Patent Publication Number: US-6985678-B2

Title: Color image forming apparatus and control method therefor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a color image forming apparatus of an electrophotographic process such as a color printer or a color copying apparatus, and a control method therefor. 
     2. Related Background Art 
     For the color image forming apparatus employing an electrophotographic process or an ink jet process such as a color printer or a color copying apparatus, there is recently required a higher image quality for the output image. In particular, a density gradation and stability thereof influence significantly on the human judgment whether an image is good or not. 
     However, in the color image forming apparatus, an obtained image shows a variation in the density, in case a variation is caused in various units of the apparatus by an environmental change or by a prolonged use. Particularly in a color image forming apparatus of an electrophotographic process, even a slight environmental change may result a density variation leading to an aberration in the color balance, so that there is required means for always maintaining a constant density. There is therefore provided such construction as to form a density detecting toner image (hereinafter called patch) with toner of each color on an intermediate transfer member or a photosensitive member, to detect the density of such unfixed toner patch with an unfixed toner density detecting sensor (hereinafter called density sensor) and to execute a density control by feedback of a result of such detection to process conditions such as an exposure amount, a developing bias, etc., thereby obtaining stable images. 
     However, the density control utilizing such density sensor is based on detecting a patch formed on the intermediate transfer member or the photosensitive drum, and cannot control an aberration in the color balance resulting from variations in a transfer and a fixation on a transfer material to be executed thereafter. Such variations cannot be coped with the aforementioned density control utilizing the density sensor. 
     Consequently, there can be conceived a color image forming apparatus equipped with a sensor (hereinafter called color sensor) for detecting a density or color of a patch formed on the transfer material. 
     Such color sensor is constituted of three or more light sources with different light emission spectra such as light-emitting elements of red (R), green (G) and blue (B) or a light source such as a light-emitting element emitting a white (W) light, and light-receiving elements bearing three or more filters of different spectral transmittances such as of red (R), green (G) and blue (B). In this manner there can be obtained three or more different outputs such as RGB outputs. 
     However, a control with such color sensor requires a patch formation on the transfer material, thus necessitating consumption of a transfer material and toners. Consequently a frequency of such control cannot be made very high. Such color sensor only is unable to achieve an effective density control while minimizing the frequency of control with such color sensor. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the aforementioned drawbacks. 
     The above-mentioned object can be attained, according to the present invention, by a color image forming apparatus including: 
     an image forming unit capable of forming a color image; 
     a first optical sensor capable of detecting an unfixed toner image; 
     a second optical sensor capable of detecting a toner image after fixation; 
     a calculation unit adapted to calculate, based on characteristics of a mixed-color toner image detected by the second optical sensor, a condition that the mixed-color toner image becomes achromatic; 
     means which causes the image forming unit to form a monochromatic toner image based on a result of calculation by the calculation unit; and 
     a setting unit adapted to set a correcting condition for an output of the first optical sensor, based on a result of detection of the monochromatic toner image detected by the first optical sensor. 
     According to the present invention, there is also provided a color image forming apparatus including: 
     an image forming unit capable of forming a color image; 
     a first optical sensor capable of detecting an unfixed toner image formed by the image forming unit; 
     a second optical sensor capable of detecting a toner image after fixation, formed in the image forming unit; 
     a calculation unit adapted to calculate, based on characteristics of a mixed-color toner image detected by the second optical sensor, a condition that the mixed-color toner image becomes achromatic; and 
     a setting unit adapted to set a correcting condition for an output of the first optical sensor, based on a result of calculation by the calculation unit. 
     According to the present invention, there is also provided a control method for controlling a color image forming apparatus capable of forming a color image and provided with a first optical sensor capable of detecting an unfixed toner image and a second optical sensor capable of detecting a toner image after fixation, the method including: 
     a calculation step of calculating, based on characteristics of a mixed-color toner image detected by the second optical sensor, a condition that the mixed-color toner image becomes achromatic; 
     a step of causing the image forming unit to form a monochromatic toner image based on a result of the calculation; and 
     a setting step of setting a correcting condition for an output of the first optical sensor, based on a result of detection of the monochromatic toner image detected by the first optical sensor. 
     According to the present invention, there is also provided a control method for a color image forming apparatus capable of forming a color image and provided with a first optical sensor capable of detecting an unfixed toner image and a second optical sensor capable of detecting a toner image after fixation, the method including: 
     a calculation step of calculating, based on characteristics of a mixed-color toner image detected by the second optical sensor, a condition that the mixed-color toner image becomes achromatic; and 
     a setting step of setting a correcting condition for an output of the first optical sensor, based on a result of the calculation. 
