JP7483389B2 - Image forming device - Google Patents

Description

The present invention relates to density control of an image formed by an image forming device.

Due to influences such as short-term fluctuations caused by changes in the environment in which the device is installed or changes in the environment inside the device, and long-term fluctuations caused by changes over time (deterioration) of the photosensitive material and developer, the density characteristics (also called gradation characteristics) of the output image may not be ideal density characteristics (ideal gradation characteristics).
Therefore, in the image forming apparatus, the image forming conditions are adjusted in order to correct the density characteristics of the output image to ideal density characteristics.

The process of appropriately correcting changes in density and color in this way is generally called calibration. A known calibration method is to form a pattern image with a uniform density on paper, a photoconductor, or an intermediate transfer body, measure the formed pattern image, and adjust the image formation conditions.

Patent Document 1 proposes a technology in which a gradation pattern is formed on paper, and conversion conditions for converting image data are generated based on the read data of the gradation pattern read by an image reading unit. This makes it possible to stabilize the density characteristics of the output image to ideal density characteristics, improving the stability of image quality.

It has also been proposed to perform calibration during an image forming job in order to maintain a certain level of color reproducibility in that job. Patent Document 2 proposes forming a pattern image during an image forming job and generating conversion conditions based on the measurement results of this pattern image.

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

As a sensor for measuring a pattern, an optical sensor is commonly known that has a light receiving section that receives reflected light from the pattern and outputs a signal value based on the light receiving section's results. However, the printing speed of image forming devices in recent years has increased, and there is a possibility that the measurement results of the optical sensor may not be accurate.

This is caused by the slow response of the optical sensor. In other words, the pattern passes through the measurement area of the optical sensor before the sensor signal value converges to the correct value. If the density of the output image is controlled based on the erroneous measurement result, it becomes impossible to form an image with the correct density.

The present invention has been made to solve the above problems. The purpose of the present invention is to suppress errors that result in incorrect measurement results.

The present invention relates to an image processing unit having a conversion unit which converts image data based on image forming conditions and a halftone processing unit which performs halftone processing on the image data, image forming means which forms a toner image on a sheet based on the image data from the image processing unit, and an image carrier on which an image pattern composed of a plurality of patch images is formed by the image forming means, the image carrier being rotated, a measurement means which measures the image pattern on the image carrier and outputs a measurement value relating to the measurement result of the image pattern as density information , and a correction means which corrects the image forming conditions based on the density information output by the measurement means, and the image pattern is formed by a toner image formed on a sheet based on the image data from the image processing unit, the image forming means being rotated, the image carrier being rotated, the second patch image is formed so that a density difference between the second patch image and the first patch image is less than a threshold value; and the third patch image is formed so that a density difference between the second patch image and the first patch image is less than a threshold value. The first patch image includes a first patch image formed at the most downstream position among the patch image group of the first color in the rotation direction, and a second patch image formed adjacent to the upstream of the first patch image in the rotation direction and subjected to a different type of halftone processing than the first patch image . the third patch image formed to have a higher density than both the first patch image and the second patch image, and the patch image group of the second color includes a fourth patch image formed at the most downstream position of the patch image group of the second color in the rotation direction, a fifth patch image formed adjacent to the upstream of the fourth patch image in the rotation direction and subjected to a different type of halftone processing than the fourth patch image, the fifth patch image being formed so that a density difference with the fourth patch image is less than the threshold value, and a sixth patch image formed at the most upstream position of the patch image group of the second color in the rotation direction, and the sixth patch image formed to have a higher density than both of the first patch image and the sixth patch image, wherein the length of the first patch image in the rotational direction is determined based on a first rule since a density difference between the first patch image and the image carrier is greater than a threshold value and indicates an increase in density, and is longer than the length of the second patch image in the rotational direction whose density difference between the first patch image and the fourth patch image is determined based on a second rule different from the first rule since a density difference between the third patch image and the image carrier is greater than the threshold value and indicates a decrease in density, and is longer than the length of the fifth patch image in the rotational direction whose density difference between the fourth patch image and the fourth patch image is smaller than the threshold value.

The present invention makes it possible to reduce errors that result in inaccurate measurement results.

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. FIG. 2 is a diagram showing an example of a software configuration of the image forming apparatus according to the embodiment. FIG. 2 is a diagram showing an example of the internal configuration of a printer image processing unit. FIG. 2 is a diagram showing a part of the internal configuration of a printer engine. 2 is a diagram for explaining the configuration of a printer engine image forming section and an image density sensor. 4 is a diagram showing the configuration of a density conversion circuit of an image density sensor and the relationship between an output value of the image density sensor and density. 11 is a flowchart showing an example of an automatic tone correction process. 6 is a diagram showing the relationship between a gradation target, an engine gamma characteristic, and a gradation correction table. FIG. 11 is a graph showing the relationship between an input signal value and a density target. 11 is a flowchart showing an example of a correction LUT creation process when adjusting image density. 11 is a flowchart showing an example of a composite correction LUT creation process when adjusting image density. 6 is a diagram showing the relationship between a density target and an initial tone correction LUT. 13 is a graph showing the relationship between a measured density plot and a successive correction LUT with respect to a density target. 11 is a diagram showing the relationship between an initial correction LUT, a sequential correction LUT, and a composite correction LUT. FIG. 4 is a graph showing a change in sensor output of an image density sensor. 5A to 5C are diagrams illustrating the sizes of patch images constituting an image pattern for image density correction in the first embodiment. 5 is a graph showing the relationship between Δ density and size of an image density correction patch according to the first embodiment. 13A to 13C are diagrams illustrating the sizes of patch images constituting an image pattern for image density correction in a second embodiment. 13A and 13B are diagrams showing the relationship between Δ density and size of an image density correction patch according to a second embodiment. 13A and 13B are diagrams showing the relationship between Δ density and size of an image density correction patch according to the third embodiment.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
First Embodiment
<Image forming apparatus>
[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 according to this embodiment has a scanner 101 which is an image input device, and a printer engine 102 which is an image output device.

