JP2017198973A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of determining whether calibration for adjusting colors of a mixed-color image is needed.SOLUTION: An image forming apparatus has: a 4DLUT 312; an image forming part which forms images of different colors on a recording medium; a reader A which reads a pattern image formed on the recording medium; an intermediate transfer belt to which a patch image formed by the image forming part is transferred; a density sensor 126 which detects the patch image; and a CPU 301 which carries out density adjustment calibration using the reader A to generate an LUTa 306, and carries out color adjustment calibration using the reader A to generate the 4DLUT 312. The CPU 301 allows the image forming part to form the patch image after the density adjustment calibration is carried out, and determines whether the color adjustment calibration is carried out based upon a last detection result and a current detection result of the density sensor 126.SELECTED DRAWING: Figure 6

Description

  The present invention relates to calibration of an image forming apparatus.

  An electrophotographic image forming apparatus charges a photoconductor, exposes the photoconductor to form an electrostatic latent image, and develops the electrostatic latent image on the photoconductor using a developer. An image is formed on. Then, the image forming apparatus transfers the image on the photoreceptor to a recording medium and outputs a printed matter.

  In recent years, the performance of electrophotographic image forming apparatuses has improved. Among recent electrophotographic image forming apparatuses, there is also an image forming apparatus that outputs a printed material having an image quality equivalent to that of a printed material printed by an offset printer.

  However, the electrophotographic image forming apparatus has instability peculiar to the electrophotographic system. Therefore, the color variation amount of the printed matter of the electrophotographic image forming apparatus is larger than the color variation amount of the printed matter of the offset printing machine. Therefore, an electrophotographic image forming apparatus forms a test image on a recording medium, detects the test image by a sensor, and sets the image forming conditions so that the image density becomes a target density based on the test image detection result. Is controlling.

  An electrophotographic image forming apparatus forms a full-color image by transferring yellow, magenta, cyan, and black images on an intermediate transfer member. However, even if the image forming conditions are controlled based on the detection result of the single color test image, there is a possibility that the color of the mixed color image does not become the target color. This is because a retransfer phenomenon occurs.

  FIG. 1 is a cross-sectional view of a main part showing a transfer nip portion for transferring an image on a photosensitive member to an intermediate transfer member. As shown in FIG. 1, when the charging polarity of the developer transferred to the intermediate transfer member is reversed, the developer having the opposite polarity is transferred from the intermediate transfer member to the photosensitive member. This is called a retransfer phenomenon. When the retransfer phenomenon occurs, the amount of the developer on the intermediate transfer member is reduced, so that the density of the output image is lowered.

  The amount of developer retransferred from the intermediate transfer member to the photoreceptor (retransfer amount) is affected by the charge amount of the developer. If the charge amount of the developer increases, the charge polarity of the developer is difficult to reverse at the transfer nip portion. Therefore, the retransfer amount decreases as the developer charge amount increases. On the other hand, as shown in FIG. 3, if the charge amount of the developer is decreased, the charge polarity of the developer is easily reversed at the transfer nip portion. Therefore, the retransfer amount increases if the charge amount of the developer decreases. That is, if the charge amount of the developer changes, the density of the output image also changes. Further, the charge amount of the developer varies depending on the surrounding environment, consumption amount, replenishment amount, stirring time, and standing time. Therefore, when the charge amount of the developer changes significantly, it is necessary to adjust the image forming conditions of the image forming apparatus by executing calibration.

  Next, the relationship between the color variation of the mixed-color image and the retransfer phenomenon described above will be described. For example, when adjusting the density of a magenta monochrome image, the magenta test image passes through the cyan transfer nip and the black transfer nip. Therefore, when the magenta image forming conditions are controlled based on the magenta test image, the image forming conditions are adjusted in consideration of the retransfer amount. That is, the magenta image forming conditions are set so that the amount of developer transferred from the photosensitive member to the intermediate transfer member is increased by the retransfer amount.

  On the other hand, as shown in FIG. 4, when adjusting the color of a mixed image of magenta and cyan, a cyan image is transferred onto the magenta image, so that magenta is retransferred from the intermediate transfer member to the cyan photosensitive member. The amount of developer is reduced. For example, when a cyan developer is uniformly transferred to a magenta image, the amount of magenta developer transferred from the intermediate transfer member to the cyan photosensitive member is almost zero.

  As described above, since the retransfer amount is affected by the charge amount of the developer, when the charge amount changes, the correspondence between the developer amount of the single color image and the developer amount of the mixed color image also changes. End up. Therefore, even if the density of the monochromatic image is the target density, the color of the mixed color image may not be the target color.

  Therefore, a method for generating a conversion condition for converting image data so that the color of the mixed color image becomes a target color is known (Patent Document 1). In the method described in Patent Document 1, a mixed color pattern image is formed on a recording medium, the pattern image is detected by a sensor, and the combination of image signal values of each color component is changed to a combination of image signal values corresponding to the target color. The conversion condition to be generated is generated based on the detection result.

JP 2011-254350 A

  However, if the amount of change in the charge amount of the developer is small, there is a high possibility that the color of the mixed color image is adjusted to the target color by adjusting the density of the single color image to the target density. Even if the amount of change in the charge amount of the developer is small, if both the calibration for adjusting the density of the monochrome image and the calibration for generating the conversion condition are executed, the downtime increases. .

  Therefore, an object of the present invention is to determine whether or not calibration for adjusting the color of a mixed-color image is necessary.

In order to solve the above-described problem, an image forming apparatus according to claim 1 includes a conversion unit that converts image data based on conversion conditions, an image formation unit that forms images of different colors on a recording medium, Reading means for reading a pattern image formed on a medium; causing the image forming means to form a monochromatic pattern image on a recording medium; causing the reading means to read the monochromatic pattern image; and reading result of the monochromatic pattern image A first generation unit that generates an image forming condition based on the image forming unit, and the image forming unit forms a mixed color pattern image on a recording medium, the reading unit reads the mixed color pattern image, and the mixed color pattern image A second generation unit that generates the conversion condition based on a reading result; an intermediate transfer body to which a measurement image formed by the image forming unit is transferred;
A measurement unit that measures the measurement image on the intermediate transfer member; and after the image forming condition is generated by the first generation unit, the image formation unit forms the measurement image, and the measurement unit Control means for measuring the measurement image and controlling whether the second generation means generates the conversion condition based on the measurement result of the previous measurement image and the measurement result of the current measurement image The image forming means forms an output image based on the image data converted by the converting means.