     Still other objects and configurations of the present invention, and advantages thereof, will become fully apparent from the following detailed description which is to be taken in conjunction with accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing an entire configuration of a first embodiment of the present invention; 
         FIG. 2  is a view showing a configuration of a density sensor  41  in the present invention; 
         FIGS. 3A and 3B  are views showing a configuration of a color sensor  42  in the present invention; 
         FIG. 4  is a flowchart showing a process in the first embodiment of the present invention; 
         FIG. 5  is a view showing an arrangement of a patch pattern on a transfer material, to be employed in the first embodiment; 
         FIG. 6  is a table explaining the patch pattern on the transfer material, to be employed in the first embodiment; 
         FIG. 7  is a three-dimensional presentation of C, M, Y coordinates of the patches shown in  FIG. 6 ; 
         FIG. 8  is a view showing density sensor correcting patches in the first embodiment; 
         FIG. 9  is a chart showing a correction table for the density sensor  41  in the first embodiment; 
         FIG. 10  is a view showing an arrangement of image gradation control patches in the first embodiment; 
         FIG. 11  is a chart showing an image gradation controlling method in the first embodiment; 
         FIG. 12  is a flowchart showing a process in a second embodiment of the present invention; 
         FIG. 13  is a view showing density sensor correcting patches in the second embodiment; 
         FIG. 14  is a chart showing a process for estimating a detection value of the density value for the patches in the second embodiment; and 
         FIG. 15  is a block diagram showing an electrical control system in the color image forming apparatus embodying the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (First Embodiment) 
       FIG. 1  is a cross-sectional view showing an entire configuration of a color image forming apparatus in a first embodiment. As shown in the drawing, the apparatus is a color image forming apparatus of a tandem type, employing an intermediate transfer member (mid transfer material)  27 , as an electrophotographic color image forming apparatus. The present color image forming apparatus is composed of an image forming unit shown in  FIG. 1  and an unrepresented image processing unit. 
     In the following, there will be explained, with reference to  FIG. 1 , an operation of the image forming unit in the electrophotographic color image forming apparatus. The image forming unit serves to form electrostatic latent images by exposing lights turned on based on exposure times converted by the image processing unit, developing such electrostatic latent images to form monochromatic toner images, superposing such monochromatic toner images to form a multi-color toner image, transferring such multi-color toner image onto a transfer material  11  and fixing such multi-color toner image on a transfer material  11 , and is constituted of a paper feed unit  21 ; a photosensitive member ( 22 Y,  22 M,  22 C,  22 K), injection charging means ( 23 Y,  23 M,  23 C,  23 K) as primary charging means, a toner cartridge ( 25 Y,  25 M,  25 C,  25 K) and developing means ( 26 Y,  26 M,  26 C,  26 K) which are provided for each of stations provided by a number of colors to be developed; an intermediate transfer member  27 ; a transfer roller  28 ; cleaning means  29 ; a fixing unit  30 ; a density sensor  41  and a color sensor  42 . 
     Each of the photosensitive drums (photosensitive members)  22 Y,  22 M,  22 C,  22 K is formed by coating an organic photoconductive layer on an external periphery of an aluminum cylinder, and is rotated by a driving force of an unrepresented driving motor, which rotates the photosensitive drums  22 Y,  22 M,  22 C,  22 K in a counterclockwise direction in the course of an image forming operation. 
     As primary charging means, the stations are respectively provided, for charging the photosensitive members of yellow (Y), magenta (M), cyan (C) and black (K), with four injection chargers  23 Y,  23 M,  23 C,  23 K, which are respectively provided with sleeves  23 YS,  23 MS,  23 CS,  23 KS. 
     Exposing lights to the photosensitive drums  22 Y,  22 M,  22 C,  22 K are supplied from scanner units  24 Y,  24 M,  24 C,  24 K and selectively expose the surfaces of the photosensitive drums  22 Y,  22 M,  22 C,  22 K, thereby forming electrostatic latent images. 
     As developing means for rending the electrostatic latent images visible, the stations are respectively provided with four developing devices  26 Y,  26 M,  26 C,  26 K, which respectively execute development of yellow (Y), magenta (M), cyan (C) and black (K) colors, and which are respectively provided with sleeves  26 YS,  26 MS,  26 CS,  26 KS. Each developing device is detachably mounted. 
     An intermediate transfer member  27  is in contact with the photosensitive drums  22 Y,  22 M,  22 C,  22 K and is rotated clockwise at the image formation along with the rotation of the photosensitive drums  22 Y,  22 M,  22 C,  22 K, thereby receiving transfers of monochromatic images. Thereafter a transfer roller  28  to be explained later is brought into contact with and supports the intermediate transfer member  27  thereby transferring a multi-color toner image thereon onto the transfer material  11 . 
     A transfer roller  28  is maintained in a position  28   a  in contact with the transfer material  11  during the transfer of the multi-color toner image, but is separated to a position  28   b  after the printing process. 
     A fixing unit  30  serves to fix the transferred multi-color toner image by fusion while the transfer material  11  is conveyed, and is provided, as shown in  FIG. 1 , with a fixing roller  31  for heating the transfer material  11  and a pressure roller  32  for contacting the transfer material  11  with the fixing roller  31  under a pressure. The fixing roller  31  and the pressure roller  32  have a hollow structure and are provided therein with heaters  33 ,  34  respectively. Thus the transfer material  11  bearing the multi-color toner image is conveyed by the fixing roller  31  and the pressure roller  32  and is given a heat and a pressure, whereby the toner is fixed to the surface. 
     The transfer material  11  after the fixation of the toner image is thereafter discharged by unrepresented discharge rollers onto an unrepresented discharge tray, whereupon the image forming operation is terminated. 
