JP2009134138A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To optimize gradation of a patch used for density control, to shorten time for density detection control by reducing the number of patches and to improve color stability. <P>SOLUTION: Density difference between a density measured by a density measuring means and a target density is calculated, at least one gradation is detected from those with great gradation differences, then the detected gradation or a prescribed area range which includes the detected gradation is detected as a gradation area with maximum density difference. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus such as a printer or a color copying machine.

  In recent years, color image forming apparatuses employing an electrophotographic system such as a color printer and a color copying machine have been required to have high output image quality. In particular, the gradation of density and its stability greatly affect the image quality.

  However, in the case of an electrophotographic color image forming apparatus, even a slight environmental fluctuation may cause a density fluctuation, which may cause a color balance to be lost. Therefore, it is necessary to always have a means for maintaining a constant density. Therefore, a gradation pattern in which a plurality of density detection toner images (hereinafter referred to as patches) are provided for each color toner is created on an intermediate transfer member or a photosensitive member, and the density of the unfixed toner patch is detected for the density of the unfixed toner. It is detected by a sensor (hereinafter referred to as a density sensor). Based on the detection result, it is configured to obtain a stable image (hereinafter, gradation control) by controlling the relationship between image data and density (hereinafter, gamma) to be a desired gamma. For example, as disclosed in Patent Document 1, there is a description of an apparatus that determines execution of density control based on the humidity in the image forming apparatus exceeding a certain set value or the time for which the photosensitive drum is left.

Further, by repeating the paper feeding, the charging state of the developer and the photoconductor changes, there is a possibility that density fluctuation occurs and color balance is lost. In particular, immediately after replacing the CRG, or when the color image apparatus is left standing for printing for a long time, the change in the charging state of the developer and the photosensitive member is large, and the density change tends to increase according to the number of sheets. Therefore, regular gradation control is performed when a predetermined number of sheets of several hundred to several thousand sheets is passed in order to maintain a constant density and to stabilize the color balance even during the sheet passing.
Japanese Patent Laid-Open No. 04-36776

  Here, performing gradation control in which all gradation patches that become gradation patterns are formed for each adjustment and the density of all patches is measured has a large time loss and causes a reduction in productivity. Furthermore, if the job is interrupted and gradation control is performed, the usability is reduced. In addition, there is a problem that the intermediate transfer member or the photosensitive member for printing the patch is deteriorated or soiled, and a lot of toner is consumed.

  In order to solve these problems, there is an embodiment in which gradation control is performed only with a specific gradation, but there is a problem that the accuracy of gradation control changes depending on whether the specific gradation is appropriately selected.

  The present invention has been made to solve the above-described problems of the prior art, and is an image that can suppress color fluctuation and achieve color stabilization without printing all gradation patches and measuring their density. An object is to provide a forming apparatus and an image forming method.

  In order to achieve the above object, in the present invention, an image forming apparatus is configured as follows.

A gradation pattern generating means for forming a gradation pattern image on the image carrier, a density measuring means for measuring the density of the gradation pattern image formed by the gradation pattern generating means, and the image gradation as a target density Image gradation control means for controlling, density difference calculating means for calculating the density difference between the density measured by the density measuring means and the target density, and the one having a large density difference calculated by the density difference calculating means A density difference maximum gradation area detecting means for detecting at least one gradation and detecting the detected gradation or a predetermined area range including the detected gradation as a density difference maximum gradation area;
An image forming apparatus comprising:

  Further, the image forming apparatus according to the present application performs image gradation control with a specific gradation pattern based on a detection result by the density difference maximum gradation area detecting means.

  Furthermore, the image forming apparatus according to the present application is characterized in that the gradation pattern generating means forms a gradation pattern image outside the actual printing time during the job.

  As described above, according to the present invention, it is possible to detect a gradation region where the density is most unstable among all gradation regions.

  Then, performing image gradation control with a specific gradation pattern based on the detection result by the density difference maximum gradation area detecting means is effective in gradation control specialized for unstable gradations. Stability will be improved. It is also possible to increase the update frequency of the gradation correction table, to obtain a more stable image, and further to change the gradation correction table without performing density measurement for all gradations. The complexity of processing associated with the change of the gradation correction table can be avoided.

  It is possible to stabilize the density without interrupting the print job.

