Image forming apparatus

Abstract

It suppresses image density unevenness in the sub-scanning direction with high precision. [Solution] The image forming apparatus 100 comprises an image forming unit PY that forms an image, a reader A that reads a measurement image including a pattern image, and a control unit 110 that determines a correction value for correcting periodic image density unevenness in the sub-scanning direction based on the reading result of the measurement image by the reader A. The image forming unit PY has a photosensitive drum 1Y that rotates in the sub-scanning direction and forms a pattern image with a length of two or more cycles of the photosensitive drum 1Y. The control unit 110 determines a correction value based on the absolute value of the image density detected based on the reading result and the direction of change of the image density in the sub-scanning direction.

Description

Technical Field

[0001] The present invention relates to image forming apparatuses such as copiers, multifunction peripherals, and printers.

Background Art

[0002] In recent years, the market for on-demand image forming apparatuses has been expanding. Such image forming apparatuses employ an electrophotographic method that is also spreading to the offset printing market, and an inkjet method that has been successful in expanding into a wide range of markets such as 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 conventional image forming apparatuses that have served this market.

[0003] In an image forming apparatus employing the electrophotographic method, variations in color occur in the color tone that affects the image quality of the output image due to fluctuations in environmental conditions such as temperature and humidity, changes over time of components, and performance degradation due to component durability. A photosensitive drum, which is a drum-shaped photoreceptor having a photosensitive layer on its surface, causes unevenness in the sensitivity of the photosensitive layer, resulting in uneven image density and color in the output image. An exposure device that irradiates the photosensitive drum with laser light causes uneven exposure amount of the laser light and lens aberration of the optical system, resulting in uneven image density and color in the output image. A developing device that develops the electrostatic latent image formed on the photosensitive drum causes uneven development, resulting in uneven image density and color in the output image. A transfer unit that transfers the toner image formed on the photosensitive drum causes uneven transfer, resulting in uneven image density and color in the output image.

[0004] Patent Document 1 discloses a technique for correcting uneven image density in the main scanning direction based on measurement results of a plurality of pattern images arranged in the main scanning direction. Patent Document 2 discloses a technique for correcting uneven image density in the sub-scanning direction that occurs at the rotation cycle of a developing sleeve. The developing sleeve is a member that rotates in accordance with the rotation of the photosensitive drum and attaches toner to the electrostatic latent image. The developing sleeve attaches toner to the electrostatic latent image by applying a development bias voltage. Uneven image density in the sub-scanning direction is caused by the rotation of a rotating member such as the photosensitive drum in addition to the developing sleeve. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2004-163216 [Patent Document 2] Japanese Patent Publication No. 2000-98675 [Overview of the project] [Problems that the invention aims to solve]

[0006] Image density unevenness in the sub-scanning direction is detected, for example, by using a band image extending in the sub-scanning direction as a pattern image and reading the result of this pattern image. In order to accurately grasp image density unevenness in the sub-scanning direction, it is necessary to read the pattern image for at least one cycle of the rotating member that is the cause of the image density unevenness in the sub-scanning direction. Image density unevenness in the sub-scanning direction occurs with a period corresponding to the rotation period of the rotating member. For this reason, depending on the circumference of the photosensitive drum, for example, the number of sheets of paper on which the pattern image is formed may increase.

[0007] Furthermore, sudden image density variations may be detected as image density variations in the sub-scanning direction. In this case, the sudden image density variations are also recognized as periodic image density variations in the sub-scanning direction, and the image density variations are excessively corrected. This leads to a further deterioration of image density variations in the sub-scanning direction. The objective of the present invention is to suppress image density variations in the sub-scanning direction with high accuracy. [Means for solving the problem]

[0008] The present invention provides an image forming apparatus comprising: an image forming means for forming an image; a reading means for reading a measurement image including a pattern image; and a determination means for determining a correction value for correcting periodic image density unevenness in a first direction based on the reading result of the measurement image by the reading means, wherein the image forming means has a rotating member that rotates in the first direction, and forms the pattern image with a length of two or more cycles of the rotating member, and the determination means determines the correction value based on the absolute value of the image density detected based on the reading result and the direction of change of the image density in the first direction. [Effects of the Invention]

[0009] According to the present invention, image density unevenness in the sub-scanning direction can be suppressed with high precision. [Brief explanation of the drawing]

[0010] [Figure 1] A diagram illustrating the configuration of an image forming apparatus. [Figure 2] Diagram illustrating the configuration of the image forming unit. [Figure 3] A flowchart illustrating the image density uniformity correction process in the sub-scanning direction. [Figure 4] (a) and (b) are illustrative diagrams of images used for sub-scanning measurements. [Figure 5] (a) and (b) are explanatory diagrams of the detection position in the sub-scan direction of the sub-scan measurement image. [Figure 6] An example diagram of a brightness density conversion table. [Figure 7] A diagram illustrating the relationship between image density and laser light exposure under different image formation conditions. [Figure 8] An example diagram of the correction coefficient database. [Figure 9] An example diagram of an image used for sub-scanning measurement. [Figure 10] A flowchart illustrating the optimization process of the correction coefficient database. [Figure 11] (a) and (b) are diagrams illustrating the image density profile. [Figure 12] (a) and (b) are illustrative diagrams of the correction coefficient database. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described with reference to the drawings. In this embodiment, an electrophotographic laser beam printer will be used as an example of an image forming apparatus. However, the image forming apparatus is not limited to a laser beam printer; it may be an electrophotographic printer or other type of printer, such as an LED (Light Emitting Diode) printer. In any case, this embodiment is effective as long as the image forming apparatus uses a rotating member for image formation.