     Cleaning means  29  serves to remove toner remaining on the intermediate transfer member  27 , and used toner remaining after the transfer of the four-color toner image formed on the intermediate transfer member  27  is stored in a cleaner container. 
     A density sensor  41  in the color image forming apparatus shown in  FIG. 1  is positioned toward the intermediate transfer member  27 , and measures a density of a patch formed on the surface of the intermediate transfer member  27 . The density sensor  41  has a configuration as exemplified in  FIG. 2 , and is constituted of an infrared light-emitting element  51  such as an LED, a light-receiving element  52  such as a photodiode, an unrepresented IC or the like for processing data of received data, and an unrepresented holder for containing these components. 
     The infrared light-emitting element  51  is positioned with an angle of 45° to a direction perpendicular to the intermediate transfer member  27 , and irradiates a toner patch  64  on the intermediate transfer member  27  with an infrared light. The light-receiving element  52  is provided in a position symmetrical to the light-emitting element  51  and detects a normally reflected light from the toner patch  64 . 
     For coupling the light-emitting element  51  and the light-receiving element  52 , there may be employed an optical element such as an unrepresented lens or the like. 
     In the present embodiment, the intermediate transfer member  27  is formed by a single-layered resinous belt of a polyimide resin, in which an appropriate amount of fine carbon particles are dispersed for regulating a resistance of the belt, and which has a black surface color. The surface of the intermediate transfer member  27  has a high smoothness and is lustrous, with a glossiness of about 100% (measured with a gloss meter IG-320 manufactured by Horiba, Ltd.). 
     In a state where the surface of the intermediate transfer member  27  is exposed (toner density  0 ), the light-receiving element  52  of the density sensor  41  detects a reflected light, because the surface of the intermediate transfer member  27  is lustrous as explained before. On the other hand, in case a toner image is formed on the intermediate transfer member  27 , the output of normal reflection decreases as the density of the toner image increases. This is because the toner covers the surface of the intermediate transfer member  27 , thereby decreasing the normally reflected light from the belt surface. 
     A color sensor  42  is provided, in the color image forming apparatus shown in  FIG. 1 , in a downstream position of the fixing unit  30  in a conveying path for the transfer material, so as to be opposed to an image forming surface of the transfer material  11 , and detects RGB outputs for a mixed-color patch after fixation, formed on the transfer material. Such positioning inside the color image forming apparatus enables an automatic detection before the image after fixation is discharged to the sheet discharge unit. 
       FIGS. 3A and 3B  illustrate an example of the configuration of the color sensor  42 . The color sensor  42  shown in  FIG. 3A  is constituted of a white-color LED  53  and a charge-accumulating sensor  54   a  with on-chip RGB filters  54   b . An output light from the white-color LED  53  is caused to enter, in a direction of an angle of 45°, the transfer material  11  on which a fixed patch  61  is formed, and a random reflectedlight intensity in a direction of an angle of 0° is detected by the charge-accumulating sensor  54   a  with on-chip RGB filters. In the charge-accumulatingsensor  54   a  with on-chip RGB filters, a light-receiving part  54  is provided with independent pixels for R, G and B colors as shown in  FIG. 3B . 
     In the charge-accumulating sensor  54   a  with on-chip RGB filters, the sensor may be composed of photodiodes, or may include several sets each including three RGB pixels. There may also be adopted a configuration with an incident angle of 0° and an exit angle of 45°, or a configuration constituted of LEDs emitting lights of RGB colors and a sensor without color filters. 
     In the following, an electrical control system of the color image forming apparatus will be explained with reference to  FIG. 15 . 
     Referring to  FIG. 15 , an image processing unit  110  for generating image data is constituted of a development unit  111  for receiving a print job from an unrepresented host computer and developing it into image data, a gamma correction unit  112  for executing various image processes based on internally stored look-up tables for respective colors, etc. There are also provided image forming units  121 – 124  for forming colored images of yellow, magenta and cyan and an achromatic black image, a fixing unit  30  for fixing a formed image to the transfer material, a motor for driving devices relating to image formation and rollers for conveying the transfer material, and a density sensor  41  and a color sensor  42  explained in the foregoing. 
     A control unit  120  controls the aforementioned color image forming units  121 – 124 , the fixing unit  30 , the motor  125 , etc. and causes these units to execute the image formation. The control unit  120  also executes a flowchart to be explained later and various image forming sequences. 
     A correction unit  126  corrects the output of the density sensor, and a table is set therein by the control unit  120  according to a flowchart to be explained later. The correction table may also be provided in an unrepresented non-volatile memory in the control unit  120 . 
     In the following there will be explained a correction process for the density sensor  41  and a color balance correction control in the present embodiment. In the present embodiment, in order to correct the density sensor  41 , it is necessary to utilize the color sensor  42 . Stated otherwise, there is required a toner image fixed on the transfer material, so that it is preferable to minimize the frequency of execution of such control. In the present embodiment, the correction control is executed by a manual operation of the user when the user desires an execution of the correction control. It is naturally possible also, as another embodiment, to execute such correction control at a predetermined interval. 
