JP2016180812A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately correct the density of an image formed by an image forming apparatus.SOLUTION: An image forming apparatus comprises: a γ correction part 62; an image forming station 10 that forms images; a density detection sensor 5 that measures a pattern image; a control part 303 that creates a γLUT on the basis of a result of measurement performed by the density detection sensor 5; and a memory 40 that stores a γLUT 211b and a γLUT 92b. The image forming apparatus determines, on the basis of the created γLUT, γLUT 211b, and γLUT 92b, a correction coefficient corresponding to the value of an input of high density to the created γLUTs, and corrects the created γLUTs on the basis of the correction coefficient.SELECTED DRAWING: Figure 12

Description

  The present invention relates to correction control for correcting characteristics of an image formed by an image forming apparatus.

  An electrophotographic image forming apparatus forms an electrostatic latent image on a photoconductor based on image data, and forms the image by developing the electrostatic latent image using a developer (toner) in a developing device. To do. In order to control the density of the image formed by the image forming apparatus to a desired density, the measurement image formed by the image forming apparatus is measured, and the correction condition is corrected based on the measurement result.

  The image forming apparatus disclosed in Patent Document 1 forms a measurement image on a photoconductor, measures the measurement image formed on the photoconductor using a sensor, and forms the image based on the measurement result of the sensor. Correct the image density.

JP 2008-139588 A

  However, even when the correction conditions are corrected, there is a possibility that the density of the image formed on the recording material does not become a desired density. This is due to the measurement error of the sensor. When an error occurs in the measurement result of the sensor, the density of the image formed by the image forming apparatus cannot be corrected with high accuracy.

  Therefore, an object of the present invention is to correct the density of an image formed by an image forming apparatus with high accuracy.

  In order to solve the above problems, an image forming apparatus of the present invention includes an image carrier, correction means for correcting image data based on correction conditions, and an image on the image carrier based on the corrected image data. An image forming unit to be formed, a measuring unit for measuring a measurement image formed on the image carrier by the image forming unit, a generating unit for generating the correction condition based on a measurement result of the measuring unit, Storage means storing a first correction condition and a second correction condition in which a correction coefficient for an input value of the image data is larger than the first correction condition; the first correction condition; the second correction condition; Determining means for determining a correction coefficient corresponding to a high density input value in the correction condition generated by the generating means based on the correction condition generated by the generating means; and Ri based on the correction coefficient corresponding to the determined input value of the high concentration, and having a changing means for changing said correction condition generated by the generating means.

  According to the present invention, the density of an image formed by the image forming apparatus can be corrected with high accuracy.

Schematic sectional view of the image forming apparatus Control block diagram of image forming apparatus Functional block diagram of the image processing unit Schematic diagram of the main part of the concentration detection sensor Graph showing conversion table A graph showing the relationship between the amount of applied toner, density on paper, and sensor output value Graph showing γLUT for correcting gradation characteristics Schematic diagram showing how correction data is corrected Flow chart of automatic gradation correction control Schematic diagram of the pattern image on the intermediate transfer belt Graph showing printer characteristics and graph showing γLUT Flow chart of γ correction process Graph showing density characteristics at the time of density reduction and graph showing γLUT Graph showing density characteristics when density is increased and graph showing γLUT Schematic showing how to estimate γLUT Diagram showing test chart and sample chart Flow chart of visual correction control Graph showing conversion table

(Image forming device)
FIG. 1 is a schematic sectional view of the image forming apparatus 100. In the image forming apparatus 100, four image forming stations 10Y, 10M, 10C, and 10K that form toner images of respective color components are arranged in a line. In each image forming station, 10Y forms a yellow toner image, 10M forms a magenta toner image, 10C forms a cyan toner image, and 10K forms a black toner image. Since the image forming stations 10Y, 10M, 10C, and 10K have the same configuration, the image forming station 10Y that forms a yellow toner image will be described below, and the other image forming stations 10M, 10C, and 10K will be described. Omitted. Here, the direction of arrow A is the direction in which the photosensitive drums 1Y, 1M, 1C, and 1K of the image forming stations 10Y, 10M, 10C, and 10K rotate.

  The image forming station 10Y includes a photosensitive drum 1Y, a charger 2Y, an exposure device 3Y, a developing device 4Y, a primary transfer roller 7Y, and a drum cleaner 8M. A photosensitive layer is provided on the surface of the photosensitive drum 1Y. A power supply unit (not shown) is electrically connected to the charger 2Y and the primary transfer roller 7Y. The exposure apparatus includes a light source that emits laser light and an ASIC that controls the light source based on an image forming unit input signal described later. The developing device 4Y contains a two-component developer including toner and carrier. The drum cleaner 8M includes a plate-like elastic member for collecting the toner on the photosensitive drum 1Y.

  The intermediate transfer belt 6 is wound around a plurality of rollers and is rotationally driven in the direction of arrow B by a driving roller. Around the intermediate transfer belt 6, a density detection sensor 5, a secondary transfer roller 9, and a belt cleaner 11 are provided. An inner roller is provided on the opposite side of the secondary transfer roller 9 via the intermediate transfer belt 6. The secondary transfer roller 9 presses the intermediate transfer belt 6 against the inner roller, and the intermediate transfer belt 6 and the secondary transfer roller 9 form a nip portion. The secondary transfer roller 9 and the inner roller are electrically connected to a power supply unit (not shown).

  The paper feeding unit 20 accommodates the recording material P. The image forming apparatus 100 includes a paper feed roller that feeds the recording material P in the paper feed unit 20, a transport roller that transports the recording material P, and a registration that controls the timing and transport speed of transporting the recording material P to the nip portion. Provide with rollers. Further, the image forming apparatus 100 includes a fixing device 30 that fixes the toner image on the recording material P. The fixing device 30 includes a heating unit 31 having a ceramic heater 33, a cylindrical high heat-resistant thin film, and a support member, and a pressure roller 32 that presses the heating unit.

  Next, an image forming operation in which the image forming apparatus 100 forms an image on the recording material P based on image data input from a not-shown PC or scanner will be described.

  In the image forming station 10Y, first, the charger 2Y uniformly charges the photosensitive drum 1Y, and the laser exposure device 3Y controls the laser beam based on the image data. Laser light from the laser exposure device 3Y scans the photosensitive drum 1Y, and an electrostatic latent image is formed on the photosensitive drum 1Y. The developing device 4Y develops the electrostatic latent image on the photosensitive drum 1Y using yellow toner. As a result, a yellow toner image is formed on the photosensitive drum 1Y. The yellow toner image is transferred from the photosensitive drum 1Y to the intermediate transfer belt 6 by a transfer voltage applied to the primary transfer roller 7Y. The toner remaining on the photosensitive drum 1Y is removed by the drum cleaner 8Y.

  The toner image transferred to the intermediate transfer belt 6 is conveyed to the nip portion between the intermediate transfer belt 6 and the secondary transfer roller 9. At this time, the recording material P fed from the paper feeding unit 20 is conveyed to the nip portion by the registration roller so as to come into contact with the toner image on the intermediate transfer belt 6. While the toner image on the intermediate transfer belt 6 and the recording material P pass through the nip portion, the power supply unit supplies a transfer voltage to the secondary transfer roller 9 and the inner roller. As a result, the toner image on the intermediate transfer belt 6 is transferred onto the recording material P. The toner remaining on the intermediate transfer belt 6 without being transferred to the recording material P is removed by the belt cleaner 11.

  The recording material P carrying the toner image is conveyed to the fixing device 30 and the fixing device 30 records the unfixed toner image by the ceramic heater 33 applying heat to the recording material P carrying the unfixed toner image. It is melt-fixed on the material P. Then, the recording material P on which the toner image is fixed is discharged from the image forming apparatus by a discharge roller (not shown).

