Image forming apparatus

Abstract

To suppress image density unevenness in a sub-scanning direction with high accuracy by suppressing consumption of paper.SOLUTION: The image forming apparatus 100 includes the image forming portion PY configured to form a yellow image, the image forming portion PM configured to form a magenta image, the reader A configured to read a first measurement image and a second measurement image including a yellow pattern image and a magenta pattern image, respectively, and the controller 110 configured to determine a correction value for correcting image density unevenness in the sub-scanning direction based on reading results of the first measurement image and the second measurement image by the reader A. In the first measurement image and the second measurement image, the positions of the yellow pattern image and the magenta pattern image are formed at different positions in the main scanning direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an image forming apparatus such as a copier, a multifunction peripheral, or a printer. [Background technology]

[0002] In recent years, the market for on-demand image forming devices has been expanding. These image forming devices use electrophotographic methods, which are also spreading to the offset printing market, and inkjet methods, which have successfully developed a wide range of markets due to their large format, low initial cost, and ultra-high speed. However, market expansion is not easy, and it is necessary to maintain the image quality (hereinafter referred to as "image quality") of the conventional image forming devices that have dominated that market.

[0003] In image forming devices that use electrophotography, fluctuations in environmental conditions such as temperature and humidity, deterioration of components over time, and performance degradation due to component durability can cause variations in color, which affect the quality of output images. In the case of a photosensitive drum, which is a drum-shaped photosensitive body with a photosensitive layer on its surface, uneven sensitivity of the photosensitive layer can cause uneven image density and color in the output image. In the exposure device that irradiates the photosensitive drum with laser light, edge dropout of the laser light exposure amount and lens aberration in the optical system can cause uneven image density and color in the output image. In the developer that develops the electrostatic latent image formed on the photosensitive drum, uneven development can cause uneven image density and color in the output image. In the transfer unit that transfers the toner image formed on the photosensitive drum, uneven transfer can cause uneven image density and color in the output image.

[0004] Patent Document 1 discloses a technique for correcting image density unevenness in the main scanning direction based on the measurement results of multiple pattern images arranged in the main scanning direction. Patent Document 2 discloses a technique for correcting image density unevenness in the sub-scanning direction that occurs with the rotation period of the developing sleeve. The developing sleeve is a member that rotates following the rotation of the photosensitive drum and adheres toner to the electrostatic latent image. The developing sleeve adheres toner to the electrostatic latent image when a development bias voltage is applied. Image density unevenness in the sub-scanning direction is caused by the rotation of the developing sleeve as well as rotating members such as the photosensitive drum. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-163216 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-98675 Summary of the Invention [Problem to be solved by the invention]

[0006] Correction of image density unevenness in the main scanning direction, as in Patent Document 1, is performed, for example, by controlling the amount of laser light for each irradiation position when forming an electrostatic latent image. In this case, the image forming apparatus forms a measurement image that allows detection of image density unevenness in the main scanning direction, and obtains a correction value for the exposure amount for each position in the main scanning direction based on the detection results of the measurement image. Correction of image density unevenness in the sub-scanning direction, as in Patent Document 2, is performed, for example, by controlling the applied development bias voltage with the rotation period of the developing sleeve. In this case, the image forming apparatus forms a measurement image that allows detection of image density unevenness in the sub-scanning direction, and obtains a correction value for the periodic development bias voltage in the sub-scanning direction based on the detection results of the measurement image. As such, there are multiple measurement images for detecting image density unevenness. Hereinafter, a measurement image that allows detection of image density unevenness in the main scanning direction will be referred to as a "main scanning measurement image," and a measurement image that allows detection of image density unevenness in the sub-scanning direction will be referred to as a "sub-scanning measurement image."

