Image forming device

Abstract

To provide an image forming apparatus that can easily adjust the deviation of dots in a main scanning direction.SOLUTION: An image forming apparatus comprises an optical scanner, a developing unit, a control unit, and a storage unit. The storage unit stores an evaluation chart. The evaluation chart has a first evaluation pattern and a second evaluation pattern. The first evaluation pattern has a plurality of first evaluation patches arranged at an equal interval in a main scanning direction and a sub scanning direction. The second evaluation pattern has a plurality of second evaluation patches arranged at an equal interval in the main scanning direction and the sub scanning direction. The first evaluation patch has a first dot row in which dots are continuously and linearly arranged in the main scanning direction, and a second dot row in which dots in a number equal to or less than the number obtained by subtracting the number of dots in the sub scanning direction of the first dot row from the number of light emitting units are continuously and linearly arranged to be adjacent to the first dot row in the sub scanning direction and deviated from the first dot row in the main scanning direction. The second evaluation patch is symmetrical to the first evaluation patch in a parallel direction of the first evaluation pattern and the second evaluation pattern.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an electrophotographic image forming apparatus. [Background technology]

[0002] Conventionally, there has been a demand for faster image output and higher resolution for electrophotographic image forming apparatuses. To meet this demand, an image forming apparatus employing a multi-beam system in which a photosensitive drum is scanned with a light beam from a multi-beam laser having multiple light emitters is known (Patent Document 1).

[0003] Patent Document 1 discloses an image forming device having a main body and a multi-beam laser with multiple light-emitting elements arranged in a line at the tip of the main body. The image forming device draws dots using light beams emitted from each light-emitting element, forming an image using the multiple dots. The spacing between each dot in the main scanning direction of the light beam can be adjusted by changing the timing of emission of the light beam (the timing of lighting each light-emitting element). The spacing between each dot in the sub-scanning direction of the light beam (the direction perpendicular to the main scanning direction) can be adjusted by changing the rotation angle of the main body. The ideal emission timing is pre-stored in a memory unit provided in the image forming device.

[0004] In conventional image forming devices, there is a risk of dots being misaligned in the main scanning direction due to jitter caused by development characteristics or vibrations in the transfer belt or other conveyance system, resulting in uneven density or density variations. In response to this, the image forming device disclosed in Patent Document 1 can detect the above-mentioned dot misalignment by forming multiple predetermined evaluation charts and comparing the density differences between the evaluation charts. The control unit of the image forming device can eliminate the detected dot misalignment by changing the light emission timing of each light-emitting element.

[0005] The evaluation chart is configured by arranging a plurality of evaluation patches, each composed of dots, at predetermined intervals in the main scanning direction and the sub-scanning direction. The evaluation patches are composed of a first dot row and a second dot row, which are linear rows of dots arranged continuously in the main scanning direction. The first dot row and the second dot row each have one dot in the sub-scanning direction. The second dot row is adjacent to the first dot row downstream in the sub-scanning direction and is shifted downstream in the main scanning direction relative to the first dot row.

[0006] If dot misalignment occurs in the main scanning direction, the area where the first dot row and the second dot row overlap changes in the main scanning direction. This causes a change in the image density of the evaluation chart. The light emission timing of each light-emitting element is adjusted according to the amount of this change. The image data of the evaluation chart and the light emission timing correction values ​​according to the magnitude of the dot misalignment are stored in advance in the memory of the control unit. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-137447 Summary of the Invention [Problem to be solved by the invention]

[0008] Incidentally, the first dot row and the second dot row in the image forming apparatus of Patent Document 1 each have one dot in the sub-scanning direction. Therefore, under relatively high resolution conditions, even if dot misalignment occurs in the main scanning direction, the developed amount of dots in the evaluation chart is small. Therefore, even if dot misalignment occurs, the change in image density of the evaluation chart is small, making it difficult to detect the dot misalignment.

