the overall configuration of a
color image forming apparatus according to embodiment 1;
[0019] FIG. 3 is a view showing the electrophotographic color image
forming apparatus in cross section according to embodiment 1;
[0020] FIG. 4 is a block diagram showing the configuration of a
color sensor and its peripheral devices according to embodiment
1;
[0021] FIG. 5 is a view showing the color sensor in cross section
according to embodiment 1;
[0022] FIG. 6 is a diagram showing one example of the test chart in
embodiment 1;
[0023] FIG. 7 is a diagram (1) showing a connection method between
color image forming apparatus and color image reading apparatus
according to an embodiment 2;
[0024] FIG. 8 is a diagram (2) showing a connection method between
color image forming apparatus and color image reading apparatus
according to embodiment 2;
[0025] FIG. 9 is a view showing one example of a color image
reading apparatus according to embodiment 2;
[0026] FIG. 10 is a view showing one example of the test chart
according to embodiment 2; and
[0027] FIG. 11 is a flowchart for converting the RGB outputs to XYZ
chromaticity values according to an embodiment 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The best mode for carrying out the present invention will be
described below in detail with reference to the accompanying
drawings.
Embodiment 1
[0029] FIG. 2 is a diagram showing the overall configuration of an
electrophotographic color image forming apparatus according to
embodiment 1. This color image forming apparatus comprises an image
processing portion and an image forming portion.
[0030] First, a process in the image processing portion will be
described below. With a color matching table 111, the RGB signals
representing the colors of an image sent from a personal computer
are converted into device RGB signals (hereafter referred to as
"DevRGB") in conformance with a color reproduction range of the
color image forming apparatus. Then, the converted DevRGB signals
are converted into the CMYK signals indicating the colors of toner
color materials for the color image forming apparatus, using a
color separation table 112. A calibration table 113 is a table for
correcting the density-gradation characteristics intrinsic to the
color image forming apparatus and employed to convert the CMYK
signals into the C'M'Y'K' signals in which the density-gradation
characteristics are corrected. Moreover, with a PWM (Pulse Width
Modulation) table 114, the C'M'Y'K' signals are converted into the
exposure times Tc, Tm, Ty and Tk of corresponding scanner portions
24C, 24M, 24Y and 24K (see FIG. 3).
[0031] Next, the image forming portion will be described. Main
means involved in the image formation include charging means 122,
exposing means 123, developing means 124, transferring means 125
and fixing means 126, which are controlled by a CPU 121. Moreover,
a color sensor 42 is connected to the CPU.
[0032] FIG. 3 is a cross-sectional view of the color image forming
apparatus. This apparatus is the color image forming apparatus of
tandem type employing an intermediate transfer member 28, which is
one example of the electrophotographic color image forming
apparatus, as shown in FIG. 3. Referring to FIG. 3, the operation
of the image forming portion in the electrophotographic color image
forming apparatus will be described below.
[0033] The image forming portion forms an electrostatic latent
image with exposing light applied based on an exposure time
converted by the image processing portion, forming the
monochromatic toner images by developing this electrostatic latent
image, forming a multi-color toner image by superposing the
monochromatic toner images, transferring this multi-color toner
image onto a recording medium 11, and fixing the multi-color toner
image on the recording medium.
[0034] The charging means 122 comprises four injection charge
devices 23Y, 23M, 23C and 23K for charging the photosensitive
members 22Y, 22M, 22C and 22K at the stations of yellow (Y),
magenta (M), cyan (C) and black (K), the injection charge devices
having respective sleeves 23YS, 23MS, 23CS and 23KS.
[0035] The photosensitive members 22Y, 22M, 22C and 22K have an
organic photoconductive layer applied on the outer periphery of an
aluminum cylinder, and are rotated by a driving force of a drive
motor, not shown. The photosensitive members 22Y, 22M, 22C and 22K
are rotated in a counterclockwise direction along with an image
forming operation by the drive motor.