  In order to solve the above problem, an image forming apparatus according to another claim includes a conversion unit that converts image data based on a conversion condition, an image formation unit that forms an image of a different color on a recording medium, Reading means for reading a pattern image formed on a recording medium; and causing the image forming means to form a monochromatic pattern image on a recording medium; causing the reading means to read the monochromatic pattern image; and reading the monochromatic pattern image A first generation unit configured to generate an image forming condition based on the result; and the image forming unit forms a mixed color pattern image on a recording medium; the reading unit reads the mixed color pattern image; and the mixed color pattern image A second generation unit that generates the conversion condition based on the read result of the image, an intermediate transfer body to which a measurement image formed by the image forming unit is transferred, and A measurement unit that measures the measurement image on the intermediary transfer member; and after the image generation condition is generated by the first generation unit, the image formation unit forms the measurement image, and the measurement unit causes the measurement unit to And a notifying unit for notifying the update timing of the conversion condition based on the measurement result of the previous measurement image and the measurement result of the current measurement image.

  According to the present invention, it is possible to determine whether or not calibration for adjusting the color of a mixed color image is necessary.

Cross-sectional view of the main part of the transfer nip part of the image forming apparatus Schematic sectional view of the image forming apparatus Diagram showing the relationship between developer charge amount and retransfer amount Schematic diagram showing the state of the image transferred to the intermediate transfer member Control block diagram of reader image processing unit Control block diagram of image forming apparatus The flowchart figure which shows the 1st calibration Graph showing the correspondence between contrast potential and image density A graph showing the correspondence between the surface potential of the photosensitive drum and the grid potential supplied to the charger 122 Schematic diagram of characteristic conversion chart The flowchart figure which shows 2nd calibration Flowchart diagram showing color adjustment calibration Schematic diagram of test print 3 Enlarged view of the main parts of the L * a * b * coordinate system Flowchart diagram showing determination processing Schematic diagram of lookup table The flowchart figure which shows another judgment processing and patch gradation control Graph showing correspondence between image signal and patch image density The figure for explaining the creation method of the correction characteristic table Schematic sectional view of a modification of the image forming apparatus Cross section of the color sensor The flowchart figure which shows another judgment processing

Example 1
An embodiment applied to an electrophotographic color copier will be described below. The present invention is applicable to any image forming apparatus that requires calibration. The image forming apparatus may be commercialized as, for example, a printing apparatus, a printer, a copier, a multifunction machine, or a facsimile. The recording medium may also be called recording paper, recording material, paper, sheet, transfer material, or transfer paper. Further, the material of the recording medium may be paper, fiber, film or resin.

  FIG. 2 is a schematic sectional view of the image forming apparatus 100. The image forming apparatus 100 includes a reader A, a printer B that forms an image on a recording medium, and an operation unit 217.

  The reader A includes an original table glass 102, a light source 103, an optical system 104, a CCD sensor 105, and a reference white plate 106. The reader A is controlled by the CPU 301 (FIG. 6). The light source 103 irradiates the document 101 placed on the document table glass 102 with light. Reflected light from the document forms an image on the CCD sensor 105 via the optical system 104. The light source 103, the optical system 104, and the CCD sensor 105 are housed in a carriage, and the carriage moves in the direction of the arrow K1. As a result, the CCD sensor 105 reads an image of a document for one page. That is, the reader A functions as a reading unit that reads the document 101 placed on the document table glass 102. The CCD sensor 105 transfers an electrical signal corresponding to the document image to the reader image processing unit 108. The reader image processing unit 108 generates an image signal based on the electrical signal. The reference white plate 106 is read by the reader A in order to perform shading correction on the reading result of the reader A. Since shading correction is a known technique, its description is omitted.

  The printer B includes image forming units 120, 130, 140, and 150, an intermediate transfer belt 113, a secondary transfer device 112, and a fixing device 114. Further, the printer unit B includes a density sensor 126 that measures a measurement image formed on the intermediate transfer belt 113. The image forming unit 120 includes a photosensitive drum 121, a charger 122, a developing device 123, a primary transfer device 124, and a laser driver 309 (hereinafter referred to as LD 309). Further, the image forming unit 120 includes a surface potential sensor 125 that measures the surface potential of the photosensitive drum 121. The surface potential measured by the surface potential sensor 125 is used to adjust the contrast potential.

  The image forming unit 120 forms a yellow (Y) image, the image forming unit 130 forms a magenta (M) image, the image forming unit 140 forms a cyan (C) image, and the image forming unit 150 A black (K) image is formed. The configurations of the image forming units 120, 130, 140, and 150 are substantially the same. Hereinafter, the configuration of the image forming unit 120 that forms a yellow image will be described.

  A photosensitive layer is formed on the surface of the photosensitive drum 121. The photosensitive drum functions as a photosensitive member. The photosensitive drum 121 is rotated by a motor (not shown). The charger 122 uniformly charges the surface of the photosensitive drum 121. The LD 309 exposes the photosensitive drum 121 charged by the charger 122 to form an electrostatic latent image. The developing device 123 develops the electrostatic latent image on the photosensitive drum 121 to form an image. The primary transfer unit 124 transfers the image on the photosensitive drum 121 to the intermediate transfer belt 113. The intermediate transfer belt 113 functions as an intermediate transfer body to which an image is transferred.

  Further, the image forming units 120, 130, 140, and 150 transfer the images so that the images of the respective colors overlap on the intermediate transfer belt 113. As a result, a full color image is carried on the intermediate transfer belt 113. The intermediate transfer belt 113 conveys the image to the secondary transfer unit 112. The secondary transfer unit 112 transfers the image on the intermediate transfer belt 113 to a recording medium. The recording medium to which the image has been transferred is conveyed toward the fixing device 114. The fixing device 114 includes a roller pair having a heater (not shown). The fixing device heats the recording medium by a pair of rollers and applies pressure to the recording medium to fix the image on the recording medium. The recording medium on which the image is fixed is discharged from the image forming apparatus 100 by a roller (not shown).