(Example 1)
FIG. 1 is a cross-sectional view illustrating the overall configuration of the color image forming apparatus according to the first embodiment. As shown in the figure, this apparatus is a tandem color image forming apparatus that employs an intermediate transfer member 27 that is an example of an electrophotographic color image forming apparatus. The color image forming apparatus includes an image forming unit shown in FIG. 1 and an image processing unit (not shown).

  Hereinafter, the operation of the image forming unit in the electrophotographic color image forming apparatus will be described with reference to FIG. When the image forming operation is started, the image forming unit forms an electrostatic latent image by illuminating exposure light based on the exposure time calculated by the image processing unit according to the image data, and developing the electrostatic latent image. Forming a single-color toner image, superimposing the single-color toner images to form a multi-color toner image, transferring the multi-color toner image to the transfer material 11, and fixing the multi-color toner image on the transfer material 11; Made in The image forming unit includes a paper feeding unit 21, photoconductors (22Y, 22M, 22C, and 22K) arranged in parallel for the development colors, charging rollers (23Y, 23M, 23C, and 23K) as primary charging units, photoconductors Cleaning means (35Y, 35M, 35C, 35K), toner container (25Y, 25M, 25C, 25K), developing means (26Y, 26M, 26C, 26K), primary transfer roller 36, intermediate transfer body 27, secondary transfer The roller 28, the intermediate transfer member 27 cleaning means 29, the fixing unit 30, the density sensor 41, and a non-volatile memory 50 that stores a density conversion table. The photosensitive drums 22 for each color, the cleaning means 42, the charging roller 23, the developing device 26, the toner container 25, and the nonvolatile memory 50 are integrated (hereinafter referred to as CRG 51) and are detachably attached.

  The photosensitive drums (photoconductors) 22Y, 22M, 22C, and 22K are configured by applying an organic optical conductive layer to the outer periphery of an aluminum cylinder, and are rotated by the driving force of a driving motor (not shown) being transmitted. Rotates the photosensitive drums 22Y, 22M, 22C, and 22K in the counterclockwise direction in accordance with the image forming operation.

  The cleaning unit 35 cleans the toner remaining on the photosensitive drum 22, and the residual toner after the toner image formed on the photosensitive drum 22 is transferred to the intermediate transfer body 27 is collected in a cleaner container. .

  As charging means, four charging rollers 23Y, 23M, 23C, and 23K for charging the photoconductors of yellow (Y), magenta (M), cyan (C), and black (K) are provided for each station. Yes.

  Exposure light to the photosensitive drums 22Y, 22M, 22C, and 22K is sent from the scanner units 24Y, 24M, 24C, and 24K, and the electrostatic latent images are selectively exposed by exposing the surfaces of the photosensitive drums 22Y, 22M, 22C, and 22K. An image is formed.

  The developing means includes four developing units 26Y, 26M for developing yellow (Y), magenta (M), cyan (C), and black (K) for each station in order to visualize the electrostatic latent image. Each developing device is provided with a sleeve 26YS, 26MS, 26CS, and 26KS.

  The intermediate transfer member 27 is in contact with the photosensitive drums 22Y, 22M, 22C, and 22K, and rotates clockwise when forming a color image. The intermediate transfer member 27 rotates as the photosensitive drums 22Y, 22M, 22C, and 22K rotate, and the monochrome toner image is transferred onto the intermediate transfer member 27 by applying a bias to the primary transfer roller. Thereafter, a secondary transfer roller 28 (described later) contacts the intermediate transfer member 27 to sandwich and transfer the transfer material 11, and the multicolor toner image on the intermediate transfer member 27 is transferred to the transfer material 11.

  The secondary transfer roller 28 contacts the transfer material 11 at the position 28a while transferring the multicolor toner image onto the transfer material 11, and is separated to the position 28b after the image forming operation.

  The fixing unit 30 melts and fixes the transferred multi-color toner image while conveying the transfer material 11, and as shown in FIG. 1, the fixing roller 31 for heating the transfer material 11 and the transfer material 11 are fixed to the fixing roller. A pressure roller 32 is provided for pressure contact with 31. The fixing roller 31 and the pressure roller 32 are formed in a hollow shape, and heaters 33 and 34 are incorporated therein. That is, the transfer material 11 holding the multicolor toner image is conveyed by the fixing roller 31 and the pressure roller 32, and heat and pressure are applied to fix the toner on the surface.

