JP6536086B2 - Image forming apparatus and test pattern generation method - Google Patents

Description

  The present invention relates to an image forming apparatus and a method of generating a test pattern.

In an electrophotographic image forming apparatus, a color material such as toner is supplied onto a photosensitive drum exposed based on original image data, and a developed image is transferred onto a sheet of paper via an intermediate transfer belt. , Forms an image on the paper.
Since unevenness in image density may occur due to the characteristics of the image forming member or changes over time, the test pattern is formed, the unevenness in density is detected from the density distribution, and the original image data is offset so as to offset the detected unevenness in density. It is common to correct the (see, for example, Patent Document 1).

JP 2007-140402 A

  Among the uneven density, there is uneven density which occurs in the same cycle as the rotating cycle of the rotating body due to the rotating body such as the photosensitive drum, the developing roller, the transfer roller, and the intermediate transfer belt. As the number of cycles of density unevenness that can be detected from the density distribution of the test pattern increases, and as the number of density levels of the test pattern increases, density unevenness can be identified more accurately, and highly accurate correction becomes possible.

Since the photosensitive drum and rollers have a length of about 20 cm at one cycle, forming a test pattern of about 60 cm is sufficient even when detecting uneven density for three cycles.
On the other hand, in the intermediate transfer belt, one cycle is as long as about 100 cm, and it is necessary to form a long test pattern of about 300 cm in order to detect density unevenness for the same three cycles. When the density level of the test pattern is two steps, the length of the required test pattern is doubled to 600 cm. The formation of the test pattern prolongs the downtime that can not be operated, and also increases the consumption of coloring materials and the like, resulting in an increase in cost.

  An object of the present invention is to generate a test pattern that can accurately correct density unevenness even if it is short.

According to the invention of claim 1,
An image forming unit for overlapping and transferring images of a plurality of individually formed colors on a rotating image carrier and further transferring the images of the plurality of colors on the image carrier onto a sheet of paper;
A test pattern generation unit that generates a test pattern for detecting density unevenness in the same cycle as the rotation cycle of the image carrier;
It said test pattern generating unit divides the regions of the same length of the test pattern as one period of the rotation period of the image bearing member N (N is an integer of 2 or more) number of intervals, further M the respective sections ( M is divided into 2 or more integer) areas, by concentration levels in the respective regions are arranged respectively M different patterns to generate the test pattern,
The test pattern generation unit generates an f-1 (f is an integer from 2 to F) cycle when the test patterns having the same number as the periodicity F of the density unevenness to be detected (F is an integer of 2 or more) are generated. In the density distribution of the test pattern, one or more areas where the density change is large is divided into sections narrower in width than the N sections, and each section is further divided into M areas, An image forming apparatus is provided characterized in that test patterns of the f-th cycle are generated by respectively arranging M patterns having different density levels .

According to the invention of claim 2,
The M patterns arranged in each section in the test pattern are arranged such that patterns of the same density level are adjacent to each other across the boundary of each section. An image forming apparatus is provided.

According to the third aspect of the invention,
The test pattern generation unit may reduce the number of test patterns to be generated when it determines that the stability of density is high according to the difference in density distribution of a plurality of test patterns generated at the time of detection of density unevenness in the past. An image forming apparatus according to claim 1 or 2 , characterized in that it is provided.

According to the invention of claim 4 ,
A density detection unit that detects the density of the test pattern formed on the image carrier by the image forming unit;
A control unit that calculates a correction value corresponding to the phase of the rotation cycle of the image carrier based on the density distribution of the test pattern detected by the density detection unit;
The image forming apparatus according to any one of claims 1 to 3 , further comprising:

According to the invention of claim 5 ,
A method of generating a test pattern for detecting density unevenness in the same cycle as a rotation cycle of an image carrier on which images of a plurality of colors formed individually are superimposed and transferred.
The region having the same length of the test pattern as one period of the rotation period of the image bearing member N (N is an integer of 2 or more) is divided into individual sections, the individual sections further M (M is an integer of 2 or more) divided into individual regions, by concentration levels in the respective regions are arranged respectively M different patterns to generate the test pattern,
When generating the same number of test patterns as the number F of cycles of density unevenness to be detected (F is an integer of 2 or more), in the density distribution of the test pattern of f−1 (f is an integer from 2 to F) The area or areas where the change in density is large is divided into sections narrower in width than the N sections, and the sections are further divided into M areas, and M levels having different density levels in each area By arranging each of the patterns of (1) to (4) , a test pattern generation method is provided that is characterized by generating an f-th cycle test pattern .

