JP5120402B2 - Image forming apparatus and image forming method - Google Patents

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a printer, and more particularly to a technique for correcting gradation of an output image in the image forming apparatus.

In an image forming apparatus such as an electrophotographic copying machine or printer, the gradation of an input image is not reproduced linearly (in a proportional relationship) in an output image transferred to a recording sheet. The data is subjected to gradation correction based on a gradation correction curve (γ correction curve) and adjusted so that the gradation of the output image is reproduced linearly with the gradation of the input image.
Further, since the amount of toner adhering to the photosensitive drum changes due to changes in temperature and humidity in the machine, deterioration of parts such as the photosensitive drum, etc., it is required to periodically update the γ correction curve.

  For this reason, the image forming apparatus is provided with a function of periodically updating the γ correction curve used for gradation correction. For example, Patent Document 1 discloses that a toner pattern for each color for gradation correction is formed on an image carrier based on image data having a predetermined gradation value, and the density of the formed toner pattern for each color is reflected. A technique for periodically updating a γ correction curve by measuring using a measurement sensor and creating a γ correction curve based on the measurement result is disclosed.

  Thereby, the gradation reproducibility in the output image can always be kept good, and the density of each color image formed on the recording sheet can be stabilized.

JP 2006-259261 A

  However, in the above-described conventional tone correction technique, when a plurality of types of single color toners are overlapped as an output image to reproduce multiple colors, a desired color tone and deviation may occur. For example, in an image forming apparatus in which a toner image is primarily transferred onto an intermediate transfer belt and then secondarily transferred onto a recording sheet to reproduce an output image, a plurality of types of single color toners are superimposed on the intermediate transfer belt. When trying to reproduce a multicolor mixed image together, the lowermost toner superimposed on the intermediate transfer belt is partially transferred after the secondary transfer as in the case of reproducing a single color image. Residual toner remains on the belt (see FIGS. 13A and 13B). As a result, the entire amount of toner in the lowermost layer is not transferred to the recording sheet, whereas the toner other than the lowermost layer that is not in contact with the intermediate transfer belt is almost entirely transferred to the recording sheet. This is because there is a difference in the transfer rate of the amount of toner transferred to the recording sheet with other toners.

  In other words, when the gradation correction of the output image is performed using the γ correction curve created for each color, the gradation value of each color of the output image is determined without considering the difference in the transfer rate. Even if the gradation values are determined so that each color has the same gradation value, in the image that is actually reproduced on the recording sheet, only the gradation of the color of the lowermost layer has a transfer rate higher than that of the other colors. The gradation is reduced by an amount corresponding to the difference, and as a result, there is a problem that the hue of the output image in which multiple colors are mixed cannot be reproduced with a desired hue.

  The present invention has been made in view of the above-described problems, and is an image forming apparatus capable of accurately reproducing a desired color even when reproducing an output image in which multiple colors are mixed. And an image forming method.

  In order to achieve the above object, an image forming apparatus according to an aspect of the present invention performs tone correction of input image data based on the density of a single-color toner pattern formed on an image carrier, and after tone correction An image forming apparatus for forming a color image by transferring a toner image of each color formed to be superimposed on an image carrier based on image data of the image to a recording sheet, and a monochrome toner for gradation correction Forming means for forming a pattern on the image carrier, density of the toner pattern before transfer on the image carrier, and the toner pattern remaining on the image carrier after the toner pattern is transferred Acquisition means for acquiring the remaining density of the image data, and first creation means for creating a first gradation correction table for correcting the gradation value of the input image data based on the density value of the toner pattern before transfer. , The second for correcting the gradation value of the input image data so that the gradation value becomes larger by the amount corresponding to the remaining density than when the gradation value is corrected using the first gradation correction table. A second creation unit that creates a gradation correction table based on the density value of the toner pattern before transfer and the remaining density; and when multi-color image data is input, the color indicated by the image data Among the density data, the tone value of the color density data excluding the color superimposed on the lowermost layer on the image carrier is corrected using the first tone correction table, and the color density data of the lowermost layer color is corrected. Correction means for correcting the gradation value using the second gradation correction table.

  Here, the image carrier is an intermediate transfer belt, and includes a secondary transfer roller that transfers a toner image on the intermediate transfer belt to a recording sheet. Based on the detected density, the detection means for detecting the density of the toner pattern and the residual density of the toner pattern remaining on the intermediate transfer belt after the toner pattern is transferred to the secondary transfer roller The image forming apparatus may include a determining unit that determines a residual density of the toner pattern when the toner pattern is transferred to a recording sheet, and a residual density acquiring unit that acquires the determined residual density.

Further, the second preparation means corrects the density value of the toner pattern before transfer to a value obtained by subtracting the amount corresponding to the remaining density, and generates the second gradation correction table based on the correction value. It can be.
In addition, an image forming method according to an aspect of the present invention performs gradation correction of input image data based on the density of a single color toner pattern formed on an image carrier, and based on the image data after gradation correction. An image forming method in an image forming apparatus for forming a color image by transferring toner images of respective colors formed so as to be superimposed on an image carrier onto a recording sheet, and a monochrome toner pattern for gradation correction Forming on the image carrier, the density of the toner pattern before transfer on the image carrier, and the toner pattern remaining on the image carrier after transferring the toner pattern. An acquisition step for acquiring a residual density, and a first creation for creating a first gradation correction table for correcting a gradation value of input image data based on a density value of the toner pattern before transfer In order to correct the gradation value of the input image data so that the gradation value becomes larger by the amount corresponding to the remaining density than when the gradation value is corrected using the step and the first gradation correction table. A second generation step of generating the second gradation correction table based on the density value of the toner pattern before transfer and the residual density, and when multicolor image data is input, the image data The color density data excluding the color superimposed on the lowermost layer on the image carrier is corrected using the first gradation correction table, and the color density data indicated by A correction step of correcting the gradation value of the color density data using the second gradation correction table;
Can be included.