  After the toner image is fixed, the transfer material 11 is discharged to a discharge tray (not shown) by a discharge roller (not shown) and the image forming operation is finished.

  The cleaning unit 29 cleans the toner remaining on the intermediate transfer body 27. The residual toner after the four-color multicolor toner image formed on the intermediate transfer body 27 is transferred to the transfer material 11 is: Collected in a cleaner container.

  In the color image forming apparatus of FIG. 1, the density sensor 41 is disposed at the center position in the longitudinal direction of the intermediate transfer body 27 toward the intermediate transfer body 27, and the density of the toner patch formed on the surface of the intermediate transfer body 27. Measure. An example of the configuration of the density sensor 41 is shown in FIG. The density sensor 41 includes an infrared light emitting element 42 such as an LED, a light receiving element 43 such as a photodiode, an IC (not shown) that processes received light data, and a holder (not shown) that accommodates these.

  The infrared light emitting element 42 is installed at an angle of 45 degrees with respect to the normal direction of the intermediate transfer member 27, and irradiates the toner patch 44 on the intermediate transfer member 27 with infrared light. The light receiving element 43 is installed at a position symmetrical to the issuing element 42 with respect to the normal direction, and detects regular reflection light from the toner patch 44. Note that an optical element such as a lens (not shown) may be used for the concentration sensor 41 in FIG.

  In this embodiment, the intermediate transfer member 27 is a single layer resin belt made of polyimide, an appropriate amount of carbon fine particles are dispersed in the resin for adjusting the belt resistance, and the surface color is black. Further, the surface of the intermediate transfer member 27 is highly smooth and glossy, and the glossiness is about 100% (measured with a gloss meter IG-320 manufactured by Horiba, Ltd.).

  In the density sensor 41, when the surface of the intermediate transfer member 27 is exposed (toner density is 0), the light receiving element 42 detects reflected light. The reason is that the surface of the intermediate transfer member 27 is glossy as described above. On the other hand, when a toner image is formed on the intermediate transfer member 27, the regular reflection light output gradually decreases as the density of the toner image increases. This is because the regular reflected light from the belt surface is reduced by the toner covering the surface of the intermediate transfer member 27.

  Next, the flow of image formation will be described with reference to FIG. The user forms an image with application software or the like that operates on the host computer 601. The image created by the user is processed as print data by a printer driver installed on the same computer as the host computer 601 on which application software is normally installed. The printer driver creates print data suitable for the image forming apparatus to be used. When a laser printer capable of high-speed printing is used, print data written in a page description language such as Adobe PostScript is created. The print data created by the printer driver is sent to the image forming apparatus 604 and converted into a bitmap by the bitmap processing unit 605. Here, the bitmapping will be briefly described. Normally, since characters and graphic images are described by commands defined in the page description language, they cannot be printed by the image forming apparatus as they are. Therefore, it is necessary to decode the command and convert it into a bitmap that can be printed by the image forming apparatus. Further, since the image that is bitmap information from the beginning has a large amount of information, the data is compressed by the printer driver and the data is expanded by the image forming apparatus.

  The color converted data is converted by the color conversion unit 606. Here, the color conversion is performed by converting an RGB signal matched to the normal monitor color into a YMCK signal matched to the image forming apparatus. The data converted into the YMCK signal is subjected to gradation correction by the gradation correction unit 607. Here, the gradation correction means that the density gradation characteristics of the image forming apparatus are prepared as a gradation correction table, and the data is stored in the gradation correction table so that the density gradation of the output image becomes a desired characteristic. Refers to correction.

  The dither processing is performed by the dither processing unit 608 on the gradation corrected data. Here, the dither processing is processing that performs area gradation suitable for the image forming apparatus, and area gradation processing using systematic dither or an error diffusion method is performed as conventionally known. The dithered data is sent to the image formation control unit 609 and image formation is performed.

  Next, tone control unique to this embodiment will be described with reference to FIG.

  The tone control peculiar to the present embodiment includes regular tone control for printing a plurality of patches and auxiliary tone control for printing a specific patch between the regular tone controls. In the description of this embodiment, the regular gradation control and the auxiliary gradation control are performed for each predetermined number of sheets. In this embodiment, the regular gradation control is performed for every 1000 prints, and the auxiliary gradation control is performed for every 100 prints. We decided to do each.

