JP4630938B2 - Color image forming apparatus and color image control method - Google Patents

Description

  The present invention relates to a color image forming apparatus that forms a color image based on an image signal, and more particularly to adjustment of the chromaticity thereof.

  In recent years, color image forming apparatuses employing an electrophotographic system such as a color printer or a color copying machine, an ink jet system, and the like have been required to improve the output image quality. In particular, the gradation of the density and its stability have a great influence on the judgment of the quality of an image given by a human.

  However, in an electrophotographic image forming apparatus, if there is a change in each part of the apparatus due to environmental changes or long-term use, the density of the obtained image will change. In particular, in the case of an electrophotographic color image forming apparatus, there is a possibility that the color balance may be lost even by a slight change in density, so it is necessary to always maintain a constant gradation-density characteristic. Therefore, the absolute humidity measured by the temperature / humidity sensor is provided for each color toner with gradation correction means such as several kinds of exposure amounts and development biases according to the absolute humidity, development bias, and lookup table (LUT). Based on the above, the process condition at that time and the optimum value for gradation correction are selected. In addition, a toner patch for density detection is created on the intermediate transfer body or drum with toner of each color so that a constant gradation-density characteristic can be obtained even if fluctuations occur in each part of the apparatus, and the unfixed toner patch The density of the toner is detected by a density detection sensor for unfixed toner, and the density control is performed by feeding back process conditions such as exposure amount and development bias based on the detection result, thereby obtaining a stable image. .

  However, density control using the density detection sensor for unfixed toner is to detect a patch by forming a patch on an intermediate transfer member, a drum, or the like, and color balance of the image by subsequent transfer to a transfer material and fixing. There is no control over changes. The color balance also changes depending on the transfer efficiency in transferring the toner image to the transfer material, and heating and pressurization by fixing. This change cannot be dealt with by density control using the density detection sensor for unfixed toner. Therefore, a density or chromaticity sensor (hereinafter referred to as a color sensor) for detecting the density of a single-color toner image on a transfer material or the chromaticity of a full-color image after transfer and fixing is installed, and a color toner patch for density or chromaticity control (hereinafter referred to as a patch). The density or chromaticity of the final output image formed on the transfer material by feeding back the detected density or chromaticity to the exposure conditions, process conditions, and process conditions such as a look-up table (LUT). An image forming apparatus that performs degree control is considered. This color sensor uses, for example, a light source that emits red (R), green (G), and blue (B) as a light emitting element in order to identify CMYK or detect density or chromaticity. Using a light source that emits white (W), three types of filters having different spectral transmittances such as red (R), green (G), and blue (B) are formed on the light receiving element. CMYK can be identified and density can be detected from three different outputs obtained by this, for example, RGB output. Further, the chromaticity can be detected by subjecting the RGB output to a mathematical process such as linear conversion or by converting the RGB output using a lookup table (LUT).

  Even in an ink jet printer, the color balance changes due to a change in ink discharge amount with time, environmental differences, and individual differences of ink cartridges, and the gradation-density characteristics cannot be kept constant. Therefore, it is considered to install a color sensor near the output unit of the printer, detect the density or chromaticity of the patch on the transfer material, and perform density or chromaticity control.

  There are various methods for controlling density or chromaticity. For example, there is a method of performing gamma characteristic control from the measured density, or correcting the color matching table or color separation table from the measured chromaticity.

  Here, in order to detect the absolute density or absolute chromaticity of a patch using a color sensor, there is a standard whose absolute value of density or chromaticity is known, such as a white reference plate for sensor output calibration, for the following reason. Necessary. The first reason is that it is necessary to calibrate the dispersion of the spectral characteristics of the light emitting element and the light receiving element constituting the sensor. The second reason is that the output may be different even if the same patch is detected due to the temporal change of the light emitting unit and the light receiving unit constituting the sensor and the ambient temperature change. The third reason is that a large amount of transfer material passes near the sensor during normal printing, so that paper dust, toner or ink scatters and accumulates on or adheres to the sensor surface, leading to a decrease in sensor output.

  However, the white reference plate that is often used as a reference for sensor output calibration is not only expensive, but, as with the sensor, paper dust, toner, or ink may scatter on the white reference plate, making it unusable as a reference plate. is there.

  On the other hand, if the density or chromaticity of the patch is detected without using the sensor output calibration reference, that is, without calibrating the sensor output, the sensor output is the actual patch density or A value different from chromaticity is output. When density or chromaticity control is executed using the result, color balance cannot be achieved and desired gradation-density characteristics cannot be obtained. In addition, the color balance may be reversed and the gradation-density characteristics may be deteriorated.

  Furthermore, in order to accurately detect absolute chromaticity, it is necessary to have a high-accuracy XYZ filter used in an expensive colorimeter, a function of splitting reflected light, and the like. Therefore, it is not realistic to install a color sensor having such a function in the printer.

  Therefore, the color sensor detects the chromaticity (L * a * b *, L * c * h *, XYZ, etc.) of the process gray patch mixed with yellow, magenta, and cyan and the gray patch of black, and the color of both patches. A method of comparing the degrees relative to each other can be considered.

  However, as described above, in order to perform the correction so that the process gray becomes an achromatic color, means for obtaining the ratio of cyan, magenta, and yellow at which the process gray becomes an achromatic color is required.

The present invention has been made under such circumstances, and a color image forming apparatus capable of obtaining a ratio of cyan, magenta, and yellow in which process gray is an achromatic color from chromaticity detected by a color sensor , It is another object of the present invention to provide a color image control method .

In order to achieve the above object, in the present invention, the color image forming apparatus is configured as the following (1) to ( 5 ) and the color image control method as described in (6) .

