Abstract:
A calibration method for calibrating a color image forming apparatus includes: a detection step of detecting the color of a patch formed on a recording medium by the color image forming apparatus using a color sensor; a conversion step of converting a color signal in the first color specification system as the detection result at the detection step into a color signal in the second color specification system; and an adjustment step of adjusting at least one of the image forming conditions for the color image forming apparatus, based on the color signal converted at the conversion step; wherein the conversion conditions at the color signal conversion step are different depending on the attributes of the patch.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for calibrating a color image forming apparatus such as a color printer or a color copier, and more particularly to a color signal conversion method for measuring a test chart outputted to improve the color stability of a color image forming apparatus using a chromaticity detecting means, and converting a color signal in the first color specification system that is the detection result of the chromaticity detecting means into a color signal in the second color specification system.  
         [0003]     2. Background of the Related Art  
         [0004]     Recently, the color image forming apparatus that adopts an electrophotography method or inkjet method such as a color printer or a color copier is demanded to have the color stability of the output image.  
         [0005]     Thus, the color image forming apparatus having a sensor for detecting the chromaticity of a patch on the recording medium after forming and fixing a monochromatic gradation patch of cyan (C), magenta (M), yellow (Y) or black (K) or a mixed color patch in which CMY are mixed on the recording medium (hereafter referred to as a color sensor) is well known (e.g., refer to US Publication No. 2003049040).  
         [0006]     In this color image forming apparatus, the color stability of a final output image formed on the recording medium is controlled by feeding back the detected result to a calibration table for correcting the exposure amount, the process conditions and the color gradation characteristics of an image forming portion. Also, the output image of the color image forming apparatus may be detected by an external image reading apparatus or a chromaticity meter to make the same control.  
         [0007]     This color sensor uses a light emitting element having three or more kinds of light sources with different emission spectra of red (R), green (G) and blue (B) and a light receiving element with sensitivity in the visible region, or a light emitting element having a light source that emits light of white color (W) as and a light receiving element formed with three or more filters of different spectral transmittances. Thereby, three or more kinds of outputs such as the RGB outputs are obtained.  
         [0008]     In the color image forming apparatus of ink jet type, since the color balance changes depending on a change in the ink discharge amount with the lapse of time, an environmental difference or the individual difference of ink cartridge, so that the color gradation characteristics can not be kept constant. Thus, some color image forming apparatuses make the color stabilization control by substituting a color sensor for the ink head and detecting the chromaticity of patch on the recording medium.  
         [0009]     In the above color stabilization control, there is a process of converting the sensor outputs of RGB into the XYZ chromaticity values as defined by International Commission on Illumination(CIE). As the prior art for this conversion, a method for using the matrix and a method for using the look-up table are provided.  
         [0010]     However, the above prior art had the following problems.  
         [0011]     Generally, the color matching functions for the spectral sensitivity of RGB outputs of the color sensor and the XYZ chromaticity values as defined by International Commission on Illumination do not have completely linear relations. Therefore, there is a problem that some differences may occur between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination.  
         [0012]     That is, the XYZ chromaticity values calculated from the RGB output signals of the sensor for a certain patch and the XYZ chromacity values calculated from the spectral reflectance for the patch as defined by International Commission on Illumination may be different in some cases. And this difference between the chromaticity values may be varied in magnitude, depending on the color material or substratum color of the patch used in forming the patch.  
       SUMMARY OF THE INVENTION  
       [0013]     This invention has been achieved in the light of the above-mentioned problems, and it is an object of the invention to provide a color signal conversion method for reducing the differences occurring between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination in a simple manner when measuring a test chart outputted by the color image forming apparatus.  
         [0014]     In order to accomplish the above object, the invention provides a color image calibration method as defined in the claim.  
         [0015]     With this invention, it is possible to reduce the differences occurring between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination by changing various kinds of parameters used in converting the RGB outputs of the sensor into the XYZ chromaticity values depending on the attribute of patch. And the precision of color stability control using the XYZ chromaticity values can be improved.  
