Image forming apparatus

Abstract

To reduce a data amount to be stored in comparison to a case where a correction amount for correcting all the density unevenness in the sub-scanning direction is stored in a correction table while enabling correction of the irregular density unevenness other than the periodical density unevenness in the sub-scanning direction of an image.SOLUTION: A sinusoidal parameter storage unit 23 stores a sinusoidal parameter expressing a correction amount for correcting the periodical density variation caused by a rotor in the sub-scanning direction. A correction table storage unit 22 stores a correction table storing for each position in the sub-scanning direction a correction amount for correcting the density variation not caused by the rotor in the sub-scanning direction. A density correction unit 21 calculates a first correction amount according to a rotation phase of the rotor on the basis of the sinusoidal parameter when an image is formed, acquires a second correction amount according to the position in the sub-scanning direction from the correction table stored in the correction table storage unit 22, and performs density correction of an image by using the calculated first correction amount and the acquired second correction amount.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to an image forming apparatus.

Background Art

[0002] In Patent Document 1, density fluctuations in the sub-scanning direction in an output image are detected by a density sensor, and a periodic pattern of a sine wave indicating the density fluctuations in the sub-scanning direction is obtained based on the output signal of the density sensor and the output signal of a drum period detection sensor, and density correction is performed using this periodic pattern. An image forming apparatus is disclosed.

[0003] In Patent Document 2, when forming an image using a device that makes periodic movements by an electrophotographic method, a correction table is created and stored in which a correction amount for correcting density fluctuations caused by this device is calculated corresponding to the rotational phase, and when forming an image, density correction is performed using the correction amount based on this correction table, and an image processing apparatus that outputs an image is disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to enable correction of irregular density unevenness other than periodic density unevenness in the sub-scanning direction of a formed image, and to reduce the amount of data that must be stored compared to the case of using a correction table in which correction amounts for correcting all density unevenness in the sub-scanning direction are stored for each position in the sub-scanning direction and the rotational phase of a rotating body. To provide an image forming apparatus.

Means for Solving the Problems

[0006] The image forming apparatus according to the first aspect of the present invention includes an output unit that forms an electrostatic latent image on an image carrier by light irradiated by a light source and outputs an image on a recording medium by developing the formed electrostatic latent image, a memory, and a processor, wherein the memory stores a sine wave setting value representing a correction amount for correcting periodic density variations caused by a rotating body in the sub-scanning direction of the image to be formed, and a correction table that stores, for each position in the sub-scanning direction, a correction amount for correcting density variations not caused by the rotating body in the sub-scanning direction of the image to be formed, and the processor when an image is formed in the output unit, calculates a first correction amount corresponding to the rotational phase of the rotating body in the output unit based on the sine wave setting value, and acquires a second correction amount corresponding to the position in the sub-scanning direction from the correction table, and performs density correction of the image formed on the recording medium using the calculated first correction amount and the acquired second correction amount.

[0007] The image forming apparatus according to the second aspect of the present invention is the image forming apparatus according to the first aspect, wherein in the correction table, a correction amount for correcting density variations in the vicinity of the maximum density among the periodic density variations caused by the rotating body in the sub-scanning direction of the image to be formed is stored in association with the rotational phase of the rotating body, and when correcting density variations in the middle tone caused by the rotating body in the sub-scanning direction, the processor performs density correction using the first correction amount calculated based on the sine wave setting value, and when correcting density variations in the vicinity of the maximum density, the processor performs density correction using a second correction amount acquired from the correction table according to the rotational phase of the rotating body.

[0008] In the image forming apparatus according to the third aspect of the present invention, in the image forming apparatus according to the second aspect, when the processor performs density correction using the first correction amount, the processor performs density correction by changing the pixel value of the image to be formed, and when performing density correction for density fluctuations caused by the rotating body using the second correction amount, the processor performs density correction by changing the exposure amount of the exposure device in the output unit.

[0009] In the image forming apparatus according to the fourth aspect of the present invention, in the image forming apparatus according to the first aspect, when the processor corrects the intermediate tone density fluctuation caused by the rotating body in the sub-scanning direction and when correcting the density fluctuation near the maximum density, the processor performs density correction using the first correction amount calculated based on the sine wave setting value.

[0010] In the image forming apparatus according to the fifth aspect of the present invention, in the image forming apparatus according to the fourth aspect, when the processor performs density correction for the intermediate tone using the first correction amount, the processor performs density correction by changing the pixel value of the image to be formed, and when performing density correction near the maximum density using the first correction amount, the processor performs density correction by changing the exposure amount of the exposure device in the output unit.

[0011] In the image forming apparatus according to the sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the rotating body is a photoreceptor roll or a developing roll in the output unit.

[0012] An image forming apparatus according to a seventh aspect of the present invention includes an output unit that forms an electrostatic latent image on an image holding body by light irradiated by a light source and outputs an image on a recording medium by developing the formed electrostatic latent image, a first correction unit that generates a correction table storing correction amounts for correcting periodic density fluctuations caused by a rotating body and density fluctuations not caused by the rotating body based on input density fluctuation data. Based on the input density fluctuation data, a sine wave set value representing a correction amount for correcting periodic density fluctuations caused by the rotating body and a correction table storing a correction amount for correcting density fluctuations not caused by the rotating body are generated by a second correction unit. A memory, Comprising a processor, The processor Displays a screen for confirming with the user which of a first density correction method that prioritizes reducing density unevenness in the middle tone and a second density correction method that prioritizes reducing density unevenness near the maximum density is to be selected. When the first density correction method is selected, the density fluctuation data generated by detecting a middle tone inspection image is input to the first correction unit, the generated correction table is stored in the memory, and when an image is formed in the output unit, a correction amount corresponding to the rotation phase of the rotating body in the output unit and a correction amount corresponding to the position in the sub-scanning direction are acquired from the correction table to perform density correction on the image formed on the recording medium. When the second density correction method is selected, the density fluctuation data generated by detecting a density inspection image near the maximum density is input to the first correction unit, the generated correction table is stored in the memory, the density fluctuation data generated by detecting a middle tone inspection image is input to the second correction unit, the generated correction table and sine wave set value are stored in the memory, and when an image is formed in the output unit, if the pixel value of the pixel to be formed is near the maximum density, a correction amount corresponding to the rotation phase of the rotating body in the output unit and a correction amount corresponding to the position in the sub-scanning direction are acquired from the correction table to perform density correction on the image formed on the recording medium. If the pixel value of the pixel to be formed is in the middle tone, a correction amount corresponding to the rotation phase of the rotating body in the output unit is calculated based on the sine wave set value, and a correction amount corresponding to the position in the sub-scanning direction is acquired from the correction table to perform density correction on the image formed on the recording medium.