(1) Image forming means for forming on a sheet a plurality of mixed color patches by a plurality of combinations of gradation values of each color in a color material of cyan, magenta, and yellow and a single color patch by a black color material ; Fixing means for fixing the plurality of formed mixed color patches and the single color patch on paper, and detecting the color values of the fixed mixed color patches and the single color patch on the paper before paper discharge A detecting unit configured to detect color values of the plurality of mixed-color patches and the single-color patch from the sheet before the paper discharge, and colors of the plurality of mixed-color patches detected by the detection unit. And the color value of the single color patch are compared, and based on the comparison, a combination of gradation values of each color in each color material close to the color value of the single color patch is obtained, and the image forming condition is compensated. Color image forming apparatus according to claim Rukoto comprises a correction unit, the for.
(2) When the plurality of mixed color patches are formed again on the paper under a predetermined condition, the image forming unit performs the gradation value of each color in each color material obtained by the correction unit last time. based on the combination of the color image forming apparatus according to (1), characterized that you form a plurality of mixed color patches on the paper.
(3) The image forming unit forms a plurality of the single-color patches corresponding to each of a plurality of black gradations, and a plurality of combinations of a plurality of combinations of gradation values of the respective colors in the respective color materials. Forming a mixed-color patch on a sheet, and the correction unit obtains, for each of the plurality of single-color patches, a combination of gradation values of each color in each color material close to each color value of the single-color patch; the color image forming apparatus according to (1) or (2), characterized that you correct the image forming conditions.
(4) The image forming unit forms the plurality of mixed color patches and the single color patch in a row so as to be sequentially read on the paper by the detecting unit, and the fixing unit is formed in a row on the paper. The plurality of mixed color patches and the single color patch are fixed, and the detection unit detects color values of the plurality of mixed color patches and the single color patch formed and fixed in a line on the sheet. The color image forming apparatus according to any one of (1) to (3).
(5) The color image forming apparatus according to any one of (1) to (4), wherein the image forming condition is a process condition or a gradation conversion table of an image signal.
(6) A method for controlling a color image in a color image forming apparatus, wherein a plurality of color mixture patches and a single black color are formed by a plurality of combinations of gradation values of each color in cyan, magenta and yellow color materials by an image forming means. A single color patch made of a color material is formed on a sheet, the plurality of mixed color patches and the single color patch formed on the sheet are fixed on a sheet by a fixing unit, and the plurality of the fixed plurality of patches on the sheet before paper discharge The color mixture patch and the color value of the single color patch are arranged so as to be detectable, and a detection means for detecting the color value of the plurality of color mixture patches and the color value of the single color patch from the sheet before the paper discharge, The color values of the plurality of mixed color patches and the single color patch are detected, and the detected color values of the plurality of mixed color patches are compared with the color values of the single color patch. The determined combination of the gradation value of each color in each color materials close to the color value of the single-color patch, a control method of a color image and correcting image forming conditions Hazuki.

  According to the present invention, it is possible to obtain the ratio of cyan, magenta, and yellow at which the process gray is an achromatic color from the chromaticity detected by the color sensor.

Sectional drawing which shows the whole structure of Example 1. The figure which shows the process in an image processing part Diagram showing the configuration of the density sensor Diagram showing the configuration of the color sensor Flow chart showing control of gradation-density characteristics Flow chart showing details of gradation-density characteristic control Diagram showing default tone-density curve The figure which shows the patch pattern formed on the intermediate transfer body The figure which shows the control of the gradation density characteristic with the output of the density sensor The figure which shows the contents of the patch pattern which is formed on the transfer material Diagram showing patch pattern to be formed on transfer material The figure which shows the C, M, Y coordinate of the patch pattern formed on a transfer material The figure which shows the control of the gradation density characteristic by the output of the color sensor The figure which shows the gradation-density characteristic of a target Flowchart showing details of gradation-density characteristic control in embodiment 2 The figure which shows the contents of the patch pattern which is formed on the transfer material Diagram showing patch pattern to be formed on transfer material The figure which shows the C, M, Y coordinate of the patch pattern formed on a transfer material Flowchart showing details of color mixture control in Embodiment 3 The figure which shows the contents of the patch pattern which is formed on the transfer material Conceptual diagram showing color mixture control in Embodiment 3 The figure which shows the content of the patch pattern formed on the transfer material in Example 4 The figure which shows the contents of the patch pattern which is formed on the transfer material

Hereinafter, embodiments of the present invention will be described in detail with reference to examples of color image forming apparatuses.
The present invention is not limited to the form of the device, but is supported by the description of the embodiments, in the form of a method, in the form of a program for realizing this method, and further, a CD-ROM or the like storing this program It can also be implemented in the form of a storage medium.

  The color sensor detects the chromaticity (L * a * b *, L * c * h *, XYZ, etc.) of the process gray patch mixed with yellow, magenta, and cyan, and the gray patch of black. A relative comparison method can be considered. Gray patches made of electrophotographic black toner and inkjet black ink are almost achromatic. Therefore, a gray patch of black is formed every time density or chromaticity control is performed, and by using this as a reference, correction is performed so that the process gray becomes an achromatic color without using the sensor output calibration reference.

  In this method, it is not necessary to have a reference for sensor output calibration and it is not necessary to have a high-accuracy filter. Furthermore, when the gray balance of cyan, magenta, and yellow is lost, it causes serious problems such as “the gray color most sensitive to human eyes appears to be colored” and “the overall color of chromatic colors also shifts”. As described above, adjusting the process gray to an achromatic color and achieving a gray balance solves the above-mentioned problem very effectively. In this method, since cyan, magenta, and yellow colors are relatively matched to black, if the density characteristics of black are shifted, the density characteristics of cyan, magenta, and yellow are also shifted accordingly. However, since the gray balance is maintained even if the density is shifted, the color tone does not change when the colors are mixed. For this reason, the whole color is stabilized in appearance.