  Next, a control block diagram of the image forming apparatus 100 will be described with reference to FIG. The control unit 303 is a control circuit that controls each unit. The ROM 90 stores various programs. The memory 40 stores a look-up table (hereinafter referred to as γLUT) for correcting gradation characteristics and image forming conditions such as the intensity of laser light from the exposure devices 3Y, 3M, 3C, and 3K. The image forming station 10 corresponds to the image forming stations 10Y, 10M, 10C, and 10K in FIG. Since the image forming station 10 has already been described, description thereof is omitted here.

  A NIC (Network Interface Card) unit 21 transmits image data input via a network to the RIP unit 22 and transmits apparatus information to the outside via the network. The RIP unit 22 analyzes image data described by using PDL (Page Description Language) and develops the image data.

  The image processing unit 60 performs various image processing on the image data to correct the image data. The image processing unit 60 may be realized by an integrated circuit such as an ASIC, or may be realized by correcting image data based on a program stored in advance by the CPU of the control unit 303.

  The operation unit 80 includes a power switch of the image forming apparatus, a mode selection button for selecting a mode of the image forming apparatus, a numeric keypad, a determination button, a liquid crystal screen, and the like. The liquid crystal screen displays information related to the remaining amount of toner stored in the developing devices 4Y, 4M, 4C, and 4K, and displays an image related to the image data.

  The density detection sensor 5 includes an LED 51 and photodiodes 52 and 53. In the density detection sensor 5, the LED 51 irradiates the measurement image with light, and the photodiodes 52 and 53 receive the reflected light from the measurement image. The photodiodes 52 and 53 of the density detection sensor 5 output sensor output values (voltage values) according to the intensity of reflected light from the measurement image. The LED 51 functions as an irradiating unit that irradiates the measurement image with light. The photodiodes 52 and 53 function as a light receiving unit that receives reflected light from the measurement image.

  The pattern generator 70 generates measurement image data for forming a measurement image. The pattern generator 70 outputs pattern image data when automatic gradation correction control for correcting the gradation characteristics of the image forming station 10 is executed, and when visual correction control for adjusting the conversion table 55b is executed. Outputs test image data. The automatic gradation correction control will be described in detail based on FIG. 9, and the visual correction control will be described in detail based on FIG. 16 and FIG.

  The A / D conversion circuit 54 converts the sensor output values (voltage values) of the photodiodes 52 and 53 into sensor output values (digital signals) of 0 to 255 levels. The density calculation circuit 55 converts the sensor output value (digital signal) into the density of the measurement image on the recording material P based on the conversion table. The conversion tables 55a and 55b correspond to conversion conditions for converting the sensor output value into the density. When the gradation correction control is executed, the density calculation circuit 55 converts the sensor output value into the on-paper density based on the conversion tables 55a and 55b. When the visual correction mode is executed, the density calculation circuit 55 converts the sensor output value into a density based on the conversion table 55a.

  The function of the image processing unit 60 will be described based on the functional block diagram of FIG. The image data input to the image processing unit 60 includes RGB data obtained by digitizing three color components of R (red), G (green), and B (blue), C (cyan), M (magenta), There is CMYK data in which four color components of Y (yellow) and K (black) are digitized. The output direct mapping unit 61 converts RGB data into CMYK data when RGB data and image area data are transferred from the control unit 303 to the image processing unit 60.

  The γ correction unit 62 corrects the gradation characteristics of the image data. The density of the image formed by the image forming apparatus does not become a desired density. Therefore, the γ correction unit 62 corrects the input value (image signal value) of the image data so that the density of the image formed by the image forming apparatus becomes a desired density. The γ correction unit 62 corrects the gradation characteristics of the image data (CMYK data) based on γLUT_A and γLUT_B stored in the memory 40. Note that the memory 40 stores γLUT_A and γLUT_B for each color component. γLUT_A and γLUT_B correspond to a gradation correction table for correcting to input values of image data.

  γLUT_A is used to correct the printer characteristics of the image forming apparatus 100 to ideal gradation characteristics when the image forming apparatus 100 operates in a reference state that is a predetermined environmental condition and a predetermined developer charge amount. This is the correction condition. γLUT_A is determined in advance by experiments. The density of an image formed by the image forming apparatus varies depending on the ambient temperature and humidity of the image forming apparatus, the number of images formed, the charge amount of the developer, and the like. Therefore, the image forming apparatus has γLUT_B that adjusts the correction result by γLUT_A according to the state of the image forming apparatus. γLUT_B is a correction condition for adjusting the image data converted based on γLUT_A to image data suitable for the current state of the image forming apparatus. γLUT_B is corrected as appropriate according to the current state of the image forming apparatus. Therefore, the image forming apparatus 100 has automatic gradation correction control for updating γLUT_B.

  The halftone processing unit 63 performs screening suitable for the image area data on the image data (CMYK data) corrected by the γ correction unit 62. As a result, in the image data (CMYK data), multi-value data for each pixel is converted into binary data for each pixel. For example, screening is performed using a dither matrix so that the character area is printed clearly. For example, a photographic image region is screened using an error diffusion method so that moire is less likely to occur. Since screening is a known technique, a detailed description thereof is omitted.

  The smoothing processing unit 64 corrects the image data so that the unevenness of the edge portion of the image becomes smooth. The smoothing processing unit 64 extracts the edge portion of the image by pattern matching, and selectively converts the data of the extracted edge portion of the image. The image data corrected by the smoothing processing unit 64 is transferred to the exposure device 3 of the image forming station 10. The exposure apparatus 3 is controlled based on the image data converted by the image processing unit 60. The exposure device 3 exposes the photosensitive drum 1, and an electrostatic latent image based on image data is formed on the photosensitive drum 1.

(Density detection sensor)
Next, the configuration of the density detection sensor 5 provided in the image forming apparatus will be described with reference to FIG. The concentration detection sensor 5 includes an LED 51, photodiodes 52 and 53, an electric board (not shown) and a case part 50 that accommodates each element, and a window part 50 provided in the case part 50. The density detection sensor 5 may further include an optical element such as a lens.

  The LED 51 is a light emitting element that irradiates light to the measurement image formed on the intermediate transfer belt 6. The wavelength of light emitted from the LED 51 is set to, for example, 800 to 850 [nm] in consideration of the spectral reflectance of the toner. The light of the LED 51 is irradiated at an angle of 45 degrees with respect to the direction orthogonal to the surface of the intermediate transfer belt 6.

  The photodiode 52 is provided on a virtual line that forms an angle of 45 degrees with respect to a direction orthogonal to the surface of the intermediate transfer belt 6. For example, the LED 51 and the photodiode 52 are disposed at positions that are symmetrical when a plane orthogonal to the surface of the intermediate transfer belt 6 is used as a reference. The photodiode 52 receives specularly reflected light from the measurement image on the intermediate transfer belt 6. The photodiode 52 outputs a sensor output value (voltage value) corresponding to the intensity of reflected light from the measurement image.

  The photodiode 53 is provided at a position where the regular reflection light from the intermediate transfer belt 6 is not received. The photodiode 53 is provided on an imaginary line having an angle of, for example, 20 degrees with respect to a direction orthogonal to the surface of the intermediate transfer belt 6. The photodiode 53 receives irregularly reflected light from the measurement image on the intermediate transfer belt 6. The photodiode 53 outputs a sensor output value (voltage value) corresponding to the intensity of reflected light from the measurement image.

  The density detection sensor 5 measures specularly reflected light from the measurement image when measuring the density of the black measurement image. Therefore, when the density detection sensor 5 detects the density of the black measurement image, the density calculation circuit 55 converts the sensor output value of the photodiode 52 into the density. On the other hand, when measuring the density of the yellow measurement image, the density of the magenta measurement image, and the density of the cyan measurement image, the irregularly reflected light from the measurement image is measured. Therefore, when the density detection sensor 5 detects the density of each measurement image of yellow, magenta, and cyan, the density calculation circuit 55 converts the sensor output value of the photodiode 53 into the density.

  The density detection sensor 5 measures a measurement image on the intermediate transfer belt 6. Therefore, the density calculation circuit 55 converts the measurement result of the density detection sensor 5 into the density of the measurement image on the recording material P (paper density) based on the conversion table. In the following description, a case will be described in which a black measurement image on the intermediate transfer belt 6 is measured by the density detection sensor 5 and the density calculation circuit 55 converts the sensor output value to the density on paper.