[0007] The sub-scanning measurement image may require printing on a large number of sheets of paper. To accurately detect image density unevenness in the sub-scanning direction, a possible configuration is to detect sub-scanning measurement images for two rotations of the rotating member to determine the period during which image density unevenness in the sub-scanning direction occurs due to the rotating member. For example, when printing the sub-scanning measurement image on A3-sized paper, the image density unevenness in the sub-scanning direction caused by a rotating member with a diameter of approximately 40 mm will be repeatedly formed over four rotations. In contrast, the image density unevenness in the sub-scanning direction caused by a rotating member with a diameter of approximately 80 mm will only be formed over one rotation on an A3-sized sheet of paper, requiring multiple sheets of paper.

[0008] The sub-scanning measurement image differs depending on the position in the main scanning direction. This is because, in the case of image density unevenness due to the shape of the rotating member, the shape of the outer diameter runout differs in the main scanning direction. Furthermore, if the rotating member is a photosensitive drum, the unevenness in the thickness of the photosensitive layer in the sub-scanning direction differs depending on the position in the main scanning direction, so the sub-scanning measurement image differs depending on the position in the main scanning direction. For this reason, image density unevenness in the sub-scanning direction needs to be determined at multiple positions (multiple points) in the main scanning direction.

[0009] When forming a color image, by printing sub-scanning measurement images for each color (yellow (Y), magenta (M), cyan (C), and black (K)) on multiple sheets of paper, it is possible to detect image density unevenness over multiple periods at multiple points in the main scanning direction. However, it is desirable that correction control for image density unevenness be easily performed while reducing the cost and control time due to paper waste.

[0010] An object of the present disclosure is to reduce paper consumption and suppress unevenness in image density in the sub-scanning direction. [Means for solving the problem]

[0011] The image forming apparatus of the present disclosure comprises a first image forming means for forming an image of a first color, a second image forming means for forming an image of a second color different from the first color, a reading means for reading a first measurement image and a second measurement image, each of which includes a first pattern image of the first color and a second pattern image of the second color, and a determination means for determining a correction value for correcting image density unevenness in a first direction based on the reading results of the first measurement image and the second measurement image by the reading means, wherein the first measurement image and the second measurement image are formed at different positions in a second direction that intersects the first direction. [Effects of the Invention]

[0012] According to the present invention, it is possible to reduce paper consumption and suppress unevenness in image density in the sub-scanning direction. [Brief explanation of the drawings]

[0013] [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] 10A and 10B are diagrams illustrating images for sub-scanning measurement. [Figure 5] 10A and 10B are explanatory diagrams of detection positions in the sub-scanning direction of the sub-scanning measurement image. [Figure 6] FIG. 10 is a diagram illustrating an example of a brightness-density conversion table. [Figure 7] 10A and 10B are diagrams illustrating images for sub-scanning measurement. [Figure 8] FIG. 10 is a diagram illustrating an image for sub-scanning measurement. DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of the present invention will be described with reference to the drawings. In this embodiment, an electrophotographic laser beam printer will be described as an example of an image forming apparatus. However, the image forming apparatus is not limited to a laser beam printer, and may be an electrophotographic printer other than a laser beam printer, such as an LED (Light Emitting Diode) printer. In any case, this embodiment is effective for any image forming apparatus that uses a rotating member for image formation.

[0015] (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 the image from paper (document G) on which the image is 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.

[0016] (Leader) The reader A includes a platen glass 102 on which the 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. The 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 101 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 101 are arranged on the edge of the platen glass 102.

[0017] 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 101 reads the image of the document G placed on the document table glass 102 line by line while moving in the direction of arrow R103.

[0018] 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.

[0019] 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.

[0020] The light receiving unit 105 generates luminance values ​​of each color of R, G, and B as an image signal from the light reflected by the original G. The image processing unit 108 converts the luminance values ​​acquired from the light receiving unit 105 into image density values. For the conversion to image density values, for example, a lookup table (luminance-density conversion table) for converting luminance values ​​into image density values, which will be described later, is used. In this embodiment, the image processing unit 108 generates density data representing 8-bit image density values.