  FIG. 1 is a cross-sectional view illustrating an overall configuration of a “color image forming apparatus” according to a first embodiment. As shown in the figure, this apparatus is a tandem color image forming apparatus employing an intermediate transfer member 27, which is an example of an electrophotographic color image forming apparatus. The color image forming apparatus includes an image forming unit shown in FIG. 1 and an image processing unit (not shown).

  First, processing in the image processing unit will be described. FIG. 2 is a flowchart illustrating an example of processing in the image processing unit of the color image forming apparatus. In step 131 (denoted as S131 in the figure), a device that matches an RGB signal representing the color of an image sent from a host computer or the like with the color reproduction range of the color image forming apparatus using a color matching table prepared in advance. It is converted into an RGB signal (hereinafter referred to as DevRGB). In step 132, the DevRGB signal is converted into a CMYK signal, which is the color of the toner color material of the color image forming apparatus, using a color separation table prepared in advance. In step 133, the CMYK signal is converted into a C′M′Y′K ′ signal obtained by correcting the gradation-density characteristic by using a density correction table for correcting the gradation-density characteristic specific to each color image forming apparatus. Convert. Thereafter, in step 134, halftone processing is performed to convert the signal into a C ″ ″ M ″ ″ ″ ″ K ″ signal. In step 135, the exposure times Tc, Tm, Ty, Tk of the scanner units 24C, 24M, 24Y, 24K corresponding to the C ″ M ″ Y ″ K ″ signals are obtained by a PWM (Pulse Width Modulation) table. Convert to

  Next, the operation of the image forming unit will be described with reference to FIG. An electrostatic latent image is formed by exposure light that is turned on based on the exposure time converted by the image processing unit, the electrostatic latent image is developed to form a single color toner image, and the single color toner image is superimposed to obtain a multicolor image. A toner image is formed, this multi-color toner image is transferred to a transfer material 11, and the multi-color toner image on the transfer material 11 is fixed. Each photosensitive member 22Y, 22M, 22C, 22K, injection charging means 23Y, 23M, 23C, 23K as primary charging means, toner cartridges 25Y, 25M, 25C, 25K, developing means 26Y, 26M, 26C, 26K, intermediate transfer The body 27, the transfer roller 28, and the fixing unit 30 are configured.

  The photosensitive drums (photoconductors) 22Y, 22M, 22C, and 22K are configured by applying an organic optical conductive layer to the outer periphery of an aluminum cylinder, and are rotated by the driving force of a driving motor (not shown) being transmitted. Rotates the photosensitive drums 22Y, 22M, 22C, and 22K in the counterclockwise direction in accordance with the image forming operation.

  As the primary charging means, four injection chargers 23Y, 23M, 23C, and 23K for charging the yellow (Y), magenta (M), cyan (C), and black (K) photoreceptors are provided for each station. In configuration, each injection charger is provided with a sleeve 23YS, 23MS, 23CS, 23KS.

  Exposure light to the photosensitive drums 22Y, 22M, 22C, and 22K is sent from the scanner units 24Y, 24M, 24C, and 24K, and the electrostatic latent images are selectively exposed by exposing the surfaces of the photosensitive drums 22Y, 22M, 22C, and 22K. An image is formed.

  As developing means, in order to visualize the electrostatic latent image, four developing devices 26Y, 26M for developing yellow (Y), magenta (M), cyan (C), and black (K) for each station, Each developing device is provided with a sleeve 26YS, 26MS, 26CS, and 26KS. Each developing device is detachably attached.

  The intermediate transfer member 27 is in contact with the photosensitive drums 22Y, 22M, 22C, and 22K, and rotates clockwise when forming a color image. The intermediate transfer member 27 rotates in accordance with the rotation of the photosensitive drums 22Y, 22M, 22C, and 22K. The toner image is transferred. Thereafter, a transfer roller 28 to be described later comes into contact with the intermediate transfer member 27 to sandwich and convey the transfer material 11, and the multicolor toner image on the intermediate transfer member 27 is transferred to the transfer material 11.

  The transfer roller 28 contacts the transfer material 11 at the position 28a while the multicolor toner image is transferred onto the transfer material 11, and is separated to the position 28b after the printing process.

  The fixing unit 30 melts and fixes the transferred multi-color toner image while conveying the transfer material 11, and as shown in FIG. 1, the fixing roller 31 for heating the transfer material 11 and the transfer material 11 are fixed to the fixing roller. A pressure roller 32 is provided for pressure contact with 31. The fixing roller 31 and the pressure roller 32 are formed in a hollow shape, and heaters 33 and 34 are incorporated therein, respectively. That is, the transfer material 11 holding the multicolor toner image is conveyed by the fixing roller 31 and the pressure roller 32, and heat and pressure are applied to fix the toner on the surface.

  After the toner image is fixed, the transfer material 11 is discharged to a discharge tray (not shown) by a discharge roller (not shown) and the image forming operation is finished.

  The cleaning unit 29 cleans the toner remaining on the intermediate transfer member 27. The waste toner after the four-color multicolor toner image formed on the intermediate transfer member 27 is transferred to the transfer material 11 is: Stored in a cleaner container.