The scanner 101 is connected to the 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. In the image forming apparatus 100, the scanner image processing unit 118 and the printer image processing unit 119 perform control for reading image data and printing out the image data. The image forming apparatus 100 also connects to the LAN 10 and the public line 104, and performs control for inputting and outputting image information and device information via the LAN 10.

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. The ROM 107 is a boot ROM in which the boot program for the system is stored. The ROM 107 is, for example, a flash ROM. The HDD 108 is a hard disk drive, and stores system software for various processes, input image data, and the like. Note that the configuration may include other storage devices such as an SSD (solid state drive) instead of or in combination with the HDD.

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

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, and inputs and outputs information with an external device (not shown). The above units are arranged on the system bus 113.

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. The image bus 115 is connected to a raster image processor (RIP) unit 116, a device I/F 117, a scanner image processing unit 118, an image processing unit for image editing 120, an image compression unit 103, an image decompression unit 121, and a color management module (CMM) 130.

The RIP unit 116 develops a page description language (PDL) code into image data.
A device I/F 117 connects to 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 image processing unit for image editing 120 performs various image processes such as rotation of the image data, color processing, binary conversion, and multi-value conversion. The image compression unit 103 encodes the image data processed by the RIP unit 116, the scanner image processing unit 118, and the image processing unit for image editing 120 using a predetermined compression method when storing the image data in the HDD 108.

The image decompression unit 121 decodes and decompresses the compressed and encoded data when image data compressed in the HDD 108 is processed as necessary in the image editing image processing unit 120 or the printer image processing unit 119 and output by the printer engine 102.
A printer image processing unit 119 performs image processing such as gamma correction and halftone processing according to the printer engine on image data to be printed out.

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.

[Software configuration]
Fig. 2 is a diagram showing an example of a software configuration of the image forming apparatus 100. Each software module shown in Fig. 2 is realized by mainly the CPU 105 executing a program stored in the HDD 108 or the like, which is a storage unit.

The job control process 201 manages and controls each software module (not shown) and controls all jobs that occur within the image forming device 100, such as copying, printing, scanning, and fax transmission and reception. In addition, the job control process 201 also controls adjustment (correction) processes (described below) such as calibration for stabilizing the color tone, density gradation, etc. of the image to be formed.

Network processing 202 is a module that mainly controls communication with the outside world via network I/F 111, and controls communication with each device on LAN 10. UI processing 203 mainly controls the operation unit 110 and operation unit I/F 109. FAX processing 204 controls the FAX function. FAX processing 204 receives and transmits FAX signals via modem 112.

The print process 207 controls the image editing image processing unit 120, the printer image processing unit 119, and the printer engine 102 based on instructions from the job control process 201 to perform print processing of the specified image. The print process 207 accepts 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 process 201. The print process 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. The print process 207 then controls the printer engine 102 to print the image data on the specified paper.

Based on instructions from job control process 201, scan process 210 controls scanner 101 and scanner image processing unit 118 to read the document on scanner 101. Scan process 210 scans the document on the platen of scanner 101 and inputs the image as digital data. Scan process 210 then notifies job control process 201 of color information of the input image. Furthermore, scan process 210 controls scanner image processing unit 118 to perform appropriate image processing on the input image, such as image compression, and then notifies job control process 201 of the input image after image processing.

The color conversion process 209 performs color conversion processing on the designated image based on an instruction from the job control process 201, and notifies the job control process 201 of the image after the color conversion processing.
The RIP process 211 interprets the PDL based on an instruction from the job control process 201, and controls the RIP unit 116 to perform rendering, thereby expanding the data into a bitmap image.

A device information transmission process 205 transmits device information to an external device via a network process 202 based on an instruction from a job control process 201. The device information includes information indicating the capabilities and characteristics of the image forming apparatus 100, such as the type of the printer engine 102 (color/monochrome), resolution, printing speed, processing time by a color conversion process 209, and an output profile.
A device information acquisition process 206 requests and acquires device information from an external device via a network process 202 based on an instruction from a job control process 201 .

With the above configuration, the image forming device 100 receives a print job from the LAN 10 and performs the operations up to printing.

[Image data processing flow]
Next, a processing flow of image data input to the printer image processing unit 119 based on the above configuration will be described.
First, as described above, the PDL transmitted from an external device via the LAN 10 is received by the network I/F 111 and input to the RIP unit 116 from the image bus I/F 114. The RIP unit 116 interprets the received PDL and converts it into code data that can be processed by the RIP unit 116. 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 at the subsequent stage and sequentially stored in the HDD 108.

The compressed data stored in the HDD 108 is then read out during a print operation in response to an instruction from the job control process 201, and the compressed data is decompressed by the image decompression unit 121. The image data decompressed by the image decompression unit 121 is input to the printer image processing unit 119 via the device I/F 117. Here, the printer image processing unit 119 will be described.

FIG. 3 is a diagram showing an example of the internal configuration of the printer image processing unit 119. As shown in FIG.
The color conversion unit 301 converts image data from luminance 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.
The multi-value image data converted to density values by the color conversion unit 301 is converted to signal values corrected for density differences within the same page by a density difference correction unit 302. The density difference correction unit 302 has a one-dimensional table that changes the same input/output signals as a gamma correction circuit 309 described below, and applies a step correction coefficient for step correction to the table in accordance with the position within the page.

The image data that has been density step corrected is converted by the gamma correction circuit 309 (hereinafter referred to as "gamma LUT") into density signals that are converted into signal values that reproduce the density in the printer engine. The gamma LUT is a table that converts input and output signals that are made to match the gamma characteristics of the printer engine, and in this embodiment, a pre-stored table is set and processed, but it may also be created using known gradation control, etc., and set and used.

