JP5949621B2 - Density correction apparatus, image forming apparatus, and program - Google Patents

Description

  The present invention relates to a density correction apparatus, an image forming apparatus, and a program.

  Japanese Patent Application Laid-Open No. 2004-133830 discloses a gradation correction unit that performs gradation correction on input image data based on predetermined gradation correction characteristics, and a development that develops a toner image on an image carrier based on the image data subjected to gradation correction. Means, a density sensor for detecting the toner density developed on the image carrier, a transfer means for transferring the toner image developed on the image carrier to recording paper, etc., and a predetermined gradation during long-term maintenance. When the tone pattern image is developed on the image carrier using the pattern data as an input image, the toner density value detected by the density sensor and the tone pattern image on the image carrier are transferred to the recording paper or the like. Correlation storage means for storing in advance the correlation between the toner density on the image carrier and the final output density, which is obtained from the image density values for each portion of the gradation pattern image that has been obtained, and during short-term maintenance The gradation pattern image is developed on the image carrier using the predetermined gradation pattern data as input image data, and the toner density value is detected by the density sensor for each part of the gradation pattern image. And the detected toner density value to obtain a correlation between the input image gradation and the toner density on the image carrier, the correlation between the input image gradation and the toner density on the image carrier thus obtained, and the above Gradation correction characteristic setting means for setting gradation correction characteristics in the gradation correction means based on the correlation between the toner density on the image carrier and the final output density stored in advance in the correlation storage means; An image forming apparatus characterized by comprising:

  In Patent Document 2, in an image forming apparatus that forms an image according to an image signal on a recording material, a signal processing unit that performs signal processing on the image signal to create an image forming control signal, and an intermediate transfer medium are disclosed. An image forming unit that forms a toner image corresponding to the image formation control signal and carries the toner image on the intermediate transfer medium, and forms the image by transferring and fixing the toner image on the recording material; Toner amount detecting means for detecting the amount of toner carried on the intermediate transfer medium as the toner image, and the signal processing by the signal processing means includes the detection result by the toner amount detecting means, and the intermediate transfer An image forming apparatus including a gradation correction process based on a transfer fixing characteristic from a medium to the recording material is disclosed. Here, the transfer and fixing characteristics are the amount of toner constituting the toner image carried on the intermediate transfer medium, and the image density on the recording material after the toner image is transferred and fixed on the recording material. It is a characteristic that represents the relationship.

  Patent Document 3 discloses an image processing unit having a first image processing mode for processing input image data and a second image processing mode for performing image processing different from the first image processing mode, Image forming means for forming a toner image on a photoconductor based on image data processed by an image processing means, and transferring and fixing the toner image to a recording medium to form an image on the recording medium; and the recording medium Density detection means for detecting the density of the upper image, and the density of the toner image formed on the photoconductor based on the image data processed in the first image processing mode and the processing in the second image processing mode Storage means for storing in advance a correlation with the density of the toner image formed on the photoconductor based on the processed image data, and image data processed in the first image processing mode in the image forming means. The image is formed on the recording medium on the basis of the image density, and the density detection unit is caused to detect the density of the image formed on the recording medium based on the image data processed in the first image processing mode. Control means for creating a first correction parameter for correcting the density of an image processed in the first image processing mode based on a detection result of the means, and the control means includes the first correction An image forming apparatus is characterized in that a second correction parameter for correcting the density of an image processed in the second image processing mode is created based on the parameter and the correlation.

JP 2001-136391 A JP 2005-316242 A JP 2009-055606 A

  An object of the present invention is to provide target values for potential control and gradation control that have become inappropriate due to changes over time, based on a correction image (final output image) formed on a recording medium in an image forming state (normal mode). The present invention provides a density correction device, an image forming apparatus, and a program that reset the image density so that density correction is performed for all gradations.

To achieve the above object, the invention according to claim 1 is a gradation that corrects the gradation of the input image information to the gradation of the output image information based on the relationship between the set input gradation and the output gradation. A correction means; a light quantity detection means for detecting the reflected light quantity from the toner image formed on the image carrier; a density measurement means for measuring the density of the toner image formed on the recording medium; and a charging voltage and a developing bias voltage. , And the control means for performing control so that each of the exposure light amounts becomes a target value set for each, and the input gradation set so that the light amount detection value for the input gradation becomes the target value set On the image carrier based on the image information before the gradation correction of the first correction image having a plurality of gradation areas, and the gradation control means for updating the relationship between the output gradation and the output gradation The light amount detecting means from each gradation region of the formed toner image Formed on the recording medium based on the detected light quantity detection value and the image information of the second correction image having a plurality of gradation areas larger than that of the first correction image. A toner image is formed on a recording medium on the basis of the measured density value measured by the density measuring unit from each gradation area of the toner image and the relationship between the set input gradation and output gradation. A correlation acquisition means for acquiring a correlation between the output density value and the light quantity detection value, and a charging voltage and a development for achieving a target density value of the maximum density gradation region based on the acquired correlation Target value acquisition means for acquiring a new target value for each of the bias voltage and exposure light amount, and a new target value for gradation control for achieving a target density value corresponding to each gradation region, and a new acquired value Reset each target value And resetting means, a density correction device equipped with.
According to a thirteenth aspect of the present invention, there is provided a gradation correcting means for correcting the gradation of the input image information to the gradation of the output image information based on the relationship between the set input gradation and the output gradation, and an image. A light amount detecting means for detecting a reflected light amount from a toner image formed on the holder, a density measuring means for measuring the density of the toner image formed on the recording medium, and a charging voltage, a developing bias voltage, or an exposure light amount. The control means for performing the control so as to achieve the set target value, and the relationship between the set input gradation and the output gradation so that the light amount detection value for the input gradation becomes the set target value is updated. Gradation control means for performing gradation control and each gradation area of the toner image formed on the image carrier based on the image information before gradation correction of the first correction image having a plurality of gradation areas From the light quantity detection value detected by the light quantity detection means, The density from each gradation area of the toner image formed on the recording medium based on the image information of the second correction image having a plurality of gradation areas larger than that of the correction image, which has been gradation corrected by the gradation correction means. Based on the measured density value measured by the measuring means and the relationship between the set input gradation and output gradation, the correlation between the output density value and the light amount detection value when forming a toner image on the recording medium. Correlation acquisition means for acquiring the relationship, and a new target value of the charging voltage, developing bias voltage, or exposure light amount for achieving the target density value of the maximum density gradation region based on the acquired correlation A target value acquisition means for acquiring a new target value for gradation control for achieving a target density value corresponding to each gradation area; a resetting means for resetting the acquired new target value; Is a density correction apparatus including

According to the second aspect of the present invention, the target value acquisition unit acquires the predicted density value of the gradation region of the maximum density based on the correlation acquired by the correlation acquisition unit, and the acquired maximum density When the predicted density value of the tone area of the current color is deviated from the target density value, a new target value for each of the charging voltage, the developing bias voltage, and the exposure light quantity for achieving the target density value of the maximum density tone area The density correction apparatus according to claim 1, wherein

  According to a third aspect of the present invention, the target value acquisition means is acquired when the predicted density value of the maximum density gradation region exceeds a predetermined allowable range based on the target density value of the maximum density. The density correction apparatus according to claim 2, wherein the predicted density value of the maximum density gradation region is deviated from the target density value.

  According to a fourth aspect of the present invention, when the target value acquisition means falls within a predetermined allowable range based on the target density value of the maximum density, the predicted density value of the maximum density gradation area is within a predetermined range. The density correction apparatus according to claim 2, wherein the obtained predicted density value of the gradation area having the maximum density is not deviated from the target density value.

  According to a fifth aspect of the invention, the predicted density value of the maximum density gradation area is obtained by extrapolation from the correlation and the light amount detection value of the maximum density gradation area. The density correction apparatus according to any one of claims 4 to 5.