     Also the present embodiment employs a CMY mixed color patch and a K monochromatic patch as the patch fixed on the transfer material, and corrects the color balance of a process gray color by comparing the CMY mixed color patch and the K monochromatic patch. 
     This is because, in a color image forming apparatus, in case the color balance becomes unstable, a variation in the hue (or coloring) tends to occur particularly in the process gray color, and the human eyes are sensitive to such variation in the hue. Consequently a correction on the process gray color can realize an effective improvement in the image quality. 
     In the following there will be explained, with reference to  FIGS. 4 and 5 , a correction process for the density sensor and a correction process for the color balance in the present embodiment.  FIG. 4  is a flowchart of a correction process for the density sensor and a correction process for the color balance in the present embodiment.  FIG. 5  is a view showing an example of a patch pattern in the present embodiment. 
     In the flowchart shown in  FIG. 4 , at first a step S 401  prepares a patch pattern on the transfer material  11 . 
       FIG. 5  shows a patch pattern formed on the transfer material  11  (which, in this case, is A3 size (297×420 mm) conveyed longitudinally). Formed patches are composed of four patch sets (SET 1 , SET 2 , SET 3 , SET 4 ), and each patch set is composed of CMY mixed-color patches  1  to  8  and a K monochromatic patch  9 , namely 9 patches in total (each 8 mm square and mutually separated by 2 mm). 
     The patches  1  to  9  in a same patch set respectively have CMY data  1 – 8  and K monochromatic data  9  as shown in  FIG. 6 . 
     In a patch set SETn (n being  1  to  4 ), C, M, Y tonalities (tonal levels of image data) corresponding to the patches are a combination of values obtained by changing the tonality by ±α from reference tonalities Cn, Mn, Yn (hereinafter represented as reference values). Also a 9th patch is a K monochromatic patch, formed with a predetermined tonality Kn. The reference values Cn, Mn, Yn and Kn are such that a mixing of Cn, Mn and Yn provides a color same as Kn in a state where the tone-density characteristics of C, M, Y and K are adjusted to a default state (most average state of the apparatus), and are selected in designing of color processing and halftone. 
     The patch sets SET 1 –SET 4  are formed with different tonality values. For example, the SET 1 , SET 2 , SET 3  and SET 4  are prepared with the tonalities of the Kn (patch  9 ) respectively at 25%, 50%, 75% and 100%. Also patches  1 – 8  are prepared with values corresponding to the tonality of Kn (patch  9 ). 
     Then, in a step S 402 , RGB outputs of the patches fixed to the transfer material in the step S 401  are detected by the color sensor  42 . 
     Then a step S 403  calculates, from the RGB outputs of the sensor, C, M, Y tonality values (mixing ratio) required in order that a process gray color formed by C, M, Y matches the color of the K patch  9 . 
     In case the image forming conditions are identical with those at the designing of color processing, the color of Kn coincides with a color obtained by mixing (Cn, Mn, Yn), but such coincidence does not happen usually and an aberration in color results because of reasons described in the explanation of the prior art.  FIG. 7  shows a three-dimensional representation of C, M, Y coordinates of the patches  1 – 8 , wherein the RGB outputs of the patches are represented by 1=(r 1 , g 1 , b 1 ), 2=(r 2 , g 2 , b 2 ) etc. In  FIG. 7 , a center of the cubic lattice has coordinate values (Cn, Mn, Yn). 
     Then, C, M, Y values required to match the RGB values of Kn from the RGB values of the patches  1  to  8  are determined by linear interpolations of 8 points shown in  FIG. 7 . More specifically, the determination is made by calculating RGB values (Rcmy, Gcmy, Bcmy) for the C, M, Y coordinates in the cubic lattice shown in  FIG. 7  according to a following formula: 
             Rcmy   =       ⁢     (         (     C   -   Cn   +   α     )     ⁢     (     M   -   Mn   +   α     )     ⁢     (     Y   -   Yn   +   α     )     ⁢   r1     +                       ⁢         (     Cn   +   α   -   C     )     ⁢     (     M   -   Mn   +   α     )     ⁢     (     Y   -   Yn   +   α     )     ⁢   r2     +                     ⁢         (     C   -   Cn   +   α     )     ⁢     (     Mn   +   α   -   M     )     ⁢     (     Y   -   Yn   +   α     )     ⁢   r3     +                     ⁢         (     C   -   Cn   +   α     )     ⁢     (     M   -   Mn   +   α     )     ⁢     (     Yn   +   α   -   Y     )     ⁢   r4     +                     ⁢         (     Cn   +   α   -   C     )     ⁢     (     Mn   +   α   -   M     )     ⁢     (     Y   -   Yn   +   α     )     ⁢   r5     +                     ⁢         (     Cn   +   α   -   C     )     ⁢     (     M   -   Mn   +   α     )     ⁢     (     Yn   +   α   -   Y     )     ⁢   r6     +                     ⁢         (     C   -   Cn   +   α     )     ⁢     (     Mn   +   α   -   M     )     ⁢     (     Yn   +   α   -   Y     )     ⁢   r7     +                       ⁢       (     Cn   +   α   -   C     )     ⁢     (     Mn   +   α   -   M     )     ⁢     (     Yn   +   α   -   Y     )     ⁢   r8     )     /     (     8   ⁢   α   ⁢           ⁢   3     )               
 
     Gcmy and Bcmy can be determined by similar formulas. 
