Image forming device

Abstract

To accurately execute correction of density unevenness in a main scanning direction even when unevenness periodically occurs in a sub scanning direction.SOLUTION: An image forming apparatus 100 forms a test pattern (Figure 7, Figure 17) on an intermediate transfer belt 1412 (S101 in Figure 6), measures the density of the formed test pattern with an image density sensor 400 (S102 in Figure 6), and corrects density unevenness in a main scanning direction of an image to be formed based on a result of measurement (S103, S104 in Figure 6). In this case, there is used a test pattern, as the test pattern, in which three or more patterns of the same color component having a plurality of measurement points in the main scanning direction are present in a sub scanning direction, and a plurality of types of intervals in the sub scanning direction of the pattern of the same color component are present.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to correction of density unevenness in the main scanning direction of an image formed by an image forming apparatus. [Background technology]

[0002] In an image forming apparatus using an electrophotographic system, an image is formed by scanning a photosensitive body with a laser beam or the like. In an electrophotographic print engine, a laser scanner or the like first irradiates a photosensitive drum charged by a charger with laser beams based on an image signal, forming an electrostatic latent image on the photosensitive drum. The electrostatic latent image formed on the photosensitive drum is then developed with toner to form a toner image on the photosensitive drum, and the toner image is transferred to a sheet and fused and thermally fixed by a fixing device, forming an image on the sheet.

[0003] In such an image forming apparatus, there may occur uneven charging due to deterioration of a charger that charges the photosensitive drum, uneven exposure due to a laser scanner that writes a latent image on the photosensitive drum, uneven development due to a developer that develops the latent image formed on the photosensitive drum, etc. These unevennesses may cause uneven density in the main scanning direction (the sheet width direction perpendicular to the sheet conveying direction) of the formed image.

[0004] Patent document 1 proposes a technology in which a sheet on which a pattern image extending in the main scanning direction is printed is output as a test print, and the density at different positions on the pattern image is read using a handy densitometer or the like, thereby correcting density unevenness in the main scanning direction.

[0005] Furthermore, Patent Document 2 proposes a technique in which a plurality of pattern images are arranged in the sub-scanning direction, and density unevenness in the main scanning direction is corrected by taking density unevenness in the sub-scanning direction into consideration. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2004-163216 A [Patent Document 2] JP 2007-264364 A Summary of the Invention [Problem to be solved by the invention]

[0007] In an image forming apparatus using an electrophotographic method, density unevenness may occur due to a rotating body such as a photosensitive drum. In other words, density unevenness with periodicity in the sub-scanning direction may occur. In terms of the density gradient in the main scanning direction (spatial density shading in the main scanning direction), this means that areas where the density is gradient (areas with shading) and areas where it is not gradient (areas with no shading) occur periodically in the sub-scanning direction.

[0008] For example, if the unevenness in density is caused by a Φ30mm photoconductor drum, the unevenness in density described above will occur at a pitch of about 94mm (≒diameter 30mm x π) in the sub-scanning direction of the sheet. Also, if the unevenness in density is caused by a Φ25mm developing sleeve, the unevenness in density will occur at a pitch of about 79mm (≒25mm x π). If multiple causes overlap, the unevenness in density will occur at multiple pitches.

[0009] When attempting to correct density unevenness as described above using the techniques of Patent Document 1 or Patent Document 2, the correction accuracy will differ between the correction results using a pattern image created in an area where the density is tilted and the correction results using a pattern image created in an area where the density is not tilted.

[0010] As in Patent Document 2, even when there are multiple pattern images of the same color in the sub-scanning direction, the intervals between the pattern images of the same color and the period of density unevenness in the sub-scanning direction may overlap or become close to each other. In this case, even if multiple pattern images of the same color are used, each of them will be created in a place where the density is inclined or not inclined. As such, conventional techniques have not been able to accurately correct density unevenness in the main scanning direction.

[0011] The present invention has been made in order to solve the above-mentioned problems, and an object of the present invention is to perform correction of density unevenness in the main scanning direction with high accuracy. [Means for solving the problem]

[0012] The present invention relates to an image forming apparatus having an image forming means for forming an image on a sheet, the image forming means including a rotating photoconductor, a charging unit for charging the photoconductor, an exposure unit for exposing the photoconductor charged by the charging unit, and a developing sleeve for developing, with toner, an electrostatic latent image formed on the photoconductor by exposure by the exposure unit, the image forming means including a pattern forming means for forming a test pattern on the sheet by the image forming means, a reading means for reading the test pattern on the sheet formed by the pattern forming means, and a correction means for correcting density unevenness in a first direction, which is an axial direction of the photoconductor, of the image formed by the image forming means based on a reading result of the test pattern read by the reading means, the test pattern being: A pattern of the same color component has a plurality of measurement points in the first direction, formed at different positions in a second direction perpendicular to the first direction same The test image includes a first test image, a second test image, and a third test image of a single color component, and is characterized in that a first distance between the first test image and the second test image in the second direction is different from a second distance between the second test image and the third test image in the second direction, and both the first distance and the second distance are different from an integer multiple of the circumference of the photoconductor. Effect of the Invention