[0009] SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that can easily and accurately adjust dot misalignment in the main scanning direction. [Means for solving the problem]

[0010] To achieve the above object, a first aspect of the present invention is an image forming apparatus including an optical scanning device, a developing unit, a control unit, and a storage unit. The optical scanning device includes a light source having three or more light-emitting elements arranged in a line at regular intervals at a predetermined angle relative to the main scanning direction, and a polygon mirror that deflects and scans the light beams emitted from each light-emitting element. The light beams form an electrostatic latent image on an image carrier. The developing unit visualizes the electrostatic latent image to form a toner image. The control unit controls the optical scanning device to switch each light-emitting element on and off to form an electrostatic latent image according to image data. The storage unit stores a predetermined evaluation chart, which is configured in dot units using the light beams of each light-emitting element and is used to determine the write timing of each light-emitting element. The evaluation chart includes a first evaluation pattern and a second evaluation pattern that is parallel to the first evaluation pattern in the main scanning direction or in the sub-scanning direction perpendicular to the main scanning direction. The first evaluation pattern has a first patch row configured by arranging a plurality of first evaluation patches at equal intervals in the sub-scanning direction, the first patch row having a first dot row in which dots in the sub-scanning direction, the number of dots being smaller than the number of light-emitting elements, are arranged consecutively in a straight line in the main scanning direction, and a second dot row in which dots the number of which is equal to or smaller than the number of light-emitting elements minus the number of dots in the first dot row in the sub-scanning direction, are adjacent to the first dot row in the sub-scanning direction and are arranged consecutively in a straight line so as to be offset from the first dot row in the main scanning direction, and the first patch row is arranged at equal intervals in the main scanning direction at a predetermined interval. The second evaluation pattern has a second patch row configured by arranging a plurality of second evaluation patches in which second evaluation patches, symmetrical to the first evaluation patches in the parallel direction of the first evaluation pattern and the second evaluation pattern, are arranged at equal intervals in the sub-scanning direction as the parallel intervals of the plurality of first evaluation patches, and the second evaluation pattern is arranged at equal intervals in the main scanning direction at a predetermined interval. [Effects of the Invention]

[0011] According to the first aspect of the present invention, at least one of the first dot row and the second dot row has two or more dots in the sub-scanning direction. Therefore, when dot misalignment occurs in the main scanning direction, the image density of the evaluation chart changes relatively significantly. This makes it easier to detect changes in image density, and provides an image forming apparatus that can easily and accurately adjust dot misalignment in the main scanning direction. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view showing the internal structure of an image forming apparatus 100 according to a first embodiment of the present invention. [Figure 2] FIG. 1 is a plan cross-sectional view showing a schematic configuration of an optical scanning device 5. [Figure 3] FIG. 1 is a perspective view showing a light source unit 26. [Figure 4] FIG. 1 is a block diagram showing an example of a control path of an image forming apparatus 100 according to a first embodiment. [Figure 5] FIG. 10 is a diagram showing the writing of dots DT1 to DT8 formed on the photosensitive drum 1a when all of the laser diodes LD1 to LD8 are turned on simultaneously and all of the light beams LB1 to LB8 are emitted. [Figure 6] FIG. 6 is a diagram showing a state in which light beams LB1 to LB8 are scanned in the main scanning direction from the state of FIG. 5. [Figure 7] FIG. 10 is a diagram showing an electrostatic latent image in a state where the light emission timing of the laser diodes LD1 to LD8 is adjusted so that the writing positions of the dots DT1 to DT8 are the same in the main scanning direction. [Figure 8] FIG. 1 is a perspective view showing an intermediate transfer belt 8 on which an evaluation chart CT is formed. [Figure 9] An enlarged plan view of the first evaluation pattern PT1 of the evaluation chart CT. [Figure 10] A magnified view of a portion of the first evaluation patch PC1 shown in Fig. 9. [Figure 11] FIG. 9 is an enlarged view of a part of the second evaluation pattern PT2 shown in FIG. 8. [Figure 12] A magnified view of a portion of the second evaluation patch PC2 shown in Figure 11. [Figure 13] FIG. 1 is a plan view showing the first evaluation patch PC1 and the second evaluation patch PC2 when dot misalignment occurs; [Figure 14] FIG. 10 is a plan view showing a modified example of the evaluation chart CT of the image forming apparatus 100 according to the first embodiment. [Figure 15] FIG. 10 is a plan view showing another modified example of the evaluation chart CT of the image forming apparatus 100 according to the first embodiment. [Figure 16] FIG. 10 is an enlarged view of a first evaluation patch PC1 constituting an evaluation chart CT of an image forming apparatus 100 according to a second embodiment. [Figure 17] FIG. 11 is a plan view showing an evaluation chart CT according to the second embodiment in a state where dot misalignment occurs. [Figure 18] FIG. 10 is an enlarged view of a plurality of first patch rows PL1 that constitute a first evaluation pattern PT1 of an image forming apparatus 100 according to a third embodiment. [Figure 19] Graph showing the relationship between the amount of misregistration and the development density difference [Figure 20] FIG. 10 is a perspective view showing a modified example of the evaluation chart CT of the image forming apparatus 100 according to each embodiment. [Figure 21] Graph showing changes in development rate in Examples 1 to 3 of the present invention and Comparative Example 1 [Figure 22] Graph showing changes in the difference values ​​of present inventions 1 to 3 according to the examples and comparative example 1 [Figure 23] Graph showing changes in development rate in Examples 4 to 6 of the present invention and Comparative Example 2 [Figure 24] Graph showing changes in the difference values ​​for Examples 4 to 6 of the present invention and Comparative Example 2 DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic cross-sectional view showing the internal structure of an image forming apparatus 100 according to an embodiment of the present invention. Within the main body of the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (the right side in Fig. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and sequentially form cyan, magenta, yellow, and black images through the respective processes of charging, exposure, development, and transfer.