[0012] (First Embodiment) Figure 1 is a diagram of the configuration of the image forming apparatus according to this embodiment. The image forming apparatus 100 consists of a reader A, a printer B, and an operation unit 20. Printer B prints an image on paper S. Reader A reads an image from the printed paper (original G). The operation unit 20 is the user interface. The operation unit 20 is equipped with various key buttons and a touch panel as input interfaces. The operation unit 20 is equipped with a display unit 218 as an output interface. The user uses the operation unit 20 to give instructions to start copying and to make various settings.

[0013] (Leader) Reader A comprises a document glass 102 on which the document G is placed, a light source 103 that illuminates the document G placed on the document glass 102, an optical system 104, a light receiving unit 105, and an image processing unit 108. Reader A also comprises a CPU (Central Processing Unit) 214, RAM (Random Access Memory) 215, and ROM (Read Only Memory) 216. The light source 103, optical system 104, and light receiving unit 105 constitute an image reading unit 101 that reads the image of the document G. A positioning member 107 that contacts one side of the document G to prevent the document G from being placed at an angle, and a reference white plate 106 used for shading correction by the image reading unit 101 are arranged on the edge of the document glass 102.

[0014] The optical system 104 forms an image of the reflected light from the original G of the light irradiated from the light source 103 on the reading surface of the light receiving unit 105. The light receiving unit 105 has a plurality of photoelectric conversion elements such as, for example, a CCD (Charge Coupled Device) sensor, and outputs an image signal obtained 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 colors R, G, and B as an image signal. The image reading unit 101 reads an image of the original G placed on the original platen glass 102 line by line while moving in the arrow direction R103.

[0015] 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.

[0016] 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 when the CPU 214 executes processing. The reader A is controlled by the CPU 214 to perform various operations for reading an image of the original G.

[0017] The light receiving unit 105 generates luminance values of each of the colors R, G, and B as an image signal from the reflected light from 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 into image density values, for example, look-up tables (luminance density conversion tables LUTid_r, LUTid_g, LUTid_b, LUTid_k) for converting luminance values into image density values, which will be described later, are used. In the present embodiment, the image processing unit 108 generates density data representing 8-bit image density values.

[0018] (Printer) Printer B comprises image forming units PY, PM, PC, PK, an intermediate transfer belt 6, a secondary transfer roller 64, a fuser 11, a paper feed cassette 65, and a printer control unit 109 for forming images of multiple colors. Printer B is a tandem-type intermediate transfer full-color printer in which image forming units PY, PM, PC, and PK are arranged along the 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 an endless belt-shaped image carrier supported by being stretched across the tension roller 61, the drive roller 62, and the 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 and rotates in the direction of arrow R2 at a predetermined process speed. The images (toner images) formed in the image forming units PY, PM, PC, and PK are sequentially transferred onto the intermediate transfer belt 6 at timings corresponding to the rotation speed of the intermediate transfer belt 6. As a result, a full-color image (toner image) is formed on the intermediate transfer belt 6.

[0020] The opposing roller 63 forms a secondary transfer section 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 section T2 and transferred to the paper S all at once. By applying a positive DC voltage to the secondary transfer roller 64, the negatively charged images (toner images) of each color carried on the intermediate transfer belt 6 are transferred to the paper S all at once. After transfer, the developer remaining on the intermediate transfer belt 6 is removed by the belt cleaner 68. The belt cleaner 68 recovers the remaining toner that has passed through the secondary transfer section T2 and remains on the intermediate transfer belt 6 by rubbing a cleaning blade against the intermediate transfer belt 6.

[0021] The paper sheets S are stored in a paper feed cassette 65 and are fed one sheet at a time. Separation rollers 66 and registration rollers 67 are provided in the transport path through which the paper sheets S are transported. The paper sheets S are fed from the paper feed cassette 65, separated one by one by the separation rollers 66, and transported to the registration rollers 67. The registration rollers 67 accept the paper sheets S in a stationary state and wait, and transport the paper sheets S to the secondary transfer section T2 in accordance with the timing when the image carried on the intermediate transfer belt 6 is transported to the secondary transfer section T2. ​​The registration rollers 67 function as a transport means for transporting the paper sheets S.

[0022] The paper S onto which the image has been transferred is transported to the fuser 11 via the transport belt 10 by the secondary transfer roller 64. The fuser 11 heats and pressurizes the paper S to melt and fix the image to the paper S. The paper S with the image fixed is then discharged outside the printer B.

[0023] On the downstream side of the image forming unit PK in the rotational direction of the intermediate transfer belt 6, an image density sensor 69 is positioned opposite the drive roller 62 across the intermediate transfer belt 6, serving as an image sensor. The image density sensor 69 is used to measure the image density of the unfixed toner image transferred to the intermediate transfer belt 6.

