Image forming apparatus

Abstract

To accurately prevent density unevenness of an image formed by an image forming apparatus.SOLUTION: An image forming apparatus 100 comprises: a printer engine 102 that forms an image on sheet; a reader 101 that reads an image for measurement for measuring the image density of the image formed on the sheet; and a CPU 105 that sets a target density in common both in a main scanning direction and sub scanning direction on the basis of a result of reading of the image for measurement performed by the reader 101, determines respective shading correction amounts of a position in the main scanning direction and a position in the sub scanning direction according to the target density, and corrects image density unevenness in the main scanning direction and the sub scanning direction according to the shading correction amounts.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to image density unevenness correction for suppressing density unevenness in an image formed on paper. [Background technology]

[0002] An image forming apparatus employing electrophotography forms an image by, for example, scanning a laser beam onto a photosensitive drum, which is a drum-shaped photosensitive body having a photosensitive layer on its surface. The image forming apparatus uniformly charges the photosensitive layer of the photosensitive drum, and then irradiates (scans) it with a laser beam to form an electrostatic latent image on the photosensitive layer of the photosensitive drum. The electrostatic latent image is developed with toner into a toner image, which is then transferred to paper. The paper onto which the toner image has been transferred is then heated and pressed, for example, to melt and fix the toner image. In this way, an image is formed (printed) on the paper.

[0003] In such an image forming apparatus, there is a possibility that charging unevenness occurs when charging the photosensitive drum, exposure unevenness occurs when scanning, and development unevenness occurs when developing. These unevennesses cause image density unevenness in a predetermined direction in the image formed on the paper. For example, image density unevenness occurs in the main scanning direction and the sub-scanning direction. The main scanning direction is the direction in which the laser light scans the photosensitive drum, and is the drum axis direction (the axial direction of the rotation shaft of the photosensitive drum). The sub-scanning direction is the direction that intersects the main scanning direction and is the rotation direction of the photosensitive drum.

[0004] In order to correct image density unevenness in a predetermined direction, the image formation area is divided into multiple regions in the predetermined direction, and the image density of each region is controlled. In order to detect image density unevenness in the predetermined direction, paper on which a test image for detecting the image density of each region is formed is used. Based on the measurement results of the test image for each region, the laser light intensity is adjusted to eliminate image density differences between regions, thereby correcting image density unevenness. For example, Patent Document 1 discloses a technique for correcting image density unevenness in the main scanning direction. Patent Documents 2 and 3 disclose techniques for correcting image density unevenness in the sub-scanning direction.

[0005] In Patent Document 1, image density unevenness in the main scanning direction is corrected based on the measurement results of multiple test images arranged in the main scanning direction. In Patent Document 2, image density unevenness in the sub-scanning direction that occurs with the rotation period of the developing sleeve is corrected based on the detection results of a toner image formed on a photosensitive belt. The developing sleeve rotates in accordance with the rotation of the photosensitive drum and is a member that adheres toner to an electrostatic latent image. In Patent Document 3, image density unevenness in the sub-scanning direction is corrected based on the measurement results of a test pattern whose longitudinal direction is the sub-scanning direction. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-52239 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-98675 [Patent Document 3] Japanese Patent Publication No. 2022-71704 Summary of the Invention [Problem to be solved by the invention]

[0007] Conventionally, as described above, image density unevenness in the main scanning direction and image density unevenness in the sub-scanning direction are corrected using separate, independent correction controls. In this case, the correction target for image density unevenness in the main scanning direction may differ from the correction target for image density unevenness in the sub-scanning direction. This makes it difficult to accurately correct image density unevenness across the entire paper surface. Specifically, if the image density that serves as the correction target for the main scanning direction is the average image density in the main scanning direction and the image density that serves as the correction target for the sub-scanning direction is the average image density in the sub-scanning direction, the average image density will differ depending on the direction, reducing correction accuracy. The same problem occurs even when the minimum image density is used instead of the average image density.

[0008] SUMMARY OF THE INVENTION In view of the above-mentioned problems, a main object of the present invention is to suppress density unevenness in an image formed by an image forming apparatus with high precision. [Means for solving the problem]

[0009] The image forming apparatus of the present invention includes an image forming unit that forms an image on a rotating photosensitive member and transfers the image formed on the photosensitive member onto a sheet of paper to form an image on the sheet of paper, a reading unit that reads a measurement image on the sheet of paper formed by the image forming unit, and a control unit that controls density unevenness of the image to be formed on the photosensitive member in the rotation direction of the photosensitive member based on the reading result of the reading unit, wherein the measurement image includes a first detection image for detecting the density of an image to be formed at a plurality of different positions on the photosensitive member in the axial direction of a rotation axis of the photosensitive member, and a second detection image for detecting density unevenness of the image to be formed at a plurality of different positions on the photosensitive member in the rotation direction of the photosensitive member. and a second detection image for detecting the density of an image to be formed at a plurality of different positions on the photosensitive body, and the control means determines target data regarding the target density based on the reading result of the first detection image read by the reading means and the reading result of the second detection image read by the reading means, and the control means suppresses density unevenness of the image to be formed on the photosensitive body in the rotation direction of the photosensitive body based on the reading result of the first detection image read by the reading means, the reading result of the second detection image read by the reading means, and the target data. An image forming apparatus according to another aspect of the present invention includes an image forming unit that forms an image on a rotating photosensitive member, a reading unit that reads a measurement image formed on a sheet of paper by the image forming unit, and a control unit that controls density unevenness of an image to be formed on the photosensitive member in an axial direction of a rotation axis of the photosensitive member based on the reading result of the reading unit, wherein the measurement image includes a first detection image for detecting the density of an image to be formed at a plurality of different positions on the photosensitive member in the axial direction of the photosensitive member, and a second detection image for detecting the density of an image to be formed at a plurality of different positions on the photosensitive member in the rotation direction of the rotation axis of the photosensitive member. and a second detection image for detecting the density of the image to be formed, and the control means determines target data regarding the target density based on the reading result of the first detection image read by the reading means and the reading result of the second detection image read by the reading means, and the control means suppresses density unevenness of the image to be formed on the photosensitive body in the axial direction of the photosensitive body based on the reading result of the first detection image read by the reading means, the reading result of the second detection image read by the reading means, and the target data. An image forming apparatus according to another aspect of the present invention includes an image forming unit that forms an image of a first color on a rotating first photoreceptor, a reading unit that reads a plurality of measurement images on a sheet formed by the image forming unit, and a control unit. The plurality of measurement images include a first measurement image for detecting the density of the image of the first color to be formed at a plurality of different positions on the first photoreceptor in the axial direction of the rotation axis of the first photoreceptor, and a position different from the position where the first measurement image is formed in the axial direction of the rotation axis of the first photoreceptor, and is formed at a plurality of different positions on the first photoreceptor in the rotation direction of the first photoreceptor. The sheet on which the plurality of measurement images are formed has a region where the image of the first color is not formed between the first measurement image and the second measurement image in the axial direction of the first photoreceptor. The control unit determines target data regarding a target density based on a reading result of the first measurement image read by the reading unit and a reading result of the second measurement image read by the reading unit. The control unit suppresses density unevenness of the image of the first color to be formed on the first photoreceptor in the rotation direction of the first photoreceptor based on the reading result of the first measurement image read by the reading unit, the reading result of the second measurement image read by the reading unit, and the target data.

Advantages of the Invention

[0010] According to the present invention, density unevenness of an image formed by an image forming apparatus can be suppressed with high precision.

Brief Description of the Drawings

[0011] [Figure 1] Configuration explanatory diagram of an image forming apparatus. [Figure 2] Functional block diagram of an image forming apparatus. [Figure 3] Exemplary configuration diagram of a printer image processing unit. [Figure 4] Explanatory diagram of a processing unit for image data. [Figure 5]FIG. 2 is a diagram illustrating the configuration of an image forming unit of a printer engine. [Figure 6] 10 is a flowchart showing an image density unevenness correction process. [Figure 7] FIG. [Figure 8] FIG. [Figure 9] 10A and 10B are explanatory diagrams of image density in the main scanning direction after image density unevenness correction. [Figure 10] 10A and 10B are explanatory diagrams of image density in the sub-scanning direction after image density unevenness correction. [Figure 11] 10A and 10B are explanatory diagrams of image density in the main scanning direction after image density unevenness correction. [Figure 12] 10A and 10B are explanatory diagrams of image density in the sub-scanning direction after image density unevenness correction. [Figure 13] 10 is a flowchart showing an image density unevenness correction process. [Figure 14] 10 is a flowchart showing an image density unevenness correction process. [Figure 15] FIG. 10 is another schematic diagram of the sub-scanning measurement image. [Figure 16] FIG. 10 is another schematic diagram of the main scanning measurement image. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

[0013] (First embodiment) In this embodiment, an electrophotographic laser beam printer will be described as an example of the image forming apparatus, but the image forming apparatus is not limited to a laser beam printer and may be any other electrophotographic printer such as an LED (Light Emitting Diode) printer.

