JP7497212B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus and a method for controlling an image forming apparatus.

Image forming devices may output images that do not have the desired density due to influences caused by fluctuations in the installation environment or internal environment. For example, short-term fluctuation factors include the influence of changes in the photoconductor and developer over time, while long-term fluctuation factors include the influence of fluctuations such as deterioration of the photoconductor and developer over time. For this reason, when forming an image, the image forming device must make corrections that take into account various fluctuations. A technique called calibration is used as a process for appropriately correcting changes in density and color. For example, in calibration, various conditions are adjusted by forming a pattern image with a uniform density on paper, a photoconductor, an intermediate transfer body, etc., measuring the density of the formed pattern, comparing it with a target value, and making corrections.

Related techniques are proposed in Patent Documents 1 to 3. The technique in Patent Document 1 reads a gradation pattern image formed on recording paper to determine density correction characteristics, and adjusts the density correction characteristics according to the relationship between the stored density and the density of the image formed on the image carrier. The technique in Patent Document 2 compensates for drift and changes within the printer by having the printer adjust and maintain a lookup table. The technique in Patent Document 3 forms a patch image on a recording medium at a set timing, detects the density or color value, and corrects the image formation conditions to approach the target value used in the image formation job.

JP 2000-238341 A Japanese Patent Application Laid-Open No. 10-224653 JP 2005-103850 A

In recent years, there has been a demand to shorten the time required for calibration and to reduce the amount of toner consumed. However, when trying to shorten the time required for calibration using the above-mentioned techniques, there is a problem that the patches used for calibration become larger and more toner is used.

An object of the present invention is to reduce the length of the patch used for calibration.

In order to achieve the above object, an image forming apparatus of the present invention includes a conversion means for converting an image signal based on a conversion condition, an image forming means for forming an image on a recording paper based on the image signal converted by the conversion means, a transfer body onto which a test pattern formed by the image forming means is transferred, an irradiation means for irradiating light onto the transfer body, a light receiving means for receiving light reflected from the test pattern on the transfer body, and a generation means for generating the conversion condition based on the reflected light from the test pattern received by the light receiving means , wherein the test pattern on the transfer body is transported so as to pass through a detection position where the irradiation means irradiates the light, and the test pattern changes from low density to high density along a rotation direction of the transfer body. a first patch row of a first color , the density of which changes from a high density to a low density along the rotation direction of the transfer body, and a second patch row of the first color , the density of which changes from a high density to a low density along the rotation direction of the transfer body, are adjacent to each other in the rotation direction of the transfer body; a third patch row of a second color, different from the first color, the density of which changes from a low density to a high density along the rotation direction of the transfer body, and a fourth patch row of the second color, the density of which changes from a high density to a low density along the rotation direction of the transfer body, are adjacent to each other in the rotation direction of the transfer body; both the first patch row and the third patch row have been subjected to a first pseudo-halftoning process, and both the second patch row and the fourth patch row have been subjected to a second pseudo-halftoning process different from the first pseudo-halftoning process .

According to the present invention, the length of the patch used for calibration can be reduced.

1 is a block diagram showing an overall configuration of an image forming apparatus; FIG. 2 illustrates an example of a software module of the image forming apparatus. 2 is a block diagram showing the internal configuration of a printer image processing unit. FIG. FIG. 2 is a diagram illustrating a part of the internal configuration of a printer engine. FIG. 2 is a diagram showing the configuration of an image forming section of a printer engine. FIG. 2 is a diagram showing a part of the structure of an image density sensor. 1 is a block diagram showing a configuration of a control unit; 5 is a diagram showing an example of the relationship between the output of an image density sensor and image density. FIG. 10 is a flowchart showing the flow of an automatic tone correction process. 11 is a diagram showing an example of a relationship between an engine gamma characteristic, a tone correction table, and a tone target. FIG. FIG. 13 is a diagram illustrating an example of a target concentration. 10 is a flowchart showing an example of a process flow for creating a tone correction LUT. 10 is a flowchart showing an example of a process flow for creating a composite correction LUT. FIG. 13 is a diagram showing the relationship between an input signal and a density. FIG. 13 is a diagram showing an example of a transition of a measurement value of a photodiode. FIG. 4 is a diagram illustrating an example of the responsiveness of a sensor. 5A and 5B are diagrams illustrating an example of an arrangement of image patterns for image density correction according to the present embodiment. FIG. 1 is a diagram showing an example of an arrangement of image patterns for conventional image density correction; FIG. 13 is a diagram showing the relationship between the measurement order, colors, and image signal values of an image pattern for image density correction in a first modified example. FIG. 13 is a diagram showing the relationship between the measurement order, colors, and image signal values of an image pattern for image density correction in a second modified example.

The present embodiment of the present invention will be described in detail below with reference to the drawings. However, the configuration described in the present embodiment below is merely an example, and the scope of the present invention is not limited to the configuration described in the embodiment. Below, the image forming apparatus will be described as an electrophotographic laser beam printer. However, the image forming apparatus of this embodiment is not limited to electrophotographic laser beam printers, and may be applied to inkjet printers, dye-sublimation printers, etc.

