Image forming apparatus

Abstract

It suppresses image density unevenness in the sub-scanning direction with high precision. [Solution] The image forming apparatus 100 includes a printer B that forms yellow, magenta, cyan, and black images on paper, a reader A that reads a measurement image including pattern images of each color formed on the paper, and a control unit 110 that determines a correction value for correcting image density unevenness in the sub-scanning direction based on the reading result of the measurement image by the reader A. In the measurement image, the width of the pattern image of each color in the main scanning direction becomes wider as the difference in brightness value between adjacent areas increases.

Description

Technical Field

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

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 tone that affect the image quality of the output image occur 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 unevenness 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 unevenness 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 unevenness 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 unevenness in the output image.

[0004] Patent Document 1 discloses a technique for correcting uneven image density in the main scanning direction based on the 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 following 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] Measurement images for detecting image density unevenness are composed of multi-color pattern images to reduce the number of sheets of paper on which the measurement image is printed. However, by arranging multi-color pattern images on a single sheet, brightness differences occur between adjacent pattern images. In addition, margins are necessary to ensure image quality. In this case, brightness differences occur between the margins and the pattern images adjacent to the margins. When an optical sensor is used in the reading device for reading the measurement image, the brightness difference in adjacent areas causes flare, narrowing the effective detection area of ​​image density. This can reduce the accuracy of detecting image density unevenness. The objective of the present invention is to suppress image density unevenness in the sub-scanning direction with high accuracy, even when there is a large brightness difference between adjacent areas. [Means for solving the problem]

[0007] The image forming apparatus of the present invention comprises: an image forming means for forming an image of a first color and an image of a second color different from the first color on a sheet of paper; a reading means for reading a measurement image including a first pattern image of the first color and a second pattern image of the second color formed on the sheet of paper; and a determination means for determining a correction value for correcting image density unevenness in a first direction based on the reading result of the measurement image by the reading means, wherein the width of the first pattern image and the second pattern image in the second direction intersecting the first direction increases as the difference in brightness values ​​between adjacent regions increases. [Effects of the Invention]

[0008] According to the present invention, image density unevenness in the sub-scanning direction can be suppressed with high precision. [Brief explanation of the drawing]

[0009] [Figure 1] Diagram showing the configuration of an image forming apparatus. [Figure 2] Diagram illustrating the configuration of the image forming unit. [Figure 3] An example diagram of an image used for measurement. [Figure 4] An example diagram of an image used for measurement. [Figure 5] (a) and (b) are diagrams illustrating the luminance profile. [Figure 6] (a) and (b) are diagrams illustrating the luminance profile. [Figure 7] A flowchart illustrating the image density uniformity correction process. [Figure 8] Diagram illustrating the detection location. [Figure 9] An example diagram of a brightness density conversion table. [Figure 10] An example diagram of an image used for measurement. [Figure 11] (a) and (b) are illustrative diagrams of image density unevenness. [Figure 12] An example diagram of an image used for measurement. [Figure 13] (a) and (b) are diagrams illustrating the luminance profile. [Modes for carrying out the invention]

[0010] 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.

[0011] (First Embodiment) FIG. 1 is a configuration diagram of the image forming apparatus according to the present 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 a sheet of paper S. The reader A reads an image from a sheet of paper (original document G) on which an image has been printed. The operation unit 20 is a user interface. The operation unit 20 includes various key buttons and a touch panel as an input interface. The operation unit 20 includes a display unit 218 as an output interface. The user gives an instruction to start copying and makes various settings using the operation unit 20.

[0012] (Reader) The reader A includes a document glass 102 on which the original document G is placed, a light source 103 that irradiates light on the original document G placed on the document 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 document G. At the edge of the document glass 102, a positioning member 107 that abuts on one side of the original document G to prevent the original document G from being obliquely arranged and a reference white plate 106 used for shading correction of the image reading unit 101 are arranged.

[0013] 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, the photoelectric conversion elements arranged in three rows corresponding to red (R), green (G), and blue (B). The light receiving unit 105 generates color component signals of each color of R, G, and B as an image signal. The image reading unit 101 reads an image of the original G placed on the document table glass 102 one line at a time while moving in the arrow direction R103. The plurality of photoelectric conversion elements are arranged linearly in a direction intersecting the arrow direction R103. The direction intersecting the arrow direction R103 in which the photoelectric conversion elements are arranged is the main scanning direction during image reading by the reader A, and the arrow direction R103 is the sub-scanning direction.