Effect of the Invention

[0013] According to the image forming apparatus of the first aspect of the present invention, while enabling correction of irregular density unevenness other than periodic density unevenness in the sub-scanning direction of the formed image, compared with the case of using a correction table in which correction amounts for correcting all density unevenness in the sub-scanning direction are stored for each position in the sub-scanning direction and the rotational phase of the rotating body, the amount of data that must be stored can be reduced.

[0014] According to the image forming apparatus of the second aspect of the present invention, for periodic density variations near the maximum density caused by the rotating body, higher-precision density correction can be performed compared with the case where density correction is performed using a sine wave set value.

[0015] According to the image forming apparatus of the third aspect of the present invention, even when performing density correction for density variations near the maximum density, the density can be increased.

[0016] According to the image forming apparatus of the fourth aspect of the present invention, when correcting density variations caused by the rotating body in the sub-scanning direction, compared with the case where density correction is performed using a correction table, the amount of data that must be stored can be further reduced.

[0017] According to the image forming apparatus of the fifth aspect of the present invention, even when performing density correction for density variations near the maximum density, the density can be increased.

[0018] According to the image forming apparatus of the sixth aspect of the present invention, while enabling correction of irregular density unevenness other than periodic density unevenness in the sub-scanning direction of the formed image, compared with the case of using a correction table in which correction amounts for correcting all density unevenness in the sub-scanning direction are stored for each position in the sub-scanning direction and the rotational phase of the rotating body, the amount of data that must be stored can be reduced.

[0019] According to the image forming apparatus of the seventh aspect of the present invention, it becomes possible for the user to select whether to prioritize reduction of density unevenness in the halftone or reduction of density unevenness near the maximum density.

Brief Description of the Drawings

[0020]

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Embodiments for Carrying Out the Invention

[0021] Next, embodiments of the present invention will be described in detail with reference to the drawings.

[0022] FIG. 1 is a diagram showing the configuration of an image forming apparatus 10 according to an embodiment of the present invention.

[0023] As shown in FIG. 1, the image forming apparatus 10 includes an image forming unit 14K, 14C, 14M, 14Y, an intermediate transfer belt 16, a paper tray 17, a paper conveyance path 18, a fixing unit 19, and a control unit 20. This image forming apparatus 10 has a printer function for printing image data received from a personal computer (not shown) or the like.

[0024] First, to explain the outline of the image forming apparatus 10, a control unit 20 is disposed above the image forming apparatus 10. The control unit 20 performs image processing such as gradation correction and resolution correction on the image data input from a personal computer (not shown) or the like via a network line such as a LAN, and outputs it to the image forming unit 14.

[0025] Then, below the control unit 20, four image forming units 14K, 14C, 14M, and 14Y are disposed corresponding to the colors constituting the color image. In the present embodiment, the four image forming units 14K, 14C, 14M, and 14Y corresponding to the respective colors of black (K), cyan (C), magenta (M), and yellow (Y) are horizontally arranged at regular intervals along the intermediate transfer belt 16. The intermediate transfer belt 16 rotates in the direction of arrow A in the figure as an intermediate transfer body, and these four image forming units 14K, 14Y, 14M, and 14C sequentially form toner images of respective colors based on the image data input from the control unit 20, and transfer (primary transfer) them to the intermediate transfer belt 16 at the timing when these plurality of toner images are superimposed on each other. Note that the order of the colors of the respective image forming units 14K, 14C, 14M, and 14Y is not limited to the order of black (K), cyan (C), magenta (M), and yellow (Y), and the order can be arbitrary, such as the order of yellow (Y), magenta (M), cyan (C), and black (K).

[0026] The paper conveyance path 18 is disposed below the intermediate transfer belt 16. The recording paper 32 supplied from the paper tray 17 is conveyed on this paper conveyance path 18, and the toner images of various colors multiply transferred onto the intermediate transfer belt 16 are collectively transferred (secondary transfer), and the transferred toner images are fixed by the fixing unit 19 and discharged to the outside along the arrow B.

[0027] Next, each component of the image forming apparatus 10 will be described in more detail.

[0028] The control unit 20 performs predetermined image processing such as shading correction, brightness / color space conversion, and gamma correction on the input image data. When the input image data is, for example, data of red (R), green (G), and blue (B) (each 8 bits), it is converted into four-color original color material gradation data of black (K), cyan (C), magenta (M), and yellow (Y) (each 8 bits) by the image processing by the control unit 20.

[0029] The image forming units 14K, 14C, 14M, and 14Y (image forming means) are arranged in parallel at regular intervals in the horizontal direction and are substantially the same in configuration except for the color of the image to be formed. Therefore, the image forming unit 14K will be described below. The configuration of each image forming unit 14 is distinguished by adding K, C, M, or Y.

[0030] The image forming unit 14K includes an exposure unit 140K that irradiates light according to the image data input from the control unit 20, and an image forming device 150K in which an electrostatic latent image is formed by the laser light scanned by the exposure unit 140K.

[0031] The exposure unit 140K exposes the photosensitive drum 152K by irradiating laser light corresponding to black (K) image data onto the photosensitive drum 152K of the image forming apparatus 150K, thereby forming an electrostatic latent image on the photosensitive drum 152K. The exposure unit 140K is composed of a plurality of rod-shaped LPHs (LED Print Heads) in which a plurality of LEDs, which are light emitting elements, are respectively arranged. Details of the configuration of this exposure unit 140K will be described later.

