JP2014026181A - Image forming apparatus, calibration method, and calibration program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the influence of density unevenness in one and the same page on the measurement result of calibration.SOLUTION: An image forming apparatus includes reading means 61 reading a test patch image printed on a printing medium and acquiring the density of the test patch image, representative value calculation means 22 calculating the representative value of the density in the printing page of the printing medium as a page representative value and calculating the representative value of the density in a measurement position corresponding line on the printing medium as a line representative value, correction coefficient calculation means 23 calculating a correction coefficient indicating the difference between the calculated page representative value and the calculated line representative value, patch density measurement means 56 measuring the density of the test patch image on a transfer belt 73, and correction table production means 24 producing a correction table for input and output characteristics, based on the density on the transfer belt 73 and the correction coefficient.

Description

  The present invention relates to an image forming apparatus that executes a printing process, a calibration method that calibrates density unevenness of an image to be printed, and a calibration program for executing this calibration method. The present invention relates to an image forming apparatus, a calibration method, and a calibration program that reduce the influence on functions.

In general, in an image forming apparatus such as a copying machine, a printer, a scanner, a facsimile, and a multifunction machine (MFP: Multi Function Peripheral) that combines these, image data is formed based on input print data, and toner density is automatically adjusted. Is printed out.
In such an image forming apparatus, so-called gamma characteristic (density gradation characteristic) correction processing is performed in order to match the preset design density with the actual density actually printed.

10 (i) to 10 (iii) are graphs for explaining a general concept of gamma correction processing.
As shown in Fig. 3 (iii), the desired gamma characteristic is that the amount of change between the input value and the output value is linear, or that the input / output characteristic changes as the designer intends. Is done.
However, in practice, there are variations among individual image forming apparatuses, and it is difficult to obtain ideal characteristics. That is, as shown in FIG. 6I, an actual image forming apparatus usually has non-linear input / output characteristics.
Therefore, in general, by incorporating a so-called gamma correction table (which may be abbreviated as “correction table” in this specification) having characteristics as shown in FIG. The gamma characteristic ((i) in the figure) is brought close to the desired gamma characteristic ((iii) in the figure).

However, the actual gamma characteristic may deviate from the original gamma characteristic, for example, the output value of the toner density adhering to the photosensitive member may decrease due to aging or environmental changes such as temperature and humidity.
Therefore, in the conventional image forming apparatus, it is necessary to correct the gamma correction table periodically or at a timing such as when an output error exceeds a threshold value.

In this correction, the density of the output pattern corresponding to several gradations is actually measured, and the gamma correction table is corrected in consideration of the density characteristics and output performance.
Specifically, a plurality of test patches having different densities are formed on an image carrier on which a toner image is formed, the density of the patches is detected by a sensor, and input / output characteristics are obtained from the obtained measured values. There is a method for generating a table to be corrected.
There is also known a method of correcting input / output characteristics by printing a test patch on paper as a recording medium and measuring it using a reading device.

By the way, when printing is performed on the entire surface of the paper uniformly with a desired color and with a single density using the image forming apparatus, the density is completely the same at all locations on the printed surface. For example, as shown in FIG. 11, irregular density irregularities occur. The numbers in the legend shown on the right side of FIG. 11 indicate the color difference from the white point.
Such density unevenness includes, for example, density unevenness in the sub-scanning direction due to the circumference of the photosensitive drum and the developing sleeve, and density unevenness in the main scanning direction due to deviation in the distance between the photosensitive drum and the developing sleeve. There is.
Since such density unevenness causes a decrease in the accuracy of the calibration measurement result, a method for reducing the influence of the density unevenness on the calibration measurement result has been proposed.

For example, a belt-like test patch is formed on the surface of the rotating image carrier along the circumferential direction of the surface of the image carrier, and the belt-like test patch is formed on the surface of the image carrier. Formed in two locations separated in the axial direction of the body, and in each of these two test patches, CMYK patches are arranged in order along the circumferential direction of the image carrier to form one test patch. The arrangement of each of the formed CMYK patches is shifted from the arrangement of each of the CMYK patches formed on the other one test patch by a relatively half cycle along the circumferential direction of the image carrier. A technique for drawing these test patches has been proposed (see, for example, Patent Document 1).
According to this method, in the two test patches, the arrangement of the CMYK patches is relatively shifted by a half cycle along the circumferential direction of the image carrier. A correction value reflecting the density unevenness can be obtained, and the influence of the density unevenness in the sub-scanning direction on the calibration measurement result can be reduced.

As another method, for example, the entire surface of the photosensitive drum is divided into minute areas vertically and horizontally, and the sensitivity unevenness correction value of the photosensitive drum is calculated for each pixel, and this is calculated as the density in the image. A technique has been proposed in which unevenness distribution is stored as two-dimensional data in the main scanning direction and sub-scanning direction, and the sensitivity unevenness is corrected by electrical means using the correction value (for example, Patent Document 2). reference.).
According to this method, since the sensitivity unevenness can be corrected using the sensitivity unevenness correction value on the entire surface of the photosensitive drum, the calibration measurement result can be corrected in both the main scanning direction and the sub-scanning direction. The influence of density unevenness can be reduced.

JP 2007-264364 A JP-A-2005-70069

However, the techniques described in Patent Documents 1 and 2 described above have the following problems.
For example, in the technique described in Patent Document 1, the arrangement of the CMYK patches in the two test patches is shifted by a relatively half cycle along the circumferential direction of the image carrier. The influence of density unevenness in the sub-scanning direction can be reduced. However, since there is no disclosure of a method considering density unevenness that occurs in the main scanning direction, there is a problem that the influence of density unevenness in the main scanning direction on the measurement result of calibration cannot be reduced.

On the other hand, the technique described in Patent Document 2 stores the density unevenness distribution in the image as two-dimensional data in the main scanning direction and the sub-scanning direction and reflects it in the correction. On the other hand, not only the influence of density unevenness in the sub-scanning direction but also the influence of density unevenness in the main scanning direction can be reduced.
However, in this technique, the entire surface of the photosensitive drum is divided vertically and horizontally, and the same number of correction values as the number of pixels are calculated. Therefore, the data amount of the correction value becomes enormous, and in order to store this correction value, There was a problem that a large memory area was required.