[0021] (Printer) Printer B includes image forming units PY, PM, PC, and PK that form images of multiple colors, an intermediate transfer belt 6, a secondary transfer roller 64, a fuser 11, a paper feed cassette 65, and a 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).

[0022] The intermediate transfer belt 6 is an endless belt-like image carrier 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.

[0023] 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 to 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 to the paper S. Any developer remaining on the intermediate transfer belt 6 after transfer is removed by a belt cleaner 68. The belt cleaner 68 collects any 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.

[0024] 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 in 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 unit that transports the sheets S.

[0025] 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 ejected outside the body of the printer B.

[0026] An image density sensor 69 serving as an image sensor is disposed downstream of the image forming unit PK in the rotation direction of the intermediate transfer belt 6, facing the drive roller 62 across the intermediate transfer belt 6. The image density sensor 69 is used to measure the image density of the unfixed toner image transferred to the intermediate transfer belt 6.

[0027] 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 that the color of the developer (here, toner) used for development differs, and the drum diameters of the photosensitive drums 1Y, 1M, and 1C and the photosensitive drum 1K differ. In this embodiment, the drum diameter of the photosensitive drum 1K is larger than the drum diameters of the photosensitive drums 1Y, 1M, and 1C. For example, the drum diameters of the photosensitive drums 1Y, 1M, and 1C are 40 mm, and the drum diameter of the photosensitive drum 1K is 80 mm. 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.

[0028] 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.

[0029] The photosensitive drum 1 of this embodiment is a drum-shaped image carrier in which a photosensitive layer with negative charging polarity is 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.

[0030] 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.

[0031] The exposure device 3 reflects laser light with a rotating mirror to scan the charged surface of the photosensitive drum 1 in the drum axial direction, forming an electrostatic latent image on the surface of the photosensitive drum 1. For this purpose, the drum axial direction (axial direction of the rotation shaft) of the photosensitive drum 1 is the main scanning direction. The sub-scanning direction, which intersects with the main scanning direction, is the rotation direction of the photosensitive drum 1. The sub-scanning direction is also parallel to the transport direction in which the paper S is transported by the registration rollers 67. 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.

[0032] The developing device 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 developing device 4 applies a developing bias voltage to the developing sleeve 41, causing 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 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, one of which is provided with a first conveying screw 42 and the other with a second conveying screw 43. 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] The primary transfer roller 7 presses the inner surface of the intermediate transfer belt 6 toward the photosensitive drum 1, 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 the corresponding color on the photosensitive drum 1, and transfers the formed toner image 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.

[0037] 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.

[0038] 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.

[0039] 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 value is obtained for the laser output signal. In this embodiment, in order to suppress unevenness in image density in the sub-scanning direction, the exposure amount setting (LPW) is managed by allowing the exposure amount to be set in units of approximately 23.59 mm in width in each direction. In addition, the exposure unit 3 outputs laser light in accordance with a 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.

[0040] The laser output signal is determined based on a gradation correction table stored in the gamma correction unit 209. The gradation correction table indicates the relationship between the laser output signal and the image density value of the image to be formed. The gradation correction table determines the laser output signal according to the image density of the image to be formed.

[0041] The printer control unit 109 acquires the image signal generated by the image processing unit 108. The printer control unit 109 pulse-width modulates (PWM) the laser light output from the exposure unit 3 based on the image signal, forming an image with image density gradation using 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 medium-width pulse signal.

[0042] The printer control unit 109 can receive not only the image signal generated by the image processing unit 108 but also an image signal from a receiving unit (not shown). This receiving unit can receive, for example, an image signal transmitted by facsimile over a telephone line or an image signal transmitted by an external device over a predetermined network. The predetermined network is, for example, a data communication network such as a LAN (Local Area Network) or a WAN (Wide Area Network). The external device is, for example, a personal computer.

[0043] 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.