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

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. When the video data request signal (VREQ_*) corresponding to each color component transmitted from the printer engine 102 is input, the data of the corresponding color component is read from the page buffer memory 306. Note that the video data request signal (VREQ_*) is VREQ_Y, VREQ_M, VREQ_C, and VREQ_K for each color component. Y indicates yellow, M indicates magenta, C indicates cyan, and K indicates black color components. The timing of exposure of each of the photosensitive drums 1401 to 1404 (FIG. 5A) corresponding to each color component in the printer engine 102 differs depending on the distance from the upstream to the downstream. Therefore, the timing of reading out the data of each color component also differs. Therefore, the printer engine 102 controls the timing by sending video data request signals for each color component (VREQ_Y, VREQ_M, VREQ_C, VREQ_K) to the printer image processing unit 119 at the timing for each color component.

[Printer engine operation]
Next, an 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.
When the printer engine 102 is ready for a printing operation, the printer I/F unit 1201 issues a video data request signal (VREQ_*) requesting data of each color component to the printer image processing unit 119. The printer I/F unit 1201 receives the color component data sequentially transmitted from the printer image processing unit 119 in response to the video data request signal (VREQ_*).

The color component data received by the printer I/F unit 1201 is input to a pulse width modulation circuit 1203. Then, the pulse width modulation circuit 1203 generates pulse signals (drive signals) for driving the downstream laser drive units 1212 to 1215 for each color based on the actual color component data, and transmits the pulse signals to the laser drive units 1212 to 1215 for each color.
Each of the laser drivers 1212 to 1215 corresponding to each color component drives a laser exposure device corresponding to each color component based on the pulse signal received from the pulse width modulation circuit 1203. After that, an image is formed as described below in FIG.

Figure 5(a) is a cross-sectional view showing an example of the configuration of the image forming portion of the printer engine 102. Below, the image forming portion for yellow (Y) will be mainly described, but the image forming portions for the other color components magenta (M), cyan (C), and black (K) have a similar configuration. Note that, in this embodiment, the printer engine 102 is intended for an image forming device using a tandem engine consisting of four colors, YMCK, but is not limited to this.

The printer engine 102 includes 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 a Y laser drive unit 1212. The primary transfer device 1408 performs primary transfer of the visualized toner image onto a transfer material (an intermediate transfer belt 1412, which is an image carrier). The secondary transfer device 1413 performs secondary transfer of the toner image formed on the intermediate transfer belt 1412 onto a recording sheet. The fixing device 1414 fixes the toner image transferred onto the recording sheet. The cleaning device 1415 removes the residual toner remaining on the intermediate transfer belt 1412 after the secondary transfer.

The developing device 1416 includes a developer container, and 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 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. Such development of the electrostatic latent image reduces the toner concentration of the developer in the developing device 1416. Therefore, the Y toner supply control unit 1204 drives the Y toner supply motor 1208 and controls the supply of yellow (Y) toner from the Y toner supply mechanism 1407 to the developing device 1416.
In addition to the configuration shown in FIG. 5A, 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.

When printing yellow in the printer engine configuration described above, 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, and the visualized toner image is transferred onto the intermediate transfer belt 1412 by the primary transfer device 1408.

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

<Image density sensor>
In this embodiment, the image density sensor 400 is described as being disposed opposite the intermediate transfer belt, but it may be disposed appropriately on a photosensitive drum or the like.
The image density sensor 400 is disposed opposite the intermediate transfer belt as shown in Fig. 5(a) and measures the density of the toner image formed on the intermediate transfer belt. Here, a part of the structure of the image density sensor 400 will be described with reference to Fig. 5(b).

Figure 5(b) is a diagram for explaining the configuration of the image density sensor 400, and corresponds to a cross-sectional view when viewed from the upstream side in the transport direction of the intermediate transfer belt 1412.

Image density sensor 400 is an optical sensor having LED 401, PD 402, and PD 403. LED 401 is a light-emitting diode arranged to irradiate infrared light at an incident angle of approximately 15°. PD 402 is a photodiode that receives the reflected light of light irradiated from LED 401 onto the intermediate transfer belt and toner image 409 at a position of a regular reflection angle. PD 403 is a photodiode that receives scattered light at a position of a diffuse reflection angle.

The shutter 410 is disposed 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 indicated by the solid line and is in an open state. When the image density sensor 400 is not in use, the shutter 410 moves to in front of the lens 404 of the image density sensor 400 as indicated by the dotted line, to prevent the lens 404 from becoming dirty.

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 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 and the light received by the PD 402 and PD 403. A shielding member 405 made of black resin is provided to prevent the light emitted by the LED 401 from being directly incident on the PD 402 and 403. However, the shielding member 405 cannot completely block the light, and there is leakage light 406 that leaks to the PD 402 and 403. The image density sensor 400 configured as described above can measure both regular reflection light and diffuse reflection light. The PD 402 that receives regular reflection light and the PD 403 that receives diffuse reflection light measure the light reflected from the intermediate transfer belt and the light reflected from the toner image 409.

Here, an example of a method for detecting the patch density will be described.
For example, a case will be described in which the density of a patch on the intermediate transfer belt 1412 is calculated by the PD 402 that receives specularly reflected light.
Since PD 402 detects both the specular reflected light component and the diffuse reflected light component, the diffuse reflected light component detected by PD 403 is removed from the reflected light component detected by PD 402, and the specular reflected light component is calculated by performing a correction calculation. Since the reflected light from the intermediate transfer belt 1412 is large and there is almost no reflected light from the toner, the specular reflected light component detected by PD 402 decreases as the toner image density increases. By storing the relationship between the toner image density and the specular reflected light in advance, the toner image density is calculated from the detected specular reflected light, and density correction is performed.

A case will be described in which the patch density on the intermediate transfer belt is calculated by the PD 403 that receives diffusely reflected light.
Only the diffuse reflection component is detected by the PD 403. Also, as the toner image density increases, the light scattered from the toner increases, and the diffuse reflection component increases. By storing the relationship between the toner image density and the diffuse reflection light in advance, the toner image density is calculated from the detected diffuse reflection light, and density correction is performed.

FIG. 6A is a diagram showing an example of a schematic configuration of a density conversion circuit of the image density sensor 400. As shown in FIG.
The reflected light (near-infrared light) from the intermediate transfer belt is input to the image density sensor 400, which converts it into an analog electric signal of 0 to 5 V and outputs it. This analog electric signal is converted into an 8-bit digital signal by an A/D conversion circuit 421 in the control unit 420 provided in the printer engine 102. This digital signal is then converted into density information by a density conversion circuit 422 in the control unit 420.