  The density sensor 41 is arranged toward the intermediate transfer body 27 in the color image forming apparatus of FIG. 1 and measures the density of the toner patch formed on the surface of the intermediate transfer body 27. An example of the configuration of the density sensor 41 is shown in FIG. An infrared light emitting element 51 such as an LED, a light receiving element 52 such as a photodiode or Cds, an IC (not shown) that processes received light data, and a holder (not shown) that accommodates these elements. The light receiving element 52a detects the intensity of irregularly reflected light from the toner patch, and the light receiving element 52b detects the intensity of regular reflected light from the toner patch. By detecting both the regular reflection light intensity and the irregular reflection light intensity, it is possible to detect the density of the toner patch from a high density to a low density. Further, a color difference from a predetermined paper can be output. An optical element (not shown) may be used for coupling the light emitting element 51 and the light receiving element 52.

  The density sensor 41 cannot distinguish the color of the toner on the intermediate transfer member 27. Therefore, a gradation patch 64 of single color toner is formed on the intermediate transfer member 27. Thereafter, the density data is fed back to a density correction table for correcting the gradation-density characteristics of the image processing unit and each process condition of the image forming unit.

  The color sensor 42 is disposed toward the image forming surface of the transfer material 11 downstream from the fixing unit 30 in the transfer material conveyance path in the color image forming apparatus of FIG. 1, and after the fixing formed on the transfer material 11. The RGB output value of the color of the mixed color patch is detected. The color sensor 42 is very similar to the density sensor 41 of FIG. 1 arranged toward the intermediate transfer member 27.

  FIG. 4 shows an example of the configuration of the color sensor 42. The color sensor 42 includes a white LED 53 and a charge storage sensor 54a with an RGB on-chip filter. The white LED 53 is incident on the transfer material 11 on which the patch after fixing is formed at an angle of 45 degrees, and the intensity of diffuse reflected light in the 0 degree direction is detected by the charge storage sensor 54a with an RGB on-chip filter. The light receiving portion of the charge storage type sensor 54a with RGB on-chip filter is a pixel independent of RGB like 54b. The charge storage sensor of the RGB on-chip filter charge storage sensor 54 may be a photodiode. There are some sets in which several sets of three RGB pixels are arranged. Further, a configuration in which the incident angle is 0 degree and the reflection angle is 45 degrees may be employed. Furthermore, you may comprise by LED which light-emits RGB3 color, and a sensor without a filter.

  Next, the concept of gradation-density characteristic control in this embodiment using these sensors will be described. FIG. 5 is a flowchart showing control of gradation-density characteristics in which the color sensor 42 and the density sensor 41 are combined. Since the control using the color sensor consumes a transfer material, the number of executions is limited compared to the control using the density sensor. Therefore, as shown in FIG. 5, first, gradation-density characteristic control (hereinafter referred to as color mixing control) using a color sensor and a density sensor is executed in step 101, and then only the density sensor is used in steps 102 to 104. Gradation-density characteristic control (hereinafter referred to as single color control) is executed a prescribed number of times, and the process returns to color mixture control again. Note that color mixing and single color control are executed between normal printing operations. The execution timing is automatically executed at a predetermined timing detected by detecting an environmental change or the like, or is executed manually by the user when the user desires to execute the control.

  FIG. 6 is a flowchart showing details of the gradation-density characteristic control combining the color mixture control and the single color control.

  First, when a new cartridge is used, that is, when the color image forming apparatus is first installed, or when the cartridge is replaced, in step 111, the target of gradation-density characteristics of each color of C, M, Y, K A predetermined default gradation-density curve is used. The default gradation-density curve is set in consideration of the characteristics of the color image forming apparatus. In this embodiment, the output density is linear with respect to the input gradation as shown in FIG. The density correction table is a so-called through table that does not change the input value.

  Next, a patch pattern is formed on the intermediate transfer member and read by the density sensor (step 112). FIG. 8 shows an example of a patch pattern formed on the intermediate transfer member. Unfixed K toner single-color gradation patches 64 are arranged, and thereafter, C, M, and Y toner single-color gradation patches (not shown) are continuously formed. At this time, the predetermined gradations of C, M, Y, and K forming the patch are used. The density of the patch pattern formed on the intermediate transfer member is detected by a density sensor, and a tone-density curve is generated by interpolation from the detected density. When the density detection result is as indicated by a black circle in FIG. 9, a tone-density curve such as 100 is generated by interpolation such as linear interpolation. Further, a reverse characteristic curve 200 is calculated based on the target density curve 300 set in step 111, and this is used as a density correction table for the input image data. By performing table conversion on the input image data using this density correction table, the input gradation degree and the output density have the relationship of the target gradation-density curve 300 (step 113).

  In step 114, the density correction table 200 generated in step 113 is used to form a CMY mixed color patch and a K single-color patch pattern on the transfer material, which are detected by a color sensor. The details of this step will be described below.

  Each of the patches (1) to (9) of the CMY mixed color patch and the K single color patch includes C, M, Y data (1) to (8) and K single color data (9) as shown in FIG. . The gradation of C, M, and Y of each patch is a combination of values obtained by changing the gradation by ± α from reference gradations (hereinafter referred to as reference values) C0, M0, and Y0. The patch (9) is a K single color patch, which is formed with a predetermined gradation K0. Here, the values of C0, M0, Y0, and K0 are adjusted so that the gradation-density characteristics of C, M, Y, and K are adjusted to the state of the default gradation-density curve 300, and under normal image forming conditions. , M0, Y0 are values that are the same as K0 when the colors are mixed, and are set during color processing and halftone design. As shown in FIG. 11, the patch patterns (1) to (9) are formed on the transfer material, and the patches formed on the transfer material are detected by the color sensor 42 after passing through the fixing device 30 and output RGB values. To do.

  Next, in step 115, C, M, and Y values (gradation levels) for matching the C, M, and Y process grays with the K patch color in (9) are calculated from the RGB output values of the sensor.