  The light from the LED 51 irradiates the intermediate transfer belt 6. A region irradiated with light from the LED 51 corresponds to a measurement position. While the measurement image (black) on the intermediate transfer belt 6 passes the measurement position, the photodiode 52 receives the reflected light from the measurement image (black). The sensor output value (voltage value) output from the photodiode 52 while the photodiode 52 receives the reflected light from the measurement image (black) corresponds to the density of the measurement image (black).

  After the A / D conversion circuit 54 converts the sensor output value (voltage value) into an 8-bit sensor output value (digital signal), the concentration calculation circuit 55 calculates the sensor output value of the photodiode 52 based on the conversion table 55a. This is converted into the density Dblack of the black measurement image. Then, the density calculation circuit 55 corrects the density Dblack based on the conversion table 55b. The density Dblack corresponds to density data.

  For example, when the relationship between the density of the measurement image formed on the recording material P and the sensor output value changes, the control unit does not change the sensor output value based on the conversion table 55a. No. 303 cannot detect the density of the measurement image with high accuracy. In order to compensate the measurement result of the measurement image with high accuracy, the density calculation circuit 55 converts the sensor output value into the density on the paper based on both the conversion tables 55a and 55b. Note that the conversion table 55b stored in advance in the memory 40 before the visual correction mode is executed is data in which the density before conversion is equal to the density after conversion. The conversion table 55b functions as correction data for correcting the conversion table when the visual correction mode is executed in order to compensate the measurement result of the high-precision measurement image.

  When the density detection sensor 5 measures the yellow measurement image, the control unit 303 calculates the sensor output value of the photodiode 53 based on the conversion tables 55a and 55b corresponding to the yellow measurement image. The yellow image for measurement is converted to the density on paper. Similarly, when the density detection sensor 5 measures a magenta measurement image, the control unit 303 outputs the sensor output value of the photodiode 53 based on the conversion tables 55a and 55b corresponding to the magenta measurement image. Is converted to the density on the paper of the magenta measurement image. When the density detection sensor 5 measures the cyan measurement image, the control unit 303 measures the sensor output value of the photodiode 53 based on the conversion tables 55a and 55b corresponding to the cyan measurement image. The image is converted to the density on paper. In the memory 40, a conversion table 55a and a conversion table 55b are stored in advance for each color.

  FIG. 5 is a graph showing the conversion table 55a. FIG. 5 shows the sensor output value of the density detection sensor 5 corresponding to the measurement image and the on-paper density of the measurement image when the density of the measurement image on the intermediate transfer belt 6 is changed stepwise according to the area gradation. The relationship is shown. As the area coverage of the toner in the measurement image increases, the density of the measurement image on the recording material P also increases. Therefore, the density of the measurement image on the recording material P increases as the output value of the sensor decreases. On the other hand, when the toner area coverage in the measurement image decreases, the density of the measurement image on the recording material P also decreases. For this reason, as the output value of the sensor increases, the density of the measurement image on the recording material P becomes thinner.

  FIG. 5 is created as follows. FIG. 6A is a graph showing a correspondence relationship between the amount of applied toner of the black measurement image and the sensor output value of the photodiode 52 when the black measurement image is measured. When the amount of applied toner in the black measurement image changes, the amount of regular reflection received by the photodiode 52 also changes. The sensor output value of the photodiode 52 decreases as the toner application amount of the measurement image increases. This is because the light emitted from the LED 51 is absorbed by the black measurement image. As the toner adhesion amount of the black measurement image increases, the intensity of the specularly reflected light received by the photodiode 52 decreases.

  FIG. 6B is a graph showing the relationship between the amount of applied toner of the black measurement image and the density of the measurement image (density on paper) when the measurement image is formed on the recording material. The density of the measurement image formed on the recording material increases as the amount of toner applied to the measurement image formed on the intermediate transfer belt 6 increases.

  The graph of FIG. 6A and the graph of FIG. 6B are determined by experiment. Further, the graph of FIG. 6A and the graph of FIG. 6B are combined to create a conversion table. The graph shown in FIG. 6C corresponds to a conversion table for converting the sensor output value corresponding to the black measurement image into the on-paper density. A conversion table is created in the same manner for yellow, magenta, and cyan. The conversion table 55a is stored in the memory 40 in advance.

  In the above description, the sensor output value of either one of the photodiodes 52 and 53 is used to determine the on-paper density of the measurement image. However, it may be configured to convert the density on paper based on both the sensor output value of the photodiode 52 and the sensor output value of the photodiode 53.

(Tone correction control)
FIG. 7A is a graph illustrating printer characteristics of the image forming apparatus 100. FIG. 7B is a graph showing a γLUT for correcting the printer characteristics of the image forming apparatus 100 of FIG. The broken line in FIG. 7A and the broken line in FIG. 7B indicate ideal gradation characteristics. In the following description, it is assumed that the ideal gradation characteristic (broken line) is directly proportional to the on-paper density with respect to the input value of the image data.

  The printer characteristic (solid line) in FIG. 7A shows the correspondence between the image signal value of the image data and the density on the paper. In the printer characteristics of the image forming apparatus 100, the density corresponding to the image signal value X is Dy. However, in the ideal gradation characteristic, the target density corresponding to the image signal value X is Dx, and it is necessary to correct the printer characteristic of the image forming apparatus 100 to the ideal gradation characteristic.

  Therefore, in order to correct the printer characteristics of the image forming apparatus 100 to the ideal gradation characteristics using the density of the pattern image detected by the density detection sensor 5, the control unit 303 creates a γLUT. For example, when the number of pattern images is 16 for each color, the density of each pattern image detected by the density detection sensor 5 is detected only for 16 data for each color. Therefore, the printer characteristics of the image forming apparatus 100 are obtained by calculating an approximate curve of each data. The control unit 303 determines the γLUT by exchanging the input value and the output value of the printer characteristics of the image forming apparatus 100. If the number of pattern images is increased, the γLUT can be created with high accuracy. However, if the number of pattern images is increased, the downtime when creating the γLUT is increased.

  Even after the γLUT is created, if the image forming apparatus 100 forms a plurality of images, or if the ambient temperature or humidity of the image forming apparatus 100 changes, the image forming apparatus 100 may Printer characteristics change. Accordingly, the image forming apparatus 100 forms a pattern image on the intermediate transfer belt 6 when a predetermined condition is satisfied, and updates the γLUT based on the measurement result of the pattern image. This is automatic gradation correction control.

  The predetermined condition for executing the automatic gradation correction control is, for example, that the number of pages of the printed image is predetermined immediately after the main power supply of the image forming apparatus 100 is turned on or after the previous automatic gradation correction control is executed. More than a page. Note that the image forming apparatus 100 may be configured such that the control unit 303 executes the automatic gradation correction control when a command for instructing execution of the automatic gradation correction control is input to the operation unit 80 by the user.

  The control unit 303 determines γLUT by combining γLUT_A and γLUT_B. γLUT_A is a γLUT created in advance by experiments and is a fixed value. In order to correct the γLUT to a γLUT that is optimal for the current printer characteristics, the control unit 303 corrects the γLUT_B.

  FIG. 8 is a schematic diagram showing how γLUT_B is corrected. FIG. 8A is a graph showing γLUT_B stored in advance. FIG. 8B is a density characteristic graph showing the relationship between the image forming unit input signal value when the pattern image is formed on the intermediate transfer belt 6 and the pattern image measurement result by the density detection sensor 5. . The pattern image includes nine pattern images, and each pattern image has a different image forming unit input signal value.