[0044] The pattern generator 192 generates an image signal of a measurement image to be formed in order to correct the image formation conditions. When forming the measurement image, the pulse width modulator 191 generates a laser output signal based on the image signal of the measurement image acquired from the pattern generator 192. The measurement image in this embodiment is, for example, an image for correcting image density unevenness in the sub-scanning direction or a band image for correcting image density.

[0045] (Shading function) In this embodiment, image density unevenness in the sub-scanning direction is corrected using the shading function of the exposure device 3. The exposure device 3 with the shading function can correct image density unevenness in the main scanning direction by adjusting the exposure amount (LPW) of the laser light during one scanning cycle. The light amount control unit 190 obtains exposure amount correction values ​​corresponding to each exposure position (position in the main scanning direction) and phase in the sub-scanning direction from the ROM 113 of the control unit 110, and controls exposure by setting the exposure amount based on the correction values. The exposure amount correction values ​​corresponding to each exposure position are obtained by image density unevenness correction, which will be described later. In this embodiment, correction values ​​for setting the exposure amount are stored in the ROM 113 at intervals of approximately 12.5 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 amount control unit 190 obtains exposure amount correction values ​​corresponding to each exposure position in the main scanning direction from the ROM 113 of the control unit 110, and controls exposure by setting the exposure amount based on the correction values.

[0046] (Image density unevenness correction) The image density unevenness correction process, which suppresses image density unevenness occurring in a predetermined direction (here, the sub-scanning direction), will be described. The control unit 110 performs, for example, exposure amount correction process for the exposure device 3 during image formation, processing of image signals (density data) acquired from the reader A, measurement image formation process for detecting image density unevenness, and image density correction control process.

[0047] 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 is caused by 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 (here, the amount of exposure to laser light) are corrected according to the rotation 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.

[0048] When the control unit 110 starts the process of correcting image density unevenness in the sub-scanning direction, it forms a sub-scanning measurement image on the paper S (S201). FIG. 4 is an example of a sub-scanning measurement image. The sub-scanning measurement image is a band image that has a predetermined width in the main scanning direction and extends 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 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 images of each color are formed using image signals that result in an image density of 40%.

[0049] There are multiple sub-scanning measurement images. In this embodiment, there are two types of sub-scanning measurement images: the measurement image shown in FIG. 4(a) and the measurement image shown in FIG. 4(b). The measurement image shown in FIG. 4(a) and the measurement image shown in FIG. 4(b) differ in the main scanning direction arrangement of the chromatic color (Y, M, C) pattern images. By using these two types of measurement images, the chromatic color pattern images are detected at different positions in the main scanning direction. As a result, image density unevenness in the sub-scanning direction can be corrected with high accuracy at different positions in the main scanning direction. The chromatic color pattern images are each formed with a length equal to or greater than two rotation periods of the photosensitive drums 1Y, 1M, and 1C. As a result, the chromatic color pattern images are read for two periods, and image density unevenness in the sub-scanning direction is also detected for two periods.

[0050] The yellow, magenta, and cyan pattern images are formed at different positions in the two sub-scanning measurement images. The photosensitive drums 1Y, 1M, and 1C each have a drum diameter of 40 mm. That is, the circumferential length of the photosensitive drums 1Y, 1M, and 1C is 125.6 mm. The short-side length of an A3-sized sheet of paper is 297 mm. On one sheet of paper S, chromatic pattern images are formed for two periods on the photosensitive drums 1Y, 1M, and 1C. Therefore, the control unit 110 controls the formation of the yellow sub-scanning measurement image so that the formation position of the yellow (Y) sub-scanning measurement image formed on the first sheet of paper differs from the formation position of the yellow (Y) sub-scanning measurement image formed on the second sheet of paper. The control unit 110 similarly controls the formation of the magenta (M) sub-scanning measurement image and the cyan (C) sub-scanning measurement image so that the formation positions of the first sheet and the second sheet are different.