[0014] Each of the image forming stations Pa through Pd is provided with photosensitive drums 1a, 1b, 1c, and 1d, each carrying a visible image (toner image) of each color. An intermediate transfer belt 8, which rotates clockwise in FIG. 1, is disposed adjacent to each of the image forming stations Pa through Pd. The toner images formed on the photosensitive drums 1a through 1d are sequentially transferred (primary transfer) onto the intermediate transfer belt 8, which moves while contacting the photosensitive drums 1a through 1d, and then superimposed on each other. The toner images primarily transferred onto the intermediate transfer belt 8 are then secondarily transferred onto a sheet of paper S (a recording medium) by a secondary transfer roller 9. The toner images are then fixed by a fixing device 13, and the sheet of paper S onto which the toner images have been secondarily transferred is then ejected from the image forming apparatus 100 main body. An image formation process is performed on each of the photosensitive drums 1a through 1d while the photosensitive drums 1a through 1d are rotated counterclockwise in FIG. 1 by a main motor 40 (see FIG. 4).

[0015] The paper S onto which the toner image is secondarily transferred is stored in a paper cassette 16 located at the bottom of the main body of the image forming apparatus 100, and is transported via a paper feed roller 12a and a pair of registration rollers 12b to the nip between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is usually used. In addition, a blade-shaped belt cleaner 19 is located downstream of the secondary transfer roller 9 to remove toner and other particles remaining on the surface of the intermediate transfer belt 8.

[0016] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photosensitive drums 1a to 1d, there are provided charging devices 2a, 2b, 2c, and 2d that charge the photosensitive drums 1a to 1d, an optical scanning device 5 that exposes image information onto each of the photosensitive drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d, cleaning devices 7a, 7b, 7c, and 7d that remove developer (toner) remaining on the photosensitive drums 1a to 1d, and an image density sensor 50 (density detection mechanism) that can detect the density of the toner image that has been primarily transferred to the intermediate transfer belt 8.

[0017] When image data is input from a host device such as a personal computer, the surfaces of the photosensitive drums 1a-1d are first uniformly charged by the charging devices 2a-2d. Next, the optical scanning device 5 irradiates light according to the image data, forming electrostatic latent images on the photosensitive drums 1a-1d in accordance with the image data. The developing devices 3a-3d are filled with a predetermined amount of two-component developer containing cyan, magenta, yellow, and black toner, respectively. If the toner content in the two-component developer in each developing device 3a-3d falls below a specified value due to the formation of a toner image (described below), toner is replenished from the toner containers 4a-4d to each developing device 3a-3d. The toner in the developer is supplied to the photosensitive drums 1a-1d by the developing devices 3a-3d and electrostatically adheres to them. This results in the formation of a toner image corresponding to the electrostatic latent image formed by the exposure from the optical scanning device 5.

[0018] Then, primary transfer rollers 6a-6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d, and the cyan, magenta, yellow, and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed in a predetermined positional relationship for forming a predetermined full-color image. After that, toner and other substances remaining on the surfaces of the photosensitive drums 1a-1d after the primary transfer are removed by cleaning devices 7a-7d in preparation for the subsequent formation of a new electrostatic latent image.