[0036] The exposing means 123 applies exposing light from the
scanner portions 24Y, 24M, 24C and 24K onto the photosensitive
members 22Y, 22M, 22C and 22K, selectively exposing the surfaces of
the photosensitive members 22Y, 22M, 22C and 22K to form the
electrostatic latent images.
[0037] The developing means 124 comprises four developing devices
26Y, 26M, 26C and 26K for developing the images of yellow (Y),
magenta (M), cyan (C) and black (K) at respective stations to
visualize the electrostatic latent images, in which the developing
devices are provided with the sleeves 26YS, 26MS, 26CS and 26KS.
Each developing device 26 can be detachably attached.
[0038] The transferring means 125 transfers the monochrome toner
images, along with the rotation of the photosensitive members 22Y,
22M, 22C and 22K and the primary transfer rollers 27Y, 27M, 27C and
27K located oppositely, by rotating the intermediate transfer
member 28 in a clockwise direction to transfer the monochrome toner
images from the photosensitive members 22 to the intermediate
transfer member 28. By applying an appropriate bias voltage to the
primary transfer rollers 27 and giving a difference between the
rotating speed of the photosensitive members 22 and the rotating
speed of the intermediate transfer member 28, the monochrome toner
images are transferred onto the intermediate transfer member 28
efficiently. This operation is called a primary transfer.
[0039] Moreover, the transferring means 125 superposes the
monochrome toner images on the intermediate transfer member 28 at
respective stations, conveys the superposed multi-color toner image
up to a secondary transfer roller 29 along with the rotation of the
intermediate transfer member 28, picks up and conveys a recording
medium 11 from a sheet feeding tray 21 to the secondary transfer
roller 29, and transfers the multi-color toner image on the
intermediate transfer member 28 onto the recording medium 11. The
toner image is electrostatically transferred by applying an
appropriate bias voltage to the secondary transfer roller 29. This
operation is called a secondary transfer. The secondary transfer
roller 29 contacts the recording medium 11 at a position 29a, while
transferring the multi-color toner image onto the recording medium
11, and is spaced to a position 29b after the printing process.
[0040] The fixing means 126 comprises a fixing roller 32 for
heating the recording medium 11 and a pressure roller 33 for
pressing the recording medium 11 onto the fixing roller 32 to fuse
and fix the transferred multi-color toner image on the recording
medium 11. The fixing roller 32 and the pressure roller 33 are
hollow and internally comprise heaters 34 and 35, respectively. A
fixing apparatus 31 conveys the recording medium 11 holding the
multi-color toner image onto the fixing roller 32 and the pressure
roller 33, and fixes the toner on the recording medium by applying
heat and pressure.
[0041] The recording medium 11 after the fixing of the toner is
then discharged to a sheet discharge tray, not shown, by a sheet
discharge roller, not shown, whereby the image forming operation is
ended.
[0042] Cleaning means 30 cleans the toner remaining on the
intermediate transfer member 28, in which after the transfer of the
multi-color toner image of four colors formed on the intermediate
transfer member 28 onto the recording medium 11, waste toner is
stored in a cleaner container.
[0043] A color sensor 42 is disposed downstream of the fixing
apparatus 31 on a conveying path of the recording medium, opposed
to an image forming face of the recording medium 11, detecting the
color of a mixed color patch that, after fixing, is formed on the
recording medium 11. This detection process is for the purpose of
outputting the RGB values. Disposed inside the color image forming
apparatus, the color sensor can automatically detect the color
before the sheet with image fixed is discharged to a sheet
discharging portion.
[0044] FIG. 4 is a block diagram showing the configuration of the
color sensor 42 and its peripheral devices. The color sensor and
its peripheral devices are a light emitting element 101, a light
receiving element 102, an A/D converter 104 and a CPU 121. The
light emitting element 101 is a light source of the color sensor
that emits light to a measurement object 103. Then, irregular light
is reflected, as the reflection factor depends on a body color of
the measurement object. The irregular reflected light enters the
light receiving element 102 that converts light into an electric
signal. Moreover, an analog electric signal is converted into a
digital electric signal by the A/D converter 104. And the digital
electric signal is taken into the CPU 121, and the XYZ chromaticity
values are outputted through a linear conversion process as shown
in formula (1).