[0014] 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, color conversion, etc. 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.

[0015] 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.

[0016] The light receiving unit 105 generates luminance values of each color of 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, a lookup table (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, is used. In the present embodiment, the image processing unit 108 generates density data representing 8-bit image density values.

[0017] (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).

[0018] 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.

[0019] 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 (transfer residue toner) remaining on the intermediate transfer belt 6 is removed by the belt cleaner 68. The belt cleaner 68 recovers the transfer residue 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.

[0020] 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.

[0021] 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.

[0022] This section describes image formation by the image forming units PY, PM, PC, and PK. The image forming units PY, PM, PC, and PK have virtually the same configuration and perform the same operation, except that the developer (in this case, toner) used for development is different. In the following description, the subscripts Y, M, C, and K are added to the end of the code when color is distinguished, and the Y, M, C, and K are omitted when color is not distinguished.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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. Furthermore, the main scanning direction and sub-scanning direction of printer B are the same as the main scanning direction and sub-scanning direction of reader A. 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.

[0027] 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. The developer container 45 in this embodiment 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 while circulating it within the developer container 45.

[0028] 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 by the power supply unit 44 is controlled by the control unit 110 (CPU 111), which will be described later.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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 for the measurement image obtained 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 an image for correcting image density.

[0040] (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 in the image density unevenness correction process described later. In this embodiment, correction values ​​for setting the exposure amount are stored in the ROM 113 at intervals of approximately 23.59 [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.

[0041] (Image for measurement) Figure 3 is an example of a measurement image for measuring image density in the sub-scanning direction. In this embodiment, as an example, the measurement image is printed on A3 size (420 [mm] × 297 [mm]) paper S. The main scanning direction and sub-scanning direction (transport direction (movement direction) of the intermediate transfer belt 6) during the formation of the measurement image are as shown by the arrows in Figure 3. Note that the size of the paper S on which the measurement image is formed is not limited to A3 size.

[0042] In this embodiment, the measurement image is composed of pattern images of yellow (Y), magenta (M), cyan (C), and black (K), arranged at predetermined intervals in the main scanning direction. In this embodiment, the pattern image of the measurement image formed on the paper is read by reader A. Reader A acquires information regarding the image density of each pattern image of the measurement image. The image density of each pattern image of the measurement image on the paper may be measured, for example, by an external colorimeter. Alternatively, an optical sensor may be provided downstream of the fuser 11 in the paper transport direction, and the image density of each pattern image of the measurement image on the paper may be detected using the optical sensor.

[0043] Each pattern image of the measurement image is a band-shaped image (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 a 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 adjacent to each other in the main scanning direction. In Figure 3, the pattern images are arranged in the order of yellow (Y), magenta (M), cyan (C), and black (K) in the main scanning direction, but this arrangement is not limited to this.

[0044] Each color pattern image has a different width in the main scanning direction. The width of the pattern image (band image) is determined based on the difference in luminance values ​​(luminance difference) between adjacent areas in the main scanning direction. When the band images arranged in the main scanning direction are read by reader A, the area in which the difference between the luminance value corresponding to the image density indicated by the image signal and the luminance value obtained from the measurement result of the measurement image is within ±2 (effective measurement area) becomes narrower due to the effect of flare. As a result, the detection error of the luminance value of the band image increases, and the accuracy of correcting image density unevenness decreases. Therefore, the measurement image in this embodiment has multiple pattern images (band images) of different widths, thereby suppressing the narrowing of the effective measurement area, reducing the detection error of the luminance value, and suppressing the decrease in the accuracy of correcting image density unevenness. Specifically, the pattern image is configured to have a wider width the greater the luminance difference with adjacent areas. In this embodiment, the threshold for the difference in luminance values ​​for determining the effective measurement area is set to ±2%, but this threshold is not limited to this value.

[0045] Figure 4 is an example of a measurement image when each pattern image has the same width. In other words, the width of the band image is not adjusted by the brightness difference of adjacent areas. Figures 5 and 6 are explanatory diagrams of the brightness profile obtained from the reading results of the measurement image.