  If the image formation conditions are exactly the same as those at the time of color processing design, the color of K0 matches the color mixed with (C0, M0, Y0), but it does not match for the reason described in the prior art, and the color Will shift. The RGB output values of each patch are (1) = (r1, g1, b1), (2) = (r2, g2, b2),..., And C, M, Y of each patch of (1) to (8) FIG. 12 shows the coordinates three-dimensionally. The coordinates of the center of the cubic lattice in the figure are (C0, M0, Y0).

From the RGB values of (1) to (8), C, M, and Y values for matching with the RGB value of K0 are obtained by linear interpolation of 8 points from FIG. Specifically, the RGB values (Rcmy, Gcmy, Bcmy) for the coordinates of C, M, and Y in the cubic lattice of FIG. 12 are calculated by the following formula.
Rcmy = [(C−C0 + α) (M−M0 + α) (Y−Y0 + α) r1 +
(C0 + α-C) (M-M0 + α) (Y-Y0 + α) r2 +
(C−C0 + α) (M0 + α−M) (Y−Y0 + α) r3 +
(C−C0 + α) (M−M0 + α) (Y0 + α−Y) r4 +
(C0 + α-C) (M0 + α-M) (Y-Y0 + α) r5 +
(C0 + α-C) (M-M0 + α) (Y0 + α-Y) r6 +
(C−C0 + α) (M0 + α−M) (Y0 + α−Y) r7 +
(C0 + α-C) ( M0 + α-M) (Y0 + α-Y) r8] / (8α 3)
Gcmy and Bcmy are also obtained by the same formula.

  The difference between (Rcmy, Gcmy, Bcmy) calculated by the above formula and the RGB value (Rk, Gk, Bk) of K is obtained by, for example, the sum of squares of the differences of each RGB. Then, the one having the smallest difference, that is, (Rcmy, Gcmy, Bcmy) closest to (Rk, Gk, Bk) is obtained, and the values of C, M, Y at this time are set as optimum values (C0 ′, M0 ′, Y0). ').

  Note that α is preferably as small as possible in order to increase the accuracy of interpolation.

  When the colors of K0 and (C0, M0, Y0) are greatly shifted, (C0 ′, M0 ′, Y0 ′) is not near the center (C0, M0, Y0) of the cubic lattice. Since (C0 ′, M0 ′, Y0 ′) must be in the cubic lattice, the cubic lattice needs to be large enough for it. Considering these two conditions, the optimum value is set.

  Further, the gradation of K is changed to have a plurality of reference values (CN, MN, YN, KN) (N = 0, 1, 2,... N), and (1) as described above for each reference value. The patches (9) to (9) are formed, and (CN ′, MN ′, YN ′, KN ′) is obtained for each (CN, MN, YN, KN). If the relationship between cyan (CN, MN, YN) and (CN ′, MN ′, YN ′) obtained in this way is as shown by the black circles in FIG. Create a curve (color correction table) like

  In step 116, the density correction target table is corrected. A tone-density curve is generated by multiplying the original target tone-density curve (FIGS. 9 and 300) by the color correction table 150 of FIG. 13, and this is used as a new cyan target tone-density curve. (FIG. 14, 400). Specifically, the input gradation degree is converted into the output density according to the target gradation-density curve after table conversion by the color correction table 150.

  Similarly, the targets for M and Y are changed. By performing density correction with this new target, the color due to the color mixture of (CN, MN, YN) matches the color of KN.

  It should be noted that the values of (CN, MN, YN, KN) are “the human eye is sensitive to highlight gray and becomes insensitive as shadows occur”, “UCR processing during normal color processing (one of CMY during color separation) In consideration of the fact that only the three colors of CMY do not appear in the shadow area, the present invention can be implemented more effectively by focusing on highlights.

  In step 117, a density correction table is generated again using the C, M, and Y targets changed in step 116 from the density detection result in step 112, and the density of the input image data is used by using this density correction table for printing thereafter. Correction is performed and the normal printing state is entered (step 118).

  When the specified number of sheets is printed in the normal printing state (step 120), monochrome density control is performed. In monochromatic density control, a patch pattern is formed on the intermediate transfer member in step 121 as in step 112, and is read by a density sensor. The density of the patch pattern formed on the intermediate transfer member is detected by a density sensor, a tone-density curve is generated by interpolation from the detected density, and the target 400 generated in step 116 is used as in step 113. The density correction table is updated by this method (step 122). Further, it is determined whether or not the specified number of times of monochrome density control has been performed (step 123). If the specified number of times has not been reached, normal printing is resumed. If the prescribed number of times has been performed, a CMY mixed color and K single-color patch pattern is again formed on the transfer material in step 114 and detected by the color sensor. At this time, the patch pattern is formed using the latest density correction table. Thereafter, processing is performed in the steps described above. However, when creating a new target, the target 400 generated in the previous step 116 is multiplied by a new inverse characteristic table.

  Further, when any color cartridge is replaced in the normal printing state (step 119), the image forming conditions greatly change, so the processing returns to step 111 again.

  In the present embodiment, the target is corrected as described above with emphasis on accuracy. However, the target may not be corrected, and the density correction table may be multiplied by the correction table shown in FIGS. .

  In this embodiment, three-dimensional linear interpolation is used to calculate the optimum C, M, and Y values. As an interpolation method, quadratic function approximation, cubic function approximation, or spline interpolation is used. A non-linear method may be used.

  In this embodiment, the same α, C, M, and Y values are used. However, different values may be used for each color.

  Furthermore, in this embodiment, the color sensor is RGB output, but it may output chromaticity such as L * a * b *, L * c * h *, and XYZ.