[0013] According to the present invention, it is possible to correct density unevenness in the main scanning direction with high precision. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a block diagram showing an example of a configuration of an image forming apparatus according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram showing an example of a software configuration of an image forming apparatus. [Diagram 3]FIG. 2 is a diagram showing an example of the configuration of a printer image processing unit. [Figure 4] FIG. 2 is a diagram showing a part of the internal configuration of a printer engine. [Diagram 5] FIG. 2 is a cross-sectional view showing an example of an image forming portion of a printer engine. [Figure 6] 11 is a flowchart showing an example of a main scanning shading process. [Figure 7] 5A and 5B are views showing an example of a correction test pattern in main-scan shading according to the first embodiment. [Figure 8] 5A and 5B are diagrams showing density measurement results before density unevenness correction in the first embodiment. [Figure 9] 5A and 5B are diagrams showing density measurement results before density unevenness correction in the first embodiment. [Figure 10] 6A and 6B are diagrams showing differential density results before density unevenness correction in the first embodiment. [Figure 11] 8A to 8C are diagrams showing calculation results of the required correction amount for each band and region in the first embodiment. [Figure 12] 5A and 5B are diagrams showing exposure correction levels when correcting density unevenness in the first embodiment. [Figure 13] 5A to 5C are diagrams showing density measurement results after density unevenness correction in the first embodiment. [Figure 14] 5A to 5C are diagrams showing density measurement results after density unevenness correction in the first embodiment. [Figure 15] 11A and 11B are diagrams showing density measurement results after conventional density unevenness correction. [Figure 16] 11A and 11B are diagrams showing density measurement results after conventional density unevenness correction. [Figure 17] FIG. 11 is a diagram showing an example of a test pattern for correcting density unevenness in the second embodiment; [Figure 18] 13 is a diagram showing an example of band intervals for each color of a test pattern for correcting density unevenness in a second embodiment. FIG. [Figure 19] 11A and 11B are diagrams showing an example of an output image in which periodic density unevenness occurs in the sub-scanning direction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] [First embodiment] In this embodiment, a method for solving the above problem will be described using an electrophotographic laser beam printer, but the present invention may be applied to electrophotographic printers other than laser beam printers, such as LED printers.

[0016] <Image forming device> [Hardware configuration] FIG. 1 is a block diagram showing an example of the configuration of an image forming apparatus according to an embodiment of the present invention. As shown in FIG. 1, an image forming apparatus 100 has a scanner 101, which is an image input device, and a printer engine 102, which is an image output device, connected internally.

[0017] The scanner 101 is connected to a device I / F 117 via a scanner image processing unit 118. Moreover, the printer engine 102 is connected to the device I / F 117 via a printer image processing unit 119. The scanner image processing unit 118 controls the reading of image data. Moreover, the printer image processing unit 119 controls the print output.

[0018] The image forming apparatus 100 also connects to the LAN 10 and the public line 104 via a network I / F 111 and a modem 112 (described later), and controls input and output of image information and device information via the LAN 10 and the public line 104 .

[0019] The CPU 105 is a central processing unit for controlling the image forming apparatus 100. The RAM 106 is a system work memory for the operation of the CPU 105, and is also an image memory for temporarily storing input image data. Furthermore, the ROM 107 is a boot ROM, and stores a boot program for the system.

[0020] The HDD 108 is a hard disk drive that stores system software for various processes, input image data, etc. Note that the configuration may include other storage devices such as a solid state drive (SSD) instead of or in combination with the hard disk drive.

[0021] The operation unit I / F 109 is an interface for an operation unit 110 having a display screen capable of displaying image data and the like, and outputs operation screen data to the operation unit 110. In addition, the operation unit I / F 109 plays a role in transmitting information input by an operator from the operation unit 110 to the CPU 105.

[0022] The network I / F 111 is realized by, for example, a LAN card, and is connected to the LAN 10 to input and output information with an external device (not shown). The modem 112 is connected to the public line 104 to input and output information with an external device (not shown). The above units are arranged on a system bus 113 .

[0023] The image bus I / F 114 is an interface for connecting the system bus 113 and an image bus 115 that transfers image data at high speed, and is a bus bridge that converts data structures. A raster image processor (RIP) unit 116, a 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 to the image bus 115.

[0024] The RIP unit 116 develops a page description language (PDL) code into image data. The device I / F 117 connects the scanner 101 and the printer engine 102 via a scanner image processing unit 118 and a printer image processing unit 119, and performs synchronous / asynchronous conversion of image data. The scanner image processing unit 118 performs various processes such as correction and editing on the image data input from the scanner 101. The printer image processing unit 119 performs image processing such as gamma correction and halftone processing according to the printer engine on the image data to be printed out.

[0025] The image processing unit 120 for image editing performs various image processing such as image data rotation, color processing, binary conversion, multi-value conversion, etc. The image compression unit 103 encodes the image data processed by the RIP unit 116, the scanner image processing unit 118, and the image processing unit 120 for image editing in a predetermined compression format when storing the image data in the HDD 108.

[0026] The image decompression unit 121 decodes and decompresses the compressed and encoded data when the image data compressed in the HDD 108 is processed as necessary in the image editing image processing unit 120 or in the printer image processing unit 119 and output by the printer engine 102.

[0027] 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. Here, a profile is information such as a function for converting color image data expressed in a device-dependent color space into a device-independent color space (such as the Lab color space). Calibration data is data for correcting the color reproduction characteristics of the scanner 101 and the printer engine 102.

[0028] [Software configuration] Fig. 2 is a diagram showing an example of a software configuration of image forming apparatus 100. Each software module shown in Fig. 2 functions by CPU 105 loading a program stored in ROM 107 or HDD 108 into RAM 106 as necessary and executing the program.

[0029] A job control processing unit 201 supervises and controls each software module (not shown) and controls all jobs that occur within the image forming apparatus 100, such as copying, printing, scanning, and fax transmission / reception.

[0030] The network processing unit 202 is a module that mainly controls communication with the outside via the network I / F 111 , and controls communication with each device on the LAN 10 . The UI processing unit 203 mainly controls the operation unit 110 and the operation unit I / F 109 . The FAX processor 204 controls the FAX function and performs FAX reception / transmission via the modem 112.