[0014] (Configuration of image forming device) 1 is a diagram illustrating the configuration of an image forming apparatus according to this embodiment. An image forming apparatus 100 has a reader 101, which is an image input device, and a printer engine 102, which is an image output device, connected internally.

[0015] The reader 101 is connected to the device interface (I / F) 117 via the reader image processing unit 118. The printer engine 102 is connected to the device I / F 117 via the printer image processing unit 119. The reader image processing unit 118 controls the image reading using the reader 101. The reader 101 is an optical reader that optically reads an image, for example, by receiving the reflected light of the light irradiated on the image. The printer image processing unit 119 controls the printing of an image on a sheet using the printer engine 102.

[0016] The image forming apparatus 100 is connected to a network 10 such as a LAN (Local Area Network) and a public line 104, and can transmit and receive the image information and device information described later via the network 10 and the public line 104. For this purpose, the image forming apparatus 100 includes a network I / F 111 and a modem 112. The network I / F 111 is realized by, for example, a NIC (Network Interface Card) or the like, and controls communication with an external device (not shown) via the network 10. The modem 112 controls communication with an external device (not shown) via the public line 104.

[0017] An operation unit 110 is connected to the image forming apparatus 100. For this purpose, the image forming apparatus 100 includes an operation unit I / F 109. The operation unit 110 is a user interface having an input interface and an output interface. The input interface is, for example, various key buttons, a touch panel, or the like. The output interface is, for example, a display, a speaker, or the like. The image forming apparatus 100 acquires instructions, settings, etc. input from the operation unit 110 via the operation unit I / F 109. The image forming apparatus 100 performs screen display such as a setting screen and sound output to the operation unit 110 via the operation unit I / F 109.

[0018] The image forming apparatus 100 includes a CPU (Central Processing Unit) 105, a RAM (Random Access Memory) 106, a ROM (Read Only Memory) 107, and a storage 108. The operation of the image forming apparatus 100 is controlled by executing a boot program stored in the CPU 105 or the ROM 107 and computer programs such as software stored in the storage 108. The RAM 106 functions as a work memory when the CPU 105 executes processing. The RAM 106 also has a storage area for temporarily storing data such as image data. The storage 108 is a large-capacity storage device such as an HDD (Hard Disk Drive) or an SSD (Solid State Drive).

[0019] The CPU 105, RAM 106, ROM 107, storage 108, operation unit I / F 109, network I / F 111, and modem 112 are connected to a system bus 113 and can communicate with each other. An image bus I / F 114 is also connected to the system bus 113. The image bus I / F 114 is an interface for connecting the system bus 113 with an image bus 115 for transferring image data at high speed, and is a bus bridge that converts data structures between the system bus 113 and the image bus 115. The image bus I / F 114 enables communication between each unit connected to the system bus 113 and each unit connected to the image bus 115.

[0020] To the image bus 115, a raster image processor (RIP) unit 116, the device I / F 117, an image processing unit 120 for image editing, an image compression unit 103, an image decompression unit 121, and a color management module (CMM) 130 are connected.

[0021] The RIP unit 116 expands page description language (PDL) data into image data. The device I / F 117 is connected to the reader 101 via the reader image processing unit 118 and to the printer engine 102 via the printer image processing unit 119. The device I / F 117 performs synchronous / asynchronous conversion of image data. The reader image processing unit 118 performs various processes such as correction and editing on the image data that is the reading result of the image acquired from the reader 101. The printer image processing unit 119 performs image processing such as gamma correction and halftone processing according to the printer engine 102 on the image data representing the image to be printed on the paper.

[0022] The image processing unit 120 for image editing performs various image processes such as rotation of image data, color processing, binary conversion, and multi-value conversion. The image compression unit 103 encodes the image data processed by the RIP unit 116, the reader image processing unit 118, and the image processing unit 120 for image editing in a predetermined compression method when temporarily storing it in the storage 108. The image decompression unit 121 decodes and decompresses the image data compressed and stored in the storage 108 when it is used for the process by the image processing unit 120 for image editing or when it is image-processed by the printer image processing unit 119 and output by the printer engine 102.

[0023] The CMM 130 is a dedicated hardware module that performs color conversion processing (also called color space conversion processing) on image data based on a profile and calibration data. A profile is information like a function for converting color image data expressed in a device-dependent color space into a device-independent color space (e.g., Lab color space, etc.). The calibration data is data for correcting the color reproduction characteristics of the reader 101 and the printer engine 102.

[0024] 2 is a functional block diagram of the image forming apparatus 100. Each functional block is realized by the CPU 105 executing a computer program. The image forming apparatus 100 functions as a job control processing unit 201 for controlling various functions that can be realized by the image forming apparatus 100. The image forming apparatus 100 functions as a network processing unit 202, a UI processing unit 203, a FAX processing unit 204, a device information transmission processing unit 205, a device information acquisition processing unit 206, a print processing unit 207, a color conversion processing unit 209, a reader processing unit 210, and a RIP processing unit 211.

[0025] A network processing unit 202 controls communication with external devices via the network I / F 111. A UI processing unit 203 controls the operation unit 110 and the operation unit I / F 109. A FAX processing unit 204 controls the facsimile function. The FAX processing unit 204 controls facsimile communication via the modem 112.

[0026] The device information transmission processing unit 205 transmits device information to a predetermined external device via the network processing unit 202 based on an instruction from the job control processing unit 201. The transmitted device information includes information indicating the capabilities and characteristics of the image forming apparatus 100. For example, the device information includes the type of printer engine 102 (color / monochrome), the resolution of the printer engine 102, the printing speed of the printer engine 102, the processing time by the color conversion processing unit 209, the output profile, etc. The device information acquisition processing unit 206 transmits a device information acquisition request to a predetermined external device via the network processing unit 202 based on an instruction from the job control processing unit 201.

[0027] The print processing unit 207 controls the image editing image processing unit 120, the printer image processing unit 119, and the printer engine 102 based on instructions from the job control processing unit 201 to print an image on paper. The print processing unit 207 acquires information such as image data, image information (image data size, color mode, resolution, etc.), layout information (offset, enlargement / reduction, imposition, etc.), and output paper information (size, print direction, etc.) from the job control processing unit 201. The print processing unit 207 controls the image compression unit 103, the image decompression unit 121, the image editing image processing unit 120, and the printer image processing unit 119 to perform appropriate image processing on the image data. The print processing unit 207 controls the printer engine 102 based on the image data after image processing to print the image on the specified paper.

[0028] The reader processing unit 210 controls the reader 101 and the reader image processing unit 118 based on instructions from the job control processing unit 201, and causes the reader 101 to read an image printed on a sheet of paper. The reader processing unit 210 scans a sheet of paper placed on a platen by the reader 101, and acquires a read image (read data) of the image printed on the sheet of paper as digital data from the reader 101. The reader processing unit 210 notifies the job control processing unit 201 of color information of the acquired read data. The reader processing unit 210 controls the reader image processing unit 118 to perform appropriate image processing such as image compression on the read data, and then transmits the image-processed read image to the job control processing unit 201.

[0029] The color conversion processing unit 209 performs color conversion processing on the specified image based on instructions from the job control processing unit 201, and notifies the job control processing unit 201 of the image after color conversion processing. The RIP processing unit 211 performs PDL interpretation based on instructions from the job control processing unit 201, and controls the RIP unit 116 to perform rendering, thereby expanding the image data into a bitmap image.

[0030] (Image data processing) The image forming apparatus 100 configured as described above receives a print job from an external device via the network 10 and prints an image corresponding to the print job on paper. The print job includes PDL data representing the image to be printed.

[0031] The print job is received by the network I / F 111 and transmitted to the RIP unit 116. The RIP unit 116 interprets the PDL data included in the acquired print job and converts it into code data that can be processed by the RIP unit 116. The RIP unit 116 performs rendering based on the converted code data to generate image data. The PDL data rendered by the RIP unit 116 is compressed by the subsequent image compression unit 103 as image data and sequentially stored in the storage 108.

