Image forming apparatus

Abstract

To correct image density unevenness with high accuracy.SOLUTION: The image forming apparatus 100 includes the image forming unit PY that prints an image on the sheet S, the image density sensor 69 that reads the measurement image printed on the sheet S together with the background portion of the sheet S on which the measurement image is not printed, and the controller 110 that suppresses the image density unevenness in the sub-scanning direction based on the reading result of the measurement image corrected by the reading result of the background portion by the image density sensor 69.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a multifunction machine, or a printer. [Background technology]

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

[0003] Such image forming apparatuses may suffer from uneven charging of the photosensitive drum when it is charged, uneven exposure when it is scanned with laser light, and uneven development when it is developed. These unevennesses can cause uneven image density in a specific direction in the image formed on the paper. For example, uneven image density can occur in the main scanning direction and the sub-scanning direction. The main scanning direction is the direction in which the laser light scans the photosensitive drum, which is the drum axial direction. The sub-scanning direction is the direction that intersects the main scanning direction and is the direction in which the photosensitive drum rotates.

[0004] To correct image density unevenness, the image forming area is divided into multiple regions, and paper is used on which a measurement image, including a pattern image for measuring image density unevenness, is formed in each region. Based on the measurement results of the pattern image for each region, the amount of laser light is adjusted to eliminate image density differences between regions, thereby correcting image density unevenness. For example, Patent Document 1 proposes a technique for correcting image density unevenness in the main scanning direction. Patent Documents 2 and 3 propose techniques for correcting image density unevenness in the sub-scanning direction.

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

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-163216 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-98675 [Patent Document 3] Japanese Patent Publication No. 2022-71704 Summary of the Invention [Problem to be solved by the invention]

[0007] When correcting image density unevenness in this way, a test chart is used, on which a measurement image for detecting image density unevenness is printed on paper. The measurement image of the test chart is read by a reading sensor such as an in-line sensor installed in the device or a scanner attached to the device. Such a reading sensor is an optical sensor that reads the measurement image by irradiating light onto the test chart and receiving reflected light.

[0008] Reading sensors can produce reading errors due to variations in the amount of light irradiated onto the test chart, differences in the sensitivity of individual reading elements, etc. Furthermore, for example, in a reading sensor that reads a test chart while transporting paper on which the test chart is printed, the reading distance can become unstable due to the paper flapping during transport, resulting in reading errors. These reading errors hinder accurate measurement of image density unevenness, making accurate correction of image density unevenness difficult.

[0009] SUMMARY OF THE INVENTION In view of the above-mentioned problems, it is a primary object of the present invention to provide an image forming apparatus that can correct image density unevenness with high precision. [Means for solving the problem]

[0010] The image forming apparatus of the present invention is characterized by comprising a conveying means for conveying paper, an image forming means for forming an image on the paper conveyed by the conveying means, a reading means for reading the paper on which a measurement image has been formed by the image forming means, and a suppression means for suppressing uneven image density in the conveying direction of the image to be formed by the image forming means based on the reading result of the measurement image read by the reading means and the reading result of the reading means reading multiple positions in the conveying direction of the conveying means in an area of ​​the paper on which the measurement image has been formed where the measurement image has not been formed. Another aspect of the image forming apparatus of the present invention comprises a conveying means for conveying paper, an image forming means for forming an image on the paper conveyed by the conveying means, a reading means for reading the paper on which a measurement image has been formed by the image forming means, and a suppression means for suppressing image density unevenness in the conveying direction of the image to be formed by the image forming means based on the reading result of the measurement image read by the reading means and the reading result of the reading means at multiple positions in the conveying direction of the conveying means in an area of ​​the paper on which the measurement image has been formed where the measurement image is not formed, wherein the suppression means corrects the reading result of the measurement image at that position based on the difference between the average value of the reading results of the area for each position in the conveying direction obtained from the result of two-dimensional linear interpolation of the reading results of the area at multiple positions in the conveying direction and the reading result of the area at each position, and suppresses image density unevenness in the conveying direction and a direction intersecting the conveying direction based on the reading result of the corrected measurement image. [Effects of the Invention]

[0011] According to the present invention, it is possible to correct image density unevenness with high precision. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an image forming apparatus. [Figure 2] FIG. 2 is a diagram illustrating the configuration of an image forming unit. [Figure 3] 10 is a flowchart showing a process for correcting image density unevenness in the sub-scanning direction. [Figure 4] FIG. 10 is a diagram illustrating an image for sub-scanning measurement. [Figure 5] FIG. 10 is an explanatory diagram of a detection position of a sub-scanning measurement image. [Figure 6] FIG. 10 is a diagram illustrating an example of luminance values ​​of a magenta pattern image. [Figure 7] 10A and 10B are diagrams illustrating the luminance difference in the background portion. [Figure 8] 10A and 10B are diagrams illustrating examples of corrected luminance values. [Figure 9] FIG. 10 is a diagram illustrating image density values. [Figure 10] FIG. 10 is a diagram illustrating an example of a brightness-density conversion table. [Figure 11] 10 is a flowchart showing a process for correcting image density unevenness in the main scanning direction. [Figure 12] FIG. 10 is a diagram illustrating an image for main scanning measurement. [Figure 13] FIG. 10 is an explanatory diagram of a detection position of a main scanning measurement image. [Figure 14] FIG. 10 is a diagram illustrating the brightness values ​​of first to third background portions. [Figure 15] FIG. [Figure 16] FIG. [Figure 17] FIG. 10 is a diagram illustrating image density values. DETAILED DESCRIPTION OF THE INVENTION

[0013] (First embodiment) FIG. 1 is a diagram showing the configuration of an image forming apparatus according to this embodiment. The image forming apparatus 100 includes a reader A, a printer B, and an operation unit 20. The printer B prints an image on paper S. The reader A reads an image from paper (document G) on which the image has been printed. The operation unit 20 is a user interface. The operation unit 20 includes various key buttons and a touch panel as an input interface. The operation unit 20 includes a display unit 218 as an output interface. A user uses the operation unit 20 to issue instructions to start copying and to make various settings.