[0046] Figure 5(a) illustrates a luminance profile based on the reading results of the measurement image from Figure 4 formed on an A3-sized sheet of paper S. Figure 5(b) illustrates a luminance profile based on the reading results of the measurement image from Figure 3 formed on an A3-sized sheet of paper S.

[0047] Based on the reading results (image density) of the measurement image, the average value of the image density in the sub-scan direction of each region—the white background of paper S, the yellow pattern image, and the magenta pattern image—is calculated. Applying a B (blue) filter to the average value generates a luminance value (detected luminance value). The luminance profiles in Figures 5(a) and (b) are plots of this detected luminance value. Note that the image data luminance value is the luminance value corresponding to the image density indicated by the image signal.

[0048] In Figure 5(a), the difference in brightness values ​​between the white background and the yellow pattern image is large, and the width of the effective detection area in the main scanning direction is narrowed due to the effect of flare. When the brightness values ​​obtained by reading the detection width X [mm] for the yellow pattern image are averaged, the image data brightness value is "80", while the detected brightness value is "105", resulting in a brightness detection error of "25". As a result, the accuracy of image density uniformity correction decreases, leading to overcorrection.

[0049] In Figure 5(b), there is a large difference in luminance values ​​between the white background and the yellow pattern image, and flare is present. However, by increasing the width of the yellow pattern image, the effect of flare is reduced compared to Figure 5(a). In this embodiment, the effect of flare is y [mm]. Therefore, by increasing the width of the yellow pattern image by y [mm] compared to the case in Figure 4, the effective detection area increases by the same amount. By increasing the width of the pattern image by an amount corresponding to the effect of flare, when the luminance values ​​obtained by reading the detection width X [mm] are averaged, the image data luminance value is "80", while the detected luminance value is "85", resulting in a luminance detection error of "5". By suppressing the luminance detection error from "25" to "5", image density uniformity correction can be performed with better accuracy than in the case of Figure 5(a).

[0050] Figure 6(a) illustrates a luminance profile based on the reading results of the measurement image from Figure 4 formed on an A3-sized sheet of paper S. Figure 6(b) illustrates a luminance profile based on the reading results of the measurement image from Figure 3 formed on an A3-sized sheet of paper S.

[0051] Based on the image density reading results (image density) from reader A, the average value of the image density in the sub-scan direction of the yellow, magenta, and black pattern images is calculated in the main scan direction. Applying a G (green) filter to the average value generates a luminance value (detected luminance value). The luminance profiles in Figures 6(a) and (b) are obtained by plotting these detected luminance values.

[0052] In Figure 6(a), there is a large difference in brightness values ​​between the yellow pattern image and the magenta pattern image, and the width of the effective detection area in the main scanning direction is narrowed due to the effect of flare. When the brightness values ​​obtained by reading the detection width X [mm] for the magenta pattern image are averaged, the image data brightness value is "80", while the detected brightness value is "94", resulting in a brightness detection error of "16".

[0053] In Figure 6(b), there is a large difference in brightness values ​​between the yellow pattern image and the magenta pattern image, and there is a flare effect. However, by increasing the width of the magenta pattern image, the flare effect is reduced compared to Figure 6(a). In this embodiment, the flare effect is z [mm]. Therefore, by increasing the width of the magenta pattern image by z [mm] compared to the case in Figure 4, the effective detection area is increased. By increasing the width of the pattern image by an amount corresponding to the flare effect, when the brightness values ​​obtained by reading the detection width X [mm] are averaged, the image data brightness value is "80", while the detected brightness value is "82", resulting in a brightness detection error of "2". By suppressing the brightness detection error from "16" to "2", image density unevenness can be measured with greater accuracy than in the case of Figure 6(a).

[0054] As described above, the area of ​​the pattern image to be measured (the width of the band image) is adjusted according to the difference in brightness values ​​between the area to be measured and the adjacent area. This suppresses detection errors caused by flare in reader A and allows for high-precision correction of image density unevenness in the sub-scanning direction.

[0055] (Image density unevenness correction) Figure 7 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.

[0056] When the control unit 110 starts image density uniformity correction processing in the sub-scanning direction, it forms a measurement image of the image density in the sub-scanning direction on the paper S (S101). The control unit 110 forms the measurement image illustrated in Figure 3 on the paper S. In this embodiment, a pattern image of each color is formed by an image signal such that the image density is 40%.

