JP2005140861A - Chromaticity stabilization control method, gradation stabilization control method, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To establish a chromaticity stabilization control system and stabilize the optimum chromaticity. <P>SOLUTION: The chromaticity of a chromaticity detection patch formed onto a transfer material with a plurality of color materials is detected by a color sensor 42. Image forming conditions are corrected so that the detected chromaticity matches predetermined target chromaticity. Control is exerted such that the image forming conditions are continuously or gradually corrected after the detection of the chromaticity of the chromaticity detection patch but before the detection of the chromaticity of the next chromaticity detection patch. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus that forms a color image on a recording material based on image information.

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

  However, in an electrophotographic image forming apparatus, if there is a change in each part of the apparatus due to a change in environment or long-term use, the density of the output image also changes. In particular, in the case of an electrophotographic color image forming apparatus, there is a risk that the color balance may be lost even by a slight change in density. Therefore, it is necessary to always maintain a constant gradation-density characteristic.

  Therefore, each color toner is provided with several kinds of exposure amounts and development biases according to absolute humidity and gradation correction means such as a lookup table (LUT), and absolute values measured by temperature and humidity sensors. Based on the humidity, the optimum process conditions and gradation correction values 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 Is detected by a density detection sensor for unfixed toner, a density deviation value is calculated from the detection result, and density control is performed by feeding back process conditions such as exposure amount and development bias according to the density deviation value. Thus, a stable image is obtained.

  However, the density control using the above-described density detection sensor for unfixed toner forms and detects a patch on an intermediate transfer body, a drum or the like, and the image is transferred and fixed onto the transfer material that is subsequently performed. There is no control over changes in color balance. Further, the color balance also changes depending on the transfer efficiency in transferring the toner image onto the transfer material and the heating and pressurization by fixing. Such a change cannot be dealt with by density control using the above-described density detection sensor for unfixed toner.

  Therefore, a density or chromaticity sensor (hereinafter referred to as a color sensor) that detects 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 controlling the density or chromaticity. (Hereinafter referred to as a patch) is formed on a transfer material, a density deviation value with respect to a reference density or a chromaticity deviation value with respect to a reference chromaticity is calculated from the detected density or chromaticity, and the calculated density or chromaticity deviation value is obtained. Accordingly, an image forming apparatus (for example, see Patent Document 1) that feeds back the exposure amount, process conditions, look-up table (LUT), etc., and controls the density or chromaticity of the final output image formed on the transfer material. Proposed.

  This color sensor uses, for example, a light source that emits red (R), green (G), and blue (B) as a light emitting element to identify CMYK or detect density or chromaticity. As 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. . This makes it possible to identify CMYK and detect density from three different outputs obtained, 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).

  Also in an ink jet printer, the color balance changes due to a change in ink discharge amount with time, an environmental difference, and an individual difference of ink cartridges, and the tone-density characteristic cannot be kept constant. Therefore, it has been proposed to install a color sensor near the output part 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 the chromaticity. For example, a process gray patch is formed by mixing a gray patch of black (K) as a reference color and cyan (C), magenta (M), and yellow (Y) as detection colors, and both patches are detected from the chromaticity detected by the color sensor. The chromaticity difference between them is detected, and this is used as the chromaticity deviation value of the detected color, and the CMY gamma characteristics, color matching table, and color separation table are corrected so that this chromaticity deviation value becomes zero. There is a control method for correcting a chromaticity shift.
JP 2003-84532 A

  However, the conventional example has the following drawbacks.

  Even if the chromaticity deviation is reduced by performing the above-described chromaticity correction control, the chromaticity deviation usually increases as time passes or the number of printed sheets increases. As a result, if an appropriate time interval, print interval, etc. are available, it is necessary to detect chromaticity deviation and perform chromaticity correction control.

  FIG. 16 is a diagram illustrating a chromaticity shift (ΔE) with respect to the print time or the number of prints. When the chromaticity deviation correction control (t1) is performed with a correction value that makes the chromaticity deviation value 0 with respect to the detected chromaticity deviation value, the time until the next chromaticity deviation correction control (t2) If the chromaticity deviation over time behavior (ΔE (t)) as shown in FIG. 16 is assumed, a value obtained by integrating the maximum value or time (number of printed sheets) with respect to the absolute value of the chromaticity deviation value (FIG. 16). The area of the inner coating) is an index of chromaticity stabilization.

Accordingly, if the chromaticity deviation increases monotonously with respect to time or the like as shown in FIG. 16, the control for correcting the chromaticity with a correction value that makes the chromaticity deviation value 0 is stable chromaticity. Since the conversion index becomes large, it is not always the optimal chromaticity stabilization control.
An object of the present invention is to construct a chromaticity stabilization control system and to achieve optimal chromaticity stabilization.

  The present invention relates to a chromaticity stabilization control method in an image forming apparatus that forms an image on a transfer material using a plurality of color materials, and detects chromaticity on a transfer material formed using a plurality of color materials Detecting the chromaticity of the patch for the image, correcting the image forming conditions so that the detected chromaticity matches the predetermined target chromaticity, and detecting the chromaticity of the chromaticity detection patch, And a step of controlling to correct the image forming conditions stepwise or continuously until the chromaticity of the detection patch is detected.

  The present invention also provides an image forming apparatus that forms an image on a transfer material using a plurality of color materials, and detects the chromaticity of a chromaticity detection patch on the transfer material formed using the plurality of color materials. Correction means for correcting an image forming condition so that the detected chromaticity matches a predetermined target chromaticity, and after detecting the chromaticity of the chromaticity detection patch, the chromaticity of the next chromaticity detection patch Control means for correcting the image forming conditions in a stepwise or continuous manner until the image is detected.

  According to the present invention, optimal chromaticity stabilization control can be performed.

  The best mode for carrying out the invention will be described below in detail with reference to the drawings. As an example of an image forming apparatus, a tandem color image forming apparatus employing an intermediate transfer member, which is an example of an electrophotographic color image forming apparatus, will be described. The color image forming apparatus includes the image forming unit shown in FIG. 1 and an image processing unit (not shown).

  FIG. 39 is a block diagram illustrating the configuration of the control unit according to the first embodiment. In FIG. 39, reference numeral 3901 denotes an image data source, which is realized by a host computer or an image scanner. A control unit 3902 controls the color image forming apparatus and includes an image processing unit 3903 that processes image data from the image data source 3901. Reference numeral 3904 denotes a laser driving unit that modulates laser light based on image data from the control unit 3902 to form an image.

  Further, the image processing unit 3903 has a memory 3905, and a gradation correction table 3906 is stored in the memory 3905.

  Further, detection signals from various sensors such as a density sensor and a color sensor are input to the control unit 3902. The arithmetic circuit 3907 performs arithmetic processing based on the input detection signal, and rewrites the gradation correction table 3906. Reference numeral 3908 denotes a memory, which stores data for the arithmetic circuit 3907 to perform arithmetic processing.

  In the first embodiment, a chromaticity stabilization control method using a color sensor will be described. First, processing in the image processing unit will be described. The image processing unit is executed by a CPU of a control unit (not shown) according to a program and control data stored in the ROM, and a work area and various tables used by the CPU when executing the processing are defined in the control unit. RAM and peripheral devices.

  FIG. 2 is a diagram illustrating an example of processing in the image processing unit of the color image forming apparatus. An RGB signal representing the color of an image sent from a host computer or the like is converted into a device RGB signal (hereinafter referred to as DevRGB) that matches the color reproduction range of the image forming apparatus by a color matching table 201 prepared in advance. The Next, this DevRGB signal is converted into a CMYK signal which is a toner color material color of the image forming apparatus by a color separation table 202 prepared in advance. The CMYK signal is corrected for gradation-density characteristics by a density correction table 203 for correcting the gradation-density characteristics specific to each color image forming apparatus, and converted to a C′M′Y′K ′ signal. Converted. After that, halftone processing is performed by the halftone table 204 and converted into C "M" Y "K" signals. Finally, PWM (PulsE Width Modulation) processing 205 converts the exposure times Tc, Tm, Ty, and Tk of the scanner units 24C, 24M, 24Y, and 24K corresponding to the respective C "M" Y "K" signals. .

  Next, the configuration and operation of the image forming unit in the electrophotographic color image forming apparatus will be described with reference to FIG.