  Here, the image carrier is an intermediate transfer belt, and includes a secondary transfer roller that transfers a toner image on the intermediate transfer belt to a recording sheet, and the acquisition step is performed before the transfer on the intermediate transfer belt. Based on the detection step for detecting the density of the toner pattern, the residual density of the toner pattern remaining on the intermediate transfer belt after the toner pattern is transferred to the secondary transfer roller, and the detected residual density A determination step of determining a residual density of the toner pattern when the toner pattern is transferred to a recording sheet, and a residual density acquisition step of acquiring the determined residual density can be included.

  Further, in the second creation step, the density value of the toner pattern before transfer is corrected to a value obtained by subtracting the residual density, and the second gradation correction table is created based on the correction value. It can be.

  By providing the above configuration, when multi-color image data is input, the color is a color excluding the color superimposed on the lowermost layer on the image carrier, and the toner does not remain on the image carrier at the time of transfer. For the color density data, tone correction is performed using the first tone correction table in which the residual density is not taken into consideration, and the color density of the lowest color in which a part of the toner remains on the image carrier at the time of transfer As for the data, the gradation value using the second gradation correction table is set so that the gradation value becomes larger by the amount corresponding to the remaining density than when the gradation correction is performed using the first gradation correction table. Correction can be performed. As a result, when reproducing an output image in which multiple colors are mixed, an error in the gradation value caused by the difference in residual toner density on the image carrier that occurs between the color of the lowermost layer and the other colors is eliminated. The output image can be accurately reproduced with a desired hue.

  Here, when the single color image data is input, the correction unit may correct the gradation value of the color density data indicated by the image data using the second gradation correction table. In the correction step, when monochrome image data is input, the gradation value of the color density data indicated by the image data may be corrected using the second gradation correction table.

As a result, for monochrome image data in which a part of the toner remains on the image carrier at the time of transfer, an amount corresponding to the remaining density is obtained as compared with the case where gradation correction is performed using the first gradation correction table. Since gradation correction is performed using the second gradation correction table so that the gradation value becomes large, the accuracy of gradation correction in single-color image data can be increased.
Here, the toner pattern may be formed in a non-sheet passing area other than the sheet passing area through which the recording sheet is passed on the image carrier.

  As a result, a tone correction toner pattern is formed in the non-sheet-passing area on the image carrier, so that the tone correction toner pattern is formed in parallel with the printing process of the toner image on the recording sheet. be able to. Therefore, the delay time of the printing process due to the formation of the tone correction toner pattern can be reduced.

1 is a diagram illustrating an overall configuration of an image forming apparatus 10. 3 is a block diagram illustrating a configuration of a control unit 14. FIG. 2 is a diagram illustrating a configuration of an image signal processing unit 105. FIG. A specific example of the γ correction LUT is shown. A specific example of the lowermost layer color identification table is shown. It is a flowchart which shows operation | movement of (gamma) correction data creation processing. FIG. 6 is a diagram showing a change in the positional relationship between the intermediate transfer belt 21 and the secondary transfer roller 35 during γ correction data creation processing. A specific example of the toner pattern of each color formed on the intermediate transfer belt 21 is shown. A specific example of the arrangement of patches constituting the toner pattern is shown. Specific examples of a measurement curve, a difference measurement curve, and a residual toner correction curve created in the γ correction data creation process are shown. A specific example of the second γ correction curve created in the γ correction data creation process is shown. It is a flowchart which shows operation | movement of an image data (gamma) correction process. FIG. 4 is a diagram illustrating a state of toner carried on an intermediate transfer belt before and after secondary transfer.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described by taking a full-color tandem image forming apparatus (hereinafter simply referred to as “image forming apparatus”) as an example.
(1) Overall Configuration of Image Forming Apparatus FIG. 1 is a diagram showing an overall configuration of an image forming apparatus 10 according to the present embodiment.
As shown in the figure, the image forming apparatus 10 forms an image by a known electrophotographic method, and includes an image processing unit 11, a paper feeding unit 12, a fixing unit 13, a control unit 14, and an image. It comprises a reading unit 15 and the like. When this image forming apparatus 10 is connected to a network (for example, a LAN) and receives a print job execution instruction from an external terminal apparatus (not shown), cyan, magenta, yellow, and black colors are received based on the instruction. The color image formation consisting of is executed.

Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are represented as Y, M, C, and K, and Y, M, C, and K are added as subscripts to the numbers of the components related to the reproduction colors.
The image processing unit 11 has image forming units 20Y, 20M, 20C, and 20K corresponding to the colors Y to K arranged in a row below the intermediate transfer belt 21 stretched by the driving roller 22 and the driven roller 23. Further, a secondary transfer roller 35 that can be pressed against and separated from the intermediate transfer belt 21 through an actuator 39 is provided. A cleaner 24 for removing toner remaining on the intermediate transfer belt 21 is disposed below the driven roller 23. The cleaner 24 is configured to be capable of being pressed and separated from the intermediate transfer belt 21, and the pressure contact and separation are controlled by the control unit 14.