  In FIG. 8B, the white circle is a target density value corresponding to the image forming unit input signal value, and the black circle is a density value detected by the density detection sensor 5. The measurement result D0 on the surface of the intermediate transfer belt 6 on which no pattern image is formed is 0 (fixed value) even if the printer characteristics of the image forming apparatus 100 change. In addition, the pattern image corresponding to the maximum value of the image forming unit input signal value is not formed, but the density value of the pattern image corresponding to the maximum value of the image forming unit input signal value is predicted from the measurement results of nine pattern images. The

  The reason why the high-density pattern image is not formed is that the density detection sensor 5 cannot measure the variation in the amount of applied toner of the high-density pattern image with high accuracy. Since the image forming apparatus 100 forms a pattern image by the area gradation method, the amount of reflected light almost changes even if the toner amount varies in a high density region where the toner amount of the pattern image covers the intermediate transfer belt 6. do not do. Therefore, the control unit 303 predicts the predicted density value Dmax of the pattern image corresponding to the maximum value of the image forming unit input signal based on the measurement result of the pattern image by the density detection sensor 5. For example, from the measurement result of the pattern image having the highest density among the nine gradation pattern images and the measurement result of the pattern image having the second highest density, the pattern image corresponding to the maximum value of the image forming unit input signal Extrapolate the measurement results.

  Next, a method for correcting γLUT_B will be described. For example, as shown in FIG. 8B, when the ideal density characteristic (broken line) of the image forming apparatus 100 is proportional to the image forming unit input signal, the input value and output of the pattern image measurement result ΓLUT_B can be corrected by replacing the value. FIG. 8C shows a graph (broken line) of γLUT_B before correction and a graph (solid line) of γLUT_B (γLUT_B ′ in the figure) after correction. As shown in FIG. 9C, the corrected γLUT_B ′ is a graph symmetrical with the actual density characteristic (solid line) with respect to the ideal density characteristic (broken line) in FIG. 9B.

  Hereinafter, automatic gradation correction control for correcting γLUT_B will be described with reference to the flowchart of FIG. In the automatic gradation correction control, the image forming station 10 forms a pattern image on the intermediate transfer belt 6, and γLUT_B is automatically corrected based on the measurement result of the pattern image by the density detection sensor 5. That is, automatic gradation correction control does not require any work by the user.

  When the automatic gradation correction is executed, the control unit 303 forms a pattern image on the intermediate transfer belt 6 (S100). The control unit 303 causes the pattern generator 70 to output pattern image data. The control unit 303 causes the γ correction unit 62 to correct the pattern image data based on γLUT_A. As a result, nine gradation pattern images with different densities are formed on the intermediate transfer belt 6. In step S100, the image forming station 10 functions as an image forming unit that forms a pattern image. Further, the intermediate transfer belt 6 corresponds to an image carrier on which a pattern image is carried.

  In step S100, the γ correction unit 62 corrects the pattern image data based on γLUT_A without using γLUT_B. Alternatively, the control unit 303 reads the initial γLUT_B in which the image signal value stored in advance and the image forming unit input signal value are equal, and corrects the pattern image data based on the γLUT obtained by combining the γLUT_A and the initial γLUT_B.

  This is because it is desired to ensure the density difference of the pattern image formed on the intermediate transfer belt 6 in the automatic gradation correction. If the density difference between the pattern image and the pattern image is too far apart, the calculation error increases and the γLUT cannot be obtained with high accuracy. For example, when the photosensitive drum and the developing device are replaced, if the pattern image data is corrected using a γLUT suitable for the print characteristics before replacement, the density of the pattern image formed on the intermediate transfer belt 6 is corrected. The difference may not be secured. This occurs when the printer characteristics of the image forming apparatus 100 are significantly changed when the photosensitive drum or the developing device is replaced. Even if the printer characteristics before the change can be controlled within the range of the density difference targeted by the pattern image, the printer characteristics after the change may not be controlled within the range of the density difference targeted by the pattern image.

  FIG. 10 is a schematic diagram of a pattern image formed on the intermediate transfer belt 6. The pattern image includes nine gradation pattern images having different densities for each color. That is, a total of 36 pattern images are formed. The size of one pattern image is 25 [mm] in the conveyance direction of the intermediate transfer belt 6 and 15 [mm] in a direction orthogonal to the conveyance direction.

  Nine pattern images are formed based on image signals of 18, 72, 126, 180, 234, 288, 342, 396, and 450, for example, with 10-bit signal values. These nine image signals correspond to pattern image data. A pattern image having an image signal value larger than 450 is not formed. This is because the density detection sensor 5 cannot detect a high density pattern image with high accuracy.

  Next, the control unit 303 measures the pattern image by the density detection sensor 5 (S101). In step S101, while the pattern image passes the measurement position of the density detection sensor 5, a sensor output value is output every 2 [msec]. The density detection sensor 5 measures one pattern image 25 times. The control unit 303 averages the sensor output values for 23 times excluding the maximum sensor output value and the minimum sensor output value from the sensor output values for 25 times. Then, the control unit 303 causes the density conversion circuit 55 to convert the average value of the sensor output values to the on-paper density based on both the conversion table 55a and the conversion table 55b.

  The control unit 303 corrects the γLUT based on the on-paper density of the pattern image measured in step S101 (S102). The control unit 303 obtains a density characteristic from the result of the on-paper density measured in step S101, and exchanges the input value and the output value of the density characteristic so that the density characteristic of the image forming apparatus 100 becomes an ideal density characteristic. To correct γLUT_B. Then, the control unit 303 synthesizes the γLUT_A stored in the memory 40 and the corrected γLUT_B to correct the γLUT. Since the pattern image is formed for each color, ideal density characteristics are also set for each color. In step S102, the control unit 303 updates γLUT_B so that the on-paper density of the pattern image becomes the target on-paper density. Then, the control unit 303 ends the automatic gradation correction.

  However, even if γLUT_B is updated, there is a possibility that the printer characteristics of the image forming apparatus 100 cannot be appropriately corrected. This is because the measurement result of the high density pattern image is predicted based on the measurement result of the pattern image by the density detection sensor 5. In particular, when the density of the pattern image formed on the intermediate transfer belt 6 decreases, the difference between the measurement result of the pattern image having the highest density among the nine gradation pattern images and the measurement result Dmax is large. For this reason, in the configuration in which the measurement result of the pattern image in the high density region is predicted when creating the γLUT, there is a problem that the density of the image in the high density region cannot be corrected with high accuracy.

  When the maximum density value Dmax predicted from the measurement result of the pattern image is lower than the maximum density value (for example, density 1.6) of the ideal gradation characteristic, the image signal value is set to be less than the image signal value. Cannot be converted to a high image forming unit input signal value. This is because γLUT_A cannot convert an image signal value to an image forming unit input signal value higher than a predetermined image forming unit input signal value. Since γLUT_A limits the image forming unit input signal value, an image forming unit input signal value higher than the limited image forming unit input signal value is not input to γLUT_B.

  Therefore, in the present embodiment, the image forming condition is set so that the image forming unit input signal value for forming an image having the maximum target paper density (for example, density 1.6) is smaller than the image signal value. Is set. When a pattern image is formed, the pattern image data is corrected based on the γLUT that corrects the image signal value to an image forming unit input signal value lower than the image signal value. Further, two γLUTs (γLUT211a and γLUT92b) having different maximum values on the paper are stored in advance, and a high density region of the combined γLUT is estimated based on γLUT211a and γLUT92b.

  Hereinafter, a method in which the image forming apparatus 100 according to the present embodiment estimates the high density region of the γLUT created by the automatic gradation correction control and obtains an appropriate γLUT will be described with reference to FIGS. The γLUT 211b functions as a first correction condition for correcting pattern image data when a pattern image is formed. On the other hand, the γLUT 92b corresponds to the second correction condition.

  FIG. 11A shows the printer characteristics of the image forming apparatus 100 obtained in advance by experiments. A broken line 92a in FIG. 11A indicates a maximum density value (for example, density value 1) having an ideal density characteristic of an image formed based on the maximum value (for example, 1023 with 10 bits) of the image signal value. .6) is the printer characteristic. A solid line 211a indicates that the density of an image formed based on the maximum value of the image signal value (for example, 1023 by 10 bits) is higher than the maximum density value (for example, the density value of 1.6) of ideal gradation characteristics. Is a printer characteristic when a large value (for example, a density value of 2.0) is obtained. When the printer characteristic of the image forming apparatus 100 is the solid line 211b, the density of the image formed based on the image signal value S1 is the ideal maximum density value (for example, density value 1.6). Note that a broken line 91a in FIG. 11A indicates ideal gradation characteristics.