[0024] Image formation by the image forming units PY, PM, PC, and PK will be described below. The image forming units PY, PM, PC, and PK have the same configuration and perform the same operation, except that the developer (in this case, toner) used for development is different in color, and the drum diameters of the photosensitive drums 1Y, 1M, and 1C and the photosensitive drum 1K are different. 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 distinguished, the subscripts Y, M, C, and K are added to the end of the code, and when colors are not distinguished, the subscripts Y, M, C, and K are omitted.

[0025] Figure 2 is an explanatory diagram of the configuration of the image forming unit P. The image forming unit P comprises a photosensitive drum 1, a charger 2, an exposure unit 3, a developer unit 4, a reflected light intensity 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, developer unit 4, reflected light intensity sensor 12, primary transfer roller 7, and drum cleaner 8 are arranged around the photosensitive drum 1.

[0026] The photosensitive drum 1 in this embodiment is a drum-shaped image carrier in which a photosensitive layer with a negative charge polarity is formed on the outer 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) photoreceptor with a reflectivity of approximately 40% for near-infrared light (960 [nm]). The photosensitive drum 1 may also be an amorphous silicon-based photoreceptor or the like with a similar reflectivity.

[0027] The charger 2 in this embodiment is a scorotron charger, which irradiates the photosensitive drum 1 with charged particles resulting from 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 section, and a grid section to which a desired voltage is applied. A predetermined charge bias voltage is applied to the wire of the charger 2 from a charge bias power supply (not shown). A predetermined grid bias voltage is applied to the grid section of the charger 2 from a grid bias power supply (not shown). Although it also depends on the voltage applied to the wire, the photosensitive drum 1 is charged to approximately the voltage applied to the grid section.

[0028] The exposure unit 3 scans the surface of the charged photosensitive drum 1 in the direction of the drum axis by reflecting laser light with a rotating mirror, thereby forming an electrostatic latent image on the surface of the photosensitive drum 1. For this purpose, the direction of the drum axis of the photosensitive drum 1 (the axis direction of the rotation axis) becomes the main scanning direction. The sub-scanning direction intersecting 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 roller 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.

[0029] The developer unit 4 applies a developing bias voltage to deposit toner onto the electrostatic latent image on the photosensitive drum 1, thereby forming an image (toner image) on the photosensitive drum 1. The developer unit 4 includes a developing sleeve 41, a first transport screw 42, and a second transport screw 43 within a developer container 45 for containing the toner. In this embodiment, the developer container 45 contains a two-component developer, which is a mixture of non-magnetic toner and a magnetic carrier. The developer container 45 is divided into two chambers by a partition wall 46, with the first transport screw 42 provided in one chamber and the second transport screw 43 in the other. The partition wall 46 has two openings, allowing toner to flow in and out between the two chambers. The first transport screw 42 and the second transport screw 43 rotate to agitate and mix the developer, circulating it within the developer container 45.

[0030] The developing sleeve 41 is positioned close to the photosensitive drum 1 and rotates in conjunction with the photosensitive drum 1. The developing sleeve 41 carries a developer mixture of toner and carrier. The developer carried in 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 the power supply unit 44. The application of the developing bias voltage to the power supply unit 44 is controlled by the control unit 110 (CPU 111), which will be described later.

[0031] The developer unit 4 is equipped with a toner quantity sensor 14 for measuring the amount of toner in the developer container 45. The toner quantity sensor 14 may be, for example, a permeability sensor that detects the magnetic permeability of the developer. The developer unit 4 is connected to the toner supply container 33 via a supply channel 32. If the toner quantity measured by the toner quantity sensor 14 is less than a predetermined amount, toner is supplied from the toner supply container 33 to the developer container 45 via the supply channel 32.

[0032] The reflected light intensity sensor 12 is an optical sensor having a light-emitting unit 12a and a light-receiving unit 12b, and is used to measure the image density of the toner image formed on the photosensitive drum 1. The reflected light intensity sensor 12 irradiates light from the light-emitting unit 12a onto the toner image on the photosensitive drum 1. The light-receiving unit 12b receives the reflected light from the toner image and outputs an output signal corresponding 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 section 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 polarity toner image supported on the photosensitive drum 1 is transferred to the intermediate transfer belt 6 as it passes through the primary transfer section T1. In this way, the image forming unit P forms a toner image of the corresponding color on 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 its cleaning blade against the photosensitive drum 1 to collect any remaining toner after the transfer to the intermediate transfer belt 6.

[0034] The operation of such an image forming unit P is controlled by a printer control unit 109 and a control unit 110 located within 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 diagram, the CPU 214 of reader A is also connected to the control unit 110.

[0035] The control unit 110 includes a CPU 111, RAM 112, and ROM 113. The CPU 111 controls the operation of the image forming apparatus 100 by executing computer programs stored in ROM 113. RAM 112 is the work memory used by the CPU 111 when it performs processing. The reader A and printer B of the image forming apparatus 100 have their various operations 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] In this embodiment, the exposure unit 3 is a laser scanner having a rotating mirror. The exposure unit 3 determines the exposure amount by the light intensity control unit 190 so that a predetermined image density value is obtained for the laser output signal. In this embodiment, in order to suppress image density unevenness in the sub-scanning direction, the exposure amount can be set in units of approximately 23.59 [mm] in each direction, and the exposure amount setting (LPW) is managed. The exposure unit 3 also outputs laser light according to the pulse width determined by the pulse width modulator 191 based on the drive signal generated using the gradation correction table (LUT) of the γ correction unit 209.