[0019] The intermediate transfer belt 8 is stretched over a driven roller 10 on the upstream side and a drive roller 11 on the downstream side, and when the intermediate transfer belt 8 starts to rotate clockwise in accordance with the rotation of the drive roller 11 by a belt drive motor 51 (see FIG. 4), the paper S is transported from the pair of registration rollers 12b to a nip portion (secondary transfer nip portion) between the drive roller 11 and a secondary transfer roller 9 provided adjacent thereto at a predetermined timing, and the full-color image on the intermediate transfer belt 8 is secondarily transferred onto the paper S. The paper S onto which the toner image has been secondarily transferred is transported to a fixing device 13.

[0020] The image density sensor 50 is disposed opposite the driven roller 10 with the intermediate transfer belt 8 sandwiched therebetween. The image density sensor 50 is, for example, a specular reflection sensor that detects reflected light. The image density sensor 50 is composed of an LED light source disposed at a predetermined angle with respect to the detection position on the surface of the intermediate transfer belt 8, and a phototransistor or the like as a light-receiving element (not shown). The LED light source irradiates the toner image on the intermediate transfer belt 8 with light, and the phototransistor detects the amount of reflected light to measure the optical density of the toner image (hereinafter simply referred to as "image density"). The image density sensor 50 converts the measurement result into an electrical signal and outputs it to the control unit 90 (described later). The image density sensor 50 may be any sensor that can detect density information of a toner image, and may be, for example, a sensor that can detect density from an image acquired by capturing a toner image.

[0021] The paper S conveyed to the fixing device 13 is heated and pressurized by a fixing belt 21 (first fixing member) and a pressure roller 22 (second fixing member), and the toner image is fixed to the surface of the paper S, forming a predetermined full-color image. The paper S on which the full-color image has been formed has its conveying direction diverted by a branching section 30 that branches in multiple directions, and is discharged directly (or after being sent to a double-sided conveying path 18 and having images formed on both sides) onto a discharge tray 17 by a pair of discharge rollers 15.

[0022] Next, the optical scanning device 5 according to the first embodiment of the present invention will be described in detail with reference to Figs. 2 and 3. Fig. 2 is a plan sectional view showing a schematic configuration of the optical scanning device 5. Fig. 3 is a perspective view showing the light source unit 26. Note that the optical scanning device 5 performs optical scanning on each of the photosensitive drums 1a to 1d, but only the optical scanning on the photosensitive drum 1a will be described here, and other descriptions will be omitted.

[0023] As shown in FIG. 2, the optical scanning device 5 includes a housing 39, a light source unit 26 housed in the housing 39, a collimator lens 41, a cylindrical lens 42, a polygon mirror 45, and a scanning lens 49.

[0024] The light source unit 26 (light source) has a tip surface 27, laser diodes LD1 to LD8 (light emitters), and a beam generating unit 20. As shown in Fig. 3, the tip surface 27 in the longitudinal direction of the light source unit 26 is a circular flat surface. The light source unit 26 is fixed by adjusting the spacing between the laser diodes LD1 to LD8 in the sub-scanning direction by rotating in the circumferential direction about an axis (central axis L1) that is normal to the tip surface 27 and passes through the center of the tip surface 27.

[0025] The laser diodes LD1 to LD8 are arranged linearly at equal intervals along the radial direction of the light source unit 26. The beam generating unit 20 generates light beams LB (hereinafter also referred to individually as light beams LB1 to LB8) to be separately emitted from the laser diodes LD1 to LD8 based on image information transmitted from a control unit 90, which will be described later.

[0026] When the light source unit 26 is rotated to adjust the spacing between the laser diodes LD1 to LD8 in the sub-scanning direction, the spacing between the laser diodes LD1 to LD8 in the main scanning direction changes. When the laser diodes LD1 to LD8 are aligned in a line parallel to the sub-scanning direction (the up-down direction in the figure), the spacing between the laser diodes LD1 to LD8 in the main scanning direction is minimum. Conversely, when the laser diodes LD1 to LD8 are aligned in a line parallel to the main scanning direction (the left-right direction in the figure), the spacing between the laser diodes LD1 to LD8 in the main scanning direction is maximum (see the dashed line portions in FIG. 3 for both).

[0027] 2, the collimator lens 41 converts the light beam LB emitted from the light source unit 26 into a substantially parallel light beam (parallel light beam). The cylindrical lens 42 has a predetermined refractive power only in the sub-scanning direction of the light beam LB. The light source unit 26, the collimator lens 41, and the cylindrical lens 42 are arranged in a straight line.