Figure 1 is a block diagram showing the overall configuration of an image forming apparatus. In Figure 1, the 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. The scanner 101 and the printer engine 102 are connected inside the image forming apparatus 100. The scanner 101 is connected to a device I/F 117 via a scanner image processing unit 118. 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. The printer image processing unit 119 controls the print output. The image forming apparatus 100 is also connected to a LAN 10 and a public line 104 so as to be able to communicate with each other, and can transmit and receive image information and device information.

The CPU 105 is a processor that controls each part of the image forming apparatus 100 and performs each process of this embodiment. The CPU 105 has the function of a correction means. The RAM 106 is a work memory for the operation of the CPU 105. The RAM 106 also functions as an image memory that temporarily stores input image data. The ROM 107 stores various programs. For example, the ROM 107 stores a boot program. The HDD 108 is a hard disk drive, and stores system software for various processes, input image data, etc. The process of this embodiment can be realized by expanding the program stored in the ROM 107 to the RAM 106 and having the CPU 105 execute the program expanded to the RAM 106.

The operation unit I/F 109 is an interface for the operation unit 110 having a display screen capable of displaying image data, etc., and outputs operation screen data to the operation unit 110. The operation unit I/F 109 also outputs information input by an operator (e.g., a user) from the operation unit 110 to the CPU 105. The network I/F 111 is realized by a LAN card or the like, and transmits and receives information to and from each device connected to the LAN 10. The modem 112 is also connected to the public line 104, and transmits and receives information to and from an external device (e.g., a server, etc.) via the public line 104. Each of the above units is connected to the system bus 113.

The image bus I/F 114 is an interface for connecting the system bus 113 and the image bus 115, and is a bus bridge that converts data structures. The image bus 115 is a bus that can transfer image data at high speed. The image compression unit 103, the RIP unit 116, the device I/F 117, the image editing image processing unit 120, the image decompression unit 121, and the CMM 130 are connected to the image bus 115. The RIP unit 116 is a raster image processor. The CMM is a color management module. The RIP unit 116 is a raster image processor that expands page description language (PDL) code into image data. The device I/F 117 connects to the scanner 101 or the printer engine 102 via the scanner image processing unit 118 or the printer image processing unit 119, and converts image data between synchronous and asynchronous systems.

The scanner image processing unit 118 performs various processes such as correction and editing on the image data input from the scanner 101. The image processing unit for image editing 120 performs various image processes such as rotation, color processing, binary conversion, and multi-value conversion of the image data. 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 for image editing 120 using a predetermined compression method when temporarily storing the image data in the HDD 108.

The image decompression unit 121 decodes and decompresses image data compressed in the HDD 108. For example, the image decompression unit 121 decodes and decompresses image data when image processing is performed on image data compressed by the image editing image processing unit 120 or the printer image processing unit 119 and the image data is output by the printer engine 102. 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. The 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 (e.g., Lab color space). The calibration data is data for correcting the color reproduction characteristics of the scanner 101 or the printer engine 102.

Next, the software modules of the image forming apparatus 100 will be described. FIG. 2 is a diagram showing an example of the software modules of the image forming apparatus 100. Each software module is stored in a storage unit such as the HDD 108. Each software module is executed by the CPU 105. The job control processing module 201 controls various jobs that occur within the image forming apparatus 100, such as copying, printing, scanning, and fax transmission and reception. The network processing module 202 is a module that mainly controls communication with the outside via the network I/F 111. The network processing module 202 also controls communication with devices connected to the LAN 10.

The UI processing module 203 mainly controls the operation unit I/F 109. For example, the UI processing module 203 controls the display of the screen of the operation unit 110. The FAX processing module 204 controls the FAX function. The FAX processing module 204 sends and receives FAX via the modem 112. The print processing module 207 controls the printer engine 102, the printer image processing unit 119, and the image editing image processing unit 120 based on instructions from the job control processing module 201, and prints the specified image. The print processing module 207 receives various information from the job control processing module 201. The various information includes, for example, image data and image information (image data size, color mode, resolution, etc.), layout information (offset, enlargement/reduction, imposition, etc.), and output paper information (size, print direction, etc.).

The print processing module 207 then controls the image compression unit 103, image decompression unit 121, image editing image processing unit 120, and printer image processing unit 119 to perform appropriate image processing on the image data. The print processing module 207 controls the printer engine 102 to print the image data that has been subjected to image processing on a specified sheet of paper. Based on instructions from the job control processing module 201, the scan processing module 210 controls the scanner 101 and scanner image processing unit 118 to read the original on the scanner 101.

The scan processing module 210 is a module for executing scanning of an original placed on the platen of the scanner 101. Scanned image data (digital data) is input to the scan processing module 210. The scan processing module 210 notifies the job control processing module 201 of color information of the input image (image data). The scan processing module 210 also controls the scanner image processing unit 118 to perform image processing such as compression on the input image, and then notifies the job control processing module 201 of the image that has been subjected to image processing. The color conversion processing module 209 performs color conversion processing on the specified image based on an instruction from the job control processing module 201. The color conversion processing module 209 notifies the job control processing module 201 of the image after color conversion processing. The RIP processing module 211 performs PDL interpretation based on an instruction from the job control processing module 201. The RIP processing module 211 then controls the RIP unit 116 to perform rendering, thereby expanding the image into a bitmap image.