  FIG. 1 is a diagram illustrating a configuration of an image forming unit in a color image forming apparatus. The image forming unit forms an electrostatic latent image on the photosensitive drum by the exposure light that is turned on based on the exposure time converted by the image processing unit, and develops the electrostatic latent image to form a single color toner image. The single color toner images are superposed to form a multicolor toner image, the multicolor toner image is transferred to the transfer material 11, and the multicolor toner image on the transfer material 11 is fixed. , Photoconductors (22Y, 22M, 22C, 22K) for each station juxtaposed for development colors, injection charging means (23Y, 23M, 23C, 23K) as primary charging means, toner cartridges (25Y, 25M, 25C, 25K) , Developing means (26Y, 26M, 26C, 26K), an intermediate transfer member 27, a transfer roller 28, and a fixing unit 30.

  Each photosensitive drum (photoconductor) 22Y, 22M, 22C, and 22K is configured by applying an organic optical conductive layer to the outer periphery of an aluminum cylinder, and rotates by receiving a driving force of a driving motor (not shown). The motor rotates the photosensitive drums 22Y, 22M, 22C, and 22K in the counterclockwise direction according to 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 above-described electrostatic latent image, four developing units 26Y and 26M that develop yellow (Y), magenta (M), cyan (C), and black (K) for each station. , 26C, 26K, and each developing device is provided with a sleeve 26YS, 26MS, 26CS, 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 multicolor toner image while conveying the transfer material 11. As shown in FIG. 1, the fixing unit 31 heats the transfer material 11 and the transfer material 11 is fixed to the fixing roller 31. A pressure roller 32 is provided for pressure contact. The fixing roller 31 and the pressure roller 32 are formed in a hollow shape, and heaters 33 and 34 are incorporated in each of them. 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 completed.

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

  Further, the density sensor 41 is disposed toward the intermediate transfer body 27 in the color image forming apparatus shown in FIG. 1 and measures the density of the toner patch formed on the surface of the intermediate transfer body 27.

  FIG. 3 is a diagram showing an example of the configuration of the density sensor 41 shown in FIG. The density sensor 41 includes an infrared light emitting element 301 such as an LED, a light receiving element 302 such as a photodiode or Cds, an IC (not shown) that processes received light data, and a holder (not shown) that accommodates these. The light receiving element 302 a detects the intensity of irregularly reflected light from the toner patch 64, and the light receiving element 302 b detects the intensity of regular reflected light from the toner patch 64.

  Thus, by detecting both the regular reflection light intensity and the irregular reflection light intensity, the density of the toner patch 64 from the high density to the low density can be detected. 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 301 and the light receiving element 302.

  The density sensor 41 described above 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. Thereafter, this density data is fed back to the density correction table for correcting the gradation-density characteristics of the image processing unit and each process condition of the image forming unit.

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

  FIG. 4 is a diagram showing an example of the configuration of the color sensor 42 shown in FIG. The color sensor 42 includes a white LED 401 and a charge storage sensor 402a with an RGB on-chip filter.

  In this configuration, the white LED 401 is incident at an angle of 45 degrees with respect to the transfer material 11 on which the patch after fixing is formed, and the intensity of diffuse reflected light in the 0 degree direction is detected by the charge storage sensor 402a with an RGB on-chip filter. . The light receiving portion of the charge storage sensor 402a with the RGB on-chip filter is a pixel independent of RGB as in 402b. Further, the charge storage sensor of the RGB charge storage sensor 402a may be a photodiode. Note that 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 emits RGB three colors, and a sensor without a filter.

  Next, the concept of gradation-density characteristic control in the first embodiment using the above-described density sensor 41 and color sensor 42 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 42 consumes the transfer material 11, the number of executions is limited compared to the control using the density sensor 41. Therefore, as shown in FIG. 5, first, gradation-density characteristic control (hereinafter referred to as color mixing control) using the color sensor 42 and the density sensor 41 is performed at 501, and then at 502 to 504, the density sensor. Gradation-density characteristic control using only 42 (hereinafter referred to as single color control) is performed a prescribed number of times (N times), and the process returns to color mixing control.

  Note that color mixing and single color control are performed between normal printing operations. The execution timing is detected by detecting an environmental change or the like, and is automatically executed at a predetermined timing set in advance, or is performed manually by the user when the user desires to execute the control.

  FIG. 6 is a flowchart showing details of gradation-density characteristic control combining color mixture control and single color control. First, in step S610, it is determined whether or not the cartridge has been replaced, and it can be determined whether to perform color mixing control 1 or to perform control combining single color control, color mixing control 2, and color mixing control 3. If the cartridge has been replaced, the process advances to step S611 to use a predetermined default gradation-density curve as a target of the gradation-density characteristics of each color of C, M, Y, K. This default gradation-density curve is set in consideration of the characteristics of the color image forming apparatus. In the first embodiment, the output density is linear with respect to the input gradation as shown in FIG. 7, and the density correction table 203 is a so-called through table that does not change the input value. Shall be used.

  In step S 612, a patch pattern is formed on the intermediate transfer member 27 and read by the density sensor 41. FIG. 8 is a diagram illustrating an example of a patch pattern formed on the intermediate transfer member 27. 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 gradations of C, M, Y, and K forming the patch are determined in advance. The density of the patch pattern 64 formed on the intermediate transfer member 27 is detected by the density sensor 41, and a gradation-density curve is generated from the density by interpolation.

  Here, when the cyan density detection result is as indicated by the black circle shown in FIG. 9, a tone-density curve such as 901 is generated by interpolation such as linear interpolation. Further, an inverse characteristic curve 903 is calculated based on the target density curve 902 set in step S611, and the vertical axis of which is the output gradation level is used as the cyan density correction table for the input image data. In step S613, the input image data is subjected to table conversion using the cyan density correction table, so that the cyan input gradation degree and the output density have the relationship of the target gradation-density curve 902. A similar density correction table is generated for M, Y, and K.

  Next, in step S614, using the density correction table for each color generated in step S613, each corrected CMY mixed color patch and K single-color patch pattern are formed on the transfer material 11 and detected by the color sensor 42. . Hereinafter, the contents of this process will be described in detail.

  First, in the first embodiment, the CMY mixed color patch pattern is composed of 64 patches in total, 8 sets each including 8 patches. The eight patches in each group will be described by taking the group 0 as an example. The seven C, M, Y data (0-0) to (0-6) shown in FIG. -7). Further, the C, M, and Y gradations of the patches (0-0) to (0-6) are the reference gradations (hereinafter referred to as reference values) C0, M0 as shown in FIG. , Y0 and their reference values are combinations of values obtained by changing the gradation of a specific color by ± α.

  On the other hand, the patch (0-7) is a K single-color patch, and is formed with a predetermined gradation K0.

  Here, the values of the reference values C0, M0, Y0, and K0 are adjusted so that the tone-density characteristics of C, M, Y, and K are adjusted to the default tone-density curve 902, and normal image forming conditions are set. Below, when the values of C0, M0, and Y0 are mixed, the value is the same color as K0, and is set during color processing and halftone design. Further, the reference values K0 to K7 of K for each set are set so as to monotonously increase from a low density to a high density, and Cn, Mn, Yn (n = 1... 7) are mixed in the same manner as described above. The value is the same color as Kn.

  As described above, 64 patch patterns (0-0) to (7-7) as shown in FIG. 11 are formed on the transfer material 11, and the patches formed on the transfer material 11 are fixed to the fixing device. After 30 passes, the color sensor 42 detects and RGB values are output from the color sensor 42.

  Next, the RGB output values from the color sensor 42 are converted into the L * a * b * color system by primary conversion using a matrix. Here, the characteristics of the RGB filter of the color sensor 42 are in a non-linear relationship with the characteristics of the ideal L * a * b * color matching function. Therefore, if conversion is performed using the same matrix in the entire color gamut, an error occurs. It becomes very big. Therefore, considering that each set of patches is in the color gamut near each Kn (n = 0... 7), an optimum matrix is prepared for each color gamut near each Kn (n = 0... 7). Each set of patches is converted by a corresponding matrix to improve the conversion accuracy.

  In this matrix, a 3 × 3 matrix An and a 1 × 3 matrix Bn (n = 0... 7) are prepared corresponding to each set of patches, and for the set 0, A0, B0, and set 1 are set. , The RGB output value of the sensor is converted into an L * a * b * value using the following equation as A1, B1,...