[0014] (Leader) Reader A includes a platen glass 102 on which an original G is placed, a light source 103 that irradiates light onto the original G placed on the platen glass 102, an optical system 104, a light receiving unit 105, and an image processing unit 108. Reader A also includes a CPU (Central Processing Unit) 214, a RAM (Random Access Memory) 215, and a ROM (Read Only Memory) 216. The light source 103, the optical system 104, and the light receiving unit 105 constitute an image reading unit that reads an image of the original G. A positioning member 107 that abuts against one side of the original G to prevent the original G from being positioned obliquely, and a reference white plate 106 that is used for shading correction of the image reading unit are arranged on the edge of the platen glass 102.

[0015] The optical system 104 forms an image of light emitted from the light source 103 and reflected by the document G on the reading surface of the light receiving unit 105. The light receiving unit 105 has a photoelectric conversion element such as a CCD (Charge Coupled Device) sensor, and outputs an image signal by converting the received reflected light into an electrical signal. The light receiving unit 105 has, for example, three rows of photoelectric conversion elements arranged corresponding to red (R), green (G), and blue (B). The light receiving unit 105 generates color component signals of each of the R, G, and B colors as an image signal. The image reading unit moves in the direction of arrow R103 and reads the image of the document G placed on the document platen glass 102 line by line. Therefore, the image signal includes a data string for each line.

[0016] The image signal generated by the light receiving unit 105 is input to the image processing unit 108. The image processing unit 108 performs image processing such as A / D conversion, shading correction, and color conversion on the image signal acquired from the light receiving unit 105. The image processing unit 108 transmits the image signal after image processing to the printer B.

[0017] The CPU 214 controls the operation of the reader A by executing a computer program stored in the ROM 216. The RAM 215 is a work memory used when the CPU 214 executes processing. In the reader A, the CPU 214 controls various operations for reading the image of the original G.

[0018] (Printer) Printer B includes image forming units PY, PM, PC, PK, intermediate transfer belt 6, secondary transfer roller 64, fuser 11, paper feed cassette 65, and printer control unit 109. Printer B is a tandem intermediate transfer type full-color printer in which image forming units PY, PM, PC, and PK are arranged along intermediate transfer belt 6. Image forming unit PY forms a yellow image (toner image). Image forming unit PM forms a magenta image (toner image). Image forming unit PC forms a cyan image (toner image). Image forming unit PK forms a black image (toner image).

[0019] The intermediate transfer belt 6 is a belt member that is supported by being stretched over a tension roller 61, a drive roller 62, and an opposing roller 63. A belt cleaner 68 is provided opposite the tension roller 61. The intermediate transfer belt 6 is driven by the drive roller 62 to rotate in the direction of arrow R2 at a predetermined process speed. Images (toner images) formed in the image forming units PY, PM, PC, and PK are transferred onto the intermediate transfer belt 6 in a superimposed order at a timing according to the rotation speed of the intermediate transfer belt 6. In this way, a full-color image (toner image) is formed on the intermediate transfer belt 6. The intermediate transfer belt 6 functions as an intermediate transfer body onto which the toner image is transferred.

[0020] The opposing roller 63 forms a secondary transfer portion T2 between itself and the secondary transfer roller 64. The images of each color transferred to the intermediate transfer belt 6 are transported to the secondary transfer portion T2 and transferred collectively onto the paper S. When a positive DC voltage is applied to the secondary transfer roller 64, the images (toner images) of each color, which are negatively charged and carried on the intermediate transfer belt 6, are transferred collectively onto the paper S. The developer (residual toner) remaining on the intermediate transfer belt 6 after transfer is removed by a belt cleaner 68. The belt cleaner 68 collects the residual toner remaining on the intermediate transfer belt 6 after passing through the secondary transfer portion T2 by rubbing a cleaning blade against the intermediate transfer belt 6.

[0021] The sheets S are stored in a paper feed cassette 65 and are fed one sheet at a time. A separation roller 66 and a registration roller 67 are provided on the transport path along which the sheets S are transported. The sheets S are fed from the paper feed cassette 65, separated one sheet at a time by the separation roller 66, and transported to the registration roller 67. The registration roller 67 receives the sheets S in a stopped state and keeps them waiting, and transports the sheets S to the secondary transfer unit T2 in accordance with the timing at which the image carried on the intermediate transfer belt 6 is transported to the secondary transfer unit T2. The registration roller 67 functions as a transport means for transporting the sheets S.

[0022] The paper S onto which the image has been transferred is transported by the secondary transfer roller 64 via the transport belt 10 to the fixing device 11. The fixing device 11 applies heat and pressure to the paper S to melt and fix the image onto the paper S. The paper S onto which the image has been fixed is discharged outside the body of the printer B via a transport path 70.

[0023] An image density sensor 69 is disposed on a transport path 70 for transporting the paper S from the fixing unit 11 to the outside of the apparatus body as a reading sensor for reading the image printed on the paper S. The image density sensor 69 is an in-line sensor used to measure the image density of a pattern image for detecting image density unevenness.

[0024] Image formation by the image forming units PY, PM, PC, and PK will be described. The image forming units PY, PM, PC, and PK have the same configuration and perform the same operations, except for the color of the developer (toner in this case) used for development. In the following description, when colors are to be distinguished, the suffixes Y, M, C, and K are added to the end of the reference numerals, and when colors are not to be distinguished, the suffixes Y, M, C, and K are omitted.