[0032] The compressed image data stored in the storage 108 is read out by a print operation according to an instruction from the job control processing unit 201, and is subjected to an expansion process by the image expansion unit 121. The image data expanded by the image expansion unit 121 is transmitted to the printer image processing unit 119 via the device I / F 117.

[0033] FIG. 3 is a configuration example diagram of the printer image processing unit 119. The printer image processing unit 119 includes a color conversion unit 301, an image density gradient correction unit 302, a γ correction unit 303, a halftone processing unit 304, a drum - to - drum delay memory control unit 305, and a page buffer memory 306.

[0034] The color conversion unit 301 converts the acquired image data into a color space corresponding to color components printable by the subsequent printer engine 102. For this purpose, the color conversion unit 301 converts the image data from luminance values (such as RGB, YUV, etc.) to density values (such as CMYK, etc.).

[0035] The image density gradation correction unit 302 converts the multi-valued image data converted into density values by the color conversion unit 301 into a density signal obtained by correcting the gradation of the image density within the same page. Similar to the subsequent γ correction unit 303, the image density gradation correction unit 302 has a one-dimensional table for changing the input / output signal. The one-dimensional table has a gradation correction coefficient for gradation correction according to the position within the page. The image density gradation correction unit 302 corrects the image data by multiplying the gradation correction coefficient by the image data.

[0036] The γ correction unit 303 has a γ LUT (Look Up Table). The γ correction unit 303 performs γ correction using the γ LUT, and converts the density signal of the image data corrected by the image density gradation correction unit 302 into a signal value for reproducing the image density by the printer engine 102. The γ LUT is a table for converting the input / output signal created according to the γ characteristics of the printer engine 102. In the present embodiment, the γ LUT is stored in advance, but it may also be created using known tone control or the like.

[0037] The halftone processing unit 304 performs halftone processing on the image data corrected by the γ correction unit 303, and converts it into image data in which each color component of one pixel is represented by two values (1 bit). Generally, the dither method, the error diffusion method, or the like is used for halftone processing. In the present embodiment, either method may be used for halftone processing. Also, the halftone processing is not limited to these methods, and may be performed by other methods. The binary image data generated by the halftone processing unit 304 is separated for each color component of each pixel in the image data via the inter-drum delay memory control unit 305, and temporarily stored in the page buffer memory 306.

[0038] The inter-drum delay memory control unit 305 reads out the image data of the corresponding color component from the page buffer memory 306 at the timing of acquiring a video data request signal (VREQ_* (* is Y / M / C / K)) corresponding to each color component from the printer engine 102. The inter-drum delay memory control unit 305 transmits the read image data to the printer engine 102.

[0039] The video data request signals are represented by VREQ_Y, VREQ_M, VREQ_C, and VREQ_K corresponding to each color component. Photosensitive drums 1401 to 1404 (described below) corresponding to each color component in the printer engine 102 are exposed to light at different times during image formation. Therefore, the printer engine 102 transmits video data request signals for each color component at different times, and also acquires image data for each color component at different times. The printer engine 102 forms an image based on the acquired image data for each color component.

[0040] (Printer engine operation) 4 is an explanatory diagram of an image data processing unit provided in the printer engine 102. The printer engine 102 includes, as processing units, a printer I / F 1201, a pulse width modulation unit 1203, a Y laser driving unit 1212, an M laser driving unit 1213, a C laser driving unit 1214, and a K laser driving unit 1215.

[0041] When the printer engine 102 is ready to print, the printer I / F 1201 transmits a video data request signal (VREQ_*) requesting image data for each color component to the printer image processing unit 119. The printer I / F 1201 receives the image data for each color component sequentially transmitted from the printer image processing unit 119. The image data for each color component received by the printer I / F 1201 is sent to the pulse width modulation unit 1203.

[0042] Based on the acquired image data for each color component, the pulse width modulation unit 1203 generates pulse signals (drive signals) for driving the Y laser drive unit 1212, M laser drive unit 1213, C laser drive unit 1214, and K laser drive unit 1215. A drive signal is generated for each color. The generated drive signals are sent to the Y laser drive unit 1212, M laser drive unit 1213, C laser drive unit 1214, and K laser drive unit 1215 to form images of the corresponding colors.

[0043] The Y laser driving unit 1212, the M laser driving unit 1213, the C laser driving unit 1214, and the K laser driving unit 1215 perform output control of the laser light of the corresponding color component based on the acquired driving signal. The Y laser driving unit 1212 acquires a driving signal based on yellow image data and performs output control of the laser light for forming a yellow image. The M laser driving unit 1213 acquires a driving signal based on magenta image data and performs output control of the laser light for forming a magenta image. The C laser driving unit 1214 acquires a driving signal based on cyan image data and performs output control of the laser light for forming a cyan image. The K laser driving unit 1215 acquires a driving signal based on black image data and performs output control of the laser light for forming a black image.

[0044] FIG. 5 is a configuration diagram of the image forming unit of the printer engine 102. As described above, the printer engine 102 of the image forming apparatus 100 according to the present embodiment employs an electrophotographic method. This type of printer engine 102 includes, for example, a photoreceptor, a charging unit that charges the photoreceptor, an exposure unit that exposes the charged photoreceptor, a developing unit that develops the electrostatic latent image formed on the photoreceptor by exposure, and a transfer unit that transfers the developed toner image onto an image carrier. The printer engine 102 of the present embodiment includes toners of a plurality of color components (here, yellow (Y), magenta (M), cyan (C), black (K)) as a developer, and can form a toner image by a plurality of color components on the image carrier as a developer image. Hereinafter, mainly the image forming portion of yellow will be described, but the image forming portions of other color components have the same configuration. Note that the printer engine 102 of the present embodiment is configured to use a tandem engine composed of four colors of yellow (Y), magenta (M), cyan (C), and black (K), but is not limited thereto.

[0045] The printer engine 102 includes a photosensitive drum 1401, a charging roller 1400, a Y laser exposure unit 1406, and a developing unit 1416 as a configuration for forming a yellow image. The printer engine 102 also includes a primary transfer unit 1408, an intermediate transfer belt 1412, a secondary transfer unit 1413, a fixing unit 1414, and a cleaner 1415.

[0046] The photosensitive drum 1401 is a drum-shaped photoreceptor having a photosensitive layer on its surface and is rotatable in the direction of the arrow around the drum axis. The drum diameter of the photosensitive drum 1401 is, for example, 32 [mm]. The charging roller 1400 is a charging member that uniformly charges the surface of the photosensitive drum 1401. The charging roller 1400 has a drum shape, and the drum diameter is, for example, 10 [mm]. The charging roller 1400 uniformly charges the surface of the photosensitive drum 1401 when a predetermined charging bias voltage is applied.

[0047] The Y laser exposure unit 1406 (light source) is driven by the Y laser drive unit 1212. The Y laser exposure unit 1406 irradiates the surface of the uniformly charged photosensitive drum 1401 with laser light modulated based on a yellow drive signal, thereby forming an electrostatic latent image on the surface of the photosensitive drum 1401. The laser light scans the surface of the rotating photosensitive drum 1401 in the drum axis direction. Therefore, an electrostatic latent image is formed with the drum axis direction of the photosensitive drum 1401 as the main scanning direction and the rotation direction of the photosensitive drum 1401 as the sub-scanning direction.

[0048] The developing unit 1416 develops the electrostatic latent image with a developer (toner here). When the electrostatic latent image is developed, a yellow toner image is formed on the surface of the photosensitive drum 1401. Similarly, a magenta toner image is formed on the surface of the photosensitive drum 1402. A cyan toner image is formed on the surface of the photosensitive drum 1403. A black toner image is formed on the surface of the photosensitive drum 1404.

[0049] The developing device 1416 of this embodiment includes a developer container that contains a two-component developer, which is a mixture of toner particles (toner) and magnetic carrier particles (carrier). The developer container is divided into two chambers, one of which is provided with an A screw 1420 and the other with a B screw 1421. The A screw 1420 and the B screw 1421 transport and mix the toner particles and magnetic carrier particles, respectively.