[0045] FIG. 5 is a cross-sectional view of the color sensor 42. The
color sensor 42 employs a white color LED 53 as the light emitting
element 101 and a charge storage type sensor 54a with on-chip
filters for three or more colors, such as RGB, as the light
receiving element 102. Light from the white color LED 53 is made
incident obliquely at an angle of 45.degree. upon the recording
medium 11 on which is formed the patch after fixing, and the
intensity of irregular reflected light in a direction of 0.degree.
is detected by the charge storage type sensor 54a with the RGB
on-chip filters. A light receiving portion of the charge storage
type sensor 54a with RGB on-chip filters has independent pixels of
RGB like 54b. The light receiving element 102 may be a photodiode.
A set of three pixels of RGB may be arranged multiply. Also,
instead of the mentioned arrangement, the angle of incidence may be
0.degree. and the angle of reflection may be 45.degree.. Moreover,
an LED for emitting light of three or more colors such as RGB and a
sensor without filter may be combined.
[0046] FIG. 6 is a diagram showing one example of a test chart
detected by the color sensor 42. A color stabilization control test
chart 60 is a gradation patch pattern of gray that is the most
important color in making the color balance, and composed of a gray
gradation patch 61 of only black (K) and a process gray gradation
patch 62 in which yellow (Y), magenta (M) and cyan (C) are mixed.
The gray gradation patch 61 of only black (K) and the process gray
gradation patch 62, which have the same chromaticity in the image
processing portion of the image forming apparatus, are paired, such
as 61a and 62a, 61b and 62b, 61c and 62c. The chromaticity of this
patch is detected by the color sensor 42, and is fed back to a
calibration table so that there may be no color difference between
the gray gradation patch 61 of only black (K) and the process gray
gradation patch 62 which are paired.
[0047] To convert the RGB outputs of the sensor into the XYZ
chromaticity values, the following formula (1) is employed:
( X Y Z ) = A ( r g b ) = ( a 11 a 12 a 13 a 21 a 22 a 23 a 31 a 32
a 33 ) ( r g b ) ( 1 ) ##EQU00001##
[0048] where XYZ are XYZ chromaticity values calculated by
converting the RGB outputs of the sensor, r, g and b are sensor
outputs, A is a conversion matrix, and a is a matrix element.
[0049] In the following, a color conversion method for reducing the
differences between the XYZ chromaticity values obtained by
converting the RGB outputs of the sensor in this embodiment and the
XYZ chromaticity values obtained from the spectral reflectance as
defined by the International Commission on Illumination is
given.
[0050] This method involves changing the matrix A in formula (1)
for every attribute of patch. A gray gradation patch matrix A1 of
only black (K) and a process gray gradation patch matrix A2 are set
up. The matrices A1 and A2 are optimized to convert the RGB outputs
of the sensor detecting the patch of each attribute into the XYZ
chromaticity values.
[0051] FIG. 1 is a flowchart for converting the RGB outputs of the
sensor into the XYZ chromaticity values.
[0052] In step 211, it is determined whether or not the detected
patch is gray gradation patch 61 of only black (K) or process gray
gradation patch 62. Since the patch format of the color
stabilization control test chart 60 is fixed in the image forming
apparatus, the determination may be made in the sequence of
detecting the patch.
[0053] If it is determined in step 211 that the detected patch is a
gray gradation patch 61 of only black (K) because the patch is
detected at an odd number in the color stabilization control test
chart 60, the RGB outputs of the sensor are converted into the XYZ
chromaticity values, using the gray gradation patch matrix A1 of
only black (K), in accordance with formula (1), in step 212.