Next, in step S615, using the converted L * a * b * values, the C, M, Y process gray and the K single-color patch colors (C, M, Y values) Furniture).
Here, the contents of step S615 will be described using the patch set 0 as an example. The gradation levels of the CMY mixed color patches shown in FIG. 10 are again set to (0-0) = (C00, M00, Y00) to (0-6) = (C06, M06, Y06). Further, the L * a * b * values of the CMY mixed color patches are (0-0) = (L * 00, a * 00, b * 00), (0-1) = (L * 01, a * 01). , B * 01), ..., (0-6) = (L * 06, a * 06, b * 06), and the L * a * b * value of the K single color patch (0-7) is (L * K0, a * K0, b * K0).

  FIG. 15 shows the reference chromaticity (L * 00, a * 00, b * 00) of the CMY mixed color patch and the chromaticity (L * K0, a * K0, b * K0) of the K single color patch. * It is the figure plotted in a * b * space. ΔE in the figure is a color difference between both patches, and is an amount obtained as follows.

In the color mixing control 1 (step S615), CMY gradation levels are obtained such that ΔE is 0 for each set of patches.
Next, for L *, as shown in FIG. 12, coefficients L0, L1, L2, and L3 of the following multiple regression equations are obtained with C, M, and Y gradations as explanatory variables and L * as a target variable.

L * = L1 * C + L2 * M + L3 * Y + L0 Formula 4
The coefficients L0, L1, L2, and L3 are obtained as follows.

  However,

Then,
L = S ⁻¹ T Formula 6
L1, L2, and L3 can be obtained.

  Furthermore,

To find L0.
Furthermore, the coefficient of the following multiple regression equation is similarly obtained for a * b *.

a * = a1 * C + a2 * M + a3 * Y + a0 Equation 8
b * = a1 * C + b2 * M + b3 * Y + b0 Equation 9
Therefore,

It becomes. Here, the values of C, M, and Y for the L * a * b * values of K (L * K0, a * K0, b * K0) are substituted into the above formula as (C0 ', M0', Y0 '). And solving this,

Thus, (C0 ′, M0 ′, Y0 ′) is obtained.
The same calculation is performed for the other sets 1 to 7, and (Cn ', Mn', Yn ') for the reference values (Cn, Mn, Yn, Kn) (n = 1, 2, ..., 7). , Kn ′). If the relationship of cyan between (Cn, Mn, Yn) and (Cn ', Mn', Yn ') obtained in this way is as shown by the black circles in FIG. 13, the value between the black circles is, for example, linear A curve (color correction table) such as 1301 is created by interpolation.

  In step S616, the density correction target table is corrected. A tone-density curve is generated by multiplying the original target tone-density curve (902 shown in FIG. 9) by the color correction table 1301 shown in FIG. 13, and this is used as the tone-density of the new cyan target. A curve is formed (1401 shown in FIG. 14). Specifically, the input gradation degree is converted into an output density according to a target gradation-density curve after table conversion with the color correction table 1301.

  Similarly, the targets for M and Y are changed. By performing density correction with this new target, the color due to the mixed color of (Cn, Mn, Yn) matches the color of Kn. The values of the reference values (Cn, Mn, Yn, Kn) are “the human eye is sensitive to highlight gray and becomes insensitive as shadows occur”, “UCR processing (during color separation) Note that gray due to only the three colors of CMY does not appear in the shadow area because a part of CMY is replaced with K), and the present invention is more effectively implemented by selecting highlights as the center. it can.

  Next, in step S617, a density correction table is generated again using the C, M, and Y targets changed in step S616 from the density detection result in step S612. Thereafter, this density correction table is used for printing. The density of the input image data is corrected and the printer is ready for printing. The above is color mixing control 1 when the cartridge is replaced.

  FIG. 16 is a diagram illustrating ΔE of an arbitrary patch set with respect to the print time or the number of prints. When the color mixing control 1 is performed so that ΔE becomes 0 (t1), the time-dependent behavior of ΔE as shown in FIG. 16 until the next color mixing control 1 or the color mixing control 2 (t2) described later is performed. (ΔE (t)), it is assumed that it is known from a study experiment or the like. As a result, although ΔE is small immediately after color mixing control 1 at time t1, ΔE increases monotonically thereafter and reaches the maximum at the next color mixing control t2.

  Next, when the cartridge has not been replaced and the specified number of sheets is printed in a printable state (step S620), the color sensor 42 detects whether the color sensor 42 previously detected the patch or not (step S621). The control flow changes. When the previous color mixture control 1 is performed, the monochrome color density control is performed.

  First, the case where the previous time was the color mixture control 1 will be described. In monochromatic density control, a patch pattern is formed on the intermediate transfer body 27 in step S625 as in step S612, and is read by the density sensor 41. The density of the patch pattern formed on the intermediate transfer body 27 is detected by the density sensor 41, a gradation-density curve is generated by interpolation based on the detected density, and the target 1401 generated in step S616 is used. The density correction table is updated by the same method as in step S613 (step S626).

  Further, it is determined whether or not the specified number of times, i.e., N times of monochromatic density control has been performed (step S627). If the specified number of times has not been reached, the printable state is entered again. If the specified number of times has been performed, color mixing control 2 is performed. Similar to the color mixture control 1 described above, the CMY color mixture and the K single-color patch pattern are again formed on the transfer material 11 in step S614 and detected by the color sensor 42. At this time, the patch pattern is formed using the latest density correction table. Processing is performed in the steps described above until step S617.

  However, when creating a new target, the new inverse characteristic table is multiplied with the target 1401 generated in the previous step S616. In step S630, the gradation data (C00, M00, Y00) to (C76, M76, Y76) of all patches detected by the color sensor, which are data for use in color mixture control 3 described later, and the chromaticity Data (L * 00, a * 00, b * 00) to (L * 76, a * 76, b * 76), (L * K0, a * K0, b * K0) to (L * K7, a * K7, b * K7) and color difference data ΔE for each gradation between the K single color patch and the CMY mixed color patch are stored in a nonvolatile memory such as an EEPROM.

  Next, returning to step S621, the chromaticity stabilization control method in the first embodiment when the previous color mixing control 2 was performed will be described.

  It is determined whether or not the monochrome density control has been performed for the specified number of times from the previous detection of the patch by the color sensor 42 to the present (step S622). The specified number of times at this time is the number obtained by dividing the specified number of times (N times) in step S627 into an appropriate stage number n. If this number is Mn times, Mn is M1 = N / (n + 1), M2 = 2 × M1. , M3 = 3 × M1,. FIG. 20 illustrates the above-mentioned prescribed number Mn applied to the case of FIG. 16 described above.

  In the figure, s1 and s2 are times at each stage (monochromatic density control M1 and M2 times) where the number of stages n = 2. If it is not the prescribed number of times Mn, the above-described monochromatic density control is performed. When the density correction table is updated in step S626, the latest target 1401 generated in step S616 or step S634 described later is used. If the specified number of times is Mn, the color difference between the CMY color mixture patch at the current stage and the target K single color patch is predicted using the data held in step S630 in the previous color mixture control 2 (step S631).

  Here, a method for predicting the color difference will be described. As described in the color mixing control 1, when performing the color mixing control 1 such that ΔE becomes 0, the behavior shown in FIG. 20 is performed. For example, in FIG. 15, the CMY color mixing patch is determined from the chromaticity of the target patch. Although the deviation of ΔE becomes the chromaticity of the target patch by the color mixing control 1, the next color mixing control 1 is expected to have the same chromaticity as the previous time, which is also shifted by ΔE from the target. It is expected that CMY mixed colors will go back and forth between both patch chromaticities each time.

  Therefore, when the next color sensor 42 detects, it can be predicted that the same ΔE shifts in the same direction. Therefore, for example, ΔE until the next color mixing control is predicted by a linear approximation equation as shown in the following equation. .

However, ΔE in Equation 12 is a value obtained by Equation 3 in each patch set, and ΔE ′ is a predicted ΔE in each patch set at time t until the next t2. FIG. 21 shows a prediction using this equation 12. Therefore, ΔE predicted at each stage is ΔE ′ (s1) and ΔE ′ (s2).
Next, in step S632, an offset value for stabilizing chromaticity is calculated from the predicted color difference ΔE ′, and is added to the chromaticity of the K single-color patch detected by the previous color mixing control 2.