  According to a sixth aspect of the present invention, the correlation is a curve obtained by mathematical regression based on a plurality of correspondences between the measured density value and the light amount detection value. The density correction apparatus according to any one of the above.

  According to a seventh aspect of the invention, the correlation is similar in shape to the reference curve based on a plurality of correspondences between the measured density value and the light quantity detection value and a reference curve set according to the light quantity detection characteristic. The density correction apparatus according to any one of claims 1 to 5, wherein the density correction apparatus is a curve acquired as described above.

According to the eighth aspect of the present invention, the maximum density gradation when a predetermined gradation area other than the maximum density gradation area is set as a control point for controlling the charging voltage, the developing bias voltage, and the exposure light amount. based on the potential sensitivity ratio in the area and the control point, the charging voltage at the control point as the target density value of the gradation region of the maximum concentration is achieved after controlled developing bias voltage, and the exposure light quantity 8. A new target value for each of the charging voltage, the developing bias voltage, and the exposure light amount is obtained based on the obtained predicted density value. 8. It is a density correction apparatus of description.

  The invention according to claim 9 is the density correction apparatus according to claim 8, wherein the potential sensitivity ratio is acquired in advance before acquisition of a new target value by the target value acquisition means.

  The invention according to claim 10 is the density correction apparatus according to claim 8, wherein the potential sensitivity ratio is acquired during acquisition of a new target value by the target value acquisition means.

According to the eleventh aspect of the present invention, an image is formed by forming a toner image on an image carrier based on image information, transferring the toner image on the image carrier onto a recording medium, and forming a toner image on the recording medium. Formed on the image carrier, forming means, gradation correcting means for correcting the gradation of the input image information to the gradation of the output image information based on the relationship between the set input gradation and the output gradation, Each of the light amount detecting means for detecting the reflected light amount from the toner image, the density measuring means for measuring the density of the toner image formed on the recording medium, the charging voltage, the developing bias voltage, and the exposure light amount are set for each. A control means for performing control so as to obtain a target value, and a relationship between the set input gradation and the output gradation so that the light amount detection value for the input gradation becomes a set target value. Gradation control means for gradation control and a plurality of gradation areas A light amount detection value detected by the light amount detection means from each gradation region of the toner image formed on the image holding body based on the image information of the first correction image before gradation correction, and the first correction image. The density measurement means from each gradation area of the toner image formed on the recording medium on the basis of the image information subjected to gradation correction by the gradation correction means of the second correction image having a plurality of gradation areas. Based on the measured measured density value and the relationship between the set input gradation and output gradation, the correlation between the output density value and the light amount detection value when forming a toner image on the recording medium is obtained. And a new target value for each of the charging voltage, the developing bias voltage, and the exposure light amount for achieving the target density value of the gradation area of the maximum density based on the acquired correlation. Corresponding eyes in each gradation area An image forming apparatus comprising: a target value acquisition unit that acquires a new target value for gradation control for achieving a density value; and a resetting unit that resets each of the acquired new target values. is there.

According to a twelfth aspect of the present invention, an image is formed by forming a toner image on an image carrier based on image information, transferring the toner image on the image carrier onto a recording medium, and forming a toner image on the recording medium. Formed on the image carrier, forming means, gradation correcting means for correcting the gradation of the input image information to the gradation of the output image information based on the relationship between the set input gradation and the output gradation, Each of the light amount detecting means for detecting the reflected light amount from the toner image, the density measuring means for measuring the density of the toner image formed on the recording medium, the charging voltage, the developing bias voltage, and the exposure light amount are set for each. A control means for performing control so as to obtain a target value, and a relationship between the set input gradation and the output gradation so that the light amount detection value for the input gradation becomes a set target value. Gradation control means for performing tone control, and image formation The toner image formed on the image carrier on the basis of the image information before gradation correction of the first correction image having a plurality of gradation areas is a program for executing density correction processing. Light intensity acquisition means for acquiring a light intensity detection value detected by the light intensity detection means from each of the gradation areas, and the gradation correction of the second correction image having a plurality of gradation areas larger than the first correction image. A density acquisition means for acquiring a measured density value measured by the density measurement means from each gradation area of the toner image formed on the recording medium based on the image information subjected to gradation correction by the means, and a light quantity acquisition means. Based on the acquired light amount detection value, the measured density value acquired by the density acquisition means, and the relationship between the set input gradation and output gradation, the output density when forming a toner image on the recording medium Value and light intensity A correlation obtaining means for obtaining the correlation between the detection value, based on the acquired the correlation charging voltage to achieve the target density value of the gradation region of maximum density, a developing bias voltage, and the exposure light quantity and a new target value for each target value obtaining means for obtaining a new target value of the gradation control to achieve the target density value corresponding with each tone region, each of the acquired new target value It is a program that functions as resetting means for resetting.

According to the inventions according to claim 1, claim 11, claim 12 , and claim 13 , based on the correction image (final output image) formed on the recording medium in the image forming state (normal mode), The target values for potential control and gradation control that have become inappropriate due to changes over time are reset, and density correction is performed for all gradations.

  According to the second, third, and fourth aspects of the invention, a new target value for potential control is acquired only when necessary, and the potential does not increase too much.

  According to the fifth, sixth, and seventh aspects of the invention, the predicted density value of the maximum density gradation region is acquired, and a new target value for potential control is acquired without actually performing potential control. The According to the seventh aspect of the present invention, the predicted density value of the gradation area having the maximum density is acquired in accordance with the detection characteristic of the light amount detecting means.

  According to the eighth aspect of the present invention, a new target value for potential control can be obtained with higher accuracy than when the present configuration is not provided. According to the ninth aspect of the present invention, the density correction processing procedure is simplified as compared with the case where this configuration is not provided. According to the tenth aspect of the present invention, the potential sensitivity ratio can be obtained with higher accuracy than when the present configuration is not provided.

1 is a schematic configuration diagram illustrating an example of a configuration of an image forming apparatus according to an embodiment of the present invention. It is a schematic diagram which shows an example of a structure of a light quantity detection part. FIG. 2 is a block diagram illustrating a configuration of a control system of the image forming apparatus illustrated in FIG. 1. It is a conceptual diagram which shows the outline of density correction operation | movement. (A) is a schematic diagram showing how the amount of reflected light is detected from the first correction image (toner image) on the image carrier. (B) is a graph showing the relationship between the input gradation and the light amount detection value. FIG. 6A is a schematic diagram illustrating a state in which the density of a second correction image (toner image) on a recording medium is measured. (B) is a graph showing the relationship between the output gradation and the measured density value. (A) is a graph showing the relationship between the input gradation, the target density value, the predicted density value (without gradation correction), and the measured density value. (B) It is a graph which shows the relationship between an input gradation and an output gradation. (C) is a table showing the relationship between the input gradation, the target density value, the predicted density value (no gradation correction), and the measured density value. 6 is a flowchart illustrating an example of a procedure of “density correction processing”. It is a flowchart which shows an example of the procedure of a "setting change process." It is a schematic diagram for demonstrating the method of acquiring the prediction density | concentration value of a solid gradation from the correlation of a light quantity detection value and an output density value. (A) And (B) is a schematic diagram for demonstrating the extrapolation method which considered the detection characteristic of the light quantity detection part. It is a schematic diagram for explaining a control method for setting the output density value of the solid gradation to the target density value. (A) is a graph which shows the relationship between the input gradation and the variation | change_quantity of a light amount detection value when an electric potential is changed. (B) is a graph showing the relationship between the input gradation and the amount of change in the output density value when the potential is changed. It is a graph which shows the relationship between an input gradation when an electric potential control is performed, and an output density value. It is a graph which shows the relationship between an input gradation and output density value when gradation control is performed after electric potential control.

  Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings.