[0031] The device information transmission processing unit 205 is a software module that 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 device information includes information that indicates the capabilities and characteristics of the image forming device 100, such as the type of the 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, and the output profile. The device information acquisition processor 206 is a software module that transmits a device information acquisition request to a predetermined external device via the network processor 202 based on an instruction from the job control processor 201 .

[0032] 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 an instruction from the job control processing unit 201 to perform printing of a specified image. The print processing unit 207 receives information on 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 then 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. Furthermore, the print processing unit 207 controls the printer engine 102 to print the image data on a specified paper.

[0033] The scan processing unit 210 controls the scanner 101 and the scanner image processing unit 118 based on instructions from the job control processing unit 201 to read the document placed on the scanner 101. The scan processing unit 210 scans the document placed on the platen of the scanner 101 and inputs the image as digital data. Color information of the input image is notified to the job control processing unit 201. Furthermore, the scan processing unit 210 controls the scanner image processing unit 118 to perform appropriate image processing such as image compression on the input image, and then notifies the job control processing unit 201 of the input image after image processing.

[0034] The color conversion processing unit 209 performs color conversion processing on the designated image based on an instruction from the job control processing unit 201, and notifies the job control processing unit 201 of the image after the color conversion processing.

[0035] The RIP processor 211 interprets the PDL based on an instruction from the job control processor 201, and controls the RIP unit 116 to perform rendering, thereby expanding the data into a bitmap image.

[0036] [Image data processing flow] With the above-mentioned configuration, the image processing system of this embodiment receives a print job from the LAN 10 and performs operations up to printing. First, as described above, PDL data transmitted from an external device via LAN 10 is received by network I / F 111 and input to RIP unit 116 via image bus I / F 114. RIP unit 116 interprets the received PDL and converts it into code data that can be processed by RIP unit 116. Furthermore, RIP unit 116 performs rendering based on the converted code data. The page data rendered by RIP unit 116 is compressed by image compression unit 103 at a subsequent stage and stored sequentially in HDD 108.

[0037] Next, the compressed data stored in HDD 108 is read out in a print operation in response to an instruction from job control processing unit 201, and the compressed data is decompressed by image decompression unit 121. The image data decompressed by image decompression unit 121 is input to printer image processing unit 119 via device I / F 117.

[0038] [Printer image processing unit] Next, a processing flow of image data input to the printer image processing unit 119 based on the above configuration will be described. FIG. 3 is a diagram showing an example of the configuration of the printer image processing unit 119. As shown in FIG. The printer image processing unit 119 includes a color conversion unit 301 , a density difference correction unit 302 , a gamma correction circuit 309 , a halftone processing unit 304 , a page buffer memory 306 , and an inter-drum delay memory control unit 305 .

[0039] The color conversion unit 301 converts image data from brightness values ​​(RGB, YUV, etc.) to density values ​​(CMYK, etc.), and converts the input image data into a color space corresponding to the color components that can be printed by the downstream printer engine 102.

[0040] The density difference correction unit 302 converts the multi-value image data converted to density values ​​by the color conversion unit 301 into signal values ​​that have been corrected for density differences within the same page. The density difference correction unit 302 has a one-dimensional table that changes the same input / output signals as the subsequent gamma correction circuit 309, and applies the table by multiplying it by a step correction coefficient for step correction according to the position within the page.

[0041] A gamma correction circuit 309 (hereinafter, "γLUT 309") converts the density signal of the image data that has been subjected to density step correction by the density step correction unit 302 into a signal value for reproducing that density in the printer engine. The γLUT is a table that converts input / output signals made in accordance with the γ characteristics of the printer engine, and in this embodiment, a pre-stored table is set for processing, but it may also be created using known gradation control or the like and set and used.

[0042] The halftone processing unit 304 performs halftone processing on the image data corrected by the γLUT 309, converting the image data into image data in which each color component of one pixel is expressed in two values ​​(1 bit). The halftone processing generally includes a dither method and an error diffusion method, and either method may be used in this embodiment. Note that the halftone processing is not limited to the above method, and other methods may be used.

[0043] The binary image data generated by the conversion process in the halftone processing unit 304 is separated into color components 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. The inter-drum delay memory control unit 305 reads out the data of the corresponding color component at the timing when a video data request signal (VREQ_* (* is any one of Y / M / C / K)) corresponding to each color component transmitted from the printer engine 102 is input. Note that the video data request signals are VREQ_Y, VREQ_M, VREQ_C, and VREQ_K for each color component. This is because the timing of exposure of each of the photosensitive drums 1401 to 1404 differs depending on the distance from the upstream to the downstream where the photosensitive drums 1401 to 1404 corresponding to each color component in the printer engine 102 are arranged, and therefore the timing of reading out the data of each color component also differs. The read-out color component data is output to the printer engine 102.

[0044] Next, the operation when the color component data output from the printer image processing unit 119 is input to the printer engine 102 will be described.

[0045] [Printer engine operation] Next, the operation when the color component data output from the printer image processing unit 119 is input to the printer engine 102 will be described. FIG. 4 is a diagram showing a part of the internal configuration of the printer engine 102. As shown in FIG. Printer I / F unit 1201 issues a video data request signal VREQ_* (* is any of Y / M / C / K) that requests data for each color component when printer engine 102 is ready for printing. Printer I / F unit 1201 receives color component data sequentially transmitted from printer image processing unit 119. The color component data received by printer I / F unit 1201 is input to pulse width modulation circuit 1203.

[0046] The pulse width modulation circuit 1203 generates pulse signals (drive signals) for driving the downstream laser drive units 1212-1215 for each color based on the input actual color component data, and transmits them to the laser drive units 1212-1215. Based on the pulse signal received from the pulse width modulation circuit 1203, the laser drivers 1212 to 1215 corresponding to the respective color components drive the laser exposure devices corresponding to the respective color components.