[0037] The laser output signal is determined based on a grayscale correction table held in the gamma correction unit 209. The grayscale correction table shows the relationship between the laser output signal and the image density value 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 exposure unit 3 to form an image with area gradation. To this end, the printer control unit 109 uses a pulse-width modulator 191 to generate and output a laser output signal with 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 indicating high image density, the laser output signal is a wide pulse signal. For an image signal indicating low image density, the laser output signal is a narrow pulse signal. For an image signal indicating intermediate image density, the laser output signal is a pulse signal of intermediate width.

[0039] Furthermore, the printer control unit 109 can acquire image signals not only from the image processing unit 108, but also from a receiving unit (not shown). This receiving unit can acquire, for example, image signals transmitted by facsimile via a telephone line, or image signals transmitted by an external device via a predetermined network. The predetermined network is a data communication network such as a LAN (Local Area Network) or WAN (Wide Area Network). The external device is an information processing device such as a personal computer.

[0040] The laser output signal (laser drive pulse signal) output from the pulse width modulator 191 is supplied to the laser light source (e.g., semiconductor data) of the exposure unit 3. The semiconductor laser outputs laser light for a duration corresponding to the pulse width of the laser output signal. For this reason, the semiconductor laser is driven for a longer time for pixels with high image density and for a shorter 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 differs depending on the image density of the pixel. The exposure unit 3 exposes a longer area in the main scanning direction for pixels with high image density and a shorter area in the main scanning direction for pixels with low image density.

[0041] The pattern generator 192 generates an image signal for a measurement image to be formed in order to correct the image formation conditions. When forming a 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. In this embodiment, the measurement image is, for example, an image for correcting image density unevenness in the sub-scanning direction or a band image for correcting image density.

[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 exposure unit 3, which has a shading function, can correct image density unevenness in the main scanning direction by adjusting the laser light exposure amount (LPW) during one scanning cycle. The light intensity control unit 190 obtains correction values ​​for the exposure amount corresponding to each exposure position (position in the main scanning direction) and the 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 these correction values. The correction values ​​for the exposure amount corresponding to each exposure position are obtained by the image density unevenness correction 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. 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 correction values ​​for the exposure amount 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 these correction values.

[0043] (Image density unevenness correction) In this embodiment, an image density unevenness correction process that 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 processing of the exposure unit 3 during image formation, processing of image signals (density data) acquired from reader A, measurement image formation processing for detecting image density unevenness, and image density correction control processing.

[0044] Figure 3 is a flowchart illustrating the image density unevenness correction process 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 periodically according to the rotation period of these rotating members. In the image density unevenness correction process in the sub-scanning direction, the image formation conditions (in this case, the amount of laser light exposure) are corrected according to the period of these rotating members, thereby correcting the image density unevenness in the sub-scanning direction. Here, we will explain the case of correcting image density unevenness in the sub-scanning direction caused by the photosensitive drum 1.

[0045] When the control unit 110 starts the image density unevenness correction process in the sub-scanning direction, it forms a sub-scanning measurement image on the paper S (S201). Figure 4 is an example of a sub-scanning measurement image. The sub-scanning measurement image is a band image having a predetermined width in the main scanning direction and extending to a predetermined length in the sub-scanning direction, and is formed based on an image signal that shows 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. The sub-scanning measurement image is an image pattern for detecting image density unevenness in the sub-scanning direction. In this embodiment, pattern images of each color are formed by an image signal such that the image density is 40%.

[0046] There are multiple sub-scan measurement images. In this embodiment, there are two types of sub-scan measurement images: the measurement image in Figure 4(a) and the measurement image in Figure 4(b). The arrangement of the chromatic (Y, M, C) pattern images in the main scanning direction differs between the measurement image in Figure 4(a) and the measurement image in Figure 4(b). By using these two types of measurement images, the chromatic pattern images are detected at different positions in the main scanning direction. Therefore, image density unevenness in the sub-scan direction can be corrected with high accuracy at different positions in the main scanning direction. Each chromatic pattern image is formed to have a length of at least two rotation periods of the photosensitive drum 1Y, 1M, and 1C. Therefore, the chromatic pattern images are read for two periods, and image density unevenness in the sub-scan direction is also detected for two periods.

[0047] The yellow, magenta, and cyan pattern images are formed at different positions in the two sub-scan measurement images. The photosensitive drums 1Y, 1M, and 1C have a drum diameter of 40 mm. That is, the circumference of the photosensitive drums 1Y, 1M, and 1C is 125.6 mm. The length of an A3 size sheet of paper in the sub-scan direction is 420 mm. Therefore, on one sheet of paper S, a chromatic pattern image can be formed for more than two cycles of the photosensitive drums 1Y, 1M, and 1C. The control unit 110 controls the formation of the yellow sub-scan measurement image so that the formation position of the yellow (Y) sub-scan measurement image formed on the first sheet of paper is different from the formation position of the yellow (Y) sub-scan measurement image formed on the second sheet of paper. Similarly, for the magenta (M) subscan measurement image and the cyan (C) subscan measurement image, the control unit 110 controls the formation of the magenta and cyan subscan measurement images so that the formation position of the first image and the formation position of the second image are different.

