JP2008048165A - Image forming apparatus and calibration method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the precision of a gray-level correction table that is generated through soft calibration processing. <P>SOLUTION: A digital copying machine includes a pattern image forming unit 51, which forms a pattern image that includes patches of a plurality of densities, a first colorimetric data receiving unit 55, which receives first colorimetric data that a colorimetry device measures and outputs, the density of a pattern image, formed in soft calibration being measured by the colorimetry device, an interpolation target part extracting unit 57 which extracts an interpolation target part from the first colorimetric data, by comparing the first colorimetric data with the second colorimetric data that a colorimetric sensor which measures the density of the pattern image and outputs in the soft calibration; a first colorimetric data correcting unit 59 which deletes the interpolation target part from the first colorimetric data, correct the first colorimetric data, and a gray-level correction table generating unit 61 which generates the gray-level correction table, based on the corrected first colorimietric data and stores the gray-level correction table. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and a calibration method, and more particularly to an image forming apparatus that corrects the gradation of image data in order to make the density of an image formed on a recording medium appropriate and a calibration executed by the image forming apparatus. It is related to the method.

  In recent years, image forming apparatuses that form images with toner or ink on a recording medium such as paper have been distributed. This image forming apparatus is affected by changes in the environment such as temperature and humidity, wear of consumables such as belts and drums, and charging of toner due to continuous operation. Change in the short term. In order to cope with this long-term or short-term change in gradation characteristics, the image forming apparatus executes two calibration processes. The first calibration processing is called soft calibration processing, and the image forming apparatus forms images of a plurality of patches having different densities on a sheet, and the colorimetric device or scanner device reads the patches and outputs them. Based on the data, a gradation correction table used for γ correction processing is generated. According to this soft calibration process, since the color measurement device or the scanner device has a high accuracy of reading an image, there is an advantage that a highly accurate gradation correction table can be generated using accurate color measurement data. There is a demerit that the user has to operate the color measurement device or the scanner device to read the patch, and the burden on the user to operate becomes large. As a technique for improving the accuracy of the soft calibration process, there is JP-A-2001-29330 (Patent Document 1).

  The second calibration process is called a device calibration process. The color measurement sensor built in the image forming apparatus reads the images of a plurality of patches with different densities formed on the drum or belt and outputs them to the color measurement data. Based on this, a gradation correction table is generated. According to this device calibration process, the colorimetric data output from the built-in colorimetric sensor is used, so there is an advantage that the burden on the user can be eliminated. However, the accuracy of the built-in colorimetric sensor is Since it is lower than the apparatus and scanner, there is a demerit that the accuracy of the gradation correction table is worse than that in the soft calibration process.

However, if the patches formed on the paper become inaccurate due to dust adhering to the paper during the soft calibration process, the color measurement data output by the color measurement device or the scanner device becomes an abnormal value. End up. For this reason, there is a problem that the gradation correction data generated by the soft calibration process is not accurate.
JP 2001-29330 A

  The present invention has been made to solve the above-described problems, and one object of the present invention is to provide an image forming apparatus capable of improving the accuracy of a gradation correction table generated by a soft calibration process. Is to provide.

  Another object of the present invention is to provide a calibration method capable of improving the accuracy of the gradation correction table generated by the soft calibration process.

  In order to achieve the above-described object, according to one aspect of the present invention, an image forming apparatus images an image forming unit that forms an image based on image data and a pattern image including a plurality of density patches on the image forming unit. A pattern image forming unit to be formed; a colorimetric data receiving unit that receives the input of the first colorimetric data to be output by the colorimetric device by measuring the density of the pattern image formed by the image forming unit during soft calibration; The color forming unit that measures the density of the pattern image formed by the image forming unit and outputs the second colorimetric data, the received first colorimetric data, and the image forming unit forms the image at the time of soft calibration. The colorimetric means measures the density of the pattern image, compares it with the second colorimetric data to be output, and extracts the interpolation target portion from the first colorimetric data. Based on the extraction means, the first colorimetric data correction means for correcting the first colorimetric data by deleting the interpolation target portion from the received first colorimetric data, and the corrected first colorimetric data. And a tone correction table generating means for generating and storing a tone correction table.

  According to this aspect, the colorimetric device measures the density of a pattern image including a plurality of density patches during soft calibration, and outputs the first colorimetric data to be output and the density of the pattern image during soft calibration. The second colorimetric data is compared, the interpolation target part is extracted from the first colorimetric data, the interpolation target part is deleted from the first colorimetric data, and the first colorimetric data is corrected. The Based on the corrected first colorimetric data, a gradation correction table is generated and stored. If a portion having a large difference between the first color measurement data and the second color measurement data is extracted as an interpolation target portion, the noise portion can be deleted from the first color measurement data. As a result, a portion that may be noisy is deleted from the first colorimetric data and a tone correction table is generated, so that the accuracy of the tone correction table generated by the soft calibration process can be improved. A possible image forming apparatus can be provided.

  Preferably, the extracting unit measures the density of the pattern image formed by the image forming unit during soft calibration, and the ratio between the second colorimetric data to be output and the first colorimetric data is predetermined. A density exceeding the value of is extracted as a part to be interpolated.

  Preferably, after the gradation correction table is stored, the color measurement unit measures the density of the pattern image formed by the image forming unit, and the history data storage unit stores the second color measurement data to be output. Detecting means for comparing the density corresponding to each of the plurality of density patches between the two or more second colorimetric data and detecting a density having a predetermined variation, and the correcting means comprises the detecting means The interpolation target part other than the density having the predetermined variation detected by the step is deleted from the received first colorimetric data.

  Preferably, when the density having a predetermined variation is detected, the pattern image forming unit changes the pattern image to be image-formed by the image forming unit in a stepwise manner with respect to the density having the predetermined variation in fine steps. The image forming apparatus includes a pattern image changing unit that changes to a pattern image including a plurality of patches, and the image forming apparatus interpolates the first colorimetric data or the gradation correction table from which the interpolation target portion is deleted by a least square method. Means are further provided.

  Preferably, a gradation correction table changing unit that generates a new gradation correction table based on the second colorimetric data, rewrites the stored gradation correction table, and the second colorimetric data or gradation correction table. Is further provided with a second interpolation means for interpolating the signal by the least square method.