FIG. 6B is a diagram showing an example of the relationship between the output value of the image density sensor 400 and the density.
6B, when the image density of the pattern 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 pattern image. In this case, the output of the image density sensor 400 changes up to 255 levels based on the output of the image density sensor 400 when no toner is attached to the intermediate transfer belt 1412.

As the area coverage rate of the toner in the pixels 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 diffuse reflection light increases.
Based on such characteristics of the image density sensor 400, a table (table 422a in FIG. 6A) dedicated to each color is prepared in advance to convert the output of the image density sensor 400 into a density signal for each color. The table 422a is stored in the memory section of the density conversion circuit 422. This allows the density conversion circuit 422 to read the pattern image density with high accuracy for each color. The density conversion circuit 422 outputs the density information to the CPU 423 in the control section 420. The CPU 423 notifies the CPU 105 of this density information via the printer image processing section 119 and the device I/F 117.

<Image Density Control>
The image density control system in this embodiment will be described.
[Get target concentration]
First, a method for obtaining a density target required for performing the image density control described in this embodiment will be described.
The density target used in this embodiment is acquired during automatic tone correction control using an output image (toner image after fixing) formed on paper, which is performed periodically or arbitrarily by the user as shown in Fig. 7, and is stored in the ROM 107. A detailed description will be given below.

Figure 7 is a flowchart showing an example of automatic tone correction processing. The processing of this flowchart is realized by the CPU 105 executing a program stored in the ROM 107, HDD 108, etc.

First, when the execution of automatic tone correction is started, the CPU 105 controls the printer engine 102 or the like to form an image pattern with 64 gradations for each color and output it onto paper (S901). Note that the number of gradations is not limited to this.
The user places the paper on which the image pattern has been output on the platen or the like of the scanner 101, and issues an instruction to start reading from the operation unit 110. In response to this instruction, the CPU 105 controls the scanner 101 to read the paper placed on the platen or the like, and the scanner image processing unit 118 to automatically detect the density of the image pattern (S902).

Then, the CPU 105 performs an interpolation process and a smoothing process from the density obtained from the image pattern to obtain the engine gamma characteristic for the entire density region. Next, the CPU 105 creates a tone correction table (LUT) which is a correction table for the input image signal, using the obtained engine gamma characteristic and a preset gradation target, and sets it in the gamma correction circuit 309. In this embodiment, as shown in FIG. 8, an inverse conversion process is performed to match the gradation target, and a tone correction table is created. A tone correction table is created for each color, and set in the gamma correction circuit 309.
FIG. 8 is a diagram showing the relationship between the gradation target, the engine γ characteristic, and the gradation correction table.
When this process is completed, the density on the paper matches the gradation target over the entire density range. Therefore, if a plurality of toner image patterns are formed under the above conditions and the density is detected on the intermediate transfer belt 1412 using the image density sensor 400, the density value becomes the target density for the input signal on the intermediate transfer belt 1412.

When the creation of the gradation correction table is instructed, the CPU 105 instructs the printer engine 102, etc. to form patch images on the intermediate transfer belt 1412, read density information using the image density sensor 400, and send it to the CPU 105. The patch images instructed to be formed here are, for example, a plurality of patch images constituting an image pattern (test pattern) of 10 gradations for each color. In response to this instruction, the printer engine 102 forms an image pattern (patch image) of 10 gradations for each color on the intermediate transfer belt 1412 (S904). In this embodiment, the 10-gradation image pattern is an image pattern with density values of 1Ah, 33h, 4Dh, 66h, 80h, 99h, B3h, CCh, E6h, and FFh, respectively ("h" indicates a hexadecimal number). Furthermore, the image density sensor 400 reads the image pattern (patch image) of 10 gradations for each color formed on the intermediate transfer belt 1412 (S905), and the control unit 420 transmits density information based on the reading results to the CPU 105 (S906).

The CPU 105 stores the density information received from the control unit 420 of the printer engine 102 in the ROM 107 as a density target (FIG. 9) (S907), and ends the processing of this flowchart.
FIG. 9 is a diagram showing the relationship between the input signal value and the density target.

The gradation pattern and number of gradations of the patch image to be created are not limited to the above examples. For example, the number of patterns in the midtone portion may be increased to focus on correcting midtone areas where density changes are large due to the gamma characteristics of the engine. The number of patterns in the high density area may also be increased to stably output the high density side. The number of patterns in the low density area may also be increased to place importance on the gradation on the highlight side. In this way, the gradation pattern and number of gradations of the patch image can be changed as needed.

[Image Density Adjustment Flow]
Next, the density correction process in this embodiment will be described.
First, when it is determined that density correction is necessary in the image forming apparatus 100, for example, when a long-term job (for example, a job with several hundred sheets of output) continues, or when the image forming environment changes, a density adjustment pattern is created. The types of density adjustment include, for example, a pattern in which the gamma characteristic of the engine is obtained from a 10-level patch and density adjustment is performed (hereinafter, "adjustment pattern A"), and a pattern in which the gamma characteristic of the engine is obtained from a 3-level patch and density adjustment is performed (hereinafter, "adjustment pattern B").

Adjustment pattern A is an adjustment for increasing the number of gradations in the patch to adjust the gradation with high precision, and is executed after a relatively large environmental change or after a large volume job of printing several thousand sheets.
Adjustment pattern B reduces the number of gradations in the patches and performs fine adjustments at high frequency, and is executed during or after a job of several tens to several hundreds of sheets.
The density adjustment can maintain highly accurate gradation characteristics by performing a combination of adjustment pattern A and adjustment pattern B. Adjustment pattern A and adjustment pattern B are merely examples, and the present invention is not limited to these.
The arrangement, order, and size of the patches will be described later.