  According to the present invention, it is possible to generate a test pattern that can accurately correct density unevenness even if it is short.

FIG. 2 is a block diagram showing the configuration of the image forming apparatus of the present embodiment for each function. It is a front view which shows schematic structure of the image formation part of FIG. It is a figure which shows the conventional test pattern. It is a figure which shows the Example of the test pattern produced | generated by the test pattern production | generation part of FIG. It is a graph which shows density distribution of the test pattern shown in FIG. It is a figure which shows the center position of each pattern at the time of arrange | positioning one pattern in one area, and at the time of arrange | positioning two patterns in one area. It is a figure which shows the modification of a test pattern. It is a figure which shows the modification of a test pattern.

  Hereinafter, embodiments of an image forming apparatus and a test pattern generation method of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing the main configuration of an image forming apparatus G of the present embodiment for each function.
As illustrated in FIG. 1, the image forming apparatus G includes a control unit 11, a storage unit 12, an operation unit 13, a display unit 14, a communication unit 15, an image generation unit 16, an image memory 17, a test pattern generation unit 18, and image processing. The image forming unit 20, the density detection unit 30, and the phase detection unit 31 are provided.

The control unit 11 reads a program stored in the storage unit 12 and controls each unit of the image forming apparatus G by executing the program. The control unit 11 can be configured by a central processing unit (CPU), a random access memory (RAM), and the like.
For example, the control unit 11 causes the image processing unit 19 to process the original image data generated by the image generation unit 16 and the image forming unit 20 performs image processing according to the pixel value of each pixel of the original image data after image processing. Form an image on a sheet of paper.

The control unit 11 can calculate a correction value of uneven density occurring in the same cycle as the rotation cycle of the image carrier used as an image forming member in the image forming unit 20.
When calculating the correction value, the control unit 11 causes the test pattern generation unit 18 to generate a test pattern for detecting uneven density, and causes the image forming unit 20 to form the test pattern. The control unit 11 calculates a correction value corresponding to the phase of the rotation cycle of the image carrier based on the density distribution of the test pattern detected by the density detection unit 30.

  The storage unit 12 stores a program readable by the control unit 11, a file used at the time of execution of the program, and the like. A large capacity memory such as a hard disk can be used as the storage unit 12.

  The operation unit 13 generates an operation signal according to the user's operation and outputs the operation signal to the control unit 11. Examples of the operation unit 13 include a key, a touch panel integrally formed with the display unit 14, and the like.

  The display unit 14 displays an operation screen or the like in accordance with an instruction from the control unit 11. As the display unit 14, an LCD (Liquid Crystal Display), an OELD (Organic Electro Luminescence Display), or the like can be used.

The communication unit 15 communicates with an external device on a network, such as a user terminal, a server, or another image forming apparatus.
For example, the communication unit 15 receives data (hereinafter referred to as PDL data) in which the content of an instruction to form an image is described in page description language (PDL) from the user terminal via the network.

  The image generation unit 16 rasterizes the PDL data received by the communication unit 15, and converts C (cyan), M (magenta), Y (yellow), and the original image data in a bitmap format having pixel values for each pixel. Generate for each color of K (black). The pixel value is a data value representing the density level of the image, and for example, an 8-bit data value represents a density level of 0 to 255 levels.

  The image forming apparatus G may include an image reading unit for copying, and the original image data may be generated by reading an image of a document set by the user using the image reading unit.

  The image memory 17 is a buffer memory that temporarily holds the image data generated by the image generation unit 16. For example, a DRAM (Dynamic RAM) or the like can be used as the image memory 17.

The test pattern generation unit 18 generates a test pattern for detecting density unevenness which occurs in the same cycle as the rotation cycle of the image carrier used in the image forming unit 20.
Specifically, the test pattern generation unit 18 divides the area of the test pattern having the same length as one cycle of the rotation cycle of the image carrier into N (N is an integer of 2 or more) sections, and sets each section. Furthermore, a test pattern is generated by dividing into M (M is an integer of 2 or more) areas and arranging M patterns having different density levels in each area. One cycle of the rotation cycle of the image carrier corresponds to the length of the outer periphery when the image carrier makes one rotation.