  According to another aspect of the present invention, an image forming apparatus includes: an image forming unit that forms an image based on image data; and a pattern image forming unit that causes the image forming unit to form a pattern image including a plurality of density patches. The colorimetric device measures the density of the pattern image formed by the image forming unit during the soft calibration, and the colorimetric data receiving unit receives the input of the first colorimetric data to be output, and the image forming unit forms the image. A gradation correction table that generates and stores a gradation correction table based on the color measurement means that measures the density of the pattern image and outputs the second color measurement data, and the received first color measurement data And a history data for storing the second colorimetric data to be output by the colorimetric means by measuring the density of the pattern image formed by the image forming means after the gradation correction table is stored. A data storage means, and a detection means for comparing the density corresponding to each of a plurality of density patches between two or more stored second colorimetric data, and detecting a density having a predetermined variation, The gradation correction table generating means includes interpolation means for interpolating the received first colorimetric data or gradation correction table by the least square method, and the pattern image forming means generates a pattern image to be formed by the image forming means. Pattern image changing means for changing to a pattern image including a plurality of patches whose density changes step by step with respect to the detected density having a predetermined variation is included.

  According to this aspect, the colorimetric means measures the density of the pattern image and stores the second colorimetric data to be output, and a plurality of density patches between the two or more stored second colorimetric data. A pattern image including a plurality of patches in which the density corresponding to each of the patches is compared, and when a density with a predetermined variation is detected, the density changes stepwise with respect to the detected density with a predetermined variation in fine steps. Changed to It is conceivable that the accuracy of image formation by the image forming means is unstable at a density having a predetermined variation. By forming a pattern image including patches of density around this density, the number of data of the first color measurement data or the second color measurement data is increased, so that the gradation generated by interpolation by the least square method The accuracy of the correction table can be improved. As a result, it is possible to provide an image forming apparatus with improved accuracy of the gradation correction table.

  According to still another aspect of the present invention, the calibration method is a calibration method executed by the image forming apparatus, and the image forming apparatus includes: an image forming unit that forms an image based on image data; A pattern image forming unit that causes the image forming unit to form a pattern image including a density patch; and a colorimetric unit that measures the density of the pattern image formed by the image forming unit and outputs colorimetric data. The step of receiving the input of the first colorimetric data to be output by the colorimetric device and measuring the density of the pattern image formed by the image forming means during the soft calibration, and the received first colorimetric data; Second colorimetric data measured and output by the colorimetric means for measuring the density of the pattern image formed by the image forming means during the soft calibration Comparing the interpolation target portion from the first colorimetric data, deleting the interpolation target portion from the received first colorimetric data, and correcting the first colorimetric data; Generating and storing a gradation correction table based on the corrected first colorimetric data.

  According to this aspect, it is possible to provide a calibration method capable of improving the accuracy of the gradation correction table.

  According to still another aspect of the present invention, the calibration method is a calibration method executed by the image forming apparatus, and the image forming apparatus includes: an image forming unit that forms an image based on image data; A pattern image forming unit that causes the image forming unit to form a pattern image including a density patch; and a colorimetric unit that measures the density of the pattern image formed by the image forming unit and outputs colorimetric data. The color measurement device measures the density of the pattern image formed by the image forming means during the soft calibration, receives the input of the first color measurement data to be output, and receives the received first color measurement data. And a step of generating and storing a gradation correction table based on the pattern image formed by the image forming means after the gradation correction table is stored. The density corresponding to each of the plurality of density patches is stored between the step of storing the second colorimetric data to be measured and outputted and the two or more stored second colorimetric data. Comparing and detecting a density with predetermined variation and a plurality of patches in which the density of the pattern image to be image-formed by the image forming means is changed stepwise with respect to the detected density with predetermined variation And the step of generating a gradation correction table includes a step of interpolating the received first colorimetric data or gradation correction table by a least square method.

  According to this aspect, it is possible to provide a calibration method capable of improving the accuracy of the gradation correction table.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

  FIG. 1 is a schematic cross-sectional view showing a schematic configuration of a digital copying machine according to one embodiment of the present invention. Referring to the figure, a digital copying machine 1 includes an image reading unit 10 for reading image data from a document, a main circuit 100 for processing image data output from the image reading unit 10, and an image on a sheet. And an image forming unit 20 for forming.

  In the image reading unit 10, an image of a document set on the document glass 11 is exposed by an exposure lamp 13 attached to a slider 12 that moves below the image. Reflected light from the document is guided to the lens 16 by the mirror 14 and the two reflecting mirrors 15 and 15A, and forms an image on a CCD (Charge Coupled Devices) sensor 18. The exposure lamp 13 and the mirror 14 are attached to the slider 12, and the slider 12 is moved by the scanner motor 17 in the direction indicated by the arrow (sub-scanning direction) at a speed V corresponding to the copying magnification. Thereby, the document set on the document glass 11 can be scanned over the entire surface. As the exposure lamp 13 and the mirror 14 move, the two reflecting mirrors 15 and 15A move in the direction of the arrow in the drawing at a speed V / 2. As a result, the optical path length from when the light applied to the document by the exposure lamp 13 is reflected by the document until it is imaged on the CCD sensor 18 is always constant.

  The reflected light imaged on the CCD sensor 18 is converted into image data as an electrical signal in the CCD sensor 18 and sent to the main circuit 100. The main circuit 100 performs A / D conversion processing, digital image processing, and the like on the received analog image data, and then outputs it to the image forming unit 20. The main circuit 100 converts the image data into print data for cyan (C), magenta (M), yellow (Y), and black (K) and outputs the data to the image forming unit 20.

  The image forming unit 20 includes yellow, magenta, cyan, and black image forming units 20Y, 20M, 20C, and 20K. At least one of the image forming units 20Y, 20M, 20C, and 20K is driven to form an image. When all of the image forming units 20Y, 20M, 20C, and 20K are driven, a full color image is formed. Printing data for yellow, magenta, cyan, and black are input to the image forming units 20Y, 20M, 20C, and 20K, respectively. Since the image forming units 20Y, 20M, 20C, and 20K differ only in the color of the toner to be handled, the image forming unit 21Y for forming a yellow image will be described here. Note that the image forming units 20Y, 20M, 20C, and 20K are each integrally formed and detachable from the digital copying machine 1.