[0047] FIG. 5 is a cross-sectional view showing an example of an image forming portion of the printer engine 102. As shown in FIG. The printer engine 102 is a so-called electrophotographic printer engine. This type of printer engine has a photoconductor, a charging unit that charges the photoconductor, an exposure unit that exposes the charged photoconductor, a development unit that develops an electrostatic latent image formed on the photoconductor by exposure with toner, and a transfer unit that transfers the developed toner image onto an image carrier. The printer engine 102 of this embodiment is equipped with toner of multiple color components (e.g., yellow (Y), magenta (M), cyan (C), and black (K)) and can form a toner image made of multiple color components on the image carrier. Hereinafter, the image forming unit for yellow (Y) will be mainly described, but the image forming units for the other color components magenta (M), cyan (C), and black (K) have the same configuration. Note that in this embodiment, the printer engine 102 is intended for an image forming apparatus using a tandem engine consisting of four colors, YMCK, but is not limited thereto.

[0048] The printer engine 102 includes, as an image forming portion for yellow (Y), a photosensitive drum 1401 having a diameter of, for example, Φ32 mm, a charging roller 1205 having a diameter of, for example, Φ10 mm, a Y laser exposure device 1406, a developing device 1416, and a primary transfer device 1408. The printer engine 102 also includes a secondary transfer device 1413, a fixing device 1414, a cleaning device 1415, and an image density sensor 400.

[0049] The Y laser exposure device 1406 is driven by the Y laser drive unit 1212, and irradiates the photosensitive drum 1401 with laser light to form an electrostatic latent image on the photosensitive drum 1401. The developing device 1416 develops the electrostatic latent image on the photosensitive drum 1401 with toner. The primary transfer device 1408 transfers the visualized toner image onto a transfer material (an intermediate transfer belt 1412 which is an image carrier) (primary transfer). The secondary transfer device 1413 transfers the toner image formed on the intermediate transfer belt 1412 onto a sheet which is an image carrier (secondary transfer).

[0050] A fixing device 1414 fixes the toner image transferred onto the sheet. A cleaning device 1415 removes any residual toner remaining on the intermediate transfer belt 1412 after the secondary transfer. The image density sensor 400 measures the density of a patch image formed on the intermediate transfer belt 1412 .

[0051] The developing device 1416 includes a developer container, which contains a developer in which toner particles (toner) and magnetic carrier particles (carrier) are mixed as a two-component developer. An A screw 1420 and a B screw 1421 transport the toner particles and mix them with the magnetic carrier particles, respectively. For example, a developing sleeve 1422 having a diameter of 13 mm is disposed close to the photosensitive drum 1401, rotates in a manner following the photosensitive drum 1401, and carries the developer in which the toner and carrier are mixed. The developer carried by the developing sleeve 1422 comes into contact with the photosensitive drum 1401, and the electrostatic latent image on the photosensitive drum 1401 is developed.

[0052] In addition to the configuration shown in FIG. 5, the printer engine 102 also includes a transport unit (not shown) for transporting print paper, etc., but a description thereof will be omitted in this embodiment.

[0053] In the printer engine configuration described above, when printing yellow, the photosensitive drum 1401 is exposed by a Y laser exposure device 1406 driven by a Y laser drive unit 1212, and an electrostatic latent image is formed on the photosensitive drum 1401. The formed electrostatic latent image is visualized as a toner image by a yellow developer carried on a developing sleeve 1422 in a developing device 1416, and the visualized toner image is transferred onto an intermediate transfer belt 1412 by a primary transfer device 1408.

[0054] In this manner, each of the magenta, cyan, and black color components is similarly developed by each developing device 1417, 1418, and 1419, and visualized as a toner image on each of the photosensitive drums 1402, 1403, and 1404. The visualized toner images are then transferred in sequence by primary transfer devices 1409, 1410, and 1411 in synchronization with the toner images of the color components transferred immediately before, and a final toner image formed of the four color toner images is formed on the intermediate transfer belt 1412. The toner image formed on the intermediate transfer belt 1412 is transferred by a secondary transfer device 1413 to a sheet that is conveyed in synchronization, and the toner image is fixed by a fixing device 1414.

[0055] [Main scan shading] Next, correction of density unevenness in the main scanning direction (hereinafter referred to as "main scanning shading") will be described. 6 is a flowchart showing an example of main-scan shading processing. The processing of this flowchart is realized by the CPU 105 loading a program stored in the ROM 107 or HDD 108 into the RAM 106 as necessary and executing it. For example, the processing may be executed by the job control processing unit 201 or another software module (not shown).

[0056] First, when the main scanning shading process is started, the CPU 105 controls the print processing unit 207 and the like to output a test pattern image for correcting the density in the main scanning direction (S101). Note that the details of the test pattern image in this embodiment will be described later.

[0057] CPU 105 gives instructions to the user via the display unit of operation unit 110, and the user performs an operation to read the sheet on which the test pattern image is formed using scanner 101. In response to this operation, scanner 101 reads the sheet and outputs read data related to the test pattern image to CPU 105. CPU 105 acquires the read data related to the test pattern image and measures the density of the test pattern image based on the read data (S102). Next, the CPU 105 calculates the density distribution in the main scanning direction from the density measured in S102, and determines the amount of shading correction so as to eliminate the density unevenness (S103). The method of determining the amount of density unevenness correction will be described in detail later.