[0025] 2 is an explanatory diagram of the configuration of the image forming unit P. The image forming unit P includes a photosensitive drum 1, a charger 2, an exposure unit 3, a developing unit 4, a reflected light amount sensor 12, a primary transfer roller 7, and a drum cleaner 8. An intermediate transfer belt 6 is sandwiched between the photosensitive drum 1 and the primary transfer roller 7. The charger 2, exposure unit 3, developing unit 4, a reflected light amount sensor 12, a primary transfer roller 7, and a drum cleaner 8 are arranged around the photosensitive drum 1.

[0026] The photosensitive drum 1 of this embodiment is a photosensitive member having a photosensitive layer formed on the outer peripheral surface (surface) of an aluminum cylinder. The photosensitive drum 1 rotates in the direction of arrow R1 around the drum axis at a predetermined process speed. The photosensitive drum 1 is, for example, an OPC (Organic Photo Conductor) photosensitive member with a reflectance of approximately 40% for near-infrared light (960 [nm]). Note that the photosensitive drum 1 may also be an amorphous silicon-based photosensitive member with a similar reflectance.

[0027] The charger 2 in this embodiment is a scorotron charger, which irradiates the photosensitive drum 1 with charged particles generated by corona discharge, thereby charging the photosensitive layer on the surface of the photosensitive drum 1 to a uniform negative potential. The scorotron charger has a wire to which a high voltage is applied, a grounded shield portion, and a grid portion to which a desired voltage is applied. A predetermined charging bias voltage is applied to the wire of the charger 2 from a charging bias power supply (not shown). A predetermined grid bias voltage is applied to the grid portion of the charger 2 from a grid bias power supply (not shown). Although it depends on the voltage applied to the wire, the photosensitive drum 1 is charged to approximately the voltage applied to the grid portion.

[0028] The exposure device 3 scans the surface of the charged photosensitive drum 1 in the drum axial direction by reflecting laser light emitted from a light source with a rotating mirror, forming an electrostatic latent image on the surface of the photosensitive drum 1. For this reason, the drum axial direction of the photosensitive drum 1 is the main scanning direction. The sub-scanning direction, which intersects with the main scanning direction, is the direction of rotation of the photosensitive drum 1. The sub-scanning direction is also parallel to the transport direction in which the paper S is transported. A potential sensor 5, which is a potential detector, is provided near the photosensitive drum 1. The potential sensor 5 can detect the potential of the electrostatic latent image formed on the photosensitive drum 1.

[0029] When a development bias voltage is applied, the developing unit 4 causes toner to adhere to the electrostatic latent image on the photosensitive drum 1, thereby forming an image (toner image) on the photosensitive drum 1. The developing unit 4 includes a developing sleeve 41, a first conveying screw 42, and a second conveying screw 43 in a developer container 45 for containing toner. The developer container 45 of this embodiment contains a two-component developer, which is a mixture of non-magnetic toner and magnetic carrier. The developer container 45 is divided into two chambers by a partition 46, with the first conveying screw 42 provided in one chamber and the second conveying screw 43 provided in the other. The partition 46 has two openings, which allow toner to flow between the two chambers. The first conveying screw 42 and the second conveying screw 43 rotate to circulate the developer within the developer container 45 while stirring and mixing the developer.

[0030] The developing sleeve 41 is disposed close to the photosensitive drum 1 and rotates in a manner following the rotation of the photosensitive drum 1. The developing sleeve 41 carries a developer in the form of a mixture of toner and carrier. The developer carried on the developing sleeve 41 develops the electrostatic latent image on the photosensitive drum 1 when a developing bias voltage is applied to the developing sleeve 41. The developing bias voltage is applied by a power supply unit 44. The application of the developing bias voltage by the power supply unit 44 is controlled by a control unit 110 (CPU 111) described below.

[0031] Developing unit 4 is equipped with toner amount sensor 14 for measuring the amount of toner in developer container 45. For example, a magnetic permeability sensor that detects the magnetic permeability of the developer is used as toner amount sensor 14. Developing unit 4 is connected to toner supply container 33 via supply path 32. If the toner amount measured by toner amount sensor 14 is less than a predetermined amount, toner is replenished from toner supply container 33 via supply path 32 to developer container 45.

[0032] The reflected light amount sensor 12 is an optical sensor having a light-emitting element 12a and a light-receiving element 12b, and is used to measure the image density of the toner image formed on the photosensitive drum 1. The reflected light amount sensor 12 irradiates light from the light-emitting element 12a onto the toner image on the photosensitive drum 1. The light-receiving element 12b receives the light reflected by the toner image and outputs an output signal according to the amount of reflected light received.

[0033] The primary transfer roller 7 presses against the inner surface of the intermediate transfer belt 6, forming a primary transfer portion T1 between the photosensitive drum 1 and the intermediate transfer belt 6. When a positive DC voltage is applied to the primary transfer roller 7, the negative toner image carried on the photosensitive drum 1 is transferred to the intermediate transfer belt 6, which passes through the primary transfer portion T1. In this way, the image forming portion P forms a toner image of a color corresponding to the photosensitive drum 1. The toner image is transferred from the photosensitive drum 1 to the intermediate transfer belt 6. The drum cleaner 8 rubs a cleaning blade against the photosensitive drum 1 to collect residual toner remaining on the photosensitive drum 1 after transfer to the intermediate transfer belt 6.