[0048] On the other hand, the formation position of the black pattern image does not change in the two sub-scan measurement images. The photosensitive drum 1K is larger than the photosensitive drums 1Y, 1M, and 1C used for chromatic images, with a drum diameter of 80 mm. In other words, the circumference of the photosensitive drum 1K is 251.2 mm. The length of an A3 size sheet of paper in the sub-scan direction is 420 mm. On one sheet of paper S, the black pattern image is formed for only one cycle of the photosensitive drum 1K. In other words, on one sheet of paper S, only one cycle of image density unevenness in the sub-scan direction is detected. Since image density unevenness in the sub-scan direction needs to be measured for two or more cycles, it is necessary to detect the pattern image at the same position. For this reason, the formation position of the black (K) pattern image does not change in the two sub-scan measurement images.

[0049] The image density unevenness in the sub-scanning direction caused by the black photosensitive drum 1K has a larger drum diameter than the chromatic photosensitive drums 1Y, 1M, and 1C, resulting in a smaller change per unit length and a longer period for the occurrence of image density unevenness in the sub-scanning direction. Therefore, considering the visual sensitivity on the printed image, it is not possible to increase the number of detections in the main scanning direction of the black sub-scanning measurement image. However, the visual quality of the image density unevenness on the corrected image can be expected to improve to the same level as that of the chromatic image. In this embodiment, the measurement image shown in Figure 4(a) is formed on the first sheet of paper S, and the measurement image shown in Figure 4(b) is formed on the second sheet of paper S. Of course, the measurement images shown in Figure 4(a) or Figure 4(b) may be formed on two sheets of paper S.

[0050] The image formation conditions in the sub-scanning direction require relating the position of the sub-scanning measurement image in the main scanning direction with the rotation phase of the rotating member, which is a factor in image density unevenness. In this embodiment, the phase of the image carrier (here, the photosensitive drum 1) is controlled so that the pattern image writing position corresponds to the home position of the rotation phase. This makes it possible to link the phase of one rotation of each color image carrier (here, the photosensitive drum 1) to which position in the image density unevenness corresponds, and to obtain image density unevenness information that represents the image density unevenness corresponding to the phase of the image carrier.

[0051] The user places a sheet of paper S on which a sub-scan measurement image has been formed on the document glass 102 and has the reader A read the sub-scan measurement image. The reader A reads the sub-scan measurement image formed on the sheet of paper S and detects the brightness value representing the unevenness of the image density. Figure 5 is an explanatory diagram of the detection position in the sub-scan direction of the sub-scan measurement image. In both the measurement images in Figure 4(a) and Figure 4(b), the detection position in the sub-scan direction is the same.

[0052] The detection position of the chromatic pattern image (measurement image) is as shown in the example in Figure 5(a). In Figure 5(a), 126 [mm], which corresponds to more than one cycle of the photosensitive drum 1Y, 1M, and 1C, is evenly divided into 10 sections, and the pattern image is detected in units of 1 to 10 at approximately 12.6 [mm] intervals from the upstream side in the transport direction (sub-scanning direction). Three cycles of the chromatic pattern image are detected on one sheet of paper S.

[0053] The detection position of the black pattern image (measurement image) is as shown in the example in Figure 5(b). In Figure 5(b), 251 [mm], which corresponds to more than one rotation of the 1K photosensitive drum, is evenly divided into 10 sections, and the pattern image is detected in units of 1 to 10 at approximately 25.1 [mm] intervals from the upstream side in the transport direction (sub-scanning direction).

[0054] The control unit 110 detects luminance values ​​as a result of reading the paper S on which the sub-scan measurement image formed by the reader A has been created (S202). The detection of luminance values ​​by the reader A is performed at each detection position as described in Figures 5(a) and 5(b). The control unit 110 converts the luminance values ​​of each detection position detected from the sub-scan measurement image into image density values ​​using the image processing unit 108 (S203). The control unit 110 acquires the image density values ​​of each detection position converted by the image processing unit 108.

[0055] Figure 6 is an example of the luminance density conversion table LUTid_r, which converts the luminance value detected by the red (R) photoelectric conversion element of reader A when reading a cyan image to the image density value of cyan. The image processing unit 108 converts the luminance value to the image density value using the luminance density conversion table LUTid_r. Similarly, the luminance value of a magenta image is converted to an image density value using the luminance density conversion table LUTid_g, which converts the luminance value detected by the green (G) photoelectric conversion element. Similarly, the luminance value of a yellow image is converted to an image density value using the luminance density conversion table LUTid_b, which converts the luminance value detected by the blue (B) photoelectric conversion element. The luminance value of a black image is converted to an image density value using the luminance density conversion table LUTid_k, which converts the luminance value detected by the green (G) photoelectric conversion element.

[0056] Furthermore, the image processing unit 108 may convert luminance values ​​to image density values ​​using a mathematical formula that represents the relationship between the luminance density conversion tables LUTid_r, LUTid_g, LUTid_b, and LUTid_k. Alternatively, the conversion from luminance values ​​to image density values ​​using the luminance density conversion tables LUTid_r, LUTid_g, LUTid_b, and LUTid_k may be performed by the control unit 110. In this case, the control unit 110 will obtain luminance values ​​from reader A and perform the conversion.