  In the flow of gradation control unique to this embodiment, first, periodic gradation control 401 is performed, and one gradation patch having the largest density difference and density difference is selected as a specific gradation. Thereafter, a predetermined number of sheets 100 are printed (402). An auxiliary gradation control 403 is performed using the image data of the selected specific gradation patch. Thereafter, auxiliary gradation control is performed every time 100 sheets are printed (404).

  In this way, until the next regular gradation control, the combination 405 of the auxiliary gradation control using the image data of the selected specific gradation patch and the predetermined number of 100 sheets is repeated. In this embodiment, the regular gradation control is performed every 1000 sheets, and the auxiliary gradation control is performed every 100 sheets. Therefore, the combination 405 of auxiliary gradation control and 100 sheets of a predetermined number of sheets is repeated nine times.

  Next, the regular gradation control and the auxiliary gradation control will be described in detail.

  First, details of the regular gradation control will be described with reference to the flowchart of FIG. 5 and schematic diagrams of FIGS. 6, 7, and 8. FIG.

  The regular gradation control is performed when a change in image density is assumed, when the power of the main body is turned on, when the CRG 51 is replaced, after a predetermined number of sheets have been printed since the previous regular gradation control.

  STEP 1: A patch pattern is formed on the intermediate transfer member 27.

  FIG. 6 is a diagram showing a patch pattern formed on the intermediate transfer member 27. 8 mm square patches are printed at 10 mm intervals at positions that can be corrected by the density sensor 41, and images are printed for each of Y, M, C, and K. The ratio (density gradation degree) is changed in 8 steps (each color has 8 patches), and a total of 32 are formed. The correspondence between each patch and the printing rate (gradation) is Y1, M1, C1, K1 = 12.5%, Y2, M2, C2, K2 = 25%, Y3, M3, C3, K3 = 37.5%, Y4, M4 , C4, K4 = 50%, Y5, M5, C5, K5 = 62.5%, Y6, M6, C6, K6 = 75%, Y7, M7, C7, K7 = 87.5%, Y8, M8, C8, K8 = 100 %, Is set to.

  STEP 2: The toner sensor formed on the intermediate transfer member 27 is detected by the density sensor 41.

  STEP 3: The detection signal of the density sensor 41 is converted into density by referring to the density conversion table stored in the nonvolatile memory 50 of the CRG 51 for each color.

  The density conversion table is a table showing the relationship between density sensor output and density when detecting a toner patch as shown in FIG.

  By referring to this density conversion table for gradation control, the density sensor output can be uniquely converted to density.

  In this embodiment, the CRG is provided with a non-volatile memory that holds the density conversion table. However, it may be provided in a toner replenishment process cartridge that holds the toner cartridge and the process cartridge separately, or in the image forming main body. It doesn't matter.

  STEP 4: Perform gradation control. Hereinafter, the gradation control will be described with reference to FIG. Here, only cyan tone correction will be described, but magenta, yellow, and black are also corrected in the same manner.

  The horizontal axis in FIG. 8 represents image data. The vertical axis represents the density detection value of the density sensor 41.

  In the figure, ◯ represents the output density value of the density sensor 41 for each patch of C1, C2, C3, C4, C5, C6, C7, and C8. Next, a straight line T represents a target gradation density characteristic for image density control. In this embodiment, the target gradation density characteristic T is determined so that the relationship between image data and density is proportional. A curve γ represents density gradation characteristics when gradation control is not performed. It should be noted that the density of gradations not forming a patch is calculated by spline interpolation through the origin and C1, C2, C3, C4, C5, C6, C7, and C8.

  A curve D represents a gradation correction curve calculated by this control, and is calculated by obtaining a symmetric point with respect to the target gradation density characteristic T of the gradation characteristic γ before correction. The calculation of the gradation correction curve D is executed by a main body CPU (not shown), and the calculated gradation correction curve D is stored in the main body memory 606 as a gradation correction table having discrete values.

  When the print image is formed, the target gradation density characteristic T can be obtained by correcting the image data with the gradation correction table.