  The contents of step S632 will be described using the patch set 0 as an example. An offset value calculated in accordance with the color difference ΔE ′ predicted in step S631 is added to the chromaticity of the K single color detected in the previous color mixture control 2, and the added chromaticity of the new target is added. The CMY color mixture is corrected again. As a result of this correction, the offset value so that the CMY mixed color coincides with the K single color patch can be obtained as shown in FIG. 17 on the straight line passing through the chromaticity between both patches detected by the previous color mixing control 2 as shown in FIG. This is a value shifted by ΔE ′ predicted in the opposite direction from CMY mixed color to CMY. That is, the offset values (L * offset0, a * offset0, b * offset0) can be obtained as follows, for example, with the offset coefficient coe = -1.

  Here, ΔE ′ in the equation is a color difference ΔE predicted at the current stage, for example, ΔE ′ (s1) at the time of M1 times. Then, this offset value is added to the chromaticity of the K single color patch to obtain new target chromaticities (L * K0 ′, a * K0 ′, b * K0 ′) of the K single color patch.

Here, (L * K0, a * K0, b * K0) in the equation is the chromaticity of K single color detected in the previous color mixture control 2. The same calculation is performed for the other sets 1 to 7, and the new target chromaticity (L * Kn ′, a * Kn ′, b * Kn ′) of K single color (n = 1, 2,..., 7). Ask for.
Next, in step S633, C, M, and Y gradations for the C, M, and Y process gray patches to match the new target chromaticity obtained in step S632 are calculated again.

  The process in step S633 will be described using the patch set 0 as an example. The method of calculating the C, M, and Y gradations to match the new target chromaticity of the K single color is obtained by using the chromaticities detected by the previous color mixing control 2 and the like in Expressions 4 to 10 in Step S615. Calculated in the same manner, and the values of C, M, and Y for the new target chromaticity (L * K0 ′, a * K0 ′, b * K0 ′) are (C0 ″, M0 ″, Y0 ″) and So, so

It becomes.
The same calculation is performed for the other sets 1 to 7, and (Cn ″, Mn ″, Yn ″ with respect to the reference values (Cn, Mn, Yn, Kn) (n = 1, 2,..., 7). , Kn ").

  Thereafter, in steps S634 and S635, processing similar to that in steps S616 and S617 of the color mixture control 2 is performed. However, when a new target is created, the new inverse characteristic table is multiplied with the target 1401 used in step S616 of the previous color mixing control 2.

  The above is the color mixture control 3 when the cartridge is not exchanged and the previous time was the color mixture control 2. Subsequently, the single-color control is performed again on the newly generated density correction target table to correct the deviation of the CMY color mixture.

  FIG. 18 is a diagram illustrating ΔE of a set of arbitrary patches with respect to the print time or the number of prints when the color mixture control 3 is performed. This is a case where the color mixture control 2 is performed at times t1 and t2, and the color mixture control 3 (s1, s2) is performed in two stages between them. At each stage of the color mixing control 3, correction is performed by the predicted ΔE ′, but there is a shift between the predicted ΔE ′ and the actual color difference ΔE (FIG. 21), so the shift remains. However, the chromaticity is stable as compared with FIG. 16 in which the color mixture control 3 is not performed.

  If any color cartridge is exchanged in the printable state (step S610), the image forming conditions change greatly, and the process returns to step S611.

  The above is the single color / color mixture control using the color sensor 42 in the first embodiment.

  In the first embodiment, chromaticity correction is performed in two stages. However, the number of stages is not limited to two, and continuous correction such that chromaticity correction is performed every time monochrome density control is performed. good.

  In the first embodiment, the color difference until the next chromaticity shift is detected is predicted by linear approximation. However, the prediction method is not limited to linear approximation, and may be a polynomial or various interpolation equations. A predetermined value may be used as the same chromaticity change can always be predicted by experiments.

As described above, if the chromaticity deviation is predicted and the chromaticity correction is performed in stages until the next chromaticity deviation is detected in the first embodiment, the chromaticity stabilization control is performed with a simple configuration. be able to.
[Modification 1]
In the first embodiment, the color mixture control 3 is performed in the case of YES in step S621 shown in FIG. 6, but in order to suppress the consumption of the transfer material 11, this color mixture control 3 is not performed, only the single color control is performed, and the color mixture is performed. Control 2 may be performed.

  FIG. 35 is a flowchart showing details of gradation-density characteristic control in which color mixture control and single color control are combined in a modification of the first embodiment. In addition, the same code | symbol is attached | subjected to the same step as each step shown in FIG. 6, and the detailed description is abbreviate | omitted. Here, the color mixture control 2 in the modification will be described.

  First, in step S3501, similarly to the color mixture control 1, a CMY color mixture and a K single-color patch pattern are formed on the transfer material, detected by the color sensor 42, and converted into L * a * b * values. At this time, however, the patch pattern is formed using the latest density correction table.

  In step S3502, an offset value for chromaticity stabilization control is calculated from the color difference between the CMY color mixture and the K single color patch using the converted L * a * b * value, and the chromaticity of the K single color patch is calculated. Append to

  The contents of step S3502 will be described using the patch set 0 as an example. Similar to the color mixture control 1, the gradation and chromaticity of each patch formed by the latest density correction table are the same as those of the CMY mixed color patches (C00, M00, Y00) to (C06, M06, Y06). The L * a * b * values are (L * 00, a * 00, b * 00) to (L * 06, a * 06, b * 06), and the chromaticity L * a * b * value of the K single color patch Is (L * K0, a * K0, b * K0).

  In the color mixture control 1, for each set of patches, the CMY gradation level such that the color difference ΔE between the CMY color mixture and the K single color is 0 is obtained, and the density correction target table is corrected. However, the color mixing control 2 corrects the CMY gradation so that ΔE is not 0 but an offset value. That is, the target for CMY chromaticity correction is not the chromaticity of the K single color, but the chromaticity obtained by adding an offset value to the chromaticity of the K single color. As described in the color mixing control 1, when performing the color mixing control 1 such that ΔE becomes 0, the behavior as shown in FIG. 16 is performed. For example, the CMY color mixture patch shown in FIG. 15 has the chromaticity of the target patch. Although the chromaticity of the target patch is changed by ΔE from the target, the color chromaticity of the target patch is expected to be the same as the previous chromaticity that is also shifted by ΔE from the target at the next color mixing control 1. It is expected that the CMY mixed color goes back and forth between both patch chromaticities each time.

  Therefore, in order to keep ΔE small for a longer period of time, that is, to stabilize the chromaticity, the CMY mixed color is not matched with the K single color as the target, but the CMY mixed color is shifted in the opposite direction as seen from the K single color. Overcorrection is performed to correct the target, and the chromaticity of the CMY mixed color passes over the chromaticity of the K single color or near the chromaticity of the K single color until the next color mixing control. Like that.

  Here, as shown in FIG. 36, the offset value is a value shifted in the opposite direction from the K single color to the CMY mixed color on a straight line passing through the chromaticity between both patches. For example, the CMY mixed color (L * 00, a * 00, b * 00) and a value shifted by a half of ΔE between K single colors (L * K0, a * K0, b * K0). That is, the offset values (L * offset0, a * offset0, b * offset0) can be obtained as follows, for example, with the offset coefficient coe = −1 / 2.

Here, (L * 00, a * 00, b * 00) and (L * K0, a * K0, b * K0) in the equation are the chromaticity of the standard gradation of the CMY mixed color and the chromaticity of the K single color. It is. Then, the obtained offset value is added to the chromaticity of the K single color patch to obtain new chromaticities (L * K0 ′, a * K0 ′, b * K0 ′) of the K single color patch. This new chromaticity is obtained by the above equation 14.
The same calculation as above is performed for the other sets 1 to 7, and new chromaticities (L * Kn ′, a * Kn ′, b * Kn ′) of K single color (n = 1, 2,..., 7 )

  Next, in step S3503, C, M, and Y gradations are calculated so that the C, M, and Y process gray patches match the new chromaticity of the K single color obtained in step S3502.