  The density sensor 126 includes an LED that emits light to the intermediate transfer belt 113 and a photodiode that receives reflected light from the intermediate transfer belt 113. The density sensor 126 measures reflected light from the measurement image formed on the intermediate transfer belt 113. The operation unit 217 includes a start button for causing the reader A to start reading a document, a numeric keypad, and a display 218. The display 218 displays, for example, the number of prints, print settings, or the state of the image forming apparatus 100.

  FIG. 5 is a control block diagram of the reader image processing unit 108. The CCD / AP circuit board 201 includes an analog image processing unit 202 and an A / D conversion unit 203. The analog image processing unit 202 amplifies the image signal transferred from the CCD sensor 105. The A / D conversion unit 203 converts the image signal from an analog value to a digital value. The CCD / AP circuit board 201 transfers the image signal to the reader controller circuit board 210. The reader controller circuit board 210 includes a CPU 211 and a shading processing unit 212. The CPU 211 controls the shading processing unit 212 to execute shading correction on the image signal. Then, the CPU 211 transfers the image signal to the printer control unit 109. The image signal output from the reader controller circuit board 210 is composed of luminance information of red (R), green (G), and blue (B).

  The printer control unit 109 generates a laser output signal based on the image signal transferred from the reader image processing unit 108. The printer control unit 109 outputs a laser output signal to the LD 309. The LD 309 controls the semiconductor laser 310 (FIG. 6) based on the laser output signal. The semiconductor laser 310 (FIG. 6) emits a laser beam based on the laser output signal. The laser beam is deflected by a polygon mirror and scans the photosensitive drum 121. Thereby, an electrostatic latent image is formed on the photosensitive drum 121. That is, the LD 309 exposes the photosensitive drum 121 based on the image signal in order to form an electrostatic latent image on the photosensitive drum 121.

  FIG. 6 is a control block diagram of the image forming apparatus 100 to which the printer server C is connected via a network. In FIG. 6, the control block diagram of the reader A shown in FIG. 5 is omitted. The CPU 301 comprehensively controls each unit of the image forming apparatus 100. The memory 302 is a ROM or RAM, and stores a control program and various data.

  The image signal processed by the reader A or the print server C is input to the color processing unit 303 of the printer control unit 109. The color processing unit 303 converts the luminance information of red (R), green (G), and blue (B) into yellow (Y), magenta (M), cyan (C), and black (K) image signals. Convert. The image signal output from the color processing unit 303 includes, for example, a 10-bit digital signal value. LUTid 304 is a luminance density conversion table for converting luminance information included in the image signal output from the reader A into density information. When the image forming apparatus 100 executes calibration based on the reading result of the reader A, the LUTid 304 of the color processing unit 303 converts the reading result of the reader A into density information. The density information and yellow (Y), magenta (M), cyan (C), and black (K) image signals are different types of parameters. The density information corresponding to the maximum density pattern image is, for example, 1.6.

  The gradation control unit 311 includes a four-dimensional LUT 312, a UCR unit 305, and an LUTa 306. The gradation control unit 311 corrects the image signal in order to correct the gradation characteristic of the output image formed by the printer B to an ideal gradation characteristic.

  The 4DLUT 312 is a color conversion table for correcting a four-dimensional color space of yellow, magenta, cyan, and black. The 4DLUT 312 is generated by the CPU 301 in color adjustment calibration described later. The 4DLUT 312 corresponds to a conversion condition for converting a combination of image signal values of yellow, magenta, cyan, and black.

  The UCR unit 305 executes an under color removal process (Under Color Removable Process) in order to suppress the adhesion amount of the developer on the image on the recording medium. Under color removal processing means that the sum of the image signal values of cyan, magenta, and yellow is the black image so that the color of the image does not change so that the integrated value of the image signal in each pixel is less than the specified value. This is a process of replacing with a signal.

  The LUTa 306 is a one-dimensional lookup table (hereinafter referred to as LUT) that converts image signal values in order to correct gradation characteristics (density characteristics) of an output image formed by the printer B. The LUTa 306 converts the image signal value of each color. For this reason, the LUTa 306 includes four types, that is, a yellow LUT, a magenta LUT, a cyan LUT, and a black LUT. The LUTa 306 is generated by the CPU 301 in density adjustment calibration described later.

  The dither processing unit 307 performs dither processing on the image signal output from the gradation control unit 311. As a result, the image signal is converted into a 4-bit digital signal. The PWM unit 308 generates a laser output signal for controlling the semiconductor laser 310 based on the image signal. The LD 309 controls the semiconductor laser 310 based on the laser output signal generated by the PWM unit 308.

  The A / D conversion circuit 181 converts the analog signal output from the density sensor 126 into a digital signal. The density conversion circuit 182 converts the digital signal output from the A / D conversion circuit 181 into an optical density value. The CPU 301 controls the image forming units 120, 130, 140, and 150 to form the measurement image on the intermediate transfer belt 113, causes the density sensor 126 to measure the measurement image, and based on the measurement result, the density of the measurement image. Get the value. Then, the CPU 301 can obtain the gradation characteristics of the printer B based on the density value of the measurement image.

  Next, density adjustment calibration and color adjustment calibration executed by the CPU 301 of the image forming apparatus 100 will be described. When the density adjustment calibration is executed, the CPU 301 determines an image forming condition such that the density of the monochromatic image becomes the target density based on the measurement result of the pattern image. On the other hand, when the color adjustment calibration is executed, the CPU 301 generates the 4DLUT 312 so that the color of the mixed color image becomes the target color.

  In the density adjustment calibration, the first calibration for controlling the contrast potential so that the maximum density of the output image becomes the target maximum density, and the LUTa 306 so that the gradation characteristics of the output image become ideal gradation characteristics. And a second calibration for generating. When the density adjustment calibration is executed, the CPU 301 first executes the first calibration, and then executes the second calibration after the contrast potential is controlled.

  When a command for executing density adjustment calibration is input from the operation unit 217, the CPU 301 executes the first calibration shown in the flowchart of FIG. Then, after the first calibration process is finished, the CPU 301 executes the second calibration shown in the flowchart of FIG. The processing of the flowcharts shown in FIGS. 7 and 11 is executed when the CPU 301 reads a program stored in the memory 302.

  Hereinafter, density adjustment calibration performed by the image forming apparatus 100 will be described with reference to FIGS.