         [0016]     Other objects, constitutions and effects of the invention will be apparent from the following detailed description of the invention and the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a flowchart for converting the RGB outputs into XYZ chromaticity values according to an embodiment 1;  
         [0018]      FIG. 2  is a diagram showing the overall configuration of a color image forming apparatus according to the embodiment 1;  
         [0019]      FIG. 3  is a view showing the electrophotographic color image forming apparatus in cross section according to the embodiment 1;  
         [0020]      FIG. 4  is a block diagram showing the configuration of a color sensor and its peripheral devices according to the embodiment 1;  
         [0021]      FIG. 5  is a view showing the color sensor in cross section according to the embodiment 1;  
         [0022]      FIG. 6  is a diagram showing one example of the test chart in the embodiment 1;  
         [0023]      FIG. 7  is a diagram (1) showing a connection method between color image forming apparatus and color image reading apparatus according to an embodiment 2;  
         [0024]      FIG. 8  is a diagram (2) showing a connection method between color image forming apparatus and color image reading apparatus according to the embodiment 2;  
         [0025]      FIG. 9  is a view showing one example of a color image reading apparatus according to the embodiment 2;  
         [0026]      FIG. 10  is a view showing one example of the test chart according to the embodiment 2; and  
         [0027]      FIG. 11  is a flowchart for converting the RGB outputs to XYZ chromaticity values according to an embodiment 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings.  
       Embodiment 1  
       [0029]      FIG. 2  is a diagram showing the overall configuration of an electrophotographic color image forming apparatus according to the embodiment 1. This color image forming apparatus comprises an image processing portion and an image forming portion.  
         [0030]     Firstly, a process in the image processing portion will be described below. With a color matching table  111 , the RGB signals representing the colors of an image sent from a personal computer are converted into the device RGB signals (hereafter referred to as DevRGB) in conformance with a color reproduction range of the color image forming apparatus. Then, the converted DevRGB signals are converted into the CMYK signals indicating the colors of toner color materials for the color image forming apparatus, using a color separation table  112 . A calibration table  113  is a table for correcting the density-gradation characteristics intrinsic to the color image forming apparatus and employed to convert the CMYK signals into the C′M′Y′K′ signals in which the density-gradation characteristics are corrected. Moreover, with a PWM (Pulse Width Modulation) table  114 , the C′M′Y′K′ signals are converted into the exposure times Tc, Tm, Ty and Tk of corresponding scanner portions  24 C,  24 M,  24 Y and  24 K (see  FIG. 3 ).  
         [0031]     Next, the image forming portion will be described. Main means involving the image formation include charging means  122 , exposing means  123 , developing means  124 , transferring means  125  and fixing means  126 , which are controlled by a CPU  121 . Moreover, a color sensor  42  is connected to the CPU.  
         [0032]      FIG. 3  is a cross-sectional view of the color image forming apparatus. This apparatus is the color image forming apparatus of tandem type employing an intermediate transfer member  28 , which is one example of the electrophotographic color image forming apparatus, as shown in  FIG. 3 . Referring to  FIG. 3 , the operation of the image forming portion in the electrophotographic color image forming apparatus will be described below.  
         [0033]     The image forming portion forms an electrostatic latent image with an exposing light applied based on an exposure time converted by the image processing portion, forming the monochromatic toner images by developing this electrostatic latent image, forming a multi-color toner image by superposing the monochromatic toner images, transferring this multi-color toner image onto a recording medium  11 , and fixing the multi-color toner image on the recording medium.  
         [0034]     The charging means  122  comprises four injection charge devices  23 Y,  23 M,  23 C and  23 K for charging the photosensitive members  22 Y,  22 M,  22 C and  22 K at the stations of yellow (Y), magenta (M), cyan (C) and black (K), the injection charge devices having respective sleeves  23 YS,  23 MS,  23 CS and  23 KS.  
         [0035]     The photosensitive members  22 Y,  22 M,  22 C and  22 K have an organic photoconductive layer applied on the outer periphery of an aluminum cylinder, and are rotated by a driving force of a drive motor, not shown. The photosensitive members  22 Y,  22 M,  22 C and  22 K are rotated in a counterclockwise direction along with an image forming operation by the drive motor.  
         [0036]     The exposing means  123  applies an exposing light from the scanner portions  24 Y,  24 M,  24 C and  24 K onto the photosensitive members  22 Y,  22 M,  22 C and  22 K, selectively exposing the surfaces of the photosensitive members  22 Y,  22 M,  22 C and  22 K to form the electrostatic latent images.  