  STEP 5: Subtract the target density of the target gradation density characteristic T for each patch from the output density values C1, C2, C3, C4, C5, C6, C7, C8 of the density sensor 41 for each patch, and the density sensor for each patch The difference between 41 and the target density is calculated. The gradation having the largest density difference between the calculated density patches is selected as the specific gradation, and the gradation data of the gradation (62.5% image data if the density value is C5) or the number of the gradation is indicated. Information that can specify gradation such as information is recorded in the nonvolatile memory 610. These subtraction processes are executed by a main body CPU (not shown).

  In FIG. 8, the image data of the gradation that maximizes the difference between the output density of the density sensor 41 and the target density is the 62.5% gradation, and this gradation will be described as a specific gradation hereinafter.

  Next, auxiliary gradation control will be described with reference to FIGS.

  In the auxiliary gradation control, an evaluation patch is generated from the specific gradation data selected in the regular gradation control, and gradation control is performed.

  A curve γ ′ is a density gradation characteristic immediately after printing 100 sheets after periodic gradation correction, and a deviation from the target density occurs. This impairs color balance.

  Here, the variation of the gradation characteristic γ depending on the number of printed sheets is patterned for each cartridge or image forming apparatus, and a gradation region having a large density difference from the target density at the time of regular gradation control is It has been experimentally confirmed that the density is most unstable when printing is performed. Therefore, only the gradation patch of a specific gradation recorded in the nonvolatile memory 610 by the regular gradation control is formed on the intermediate transfer body 27, the density is detected by the density detection sensor 41, and the target gradation density characteristic is obtained. A variation ΔD between T and its detected density is calculated.

  Next, the gradation value C5 of the gradation correction table data 62.5% generated from the gradation correction curve D composed of the symmetrical points with respect to the target gradation density characteristic T of the gradation characteristic γ before the regular gradation correction is obtained. A variation amount optimum for canceling the density variation amount ΔD is added, and spline interpolation is performed for the density value of each gradation. A curve subjected to spline interpolation becomes a new gradation correction curve D ′. The calculation of the gradation correction curve D ′ is executed by a main body CPU (not shown), and a new gradation correction table generated from the gradation correction curve D ′ is stored in the main body memory 606. By using the gradation correction table generated from the gradation correction curve D ′ for the gradation correction 607, the density gradation characteristic γ ″ approximated to the target gradation density characteristic T is obtained (FIG. 10).

  The above is the description of the gradation gradation control in this embodiment.

  As described above, in this embodiment, the gradation region where the density is most unstable among all gradation regions is detected. By utilizing this detection result, gradation control specialized for unstable gradation can efficiently improve density stability and obtain an image with more stable color balance. In addition, it is possible to increase the frequency of updating the tone correction table while minimizing the interruption time of the print job, so that a more stable image can be obtained, and furthermore, density measurement of all tones is performed. Therefore, the gradation correction table can be changed, and the complexity of processing associated with the gradation correction table change can be avoided.

  In addition, by limiting the gradation correction pattern, it is possible to suppress the decrease in productivity by shortening the time required for the gradation correction, to prevent deterioration and contamination of the intermediate transfer member and the photosensitive member for printing patches, and to reduce toner consumption. Effective for reduction.

  In this embodiment, an example of an image forming apparatus using an intermediate transfer member has been described as an example of a color image forming apparatus. However, the present invention can also be applied to color image forming apparatuses of other forms. For example, a color image forming apparatus in which a toner image on a photosensitive member is directly transferred onto a transfer material on a transfer material carrier (such as a transfer belt), and a toner patch is formed on the transfer material carrier to form a density. The present invention can also be applied to a color image forming apparatus that can detect the patch density with a sensor.

  In this embodiment, the density is detected during gradation correction. However, the chromaticity and brightness are used instead of the density, and the chromaticity sensor and brightness sensor are used instead of the density sensor 41. Needless to say, similar effects can be obtained by using chromaticity conversion and lightness conversion tables instead.

(Example 2)
There is real-time gradation control in which patches are formed in an inter-sheet area on an intermediate transfer member. This gradation control can be executed without interrupting image formation.

  However, since real-time gradation control forms patches in the inter-sheet area on the intermediate transfer member, the number of patches that can be printed is limited by the space in the inter-sheet area. Conventionally, specific gradations are set for these patches according to the characteristics specific to the model, but it is desirable that the patches used for control are appropriately selected according to the characteristics, environment, and durability of the engine and CRG solids. .

  Therefore, in this embodiment, the auxiliary gradation control of the first embodiment is performed as real-time gradation control.