[0050] A developing sleeve 1422 is provided on the photosensitive drum 1401 side of the A screw 1420. The developing sleeve 1422 is drum-shaped with a drum diameter of, for example, 13 mm. The developing sleeve 1422 is disposed close to the photosensitive drum 1401 and rotates so as to follow the rotation of the photosensitive drum 1401. The developing sleeve 1422 carries a developer that is a mixture of toner and carrier. The developer carried by the developing sleeve 1422 develops the electrostatic latent image on the photosensitive drum 1401 when a developing bias voltage is applied to the developing sleeve 1422.

[0051] When a transfer bias voltage is applied, primary transfer unit 1408 transfers the yellow toner image on photosensitive drum 1401 to intermediate transfer belt 1412, which is an image carrier. When a transfer bias voltage is applied, primary transfer unit 1409 transfers the magenta toner image on photosensitive drum 1402 to intermediate transfer belt 1412. When a transfer bias voltage is applied, primary transfer unit 1410 transfers the cyan toner image on photosensitive drum 1403 to intermediate transfer belt 1412. When a transfer bias voltage is applied, primary transfer unit 1411 transfers the black toner image on photosensitive drum 1404 to intermediate transfer belt 1412.

[0052] The intermediate transfer belt 1412 is an endless belt-like transfer body that rotates in the direction of the arrow. The toner images of each color are transferred at a timing that corresponds to the rotation speed of the intermediate transfer belt 1412 and the intervals between the photosensitive drums 1401, 1402, 1403, and 1404, and are carried on the intermediate transfer belt 1412 in a superimposed state.

[0053] As the intermediate transfer belt 1412 rotates, the toner images of each color are transported to a secondary transfer unit 1413. A feeding mechanism (not shown) feeds paper to the secondary transfer unit 1413 in synchronization with the transport of the toner images. When a transfer bias voltage is applied, the secondary transfer unit 1413 transfers the toner images of each color carried by the intermediate transfer belt 1412 all at once onto the paper. The paper onto which the toner images have been transferred is transported to a fixing unit 1414. Any toner remaining on the intermediate transfer belt 1412 after transfer is removed by a cleaner 1415.

[0054] The fuser 1414 fuses the toner image transferred onto the paper. The fuser 1414 fuses and fixes the toner image to the paper, for example, by heating and pressurizing the paper carrying the toner image. In this way, an image is printed on the paper. The paper is also an example of an image carrier that carries an image.

[0055] An image density sensor 400 is disposed downstream in the rotation direction of the intermediate transfer belt 1412 from the photosensitive drum 1404 used for black image formation. The image density sensor 400 is an optical sensor used to measure the image density of the toner image carried by the intermediate transfer belt 1412.

[0056] A toner bottle 1407 for supplying toner is attached to the developing device 1416 used for forming a yellow image. The toner bottle 1407 is rotated by a motor 1208 to supply yellow toner to the developing device 1416. The operation of the motor 1208 is controlled by a Y toner supply unit 1204. When the toner in the developing device 1416 becomes less than a predetermined amount, the Y toner supply unit 1204 drives the motor 1208 in response to an instruction from the CPU 105, thereby controlling the supply of toner to the developing device 1416.

[0057] Developing units 1417, 1418, and 1419 used for forming images of other colors also have toner bottles attached to them in the same way. The rotation of the toner bottles of each color is controlled by motors 1209, 1210, and 1211. The operation of motor 1209 is controlled by M toner supply unit 1205. The operation of motor 1210 is controlled by C toner supply unit 1206. The operation of motor 1211 is controlled by K toner supply unit 1207. The operation of M toner supply unit 1205, C toner supply unit 1206, and K toner supply unit 1207 is also controlled by CPU 105, similar to Y toner supply unit 1204.

[0058] (Image density unevenness correction) FIG. 6 is a flowchart showing the image density unevenness correction process performed by the image forming apparatus 100. In the following description, the image density unevenness correction may also be referred to as "shading correction." The shading correction is performed by the CPU 105 executing a computer program. For example, the shading correction is performed by the job control processing unit 201 or another functional block.

[0059] The CPU 105 controls the printer engine 102 by the print processing unit 207 or the like to form a main scanning measurement image for measuring the image density in the main scanning direction on the paper (S101). The main scanning measurement image functions as a first detection image used to detect the density of images to be formed at a plurality of different positions on the photosensitive drum 1402 in the rotation direction of the photosensitive drum 1402. Details of the main scanning measurement image will be described later. The CPU 105 displays a message on the screen of the operation unit 110 to prompt the reading of the main scanning measurement image using the reader 101. When an instruction to start reading is input from the user, the reader 101 transmits the reading data A, which is the reading result (read image) of the main scanning measurement image, to the CPU 105. Note that the reading of the measurement image may be performed not by the reader 101 but by an in-line sensor (not shown) provided downstream of the fixing unit 1414 in the conveyance direction in which the paper is conveyed. In this case, the main scanning measurement image printed on the paper is read by the in-line sensor while the paper is being conveyed. Next, when the CPU 105 acquires the reading data A of the main scanning measurement image, it measures the image density of the main scanning measurement image based on the reading data A (S102). Thereby, the CPU 105 acquires the image density in the main scanning direction.

[0060] Next, the CPU 105 controls the printer engine 102 via the print processing unit 207 and the like to form a sub-scanning measurement image on paper for measuring image density in the sub-scanning direction (S103). This serves as a second detection image used to detect the density of images to be formed at multiple different positions on the photosensitive drum 1402 in the axial direction of the rotation shaft of the photosensitive drum 1402. Details of the sub-scanning measurement image will be described later. The CPU 105 displays a message on the screen of the operation unit 110 prompting the reader 101 to read the sub-scanning measurement image. When a command to start reading is input by the user, the reader 101 reads the sub-scanning measurement image printed on paper and transmits read data B, which is the read result (read image) of the sub-scanning measurement image, to the CPU 105. When the CPU 105 acquires read data B, which is the read result of the sub-scanning measurement image from the reader 101, it measures the image density of the sub-scanning measurement image based on the read data B (S104). As a result, the CPU 105 acquires the image density in the sub-scanning direction.

[0061] CPU 105 acquires the image density distribution in the main scanning direction based on the image density of the main scanning measurement image measured in the process of S102. CPU 105 acquires the image density distribution in the sub-scanning direction based on the image density of the sub-scanning measurement image measured in the process of S104. CPU 105 determines, for each color component, a shading correction amount (image density unevenness correction amount) that suppresses image density unevenness at each position (each region) in the main scanning direction and the sub-scanning direction based on the acquired image density distributions in both directions (S105). The method for determining the image density unevenness correction amount will be described in detail later.

[0062] The CPU 105 performs shading correction by setting the shading correction amount for each color component in the printer engine 102 via the printer image processing unit 119 (S106). Known shading correction methods include a method of changing the modulation degree of pulse width modulation (PWM) of laser light depending on the exposure position based on the shading correction amount, and a method of changing the intensity of laser light depending on the exposure position, but the method is not limited to these two methods.

[0063] In the case of a method of changing the modulation degree of the pulse width modulation of the laser beam according to the exposure position, the printer engine 102 stores the shading correction amount for each color component in a memory (not shown) in the pulse width modulation unit 1203. Thereby, at the time of image formation, the pulse width modulation unit 1203 can change the modulation degree of the pulse width modulation of the laser beam according to the exposure position based on the shading correction amount for each color component. As a result, the image density unevenness in the main scanning direction and the sub-scanning direction is corrected.

[0064] The image density unevenness in the main scanning direction is corrected by changing the modulation degree of the pulse width modulation of the laser beam according to the scanning position in the main scanning direction. The image density unevenness in the sub-scanning direction is corrected by changing the modulation degree of the pulse width modulation of the laser beam according to the scanning position in the sub-scanning direction.

[0065] The scanning position in the sub-scanning direction corresponds to the respective rotation phases of the photosensitive drums 1401, 1402, 1403, and 1404. For this purpose, each of the photosensitive drums 1401, 1402, 1403, and 1404 is provided with, for example, a rotation phase detection sensor for detecting the rotation phase. The formation of the sub-scanning measurement image is started based on the timing when the phase detected by the rotation phase detection sensor becomes the reference phase. In this way, the position in the sub-scanning direction of the sub-scanning measurement image on the paper corresponds to the respective rotation phases of the photosensitive drums 1401, 1402, 1403, and 1404. The image density is measured from the read data B of the sub-scanning measurement image corresponding to the respective rotation phases of the photosensitive drums 1401, 1402, 1403, and 1404. Therefore, the shading correction amount corresponding to the scanning position in the sub-scanning direction is determined based on the image density distribution in the sub-scanning direction by the measured image density.