[0058] Next, the CPU 105 controls the printer engine 102 to transmit the shading correction amount for each color component determined in S103 above via the printer image processor 119 and set it (S104). Known shading correction methods include a method of changing the modulation degree of pulse width modulation (PWM) of the exposure light depending on the scanning position, and a method of changing the modulation degree of the intensity of the exposure light depending on the scanning position, but the method is not limited to these two methods. For example, in the case of a method of changing the modulation degree of the pulse width modulation of the exposure light depending on the scanning position, the shading correction amount for each color component determined in S103 above is transmitted to the pulse width modulation circuit 1203 and stored in a memory (not shown) in the pulse width modulation circuit 1203. As a result, the pulse width modulation circuit 1203 changes the modulation degree of the pulse width modulation of the exposure light depending on the scanning position based on the shading correction amount for each color component when printing is performed. As a result, density unevenness in the main scanning direction is corrected.

[0059] [Test Pattern] Next, a correction test pattern in main-scan shading will be described with reference to FIG. 7 is a diagram showing an example of a correction test pattern in main-scan shading according to the first embodiment. Here, a cyan test pattern will be described as an example, but the same effect can be obtained by using similar test patterns for other color components.

[0060] In this embodiment, a test pattern is formed on an A4 size (210 mm×297 mm) sheet. The main scanning direction and sub-scanning direction (the conveying direction (moving direction) of the intermediate transfer belt 1412) when forming the test pattern are as shown by the arrows in the figure. The main scanning direction and the sub-scanning direction are perpendicular to each other. Note that the size of the sheet on which the test pattern is formed is not limited to A4 size. Also, the image forming apparatus of this embodiment forms a test pattern on a sheet and causes the scanner 101 to read the test pattern on the sheet. This allows information regarding the density of the test pattern to be obtained. However, the test pattern may be formed on a sheet, and the image density of the test pattern on the sheet may be measured, for example, by an external colorimeter.

[0061] As shown in Fig. 7, the test pattern of this embodiment includes a plurality of band images (patterns) extending along the main scanning direction. In order to achieve the effects of this embodiment, it is necessary that there are a plurality of intervals between these band images (intervals in the sub-scanning direction between adjacent band images). The test pattern of this embodiment includes three band images. However, the number of band images formed on one sheet is not limited to that of this embodiment.

[0062] FIG. 7 shows, as an example, a test pattern in which three band images are arranged, the interval between the first and second bands (the interval between the center positions of the band images) is 30 mm, and the interval between the second and third bands is 50 mm. Density unevenness is detected from the density of multiple areas (density measurement points) of the test pattern in the main scanning direction. In the test pattern of this embodiment, for example, the density of five areas from area A to area E in the main scanning direction is obtained. The CPU 105 obtains the density for each area from the read data of the scanner 101. Note that the number of divisions of the areas in the main scanning direction is not limited to this. Also, the dimensions of the band images are 20 mm x 280 mm, but are not limited to this.

[0063] The reason why multiple intervals between the band images are required is that, as described above, in an image forming apparatus using electrophotography, density unevenness having a periodicity in the sub-scanning direction is likely to occur. An example will be described below with reference to FIG.

[0064] Fig. 19 shows an output image printed on an A3 size paper, captured by a scanner, in which uneven density is expressed by light and dark. Special image processing is performed to make the uneven density more obvious. The image shown in Fig. 19 shows an example of an output image in which the density periodically fluctuates in the sub-scanning direction, and uneven density occurs in the main scanning direction.

[0065] Periodic density unevenness occurring in an output image occurs in a period corresponding to the circumferential length of the photosensitive drum, developing sleeve, and charging roller. For example, the image in FIG. 19 is an image in which unevenness with a period of about 40 mm and unevenness with a period of about 102 mm are mixed. In the image forming apparatus 100 of this embodiment, the developing sleeve is Φ13 mm and the photosensitive drum is Φ32 mm. Therefore, the circumferential length of the developing sleeve is about 41 mm (≒13 mm×π) and the circumferential length of the photosensitive drum is about 101 mm (≒32 mm×π), which are close to (almost identical to) the above-mentioned unevenness periods of about 40 mm and about 102 mm. Therefore, it is considered that these rotating bodies are the cause of the unevenness with a period of about 40 mm and the unevenness with a period of about 102 mm.

[0066] Below, a test pattern required for executing shading correction for an image having such periodic unevenness so as to reduce uneven density in the main scanning direction will be considered.

[0067] First, consider the case of a correction test pattern having one band image of one color as pattern 1. The measured density differs when a band image is formed in a location where density unevenness in the main scanning direction is strong (for example, the solid line portion and dotted line portion in FIG. 19) and when a band image is formed in a location where density unevenness in the main scanning direction is not strong. Therefore, the amount of shading correction also differs depending on the location where the band image is formed. If a band image is formed in a location where density unevenness is strong, the amount of shading correction is determined so as to correct the unevenness. Therefore, the amount of deviation is suppressed in locations where the amount of deviation in density is originally large, but in locations where the amount of deviation in density is originally small, a large density deviation occurs due to an excessive amount of correction.

[0068] Next, consider pattern 2, where the band image has two or more equally spaced bands of one color (including two). In this case, if there is a sudden density gradient in only a certain part, the amount of correction may be averaged, or, depending on the processing method, the amount of correction may be optimized by processing such as removing the sudden part. On the other hand, for periodic density unevenness in the sub-scanning direction, the amount of correction also differs greatly depending on whether the interval between the band images roughly matches the period (or an integer multiple of the period) or not, resulting in the same tendency as pattern 1.