     Then, there is determined a difference between thus calculated (Rcmy, Gcmy, Bcmy) and the RGB values (Rk, Gk, Bk) of K by, for example, squared sum of RGB differences. Then there is determined a smallest difference, namely (Rcmy, Gcmy, Bcmy) closest to (Rk, Gk, Bk) and C, M, Y values in such state are selected as optimum values (Cn′, Mn′, Yn′). 
     α is selected at an optimum value taking into consideration following two conditions that:
         1) the dimension of the cubic lattice should as small as possible in order to increase the precision of interpolation; and   2) in case Kn and (Cn, Mn, Yn) are significantly aberrated, the point (Cn′, Mn′, Yn′) is not present in the vicinity of the cubic lattice center (Cn, Mn, Yn), but it has to be contained in the cubic lattice and the cubic lattice should be large enough for this purpose.       

     Then a step S 404  forms correction patches for the density sensor  41  on the intermediate transfer member  27 .  FIG. 8  shows a patch pattern to be formed on the intermediate transfer member  27 , and, corresponding to the position of the density sensor  41 , 8 mm square patches are formed with a gap of 12 mm and, for each of C, M, Y, with an image print rate (density tonality) in 4 levels (4 patches for each color), thus 12 patches in total. The print rates (tonality levels) of the patches correspond to Cn′, Mn′, Yn′ of 4 tonality levels (SET 1 –SET 4 ) calculated in the step S 403 . More specifically, C 1 , M 1 , Y 1  respectively correspond to Cn′ 1 , Mn′ 1 , Yn′ 1  of the SET 1 ; C 2 , M 2 , Y 2  respectively correspond to Cn′ 2 , Mn′ 2 , Yn′ 2  of the SET 2 ; C 3 , M 3 , Y 3  respectively correspond to Cn′ 3 , Mn′ 3 , Yn′ 3  of the SET 3 ; and C 4 , M 4 , Y 4  respectively correspond to Cn′ 4 , Mn′ 4 , Yn′ 4  of the SET 4 . 
     Then a step S 405  causes the density sensor  401  to detect the density of the correction patches formed in the step S 404 . For converting a detection signal of the density sensor  41  into a density, there can be employed, for example, a detection signal-density conversion table (density conversion table) which is already known in the art. Details of such density conversion table will not be explained further. 
     Then a step S 406  sets a correction table for each of YMC color components stored in the correction unit  126  for correcting the output of the density sensor  41 . 
     In the following there will be explained, with reference to  FIG. 9 , a correction method for the density sensor  41 .  FIG. 9  is a chart representing a correction table for correcting the output of the density sensor  41  in the present embodiment. Referring to a chart shown in  FIG. 9 , the abscissa represents detection values of the density sensor  41  for patches C 1 , C 2 , C 3  and C 4 , while the ordinate represents an output density (DCn) corresponding to Cn in each of the 4 tonality values (SET 1  to SET 4 ) in the step S 401 . 
     In  FIG. 9 , a curve C 901  represents a correction table for the density sensor  41 . The correction table C 901  passes black circle points (P 1 ′ to P 4 ′; each corresponding to an output density for Cn in the step S 401  and a detection result of the density sensor  41  in the step S 405 ), and, any density not corresponding to a patch (tonality between patches) is calculated by a spline interpolation of the original point, the points  902  and a point of a maximum output of the density sensor (maximum value of the density conversion table). Thus calculated correction table C 901  is used in an image tone control (tone correction) to be explained in a step S 407  and thereafter. 
     In the following, there will be explained, in more specific manner, a correction method for the output density of the density sensor  41 , utilizing the correction table C 901 . A relationship between the detection value of the density sensor  41  and output density prior to correction is represented by a broken line  903 , connecting while circle points P 1  to P 4 . Thus, let us consider for example a detection value O2 of the density sensor corresponding to P 2 ′ and P 2 . The output density prior to correction is P 2 , corresponding to the detection value O2, but the output density can be determined as P 2 ′ based on the correction table C 901 . In this manner it is rendered possible to correct the output density of the density sensor  41 . 
     The correction table C 901  is calculated not only for cyan color but also for magenta and yellow colors in a similar manner. The correction table C 901  is calculated by an unrepresented CPU in a main body, and is stored in an unrepresented memory in the main body (a non-volatile memory being used in the present embodiment). The correction process for the density sensor  41  in the present embodiment is executed as explained above. 
     Then, steps S 407  to S 409  execute an image tone control (tone correction) by detecting reflective characteristics of each of YMCK single-color patches by the density sensor  41  and setting a tone correction table (gamma correction table) for each of YMCK colors stored in a gamma correction unit  112 . In the following there will be explained such image tone control (tone correction). 
     At first a step S 407  forms patches for the image tone control (tone correction) on the intermediate transfer member  27 . 