[0028] The polygon mirror 45 is a regular polygonal prism (here, a regular hexagonal prism) with deflection surfaces 63 formed on each side. Each deflection surface 63 is a mirror surface and is capable of reflecting and deflecting the light beam LB emitted from the light source unit 26. The polygon mirror 45 is supported so as to be rotatable about a central axis (not shown) extending in the vertical direction (the direction of the paper in FIG. 2). The polygon mirror 45 is connected to a polygon motor (not shown) and rotates by the rotary driving force of the polygon motor.

[0029] The scanning lens 49 is a lens having fθ characteristics. The scanning lens 49 is disposed between the photosensitive drum 1a and the polygon mirror 45. The light beam LB emitted from the light source unit 26 is incident on the collimator lens 41 and then the cylindrical lens 42, and is focused as a line image on the deflection surface 63. The light beam LB focused on the deflection surface 63 is deflected and passes through the scanning lens 49, and is focused on the photosensitive drum 1a with a spot diameter of a predetermined size.

[0030] The polygon mirror 45 is rotated at a constant speed in the clockwise direction by a polygon motor. Therefore, the light beam LB scans the scanned surface of the photosensitive drum 1a at a constant speed in the main scanning direction (the direction of the arrow X' in the figure). As a result, a scanning line SL extending linearly in the main scanning direction is formed on the scanned surface of the photosensitive drum 1a. If the light source unit has one laser diode, one scanning line SL is drawn per deflection surface 63. If the light source unit has multiple laser diodes, multiple scanning lines SL are drawn per deflection surface 63. As the polygon mirror 45 rotates, the light beam LB is sequentially focused on adjacent deflection surfaces 63. As the photosensitive drum 1a rotates, multiple scanning lines SL are formed in the sub-scanning direction, forming an electrostatic latent image.

[0031] 4 is a block diagram showing an example of a control path of the image forming apparatus 100 of this embodiment. Note that, since various controls are performed on each unit of the image forming apparatus 100 when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complex. Therefore, the following description will focus on the parts of the control path that are necessary for implementing the present invention.

[0032] The control unit 90 includes a CPU (Central Processing Unit) 91, a ROM (Read Only Memory) 92 (storage unit), a RAM (Random Access Memory) 93, a temporary storage unit 94, a counter 95, an I / F (Interface) 96, and a color shift correction unit 97. The CPU 91 functions as a central processing unit. The ROM 92 is a read-only storage unit. The RAM 93 is a readable and writable storage unit. The temporary storage unit 94 temporarily stores image data and the like. The counter 95 accumulates and counts the number of printed pages. The I / F 96 transmits control signals to each device in the image forming apparatus 100 and receives input signals from the operation unit 80. Multiple I / Fs 96 (two in this example) are provided. The color shift correction unit 97 corrects the color shift of the output image by correcting the shift of the electrostatic latent image drawn on the photosensitive drum 1a. The control unit 90 can be located anywhere inside the main body of the image forming apparatus 100.

[0033] The ROM 92 stores data such as a control program for the image forming apparatus 100, numerical values ​​necessary for control, and other data that will not be changed while the image forming apparatus 100 is in use. The ROM 92 stores an evaluation chart CT (image data used for calibration) for performing color misregistration correction (calibration). The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for controlling the image forming apparatus 100, and the like. The RAM 93 (or ROM 92) also stores a density correction table used for color misregistration correction, and the like.

[0034] Furthermore, the control unit 90 transmits control signals from the CPU 91 to each part and device in the image forming apparatus 100 via the I / F 96. Furthermore, signals indicating the state of each part and device and input signals are transmitted from each part and device to the CPU 91 via the I / F 96. Examples of each part and device controlled by the control unit 90 include image forming units Pa to Pd, optical scanning device 5, primary transfer rollers 6a to 6d, secondary transfer roller 9, main motor 40, image density sensor 50, belt drive motor 51, transfer roller drive motor 64, image input unit 70, voltage control circuit 71, and operation unit 80.

[0035] The image density sensor 50 emits measurement light from the light-emitting element toward the evaluation chart CT formed on the intermediate transfer belt 8, and measures the luminous intensity, etc. of the measurement light (including light reflected by the toner and light reflected by the belt surface) that is reflected and enters the light-receiving element.

[0036] The light reflected from the toner and belt surface includes specularly reflected light and diffusely reflected light. This specularly reflected light and diffusely reflected light are separated by a polarizing separation prism and then incident on separate light receiving elements. Each light receiving element photoelectrically converts the received specularly reflected light and diffusely reflected light and outputs an output signal to the control unit 90 (color misregistration correction unit 97).