  Further, in this embodiment, the color of the mixed color patch of C, M, and Y is matched to the color of the patch of K. However, the L * a * b * value of the mixed color patch of C, M, and Y is measured by the color sensor. For example, an optimum gradation degree at which the mixed color of C, M, and Y becomes an achromatic color may be calculated by targeting the achromatic axis of a = 0 and b = 0, and may be fed back to the monochromatic control.

  The configuration of the “color image forming apparatus” of the second embodiment is the same as that of the first embodiment. A flowchart in this embodiment is shown in FIG.

  First, when a new cartridge is used, that is, when the color image forming apparatus is first installed, or when the cartridge is replaced, the gradation determined in advance as the target of the K gradation-density characteristic in step 211. -Concentration curves are used. The default gradation-density curve is set in consideration of the characteristics of the color image forming apparatus. In this embodiment, the output density is linear with respect to the input gradation as shown in FIG. The density correction table 133 is a so-called through table that does not change the input value.

  Next, a patch pattern is formed on the intermediate transfer member and read by the density sensor (step 212). The patch pattern formed on the intermediate transfer member is the same as that of the first embodiment and FIG. 8, but in this embodiment, only the unfixed K toner single-color gradation patches 64 are arranged. At this time, a predetermined gradation is used for K to form a patch. The density of the patch pattern formed on the intermediate transfer member is detected by a density sensor, and a tone-density curve is generated by interpolation from the detected density. When the density detection result is as indicated by a black circle in FIG. 9, a tone-density curve such as 100 is generated by interpolation such as linear interpolation. Further, a reverse characteristic curve 200 is calculated based on the target density curve, and this is used as a density correction table for the input image data. By converting the input image data using the density correction table, the input data and the output density have the relationship of the target tone-density curve 300 (step 213).

  Next, in step 214, a predetermined default value (Cd, Md, Yd) is set as a reference value (C0, M0, Y0) for the next color mixture control patch.

  In step 215, a KMY mixed color and K single-color patch pattern is formed on the transfer material using the density correction table 300 generated in step 213 for K, and a through table for C, M, and Y. Detect with color sensor. The details of this step will be described below.

  Each patch of the CMY mixed color patch and the K single color patch includes C, M, Y data (1) to (6) and K single color data (7) as shown in FIG. In this embodiment, the values of C, M, and Y of each patch are values obtained by changing only a specific color by ± α from the reference values C0, M0, and Y0. The patch (7) is a K single-color patch and is formed with a predetermined value K0. As described above, default values (Cd, Md, Yd) are initially set in C0, M0, Y0. (Cd, Md, Yd) is a state in which the density characteristic of K is adjusted to the state of the gradation-density curve 300, and the values of Cd, Md, Yd are obtained with C, M, Y in the state of a typical gradation-density curve. When the colors are mixed, the value is such that the color is almost the same as K0. As shown in FIG. 17, the patch patterns (1) to (7) are formed on the transfer material, and the patches formed on the transfer material are detected by the color sensor 42 after passing through the fixing device 30 and output RGB values. (Step 215).

  In step 216, C, M, and Y values (gradation levels) for matching the C, M, and Y process grays with the K color are calculated from the RGB output values of the sensor.

  If the image formation conditions are exactly the same as those at the time of color processing design, the color of K0 matches the color mixed with (C0, M0, Y0), but it does not match for the reason described in the prior art, and the color Will shift. The RGB output values of each patch are (1) = (r1, g1, b1), (2) = (r2, g2, b2),..., And C, M, Y of each patch of (1) to (8) FIG. 18 shows the coordinates in a three-dimensional manner. The coordinates of the intersection of the three axes in the figure are (C0, M0, Y0).

From the RGB values of (1) to (6), C, M, and Y values for matching with the RGB value of K0 are obtained from FIG. Specifically, first, the value of C is changed on the axis between (1) and (2), and the value of each C (Rc, Gc, Bc) is obtained by linear interpolation using the following equation.
Rc = [(C−C0 + α) r1 + (C0 + α−C) r2] / (2α)
Gc = [(C−C0 + α) g1 + (C0 + α−C) g2] / (2α)
Bc = [(C−C0 + α) b1 + (C0 + α−C) g2] / (2α)
The difference between (Rc, Gc, Bc) calculated by the above formula and the RGB value (Rk, Gk, Bk) of K is obtained by, for example, the sum of squares of the differences of RGB. Then, the one with the smallest difference, that is, (Rc, Gc, Bc) closest to (Rk, Gk, Bk) is obtained, and the value of C at this time is set as the optimum value and is set as C0 ′.

  Similarly, optimum values M0 'and Y0' are obtained for M and Y, and (C0 ', M0', Y0 ') are set as the optimum values of C, M, and Y that form a color closest to K0.

  The subsequent flow is the same as that of the flowchart of FIG. 6 of the first embodiment. However, in this embodiment, when the single color control is performed a predetermined number of times (step 224), the optimum value (C0 ′, M0 ′, Y0 ′) is set to the reference value (C0, M0, Y0). Then, a patch is formed with a new reference value (C0, M0, Y0) and detected by the color sensor (step 215).

  When the cartridge of any color is replaced in the normal printing state (step 220), the process returns to the process of step 211 again, and after the K density control is performed, the default reference values (Cd, Md, Yd) are again obtained. The color mixing control is performed at.

  As described above, in this embodiment, the number of combinations of one set of mixed color patches is as small as seven, and a larger number of sets of patches can be formed on the transfer material. Further, it is possible to perform control with higher accuracy by using the previous optimum value as the reference value.

  In this embodiment, as in the first embodiment, the same α value is used for C, M, and Y, but different values may be used for each color.