The image forming device 100 configured as described above receives a print job and prints it. Below, an example of the operation of the image forming device 100 from receiving a print job from the LAN 10 to printing will be described. The processing flow of image data input to the printer image processing unit 119 will also be described. Note that the image forming device 100 may receive a print job from a source other than the LAN 10.

As described above, the network I/F 111 receives the PDL transmitted by the external device via the LAN 10. The received PDL is input to the RIP unit 116 from the image bus I/F 114. The RIP unit 116 interprets the input PDL and converts it into code data that the RIP unit 116 can process. The RIP unit 116 then performs rendering based on the converted code data. The page data rendered by the RIP unit 116 is compressed by the image compression unit 103 in the subsequent stage, and the compressed page data (compressed data) is stored sequentially in the HDD 108. The compressed data stored in the HDD 108 is read out during a print operation in response to an instruction from the job control processing module 201. The image decompression unit 121 performs decompression processing on the read compressed data. The decompressed image data is input to the printer image processing unit 119 via the device I/F 117.

Next, the printer image processing unit 119 will be described. FIG. 3 is a block diagram showing the internal configuration of the printer image processing unit 119. The color conversion unit 301 converts image data from luminance values (RGB, YUV, etc.) to density values (CMYK, etc.). The color conversion unit 301 converts the input image data into a color space corresponding to the color components that can be printed by the printer engine 102 at the subsequent stage. The density step correction unit 302 converts the multi-value image data converted into density values by the color conversion unit 301 into signal values that have corrected the density steps within the same page. The density step correction unit 302 has a one-dimensional table that changes the same input/output signals as the gamma correction circuit 309, and multiplies the one-dimensional table by a step correction coefficient for step correction according to the position within the page. The image data corrected for density steps by the density step correction unit 302 is input to the gamma correction circuit 309. The gamma correction circuit 309 is a correction circuit having a gamma LUT. The gamma correction circuit 309 converts the density signal into a signal value for reproducing the density of the printer engine 102. The gamma LUT is a table (lookup table) that converts an output signal created in accordance with the gamma characteristics of the printer engine 102. In this embodiment, the gamma correction circuit 309 performs processing using a table stored in advance, but a table created using known gradation control, etc. may also be used.

The position information generating unit 303 generates correction position information by adding information on the position where the density value is to be corrected to the image position information in the page of the image data input from the device I/F 117. The position information generating unit 303 notifies the density step correction unit 302 of the generated correction position information. The density step correction unit 302 changes the γLUT according to the correction position information. The image data corrected by the γ correction circuit 309 is input to the halftone processing unit 304. The halftone processing unit 304 performs halftone processing on the input image data and converts it into image data in which each color component of one pixel is expressed in binary (1 bit). Any method such as a dither method or an error diffusion method may be used as the halftone processing performed by the halftone processing unit 304. The binary image data generated by the halftone processing unit 304 performing the conversion process 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. When a video data request signal corresponding to each color component sent from the printer engine 102 is input, image data of the corresponding color component is read from the page buffer memory 306. The read color component data is sent to the printer engine 102. The video data request signals are VREQ_Y, VREQ_M, VREQ_C, and VREQ_K, corresponding to each color component. The timing of exposure of each of the photosensitive drums 1401-1404 as image carriers differs depending on the distance from upstream to downstream where the photosensitive drums 1401-1404 corresponding to each color component in the printer engine 102 are arranged. Therefore, the timing of reading data for each color component also differs. The photosensitive drums 1401-1404 will be described later.

Next, the operation of the printer engine 102 will be described. FIG. 4 is a diagram showing a part of the internal configuration of the printer engine 102. The printer I/F unit 1201 receives color component data sequentially output by the printer image processing unit 119. When the printer engine 102 is ready for printing, the printer I/F unit 1201 outputs a video data request signal VREQ_* (* is any of Y/M/C/K) that requests data for each color component. The color component data is input to the pulse width modulation circuit 1203. The pulse width modulation circuit 1203 then generates a pulse signal (drive signal) for driving the laser drive units 1212 to 1215 of each color in the subsequent stage based on the actual color component data. The pulse width modulation circuit 1203 then outputs the generated pulse signal to each laser drive unit 1212 to 1215. Each laser drive unit 1212 to 1215 corresponding to each color component drives a laser exposure device corresponding to each color component based on the pulse signal input from the pulse width modulation circuit 1203.

Figure 5 is a diagram showing the configuration of the image forming part of the printer engine 102. The printer engine 102 corresponds to an image forming means that forms a toner image on an image carrier. The image forming part for yellow will be mainly described below, but the image forming parts for other color components (magenta, cyan and black) are similar. In this embodiment, the printer engine 102 will be described as an image forming apparatus using a tandem engine consisting of four colors, YMCK. However, this embodiment can also be applied to an image forming apparatus using any printer engine. The printer engine 102 has a photosensitive drum 1401, which is an image carrier, a charging roller 1405, a Y laser exposure device 1406, a primary transfer device 1408, a secondary transfer device 1413, a fixing device 1414 and a cleaning device 1415. The Y laser exposure device 1406 is driven by the Y laser drive unit 1212. The primary transfer device 1408 transfers the visualized toner image onto the transfer material. In the following description, the transfer material is the intermediate transfer belt 1412, but the transfer material is not limited to the intermediate transfer belt. The secondary transfer device 1413 performs secondary transfer of the toner image formed on the intermediate transfer belt 1412 onto recording paper. The fixing device 1414 fixes the toner image transferred onto the recording paper. The cleaning device 1415 removes residual toner remaining on the intermediate transfer belt 1412 after the secondary transfer.