  The present invention has been considered in view of the above circumstances, and reduces the influence of density unevenness in both directions in the sub-operation direction and the main operation direction on the calibration measurement result, and provides correction data. An object of the present invention is to provide an image forming apparatus, a calibration method, and a calibration program capable of reducing a memory area of a storage unit for storing.

  In order to achieve this object, an image forming apparatus of the present invention is an image forming apparatus that forms an image on a print medium by transferring an image formed on an image carrier, and is an aspect of the print medium. A patch generation unit configured to generate image data of a first test patch image printed at a single density with a single color on the entire surface of the print page or within a predetermined range; The engine unit that forms the first test patch image on the image carrier based on the image data, the transfer belt that receives the first test patch image from the image carrier, and the print from the transfer belt printed on the print medium A reading unit that reads the first test patch image and acquires the density for each predetermined area in the first test patch image, and the density for each predetermined area acquired by the reading unit Accordingly, the representative value of the density in the print page of the print medium is calculated as the page representative value, and the measurement position on the print medium at the position corresponding to the measurement line on the transfer belt whose density is measured by the patch density measurement means A representative value calculating means for calculating a representative value of density in the corresponding line as a line representative value, a correction coefficient calculating means for calculating a correction coefficient representing a difference between the calculated page representative value and the line representative value, and storing the correction coefficient Storage means, patch density measuring means for measuring the density of the second test patch image formed on the measurement line of the transfer belt, density measured by the patch density measuring means, and correction coefficient calculating means. Correction table generation means for generating an input / output characteristic correction table based on the correction coefficient and the correction table are set in the image forming apparatus. A configuration equipped with a positive table setting means.

  The calibration method of the present invention is a calibration method having a process of transferring an image formed on an image carrier to form an image on a print medium, which is a printing surface that is one surface of the print medium. A process for generating image data of a first test patch image printed at a single density with a single color on the entire surface of the page or in a predetermined range for each of a plurality of densities and a plurality of densities. First, the first test patch image printed on the printing medium by the process of forming the first test patch image on the image carrier and the transfer from the transfer belt that received the first test patch image from the image carrier Processing to read and acquire the density for each predetermined area in the first test patch image, and the representative value of the density in the print page of the print medium based on the acquired density for each predetermined area In addition to calculating the page representative value, the representative value of the density in the measurement position corresponding line on the print medium at the position corresponding to the measurement line on the transfer belt whose density is measured by the patch density measuring unit is calculated as the line representative value. Processing, processing for calculating a correction coefficient representing the difference between the calculated page representative value and line representative value, processing for storing the correction coefficient, and the second test patch image formed on the measurement line of the transfer belt. A process for measuring the density by the patch density measuring means, a process for generating a correction table for the input / output characteristics based on the density measured by the patch density measuring means and the correction coefficient calculated by the correction coefficient calculating means; And a process of setting a correction table in the image forming apparatus.

  The calibration program of the present invention is a calibration program for causing an image forming apparatus to execute a process of transferring an image formed on an image carrier and forming an image on a print medium. A process for generating image data of a first test patch image printed at a single density with a single color on the entire surface or a predetermined range of one surface for each of a plurality of densities for each of a plurality of colors; Based on the generated image data, a first test patch image is formed on the image carrier, and the first test patch image is printed on the print medium by transfer from the transfer belt received from the image carrier. One test patch image is read and the density for each predetermined area in the first test patch image is acquired, and based on the acquired density for each predetermined area The representative value of the density in the print page of the print medium is calculated as the page representative value, and the measurement position corresponding line on the print medium is located at the position corresponding to the measurement line on the transfer belt whose density is measured by the patch density measuring means. Processing for calculating a representative value of the density as a line representative value, processing for calculating a correction coefficient representing a difference between the calculated page representative value and the line representative value, processing for storing the correction coefficient, and measurement line of the transfer belt Based on the processing in which the patch density measuring unit measures the density of the second test patch image formed above, the density measured by the patch density measuring unit, and the correction coefficient calculated by the correction coefficient calculating unit, The image forming apparatus is configured to execute a process for generating an input / output characteristic correction table and a process for setting the correction table in the image forming apparatus.

According to the image forming apparatus, the calibration method, and the calibration program of the present invention, the representative value (page representative value) in the page measured using the reading device and the representative of the detection position of the density sensor that is the patch density measuring unit. By comparing the values (line representative values), the representative value of the density in the page can be estimated from the detection result of the density sensor. Thereby, it is possible to reduce the influence of density unevenness in both directions in the sub-operation direction and the main operation direction on the calibration measurement result.
Further, since the data to be stored in the storage means may be several pieces of data such as page representative values and line representative values, not correction data for each pixel, the memory area of the storage means for storing these can be reduced.

1 is a block diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a block diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. FIGS. 4A and 4B are diagrams illustrating examples of measurement test patches, where FIG. 4A is a diagram illustrating an example of a test patch printed on a print medium, and FIG. 4I is a diagram illustrating an example of a test patch formed on a transfer belt. It is. FIG. 3 is an external perspective view showing a structure of a photosensitive drum and a developing sleeve. It is a flowchart which shows the procedure of calculation of a density representative value. It is a flowchart which shows the procedure of the production | generation of a correction table. 3 is a graph showing input / output characteristics of an image forming apparatus. It is a graph which shows the input-output characteristic for demonstrating the production | generation method of a gamma correction table. It is a graph which shows a gamma correction table. It is a principle figure for demonstrating the correction principle of an input / output characteristic. It is a figure which shows the example of the density nonuniformity in a page.

  Hereinafter, preferred embodiments of an image forming apparatus, a calibration method, and a calibration program according to the present invention will be described with reference to the drawings.