Figure 10 is a flowchart showing an example of a correction LUT creation process when adjusting image density. The process of this flowchart is realized by the CPU 105 executing a program stored in the ROM 107, HDD 108, etc.

When the image density adjustment starts, the CPU 105 instructs the printer engine 102 and the like to form a density adjustment gradation patch on the intermediate transfer belt 1412, read it with the image density sensor 400, and transmit the read density information to the CPU 105. The density adjustment gradation patch instructed to be formed here is an image pattern consisting of multiple patch images whose lengths in the sub-scanning direction differ according to the density difference between adjacent patch images, as described later in FIG. 16 and FIG. 17. Here, as an example, a 10-tone image pattern for image density adjustment is used, but this is not limited to this, and a 3-tone image pattern such as adjustment pattern B or other patterns may be used. In response to the above instruction, the printer engine 102 forms the density adjustment gradation patch as described above on the intermediate transfer belt 1412 (S101). Furthermore, the image density sensor 400 reads the density adjustment gradation patch formed on the intermediate transfer belt 1412 (S102), and the control unit 420 transmits density information based on the reading result to the CPU 105.

Next, the CPU 105 compares the detected density information with a predetermined target density, creates a tone correction table (LUT) based on the comparison result, and sets the γ correction circuit 309 so that the LUT is reflected during image formation (S103). The method of creating the tone correction table (LUT) is described below.

[LUT creation method]
A method for reflecting the detected concentration in the LUT in this embodiment will be described below.
First, when the automatic tone correction (FIG. 7) is performed arbitrarily by the user, a tone correction table (hereinafter referred to as "initial correction LUT") as shown in FIG. 8 is formed in accordance with the engine γ characteristics so as to become a preset tone target (hereinafter referred to as "tone LUT"). Then, the density target value of the 10 tones of each color described above is acquired. After the above-mentioned automatic tone correction, the printer image processing unit 119 converts the input image data based on the initial correction LUT and transfers the converted image data to the printer engine 102. The printer engine 102 forms an image based on the input image data. Also, as shown in FIG. 10, the image forming apparatus 100 creates a pattern for image density adjustment, detects the created pattern image with an image density sensor, and generates a tone correction table based on the detection result. Although details will be described later, a correction table (hereinafter referred to as "successive correction LUT") is generated from the measurement result of the pattern in the image density adjustment, and a tone correction table (synthetic correction LUT) is newly generated by combining the initial correction LUT and the successive correction LUT. Immediately after the automatic tone correction (FIG. 7), the initial correction LUT and the successive correction LUT are the same.

The first composite correction LUT creation method will be described below with reference to Figures 11, 12, 13, and 14. Note that the composite correction LUT creation method will be described using the above-mentioned adjustment pattern A, but it is also possible to create a composite LUT for adjustment pattern B using a similar correction method.

Figure 11 is a flowchart showing an example of a composite correction LUT creation process when adjusting image density. The process of this flowchart is realized by the CPU 105 executing a program stored in the ROM 107, HDD 108, etc. Note that S201 in Figure 11 corresponds to S101 in Figure 10, S202 corresponds to S102, and S203 to S205 correspond to S103.

The CPU 105 instructs the printer engine 102 and the like to form an image pattern for image density adjustment as described above on the intermediate transfer belt 1412, read it with the image density sensor 400, and transmit the read density information to the CPU 105. After the automatic tone correction, when forming the first output image and the image pattern used in the image density adjustment, the image is formed by applying the initial correction LUT (1500) as shown in Fig. 12 set in the gamma correction circuit 309 by the automatic tone correction (S201). Then, the image density sensor 400 detects the density information of the image pattern thus formed (S202).
FIG. 12 is a diagram showing the relationship between the density target and the initial tone correction LUT.

Then, the CPU 105 creates a density curve 1501 (FIG. 13) from the detection results by the image density sensor 400 (S203). Specifically, as shown by the circles in FIG. 13, 1Ah, 33h, 4Dh, 66h, 80h, 99h, B3h, CCh, E6h, and FFh of the detection results (actual measured values of the image pattern) are plotted, and a density curve (1501) as shown in FIG. 13 is created using these 10 points. The method of creating this density curve may be a commonly used approximation method, such as using an approximation formula that connects the 10 points.
FIG. 13 is a diagram showing the relationship between the measured density plot and the successive correction LUT with respect to the density target.

Next, in S203, the CPU 105 performs an inverse transformation to correct the current density curve (1501) created in S203 to the initial density curve (1502), and creates a sequential correction LUT (1503) as shown in FIG. 13 (S204).

Finally, the CPU 105 creates a composite correction LUT (1601) as shown in Fig. 14 by multiplying the sequential correction LUT (1503) and the initial correction LUT (1500), sets the composite correction LUT (1601) in the gamma correction circuit 309 (S205), and thereafter controls the output image to be reflected in the output image. That is, the CPU 105 corrects the image forming conditions of the printer engine 102 based on the density information detected in S202. After the composite correction LUT (1601) is reflected, the output image and the next image density correction gradation pattern are output in a state where they are multiplied by the composite correction LUT (1601).
FIG. 14 is a diagram showing the relationship between the initial correction LUT, the sequential correction LUT, and the composite correction LUT.
Even if it is not immediately after the automatic tone correction, density adjustment is possible with the same image adjustment flow by multiplying the composite correction LUT by a new sequential LUT.

[Image density sensor response]
Next, the responsiveness of the image density sensor 400 will be described.
The responsiveness referred to here means the speed of reaction when the image density sensor 400 detects an image pattern for image density correction formed on the intermediate transfer belt 1412. For example, when detecting an image pattern formed on the intermediate transfer belt 1412, if the time it takes for a stable measurement to be achieved is short, the sensor has a high response, whereas if the time it takes is long, the sensor has a low response.

Figure 15 (a) shows the change in the measurement value of PD 403, which receives diffuse reflection, when an image pattern for image density correction is formed on intermediate transfer belt 1412 and intermediate transfer belt 1412 and the image pattern are measured by image density sensor 400.