  FIG. 11B is a graph showing a γLUT created based on the two printer characteristics shown in FIG. A broken line 92b in FIG. 11B is a γLUT for correcting the printer characteristic of the broken line 92a in FIG. 11A to an ideal gradation characteristic 91a. A solid line 211b in FIG. 11B is a γLUT for correcting the printer characteristic of the solid line 211a in FIG. 11A to an ideal gradation characteristic 91a. The two γLUTs 92b and 211b are stored in the memory 40 in advance.

  As shown in FIG. 11A, since the image forming unit input signal value corresponding to the maximum density value of the ideal gradation characteristic is S1, the γLUT 211b is the maximum value (for example, 10 bits) of the image signal value. 1023) is converted into an image forming unit input signal value S1 smaller than the maximum value. That is, the correction coefficient by which the γLUT 211b corrects the image signal to the image forming unit input signal is smaller than the correction coefficient by which the γLUT 92b corrects the image signal to the image forming unit input signal. Note that a broken line 91b in FIG. 11B indicates ideal gradation characteristics. A broken line 91a and a broken line 91b indicate the same ideal gradation characteristics.

  Here, FIG. 12 shows a subroutine for the γLUT correction processing in step S102 of FIG. When correcting the γLUT, first, the control unit 303 acquires the current density characteristics of the image forming apparatus 100 based on the pattern image measurement result by the density detection sensor 5 in step S101 (S01).

  Next, the control unit 303 updates the γLUT_B by switching the input value and the output value of the density characteristic determined in step S01 (S02). γLUT_B is updated by the method described above.

  Next, the control unit 303 determines whether or not the maximum density value Dmax in the density characteristic acquired in step S01 is thinner than the ideal maximum density value (corresponding to the maximum density value of the target on-paper density). Determine (S03). This is whether the image signal value may be limited to the image forming unit input signal value S1 in the synthesized γLUT when the maximum density value Dmax is lower than the target paper density (for example, 1.6). This is for determining. In step S03, the control unit 303 functions as a determination unit that determines whether the maximum density value Dmax is lower than a predetermined density.

  In step S03, if the maximum density value Dmax is not thinner than the ideal density characteristic maximum density value, the γLUT_B generated in step S02 and the previously stored γLUT_A are combined to correct the γLUT. Complete the process.

  On the other hand, if the maximum density value is thinner than the ideal density characteristic maximum density value in step S03, the high density region of γLUT is corrected according to the following method (S04). After correcting the high density region of γLUT in step S04, the control unit 303 completes the correction processing of γLUT.

  When the predicted maximum density value Dmax is lower than the maximum density value of ideal gradation characteristics (for example, density 1.6), the control unit 303 has the highest density among the nine gradation pattern images. It is determined that the difference between the measurement result of the high pattern image and the measurement result Dmax is large. Further, when the predicted maximum density value Dmax is lower than the maximum density value (for example, density 1.6) of the ideal gradation characteristic, the image signal value in the combined γLUT is the image forming unit input signal value S1. It is determined that it is restricted to.

  FIG. 13A shows the density characteristics of the image forming apparatus 100 when the predicted maximum density value Dmax is lower than the target on-paper density. When the image forming apparatus 100 has the density characteristics shown in FIG. 13A, in order to form an image having the maximum density value with ideal gradation characteristics, the image forming section input signal value S1 is used as the image forming section input signal. It is necessary to create a γLUT that corrects the value S2. Note that the γLUT 211b when the pattern image is formed converts the maximum value of the image signal value (for example, 1023 in 10 bits) into the image forming unit input signal value S1 smaller than the maximum value. That is, since the image forming unit input signal value S1 is smaller than the maximum value of the convertible image forming unit input signal (for example, 1023 in 10 bits), the γLUT 211b sets the maximum value of the image forming unit input signal value from S1 to S2. Can be corrected.

  FIG. 13B is a graph showing a γLUT 221b created by synthesizing γLUT_B and γLUT_A (γLUT211b) created based on the density characteristics of FIG. 13A. In order for the γLUT 221b to correct the image forming unit input signal value from S1 to S2, not only the γLUT_B but also the γLUT 92b and the γLUT 211b are used. A method for estimating the high density region of γLUT 221b will be described with reference to FIG.

  On the other hand, FIG. 14A shows density characteristics of the image forming apparatus 100 when the predicted maximum density value Dmax is higher than the target on-paper density. When the image forming apparatus 100 has the density characteristics shown in FIG. 14A, in order to form an image having the maximum density value with ideal gradation characteristics, the image forming section input signal value S1 is used as the image forming section input signal. It is necessary to create a γLUT that corrects the value S3. Since the image forming unit input signal S3 is smaller than the image forming unit input signal S1, the γLUT 211b can correct the image forming unit input signal value from S1 to S3.

  FIG. 14B is a graph showing a γLUT 231b created by synthesizing γLUT_B and γLUT_A (γLUT211b) created based on the density characteristics of FIG. 14A. Since the image forming unit input signal value in the γLUT 231b is not limited to S1, the γLUT 231b corrects the printer characteristics of the image forming apparatus 100 to ideal gradation characteristics without using the γLUT 92b and the γLUT 211b to estimate the high density range. it can.

  A method for predicting the high density region of the γLUT 211b shown in FIG. 13B by using the γLUT 92b and the γLUT 211b shown in FIG. 11B will be described with reference to FIG.

First, the control unit 303 reads an input value P (for example, 450) having the largest image signal from the pattern image actually measured by the density detection sensor 5 from the memory 40. Then, the control unit 303 estimates the γLUT 221b in a higher density region than the image signal value P based on the following Expression 1, Expression 2, and Expression 3.
γLUT92b (P) −γLUT221b (P) = m1 (Formula 1)
γLUT221b (P) −γLUT211b (P) = n1 (Expression 2)
γLUT242 (i) = γLUT211b (i) + {γLUT92b (i) −γLUT211b (i) × n1 / (m1 + n1)} (Expression 3)
Note that i = P to 1023.

  That is, in the high density region of γLUT 242, the ratio between the correction amount difference between γLUT 221 b and γLUT 242 and the correction amount difference between γLUT 242 and γLUT 92 b is maintained regardless of the image signal. In the high density region of γLUT 242, as shown in FIG. 15, the ratio of m1 and n1, the ratio of m2 and n2, and the ratio of m3 and n3 are equal. Furthermore, in the range below the image signal value P of the γLUT 242, it means that the γLUT 221b is created based on the measurement result of the density detection sensor 5.

  In step S04 in FIG. 12, the control unit 303 functions as a changing unit that changes the high density side of the γLUT based on the γLUT 211b (P), the γLUT 92b (P), and the γLUT 221b (P). Here, γLUT 211b (P) corresponds to the first correction coefficient, γLUT 92b (P) corresponds to the second correction coefficient, and γLUT 221b (P) corresponds to the third correction coefficient.

  In step S03, for example, the control unit 303 may determine whether or not the maximum density value Dmax is lower than a density 1.5 that is lower than the target on-paper density. That is, the predetermined density used for determining whether to correct the high density region of γLUT may be lower than the target on-paper density.

  As a result, since the high density region of the γLUT used for image formation is estimated, the image density can be corrected from the low density region to the high density region. In particular, even if the output density of the image forming apparatus 100 is lowered, it is possible to suppress the gradation of the image in the high density region from being lowered.

(Visual correction control)
Incidentally, when toner or paper dust adheres to the window 50 of the density detection sensor 5, an error occurs in the sensor output value. This is because when toner or paper dust adheres to the window 50, the intensity of light emitted from the LED 51 to the measurement image decreases, or the intensity of reflected light received by the photodiodes 52 and 53 decreases. Because it does. When toner or paper dust adheres to the window 50, an error occurs in the sensor output values of the photodiodes 52 and 53, so that the on-paper density of the measurement image cannot be detected with high accuracy.