         [0037]     The developing means  124  comprises four developing devices  26 Y,  26 M,  26 C and  26 K for developing the images of yellow (Y), magenta (M), cyan (C) and black (K) at respective stations to visualize the electrostatic latent images, in which the developing devices are provided with the sleeves  26 YS,  26 MS,  26 CS and  26 KS. Each developing device  26  can be detachably attached.  
         [0038]     The transferring means  125  transfers the monochrome toner images, along with the rotation of the photosensitive members  22 Y,  22 M,  22 C and  22 K and the primary transfer rollers  27 Y,  27 M,  27 C and  27 K located oppositely, by rotating the intermediate transfer member  28  in a clockwise direction to transfer the monochrome toner images from the photosensitive members  22  to the intermediate transfer member  28 . By applying an appropriate bias voltage to the primary transfer rollers  27  and giving a difference between the rotating speed of the photosensitive members  22  and the rotating speed of the intermediate transfer member  28 , the monochrome toner images are transferred onto the intermediate transfer member  28  efficiently. This operation is called a primary transfer.  
         [0039]     Moreover, the transferring means  125  superposes the monochrome toner images on the intermediate transfer member  28  at respective stations, conveys the superposed multi-color toner image up to a secondary transfer roller  29  along with the rotation of the intermediate transfer member  28 , picks up and conveys a recording medium  11  from a sheet feeding tray  21  to the secondary transfer roller  29 , and transfers the multi-color toner image on the intermediate transfer member  28  onto the recording medium  11 . The toner image is electrostatically transferred by applying an appropriate bias voltage to the secondary transfer roller  29 . This operation is called a secondary transfer. The secondary transfer roller  29  contacts the recording medium  11  at a position  29   a , while transferring the multi-color toner image onto the recording medium  11 , and is spaced to a position  29   b  after the printing process.  
         [0040]     The fixing means  126  comprises a fixing roller  32  for heating the recording medium  11  and a pressure roller  33  for pressing the recording medium  11  onto the fixing roller  32  to fuse and fix the transferred multi-color toner image on the recording medium  11 . The fixing roller  32  and the pressure roller  33  are formed hollow and internally comprise the heaters  34  and  35 , respectively. A fixing apparatus  31  conveys the recording medium  11  holding the multi-color toner image on the fixing roller  32  and the pressure roller  33 , and fixes the toner on the recording medium by applying a heat and pressure.  
         [0041]     The recording medium  11  after fixing the toner is then discharged to a sheet discharge tray, not show, by a sheet discharge roller, not shown, whereby the image forming operation is ended.  
         [0042]     Cleaning means  30  cleans the toner remaining on the intermediate transfer member  28 , in which after transferring the multi-color toner image of four colors formed on the intermediate transfer member  28  onto the recording medium  11 , waste toner is stored in a cleaner container.  
         [0043]     A color sensor  42  is disposed downstream of the fixing apparatus  31  on a conveying path of the recording medium, opposed to an image forming face of the recording medium  11 , detecting the color of a mixed color patch after fixing formed on the recording medium  11  to output the RGB values. Disposed inside the color image forming apparatus, the color sensor can automatically detect the color before the sheet with image fixed is discharged to a sheet discharging portion.  
         [0044]      FIG. 4  is a block diagram showing the configuration of the color sensor  42  and its peripheral devices. The color sensor and its peripheral devices are a light emitting element  101 , a light receiving element  102 , an A/D converter  104  and a CPU  121 . The light emitting element  101  is a light source of the color sensor that emits light to a measurement object  103 . Then, an irregular light is reflected in which the reflection factor depends on a body color of the measurement object. The irregular reflected light enters the light receiving element  102  that converts light into an electric signal. Moreover, an analog electric signal is converted into a digital electric signal by the A/D converter  104 . And the digital electric signal is taken into the CPU  121 , and the XYZ chromaticity values are outputted through a linear conversion process as shown in formula (1).  