  The image forming unit 20Y includes an exposure head 21Y to which yellow print data is input, a photosensitive drum 23Y, a charging charger 22Y, a developing device 24Y, and a transfer charger 25Y. The exposure head 21Y emits laser light according to the received printing data (electrical signal). The emitted laser light is one-dimensionally scanned by a polygon mirror provided in the exposure head 21Y to expose the photosensitive drum 23Y. The direction in which the photosensitive drum is one-dimensionally scanned is the main scanning direction.

  The photosensitive drum 23Y is charged by the charging charger 22Y, and then irradiated with laser light emitted from the exposure head 21Y. Thereby, an electrostatic latent image is formed on the photosensitive drum 23Y. Subsequently, toner is placed on the electrostatic latent image by the developing device 24Y to form a toner image. The toner image formed on the photoreceptor drum 23Y is transferred onto the endless belt 30 by the transfer charger 25Y.

  On the other hand, the endless belt 30 is suspended by the driving roller 33C and the rollers 33A and 33B so as not to be loosened. When the drive roller 33C rotates counterclockwise in the figure, the endless belt 30 rotates counterclockwise in the figure at a predetermined speed. As the endless belt 30 rotates, the rollers 33A and 33B rotate counterclockwise.

  As a result, the image forming units 20Y, 20M, 20C, and 20K sequentially transfer the toner images onto the endless belt. The timing at which each of the image forming units 20Y, 20M, 20C, and 20K transfers the toner image onto the endless belt is adjusted by detecting a reference mark attached to the endless belt 30. As a result, yellow, magenta, cyan, and black toner images are superimposed on the endless belt 30.

  In the paper feed cassettes 35, 35A, and 35B, sheets of different sizes are set. A sheet of a desired size is supplied to the conveyance path by paper feed rollers 36, 36A, 36B attached to the paper feed cassettes 35, 35A, 35B. The paper supplied to the conveyance path is sent to the timing roller 31 by the conveyance roller pair 37.

  A timing sensor for detecting the reference mark of the endless belt 30 is installed. When the timing sensor detects the reference mark of the endless belt 30, the timing roller 31 supplies paper to the endless belt 30 in synchronization therewith. The paper is pressed against the endless belt by the transfer roller 26, and the yellow, magenta, cyan, and black toner images formed on the endless belt 30 are transferred onto the paper.

  The sheet on which the toner image has been transferred is conveyed to the fixing roller pair 32 and heated by the fixing roller pair 32. As a result, the toner is melted and fixed on the paper. Thereafter, the paper is discharged to a paper discharge tray 39. Here, the tandem type digital copier 1 having the image forming units 20Y, 20M, 20C, and 20K that form the four color toners on the paper will be described. May be a four-cycle digital copying machine that sequentially transfers images to a sheet.

  A colorimetric sensor 27 is disposed facing the endless belt 30 upstream in the surface movement direction from the position where the transfer roller 26 presses the sheet against the endless belt 30. The digital copying machine 1 executes a soft calibration process or a device calibration process, which will be described later, in the image stabilization mode. At this time, the image forming units 20Y, 20M, 20C, and 20K form a test pattern image toner image on the endless belt 30. The colorimetric sensor 27 optically reads a pattern image formed on the endless belt 30 and outputs colorimetric data to the main circuit 100. A cleaner 28 is disposed on the downstream side of the colorimetric sensor 27 in the surface movement direction of the endless belt 30, and the cleaner 28 removes the toner remaining on the endless belt 30.

  FIG. 2 is a block diagram showing a detailed hardware configuration of the main circuit of the digital copying machine. Referring to FIG. 2, main circuit 100 includes a CPU (Central Processing Unit) 101 connected to bus 111, a storage unit 102, and an input image processing unit that processes image data output from image reading unit 10. 104, an output image processing unit 105 that performs processing for converting image data into data suitable for output to the image forming unit 20, a communication I / F 106, a network interface (I / F) 107, a modem 108, , An operation panel 109 and a card I / F 110.

  The CPU 101 controls the entire main circuit 100. The storage unit 102 includes a semiconductor memory such as a ROM (Read Only Memory), a RAM (Random Access Memory), and an EEPROM (Electrically Erasable / Programmable ROM), and a magnetic storage device such as a hard disk drive (HDD).

  The input image processing unit 104 receives image data output from the image reading unit 10. The input image processing unit 104 performs image processing such as color correction processing and resolution conversion processing on the input image data, and stores the processed image data in the storage unit 102. The output image processing unit 105 reads out the image data from the storage unit 102, executes image processing such as color conversion processing, screen control processing, smoothing processing, gradation correction processing, and the like, and outputs the processed image data to the image forming unit. 20 is output. In the gradation correction process, the gradation of the image data is corrected using a gradation correction table stored in the EEPROM of the storage unit. The gradation correction process is known as γ correction.

  The CPU 101, the input image processing unit 104, and the output image processing unit 105 execute programs stored in the storage unit 102. The storage unit 102 stores programs to be executed by the CPU 101, the input image processing unit 104, and the output image processing unit 105, respectively.

  The communication I / F 106 connects the digital copying machine 1 to the color measurement device 2. The communication I / F 106 communicates with the color measurement device 2 and receives color measurement data (first color measurement data) from the color measurement device 2. A network I / F 107 connects the digital copying machine 1 to the network 112. The network I / F 107 outputs data to the network according to a predetermined communication protocol, and receives data from the network 112 according to a predetermined communication protocol. The CPU 101 can communicate with other computers connected to the network 112 via the network I / F 107. For this reason, for example, image data can be transmitted and received using e-mail.

  The modem 108 connects the digital copying machine 1 to the telephone line 113. The modem 108 enables facsimile communication of data according to a facsimile communication protocol. The modem 108 stores the data received by facsimile in the HDD of the storage unit 102. The modem 108 also includes an NCU (Network Control Unit) to enable communication with a computer connected to the telephone line 113. Data received from another computer by the modem 108 is stored in the HDD of the storage unit 102.

  Operation panel 109 includes an input unit 109A and a display unit 109B. The input unit 109 </ b> A is an input device such as a touch panel, a keyboard, or a mouse for accepting an operation input by the user of the digital copying machine 1. The display unit 109B is a liquid crystal display device or an organic EL (electro-luminescence) display panel. When a touch panel made of a transparent member is used as the input unit 109A, an instruction of a button displayed on the display unit 109B can be detected by placing the touch panel on the display unit 109B. Thereby, various operations can be input.

  An IC card 110A is attached to the card I / F 110. The CPU 101 can access the IC card 110 </ b> A via the card I / F 110.