I. First Calibration First, the CPU 301 controls the printer B to output the test print 1 and measures the surface potential of the photosensitive drum (S101).

  In step S <b> 101, the CPU 301 inputs the first pattern image data to the printer control unit 109 and controls the printer B to form the first pattern image on the recording medium. Hereinafter, the recording medium on which the first pattern image is formed is referred to as test print 1. Note that the contrast potential when the printer B outputs the test print 1 is set to a value corresponding to the environmental information. The environmental information is, for example, the absolute water content around the image forming apparatus 100. For example, the CPU 301 sets an initial value of the contrast potential from the absolute moisture amount detected by a sensor (not shown) with reference to the correspondence relationship between the absolute moisture amount and the contrast potential. Here, the correspondence between the absolute water content and the contrast potential is stored in the memory 302 in advance.

  The first pattern image includes, for example, a yellow, magenta, cyan, and black band pattern and a maximum density pattern of yellow, magenta, cyan, and black. The band pattern is formed based on, for example, 128 image signal values. The maximum density pattern is formed based on, for example, 255 image signal values.

  In step S101, the CPU 301 measures the surface potential of the photosensitive drum. The CPU 301 causes the surface potential sensor 125 to measure the surface potential of the photosensitive drum 121 corresponding to the region where the electrostatic latent image having the yellow maximum density pattern is formed. The CPU 301 measures the surface potentials of the photosensitive drums of other colors in the same manner. The CPU 301 stores the measurement result of the surface potential of the photosensitive drum corresponding to the area where the electrostatic latent image having the maximum density pattern is formed in the memory 302.

  The test print 1 is placed on the platen glass 102 by the user. If the user presses the start button of the operation unit 217, the CPU 301 controls the reader A to read the test print 1 (S102). In step S102, the CPU 301 converts the luminance information of the first pattern image into the optical density of the first pattern image using the LUTid 304.

  Next, the CPU 301 calculates a contrast potential b corresponding to the target maximum density (S103). The contrast potential is a potential difference between the surface potential of the photosensitive drum corresponding to the area where the electrostatic latent image is formed and the developing bias used to develop the electrostatic latent image. The CPU 301 determines the contrast potential a from the difference between the surface potential of the photosensitive drum corresponding to the area where the electrostatic latent image having the maximum density pattern is formed and the developing bias used to develop the maximum density pattern.

  Here, FIG. 8 is a graph showing the correspondence between the contrast potential and the image density. In FIG. 8, the horizontal axis represents the contrast potential, and the vertical axis represents the image density. As shown in FIG. 8, the relationship between the contrast potential and the image density is linear as shown by the solid line L in the range of the image density 0.8 to 2.0 including the target maximum density. Therefore, the CPU 301 predicts a contrast potential b for forming an image having a target maximum density based on the contrast potential a of the maximum density pattern and the density Da of the maximum density pattern. The target maximum density is 1.6.

The CPU 301 calculates the contrast potential b corresponding to the target maximum density based on the solid line L. A table or function corresponding to the solid line L is stored in the memory 302 in advance. The CPU 301 calculates the contrast potential b using, for example, Expression (1).
b = (a + ka) × 1.6 / Da (1)

Here, ka is a correction coefficient, and is determined by the type of development method.

  Next, the CPU 301 sets a grid potential Vg and a developing bias potential Vds from the contrast potential b (S104). FIG. 9 is a graph showing the correspondence between the surface potential of the photosensitive drum and the grid potential supplied to the charger 122. In step S104, the CPU 301 controls the grid potential and the light emission intensity of the semiconductor laser 310 to predetermined conditions to form an electrostatic latent image on the photosensitive drum, and the surface potential sensor measures the area corresponding to the electrostatic latent image. The correspondence shown in FIG.

  First, the CPU 301 sets the grid potential Vg to −300 V, controls the semiconductor laser 310 based on the laser output signal of the minimum light emission level, and causes the surface potential sensor to measure the surface potential Vd1 of the photosensitive drum. Further, the CPU 301 sets the grid potential Vg to −300 V, controls each color semiconductor laser 310 based on the laser output signal of the maximum light emission level, and causes the surface potential sensor to measure the surface potential Vl1 of the photosensitive drum. The CPU 301 sets the grid potential Vg to −700 V, controls the semiconductor laser 310 in the same manner as described above, and measures Vd2 and Vl2 as the surface electricity of the photosensitive drum.

  Then, the CPU 301 obtains the correspondence between the grid potential and the photosensitive drum surface potential by linearly interpolating or extrapolating the surface potentials Vd1, Vd2, Vl1, and Vl2. As shown in FIG. 9, the contrast potential Vcont is a difference between the developing bias Vdc and the surface potential Vl. Here, the developing bias Vdc is set to a value larger than the surface potential Vd by a predetermined value Vback. The predetermined value Vback is, for example, 150V. Therefore, the CPU 301 obtains the grid potential and the developing bias that become the contrast potential b based on the correspondence relationship shown in FIG. The predetermined value Vback is a value set so that the developer does not adhere to a region on the photosensitive drum where the laser beam is not irradiated.

II. Second Calibration FIG. 10 is a characteristic conversion chart. The I region indicates the characteristics of the reader A that converts the document density into a density signal. Region II shows the characteristics of the gradation control unit 311 (LUTa 306) for converting the density signal into a laser output signal. Region III shows the gradation characteristics of printer B that converts laser output signals into output densities. The IV region shows the correspondence between the document density and the output density. The correspondence between the document density and the output density represents the overall gradation characteristics of the image forming apparatus 100.

  The image forming apparatus 100 corrects the gradation characteristics of the printer B shown in the III region in order to correct the gradation characteristics of the printer B to the ideal gradation characteristics. In the second calibration, the printer B forms a pattern image without causing the gradation control unit 311 to perform gradation correction, a laser output signal for forming this pattern image, and a laser having an ideal density. An LUTa 306 is generated by replacing the output signal.

  Hereinafter, the second calibration will be described with reference to FIG. The CPU 301 controls the printer B and outputs a test print 2 (S201). In step S201, the CPU 301 inputs the second pattern image data to the printer control unit 109, and controls the printer B to form a second pattern image on the recording medium. At this time, the CPU 301 causes the printer B to form the second pattern image without causing the LUTa 306 to convert the second pattern image data. Hereinafter, the recording medium on which the second pattern image is formed is referred to as test print 2. Note that the values obtained in the first calibration are set as the grid potential and the developing bias when the printer B outputs the test print 2.