[Image forming apparatus]
First, an embodiment of an image forming apparatus of the present invention will be described with reference to FIGS.
1 and 2 are block diagrams showing the configuration of the image forming apparatus of the present embodiment.
It is assumed that the image forming apparatus shown in FIG. 1 and the image forming apparatus shown in FIG. 2 are both provided in one image forming apparatus.

  The image forming apparatus 1 is a computer that operates under program control. As illustrated in FIGS. 1 and 2, the print data acquisition unit 10, the image processing unit 20, the storage unit 30, the engine control unit 40, The engine unit 50, the reading unit 61, the operation display unit 62, the paper feeding unit 71, the conveyance belt 72, the transfer belt 73, the transfer roller 74, the fixing unit 75, and the paper discharge holding unit 76 are provided. ing.

Here, the print data acquisition unit 10 is an interface for receiving print data transmitted from the host computer 2.
The host computer 2 is a host device that issues a print request to the image forming apparatus 1, and transmits print data including image data and control commands to the image forming apparatus 1 based on a user operation such as a keyboard and a mouse.
The host computer 2 can be composed of, for example, a personal computer.

The image processing unit 20 includes a CPU (Central Processing Unit) and the like, and controls various processes performed in the image forming apparatus 1.
In particular, the image processing unit 20 interprets a process for storing image data included in the print data received by the print data acquisition unit 10 in the storage unit 30, a control command included in the print data, and sends it to the engine control unit 40. Processing for instructing appropriate printing processing, processing for performing predetermined processing on image data included in the printing data and providing the processing to the engine control unit 40, processing for controlling the operation display means 62 that forms an interface with the user Etc.

In addition, the image processing unit 20 includes a patch generation unit 21, a representative value calculation unit 22, a correction coefficient calculation unit 23, and a correction table generation unit 24.
The patch generation unit 21 is a patch generation unit that generates image data of a test patch image.
The generated image data includes image data of a test patch image (first test patch image) printed on a sheet as a printing medium, and a test patch image (second test patch) formed on the transfer belt 73. Image).
As shown in FIG. 3I, the test patch image (first test patch image) printed on the sheet is a test patch image printed on almost the entire surface or a predetermined range of one side (printed page) of the sheet. It is. That is, the test patch image printed on one sheet is a test patch image having one color and one density. However, the patch generation unit 21 generates image data of a test patch image for each color and for each of a plurality of levels of density. Therefore, test patch images of different colors or different densities are printed on each of a plurality of sheets.
The range where the test patch image is printed on the print page of the paper may be the entire print page or may be within a predetermined range. Examples of the case where it is within the predetermined range include a case where a margin is provided at the edge of the paper. However, when it is within the predetermined range, it is desirable to print the test patch image in the widest possible range on the print page.

As shown in FIG. 3 (ii), the test patch image (second test patch image) formed on the transfer belt 73 has a density detected by the density sensor 56 on the transfer belt 73 out of the surface of the transfer belt 73. Is a test patch image printed in a band on a range (measurement line) through which a position (sensor reading position) is read. Usually, one image forming apparatus 1 is provided with one or two density sensors 56. The example shown in FIG. 3 (ii) is a case where two density sensors 56 are provided, and each of the two measurement lines passing through the sensor reading position where the two density sensors 56 measure density on the transfer belt 73. The patch generation unit 21 generates image data of the test patch image so that the test patch images are formed in a total of two rows.
In addition, the patch generation unit 21 generates image data of the test patch images so that the CMYK patches are arranged in order along the moving direction of the transfer belt 73 in the test patch images in one row.
Further, the patch generation unit 21 generates image data of a test patch image in which patches whose density gradually changes are arranged for each color of CMYK.

The representative value calculating unit 22 is a representative value calculating unit that calculates a page representative value and a line representative value based on density data acquired by the reading unit 61 reading a test patch image printed on a sheet. .
The page representative value has an XYZ value that is the same tristimulus value among patches for each pixel within the entire patch area with respect to the entire patch area (the area in the range where the test patch image is printed on the paper). It is a representative value calculated from the area ratio occupied by the patch.
The line representative value is not the total patch area, but the portion on the sheet (measurement position) at the position corresponding to the range (measurement line) through which the sensor reading position (position where the density sensor 56 reads the density on the transfer belt 73) passes. Of the corresponding line, see FIG. 3 (i)), the representative value calculated from the area ratio occupied by patches having the same tristimulus value XYZ value among the patches for each pixel in the measurement position corresponding line. That is.
The portion on the paper at the position corresponding to the measurement line is the second test patch image formed on the measurement line of the transfer belt 73 when the second test patch image is transferred to the paper. The area where the patch image is transferred.

The correction coefficient calculator 23 is a correction coefficient calculator that calculates a correction coefficient using the page representative value and the line representative value calculated by the representative value calculator 22.
The correction coefficient is a numerical value representing the difference between the page representative value and the line representative value. For example, a correction coefficient can be obtained by expressing the difference between the page representative value and the line representative value as a ratio.

The correction table generation unit 24 refers to the density data obtained by the density sensor 56 measuring the test patch image on the transfer belt 73 and the correction coefficient stored in the storage unit 30 to generate a correction table. It is a generation means. The generated correction table is stored in the storage unit 30.
In addition, when a correction table is newly generated and stored in the storage unit 30, the image processing unit 20 functions as a correction table setting unit when the printing process is subsequently executed, and controls the printing process. Is used (setting of a correction table in the image processing apparatus 1).

  The storage unit 30 stores various programs for controlling the image forming apparatus 1. The storage unit 30 stores the print data received by the print data acquisition unit 10. Further, the storage unit 30 stores image data of a test patch image formed at the time of density gradation adjustment. The image data of each page to be printed by the engine control unit 40 is transferred from the storage unit 30 to the engine control unit 40.