[0034] The operation of such image forming unit P is controlled by a printer control unit 109 and a control unit 110 provided in printer A. The printer control unit 109 controls the operation of printer B. The control unit 110 controls the operation of the entire image forming apparatus 100. The control unit 110 is connected to the printer control unit 109 and the image processing unit 108 of reader A. An operation unit 20 is also connected to the control unit 110. The operation unit 20 is also connected to the CPU 214 of reader A. Although not shown in the figure, the CPU 214 of reader A is also connected to the control unit 110.

[0035] The control unit 110 includes a CPU 111, a RAM 112, and a ROM 113. The CPU 111 controls the operation of the image forming apparatus 100 by executing a computer program stored in the ROM 113. The RAM 112 is a work memory used by the CPU 111 when executing processing. Various operations of the reader A and printer B of the image forming apparatus 100 are controlled by the CPU 111. The printer control unit 109 includes a light intensity control unit 190, a pattern generator 192, and a pulse width modulator 191. The image processing unit 108 includes a video counter 220 and a gamma correction unit 209.

[0036] The exposure unit 3 in this embodiment is a laser scanner having a rotating mirror. The exposure amount of the exposure unit 3 is determined by a light amount control unit 190 so that a predetermined image density level is obtained for the laser output signal. In this embodiment, in order to suppress uneven image density in the main scanning direction and the sub-scanning direction, the exposure amount setting (LPW) is managed by allowing the exposure amount to be set in units of approximately 30 mm in width in each direction. In addition, the exposure unit 3 controls the blinking of the laser light in accordance with the pulse width determined by a pulse width modulator 191 based on a drive signal generated using a tone correction table (LUT) in a gamma correction unit 209.

[0037] The laser output signal is determined based on a gradation correction table held in the gamma correction unit 209. The gradation correction table indicates the relationship between the laser output signal and the image density level of the image to be formed, and the laser output signal is determined according to the image density of the image to be formed.

[0038] The printer control unit 109 acquires the image signal generated by the image processing unit 108. Based on the image signal, the printer control unit 109 pulse-width modulates (PWM) the laser light output from the light source of the exposure unit 3 to form an image with image density gradation based on area modulation. To this end, the printer control unit 109 uses the pulse-width modulator 191 to generate and output a laser output signal having a width (time width) corresponding to the level of the image signal for each pixel. The laser output signal is a laser drive pulse signal. For an image signal specifying a high image density, the laser output signal is a wide pulse signal. For an image signal specifying a low image density, the laser output signal is a narrow pulse signal. For an image signal specifying an intermediate image density, the laser output signal is a pulse signal with an intermediate width.

[0039] The laser output signal (laser drive pulse signal) output from the pulse width modulator 191 is supplied to a laser light source (e.g., a semiconductor laser) of the exposure device 3. The semiconductor laser outputs laser light for a period of time corresponding to the pulse width of the laser output signal. Therefore, the semiconductor laser is driven for a long time for pixels with high image density and for a short time for pixels with low image density. As a result, the dot size (area) of the electrostatic latent image formed on the photosensitive drum 1 varies depending on the image density of the pixel. The exposure device 3 exposes a longer range in the main scanning direction for pixels with high image density and a shorter range in the main scanning direction for pixels with low image density.

[0040] The pattern generator 192 generates an image signal of a pattern image to be formed in order to correct the image formation conditions. Here, the image formation conditions are, for example, the amount of light (exposure amount) emitted from the light source of the exposure device 3, the charging bias voltage applied to the charger 2, and the developing bias voltage applied to the developer 4. The image formation conditions may be any one, two, or all three of the exposure amount, charging bias voltage, and developing bias voltage. When forming a pattern image to correct density unevenness in the sub-scanning direction, the pulse width modulator 191 generates a laser output signal based on the image signal of the pattern image obtained from the pattern generator 192.

[0041] It should be noted that the printer control unit 109 may acquire an image signal from an external device other than the reader A. For example, the printer control unit 109 may acquire an image signal via a telephone line using a receiving unit (not shown). The printer control unit 109 may also acquire an image signal via a network (not shown). The printer control unit 109 performs the above-described processing on image signals acquired via any route. The image forming apparatus 100 functions as a copier when printing an image based on an image signal acquired from the reader A. The image forming apparatus 100 functions as a facsimile when printing an image based on an image signal acquired via a telephone line. The image forming apparatus 100 functions as a printer when printing an image based on an image signal acquired via a network.

[0042] (Shading function) In this embodiment, image density unevenness in the sub-scanning direction is corrected using the shading function of the exposure unit 3. The light intensity control unit 190 obtains exposure intensity correction values ​​corresponding to each exposure position and phase in the sub-scanning direction from the ROM 113 of the control unit 110, and controls exposure with an exposure intensity setting (LPW) based on the correction values. The exposure intensity correction values ​​corresponding to each exposure position are obtained through image density unevenness correction, which will be described later. In this embodiment, the ROM 113 stores correction values ​​for exposure intensity setting at intervals of approximately 10 mm in the main scanning direction and approximately 30 mm in the sub-scanning direction. Note that image density unevenness in the main scanning direction is addressed by shading correction in the main scanning direction. In shading correction in the main scanning direction, the light intensity control unit 190 obtains light intensity correction values ​​corresponding to each exposure position in the main scanning direction from the ROM 113 of the control unit 110, and controls exposure with a light intensity setting based on the correction values.

[0043] (Image density unevenness correction) The following describes the image density unevenness correction process for suppressing image density unevenness in the sub-scanning direction. In the image density unevenness correction process, the control unit 110 performs, for example, exposure amount correction process for the exposure device 3 during image formation, pattern image formation process for detecting image density unevenness, image density unevenness detection process, and image density correction calculation process.