The developing device 1416 has a developer container. The developing device 1416 contains a developer in which toner particles (toner) and magnetic carrier particles (carrier) are mixed as a two-component developer. The A screw 1420 and the B screw 1421 transport the toner particles and mix them with the magnetic carrier particles, respectively. The developing sleeve 1422 is disposed close to the photosensitive drum 1401, rotates 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. The printer engine 102 is not limited to the example in FIG. 5. For example, the printer engine 102 has a transport unit that transports printing paper, etc.

In the printer engine 102 described above, when printing yellow, the photosensitive drum 1401 is exposed by the Y laser exposure device 1406 driven by the 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 the yellow developer carried on the developing sleeve 1422 in the developing device 1416. The visualized toner image is transferred onto the intermediate transfer belt 1412 by the primary transfer device 1408. Similarly, each of the magenta, cyan, and black color components is developed by each developing device 1417, 1418, and 1419, and visualized as a toner image on the photosensitive drums 1402, 1403, and 1404, respectively. The visualized toner images are transferred sequentially by the primary transfer devices 1409, 1410, and 1411 in synchronization with the toner image of the color component transferred immediately before. Then, a final toner image formed by the four color toner images is formed on the intermediate transfer belt 1412. The toner image formed on the intermediate transfer belt 1412 is secondarily transferred to the recording paper that is transported in synchronization by the secondary transfer device 1413, and the toner image is fixed by the fixing device 1414. Note that a description of the transport mechanism that transports the recording paper will be omitted.

Next, the image density sensor 400 will be described. In this embodiment, as shown in FIG. 5, the image density sensor 400 is disposed in a positional relationship facing the intermediate transfer belt 1412 serving as a transfer material. However, the image density sensor 400 may be disposed at any position (for example, on a photosensitive drum). The image density sensor 400 is a sensor that measures the density of a toner image formed on the intermediate transfer belt 1412 serving as a transfer material. The image density sensor 400 corresponds to a density detection means.

6 is a diagram showing a part of the structure of the image density sensor 400. The image density sensor 400 has a light emitting diode (LED) 401, a photodiode (PD) 402, and a photodiode (PD) 403. The LED 401 is arranged to irradiate infrared rays at an incident angle of about 15°. The PD 402 receives the reflected light of the light irradiated from the LED 401 to the intermediate transfer belt 1412 and the toner image 409 at a position of a regular reflection angle. The PD 403 receives the scattered light at a position of a diffuse reflection angle. The shutter 410 is arranged between the image density sensor 400 and the intermediate transfer belt 1412. When detecting the intermediate transfer belt 1412 and the toner image, the shutter 410 moves to the position shown by the solid line and is in an open state. When the image density sensor 400 is not used, the shutter 410 moves in front of the lens 404 of the image density sensor 400 as shown by the dotted line. This makes it possible to prevent each part of the image density sensor 400 from becoming dirty.

The image density sensor 400 has an electric board 407. The electric board 407 is equipped with a light receiving circuit having an IV conversion function that converts the current flowing according to the amount of light received by the PD 402 and the PD 403 into a voltage. The lens 404 is an optical component molded from epoxy resin to create a path for the light emitted from the LED 401, the light received by the PD 402, and the light received by the PD 403. The shielding member 405 is provided to prevent the light emitted from the LED 401 from being directly incident on the PD 402 and the PD 403. The shielding member 405 is made of, for example, black resin. If the shielding member 405 cannot completely shield the light emitted by the LED 401, light (leakage light 406) may leak to the PD 402 and the PD 403. The image density sensor 400 is capable of measuring both specularly reflected light and scattered light (scattered light). As described above, PD 402, which receives the specularly reflected light, and PD 403, which receives the scattered light, detect the reflected light from the intermediate transfer belt 1412 and the toner image 409.

Next, an example of a method for calculating the patch density will be described. First, a case will be described in which PD 402, which receives specularly reflected light, calculates the patch density on the intermediate transfer belt 1412. PD 402 detects both the specularly reflected light component and the scattered light component. Therefore, the specularly reflected light component can be calculated by performing a correction calculation to remove the scattered light component detected by PD 403 from the reflected light component detected by PD 402. The reflected light from the intermediate transfer belt 1412 is large, and the reflected light from the toner image 409 is very small. When the density of the toner image 409 increases, the specularly reflected light component detected by PD 402 decreases. By storing the relationship between the density of the toner image 409 and the specularly reflected light in advance, the density of the toner image 409 can be calculated from the detected specularly reflected light, and density correction can be performed.

Next, a case will be described in which the patch density on the intermediate transfer belt 1412 is calculated using PD 403, which receives scattered light. PD 403 detects only the diffuse reflection component. Furthermore, as the density of the toner image 409 increases, the scattered light from the toner image 409 increases, and the scattered light component increases. By storing the relationship between the density of the toner image 409 and the scattered light in advance, it is possible to calculate the density of the toner image 409 from the detected scattered light and perform density correction. As the scattered light from the toner image 409 increases as the density of the toner image 409 increases, it is preferable to use PD 403 to perform density correction.