  The test print 2 includes, for example, a pattern image in which the density is changed to 64 levels (64 gradations) for each color. Here, in the second pattern image, the number of pattern images in the low density region is larger than the number of pattern images in the high density region. Thereby, the gradation characteristics of the low density region can be adjusted favorably.

  The test print 2 may be configured to include, for example, a low-resolution second pattern image and a high-resolution second pattern image. The low resolution is, for example, a range from 160 lpi to 180 lpi. The high resolution is, for example, a range from 250 lpi to 300 lpi. The dither processing unit 307 performs dither processing for low resolution and dither processing for high resolution on the second pattern image data. Here, the dither processing for low resolution is suitable for a gradation image, and the dither processing for high resolution is suitable for a character image. Even if the image signal values are the same, there is a possibility that the densities of images that have been subjected to different dithering processing have different values. Therefore, the CPU 301 may control the image forming condition corresponding to the type of dither processing based on the reading result of the second pattern image corresponding to each different dither processing on the test print 2. Note that when the gradation characteristics are remarkably different for each resolution, the second pattern image data may be set to different data for each resolution.

  Further, when the printer B can form images having three or more types of resolutions, the printer B may output the test print 2 by dividing it into a plurality of pages.

  The test print 2 is placed on the platen glass 102 by the user. When the user presses the start button on the operation unit 217, the CPU 301 controls the reader A to read the test print 2 (S202). In step S202, the CPU 301 converts the luminance information of the second pattern image into the optical density of the second pattern image using the LUTid 304.

  Next, the CPU 301 generates an LUTa 306 (S203). In step S203, first, the CPU 301 obtains the gradation characteristics of the printer B based on the image signal value of the second pattern image data and the density value of the second pattern image. Here, in order to convert the density Di of the image signal value i into the target density Ditgt, the image signal value i may be converted into the image signal value itgt corresponding to the target density Ditgt of the image signal value i. The CPU 301 generates a table for converting the image signal value i into the image signal value itgt. A table for converting the image signal value i into the image signal value itgt corresponds to the LUTa 306. The CPU 301 generates the LUTa 306 based on a table for converting the image signal value i into the image signal value itgt.

  Note that the second pattern image has only a 64-tone pattern image. Since the number of gradations of the image formed by the printer B is 256, the density data of the pattern image is insufficient to obtain the LUTa 306. Therefore, the CPU 301 calculates the missing density data by linear interpolation. As a result, the CPU 301 can obtain the gradation characteristics of the printer B in the range of 256 gradations.

  The CPU 301 executes density adjustment calibration to correct fluctuations in the image density of the monochrome image and the gradation characteristics of the monochrome image.

III. Color Adjustment Calibration Next, the CPU 301 will be described with reference to FIGS. 12 to 14 for color adjustment calibration for generating the 4DLUT 312 so that the color of the mixed color image becomes the target color. In the color adjustment calibration, a pattern image of a mixed color image is formed on a recording medium, and a 4DLUT 312 for converting a combination of image signal values of yellow, magenta, cyan, and black so that the color of the pattern image becomes a target color. Is generated.

  When a command for executing color adjustment calibration is input from the operation unit 217, the CPU 301 executes color adjustment calibration shown in the flowchart of FIG. The process of the flowchart shown in FIG. 12 is executed when the CPU 301 reads a program stored in the memory 302.

  First, the CPU 301 controls the printer B to output the test print 3 (S301). In step S301, the CPU 301 inputs the third pattern image data to the printer control unit 109, and controls the printer B to form the third pattern image on the recording medium. At this time, the CPU 301 causes the LUTa 306 to convert the third pattern image data, and causes the printer B to form a third pattern image based on the converted third pattern image data. Hereinafter, the recording medium on which the third pattern image is formed is referred to as a test print 3. Note that the values obtained in the first calibration are set as the grid potential and the developing bias when the printer B outputs the test print 3.

  FIG. 13 is a schematic diagram of the test print 3. The test print 3 includes a mixed color pattern image of yellow and magenta, a mixed color pattern image of yellow and cyan, and a mixed color pattern image of magenta and cyan. The test print 3 may further include a mixed color pattern image of black and chromatic colors. Alternatively, the test print 3 may further include a mixed color pattern image of yellow, magenta, and cyan.

  The mixed color pattern image of yellow and magenta shown in FIG. 13 is formed using, for example, a plurality of levels of yellow image signal values and a plurality of levels of magenta image signal values. The number of mixed color pattern images may be an appropriate number for generating the 4DLUT 312.

  The test print 3 is placed on the platen glass 102 by the user. If the user presses the start button of the operation unit 217, the CPU 301 controls the reader A to read the test print 3 (S302). In step S302, the CPU 301 acquires luminance information of the mixed color pattern image. The CPU 301 converts the luminance information (RGB luminance data) of the mixed color pattern image into color data (L * a * b * data) (S303). Here, L * a * b * data is data representing a color in a device-independent color space. The L * a * b * data includes three values. The L * a * b * data is a parameter (a * and b *) representing the luminance L *, hue, and saturation.

  The CPU 301 generates the 4DLUT 312 so that the difference (color difference) between the L *, a *, and b * values of the mixed color pattern image and the target value is less than a predetermined value (S304). Here, FIG. 14 shows the coordinates of a * and b * at a predetermined luminance in the L * a * b * color space. In step S304, the CPU 301 generates the 4DLUT 312 so that the values of L *, a *, and b * of the mixed color pattern image X2 become the target value X1. The CPU 301 determines yellow, magenta, cyan, and black image signal values to be the target value X1. Next, the CPU 301 converts the values of yellow, magenta, cyan, and black so that the combination of image signal values corresponding to the mixed color pattern image X2 becomes a combination of image signal values corresponding to the target value X1. A 4DLUT 312 is generated. The CPU 301 sets the 4DLUT 312 (S305) and ends the color adjustment calibration process.

  Next, determination processing for determining whether the CPU 301 executes color adjustment calibration processing will be described. If the above-described density adjustment calibration is executed, the density of the monochrome image is corrected to the target density. However, even if the density adjustment calibration is executed, there is a possibility that the color of the mixed color image does not become the target color. Therefore, the user cannot appropriately determine whether or not the color adjustment calibration needs to be executed after the density adjustment calibration is executed. Therefore, hereinafter, a determination process for automatically determining whether or not color adjustment calibration needs to be performed will be described.