Each of the image forming units 20Y to 20K includes photosensitive drums 1Y to 1K, chargers 2Y to 2K, exposure units 3Y to 3K, developing units 4Y to 4K, primary transfer rollers 5Y to 5K disposed around the photosensitive drums 1Y to 1K. It consists of cleaners 6Y to 6K for cleaning the photosensitive drum.
The developing devices 4Y to 4K are provided with developing rollers 41Y to 41K for supplying toner to the corresponding photosensitive drums. The developing rollers 41Y to 41K are individually biased by a developing bias power supply unit (not shown). A voltage is applied.

In the configuration of the image processing unit 11 as described above, for example, in the black toner image formation on the photosensitive drum 1K, the photosensitive drum 1K is uniformly charged by the charger 2K, and then the exposure unit 3K. After an electrostatic latent image is formed by exposure scanning with the laser beam L emitted from the toner, the toner is supplied from the developing roller 41K of the developing device 4K to be visualized.
The black toner image thus formed is primarily transferred onto the intermediate transfer belt 21 by the electrostatic force acting between the primary transfer roller 5K and the photosensitive drum 1K. The toner image forming operation by the other color image forming units 20Y, 20M, and 20C is performed in the same manner as described above. The image forming operation in each image forming unit is executed while being shifted by a predetermined timing so that the toner images of the respective colors are multiplex-transferred to the same position on the intermediate transfer belt 21.

  The secondary transfer roller 35 is rotatably supported at the tip of a drive rod (see drive rod 391 in FIG. 7 described later) of the actuator 39 which is not shown in FIG. It is brought into pressure contact with the intermediate transfer belt 21 or separated from the intermediate transfer belt 21. The driving of the actuator 39 is controlled by the control unit 14. As the actuator 39, for example, a solenoid or a linear motion motor can be used, but the actuator 39 is not limited to this. For example, an eccentric cam can be used.

A set of pattern detection sensors for detecting a toner pattern formed on the intermediate transfer belt 21 for generating a γ correction curve at a position along the intermediate transfer belt 21 downstream of the secondary transfer position 351. 25 are arranged on both sides in the width direction of the intermediate transfer belt 21 (direction orthogonal to the running direction of the intermediate transfer belt 21). As the pattern detection sensor 25, for example, a reflective photoelectric sensor that includes a light emitting element such as an LED and a light receiving element such as a photodiode and detects the reflection density of the toner pattern can be used. Here, the reflection density is D represented by the equation D = −log I / I ₀ where I ₀ is the amount of light projected onto the measurement object and I is the amount of light reflected from the measurement object. Say.

When the intermediate transfer belt 21 is made of a translucent material, a transmission type density detection sensor composed of a light emitting part and a light receiving part can be used as the pattern detection sensor 25.
The paper feed unit 12 holds a recording sheet S such as transfer paper, and transports the held recording sheet S to the secondary transfer position 351, and includes a paper feed cassette 31 that stores the recording sheet S From the feeding roller 32 that feeds the recording sheets S in the paper feed cassette 31 one by one onto the conveyance path 37, the intermediate conveyance roller pair 33, and the timing roller pair 34 for taking the timing to send them to the secondary transfer position 351. Become.

  The recording sheet S is conveyed from the paper feeding unit 12 to the secondary transfer position 351 in accordance with the timing of the toner image formation, and an intermediate transfer belt is generated by an electrostatic force acting between the secondary transfer roller 35 and the drive roller 22. The toner image on 21 is secondarily transferred onto the recording sheet S. The recording sheet S to which the toner image has been transferred is thermally fixed by the fixing unit 13 and then discharged onto the discharge tray 38 via the discharge roller pair 36.

  As the image reading unit 15, a known unit that reads a document image with a CCD sensor and generates R, G, B image data is used, and the document placed on the document table is scanned by moving the scanner. There are a mirror scan type and a sheet-through type that reads a document image while moving the document with a document conveying device with a scanner fixed, but there is no particular limitation as long as the document image can be read in color.

(2) Configuration of Control Unit 14 FIG. 2 is a block diagram showing the configuration of the control unit 14. As shown in the figure, the control unit 14 includes, as main components, a CPU 101, a communication interface (I / F) unit 102, a ROM 103, a RAM 104, an image signal processing unit 105, a γ correction data creation unit 106, and a γ correction unit. An LUT storage unit 107, a pattern image storage unit 108, a density conversion LUT storage unit 109, a lowermost layer color specification table storage unit 110, an image data storage unit 111, and the like are provided.

  The communication I / F unit 102 is an interface for connecting to a LAN such as a LAN card or a LAN board. The ROM 103 stores a program necessary for executing an image forming operation, a program for executing a later-described γ correction data creation process, a later-described correction coefficient used in the process, and an later-described image data γ correction process. Stores programs to be executed. The RAM 104 is used as a work area when the CPU 101 executes a program.

  The image signal processing unit 105 converts R, G, and B electrical signals obtained by scanning the document by the image reading unit 15 into digital signals, and outputs image data of reproduction colors of Y, M, C, and K, respectively. Generate. FIG. 3 is a diagram illustrating a configuration of the image signal processing unit 105. As shown in the figure, the image signal processing unit 105 includes an A / D conversion unit 1051, a shading correction unit 1052, a LOG conversion unit 1053, a UCR (under color removal) / BP (black ink printing) processing unit 1054, and a color correction. Unit 1055, γ correction unit 1056, and spatial frequency correction unit 1057.