The image processing unit 19 performs image processing such as tone correction processing and halftone processing on the original image data generated by the image generation unit 16. The image processing unit 19 can perform similar image processing on the test pattern generated by the test pattern generation unit 18.
The gradation correction process is a process of converting the pixel value of each pixel of the original image data into a pixel value corrected so that the density of the image formed on the sheet matches the target density. Halftone processing is, for example, error diffusion processing, screen processing using systematic dither method, or the like.

  Further, the image processing unit 19 can use the correction value of density unevenness calculated by the control unit 11 based on the density distribution of the test pattern, and perform correction processing of density unevenness on the original image data.

The image forming unit 20 forms an image of a plurality of colors on a sheet in accordance with the pixel value of each pixel of the original image data subjected to image processing by the image processing unit 19.
FIG. 2 shows a schematic configuration of the image forming unit 20. As shown in FIG.
As illustrated in FIG. 2, the image forming unit 20 includes four writing units 21, an image carrier 22, a secondary transfer roller 23, a fixing device 24, a sheet feeding tray 25, and a reverse path 26.

As shown in FIG. 2, the respective writing units 21 are arranged in series along the belt surface of the image carrier 22.
The image carrier 22 is a belt or the like which is wound and rotated by a plurality of rollers, and is generally called an intermediate transfer belt. Among the plurality of rollers, a primary transfer roller 2 f and a secondary transfer roller 23 are included. Since the image of C, M, Y and K colors individually formed by the four writing units 21 is transferred in an overlapping manner on the surface of the image carrier 22, the images of the respective colors are further transferred onto the paper. Do.

The four writing units 21 form images of C, M, Y and K colors, respectively. The configuration of each writing unit 21 is the same, and as shown in FIG. 2, it includes an exposure unit 2a, a photosensitive drum 2b, a developing unit 2c, a charging unit 2d, a cleaning unit 2e and a primary transfer roller 2f.
At the time of image formation, in each writing unit 21, a voltage is applied to the photosensitive drum 2 b by the charging unit 2 d to charge it. Each writing unit 21 irradiates a laser beam from the exposure unit 2a according to the corresponding modulated original image data of colors C, M, Y and K, and repeatedly scans the photosensitive drum 2b in the main scanning direction. During scanning, the photosensitive drum 2b rotates and the position of each line to be scanned is shifted in the sub-scanning direction, so that an electrostatic latent image consisting of a plurality of lines is formed on the photosensitive drum 2b. The exposure unit 2a detects a laser beam scanning the leading end side in the main scanning direction, and outputs a synchronization signal indicating the start position of the image of each line.

When a color material such as toner is supplied by the developing unit 2 c and the electrostatic latent image on the photosensitive drum 2 b is developed, an image of each color is formed on the photosensitive drum 2 b of each writing unit 21.
The images on the respective photosensitive drums 2b are sequentially superimposed and transferred onto the image carrier 22 by the respective primary transfer rollers 2f, and images of a plurality of colors are formed on the image carrier 22. After the transfer of the image, in each writing unit 21, the cleaning unit 2e removes the color material remaining on the photosensitive drum 2b.

Thereafter, the sheet is fed by the sheet feeding tray 25, and the image on the image carrier 22 is transferred onto the sheet by the secondary transfer roller 23. The sheet after transfer is heated and pressed by the fixing device 24 to fix the image on the sheet.
When an image is formed on both sides of one sheet of paper, the sheet is conveyed to the reversing path 26 to reverse the sheet surface, and then the sheet is conveyed to the secondary transfer roller 23 again.

As shown in FIG. 2, the density detection unit 30 is disposed at a position facing the surface of the image carrier 22 onto which the image is to be transferred, and the density of the image formed on the image carrier 22 by the image forming unit 20 is To detect.
As the concentration detection unit 30, an optical sensor whose detection range is a spot may be used, or a line sensor, an area sensor or the like may be used when a wider detection range is required.