  The CPU 301 controls whether or not the color adjustment calibration needs to be executed based on the measurement result of the patch image measured by the density sensor 126. The CPU 301 causes the image forming units 120, 130, 140, and 150 to form a patch image based on the patch image data and causes the density sensor 126 to measure the patch image every time density adjustment calibration is executed, and the patch image Is stored in the memory 302. Then, if the difference between the density of the patch image acquired after the previous execution of the density adjustment calibration and the density of the patch image acquired this time is larger than a predetermined value, the CPU 301 needs to execute the color adjustment calibration. It is determined that

  As the image forming condition of the printer B, the image forming condition determined in the immediately preceding density adjustment calibration process is set. Therefore, the density of the monochrome image on the recording medium is controlled to the target density. However, even if the density of the monochrome image formed on the recording medium is the target density, the density of the monochrome patch image on the intermediate transfer belt 113 is not necessarily the same as the target density. The density sensor 126 detects a patch image on the intermediate transfer belt 113. Therefore, the detection result of the patch image is not affected by the retransfer phenomenon, the secondary transfer efficiency, and the heat supplied from the fixing device. That is, when the density of the patch image on the intermediate transfer belt 113 is fluctuating, it is considered that any one of the retransfer phenomenon, the secondary transfer efficiency, and the heat supplied from the fixing device has an influence.

  After the density adjustment calibration is executed, the CPU 301 executes a determination process shown in FIG. The processing of the flowchart shown in FIG. 15 is executed when the CPU 301 reads a program stored in the memory 302.

  After the nth density adjustment calibration is executed (S401), the CPU 301 causes the printer B to form a patch image on the intermediate transfer belt 113 and causes the density sensor 126 to measure the patch image (S402). In step S402, a patch image is formed for each color of yellow, magenta, cyan, and black. The image signal value for forming the patch image is 96, for example. The CPU 301 causes the LUTa 306 to correct the image signal value, causes the PWM unit 308 to generate a laser output signal from the corrected image signal value, and causes the LD 309 to emit the semiconductor laser 310 based on the laser output signal.

  For example, when the LUTa 306 is the LUT indicated by the solid line in FIG. 16, the image signal value 96 is corrected to the image signal value 120. The LUTa 306 is set for each color. Therefore, the image signal value for each color is not always the same value. The detection value of the density sensor 126 is converted into a density value Dn by the A / D conversion circuit 181 and the density conversion circuit 182. This density value Dn is compared with the density value Dn-1 detected in the same manner after the (n-1) th density adjustment calibration, and if the difference is equal to or greater than the threshold value, it is determined that the color of the mixed color image has changed ( S403). The threshold is set to 0.02, for example. The threshold value is appropriately determined in consideration of the detection accuracy of the density sensor 126. As a result, the density sensor 126 erroneously detects the density of the patch image, and color adjustment calibration is prevented from being performed wastefully.

  If the CPU 301 determines in step S403 that the color of the mixed-color image has changed, the CPU 301 displays a color adjustment calibration execution instruction on the display 218 of the operation unit 217 (S404). That is, in step S404, the display 218 notifies that the 4DLUT 312 needs to be updated. Alternatively, the display 218 may notify the user of the update time of the 4DLUT 312. When a command for executing color adjustment calibration is input from the operation unit 217, the CPU 301 executes color adjustment calibration shown in the flowchart of FIG. Then, the CPU 301 ends the determination process.

  On the other hand, if the difference between the density value Dn and the density value Dn−1 is less than the threshold value in step S403, the CPU 301 determines that the color of the mixed color image has not changed, and ends the determination process.

  Further, since the CPU 301 does not automatically execute the color adjustment calibration, the color adjustment calibration can be prevented from being executed at a timing unnecessary for the user. According to the present embodiment, the CPU 301 can notify the user that it is necessary to perform color adjustment calibration, and can prompt the user to perform color adjustment calibration.

  Although the configuration of density adjustment calibration and color adjustment calibration has been described as the correction method for the single color image and the mixed color image, the correction method is not limited to this, and other methods may be used. For example, the color conversion may be realized by an arithmetic process such as masking instead of the LUT.

(Example 2)
Hereinafter, a configuration in which the density of the patch image formed on the intermediate transfer belt 113 in the above-described determination process (FIG. 15) is used as the reference density value will be described.

  In the density adjustment calibration, since the reader A reads the pattern image on the recording medium, the CPU 301 cannot execute the determination process unless the density adjustment calibration is executed. Therefore, the CPU 301 causes the printer B to form a patch image on the intermediate transfer belt 113, causes the density sensor 126 to measure the patch image, and executes patch gradation control for controlling image forming conditions based on the measurement result of the density sensor 126. To do. Thereby, even when the density adjustment calibration is not executed, the density of the output image formed by the image forming apparatus 100 is maintained at the reference density value.

  The patch gradation control does not require the user to place a test print on the reader A, so that usability is good and downtime is less than that of density adjustment calibration. On the other hand, since the patch tone control controls the image forming conditions based on the density of the patch image on the intermediate transfer belt 113, it is less accurate than the density adjustment calibration.

  In the patch tone control, a change in tone characteristics is predicted from the difference between the density of the patch image and the reference density value, and the LUTa 306 is corrected so that the tone characteristics become ideal tone characteristics. The reference density value is determined based on the measurement result of the patch image formed on the intermediate transfer belt 113 in the determination process.

  After the density adjustment calibration is executed, the CPU 301 executes a determination process shown in FIG. The process of the flowchart shown in FIG. 17A is executed when the CPU 301 reads a program stored in the memory 302.

  After the nth density adjustment calibration is executed (S501), the CPU 301 causes the printer B to form a patch image on the intermediate transfer belt 113 and causes the density sensor 126 to measure the patch image (S502).

  In step S502, the detection value of the density sensor 126 is converted into a density value Dn by the A / D conversion circuit 181 and the density conversion circuit 182. This density value Dn is compared with the density value Dn-1 detected in the same manner after the (n-1) th density adjustment calibration, and if the difference is equal to or greater than the threshold value, it is determined that the color of the mixed color image has changed ( S503). The threshold is set to 0.02, for example.