  The engine control unit 40 includes a CPU, a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, and is stored in the storage unit 30 at a predetermined timing when the engine unit 50 executes printing. Pixel data is read out, screening processing (halftone processing) is performed on the pixel data, and the density gradation value data of each color of each pixel is attached to the paper surface of each pixel. The data is converted into output gradation value data from the screening, and based on the output gradation value data, a pulse width modulation signal (laser lighting / non-lighting signal for each raster) is generated and sent to the engine unit 50. . In screening (halftoning) processing for converting pixel data composed of density gradation values of each pixel into binary signals indicating laser on / off, which is performed by the image processing means 20 and the engine control unit 40 The density gradation correction suitable for the characteristics of the engine control unit 40 is also performed. Specifically, the engine control unit 40 is configured by an ASIC.

The engine unit 50 includes a mechanism unit that forms a toner image for each color and transfers the toner image to the transfer belt 73 in accordance with an instruction from the engine control unit 40.
The mechanism section includes a yellow image forming section Y, a magenta image forming section M, a cyan image forming section C, and a monochrome (black) image forming section K. In each of the image forming units Y to K, a photosensitive drum 51, a main charger 52, an exposure device 53, a developing unit 54 (54-1 to 54-4), a static eliminator 55, and the like are arranged.

The photosensitive drum 51 is an image carrier having a cylindrical conductive substrate and a photosensitive layer formed on the outer peripheral surface thereof, on which an electrostatic latent image or a toner image is formed. The photosensitive drum 51 is rotatable around a cylindrical central axis. For example, the photosensitive drum 51 shown in FIG. 4 rotates clockwise. Further, on the circumferential surface of the photosensitive drum 51, the axial direction of the central axis is the main scanning direction, and the circumferential direction is the sub scanning direction.
The main charger 52 uniformly charges the peripheral surface of the photosensitive drum 51 to a predetermined polarity by corona charging.

The exposure device 53 irradiates the charged photosensitive drum 51 with a beam from a light source such as a built-in laser or LED array to expose the image. Thereby, a latent image due to static electricity is formed on the peripheral surface of the photosensitive drum 51.
Beam irradiation of the exposure device 53 is controlled by a drive signal modulated based on image data input from the host computer 2.

The developing device 54 develops the electrostatic latent image formed on the peripheral surface of the photosensitive drum 51 using toner of a color corresponding to each of the image forming units Y to K, and forms a toner image.
The yellow image forming unit Y is provided with a yellow developing unit 54-1 filled with yellow toner. The magenta image forming unit M includes a magenta developing unit 54-2 filled with magenta toner, and the cyan image forming unit C includes a cyan developing unit 54-3 filled with cyan toner. Each of K is provided with a monochrome developing unit 54-4 filled with black toner.

When a full color image is formed in the engine unit 50 having such a configuration, image formation is performed in each of the image forming units Y, M, C, and K, and a single color toner image of each color is formed on the transfer belt 73. After being overlaid on each other, the full color image is transferred onto the paper.
The order of the image forming units is Y → M → C → K in the present embodiment, but is not limited to Y → M → C → K, and can be in any order.
When a monochrome image is formed, image formation is performed only in the monochrome image forming unit K, a black toner image is copied onto the transfer belt 73, and the monochrome image is transferred onto a sheet.

  The static eliminator 55 removes charge remaining on the photosensitive drum 51 after the toner image is transferred to the transfer belt 73. As a result, excess toner adhering to the photosensitive drum 51 is removed. Further, excess toner is mechanically removed (cleaning).

The density sensor 56 is a patch density measuring unit that is provided in the vicinity of the transfer belt 73 and measures the density of a test patch image formed on the transfer belt 73.
The density data indicating the density of the test patch image measured by the density sensor 56 is sent to the storage means 30 via the engine control unit 40 and stored therein.

  The reading means 61 reads an image represented on a sheet of paper that is a printing medium placed on an original table (contact glass, not shown) by an image sensor (not shown) having a CCD or the like, and reads the image. The data is sent to the image processing means 20 as data.

The operation display means 62 includes a display unit 62a that displays an operation screen, and an operation unit 62b that can be operated by a general user or an administrator.
The display unit 62a is a display device including a liquid crystal display or the like having a touch panel function, and displays an operation screen.
The operation screen is a screen on which various items related to the functions of the image forming apparatus 1 are arranged and displayed, and items selected and set by a general user or an administrator are displayed as software keys.

This operation screen includes, for example, a patch image print setting screen.
The patch image print setting screen is an operation screen for setting the contents and starting the printing when trying to print a test patch image on paper.
On the patch image print setting screen, for example, numerical values indicating the color (C, M, Y, K) and density of the test patch image generated by the patch generation unit 21 of the image processing means 20 are displayed. It is possible to display a GUI widget (for example, a radio button, a check box, a text box, etc.) for setting a color or density to be actually printed from among colors and densities.

The operation unit 62b is an input unit that includes software keys displayed on the display unit 62a and hardware keys provided at locations other than the display unit 62a. When operated by a general user or administrator, the operation unit 62b accepts information input by this operation. Also, various commands corresponding to the contents of the operation are output to the image processing means 20.
The operation unit 62b includes, for example, a setting screen display key for displaying a patch image print setting screen on the display unit 62a, and a process for causing the image forming apparatus 1 to execute processing for printing a predetermined test patch image on a sheet. A patch image print start key and the like are included. The setting screen display key and the patch image print start key can be provided as software keys displayed on the display unit 62a, or can be provided as hardware keys other than the display unit 62a.

The paper feeding unit 71 stocks paper before printing processing, takes out the paper one by one, and feeds it to the conveyor belt 72. The paper feed unit 71 includes a paper feed cassette that mainly stocks standard size paper, a manual paper feed unit (not shown) for feeding small amounts of non-standard size paper one by one, and the like. included.
The transport belt 72 is a belt stretched between a pair of rollers, and transports the paper fed from the paper feed unit 71 in a predetermined direction.

The transfer belt 73 copies the toner images of the respective colors formed by the image forming units Y to K of the engine unit 50 and transfers them to a sheet.
The transfer roller 74 is composed of a pair of rollers that sandwich the transfer belt 73 and the conveyance belt 72. The toner images of the respective colors formed on the transfer belt 73 are placed on the sheet conveyed by the conveyance belt 72. It is designed to be transferred with one pass of paper.