[0032] The image forming apparatus 150K includes a photosensitive drum 152K that rotates at a predetermined rotational speed along the direction of arrow A, a charging device 154K as charging means for uniformly charging the surface of the photosensitive drum 152K, a developing device 156K for developing the electrostatic latent image formed on the photosensitive drum 152K, and a cleaning device 158K. The photosensitive drum 152K is a cylindrical image holding member that holds an image developed with a developer such as toner. It is uniformly charged by the charging device 154K and an electrostatic latent image is formed by the laser light irradiated by the exposure unit 140K. The electrostatic latent image formed on the photosensitive drum 152K is developed with a developer such as black (K) toner by the developing device 156K and transferred to the intermediate transfer belt 16. Residual toner, paper dust, etc. adhering to the photosensitive drum 152K after the transfer process of the toner image (developer image) are removed by the cleaning device 158K.

[0033] The other image forming units 14C, 14M, and 14Y also similarly have photosensitive drums 152C, 152M, 152Y and developing devices 156C, 156M, 156Y respectively, form toner images of each color of cyan (C), magenta (M), and yellow (Y), and transfer the formed toner images of each color to the intermediate transfer belt 16.

[0034] The intermediate transfer belt 16 is looped with a certain tension between the drive roll 164, the idle rolls 165, 166, 167, the backup roll 168, and the idle roll 169. The drive roll 164 is rotationally driven by a drive motor (not shown), and thus the intermediate transfer belt 16 is circulated and driven at a predetermined speed in the direction of arrow A. This intermediate transfer belt 16 is formed, for example, by forming a synthetic resin film such as flexible polyimide into a belt shape, and connecting both ends of the synthetic resin film formed in this belt shape by welding or the like to form an endless belt shape.

[0035] Also, primary transfer rolls 162K, 162C, 162M, and 162Y are respectively disposed at positions facing the respective image forming units 14K, 14C, 14M, and 14Y on the intermediate transfer belt 16. The toner images of each color formed on the photoreceptor rolls 152K, 152C, 152M, and 152Y are multiply transferred onto the intermediate transfer belt 16 by these primary transfer rolls 162. The residual toner attached to the intermediate transfer belt 16 is removed by a cleaning blade or brush of a belt cleaning device 189 provided downstream of the secondary transfer position.

[0036] Also, a density sensor 170 is provided in the vicinity of the intermediate transfer belt 16. This density sensor 170 is a density detection unit that detects the density of the toner image transferred onto the intermediate transfer belt 16. In this embodiment, a plurality of density sensors 170 are arranged, and the positions where the density sensors 170 are arranged will be described later.

[0037] In the paper conveyance path 18, a paper feed roll 181 for taking out the recording paper 32 from the paper tray 17, first roll pairs 182, second roll pairs 183, and third roll pairs 184 for paper conveyance, and a registration roll 185 for conveying the recording paper 32 to the secondary transfer position at a predetermined timing are arranged.

[0038] Also, at the secondary transfer position on the paper conveyance path 18, a secondary transfer roll 186 that presses against the backup roll 168 is disposed. The toner images of each color multiply transferred onto the intermediate transfer belt 16 are secondarily transferred onto the recording paper 32 by the pressing force and electrostatic force of this secondary transfer roll 186. The recording paper 32 onto which the toner images of each color are transferred is conveyed to the fixing unit 19 by the conveyance belts 187 and 188.

[0039] The fixing unit 19 melts and fixes the toner to the recording paper 32 by performing a heat treatment and a pressure treatment on the recording paper 32 onto which the toner images of the above-mentioned respective colors are transferred.

[0040] Note that the developing device 156K has a cylindrical developing roll (developer conveyance unit) 157K that conveys the developer to the photosensitive drum 152K by rotating and forms a developer image on the photosensitive drum 152K. Note that the developing devices 156C, 156M, and 156Y for forming images of other colors are also provided with developing rolls therein in the same manner.

[0041] In the image forming apparatus 10 of the present embodiment, an image is formed on a recording medium such as printing paper by the electrophotographic method having the configuration as described above. The output unit is configured by the image forming unit 14, the intermediate transfer belt 16, the fixing unit 19, etc. described above. This output unit forms an electrostatic latent image on the photosensitive drum 152 which is an image holding body by the light irradiated by the light source, and outputs an image on the recording medium by developing the formed electrostatic latent image.

[0042] However, in the image forming apparatus 10 of the present embodiment, since an image is formed using rotating bodies such as the photosensitive drum 152 and the developing roll 157, periodic density unevenness (density variation) may occur in the sub-scanning direction which is the paper conveyance direction.

[0043] For example, the positional relationship between the photosensitive drum 152 and the developing roll 157 is shown in FIG. 2.

[0044] As can be seen with reference to FIG. 2, the photoreceptor roll 152 and the developing roll 157 are provided to face each other with a certain gap therebetween. The developing roll 157 holds the developer on its surface by the magnetic force of a magnet provided inside, and performs a rotational movement to convey the developer held in the gap formed between the developing roll 157 and the photoreceptor roll 152, thereby visually developing the electrostatic latent image formed on the surface of the photoreceptor roll 152.

[0045] However, when the rotation axes of the photoreceptor roll 152 and the developing roll 157 are displaced from the ideal rotation axes and are eccentric, the gap between the photoreceptor roll 152 and the developing roll 157 will periodically change. Also, the same problem occurs when the photoreceptor roll 152 and the developing roll 157 are not arranged completely parallel. The same problem also occurs when the shapes of the photoreceptor roll 152 and the developing roll 157 themselves are distorted or deflected.

[0046] Due to such causes, periodic density unevenness may occur in the sub-scanning direction in the formed image.

[0047] Here, with reference to FIG. 3, the names of the directions in the formed image and the arrangement of the image forming apparatus 10 will be described. As shown in FIG. 3, the direction in which the laser beam is scanned by the exposure unit 140, that is, the longitudinal direction of the photoreceptor roll 152, is defined as the main scanning direction. And the direction orthogonal to this main scanning direction, that is, the paper conveyance direction in which printing paper or the like is conveyed, is defined as the sub-scanning direction.