  If the CPU 301 determines in step S503 that the color of the mixed-color image has changed, the CPU 301 displays an instruction to perform color adjustment calibration on the display 218 of the operation unit 217 (S504). When a command for executing color adjustment calibration is input from the operation unit 217, the CPU 301 executes color adjustment calibration shown in the flowchart of FIG.

  Then, the CPU 301 stores the density value Dn of the patch image in the memory 302 as a reference density value (S505), and ends the determination process.

  On the other hand, if the difference between the density value Dn and the density value Dn−1 is less than the threshold value in step S503, the CPU 301 determines that the color of the mixed color image has not changed, and the process proceeds to step S505. In step S505, the CPU 301 determines a reference density value and ends the determination process.

  The CPU 301 executes patch gradation control when a predetermined condition is satisfied. For example, the CPU 301 executes patch gradation control when the number of recording media on which images are formed by the image forming apparatus 100 exceeds a predetermined number.

  In the patch gradation control, the CPU 301 obtains a change amount of the density value of the patch image on the intermediate transfer belt 113, creates a correction table based on the change amount, and updates the LUTa 306 based on the correction table. Therefore, in the patch tone control, the CPU 301 corrects the patch image data based on the LUTa 306, and causes the printer B to form a patch image based on the corrected patch image data.

  Further, the tone characteristics immediately after the density adjustment calibration is performed are ideal tone characteristics. Therefore, the CPU 301 stores the density of the patch image formed on the intermediate transfer belt 113 immediately after the density adjustment calibration is performed in the memory 302 as a reference density value.

  It should be noted that if the gradation characteristics change significantly due to environmental changes, it is necessary to execute density adjustment calibration.

  When the predetermined condition is satisfied, the CPU 301 executes the patch gradation control shown in FIG. The process of the flowchart shown in FIG. 17B is executed when the CPU 301 reads a program stored in the memory 302.

  The CPU 301 controls the image forming units 120, 130, 140, and 150 to form a patch image on the intermediate transfer belt 113 (S601). In step S601, a patch image is formed for each of yellow, magenta, cyan, and black colors. The image signal value for forming the patch image is the same image signal value 96 as in the determination process. The CPU 301 causes the LUTa 306 to correct the image signal value, causes the PWM unit 308 to generate a laser output signal from the corrected image signal value, and causes the LD 309 to emit the semiconductor laser 310 based on the laser output signal.

  The CPU 301 causes the density sensor 126 to measure the patch image on the intermediate transfer belt 113 (S602). The detection value of the density sensor 126 is converted into a density value by the A / D conversion circuit 181 and the density conversion circuit 182. Then, the CPU 301 calculates a difference between the density value of the patch image and the reference density value, and creates a correction table (S603). FIG. 18 is a graph showing the correspondence between the image signal value of the patch image and the density of the patch image. In FIG. 18, the density value A is the reference density value, and the patch density is the density value B. The amount of change from the reference density corresponds to the difference between the actual density value B and the reference density value A, as shown in FIG.

Here, a method for creating a correction table will be described. FIG. 19A is a correction coefficient table showing the correspondence between image signal values and correction gains. In the correction coefficient table, the correction gain corresponding to the input value 96 of the image signal is larger than the correction gains corresponding to other input values. The correction gain corresponding to the input value 96 of the image signal is 48, for example. The correction amount of the output value of the image signal with respect to the input value of the image signal is calculated by the following formula 1.
Correction amount = correction gain × {− (density change amount) / maximum correction gain} Equation 1

  The correction gain for each input value is determined with reference to the correction coefficient table. The density change amount is a difference between the density value of the patch image and the reference density value. The maximum correction gain corresponds to the maximum value of the correction gain in the correction coefficient table. In FIG. 19A, the maximum value of the correction gain is 48. Note that the correction coefficient table is an example, and the value of the correction gain is not limited to the value shown in FIG.

  FIG. 19B is a schematic diagram of a basic LUT that converts an input value of an image signal into an output value of the image signal. A correction table is generated by adding the aforementioned correction amount to the output value of the basic LUT. FIG. 19C is a schematic diagram of a correction table generated based on the correction amount and the basic LUT. The correction table shown in FIG. 19C corrects the gradation characteristics of the image so that the density value of the patch image becomes the target density value. The CPU 301 combines the LUTa 306 and the correction table, and updates the LUTa 306. Here, for example, when the input value of the image signal is 48, the correction gain corresponding to the input value is 40, and the density change amount is 10, the correction amount is 40 × −10 / 48 = −8.3 from Equation 1. . When the correction amount corresponding to the input value 48 of the image signal is −8.3, the output value of the image signal is 48−8.3 = 39.7≈40.

  The CPU 301 combines the LUTa 306 with the LUTa 306 and the correction table to update the LUTa 306 (S604), and ends the patch gradation control process. When the image forming apparatus 100 forms an output image based on the image data, the image data (image signal) is corrected based on the LUTa 306.

(Example 3)
Hereinafter, the configuration of the image forming apparatus 100 in which the color sensor is provided in the conveyance path through which the recording medium is conveyed will be described. If the image forming apparatus 100 includes a color sensor in the conveyance path, the CPU 301 can automatically execute density adjustment calibration and color adjustment calibration. That is, the user does not have to place the test print on the reader A.

  FIG. 20 is a schematic cross-sectional view of the image forming apparatus 100 including the color sensor 3000. The image forming apparatus 100 illustrated in FIG. 20 includes a color sensor 3000 on the downstream side of the fixing device 114 in the conveyance path where the recording medium is conveyed. The color sensor 3000 measures the pattern image fixed on the recording medium by the fixing device 114. That is, the color sensor 3000 reads the test prints 1, 2, and 3. When a command for executing density adjustment calibration is input from the operation unit 217, the CPU 301 causes the color sensor 3000 to read the first pattern image and the second pattern image. Further, when a command for executing color adjustment calibration is input from the operation unit 217, the CPU 301 causes the color sensor 3000 to read the third pattern image. As a result, the user can input the density adjustment calibration or the color adjustment calibration only by inputting a command from the operation unit 217, and the user can save the trouble of placing the test print on the reader A.