  The A / D conversion unit 1051 converts the R, G, and B electrical signals (reflectance data) obtained from the image reading unit 15 into R, G, and B multilevel digital signals. The shading correction unit 1052 is provided in order to eliminate the unevenness of irradiation of the exposure lamp of the scanner of the image reading unit 15 and the unevenness of sensitivity of the CCD sensor. Specifically, the shading correction unit 1052 reads a white reference plate placed on the original glass plate during pre-scanning, determines the multiplication ratio of each pixel from the read value of the image data at this time, and stores it in the RAM 104. At the time of reading the document, the image data is corrected by multiplying each image data by the multiplication ratio stored in the RAM 104.

The LOG conversion unit 1053 converts the image data corrected by the shading correction unit 1052 into density data. When image data is received from an external terminal, they are often R, G, and B color density data, so the data in the A / D conversion unit 1051, shading correction unit 1052, and LOG conversion unit 1053 described above. No special treatment is required.
In order to improve black reproduction, the UCR / BP processing unit 1054 calculates the common part of the R, G, and B color data as black data, and calculates the black data from each value of the three color data. The value obtained by subtracting the value is multiplied by a predetermined coefficient and output to the color correction unit 1055.

  The color correction unit 1055 multiplies the input three-color data by a predetermined masking coefficient to generate C, M, and Y color density data. R, G, B and C, M, Y are complementary to each other and should have the same density. Actually, however, the transmission characteristics of the filters R, G, B in the CCD sensor and each toner of the image forming apparatus Since the reflection characteristics of C, M, and Y do not change linearly, the color correction unit 1055 applies linear correction by multiplying the input three-color data by a predetermined masking coefficient to obtain C, M, and Y. Color density data is generated and output to the γ correction unit 1056 together with the K color density data. The γ correction unit 1056 corrects the tone values of the Y, M, C, and K color density data of the image data by performing image data γ correction processing described later. The Y, M, C, and K color density data after correction are stored in the image data storage unit 111 after the spatial frequency correction unit 1057 performs data smoothing or the like.

  Returning to the description of FIG. 2, the γ correction data creation unit 106 performs γ correction data creation processing described later. The γ correction LUT storage unit 107 stores a γ correction LUT created by a γ correction data creation process described later. Here, the γ correction LUT is a conversion of gradation values of document density into correction values so that the read document density and the output image density output as an image satisfy a linear relationship (proportional relationship). This table is created and stored for each color of Y, M, C, and K. FIG. 4 shows a specific example of the γ correction LUT.

  In the γ correction LUT, when the image data of the document is multicolor (image data in which two or more gradation values are larger than 0 among Y, M, C, and K), it is on the intermediate transfer belt 21. The first LUT for γ correction used for the color density data (tone value) of the color excluding the lowermost layer color, and the original image data is a single color (Y, M, C, K, 3 component levels) The second LUT for γ correction used for the color density data of each single color and the color density data of the lowermost color of the multi-color when the tone value is 0). A method of creating the first and second LUTs for γ correction will be described in detail in the explanation of the γ correction data creation process described later.

  Returning to the description of FIG. 2, the pattern image storage unit 108 stores image data (hereinafter, “toner pattern data”) indicating the toner pattern of each color (Y, M, C, and K) for creating a γ correction curve. I remember.) The toner pattern data for each color is composed of image data indicating at least two gradation values. For example, as toner pattern data, image data indicating a toner pattern formed by continuously arranging patches having a predetermined width indicating gradation values of 0, 30, 60, 90, 120, 150, 180, 210, and 240. Can be used.

  Each color toner pattern is formed on the intermediate transfer belt 21 in the sub-scanning direction (direction in which the intermediate transfer belt 21 travels) in γ correction data creation processing, which will be described later, and in the sub-scanning direction of one patch constituting the toner pattern. The width is equal to or greater than the detection width of the pattern detection sensor 25 in the sub-scanning direction, and the density of each patch constituting the toner pattern is detected by the pattern detection sensor 25 while the toner pattern travels on the intermediate transfer belt 21. Is set to the required width. The density conversion LUT storage unit 109 stores a density conversion lookup table (density conversion LUT) indicating the correspondence between the reflection density and the gradation value for each of the colors Y, M, C, and K. The lowest layer color specification table storage unit 110 stores a lowest layer color specification table.

  Here, the “lowermost layer color specification table” is the lowermost layer when a combination of a plurality of types of color density data and the toner of each color density data of the combination are superimposed on the intermediate transfer belt 21. A table indicating the correspondence with toner colors (hereinafter referred to as “lowermost layer color”). FIG. 5 shows a specific example of the lowermost layer color identification table. Returning to the description of FIG. 2, the image data storage unit 111 stores image data in which the gradation value is corrected by the image data γ correction processing.

(3) γ Correction Data Creation Processing FIG. 6 is a flowchart showing the operation of γ correction data creation processing performed by the γ correction data creation unit 106. The γ correction data creation process is performed, for example, when the image forming apparatus 10 is turned on, when a predetermined time elapses after the power is turned on, when a predetermined number of printing processes are completed, or any of the developing devices 4Y to 4K. This is executed when the toner is replaced. FIG. 7A is a diagram showing a positional relationship between the intermediate transfer belt 21 and the secondary transfer roller 35 when the γ correction data creation process is started.