  The contents of step S3503 will be described using the patch set 0 as an example. The method of calculating the gradation of C, M, and Y to match the new chromaticity of the K single color is calculated by the same method as Expression 4 to Expression 10 in Step S615 of the color mixing control 1, and the new K single color patch is calculated. The values of C, M, and Y with respect to the chromaticity (L * K0 ′, a * K0 ′, b * K0 ′) are obtained from Equation 10 as (C0 ″, M0 ″, Y0 ″). .

  The same calculation is performed for the other sets 1 to 7, and (Cn ″, Mn ″, Yn ″ with respect to the reference values (Cn, Mn, Yn, Kn) (n = 1, 2,..., 7). , Kn ").

  Thereafter, in steps S3504 and S3505, processing similar to that in steps S616 and S617 of the color mixture control 1 is performed. However, when creating a new target, the new inverse characteristic table is multiplied with the target 1401 generated in the previous step S3504.

  The above is the single color control / color mixing control 2 when the cartridge is not exchanged.

  FIG. 37 is a diagram illustrating ΔE of a set of arbitrary patches with respect to the print time or the number of prints when the color mixture control 2 is performed. When color mixing control 2 is performed so that ΔE becomes the offset value ΔE / 2 (t1), the color mixing control is performed because the chromaticity of the K single-color patch passes until the next color mixing control 1 or 2 (t2) is performed. Compared to FIG. 16 in the case of 1, the chromaticity is stable.

  According to this modification, chromaticity stabilization control can be performed with a simple configuration by performing chromaticity correction by adding an offset value to the chromaticity of the target.

  The number of patches formed on the transfer material 11 is not limited to the number used in the first embodiment and the modification.

  Further, although the same α, C, M, and Y values are used, different values may be used for each color.

  Furthermore, although the color sensor 42 has RGB output, the shape of the filter is not limited to RGB.

  Furthermore, although the matrix is a conversion matrix from RGB to L * a * b *, the conversion source data may be four data including sensor output values that do not pass through the filter in addition to RGB. This data may be data on a color space other than L * a * b *.

  Furthermore, the color of the mixed color patch of C, M, and Y is matched with the color of the K patch. The chromaticity L * a * b * of the mixed color patch of C, M, and Y detected by the color sensor 42 is 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, b * = 0, and fed back to the single color control.

  Furthermore, although the offset value is set to a value proportional to the predicted color difference, the offset value may be a value that minimizes the above-described chromaticity stabilization index (Equation 1) from the predicted color difference.

  Further, although the offset value is added to the target chromaticity, the chromaticity is stabilized so that the chromaticity is stabilized after calculating the chromaticity such that the mixed color of C, M, and Y matches the chromaticity of the target. An offset value may be added, and an offset value may be added to any correction value as long as the correction value controls chromaticity.

  As described above, according to the first embodiment and the modification, it is possible to construct a chromaticity deviation stabilization control system.

  In the first embodiment, the offset value to be added to the chromaticity of the target K single color in the color mixing control 3 is determined by prediction from the color difference between the K single color detected in the color mixing control 2 and the CMY mixed color, but in the second embodiment, The offset value is determined from the previous target chromaticity when the CMY color mixture patch is formed by the color mixture control 2 and the color difference between the detected CMY color mixture.

  That is, with reference to FIG. 21, when the color mixing control 2 for forming the CMY color mixing patch is time t2, the offset value newly calculated by the color mixing control 3 at time s2 is added to the previous target chromaticity. 19, the target chromaticity obtained by adding an offset value to the chromaticity of the K single-color patch each time in the color mixture control 3 is held in a non-volatile memory such as an EEPROM (not shown). As shown in FIG. 5, the same as that detected in the next color mixing control 2 from the chromaticity ΔE between the chromaticity of the CMY color mixing patch detected in the color mixing control 2 and the target chromaticity held when the color mixing patch is formed. It is also predicted that the chromaticity of the CMY mixed color is shifted by the same ΔE.

  Here, differences from the first embodiment will be described. First, in step S631 of the color mixture control 3, if the patch set 0 is taken as an example, from the chromaticity (L * 00, a * 00, b * 00) of the CMY color mixture patch detected in the previous color mixture control 2, If the chromaticity of the target to which the offset value is added when the mixed color patch is formed is (L * K0 ″, a * K0 ″, b * K0 ″), the prediction is made until the next color mixing control 2 is performed. ΔE ′ is

It becomes.
Then, as in step S632 of the first embodiment, the obtained offset value is added to the K chromaticity of the K single color detected in the previous color mixture control 2, and the new chromaticity of the target (L * K0 ′, a * K0 ′). B * K0 ′). Similar calculations are performed for the other sets 1 to 7 to obtain new chromaticities (L * Kn ′, a * Kn ′, b * Kn ′) (n = 1, 2,..., 7) of the target. . Then, for the color mixing control 3 in the second embodiment after the next color mixing control 2, the obtained new chromaticities (L * Kn ′, a * Kn ′, b * Kn ′) of the target are held in the memory.

  As described above, if the prediction of ΔE until the next color mixing control 2 in the second embodiment is performed in consideration of the chromaticity of the target when the color mixing patch is formed by the color mixing control 2, high-precision color can be obtained. The degree deviation can be predicted, and therefore, highly accurate chromaticity stabilization control can be performed.

  The data stored in the memory for predicting the chromaticity deviation ΔE by the color mixing control 3 is not limited to the target chromaticity, and may be an offset value or other correction values for correcting the chromaticity.

In the second embodiment, in order to predict a chromaticity shift, the previous chromaticity between the chromaticity of the CMY mixed color patch detected in the previous mixed color control 2 and the chromaticity of the target held when the mixed color patch is formed are recorded once. However, the color difference data used for predicting the chromaticity deviation until the next color mixing control 2 is not necessarily the previous one. For example, the chromaticity deviation ΔE until the next color mixing control 2 is predicted from the average value of the last three data of the target chromaticity to which the offset value is added or from the linear interpolation of the color difference ΔE data of the previous two times. May be.
[Modification 2]
In the first modification of the first embodiment, the offset value added to the chromaticity of the target K single color is determined from the color difference between the detected K single color and the CMY mixed color, but in the second modification, the previous color mixing control 2 It is characterized in that it is determined from the chromaticity of the previous target to which the offset value is added and the color difference between the detected CMY mixed colors.

  This is because the chromaticity of the target in which the offset value is added to the chromaticity of the K single-color patch is held in a nonvolatile memory such as an EEPROM (not shown), and the held previous offset value is added as shown in FIG. From the color difference ΔE between the detected chromaticity of the target and the detected CMY color mixture, the same color difference detected in the next color mixing control 1 or 2 is also predicted to shift the chromaticity of the CMY color mixture by the same ΔE. On the straight line passing through the chromaticity between both detected patches so that the chromaticity of the CMY mixed color crosses the chromaticity of the K single color or passes close to the chromaticity of the K single color before the mixed color control. A value obtained by shifting the offset value in the direction opposite to the CMY mixed color as viewed from the K single color, for example, a value shifted by half of ΔE.

  Here, differences from Modification 1 will be described. In step S3502 of color mixture control 2, for example, when patch set 0 is used, the chromaticity of the K single color to which the offset value in the previous color mixture control 2 held is added (L * K0 ", a * K0", b * K0 "), from the chromaticity of the detected CMY mixed color patches (L * 00, a * 00, b * 00) and the chromaticity of the K single color patch (L * K0, a * K0, b * K0) The offset values (L * offset0, a * offset0, b * offset0) can be obtained as follows with the offset coefficient coe = −1 / 2.

Then, similarly to the above-described Expression 14 in the first embodiment and the first modification, the obtained offset value is added to the chromaticity of the K monochrome, and the new chromaticity of the target K monochrome (L * K0 ′, a * K0 ′, b * K0 ').
The same calculation as above is performed for the other sets 1 to 7, and new chromaticities (L * Kn ′, a * Kn ′, b * Kn ′) of K single color (n = 1, 2,..., 7 ) Then, for the next color mixture control 2, the obtained new chromaticity of the target K single color is held in the memory.

  In this way, if the calculation is performed by adding the previous offset value by the calculation method of the offset value added to the chromaticity of the target, more accurate chromaticity stabilization control can be performed.

  The data held in the memory for predicting the chromaticity deviation may be an offset value or other correction values for correcting the chromaticity.