[0057] The control unit 110 calculates the average value of 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 value of the image density values ​​and the respective image density values ​​of each detection position (regions 1 to 10) (S205). The control unit 110 calculates a correction value (ΔLPW) corresponding to the calculated density difference Δ (S206). The correction value (ΔLPW) corresponding to the density difference Δ is calculated, for example, using a correction coefficient database that is stored in advance and described later.

[0058] The relationship between the density difference Δ and the correction value (ΔLPW) will be explained. Figure 7 is a diagram showing the relationship between image density and laser light exposure (LPW) for each condition during image formation by the image forming apparatus 100. Here, the conditions during image formation are environmental conditions such as temperature and humidity, and the ratio of the printed image to the printed surface of the paper S (image ratio). The first condition is a temperature of 32°C, humidity of 85%, and an image ratio of 30%. The second condition is a temperature of 24°C, humidity of 55%, and an image ratio of 10%. The third condition is a temperature of 20°C, humidity of 10%, and an image ratio of 3%. Figure 7 shows the relationship between image density and exposure when images are repeatedly formed under each condition.

[0059] As shown in Figure 7, the relationship (graph) between image density and exposure differs depending on the conditions. Furthermore, the slope of the relationship (graph) between exposure and image density also changes depending on the absolute value of the image density.

[0060] Conventionally, even when the density difference Δ indicates the direction of change in image density (increase or decrease), the correction value (ΔLPW) is determined solely by the absolute value of the density difference Δ. However, in reality, the slope of image density with respect to exposure is not constant, so it is preferable to adjust the exposure according to the direction of change in image density (positive or negative of the density difference Δ: increase or decrease). Furthermore, the slope of image density with respect to exposure also differs depending on the absolute value of the density difference Δ.

[0061] In this embodiment, a correction coefficient database is generated to derive a correction value (ΔLPW) corresponding to the density difference Δ, based on a basic database showing the relationship between image density and exposure (LPW) under multiple conditions during image formation. The basic database may be affected not only by environmental conditions and image ratio, but also by the capacitance of the photosensitive drum 1 (change from the initial state), the charge amount of the developer, and the number of prints used. If these influences are significant, the influence of these elements must also be reflected in the basic database. The correction coefficient database is generated, for example, by the control unit 110.

[0062] Figure 8 is an example of a correction coefficient database. Image density is divided into categories A (absolute value less than ~0.4), category B (between 0.4 and less than 0.8), category C (between 0.8 and less than 1.2), and category D (1.2 and above). Density difference Δ is divided into categories based on whether the image density is increasing or decreasing, and further divided into large, medium, and small based on the magnitude of the density difference Δ. Specifically, a density difference Δ of 0.05 or more is classified as "large," 0.02 or more but less than 0.05 is classified as "medium," and less than 0.02 is classified as "small." A correction value (ΔLPW) is set according to these categories. The control unit 110 calculates the correction value (ΔLPW) for the density difference Δ by referring to such a correction coefficient database.

[0063] The control unit 110 determines an exposure amount correction value (ΔLPW) to correct image density unevenness in the sub-scanning direction caused by the photosensitive drum 1 by averaging the correction value (ΔLPW) calculated for each cycle of the photosensitive drum 1 over the acquired cycles (S207).

[0064] The above processing is performed for each color pattern image formed at each position in the main scanning direction. Ultimately, exposure correction values ​​for forming each color image corresponding to each position in the main scanning direction are determined. The paper S on which the sub-scan measurement image is formed in processing S201 consists of two sheets: one with the measurement image shown in Figure 4(a) printed on it, and another with the measurement image shown in Figure 4(b) printed on it. For this reason, processing S202 to S207 is performed twice. The chromatic pattern images are formed at different positions in the main scanning direction. For this reason, processing S202 to S207 is performed twice to determine the exposure correction value (ΔLPW) for the sub-scan direction at different positions in the main scanning direction. For the black pattern image, processing S202 to S207 is performed twice using the two sheets S on which the sub-scan measurement image is formed, thereby determining the exposure correction value (ΔLPW) for the sub-scan direction.

[0065] For each sheet of paper S on which a measurement image is formed, a pattern image of each color of the sub-scan measurement image is formed by changing its position in the main scanning direction. By using such measurement images, the cost and time incurred due to paper waste caused by correcting image density unevenness are suppressed, and high-precision image density unevenness correction is achieved.

[0066] (Second Embodiment) In the first embodiment, a technique for correcting image density unevenness by forming a measurement image on paper S was described. In the second embodiment, a technique for correcting image density unevenness based on a measurement image formed on an intermediate transfer belt 6 will be described. The configuration of the image forming apparatus 100 is the same as in the first embodiment, so its description will be omitted.

[0067] The measurement image formed on the intermediate transfer belt 6 is read by the image density sensor 69 to detect the image density. The rotation direction of the intermediate transfer belt 6 is the sub-scanning direction. Figure 9 is an example of a sub-scanning measurement image. The sub-scanning measurement image in Figure 9 is composed of the sub-scanning measurement image (first section) of Figure 4(a) and the sub-scanning measurement image (second section) of Figure 4(b), which are arranged continuously in the sub-scanning direction.