In FIG. 15A, section A corresponds to an area where the image density sensor 400 measures the surface of the intermediate transfer belt 1412 on which no image pattern is formed (hereinafter referred to as the “base”).
Section B corresponds to the area where the image pattern was measured.
Section C corresponds to the region where the base was measured, similar to section A.

In sections A and C, when the intermediate transfer belt 1412 is irradiated with light from the LED of the image density sensor 400, the PD 403 receives a small amount of scattered light.
In section B, the scattered light received by the PD 403 changes over time in the area where the image pattern is being measured. Section B1 indicates a gradual change from the start of reading the image pattern area until it reaches an output value corresponding to the actual image density (hereinafter referred to as the "sensor rise time"), which takes about 10 ms in FIG. 15(a). Section B2 is an area where the output value is stable after it reaches an output value corresponding to the actual image density, and image density control is performed using the sensor output value in this area. Section B3 is an area where the sensor output value is stable from the end of reading the image pattern area until it reaches the output value from the background area (hereinafter referred to as the "sensor fall time"), which takes about 13 ms in FIG. 15(a).

Thus, when measuring the image pattern formed on the transfer belt 1412, an area required for the sensor rise time and sensor fall time is required within the image pattern. For example, in the graph of FIG. 15(a), if the process speed of the transfer belt is 300 mm/s, the area required for sensor rise is "300 x 0.015 = 4.5 mm". Additionally, the area required for sensor fall is "300 x 0.019 = 5.7 mm". Thus, the size of the actual patch (image pattern) must be created taking this area into account.

<Effectiveness verification>
[Patch arrangement and patch size configuration]
Next, the arrangement and size of patch images constituting an image pattern, which is a feature of this embodiment, will be described.
First, the present embodiment is characterized in that the patch size is increased when the density difference between adjacent patches in an image pattern for image density correction becomes large. The necessity for this will be explained.

FIG. 15B is a diagram showing the density difference in the transition of the sensor output of the image density sensor 400.
15B shows the transition of the sensor output value when three image patterns of different densities are measured by the image density sensor 400. In the figure, △ shows the transition of the sensor output value when a pattern with a density difference from the background of 0.35 is measured, ○ shows the pattern with a density difference of 0.91, and □ shows the pattern with a density difference of 1.51 is measured.

As can be seen from Figure 15 (b), the sensor rise time and sensor fall time differ depending on the density difference of the image pattern. If the boundary between the sensor rise time and fall time is set to the point where the sensor output value is 95% from the value at which it becomes stable, the sensor rise time and fall time for each density difference are as shown in Table 1 below.

As can be seen from FIG. 15B and Table 1, as the concentration difference increases, the sensor rise time and fall time, that is, the sensor response, becomes longer.
When the sensor response time is increased, the area required for measurement increases, resulting in a problem that the correction image pattern becomes larger, the correction time becomes longer, or the amount of toner required for correction increases.

As mentioned above, there are two types of density adjustment: adjustment pattern A, which uses 10-level patches for high-precision adjustment, and adjustment pattern B, which reduces the number of gradations in the patches and makes fine adjustments more frequently. When there are a large number of patches, as in adjustment pattern A, it may be possible to prevent the density difference between adjacent patches from becoming large by arranging the patches in a clever way. However, when there are a small number of gradations, as in adjustment pattern B, the density difference will become large even if the arrangement is devised.

Therefore, in this embodiment, an array pattern is used in which the patch size of the image pattern for image density adjustment is larger when the density step between adjacent density adjustment patches is large than when the density step is not large.

Using Figure 16, we will explain the patch arrangement of an image pattern for image density correction in three gradations (4Dh, 99h, FFh) in adjustment pattern B of density adjustment in this embodiment. Note that while adjustment pattern B is explained here, an image pattern consisting of multiple patch images formed in different sizes according to the density difference between adjacent patch images may also be used in the case of adjustment pattern A.

In this embodiment, as an example, the density difference between adjacent image patterns is set to ΔD = 0.35 as the standard. If ΔD < 0.35 is assumed, the patch size is determined to produce a small patch (here, the patch size assumes the rise time and fall time when ΔD = 0.35 in Table 1). If ΔD ≥ 0.35 is assumed, the patch size is determined to produce a large patch (here, the patch size assumes the rise time and fall time when ΔD = 1.51).

Note that the gradation pattern, reference density step ΔD, and patch size used for density adjustment in this embodiment are merely examples and are not limited to these.

Figure 16(a) is a diagram showing an example of a small-sized patch image (hereinafter, "patch size X") among the image patterns for image density correction in the first embodiment. Note that this patch image is read by the image density sensor 400 from the left to the right in the figure (from downstream to upstream in the direction of movement of the intermediate transfer belt).

The patch size X is an image pattern that is 16 mm in the movement direction (X direction) of the intermediate transfer belt 1412 and 16.3 mm in the direction perpendicular to the movement direction (Y direction) of the intermediate transfer belt 1412. The length in the X direction is 300 mm/s x 9 ms = 2.7 mm for the sensor rise time, 300 mm/s x 12 ms = 3.6 mm for the sensor fall time, and 10 mm for the length at which the sensor output can be stably measured.

Figure 16(b) is a diagram showing an example of a large-sized patch (hereinafter, "patch size Z") among the image patterns for image density correction in the first embodiment. This patch image is read by the image density sensor 400 from the left to the right in the figure (from the downstream side to the upstream side in the movement direction of the intermediate transfer belt).

The patch size Z is an image pattern that is 16 mm in the Y direction and 20.2 mm in the X direction. The length in the X direction is 300 mm/s x 14 ms = 4.2 mm for the sensor rise time, 300 mm/s x 20 ms = 6.0 mm for the sensor fall time, and 10 mm for the length at which the sensor output can be stably measured.

Figure 16 (c) shows an example of a patch arrangement of an image pattern for image density correction of three gradations (4Dh, 99h, FFh) for each color in the first embodiment.