  Further, although the previous correction value data is used to predict the chromaticity shift and obtain the offset value, the correction data used to obtain the offset value is not necessarily the previous correction data. For example, the offset value is obtained from the previous three chromaticity data of the K single color to which the offset value is added, from the average value of the three chromaticities, or from the linear interpolation of the previous two color difference ΔE data. May be.

  As described above, according to the second embodiment and the modification, it is possible to construct a highly accurate chromaticity deviation stabilization control system.

  The third embodiment is characterized by comprising means for selecting whether or not to add an offset value for measuring chromaticity stabilization in the color mixture control 3. This is because the environment in the color mixture control 3 is significantly different from the environment in which the CMY color mixture patch in the previous color mixture control 2 is detected. For example, if the temperature becomes too high, the environment in the previous color mixture control 2 This is because it is difficult to predict using the chromaticity shift ΔE.

  Specifically, it is configured such that the user can select whether or not to add an offset value using an operation panel attached to the image forming apparatus.

  In this way, when the prediction is likely to deviate greatly, the user can determine the chromaticity stabilization with higher accuracy by making a determination from the operation panel.

  As described above, according to the third embodiment, a more accurate chromaticity deviation stabilization control system can be constructed.

  In the first to third embodiments described above, the chromaticity stabilization control using the color sensor 42 has been described. In the fourth embodiment, the gradation stabilization control using the density sensor 41 will be described.

  As described above, a plurality of gradation patches from low density to high density with toners and inks of each color can be transferred to an intermediate transfer member or drum so that a constant density gradation can be obtained even if fluctuations occur in each part of the apparatus. And the density of the gradation patch is detected by a density sensor such as an optical sensor. From the detection result, the relationship between the image signal that is the input gradation level and the image density that is the output gradation level (hereinafter, The gradation characteristic is shifted from the target by performing gradation correction control (so-called gamma correction) for performing correction so that the gradation characteristic is a desired relationship (hereinafter referred to as a target), for example, a linear relationship. Even if the value is reduced, the deviation of the gradation characteristics increases with time or as the number of printed sheets increases. As a result, if an appropriate time interval, printed sheet interval, etc. are available, it is necessary to generate gradation patches again, detect gradation deviation, and perform gradation correction control.

  FIG. 22 is a diagram showing the gradation characteristics of the output density y with respect to the input gradation degree x. A curve 2201 shown in FIG. 22 is a target d (denoted as d (x) in function notation), and a curve 2202 is a gradation characteristic f (denoted as f (x) in function notation) obtained from patch density detection. is there. Here, a gradation error Δ of density calculated by the following equation is defined as indicating how much gradation the gradation characteristic f is from the target d.

This gradation error Δ corresponds to an area 2203 surrounded by the curves 2202 and 2201. Note that this gradation error Δ has a sign.
FIG. 23 is a diagram illustrating the gradation error Δ with respect to the print time or the number of prints. Here, when the gradation correction control (t1) is performed so that the gradation error Δ obtained by detection is 0, as shown in FIG. 23, until the next gradation correction control (t2). If the gradation error Δ behaves with time (Δ (t)), the maximum value or time (number of printed sheets) with respect to the absolute value of the gradation error Δ is integrated (the area of the painted portion in the figure). This is an index for gradation stabilization.

Thus, if the gradation error Δ has a monotonous increase as shown in FIG. 23 with respect to time or the like, the control for correcting the gradation with a correction value that makes the gradation error Δ zero is as follows: Since the gradation stabilization index becomes large, optimal gradation stabilization control is not necessarily performed.
Therefore, the purpose of the fourth embodiment is to construct a gradation stabilization control system, and to achieve optimum gradation stabilization.

  The configuration and operation of the image forming unit in the fourth embodiment are the same as those in the first embodiment described with reference to FIG. Further, the processing in the image processing unit is the same as that in the first embodiment described with reference to FIG.

  Here, the density sensor 41 shown in FIG. 3 detects the density of the gradation patch of the single color toner formed on the intermediate transfer member 27, and corrects the gradation characteristics of the image processing unit based on the detection result. The tone correction control and the gradation stabilization control will be described.

  FIG. 24 is a flowchart showing details of tone correction control and tone stabilization control. First, in step S2410, it is determined whether or not the cartridge has been replaced. If the cartridge has been replaced, the process proceeds to step S2411 to perform tone correction control. If the cartridge has not been replaced, the process proceeds to step S2420, where it is determined whether a predetermined number of sheets have been printed. If printing has been performed, the process proceeds to step S2421, and gradation stabilization control is performed.

  The gradation correction control when the cartridge is replaced will be described. In step S2411, a default gradation characteristic (hereinafter, default target) determined in advance is used as a target of gradation characteristics of each color of CMYK. The default target is set in consideration of the characteristics of the image forming unit. The tone correction table (corresponding to the density correction table 203 shown in FIG. 2) is a so-called through table that does not change the input value.

  In step S 2412, a patch pattern is formed on the intermediate transfer body 27 and read by the density sensor 41. This patch pattern is the same as that of the first embodiment described with reference to FIG. 8, and the description thereof is omitted. The density of the patch pattern formed on the intermediate transfer member 27 is detected by the density sensor 41, and a curve of gradation characteristics is generated from the density by interpolation. If the cyan density detection result is indicated by a black circle shown in FIG. 25, a curve having a gradation characteristic such as 2502 is generated by interpolation such as spline interpolation.

  In step S2413, a reverse characteristic curve 2503 is calculated with reference to the default target density curve 2501 set in step S2411, and the vertical gradation is used as the output gradation, and the cyan gradation correction for the input image data is performed. A table. Then, the input image data is subjected to table conversion using the cyan gradation correction table, whereby the cyan input gradation degree and the output density have a relationship between the default target 2501. A tone correction table is similarly generated for M, Y, and K, and thereafter, at the time of printing, tone correction of input image data is performed using the generated tone correction table, and a printable state is entered. And The above is the gradation correction control when the cartridge is replaced.

  In order to simplify the following description, the above-described gradation correction control will be described again using mathematical expressions. As shown in FIG. 26, the input gradation degree is x, the output density is y, and the gradation characteristic 2502 shown in FIG. 25 is expressed by a function notation y = f (x), and the default target 2501 is y = d (x). If the input gradation degree is converted by the gradation correction table and the converted input gradation degree x ′ is x ′ = h (x), the gradation characteristic of the default target currently set as the target is obtained. For such gradation correction, the following two expressions must be equal.

y = f (x ′) = f (h (x))
y = d (x)
Therefore, the tone correction function h uses an inverse characteristic function f ⁻¹ of the function f,
h (x) = f ⁻¹ (d (x))
Then, it is possible to perform gradation correction to a default target that has desired gradation characteristics. This will be confirmed through an appropriate input gradation x1.

2601 shown in FIG. 26 is a so-called through conversion of x ′ = x. From this, the desired gradation characteristic at the input gradation degree x1 is
y1 = d (x1)
It becomes. On the other hand, in the image formation by the above gradation correction, first, the input gradation degree is
x1 ′ = h (x1) = f ⁻¹ (d (x1))
And then the gradation characteristics of the image forming apparatus are applied.
y = f (f ⁻¹ (d (x1))) = d (x1) = y1
As a result, it matches the desired gradation. The coincidence at the input gradation degree x1 also coincides at all input gradation degrees, so that it can be confirmed that gradation correction is performed to a desired gradation characteristic (default target).

  Next, when the cartridge has not been exchanged, gradation stabilization control is performed when a specified number of sheets are printed in a printable state. In this gradation stabilization control, in step S2421, a patch pattern is formed on the intermediate transfer member 27 and read by the density sensor 41, as in step S2412. However, the patch at this time is formed by the latest gradation correction table generated in step S2413 or step S2425.

  Next, in step S2422, an offset function for gradation stabilization control is calculated from the gradation characteristic of the image forming apparatus detected by the density sensor 41 and the gradation difference between the default target and added to the default target. To do.

  Here, the processing content of step S2422 will be described in detail. In the gradation correction control described above, a gradation correction function h (x) is obtained so that the gradation error Δ representing the amount of deviation between the gradation characteristics of the image forming unit and the default target is zero, and the gradation correction is performed. The table was corrected. When such gradation correction control is performed, if the subsequent gradation error behaves monotonously as shown in FIG. 23, for example, the image forming unit in FIG. Although the characteristic f is corrected to the default target by the gradation correction control, it is expected that the gradation characteristic f will be the same level as the previous time at the next gradation correction control. That is, it is expected that the gradation will reciprocate between the default target and the gradation characteristic f each time gradation correction control is performed.