[0048] Next, the hardware configuration of the control device 20 that controls the operation of the image forming apparatus 10 of the present embodiment is shown in FIG. 4.

[0049] As shown in FIG. 4, the control device 20 includes a CPU 41, a memory 42, a storage device 43 such as a hard disk drive, a communication interface (abbreviated as IF) 44 that transmits and receives data to and from external devices via a network, and a user interface (abbreviated as UI) device 45 that includes a touch panel or a liquid crystal display and a keyboard. These components are connected to each other via a control bus 46.

[0050] The CPU 41 is a processor that executes predetermined processing based on a control program stored in the memory 42 or the storage device 43 to control the operation of the control device 20. In this embodiment, the CPU 41 is described as reading and executing a control program stored in the memory 42 or the storage device 43, but it is also possible to store the program in a storage medium such as a CD-ROM and provide it to the CPU 41.

[0051] FIG. 5 is a block diagram showing the functional configuration of the control unit 20 realized by executing the above control program.

[0052] As shown in FIG. 5, the control unit 20 includes a density correction unit 21, a correction table storage unit 22, and a sine wave parameter storage unit 23.

[0053] The density correction unit 21 detects density unevenness in the output image based on density information such as the density value detected by the density sensor 170, and adjusts the exposure amount in the exposure unit 140 or changes the pixel value when forming an image so as to suppress the detected density unevenness. The density correction unit 21 determines the position in the image when performing such density correction based on rotation phase information such as the Z-phase signal of the photoreceptor roll 152, rotation phase information such as the Z-phase signal of the developing roll 157, the page start signal, and the scan start signal.

[0054] Here, by performing density unevenness correction using a correction table in which correction amounts are stored for each position in the sub-scanning direction, it becomes possible to accurately correct even finer density fluctuations. However, if correction for all density unevenness in the sub-scanning direction is performed using such a correction table, it is necessary to store a correction table corresponding to the rotation phase of the photoreceptor roll 152, a correction table corresponding to the rotation phase of the developing roll 157, a correction table corresponding to the page position, and so on. Therefore, there was a drawback that the amount of data to be stored became large.

[0055] On the other hand, by expressing and storing the correction amount for correcting periodic density fluctuations by sine wave parameters, it becomes possible to reduce the amount of data to be stored. However, when performing density unevenness correction using the correction amount expressed by sine wave parameters, the correction accuracy of density unevenness may deteriorate compared to the case of performing density unevenness correction using a correction table. Also, it is difficult to correct more irregular density unevenness caused by collisions between the paper and members using sine wave parameters.

[0056] Therefore, in the present embodiment, for periodic density fluctuations caused by a rotating body in the sub-scanning direction, density correction is performed using sine wave parameters (sine wave setting values), and for density fluctuations not caused by a rotating body in the sub-scanning direction, density correction is performed using a correction table.

[0057] The sine wave parameter storage unit 23 stores sine wave parameters (sine wave setting values) that represent correction amounts for correcting periodic density fluctuations caused by a rotating body in the sub-scanning direction of the formed image.

[0058] The correction table storage unit 22 stores a correction table in which correction amounts for correcting density fluctuations not caused by a rotating body in the sub-scanning direction of the formed image are stored for each position in the sub-scanning direction.

[0059] Based on the density value from the density sensor 170, the density correction unit 21 extracts the sine wave parameters representing the periodic variations among the detected density non-uniformities, and stores in the sine wave parameter storage unit 23, as the correction amount for correcting the density non-uniformity, the sine wave parameters with characteristics to cancel out the extracted sine wave parameters.

[0060] Also, the density correction unit 21 generates a correction table storing, for each position in the sub-scanning direction, the correction amount for correcting the density variations other than the periodic density variations among the detected density non-uniformities based on the density value from the density sensor 170, and stores it in the correction table storage unit 22.

[0061] Then, when an image is formed in the image output unit, the density correction unit 21 calculates the first correction amount corresponding to the rotational phase of the rotating body such as the photoreceptor roll 152 or the developing roll 157 based on the sine wave parameters, and acquires the second correction amount corresponding to the position in the sub-scanning direction from the correction table stored in the correction table storage unit 22.

[0062] When an image is formed on the recording medium, the density correction unit 21 corrects the density of the image formed on the recording medium using the calculated first correction amount and the acquired second correction amount.

[0063] Note that in the density correction for correcting the density non-uniformity, different density corrections are performed depending on whether correcting the density variations near the maximum density or correcting the density variations in the middle tone.

[0064] Therefore, when minimizing the amount of data to be stored, for the periodic density non-uniformity caused by the rotating body in the sub-scanning direction, density correction may be performed for both the density variations near the maximum density and the density variations in the middle tone using the correction amount using the sine wave parameters.

[0065] Specifically, when correcting the density variation in the halftone due to the rotating body in the sub-scanning direction and when correcting the density variation near the maximum density, the density correction unit 21 performs density correction using the first correction amount calculated based on the sine wave parameters in both cases.

[0066] When the density correction unit 21 performs density correction for the halftone using the first correction amount, it corrects the density by changing the pixel value of the image to be formed. When performing density correction near the maximum density using the first correction amount, it corrects the density by changing the exposure amount of the exposure unit 140.

[0067] Also, when it is desired to correct the density variation near the maximum density with higher accuracy, in the correction table stored in the correction table storage unit 22, the correction amount for correcting the density variation near the maximum density among the periodic density variations caused by the rotating body in the sub-scanning direction of the image to be formed is stored in association with the rotation phase of the rotating body.

[0068] In this case, when the density correction unit 21 corrects the density variation in the halftone due to the rotating body in the sub-scanning direction, it performs density correction using the first correction amount calculated based on the sine wave parameters. When correcting the density variation near the maximum density, it performs density correction using the second correction amount obtained from the correction table according to the rotation phase of the rotating body.

[0069] When the density correction unit 21 performs density correction using the first correction amount, it corrects the density by changing the pixel value of the image to be formed. When performing density correction for the density variation caused by the rotating body using the second correction amount, it corrects the density by changing the exposure amount of the exposure device in the image output unit.