Figure 7 is a block diagram showing the configuration of the control unit 400C connected to the image density sensor 400. The image density sensor 400 receives reflected light (near-infrared light) from the intermediate transfer belt 1412, generates an analog electrical signal of 0 to 5 V, and outputs the generated analog electrical signal to the control unit 400C. The A/D conversion circuit 411 converts the analog electrical signal output by the image density sensor 400 into an 8-bit digital signal. The density conversion circuit 412 converts the 8-bit digital signal into density information using a table 412T. The density conversion circuit 412 outputs the converted digital signal to the CPU 413.

Figure 8 is a diagram showing an example of the relationship between the output of the image density sensor 400 and image density. When the image density of the image formed on the intermediate transfer belt 1412 is changed stepwise by area gradation, the output of the image density sensor 400 changes according to the density of the formed image. In the example of Figure 8, the image density level changes from "0" to "255" based on the output of the image density sensor 400 when no toner image is attached to the intermediate transfer belt 1412.

As the area coverage rate of the toner image in the pixel formed on the intermediate transfer belt 1412 increases and the image density increases, the output of the regular reflection light decreases and the output of the scattered light increases. Based on the characteristics of the image density sensor 400 as shown in FIG. 8, the above-mentioned table 412T is a dedicated table for each color for converting the output of the image density sensor 400 into a density signal for each color. The table 412T is stored in the density conversion circuit 412, for example. This allows the density conversion circuit 412 to read the image density with good accuracy for each color. The density conversion circuit 412 then outputs the generated density information to the CPU 413. The CPU 413 may output the input density information to the CPU 105. The density conversion circuit 412 may also output the generated density information to the CPU 105.

Next, image density control will be described. First, a method for acquiring a target density required for image density control will be described. The target density is acquired during automatic tone correction control using an output image (toner image after fixing) formed on paper, which is performed periodically. The acquired target density is stored in ROM 107. FIG. 9 is a flowchart showing the flow of automatic tone correction processing. When automatic tone correction is performed based on a user operation, the image forming device 100 outputs a predetermined number of image patterns of each color of YMCK as patch images onto a sheet of paper such as a recording sheet (S901). The predetermined sheet of paper onto which the patch images have been output is set by the user in the reader unit, and the reader unit detects the density of the image pattern (S902). The reader unit is, for example, an image reading unit having a CCD sensor and an image processing unit. A configuration may be adopted in which the CCD sensor reads the luminance signal of the patch image formed on the predetermined sheet of paper, and the image processing unit detects the density of the image pattern.

The CPU 105 performs an interpolation process and a smoothing process from the density of the detected image pattern to obtain the engine gamma characteristic for the entire density range. The CPU 105 then creates a tone correction table (LUT) for the input image signal using the obtained engine gamma characteristic and a preset gradation target (S903). At this time, the CPU 105 performs an inverse conversion process to create the tone correction table (LUT). FIG. 10 is a diagram showing an example of the relationship between the engine gamma characteristic, the tone correction table, and the gradation target. The image forming apparatus 100 performs an inverse conversion process on the gradation target so that it matches the engine gamma characteristic, and creates the tone correction table. As a result, the density on a specified sheet of paper matches the gradation target over the entire density range.

As shown in FIG. 9, the CPU 105 performs control to form a plurality of toner image patterns under the above-mentioned conditions (S904). For example, the CPU 105 performs control to form a five-level image pattern for each color of YMCK. Then, the control unit 400C generates density information based on the image density on the intermediate transfer belt 1412 detected by the image density sensor 400 (S905). The generated density information is output to the CPU 105 (S906). The CPU 105 stores the input density information in the RAM 106, ROM 107, etc. (S907). By performing the process of FIG. 9, the density on the paper matches the gradation target in all areas (density areas). As a result, the generated density information becomes the target density for the input signal on the intermediate transfer belt 1412.

In this embodiment, the CPU 105 forms an image pattern of five gradations (30H, 60H, 90H, C0H, FFH) for each color after the gradation correction table is created. The image density sensor 400 detects the formed image pattern, and the CPU 105 stores the detection result as a target density in the RAM 106, ROM 107, etc. FIG. 11 is a diagram showing an example of the target density. Here, regarding the gradation pattern and number of gradations to be created, for example, the number of patterns in the midtone part may be increased in order to focus on correcting the midtone area (the gradation area where the density change is a predetermined amount or more) where the density change is large due to the gamma characteristic of the engine. In addition, the number of patterns in the high density area may be increased in order to stably output the high density side, and the number of patterns in the low density area may be increased in order to emphasize the gradation on the highlight side. In other words, the gradation pattern and the number of gradations can be changed as necessary.

Next, the flow of the density adjustment (density correction) process will be described. When the CPU 105 determines that image density correction is necessary, it creates an image pattern (test pattern) for density correction. For example, the CPU 105 determines that density correction is necessary when a long job continues or when the image formation environment changes. The image pattern for density correction has the above-mentioned five gradations of 30H, 60H, 90H, C0H, and FFH, but is not limited to this. Details of the arrangement and order of the patches will be described later. FIG. 12 is a flowchart showing an example of the process flow of creating a gradation correction LUT. When the image density correction process starts, a pattern for image density correction is formed (S1201). The pattern for image density correction is a density adjustment gradation patch. Then, the image density sensor 400 detects the image density of the formed pattern for image density correction (S1202). The CPU 105 compares the detected image density with a predetermined target density, creates a gradation correction LUT, and reflects it in the LUT during image formation (S1203).