  The γ correction data creation unit 106 controls the actuator 39 so that the intermediate transfer belt 21 and the secondary transfer roller 35 are in a separated state as shown in FIG. 7A when the γ correction data creation process is started. To do. 7A and 7B, reference numerals 1Y to 1K denote photosensitive drums, reference numerals 5Y to 5K denote primary transfer rollers, reference numeral 21 denotes an intermediate transfer belt, reference numeral 22 denotes a driving roller, Reference numeral 23 denotes a driven roller, reference numeral 24 denotes a cleaner, reference numeral 25 denotes a pattern detection sensor, reference numeral 35 denotes a secondary transfer roller, reference numeral 39 denotes an actuator, and reference numeral 391 denotes a drive rod. .

The γ correction data creation unit 106 reads out the toner pattern data of each color from the pattern image storage unit 108 and causes the image processing unit 11 to form a toner pattern based on the toner pattern data of each color (step S601). Specifically, a plurality of toner patterns are formed on the intermediate transfer belt 21.
FIG. 8 shows a specific example of the toner pattern of each color formed on the intermediate transfer belt 21. Reference numeral 21 in the figure denotes the intermediate transfer belt 21, reference numerals Y1, M1, C1, K1, Y2, M2, C2, and K2 denote toner patterns having colors Y, M, C, and K, respectively. Is a non-sheet passing area where the recording sheet S is not passed and the toner image on the intermediate transfer belt 21 is not transferred to the recording sheet S. Reference numeral 212 is an intermediate transfer to the recording sheet S where the recording sheet S is passed. Each of the paper passing regions to which the toner images on the belt 21 are transferred is represented. As shown in the figure, each color toner pattern is formed in a non-sheet passing area 211 on both sides of the intermediate transfer belt 21. Here, with respect to the first, Y1, M1, C1, and K1 toner patterns of each color, the reflection density is detected by the pattern detection sensor 25 in a state where the toner pattern is not secondarily transferred to the secondary transfer roller 35, and two sets of each color are detected. For the toner patterns of the eyes Y2, M2, C2, and K2, the reflection density of the residual toner pattern remaining on the intermediate transfer belt 21 after the secondary transfer to the secondary transfer roller 35 is detected by the pattern detection sensor 25. And

  As shown in FIG. 9, the patches constituting each color toner pattern are arranged at a predetermined interval so as not to overlap the front and rear patches even after the secondary transfer. The numbers in each patch shown in the figure indicate the tone values of the patch. The length of each color toner pattern is shorter than the peripheral length of the secondary transfer roller 35. If the toner pattern is longer than the peripheral length of the secondary transfer roller 35, the toner pattern corresponding to the peripheral length of the secondary transfer roller 35 is secondarily transferred to the secondary transfer roller 35 and then secondary to the secondary transfer roller 35. This is because the transferred toner pattern is retransferred onto the remaining toner pattern on the intermediate transfer belt 21 and the reflection density is not detected correctly.

Returning to the explanation of FIG. 6, the γ correction data creation unit 106 is formed on the intermediate transfer belt 21 by monitoring whether or not a predetermined time has elapsed after the image processing unit 11 is instructed to form the toner pattern. It is then determined whether the first set of toner patterns of each color has passed the secondary transfer position 351 (step S602).
Here, the predetermined time starts from the point of time when the image processing unit 11 is instructed to form the toner pattern, and thereafter, as shown in FIG. It is assumed that all the toner patterns of each color of the set correspond to the time when the time when the toner pattern passes through the secondary transfer position 351 is the end point, and the time is obtained in advance and stored in the ROM 103. Reference numeral 702 in FIG. 7B represents a second set of toner patterns of each color.

  When the first toner pattern of each color passes the secondary transfer position 351 (step S602: YES), the γ correction data creation unit 106, as shown in FIG. 35 is brought into pressure contact with the intermediate transfer belt 21 (step S603), and the detection result of the reflection density of each color toner pattern of the first set and the remaining toner of each color of the second set after the secondary transfer are detected by the pattern detection sensor 25. Pattern reflection density detection results are acquired (steps S604 and S605).

Next, the γ correction data creation unit 106 determines each color based on the correspondence between the reflection density detection result of the patch of each tone value constituting the toner pattern of each color of the first set and the tone value of the patch. A measurement curve is created for (Step S606), and a curve that is an inverse function of each created measurement curve is created as a first γ correction curve (Step S607).
Further, the γ correction data creation unit 106 refers to the density conversion LUT stored in the density conversion LUT storage unit 109 and converts the reflection density of the first γ correction curve for each color into a gradation value. Then, for each color, a first γ correction LUT indicating the correspondence relationship between the gradation value of the input image data (the gradation value of the original density) and the corrected gradation value (correction value) is created. The data is stored in the LUT storage unit 107 (step S608).

  Next, the γ correction data creation unit 106 multiplies the detection result of the reflection density of the patch of each gradation value constituting the residual toner pattern of each color of the second set by a correction coefficient stored in advance in the ROM 103, When the toner pattern of each color is secondarily transferred to the recording sheet instead of the secondary transfer roller 35, it is calculated as an index value of the reflection density of the patch of each gradation value constituting the residual toner pattern of each color (step S609).

  Here, the correction coefficient indicates the toner pattern on the intermediate transfer belt 21 after the secondary transfer when the toner pattern of each color is secondarily transferred to the secondary transfer roller 35 and when it is secondarily transferred to the recording sheet. Remaining densities are measured in advance using the pattern detection sensor 25, and the ratio between the two remaining densities (residual density when secondary transferred to recording sheet / remaining density when secondary transferred to secondary transfer roller 35) Density) can be obtained as a correction coefficient. The correction coefficient may be determined separately for each color toner pattern, or the average value of correction coefficients obtained for each color may be determined uniformly regardless of the color of the toner pattern.