The phase detection unit 31 generates and outputs a detection signal of the reference phase in a rotation cycle of the image carrier 22. For example, an optical sensor or the like that detects one or more markers provided in a non-image area on the surface of the image carrier 22 can be used as the phase detection unit 31.
FIG. 2 shows an arrangement example of the phase detection unit 31 and the markers 22 a on the image carrier 22.
According to the phase detection unit 31 illustrated in FIG. 2, when the phase in one cycle is represented in the range of 0 to 1, the marker 22 a provided on the image carrier 22 can be detected as the reference phase of 0.
A plurality of reference phases may be detected, and two markers provided for each half of the belt of the image carrier 22 may be detected as the reference phases of 0 and 1/2, respectively.

  The image forming apparatus G forms a test pattern and analyzes the density distribution to detect density unevenness occurring in the same cycle as the rotation cycle of the image carrier 22 due to the image carrier 22 it can. As a rotating body used as an image forming member of the image forming unit 20, in addition to the image carrier 22, there are a photosensitive drum 2b, a primary transfer roller 2f, a secondary transfer roller 23 and the like. The length of the cycle is generally 5 times or more as long as other rotating bodies.

When the length of one rotation cycle of the image carrier 22 is L (cm), in order to detect density unevenness of the cycle number F (F is an integer of 1 or more) occurring in the same cycle, the sub scanning direction It is necessary to form a test pattern having a length of F × L (cm). If the number M of density levels of the test pattern is set to 2 or more in order to enhance the accuracy of correction, the length in the sub-scanning direction of the test pattern is M × F × L (cm) in total.
For example, when forming test patterns of high density level and low density level as level number 2 in order to detect density unevenness of cycle number 3, a total of six test patterns are required as shown in FIG. The total length of the test patterns in the sub-scanning direction y is 2 × 3 × L (cm). When L is 100 (cm), a test pattern having a length of 600 cm must be formed, and the down time (non-operating time of the image forming apparatus G) due to the formation of the test pattern becomes long and the toner Etc. consumption increases and costs rise.

In order to reduce such down time and cost, the image forming apparatus G generates a test pattern with high correction accuracy even if the test pattern generation unit 18 has a short length.
Specifically, the test pattern generation unit 18 divides the area of the test pattern having the same length L as one cycle of the image carrier 22 into N sections, and further divides each section into M areas. A test pattern is generated by respectively arranging M patterns having different density levels in each area. N and M can be selected arbitrarily as long as they are integers of 2 or more. If N and M are powers of 2, it is easy to cope with hardware resources used for correction processing of density unevenness, which is preferable.

The test pattern generation unit 18 can generate the same number of test patterns as the number F of cycles of density unevenness to be detected.
FIG. 4 shows an example of three test patterns T11, T12 and T13 generated when F = 3.
As shown in FIG. 4, the length of the test pattern T11 to T13 in the sub scanning direction y is the same as the length L (cm) of one cycle of the image carrier 22. The length in the main scanning direction x of each of the test patterns T11 to T13 is not particularly limited, and can be any length.

The area of each of the test patterns T11 to T13 is divided into N sections Db, and the length of the sub scanning direction y of each section Db is L / N (cm). The section Db is a unit for calculating a correction value, and one correction value is calculated in each section Db.
Each section Db is divided into two areas Dc, and the length of the sub-scanning direction y of each area Dc is L / (N × M) (cm).
For example, in the case of N = 64 and M = 2, the lengths of the section Db and the area Dc in the sub scanning direction y are L / 64 and L / 128 (cm), respectively.

  In each area Dc of each section Db, two patterns g11 and g12 having different density levels are alternately arranged. For example, when the density level of the image is expressed in 0 to 255 steps, the density levels of the patterns g11 and g12 can be 192 and 64, respectively.

  When calculating the correction value, the image forming unit 20 forms each of the test patterns T11 to T13 on the image carrier 22, and the density detection unit 30 detects the density of each of the test patterns T11 to T13. The phase of each of the rotation cycles 0 to 1 corresponding to each test pattern T11 to T13 on the image carrier 22 corresponds to the reference phase of the rotation cycle of the image carrier 22 detected by the phase detector 31. It can be identified by counting how many lines of the image have been formed. For counting, use a counter that counts up each time a synchronization signal indicating the start position of each line is output from the exposure unit 2a, and resets the count value each time a reference phase is detected by the phase detection unit 31. it can.