         [0045]      FIG. 5  is a cross-sectional view of the color sensor  42 . The color sensor  42  employs a white color LED  53  as the light emitting element  101  and a charge storage type sensor  54   a  with on-chip filters for three or more colors, such as RGB, as the light receiving element  102 . Light from the white color LED  53  is made incident obliquely at an angle of 45° upon the recording medium  11  formed with the patch after fixing, and the intensity of irregular reflected light in a direction of 0° is detected by the charge storage type sensor  54   a  with RGB on-chip filters. A light receiving portion of the charge storage type sensor  54   a  with RGB on-chip filters has independent pixels of RGB like  54   b . The light receiving element  102  may be a photo diode. A set of three pixels of RGB may be arranged multiply. Also, the angle of incidence may be 0° and the angle of reflection may be 45°. Moreover, an LED for emitting lights of three or more colors such as RGB and a sensor without filter may be combined.  
         [0046]      FIG. 6  is a diagram showing one example of a test chart detected by the color sensor  42 . A color stabilization control test chart  60  is a gradation patch pattern of gray that is the most important color in making the color balance, and composed of a gray gradation patch  61  of only black (K) and a process gray gradation patch  62  in which yellow (Y), magenta (M) and cyan (C) are mixed. The gray gradation patch  61  of only black (K) and the process gray gradation patch  62 , which have the same chromaticity in the image processing portion of the image forming apparatus, are paired, such as  61   a  and  62   a ,  61   b  and  62   b ,  61   c  and  62   c . The chromaticity of this patch is detected by the color sensor  42 , and fed back to a calibration table so that there may be no color difference between the gray gradation patch  61  of only black (K) and the process gray gradation patch  62  which are paired.  
         [0047]     To convert the RGB outputs of the sensor into the XYZ chromaticity values, the following formula (1) is employed.  
               (         X           Y           Z         )     =       A   ·     (         r           g           b         )       =       (           a   11           a   12           a   13               a   21           a   22           a   23               a   31           a   32           a   33           )     ·     (         r           g           b         )                 (   1   )             
 
         [0048]     Where XYZ are XYZ chromaticity values calculated by converting the RGB outputs of the sensor, r, g and b are sensor outputs, A is a conversion matrix, and a is a matrix element.  
         [0049]     In the following, a color conversion method for reducing the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor in this embodiment and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination is given.  
         [0050]     This method involves changing the matrix A in the formula (1) for every attribute of patch. A gray gradation patch matrix of only black (K) A 1  and a process gray gradation patch matrix A 2  are set up. The matrixes A 1  and A 2  are optimized to convert the RGB outputs of the sensor detecting the patch of each attribute into the XYZ chromaticity values.  
         [0051]      FIG. 1  is a flowchart for converting the RGB outputs of the sensor into the XYZ chromaticity values.  
         [0052]     At step  211 , it is determined whether or not the detected patch is gray gradation patch  61  of only black (K) or process gray gradation patch  62 . Since the patch format of the color stabilization control test chart  60  is fixed in the image forming apparatus, the determination may be made in the sequence of detecting the patch.  
         [0053]     If it is determined at step  211  that the detected patch is the gray gradation patch of only black (K)  61  because the patch is detected at odd number in the color stabilization control test chart  60 , the RGB outputs of the sensor are converted into the XYZ chromaticity values, using the gray gradation patch matrix of only black (K) A 1 , in accordance with the formula (1) at step  212 .  
         [0054]     If it is determined at step  211  that the detected patch is the process gray gradation patch  62  because the patch is detected at even number in the color stabilization control test chart  60 , the RGB outputs of the sensor are converted into the XYZ chromaticity values, using the process gray gradation patch matrix A 2 , in accordance with the formula (1) at step  213 .  
         [0055]     Next, the experimental results are shown below in which the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination could be reduced by changing the matrix A in the formula (1) for every attribute of patch.  
         [0056]     In the experiment, because there are not complete linear relations between the spectral sensitivity of the RGB outputs of the color sensor and the XYZ chromaticity values as defined by International Commission on Illumination, a formula (2) using the RGB outputs of the sensor up to the third order is employed, instead of the formula (1′), to decrease the influence of incomplete linear relations as much as possible.  