[0051] On the other hand, the formation position of the black pattern image does not change between the two sub-scanning measurement images. The photosensitive drum 1K has a drum diameter of 80 mm. This means that the circumferential length of the photosensitive drum 1K is 251.2 mm. The length of an A3-sized sheet of paper in the short direction is 297 mm. Therefore, for one sheet of paper S, the black pattern image is formed for only one cycle on the photosensitive drum 1K, and only one cycle of image density unevenness in the sub-scanning direction is detected. Since image density unevenness in the sub-scanning direction must be measured for two or more cycles, the pattern image must be detected at the same position. For this reason, the formation position of the black (K) pattern image does not change between the two sub-scanning measurement images.

[0052] The image density unevenness in the sub-scanning direction caused by the black photosensitive drum 1K has a small amount of change per unit length due to the large drum diameter, and the occurrence period of the image density unevenness in the sub-scanning direction is long. For this reason, when considering the visual sensitivity on the printed image, although it is not possible to increase the number of detections in the main scanning direction of the black sub-scanning measurement image, it is expected that the visual quality of the image density unevenness on the corrected image will improve to the same level as that of a chromatic color image.

[0053] The image formation conditions in the sub-scanning direction require that the position of the sub-scanning measurement image in the main scanning direction be associated with the rotational phase of the rotating member, which is a cause of image density unevenness. In this embodiment, the phase of the image carrier (here, photosensitive drum 1) is controlled so that the pattern image writing position matches the home position of the rotational phase. This makes it possible to obtain image density unevenness information that accurately corresponds to the phase of one rotation of the image carrier (here, photosensitive drum 1) for each color.

[0054] The user places the paper S on which the sub-scanning measurement image is formed on the platen glass 102 and causes the reader A to read the sub-scanning measurement image. The reader A reads the sub-scanning measurement image formed on the paper S and detects a luminance value that represents image density unevenness. FIG. 5 is an explanatory diagram of the detection position in the sub-scanning direction of the sub-scanning measurement image. The detection position in the sub-scanning direction is the same for both measurement images in FIGS. 4(a) and 4(b).

[0055] The detection positions of the chromatic pattern images (measurement images) are as shown in Fig. 5(a). In Fig. 5(a), 126 mm, which corresponds to at least one cycle of the photosensitive drums 1Y, 1M, and 1C, is divided into 10 equal parts, and the pattern images are detected in units of 1 to 10, each approximately 12.6 mm apart from the upstream side in the transport direction (sub-scanning direction). Two cycles of chromatic pattern images are detected on one sheet of paper S.

[0056] The detection position of the black pattern image (measurement image) is the position shown in Fig. 5(b) as an example. In Fig. 5(b), 251 mm, which corresponds to more than one rotation of the photosensitive drum 1K, is divided into 10 equal parts, and the pattern image is detected in units of 1 to 10, each divided approximately every 25.1 mm from the upstream side in the transport direction (sub-scanning direction).

[0057] The control unit 110 reads the paper S on which the sub-scanning measurement image is formed by the reader A, and detects luminance values ​​as the reading results (S202). The detection of luminance values ​​by the reader A is performed at each detection position described in FIGS. 5(a) and 5(b). The control unit 110 converts the luminance values ​​at each detection position detected from the sub-scanning measurement image into image density values ​​by the image processing unit 108 (S203). The control unit 110 acquires the image density values ​​at each detection position converted by the image processing unit 108.

[0058] FIG. 6 is a diagram illustrating an example of a brightness-density conversion table LUTid_r that converts brightness values ​​detected by a red (R) photoelectric conversion element of reader A when a cyan image is read into a cyan image density value. The image processing unit 108 converts brightness values ​​into image density values ​​using the brightness-density conversion table LUTid_r. Similarly, brightness values ​​of a magenta image are converted into image density values ​​using a brightness-density conversion table LUTid_g for converting brightness values ​​detected by a green (G) photoelectric conversion element. Similarly, brightness values ​​of a yellow image are converted into image density values ​​using a brightness-density conversion table LUTid_b for converting brightness values ​​detected by a blue (B) photoelectric conversion element. Note that brightness values ​​of a black image are converted into image density values ​​using a brightness-density conversion table LUTid_k for converting brightness values ​​detected by a green (G) photoelectric conversion element. The image processing unit 108 may also convert brightness values ​​into image density values ​​using a mathematical formula that represents the relationship of the brightness-density conversion table. Note that the conversion from brightness values ​​to image density values ​​using the brightness-density conversion table may be performed by the control unit 110. In this case, the control unit 110 acquires the luminance value from the reader A and performs the conversion process.