[0068] The differences between the flowchart in Figure 3 and the first embodiment will be explained. In the first embodiment, reader A is used as the sensor (reading unit) for reading the measurement image, but in the second embodiment, image density sensor 69 is used as the sensor (reading unit) for reading the measurement image.

[0069] To this end, in the S202 process, the control unit 110 obtains the brightness value of the sub-scan measurement image from the image density sensor 69. In the S203 process, the control unit 110 converts the obtained brightness value into an image density value using a brightness density conversion table (S203). The brightness density conversion table is the same as the table exemplified in Figure 6. Alternatively, the control unit 110 may convert the brightness value into an image density value using a mathematical formula that represents the relationship in the brightness density conversion table.

[0070] In the configuration of the second embodiment, paper S for printing the measurement image is not required. Therefore, image density unevenness can be corrected without generating paper waste, and costs can be reduced. In addition, since reading the measurement image by the reader A is not required, the user does not need to place the paper S with the printed measurement image on the document glass 102 of the reader A. This reduces working time and allows for efficient correction of image density unevenness. It also reduces the effort required from the user.

[0071] (Third embodiment) In the first embodiment, an optimal correction value corresponding to the density value Δ is determined based on a pre-stored basic database and a correction coefficient database. In the third embodiment, the accuracy of the basic database is improved, and the correction of image density unevenness in the sub-scanning direction is performed with higher accuracy. The accuracy of the basic database is improved by actually measuring the relationship between the laser light exposure amount (LPW) and the density difference Δ according to the usage conditions of the image forming apparatus 100. The configuration of the image forming apparatus 100 is the same as in the first embodiment, so a description is omitted.

[0072] Figure 10 is a flowchart illustrating the optimization process of the correction coefficient database. By updating the basic database, the correction coefficient database is optimized, and the image density uniformity correction in the sub-scanning direction is optimized. This process is performed at the timing of the image density uniformity correction process in the sub-scanning direction, the timing of the power-on of the image forming apparatus 100, and the timing of a predetermined time elapsed since the previous image density correction process in the sub-scanning direction.

[0073] The control unit 110 starts the optimization process at a time that does not affect the user's productivity. The control unit 110 determines whether or not it is necessary to measure the relationship between image density and laser light exposure (S301). The control unit 110 determines that it is necessary to measure the image density and laser light exposure if, for example, any of the following conditions apply. (1) A predetermined amount of time has elapsed since the previous sub-scanning direction image density uniformity correction process (for example, 10 hours or more). (2) The environmental conditions of the image forming apparatus 100 have changed since the previous sub-scanning direction image density uniformity correction process (for example, the ambient temperature has changed by 3°C or more, or the humidity has changed by 15% or more). (3) If the number of images formed on the same photosensitive drum 1 exceeds a predetermined number (for example, 10,000 images) (4) Replacement of photosensitive drum 1 or developer (5) The ratio of non-magnetic toner to magnetic carriers in the developer has changed by more than a predetermined amount (e.g., more than 2%) since the previous sub-scanning direction image density uniformity correction process.

[0074] If actual measurement is required (S301:Y), the control unit 110 measures the image density while changing the laser light exposure amount (LPW) in 10-level increments, and records the relationship between the image density and the laser light exposure amount as measurement data (S302). If actual measurement is not required (S301:N), the control unit 110 acquires data on environmental conditions (temperature, humidity), developer toner, the ratio of non-magnetic toner to magnetic carriers, the number of images formed, and the image ratio without actually measuring the image density and exposure amount (S303). The basic database is optimized by the processing in S302 or S303.

[0075] Subsequently, the control unit 110 updates the correction coefficient database (S304). If the image density and exposure amount have been measured, the control unit 110 updates the correction coefficient database based on the basic database obtained from the measurement results. If the measurements have not been taken, the control unit 110 updates the correction coefficient database based on the data obtained in the processing of S303. The control unit 110 terminates the optimization process by recording the updated correction coefficient database as the latest correction coefficient database. By performing image density uniformity correction in the sub-scanning direction using the updated correction coefficient database, optimal correction of image density uniformity in the sub-scanning direction becomes possible.

[0076] (Fourth Embodiment) In the fourth embodiment, when correcting image density unevenness in the sub-scanning direction, excessive correction of image density unevenness due to suddenly occurring image density unevenness is prevented. The configuration of the image forming apparatus 100 is the same as in the first embodiment, so a description is omitted.

[0077] For example, the image density difference ΔD obtained from the measurement results of the image density of adjacent pixels in the sub-scanning direction is compared between the first cycle and the second cycle and beyond at the same phase of the photosensitive drum 1. In this case, if the difference in image density difference ΔD at the same phase between the first cycle and the second cycle and beyond is greater than a predetermined value (e.g., 0.02 or more), it is determined that there was a sudden change in density difference Δ. The image density at this phase is treated as an abnormal image density profile. However, if the difference in image density difference ΔD between the first cycle and the second cycle and beyond is greater than a predetermined value over almost the entire sub-scanning direction, it is determined that the image density is shifted overall between the first cycle and the second cycle and beyond. In this case, since it is an overall offset rather than a sudden change in image density difference ΔD, the image density over the entire sub-scanning direction is treated as a normal image density profile.