<Image forming apparatus>
(Whole image forming apparatus)
First, the entire image forming apparatus will be described.
The image forming apparatus is an electrophotographic image forming apparatus that forms an image on a recording medium using an electrophotographic developer containing toner. In the present embodiment, a so-called tandem type intermediate transfer type image forming apparatus will be described. An image forming apparatus is an image in which density correction of all gradations is performed by potential control and gradation control based on a correction image (final output image) formed on a recording medium in an image formation state (normal mode). Any image forming apparatus may be used as long as it is a forming apparatus. Hereinafter, the recording medium may be abbreviated as “medium”.

  FIG. 1 is a schematic configuration diagram showing an example of the configuration of an image forming apparatus according to an embodiment of the present invention. As shown in FIG. 1, the image forming apparatus according to the present embodiment includes an operation display unit 10, an image reading unit 20, an image forming unit 30, a medium supply unit 40, a medium discharge unit 50, a light amount detection unit 60, and a density. A measurement unit 70 is provided. Each of the medium supply unit 40, the image forming unit 30, the density measurement unit 70, and the medium discharge unit 50 is arranged in the described order along the medium conveyance path illustrated by a dotted line.

  The operation display unit 10 includes various buttons such as a start button and a numeric keypad, and a touch panel for displaying various screens such as a warning screen and a setting screen. With the above configuration, the operation display unit 10 accepts user operations and displays various types of information to the user. The image reading unit 20 includes an image reading device that optically reads an image formed on a recording medium, such as an image sensor, and a scanning mechanism for scanning the recording medium. With the above configuration, the image reading unit 20 reads an image on a recording medium placed on the image reading unit 20 and generates image information.

  The image forming unit 30 forms an image on a recording medium by electrophotography. The image forming unit 30 includes an image forming unit 32K that forms a K toner image, an image forming unit 32C that forms a C toner image, an image forming unit 32M that forms an M toner image, and a Y toner. An image forming unit 32Y for forming an image is provided. The image forming unit 30 also includes an intermediate transfer belt 36 wound around a plurality of rollers 34 so as to move in the direction of arrow B, and a secondary transfer device that collectively transfers a toner image on the intermediate transfer belt 36 onto a recording medium. 38, and a fixing device 39 for fixing the second-transferred toner image.

  Each of the image forming units 32K, 32C, 32M, and 32Y is arranged in the order of Y, M, C, and K colors on the intermediate transfer belt 36 when the intermediate transfer belt 36 moves in the arrow B direction. The toner images are arranged in the order shown so as to form a toner image. Hereinafter, when it is not necessary to distinguish each color, they are collectively referred to as an image forming unit 32. The image forming unit 32 includes a photosensitive drum, a charging device, an exposure device, a developing device, a transfer device, a cleaning device, and the like. The photosensitive drum is configured to rotate in the direction of the arrow.

  The intermediate transfer belt 36 is wound around a driving roller 34A, a back support roller 34B, a tension applying roller 34C, and a driven roller 34D. When it is not necessary to distinguish between these rollers, they are collectively referred to as a plurality of rollers 34. The plurality of rollers 34 are driven by a drive mechanism (not shown). When the drive roller 34A is driven to rotate by the drive mechanism, the intermediate transfer belt 36 moves in the arrow B direction at a predetermined speed. Further, the tension applying roller 34C is moved outward by the driving mechanism, whereby a predetermined force tension is applied to the intermediate transfer belt 36.

  The medium supply unit 40 includes a storage unit 42 that stores a recording medium, a supply mechanism that supplies the recording medium from the storage unit 42 to the image forming unit 30, and the like. The supply mechanism includes a take-out roller 44 for taking out the recording medium from the storage unit 42, a transport roller 46, and the like. A plurality of storage portions 42 are provided according to the type and size of the recording medium. The medium supply unit 40 takes out the recording medium from one of the storage units 42 and supplies it to the image forming unit 30. The medium discharge unit 50 includes a discharge unit 54 for discharging the recording medium, a discharge mechanism for discharging the recording medium onto the discharge unit 54, and the like.

  The light amount detection unit 60 is disposed around the image holding member constituting the image forming unit 30 so as to face the image holding member. In the present embodiment, the image carrier is the intermediate transfer belt 36. The light amount detection unit 60 is disposed downstream of the image forming unit 32 constituting the image forming unit 30 in the moving direction (arrow B direction) of the intermediate transfer belt 36, and is placed on the intermediate transfer belt 36 by the image forming unit 30. The amount of reflected light from each gradation region or the like of the formed first correction image (toner image) is measured.

  The density measurement unit 70 is disposed along the medium conveyance path so as to face the medium conveyance path. The density measuring unit 70 is disposed downstream of the fixing device 39 in the medium conveyance direction (arrow A direction), and the second correction image (toner image) formed on the recording medium M by the image forming unit 30 is displayed. Read and measure the density of each gradation region. The optical density (OD: Optical Density) is measured here. The density measuring unit 70 is an image reading device that optically reads an image (toner image) formed on a recording medium, such as an image sensor. For example, a contact image sensor (CIS: Contact Image Sensor) that reads an image in close contact with a recording medium is used.

  The image forming apparatus includes a plurality of transport rollers 46 in addition to the transport rollers 46 of the medium supply unit 40. The plurality of transport rollers 46 are arranged along the medium transport path illustrated by dotted lines. The plurality of transport rollers 46 are driven by a drive mechanism (not shown) and transport the recording medium according to an image forming operation.

(Light intensity detector)
Next, the light quantity detection unit will be described.
The light amount detection unit 60 is an optical sensor that irradiates the detection object with detection light and detects the amount of reflected light reflected from the detection object. The detection signal output from the light amount detection unit 60 represents the amount of reflected light from the object to be detected. The detected object is the intermediate transfer belt 36 on which the correction image is not formed, or the first correction image G1 formed on the intermediate transfer belt 36 (see FIG. 5A). The density correction process and the correction image will be described in detail later.

  FIG. 2 is a schematic diagram illustrating an example of the configuration of the light amount detection unit. As shown in FIG. 2, the light amount detection unit 60 includes a light emitting element 62 that emits detection light that irradiates a detection object, and a light receiving element 64 that receives reflected light. As the light emitting element 62, a light emitting element that emits light in a visible region or an infrared region such as an LED (Light Emitting Diode) is used. As the light receiving element 64, a light receiving element having sensitivity to detection light such as PD (Photo Diode) is used.

  Each of the light emitting element 62 and the light receiving element 64 is supported by a support member (not shown) and accommodated in the housing 61. In the example illustrated in FIG. 2, the housing 61 includes a light guide path 66 that guides detection light and a light guide path 68 that guides reflected light. The detection light emitted from the light emitting element 62 propagates through the light guide path 66 and is applied to the first correction image G1 on the intermediate transfer belt 36. The reflected light reflected by the first correction image G <b> 1 propagates through the light guide path 68 and is received by the light receiving element 64.

  In the illustrated light quantity detection unit 60, the light emitting element 62 and the light receiving element 64 are arranged so that the specularly reflected light of the detection light is received. That is, the light quantity detection unit 60 is a regular reflection type optical sensor. The regular reflection type optical sensor is used to measure the amount of reflected light of regular reflection light from the first K-color correction image G1. In the present embodiment, in addition to the regular reflection type optical sensor, a diffuse reflection type optical sensor is also disposed as the light amount detection unit 60. In the diffuse reflection type optical sensor, the light emitting element 62 and the light receiving element 64 are arranged so that the diffuse reflected light of the detection light is received, and the amount of reflected reflected light from the first correction image G1 for each color of YMC is measured. Used to do. The diffuse reflection type optical sensor is arranged corresponding to each color of YMC.

  The light emitting element 62 is driven to be lit by a driver (not shown) in accordance with a control signal from the control unit 100 described later. The light receiving element 64 is connected to the control unit 100 via an A / D converter (not shown), and outputs a digitally converted detection signal to the control unit 100.