[0044] In this embodiment, the reading sensor used in the image density unevenness detection process is an image density sensor 69. The image density sensor 69 reads a pattern image (measurement image) printed on the paper S, with a direction intersecting the transport direction of the paper S transported on the transport path 70 as one line. The image density sensor 69 can measure the brightness value of the entire paper S by reading the paper S during transport.

[0045] FIG. 3 is a flowchart showing the process of correcting image density unevenness in the sub-scanning direction. Image density unevenness in the sub-scanning direction occurs due to eccentricity of rotating members involved in image formation, such as the photosensitive drum 1, developing sleeve 41, and primary transfer roller 7, and occurs according to the rotation period of the rotating members. In the process of correcting image density unevenness in the sub-scanning direction, the image formation conditions are corrected according to the period of such rotating members, thereby correcting image density unevenness in the sub-scanning direction. Here, a case of correcting image density unevenness in the sub-scanning direction caused by the photosensitive drum 1 will be described.

[0046] When the control unit 110 starts the process of correcting image density unevenness in the sub-scanning direction, it forms a pattern image (sub-scanning measurement image) for correcting image density unevenness in the sub-scanning direction on the paper S (S101). FIG. 4 is an example of the sub-scanning measurement image. The sub-scanning measurement image is a band-shaped image having a predetermined width in the main scanning direction and extending a predetermined length in the sub-scanning direction, and is formed based on an image signal that indicates uniform image density. This image signal is generated by the pattern generator 192. Band-shaped images (pattern images) of each color (Y, M, C, K) are arranged at predetermined intervals in the main scanning direction. In this embodiment, the pattern image of each color is formed using an image signal that results in an image density of 40%.

[0047] The image formation conditions in the sub-scanning direction require that the position of the sub-scanning measurement image be associated with the rotational phase of the rotating member (photosensitive drum 1), which is a cause of image density unevenness. In this embodiment, the phase of the photosensitive drum 1 is controlled so that the pattern image writing position matches the home position of the rotational phase. This allows the control unit 110 to obtain image density unevenness information that accurately corresponds to the phase of one rotation of the photosensitive drum 1 for each color.

[0048] Fig. 5 is an explanatory diagram of the detection position of the sub-scanning measurement image. In Fig. 5, approximately 300 mm, which corresponds to at least one cycle of the photosensitive drum 1, is divided into 10 equal parts, and pattern images are detected in units of 1 to 10, each divided into approximately 30 mm from the upstream side in the transport direction (sub-scanning direction).

[0049] The control unit 110 reads the paper S on which the sub-scanning measurement image is printed using the image density sensor 69, and detects a luminance value as the reading result (S102). The control unit 110 detects the luminance value I of the sub-scanning measurement image for each segment based on the reading result of the sub-scanning measurement image by the image density sensor 69. The control unit 110 also detects the luminance value Ib of the background portion of the paper S for each segment based on the reading result of the image density sensor 69 of the area (background portion) on the paper S where no pattern image is formed between the pattern images of each color on which the sub-scanning measurement image is not printed. FIG. 6 is an example diagram of the luminance value I of the magenta pattern image. As shown in FIG. 6, the luminance value I is detected for each segment (each region).

[0050] The control unit 110 calculates an average value Ibave of the luminance values ​​Ibn (n=1 to 10) of the background portion for each division unit (each region) (S103). The control unit 110 calculates the difference (luminance difference ΔIbn) between the calculated average value Ibave and the luminance value Ibn of the background portion of each region (S104). Equation 1 is a calculation formula for the luminance difference ΔIbn. Fig. 7 is a diagram illustrating the luminance difference ΔIbn of the background portion. ΔIbn=Ibn-Ibave …(Equation 1) (n:1~10)

[0051] In the pattern image of FIG. 4, the pattern images of each color are arranged at a predetermined interval in the main scanning direction, so multiple luminance values ​​are detected in the same region in the sub-scanning direction. For example, in region 1 in the sub-scanning direction (see FIG. 5), 15 luminance values ​​are detected between the pattern images of each color. In this case, the luminance value Ibn is determined to be, for example, the average, median, maximum, or minimum value of the multiple luminance values ​​detected in the same region in the sub-scanning direction. If the pattern image is an image in which only one background portion can be detected, that single luminance value is used as the luminance value Ibn. For example, if the pattern images of each color are arranged without any intervals in the main scanning direction and only one background portion is provided between any of the pattern images in the main scanning direction, the control unit 110 can detect only one background portion.

[0052] When detecting the brightness value of the sub-scanning measurement image from the paper S being transported, the distance between the image density sensor 69 and the paper S fluctuates due to the paper S flapping during transport. The output value (brightness value) of the image density sensor 69 changes due to fluctuations in the distance from the object to be measured. As a result, brightness unevenness occurs in the brightness value detected by the image density sensor 69. The brightness value obtained from the sub-scanning measurement image also includes the image density unevenness of the sub-scanning measurement image. The brightness value detected in this way includes the detection error of the image density sensor 69 and the image density unevenness.

[0053] The brightness difference ΔIbn between the brightness value Ibn of each region in the background portion of the paper S and the average value Ibav only extracts brightness unevenness caused by changes in detection error of the image density sensor 69 due to fluttering of the paper S. Detection errors caused by fluttering of the paper S tend to be roughly the same in the main scanning direction. The control unit 110 corrects the brightness value I obtained from the reading results of the pattern image of each color by the brightness difference ΔIbn obtained from the reading results of the background portion, thereby removing the influence of detection error caused by fluttering of the paper S from the brightness value I (S105). By correcting using the brightness difference ΔIbn, only density unevenness of the pattern image of each color is extracted. The correction of the brightness value I of each region using the brightness difference ΔIbn is performed using, for example, (Equation 2). Corrected luminance value I'n = luminance value In - luminance difference ΔIbn ... (Equation 2) (n:1~10)

[0054] Fig. 8 is an example diagram of the corrected luminance value I'n of each region obtained by correcting the luminance value In of each region detected from the magenta pattern image by the luminance difference ΔIbn. Fig. 8 shows the result of correcting the luminance values ​​of Fig. 6 by the luminance difference of Fig. 7. The corrected luminance value I'n in Fig. 8 becomes the image density unevenness of the sub-scanning measurement image that is the original target of correction.