[0057] The image formation conditions in the sub-scanning direction require relating the position of the pattern image in the main scanning direction of the measurement image 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 writing position of the pattern image 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.

[0058] The user places a sheet of paper S on which a measurement image has been formed on the document glass 102 and has the reader A read the measurement image. The reader A reads the measurement image formed on the sheet of paper S and detects the brightness value representing the unevenness of the image density. Figure 8 is an explanatory diagram of the detection position in the sub-scanning direction of the measurement image. The detection of the pattern image (measurement image) is performed by dividing 300 [mm], which corresponds to more than one cycle of the image carrier (photosensitive drum 1), into 10 equal parts, and dividing them into units of 1 to 10 at intervals of approximately 30 [mm] from the upstream side in the transport direction (sub-scanning direction).

[0059] The control unit 110, using the image processing unit 108, detects the brightness value as a result of reading the paper S on which the measurement image has been formed by the reader A (S102). The brightness value detection by the reader A is performed at each detection position as described in Figure 8. The control unit 110, using the image processing unit 108, converts the brightness value of each detection position detected from the sub-scan measurement image into an image density value (S103). The control unit 110 acquires the image density value of each detection position converted by the image processing unit 108.

[0060] Figure 9 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.

[0061] 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.

[0062] 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 (S104). 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) (S105). The control unit 110 calculates a correction value (ΔLPW) corresponding to the calculated density difference Δ (S106). 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.

[0063] The control unit 110 determines an exposure correction value (ΔLPW) to correct for uneven image density 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 (S107).

[0064] (Second Embodiment) The configuration of the image forming apparatus 100 in the second embodiment is the same as in the first embodiment, so a description will be omitted. The second embodiment differs from the first embodiment in that it uses different measurement images.

[0065] The measurement image of the first embodiment illustrated in Figure 3 has one pattern image (band image) each of yellow, magenta, cyan, and black arranged, and the width of the pattern images differs according to the brightness difference of adjacent areas in order to prevent the effects of flare. In the measurement image of the first embodiment, increasing the width of the bands increases the amount of toner consumed, which increases the running costs.

[0066] Figure 10 is an illustrative diagram of a measurement image of the second embodiment. In the measurement image of the second embodiment, the band image adjacent to the white area is the band image of the color with the smallest difference in brightness value from the white area among the four color band images. Specifically, the pattern images adjacent to the white area of ​​the paper in the main scanning direction are yellow (Y) and cyan (C). The pattern images of the other colors are sandwiched between the yellow (Y) and cyan (C) pattern images. The width of each color pattern image in the main scanning direction is individually set to take into account the effect of flare. However, compared to the measurement image of the first embodiment, the width of the pattern images tends to be narrower. By using such pattern images, it is possible to suppress the increase in toner consumption due to an increase in band width. Therefore, it is possible to suppress the increase in running costs.

[0067] (Third embodiment) The configuration of the image forming apparatus 100 in the third embodiment is the same as in the first embodiment, so a description will be omitted.

[0068] The periodic image density unevenness in the sub-scanning direction of the image formed by the image forming apparatus 100 may have different characteristics depending on the position in the main scanning direction. Figure 11 is an example diagram of the image density unevenness in the sub-scanning direction for each position in the main scanning direction. Figure 11(a) shows the image density unevenness in the main scanning direction and the sub-scanning direction. Figure 11(b) shows the displacement of the brightness value for each sub-scanning direction position for positions F, C, and R in the main scanning direction in Figure 11(a).

[0069] Because the characteristics of image density unevenness in the sub-scanning direction differ depending on the position in the main scanning direction, it is necessary to place multiple pattern images (band images) of the same color at different positions in the main scanning direction. Figure 12 is an example of a measurement image of the third embodiment. In the measurement image of the third embodiment, the color sequence of the pattern image is a repeating pattern of yellow (Y), black (K), cyan (C), and magenta (M) in the main scanning direction. This color sequence is designed to minimize the difference in brightness values ​​between adjacent regions. In the measurement image of the third embodiment, pattern images of each color are repeatedly arranged in the main scanning direction with this color sequence.

[0070] The width of each color pattern image in the main scanning direction is individually determined according to the brightness value of the adjacent area. The filter used to acquire the brightness value is the complementary color filter of each color. This arrangement of pattern images minimizes the difference in brightness values ​​between adjacent areas.