The fixing unit 75 includes a fixing roller, a pressure roller, and the like, and each color toner image transferred onto the sheet is melted and attached to the sheet by heat and mechanical pressure (thermal fixing process).
The paper discharge holding unit 76 holds paper discharged after the printing process.

[Calibration method]
Next, a calibration method executed by the image forming apparatus of the present embodiment will be described with reference to FIGS.
FIG. 5 is a flowchart illustrating an operation procedure of a method for calculating a representative density value among the calibration methods. FIG. 6 is a flowchart showing an operation procedure of a method of generating a correction table and setting it in the image forming apparatus among the calibration methods.
As shown in FIGS. 5 and 6, the calibration method includes a density representative value calculation method and a correction table generation method.
Here, a description will be given in the order of “a density representative value calculation method” and “a correction table generation method”.

(Concentration value calculation method)
First, the procedure of the density representative value calculation method will be described with reference to FIG.
When the administrator of the image forming apparatus 1 operates the setting screen display key of the operation unit 62b, the patch image print setting screen is displayed on the display unit 62a. Then, according to the display content of the patch image print setting screen displayed on the display unit 62a, the color and density of the test patch image are set by the operation of the operation unit 62b by the administrator, and the patch image printing of the operation unit 62b is further performed. When the start key is operated, the operation display unit 62 receives these operations and sends a signal indicating that the execution of the patch image printing function is started to the image processing unit 20.

As described above, the administrator can set the color and gradation of the test patch image according to the display content of the patch image print setting screen displayed on the display unit 62a.
Originally, it is ideal that the image data of the generated test patch image is generated for all the gradations (for example, all of the 256 gradations) for each color of CMYK. However, since this is practically difficult, image data of a predetermined number of test patch images is generated at different densities for each color of CMYK. On the patch image print setting screen, a specific numerical value indicating the density can be set.

The patch generation unit 21 of the image processing unit 20 generates image data of a test patch image for density measurement (S10).
At this time, as shown in FIG. 3I, the patch generation unit 21 prints a test patch image of one color with one color on almost the entire surface (print page) of one sheet. Then, image data of the test patch image for each color and each density set on the patch image print setting screen is generated.

The engine unit 50 prints the test patch images on paper based on the image data of the test patch images generated by the patch generation unit 21 (S11).
Here, when image data of a plurality of test patch images is generated by the patch generation unit 21, the engine unit 50 prints on a different sheet for each of the plurality of test patch images. As a result, test patch images of different colors and densities are printed for each sheet.
The paper on which the test patch image is printed by the engine unit 50 is discharged to the paper discharge holding unit 76.

The administrator takes out the paper discharged to the paper discharge holding unit 76 and places it on the document table (contact glass) of the reading means 61.
The reading unit 61 reads the test patch image printed on the paper (S12). Thereby, the density of the test patch image is measured for each predetermined area of the test patch image (for example, for each pixel when the printed page of the paper is divided into pixels that are minute areas based on the resolution of the reading unit 61). And get. The obtained density for each pixel of the test patch image is stored in the storage unit 30.

The representative value calculation unit 22 of the image processing unit 20 calculates the page representative value and the line representative value based on the density of the test patch image acquired by the reading unit 61 (the density of the test patch image extracted from the storage unit 30). (S13).
The calculation of the page representative value and the line representative value executed by the representative value calculation unit 22 in S13 will be described in detail later in [Specific examples of calculation of representative value and correction coefficient].

The correction coefficient calculation unit 23 calculates a correction coefficient that represents the difference between the page representative value and the line representative value calculated by the representative value calculation unit 22 (S14).
The calculation of the correction coefficient executed by the correction coefficient calculation unit 23 in S14 will be described in detail later in [Specific examples of calculation of representative value and correction coefficient].
The storage means 30 stores the correction coefficient calculated by the correction coefficient calculation unit 23 (S15).

(Generation of correction table)
Next, a procedure for generating a correction table will be described with reference to FIG.
Note that the correction table is generated when the image forming apparatus 1 is turned on, for example, when the cumulative start-up time of the image forming apparatus 1 exceeds a predetermined time, or when the printing process is executed or the number of printed sheets. When the cumulative total exceeds the threshold, it may be any time when an instruction to generate a correction table is received by an operation of the operation unit 62b by the administrator, or a timing other than these.

The patch generation unit 21 of the image processing means 20 generates image data of a test patch for density measurement (S20).
Here, the patch generation unit 21 generates image data on which the test patch image shown in FIG. 3 (ii) is formed on the transfer belt 73.
That is, the patch generator 21 forms a test patch image in a belt-like shape in two rows on the surface of the transfer belt 73, and the two rows of test patch images are formed in a position that can be read by the density sensor 56. As described above, the image data of the test patch image is generated.

The engine unit 50 forms a test patch image on the transfer belt 73 based on the image data of the test patch image generated by the patch generation unit 21 (S21).
The density sensor 56 measures the density of the test patch image formed on the transfer belt 73 (S22).

The correction table generator 24 generates a correction table with reference to the data indicating the density measured by the density sensor 56 and the correction coefficient stored in the storage means 30 (S23).
The correction table generation executed by the correction table generation unit 24 in S23 will be described in detail later in [Specific example of generation of gamma correction table].

The storage unit 30 stores the correction table generated by the correction table generation unit 24 (S24).
The image processing unit 20 functions as a correction table setting unit, and sets the correction table generated by the correction table generation unit 24 and stored in the storage unit 30 in the image forming apparatus 1 (S25). That is, the image processing unit 20 refers to the correction table stored in the storage unit 30 when performing gamma correction when the printing process is subsequently executed.