               (         X           Y           Z         )     =       A   ·     (         r           g           b             r   2               g   2               b   2               r   3               g   3               b   3           )       =       (           a   11           a   12           a   13           a   14           a   15           a   16           a   17           a   18           a   19               a   21           a   22           a   23           a   24           a   25           a   26           a   27           a   28           a   29               a   31           a   32           a   33           a   34           a   35           a   36           a   37           a   38           a   39           )     ·     (         r           g           b             r   2               g   2               b   2               r   3               g   3               b   3           )                 (   2   )             
 
         [0057]     The number of measured patches is about 250 for the gray gradation patch of only black and about 500 for the process gray gradation patch. The results are shown in Table 1.  
                                 TABLE 1                           Experimental results                    Gray gradation   Process gray           ΔE(Ave.)   patch of only black   gradation patch                       Matrix A is changed   0.53   0.99           for every attribute           of patch           Matrix A is common   3.90   1.53           for all patches                      
 
         [0058]     The numerical values as listed in the table are average values of chromaticity value differences occurring between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination. Both the XYZ values are converted into L*a*b* as defined by International Commission on Illumination and then calculated as the color difference (ΔE).  
         [0059]     The above results reveal that the method for changing the matrix A for every attribute of patch that is the kind of color material for use, whether the measured patch is gray gradation patch of only black or process gray gradation patch, can make smaller the color difference (ΔE) occurring between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination. This is because the non-linearity of the color matching functions for the spectral sensitivity of the RGB outputs of the color sensor and the XYZ chromaticity values as defined by International Commission on Illumination is reduced in the extent of influence by changing the matrix A for every attribute of patch.  
         [0060]     Accordingly, it is possible to reduce the color differences between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination by employing the color conversion method for changing the matrix A for every attribute of patch.  
         [0061]     This method is on the premise that the attribute of patch can be determined when detecting the patch. In the image forming apparatus mounting the color sensor, because the patch is detected in the sequence of forming the image, the attribute of patch detected by the color sensor can be judged, whereby this method is applicable.  
         [0062]     Though the RGB outputs of the sensor are converted into the XYZ chromaticity values here, it is apparent that the method as described in this embodiment is effective when the color matching function in two different color specifications systems has non-linearity.  
         [0063]     Moreover, there are two attributes here, including the gray gradation patch of only black (K) and the process gray gradation patch in which yellow (Y), magenta (M) and cyan (C) are mixed, but the classification method for attributes is not limited to the combination of color materials as indicated here.  
         [0064]     Moreover, the methods for converting the RGB outputs of the sensor into the XYZ chromaticity values include a linear conversion method by matrix, a neural network method and a method using the look-up table. In any method, if the attribute of patch is judged, it is possible to reduce the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination by changing the weight of connection between neurons for use in the neural network for every attribute of patch, or changing the look-up table for use.  
         [0065]     As described above, by changing various parameters for use in converting the RGB outputs of the sensor into the XYZ chromaticity values for every attribute of patch, it is possible to reduce the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination, whereby the precision of color stabilization control using the XYZ chromaticity values can be improved.  
       Embodiment 2  
       [0066]     In an embodiment 2, an image forming apparatus not mounting the color sensor  42  implements the color stabilization control equivalent to that as described in the embodiment 1, employing an external image reading apparatus, instead of the color sensor  42 , and using a color conversion method that reduces the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the external image reading apparatus and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination.  
         [0067]      FIG. 7  and  FIG. 8  are diagrams showing a connection method between color image forming apparatus and color image reading apparatus. In  FIG. 7 , the color image forming apparatus and color image reading apparatus are directly connected, but in  FIG. 8 , they are connected via a network. The color image reading apparatus, like the color sensor, mounts a white color source and a sensor with the filters of three or more colors such as RGB, or the light sources of three or more colors such as RGB and a sensor having sensitivity in the visible region, and outputs the RGB sensor signals.  
         [0068]      FIG. 9  is a view showing a flat bed image scanner as seen from the above as one example of the color image reading apparatus. The operation will be described below. The user of the image reading apparatus sets a medium formed with the image on a platen  402 . If the medium is set, the sensor  401  is moved in a direction as indicated by the arrow. The sensor proceeds in step operation, and reads the image of one line at each step. By integrating the images of one line, the entire image formed on one medium can be read.  