[0066] When correcting the image density unevenness in the sub-scanning direction, based on the timing when the phase detected by the rotation phase detection sensor becomes the reference phase, the modulation degree of the pulse width modulation of the laser beam is changed by an amount corresponding to the shading correction amount according to the scanning position in the sub-scanning direction. Note that the correction timing may be predicted by a timer and the image density unevenness.

[0067] FIG. 7 is an explanatory diagram of a main scanning measurement image. In this embodiment, the main scanning measurement image is printed on A4 size (210 mm × 297 mm) paper. The main scanning measurement image is a band-shaped image that has a constant width in the sub-scanning direction for each color component, and the main scanning direction is the longitudinal direction. The dimensions of the band-shaped image are 20 mm in the sub-scanning direction and 280 mm in the main scanning direction. Images of each color component of the main scanning measurement image are arranged at a predetermined interval in the sub-scanning direction. The main scanning measurement image is formed with a uniform image signal value, and ideally has a constant image density. In this embodiment, the image signal value is a value that results in an image density of, for example, 40%.

[0068] As an example, the main scanning measurement image shown in FIG. 7 is divided into five areas, area A to area E, in the main scanning direction, and the image density of each area is measured. The CPU 105 obtains the image density for each area from the read data A of the reader 101. Note that the number of areas divided in the main scanning direction is not limited to this. Also, the size of the paper on which the main scanning measurement image is printed is not limited to A4 size. The dimensions of the strip-shaped image of each color are 20 [mm] x 280 [mm], but are not limited to this.

[0069] The main scanning direction and the sub-scanning direction are the directions indicated by the arrows in the figure. As described above, the main scanning direction is the scanning direction of the laser light, and the sub-scanning direction is the direction intersecting the scanning direction of the laser light. The sub-scanning direction is also the same direction as the rotation direction of the intermediate transfer belt 1412 (the transport direction of the toner image). The image density of the main scanning measurement image printed on the paper is measured using the reader 101, and the image density may also be measured using, for example, an external colorimeter.

[0070] FIG. 8 is an explanatory diagram of a sub-scanning measurement image. In this embodiment, the sub-scanning measurement image is printed on A3 size (297 mm x 420 mm) paper. The sub-scanning measurement image is a band-shaped image that has a constant width in the main scanning direction for each color component, and the sub-scanning direction is the longitudinal direction. Images of each color component of the sub-scanning measurement image are arranged at predetermined intervals in the main scanning direction. The sub-scanning measurement image is formed with a uniform image signal value, and ideally has a constant image density. In this embodiment, the image signal value is a value that results in an image density of, for example, 40%.

[0071] As an example, the sub-scanning measurement image shown in FIG. 8 is divided into six regions, region F to region K, in the sub-scanning direction, and the image density of each region is measured. Here, the rotational phase of the photosensitive drum 1403 when forming the sub-scanning measurement image is adjusted so that region F of the cyan sub-scanning measurement image is the same region on the photosensitive drum 1403 when forming the cyan region A (see FIG. 7). The rotational phase of the photosensitive drum 1402 when forming the sub-scanning measurement image is adjusted so that region G of the magenta sub-scanning measurement image is the same region on the photosensitive drum 1402 when forming the magenta region B (see FIG. 7). The rotational phase of the photosensitive drum 1401 when forming the sub-scanning measurement image is adjusted so that region H of the yellow sub-scanning measurement image is the same region on the photosensitive drum 1401 when forming the yellow region C (see FIG. 7). The rotation phase of the photosensitive drum 1401 when forming the sub-scanning measurement image is adjusted so that area I of the black sub-scanning measurement image is the same area on the photosensitive drum 1401 when forming black area D (see FIG. 7). The CPU 105 obtains the image density for each area from the read data B of the reader 101. Note that the number of areas divided in the sub-scanning direction is not limited to this. Furthermore, the size of the paper on which the sub-scanning measurement image is printed is not limited to A3 size.

[0072] When reading the sub-scanning measurement image, the length of the sheet in the sub-scanning direction may exceed the size of the reader 101. In this case, an ADF (Automatic Document Feeder) (not shown) that can be attached to the reader 101 may be used, and the sub-scanning measurement image may be read from the sheet automatically fed by the ADF.

[0073] (Determination of shading correction amount) The determination process of the shading correction amount (image density unevenness correction amount) in S105 of FIG. 6 will be described. That is, the image density unevenness correction method in the main scanning direction and the sub-scanning direction of the image forming apparatus 100 will be described.

[0074] As described with reference to FIG. 7, for the main scanning measurement image formed in the process of S101 in FIG. 6, the image density of each region A to region E is measured for each color component. The CPU 105 calculates the average value (average density) DSA of the measured image density for each of the regions A to region E for each color component.

[0075] As described with reference to FIG. 8, for the sub-scanning measurement image formed in the process of S103 in FIG. 6, the image density of each region F to region K is measured for each color component. The CPU 105 calculates the average value (average density) DFA of the measured image density for each of the regions F to region K for each color component.

[0076] The CPU 105 calculates the average density DZA of the average density DSA in the main scanning direction and the average density DFA in the sub-scanning direction (DZA = (DSA + DFA) / 2). The CPU 105 determines the average density DZA as target data regarding the target density. The CPU 105 calculates the difference (density difference) ΔDens between the average density DZA and the image density of each of the regions A to region E in the main scanning direction and the regions F to region K in the sub-scanning direction. Here, the density difference of the cyan region A is equal to the density difference of the cyan region F. The density difference of the magenta region B is equal to the density difference of the magenta region G. The density difference of the yellow region C is equal to the density difference of the yellow region H. The density difference of the black region D is equal to the density difference of the black region I.

[0077] In the image forming apparatus 100 of this embodiment, even if the main scanning measurement image and the sub-scanning measurement image are different, the density difference ΔDens is calculated from the average density DZA. The average density DZA becomes the correction target (target density). This eliminates the reduction in correction accuracy due to the difference in average density between the image density in the main scanning direction and the image density in the sub-scanning direction. Note that the image forming apparatus 100 determines the shading correction amount for each of regions A to E in the main scanning direction and each of regions F to K in the sub-scanning direction so as to correct image density unevenness in the main scanning direction.

[0078] The CPU 105 calculates the amount of shading correction for each area based on the density difference ΔDens. As described above, the density difference for cyan area A is equal to the density difference for cyan area F. The density difference for magenta area B is equal to the density difference for magenta area G. The density difference for yellow area C is equal to the density difference for yellow area H. The density difference for black area D is equal to the density difference for black area I. The amount of shading correction for each area is calculated using the following formula. (Shading correction amount) = ΔDens × (correction coefficient N) ... (Equation 1) The correction coefficient N is a coefficient for determining the amount of change in the PWM modulation amount of the laser light (or the amount of change in the intensity of the laser light) relative to the density difference ΔDens. For example, if the correction coefficient N is "100" and the density difference ΔDens is "0.01", this indicates that the modulation level changes by "1". In this embodiment, the correction coefficient N is "200", but the correction coefficient N is not limited to this. Note that here, the shading correction amount is calculated by rounding off the calculation result of the right side of the above equation, but the invention is not limited to this.

[0079] In this embodiment, the amount of shading correction is calculated as the average value of each of regions A to K. In addition to the average value, the amount of shading correction may be an intermediate value or a value obtained by other statistical processing. It is important to determine the amount of shading correction so that the target density in the main scanning direction and the target density in the sub-scanning direction coincide with each other. In this embodiment, the average density DZA is used as the target density (correction target). The CPU 105 can correct uneven image density in the main scanning direction and the sub-scanning direction by adjusting the laser light for each exposure position based on the amount of shading correction determined in this manner.

[0080] The CPU 105 suppresses density unevenness in the sub-scanning direction of the image to be formed on the photosensitive drum 1402 based on the reading result of the main scanning measurement image by the reader 101, the reading result of the sub-scanning measurement image by the reader 101, and the average density DZA. The CPU 105 also suppresses density unevenness in the main scanning direction of the image to be formed on the photosensitive drum 1402 based on the reading result of the main scanning measurement image by the reader 101, the reading result of the sub-scanning measurement image by the reader 101, and the average density DZA.