  Here, an example is shown in which the CPU 101, the input image processing unit 104, and the output image processing unit 105 each execute a program stored in the ROM of the storage unit 102. However, the CPU 101, the input image processing unit 104, and the output image processing unit 105, respectively. Is not limited to the program stored in the ROM, but may be stored in the EEPROM. If stored in the EEPROM, the program can be rewritten or additionally written. For this reason, another computer connected to the network 112 can rewrite the program stored in the EEPROM of the digital copying machine 1, or can write a new program. Further, the digital copying machine 1 can download a program from another computer connected to the network 112 and store the program in the EEPROM. Further, the program is stored in the IC card 110A, and the CPU 101, the input image processing unit 104, and the output image processing unit 105 each load and execute the program recorded on the IC card 110A mounted on the card I / F 110. You may do it. The recording medium for recording the program is not limited to the IC card 110A, and other recording media may be used. The programs referred to here include not only programs that can be directly executed by the CPU 101, the input image processing unit 104, and the output image processing unit 105, but also include programs in a source program format, compressed programs, encrypted programs, and the like. .

  In the digital copying machine 1 according to the present embodiment, the image forming unit 20 forms a pattern image on a sheet when executing the soft calibration process. When the user inputs an instruction to execute a soft calibration process to the input unit 109A, the digital copying machine 1 forms a pattern image on a sheet and outputs it. The pattern image is an image of a plurality of patches having different colors and densities. When the user scans the pattern image formed on the sheet with the color measurement device 2 and inputs the color measurement data (first color measurement data) output from the color measurement device 2 to the digital copying machine 1, the digital copying machine 1. Generates a gradation correction table using the colorimetric data. In the digital copying machine 1, when the device calibration process is executed, the image forming unit 20 forms a pattern image on the endless belt 30. The digital copying machine 1 executes device calibration processing at a predetermined timing, for example, when the image forming unit 20 forms an image a predetermined number of times. In the digital copying machine 1, the colorimetric sensor 27 reads a pattern image formed on the endless belt 30, and generates a gradation correction table using the colorimetric data (second colorimetric data) to be output.

  FIG. 3 is a diagram schematically showing a pattern image formed on the endless belt. Referring to FIG. 3, the pattern image includes a multi-gradation patch whose density changes in a 10% step from a high gradation (100%) to a low gradation (0%). In the figure, a one-color pattern image is shown, but the pattern image actually includes a plurality of color patches. When the color measurement sensor 27 reads the pattern image, the color measurement sensor 27 outputs the second color measurement data. The second colorimetric data associates the color and output gradation associated with each of the plurality of patches included in the patch formed by the image forming unit 20 with the value output from the colorimetric sensor 27. This pattern image may cause density unevenness partially. Since the density unevenness occurs due to the characteristics of the image forming unit 20, the density tends to appear every time the image forming unit 20 forms a pattern image. For this reason, the value of the second colorimetric data output every time the colorimetric sensor 27 reads the pattern image varies in the density unevenness portion.

  FIG. 4 is a functional block diagram showing an outline of CPU functions together with data stored in the storage unit. Referring to FIG. 4, the CPU 101 includes a pattern image forming unit 51 that forms a pattern image, a first colorimetric data receiving unit 55 that receives first colorimetric data output from the colorimetric device 2, and a colorimetric sensor 27. A second colorimetric data receiving unit 53 for receiving the second colorimetric data, an interpolation target part extracting unit 57 for extracting an interpolation reference part from the first colorimetric data, and a first for correcting the first colorimetric data. A colorimetric data correction unit 59, a gradation correction table generation unit 61 that generates a gradation correction table, and a nonuniformity detection unit 63 are included.

  The pattern image forming unit 51 controls the image forming unit 20 to form a pattern image when the CPU 101 executes a soft calibration process or a device calibration process. The pattern image forming unit 51 has a plurality of steps in which the density changes stepwise in fine steps with respect to the density corresponding to the output gradation of the detected uneven portion when the uneven portion detecting unit 63 detects the uneven portion. The pattern image formed so far is changed to the pattern image including the patch. For example, in FIG. 3, when the density varies with a patch having a density of 20% and the density of 20% is detected as a non-uniform portion, for example, a patch having a density of 15% and a patch having a density of 25% are detected before and after the patch with a density of 20%. Change to a pattern image containing density patches. Therefore, when the soft calibration process or the device calibration process is executed after the uneven part is detected by the uneven part detecting unit 63, the pattern image forming unit 51 forms the changed pattern image.

  When the communication I / F 106 receives the first color measurement data from the color measurement device 2 connected thereto, the first color measurement data receiving unit 55 receives the first color measurement data from the communication I / F 106. The first colorimetric data is obtained by reading the pattern image formed on the sheet by the pattern image forming unit 51 controlling the image forming unit 20 with the colorimetric device 2 when the CPU 101 executes the soft calibration process. Measured colorimetric data. The first colorimetric data is data corresponding to patches of a plurality of colors and a plurality of densities included in the pattern image. For example, the color and the output gradation corresponding to each patch are attached, and the color and the output gradation are associated with values output by the colorimetry device measuring the density of the patch. That is, the color measurement data is data in which the color and output gradation corresponding to each of the plurality of patches included in the pattern image by the image forming unit 20 are associated with the values measured by the color measurement device 2. The first color measurement data may not correspond to all the patches of the pattern image, and may be a value obtained by measuring a part of the color. The first colorimetric data receiving unit 55 outputs the received first colorimetric data to the first colorimetric data correcting unit 59 and the interpolation target part extracting unit 57. Here, an example in which the first color measurement data is received from the color measurement device 2 connected to the communication I / F 106 will be described. However, the user inputs a color measurement value obtained by measuring the pattern image with the color measurement device 2. It may be input to the unit 109A. In this case, the first color measurement data receiving unit 55 receives the first color measurement data from the input unit 109A.