         [0069]     In this embodiment, a monochrome gradation patch of cyan (C), magenta (M), yellow (Y) and black (K) or a patch in which CMY are mixed is formed on the recording medium, and the outputted recording medium formed with the patch is set on the color image reading apparatus to detect the chromaticity of the patch. By feeding back its detection result to the calibration table for correcting the exposure amount, process conditions and the color gradation characteristics of the image forming portion, the color stabilization control of the final output image equivalent to that as described in the embodiment 1 can be performed.  
         [0070]      FIG. 10  is a view showing one example of the test chart detected by the color image reading apparatus. A color stabilization control test chart  63  is a gradation patch pattern of gray that is the most important color in making the color balance, and composed of a gray gradation patch of only black (K)  61  and a process gray gradation patch  62  in which yellow (Y), magenta (M) and cyan (C) are mixed. The gray gradation patch of only black (K)  61  and the process gray gradation patch  62 , which have the same chromaticity in the image processing portion of the image forming apparatus, are paired, such as  61   a  and  62   a ,  61   b  and  62   b ,  61   c  and  62   c . A different point from the color sensor test chart as shown in  FIG. 6  in the embodiment 1 is that the patches  61 ,  62  are arranged over the entire face of the recording medium  1 . Because the color image reading apparatus can read the images on the overall face of the recording medium at a time.  
         [0071]     In this embodiment, a color conversion method for reducing the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination is identical to the method as shown in  FIG. 1  in the embodiment 1. At step  211  in  FIG. 1 , it is determined whether or not the detected patch is gray gradation patch of only black (K)  61  or process gray gradation patch  62 .  
         [0072]     The image reading apparatus can selectively detect the gray gradation patch of only black (K)  61  or process gray gradation patch  62  if there is the patch position coordinate information of the color stabilization control test chart  63 . The image reading apparatus is connected to the image forming apparatus, and notifies the patch position coordinate information of the color stabilization control test chart  63  from the image forming apparatus to the image reading apparatus. Accordingly, the determination at step  211  in  FIG. 1  can be made.  
         [0073]     Though the RGB outputs of the sensor are converted into the XYZ chromaticity values here, it is apparent that the method as described in this embodiment is effective when the color matching function in two different color specifications systems has non-linearity.  
         [0074]     Moreover, there are two attributes here, including the gray gradation patch of only black (K) and process gray gradation patch in which yellow (Y), magenta (M) and cyan (C) are mixed, but the classification method for attributes is not limited to the combination of color materials as indicated here.  
         [0075]     Moreover, the methods for converting the RGB outputs of the sensor into the XYZ chromaticity values include a linear conversion method by matrix, a neural network method and a method using the look-up table. In any method, if the attribute of patch is judged, it is possible to reduce the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination by changing the weight of connection between neurons for use in the neural network for every attribute of patch, or changing the look-up table for use.  
         [0076]     As described above, the color image forming apparatus having no color sensor is connected to the color image reading apparatus, and by changing various parameters for use in converting the RGB outputs of the color image reading apparatus into the XYZ chromaticity values for every attribute of patch, it is possible to reduce the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the color reading apparatus and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination, whereby the precision of color stabilization control using the XYZ chromaticity values can be improved.  
       Embodiment 3  
       [0077]     In an embodiment 3, a method for changing the matrix for use in converting the RGB outputs into the XYZ chromaticity values as described in the embodiments 1 and 2 is shown, in which the matrix is changed depending on the attribute of patch, whether the patch is the gray gradation patch of only black (K) or the process gray gradation patch, and by detecting the substratum color of patch, namely, the color of the recording medium, to judge the color area. Thereby, it is possible to further reduce the differences between the XYZ chromaticity values obtained by converting the RGB outputs and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination.  
         [0078]      FIG. 11  is a flowchart for converting the RGB outputs into the XYZ chromaticity values in the embodiment 3.  
         [0079]     At step  221 , the recording medium  11  in an area where the patch is not formed as the substratum color of patch is detected, and it is judged into which of (1) to (6) the sizes of the RGB outputs are classified. Herein, (1) R&gt;G&gt;B, (2) R&gt;B&gt;G, (3) B&gt;R&gt;G, (4) B&gt;G&gt;R, (5) G&gt;R&gt;B and (6) G&gt;B&gt;R.  