By executing the calibration method with such contents, the following effects can be obtained.
For example, the representative value of the density (page representative value) in the page in the test patch image printed on the paper, and the representative value (line representative value) of the density in the measurement position corresponding line of the test patch image printed on the paper By using the correction coefficient obtained by comparing the two, the representative density value in the page can be estimated from the density of the test patch image formed on the transfer belt 73 detected by the density sensor 56. .

Since the correction coefficient is reflected when generating the correction table, the density difference between the reading position of the density sensor 56 and the entire page can be reflected as a correction coefficient in the correction table. Thereby, it is possible to reduce the influence of density unevenness in both directions in the sub-operation direction and the main operation direction on the calibration measurement result.
Further, before generating the correction table based on the density obtained by measuring the test patch image formed on the transfer belt 73, the data calculated in advance includes the page representative value, the line representative value, and the correction coefficient. Therefore, the memory area of the storage means 30 for storing these data can be minimized.

[Specific example of calculation of representative value and correction coefficient]
Next, a specific example of calculating the page representative value, the line representative value, and the correction coefficient will be described.
Here, the page representative value is “Vp” and the line representative value is “Vl”.
Also, Black and Magenta may use the XYZ Y value, Cyan may use the XYZ X value, and Yellow may use the XYZ Z value.

(1) Example of calculating the page representative value As described above, the page representative value is the total patch area (the area in the range where the test patch image is printed on the paper) for each pixel within the total patch area. It is a representative value calculated from the area ratio occupied by patches having the same tristimulus value XYZ value.

Here, when a test patch image having a color of Black and a color value of 50% is printed on a sheet, the Y value for each measured patch (for each pixel) and the area of the patch for each Y value with respect to the total patch area It is assumed that the ratio (%) is as follows.
Patch area with a Y value of 25 is 15% of the total patch area
The patch area with a Y value of 20 is 80% of the total patch area
The area of the patch with a Y value of 15 is 5% of the total patch area.

In this case, the page representative value Vp can be calculated using the following equation.
Page representative value Vp = 25 × 0.15 + 20 × 0.8 + 15 × 0.05
= 20.5 (Formula 1)

(2) Calculation Example of Line Representative Value As described above, the line representative value is a portion (measurement position corresponding line) on the paper corresponding to the sensor reading position (position where the density sensor 56 reads the density on the transfer belt 73). ) Is a representative value calculated from an area ratio occupied by patches having the same tristimulus value XYZ value among patches for each pixel in the measurement position corresponding line.

Here, when a test patch image having a color of Black and a color value of 50% is printed on a sheet, the Y value for each patch (for each pixel) in the measurement position corresponding line and the patch for each Y value with respect to the line area Assume that the area ratio (%) is as follows.
Patch area with Y value 25 is 30% of line area
The area of the patch with a Y value of 20 is 60% of the line area
The area of the patch with a Y value of 15 is 10% of the line area

In this case, the line representative value Vl can be calculated using the following equation.
Line representative value Vl = 25 × 0.3 + 20 × 0.6 + 15 × 0.1
= 21 (Formula 2)

(3) Correction coefficient calculation example The correction coefficient is a numerical value representing the difference between the page representative value and the line representative value.
Here, a correction coefficient is obtained by expressing the difference between the page representative value and the line representative value as a ratio. Therefore, the correction coefficient k can be calculated by the following equation.
k = Vp / Vl (Formula 3)

Substituting the numerical values calculated in (Equation 1) and (Equation 2) into (Equation 3) results in the following.
Correction coefficient k = 20.5 / 21 = 0.976 (Expression 4)

With the above procedure, the page representative value, the line representative value, and the correction coefficient can be calculated.
For each of a plurality of color values (e.g., eight color values of 12.5%, 25%, 37.5%, 50%, 62.5%, 75%, 87.5%, 100%) A correction coefficient k is calculated.
The calculated correction coefficient k is used when generating the gamma correction table. The generation of a gamma correction table using this correction coefficient will be described in the following [Specific Example of Generation of Gamma Correction Table].

[Specific example of gamma correction table generation]
Next, a specific example of generating a gamma correction table will be described.
Here, the following items will be described in order.
(1) Correction of density using correction coefficient (2) Generation of gamma correction table based on corrected density

(1) Density Correction Using Correction Coefficient The correction coefficient is used when correcting the density obtained by measuring a patch drawn on the transfer belt 51.
The density correction is performed using the following equation.
Density after correction = density obtained by measuring a patch drawn on the transfer belt 51 × correction coefficient
... (Formula 5)
In this way, the density can be corrected by multiplying the density obtained by measuring the patch drawn on the transfer belt 51 by the correction coefficient.

Here, the correction of the density will be described using specific numerical values.
For example, in “(3) Calculation example of correction coefficient” in [Specific example of calculation of representative value and correction coefficient] described above, when a test patch image having a color of Black and a color value of 50% is printed on paper. The correction coefficient k was calculated. Specifically, in “(3) Correction coefficient calculation example”, the correction coefficient k = 0.976 was obtained.
As described above, the correction coefficient k is calculated based on the density of the test patch image having a color value of 50%. For this reason, for example, the correction coefficient k is applied to the portion of the color value of 50% when the current input / output characteristic curve is obtained as a curve shown by a solid line in FIG.
In the input / output characteristic curve of the figure, when the color value is 50%, the density is about 11 (dots shown in the figure). Therefore, the density can be corrected by substituting the density and the correction coefficient k into (Equation 5). Specifically, it is calculated as follows.
Density after correction = 11 × 0.976 = 10.736 (Expression 6)

(Expression 6) is an expression for correcting the density using the correction coefficient k calculated based on the density of the test patch image having a color value of 50%, but similarly for other color values (Expression 5). Is used to correct the density.
For example, a test patch image is generated for each of eight color values of 12.5%, 25%, 37.5%, 50%, 62.5%, 75%, 87.5%, and 100%, When printing is performed on paper, the density for each pixel (for each patch) is measured, and the correction coefficient k is calculated, correction using the correction coefficient k is performed for each of the densities corresponding to the eight color values. .
The “current input / output characteristic curve” is corrected by interpolating the eight points after the correction of the density by spline interpolation or the like.
Based on the corrected current input / output characteristic curve, a gamma correction table is generated as described below.