(Control system configuration)
Next, a control system of the image forming apparatus will be described.
FIG. 3 is a block diagram showing the configuration of the control system of the image forming apparatus shown in FIG. As shown in FIG. 3, the control unit 100 is configured as a computer that controls the entire apparatus and performs various calculations. That is, the control unit 100 includes a CPU (Central Processing Unit) 100A, a ROM (Read Only Memory) 100B storing various programs, a RAM (Random Access Memory) 100C used as a work area when executing the programs, A nonvolatile memory 100D for storing various information and an input / output interface (I / O) 100E are provided. Each of CPU 100A, ROM 100B, RAM 100C, nonvolatile memory 100D, and I / O 100E is connected via a bus 100F.

  The I / O 100E of the control unit 100 includes an operation display unit 10, an image reading unit 20, an image forming unit 30, a medium supply unit 40, a medium discharge unit 50, a light amount detection unit 60, a density measurement unit 70, a communication unit 80, and Each unit of the storage unit 90 is connected. The control unit 100 includes the operation display unit 10, the image reading unit 20, the image forming unit 30, the medium supply unit 40, the medium discharge unit 50, the light amount detection unit 60, the density measurement unit 70, the communication unit 80, and the storage unit 90. To control. Further, the control unit 100 acquires detection results output as detection signals from each of the light amount detection unit 60 and the concentration measurement unit 70. That is, the control unit 100 acquires the light amount detection value RADC from the light amount detection unit 60 and acquires the measurement density value Dout from the concentration measurement unit 70.

  The communication unit 80 is an interface for communicating with an external device via a wired or wireless communication line. The communication unit 80 acquires image forming parameters including image forming attributes such as pages and the number of copies, together with image forming instructions and image information of the electronic document, from an external device. The storage unit 90 includes a storage device such as a hard disk. The storage unit 90 stores various data such as log data, control programs, and the like.

  In the present embodiment, a case will be described in which a control program for “density correction processing” described later is stored in advance in the ROM 100B of the control unit 100. The control program stored in advance is read and executed by the CPU 100A. In the present embodiment, the ROM 100B stores in advance a set-up potential V, which will be described later, and a look-up table (setting LUT) that represents the relationship between the input gradation and the output gradation. Note that the control program and various types of information may be stored in other storage devices such as the storage unit 90.

  Various drives may be connected to the control unit 100. Each type of drive is a device that reads data from a computer-readable portable recording medium such as a flexible disk, a magneto-optical disk, or a CD-ROM, and writes data to the recording medium. When various types of drives are provided, a control program may be recorded on a portable recording medium, and this may be read and executed by a corresponding drive.

(Image forming operation)
Specifically, the image forming unit 30 forms an image according to the following procedure.
The toner image of K color is transferred onto the intermediate transfer belt 36 by the image forming unit 32K. In the image forming unit 32K, the photosensitive drum is charged by a charging device. The exposure apparatus exposes the charged photosensitive drum with light corresponding to the K color image. As a result, an electrostatic latent image corresponding to the K-color image is formed on the photosensitive drum. The developing device develops the electrostatic latent image formed on the photosensitive drum with K color toner. The transfer device transfers the K-color toner image formed on the photosensitive drum onto the intermediate transfer belt 36.

  Similarly, the C-color toner image is transferred onto the intermediate transfer belt 36 by the image forming unit 32C. Further, the M color toner image is transferred onto the intermediate transfer belt 36 by the image forming unit 32M. Further, the Y-color toner image is transferred onto the intermediate transfer belt 36 by the image forming unit 32Y. When an image having each color of YMCK is formed, toner images of K color, C color, M color, and Y color are superimposed on the intermediate transfer belt 36 to form a “superimposed toner image”. The secondary transfer device 38 collectively transfers the “superimposed toner image” on the intermediate transfer belt 36 onto a recording medium. The fixing device 39 fixes the “superimposed toner image” collectively transferred onto the recording medium by heating or heating.

<Correction image>
Here, the “correction image” will be described. A correction image formed on an image carrier such as an intermediate transfer belt is a “first correction image”, and a correction image formed on a recording medium such as paper is a “second correction image”. . As the “second correction image”, a multi-gradation image having a larger number of gradation areas than the “first correction image” is used.

  As shown in FIG. 5A, the first correction image G1 has a plurality of gradation regions P (hereinafter referred to as “patch images P”). The first correction image G1 is formed in a non-image area (inter-image) between adjacent image areas. For this reason, there is a restriction on control, and generally, the number of the plurality of patch images P is about 10. The plurality of patch images P are toner images formed by one specific color (for example, K color). The plurality of patch images P are arranged in a one-dimensional manner on the intermediate transfer belt 36 along the moving direction (arrow B direction) of the intermediate transfer belt 36. That is, an image group in which a plurality of patch images P are arranged is the first correction image G1. When density correction processing is performed for each color of YMCK, a first correction image G1 is formed for each color.

  One patch image P is an image formed with a predetermined gradation in a predetermined area. In the present embodiment, the plurality of patch images P have different gradations. The plurality of patch images P are arranged so that the gradation increases or decreases along the arrangement direction. The “gradation” of the patch image P is represented by the toner coverage per unit area, that is, “area ratio”. “100%” is the maximum density gradation (so-called solid gradation), and “50%” is the halftone gradation. In this example, the first correction image G1 has 11 patch images P, and the gradation (area ratio) is 100% → 90% → 80% → 70% → 60 from the left to the right in the drawing. % → 50% → 40% → 30% → 20% → 10% → 0%

  As shown in FIG. 6B, the second correction image G2 also has a plurality of gradation regions P. The second correction image G2 has a larger number of gradation regions P than the first correction image G1. Since there is no restriction like the first correction image G1, the number of gradation regions P is not particularly limited. The second correction image G2 may be a continuous gradation image, for example, having 101 gradation regions P in increments of 1% from 0% to 100%.

  In the following description, the “first correction image G1” formed on the image holding member and the “second correction image G2” formed on the recording medium are both “toner images” even when not specifically described. is there. In the present embodiment, the gradation value in the input image information before gradation correction is “input gradation Cin”, and the gradation value in the output image information after gradation correction is “output gradation Cout”. is there. The gradation correction characteristic is represented by the relationship between “input gradation Cin” and “output gradation Cout” desired to be obtained for the input gradation Cin. In the present embodiment, the relationship between “input gradation Cin” and “output gradation Cout” is given by a look-up table (LUT).

<Outline of density correction operation>
Next, an outline of the “density correction operation” performed in the present embodiment will be described.
In the image forming apparatus, the reflected light amount from the first correction image formed on the image carrier is detected to obtain the light amount detection value RADC, and the LUT is updated so that the light amount detection value RADC becomes the target value RADCt. Gradation control is performed. Here, the subscript “t” means a target value (target). By repeating gradation control based on the light amount detection value RADC, the deviation amount ΔRADC of the light amount detection value RADC from the target value RADCt is eliminated, and the light amount detection value RADC converges to the target value RADCt.

  However, there may be a case where the sensitivity of the sensor that detects the amount of reflected light is deviated and the transfer of the toner image from the image carrier to the recording medium is poor during long-term use. In this case, even if the light amount detection value RADC is the target value RADCt, the measured density value Dout measured from the second correction image formed on the recording medium does not become the target density value Dt. In addition, since the solid tone light amount detection value RADC is not accurately read, the solid tone measurement density value Dout may not become the target density value Dt only by tone control.

  As described above, if the target value for gradation control becomes inappropriate due to changes over time, the target value for potential control and the target value for gradation control are reset and It is necessary to perform density correction. Density correction processing corresponding to long-term use, which is referred to as so-called long-term maintenance, is required. In the present embodiment, density correction processing corresponding to long-term use is performed on the assumption that gradation control based on the light amount detection value RADC is repeatedly performed.

  In the density correction processing according to the conventional long-term use, the mode is shifted to the setup mode, a second correction image is formed on the recording medium without gradation correction, and the measured density value Dout is formed from the formed second correction image. Was getting. When the measured density value Dout deviates from the target density value Dt, (1) potential control is performed to adjust the potential state so that the output density value Dout of the solid gradation becomes the target density value Dt. 2) After the potential control, the second correction image is formed again on the recording medium without gradation correction, and the measured density value Dout is obtained from the formed second correction image. (3) Each gradation image A plurality of steps of adjusting the gradation so that the output density value Dout is equal to the target density value Dt.