[0081] Although the above description has been given of a configuration in which the main scanning measurement image and the sub-scanning measurement image printed on paper are read by the reader 101, a configuration in which each measurement image is read by the image density sensor 400 may also be used. In this case, the measurement images for each direction are read by the image density sensor 400 while carried on the intermediate transfer belt 1412. Image density unevenness correction based on the read results is the same as when paper is used. With this configuration, it is possible to measure image density unevenness in the sub-scanning direction regardless of paper size.

[0082] The effect of the image density unevenness correction according to this embodiment will now be described. As a comparative example, a case where image density unevenness correction is performed separately in the main scanning direction and the sub-scanning direction will be shown. Here, image density unevenness is defined as the difference between the maximum and minimum image density values.

[0083] When individually performing image density unevenness correction in the main scanning direction and the sub-scanning direction, for example, after image density unevenness correction in the main scanning direction, image density unevenness correction in the sub-scanning direction is performed. FIG. 9 is an explanatory diagram of the image density in the main scanning direction after image density unevenness correction. FIG. 10 is an explanatory diagram of the image density in the sub-scanning direction after image density unevenness correction. The average density of the image density before correction in the main scanning direction is "0.45", and the difference in the image density after correction in the main scanning direction is "0.04" from the maximum value and the minimum value. The average density of the image density before correction in the sub-scanning direction is "0.41", and the difference in the image density after correction in the sub-scanning direction is "0.04" from the maximum value and the minimum value. The target densities are different because the target density in the main scanning direction is "0.45" and the target density in the sub-scanning direction is "0.41".

[0084] FIG. 11 is an explanatory diagram of the image density in the main scanning direction after image density unevenness correction according to the present embodiment. FIG. 12 is an explanatory diagram of the image density in the sub-scanning direction after image density unevenness correction according to the present embodiment. The difference in the image density after correction in the main scanning direction is "0.02" from the maximum value and the minimum value. The difference in the image density after correction in the sub-scanning direction is "0.02" from the maximum value and the minimum value. Compared with FIGS. 9 and 10, in FIGS. 11 and 12, the difference in the image density after correction is smaller in both the main scanning direction and the sub-scanning direction.

[0085] In the present embodiment, the target density in each of the main scanning direction and the sub-scanning direction is not set individually in each direction, but is commonly set based on the average density in each direction. Therefore, the difference in the image density after correction is smaller in the image density unevenness correction of the present embodiment than when performing image density unevenness correction individually in each direction. As described above, by determining the shading correction amount based on a common target density for the main scanning direction and the sub-scanning direction, image density unevenness correction is performed with high accuracy in both the main scanning direction and the sub-scanning direction. That is, by performing the image density unevenness correction of the present embodiment, correction of image density unevenness in different directions is realized with high accuracy.

[0086] (Second Embodiment) Since the configuration of the image forming apparatus 100 according to the second embodiment is the same as that of the first embodiment, the description of the configuration will be omitted. In the first embodiment, both the image density unevenness in the main scanning direction and the image density unevenness in the sub-scanning direction are detected. That is, the main scanning measurement image and the sub-scanning measurement image are printed on different sheets. Therefore, a plurality of sheets are required for the image density unevenness correction process. Also, the image density unevenness correction process takes time because it is printed on a plurality of sheets.

[0087] In the second embodiment, during the image density unevenness correction process, either the detection of the image density unevenness in the main scanning direction or the detection of the image density unevenness in the sub-scanning direction is performed, and the detection result of the other image density unevenness uses the detection result obtained in the past. In such a process, the sheets used for the image density unevenness correction process can be reduced, and the processing time can be shortened.

[0088] FIG. 13 is a flowchart showing the image density unevenness correction process according to the second embodiment. Here, the case where the image density unevenness in the main scanning direction is detected by the same process as in the first embodiment and the detection result of the previous image density is used for the image density unevenness in the sub-scanning direction will be described, but this may be reversed. Note that the detection result of the previous image density is stored in the storage 108 in this embodiment, but it may be stored in an external device such as a server.

[0089] The CPU 105 measures the image density of the main scanning measurement image by the same process as the processes of S101 and S102 in FIG. 6, and obtains the image density distribution in the main scanning direction (S201, S202). The CPU 105 reads the detection result of the previous image density in the sub-scanning direction from the storage 108, and obtains the image density distribution in the sub-scanning direction (S203). The CPU 105 determines the shading correction amount by the same process as S105 and S106 in FIG. 6, and performs shading correction (S204, S205). Note that the storage 108 may store the image density distribution in the sub-scanning direction obtained from the detection result of the previous image density in the sub-scanning direction. In this case, the CPU 105 reads the previous image density distribution in the sub-scanning direction from the storage 108 and shifts the process to step S204.

[0090] In the above explanation, the previous image density detection result is used as an example, but it is not limited to the previous one, and the result two times before may also be used. Furthermore, the information used may be a statistical value of the previous detection result (for example, the average value of the previous and second previous detection results).

[0091] In the image forming apparatus 100 of the second embodiment, the laser light is corrected in accordance with the amount of shading correction, as in the first embodiment, and therefore, uneven image density is corrected, as in the first embodiment.

[0092] (Third embodiment) The configuration of the image forming apparatus 100 of the third embodiment is the same as that of the first embodiment, so a description of the configuration will be omitted. In the third embodiment, image density unevenness is measured based on the surface potential of the photosensitive drums 1401, 1402, 1403, and 1404, rather than the measurement results of a measurement image printed on paper. For example, image density unevenness is measured based on the surface potential after a toner image of the measurement image is formed on the surface of the photosensitive drums 1401, 1402, 1403, and 1404. For this purpose, potential sensors are provided around the photosensitive drums 1401, 1402, 1403, and 1404. The photosensitive drums 1401, 1402, 1403, and 1404 function as image carriers like the intermediate transfer belt 1412 and paper.

[0093] In this case, the average potential of the photosensitive drums 1401, 1402, 1403, and 1404 in the main scanning direction and the sub-scanning direction is averaged to determine the average potential that corresponds to the target density. Instead of the density difference ΔDens, a potential difference ΔV is calculated using the average potential, the potential of each region in the main scanning direction, and the potential of each region in the sub-scanning direction. The potential difference V is the difference between the average potential of the toner image of the measurement image formed on the photosensitive drums 1401, 1402, 1403, and 1404 and the potential of each region on the surface. The shading correction amount is calculated by multiplying the potential difference ΔV by a correction coefficient N. The laser beam is corrected according to the shading correction amount, thereby correcting image density unevenness. Furthermore, as in the second embodiment, image density unevenness correction processing may be performed using a previously measured potential in either the main scanning direction or the sub-scanning direction.

[0094] (Fourth embodiment) The configuration of the image forming apparatus 100 of the fourth embodiment is similar to that of the first embodiment, and therefore a description of the configuration will be omitted. In the first embodiment, the image density information in the sub-scanning direction actually measured from the yellow main scanning measurement image is only for region H in the sub-scanning direction. The image density information in the sub-scanning direction actually measured from the main scanning measurement images of magenta, cyan, and black other than yellow is also only for one region in the sub-scanning direction. The density unevenness (distribution) in the sub-scanning direction of regions A to E in the main scanning direction is determined by offsetting the measured density of the region in the main scanning direction where the sub-scanning measurement image is formed by the density difference between each of regions A to E of the main scanning measurement image formed in one region in the sub-scanning direction. Therefore, if the density distribution in the sub-scanning direction differs for each of regions A to E in the main scanning direction, the density unevenness in the sub-scanning direction cannot be suppressed with high accuracy.

[0095] In the density unevenness correction of this embodiment, multiple measurement images of each color are formed in the main scanning direction, and the image densities of multiple different regions are detected in both the main scanning direction and the sub-scanning direction, thereby suppressing density unevenness with higher accuracy than in the density unevenness correction of the first embodiment.

[0096] 14 is a flowchart showing the process of correcting image density unevenness in the sub-scanning direction by image forming apparatus 100 according to the fourth embodiment. In the following description, the correction of image density unevenness in the sub-scanning direction is referred to as "sub-scanning shading correction." The sub-scanning shading correction is performed by CPU 105 executing a computer program.

[0097] The CPU 105 controls the printer engine 102 by the print processing unit 207 or the like to form a sub-scanning measurement image (FIG. 15) on the paper (S301). The CPU 105 displays a message on the screen of the operation unit 110 to prompt the reading of the sub-scanning measurement image using the reader 101. When an instruction to start reading is input from the user, the reader 101 transmits the read data B, which is the reading result (read image) of the sub-scanning measurement image, to the CPU 105. Note that the reading of the sub-scanning measurement image may be performed not by the reader 101 but by an in-line sensor (not shown) provided downstream of the fixing unit 1414 in the conveyance direction in which the paper is conveyed.