[0069] Finally, consider a correction test pattern as pattern 3, which has three or more band images of one color and multiple band image intervals as in this embodiment. In this case, the intervals between the band images are changed, so that all band images do not substantially match the periodic density unevenness in the sub-scanning direction as in patterns 1 and 2. In the test pattern of this embodiment, the intervals between adjacent band images are multiple different intervals, including intervals that do not correspond to integer multiples of the circumferential lengths of the photosensitive drum, charging roller, and developing sleeve. Therefore, it is possible to appropriately correct density unevenness in the main scanning direction without overcorrecting it.

[0070] As described above, the main scanning shading correction test pattern in this embodiment has been described as a test pattern with three band images of one color and two types of intervals, but is not limited to this and may have more numbers and types of intervals. Furthermore, the main scanning shading correction test pattern may be a test pattern with three or more band images of one color, and the intervals in the sub-scanning direction of the three or more band images may be two or more types of intervals that are mutually prime (the common divisor is only 1). For example, the interval between the first band and the second band in FIG. 7 may be 30 mm, and the interval between the second band and the third band may be 49 mm, so that the two intervals formed by the three band images are mutually prime (the common divisor between 30 and 49 is only 1). This prevents all band images from substantially matching the periodic density unevenness in the sub-scanning direction.

[0071] [Shading correction amount determination method] Next, a method for determining the amount of shading correction in step S103 in Fig. 6 will be described. That is, a method for correcting density unevenness in the main scanning direction of the image forming apparatus will be described. Note that, as a correction method, a method for changing the modulation degree of the PWM modulation of the exposure light depending on the scanning position and a method for changing the modulation degree of the intensity of the exposure light depending on the scanning position are known, but the correction method is not limited to these two methods.

[0072] The main scanning shading correction chart output in step S101 in Fig. 6 is divided into areas in the main scanning direction as shown in Fig. 7, and density measurement is performed for each band in each area. Examples of the measurement results are shown in Figs. 8 and 9. FIG. 8 is a graph showing an example of the results of measuring the density of the test pattern in step S102. FIG. 9 is a diagram showing an example of the result of measuring the density of the test pattern in step S102.

[0073] Next, the average density of each band is calculated, and the density difference ΔDens of each region with respect to the average density is calculated. An example of the calculation result is shown in FIG. FIG. 10 is a diagram showing an example of a differential density result before density unevenness correction in the first embodiment.

[0074] Next, the necessary correction amount for the density difference result is calculated by Equation 1.

number

[0075] The correction coefficient N in Equation 1 is a coefficient that determines how much the PWM modulation of the exposure light or the level of the modulation degree of the exposure light intensity is changed with respect to the density difference. For example, if the correction coefficient N=100, it indicates that when the density difference ΔDens is "0.01", the level of the modulation degree is changed by "1". In this embodiment, the correction coefficient N is set to 200. Note that the correction coefficient N is not limited to this. Note that, in this embodiment, the calculation result of the right side of Equation 1 is rounded off to the nearest whole number and calculated as the required correction amount, but this is not limited to this. FIG. 11 is a diagram showing an example of the calculation result of the required correction amount for each band and area in the first embodiment.

[0076] Next, the exposure correction level for each region is determined from the necessary correction amount for each band and region calculated as described above. The exposure correction level is calculated as the average value of the density difference results for each band in the same region. FIG. 12 is a diagram showing an example of exposure correction levels when correcting density unevenness in the first embodiment.

[0077] In this embodiment, the exposure correction level is calculated as an average value for each region, but this is not limiting. For example, it may be an intermediate value or a value obtained by other statistical processing. In this manner, the modulation degree of the exposure level required for correction is calculated, and the exposure level is adjusted, thereby making it possible to correct density unevenness in the main scanning direction.

[0078] [Effect description] Next, the effect of correcting density unevenness using the main scanning shading pattern as in this embodiment will be described. After the above-mentioned correction was performed, the same test pattern was output again, and the measured densities of each band and region are shown in FIGS. FIG. 13 is a graph showing an example of a density measurement result after density unevenness correction in the first embodiment. FIG. 14 is a diagram showing an example of a density measurement result after density unevenness correction in the first embodiment.

[0079] As shown in FIGS. 8 and 9, before the main scanning shading correction of this embodiment is performed, Maximum concentration Dmax=0.83 Minimum concentration Dmin=0.72 Difference density ΔD=0.11 Density gradient of each band: 1st band = 0.11 / 2nd band = 0.03 / 3rd band = 0.05 It is.

[0080] In contrast, after the main scanning shading correction of this embodiment is performed, as shown in FIGS. Maximum concentration Dmax=0.795 Minimum concentration Dmin=0.73 Difference density ΔD=0.065 Density gradient of each band: 1st band = 0.055 / 2nd band = 0.025 / 3rd band = 0.03 It can be seen that the density unevenness in the main scanning direction can be reduced.

[0081] As a comparative example 1, the bands are spaced at equal intervals, and main-scanning shading is performed using a test pattern formed in an area with a large density gradient. FIG. 15 shows the density results after correction.

[0082] As shown in Figure 15, after conventional density unevenness correction, when main-scanning shading is performed using a test pattern formed in an area with a large density gradient, Maximum concentration Dmax=0.8 Minimum concentration Dmin=0.71 Difference density ΔD=0.09 Density gradient of each band: 1st band = 0 / 2nd band = 0.08 / 3rd band = 0.08 It can be seen that although appropriate correction is performed in some areas, over-correction occurs in other areas, resulting in little effect of correction.

[0083] As a comparative example 2, FIG. 16 shows the density results after correction when main-scanning shading is performed using a test pattern in which the bands are created with equal intervals and the test pattern is formed in a location where the density gradient is small.