[0054] If it is determined in step 211 that the detected patch is
the process gray gradation patch 62, because the patch is detected
at an even number in the color stabilization control test chart 60,
the RGB outputs of the sensor are converted into the XYZ
chromaticity values, using the process gray gradation patch matrix
A2, in accordance with formula (1), in step 213.
[0055] Next, the experimental results are shown below in which the
differences between the XYZ chromaticity values obtained by
converting the RGB outputs of the sensor and the XYZ chromaticity
values obtained from the spectral reflectance as defined by the
International Commission on Illumination could be reduced by
changing the matrix A in formula (1) for every attribute of
patch.
[0056] In the experiment, because the relations between the
spectral sensitivity of the RGB outputs of the color sensor and the
XYZ chromaticity values as defined by the International Commission
on Illumination are not completely linear, formula (2) using the
RGB outputs of the sensor up to the third order is employed,
instead of formula (1), to decrease the influence of not completely
linear relations as much as possible:
( X Y Z ) = A ( r g b r 2 g 2 b 2 r 3 g 3 b 3 ) = ( a 11 a 12 a 13
a 14 a 15 a 16 a 17 a 18 a 19 a 21 a 22 a 23 a 24 a 25 a 26 a 27 a
28 a 29 a 31 a 32 a 33 a 34 a 35 a 36 a 37 a 38 a 39 ) ( r g b r 2
g 2 b 2 r 3 g 3 b 3 ) ( 2 ) ##EQU00002##
[0057] The number of measured patches is about 250 for the gray
gradation patch of only black, and about 500 for the process gray
gradation patch. The results are shown in Table 1:
TABLE-US-00001 TABLE 1 Experimental results Gray gradation patch
Process gray .DELTA.E(Ave.) of only black gradation patch Matrix A
is changed for 0.53 0.99 every attribute of patch Matrix A is
common for 3.90 1.53 all patches
[0058] The numerical values as listed in the table are average
values of chromaticity value differences occurring between the XYZ
chromaticity values obtained by converting the RGB outputs of the
sensor and the XYZ chromaticity values obtained from the spectral
reflectance as defined by the International Commission on
Illumination. Both the XYZ values are converted into L*a*b* as
defined by the International Commission on Illumination and then
calculated as the color difference (.DELTA.E).
[0059] The above results reveal that the method for changing the
matrix A for every attribute of patch that is the kind of color
material for use, whether the measured patch is gray gradation
patch of only black or process gray gradation patch, can make
smaller the color difference (.DELTA.E) occurring between the XYZ
chromaticity values obtained by converting the RGB outputs of the
sensor and the XYZ chromaticity values obtained from the spectral
reflectance as defined by the International Commission on
Illumination. This is because the non-linearity of the color
matching functions for the spectral sensitivity of the RGB outputs
of the color sensor and the XYZ chromaticity values as defined by
the International Commission on Illumination is reduced in the
extent of influence by changing the matrix A for every attribute of
patch.
[0060] Accordingly, it is possible to reduce the color differences
between the XYZ chromaticity values obtained by converting the RGB
outputs of the sensor and the XYZ chromaticity values obtained from
the spectral reflectance as defined by the International Commission
on Illumination by employing the color conversion method for
changing the matrix A for every attribute of patch.
[0061] This method is based on the premise that the attribute of a
patch can be determined when detecting the patch. In an image
forming apparatus having a color sensor, as described, because the
patch is detected in the sequence of forming the image, the
attribute of patch detected by the color sensor can be judged, and
thus this method is applicable.
[0062] Though the RGB outputs of the sensor are converted into the
XYZ chromaticity values here, it is apparent that the method as
described in connection with this embodiment is effective when the
color matching function in two different color specifications
systems has non-linearity.
[0063] Moreover, there are two attributes here, including the gray
gradation patch of only black (K) and the process gray gradation
patch in which yellow (Y), magenta (M) and cyan (C) are mixed, but
the classification method for attributes is not limited to the
combination of color materials as indicated here.