[0055] The control unit 110 converts the corrected luminance value I'n of each region into an image density value (S106). The control unit 110 converts the corrected luminance value I'n into an image density value using, for example, a conversion table or a conversion formula. The control unit 110 calculates the average value of the 10 image density values ​​of each region (S107). The control unit 110 calculates the image density difference Δ between the average image density value and the image density value of each region (S108). The control unit 110 calculates a correction value (ΔLPW) corresponding to the calculated image density difference Δ (S109). The control unit 110 determines the exposure amount of each region based on the calculated correction value (ΔLPW) (S110).

[0056] The above image density unevenness correction process is performed for each pattern image of each color formed at each position in the main scanning direction. As a result, the exposure amount for forming an image of each color corresponding to each position in the main scanning direction is determined. The control unit 110 suppresses image density unevenness by forming an image with the exposure amount determined by the combination of the position in the main scanning direction and the rotation phase.

[0057] The effect of suppressing image density unevenness by the image density unevenness correction process described above will now be described. The sub-scanning measurement image used to confirm the effect is the image shown in FIG. 4. The reading position is the position shown in FIG. 5. Under these conditions, the image density sensor 69 was used to detect the image density of the sub-scanning measurement image printed on paper S to verify the effect.

[0058] Fig. 9 is an example diagram of image density values ​​obtained from a magenta pattern image through verification. Fig. 9 illustrates image density values ​​before image density unevenness correction processing (thin line), image density values ​​after image density unevenness correction processing (thick line), and image density values ​​when image density unevenness correction processing is performed without correction using the luminance value of the background portion (dotted line). The maximum image density value, minimum image density value, and differential image density value are shown as indices representing image density unevenness.

[0059] The image density values ​​before the image density unevenness correction process have the following image density unevenness. Maximum image density value Dmax=0.447 Minimum image density value Dmin=0.415 Differential image density value ΔD=0.032

[0060] The image density values ​​after the image density unevenness correction process have the following image density unevenness: The image density values ​​after the image density unevenness correction process have a smaller differential image density value ΔD than the image density values ​​before the image density unevenness correction process. Maximum image density value Dmax=0.435 Minimum image density value Dmin=0.425 Differential image density value ΔD=0.01

[0061] The image density values ​​obtained without correction based on the luminance value of the background portion will have the following image density unevenness: In this case, the degree of correction for the image density values ​​will differ for each region, resulting in a decrease in the effectiveness of the image density unevenness correction process. Maximum image density value Dmax=0.439 Minimum image density value Dmin=0.423 Differential image density value ΔD=0.016

[0062] In this way, the image forming apparatus 100 of the first embodiment can reduce the influence of image density unevenness caused by the image density sensor 69 and accurately detect image density unevenness in the sub-scanning direction. Therefore, the exposure amount can be determined based on an optimal correction value, and a high-quality image with reduced image density unevenness can be formed. Such high-precision correction of image density unevenness is realized.

[0063] (Second embodiment) In the first embodiment, a technique for correcting image density unevenness in the sub-scanning direction was described. In the second embodiment, a technique for correcting image density unevenness in the main scanning direction will be described. The configuration of the image forming apparatus 100 is the same as in the first embodiment, so a description thereof will be omitted. In the second embodiment, the pattern image is read by a reader A instead of an image density sensor 69.

[0064] As described above, reader A optically reads the image of the entire area of ​​document G placed on document glass 102, line by line. Reader A generates an image signal including a data string for each line as a result of the reading. The image signal is subjected to image processing in image processing unit 108 and sent to printer control unit 109 of printer B. Printer control unit 109 performs predetermined image processing on the acquired image signal.

[0065] Reader A detects luminance values ​​by reading document G. The luminance values ​​are converted into image density values ​​by image processing unit 108. FIG. 10 is an example of a luminance-density conversion table for converting luminance values ​​into image density values. This luminance-density conversion table LUTid_r converts luminance values ​​into 8-bit image density values. The converted image density values ​​are sent to printer control unit 109.

[0066] 11 is a flowchart showing the process of correcting image density unevenness in the main scanning direction. The control unit 110 performs, for example, a process of correcting the amount of exposure of the exposure device 3 during image formation, a process of forming a pattern image for detecting image density unevenness, a process of detecting image density unevenness, a process of correcting the background of the paper S based on the detection results, and a process of calculating image density correction.

[0067] When the control unit 110 starts the process of correcting image density unevenness in the main scanning direction, it forms a pattern image (main scanning measurement image) for correcting image density unevenness in the main scanning direction on the paper S (S201). FIG. 12 is an example of the main scanning measurement image. The main scanning measurement image is a belt-shaped image having a predetermined width in the sub-scanning direction and extending a predetermined length in the main scanning direction, and is formed based on an image signal that indicates a uniform image density. This image signal is generated by the pattern generator 192. Belt-shaped images (pattern images) of each color (Y, M, C, K) are arranged in the sub-scanning direction. In this embodiment, the pattern image of each color is formed using an image signal that results in an image density of 40%.