Next, a process of reflecting the image density detected by the image density sensor 400 in the LUT will be described. During automatic gradation correction performed based on user operation, the image forming apparatus 100 forms a gradation correction table (initial correction LUT) in accordance with the engine γ characteristics so as to become a preset gradation target (gradation LUT). Then, the image forming apparatus 100 acquires target density values of five gradations for each color. After automatic gradation correction, the initial correction LUT is applied to the input image data and input to the printer engine 102, and the engine γ characteristics are combined and output to become the target gradation LUT. Then, the CPU 105 creates a pattern for image density correction, and the image density sensor 400 detects an image for image density correction. The image forming apparatus 100 sequentially creates a correction table (sequential correction LUT) based on the detection result by the image density sensor 400. Immediately after automatic gradation correction, the initial correction LUT and the sequential correction LUT are the same.

Next, the creation of the composite correction LUT will be described. FIG. 13 is a flowchart showing an example of the process flow of creating the composite correction LUT. After the automatic tone correction, the CPU 105 creates an image pattern to be used in the initial output image and image density correction by multiplying the initial correction LUT as shown in FIG. 14(A) obtained during the automatic tone correction (S1301). The image density sensor 400 detects the created image pattern (S1302). At this time, the CPU 105 plots five points of the initial target density values of 30H, 60H, 90H, C0H, and FFH as shown by white circles in FIG. 14(B). The image forming apparatus 100 uses the five plotted points to create a density curve as shown by the two-dot chain line in FIG. 14(B) (S1303). The density curve can be created by a commonly used approximation method, such as using an approximation formula that connects five points.

Next, the CPU 105 performs an inverse conversion to correct the density curve created at the time of S1303 to the initial density curve, and creates a sequential correction LUT as shown by the dashed line in FIG. 14(B) (S1304). Then, the CPU 105 multiplies the sequential correction LUT and the initial correction LUT to create a composite correction LUT as shown by the solid line in FIG. 14(C) (S205). The CPU 105 reflects the composite correction LUT in the output image and performs image output. The output image after the composite correction LUT is reflected and the next image density correction gradation pattern are output in a state where they are multiplied by the composite correction LUT. Note that even if it is not immediately after the automatic gradation correction, density correction is possible with the same image correction process by multiplying the composite correction LUT by a new sequential LUT.

Next, the responsiveness of the image density sensor that detects the patch will be described. Responsiveness is the speed of reaction when the image density sensor 400 detects an image pattern (test pattern) for image density correction formed on the intermediate transfer belt 1412. For example, when an image pattern formed on the intermediate transfer belt 1412 is detected, the responsiveness increases as the time required for stable measurement decreases. Figure 15 shows the transition of the measurement value of PD 403, which receives diffuse reflection, when an image pattern for image density correction is formed on the intermediate transfer belt 1412 and the intermediate transfer belt 1412 and the image pattern are measured by the image density sensor 400.

In FIG. 15, section A is the area where the image density sensor 400 measured the background portion on the intermediate transfer belt 1412 where no image pattern is formed, and section B is the area where the image pattern is measured. Section C is the area where the background portion is measured, just like section A. In sections A and C, when the LED of the image density sensor 400 irradiates the intermediate transfer belt 1412 with infrared light, the PD 403 receives a small amount of scattered light.

Section B is the area where the image pattern is measured, and the scattered light received by PD403 changes over time. Section B1 is the time from when the image pattern area starts to be read until the output value corresponding to the actual image density is reached (sensor rise time), and indicates that the sensor output value changes gradually. In FIG. 15, the sensor rise time takes about 10 ms. Section B2 is the area where the output value stabilizes after reaching the output value corresponding to the actual image density, and image density control is performed using the sensor output value of section B2. Section B3 is the time from when the image pattern area is read until the sensor output value stabilizes to the output value of the background area (sensor fall time). In the example of FIG. 15, the sensor fall time takes about 13 ms.

As described above, when measuring an image pattern formed on the intermediate transfer belt 1412, an area is required within the image pattern for the sensor rise time and the sensor fall time. For example, if the process speed of the intermediate transfer belt 1412 is 300 mm/s, the area required for the sensor rise is "4.5 mm = 300 x 0.015", and the area required for the sensor fall is "5.7 mm = 300 x 0.019". Therefore, when actually creating an image pattern (test pattern) for image density correction, it is necessary to create it taking the above areas into consideration.

Next, the arrangement of the image patterns in this embodiment will be described. In this embodiment, the image patterns for image density correction are arranged so that they change from a low density pattern to a high density pattern, or from a high density pattern to a low density pattern. FIG. 16 is a diagram showing the responsiveness of the sensor (progression of output value) when three image patterns with different densities are measured by the image density sensor 400. FIG. 16 shows the measurement results of an image pattern with a density difference from the background of "0.35", "0.91", and "1.51" as the responsiveness of the sensor. As shown in FIG. 16, the sensor rise time and the sensor fall time differ depending on the density difference of the image pattern. If the boundaries of the sensor rise time and the fall time are set to 95% from the value at the time when the sensor output value becomes stable, the sensor rise time and the fall time at each density difference are as shown in Table 1 below. Note that ΔD indicates the density difference. Also, the density difference "0.35" described above corresponds to a predetermined threshold value.