[0084] As shown in Figure 16, after conventional density unevenness correction, when main scanning shading is performed using a test pattern formed in an area with a small density gradient, Maximum concentration Dmax=0.82 Minimum concentration Dmin=0.73 Difference density ΔD=0.09 Density gradient of each band: 1st band = 0.08 / 2nd band = 0 / 3rd band = 0.03 It can be seen that, although appropriate correction is performed in some areas, correction is insufficient in other areas, and as a result, the effect of correction is small.

[0085] As described above, the test pattern for correcting the density unevenness in the main scanning direction has three or more band images in the sub-scanning direction for measuring the density, and the test pattern has two or more types of intervals between the band images, and the test pattern is used to correct the density unevenness in the main scanning direction. In this way, the test pattern is composed of three or more band images for one color component, and the interval between adjacent band images includes an interval that does not correspond to an integer multiple of the circumference of the rotating body (photoconductor, charging roller, developing sleeve, etc.) in the imaging unit. Therefore, it is possible to prevent the density unevenness that occurs periodically in the sub-scanning direction due to the rotating body in the imaging unit from coinciding with the formation positions of each band image in the test pattern used for main scanning shading. This makes it possible to perform the density unevenness correction in the main scanning direction with high accuracy, even when the density unevenness occurs periodically in the sub-scanning direction.

[0086] Second Embodiment In the first embodiment, a monochromatic (single-color component) test pattern was used in which the test pattern for correcting density unevenness in the main scanning direction had three or more band images in the sub-scanning direction for density measurement and two or more types of intervals between the band images. A configuration was then described in which the monochromatic test pattern was used to set the amount of shading correction for a color component requiring main scanning shading. In the second embodiment, a case in which four colors, Y, M, C, and K, were used as the test pattern was described. Note that the description of the image forming apparatus, the flow chart for main scanning shading, and the correction method described in the first embodiment are the same in the second embodiment, and therefore will not be repeated.

[0087] [Test Pattern] The main-scanning shading correction test pattern in the second embodiment will be described with reference to FIG. 17 is a diagram showing an example of a correction test pattern in main-scanning shading according to the second embodiment. Here, as an example, a test pattern in which four band images for each color are arranged in the sub-scanning direction will be described, but the present invention is not limited to this.

[0088] In the image forming apparatus of this embodiment, a test pattern is formed on an A4 size (210 mm x 297 mm) sheet. The main scanning direction and sub-scanning direction (the conveying direction (moving direction) of the intermediate transfer belt 1412) when forming the test pattern are as shown by the arrows in the figure. Note that the size of the sheet on which the test pattern is formed is not limited to A4 size.

[0089] As shown in Figure 17, the test pattern is arranged as follows, starting from the top of the feed direction: Y→M→C→K→M→Y→K→C→M→Y→C→K→Y→M→C→K A total of 16 band images are arranged in the following order: This test pattern has band images of multiple color components arranged in the sub-scanning direction, four for each color component, and the order of the arrangement is such that there are multiple types of intervals between the band images of the same color component.

[0090] FIG. 18 shows the intervals between the band images of each color. FIG. 18 is a table showing an example of band intervals for each color of a correction test pattern in main-scan shading according to the second embodiment.

[0091] As can be seen from Fig. 18, the intervals between the band images of each color are set so that the interval between the first band and the second band, the interval between the second band and the third band, and the interval between the third band and the fourth band are different from each other. Note that the band images are 10 mm x 280 mm, but are not limited to this. The test pattern of this embodiment is an arrangement of three or more patterns of multiple color components per color component in the sub-scanning direction, and the arrangement order is such that there are multiple types of intervals between patterns of the same color component in the sub-scanning direction.

[0092] As explained in the first embodiment, in an image forming apparatus using electrophotography, density unevenness that has a period in the sub-scanning direction is likely to occur. Therefore, when correcting density unevenness in the main scanning direction, if you try to correct it with one or two band images for each color in the sub-scanning direction, it will be easily affected by the period in the sub-scanning direction, and even if there are three or more band images, it will be easily affected by the period in the sub-scanning direction if the intervals between the band images are the same. Therefore, by using a correction test pattern formed with an arrangement of band images as shown in the second embodiment, even when density unevenness occurs periodically in the sub-scanning direction, the effect of the intensity of density unevenness is mitigated, making it possible to perform density unevenness correction appropriately for all colors.

[0093] In this embodiment, the correction pattern has been described in which the arrangement of the band images is in the order of Y→M→C→K→M→Y→K→C→M→Y→C→K→Y→M→C→K. However, the arrangement is not limited to this, and the same effect can be obtained as long as the arrangement is in an order in which there are multiple types of intervals between the band images of each color.

[0094] In addition, in this embodiment, the correction pattern in which all colors (here, the four colors YMCK) are arranged in an area of ​​one A4 size sheet has been described, but this is not limited to this. For example, it is also possible to use a test pattern formed over the area of ​​multiple A4 size sheets, and the same effect can be obtained if there are multiple intervals between the band images of each color.

[0095] Also, even if the correction pattern has two or three colors, the same effect can be obtained for the colors as long as there are multiple intervals between the band images of each color. CPU 105 performs the main scanning shading process shown in Fig. 6 using a correction pattern in which band images of the color components for which main scanning shading is performed are arranged. For example, when main scanning shading of two colors, Y and C, CPU 105 performs the main scanning shading process shown in Fig. 6 using a correction pattern in which the two colors, Y and C, are arranged. In the case of an image forming apparatus that forms an image using five or more color components, correction patterns of five or more colors may be used.

[0096] As described above, according to each embodiment, even when unevenness occurs periodically in the sub-scanning direction, it is possible to correct uneven density in the main scanning direction with high accuracy.