FIG. 5 shows a part of the concentration distribution obtained by detecting the concentration of the test pattern T11.
In the test pattern T11, since the patterns g11 and g12 are alternately arranged, the density distribution has a sinusoidal waveform as shown in FIG. 5 if there is no density unevenness. The length of one cycle of a sine wave is 1 / N, and the length of a half cycle is 1 / (N × M).

The control unit 11 analyzes the density distribution of each of the test patterns T11 to T13 obtained by the density detection unit 30, and averages the average value of density values of the pattern g11 detected in each section Db and the average value of density values of the pattern g12. Calculate each.
When the pattern g11 and the pattern g12 are included in the range of the spot diameter of the light source of the density detection unit 30, the density values of both the patterns g11 and g12 are included in the detected density value. Therefore, among the density values detected by the density detection unit 30, the density values detected within the range of the spot diameter of the light source of the density detection unit 30 are excluded from the boundaries of the patterns g11 and g12. It is preferable to calculate an average value.

  The control unit 11 normalizes the average value of the density values of the patterns g11 and g12 calculated for each section Db to 0 to 255 density levels. The control unit 11 calculates a difference obtained by subtracting the density level obtained by normalization from the density levels of the patterns g11 and g12 as a correction value of each section Db. The calculated correction value of each section Db is a correction value corresponding to the phase of the center position of each section Db.

Since the central position of each of the patterns g11 and g12 is shifted from the central position of the section Db, the phase to which the correction value of each section Db corresponds according to the shift of the central position in order to carry out the correction with high accuracy. It is preferable to shift the
For example, when N = 64 and M = 2, as described above, the length of one section Db is L / 64 (cm), and the length of one area Dc is L / 128 (cm) It is.
As shown in FIG. 6, when only one pattern g11 is arranged in one section Db, the center position of the pattern g11 is L / 128 (cm) from the start end in the sub scanning direction of the section Db. And coincides with the center position of the section Db. Therefore, the correction value calculated from the pattern g11 can be used as a correction value corresponding to the phase of L / 128 from the start of the section Db.

  On the other hand, when two patterns g11 and g12 are arranged in the same section Db, the central positions of the patterns g11 and g12 are L / 256 and L / 48, respectively, at the distance from the start end of the section Db in the subscanning direction. (cm) position, which is shifted by L / 256 (cm) to the start and end sides of the center position of the section Db. That is, the correction value of section Db calculated from pattern g11 corresponds to the phase of L / 256 from the start end, and the correction value of section Db calculated from pattern g12 corresponds to the phase of L / 48 from the start end Correction value. Therefore, to correct more accurately, the correction value of each section Db corresponding to the density level of the pattern g11 is used as the correction value corresponding to the phase shifted by L / 256 toward the start end side, and the density level of the pattern g12 The correction value of each section Db corresponding to may be used as a correction value corresponding to the phase shifted by L / 256 toward the end.

When performing correction processing of uneven density on original image data using the correction value, the reference phase of the rotation cycle of the image carrier 22 detected by the phase detection unit 31 and the count of the lines of the image formed from the reference image Based on the original image data, the value specifies which phase of the rotation cycle of 0 to 1 each line of the image corresponds to when the image is formed on the image carrier 22. The image processing unit 19 adds a correction value corresponding to the phase of each specified line among the correction values of each section Db calculated by the control unit 11 to the pixel value of each line of the original image data.
Since correction values are calculated for each density level M by the patterns g11 and g12, more accurate correction is performed by using the correction value of the closest density level among the density levels. be able to.
Further, as described above, the correction value of each section Db is a correction value corresponding to the phase of the center position of each of the patterns g11 and g12 arranged in each section Db, but the phase of each specified line is 2 When the phase is between the central positions of the patterns g11 and g12 of one section Db, the correction value corresponding to the phase of each line can be obtained by linearly interpolating the correction value of each section Db.