       FIG. 10  shows a patch pattern formed on the intermediate transfer member, and, corresponding to the position of the density sensor  41 , 8 mm square patches are formed with a gap of 2 mm and, for each of Y, M, C, K with an image print rate (density tonality) in 8 levels (8 patches for each color), thus 32 patches in total. In the present embodiment, the patches are formed with following print rates (tonality values): Y 1 , M 1 , C 1 , K 1 =12.5%; Y 2 , M 2 , C 2 , K 2 =25%; Y 3 , M 3 , C 3 , K 3 =37.5%; Y 4 , M 4 , C 4 , K 4 =50%; Y 5 , M 5 , C 5 , K 5 =62.5%; Y 6 , M 6 , C 6 , K 6 =75%; Y 7 , M 7 , C 7 , K 7 =87.5%; and Y 8 , M 8 , C 8 , K 8 =100%. 
     Then a step S 408  causes the density sensor  41  to detect the density of such patches. In this operation, the density output of the density sensor  41  is corrected by the density sensor correction table C 901  shown in  FIG. 9 . 
     Then a step S 409  executes an image tone control (tone correction), which will be explained in the following with reference to  FIG. 11 . In the following there will be only explained the tone control for cyan color, but the correction is executed also for magenta, yellow and black colors in a similar manner. 
     Referring to a chart shown in  FIG. 11 , the abscissa represents a tonality value of the image data, while an ordinate represents a detected density (detection value corrected by the correction table). Also an ordinate represents a tonality value of the image data after tone correction. 
     In  FIG. 11 , each of white circles C 1 , C 2 , C 3 , C 4 , C 5 , C 6 , C 7 , C 8  indicates an output density of the density sensor  41  corresponding to each patch. A straight line T  1101  indicates a target density-tone characteristics of the image density control. In the present embodiment, the target density-tone characteristics is so determined that the image data and the density are proportional. A curve γ  1102  represents density-tone characteristics in a state where the density control (tone correction control) is not executed. Densities of tonality levels not corresponding to patches are calculated by such a spline interpolation as to pass through the original point and the points C 1  to C 8 . 
     A curve D 1103  represents a tone correction table calculated in the present control, and is calculated by determining points symmetrical to the tone characteristics γ  1102  with respect to the target tone-characteristics T  1101 . The tone control table D 1103  is calculated by the unrepresented image processing unit  120 , and is stored in the gamma correction unit  112  (utilizing a non-volatile memory in the present embodiment) in the image processing unit  110 . In printing an image, the target tone characteristics can be obtained by correcting the image data with the tone correction table D 1103 . 
     In the following, there will be given a more specific explanation on the correction method for the image data, utilizing the tone correction table D 1103  at the print image formation. For example, let us consider a C 4  patch shown in  FIG. 11 . The C 4  patch before correction has a density of about 0.7 for a print rate (tonality) of 50%. Since the target density for the C 4  patch is 0.6 according to the straight line T 1101 , there is required a tone control of about 0.1. On the image data axis  1105  of the tone correction table D 1103 , a tonality of 50% provides a point C 4 ′ which corresponds to a tonality of about 46% on the image data axis  1106  after the tone correction, thus providing the tonality after the correction. It is thus identified that, for the patch C 4 , the formation of the print image should be made by correcting the tonality from 50% to 46%. 
     The corrections for the density sensor and for the color balance in the present embodiment are executed as explained in the foregoing. 
     The image tone control (tone correction) explained in the steps S 407  to S 409  is executed periodically, utilizing the density sensor  41 . The output of the density sensor is corrected every time by the already calculated correction table C 901 . In the color image forming apparatus of the present embodiment, the image tone control (tone correction) is executed when the power supply is turned on, also when a developing apparatus or a photosensitive drum is replaced, and after printing operations of a predetermined number, namely in a situation where a density variation is anticipated. The apparatus can always maintain a satisfactory color balance by executing such image tone control (tone correction) periodically. 
     Also in case a variation in the transfer condition or in the fixing condition is anticipated (for example, when an intermediate transfer member or a fixing apparatus is replaced or when a installed location of the apparatus, namely an environment of use thereof, is changed), the user executes the aforementioned correction of the color sensor  42  (by the aforementioned steps S 401  to S 406 ), thereby renewing the correction table C 901 . 
     In this manner it is rendered possible to reduce the number of execution of the density control utilizing the color sensor thereby suppressing the consumption of the transfer material, and to provide a color image forming apparatus superior in the density stability in comparison with a prior density control utilizing only a density sensor. 
     The present embodiment has been explained by a color image forming apparatus utilizing an intermediate transfer member, but the present invention is applicable also to color image forming apparatus of other configurations. For example, the present invention is applicable also to a color image forming apparatus in which a toner image on a photosensitive member is directly transferred to a transfer material supported on a transfer material carrying member (such as a transfer belt) and which executes a density control by forming a toner patch on the transfer material carrying member. 
     As explained in the foregoing, the present embodiment allows to suppress the consumption of the transfer material, and to provide a color image superior in the density stability in comparison with a prior density control utilizing only a density sensor, by forming a mixed toner image containing a cyan toner, a magenta toner and a yellow toner on a transfer material, detecting the reflective characteristics of the mixed toner image with a color sensor, calculating a toner mixing ratio for bringing the mixed toner image to a achromatic state based on the result of such detection, detecting a density of a monochromatic toner image corresponding to the calculated toner mixing ratio by a density sensor, executing a correction of the density sensor based on the result of such detection and further executing an image tone control (tone correction) utilizing the density sensor. 