[0059] The control unit 110 calculates the average value of the 10 image density values ​​at each detection position for each cycle of the photosensitive drum 1 (S204). The control unit 110 calculates the density difference Δ between the average image density value and the image density value of each detection region (regions 1 to 10) (S205). The control unit 110 calculates a correction value (ΔLPW) corresponding to the calculated density difference Δ (S206). The control unit 110 averages the correction values ​​(ΔLPW) calculated for each cycle of the photosensitive drum 1 over the acquired cycles to determine the exposure correction value (ΔLPW) for correcting image density unevenness in the sub-scanning direction caused by the photosensitive drum 1 (S207).

[0060] The above process is performed for each pattern image of each color formed at each position in the main scanning direction. Ultimately, the exposure correction value for forming an image of each color corresponding to each position in the main scanning direction is determined. The paper S on which the sub-scanning measurement image is formed in the process of S201 is two sheets: one with the measurement image of FIG. 4(a) printed and one with the measurement image of FIG. 4(b) printed. Therefore, the processes of S202 to S207 are performed twice. The chromatic color pattern images are formed at different positions in the main scanning direction. Therefore, by performing the processes of S202 to S207 twice, the exposure correction value (ΔLPW) in the sub-scanning direction is determined at different positions in the main scanning direction. For the black pattern image, the processes of S202 to S207 are performed twice for the two sheets of paper S on which the sub-scanning measurement image is formed, and the exposure correction value (ΔLPW) in the sub-scanning direction is determined.

[0061] In this way, for each sheet S on which a measurement image is formed, a pattern image of each color of the sub-scanning measurement image is formed by changing the position in the main scanning direction. By using such measurement images, the cost and time caused by paper waste resulting from correction of image density unevenness is reduced, and high-precision correction of image density unevenness is achieved.

[0062] (Second embodiment) In the image forming apparatus 100 described in the first embodiment, the diameter of the black photosensitive drum 1K is 80 mm, and it is not possible to form two rounds of black pattern images (band images) on one sheet of paper. Therefore, the position in the main scanning direction of the black pattern image formed on the first sheet of paper is the same as the position in the main scanning direction of the black pattern image formed on the second sheet of paper.

[0063] In the image forming apparatus 100 described in the second embodiment, the diameter of the black photosensitive drum 1K is 40 mm, and two rotations of a black pattern image (band image) can be formed on one sheet of paper. The photosensitive drums 1Y, 1M, and 1C also have a diameter of 40 mm, as in the first embodiment. In this case, the control unit 110 may control the formation of the black pattern image so that the position in the main scanning direction of the black pattern image formed on the first sheet of paper differs from the position in the main scanning direction of the black pattern image formed on the second sheet of paper.

[0064] 7A and 7B are illustrations of sub-scanning measurement images formed in this manner. The positions in the main scanning direction of the pattern images of yellow, magenta, cyan, and black on the first sheet shown in FIG. 7A are different from the positions in the main scanning direction of the pattern images of yellow, magenta, cyan, and black on the second sheet shown in FIG. 7B. The process from sub-scanning measurement image formation to correction value determination in the second embodiment is represented by the flowchart in FIG. 3, which was described in the first embodiment.

[0065] According to this configuration, paper consumption can be reduced and density unevenness in the sub-scanning direction of yellow, magenta, cyan, and black images can be reduced with high precision.