[0037] The color shift correction unit 97 determines the image density (toner amount) and image position of the evaluation chart CT from the detection results of the image density sensor 50 (characteristic changes in the output signals of specularly reflected light and diffusely reflected light).The color shift correction unit 97 compares this determination result with a reference density and reference position pre-stored in ROM 92 and adjusts the characteristic values ​​of the development voltage, the light emission timing of the laser diodes LD1 to LD8, etc., to correct the positions of the dots DT1 to DT8 (see FIG. 5) drawn by the light beams LB1 to LB8, thereby performing density correction and color shift correction for each color.Hereinafter, the dots drawn by the light beams LB1 to LB8 will be referred to as "dots DT1 to DT8."

[0038] Furthermore, the color misregistration correction unit 97 determines whether misregistration (dot misregistration) has occurred in the dots DT1 to DT8 based on the determination result of the image density of the evaluation chart CT. If dot misregistration has occurred, a dot misregistration correction value is calculated based on the determination result of the image density of the evaluation chart CT. The control unit 90 adjusts the light emission timing of the laser diodes LD1 to LD8 (the amount of shift in the emission timing of the light beams LB1 to LB8) based on this dot misregistration correction value. This adjusts the positions of the dots DT1 to DT8 in the main scanning direction, thereby correcting the dot misregistration.

[0039] The image input unit 70 is a receiving unit that receives image data transmitted from a host device such as a personal computer to the image forming apparatus 100. The image signal input from the image input unit 70 is converted into a digital signal and then sent to the temporary storage unit 94.

[0040] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 that indicate various states. The user operates the stop / clear button on the operation unit 80 to stop image formation, and operates the reset button to reset various settings of the image forming apparatus 100 to their default states. The liquid crystal display unit 81 indicates the status of the image forming apparatus 100, as well as the image formation status and the number of copies to be printed. Various settings of the image forming apparatus 100 are made using a printer driver on a personal computer.

[0041] Fig. 5 is a diagram showing the start of writing dots DT1 to DT8 formed on the photosensitive drum 1a when all of the laser diodes LD1 to LD8 are turned on simultaneously and all of the light beams LB1 to LB8 are emitted. Fig. 6 is a diagram showing the state after scanning with the light beams LB1 to LB8 in the main scanning direction from the state shown in Fig. 5. Fig. 7 is a diagram showing the electrostatic latent image in a state where the light emission timing of the laser diodes LD1 to LD8 is adjusted so that the writing positions of the dots DT1 to DT8 are the same in the main scanning direction.

[0042] The light beams LB1 to LB8 draw dots DT1 to DT8 on the scanned surface of the photosensitive drum 1a (see FIG. 5). When the light source unit 26, which is rotated at a predetermined angle, causes all of the laser diodes LD1 to LD8 to emit light at the same timing to emit the light beams LB1 to LB8, a straight line (a row of dots DT1 to DT8) inclined with respect to the sub-scanning direction is drawn on the scanned surface, as shown in FIG. 5. In this state, when the light beams LB1 to LB8 are scanned in the main scanning direction on the scanned surface of the photosensitive drum 1a by the rotation of the polygon mirror 45, an electrostatic latent image is drawn with the writing start portion inclined obliquely in the sub-scanning direction, as shown in FIG.

[0043] In order to align the writing start positions of the electrostatic latent images in the main scanning direction, the color misregistration correction unit 97 controls the timing of light emission of the laser diodes LD1 to LD8. For example, in order to draw an electrostatic latent image such that the writing start positions of the light beams LB1 to LB8 are the same in the main scanning direction, the timing of light emission of the laser diodes LD1 to LD8 is set in the order of laser diodes LD8, LD7, LD6, LD5, LD4, LD3, LD2, LD1, and the light beams LB1 to LB8 are emitted so that writing starts in order from the dot DT1 to DT8 that is shifted furthest downstream in the main scanning direction.

[0044] Conversely, when the writing ends in the main scanning direction are to be aligned, the timing of turning off the laser diodes LD1 to LD8 is set in the same order as above (here, the order of laser diodes LD8, LD7, LD6, LD5, LD4, LD3, LD2, LD1) (not shown). The color misregistration correction unit 97 calculates the on / off timing of the laser diodes LD1 to LD8 from the image density of the evaluation chart CT detected by the image density sensor 50, and outputs an output signal to the light source unit 26.