By storing the correction coefficient determined in this way in the ROM 103, it is possible to calculate the index value of the reflection density of the patch of each gradation value constituting the residual toner pattern of each color using the correction coefficient.
A residual toner correction curve is created for each color based on the correspondence relationship between the reflection density index value of the patch of each gradation value constituting the calculated residual toner pattern of each color and the gradation value of the patch (step S610). Further, the γ correction data creation unit 106 calculates a difference between the measurement curve created for each color and the residual toner correction curve created for the color, and creates a difference measurement curve based on the calculated value (step S611). A curve that is an inverse function of the created difference measurement curves is created as a second γ correction curve (step S612).

  Next, the γ correction data creation unit 106 refers to the density conversion LUT for each color stored in the density conversion LUT storage unit 109, and uses the reflection density of the second γ correction curve for each color as a gradation value. By converting, for each color, a second γ correction LUT indicating the correspondence between the gradation value of the input image data (the gradation value of the original density) and the corrected gradation value (correction value) is created, and γ correction is performed. The data is stored in the LUT storage unit 107 (step S613).

  As a result, a γ correction curve is created in a state in which the reflection density of the residual toner that is not secondarily transferred to the secondary transfer roller 35 is subtracted. Therefore, when reproducing an output image based on multicolor image data, To reduce the error of the gradation correction value caused by the residual toner density in the gradation correction using the γ correction curve for the color density data of the lowermost layer color in which a part of the toner remains on the intermediate transfer belt 21. Can do. As a result, it is possible to improve the accuracy of gradation correction in multicolor image data.

  FIG. 10 shows specific examples of a measurement curve, a difference measurement curve, and a residual toner correction curve created in the γ correction data creation process. The vertical axis of the graph shown in the figure represents the reflection density, and the horizontal axis represents the gradation value. In the figure, reference numeral 901 denotes a difference measurement curve, reference numeral 902 denotes a measurement curve, and reference numeral 903 denotes a remaining toner correction curve. The difference measurement curve 901 is created by taking the difference between the measurement curve 902 and the remaining toner correction curve 903.

  FIG. 11 shows a specific example of the second γ correction curve created in the γ correction data creation process. The vertical axis of the graph shown in the figure represents the reflection density, and the horizontal axis represents the gradation value. 10, reference numeral 901 denotes the difference measurement curve shown in FIG. 10, reference numeral 1001 denotes a straight line indicating a target gradation in which the reflection density of the input image and the output image has a linear relationship, and reference numeral 911 denotes the difference measurement curve 901. A second γ correction curve that is an inverse function of is shown. Further, reference numeral 902 in FIG. 11 indicates the measurement curve shown in FIG. 10, and reference numeral 912 indicates a first γ correction curve that is an inverse function of the measurement curve 902.

  As shown in FIG. 11, when the first γ correction curve 912 created without correcting the residual toner density remaining on the intermediate transfer belt 21 during the secondary transfer is used, the required reflection density of the output image is obtained. In contrast, when the second γ correction curve 911 created by correcting the residual toner density is used, the reflection density of the output image to be obtained is y + Δy, which corresponds to the residual toner density. The required reflection density is increased by Δy.

  Therefore, when performing tone correction of the color density data of the single-color or multi-color lowermost layer color in which a part of the toner remains on the intermediate transfer belt 21 at the time of secondary transfer, it is created based on the second γ correction curve. Further, by using the second LUT for γ correction, it is possible to prevent the gradation in the reproduced image from becoming smaller than the desired gradation. As a result, it is possible to effectively prevent a problem that the hue of the output image in which multiple colors are mixed is not reproduced with a desired hue.

(4) Image Data γ Correction Process FIG. 12 is a flowchart showing the operation of the image data γ correction process performed by the γ correction unit 1056. When the Y, M, C, and K color density data is input as image data from the color correction unit 1055 (step S1201), the γ correction unit 1056 is input based on the gradation value indicated by each color density data. It is determined whether the image data is a single color (step S1202).

If it is not a single color (step S1202: NO), the γ correction unit 1056 refers to the lowermost layer color specification table stored in the lowermost layer color specification table storage unit 110, and displays multiple colors ( The lowermost layer color corresponding to the color density data of which the gradation value is not 0 is specified (step S1203).
Further, the γ correction unit 1056 stores the color density data corresponding to the lowermost layer color among the Y, M, C, and K color density data indicated by the image data in the γ correction LUT storage unit 107. Using the second LUT for γ correction, the gradation value is converted into a corresponding correction value to correct the gradation value. For color density data not corresponding to the lowermost layer color, the first LUT for γ correction is used to convert the gradation value. The gradation value is corrected by converting the tone value to the corresponding correction value (step S1204).

If the input image data is a single color in step S1202, the γ correction unit 1056 uses the second γ correction LUT stored in the γ correction LUT storage unit 107 to correspond to the gradation value. The gradation value is corrected by converting into a correction value (step S1205).
Thus, based on the multicolor image data, when a plurality of types of single color toners are superimposed on the intermediate transfer belt to reproduce an output image in which multiple colors are mixed, one of the toners during secondary transfer is reproduced. For the lowermost layer color whose portion remains on the intermediate transfer belt 21, the second LUT for γ correction capable of correcting the gradation value so as to increase the gradation value by the amount corresponding to the remaining toner is used. Tone correction. Therefore, it is possible to perform correction so that the deviation of the gradation value due to the residual toner is smaller than that in the case where the gradation correction is performed using the first γ correction LUT, and the gradation value of the image data of the color of the lowermost layer can be corrected. The accuracy of gradation correction can be improved. As a result, an output image in which multiple colors are mixed can be accurately reproduced with a desired hue.
<Supplement>
As described above, the image forming apparatus according to the present invention has been described based on the embodiment, but the present invention is not limited to this embodiment.