[0064] Moreover, the methods for converting the RGB outputs of the
sensor into XYZ chromaticity values include a linear conversion
method by matrix, a neural network method and a method using a
look-up table. In any method, if the attribute of the patch is
judged, it is possible to reduce the differences between the XYZ
chromaticity values obtained by converting the RGB outputs of the
sensor and the XYZ chromaticity values obtained from the spectral
reflectance as defined by the International Commission on
Illumination, by changing the weight of connection between neurons
for use in the neural network for every attribute of patch, or by
changing the contents of the look-up table.
[0065] As described above, by changing various parameters for use
in converting the RGB outputs of the sensor into the XYZ
chromaticity values for every attribute of patch, it is possible to
reduce the differences between the XYZ chromaticity values obtained
by converting the RGB outputs of the sensor and the XYZ
chromaticity values obtained from the spectral reflectance as
defined by the International Commission on Illumination, whereby
the precision of color stabilization control using the XYZ
chromaticity values can be improved.
Embodiment 2
[0066] In embodiment 2, an image forming apparatus not having a
color sensor like color sensor 42 above implements color
stabilization control equivalent to that described in embodiment 1,
employing an external image reading apparatus, instead of color
sensor 42, and using a color conversion method that reduces the
differences between the XYZ chromaticity values obtained by
converting the RGB outputs of the external image reading apparatus
and the XYZ chromaticity values obtained from the spectral
reflectance as defined by the International Commission on
Illumination.
[0067] FIG. 7 and FIG. 8 are diagrams showing a connection method
between color image forming apparatus and color image reading
apparatus. In FIG. 7, the color image forming apparatus and color
image reading apparatus are directly connected, but in FIG. 8, they
are connected via a network. The color image reading apparatus,
like the color sensor, mounts a white color source and a sensor
with the filters of three or more colors such as RGB, or light
sources of three or more colors such as RGB and a sensor having
sensitivity in the visible region, and outputs the RGB sensor
signals.
[0068] FIG. 9 is a view showing a flat bed image scanner as seen
from above as one example of the color image reading apparatus. The
of this apparatus operation will be described below. The user of
the image reading apparatus sets a medium formed with the image on
a platen 402. Once the medium is set, the sensor 401 is moved in a
direction as indicated by the arrow. The sensor proceeds in step
operation, and reads the image one line at each step. By
integrating these images of one line each, the entire image formed
on one medium can be read.
[0069] In this embodiment, a monochrome gradation patch of cyan
(C), magenta (M), yellow (Y) and black (K) or a patch in which CMY
are mixed is formed on the recording medium, and the outputted
recording medium formed with the patch is set on the color image
reading apparatus to detect the chromaticity of the patch. By
feeding back the detection result to the calibration table for
correcting the exposure amount, process conditions and the color
gradation characteristics of the image forming portion, the color
stabilization control of the final output image equivalent to that
described in connection with embodiment 1 can be achieved.
[0070] FIG. 10 is a view showing one example of the test chart
detected by the color image reading apparatus. A color
stabilization control test chart 63 is a gradation patch pattern of
gray, which is the most important color in making the color
balance, and is composed of a gray gradation patch of only black
(K) 61 and a process gray gradation patch 62 in which yellow (Y),
magenta (M) and cyan (C) are mixed. The gray gradation patch 61 of
only black (K) and the process gray gradation patch 62, which have
the same chromaticity in the image processing portion of the image
forming apparatus, are paired, such as 61a and 62a, 61b and 62b,
61c and 62c. A different point from the color sensor test chart as
shown in FIG. 6 in embodiment 1 is that the patches 61, 62 are
arranged over the entire face of the recording medium 1, since the
color image reading apparatus can read the images on the overall
face of the recording medium at one time.