  In the “density correction processing” of the present embodiment, target values for potential control and gradation control that have become inappropriate due to changes over time are reset based on the final output image formed on the recording medium in the normal mode. Thus, density correction for all gradations is performed. The density correction processing is executed in the normal mode used by the user without shifting to the setup mode or the like. Further, the target value for potential control and the target value for gradation control are acquired in a series of density correction processes without performing multi-step adjustment. By setting each of the acquired target values, potential control and gradation control are performed so that the target values converge until the next image formation.

  FIG. 4 is a conceptual diagram showing an outline of the “density correction operation” of the present embodiment. As shown in FIG. 4, when a toner image (second correction image) is formed on a recording medium, the image information of the second correction image is subjected to gradation correction, and the set LUT is used. The input gradation Cin is converted into the output gradation Cout. For each of the plurality of gradation regions of the second correction image, the input gradation Cin is converted to the output gradation Cout.

  Next, a second correction image (final output image) is formed on the recording medium using a preset potential V based on the image information of the second correction image that has undergone gradation correction. Here, the “set potential V” is a generic term for a target value of potential control for controlling the potential state of the image forming unit, and specifically, for controlling a charging voltage, a developing bias voltage, an exposure light amount, and the like. Is the target value. The “set potential V” is a target value before a new target value for potential control is reset. The final output image is a toner image formed on the recording medium in an image forming system (normal mode) using the set potential V and the set LUT. Next, the density of the second correction image formed on the recording medium is measured, and a measured density value Dout is obtained for each of the plurality of gradation areas.

  On the other hand, when patch image formation is performed on the image carrier, the first correction image is formed on the image carrier based on the image information of the first correction image before gradation correction. . That is, in the case of patch image formation, the first correction image is formed without performing gradation correction. Note that patch image formation is performed by the image forming unit. Next, the light amount detection unit detects the reflected light amount from the first correction image formed on the image carrier, and obtains the light amount detection value RADC for each of the plurality of patch images. The input gradation Cin of a plurality of patch images is a control point for gradation control.

  In the present embodiment, the measured density value Dout and the light quantity detection value RADC are associated with each other based on the set LUT information, and the correlation (Dout vs. RADC) between the output density value Dout and the light quantity detection value RADC is obtained. To be acquired. The measured density value Dout is an actually measured value, and the output density value Dout includes a predicted value. As will be described in detail later, a new target value Vr for potential control for obtaining a solid tone patch image as a target density value Dt is acquired using the correlation (Dout vs. RADC). In addition, a new target value RADCt− for gradation control at each control point Cin for making each patch image other than the solid gradation a target density value Dt using the above correlation (Dout vs. RADC). r is obtained. That is, a new target value for potential control and a new target value for gradation control are acquired in a series of density correction processes.

(Detection of light quantity from toner image on image carrier)
FIG. 5A is a schematic diagram showing how the amount of reflected light is detected from the first correction image G1 (toner image) on the image carrier. As shown in FIG. 5A, the first correction image G1 formed on the intermediate transfer belt 36, which is an image carrier, is moved by the intermediate transfer belt 36 in the direction of arrow B, whereby the light amount detection unit 60 is detected. Pass under. The light amount detection unit 60 detects the reflected light amount from the first correction image G1 and acquires the light amount detection value RADC for the plurality of patch images P.

  FIG. 5B is a graph showing the relationship between the input gradation and the light amount detection value. In the example shown in FIG. 5B, the reflected light amount of the regular reflection light from the K-color first correction image G1 is detected. In this case, the value of the light amount detection value RADC decreases as the input gradation Cin increases. In the illustrated example, the light amount detection value RADC corresponding to the input gradation Cin is plotted with black dots for 11 points with the input gradation Cin ranging from 0% to 100%. In the present embodiment, the light amount detection value RADC is a standardized value with the detection range represented by 10 bits (0 to 1023) and the maximum detected light amount as 1023. When the input gradation Cin is “0%”, the light amount detection value RADC becomes “1023” at the maximum.

  When detecting the amount of diffuse reflected light from the correction image G for each color of YMC, the value of the light amount detection value RADC increases as the input gradation Cin increases. In this case, the input gradation Cin is “100%” and the light amount detection value RADC is “1023”, which is the maximum.

(Measurement of toner image density on recording medium)
FIG. 6A is a schematic diagram showing how the density of the second correction image G2 (toner image) on the recording medium is measured. As shown in FIG. 6A, the second correction image G2 formed on the recording medium M passes under the density measuring unit 70 as the recording medium M is conveyed in the arrow A direction. The density measuring unit 70 reads the second correction image G2 and acquires the measured density value Dout for each of the plurality of gradation areas P. As described above, the measured density value Dout is “optical density”.

  FIG. 6B is a graph showing the relationship between the output gradation and the measured density value. As described above, the second correction image G2 formed on the recording medium M is the final output image formed on the recording medium in the normal mode, and the image information of the second correction image G2 subjected to gradation correction. Is a toner image formed based on the above. In the illustrated example, the measured density value Dout is continuously acquired for the output gradation Cout from 0% to 90%, and the measured density value Dout corresponding to the output gradation Cout is represented by a solid line.

(Difference in concentration subject to potential control)
FIG. 7A is a graph showing the relationship between the input gradation, the target density value, the predicted density value (without gradation correction), and the measured density value. FIG. 7B is a graph showing the relationship between the input gradation and the output gradation. FIG. 7C is a table showing the relationship between the input gradation, the target density value, the predicted density value (without gradation correction), and the measured density value. In the graph shown in FIG. 7A, the measured density value Dout corresponding to the input gradation Cin is plotted with black dots. Further, the target density value Dt corresponding to the input gradation Cin is shown by a solid line.

  As described above, the final output image from which the measured density value Dout has been acquired is a toner image formed based on the tone-corrected image information. Here, gradation correction is performed using the relationship (LUT) shown in FIG. For example, the input density Cin100% is converted to the output gradation Cout90% by the LUT, and the gradation correction is performed, so that the measured density value Dout of the solid gradation patch image is derived from the target density value Dt indicated by the solid line. It will be a big difference.

  In the graph shown in FIG. 7A, the predicted density value Dout-pre corresponding to the input gradation Cin is plotted with a white square. The predicted density value Dout-pre is an output density value predicted to be acquired from a correction image formed on a recording medium without gradation correction (LUT open state). In the illustrated example, the predicted density value Dout-pre substantially matches the target density value Dt at each input gradation Cin. Here, for convenience of explanation, the predicted density value Dout-pre at each input gradation Cin is illustrated, but as will be described later, in the “density correction processing” according to the present embodiment, a solid gradation patch image. The predicted density value Dout-pre is acquired only for.

  Also from the table shown in FIG. 7C, when tone correction is performed (LUT application state), the measured density value Dout of the solid tone patch image with the input tone Cin of 100% is “1.55”, and the target density. There is a difference of “0.25” from the value Dt “1.80”. On the other hand, without tone correction (LUT open state), the predicted density value Dout-pre of the solid tone patch image is “1.71”, and the density difference ΔDout from the target density value Dt is “0. 09 ”.

  In the density correction process, a new target value for potential control is acquired in order to set the output density value Dout of the solid tone patch image to the target density value Dt. Also, a new target value for gradation control is acquired in order to set the output density value Dout of the patch image having a gradation other than the solid gradation to the target density value Dt. Therefore, on the premise that gradation control based on the light amount detection value RADC is repeatedly performed, a predicted density value Dout-pre of a solid gradation patch image without gradation correction (LUT open state) is acquired, and the predicted density is obtained. What is necessary is just to acquire the new target value Vr of potential control so that the value Dout-pre becomes the target density value Dt. In the example shown in the figure, the concentration difference ΔDout to be subjected to potential control is “0.09”, and the potential control may be performed so as to eliminate this concentration difference ΔDout.