(2) Generation of gamma correction table based on corrected density When the current input / output characteristics are corrected according to the contents described in (1), a gamma correction table is generated from the corrected input / output characteristics curve. . The generation of this gamma correction table will be described with reference to FIGS.
In FIG. 8, the solid curve shows the current input / output characteristics after correction using the correction coefficient k. Further, in the figure, a broken curve indicates a target input / output characteristic. Further, in FIG. 9, a solid curve indicates the generated gamma correction table. In the figure, the broken line shows the gamma correction table before the solid gamma correction table is generated.

A specific example of generating a gamma correction table will be described with reference to these drawings. For example, in the current input / output characteristics shown in FIG. 8, when the color value is 25% (about 64), the density is about 70 (the “current” position shown in FIG. 8). This density is brought close to 48 which is the target density (the position of “target” shown in the figure).
As indicated by the arrows in the figure, it can be seen that the color value should be 35% (about 90) in order to set the density to 48 at present.
Therefore, the input tone value (In) = output tone value (Out) is currently set for the gamma correction table shown in FIG. 9 (broken line in FIG. 9), but the input tone value (In) is 64. In this case, the output gradation value (Out) may be set to 90 (white circles shown in the figure).
Similarly, by measuring a plurality of locations (for example, 8 locations) and performing linear interpolation or spline interpolation based on the obtained plot (black dots in FIG. 9), the number of points is increased to 256 gradations. A gamma correction table can be generated (solid curve shown in FIG. 9).

[Calibration program]
Next, the calibration program will be described.
The calibration function (function for executing the calibration method) of the computer (image forming apparatus 1) in the above embodiment is the calibration stored in the storage means 30 (for example, ROM (Read Only Memory) or hard disk). Realized by the application program.

The calibration program is read by a computer control means (such as a CPU provided in the image processing means 20, the engine control unit 40, etc.), so that a command is sent to each component of the computer to perform predetermined processing such as image formation. Image reading processing of the reading unit 61 of the apparatus 1, toner image formation processing of the engine control unit 40 and engine unit 50, density measurement processing of the density sensor 56, patch generation processing of the patch generation unit 21, and representative value of the representative value calculation unit 22 Calculation processing, correction coefficient calculation processing of the correction coefficient calculation unit 23, correction table generation processing of the correction table generation unit 24, display processing of the patch image print setting screen of the operation display means 62, and the like are performed.
Thus, the calibration function is realized by the cooperation of the calibration program as software and each component of the computer (image forming apparatus 1) as hardware resources.

A calibration program for realizing the calibration function is stored in a computer ROM, hard disk, or the like, or may be stored in a computer-readable recording medium, such as an external storage device or a portable recording medium. it can.
The external storage device refers to a memory expansion device that incorporates a storage medium such as a CD-ROM (Compact Disk-ROM) and is externally connected to the calibration device. On the other hand, the portable recording medium is a recording medium that can be mounted on a recording medium driving device (drive device) and is portable, and refers to, for example, a flexible disk, a memory card, a magneto-optical disk, and the like.

The program recorded on the recording medium is loaded into the RAM of the computer and executed by the CPU (control means). By this execution, the function of the calibration device of the above-described embodiment is realized.
Further, when the calibration program is loaded by the computer, the calibration program held by another computer can be downloaded to its own RAM or external storage device using a communication line. This downloaded calibration program is also executed by the CPU, and realizes the calibration function of the calibration device of the above-described embodiment.

  As described above, according to the image forming apparatus, the calibration method, and the calibration program of the present embodiment, by storing in advance the coefficient for correcting the difference between the page representative value and the line representative value, It is possible to infer a page representative value that is a representative value of density unevenness in the actual printing from the measured value of the test patch image.

Since the correction coefficient is reflected when generating the correction table, the density difference between the reading position of the density sensor and the entire page can be reflected as a correction coefficient in the correction table. Thereby, it is possible to reduce the influence of density unevenness in both directions in the sub-operation direction and the main operation direction on the calibration measurement result.
Furthermore, before generating the correction table based on the density obtained by measuring the test patch image formed on the transfer belt, the data calculated in advance is the page representative value, the line representative value, and the correction coefficient. Therefore, the memory area of the storage means for storing these can be minimized.

  The preferred embodiments of the image forming apparatus, calibration method, and calibration program of the present invention have been described above. However, the image forming apparatus, calibration method, and calibration program according to the present invention are limited to the above-described embodiments. It goes without saying that various modifications can be made within the scope of the present invention.

  For example, in the above-described embodiment, the reading unit of the image forming apparatus reads the density of the test patch image printed on the paper, but the reading unit is a reading unit incorporated in the main body of the image forming apparatus. Alternatively, a reading unit such as a scanner externally connected to the main body of the image forming apparatus may be used. The reading unit may be a handy type reading unit that is independent from the main body of the image forming apparatus. Here, when the reading unit is externally connected to the main body of the image forming apparatus, or is not connected to the main body of the image forming apparatus and is independent, the reading unit reads the test patch image from the sheet. After reading the density, the density data can be transmitted to the main body of the image processing apparatus via an externally connected cable or by connecting a cable. Further, after the density of the test patch image is read from the paper, the storage medium is removed from the reading unit, and the storage medium is transferred to the image forming apparatus or transferred to be stored in the storage medium. The density data may be acquired by reading the main body of the image forming apparatus.

In the above-described embodiment, it is assumed that two density sensors are provided in one image forming apparatus. For this reason, for example, as shown in FIGS. 3 (i) and (ii), a second test patch image is formed on each of the two measurement lines on the transfer belt, and two measurements are made on the sheet. A position corresponding line is set. However, the number of density sensors provided in one image forming apparatus is not limited to two, and may be one, for example. In this case, there is a single measurement line on the transfer belt, and a second test patch image is formed on the single measurement line, and a single measurement position is formed on the paper at a position corresponding to the measurement line. Corresponding lines are set. For the line representative value, the representative value of the density in the single measurement position corresponding line is used as the line representative value.
Further, when the reading unit of the image forming apparatus reads the density of the test patch image printed on the paper, the paper can be acquired as to what color and density are set in the image data of the test patch image. It is desirable to print the color and density of these test patch images with characters or the like at arbitrary or specific locations in FIG.