[0098] In this case, the sub-scanning measurement image printed on the paper is read by the in-line sensor while the paper is being conveyed. Next, when the CPU 105 acquires the reading data B of the sub-scanning measurement image, it measures the image density of the sub-scanning measurement image based on the reading data B (S302). Thereby, the CPU 105 acquires the image density distribution in the sub-scanning direction at a plurality of different positions in the main scanning direction.

[0099] The CPU 105 determines a shading correction amount (image density unevenness correction amount) for suppressing the image density unevenness at each position (each region) in the sub-scanning direction for each color component based on the image density distributions in the sub-scanning direction at a plurality of different positions in the main scanning direction (S303). Details of the method for determining the image density unevenness correction amount will be described later.

[0100] The CPU 105 performs shading correction by setting the shading correction amount for each color component to the printer engine 102 via the printer image processing unit 119 (S304). As methods of shading correction, a method of changing the modulation degree of pulse width modulation (PWM) of the laser beam according to the exposure position and a method of changing the intensity of the laser beam according to the exposure position are known based on the shading correction amount, but the methods are not limited to these two.

[0101] The image density unevenness in the sub-scanning direction is corrected by changing the modulation degree of the pulse width modulation of the laser beam according to the scanning position in the sub-scanning direction. When correcting the image density unevenness in the sub-scanning direction, based on the timing when the phase detected by the rotational phase detection sensor becomes the reference phase, the modulation degree of the pulse width modulation of the laser beam is changed by an amount corresponding to the shading correction amount according to the scanning position in the sub-scanning direction. Note that the correction timing may be predicted by a timer and the image density unevenness.

[0102] FIG. 15 is an explanatory diagram of an image for sub-scanning measurement. In the present embodiment, an image for sub-scanning measurement is printed on a sheet of A3 size (297 [mm] × 420 [mm]). The image for sub-scanning measurement has three strip-shaped images whose sub-scanning direction is the longitudinal direction for each color component. The image for sub-scanning measurement has a constant width in the main scanning direction for each color component. Each of the strip-shaped images is arranged at a predetermined interval in the main scanning direction. The sheet on which a plurality of strip-shaped images are formed has a region where no image of any color is formed between the strip-shaped images of the same color in the main scanning direction. The three strip-shaped images of yellow are formed with the same image signal value, and ideally all have the same image density. Similarly, the strip-shaped images of magenta, cyan, and black are the same. In the present embodiment, the image signal value is, for example, an image density of 40% regardless of the color. Note that the image signal value may be different for each color, and the ideal image density may be different for each color. For example, in FIG. 15, a plurality of first-color strip images on the sheet are the first measurement image and the second measurement image, and the strip image of the second color different from yellow on the sheet is the third measurement image and the fourth measurement image. Here, for example, if the first color is cyan, the second color is yellow or magenta or black other than cyan, and if the first color is yellow, the second color is cyan or magenta or black other than yellow.

[0103] As an example, the image for sub-scanning measurement shown in FIG. 15 is divided into six regions, namely regions F to K, in the sub-scanning direction, and the image density of each region is measured. The CPU 105 acquires the image density for each region from the read data B of the reader 101. Note that the number of divided regions in the sub-scanning direction is not limited to this. Also, the size of the paper on which the image for sub-scanning measurement is printed is not limited to the A3 size.

[0104] When reading the image for sub-scanning measurement, the length of the paper in the sub-scanning direction may exceed the size of the reader 101. In this case, an ADF (not shown) that can be attached to the reader 101 may be used to read the image for sub-scanning measurement from the paper automatically fed by the ADF.

[0105] (Determination of shading correction amount) The determination process of the shading correction amount (image density unevenness correction amount) in S304 of FIG. 14 will be described. The method for correcting image density unevenness in the sub-scanning direction of the image forming apparatus 100 will be described.

[0106] As described with reference to FIG. 15, for the image for sub-scanning measurement formed in the process of S301 in FIG. 14, the image density of each of the regions F to K is measured three by three for each color component. The CPU 105 acquires the density of each of the regions F to K for the three strip-shaped images. The CPU 105 acquires the density DF1 of region F, the density DG1 of region G, the density DH1 of region H, the density DI1 of region I, the density DJ1 of region J, and the density DK1 of region K from the first strip-shaped image. The CPU 105 acquires the density DF2 of region F, the density DG2 of region G, the density DH2 of region H, the density DI2 of region I, the density DJ2 of region J, and the density DK2 of region K from the second strip-shaped image. The CPU 105 acquires the density DF3 of region F, the density DG3 of region G, the density DH3 of region H, the density DI3 of region I, the density DJ3 of region J, and the density DK3 of region K from the third strip-shaped image.

[0107] The CPU 105 calculates the average density (average value) DFA' of the densities of each region F to region K of the belt-shaped image for each color component. The sub-scanning measurement image is three belt-shaped images formed in multiple different regions in the main scanning direction. Therefore, the average density (average value) DFA' of the densities of each region F to region K of the three belt-shaped images corresponds to the average density DZA described in the first embodiment. Next, the CPU 105 calculates the average densities DFave to DKave of each region F to region K in the sub-scanning direction from the densities of each region F to region K of the three belt-shaped images. For example, the average density DFave of region F is the average value of densities DF1, DF2, and DF3.

[0108] Then, CPU 105 calculates the difference (density difference) ΔDens between the average density DFA' and the average densities DFave to DKave of regions F to K in the sub-scanning direction. CPU 105 calculates the amount of shading correction for each of regions F to K in the sub-scanning direction based on the density difference ΔDens. Next, CPU 105 corrects image density unevenness in the sub-scanning direction by adjusting the intensity of the laser light for each exposed position (regions F to K in the sub-scanning direction) based on the shading correction amount determined in this way.

[0109] In this embodiment, the amount of shading correction in the sub-scanning direction is calculated as an average value using band-shaped images formed at multiple positions in the main scanning direction. The image forming apparatus 100 of this embodiment also determines the amount of shading correction by matching the target density in the main scanning direction with the target density in the sub-scanning direction. The CPU 105 adjusts the laser light for each exposure position based on the amount of shading correction thus determined, thereby correcting density unevenness in the main scanning direction and image density unevenness in the sub-scanning direction.

[0110] FIG. 16 is an explanatory diagram of the main scanning measurement image. The main scanning measurement image is printed on a sheet of A4 size (210 [mm] × 297 [mm]). The main scanning measurement image has two strip-shaped images for each color component, with the main scanning direction being the longitudinal direction. The main scanning measurement image has a certain width in the main scanning direction for each color component. Each of the strip-shaped images is arranged at a predetermined interval in the sub-scanning direction. The two yellow strip-shaped images are formed with the same image signal value, and ideally, all have the same image density. Similarly, the strip-shaped images of magenta, cyan, and black are the same. In this embodiment, the image signal value is set to an image density of 40% regardless of color, for example. Note that the image signal values may be different for each color, and the ideal image densities may be different for each color.

[0111] Even when using the main scanning measurement image shown in FIG. 16, the shading correction amount in the main scanning direction is calculated as an average value using the strip-shaped images formed at a plurality of positions in the sub-scanning direction. This image forming apparatus 100 also determines the shading correction amount by matching the target density in the main scanning direction and the target density in the sub-scanning direction. The CPU 105 can correct the density unevenness in the main scanning direction and the image density unevenness in the sub-scanning direction by adjusting the laser light for each exposure position based on the shading correction amount determined in this way.

[0112] In the first to fourth embodiments described above, the intensity (or modulation amount) of the laser beam is adjusted to correct density unevenness in the sub-scanning direction. However, the parameter for correcting density unevenness in the sub-scanning direction is not limited to the intensity (or modulation amount) of the laser beam. For example, density unevenness in the sub-scanning direction may be corrected by adjusting the charging bias voltage applied to the charging roller 1400 in synchronization with the rotational phase of the photosensitive drum 1401. Furthermore, density unevenness in the sub-scanning direction may be corrected by adjusting the developing bias voltage applied to the developing sleeve 1422 in synchronization with the rotational phase of the photosensitive drum 1401. Alternatively, density unevenness in the sub-scanning direction may be corrected by adjusting both the charging bias voltage and the developing bias voltage in synchronization with the rotational phase of the photosensitive drum 1401. The intensity (or modulation amount) of the laser beam, the charging bias voltage, and the developing bias voltage are examples of image formation conditions for adjusting the density of images to be formed at multiple different positions on the photosensitive drum 1402 in the sub-scanning direction.