<Density correction processing>
Next, “density correction processing” will be described.
FIG. 8 is a flowchart showing an example of the procedure of “density correction processing”. The control program for “density correction processing” is read from the ROM 100B of the control unit 100 and executed by the CPU 100A.

  The “density correction process” according to the present embodiment is implemented as a density correction process corresponding to long-term use, such as so-called long-term maintenance. The image forming apparatus starts “density correction processing” when a predetermined condition is satisfied. A normal image forming operation is not performed during the density correction process. For example, the “density correction process” may be started when the number of image formations is counted and the number of image formations exceeds the limit number. The conditions for starting the density correction process may be other conditions. For example, the “density correction process” may be started when a predetermined period has elapsed.

  As shown in FIG. 8, first, in step 100, a first correction image (toner image) is formed on the image carrier based on the image information of the first correction image before gradation correction. That is, the image forming unit is instructed to form an image. Next, in step 102, the light amount detection unit detects the reflected light amount from the first correction image, and obtains a light amount detection value RADC without gradation correction for a plurality of patch images.

  Next, in step 104, a second correction image (final output image) is formed on the recording medium based on the image information of the second correction image subjected to gradation correction. That is, the image forming unit is instructed to form an image. Next, in step 106, the density measurement unit reads the second correction image formed on the recording medium, and obtains measured density values Dout with gradation correction for a plurality of gradation areas.

  Next, in step 108, whether the measured density value Dout is equal to or higher than the target density value Dt at a plurality of gradations set as control points, that is, at each input gradation Cin of the plurality of patch images of the first correction image. Judge whether or not. If there is a control point at which the measured density value Dout is less than the target density value Dt, the process proceeds to step 110 and “setting change processing” described below is performed. If the measured density value Dout is greater than or equal to the target density value Dt at each control point, it is not necessary to change the set potential or the set LUT, and the routine ends.

(Setting change processing)
FIG. 9 is a flowchart illustrating an example of the procedure of “setting change processing”. As shown in FIG. 9, first, in step 200, a correlation (Dout vs. RADC) between the output density value Dout and the light amount detection value RADC is acquired. The measured density value Dout and the light quantity detection value RADC are associated with each other based on the set LUT information, and a curve representing the correlation (Dout vs RADC) between the output density value Dout and the light quantity detection value RADC is acquired. . The correlation (Dout vs. RADC) after the change with time is acquired. The setting LUT here is the LUT shown in FIG. 7B and will be described as having a relationship of Cin / Cout = 0.9.

  The light amount detection value RADC is the light amount detection value RADC without gradation correction. As shown in FIGS. 5A and 5B, the relationship between the input gradation Cin and the light amount detection value RADC is acquired by acquiring the light amount detection value RADC for a plurality of patch images. The measured density value Dout is the measured density value Dout with gradation correction. As shown in FIGS. 6A and 6B, the relationship between the output gradation Cout and the measured density value Dout is obtained by obtaining the measured density value Dout for a plurality of gradation areas.

  Based on the set LUT information, the measured density value Dout and the light amount detection value RADC for the same gradation are associated with each other. For example, as shown in FIG. 6B, the measured density value Dout “1.2” for the output gradation Cout 70% and the light amount detection value RADC “200” for the input gradation Cin 70% are associated with each other. However, what the user actually knows is the input image information Cin and the set LUT information. From the set LUT information, the input gradation Cin corresponding to the output gradation Cout when the measured density value Dout is acquired is known. Therefore, when converted in reverse from the relationship of Cin / Cout = 0.9, it can be seen that the measured density value Dout “1.2” is a measured value for the input gradation Cin of 78% (= 70% ÷ 0.9). Therefore, the measured density value Dout “1.2” for the input gradation Cin 78% may be associated with the light amount detection value RADC “200” for the input gradation Cin 70%.

  Similarly for other control points, the measured density value Dout and the light amount detection value RADC for the same gradation are associated with each other. A correlation (Dout vs. RADC) between the output density value Dout and the light amount detection value RADC is acquired from the plurality of correspondences acquired in this way. In the present embodiment, the measured density value Dout with gradation correction is associated with the light amount detection value RADC without gradation correction. However, since the measured density value Dout and the light amount detection value RADC for the same gradation are associated with each other, the correlation (Dout vs. RADC) is acquired regardless of the presence or absence of gradation correction. That is, the same correlation (Dout vs. RADC) is obtained as the correlation (Dout vs. RADC) without tone correction or with tone correction.

  On the other hand, the measured density value Dout with gradation correction and the light quantity detection value RADC without gradation correction are associated with each other. Therefore, as described below, the light quantity detection value RADC ( Only Cin100%) is known, and the predicted concentration value Dout-pre (Cin100%) is acquired from the correlation (Dout vs. RADC).

  Next, in step 202, based on the above correlation (Dout vs RADC), a predicted density value Dout-pre (Cin100%) of a solid gradation patch image of the input gradation Cin100% is acquired. FIG. 10 is a schematic diagram for explaining a method for obtaining a predicted density value of solid gradation from the correlation between the light amount detection value and the output density value. As described above, the measured density value Dout and the light amount detection value RADC for the same gradation are associated with each other. Therefore, actually, the measured density value Dout and the light amount detection value RADC have a one-to-one correspondence only up to the input gradation Cin corresponding to the upper limit value of the output gradation Cout.

  In FIG. 10, the one-to-one correspondence between the measured density value Dout and the light amount detection value RADC is plotted with black dots. In this example, the measured density value Dout and the light amount detection value RADC have a one-to-one correspondence up to the input gradation Cin of 90%. That is, the light amount detection value RADC corresponding to the input gradation Cin100% exists, but the measured density value Dout corresponding to the input gradation Cin100% does not exist. Therefore, the predicted density value Dout-pre (Cin100%) corresponding to the light amount detection value RADC at the input gradation Cin100% is acquired by extrapolation using the curve representing the correlation (Dout vs RADC) acquired in step 200. To do.

  The curve used for extrapolation may be obtained by mathematical regression from a plurality of correspondence relationships. Alternatively, a “reference curve” may be prepared according to the detection characteristics of the optical sensor used in the light amount detection unit, and a curve having a shape similar to the reference curve may be acquired from a plurality of correspondence relationships and reference curves. Good. FIGS. 11A and 11B are schematic diagrams for explaining an extrapolation method in consideration of the detection characteristics of the light quantity detection unit. As shown in FIG. 11A, when a regular reflection type optical sensor is used, the curve representing the correlation (Dout vs. RADC) is convex downward, and the RADC decreases as Dout increases. The curve asymptotically approaches 0. When a diffuse reflection type optical sensor is used, the curve representing the correlation (Dout vs. RADC) is convex upward, and as Dout decreases, RADC increases and becomes a curve asymptotic to 1023. .

  Here, an example of a method for acquiring a curve representing a correlation (Dout vs. RADC) when using a regular reflection type optical sensor using a reference curve will be described. In the illustrated example, there are only four measured points (corresponding relationships) plotted. As described above, when the number of correspondences is small, mathematical regression can obtain only a distorted correlation as shown by the dotted line. In such a case, when the reference curve is used, a curve suitable for the correlation (Dout vs. RADC) of the regular reflection type optical sensor is obtained as shown by the solid line.

In the illustrated example, the reference curve is given as a function of x as shown in the following equation.
RADC (x) = a + bx + cx ² + dx ³
(In the above formula, x is Dout, and a, b, c and d are constants (a = 1105.4, b = −1106.6, c = 244.49, d = 22.73).)

  Dividing into sections between adjacent measured points, the reference curve is moved so as to fill the difference with the measured points for each section, and the correspondence (Dout vs RADC) at a plurality of points in the section is acquired. The complemented correlation (Dout vs RADC) is acquired using the correspondences at these multiple points as pre-complementation data.