  The present embodiment will be described with reference to FIGS. 1, 9, 10, and 11.

  An optical sensor 41 is opposed to the intermediate transfer member 27 serving as a transfer image carrier and is located downstream of the four-color image forming unit in the running direction of the intermediate transfer member 27, that is, on the most downstream side of the transfer unit. Is provided.

  FIG. 11 shows an image forming state on the intermediate transfer member, where an area In is an image area where image formation has been completed, an area In + 1 is an image area where image formation is refrained, and an area Bn is an image area In and an image In + 1. Between the sheets, which is a non-image forming area.

  The image forming apparatus according to the present embodiment forms specific gradation patches Yk, Ml, Cm, and Kn in the inter-paper area Bn on the ITB according to the same periodical gradation control as in the first embodiment. These image densities are detected by a density sensor 41 as density detection means. The infrared light emitted from the infrared light emitting element 42 is reflected by the surface of the intermediate transfer body 27, and the reflected light is measured by the light receiving element 43. The reflected light measured by the light receiving element 43 is used for auxiliary gradation control similar to that in the first embodiment, and a gradation correction table D ′ is obtained.

  If there is no image being formed in the uppermost image forming unit at the timing when the gradation correction table D ′ is obtained, gradation correction using the gradation correction table D ′ is performed for the next image formation. Do. When there is an image being formed in the uppermost image forming unit, the gradation correction table D ′ is used for the next image formation.

  The gradation patches Yk, Ml, Cm, Kn on the inter-paper area Bn that have passed through the density detection sensor 41 pass through the secondary transfer roller portion spaced apart at the position 28b, as in the gradation patch for regular gradation control. The toner is collected as waste toner by the cleaning unit 29.

  As described above, the optimum gradation patch is detected by the regular gradation control, the gradation patch is formed in the inter-paper area on the intermediate transfer member, and the auxiliary gradation control is performed in real time. Do not cause interruption.

  In this embodiment, an example of an image forming apparatus using an intermediate transfer member has been described as an example of a color image forming apparatus. However, a toner image on a photosensitive member is transferred to a transfer material on a transfer material carrier (such as a transfer belt). A color image forming apparatus that directly transfers images can also be implemented by forming toner patches in the inter-paper area on the transfer material carrier.

FIG. 3 is a cross-sectional view showing the overall configuration of Example 1. FIG. 2 is a block diagram illustrating a configuration of a control unit as an image forming apparatus. The figure which shows the structure of the density sensor 41. FIG. 5 is a flowchart for explaining gradation control in the first embodiment. 6 is a flowchart for explaining regular gradation control in the first embodiment. FIG. 3 is a layout diagram of patch patterns on an intermediate transfer member used for gradation control in the first embodiment. The figure which shows the relationship between the output and density | concentration of a density | concentration sensor. FIG. 5 is a diagram illustrating a gradation control method according to the first embodiment. FIG. 5 is a diagram illustrating a gradation control method according to the first embodiment. FIG. 5 is a diagram illustrating a gradation control method according to the first embodiment. FIG. 10 is a layout diagram of patch patterns on an intermediate transfer member used for gradation control in the second embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Transfer material 22 Photoconductor, photosensitive drum 26 Developing means 27 Intermediate transfer body 30 Fixing device 41 Density sensor 50 Non-volatile memory 51 CRG

Claims (3)

A gradation pattern generating means for forming a gradation pattern image on the image carrier;
A density measuring means for measuring the density of a first gradation pattern image comprising a plurality of gradation patches formed by the gradation pattern generating means; an image gradation control means for controlling the image gradation to a target density;
A density difference calculating means for calculating a density difference between the density measured by the density measuring means and the target density;
At least one gradation is detected from those having a large density difference calculated by the density difference calculating means, and the detected gradation or a predetermined area range including the detected gradation is detected as a maximum density difference gradation area. Density difference maximum gradation region detecting means;
An image forming apparatus comprising:   2. The image forming apparatus according to claim 1, wherein image gradation control is performed with a second gradation pattern made up of a specific gradation patch based on a detection result by said density difference maximum gradation area detecting means. Image forming apparatus.   3. The image forming apparatus according to claim 2, wherein the gradation pattern generation unit forms the second gradation pattern image outside the actual printing time during the job.

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