[0071] In this embodiment, the color conversion method for reducing
the differences between the XYZ chromaticity values obtained by
converting the RGB outputs of the sensor and the XYZ chromaticity
values obtained from the spectral reflectance as defined by the
International Commission on Illumination is identical to the method
shown in FIG. 1 in embodiment 1. In step 211 in FIG. 1, it is
determined whether the detected patch is a gray gradation patch of
only black (K) like patch 61 or a process gray gradation patch like
patch 62.
[0072] The image reading apparatus can selectively detect the gray
gradation patch of only black (K) 61 or process gray gradation
patch 62 using patch position coordinate information of the color
stabilization control test chart 63. The image reading apparatus is
connected to the image forming apparatus, and notifies the image
reading apparatus of the patch position coordinate information of
the color stabilization control test chart 63. In this way the
determination in step 211 in FIG. 1 is made.
[0073] Though the RGB outputs of the sensor are converted into XYZ
chromaticity values here, it is apparent that the method described
in this embodiment is effective also in other situations where the
color matching function in two different color specifications
systems is non-linear.
[0074] Moreover, there are two attributes here, including the gray
gradation patch of only black (K) and the process gray gradation
patch in which yellow (Y), magenta (M) and cyan (C) are mixed, but
the classification method for attributes is not limited to this
combination of color materials.
[0075] Moreover, the methods for converting the RGB outputs of the
sensor into XYZ chromaticity values include a linear conversion
method using a matrix, a neural network method and a method using a
look-up table. In any method, if the attribute of the patch is
judged, it is possible to reduce the differences between the XYZ
chromaticity values obtained by converting the RGB outputs of the
sensor and the XYZ chromaticity values obtained from the spectral
reflectance as defined by the International Commission on
Illumination by changing the weight of connection between neurons
for use in the neural network for every attribute of patch, or by
changing the look-up table that is used.
[0076] As described above, the color image forming apparatus having
no color sensor is connected to the color image reading apparatus,
and by changing various parameters for use in converting the RGB
outputs of the color image reading apparatus into XYZ chromaticity
values for every attribute of patch, it is possible to reduce the
differences between the XYZ chromaticity values obtained by
converting the RGB outputs of the color reading apparatus and the
XYZ chromaticity values obtained from the spectral reflectance as
defined by the International Commission on Illumination, whereby
the precision of color stabilization control using the XYZ
chromaticity values can be improved.
Embodiment 3
[0077] In embodiment 3, the method for changing the matrix for use
in converting the RGB outputs into XYZ chromaticity values as
described in embodiments 1 and 2 is shown, in which the matrix is
changed depending on the attribute of the patch, i.e., whether the
patch is a gray gradation patch of only black (K) or a process gray
gradation patch, and by detecting the substratum color of patch,
namely, the color of the recording medium, to judge the color area.
Thereby, it is possible further to reduce the differences between
the XYZ chromaticity values obtained by converting the RGB outputs
and the XYZ chromaticity values obtained from the spectral
reflectance as defined by the International Commission on
Illumination.
[0078] FIG. 11 is a flowchart for converting the RGB outputs into
XYZ chromaticity values in embodiment 3.
[0079] In step 221, the recording medium 11 in an area where the
patch is not formed as the substratum color of patch is detected,
and it is judged into which of (1) to (6) the sizes of the RGB
outputs are classified: (1) R>G>B, (2) R>B>G, (3)
B>R>G, (4) B>G>R, (5) G>R>B and (6)
G>B>R.
[0080] Since the following steps are common except that the matrix
A is different in each of (1) to (6), the flow F301 for the
classification of (1) will be described below.
[0081] In step 222, it is determined whether the detected patch is
a gray gradation patch of only black (K) like patch 61 or a process
gray gradation patch like patch 62. The patch format of color
stabilization control test charts 60, 63 is fixed in the image
forming apparatus, whereby the determination is made based on the
sequence or position of detecting the patch.