  Next, in step 204, it is determined whether or not the predicted density value Dout-pre (Cin 100%) is within an allowable range. When the predicted density value Dout-pre (Cin 100%) is within the allowable range, it is not necessary to perform potential control so that the output density value Dout of the solid tone patch image becomes the target density value Dt. Steps 206, 208, and 210 are omitted, and the process proceeds to step 212. In this case, the target value of the potential control remains “set potential V”. On the other hand, if the predicted density value Dout-pre (Cin 100%) is outside the allowable range, the process proceeds to step 206 to start a procedure for acquiring a new target value for potential control.

  The gradation control becomes easier as the potential is increased by the potential control. However, if the potential becomes too high, the image quality may be deteriorated, for example, the toner density increases and the line width increases. If the predicted concentration value Dout-pre (Cin 100%) is too high, the potential needs to be lowered by potential control. By setting an allowable range with respect to the predicted density value Dout-pre (Cin 100%) based on the target density value Dt, a new target value for potential control is acquired only when necessary.

  In step 206, the potential sensitivity ratio between the solid and the potential control point is acquired. FIG. 12 is a schematic diagram for explaining a control method for setting the output density value of the solid gradation to the target density value. As shown in FIG. 12, when the predicted density value Dout-pre (Cin 100%) of the solid tone patch image is low, the output density at the potential control point is determined using the halftone patch image as the potential control point. The value Dout is increased, and the output density value Dout (Cin 100%) of the solid gradation patch image is set to the target density value Dt. The reason why the halftone patch image is used as the potential control point is that the reproducibility is better than that of the solid tone patch image, and the measured density value Dout can be obtained accurately. As a result, the target value of the potential control is obtained with high accuracy, and the correction accuracy is improved.

  Specifically, the density difference ΔDout at the potential control point necessary for eliminating the density difference ΔDout of the solid gradation patch image is obtained. Here, the “potential sensitivity ratio” is required. In the following, a patch image having an input gradation Cin of 50% is set as a potential control point.

  FIG. 13A is a graph showing the relationship between the input gradation Cin and the change amount ΔRADC of the light amount detection value RADC when the potential is changed. When the potential is changed so as to increase the light amount detection value RADC, the amount of change ΔRADC of the light amount detection value RADC differs depending on the input gradation Cin even if the potential change is the same. When the above correlation (Dout vs. RADC) is used, as shown in FIG. 13B, the relationship between the input gradation Cin and the change amount ΔDout of the output density value Dout is acquired. From FIG. 13B, in order to increase the output density value Dout (Cin 100%) in the solid tone patch image by “0.1”, the output density value Dout (Cin 50%) at the potential control point is set to “0”. .14 ”, it is necessary to raise. That is, the potential sensitivity ratio between the solid and the potential control point is “1: 1.4”.

  The potential sensitivity ratio may be acquired in advance before executing the “density correction process”, or may be acquired in a series of processes each time the “density correction process” is executed. The potential sensitivity ratio acquired in advance is stored in a storage device such as the storage unit 90, and is read out and acquired from the storage device when the “density correction process” is executed. In the case where the potential sensitivity ratio is acquired in a series of processes, for example, when forming the first correction image on the image carrier in step 100 of FIG. Another first correction image may be formed to acquire the potential sensitivity ratio. The potential sensitivity ratio acquired in the “density correction process” is more accurate, and using this increases the correction accuracy.

  Next, in step 208, the potential sensitivity ratio is used to set the output density value Dout (Cin 100%) of the solid gradation patch image without gradation correction to the target density value Dt. Calculation target concentration value Dout-calc (Cin 50%) is calculated.

  For example, as shown in FIG. 7C, the predicted density value Dout-pre (Cin100%) of the solid gradation patch image without gradation correction is “1.71”, and the target density value Dt (Cin100%). Is 1.8 and the output density value Dout (Cin100%) is increased by “0.09” in order to achieve the target density value Dt (Cin100%), the calculated target density value Dout− at the potential control point calc (Cin 50%) is obtained as follows. Since the potential sensitivity ratio between the solid and the potential control point is “1: 1.4”, it is necessary to increase the output density value Dout by “0.126 (= 0.09 × 1.4)” at the potential control point. is there. Since the current predicted concentration value Dout-pre (Cin50%) is “0.89”, the calculated target concentration value Dout-calc (Cin50%) at the potential control point is “1.016 (= 0.89 + 0.126). ) ”.

  Next, in step 210, a new target value Vr of potential control for achieving the calculated target concentration value Dout-calc (Cin50%) at the potential control point is acquired. That is, a new target value Vr for potential control is acquired without actually setting a different potential and forming the second correction image on the recording medium. FIG. 14 is a graph showing the relationship between the input gradation and the output density value when potential control is performed. By performing the potential control using the new target value Vr, the output density value Dout (Cin50%) at the potential control point becomes the calculated target density value Dout-calc (Cin50%), which is a solid tone patch image. The output density value Dout (Cin100%) is predicted to be the target density value Dt (Cin100%).

  Next, in step 212, for each control point including the potential control point, a new target value RADCt− for gradation control for achieving the target density value Dt based on the above correlation (Dout vs. RADC). Get r. Even if the target value of potential control changes, the above correlation (Dout vs. RADC) does not change. Accordingly, in the above correlation (Dout vs. RADC), the light amount detection value RADC corresponding to the target density value Dt becomes a new target value RADCt-r. Next, in step 214, the acquired new target value Vr for potential control and the new target value RADCt-r for gradation control are set as control target values, and the routine is terminated.

  Each of the acquired target values is reset (stored as a target value), whereby potential control and gradation control are performed so as to converge to a new control target value until the next image formation. As a result, as shown in FIG. 15, density correction is performed for all gradations, and the output density value Dout converges to the target density value Dt at each control point.

<Modification>
Note that the configurations of the density correction apparatus, the image forming apparatus, and the program described in the above embodiments are examples, and it goes without saying that the configurations may be changed without departing from the gist of the present invention. For example, the image carrier may be a drum, the order of steps in the flowchart may be changed, some steps may be omitted, and the like.

  Further, in the above embodiment, the predicted density value Dout-pre (Cin 100%) of the solid gradation patch image without gradation correction is acquired, and the obtained density measurement value Dout-pre (Cin 100%) is acquired. Is determined to be within the allowable range, but it is only necessary to determine whether the predicted density of the solid gradation patch image without gradation correction is within the allowable range, and the predicted density value Dout-pre (Cin100 %) Is not mandatory.

  In the above embodiment, the example in which the light amount detection value RADC of each patch image is acquired before the measurement density value Dout is acquired has been described. However, the light amount detection value RADC is the acquisition of the measurement density value Dout of the final output image. It is only necessary to be acquired before and after (within a predetermined period). The light amount detection value RADC may be obtained after obtaining the measured density value Dout. Further, the light amount detection value RADC acquired before the start of the “density correction process” may be used.