If the measurement start position is the left edge, the patches are arranged from the left edge in order of yellow (Y) densities 4Dh, 99h, FFh, magenta (M) densities 4Dh, 99h, FFh, cyan (C) densities 4Dh, 99h, FFh, and black (K) densities 4Dh, 99h, FFh. In other words, the image density sensor 400 detects the density information of each of these patches sequentially from the leftmost patch to the rightmost patch (from downstream to upstream in the direction of movement of the intermediate transfer belt 1412).

17 is a diagram showing the image level, expected density, density difference from the previous patch, patch size relationship, and total patch size of each patch shown in FIG. 16C. Note that, as a comparative example, a case where all patch sizes are large patch size Z is also shown.
The assumed density is the target density for each image level shown in FIG. 9, and is the density when it matches the target density, and is not limited to the density shown in FIG.

17, when all patches are formed with the large patch size Z as in the conventional comparative example, the total patch size is 242.4 mm. On the other hand, when patches with a small patch size X and a large patch size Z are combined according to the density difference between adjacent patches as in this embodiment, the total patch size is 222.9 mm. In this way, this embodiment can achieve a patch size reduction effect of just under 10% compared to the conventional method.
In addition, among the multiple patch images that make up the image pattern, the Δ density difference of the row of the patch image (patch image of patch order 1) that is furthest downstream in the movement direction of the intermediate transfer belt 1412 is, for example, the density difference between the patch image and the surface (base) of the intermediate transfer belt 1412.

As described above, the patch arrangement of the image pattern for image density correction formed on the image carrier used in the first embodiment is composed of a combination of patches of small patch size X and large patch size Z selected according to the density difference between adjacent patches. In other words, calibration (correction) is performed using an image pattern (test pattern) composed of multiple patch images formed in different sizes according to the density difference between adjacent patch images. With this configuration, it is possible to perform calibration for color and density gradation stabilization control in a short time and with high accuracy, and to suppress an increase in the amount of toner consumed.

Second Embodiment
In the second embodiment, a case will be described in which the standard for the density step between adjacent patches is set to ΔD=1.0 as standard B in addition to ΔD=0.35 as shown in the first embodiment, with respect to the patch arrangement of the image density correction pattern. Note that, although the case will be described here in which two density step reference points (threshold values) are received, the same effect can be obtained even if the number is increased to three or more. For example, reference points may be provided at intervals of 0.1, and the length of the patch image in the sub-scanning direction may be changed according to the magnitude relationship with each reference point. Note that the configuration of the image pattern used in this embodiment other than the patch arrangement is the same as that of the first embodiment.

FIG. 18 is a diagram showing an example of a patch (hereinafter, "patch size Y") that is larger than patch size X and smaller than patch size Z, among image patterns for image density correction in the second embodiment.
The patch size Y is an image pattern with a size of 16 mm in the main scanning direction and 18.4 mm in the sub-scanning direction. In the sub-scanning direction, an area of 300 mm/s x 11 ms = 3.3 mm is required for the sensor rise time. Also, an area of 300 mm/s x 17 ms = 5.1 mm is required for the sensor fall time. In addition, an area of 10 mm is required for stable measurement of the sensor output.

Figure 19 shows the image level, expected density, density difference from the previous patch, patch size relationship, and total patch size of each patch when patches of patch sizes X, Y, and Z in the second embodiment are set according to the density step between adjacent patches.

As can be seen from Fig. 19, when combining patches of patch sizes X, Y, and Z according to the density difference between adjacent patches as in this embodiment, the total patch size is 215.7 mm. Also, as can be seen from the comparative example in Fig. 17, when all patches are formed with the large patch size Z as in the conventional method, the total patch size is 242.4 mm. In this way, in this embodiment, it is possible to obtain a patch size reduction effect of about 10% compared to the conventional method.
An image pattern constituted by patch images as described above is formed at the time of an image pattern such as S101 in FIG. 10 (S201 in FIG. 11) described above, and image density adjustment is performed.

As described above, the patch arrangement of the image pattern for image density correction formed on the image carrier used in the second embodiment is composed of a combination of patches of appropriate patch sizes determined according to the density difference between adjacent patches. By using such an image pattern, it is possible to perform calibration for color and density gradation stabilization control in a short time with high accuracy, and to more precisely suppress increases in the amount of toner used.

Third Embodiment
In the third embodiment, a different pattern from the gradation patterns shown in the first and second embodiments will be described. That is, the patch size when forming a plurality of dither processing patterns will be described. Note that the other configurations are the same as those of the first and second embodiments.

In the third embodiment, the pattern for adjusting image density is a correction patch with two gradations (99h, FFh) for each color, and further a pattern in which error diffusion, low line frequency, and high line frequency halftone dithering are performed for each color. This pattern has a patch order, for example, as shown in FIG. 20.

FIG. 20 is a diagram showing the image level, expected density, density difference from the previous patch, patch size relationship, and total patch size of each patch in the third embodiment.
In this example, as in the first embodiment, the density difference between adjacent image patterns is ΔD = 0.35, and if ΔD < 0.35 is expected, a patch of patch size X is used, and if ΔD ≧ 0.35 is expected, a patch of patch size Z is used.
As a comparative example, a case in which all patches for density adjustment are created with patches of patch size Z is also shown.

20, when all patches are formed with the large patch size Z as in the conventional comparative example, the total patch size is 323.2 mm. On the other hand, when patches with a small patch size X and a large patch size Z are combined according to the density difference between adjacent patches as in this embodiment, the total patch size is 292 mm. In this way, this embodiment can achieve a patch size reduction effect of about 10% compared to the conventional method.
As in the second embodiment, for example, two or more threshold values may be provided and an appropriate patch size may be determined according to the density difference between adjacent patches.

As described above, in the third embodiment, an image pattern is used that is composed of patch images that have been subjected to halftone dithering for each color, error diffusion, and low and high line counts. In other words, an image pattern is used that is composed of adjacent patch images across multiple pseudo halftone processing patterns. The size of each patch image that composes this image pattern is determined according to the density difference between adjacent patch images, as in the first and second embodiments. By using such an image pattern, it is possible to perform calibration for color and density gradation stabilization control in a short time and with high accuracy, and to suppress an increase in the amount of toner consumed.