  The second colorimetric data receiving unit 53 controls the colorimetric sensor 27 to read the density of the pattern image formed on the endless belt 30 by the pattern image forming unit 51, and the second colorimetric data output by the colorimetric sensor 27 is output. Accept color data. The second colorimetric data is a pattern image formed on the endless belt 30 by the pattern image forming unit 51 controlling the image forming unit 20 when the CPU 101 executes the soft calibration process or the device calibration process. Colorimetric data read and output by the colorimetric sensor 27. The second colorimetric data is data corresponding to a plurality of colors and a plurality of density patches included in the pattern image. For example, the color and output gradation are associated with each patch, and the color and output gradation are associated with the value output by the colorimetric sensor 27 measuring the density of the patch. The second color measurement data receiving unit 53 outputs the second color measurement data acquired from the color measurement sensor 27 to the interpolation target portion extraction unit 57 and the gradation correction table generation unit 61 and stores the second color measurement data in the storage unit 102. The second colorimetric data stored in the storage unit 102 is stored as history data 71. The second color measurement data receiving unit 53 outputs the second color measurement data interpolated by the least square method to the gradation correction table generation unit 61.

  The interpolation target part extraction unit 57 compares the density of the first colorimetric data and the second colorimetric data for each color and output gradation, and uses the color and output gradation having a predetermined difference between them as the interpolation target part. Extract. Then, the extracted interpolation target part is output to the first colorimetric data correction unit 59. Based on the history data 71 stored in the storage unit 102, the unevenness portion detection unit 63 uses, as uneven portions, colors and output gradations having a variation in the density of history data among a plurality of colors and output gradations. Extract. Then, the extracted uneven portion is output to the first colorimetric data correction unit 59 and the pattern image forming unit 51. FIG. 5 is a diagram for explaining a non-uniformity portion in which the density of history data varies. Referring to FIG. 5, the difference between the maximum value and the minimum value of the history data exceeds the predetermined range in the two output gradations.

  Returning to FIG. 4, the first colorimetric data correction unit 59 corrects the first colorimetric data. Specifically, the density of the output gradation of the interpolation target portion other than the uneven portion is deleted from the first colorimetric data. This is because the interpolated portion is a portion having a large difference between the first colorimetric data and the second colorimetric data, and therefore the first colorimetric data may contain noise. In addition, the reason why the interpolation target part to be deleted is other than the uneven part is that the first colorimetric data is highly likely to be uneven in the output gradation in the part where the second colorimetric data is uneven. is there. The first colorimetric data correction unit 59 interpolates the first colorimetric data from which the interpolation target part other than the uneven part is deleted using the least square method for each color. Then, the interpolated first colorimetric data is output to the gradation correction table generation unit 61. The first colorimetric data interpolated by the least square method defines the density corresponding to all output gradations.

  FIG. 6 is a diagram for explaining interpolation by the least square method. Referring to FIG. 6, consider the case where output gradations D1 and D2 are uneven portions. In this case, the first colorimetric data may have an abnormal density value in the output gradations D1 and D2. A line 301 indicates the first colorimetric data corrected by the least square method, and a line 301A indicates the first colorimetric data corrected by the spline interpolation. The line 301A is distorted upward as a whole due to the influence of noise. On the other hand, the line 301A is a line in which the influence of noise is minimized. As a characteristic of the image forming unit 20, unevenness occurs in the density of the image to be formed in the uneven portion of the output gradation, but unevenness may not occur in the peripheral output gradation. As described above, by interpolating the first colorimetric data by the least square method, it is possible to reduce the influence of the noise of the uneven portion on the output gradation around the uneven portion. A similar effect can be achieved when the second colorimetric data is interpolated by the least square method.

  Further, when the unevenness portion is detected by the unevenness portion detection unit 63, the pattern image forming unit 51 has a step in which the density of the pattern image formed so far is fine with respect to the density corresponding to the output gradation of the unevenness portion. To change to a pattern image including a plurality of patches that change step by step. For this reason, the first colorimetric data includes data defining the density corresponding to the output gradation before and after the output gradation of the uneven portion. By interpolating such first colorimetric data by the least square method, it is possible to further reduce the influence of the noise of the uneven portion on the output gradation around the uneven portion. The same effect can be obtained when the second colorimetric data is interpolated by the least square method.

  Returning to FIG. 4, the gradation correction table generation unit 61 generates a gradation correction table based on the first colorimetric data when executing the software calibration process, and executes the second when the device calibration process is executed. A gradation correction table is generated based on the colorimetric data. Then, the generated gradation correction table is stored in the storage unit 102. As a result, the gradation correction table 72 is stored in the EEPROM of the storage unit 102. The gradation correction table 72 is referred to when the output image processing unit 105 executes gradation correction processing (γ correction processing). Here, when generating the gradation correction table, the gradation correction table is generated based on the first colorimetric data after interpolation by the least square method or the second colorimetric data after interpolation by the least square method. Although an example of generation will be described, a gradation correction table is generated without interpolating the first color measurement data or the second color measurement data by the least square method, and the generated gradation correction table is interpolated by the least square method. You may do it.

  Here, generation of the gradation correction table will be described. FIG. 7 is a diagram showing the relationship between the colorimetric data and the gradation conversion curve. Referring to FIG. 7, the horizontal axis represents output gradation, the vertical axis represents density and output gradation, and all axes represent a positive axis with the origin as 0. The first quadrant indicates colorimetric data. The color measurement data defines a density which is a color measurement value of the color measurement device 2 or the color measurement sensor 27 with respect to an output gradation determined by a pattern image number. The second quadrant is a gradation characteristic defined by an ICC (International Color Consortium) profile of the digital copying machine 1. This gradation characteristic is a color reproduction characteristic depending on the digital copying machine 1, and is a predetermined characteristic. The gradation characteristic defines the density with respect to the output gradation. The fourth quadrant shows a gradation conversion curve. The gradation conversion curve 303 is used to correct gradation characteristics that change due to a change in the usage environment of the digital copying machine 1 and aged deterioration of consumables. The gradation conversion curve 303 defines an output gradation appropriate for forming an image with a density corresponding to the output gradation using the gradation characteristic 302 defined in the ICC profile corresponding to the output gradation. . FIG. 8 is a diagram illustrating an example of a gradation conversion curve obtained from the colorimetric data and the ICC profile. A gradation correction table represents a gradation conversion curve in a table.

  FIG. 9 is a flowchart illustrating an example of device calibration processing executed by the CPU. This device calibration process is a process executed by the CPU 101 when the CPU 101 executes a calibration program stored in the ROM of the storage unit 102. Referring to FIG. 9, CPU 101 determines whether it is time to execute device calibration (step S51). The process waits until the timing for executing device calibration (NO in step S51). If it is the timing for executing device calibration (YES in step S51), the process proceeds to step S51. That is, the device calibration process is a process that is executed on the condition that it is time to execute device calibration. In the digital copying machine 1 according to the present embodiment, device calibration is executed every time the image forming unit 20 forms a predetermined number of images, here, 1000 times. For this reason, the CPU 101 counts the number of times that the image forming unit 20 forms an image, and determines whether or not the count value is a multiple of 1000.