     (Second Embodiment) 
     In this embodiment, there will be explained a method of simultaneously forming patches of two kinds to be used for correcting the density sensor, namely patches for detection by the color sensor and patches for detection by the density sensor, thereby reducing a correction time for the density sensor and improving a precision of correction of the density sensor. 
     The present embodiment is an extension of the first embodiment, and is different therefrom in a timing and a pattern of formation of the patches to be detected by the density sensor for the correction thereof, and in a method for calculating the sensor correction table. An entire configuration of the color image forming apparatus to be employed in the present embodiment is similar to that of the color image forming apparatus explained in  FIG. 1 , and will not, therefore, be explained further. 
     In the following there will be explained, with reference to a flowchart shown in  FIG. 12 , correction methods for the density sensor and for the color balance in the present embodiment. 
     At first, a step S 1201  forms a patch pattern on the intermediate transfer member  27 .  FIG. 13  shows the patch pattern, formed on the intermediate transfer member  27  and including a pattern A 1301  for detection by the color sensor and a pattern B 1302  for detection by the density sensor. The pattern B 1302  is so positioned as to correspond to the detecting position of the density sensor  41 , while the pattern A 1301  is so positioned, when the pattern on the intermediate transfer member  27  is transferred onto a transfer material, as to correspond to the detecting position of the color sensor  42 . 
     The pattern A 1301  is composed of four patch sets (SET 1 , SET 2 , SET 3 , SET 4 ), and each patch set is composed of CMY mixed-color patches  1  to  8  and a K monochromatic patch  9 , namely 9 patches in total. 
     The patches  1  to  9  in a same patch set respectively have CMY data  1 – 8  and K monochromatic data  9  as shown in  FIG. 6 . 
     In a patch set SETn (n being 1 to 4), C, M, Y tonalities corresponding to the patches are a combination of values obtained by changing the tonality by ±α from reference tonalities Cn, Mn, Yn (hereinafter represented as reference values). Also a 9th patch is a K monochromatic patch, formed with a predetermined tonality Kn. The reference values Cn, Mn, Yn and Kn are such that a mixing of Cn, Mn and Yn provides a color same as Kn in a state where the tone-density characteristics of C, M, Y and K are adjusted to a default state (most average state of the apparatus), and are selected in designing of color processing and halftone. 
     The patch sets SET 1 –SET 4  are formed with different tonality values. More specifically, the SET 1 , SET 2 , SET 3  and SET 4  are prepared, for example, with the tonality of the Kn (patch  9 ) respectively at 25%, 50%, 75% and 100%. Also patches  1 – 8  are prepared with values corresponding to the tonality of Kn (patch  9 ). 
     The pattern B 1302  is formed by monochromatic component patches (monochromatic patches) of the CMY mixed patches in the pattern A 1301 . More specifically it is composed of four tonality sets SET 1 , SET 2 , SET 3  and SET 4 , and each tonality set includes 6 monochromatic patches of Cn−α, Cn+α, Mn−α, Mn+α, Yn−α, and Yn+α, corresponding to such tonality. 
     Then, in a step S 1202 , the density sensor  41  detects the patch density of the pattern B 1302  formed on the intermediate transfer member  27  in the step S 1201 . Then a step S 1203  transfers the patch pattern from the intermediate transfer member  27  to the transfer material  11  and executes a fixation by the fixing unit  30 . 
     Then, in a step S 1204 , the color sensor  42  detects the RGB outputs of the patches of the pattern A 1301  fixed on the transfer material  11  in the step S 1203 . Then a step S 1205  calculates, based on the RGB outputs of the color sensor  42 , C, M, Y values (tonalities) required for matching the C, M, Y process gray color with the K color of the patch  9 , namely Cn′, Mn′ and Yn′ values. A method for calculating the Cn′, Mn′ and Yn′ values is similar to that in the first embodiment and will not be explained further. 
     A next step S 1206  executes a correction on the output of the density sensor  41 . In the present embodiment, different from the first embodiment, the patches for detection by the color sensor and the patches for detection by the density sensor are formed simultaneously, so that the Cn′, Mn′ and Yn′ values are not determined at the formation of the patches for detection by the density sensor. It is therefore necessary to estimate, by calculation, the detection values of the Cn′, Mn′ and Yn′ patches by the density sensor. 
     In the following there will be explained, with reference to  FIG. 14 , a method for estimating the detection values of the density sensor for the Cn′, Mn′ and Yn′ patches. In the following there will be explained a method for a tonality level of Cn′ (value for cyan toner), but a similar method can be used for other tonality levels or for magenta or yellow toner. 
     Referring to  FIG. 14 , the ordinate represents a detection result of the density sensor  41  on a patch, while the abscissa represents toner densities corresponding to Cn−α, Cn and Cn+α when the apparatus is in a most average state, namely densities corresponding to Cn−α, Cn and Cn+α at the designing of the color processing. 