[0097] It goes without saying that the configurations and contents of the various data described above are not limited to those described above, and the data may have various configurations and contents depending on the application and purpose. Although one embodiment has been described above, the present invention can be embodied, for example, as a system, an apparatus, a method, a program, a storage medium, etc. Specifically, the present invention may be applied to a system composed of multiple devices, or may be applied to an apparatus composed of a single device. Furthermore, any combination of the above-described embodiments is also included in the present invention.

[0098] (Other embodiments) The present invention can also be realized by a process in which a program for implementing one or more of the functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., ASIC) that implements one or more of the functions. Furthermore, the present invention may be applied to a system made up of a plurality of devices, or to an apparatus made up of a single device. The present invention is not limited to the above-mentioned embodiment, and various modifications (including organic combinations of the embodiments) are possible based on the spirit of the present invention, and are not excluded from the scope of the present invention. In other words, the present invention includes all configurations that combine the above-mentioned embodiments and their modifications. [Explanation of symbols]

[0099] 100 Image forming device 102 Printer Engine 105 CPU 400 Image Density Sensor 1203 Pulse width conversion circuit 1412 Intermediate transfer belt (transfer material)

Claims

1. An image forming apparatus having an image forming means for forming an image on a sheet, the image forming means comprising: a rotating photoconductor; a charging section for charging the photoconductor; an exposure section for exposing the photoconductor charged by the charging section to light; and a developing sleeve for developing, with toner, an electrostatic latent image formed on the photoconductor by exposure by the exposure section, a pattern forming means for forming a test pattern on a sheet by the image forming means; a reading means for reading the test pattern formed on the sheet by the pattern forming means; a correction unit that corrects density unevenness in a first direction, which is an axial direction of the photoconductor, of the image formed by the image forming unit based on a reading result of the test pattern read by the reading unit, the test pattern includes a pattern of a same color component having a plurality of measurement points in the first direction, and a first test image, a second test image, and a third test image of the same color component formed at different positions in a second direction perpendicular to the first direction; An image forming apparatus, characterized in that a first interval between the first test image and the second test image in the second direction is different from a second interval between the second test image and the third test image in the second direction, and both of the first interval and the second interval are different from an integer multiple of a circumference of the photoconductor.

2. the image forming means is capable of forming a toner image made of toners of a plurality of color components, 2. The image forming apparatus according to claim 1, wherein the test pattern is composed of a pattern of one color component.

3. the image forming means is capable of forming a toner image made of toners of a plurality of color components, 2. The image forming apparatus according to claim 1, wherein the test pattern is made up of patterns of a plurality of color components.

4. 4. The image forming apparatus according to claim 3, wherein the test pattern is an arrangement of three or more patterns of multiple color components per color component in the second direction, and the order of the arrangement is an order in which the spacing between patterns of the same color component in the second direction is of multiple types.

5. An image forming apparatus having an image forming means for forming an image on a sheet, the image forming means comprising: a rotating photoconductor; a charging section for charging the photoconductor; an exposure section for exposing the photoconductor charged by the charging section to light; and a developing sleeve for developing, with toner, an electrostatic latent image formed on the photoconductor by exposure by the exposure section, a pattern forming means for forming a test pattern on a sheet by the image forming means; a reading means for reading the test pattern formed on the sheet by the pattern forming means; a correction unit that corrects density unevenness in a first direction, which is an axial direction of the photoconductor, of the image formed by the image forming unit based on a reading result of the test pattern read by the reading unit, the test pattern includes a pattern of a same color component having a plurality of measurement points in the first direction, and a first test image, a second test image, and a third test image of the same color component formed at different positions in a second direction perpendicular to the first direction; a first distance between the first test image and the second test image in the second direction is different from a second distance between the second test image and the third test image in the second direction; 2. An image forming apparatus according to claim 1, wherein both of the first interval and the second interval are different from an integer multiple of a circumferential length of the developing sleeve.

6. the image forming means is capable of forming a toner image made of toners of a plurality of color components, 6. The image forming apparatus according to claim 5, wherein the test pattern is made up of patterns of a plurality of color components.

7. 7. The image forming apparatus according to claim 6, wherein the test pattern is an arrangement of three or more patterns of multiple color components per color component in the second direction, and the order of the arrangement is an order in which the spacing between patterns of the same color component in the second direction is of multiple types.

8. An image forming apparatus having an image forming means for forming an image on a sheet, the image forming means comprising: a rotating photoconductor; a charging roller for charging the photoconductor; an exposure unit for exposing the photoconductor charged by the charging roller; and a developing sleeve for developing, with toner, an electrostatic latent image formed on the photoconductor by exposure by the exposure unit, a pattern forming means for forming a test pattern on a sheet by the image forming means; a reading means for reading the test pattern formed on the sheet by the pattern forming means; a correction unit that corrects density unevenness in a first direction, which is an axial direction of the photoconductor, of the image formed by the image forming unit based on a reading result of the test pattern read by the reading unit, the test pattern includes a pattern of a same color component having a plurality of measurement points in the first direction, and a first test image, a second test image, and a third test image of the same color component formed at different positions in a second direction perpendicular to the first direction; a first distance between the first test image and the second test image in the second direction is different from a second distance between the second test image and the third test image in the second direction; 2. An image forming apparatus comprising: a charging roller that is electrically connected to a charging member; a charging member that is electrically connected to a charging roller;

9. the image forming means is capable of forming a toner image made of toners of a plurality of color components, 9. The image forming apparatus according to claim 8, wherein the test pattern is made up of patterns of a plurality of color components.

10. 10. The image forming apparatus according to claim 9, wherein the test pattern is an arrangement of three or more patterns of multiple color components per color component in the second direction, and the order of the arrangement is an order in which the spacing between patterns of the same color component in the second direction is of multiple types.

Images