The length of the image pattern in the sub-scanning direction may be controlled to change depending on whether the density difference between adjacent image patterns is an increase or decrease in density. That is, when the density difference between adjacent patch images is due to an increase in density, the length of the patch image in the sub-scanning direction is determined based on a first rule in accordance with the amount of the increase. When the density difference is due to a decrease in density, the length of the patch image in the sub-scanning direction is determined based on a second rule (a rule different from the first rule) in accordance with the amount of the decrease. Then, calibration is performed using an image pattern that combines the patch images determined in this way. This makes it possible to more precisely control the increase in the amount of toner consumed during calibration, thereby further reducing the amount of toner consumed.

According to each embodiment described above, the arrangement of patches used for correction such as density adjustment is configured with patches in which the patch length in the sub-scanning direction is changed (made longer than normal) only for patches adjacent to a patch with a large density difference, and the patches are measured by an optical sensor and correction is performed based on the measurement results. This makes it possible to prevent the size of each patch from becoming larger than necessary and to suppress errors in which the measurement results are not obtained correctly. This makes it possible to reduce the amount of toner consumed by correction and shorten the correction time. Therefore, it becomes possible to perform calibration for color and density gradation stabilization control in a short time with high accuracy and to suppress an increase in toner consumption.
Note that in S904 in FIG. 8 described above, an image pattern may be used that is configured with a plurality of patch images that are formed in different sizes according to the density difference between adjacent patch images, as described in each of the above embodiments.

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 embodiments is also included in the present invention.

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-described 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-described embodiments and their modifications.

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

Claims (5)

an image forming means having an image processing section including a conversion section that converts image data based on image forming conditions and a halftone processing section that performs halftone processing on the image data, and forming a toner image on a sheet based on the image data from the image processing section;
an image carrier on which an image pattern composed of a plurality of patch images is formed by the image forming means, the image carrier being rotated;
a measuring means for measuring the image pattern on the image carrier and outputting a measurement value relating to the measurement result of the image pattern as density information ;
a correction unit that corrects the image forming conditions based on the density information output by the measurement unit,
the image pattern includes a patch image group of a first color formed at the front in a rotation direction of the image carrier, and a patch image group of a second color different from the first color formed adjacent to the upstream of the patch image group of the first color in the rotation direction,
The patch images of the first color include
a first patch image formed at the most downstream position of the group of patch images of the first color in the rotation direction;
a second patch image that is formed adjacent to the upstream side of the first patch image in the rotation direction and that is subjected to a different type of halftone processing from that of the first patch image, the second patch image being formed such that a density difference from the first patch image is less than a threshold value;
a third patch image that is formed most upstream of the group of patch images of the first color in the rotation direction, the third patch image being formed to have a higher density than both of the first patch image and the second patch image;
The group of patch images of the second color is
a fourth patch image formed at the most downstream position of the group of patch images of the second color in the rotation direction;
a fifth patch image that is formed adjacent to the upstream side of the fourth patch image in the rotation direction and that is subjected to a different type of halftone processing from that of the fourth patch image, the fifth patch image being formed such that a density difference between the fifth patch image and the fourth patch image is less than the threshold value;
a sixth patch image that is formed at the most upstream position of the group of patch images of the second color in the rotation direction, the sixth patch image being formed to have a higher density than both of the fourth patch image and the fifth patch image;
a length of the first patch image in the rotation direction is determined based on a first rule because a density difference between the first patch image and the image carrier is greater than a threshold value and indicates an increase in density, and is longer than a length of the second patch image in the rotation direction, the density difference between the first patch image and the second patch image being smaller than the threshold value;
an image forming apparatus characterized in that the length of the fourth patch image in the rotational direction is determined based on a second rule different from the first rule because the density difference between the fourth patch image and the third patch image is greater than the threshold value and indicates a decrease in density, and is longer than the length of the fifth patch image in the rotational direction, the density difference between the fourth patch image and the third patch image being smaller than the threshold value. 2. The image forming apparatus according to claim 1, wherein the length of the third patch image in the rotational direction is determined based on the first rule because the density difference between the third patch image and another patch image arranged downstream of the third patch image in the rotational direction is greater than the threshold value and indicates an increase in density, and is longer than the length of the second patch image in the rotational direction, the density difference between the third patch image and the first patch image being smaller than the threshold value. the image pattern further includes a patch image group of a third color different from the second color formed adjacent to the upstream of the patch image group of the second color in the rotation direction of the image carrier,
The group of patch images of the third color is
a seventh patch image formed most downstream in the group of patch images of the third color in the rotation direction;
an eighth patch image that is formed adjacent to the seventh patch image upstream in the rotation direction and that is subjected to a different type of halftone processing from that of the seventh patch image, the eighth patch image being formed such that a density difference between the eighth patch image and the seventh patch image is less than a threshold;
a ninth patch image that is formed most upstream of the group of patch images of the third color in the rotation direction, the ninth patch image being formed to have a higher density than both of the seventh patch image and the eighth patch image,
2. The image forming apparatus according to claim 1, wherein the length of the seventh patch image in the rotational direction is determined based on the second rule because the density difference between the seventh patch image and the sixth patch image is greater than the threshold value and indicates a decrease in density, and is longer than the length of the eighth patch image in the rotational direction, the density difference between the seventh patch image and the eighth patch image being smaller than the threshold value. The image forming apparatus of claim 3, characterized in that the length of the 9th patch image in the rotational direction is determined based on the first rule because the density difference between the 9th patch image and other patch images arranged downstream of the 9th patch image in the rotational direction is greater than the threshold value and indicates an increase in density, and is longer than the length of the 8th patch image in the rotational direction, the density difference between the 8th patch image and the 7th patch image being smaller than the threshold value. 2. The image forming apparatus according to claim 1, wherein the measuring means comprises: a light emitting unit; a first light receiving unit that receives specularly reflected light from the image pattern ; and a second light receiving unit that is disposed at a position not receiving specularly reflected light from the image pattern and receives diffusely reflected light from the image pattern.

Images