[0070] Specifically, the rotating body that causes density unevenness in the sub-scanning direction of the image to be formed is the photoreceptor roll 152 or the developing roll 157.

[0071] Next, with reference to the drawings, the operation of the density correction unit 21 in the pixel 10 of the present embodiment during density correction will be described in detail.

[0072] First, the operation when the density correction unit 21 creates a sine wave parameter and a correction table for density correction is shown in the flowchart of FIG. 6.

[0073] First, in step S101, the image output unit forms a patch image with a density near the maximum density (Cin≈100%) (hereinafter referred to as a solid density) on the intermediate transfer belt 16, and the density correction unit 21 obtains a density variation profile continuous in the sub-scanning direction, which is the paper conveyance direction, by detecting the density of this patch image with the density sensor 170.

[0074] Here, the density sensor 170 may be configured to detect the density of the unfixed image not only on the intermediate transfer belt 16 but also on the photoreceptor roll 152. Then, the density sensor 170 irradiates light on the unfixed image on the intermediate transfer belt 16 or the photoreceptor roll 152 and detects the reflection intensity. Note that the reflection intensity of the image fixed on the paper may be detected by an in-line sensor or a scanner.

[0075] Note that Cin is the tone area ratio representing the tone by the toner amount when the maximum available toner amount of a certain color is set to 100%. That is, Cin = 100% means the maximum density. Also, the vicinity of the maximum density means the range of Cin = 80 to 100%.

[0076] Next, in step S102, the density correction unit 21 creates a sine wave parameter for density correction based on the obtained density variation profile.

[0077] Specifically, the density correction unit 21 performs an inner product operation of the obtained density fluctuation profile with sine waves and cosine waves of a period and its higher-order components determined according to the circumferences of rotating bodies such as the photoreceptor roll 152 and the developing roll 157, and the peripheral speed ratio between members. From this inner product result, the phase and amplitude when a sine wave is applied to the density fluctuation profile are obtained. A waveform example of the result when a sine wave is applied to the density fluctuation profile obtained in this way is shown in FIG. 7.

[0078] Then, the density correction unit 21 obtains a phase difference from the distance and period between the head of the density fluctuation profile and the Z-phase signal of the rotating body, and converts the obtained phase into a phase with respect to the Z-phase of the rotating member. Note that it is also possible to use the least squares method more precisely instead of performing the inner product operation to obtain the inner product result.

[0079] Then, the density correction unit 21 shifts the obtained phase by π with respect to the phase and amplitude, corrects the amplitude according to the input / output response when performing density correction, and generates sine wave parameters for density correction. The generated sine wave parameters are stored in the sine wave parameter storage unit 23.

[0080] Next, in step S103, the image output unit forms patch images with intermediate densities, for example, Cin ≒ 25%, 50%, 75% on the intermediate transfer belt 16, and the density correction unit 21 obtains a density fluctuation profile continuous in the sub-scanning direction, which is the paper conveyance direction, by detecting the density of this patch image with the density sensor 170.

[0081] When obtaining this intermediate-density density fluctuation profile, the patch image may be formed while performing density correction using the sine wave parameters obtained in step S102.

[0082] Then, in step S104, the density correction unit 21 creates sine wave parameters for intermediate-density density correction based on the intermediate-density density fluctuation profile obtained in step S103. The specific generation method of the sine wave parameters is the same as the generation method described in step S102.

[0083] Next, in step S105, the image output unit forms a solid density patch image near the maximum density on the intermediate transfer belt 16, and the density correction unit 21 obtains a density variation profile continuous in the sub-scanning direction, which is the paper conveyance direction, by detecting the density of this patch image with the density sensor 170.

[0084] Note that in this step S105, a density variation profile for correcting density unevenness caused by a cause other than the periodic density unevenness due to the rotating body is obtained. For example, impulse binding generated by vibrations or the like that occur when the paper enters a member is a correction target.

[0085] Also in this step S105, a solid density patch image is formed to obtain a density variation profile. However, the density variation profile here can be considered as a part of a signal having a period from the page start signal to the page start signal of the next sheet, for example, starting from the page start signal.

[0086] The density correction unit 21 may obtain the position from the page start signal of the density variation profile and repeatedly form patch images to obtain a plurality of density variation profiles.

[0087] Note that when obtaining the density variation profile in step S105, the patch image is formed in a state where density correction of periodic density unevenness is performed using the sine wave parameters generated in steps S102 and S104.

[0088] Then, the density correction unit 21 averages the obtained density variation profiles based on the position from the page start signal. An example of the density variation profile obtained in this way is shown in FIG. 8.

[0089] Then, in step S106, the density correction unit 21 obtains a density correction amount by inverting the sign of the density fluctuation profile thus obtained, and generates a correction table based on this density correction amount.

[0090] An example of obtaining the density correction amount by inverting the sign of the density fluctuation profile is shown in FIG. 9. FIG. 9(A) is a density fluctuation profile showing the density fluctuation amount for each position in the sub-scanning direction, and FIG. 9(B) is a density correction amount profile in which the density fluctuation is canceled by inverting the sign of FIG. 9(A).

[0091] Note that when performing this density correction, the density correction unit 21 adjusts the exposure amount of the exposure unit 140 to perform density correction. Therefore, in the actual correction table, a value obtained by multiplying the sensitivity when changing the exposure amount is stored in the correction table.

[0092] Next, in step S107, the image output unit forms a halftone density patch image on the intermediate transfer belt 16, and the density sensor 170 detects the density of this patch image, whereby the density correction unit 21 obtains a halftone density fluctuation profile continuous in the sub-scanning direction, which is the paper conveyance direction. Note that the method for obtaining the density fluctuation profile is the same as the method described in step S105 except that the density of the patch image to be formed is different.

[0093] Note that when obtaining the density fluctuation profile in step S107, the patch image is formed in a state where density correction for density unevenness is performed using the sine wave parameters generated in steps S102 and S104 and the correction table generated in step S106.