     In  FIG. 14 , white circle points  1403  and  1404  indicate the detection densities of the density sensor  41  for the patches Cn−α and Cn+α. An estimated detection value of the density sensor for a patch Cn′ is calculated by a linear interpolation. More specifically, a value of a point  1405  is calculated on a straight line connecting the points of Cn−α and Cn+α. 
     Thus, in  FIG. 14 , the detection value of the density sensor for the Cn′ patch is given by X. Such calculation allows to provide the detection values of the density sensor for the Cn′, Mn′, Yn′ patches. 
     Then a correction table C for the density sensor  41  is calculated, utilizing the values calculated by the above-described method (estimated detection values of the density sensor for the Cn′, Mn′, Yn′ patches at each tonality. The correction table is calculated in a similar manner as in the first embodiment. 
     Then a step S 1207  executes an image tone control (tone correction) utilizing the density sensor  41 , thereby correcting the color balance. The image tone control (tone correction) is similar to that in the first embodiment. More specifically, patches of image print rates (density tonality values) varied in 8 levels are formed on the intermediate transfer member  27 , then the densities of the patches are detected by the density sensor  41 , and a tone correction table D is calculated based on the result of detection. 
     The corrections for the density sensor and for the color balance in the present embodiment are executed as explained in the foregoing. 
     The image tone control (tone correction) is executed periodically, utilizing the density sensor  41 . The output of the density sensor is corrected every time by the table C. Also in case a variation in the transfer condition or in the fixing condition is anticipated, the user executes the aforementioned correction of the color sensor  42 , thereby renewing the correction table C. 
     In this manner, it is rendered possible to reduce the number of execution of the density control utilizing the color sensor thereby suppressing the consumption of the transfer material, and to provide a color image forming apparatus superior in the density stability in comparison with a prior density control utilizing only a density sensor. 
     The present embodiment is suitable and effective in a color image forming apparatus capable of simultaneously forming two types of patches to be used for correcting the density sensor, namely the patches for detection by the color sensor and the patches for detection by the density sensor, namely a color image forming apparatus utilizing an intermediate transfer member as in the present embodiment. 
     Also the present embodiment, adapted to simultaneously form the patches for detection by the color sensor and the patches for detection by the density sensor, when applied to an image forming apparatus showing a significant variation in the density for example after a prolonged pause, can avoid the influence of density variation in time between the patches of two types (patches for detection by the color sensor and patches for detection by the density sensor), thereby allowing to improve the precision of correction of the density sensor and to further stabilize the color balance. 
     As explained in the foregoing, the present embodiment is capable, by simultaneously forming the patches of two types used for correcting the density sensor, namely patches for detection by the color sensor and patches for detection by the density sensor, of reducing the time required for correcting the density sensor and improving the precision of correction thereof. 
     In the first embodiment and the second embodiment, the output density value of the density sensor is corrected by the correction table C 901 , but, in case a density conversion table is provided in advance for the relationship between the output voltage of the density sensor and the density, it is also possible to apply the correction table C 901  to such density conversion table thereby preparing a new density conversion table. 
     Also in the first embodiment and the second embodiment, there has been explained a case of utilizing a density as the optical reflection characteristics at the detection of the toner patch by the density sensor, but the optical reflection characteristics to be detected by the density sensor is not limited to such case, and it is also possible, for example, to utilize a color hue, an optical reflectance or a toner amount (toner weight) calculated from the optical reflectance. Stated differently, the present invention naturally includes the detection by an optical sensor of any physical amount convertible from the optical reflection characteristics of the toner patch. 
     (Other Embodiments) 
     The present invention is applicable not only to a system formed by plural equipment (for example a host computer, an interface device, a reader, a printer, etc.) but also to an apparatus formed by a single equipment (for example, a copying machine, a facsimile apparatus, etc.). 
     Also the objects of the present invention can naturally be attained also in a case where a memory medium (or a recording medium) storing program codes of a software realizing the functions of the aforementioned embodiments is supplied to a system or an apparatus and a computer (or CPU or MPU) of such system or apparatus reads and executes the program codes stored in the memory medium. In such case, the program codes themselves realize the functions of the aforementioned embodiments, and the memory medium storing the program codes constitute the present invention. The present invention naturally includes not only a case where the functions of the aforementioned embodiments are realized by the execution of the read program codes by the computer, but also a case where an operating system (OS) or the like functioning on the computer executes all the actual processes or a part thereof under the instructions of the program codes thereby realizing the functions of the aforementioned embodiments. 
     Further, the present invention naturally includes a case where program codes read from the memory medium are written into a memory provided in a function expanding card inserted into the computer or in a function expanding unit connected to the computer and a CPU or the like provided in such function expanding card or function expanding unit executes all the actual processes or a part thereof under the instruction of such program codes thereby realizing the functions of the aforementioned embodiments. 
     According to the embodiments explained in the foregoing, it is rendered possible to suppress the consumption of the transfer material required for the density control and to obtain a color image with superior density stability in comparison with the prior density control utilizing the density sensor only. 
     It is also rendered possible to reduce the time required for correction of the density sensor and to improve the precision of correction therefor. 
     The present invention has been explained by certain preferred embodiments, but the present invention is not limited to such embodiments and is subject to various modifications and applications within the scope and spirit of the appended claims.