[0045] FIG. 8 is a diagram showing the intermediate transfer belt 8 on which the evaluation chart CT is formed. FIG. 9 is an enlarged plan view of the first evaluation pattern PT1 of the evaluation chart CT. In the enlarged views of the evaluation chart CT, including FIG. 9, the left-right direction on the paper (the direction of the arrow XX' in the figure) is the main scanning direction, and the up-down direction on the paper (the direction of the arrow YY' in the figure) is the sub-scanning direction. As shown in FIG. 8, the evaluation chart CT is visualized by the developing devices 3a to 3d and formed on the intermediate transfer belt 8. The evaluation chart CT is drawn in magenta, cyan, yellow, and black, respectively.

[0046] The evaluation chart CT is drawn in multiple lines at predetermined intervals in the circumferential direction (sub-scanning direction (direction of arrow YY' in Figure 8)) of the intermediate transfer belt 8. The evaluation chart CT is arranged in a position overlapping the image density sensor 50 in the width direction (main scanning direction (direction of arrow XX' in Figure 8)) of the intermediate transfer belt 8. As the intermediate transfer belt 8 rotates, the image density sensor 50 can measure the evaluation chart CT multiple times, and the deviation of dots DT1 to DT8 (dot deviation) is calculated from the average value of the multiple measurement results, thereby reducing detection unevenness.

[0047] The evaluation chart CT is composed of a first evaluation pattern PT1 and a second evaluation pattern PT2, which are depicted in a rectangular shape. The first evaluation pattern PT1 and the second evaluation pattern PT2 are arranged adjacent to each other in the main scanning direction.

[0048] As shown in Fig. 9, the first evaluation pattern PT1 is composed of a plurality of first evaluation patches PC1. The first evaluation patches PC1 are arranged at predetermined intervals in the main scanning direction (the direction of the arrow XX' in the figure) and the sub-scanning direction (the direction of the arrow YY' in the figure). A plurality of first evaluation patches PC1 are arranged at predetermined intervals in the sub-scanning direction to form a first patch row PL1. A plurality of first patch rows PL1 are arranged at predetermined intervals in the main scanning direction to form the first evaluation pattern PT1.

[0049] Fig. 10 is an enlarged view of a portion of the first evaluation patch PC1 shown in Fig. 9. The first evaluation patch PC1 is drawn with light beams LB1 to LB4 emitted by laser diodes LD1 to LD4, each of which is turned on or off for one dot at a time. The first evaluation patch PC1 is made up of a first dot row DL1 and a second dot row DL2. The first dot row DL1 and the second dot row DL2 are rows of dots DT1 to DT4 drawn consecutively in a straight line, each having a length of two dots in the sub-scanning direction and four dots in the main scanning direction.

[0050] The first dot row DL1 is drawn by light beams LB1 and LB2 emitted from laser diodes LD1 and LD2 (see FIG. 3). That is, the first dot row DL1 is made up of a plurality of dots DT1 and DT2 arranged consecutively in a straight line in the main scanning direction. The dots DT1 and DT2 start writing at the same position in the main scanning direction.

[0051] The second dot row DL2 is drawn by light beams LB3 and LB4 emitted from laser diodes LD3 and LD4 (see FIG. 3). That is, the second dot row DL2 is made up of multiple dots DT3 and DT4 arranged consecutively in a straight line in the main scanning direction. The dots DT3 and DT4 start writing at the same position in the main scanning direction.

[0052] The second dot row DL2 is adjacent to and below the first dot row DL1 in the sub-scanning direction. The second dot row DL2 is shifted by a predetermined number of dots (here, two dots) downstream in the main scanning direction from the first dot row DL1.

[0053] 11 is an enlarged view of a portion of the second evaluation pattern PT2 shown in FIG. 8. The second evaluation pattern PT2 is composed of a plurality of second evaluation patches PC2. Each second evaluation patch PC2 is arranged at a predetermined interval in the main scanning direction and the sub-scanning direction. A plurality of second evaluation patches PC2 are arranged at a predetermined interval in the sub-scanning direction to form a second patch row PL2. A plurality of second patch rows PL2 are arranged at a predetermined interval in the main scanning direction to form the second evaluation pattern PT2.