  In step S52, the image forming unit 20 outputs an instruction to form a pattern image. The pattern image to be formed in the image forming unit 20 is the pattern image after the change when the pattern image is changed (step S26 in FIG. 11) in the unevenness detection process described later (see FIG. 11). The image forming unit 20 forms a pattern image on the endless belt 30 when an instruction to form a pattern image is input. In step S53, the colorimetric sensor 27 is caused to read the pattern image formed on the endless belt 30, and second colorimetric data output from the colorimetric sensor 27 is acquired.

  In the next step S54, a gradation correction table is generated using the second colorimetric data. Then, by storing the generated gradation correction table in the EEPROM of the storage unit 102, the gradation correction table 72 stored in the EEPROM of the storage unit 102 is updated (step S55). Then, the second colorimetric data acquired in step S53 is stored as history data in the EEPROM of the storage unit 102 (step S56).

  FIG. 10 is a flowchart illustrating an example of the soft calibration process executed by the CPU. The soft calibration process is a process executed by the CPU 101 when the CPU 101 executes a calibration program stored in the ROM of the storage unit 102. Referring to FIG. 10, CPU 101 determines whether a soft calibration instruction has been received (step S01). When the user inputs an instruction to execute the soft calibration process to the input unit 109A, the CPU 101 receives the soft calibration instruction from the input unit 109A. The process waits until a soft calibration instruction is accepted (NO in step S01). If a soft calibration instruction is accepted (YES in step S01), the process proceeds to step S02. That is, the soft calibration process is a process that is executed on condition that a soft calibration instruction is input.

  In step S02, the image forming unit 20 outputs an instruction to form a pattern image. The pattern image to be formed in the image forming unit 20 is the pattern image after the change when the pattern image is changed (step S26 in FIG. 11) in the unevenness detection process described later (see FIG. 11). The image forming unit 20 forms a pattern image on the endless belt 30 when an instruction to form a pattern image is input. In step S03, the colorimetric sensor 27 is caused to read the pattern image formed on the endless belt 30, and second colorimetric data output from the colorimetric sensor 27 is acquired.

  Next, the image forming unit 20 is controlled to transfer the pattern image formed on the endless belt to the sheet, thereby forming the pattern image on the sheet (step S04). Thereby, the user can measure the pattern image formed on the sheet using the color measuring device 2. The color measurement device 2 reads a pattern image formed on a sheet and generates first color measurement data.

  Then, it is determined whether or not the first color measurement data is received from the color measurement device 2 (step S05). The user connects the color measurement device 2 to the communication I / F 106 and determines whether or not the first color measurement data is received from the color measurement device 2 through the communication I / F 106. The process waits until the second color measurement data is received (NO in step S05). If the second color measurement data is received (YES in step S05), the process proceeds to step S06. In step S06, unevenness portion detection processing for detecting unevenness portions from the history data 71 is executed, and in the next step S07, interpolation target portion extraction for extracting the interpolation target portion from the first colorimetric data received in step S05. Execute the process. Further, in the next step S08, a first colorimetric data correction process for correcting the first colorimetric data received in step S05 is executed. The unevenness part detection process, the interpolation target part extraction process, and the first colorimetric data correction process will be described later.

  In step S09, a gradation correction table is generated from the corrected first colorimetric data. The generated gradation correction table is stored in the EEPROM of the storage unit 102 (step S10), and the second colorimetric data acquired in step S03 is stored as history data in the EEPROM of the storage unit 102 (step S11).

  FIG. 11 is a flowchart illustrating an example of the flow of unevenness detection processing. The unevenness detection process is a process executed in step S06 in FIG. Referring to FIG. 11, history data 71 is read from the EEPROM of storage unit 102 (step S21). Here, the history data is indicated by H. The history data may be read out from all stored in the storage unit 102 or may be read out for a predetermined period. Since the old history data may differ from the current characteristics of the image forming unit 20, it is possible to improve the accuracy of unevenness detection using only new history data. The history data may be deleted when the soft calibration process is executed, and the second colorimetric data measured after the soft calibration process may be stored as history data. .

  In step S22, a processing target density is selected. Next, the range R is calculated from the difference between the maximum value MAX (H) and the minimum value MIN (H) of the selected processing target density (step S23). Then, it is determined whether or not the calculated range R exceeds the threshold value T1 (step S24). If range R exceeds threshold value T1 (YES in step S24), the process proceeds to step S25. If range R does not exceed threshold value T1 (NO in step S24), the selected concentration to be processed is set. The process proceeds to step S27 without determining the uneven portion. In step S25, the selected processing target density is determined as the uneven portion. Then, the pattern image is changed, and the process proceeds to step S27 (step S26). Specifically, the pattern image used so far is changed to a pattern image including a plurality of patches whose density changes stepwise with respect to the density of the uneven portion. In step S27, it is determined whether there is an unprocessed concentration to be processed. If there is an unprocessed pixel to be processed (YES in step S26), the process returns to step S22, and if not (NO in step S26), the process returns to the soft calibration process. By executing the unevenness detection process, the density of the history data whose range exceeds the predetermined threshold value T1 is detected as the unevenness portion.

  FIG. 12 is a flowchart illustrating an example of the flow of the interpolation target portion extraction process. The interpolation target part extraction process is a process executed in step S07 of FIG. Referring to FIG. 12, the processing target density is selected (step S31). Next, the second colorimetric data (B) of the selected processing target density is divided by the first colorimetric data (A) to calculate a comparison value D (step S32). Then, it is determined whether or not the absolute value obtained by subtracting 1 from the calculated comparison value D exceeds the threshold value T2 (step S33). For example, if T2 is 0.2, it is determined whether or not the second colorimetric data B exceeds a value that is 20% higher than the first colorimetric data or is smaller than a value that is 20% lower than the first colorimetric data. If the absolute value obtained by subtracting 1 from the comparison value D exceeds the threshold value T2 (YES in step S33), the selected processing target density is determined as the interpolation portion (step S34). If the absolute value obtained by subtracting 1 from the comparison value D does not exceed the threshold value T2 (NO in step S33), the process proceeds to step S35 without determining the selected processing target density as the interpolation portion. In step S35, it is determined whether there is an unprocessed concentration to be processed. If there is an unprocessed pixel to be processed (YES in step S35), the process returns to step S31, and if not, the process returns to the soft calibration process.