  When the surface of the intermediate transfer belt 6 is rough due to the formation of a large amount of images, the correspondence between the sensor output value of the measurement image on the intermediate transfer belt 6 and the density of the measurement image on the paper changes. Resulting in. When the surface of the intermediate transfer belt 6 is rough, the reflected light from the surface of the intermediate transfer belt 6 changes. Therefore, in particular, an error occurs in the sensor output value when a low-density measurement image with a low toner coverage is measured. When the surface of the intermediate transfer belt 6 is rough, an error occurs in the sensor output values of the photodiodes 52 and 53, so that the on-paper density of the measurement image cannot be detected with high accuracy.

  Further, the ratio (transfer efficiency) of toner transferred from the intermediate transfer belt 6 to the recording material P in the secondary transfer nip T2 varies depending on the ambient temperature and humidity of the image forming apparatus. Therefore, when the ambient temperature or humidity of the image forming apparatus changes, the amount of toner of the measurement image formed on the intermediate transfer belt 6 and the density of the measurement image formed on the recording material P (the density on paper) ) Could change. Further, the ratio (transfer efficiency) of the toner transferred from the intermediate transfer belt 6 to the recording material P in the secondary transfer nip T2 also changes when the secondary transfer roller pair 9 deteriorates over time. This is because the resistance value of the secondary transfer roller pair 9 changes as the secondary transfer roller pair 9 deteriorates over time. That is, even when the secondary transfer roller pair 9 has deteriorated over time, the amount of toner in the measurement image formed on the intermediate transfer belt 6 and the density (on-paper density) of the measurement image formed on the recording material P The relationship could change. For this reason, when the transfer efficiency changes, the correspondence between the sensor output value and the on-paper density differs from the preset correspondence, so that the on-paper density of the measurement image cannot be detected with high accuracy.

  Therefore, in order to detect the on-paper density of the measurement image with high accuracy based on the measurement result of the measurement image formed on the intermediate transfer belt 6, the control unit 303 density of the measurement image formed on the recording material P. The conversion table 55b is corrected based on the information and the sensor output value of the measurement image. As a result, even if the window 50 of the density detection sensor 5 is dirty, the corrected conversion table 55b reduces the measurement error of the density detection sensor 5, so that the measurement result of the high-precision measurement image is compensated. it can. Furthermore, even when the surface of the intermediate transfer belt 6 is rough, the corrected conversion table 55b reduces the measurement error of the density detection sensor 5, so that the measurement result of the high-precision measurement image can be compensated. . Furthermore, even if the transfer efficiency changes, the corrected conversion table 55b converts the sensor output value to the on-paper density of the measurement image with high accuracy, so that the on-paper density of the measurement image can be detected with high accuracy. .

  Here, the visual correction control for updating the conversion table 55b will be described with reference to FIGS.

  FIG. 16A is a schematic diagram of a test sheet. The test sheet is printed by the image forming station 10 when visual correction control is executed. The test sheet is a recording material P on which a test image as a measurement image is formed. The test sheet has a plurality of test images A, B, and C for each color. For example, a test sheet is formed by forming yellow test images A, B, and C on one recording material P. The target density (for example, 0.4) of the test image A, the target density (for example, 0.8) of the test image B, and the target density (for example, 1.2) of the test image C are different. That is, the test images A, B, and C are formed based on test image data having different signal values.

  The test sheet is formed for each color. That is, in the visual correction control, the image forming apparatus 100 performs a test sheet on which magenta test images A, B, and C are formed, a test sheet on which cyan test images A, B, and C are formed, and a black test. A test sheet on which images A, B, and C are formed is also printed. In the following description, a case where a yellow test sheet is printed and the yellow conversion table 55b is corrected will be described.

  FIG. 16B is a sample chart prepared in advance. Sample charts are also available for each color. In the sample chart, a yellow image of 15 gradations with a density of 0.1 to 1.5 and a number of 0 to 14 are written alongside the image. The user visually determines whether the image of the sample chart has the closest density to the output test image, and inputs the number assigned to the image to the operation unit 80. For example, when the density of the “10” th image in the sample chart is closest to the density of the test image B, the user inputs “10” from the numeric keypad of the operation unit 80. The control unit 303 refers to data indicating the correspondence relationship between the sample chart number and the density, and determines the density corresponding to the image “10” on the sample chart. Thereby, the control unit 303 acquires the density of the test image B. The number input from the operation unit 80 corresponds to the density information of the test images A, B, and C. Data indicating the correspondence between the sample chart number and the density is stored in the memory 40 in advance.

  The operation of each unit of the image forming apparatus when the visual correction control is executed will be described based on the flowchart of FIG. When the user presses the visual correction control button of the operation unit 80, the control unit 303 executes visual correction control. Note that the control unit 303 executes the visual correction control of FIG. 17 based on a program stored in the ROM 90.

  First, the control unit 303 forms a test image on the recording material P (S131). The control unit 303 causes the pattern generator 70 to output test image data, and causes the γ correction unit 62 to correct the test image data based on the γLUT. In step S131, the control unit 303 controls the image forming station 10 to form test images A, B, and C on the intermediate transfer belt 6 based on the corrected test image data.

  Then, the control unit 303 causes the power supply unit to apply a transfer voltage to the secondary transfer roller 9 and the inner roller. As a result, the test images A, B, and C on the intermediate transfer belt 6 are transferred to the recording material P. The control unit 303 controls the fixing device 30 to fix the test images A, B, and C on the recording material P, and discharges them from the image forming apparatus 100. The secondary transfer roller 9 functions as a transfer unit that transfers the test images A, B, and C on the intermediate transfer belt 6 to the recording material P. The fixing unit 30 functions as a fixing unit that fixes the test images A, B, and C to the recording material P.

  Here, the γ correction unit 62 corrects the test image data based on the latest γLUT. For example, when tone correction is not performed, the γ correction unit 62 corrects image data based on γLUT_A stored in advance in the memory 40. On the other hand, when the tone correction is performed before step S131 is executed, the γ correction unit 62 is based on the γLUT (γLUT obtained by combining γLUT_A and γLUT_B) created in the tone correction control. Correct the test image data.

  This is because it is desired to control the density of the test images A, B, and C on the recording material P to the target density. When the on-paper density of the test image A is higher than the target density (for example, 0.4), the low density area of the conversion table 55b cannot be corrected with high accuracy. Similarly, when the on-paper density of the test image C is lower than the target density (for example, 1.2), the high density area of the conversion table 55b cannot be corrected with high accuracy. In particular, the high-density region has low human eye identification accuracy. Therefore, it is desirable that the target density of the test image C is the highest density in a range where the human eye can recognize the difference in density. Accordingly, the γ correction unit 62 corrects the test image data based on the latest γLUT in order to control the on-paper density of the test images A, B, and C to the target density in step S131.

  In step 131, the image forming station 10 functions as an image forming unit that forms test images A, B, and C on the recording material P. Further, in step S131, the γ correction unit 62 functions as a correction unit that corrects the test image data based on the γLUT. In step S131, the control unit 303 functions as a control unit that causes the γ correction unit 62 to correct the test image data and causes the image forming station 10 to form the test images A, B, and C on the recording material P.

  Next, the control unit 303 forms test images A, B, and C on the intermediate transfer belt 6 (S132). The control unit 303 causes the pattern generator 70 to output test image data, and causes the γ correction unit 62 to correct the test image data based on the γLUT. The image forming conditions for forming the test images A, B, and C on the intermediate transfer belt 6 in step S132 are the images when the test images A, B, and C are formed on the intermediate transfer belt 6 in step S131. The formation conditions are the same. The γLUT for the γ correction unit 62 to correct the test image data in step S132 is the same as the γLUT for the γ correction unit 62 to correct the test image data in step S131. The image forming conditions include the charging voltages of the chargers 2Y, 2M, 2C, and 2K, the light intensity of the laser beams of the exposure devices 3Y, 3M, 3C, and 3K, the primary transfer rollers 7Y, 7M, 7C, and 7K. Transfer voltage applied to the.