As shown in FIG. 16 and Table 1, it can be seen that as the density difference increases, the sensor rise time and fall time become longer. As described above, as the sensor responsiveness slows, the area required for measurement increases. As a result, problems occur in that the image pattern for image density correction becomes larger, the density correction time becomes longer, and the amount of toner required for density correction increases. Therefore, in this embodiment, the image pattern for image density correction is arranged in such a way that it changes from a low density pattern to a high density pattern, or from a high density pattern to a low density pattern, so that the density difference between adjacent image patterns becomes smaller.

Figure 17 is a diagram showing an example of the arrangement of image patterns for image density correction in this embodiment. In this embodiment, the gradation patterns of each color are allocated so that the density difference between adjacent image patterns is "ΔD = 0.35 or less". Therefore, the time required for the sensor to rise is 9 ms, and the time required for the sensor to fall is 12 ms. Figure 17 (B) is an enlarged view of one patch (section) of the image pattern for image density correction in Figure 17 (A). The image pattern in Figure 17 (B) is an image pattern that is 16 mm in the main scanning direction and 16.3 mm in the sub-scanning direction. In the sub-scanning direction, an area of "2.7 mm = 300 mm/s x 9 ms" is required for the sensor rise time, and an area of "3.6 mm = 300 mm/s x 12 ms" is required for the sensor fall time. In addition, the area in which the sensor output can be measured stably is 10 mm.

Figure 17 (A) is a diagram showing the arrangement of an image pattern (test pattern) for image density correction of five gradations (30H, 60H, 90H, C0H, FFH) for each color in this embodiment. In this test pattern, multiple colors are continuous, and each color is expressed in five gradations. If the measurement start position is the left end (Y_30), the first color Y is arranged from the left end in order from a light density patch to a dark density patch. The color M after Y is arranged from the left in order from a dark density patch to a light density patch. The color C after M is arranged from the left in order from a light density patch to a dark density patch. The color K after C is arranged from the left in order from a dark density patch to a light density patch.

That is, the multiple patches in one of the two adjacent colors of each color in the test pattern change continuously from high density to low density, and the multiple patches in the adjacent color change continuously from low density to high density. The test pattern is arranged so that the density step (density difference) between adjacent patches is equal to or less than a predetermined threshold. Similarly, the density step (density difference) at the boundary between the two adjacent colors is also arranged so that it is equal to or less than a predetermined threshold. By adopting the above-mentioned test pattern arrangement, there is no large density step between the base and the patch, and between the patches. As a result, the area required for the responsiveness of the sensor is reduced, the density correction time can be shortened, and the amount of toner used can be reduced. As described above, in this embodiment, the PD 403 that detects scattered light is used to perform density correction, but the PD 402 that detects regular reflection light may be used. Also, in the test pattern arrangement in FIG. 17(A), the density of one or both of the patches at both ends of the test pattern may be equal to or less than the density of the intermediate transfer belt 1412 and the above-mentioned predetermined threshold. This makes it possible to reduce the density step between the base and the patch.

Figure 18 (A) is a diagram showing the arrangement of image patterns for conventional image density correction. Figure 18 (B) is a diagram showing an enlarged view of one patch (section) of the image pattern for image density correction in Figure 18 (A). The image pattern in Figure 18 (B) is an image pattern of 16 mm in the main scanning direction and 20.2 mm in the sub-scanning direction. The arrangement of the image pattern (test pattern) for image density correction in Figure 18 (A) is assumed to have a density step between adjacent patches of about "1.5". In the sub-scanning direction, "4.2 mm = 300 mm/s x 14 ms" is required as an area for the sensor rise time, and "6 mm = 300 mm/s x 20 ms" is required as an area for the sensor fall time. Also, as in Figure 17 (B), the area where the sensor output can be stably measured is 10 mm. Figure 18 (A) is a diagram showing the arrangement of image patterns for image density correction of five gradations (30H, 60H, 90H, C0H, FFH) for each color. In the example of FIG. 18A, if the measurement start position is the left end (Y_30H), YMCK are arranged from the left end in order from light density patches to dark density patches.

Table 2 below shows the length of the image pattern (total length of the image pattern) and the measurement time required to measure the image pattern for this embodiment and a comparative example. The time required to measure the image pattern is the time when the process speed is 300 mm/s.

As shown in Table 2, the length of the image pattern and the measurement time are reduced by about 20% in this embodiment compared to the conventional comparative example. As described above, the image pattern (test pattern) for image density correction adopts an arrangement that changes from a low density pattern to a high density pattern and from a high density pattern to a low density pattern. This allows calibration for stabilizing control of color tone and density gradation to be performed in a short time and with high accuracy, and also suppresses an increase in the amount of toner consumed.

Next, the first modified example will be described. In the first modified example, the arrangement of the pattern for image density correction differs from that of the above-mentioned embodiment, but other points are similar to those of the above-mentioned embodiment. Figure 19 is a diagram showing the measurement order of the image pattern for image density correction in the first modified example and the relationship between the color and the image signal value. The color order and signal values are not limited to the example of Figure 19. In the above-mentioned embodiment, an arrangement is adopted for the same direction of density change (for example, the direction of increasing density for Y and the direction of decreasing density for M) among the colors YMCK. On the other hand, in the first modified example, even within the same color, there is a gradient in both the direction of increasing density and the direction of decreasing density.