  FIG. 13 is a flowchart illustrating an example of the flow of the first colorimetric data correction process. The first colorimetric data correction process is a process executed in step S08 of FIG. Referring to FIG. 13, the processing target density is selected (step S41). Next, it is determined whether the processing target density is an interpolation target part (step S42). If the processing target density is the interpolation target part, the process proceeds to step S43; otherwise, the process proceeds to step S45. In step S43, it is determined whether or not the processing target density is a non-uniform portion. If the density to be processed is an uneven portion, the process proceeds to step S45; otherwise, the process proceeds to step S44. In step S44, the value of the processing target density is deleted from the first colorimetric data. That is, the value of the interpolation target portion other than the uneven portion is deleted from the first colorimetric data.

  In step S45, it is determined whether an unprocessed density exists. If there is an unprocessed density (YES in step S45), the process returns to step S41, and if not (NO in step S45), the process returns to the soft calibration process.

  In the present embodiment, the uneven portion detection unit 63 is provided, but the uneven portion detection unit 63 may not be provided. In this case, the first colorimetric data correction unit 59 deletes the density of the output gradation of the interpolation target portion from the first colorimetric data. This is because the interpolation target portion is a portion having a large difference between the first colorimetric data and the second colorimetric data, and thus the first colorimetric data may contain noise. Then, the first colorimetric data correction unit 59 interpolates the first colorimetric data from which the interpolation target portion has been deleted using the least square method, and the interpolated first colorimetric data is supplied to the gradation correction table generation unit 61. Output. Even with this configuration, since the interpolation target portion that may contain noise is deleted from the first colorimetric data, the accuracy of the gradation correction table generated in the soft calibration process can be improved. it can.

  As described above, in the digital copying machine 1 according to this embodiment, the colorimetric device 2 measures and outputs the density of a pattern image including a plurality of density patches during the soft calibration process. The colorimetric sensor 27 measures the density of the pattern image during the soft calibration process, compares it with the second colorimetric data to be output, extracts the portion to be interpolated from the first colorimetric data, and performs the interpolation. A gradation correction table is generated based on the first colorimetric data from which the target portion has been deleted. If a portion having a large difference between the first colorimetric data and the second colorimetric data is extracted as an interpolation target portion, the noise portion can be deleted from the first colorimetric data. The accuracy of the correction table can be improved.

  In the device calibration process after the gradation correction table is stored by the soft calibration process, the colorimetric sensor 27 measures the density of the pattern image and creates a new floor based on the second colorimetric data to be output. A tone correction table is generated, the tone correction table is rewritten, and the second colorimetric data is stored. Then, the density corresponding to each of the plurality of density patches is compared between two or more second colorimetric data, and if a density with a predetermined variation is detected, an interpolation target other than the density with the predetermined variation is detected. The part is deleted from the first colorimetric data. Since there is a high possibility that the first colorimetric data also has unevenness in the output gradation in the part where the second colorimetric data is uneven, the first measurement is performed by making the interpolation target part other than the uneven part. The noise portion can be deleted from the color data. For this reason, the accuracy of the gradation correction table can be improved.

  Further, the density corresponding to each of a plurality of density patches is compared between two or more second colorimetric data, and when a density with a predetermined variation is detected, the density is compared with the density with a predetermined variation. The pattern image used so far is changed to a pattern image including a plurality of patches that gradually change in small steps. Then, the first colorimetric data or the gradation correction table obtained by reading the pattern image and from which the interpolation target part is deleted is interpolated by the least square method, so that the noise included in the first colorimetric data is The influence on the output gradation can be reduced, and the accuracy of the gradation correction table can be improved.

  Further, since the second colorimetric data or the gradation correction table is interpolated by the least square method, the influence of noise included in the second colorimetric data on the peripheral output gradation is reduced during the device calibration process. And the accuracy of the gradation correction table can be improved.

  In the digital copying machine 1, the colorimetric sensor 27 measures the density of the pattern image, stores the second colorimetric data to be output as the history data 71, and between the two or more stored second colorimetric data. The density corresponding to each of a plurality of density patches is compared, and when a density with a predetermined variation is detected, the density changes in stages in fine steps with respect to the detected density with a predetermined variation. The pattern image used so far is changed to the pattern image including the patch. It is conceivable that the accuracy of image formation of the image forming unit 20 is unstable at a density having a predetermined variation. By forming a pattern image including a patch having a density around this density, the measurement value increases at a density around the density having a predetermined variation in the first color measurement data or the second color measurement data. The accuracy of the gradation correction table generated by interpolation using the square method can be improved.

  In the above-described embodiment, the digital copying machine 1 has been described. However, the invention is understood as a calibration method or a calibration program for causing the digital copying machine 1 to execute the processes shown in FIGS. Needless to say, you can.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

1 is a schematic cross-sectional view showing a schematic configuration of a digital copying machine according to one embodiment of the present invention. 2 is a block diagram illustrating a detailed hardware configuration of a main circuit of the digital copying machine. FIG. It is a figure which shows typically the pattern image formed on the endless belt. It is a functional block diagram which shows the outline | summary of the function of CPU with the data memorize | stored in a memory | storage part. It is a figure for demonstrating the nonuniformity part in which the density | concentration of log | history data has dispersion | variation. It is a figure for demonstrating the interpolation by the least squares method. It is a figure which shows the relationship between colorimetric data and a gradation conversion curve. It is a figure which shows an example of the gradation conversion curve calculated | required from colorimetric data and an ICC profile. It is a flowchart which shows an example of the device calibration process performed with CPU. It is a flowchart which shows an example of the soft calibration process performed with CPU. It is a flowchart which shows an example of the flow of a nonuniformity detection process. It is a flowchart which shows an example of the flow of an interpolation object part extraction process. It is a flowchart which shows an example of the flow of a 1st colorimetric data correction process.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Digital copying machine, 2 Color measuring device, 10 Image reading part, 11 Original glass, 12 Slider, 13 Exposure lamp, 14 Mirror, 15, 15A Reflection mirror, 16 Lens, 17 Scanner motor, 18 CCD sensor, 20 Image formation part , 20Y, 20M, 20C, 20K Image forming unit, 21Y, 21M, 21C, 21K Exposure head, 22Y, 22M, 22C, 22K Charger charger, 23Y, 23M, 23C, 23K Photosensitive drum, 24Y, 24M, 24C, 24K Developing machine, 25Y, 25M, 25C, 25K Transfer charger, 26 Transfer roller, 27 Colorimetric sensor, 28 Cleaner, 30 Endless belt, 31 Timing roller, 32 Fixing roller pair, 35, 35A, 35B Paper feed cassette, 36, 36A , 36B Paper feed roller, 37 Feed roller pair, 39 paper discharge tray, 51 pattern image forming unit, 53 second colorimetric data receiving unit, 55 first colorimetric data receiving unit, 57 interpolation target part extracting unit, 59 colorimetric data correcting unit, 61 gradation Correction table generation unit, 63 Uneven portion detection unit, 71 History data, 72 gradation correction table, 100 main circuit, 101 CPU, 102 storage unit, 104 input image processing unit, 105 output image processing unit, 106 communication I / F, 107 Network I / F, 108 Modem, 109 Operation Panel, 109A Input Unit, 109B Display Unit, 110 Card I / F, 110A IC Card, 111 Bus, 112 Network, 113 Telephone Line.