[0078] Figure 11 is an explanatory diagram of such image density profiles. Figure 11(a) is a normal image density profile without sudden changes in image density difference ΔD. Figure 11(b) is an abnormal density profile with sudden changes in image density difference ΔD. According to Figure 11(b), a sudden change in image density difference ΔD occurs at position Z in the subscan direction. Therefore, the image density profile at position Z differs between normal and abnormal cases.

[0079] Figure 12 is an example of a correction coefficient database. Figure 12(a) is a correction coefficient database used when the image density profile is determined to be normal. Figure 12(b) is a correction coefficient database used when the image density profile is determined to be abnormal.

[0080] Image density is divided into four categories: Category A for absolute values ​​less than ~0.4, Category B for values ​​between 0.4 and less than 0.8, Category C for values ​​between 0.8 and less than 1.2, and Category D for values ​​above 1.2. The image density difference ΔD is divided into categories based on whether the image density is increasing or decreasing, and further divided into large, medium, and small based on the magnitude of the image density difference ΔD. Specifically, an image density difference ΔD of 0.05 or more is classified as "large," 0.02 or more but less than 0.05 is classified as "medium," and less than 0.02 is classified as "small." A correction value (ΔLPW) is set according to these categories. The control unit 110 calculates the correction value (ΔLPW) for the image density difference ΔD by referring to such a correction coefficient database.

[0081] In the image density profile, the correction coefficient database in Figure 12(b) is used only for position Z, while the correction coefficient database in Figure 12(a) is used for other positions. For abnormal image density profiles with sudden changes in image density difference ΔD, the correction coefficient database is set to less than 1 / 3 of the normal correction coefficient, or to zero, so that there is almost no correction. In other words, the correction value for abnormal positions (phases) is determined to be smaller than the correction value for normal positions (phases). The correction value set in the correction coefficient database in Figure 12(b) is smaller than the correction value set in the correction coefficient database in Figure 12(a). As described above, it is determined whether the image density unevenness is a periodic unevenness in the sub-scanning direction inherent in the rotating member such as the photosensitive drum 1, or an abnormal unevenness in image density. In the case of an abnormal unevenness in image density, the unevenness in image density is corrected using the correction coefficient database in Figure 12(b). This prevents overcorrection due to abnormal unevenness in image density.

[0082] As described above, each embodiment suppresses the cost and time incurred due to paper waste caused by correcting image density unevenness, and achieves high-precision image density unevenness correction. Furthermore, since high-precision image density unevenness correction can be performed even for sudden image density unevenness, it is possible to provide a correction function that suits the user's needs.

Claims

1. Image forming means for forming an image, A reading means for reading a measurement image including a pattern image, The system comprises a determination means for determining a correction value for correcting periodic image density unevenness in a first direction based on the reading result of the measurement image by the reading means, The image forming means has a rotating member that rotates in the first direction, and forms a pattern image with a length of two or more cycles of the rotating member. The determination means is characterized by determining the correction value based on the absolute value of the image density detected based on the reading result and the direction of change of the image density in the first direction. Image forming apparatus.

2. The reading means reads the pattern image in the first direction for two rotation periods of the rotating member. The determination means is characterized by determining the correction value based on the reading results for two cycles. The image forming apparatus according to claim 1.

3. The determination means is characterized by generating a first database for deriving the correction value in correspondence with the average value of the reading results at each position in the first direction for each cycle and the density difference between the average value and the reading results at each position, and determining the correction value based on the first database. The image forming apparatus according to claim 2.

4. The first database is characterized in that the correction value is set according to the sign and absolute value of the concentration difference. The image forming apparatus according to claim 3.

5. The image forming means forms an image by exposing a photoreceptor to light, The determination means is characterized by generating the first database based on a second database showing the relationship between image density and exposure under multiple conditions during image formation. The image forming apparatus according to claim 3.

6. The conditions for image formation are characterized by environmental conditions and the ratio of the printed image to the printed surface. The image forming apparatus according to claim 5.

7. The determination means is characterized by updating the first database based on the second database obtained from the results of actual measurements of exposure amount and image density by the image forming means. The image forming apparatus according to claim 5.

8. The determination means is characterized by acquiring the conditions during image formation and updating the first database based on the second database which is based on the acquired conditions during image formation. The image forming apparatus according to claim 5.

9. The determination means is characterized by comparing the reading result of the first cycle and the reading result of the second cycle in the same phase, and determining a phase correction value for the difference in the reading results that is greater than or equal to a predetermined value, based on a third database in which a correction value smaller than that of the first database is set. The image forming apparatus according to claim 3.

10. The determination means is characterized by comparing the reading result of the first cycle and the reading result of the second cycle in the same phase, and determining the correction value based on the first database if the difference in the reading results is greater than or equal to a predetermined value across the entire range. The image forming apparatus according to claim 9.

11. The image forming means forms an image on paper, The reading means is characterized by reading the measurement image formed on the paper. The image forming apparatus according to claim 1.

12. The image forming means forms an image on the image carrier. The reading means is characterized by reading the measurement image formed on the image carrier. The image forming apparatus according to claim 1.

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