  In step S132, it functions as an image forming unit that forms test images A, B, and C on the intermediate transfer belt 6. In step S132, the γ correction unit 62 functions as a correction unit that corrects the test image data based on the γLUT. Further, in step S132, the control unit 303 functions as a control unit that causes the γ correction unit 62 to correct the test image data and causes the image forming station 10 to form the test images A, B, and C on the intermediate transfer belt 6. .

  Next, the control unit 303 measures the test images A, B, and C on the intermediate transfer belt 6 by the density detection sensor 5 (S133). In step S133, the control unit 303 starts light emission of the LED 51 before each of the test images A, B, and C on the intermediate transfer belt 6 reaches the measurement position. Then, the control unit 303 acquires the output voltage of the photodiode 52 (or 53) at the timing when the test images A, B, and C on the intermediate transfer belt 6 pass through the measurement position.

  At this time, the A / D conversion circuit 54 converts the output voltage of the photodiode 52 (or 53) into a digital signal. The density calculation circuit 55 uses the digital signal output from the A / D conversion circuit 54 at the timing when each of the test images A, B, and C on the intermediate transfer belt 6 passes the measurement position based on the conversion table 55a alone. Convert to density value. The control unit 303 acquires the density values of the test images A, B, and C converted by the density calculation circuit 55.

  In the steps from step S131 to step S133, the image forming station 10 forms the test images A, B, and C twice, but the test images A, B, and C may be formed only once. In this configuration, the density detection sensor 5 measures the test images A, B, and C on the intermediate transfer belt 6 before the test images A, B, and C are transferred to the recording material P.

  Next, the control unit 303 waits until the user inputs the result of the visual comparison between the test image and the sample chart to the operation unit 80 (S134). When information regarding the density of the test images A, B, and C for each color is input to the liquid crystal screen of the operation unit 80, the control unit 303 refers to the density conversion data stored in the ROM 90 to thereby test the test image A for each color. , B, and C concentrations are determined.

  Next, the control unit 303 corrects the conversion table 55b (S136). The control unit 303 converts the density values of the test images A, B, and C converted by the conversion table 55a in step S133 into the density values of the test images A, B, and C acquired in step S134. Then, the conversion table 55b is created. After the conversion table 55b is corrected in step S136, the control unit 303 ends the visual correction control.

Hereinafter, a method for creating the conversion table 55b will be described. The control unit 303 corrects the conversion table 55b so that the density value of the test image A measured by the density detection sensor 5 becomes the on-paper density determined by the user's visual observation. Here, the range from the density value 0 to the density value DA of the test image A determined using the density detection sensor 5 and the conversion table 55a is referred to as a first area. The control unit 303 corrects the first area of the conversion table 55b based on Expression 4 and Expression 5.
D ′ = α1 × D (Formula 4)
α1 = (DA2-0) / (DA1-0) (Formula 5)

  Here, D is a density value before being converted based on the conversion table 55b, and D ′ is a density value after being converted based on the conversion table 55b. Further, α1 is a correction coefficient for the conversion table 55b to correct the density value in the first region. DA1 is a density value of the test image A on the intermediate transfer belt 6 measured by the density detection sensor 5. DA1 is determined in step S133. DA2 is the density value of the test image A on the recording material P determined by the user's visual observation. DA2 is acquired in step S134.

  The control unit 303 divides the density value of the test image A visually determined by the user by the density value of the test image A measured by the density detection sensor 5, and calculates the correction coefficient α1 of the first region. As a result, in the first region, a conversion table 55b for correcting a deviation between the density on the paper visually observed by the user and the density value determined by the density detection sensor 5 and the conversion table 55a is created.

Further, the control unit 303 corrects the conversion table 55b so that the density value of the test image B measured by the density detection sensor 5 becomes the on-paper density determined by the user's visual observation. Here, the range from the density value DA determined using the density detection sensor 5 and the conversion table 55a to the density value DB of the test image B determined using the density detection sensor 5 and the conversion table 55a is the first. This is called two regions. The control unit 303 modifies the second area of the conversion table 55b based on Expression 6, Expression 7, and Expression 8.
D ′ = α2 × D + β1 (Formula 6)
α2 = (DB2-DA2) / (DB1-DA1) (Expression 7)
β1 = DB2−α2 × DB1 (Equation 8)

  Here, D is a density value before being converted based on the conversion table 55b, and D ′ is a density value after being converted based on the conversion table 55b. Further, α2 and β1 are correction coefficients for the conversion table 55b to correct the density value in the second region. DB1 is the density value of the test image B on the intermediate transfer belt 6 measured by the density detection sensor 5. DB1 is determined in step S133. DB2 is a density value of the test image B on the recording material P determined by visual observation of the user. DB2 is acquired in step S134.

  The control unit 303 divides the density value of the test image B visually determined by the user by the density value of the test image B measured by the density detection sensor 5, and calculates the correction coefficient α2 of the second region. The control unit 303 calculates the correction coefficient β1 based on the correction coefficient α2, the density value of the test image B measured by the density detection sensor 5, and the density value of the test image B determined by the user's visual observation. Thereby, in the second region, a conversion table 55b for correcting a deviation between the density on the paper visually observed by the user and the density value determined by the density detection sensor 5 and the conversion table 55a is created. The control unit 303 calculates the correction coefficient β1 based on the correction coefficient α2, the density value of the test image A measured by the density detection sensor 5, and the density value of the test image A determined by the user's visual observation. May be.

Further, the control unit 303 corrects the conversion table 55b so that the density value of the test image C measured by the density detection sensor 5 becomes the on-paper density determined by the user's visual observation. Here, the range from the density value DB determined using the density detection sensor 5 and the conversion table 55a to the density value DC of the test image C determined using the density detection sensor 5 and the conversion table 55a is the first. This is called 3 regions. The control unit 303 corrects the third area of the conversion table 55b based on Expression 9, Expression 10, and Expression 11.
D ′ = α3 × D + β2 (Formula 9)
α3 = (DC2-DB2) / (DC1-DB1) (Equation 10)
β2 = DC2−α3 × DC1 (Formula 11)

  Here, D is a density value before being converted based on the conversion table 55b, and D ′ is a density value after being converted based on the conversion table 55b. Furthermore, α3 and β2 are correction coefficients for the conversion table 55b to correct the density value in the third region. DC 1 is a density value of the test image C on the intermediate transfer belt 6 measured by the density detection sensor 5. DC1 is determined in step S133. DC2 is a density value of the test image C on the recording material P determined by visual observation by the user. DC2 is acquired in step S134.

  The control unit 303 divides the density value of the test image C determined by the user's visual observation by the density value of the test image C measured by the density detection sensor 5, and calculates the correction coefficient α3 of the third region. The control unit 303 calculates the correction coefficient β2 based on the correction coefficient α3, the density value of the test image C measured by the density detection sensor 5, and the density value of the test image C determined by the user's visual observation. As a result, in the third region, a conversion table 55b that corrects the deviation between the density on the paper visually observed by the user and the density value determined by the density detection sensor 5 and the conversion table 55a is created. The control unit 303 calculates the correction coefficient β2 based on the correction coefficient α3, the density value of the test image B measured by the density detection sensor 5, and the density value of the test image B determined by the user's visual observation. May be.

Further, the control unit 303 modifies the conversion table 55b in order to determine a density higher than the density value of the test image C measured by the density detection sensor 5. Here, a range of density higher than the density value DC of the test image C determined using the density detection sensor 5 and the conversion table 55a is referred to as a fourth area. The control unit 303 modifies the fourth area of the conversion table 55b based on Expression 12 and Expression 13.
D ′ = D + β3 (Expression 12)
β3 = DC2−DC1 (Formula 13)

  Here, D is a density value before being converted based on the conversion table 55b, and D ′ is a density value after being converted based on the conversion table 55b. Further, β3 is a correction coefficient for the conversion table 55b to correct the density value in the third region. DC 1 is a density value of the test image C on the intermediate transfer belt 6 measured by the density detection sensor 5. DC1 is determined in step S133. DC2 is a density value of the test image C on the recording material P determined by visual observation by the user. DC2 is acquired in step S134.