  21A is a cross-sectional view of the main part of the color sensor 3000, and FIG. 21B is a schematic configuration diagram of a light receiving element of the color sensor 3000. The color sensor 3000 includes a white LED 53 and a charge storage sensor 54 with an RGB on-chip filter. The white LED 53 functions as a light emitting element, and the charge storage type sensor 54 functions as a light receiving element. The color sensor 3000 reads a pattern image fixed on the recording medium P and outputs luminance signals of red (R), green (G), and blue (B).

  The color sensor 3000 causes the light emitted from the white LED 53 to enter the recording medium P on which the pattern image after fixing is formed at an angle of 45 degrees, and the intensity of diffusely reflected light in the 0 degree direction is measured by the charge storage sensor 54. To detect. As shown in FIG. 21B, the charge storage sensor 54 has independent red (R), green (G), and blue (B) pixels.

  The charge storage sensor 54 may be a photodiode, for example. A line sensor in which several sets of red (R), green (G), and blue (B) pixels are arranged may be used. The color sensor 3000 may have a configuration in which a light emitting element and a light receiving element are arranged so that an incident angle is 0 degree and a reflection angle is 45 degrees. Further, the color sensor 3000 may include a LED and a photodiode that emit red, green, and blue light. The color sensor 3000 transfers the luminance signal of the pattern image on the recording medium to the CPU 301. The CPU 301 executes density adjustment calibration or color adjustment calibration based on the luminance signal of the color sensor 3000.

  In the image forming apparatus 100, the color print 3000 can automatically measure the test print 1. For this reason, the CPU 301 may automatically perform color adjustment calibration when it is determined in the determination process that the color of the mixed-color image has changed. That is, the image forming apparatus 100 may be configured to execute the determination process shown in FIG. 22 instead of the determination process of FIG. FIG. 22 is a flowchart illustrating a modification of the determination process. The process of the flowchart shown in FIG. 22 is executed when the CPU 301 reads a program stored in the memory 302.

  After the nth density adjustment calibration is executed (S701), the CPU 301 causes the printer B to form a patch image on the intermediate transfer belt 113 and causes the density sensor 126 to measure the patch image (S702).

  The detection value of the density sensor 126 is converted into a density value Dn by the A / D conversion circuit 181 and the density conversion circuit 182. This density value Dn is compared with the density value Dn-1 detected in the same manner after the (n-1) th density adjustment calibration, and if the difference is equal to or greater than the threshold value, it is determined that the color of the mixed color image has changed ( S703).

  If the CPU 301 determines in step S703 that the color of the mixed-color image has changed, the CPU 301 executes color adjustment calibration to generate a 4DLUT 312 (S704), and the determination process ends.

  On the other hand, if the difference between the density value Dn and the density value Dn−1 is less than the threshold value in step S703, the CPU 301 determines that the color of the mixed color image has not changed, and ends the determination process.

  According to the present embodiment, since the color adjustment calibration is executed at an appropriate timing, it is possible to prevent the color adjustment calibration from being executed excessively. According to this embodiment, when both the density adjustment calibration and the color adjustment calibration are continuously executed, the downtime is 5 minutes, but when only the density adjustment calibration is executed. Had a downtime of 3 minutes.

A reader B printer 109 printer control unit 113 intermediate transfer belt 126 density sensor 311 gradation control unit 312 4DLUT

Claims (8)

Conversion means for converting image data based on conversion conditions;
Image forming means for forming images of different colors on a recording medium;
Reading means for reading a pattern image formed on a recording medium;
First generation means for causing the image forming means to form a monochromatic pattern image on a recording medium, causing the reading means to read the monochromatic pattern image, and generating image forming conditions based on the reading result of the monochromatic pattern image When,
Second generation means for causing the image forming means to form a mixed color pattern image on a recording medium, causing the reading means to read the mixed color pattern image, and generating the conversion condition based on the reading result of the mixed color pattern image When,
An intermediate transfer body to which a measurement image formed by the image forming means is transferred;
Measuring means for measuring the measurement image on the intermediate transfer member;
After the image forming conditions are generated by the first generation unit, the image forming unit forms the measurement image, the measurement unit measures the measurement image, and the measurement result of the previous measurement image Control means for controlling whether to cause the second generation means to generate the conversion condition based on the measurement result of the current measurement image,
The image forming apparatus, wherein the image forming unit forms an output image based on the image data converted by the converting unit. The previous measurement image is formed based on the previous image forming condition generated by the first generation unit,
The image forming apparatus according to claim 1, wherein the current measurement image is formed based on a current image forming condition generated by the first generation unit.   The control means causes the second generation means to generate the conversion condition if the difference between the density of the previous measurement image and the density of the current measurement image is greater than a threshold value. The image forming apparatus according to 1.   The image forming apparatus according to claim 1, wherein the control unit causes the image forming unit to form the measurement image every time the image forming condition is generated by the first generating unit. Conversion means for converting image data based on conversion conditions;
Image forming means for forming images of different colors on a recording medium;
Reading means for reading a pattern image formed on a recording medium;
First generation means for causing the image forming means to form a monochromatic pattern image on a recording medium, causing the reading means to read the monochromatic pattern image, and generating image forming conditions based on the reading result of the monochromatic pattern image When,
Second generation means for causing the image forming means to form a mixed color pattern image on a recording medium, causing the reading means to read the mixed color pattern image, and generating the conversion condition based on the reading result of the mixed color pattern image When,
An intermediate transfer body to which a measurement image formed by the image forming means is transferred;
Measuring means for measuring the measurement image on the intermediate transfer member;
After the image forming conditions are generated by the first generation unit, the image forming unit forms the measurement image, the measurement unit measures the measurement image, and the measurement result of the previous measurement image An image forming apparatus comprising: notification means for notifying the update timing of the conversion condition based on the measurement result of the current measurement image. The previous measurement image is formed based on the previous image forming condition generated by the first generation unit,
6. The image forming apparatus according to claim 5, wherein the current measurement image is formed based on a current image forming condition generated by the first generation unit.   6. The notification unit according to claim 5, wherein if the difference between the density of the previous measurement image and the density of the current measurement image is greater than a threshold value, the notification unit notifies the update timing of the conversion condition. Image forming apparatus.   The image forming apparatus according to claim 5, wherein the notification unit causes the image forming unit to form the measurement image every time the image forming condition is generated by the first generating unit.

Images