[0094] Next, in step S108, the density correction unit 21 obtains a density correction amount by inverting the sign of the obtained density fluctuation profile of the halftone, and generates a correction table based on this density correction amount. Here, the specific method for generating the correction table is the same as the method described in step S106 above. However, the density correction amount stored in the correction table is converted into a correction amount for changing the pixel value of the image to be formed, rather than the correction amount for adjusting the exposure amount.

[0095] Next, in step S109, a density fluctuation profile of solid density is obtained in the main scanning direction orthogonal to the sub-scanning direction. Here, the object of density correction is the density unevenness in the main scanning direction that is repeated for each page due to uneven wear, cutting marks of the photoreceptor roll 152, light amount unevenness of the exposure unit 140, and the like.

[0096] Note that when obtaining the density fluctuation profile in step S109, a patch image is formed in a state where density correction of density unevenness is performed using the sine wave parameters generated in steps S102 and S104, and the correction table generated in steps S106 and S108.

[0097] Then, in step S110, the density correction unit 21 obtains a density correction amount by inverting the sign of the obtained density fluctuation profile of solid density, and generates a correction table based on this density correction amount. Here, the specific method for generating the correction table is the same as the method described in step S106 above. Also, the density correction amount stored in the correction table stores the correction amount for adjusting the exposure amount as in step S106.

[0098] Finally, in step S111, a density fluctuation profile of halftone is obtained in the main scanning direction orthogonal to the sub-scanning direction.

[0099] Then, in step S112, the density correction unit 21 obtains a density correction amount by inverting the sign of the obtained density fluctuation profile of the halftone, and generates a correction table based on this density correction amount. Here, the specific method for generating the correction table is the same as the method described in step S106 above. Also, as the density correction amount to be stored in the correction table, a correction amount for changing the pixel value of the image to be formed is stored.

[0100] Note that the generation of the correction table and the generation of the sine wave parameters described above may be performed at a plurality of locations in the main scanning direction.

[0101] FIG. 10 shows the combination of the direction of density unevenness, the density to be corrected, the storage method of the correction amount, and the correction method in the density unevenness correction described above.

[0102] In the combination example shown in FIG. 10, it can be seen that the correction amount for correcting the periodic density unevenness in the sub-scanning direction caused by rotating bodies such as the photoreceptor roll 152 and the developing roll 157 is stored as sine wave parameters, and the correction amount for correcting the density unevenness in the sub-scanning direction caused by causes other than the rotating body is stored as a correction table.

[0103] Note that as for the method of correcting density unevenness, two methods are shown: a method of changing the exposure amount and a method of changing the pixel value. However, the method of changing the pixel value cannot be used when performing density correction of solid density. This is because even if an attempt is made to increase the pixel value of a certain image to correct density unevenness, the tone area ratio Cin cannot be made 100% or more, so correction to darken the density cannot be performed. That is, when performing density correction of solid density, density correction is performed by changing the exposure amount.

[0104] Finally, the process of actually forming an image using the correction amount for density correction obtained as described above will be described with reference to the flowchart of FIG. 11.

[0105] First, in step S201, when image data for image formation is input, the density correction unit 21 determines a processing pixel.

[0106] Then, in step S202, the density correction unit 21 obtains a correction amount for correcting a pixel value from a correction table stored in the correction table storage unit 22 based on the position in the sub-scanning direction, the position in the main scanning direction, and the Z-phase signal of the rotating body, and adds it to the pixel value of the processing pixel.

[0107] Next, in step S203, the density correction unit 21 calculates a correction amount corresponding to the Z-phase signal of the rotating body based on the sine wave parameters stored in the sine wave parameter storage unit 23. Then, the density correction unit 21 adds the correction amount calculated based on the sine wave parameters to the pixel value corrected by the correction amount obtained from the correction table.

[0108] Next, in step S204, the density correction unit 21 obtains a correction amount for correcting the exposure amount from the correction table stored in the correction table storage unit 22 based on the position of the processing pixel in the sub-scanning direction, the position in the main scanning direction, and the Z-phase signal of the rotating body.

[0109] Furthermore, in step S205, the density correction unit 21 calculates a correction amount for exposure amount correction corresponding to the Z-phase signal of the rotating body based on the sine wave parameters stored in the sine wave parameter storage unit 23.

[0110] Then, in step S206, the density correction unit 21 adds the correction amount for correcting the exposure amount obtained from the correction table and the correction amount for exposure amount correction calculated based on the sine wave parameters.

[0111] Then, in step S207, the density correction unit 21 corrects the exposure amount when exposing the processing pixel by the exposure unit 140 with the correction amount obtained by addition.

[0112] In the density correction method for density unevenness in the sub-scanning direction described above, both the density unevenness of solid density and the density unevenness of halftone were corrected by the correction amount calculated based on the sine wave parameters. In the following, for the density unevenness of solid density, the case of correcting by the correction amount based on the correction table will be described.

[0113] The operation of the density correction unit 21 when performing density correction of periodic density unevenness in the sub-scanning direction of solid density using a correction table will be described with reference to the flowchart of FIG. 12.

[0114] Note that the flowchart of FIG. 12 is the same as the flowchart shown in FIG. 6 except that step S102 is replaced with step S102a. Therefore, only step S102a will be described here.

[0115] Also in the flowchart of FIG. 12, first, in step S101, the image output unit forms a solid density patch image on the intermediate transfer belt 16, and the density correction unit 21 acquires a density variation profile continuous in the sub-scanning direction. At this time, the density variation profile is acquired such that the period of the rotating body is included a plurality of times in the density variation profile.

[0116] Then, based on the acquired density variation profile, the density correction unit 21 generates a correction table in which the correction amount for correcting the density unevenness in the sub-scanning direction is stored.

[0117] Specifically, the density correction unit 21 refers to the Z-phase signal of the rotating body, divides the density variation profile for each period of the rotating body, and averages the divided density variation profiles based on the position from the Z-phase.

[0118] For example, an example of generating a correction table for storing the correction amount for correcting the density unevenness caused by the photoreceptor roll 152 is shown in FIGS. 13 to 15.

[0119] First, an example of the density fluctuation profile acquired by the density sensor 170 is shown in FIG. 13. The density fluctuation profile shown in FIG. 13 includes a plurality of cycles T of the photoreceptor roll 152.