  The control unit 303 calculates a correction coefficient β3 by subtracting the density value of the test image C determined by the user's visual observation from the density value of the test image C measured by the density detection sensor 5.

  In step S136, the control unit 303 stores the relationship between the density value D before conversion and the density value D ′ after conversion in the first area, the second area, the third area, and the fourth area as the conversion table 55b. To store.

  FIG. 18 shows a graph of the conversion table 55b before correction and a graph of the conversion table 55b after correction. A broken line in FIG. 18 represents a graph of the conversion table 55b before correction. The solid line in FIG. 18 represents the graph of the conversion table 55b after correction. The density information measured by the user's visual observation is that each correction coefficient α1 is obtained when the density of the test image A is 0.3, the density of the test image B is 0.7, and the density of the test image C is 1.3. , Α2, α3, β1, β2, and β3 are determined as follows.

  In the first region where the density before conversion is 0 or more and 0.4 or less, the correction coefficient α1 is 3/4. In the second region where the density before conversion is higher than 0.4 and lower than or equal to 0.8, the correction coefficient α2 is 1 and the correction coefficient β1 is −0.1. In the third region where the density before conversion is higher than 0.8 and lower than or equal to 1.2, the correction coefficient α3 is 1.5 and the correction coefficient β2 is −0.5. In the fourth region where the density before conversion is higher than 1.2, the correction coefficient β3 is 0.1.

  In the present embodiment, the conversion table 55b is corrected based on the density information of the test image input by the user's visual observation and the sensor output value of the density detection sensor 5. Therefore, the measurement error of the density detection sensor 5 can be reduced, and a highly accurate measurement result can be compensated. Further, in the present embodiment, the γLUT is corrected based on the measurement result of the pattern image by the density detection sensor 5 converted based on the corrected conversion table 55b. Therefore, the density characteristics of the image formed on the recording material by the image forming apparatus can be corrected with high accuracy.

In the above description, the measurement images formed in the visual correction control are the test images A, B, and C having three different densities. However, the number of test images formed on the recording material P in order to correct the conversion table 55b may be any number. For example, when there is one test image, density 0 before conversion is converted to density 0 after conversion, and density values converted by the density detection sensor 5 and the conversion table 55a are determined by the user's visual density. The conversion table 55b is corrected so that When the density value converted by the density detection sensor 5 and the conversion table 55a is V, and the on-paper density determined visually by the user is W, Expression 14 becomes the conversion table 55b.
D ′ = (W / V) × D (Expression 14)

  Note that D is a density value converted by the conversion table 55a, and D ′ is a density value converted by the conversion table 55b. It is assumed that the density value converted by the density detection sensor 5 and the conversion table 55a is a directly proportional function passing through the intersection of the density on paper determined by the user's visual observation.

  Alternatively, the automatic gradation correction may be automatically executed after the visual correction control is executed. As a result, the density of the pattern image formed on the intermediate transfer belt 6 can be converted to the on-paper density with high accuracy using the conversion table corrected by the visual correction control. Therefore, a γLUT for controlling the on-paper density to a desired density can be created with high accuracy.

  According to the present invention, among the pattern images formed on the intermediate transfer belt 6, the density higher than the detected density of the pattern image having the highest image signal value is set as the high density area, and the γLUT 211b and γLUT 92b stored in advance are used. Based on this, the high density region of γLUT is corrected. However, the high concentration region may be a region higher than an arbitrary concentration.

  The control unit 303 estimates a high density region of the γLUT created based on the pattern image measurement result by the density detection sensor 5 based on a plurality of γLUTs suitable for printer characteristics having different maximum output densities. That is, the correction amount for the low density region in the γLUT is determined based on the measurement result of the pattern image by the density detection sensor 5, and the correction amount for the high density region is set to the γLUT set by experiment. Predicted based on. According to the present invention, since the high density region of the γLUT created based on the measurement result of the density detection sensor 5 is estimated with high accuracy, the gradation characteristics can be corrected with high accuracy from low density to high density.

5 Density Detection Sensor 6 Intermediate Transfer Belt 10 Image Forming Station 40 Memory 62 Gamma Correction Unit 80 Operation Unit 303 Control Unit

Claims (12)

An image carrier;
Correction means for correcting image data based on correction conditions;
Image forming means for forming an image on the image carrier based on the corrected image data;
Measuring means for measuring a measurement image formed on the image carrier by the image forming means;
Generating means for generating the correction condition based on the measurement result of the measuring means;
Storage means for storing a first correction condition and a second correction condition in which a correction coefficient for an input value of the image data is larger than the first correction condition;
Based on the first correction condition, the second correction condition, and the correction condition generated by the generation unit, a correction coefficient corresponding to a high density input value in the correction condition generated by the generation unit is obtained. A decision means to decide;
An image forming apparatus comprising: a changing unit configured to change the correction condition generated by the generating unit based on a correction coefficient corresponding to the high density input value determined by the determining unit.   The determining means includes a first correction coefficient corresponding to a predetermined input value of the image data in the first correction condition, a second correction coefficient corresponding to the predetermined input value in the second correction condition, and The correction coefficient corresponding to the high density input value is determined based on a third correction coefficient corresponding to the predetermined input value in the correction condition generated by the generation means. The image forming apparatus according to 1. The determining unit determines a correction coefficient corresponding to an input value on a higher density side than the predetermined input value in the correction condition generated by the generating unit;
The said change means changes the said correction conditions produced | generated by the said production | generation means based on the correction coefficient with respect to the said high density side input value determined by the said determination means. Image forming apparatus. The determination unit includes a determination unit that determines whether the density of an image formed by the image forming unit is changed from a predetermined density based on a measurement result of the measurement unit;
When the determination unit determines that the density of the image formed by the image forming unit is changed from a predetermined density by the determination unit, the first correction condition, the second correction condition, and The image forming apparatus according to claim 1, wherein the correction coefficient corresponding to the high density input value is determined based on the correction condition generated by the generation unit. . The determination unit determines whether or not the density of the image formed by the image forming unit has decreased,
The determining means determines the first correction condition, the second correction condition, and the correction condition generated by the generating means when the determining unit determines that the density of the image is reduced. The image forming apparatus according to claim 4, wherein the correction coefficient corresponding to the input value of the high density is determined based on the input value.   2. The measurement image is formed on the image carrier by the image forming unit based on measurement image data corrected by the correction unit based on the first correction condition. The image forming apparatus according to any one of claims 1 to 5. Conversion means for converting the measurement result of the measurement image by the measurement means into density data based on conversion conditions;
Causing the correction means to correct the test image data based on the correction condition; causing the image forming means to form a test image on a recording material based on the corrected test image data; and Control means for acquiring the measurement result of the image;
Input means for inputting information on the density of the test image determined by the user visually;
Update means for updating the conversion condition based on the information input by the input means and the measurement result of the test image;
The image forming apparatus according to claim 1, wherein the generation unit generates the correction condition based on the density data converted by the conversion unit.   The image forming unit includes a transfer unit that transfers the image formed on the image carrier to the recording material, and a fixing unit that fixes the image transferred to the recording material to the recording material. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   The image forming apparatus according to claim 7, wherein the test image includes a plurality of test images having different densities.   The image forming apparatus according to claim 1, wherein the correction condition is a gradation correction table for correcting gradation characteristics of the image forming unit.   The said measurement means has an irradiation part which irradiates light to the said image for a measurement, and a light-receiving part which receives the light reflected from the said image for a measurement, The any one of Claim 1 thru | or 10 characterized by the above-mentioned. The image forming apparatus described in 1. The image forming unit includes a first image forming unit that forms an image of a first color, and a second image forming unit that forms an image of a second color different from the first color,
12. The test image according to claim 1, wherein the test image includes a first test image formed by the first image forming unit and a second test image formed by the second image forming unit. The image forming apparatus according to claim 1.

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