DESCRIPTION OF SYMBOLS 10 Operation display part 20 Image reading part 30 Image formation part 32 Image formation unit 34 Roller 36 Intermediate transfer belt 38 Secondary transfer apparatus 39 Fixing apparatus 40 Medium supply part 42 Storage part 44 Extraction roller 46 Conveyance roller 50 Medium discharge part 54 Discharge part 60 Light quantity detection unit 61 Housing 62 Light emitting element 64 Light receiving element 66 Light guide path 68 Light guide path 70 Density measuring section 80 Communication section 90 Storage section 100 Control section G Correction image M Recording medium P Patch image (gradation area)

Claims (13)

Gradation correction means for correcting the gradation of the input image information to the gradation of the output image information based on the relationship between the set input gradation and the output gradation;
A light amount detecting means for detecting a reflected light amount from a toner image formed on the image holding member;
Density measuring means for measuring the density of the toner image formed on the recording medium;
Control means for performing control so that each of the charging voltage, the developing bias voltage, and the exposure light amount becomes a target value set for each;
Gradation control means for performing gradation control by updating the relationship between the set input gradation and the output gradation so that the light amount detection value for the input gradation becomes a set target value;
A light amount detection value detected by the light amount detection means from each gradation region of the toner image formed on the image carrier based on the image information before gradation correction of the first correction image having a plurality of gradation regions; Each gradation area of the toner image formed on the recording medium based on the image information subjected to gradation correction by the gradation correction means of the second correction image having a plurality of gradation areas larger than the first correction image From the measured density value measured by the density measuring means and the relationship between the set input gradation and output gradation, the output density value and the light amount detection value when forming a toner image on the recording medium A correlation acquisition means for acquiring a correlation with
Based on the acquired correlation, a new target value for each of the charging voltage, the developing bias voltage, and the exposure light amount for achieving the target density value of the maximum density gradation area , and the target corresponding to each gradation area Target value acquisition means for acquiring a new target value of gradation control for achieving the density value;
Resetting means for resetting each of the acquired new target values;
Density correction device with The target value acquisition unit acquires the predicted density value of the maximum density gradation region based on the correlation acquired by the correlation acquisition unit, and the acquired predicted density value of the maximum density gradation region is 2. A new target value for each of a charging voltage, a developing bias voltage, and an exposure light amount for achieving a target density value in a gradation region having a maximum density is obtained when there is a deviation from the target density value. Density correction device.   The target value acquisition means predicts the acquired maximum density gradation area when the predicted density value of the maximum density gradation area exceeds a predetermined allowable range based on the maximum density target density value. The density correction apparatus according to claim 2, wherein the density value is deviated from the target density value.   When the target density acquisition means has the predicted density value of the maximum density gradation area within a predetermined allowable range based on the maximum density target density value, the acquired maximum density gradation area The density correction apparatus according to claim 2, wherein the predicted density value is not deviated from the target density value.   5. The predicted density value of the maximum density gradation area is obtained by extrapolation from the correlation and the light amount detection value of the maximum density gradation area. 6. The density correction apparatus described in 1.   The density according to any one of claims 1 to 5, wherein the correlation is a curve acquired by mathematical regression based on a plurality of correspondences between the measured density value and the light amount detection value. Correction device.   The correlation is a curve acquired so as to have a shape similar to the reference curve based on a plurality of correspondence relationships between the measured density value and the light amount detection value and a reference curve set according to the light amount detection characteristic. The density correction apparatus according to any one of claims 1 to 5. When a predetermined gradation area other than the maximum density gradation area is a control point for controlling the charging voltage, the developing bias voltage, and the exposure light amount , the maximum density gradation area and the potential sensitivity at the control point. Based on the ratio , the estimated density value after controlling the charging voltage, the developing bias voltage, and the amount of exposure light at the control point is obtained so that the target density value of the gradation area of the maximum density is achieved. The density correction apparatus according to claim 1, wherein a new target value for each of the charging voltage, the developing bias voltage, and the exposure light amount is acquired based on the predicted density value.   The density correction apparatus according to claim 8, wherein the potential sensitivity ratio is acquired in advance before acquisition of a new target value by the target value acquisition unit.   The density correction apparatus according to claim 8, wherein the potential sensitivity ratio is acquired during acquisition of a new target value by the target value acquisition unit. Image forming means for forming a toner image on the image carrier based on the image information, transferring the toner image on the image carrier onto the recording medium, and forming a toner image on the recording medium;
Gradation correction means for correcting the gradation of the input image information to the gradation of the output image information based on the relationship between the set input gradation and the output gradation;
A light amount detecting means for detecting a reflected light amount from a toner image formed on the image holding member;
Density measuring means for measuring the density of the toner image formed on the recording medium;
Control means for performing control so that each of the charging voltage, the developing bias voltage, and the exposure light amount becomes a target value set for each;
Gradation control means for performing gradation control by updating the relationship between the set input gradation and the output gradation so that the light amount detection value for the input gradation becomes a set target value;
A light amount detection value detected by the light amount detection means from each gradation region of the toner image formed on the image carrier based on the image information before gradation correction of the first correction image having a plurality of gradation regions; Each gradation area of the toner image formed on the recording medium based on the image information subjected to gradation correction by the gradation correction means of the second correction image having a plurality of gradation areas larger than the first correction image From the measured density value measured by the density measuring means and the relationship between the set input gradation and output gradation, the output density value and the light amount detection value when forming a toner image on the recording medium A correlation acquisition means for acquiring a correlation with
Based on the acquired correlation, a new target value for each of the charging voltage, the developing bias voltage, and the exposure light amount for achieving the target density value of the maximum density gradation area , and the target corresponding to each gradation area Target value acquisition means for acquiring a new target value of gradation control for achieving the density value;
Resetting means for resetting each of the acquired new target values;
An image forming apparatus. An image forming means for forming a toner image on the image carrier based on the image information, transferring the toner image on the image carrier onto the recording medium, and forming the toner image on the recording medium, and a set input level Tone correction means for correcting the tone of the input image information to the tone of the output image information based on the relationship between the tone and the output tone, and the amount of light for detecting the amount of reflected light from the toner image formed on the image carrier Control is performed so that each of the detection means, the density measurement means for measuring the density of the toner image formed on the recording medium, and the charging voltage, the developing bias voltage, and the exposure light amount become the target values set for each. And a gradation control means for performing gradation control by updating a relationship between the set input gradation and the output gradation so that a light amount detection value for the input gradation becomes a set target value. , Density correction processing in an image forming apparatus A program to be executed,
Computer
A light amount detection value detected by the light amount detection means from each gradation region of the toner image formed on the image carrier based on the image information before gradation correction of the first correction image having a plurality of gradation regions. A light quantity acquisition means for acquiring
Each gradation area of the toner image formed on the recording medium based on the image information subjected to gradation correction by the gradation correction means of the second correction image having a plurality of gradation areas larger than the first correction image From the concentration acquisition means for acquiring the measured concentration value measured by the concentration measurement means,
A toner image is formed on the recording medium based on the light amount detection value acquired by the light amount acquisition unit, the measured density value acquired by the density acquisition unit, and the relationship between the set input gradation and output gradation. Correlation acquisition means for acquiring the correlation between the output density value and the light amount detection value in the case,
Based on the acquired correlation, a new target value for each of the charging voltage, the developing bias voltage, and the exposure light amount for achieving the target density value of the maximum density gradation area , and the target corresponding to each gradation area Target value acquisition means for acquiring a new target value of gradation control for achieving the density value;
Resetting means for resetting each of the acquired new target values;
Program to make it work. Gradation correction means for correcting the gradation of the input image information to the gradation of the output image information based on the relationship between the set input gradation and the output gradation;
A light amount detecting means for detecting a reflected light amount from a toner image formed on the image holding member;
Density measuring means for measuring the density of the toner image formed on the recording medium;
Control means for performing control so that the charging voltage, the developing bias voltage, or the exposure light amount becomes a set target value;
Gradation control means for performing gradation control by updating the relationship between the set input gradation and the output gradation so that the light amount detection value for the input gradation becomes a set target value;
A light amount detection value detected by the light amount detection means from each gradation region of the toner image formed on the image carrier based on the image information before gradation correction of the first correction image having a plurality of gradation regions; Each gradation area of the toner image formed on the recording medium based on the image information subjected to gradation correction by the gradation correction means of the second correction image having a plurality of gradation areas larger than the first correction image From the measured density value measured by the density measuring means and the relationship between the set input gradation and output gradation, the output density value and the light amount detection value when forming a toner image on the recording medium A correlation acquisition means for acquiring a correlation with
Based on the acquired correlation, a new target value of the charging voltage, developing bias voltage, or exposure light amount for achieving the target density value of the gradation area of the maximum density, and the target density corresponding to each gradation area Target value acquisition means for acquiring a new target value of gradation control for achieving the value;
Resetting means for resetting the acquired new target value;
Density correction device with