  The configuration of the “color image forming apparatus” of the third embodiment is the same as that of the first embodiment.

  In the first and second embodiments, the patch for color mixture control is formed only once, but this embodiment is characterized in that the patch pattern is formed a plurality of times.

  The processing flow in the present embodiment is the same as the flowchart of FIG. 6 in the first embodiment except for the color mixture control. FIG. 19 shows a flowchart of color mixture control in this embodiment.

  When color mixing control is started in step 311, predetermined gradation levels (C0, M0, Y0) of C, M, and Y are set to reference values (Cs, Ms, Ys) as initial values (steps). 312). However, (C0, M0, Y0) is a value determined in the same manner as in the first embodiment. In step 313, the initial value α0 is set to α. The density correction table for C, M, Y, and K uses a density controlled by monochromatic control (not shown).

  In step 314, a CMY mixed color and K single-color patch pattern is formed. FIG. 20 shows the contents of the patch pattern in this embodiment.

  The values of C, M, and Y of each patch are a combination of values obtained by changing ± α from the reference values Cs, Ms, and Ys. The patch (9) is a K single-color patch and is formed with a predetermined value K0. As in the first embodiment, patch patterns (1) to (9) are formed on the transfer material as shown in FIG. 11, and the patches formed on the transfer material are detected by the color sensor 42 after passing through the fixing device 30. Then, RGB values are output.

  Next, in step 315, optimum values (C0 ', M0', Y0 ') are obtained by the same method as in the first embodiment.

  In step 316, a patch is formed again on the transfer material using (C0 ', M0', Y0 ') obtained in step 315. At this time, a K0 monochrome patch is also formed. The patch formed on the transfer material is detected by the color sensor 42 after passing through the fixing device 30 and outputs RGB values.

  In step 317, the difference between the RGB values of the mixed color patch (C0 ′, M0 ′, Y0 ′) output in step 316 and the single color patch of K0 is calculated as, for example, the square sum of the differences of R, G, B, In step 318, it is determined whether the difference is larger than a predetermined threshold value. When the difference is larger, it is determined that the difference exceeds the allowable range, and the process proceeds to step 319.

  In step 319, (C0 ′, M0 ′, Y0 ′) calculated in step 315 is set to a reference value (Cs, Ms, Ys). In step 320, the value of α is set to the value of α used in the previous steps 314 to 315. In step 314, a CMY mixed color and K single-color patch pattern is formed with the new reference value and the value of α, and the RGB value is detected by the color sensor.

  The above steps 314 to 320 are repeated until the difference is within the allowable range in step 318 while updating the reference value (Cs, Ms, Ys) and α.

  FIG. 21 is a conceptual diagram for searching for the optimum value in the processing of steps 314 to 320. In FIG. 21, for simplicity, the Y coordinate is omitted, and the C and M coordinates are used. In the figure, the horizontal axis is C, the vertical axis is M, and the coordinates of (C0, M0) set as initial values are located at 411a. When color mixing control is started, patches are first formed at four corners of a rectangle (411b) in the range of ± α0 in the C and M directions centering on (C0, M0) (steps 312 to 314). By interpolation from the values, the C and M coordinates (C0 ′, M0 ′) (412a) whose sensor output values are closest to K0 are calculated (step 315). Next, in steps 316 to 317, patches of (C0 ', M0') and K0 are formed, and it is determined whether or not the difference between the sensor output values is within an allowable range (step 318). If it is not within the allowable range, the second patch is formed at the four corners of the rectangle (412b) of ± α0 / 2 around the coordinates of 412a (steps 319 to 314), and again from the value of each patch. By interpolation, C and M coordinates (C0 ″, M0 ″) (413a) whose sensor output values are closest to K0 are calculated (step 315).

  Next, in the same manner as described above, patches of (C0 ″, M0 ″) and K0 are formed in steps 316 to 317, and it is determined whether the difference between the sensor output values is within an allowable range (step 318). If not within the allowable range, a third patch is formed at four corners of a rectangle (413b) of ± α0 / 4 around the coordinates of 413a (steps 319 to 314), and interpolation from the values of the respective patches is performed. Thus, the C and M coordinates (C0 ′ ″, M0 ′ ″) (not shown) whose sensor output value is closest to K0 are calculated (step 315).

  The optimum values of C, M, and Y are obtained while narrowing the search range by repeating the above processing.

  If the difference is smaller than the threshold value in step 318, it is determined that the difference is within the allowable range, and the values of C, M, and Y obtained in step 315 are set as optimum values. Steps 321 to 322 perform the same processing as steps 116 to 117 in the first embodiment.

  In the above description, (Cs, Ms, Ys) is performed by only one set. As in the first embodiment, a patch of (CN, MN, YN) for a plurality of K values KN (N = 0,... N). And the loop of steps 314 to 320 may be repeated only for patches (CN, MN, YN) that are not within the allowable range in step 318.

  Further, in order to reduce the consumption of the transfer material and reduce the number of patch formations, when the mixed color patch is formed in step 316, the mixed color patch in the next step 314 is formed in advance at the same time assuming the case of NO in step 318. May be.

  The configuration of the “color image forming apparatus” of the fourth embodiment is the same as that of the first embodiment. The processing flow in the present embodiment is the same as the flowchart in FIG. 6 of the first embodiment. In this embodiment, multiple regression analysis is used to determine the optimum value in step 115. Hereinafter, a method for obtaining the optimum value in the present embodiment will be described.

  First, the patch pattern formed in step 114 will be described.