         [0080]     Since the following steps are common except that the matrix A is different in each of (1) to (6), a flow F 301  for the classification of (1) will be described below.  
         [0081]     At step  222 , it is determined whether the detected patch is gray gradation patch of only black (K)  61  or process gray gradation patch  62 . The patch format of color stabilization control test charts  60 ,  63  is fixed in the image forming apparatus, whereby the determination is made based on the sequence or position of detecting the patch.  
         [0082]     If it is determined at step  222  that the detected patch is gray gradation patch of only black (K)  61 , the RGB outputs of the sensor are converted into the XYZ chromaticity values in accordance with the formula (1), employing the matrix A 1  for gray gradation patch of only black (K) at step  223 .  
         [0083]     If it is determined at step  222  that the detected patch is process gray gradation patch  62 , the RGB outputs of the sensor are converted into the XYZ chromaticity values in accordance with the formula (1), employing the matrix A 2  for process gray gradation patch at step  224 .  
         [0084]     If it is determined that the color area is classified into any one of (2) to (6) at step  221 , the procedure goes to a respective flow of F 302  to F 306 . In F 302 , the matrix for gray gradation patch of only black (K) is A 3 , and the matrix for process gray gradation patch is A 4 , in which these matrixes are different from A 1  and A 2  in F 301 . Others are the same. Likewise, in F 303 , the matrix for gray gradation patch of only black (K) is A 5 , and the matrix for process gray gradation patch is A 6 , in F 304 , the matrix for gray gradation patch of only black (K) is A 7 , and the matrix for process gray gradation patch is A 8 , in F 305 , the matrix for gray gradation patch of only black (K) is A 9 , and the matrix for process gray gradation patch is A 10 , and in F 306 , the matrix for gray gradation patch of only black (K) is A 11 , and the matrix for process gray gradation patch is A 12 .  
         [0085]     The color of patch formed on the recording medium  11  is affected by the substratum color of patch, namely, the color of the recording medium  11 , without regard to the color material used for the gray gradation patch of only black (K) or process gray gradation patch. For example, if the recording medium is red, the color of patch formed on the red recording medium is redder than the color of the same patch formed on the white recording medium. That is, even when the color image forming apparatus forms the same patch, the spectral reflectance of patch may be different, if the color of the recording medium formed with the patch is different. Accordingly, it is possible to further reduce the differences between the XYZ chromaticity values obtained by converting the RGB outputs and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination by changing the matrix A for use depending on the color area of the recording medium  11 .  
         [0086]     Also, the color areas of substratum color of patch are classified into six attributes, but the classification method for attributes is not limited to the method as shown herein.  
         [0087]     Through the RGB outputs of the sensor are converted into the XYZ chromaticity values, it is apparent that the method as described in this embodiment is effective when the color matching function in two different color specification systems has non-linearity.  
         [0088]     Moreover, there are two attributes here, including the gray gradation patch of only black (K) and process gray gradation patch in which yellow (Y), magenta (M) and cyan (C) are mixed, but the classification method for attributes is not limited to the combination of color materials as indicated here.  
         [0089]     Moreover, the methods for converting the RGB outputs of the sensor into the XYZ chromaticity values include a linear conversion method by matrix, a neural network method and a method using the look-up table. In any method, if the color area of the recording medium and the attribute of patch are judged, it is possible to reduce the differences between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination by changing the weight of connection between neurons for use in the neural network for every color area of the recording medium and every attribute of patch, or using the look-up table.  
         [0090]     As described above, by changing various parameters for use in converting the RGB outputs of the sensor into the XYZ chromaticity values according to the combination of the attribute of patch and the color area of the recording medium, it is possible to further reduce the difference between the XYZ chromaticity values obtained by converting the RGB outputs of the sensor and the XYZ chromaticity values obtained from the spectral reflectance as defined by International Commission on Illumination, whereby the precision of color stabilization control using the XYZ chromaticity values can be improved.  
         [0091]     While the invention has been described in terms of its preferred embodiments, various modifications may be made thereto without departing from the spirit and scope of the invention as defined by the appended claims.  
         [0092]     This application claims priority from Japanese Patent Application No. 2004-219971 filed Jul. 28, 2004, which is hereby incorporated by reference herein.