[0120] Then, FIG. 14 shows how the density correction unit 21 divides the density fluctuation profile shown in FIG. 13 by the cycle T of the photoreceptor roll 152 and performs an averaging process. Referring to FIG. 14, it can be seen that a plurality of density fluctuation profiles divided by the cycle T are averaged to calculate one density fluctuation profile.

[0121] After that, the density correction unit 21 generates a density correction profile that cancels out the density fluctuations caused by the photoreceptor roll 152 by inverting the positive and negative of the density fluctuation profile after the averaging process.

[0122] An example of obtaining the density correction amount by inverting the positive and negative of the density fluctuation profile is shown in FIG. 15. FIG. 15(A) is a density fluctuation profile showing the density fluctuation amount corresponding to the rotation phase of the photoreceptor roll 152, and FIG. 15(B) is a density correction amount profile that cancels out the density fluctuations by inverting the positive and negative of FIG. 15(A).

[0123] Note that when performing this density correction, the density correction unit 21 adjusts the exposure amount of the exposure unit 140 to perform density correction. Therefore, in the actual correction table, a value obtained by multiplying the sensitivity when changing the exposure amount is stored in the correction table.

[0124] FIG. 16 shows the combination of the direction of density unevenness, the density to be corrected, the storage method of the correction amount, and the correction method during the density unevenness correction described above.

[0125] In the combination example shown in FIG. 16, regarding the correction amount for correcting the solid density unevenness among the periodic density unevenness in the sub-scanning direction caused by rotating bodies such as the photoreceptor roll 152 and the developing roll 157, it is stored as a correction table, and regarding the correction amount for correcting the density unevenness of the intermediate tone, it is stored as a sine wave parameter.

[0126] Note that by storing the correction amount for correcting the density unevenness of the solid density in a correction table, the amount of data becomes larger than when storing it as a sine wave parameter, but even finer density unevenness can be corrected with high accuracy.

[0127] In this way, only a part of the correction amount for correcting the periodic density unevenness caused by the rotating body in the sub-scanning direction may be stored as a sine wave parameter, and the other correction amount may be stored in a correction table.

[0128] Next, an example in the case where a user or a CE (Customer Engineer) who is a maintenance person can switch the density unevenness correction method will be described.

[0129] For example, it will be described assuming that the density correction unit 21 includes a first correction unit 51 and a second correction unit 52 as shown in FIG. 17.

[0130] Here, when the density fluctuation profile, which is density fluctuation data obtained by detecting the density of the patch image that is the inspection image by the density sensor 170, is input to the first correction unit 51, based on the input density fluctuation profile, a correction table storing the correction amount for correcting the periodic density unevenness caused by the rotating body and the density unevenness not caused by the rotating body is generated.

[0131] Also, the second correction unit 52 generates a correction table storing sine wave parameters representing the correction amount for correcting the periodic density unevenness caused by the rotating body and the correction amount for correcting the density fluctuation not caused by the rotating body based on the input density fluctuation profile.

[0132] Then, the CPU 41 in the control unit 20 displays, for example, an operation screen as shown in FIG. 18 on the UI device 45, thereby displaying a screen for confirming to the user which of the first density correction method that prioritizes reducing the density unevenness of the middle tone and the second density correction method that prioritizes reducing the density unevenness near the maximum density is to be selected.

[0133] When the first density correction method is selected on the displayed operation screen, the density correction unit 21 inputs the density variation profile generated by detecting the halftone patch image to the first correction unit 51, and stores the generated correction table in the correction table storage unit 22. Then, when an image is formed in the image output unit, the density correction unit 21 obtains, from the correction table, the correction amount corresponding to the rotation phase of the rotating body in the image output unit and the correction amount corresponding to the position in the sub-scanning direction for all pixels of the entire tone, and performs density correction on the image formed on the recording medium.

[0134] The operation of the density correction unit 21 when the first density correction method that prioritizes reduction of density unevenness in the halftone is selected is shown in FIG. 19. As can be seen by referring to FIG. 19, it can be seen that only the first correction unit 51 in the density correction unit 21 executes the process of correcting density unevenness.

[0135] Also, when the second density correction method is selected on the displayed operation screen, the density display unit 21 inputs the density variation profile generated by detecting the patch image of the density near the solid density to the first correction unit 51, stores the generated correction table in the correction table storage unit 22, inputs the density variation profile generated by detecting the halftone patch image to the second correction unit 52, and stores the generated correction table and the sine wave setting value in the correction table storage unit 22 and the sine wave parameter storage unit 23, respectively. Then, when an image is formed in the image output unit, when the pixel value of the pixel to be formed is near the solid density, the density correction unit 21 obtains, from the correction table, the correction amount corresponding to the rotation phase of the rotating body in the image output unit and the correction amount corresponding to the position in the sub-scanning direction, and performs density correction on the image formed on the recording medium. Also, when the pixel value of the pixel to be formed is in the halftone, the density correction unit 21 calculates the correction amount corresponding to the rotation phase of the rotating body in the image output unit based on the sine wave parameter, and obtains the correction amount corresponding to the position in the sub-scanning direction from the correction table, and performs density correction on the image formed on the recording medium.

[0136] The operation of the density correction unit 21 when the second density correction method that prioritizes reducing density unevenness near the solid density is selected is shown in FIG. 20. As can be seen with reference to FIG. 20, in the density correction unit 21, while the first correction unit 51 corrects the periodic density unevenness and aperiodic density unevenness of the solid density using a correction table, it can be understood that the second correction unit 52 corrects the periodic density unevenness of the halftone density using sine wave parameters and corrects the aperiodic density unevenness using a correction table.

[0137] In each of the above embodiments, the processor refers to a processor in a broad sense and includes a general-purpose processor (for example, CPU: Central Processing Unit, etc.) and a dedicated processor (for example, GPU: Graphics Processing Unit, ASIC: Application Specific Integrated Circuit, FPGA: Field Programmable Gate Array, programmable logic device, etc.).