Claims (8)

Image forming means for forming an image based on the image data;
A pattern image forming unit that causes the image forming unit to form a pattern image including a plurality of density patches;
A colorimetric data receiving unit that receives the input of the first colorimetric data to be measured and output by the colorimetric device, and the density of the pattern image formed by the image forming unit during soft calibration;
A colorimetric unit that measures the density of the pattern image formed by the image forming unit and outputs second colorimetric data;
The received first colorimetric data is compared with the second colorimetric data output by the colorimetric unit that measures the density of the pattern image formed by the image forming unit during soft calibration. Extracting means for extracting an interpolation target portion from the first colorimetric data;
First colorimetric data correcting means for deleting the interpolation target portion from the received first colorimetric data and correcting the first colorimetric data;
An image forming apparatus comprising: a gradation correction table generating unit that generates and stores a gradation correction table based on the corrected first colorimetric data.   The extraction means measures the density of the pattern image formed by the image forming means during soft calibration, and the ratio between the second color measurement data and the first color measurement data to be output is determined by the color measurement means. The image forming apparatus according to claim 1, wherein a density exceeding a predetermined value is extracted as the interpolation target portion. A history data storage means for storing the second colorimetric data to be measured and output by the colorimetric means after the gradation correction table is stored, and the density of the pattern image formed by the image forming means is measured;
Detecting means for comparing the density corresponding to each of the plurality of density patches between the two or more stored second colorimetric data and detecting a density having a predetermined variation;
2. The image forming apparatus according to claim 1, wherein the correction unit deletes the interpolation target part other than the density having a predetermined variation detected by the detection unit from the received first colorimetric data. When the density having the predetermined variation is detected, the pattern image forming unit is configured to step-wise in a step in which the density is fine with respect to the density having the predetermined variation. Including a pattern image changing means for changing to a pattern image including a plurality of changing patches;
The image forming apparatus according to claim 3, further comprising a first interpolation unit configured to interpolate the first colorimetric data or the gradation correction table from which the interpolation target portion has been deleted by a least square method. Forming equipment. A gradation correction table changing means for generating a new gradation correction table based on the second colorimetric data and rewriting the stored gradation correction table;
The image forming apparatus according to claim 3, further comprising a second interpolation unit configured to interpolate the second colorimetric data or the gradation correction table by a least square method. Image forming means for forming an image based on the image data;
A pattern image forming unit that causes the image forming unit to form a pattern image including a plurality of density patches;
A colorimetric data receiving unit that receives the input of the first colorimetric data to be measured and output by the colorimetric device, and the density of the pattern image formed by the image forming unit during soft calibration;
A colorimetric unit that measures the density of the pattern image formed by the image forming unit and outputs second colorimetric data;
A gradation correction table generating means for generating and storing a gradation correction table based on the received first colorimetric data;
A history data storage unit for storing the second colorimetric data to be measured and output by the colorimetric unit by measuring the density of the pattern image formed by the image forming unit after the gradation correction table is stored;
Detecting means for comparing the density corresponding to each of the plurality of density patches between the two or more stored second colorimetric data and detecting a density having a predetermined variation;
The gradation correction table generating means includes interpolation means for interpolating the received first colorimetric data or the gradation correction table by a least square method,
The pattern image forming unit includes a plurality of patches in which the density of the pattern image to be formed by the image forming unit changes stepwise in fine steps with respect to the detected density having a predetermined variation. An image forming apparatus including pattern image changing means for changing to A calibration method executed by an image forming apparatus,
The image forming apparatus includes an image forming unit that forms an image based on image data;
A pattern image forming unit that causes the image forming unit to form a pattern image including a plurality of density patches;
A colorimetric unit that measures the density of the pattern image formed by the image forming unit and outputs colorimetric data; and
A step of measuring the density of a pattern image formed by the image forming means during soft calibration, and receiving input of first colorimetric data to be output;
The received first colorimetric data is compared with the second colorimetric data output by the colorimetric unit that measures the density of the pattern image formed by the image forming unit during soft calibration. Extracting an interpolation target portion from the first colorimetric data;
Deleting the interpolation target portion from the received first colorimetric data and correcting the first colorimetric data;
And a step of generating and storing a gradation correction table based on the corrected first colorimetric data. A calibration method executed by an image forming apparatus,
The image forming apparatus includes an image forming unit that forms an image based on image data;
A pattern image forming unit that causes the image forming unit to form a pattern image including a plurality of density patches;
A colorimetric unit that measures the density of the pattern image formed by the image forming unit and outputs colorimetric data; and
A step of measuring the density of a pattern image formed by the image forming means during soft calibration, and receiving input of first colorimetric data to be output;
Generating and storing a gradation correction table based on the received first colorimetric data;
Storing the second colorimetric data to be measured and output by the colorimetric means after the gradation correction table is stored and the density of the pattern image formed by the image forming means is measured;
Comparing the density corresponding to each of the plurality of density patches between the two or more stored second colorimetric data, and detecting a density having a predetermined variation;
Changing the pattern image to be image-formed by the image forming means to a pattern image including a plurality of patches whose density is changed stepwise in fine steps with respect to the detected density having a predetermined variation. Including
The gradation correction table generating step includes a step of interpolating the received first colorimetric data or the gradation correction table by a least square method.

Images