  (1) In the present embodiment, the intermediate transfer belt 21 after the secondary transfer of each toner pattern to the secondary transfer roller 35 is performed on the reflection density of the residual toner pattern in the toner patterns of Y, M, C, and K colors. The reflection density of the toner pattern remaining on the intermediate transfer belt 21 after the secondary transfer of each toner pattern to the recording sheet is detected by measuring the reflection density of the toner pattern remaining on the recording sheet. It is good also as detecting by measuring. In this case, in the γ correction data creation process shown in FIG. 6, the process of correcting the detection value using the correction coefficient (step S609) can be omitted.

  (2) In the present embodiment, in order to detect the reflection density before the secondary transfer in the toner patterns of each color Y, M, C, and K and the reflection density in the residual toner pattern after the secondary transfer, Although two toner patterns are formed for each color, both reflection densities may be detected by one toner pattern. Specifically, the reflection density of the toner pattern before the secondary transfer is detected by the pattern detection sensor 25 in a state where the secondary transfer roller 35 and the cleaner 24 are separated from the intermediate transfer belt 21, and then the secondary transfer roller 35 is pressed against the intermediate transfer belt 21, and the toner pattern on the intermediate transfer belt 21 is circulated to pass through the secondary transfer position 351, and then the reflection density of the residual toner pattern after the secondary transfer is detected by the pattern detection sensor 25. It is good to do.

  Further, a toner pattern of each color is formed in a non-sheet passing region on the intermediate transfer belt 21 during the printing process of the recording sheet, and the reflection density in the residual toner pattern of each color is detected by the pattern detection sensor 25 while executing the printing process. It is good. In this case, the cleaning operation of the toner pattern transferred to the secondary transfer roller 35 is performed between the time when the recording sheet is conveyed to the secondary transfer position 351 and the time when the next recording sheet is conveyed to the secondary transfer position. I will do it. The cleaning operation is performed by applying a reverse bias voltage to the secondary transfer roller 35 during the secondary transfer, and the application of the voltage is controlled by the control unit 14.

Further, the area where the toner pattern of each color is formed is not limited to the non-sheet passing area on the transfer belt 21 and may be formed in the sheet passing area.
(3) The type of image forming apparatus to which the γ correction data creation process and the image data γ correction process according to this embodiment can be applied is not limited to a full-color tandem image forming apparatus. For example, an image forming apparatus disclosed in a patent document (Japanese Patent Laid-Open No. 2000-172043), specifically, a photosensitive drum carrying a latent image and a plurality of developing units carrying and carrying a developer. The present invention can also be applied to a color image forming apparatus (see FIG. 1 of the patent document) in which a plurality of color toner images are formed on a photosensitive drum so as to overlap each other and then transferred onto a transfer material.

  (4) In the present embodiment, the correction coefficient used in step S609 in FIG. 6 is determined regardless of the type of recording sheet. However, the recording coefficient of cardboard, plain paper, high-quality photographic paper, etc. A correction coefficient may be determined for each type. In this case, using the correction coefficient corresponding to each type of recording sheet, the index value of the reflection density of the patch of each gradation value constituting the residual toner pattern of each color is calculated, and the recording value is recorded based on the calculated index value. A residual toner correction curve, a second γ correction curve, and a second γ correction LUT are created for each sheet type, and the second γ correction LUT corresponding to the type of recording sheet on which an image based on input image data is printed is used. Thus, the gradation value correction processing in steps S1204 and S1205 in FIG. 12 may be performed.

(5) In the present embodiment, the reflection density of the remaining toner pattern is detected for each of the colors Y, M, C, and K. However, only one of the colors is detected, and the detected color is detected. A residual toner correction curve common to each color may be created based on the detection result of the residual toner pattern.
Alternatively, the average value of the reflection density of the residual toner pattern detected for each color may be calculated, and a residual toner correction curve common to each color may be created based on the calculation result. Alternatively, the residual toner pattern is not directly detected. For example, the manufacturer of the image forming apparatus 10 detects the reflection density of the residual toner pattern in advance for each color or any one of Y, M, C, and K. A residual toner correction curve may be created based on the detection result and stored in a recording medium such as the ROM 103. In this case, the residual toner correction curve is created by reading out the residual toner correction curve from the recording medium such as the ROM 103 without performing the process of detecting the reflection density of the residual toner pattern, and the γ correction data creation process is executed. be able to.
(6) In this embodiment, a difference measurement curve is created for each color, and the second γ correction LUT is created based on the created difference measurement curve. The method is not limited to the above method. For example, instead of creating a difference measurement curve for each color, a γ correction curve of the remaining toner correction curve is created by taking an inverse function of the remaining toner correction curve created for each color, and the first γ correction curve and the remaining toner correction curve are created. The difference γ correction curve may be taken to create a difference γ correction curve, and the second γ correction LUT may be created based on the created difference γ correction curve.

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or printer, and in particular, can be used as a technique for correcting gradation of an output image in the image forming apparatus.