  Each patch of the CMY mixed color patch and the K single color patch includes C, M, Y data (1) to (6) and K single color data (7) as shown in FIG. The values of C00 to C05, M00 to M05, and Y00 to Y05 are values obtained by changing only a specific color by ± α from the reference values C0, M0, and Y0 as in the second embodiment. The patch (7) is a K single-color patch and is formed with a predetermined value K0. As in the second embodiment, the reference values (C0, M0, Y0) are adjusted such that the K density characteristic is adjusted to the state of the gradation-density curve 300, and C, M, Y are the typical gradation-density curve states. When the values of C0, M0, and Y0 are mixed, the color is almost the same as K0. As shown in FIG. 17, the patch patterns (1) to (7) are formed on the transfer material, and the patches formed on the transfer material are detected by the color sensor 42 after passing through the fixing device 30 and output RGB values. To do.

  Next, in step 115, C, M, and Y values (gradation levels) for matching the C, M, and Y process grays with the K patch color in (7) are calculated from the RGB output values of the sensor.

  The RGB output values of each patch are (1) = (r00, g00, b00), (2) = (r01, g01, b01),... (6) = (r05, g05, b05), and K in (7) The RGB output value of the single color patch is (rk0, gk0, bk0).

Here, as shown in FIG. 23, the following regression coefficients rc0, rc1, rc2, and rc3 are obtained for R with the gradations of C, M, and Y as explanatory variables and R as a target variable.
R = rc1 * C + rc2 * M + rc3 * Y + rc0
The coefficients rc0, rc1, rc2, and rc3 are obtained as follows.

However,

Then,

To find rc0.
Further, the coefficients of the following multiple regression equations are similarly obtained for G and B.
G = gc1 * C + gc2 * M + gc3 * Y + gc0
B = bc1 * C + bc2 * M + bc3 * Y + bc0
Here, the values of C, M, and Y for the output values of K (rk0, gk0, bk0) are substituted into the above equation as (C0 ′, M0 ′, Y0 ′), and this is written in a matrix.

(C0 ', M0', Y0 ') is obtained by

  Further, the gradation of K is changed to have a plurality of reference values (CN, MN, YN, KN) (N = 0, 1, 2,... N), and (1) as described above for each reference value. The patches (7) to (7) are formed, and (CN ′, MN ′, YN ′, KN ′) is obtained for each (CN, MN, YN, KN).

  The subsequent processing is the same as that after step 116 in FIG.

  In the present embodiment, the number of patches and the patch value are the same as in the second embodiment, but the number of patches and how to select the value are not limited to this. Further, as a feature of the present embodiment, the present embodiment does not assume the lattice as in the first to third embodiments (FIG. 12), so the number of patches and the value of the patches can be selected relatively freely. The optimum value can be obtained with high accuracy regardless of the relationship between the value and the value of the patch.

11 Transfer material 22 Photoconductor, photosensitive drum 26 Developing means 27 Intermediate transfer body 41 Concentration sensor 42 Color sensor

Claims (6)

Image forming means for forming a plurality of mixed color patches by a plurality of combinations of gradation values of each color in cyan, magenta and yellow color materials and a single color patch by black color material on a sheet;
Fixing means for fixing the formed mixed color patches and the single color patch on a sheet;
The color values of the fixed mixed color patches and the single color patch on the paper before paper discharge are arranged so as to be detectable, and the color values of the plurality of color mixture patches and the color values from the paper before paper discharge are arranged. Detection means for detecting a color value with a single color patch;
The color values of the plurality of mixed color patches detected by the detection means are compared with the color values of the single color patch, and based on the comparison, the gradation values of the colors in the color materials close to the color value of the single color patch Correction means for obtaining a combination of the above and correcting the image forming conditions;
Color image forming apparatus according to claim Rukoto equipped with. When the plurality of mixed color patches are formed again on the paper under a predetermined condition, the image forming unit uses a combination of gradation values of each color in each color material obtained by the correction unit last time. based, color image forming apparatus according to claim 1, characterized that you form a plurality of mixed color patches on the paper. The image forming unit forms a plurality of the single-color patches corresponding to each of a plurality of black gradations, and forms a plurality of mixed-color patches based on a plurality of combinations of gradation values of each color in each color material. Formed on paper,
It said correction means for each of said plurality of said single-color patch, determined combination of the gradation value of each color in each color materials close to the color value of each of the single-color patch, that you correct the image forming conditions The color image forming apparatus according to claim 1, wherein the color image forming apparatus is a color image forming apparatus.   The image forming unit forms the plurality of mixed color patches and the single color patch in a row so as to be sequentially read on the paper by the detection unit, and the fixing unit forms the plurality of the plurality of color patches formed in a row on the paper. 2. The mixed color patch and the single color patch are fixed, and the detection unit detects color values of the plurality of mixed color patches and the single color patch formed and fixed in a line on the sheet. 4. A color image forming apparatus according to any one of items 1 to 3.   The color image forming apparatus according to claim 1, wherein the image forming condition is a process condition or a gradation conversion table of an image signal.   A color image control method in a color image forming apparatus,
  The image forming means forms on the paper a plurality of mixed color patches by a plurality of combinations of gradation values of each color in cyan, magenta and yellow color materials, and a single color patch by a black color material, and the fixing means Fixing the plurality of mixed color patches and the single color patch formed on paper;
  The color values of the fixed mixed color patches and the single color patch on the paper before paper discharge are arranged so as to be detectable, and the color values of the plurality of color mixture patches and the color values from the paper before paper discharge are arranged. By detecting means for detecting a color value with a single color patch, the color values of the plurality of mixed color patches and the single color patch are detected,
  The detected color value of the plurality of mixed color patches is compared with the color value of the single color patch, and based on the comparison, a combination of gradation values of each color in each color material close to the color value of the single color patch is determined. A method for controlling a color image, comprising: obtaining and correcting an image forming condition.

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