  Therefore, in order to reduce the gradation error Δ for a longer period of time, that is, to stabilize the gradation, the gradation characteristic f is set to the default target instead of adjusting the gradation to the default target. Change to a new target that is shifted in the opposite direction from the viewpoint, and overcorrect to compensate for this, so that the gradation characteristics of the image forming unit cross the default target until the next gradation correction .

  As shown in FIG. 27, the offset function is a gradation difference δ (x) = f (x) − for each input gradation degree between the gradation characteristic of the image forming unit detected by the density sensor 41 and the default target. It is a function of an offset value corresponding to d (x), and the offset value is a value shifted in the opposite direction from the default target to the gradation characteristic f. For example, it is a value that shifts the default target by -δ / 2, which is half the gradation difference δ in the figure and has the opposite sign.

  That is, as in step S2413 of gradation correction control, the image formation formed by the latest gradation correction table with the input gradation degree x, the input gradation degree converted by the gradation correction table as x ', and the output density as y. Assuming that the gradation characteristic of the part is y = f (x) in function notation, the offset function s (x) can be obtained as follows with the offset coefficient coe = −1 / 2.

s (x) = coe * (f (x) -d (x)) Equation 4-1
Here, d (x) is a standard default target. Then, the obtained offset function is added to the default target, and the new target is set to g (x).

g (x) = d (x) + s (x) Equation 4-2
Next, in step S2423, the suitability of the target obtained in step S2422 is checked. As shown in FIG. 28, when the gradation characteristic f (x) greatly deviates from the default target, g (x) of the target 103 calculated by the above equations 4-1 and 4-2. This is to avoid using such an inappropriate target because it may be inappropriate as a target (having a negative output gradation).

  Specifically, it is determined whether or not the target g obtained in step S2422 is within the output density Dmin ≦ g (x) ≦ Dmax, which is appropriate for all input gradation levels x. These Dmin and Dmax are, for example, the minimum density and the maximum density of the image forming unit. If it is determined that the target is an appropriate target, g is set as an official target, and the process advances to step S2425. If it is determined to be inappropriate, the process proceeds to step S2424, and in order not to use an inappropriate target, the target is set as a default target (g (x) = d (x)), and the process proceeds to step S2425.

In step S2425, using the target set in step S2424, a tone correction function is obtained in the same manner as in step S2413, and the tone correction table is updated. In other words, the tone correction function h (x) is obtained from the inverse characteristic function f ⁻¹ (x) of the function f (x) to h (x) = f ⁻¹ (g (x)).
It becomes. Thereafter, at the time of printing, the tone correction of the input image data is performed using the tone correction table created in step S2425, and a printable state is entered. The above is the gradation stabilization control.

  FIG. 29 is a diagram illustrating the tone error Δ of the image forming unit with respect to the print time or the number of prints when the tone stabilization control is performed. When the above-described gradation stabilization control is performed, the level of the image forming apparatus is not changed between the previous gradation stabilization control (t1) and the next gradation correction control or gradation stabilization control (t2). Since the tone characteristics cross the default target, the tone stabilization index (maximum value of tone error Δ or area integrated over time) is smaller than in the case of tone correction control (FIG. 23), and the tone is stable. To do.

  Further, when any color cartridge is exchanged in a printable state, the image forming conditions change greatly, so that the process proceeds from step S2410 to step S2411 and the above-described processing is executed.

  The above is the gradation stabilization control using the density sensor 41 in the fourth embodiment.

  The number of patches formed on the intermediate transfer member 27 is not limited to the number used in the fourth embodiment.

  In the fourth embodiment, the offset function is set to an offset value proportional to the gradation difference with respect to the input gradation. However, the offset function is fixed because the same gradation change can always be predicted by a study experiment or the like. Alternatively, a function that minimizes the above-described tone stabilization index may be used.

  Further, in the fourth embodiment, when the gradation characteristic f of the image forming unit is greatly deviated from the default target, the target is set as the default target (g (x) = d (x)). A target that does not become an inappropriate target even with a characteristic f may be set.

  In this case, for example, g ′ obtained by changing the following target function is obtained by setting the offset coefficients of the above equations 4-1 and 4-2 to a positive value (coe ′ = 1/2).

g ′ (x) = coe ′ * (f (x) −d (x)) + d (x)
This g ′ (x) has gradation characteristics as shown in FIG. 30, and is exactly halfway between the patch gradation f (x) and the default target d (x) when coe ′ = ½. . Then, the following function g (x) is obtained so that this g ′ (x) further has a reverse characteristic with respect to the default target, and if this is the target, an inappropriate target as shown in FIG. Never become.

g (x) = g ′ ⁻¹ (d (x))
= Coe '* f ^-1 (d (x)) + (1-coe') x
As described above, according to the fourth embodiment, a gradation stabilization control system can be constructed.

  In the fourth embodiment, the offset gradation to be added to the default target is determined from the gradation difference between the detected gradation characteristic f of the image forming unit and the default target d, but is detected in the fifth embodiment. The tone characteristic is determined from the tone difference between the tone characteristic f of the patch and the previous target g.

  FIG. 31 is a block diagram illustrating a configuration of a control unit according to the fifth embodiment. 31 is stored as a previous target 3122 in a non-volatile memory 3120 such as an EEPROM, and the previous target 3122 held (g (x in the function notation shown in FIG. )) And the tone difference of each input tone between the tone characteristics of the patch detected by the density sensor detection signal 3101 (f (x) in the function notation shown in FIG. 32), and the next tone The patch gradation difference detected by the stabilization control is also predicted to be shifted by the same level of δ, and the gradation of the image forming unit is changed to the default target 3121 (see FIG. 32) until the next gradation stabilization system. An offset function corresponding to the gradation difference δ is added to the default target 3121 so as to cross d (x)) in the function notation shown. For example, δ for each input gradation is reduced by half. It is obtained by taking the opposite sign in the.

  Here, differences from the fourth embodiment will be described. In step S2422 of the gradation stabilization control, the offset function s (x) can be obtained as follows with the offset coefficient coe = −1 / 2.

s (x) = coe * (f (x) -g (x))
Then, the obtained offset function is added to the default target to obtain a new target (gnew (x) in the function notation shown in FIG. 32).

gnew (x) = d (x) + s (x)
Next, in step S2425, using the target set in step S2423, a tone correction function is obtained in the same manner as in step S2413, and the tone correction table is updated. That is, the tone correction function h (x) is obtained from the inverse characteristic function f ⁻¹ (x) of the function f (x).
h (x) = f ⁻¹ (gnew (x))
It becomes. Then, the target gnew (x) obtained in step S2422 is held in the memory 3130 for gradation stabilization control in the next embodiment 5.

  In the fifth embodiment, the target for the previous time is used to predict the gradation shift and obtain the offset function. However, the target used for obtaining the offset function is not limited to the last time. For example, a new target may be obtained from the average gradation characteristics of the three targets from the previous three target data.

  Further, the data held in the memory 3130 for predicting gradation deviation may be an offset function, a gradation correction table, or other correction values for correcting gradation.

  As described above, according to the fifth embodiment, a more accurate gradation stabilization control system can be constructed.

  The sixth embodiment is characterized by comprising means for selecting whether or not to add an offset function for measuring gradation stabilization. This not only measures the stabilization of the gradation shift as described in the fourth embodiment, but there is a strong demand for eliminating or minimizing the gradation deviation immediately after the gradation correction depending on the user. Because.

  Specifically, the density gradation correction can be selected by the user using an operation panel or the like attached to the color image forming apparatus as shown in FIG. That is, step S3401 as shown in FIG. 34 is newly provided between step S2420 and step S2421 shown in FIG. 24, and whether the user performs gradation correction control from the operation panel described above (depressing 3301 in FIG. 33). The processing is switched depending on whether gradation stabilization control is performed (3302 in FIG. 33 is pressed).

  In the sixth embodiment, a message prompting the user to select is displayed on the operation panel of the image forming apparatus, but the same display may be performed on the host PC or the like.

  As described above, according to the sixth embodiment, it is possible to construct a flexible gradation correction control system that can meet the needs of the user.