[0066] (Third embodiment) The image forming apparatus 100 described in the first and second embodiments forms sub-scanning measurement images for two cycles of the photosensitive drums 1Y, 1M, 1C, and 1K in order to grasp the occurrence cycle of image density unevenness in the sub-scanning direction. However, a configuration may be adopted in which sub-scanning measurement images for one cycle are formed in order to suppress image density unevenness in the sub-scanning direction.

[0067] Like the image forming apparatus 100 described in the first embodiment, the image forming apparatus 100 described in the third embodiment has yellow, magenta, and cyan photosensitive drums 1Y, 1M, and 1C each having a diameter of 40 mm, and a black photosensitive drum 1K having a diameter of 80 mm. However, to determine the occurrence cycle of image density unevenness in the sub-scanning direction, it is necessary to form sub-scanning measurement images for at least one cycle of the photosensitive drums 1Y, 1M, 1C, and 1K. Therefore, as shown in FIGS. 7(a) and 7(b) exemplified in the second embodiment, it is sufficient that the first sub-scanning measurement images for yellow, magenta, cyan, and black and the second sub-scanning measurement images for yellow, magenta, cyan, and black are formed at different positions.

[0068] This configuration also makes it possible to reduce paper consumption and to accurately reduce image density unevenness in the sub-scanning direction of yellow, magenta, cyan, and black images.

[0069] (Fourth embodiment) In the first embodiment, a technique for correcting image density unevenness by forming a measurement image on paper S is described. In the fourth embodiment, a technique for correcting image density unevenness based on a measurement image formed on intermediate transfer belt 6 is described. The configuration of image forming apparatus 100 is the same as in the first embodiment, so a description thereof will be omitted.

[0070] The measurement image formed on the intermediate transfer belt 6 is read by an image density sensor 69 to detect the image density. The rotation direction of the intermediate transfer belt 6 is the sub-scanning direction. FIG. 8 is an example of a sub-scanning measurement image formed on the intermediate transfer belt 6. The sub-scanning measurement image in FIG. 8 is configured by continuously forming the sub-scanning measurement image (first section) of FIG. 4(a) and the sub-scanning measurement image (second section) of FIG. 4(b) in the sub-scanning direction.

[0071] The following describes the differences from the first embodiment regarding the processing according to the flowchart in Fig. 3. In the first embodiment, a reader A is used as a sensor (reading unit) for reading the measurement image, while in the second embodiment, an image density sensor 69 is used as the sensor (reading unit) for reading the measurement image.

[0072] To this end, in the process of S202, the control unit 110 acquires the luminance value of the sub-scanning measurement image from the image density sensor 69. In the process of S203, the control unit 110 converts the acquired luminance value into an image density value using a luminance-density conversion table (S203). Note that the control unit 110 may convert the luminance value into an image density value using a mathematical expression that represents the relationship of the luminance-density conversion table LUTid_r.

[0073] The configuration of the fourth embodiment does not require paper S for printing a measurement image. This makes it possible to correct image density unevenness without generating paper waste, thereby reducing costs. Furthermore, since it is no longer necessary for the reader A to read the measurement image, the user does not need to place the paper S on which the measurement image is printed on the platen glass 102 of the reader A. This reduces the work time and makes it possible to efficiently correct image density unevenness. Furthermore, it also saves the user effort.

[0074] (Other embodiments) The sensor (reading unit) described in the first to third embodiments is exemplified by a reader A. However, for example, an image sensor provided downstream of the fixing device 11 of the image forming apparatus 100 in the transport direction in which the paper S is transported may be configured to read the measurement image on the paper S. For example, the image sensor may be a CIS that reads the measurement image on the paper S while transporting the paper S. This configuration eliminates the need for a user to place the paper S, on which the measurement image is printed, on the platen glass 102 of the reader A. Therefore, compared to a configuration using a reader A, image density unevenness can be suppressed with less effort.