[0082] If it is determined in step 222 that the detected patch is a
gray gradation patch of only black (K) like patch 61, the RGB
outputs of the sensor are converted into XYZ chromaticity values in
accordance with formula (1), employing matrix A1 for gray gradation
patch of only black (K) in step 223.
[0083] If it is determined in step 222 that the detected patch is a
process gray gradation patch like patch 62, the RGB outputs of the
sensor are converted into XYZ chromaticity values in accordance
with formula (1), employing matrix A2 for process gray gradation
patch in step 224.
[0084] If it is determined that the color area is classified into
any one of (2) to (6) in step 221, the procedure goes respectively
tone of flows F302 to F306. In F302, the matrix for gray gradation
patch of only black (K) is A3, and the matrix for process gray
gradation patch is A4, these matrixes differing from A1 and A2 in
F301, and the others being the same. Likewise, in F303, the matrix
for gray gradation patch of only black (K) is A5, and the matrix
for process gray gradation patch is A6, in F304, the matrix for
gray gradation patch of only black (K) is A7, and the matrix for
process gray gradation patch is A8, in F305, the matrix for gray
gradation patch of only black (K) is A9, and the matrix for process
gray gradation patch is A10, and in F306, the matrix for gray
gradation patch of only black (K) is A11, and the matrix for
process gray gradation patch is A12.
[0085] The color of patch formed on the recording medium 11 is
affected by the substratum color of patch, namely, the color of the
recording medium 11, without regard to the color material used for
the gray gradation patch of only black (K) or process gray
gradation patch. For example, if the recording medium is red, the
color of patch formed on a red recording medium is redder than the
color of the same patch formed on a white recording medium. That
is, even when the color image forming apparatus forms the same
patch, the spectral reflectance of the patch may differ according
to the color of the recording medium on which the patch is formed.
Accordingly, it is possible further to reduce the differences
between the XYZ chromaticity values obtained by converting the RGB
outputs and the XYZ chromaticity values obtained from the spectral
reflectance as defined by the International Commission on
Illumination by changing the matrix A that is used, depending on
the color area of the recording medium 11.
[0086] Also, the color areas of substratum color of patch are
classified into six attributes, but the classification method for
attributes is not limited to the method shown herein.
[0087] Through the RGB outputs of the sensor are converted into the
XYZ chromaticity values, it is apparent that the method described
in this embodiment is effective in other situations where the color
matching function in two different color specification systems is
non-linear.
[0088] Moreover, there are two attributes here, including the gray
gradation patch of only black (K) and the process gray gradation
patch in which yellow (Y), magenta (M) and cyan (C) are mixed, but
the classification method for attributes is not limited to this
combination of color materials.
[0089] Moreover, the methods for converting the RGB outputs of the
sensor into XYZ chromaticity values include a linear conversion
method by means of a matrix, a neural network method and a method
using a look-up table. In any method, if the color area of the
recording medium and the attribute of the patch are judged, it is
possible to reduce the differences between the XYZ chromaticity
values obtained by converting the RGB outputs of the sensor and the
XYZ chromaticity values obtained from the spectral reflectance as
defined by the International Commission on Illumination by changing
the weight of connection between neurons for use in the neural
network for every color area of the recording medium and every
attribute of patch, or using the look-up table.
[0090] As described above, by changing various parameters for use
in converting the RGB outputs of the sensor into the XYZ
chromaticity values according to the combination of the attribute
of patch and the color area of the recording medium, it is possible
further to reduce the difference between the XYZ chromaticity
values obtained by converting the RGB outputs of the sensor and the
XYZ chromaticity values obtained from the spectral reflectance as
defined by the International Commission on Illumination, whereby
the precision of color stabilization control using the XYZ
chromaticity values can be improved.
[0091] While the invention has been described in terms of its
preferred embodiments, various modifications may be made thereto
without departing from the spirit and scope of the invention as
defined by the appended claims.
[0092] This application claims priority from Japanese Patent
Application No. 2004-219971, filed Jul. 28, 2004, which is hereby
incorporated by reference herein.
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