[0068] To mitigate the effects of image density unevenness in the sub-scanning direction, the band-shaped images of each color are arranged in two different locations in the sub-scanning direction. In Fig. 12, the band-shaped image on the upstream side in the sub-scanning direction is referred to as the first band-shaped image, and the band-shaped image on the downstream side is referred to as the second band-shaped image. A first background portion is provided upstream of the first band-shaped image in the sub-scanning direction. A second background portion is provided between the first and second band-shaped images. A third background portion is provided downstream of the second band-shaped image in the sub-scanning direction.

[0069] Fig. 13 is an explanatory diagram of the detection position of the main scanning measurement image. In Fig. 13, a strip-shaped image extending in the main scanning direction is divided into 30 equal parts in the main scanning direction, each 30 mm wide, and the main scanning measurement image is detected in units of 1 to 30 from the upstream side in the main scanning direction.

[0070] The user places the paper S on which the main scanning measurement image is formed on the platen glass 102 and issues an instruction to read the main scanning measurement image via the operation unit 20. In response to the instruction, the control unit 110 causes the reader A to read the paper S on which the main scanning measurement image is printed and detects a luminance value as the reading result (S202). The reader A detects the luminance value I of the main scanning measurement image for each segment based on the reading result of the main scanning measurement image. Furthermore, the reader A detects the luminance values ​​Ib of the first to third background portions of the paper S for each segment based on the reading result of the first to third background portions on which the main scanning measurement image is not printed. FIG. 14 is an example diagram of the luminance values ​​Ib of the first to third background portions. As shown in FIG. 14, the luminance value Ib is detected for each segment (each region).

[0071] The control unit 110 generates a luminance value Ib' by two-dimensionally linearly interpolating the luminance values ​​Ibn (n=1 to 30) of each division unit (each region) of the first to third background portions using the image processing unit 108 of reader A (S203). Fig. 15 is a diagram showing an example of the luminance value Ib'. The luminance values ​​of the background portions in the main scanning direction and the sub-scanning direction are calculated by linear interpolation.

[0072] The control unit 110 calculates the average value Ib'ave of the luminance values ​​Ib'n for each position in the sub-scanning direction from the results of two-dimensional linear interpolation of the background portion by the image processing unit 108 of reader A (S204). The control unit 110 calculates the difference (luminance difference ΔIb'n) between the calculated average value Ib'ave and the luminance value Ibn of the background portion of each region by the image processing unit 108 (S205). Equation 3 is the calculation formula for the luminance difference ΔIb'. Luminance difference ΔIb'n=Ib'n-Ibave ... (Equation 3) (n:1~30)

[0073] The control unit 110 causes the image processing unit 108 to correct the luminance value In of each region of the first band-shaped image and the second band-shaped image of each color using the luminance difference ΔIb'n of the region (S206). In this embodiment, the correction is performed using a coefficient a obtained by experimentally obtaining in advance the relationship between the luminance difference when the first to third background portions are detected and the luminance difference when a pattern image of 40% image density of each color is detected. Equation 4 is a calculation formula for the correction. FIG. 16 is a diagram illustrating an example of the coefficient a. The coefficient a is set for each color. Corrected luminance value I'n = luminance value In-a × luminance difference ΔIb'n (Equation 4) (n:1~30)

[0074] The control unit 110 converts the corrected luminance values ​​I'n of each region into image density values ​​using the image processing unit 108 (S207). The image processing unit 108 converts the corrected luminance values ​​I'n into image density values ​​using, for example, the luminance-density conversion table LUTid_r of FIG. 10 or a conversion formula. The control unit 110 acquires 30 image density values ​​of each region from the image processing unit 108 and calculates the average value of the 30 image density values ​​(S208). The control unit 110 calculates the image density difference Δ between the average image density value and each image density value of each region (S209).

[0075] The control unit 110 averages the image density difference Δ calculated from each of the first band image and the second band image. By averaging the image density difference Δ of each region of the two band images separated in the sub-scanning direction, the influence of image density unevenness in the rotation direction (sub-scanning direction) of the rotating member can be suppressed. The control unit 110 calculates a correction value (ΔLPW) corresponding to the average image density difference Δ (S210). The control unit 110 determines the exposure amount for each region based on the calculated correction value (ΔLPW) (S211).

[0076] The effect of suppressing image density unevenness by the image density unevenness correction process described above will now be described. The measurement image used to confirm the effect is the image shown in FIG. 12. The reading position is the position shown in FIG. 13. The effect was verified by detecting the image density of the measurement image printed on paper S by reader A under these conditions.

[0077] FIG. 17 is an example diagram of image density values ​​obtained from a magenta pattern image through verification. The image density values ​​in FIG. 17 are the average values ​​of the image density values ​​of the first and second magenta band images. FIG. 17 illustrates the image density values ​​before image density unevenness correction processing (thin line), the image density values ​​after image density unevenness correction processing (thick line), and the image density values ​​when image density unevenness correction processing is performed without correction using the luminance value of the background portion (dotted line). The maximum image density value, minimum image density value, and differential image density value are shown as indices representing image density unevenness.

[0078] The image density values ​​before the image density unevenness correction process have the following image density unevenness. Maximum image density value Dmax=0.447 Minimum image density value Dmin=0.396 Differential image density value ΔD=0.051

[0079] The image density values ​​after the image density unevenness correction process have the following image density unevenness: The image density values ​​after the image density unevenness correction process have a smaller differential image density value ΔD than the image density values ​​before the image density unevenness correction process. Maximum image density value Dmax=0.427 Minimum image density value Dmin=0.414 Differential image density value ΔD=0.013

[0080] The image density values ​​obtained without correction based on the luminance value of the background portion will have the following image density unevenness: In this case, the degree of correction for the image density values ​​will differ for each region, resulting in a decrease in the effectiveness of the image density unevenness correction process. Maximum image density value Dmax=0.440 Minimum image density value Dmin=0.403 Differential image density value ΔD=0.037

[0081] In this way, the image forming apparatus 100 of the second embodiment can reduce the influence of image density unevenness caused by the image density sensor 69 and accurately detect image density unevenness in the main scanning direction. Therefore, the exposure amount can be determined based on an optimal correction value, and a high-quality image with reduced image density unevenness can be formed. Such high-precision correction of image density unevenness is realized.