  As described above, the image forming apparatus G according to the present embodiment transfers the images of a plurality of colors formed individually onto the rotating image carrier 22 so as to be transferred, and from the plurality of colors on the image carrier 22 And a test pattern generation unit 18 for generating a test pattern for detecting density unevenness in the same cycle as the rotation cycle of the image carrier 22. The test pattern generation unit 18 divides the area of the test pattern having the same length as one cycle of the rotation cycle of the image carrier 22 into N sections, further divides each section into M areas, and The above test patterns are generated by respectively arranging M patterns having different density levels.

As a result, the density distribution of M density levels can be obtained in a length of 1 / M of the conventional test pattern shown in FIG. 3 in the same manner as the conventional test pattern. Test patterns T11 to T13 that can correct the error can be generated.
In order to sequentially transfer the images formed individually by the plurality of writing units 21, the rotation cycle of the image carrier 22 is longer than that of the other rotating members, but is high by using the above-described short test patterns T11 to T13. While maintaining the accuracy of correction, it is possible to reduce the down time during the formation of the test pattern, and the productivity is improved. In addition, the consumption of toner and the like for forming the test pattern can be reduced, and the cost can be reduced.

[Modification 1]
It is preferable that M patterns be arranged in each section of the test pattern so that the patterns of the same density level are adjacent to each other across the boundary of each section because the accuracy of correction can be further enhanced.

FIG. 7 shows three test patterns T21 in which patterns g11 and g12 arranged in each section Db are arranged such that patterns g11 or g12 with the same density level are adjacent to each other across the boundary of each section Db. An example of T23 is shown.
As shown in FIG. 7, the pattern g11 of each section Db is adjacent to the pattern g11 of the adjacent section Db. Similarly, the pattern g12 is adjacent to the pattern g12 of the section Db adjacent to the pattern g12 of each section Db.

As described above, since the densities of both patterns are detected near the boundary between the patterns g11 and g12 having different density levels, the density values of the patterns g11 and g12 are excluded excluding the density value near the boundary of each section Db. It is necessary to calculate the average value of
However, since each test pattern T21 to T23 has the same density level of the adjacent pattern g11 or g12 in each section Db, the density near the boundary can also be used for calculation of the correction value. Since the number of density value samples used to calculate the correction value increases, the accuracy of correction improves.

[Modification 2]
When generating a test pattern of the same number F as the number F of cycles of density unevenness to be detected, the test pattern generation unit 18 determines the density distribution of the test pattern of the period f-1 (f represents an integer from 2 to F). Divide one or more areas where the density change was large in N into N sections, further divide each section into M areas, and arrange M patterns with different density levels in each area Thus, the f-th cycle test pattern can be generated.

  For example, as shown in FIG. 8, the test pattern generation unit 18 generates a test pattern T31 of the first cycle in the same manner as the test pattern T11 shown in FIG. Form on the body 22. When the density distribution of the test pattern T31 is obtained by the density detection section 30, the test pattern generation section 18 detects a density value whose difference with the reference value of each of the patterns g11 and g12 exceeds a threshold in this density distribution. Alternatively, a plurality of sections are determined as sections having large changes in concentration distribution. The reference values of the patterns g11 and g12 are the density values of the patterns g11 and g12 when there is no change in density.

  As shown in FIG. 8, when the two sections De1 and De2 are determined as sections having large concentration distributions, the test pattern generation unit 18 divides each of the sections De1 and De2 into N sections Df. The test pattern generation unit 18 further divides the N sections Df into M areas Dg, arranges M patterns g21 and g22 having different density levels in each area Dg, and generates a test pattern of the second cycle. Generate T32. The two patterns g21 and g22 have the same density level as the patterns g11 and g12 in the test pattern T31 of the first cycle except that the length in the sub scanning direction y is short.

The patterns g21 and g22 of the test pattern T32 have shorter arrangement intervals than the patterns g11 and g12, and it is possible to obtain correction values of finer correction units in the sections De1 and De2 where the density change is large. As a result, the resolution of the correction of the phase part that needs to be corrected can be enhanced, and the accuracy of the correction can be enhanced.
Further, since the patterns g21 and g22 are arranged only in the sections De1 and De2 requiring correction, the amount of toner consumed by the formation of the test pattern T32 can be further reduced, and the cost can be reduced.