  The present invention can be used for an image forming apparatus having a calibration function.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 20 Image processing means 21 Patch generation part (patch generation means)
22 representative value calculation unit (representative value calculation means)
23 Correction coefficient calculation unit (correction coefficient calculation means)
24 Correction table generation unit (correction table generation means)
30 Storage means 50 Engine section 51 Photosensitive drum (image carrier)
56 Density sensor (Patch density measuring means)
73 Transfer belt 61 Reading means

Claims (7)

An image forming apparatus for transferring an image formed on an image carrier to form an image on a print medium,
Image data of a first test patch image printed at one density with one color on the entire printed page or a predetermined range, which is one surface of the print medium, is generated for each of a plurality of densities for each of a plurality of colors. Patch generating means to perform,
An engine unit for forming the first test patch image on the image carrier based on the generated image data;
A transfer belt for receiving the first test patch image from the image carrier;
Reading means for reading the first test patch image printed on the printing medium by transfer from the transfer belt, and acquiring the density of each predetermined area in the first test patch image;
Based on the density for each predetermined area acquired by the reading means, the representative value of the density in the print page of the print medium is calculated as a page representative value, and the density is measured by the patch density measuring means. Representative value calculating means for calculating a representative value of density in the measurement position corresponding line on the print medium at a position corresponding to the measurement line on the transfer belt as a line representative value;
Correction coefficient calculation means for calculating a correction coefficient representing a difference between the calculated page representative value and the line representative value;
Storage means for storing the correction coefficient;
Patch density measuring means for measuring the density of a second test patch image formed on the measurement line of the transfer belt;
A correction table generating means for generating an input / output characteristic correction table based on the density measured by the patch density measuring means and the correction coefficient calculated by the correction coefficient calculating means;
An image forming apparatus comprising correction table setting means for setting the correction table in the image forming apparatus. The page representative value is an area ratio occupied by patches having XYZ values that are the same tristimulus values with respect to the area of the test patch image printed on the print page of the print medium,
A patch in which the line representative value has the same XYZ value as the tristimulus value with respect to the area of the measurement position corresponding line on the paper corresponding to the position where the patch density measurement unit reads the density on the transfer belt. The image forming apparatus according to claim 1, wherein the image forming apparatus is an area ratio occupied by. The image forming apparatus according to claim 1, wherein the correction coefficient is a numerical value representing a difference between the page representative value and the line representative value as a ratio. The second test patch image formed on the measurement line of the transfer belt,
A test patch image formed in a strip shape on the measurement line,
A plurality of color patches are arranged in order on the measurement line,
The image forming apparatus according to any one of claims 1 to 3, wherein a patch image in which a patch whose density changes in stages is arranged for each of the plurality of colors. The reading unit is incorporated in the main body of the image forming apparatus, or is installed separately from the image forming apparatus.
When the reading unit is installed separately and independently from the main body of the image forming apparatus, data indicating the density is transmitted from the reading unit to the main body of the image forming apparatus, or the density is indicated. 5. The image forming apparatus acquires data indicating the density by transferring a storage medium storing data from the reading unit to the main body of the image forming apparatus. The image forming apparatus described in 1. A calibration method having a process of transferring an image formed on an image carrier to form an image on a print medium,
Image data of a first test patch image printed at one density with one color on the entire printed page or a predetermined range, which is one surface of the print medium, is generated for each of a plurality of densities for each of a plurality of colors. Processing to
Processing to form the first test patch image on the image carrier based on the generated image data;
For each predetermined area in the first test patch image, the first test patch image is read from the transfer belt that has been received from the image carrier and the first test patch image printed on the print medium is read. Processing to obtain the concentration of
Based on the acquired density for each predetermined area, a representative value of the density in the print page of the print medium is calculated as a page representative value, and the density on the transfer belt is measured by a patch density measuring unit. A process of calculating a representative value of density in a measurement position corresponding line on the print medium at a position corresponding to a line as a line representative value;
A process of calculating a correction coefficient representing a difference between the calculated page representative value and the line representative value;
Storing the correction coefficient;
A process in which the patch density measuring means measures the density of a second test patch image formed on the measurement line of the transfer belt;
A process of generating an input / output characteristic correction table based on the density measured by the patch density measuring means and the correction coefficient calculated by the correction coefficient calculating means;
And a process for setting the correction table in the image forming apparatus. A calibration program for causing an image forming apparatus to execute a process of transferring an image formed on an image carrier and forming an image on a print medium,
Image data of a first test patch image printed at one density with one color on the entire printed page or a predetermined range, which is one surface of the print medium, is generated for each of a plurality of densities for each of a plurality of colors. Processing to
Processing to form the first test patch image on the image carrier based on the generated image data;
For each predetermined area in the first test patch image, the first test patch image is read from the transfer belt that has been received from the image carrier and the first test patch image printed on the print medium is read. Processing to obtain the concentration of
Based on the acquired density for each predetermined area, a representative value of the density in the print page of the print medium is calculated as a page representative value, and the density on the transfer belt is measured by a patch density measuring unit. A process of calculating a representative value of density in a measurement position corresponding line on the print medium at a position corresponding to a line as a line representative value;
A process of calculating a correction coefficient representing a difference between the calculated page representative value and the line representative value;
Storing the correction coefficient;
A process in which the patch density measuring means measures the density of a second test patch image formed on the measurement line of the transfer belt;
A process of generating an input / output characteristic correction table based on the density measured by the patch density measuring means and the correction coefficient calculated by the correction coefficient calculating means;
A calibration program for causing the image forming apparatus to execute processing for setting the correction table in the image forming apparatus.

Images