[0082] In the above explanation, image density unevenness was detected in the sub-scanning direction in the first embodiment, and in the main scanning direction in the second embodiment, but this embodiment can also be applied to the case where image density unevenness in the main scanning direction and the sub-scanning direction is detected simultaneously. In this case, as in the second embodiment, two-dimensional interpolated data is created based on the detected values ​​of multiple paper background regions, and the detection results of the measurement image are corrected.

[0083] In the first and second embodiments, the image forming apparatus 100 is configured to form a color image, but the processes of the first and second embodiments are also effective in a configuration that forms a monochrome image.

Claims

1. a conveying means for conveying paper; an image forming means for forming an image on the paper conveyed by the conveying means; a reading means for reading the paper on which the measurement image is formed by the image forming means; and a suppression means for suppressing uneven image density in the transport direction of the image to be formed by the image forming means, based on the reading result of the measurement image read by the reading means and the reading result of the reading means reading a plurality of positions in the transport direction of the transport means in an area of ​​the paper on which the measurement image is formed where the measurement image is not formed. Image forming device.

2. the reading means reads the measurement image and the area at a plurality of positions in the transport direction; the suppression unit corrects the reading result of the measurement image at the corresponding position based on the reading result of the region at each of the plurality of positions, and suppresses image density unevenness based on the reading result of the corrected measurement image.

2. The image forming apparatus according to claim 1.

3. the suppression unit corrects the reading result of the measurement image at a position based on a difference between an average value of the reading results of the region at a plurality of positions in the transport direction and the reading result of the region at each position, and suppresses image density unevenness based on the reading result of the corrected measurement image.

3. The image forming apparatus according to claim 2.

4. the reading result of the region and the reading result of the measurement image are both luminance values, the suppression means corrects the luminance value of the measurement image using the luminance value of the region, converts the corrected luminance value of the measurement image into an image density value, and suppresses image density unevenness based on the image density value.

2. The image forming apparatus according to claim 1.

5. the reading result of the region and the reading result of the measurement image are both luminance values, the suppression unit corrects the luminance value of the measurement image at a position based on a difference between an average value of the luminance values ​​of the region at a plurality of positions in the transport direction and the luminance value of the region at each position; the suppression means converts the corrected luminance values ​​of the measurement image at each position into image density values, and suppresses image density unevenness based on a difference between an average value of the image density values ​​at each position and the image density value at each position.

2. The image forming apparatus according to claim 1.

6. the measurement images are a plurality of belt-shaped images extending in the conveying direction and arranged in a direction intersecting the conveying direction, and the region is provided between the belt-shaped images.

4. The image forming apparatus according to claim 3.

7. the image forming means includes a rotating member for forming an image; The conveying direction is the rotation direction of the rotating member. The image forming apparatus according to any one of claims 1 to 6.

8. the suppression means corrects the reading result of the measurement image at the position based on the difference between the reading result of the area at each position in the conveying direction and an average value of the reading result of the area at each position, which is obtained from a result of two-dimensionally linearly interpolating the reading results of the area at a plurality of positions in the direction intersecting the conveying direction, and the reading result of the area at each position, and suppresses image density unevenness based on the reading result of the corrected measurement image.

3. The image forming apparatus according to claim 2.

9. the reading result of the region and the reading result of the measurement image are both luminance values, the suppression means corrects the luminance value of the measurement image at the position based on a difference between an average value of the luminance values ​​of the region at each position in the transport direction, the average value being obtained by two-dimensionally linearly interpolating the luminance values ​​of the region at a plurality of positions in the direction intersecting the transport direction, and the luminance value of the region at each position; the suppression means converts the corrected luminance values ​​of the measurement image at each position into image density values, and suppresses image density unevenness based on the difference between the average image density values ​​at each position and the image density values ​​at each position.

9. The image forming apparatus according to claim 8.

10. the image forming means includes a rotating member for forming an image; The conveying direction is the rotation direction of the rotating member.

10. The image forming apparatus according to claim 8.

11. the measurement images are a plurality of belt-shaped images extending in a direction intersecting the transport direction and arranged in the transport direction, the areas are provided between the belt-shaped images, on the upstream side of the plurality of belt-shaped images in the transport direction, and on the downstream side of the plurality of belt-shaped images in the transport direction.

9. The image forming apparatus according to claim 8.

12. a conveying means for conveying paper; an image forming means for forming an image on the paper conveyed by the conveying means; a reading means for reading the paper on which the measurement image is formed by the image forming means; a suppression unit that suppresses image density unevenness in the transport direction of the image to be formed by the image forming unit, based on the reading result of the measurement image read by the reading unit and the reading result of the reading unit reading a plurality of positions in the transport direction of the transport unit in an area of ​​the paper on which the measurement image is formed where the measurement image is not formed, the suppression means corrects the reading result of the measurement image at the position based on the difference between the reading result of the area at each position and an average value of the reading results of the area at each position in the conveying direction, which is obtained from a result of two-dimensionally linearly interpolating the reading results of the area at a plurality of positions in the conveying direction, and the reading result of the area at each position, and suppresses unevenness in image density in the conveying direction and in a direction intersecting the conveying direction based on the reading result of the corrected measurement image. Image forming device.

Images