[0138] Also, the operation of the processor in each of the above embodiments may be achieved not only by one processor but also by a plurality of physically separated processors cooperating. Further, the order of each operation of the processor is not limited to the order described in each of the above embodiments and may be changed as appropriate.

Explanation of Reference Numerals

[0139] 10 Image forming apparatus 14 Image forming unit 16 Intermediate transfer belt 17 Paper tray 18 Paper conveyance path 19 Fixer 20 Control unit 21 Density correction unit 22 Correction table storage unit 23 Sine wave parameter storage unit 32 Recording paper 41 CPU 42 Memory 43 Storage device 44 Communication interface 45 User interface device 46 Scanner 47 Print engine 48 Control bus 51 First correction unit 52 Second correction unit 140 Exposure unit 150 Image forming apparatus 152 Photoconductor roll 154 Charging device 156 Developing device 158 Cleaning device 162 Primary transfer roll 164 Drive roll 165, 166, 167 Idle roll 168 Backup roll 169 Idle roll 170 Density sensor 189 Cleaning device 168 Backup roll 181 Paper feed roll 182, 183, 184 Roll pair 185 Registration roll 186 Secondary transfer roll 187, 188 Conveyor belt

Claims

1. An output unit that forms an electrostatic latent image on an image holding body by light irradiated by a light source and outputs an image on a recording medium by developing the formed electrostatic latent image; a memory; a processor, wherein the memory stores a sine wave set value representing a correction amount for correcting periodic density variations caused by a rotating body in the sub-scanning direction of the image to be formed; and a correction table that stores, for each position in the sub-scanning direction, a correction amount for correcting density variations not caused by the rotating body in the sub-scanning direction of the image to be formed, and the processor when an image is formed in the output unit, calculates a first correction amount corresponding to the rotation phase of the rotating body in the output unit based on the sine wave set value, and acquires a second correction amount corresponding to the position in the sub-scanning direction from the correction table, performs density correction on the image formed on the recording medium using the calculated first correction amount and the acquired second correction amount, in the correction table, a correction amount for correcting density variations near the maximum density among the periodic density variations caused by the rotating body in the sub-scanning direction of the image to be formed is stored in association with the rotation phase of the rotating body, when the processor corrects intermediate tone density variations caused by the rotating body in the sub-scanning direction, it performs density correction using the first correction amount calculated based on the sine wave set value, and when correcting density variations near the maximum density, it performs density correction using the second correction amount acquired from the correction table according to the rotation phase of the rotating body, An image forming apparatus.

2. The image forming apparatus according to claim 1, wherein when the processor performs density correction using the first correction amount, it performs density correction by changing the pixel value of the image to be formed, and when performing density correction on density variations caused by the rotating body using the second correction amount, it performs density correction by changing the exposure amount of the exposure device in the output unit.

3. An output unit that forms an electrostatic latent image on an image holding body by light irradiated by a light source and outputs an image on a recording medium by developing the formed electrostatic latent image; a memory; a processor, wherein the memory stores a sine wave set value representing a correction amount for correcting periodic density variations caused by a rotating body in the sub-scanning direction of the image to be formed; A correction table that stores, for each position in the sub-scanning direction, a correction amount for correcting density fluctuations not caused by a rotating body in the sub-scanning direction of the formed image. The processor When an image is formed in the output unit, a first correction amount corresponding to the rotation phase of the rotating body in the output unit is calculated based on the sine wave setting value, and a second correction amount corresponding to the position in the sub-scanning direction is obtained from the correction table. Using the calculated first correction amount and the obtained second correction amount, density correction of the image formed on the recording medium is performed. When correcting density fluctuations in the middle tone caused by the rotating body in the sub-scanning direction and when correcting density fluctuations near the maximum density, density correction is performed using the first correction amount calculated based on the sine wave setting value. When performing density correction in the middle tone using the first correction amount, density correction is performed by changing the pixel value of the image to be formed. When performing density correction near the maximum density using the first correction amount, density correction is performed by changing the exposure amount of the exposure device in the output unit. An image forming apparatus.

4. The image forming apparatus according to any one of claims 1 to 3, wherein the rotating body is a photoreceptor roll or a developing roll in the output unit.

5. An output unit that forms an electrostatic latent image on an image carrier by light irradiated by a light source and outputs an image on a recording medium by developing the formed electrostatic latent image. A first correction unit that generates a correction table storing correction amounts for correcting periodic density fluctuations caused by a rotating body and density fluctuations not caused by the rotating body based on the input density fluctuation data. A second correction unit that generates a sine wave setting value representing a correction amount for correcting periodic density fluctuations caused by a rotating body and a correction table storing correction amounts for correcting density fluctuations not caused by the rotating body based on the input density fluctuation data. A memory A processor is provided. The processor Displays a screen for confirming with the user which of a first density correction method that prioritizes reducing density unevenness in the middle tone and a second density correction method that prioritizes reducing density unevenness near the maximum density is to be selected. When the first density correction method is selected, the density fluctuation data generated by detecting the halftone inspection image is input to the first correction unit, and the generated correction table is stored in the memory. When an image is formed in the output unit, the correction amount corresponding to the rotational phase of the rotating body in the output unit and the correction amount corresponding to the position in the sub-scanning direction are obtained from the correction table, and density correction of the image formed on the recording medium is performed. When the second density correction method is selected, the density fluctuation data generated by detecting the inspection image of the density near the maximum density is input to the first correction unit, and the generated correction table is stored in the memory. The density fluctuation data generated by detecting the halftone inspection image is input to the second correction unit, and the generated correction table and the sine wave setting value are stored in the memory. When an image is formed in the output unit, when the pixel value of the pixel to be formed is near the maximum density, the correction amount corresponding to the rotational phase of the rotating body in the output unit and the correction amount corresponding to the position in the sub-scanning direction are obtained from the correction table, and density correction of the image formed on the recording medium is performed. When the pixel value of the pixel to be formed is halftone, the correction amount corresponding to the rotational phase of the rotating body in the output unit is calculated based on the sine wave setting value, and the correction amount corresponding to the position in the sub-scanning direction is obtained from the correction table, and density correction of the image formed on the recording medium is performed. Image forming apparatus.

Images