[Modification 4]
When generating test patterns of the same number F as the number F of cycles of density unevenness to be detected, the test pattern generation unit 18 determines the density according to the difference in density distribution of a plurality of test patterns generated at the time of detection of density unevenness in the past. The number of test patterns to be generated can be reduced if it is determined that the stability of is high.

  For example, in the case where F = 3 and three test patterns are respectively generated at the past two times of detection of density unevenness, the test pattern generation unit 18 compares the density distributions of the six test patterns. If the difference in density distribution of each test pattern is less than the threshold value in any phase, the test pattern generation unit 18 determines that the density stability is high, and reduces the number of test patterns from three to two.

  When the number of test patterns is reduced, the number of samples of density values is reduced. However, when the stability of density is high, the change in density unevenness is small, and the reduction in the number of test patterns does not affect the correction accuracy. Therefore, as described above, by reducing the number of test patterns, shorter test patterns can be formed while maintaining high correction accuracy.

The above embodiment is a preferred example of the present invention, and is not limited thereto. It can change suitably in the range which does not deviate from the main point of the present invention.
For example, when the control unit 11 reads a program, the processing procedure of the test pattern generation unit 18 can be executed by the control unit 11. Further, the program can be read by a computer such as a general-purpose PC as well as the image forming apparatus G to execute the above-described processing procedure.
As a computer readable medium of the program, a non-volatile memory such as a ROM and a flash memory, and a portable recording medium such as a CD-ROM can be applied. Also, as a medium for providing program data via a communication line, carrier wave (carrier wave) is also applied.

G image forming apparatus 11 control unit 12 storage unit 18 test pattern generation unit 20 image formation unit 30 density detection unit 31 phase detection unit

Claims (5)

An image forming unit for overlapping and transferring images of a plurality of individually formed colors on a rotating image carrier and further transferring the images of the plurality of colors on the image carrier onto a sheet of paper;
A test pattern generation unit that generates a test pattern for detecting density unevenness in the same cycle as the rotation cycle of the image carrier;
It said test pattern generating unit divides the regions of the same length of the test pattern as one period of the rotation period of the image bearing member N (N is an integer of 2 or more) number of intervals, further M the respective sections ( M is divided into 2 or more integer) areas, by concentration levels in the respective regions are arranged respectively M different patterns to generate the test pattern,
The test pattern generation unit generates an f-1 (f is an integer from 2 to F) cycle when the test patterns having the same number as the periodicity F of the density unevenness to be detected (F is an integer of 2 or more) are generated. In the density distribution of the test pattern, one or more areas where the density change is large is divided into sections narrower in width than the N sections, and each section is further divided into M areas, An image forming apparatus characterized in that a test pattern of the f-th cycle is generated by respectively arranging M patterns having different density levels .   The M patterns arranged in each section in the test pattern are arranged such that patterns of the same density level are adjacent to each other across the boundary of each section. Image forming device. The test pattern generation unit may reduce the number of test patterns to be generated when it determines that the stability of density is high according to the difference in density distribution of a plurality of test patterns generated at the time of detection of density unevenness in the past. An image forming apparatus according to claim 1 or 2 , characterized in. A density detection unit that detects the density of the test pattern formed on the image carrier by the image forming unit;
A control unit that calculates a correction value corresponding to the phase of the rotation cycle of the image carrier based on the density distribution of the test pattern detected by the density detection unit;
The image forming apparatus according to any one of claims 1 to 3 , further comprising: A method of generating a test pattern for detecting density unevenness in the same cycle as a rotation cycle of an image carrier on which images of a plurality of colors formed individually are superimposed and transferred.
The region having the same length of the test pattern as one period of the rotation period of the image bearing member N (N is an integer of 2 or more) is divided into individual sections, the individual sections further M (M is an integer of 2 or more) divided into individual regions, by concentration levels in the respective regions are arranged respectively M different patterns to generate the test pattern,
When generating the same number of test patterns as the number F of cycles of density unevenness to be detected (F is an integer of 2 or more), in the density distribution of the test pattern of f−1 (f is an integer from 2 to F) The area or areas where the change in density is large is divided into sections narrower in width than the N sections, and the sections are further divided into M areas, and M levels having different density levels in each area A test pattern generation method characterized by generating an f-th cycle test pattern by arranging each of the patterns of (4) .

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