[0071] Figure 13 is an explanatory diagram of the luminance profile obtained from the reading results of the measurement image. Here, we will explain the case where a complementary color R (red) filter is used to obtain the luminance value of a cyan pattern image.

[0072] Figure 13(a) shows the luminance profile when the pattern images are arranged in the order of black, cyan, and magenta. Figure 13(b) shows the luminance profile when the pattern images are arranged in the order of yellow, cyan, and magenta. As shown in Figure 13(a), by arranging the pattern images while considering the difference in luminance values ​​detected from adjacent areas, the effective detection area is widened compared to when the pattern images are arranged without considering the luminance difference, as shown in Figure 13(b). As a result, the effect of flare from reader A is suppressed, and image density uniformity correction can be performed with high accuracy.

[0073] In the example in Figure 12, the pattern image is arranged in the order of yellow, black, cyan, and magenta from one direction (left) in the main scanning direction. Alternatively, the pattern image may be arranged as black (K), cyan (C), magenta (M), and yellow (Y) from one direction (left) in the main scanning direction. The same effect can also be obtained with the order of cyan (C), magenta (M), yellow (Y), and black (K), magenta (M), yellow (Y), black (K), and cyan (C), or yellow (Y), magenta (M), cyan (C), and black (K).

[0074] By using the measurement images described in the first to third embodiments, the effects of flare from an optically reading image reader such as Reader A can be suppressed, enabling accurate measurement of the image density of the measurement images. For example, this makes it possible to perform high-precision correction of image density unevenness with a periodicity in the sub-scanning direction.

Claims

1. Image forming means for forming a first-color image and a second-color image different from the first color on paper, A reading means for reading a measurement image including a first pattern image of the first color and a second pattern image of the second color formed on paper, The system includes a determination means for determining a correction value for correcting image density unevenness in a first direction based on the reading result of the measurement image by the reading means, The measurement image is characterized in that the width of the first pattern image and the second pattern image in the second direction intersecting the first direction increases as the difference in brightness values ​​between adjacent regions increases. Image forming apparatus.

2. The image forming means forms an image on the paper with a third color different from the first and second colors. The measurement image is characterized in that the first pattern image, the second pattern image, and the third pattern image of the third color are arranged adjacent to each other in the second direction, and the pattern image of the color with a small difference in brightness value from the white area of ​​the paper is arranged adjacent to the white area in the second direction. The image forming apparatus according to claim 1.

3. The image forming means forms an image on the paper with a fourth color different from the first, second, and third colors. The first color is yellow, the second color is cyan, the third color is magenta, and the fourth color is black. The measurement image is characterized in that the first pattern image and the second pattern image are arranged adjacent to the white area of ​​the paper in the second direction, and the third pattern image and the fourth pattern image of the fourth color are arranged between the first pattern image and the second pattern image. The image forming apparatus according to claim 2.

4. The image forming means forms an image on the paper with a third color different from the first and second colors. The measurement image is characterized in that the first pattern image, the second pattern image, and the third pattern image of the third color are arranged adjacent to each other in the second direction such that the difference in brightness values ​​between each adjacent region is minimized. The image forming apparatus according to claim 1.

5. The determination means is characterized by detecting the brightness value of the pattern image of the color to be detected using a complementary color filter based on the reading result by the reading means. The image forming apparatus according to claim 4.

6. The image forming means forms an image on the paper with a fourth color different from the first, second, and third colors. The first color is yellow, the second color is cyan, the third color is magenta, and the fourth color is black. The measurement image is in the second direction. The first pattern image is adjacent to the third pattern image and the fourth pattern image of the fourth color. The first pattern image and the second pattern image are adjacent to the fourth pattern image. The second pattern image is adjacent to the fourth pattern image and the third pattern image. The third pattern image is characterized in that the second pattern image and the first pattern image are arranged adjacent to each other. The image forming apparatus according to claim 4.

7. The measurement image is characterized in that the first pattern image, the second pattern image, and the third pattern image are repeatedly arranged in the second direction. The image forming apparatus according to claim 6.

8. The first pattern image and the second pattern image are characterized by being strip-shaped images extending for a predetermined length in the first direction. The image forming apparatus according to claim 1.

9. The first pattern image and the second pattern image are formed based on an image signal exhibiting uniform image density, characterized in that The image forming apparatus according to claim 1.

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