1Y to 1K Photosensitive drums 2Y to 2K Chargers 3Y to 3K Exposure units 4Y to 4K Developers 5Y to 5K Primary transfer roller 10 Image forming apparatus 11 Image processing unit 12 Paper feeding unit 13 Fixing unit 14 Control unit 15 Image reading unit 20Y to 20K Image forming unit 21 Intermediate transfer belt 22 Drive roller 23 Driven roller 24 Cleaner 25 Pattern detection sensor 32 Feeding roller 33 Intermediate conveyance roller pair 34 Timing roller pair 35 Secondary transfer roller 36 Discharge roller pair 37 Conveyance path 38 Discharge tray 39 Actuator 41Y-41K Developing roller 101 CPU
102 Communication interface (I / F) section 103 ROM
104 RAM
105 Image signal processing unit 106 γ correction data creation unit 107 γ correction LUT storage unit 108 Pattern image storage unit 109 Density conversion LUT storage unit 110 Lowermost layer color specification table storage unit 111 Image data storage unit

Claims (10)

Each color formed so that the tone of the input image data is corrected based on the density of the single color toner pattern formed on the image carrier, and is superimposed on the image carrier based on the image data after the tone correction An image forming apparatus for transferring the toner image to a recording sheet to form a color image,
Forming means for forming a monochrome toner pattern for gradation correction on the image carrier;
Obtaining means for obtaining a density of the toner pattern before transfer on the image carrier and a residual density of the toner pattern remaining on the image carrier after the toner pattern is transferred;
First creating means for creating a first gradation correction table for correcting the gradation value of the input image data based on the density value of the toner pattern before transfer;
A second value for correcting the gradation value of the input image data so that the gradation value becomes larger by an amount corresponding to the remaining density than when the gradation value is corrected using the first gradation correction table. A second creating means for creating a gradation correction table based on the density value of the toner pattern before transfer and the residual density;
When multi-color image data is input, the tone value of the color density data excluding the color superimposed on the lowest layer on the image carrier among the color density data indicated by the image data is the first density value. Correction means for correcting using the gradation correction table and correcting the gradation value of the color density data of the color of the lowest layer using the second gradation correction table;
An image forming apparatus comprising: The image carrier is an intermediate transfer belt,
A secondary transfer roller for transferring the toner image on the intermediate transfer belt to a recording sheet;
The acquisition means includes
Detection means for detecting the density of the toner pattern before transfer on the intermediate transfer belt and the residual density of the toner pattern remaining on the intermediate transfer belt after the toner pattern is transferred to the secondary transfer roller When,
Determining means for determining the residual density of the toner pattern when the toner pattern is transferred to the recording sheet based on the detected residual density;
A residual concentration acquisition means for acquiring the determined residual concentration;
The image forming apparatus according to claim 1, further comprising: The second creating means includes
The density value of the toner pattern before transfer is corrected to a value obtained by subtracting the residual density, and the second gradation correction table is created based on the correction value. The image forming apparatus described in 1. The correction means corrects the gradation value of color density data indicated by the image data using the second gradation correction table when monochrome image data is input. The image forming apparatus according to any one of? The image forming apparatus according to claim 1, wherein the toner pattern is formed in a non-sheet passing area other than a sheet passing area through which the recording sheet is passed on the image carrier. . Each color formed so that the tone of the input image data is corrected based on the density of the single color toner pattern formed on the image carrier, and is superimposed on the image carrier based on the image data after the tone correction The toner image is transferred to a recording sheet to form a color image, and an image forming method in an image forming apparatus,
Forming a monochrome toner pattern for gradation correction on the image carrier;
Obtaining the density of the toner pattern before transfer on the image carrier and the residual density of the toner pattern remaining on the image carrier after transferring the toner pattern;
A first creation step of creating a first tone correction table for correcting the tone value of the input image data based on the density value of the toner pattern before transfer;
A second value for correcting the gradation value of the input image data so that the gradation value becomes larger by an amount corresponding to the remaining density than when the gradation value is corrected using the first gradation correction table. A second creation step of creating a gradation correction table based on the density value of the toner pattern before transfer and the residual density;
When multi-color image data is input, the tone value of the color density data excluding the color superimposed on the lowest layer on the image carrier among the color density data indicated by the image data is the first density value. A correction step of correcting using the gradation correction table and correcting the gradation value of the color density data of the color of the lowest layer using the second gradation correction table;
An image forming method comprising: The image carrier is an intermediate transfer belt,
A secondary transfer roller for transferring the toner image on the intermediate transfer belt to a recording sheet;
The obtaining step includes
Detection step of detecting the density of the toner pattern before transfer on the intermediate transfer belt and the residual density of the toner pattern remaining on the intermediate transfer belt after the toner pattern is transferred to the secondary transfer roller. When,
A determination step of determining a residual density of the toner pattern when the toner pattern is transferred to a recording sheet based on the detected residual density;
A residual concentration acquisition step of acquiring the determined residual concentration;
The image forming method according to claim 6, further comprising: In the second creation step,
The density value of the toner pattern before transfer is corrected to a value obtained by subtracting the residual density, and the second gradation correction table is created based on the correction value. The image forming method described in 1. 7. In the correction step, when monochrome image data is input, a gradation value of color density data indicated by the image data is corrected using the second gradation correction table. The image forming method according to any one of -8. The image forming method according to claim 6, wherein the toner pattern is formed in a non-sheet passing area other than a sheet passing area through which a recording sheet is passed on the image carrier. .

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