  Even if the present invention is applied to a system composed of a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), it is applied to an apparatus (for example, a copier, a facsimile machine, etc.) composed of a single device. It may be applied.

  Another object of the present invention is to supply a recording medium in which a program code of software realizing the functions of the above-described embodiments is recorded to a system or apparatus, and the computer (CPU or MPU) of the system or apparatus stores the recording medium in the recording medium. Needless to say, this can also be achieved by reading and executing the programmed program code.

  In this case, the program code itself read from the recording medium realizes the functions of the above-described embodiment, and the recording medium storing the program code constitutes the present invention.

  As a recording medium for supplying the program code, for example, a floppy (registered trademark) disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, a ROM, or the like is used. be able to.

  Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) operating on the computer based on the instruction of the program code. It goes without saying that a case where the function of the above-described embodiment is realized by performing part or all of the actual processing and the processing is included.

  Further, after the program code read from the recording medium is written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. It goes without saying that the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing and the functions of the above-described embodiments are realized by the processing.

FIG. 2 is a diagram illustrating a configuration of an image forming unit in a color image forming apparatus. It is a figure which shows an example of the process in the image process part of a color image forming apparatus. It is a figure which shows an example of a structure of the density sensor 41 shown in FIG. It is a figure which shows an example of a structure of the color sensor 42 shown in FIG. 5 is a flowchart showing control of gradation-density characteristics combining a color sensor and a density sensor. 5 is a flowchart showing details of gradation-density characteristic control combining color mixture control and single color control. It is a figure which shows the characteristic that an output density becomes linear with respect to an input gradation degree. FIG. 4 is a diagram illustrating an example of a patch pattern formed on an intermediate transfer body 27. It is a figure which shows the curve of the reverse characteristic with respect to a gradation density curve, a target density curve, and a target density curve. It is a figure which shows each patch and the gradation degree of CMYK. 3 is a diagram illustrating an example of a patch pattern formed on a transfer material 11. FIG. It is a figure which shows the explanatory variable and objective variable with respect to each patch. It is a figure which shows the cyan color correction table produced by the linear interpolation. It is a figure which shows the gradation-density curve of the target of cyan. FIG. 6 is a diagram in which the chromaticity of the reference gradation of the CMY mixed color patch and the chromaticity of the K single color patch are plotted in the L * a * b * space. FIG. 6 is a diagram illustrating a chromaticity shift (ΔE) with respect to a print time or the number of prints. FIG. 6 is a color space diagram illustrating color mixing control 3 in the first embodiment. FIG. 6 is a diagram illustrating a change with time in chromaticity deviation between the color mixing control 2 and the color mixing control 3 in the first embodiment. FIG. 10 is a color space diagram illustrating color mixture control 3 in the second embodiment. 6 is a diagram illustrating timing for performing color mixture control 3 in Embodiment 1. FIG. 6 is a diagram illustrating a color difference ΔE predicted by the color mixture control 3 in Embodiment 1. FIG. It is a figure which shows the gradation characteristic of the output density y with respect to the input gradation degree x. It is a figure showing the gradation error (DELTA) with respect to printing time or the number of printed sheets. It is a flowchart which shows the detail of gradation correction control and gradation stabilization control. FIG. 10 is a diagram illustrating gradation correction control in Embodiment 4. FIG. 10 is a diagram for explaining details of tone correction control in Embodiment 4. FIG. 10 is a diagram for explaining gradation stabilization control in Embodiment 4. FIG. 10 is a diagram illustrating a case where gradation stabilization control is inappropriate in the fourth embodiment. FIG. 6 is a diagram illustrating a gradation error Δ of an image forming unit with respect to a printing time or the number of printed sheets when gradation stabilization control is performed. FIG. 10 is a diagram for explaining a method of calculating an appropriate target for gradation stabilization control in the fourth embodiment. FIG. 10 is a block diagram illustrating a configuration of a control unit according to a fifth embodiment. FIG. 10 is a diagram for explaining gradation stabilization control in Embodiment 5. FIG. 10 is a diagram illustrating an operation panel in a sixth embodiment. 14 is a flowchart illustrating details of tone correction control and tone stabilization control in the sixth embodiment. 6 is a flowchart illustrating details of gradation-density characteristic control in which color mixture control and single color control are combined in a modification of the first embodiment. FIG. 10 is a color space diagram showing color mixture control 2 in Modification 1; FIG. 9 is a diagram showing a change with time of a chromaticity shift between color mixing control 1 and color mixing control 2 in Modification 1; FIG. 10 is a color space diagram showing color mixture control 2 in Modification 2. FIG. 3 is a block diagram illustrating a configuration of a control unit according to the first embodiment.

Claims (19)

A chromaticity stabilization control method in an image forming apparatus that forms an image on a transfer material using a plurality of color materials,
Detecting the chromaticity of a chromaticity detection patch on a transfer material formed using a plurality of color materials, and correcting image forming conditions so that the detected chromaticity matches a predetermined target chromaticity;
A step of controlling the image forming condition to be corrected stepwise or continuously after the chromaticity of the chromaticity detection patch is detected and before the next chromaticity detection patch is detected. A chromaticity stabilization control method comprising:   2. The correction step includes obtaining an offset value based on the detected chromaticity and adding the offset value to the predetermined target chromaticity to obtain a new target chromaticity. Chromaticity stabilization control method.   The chromaticity stabilization control method according to claim 2, wherein the offset value is a variable value according to timing.   The chromaticity stabilization control method according to claim 2, wherein the offset value is a value corresponding to a color difference between the detected chromaticity and a chromaticity for which a color is desired to be matched.   The chromaticity stabilization control method according to claim 2, wherein the offset value is a value corresponding to a color difference between the detected chromaticity and a previous target chromaticity.   5. The offset value according to claim 4, wherein the offset value is a value calculated by predicting a color difference over time from the color difference until the next chromaticity detection patch chromaticity is detected by a polynomial. 5. The chromaticity stabilization control method according to 5.   6. The chromaticity stabilization control method according to claim 4, wherein the offset value is a half value of the color difference and has a reverse sign value.   The chromaticity stabilization control method according to claim 2, further comprising a step of selecting whether or not to add the offset value. A gradation stabilization method in an image forming apparatus for forming an image,
A step of detecting the density of the tone detection patch formed on the transfer body and correcting the image forming condition based on the detected density so that the tone characteristic of the image forming unit matches a predetermined target tone characteristic. When,
And a step of updating the predetermined target gradation characteristic.   The updating step includes updating by adding an offset value calculated according to a difference between a gradation characteristic of an image forming unit obtained from the detected density and the target gradation characteristic. 10. The gradation stabilizing method according to 9.   In the updating step, an offset value calculated according to the difference between the gradation characteristic of the image forming unit obtained from the detected density and the target gradation characteristic changed by the result of the previous density detection is added. The gradation stabilization method according to claim 9, wherein the gradation stabilization method is updated.   The gradation stabilization method according to claim 10 or 11, wherein the offset value is a value proportional to a value that is an inverse sign of the difference with respect to an input gradation degree.   12. The gradation stabilization method according to claim 10 or 11, wherein the offset value is a value having an opposite sign at half of the difference with respect to an input gradation degree.   The gradation stabilization method according to claim 10 or 11, further comprising a step of selecting whether or not to add the offset value. In an image forming apparatus that forms an image on a transfer material using a plurality of color materials,
Correction means for detecting the chromaticity of a chromaticity detection patch on a transfer material formed using a plurality of color materials, and correcting image forming conditions so that the detected chromaticity matches a predetermined target chromaticity; ,
Control means for controlling to correct the image forming condition stepwise or continuously after detecting the chromaticity of the chromaticity detection patch until detecting the chromaticity of the next chromaticity detection patch; An image forming apparatus comprising: In an image forming apparatus for forming an image,
A correction that detects the density of the tone detection patch formed on the transfer body and corrects the image forming conditions based on the detected density so that the tone characteristics of the image forming means match the predetermined target tone characteristics. Means,
An image forming apparatus comprising: update means for updating the predetermined target gradation characteristic.   The program for making a computer perform each procedure of the chromaticity stabilization control method of Claim 1.   The program for making a computer perform each procedure of the gradation stabilization control method of Claim 9.   The computer-readable recording medium which recorded the program of Claim 17 or Claim 18.

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