[0075] The image forming apparatuses described in the first to fourth embodiments adjust the exposure amount (LPW) as an image formation condition for suppressing image density unevenness. However, the image formation condition may be, for example, the charging bias voltage of the charger 2 or the developing bias voltage of the developer 4. Alternatively, to suppress image density unevenness, the control unit 110 (CPU 111) may adjust two or more correction values ​​of the exposure amount, the charging bias voltage, and the developing bias voltage in combination in response to the density difference Δ, thereby suppressing image density unevenness.

Claims

1. a first image forming means for forming an image of a first color; a second image forming means for forming an image in a second color different from the first color; a reading means for reading a first measurement image and a second measurement image, each of which includes a first pattern image in the first color and a second pattern image in the second color; a determination unit that determines a correction value for correcting image density unevenness in the first direction based on the results of reading the first measurement image and the second measurement image by the reading unit, the first measurement image and the second measurement image are formed at different positions in a second direction intersecting the first direction, the first pattern image and the second pattern image being different from each other in a second direction intersecting the first direction. Image forming device.

2. The first pattern image and the second pattern image are band images of uniform image density extending a predetermined length in the first direction.

2. The image forming apparatus according to claim 1.

3. the first pattern image and the second pattern image are arranged at a predetermined interval in the second direction.

3. The image forming apparatus according to claim 2.

4. the first image forming means and the second image forming means each have a rotating member for forming an image, the first pattern image and the second pattern image are formed in the first direction with lengths corresponding to at least two rotation periods of the corresponding rotary members, 3. The image forming apparatus according to claim 2.

5. the reading means reads the first pattern image and the second pattern image in the first direction for two rotation periods of the corresponding rotary members; The determination means determines the correction value based on reading results for two cycles.

5. The image forming apparatus according to claim 4.

6. the reading means detects a first luminance value of the first pattern image from a reading result of the first measurement image; the determining means converts the first luminance value into a first image density value, and determines the correction value at a first position in the second direction of the image of the first color based on the first image density value; the reading means detects a second luminance value of the first pattern image from a reading result of the second measurement image; the determining means converts the second luminance value into a second image density value, and determines the correction value at the second position in the second direction of the image of the first color based on the second image density value.

2. The image forming apparatus according to claim 1.

7. the reading means detects a third luminance value of the second pattern image from a reading result of the first measurement image; the determining means converts the third luminance value into a third image density value, and determines the correction value at a third position in the second direction of the image of the second color based on the third image density value; the reading means detects a fourth luminance value of the second pattern image from a reading result of the second measurement image; the determining means converts the fourth luminance value into a fourth image density value, and determines the correction value at a fourth position in the second direction of the image of the second color based on the fourth image density value.

7. The image forming apparatus according to claim 6.

8. the first image forming means and the second image forming means form the first measurement image on a first sheet and the second measurement image on a second sheet; The reading means reads the first measurement image formed on the first sheet of paper, and reads the second measurement image formed on the second sheet of paper.

2. The image forming apparatus according to claim 1.

9. the first image forming means and the second image forming means form the first measurement image on an image carrier, and form the second measurement image on the image carrier; the reading means reads the first measurement image and the second measurement image formed on the image carrier, 2. The image forming apparatus according to claim 1.

10. the first measurement image and the second measurement image are formed on the image carrier continuously in the first direction.

10. The image forming apparatus according to claim 9.

11. a third image forming means for forming an image in a third color different from the first color and the second color; the first image forming means forms an image on a first image carrier having a drum shape with a first drum diameter; the second image forming means forms an image on a second image carrier having a drum shape with the first drum diameter; the third image forming means forms an image on a third image carrier having a drum shape and a second drum diameter larger than the first drum diameter; the first measurement image and the second measurement image include the first pattern image, the second pattern image, and the third pattern image of the third color, respectively; The first measurement image and the second measurement image are characterized in that the third pattern image is formed at the same position.

2. The image forming apparatus according to claim 1.

12. The first color and the second color are chromatic colors, and the third color is black. The image forming apparatus according to claim 11.

Images