In the first modified example, the image patterns for image density correction of 10 gradations (18H, 30H, 48H, 60H, 78H, 90H, A8H, C0H, D8H, FFH) for each color are used as the image patterns for image density correction. FIG. 19 is a diagram showing the arrangement of the image patterns for image density correction of the first modified example. As shown in FIG. 19, the image patterns for image density correction of a certain color among the colors are arranged so that the image patterns change from the lightest density to the darkest density and from the darkest density to the lightest density. Also, the image patterns for image density correction of other colors are arranged so that the image patterns for image density correction of the other colors change from the darkest density to the lightest density and from the lightest density to the darkest density. By adopting the arrangement of the image patterns for image density correction of the first modified example, the density difference between adjacent patches can be reduced. As a result, calibration for stabilizing control of color tone and density gradation can be performed in a short time with high accuracy, and an increase in the amount of toner consumed can be suppressed.

Next, the second modified example will be described. FIG. 20 is a diagram showing the relationship between the measurement order, color, and image signal value of the image pattern for image density correction in the second modified example. The color order and the signal value are not limited to the example in FIG. 20. In the second modified example, an arrangement of the image pattern (test pattern) for image density correction is adopted when forming a pattern of multiple dither processing for each color of YMCK. The image pattern for image density correction in the second modified example is a correction patch of 10 gradations (18H, 30H, 48H, 60H, 78H, 90H, A8H, C0H, D8H, FFH) for each color. FIG. 20 is a diagram showing the arrangement of the image pattern for image density correction in the second modified example. As shown in FIG. 20, the arrangement of the image pattern for image density correction in the second modified example is a pattern in which error diffusion, low-line and high-line dither processing (pseudo-halftone processing) is performed on each color. As the pseudo-halftone processing, processing other than dither processing may be applied.

In other words, the image patterns for image density correction in the second modified example are arranged in a repeating pattern of going from the lightest density pattern to the darkest pattern and then back from the darkest pattern to the lightest density pattern for each color and dither processing pattern. The density difference between adjacent patches between error diffusion and low line count, the density difference between adjacent patches between low line count and high line count, and the density difference between adjacent patches between high line count and error diffusion are below a predetermined threshold. This makes it possible to reduce the density difference between adjacent patches that have been subjected to different pseudo-halftone processing. Therefore, calibration for stabilizing control of color tone and density gradation can be performed in a short time and with high accuracy, and an increase in the amount of toner consumed can be suppressed.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-mentioned embodiments, and various modifications and variations are possible within the scope of the gist of the present invention. The present invention can also be realized by supplying a program that realizes one or more functions of the above-mentioned embodiments to a system or device via a network or storage medium, and having one or more processors of a computer in the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., an ASIC) that realizes one or more functions.

100 Image forming apparatus 102 Printer engine 105 CPU
119 Printer image processing unit 309 γ correction circuit 400 Image density sensor 400C Control unit 1401 Photosensitive drum 1412 Intermediate transfer belt

Claims (5)

A conversion means for converting an image signal based on a conversion condition;
an image forming means for forming an image on a recording sheet based on the image signal converted by the conversion means;
a transfer body onto which a test pattern formed by the image forming means is transferred;
An irradiation means for irradiating the transfer body with light;
a light receiving means for receiving reflected light from the test pattern on the transfer body;
generating means for generating the conversion condition based on the reflected light from the test pattern received by the light receiving means;
the test pattern on the transfer body is transported so as to pass through a detection position where the irradiation means irradiates the light;
The test pattern is:
a first patch row of a first color , the density of which changes from low density to high density along a rotation direction of the transfer body, and a second patch row of the first color , the density of which changes from high density to low density along the rotation direction of the transfer body, are adjacent to each other in the rotation direction of the transfer body ,
a third patch row of a second color different from the first color, the density of which changes from low density to high density along the rotation direction of the transfer body, and a fourth patch row of the second color, the density of which changes from high density to low density along the rotation direction of the transfer body, are adjacent to each other in the rotation direction of the transfer body,
Both the first patch row and the third patch row are subjected to a first pseudo-halftoning process;
an image forming apparatus, characterized in that both of the second patch row and the fourth patch row have been subjected to a second pseudo-halftone process different from the first pseudo-halftone process ; an image signal value for forming each patch of the first patch row is determined so that a density difference between adjacent patches in the rotation direction is equal to or smaller than a threshold value;
2. The image forming apparatus according to claim 1, wherein image signal values for forming each patch in the second patch row are determined so that a density difference between adjacent patches in the rotation direction is equal to or less than a threshold value. 3. The image forming apparatus according to claim 2, wherein an image signal value for forming the last patch in the first patch row and an image signal value for forming the first patch in the second patch row are determined so that the density difference between the last patch in the first patch row and the first patch in the second patch row is equal to or less than the threshold value. The image forming apparatus according to claim 1, characterized in that the light receiving means is disposed at a position where it receives only diffusely reflected light from the test pattern. The image forming apparatus according to claim 1, characterized in that the conversion condition is a tone correction table for converting the image signal so that the density of the image formed by the image forming means becomes a target density for each tone.