[0113] The configurations and contents of the various data in the first to fourth embodiments described above are not limited to these, and it goes without saying that various configurations and contents can be used depending on the application and purpose. The present invention can be embodied as, for example, a system, an apparatus, a method, a program, or a storage medium. Specifically, the present invention may be applied to a system consisting of multiple devices, or may be applied to an apparatus consisting of a single device. Furthermore, configurations that combine the above embodiments are also included in the present invention.

[0114] The image forming apparatus 100 described above has a configuration capable of forming color images, but the present invention is also effective for a monochrome printer that forms monochrome images.

Claims

1. Image forming means for forming an image on a rotating photoreceptor, transferring the image formed on the photoreceptor onto a sheet of paper, and forming an image on the sheet of paper; Reading means for reading a measurement image on the sheet of paper formed by the image forming means; Control means for controlling density unevenness of an image to be formed on the photoreceptor in the rotation direction of the photoreceptor based on a reading result of the reading means, The measurement image includes a first detection image for detecting the density of images to be formed at a plurality of different positions on the photoreceptor in the axial direction of the rotation axis of the photoreceptor, and a second detection image for detecting the density of images to be formed at a plurality of different positions on the photoreceptor in the rotation direction of the photoreceptor; The control means determines target data regarding a target density based on a reading result of the first detection image read by the reading means and a reading result of the second detection image read by the reading means; The control means suppresses density unevenness of an image to be formed on the photoreceptor in the rotation direction of the photoreceptor based on a reading result of the first detection image read by the reading means, a reading result of the second detection image read by the reading means, and the target data. An image forming apparatus.

2. The image forming means is controlled based on image forming conditions for adjusting the density of the image to be formed at a plurality of different positions on the photoreceptor in the rotation direction of the photoreceptor; The control means generates the image forming conditions based on a reading result of the first detection image read by the reading means, a reading result of the second detection image read by the reading means, and the target data. The image forming apparatus according to Claim 1.

3. The image forming means includes a charging member for charging the photoreceptor based on a charging bias voltage, a light source for exposing the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing sleeve for developing the electrostatic latent image on the photoreceptor based on a developing bias voltage; The image forming conditions are the intensity of light from the light source. The image forming apparatus according to Claim 2.

4. The image forming means includes a charging member that charges the photoreceptor based on a charging bias voltage, a light source that exposes the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing sleeve that develops the electrostatic latent image on the photoreceptor based on a developing bias voltage. The image forming condition is the charging bias voltage. The image forming apparatus according to claim 2.

5. The image forming means includes a charging member that charges the photoreceptor based on a charging bias voltage, a light source that exposes the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing sleeve that develops the electrostatic latent image on the photoreceptor based on a developing bias voltage. The image forming condition is the developing bias voltage. The image forming apparatus according to claim 2.

6. Image forming means for forming an image on a rotating photoreceptor. Reading means for reading a measurement image on a sheet formed by the image forming means. Control means for controlling density unevenness of an image to be formed on the photoreceptor in an axial direction of a rotation axis of the photoreceptor based on a reading result of the reading means. The measurement image includes a first detection image for detecting densities of images to be formed at a plurality of different positions on the photoreceptor in the axial direction of the photoreceptor, and a second detection image for detecting densities of images to be formed at a plurality of different positions on the photoreceptor in a rotation direction of the rotation axis of the photoreceptor. The control means determines target data regarding a target density based on a reading result of the first detection image read by the reading means and a reading result of the second detection image read by the reading means. The control means suppresses density unevenness of an image to be formed on the photoreceptor in the axial direction of the photoreceptor based on a reading result of the first detection image read by the reading means, a reading result of the second detection image read by the reading means, and the target data. Image forming apparatus.

7. The image forming means is controlled based on an image forming condition for adjusting the density of the image to be formed at a plurality of different positions on the photoreceptor in the axial direction of the photoreceptor. The control means generates the image forming conditions based on the reading result of the first detection image read by the reading means, the reading result of the second detection image read by the reading means, and the target data. The image forming apparatus according to claim 6.

8. The image forming means includes a charging member that charges the photoreceptor, a light source that exposes the photoreceptor charged by the charging member to form an electrostatic latent image on the photoreceptor, and a developing sleeve that develops the electrostatic latent image on the photoreceptor. The image forming conditions are the intensity of light from the light source. The image forming apparatus according to claim 7.

9. Image forming means for forming an image of a first color on a rotating first photoreceptor, Reading means for reading a plurality of measurement images on a sheet formed by the image forming means, Control means, and The plurality of measurement images include a first measurement image for detecting the density of the image of the first color to be formed at a plurality of different positions on the first photoreceptor in the axial direction of the rotation axis of the first photoreceptor, and a position different from the position where the first measurement image is formed in the axial direction of the rotation axis of the first photoreceptor, and is formed at a plurality of different positions on the first photoreceptor in the rotation direction of the first photoreceptor, and a second measurement image for detecting the density of the image of the first color to be formed. The sheet on which the plurality of measurement images are formed has a region where no image of the first color is formed between the first measurement image and the second measurement image in the axial direction of the first photoreceptor. The control means determines target data regarding the target density based on the reading result of the first measurement image read by the reading means and the reading result of the second measurement image read by the reading means. The control means suppresses density unevenness of the image of the first color to be formed on the first photoreceptor in the rotation direction of the first photoreceptor based on the reading result of the first measurement image read by the reading means, the reading result of the second measurement image read by the reading means, and the target data. Image forming apparatus.

10. The image forming means forms an image of a second color different from the first color on a rotating second photoreceptor The plurality of measurement images are a third measurement image used to detect the density of an image of a second color to be formed at a plurality of different positions on the second photoreceptor in the rotation direction of the second photoreceptor, and are formed at a position different from the position where the third measurement image is formed in the axial direction of the rotation axis of the second photoreceptor, and are formed at a plurality of different positions on the second photoreceptor in the rotation direction of the second photoreceptor. The plurality of measurement images further include a fourth measurement image used to detect the density of the image of the second color to be formed, The sheet on which the plurality of measurement images are formed has the third measurement image in the axial direction of the first photoreceptor in the region between the first measurement image and the second measurement image, The sheet on which the plurality of measurement images are formed is characterized by having the second measurement image between the third measurement image and the fourth measurement image in the axial direction of the first photoreceptor, The image forming apparatus according to claim 9.

11. The image forming means is controlled based on image forming conditions for adjusting the density of the image of the first color to be formed at a plurality of different positions on the first photoreceptor in the rotation direction of the first photoreceptor, The control means generates the image forming conditions based on the reading result of the first measurement image read by the reading means, the reading result of the second measurement image read by the reading means, and the target data, The image forming apparatus according to claim 9.

12. The image forming means includes a charging member that charges the first photoreceptor based on a charging bias voltage, a light source that exposes the first photoreceptor charged by the charging member to form an electrostatic latent image on the first photoreceptor, and a developing sleeve that develops the electrostatic latent image on the first photoreceptor based on a developing bias voltage, The image forming conditions are the intensity of light from the light source, The image forming apparatus according to claim 11.

13. The image forming means includes a charging member that charges the first photoreceptor based on a charging bias voltage, a light source that exposes the first photoreceptor charged by the charging member to form an electrostatic latent image on the first photoreceptor, and a developing sleeve that develops the electrostatic latent image on the first photoreceptor based on a developing bias voltage, The image forming conditions are the charging bias voltage, The image forming apparatus according to claim 11.

14. The image forming means includes a charging member that charges the first photoreceptor based on a charging bias voltage, a light source that exposes the first photoreceptor charged by the charging member to form an electrostatic latent image on the first photoreceptor, and a developing sleeve that develops the electrostatic latent image on the first photoreceptor based on a developing bias voltage. The image forming condition is the developing bias voltage. The image forming apparatus according to claim 11.

15. The control means determines data regarding the density for each of a plurality of positions in the rotational direction of the first photoreceptor based on the reading result of the first measurement image read by the reading means and the reading result of the second measurement image read by the reading means. The control means generates the image forming condition based on the data and the target data. The image forming apparatus according to claim 11.

Images