[0054] Fig. 12 is an enlarged view of a portion of the second evaluation patch PC2 shown in Fig. 11. Similar to the first evaluation patch PC1, the second evaluation patch PC2 is drawn by light beams LB1 to LB4 emitted from laser diodes LD1 to LD4 by repeatedly turning on and off one dot at a time.

[0055] 12, the second evaluation patch PC2 is composed of a third dot row DL3 and a fourth dot row DL4. The third dot row DL3 and the fourth dot row DL4 are rows of dots drawn consecutively in a straight line, with a length of two dots in the sub-scanning direction and four dots in the main scanning direction. The arrangement of the dots DT1 to DT4 in the second evaluation patch PC2 is symmetrical to that of the first evaluation patch PC1 in the main scanning direction, so a description thereof will be omitted.

[0056] 13, if dot misalignment occurs downstream in the main scanning direction (to the right in the figure) in the light beams LB2-LB8, the dots DT2-DT4 of the first evaluation patch PC1 and the second evaluation patch PC2 will be misaligned downstream in the main scanning direction. As a result, the amount of dot misalignment of the second dot array DL2 relative to the first dot array DL1 will increase, while the amount of dot misalignment of the fourth dot array DL4 relative to the third dot array DL3 will decrease. Therefore, the first evaluation patch PC1 will be deformed as if it were expanded in the main scanning direction, and the second evaluation patch PC2 will be deformed as if it were contracted in the main scanning direction. In other words, the area of ​​the overlapping portion between the first dot array DL1 and the second dot array DL2 will decrease in the main scanning direction, while the area of ​​the overlapping portion between the third dot array DL3 and the fourth dot array DL4 will increase.

[0057] Therefore, in this case, the first evaluation patch PC1 and the second evaluation patch PC2 have asymmetric shapes with respect to the main scanning direction, resulting in a density difference between the image density of the first evaluation patch PC1 (the ratio of the total area of ​​the dots DT1 to DT4 drawn within a rectangular area enclosed by lines overlapping both ends of the first evaluation patch PC1 in the main scanning direction and lines overlapping both ends of the first evaluation patch PC1 in the sub-scanning direction to the total area of ​​the dots DT1 to DT4 drawn within this area) and the image density of the second evaluation patch PC2. This results in a difference in image density between the evaluation patterns PT1 and PT2. In the example shown in FIG. 13, the first evaluation patch PC1 has a lower density than the second evaluation patch PC2. By detecting this density difference using the image density sensor 50, the control unit 90 can detect that dot misalignment has occurred in the main scanning direction among the dots DT1 to DT8. Furthermore, the user can visually confirm the density difference due to the loss of symmetry between the first evaluation pattern PT1 and the second evaluation pattern PT2, thereby confirming that dot misalignment has occurred among the dots DT1 to DT8.

[0058] When dot misalignment occurs, the amount of misalignment of each of the dots DT2 to DT8 increases in the order of dots DT2 to DT8 (in the order of increasing distance from dot DT1). In other words, the amount of dot misalignment of dot DT8 is the largest compared to the other dots DT2 to DT7. This is because the amount of dot misalignment of each of the dots DT2 to DT7 is accumulated in sequence.

[0059] The first evaluation patch PC1 and the second evaluation patch PC2 of this embodiment can be, for example, a patch in which the first dot row DL1 and the third dot row DL3 are a single dot row, as shown in Fig. 14. In this case, for example, the first dot row DL1 and the third dot row DL3 can be drawn by the laser diode LD1, and the second dot row DL2 and the fourth dot row DL4 can be drawn by the laser diodes LD2 and LD3.

[0060] 15, a dot array having three dots in the sub-scanning direction can be used, with the first dot array DL1, second dot array DL2, third dot array DL3, and fourth dot array DL4. In this case, the first dot array DL1 and the third dot array DL3 can be drawn by laser diodes LD1 to LD3, and the second dot array DL2 and the fourth dot array DL4 can be drawn by laser diodes LD4 to LD6. The first dot array DL1 and the third dot array DL3 can be drawn by laser diodes LD4 to LD6, and the second dot array DL2 and the fourth dot array DL4 can be drawn by laser diodes LD1 to LD3.

[0061] Next, an image forming apparatus 100 according to a second embodiment will be described. Fig. 16 is an enlarged view of a first evaluation patch PC1 constituting an evaluation chart CT according to the second embodiment. Note that, below, differences from the first embodiment will be described, and the same components as those in the first embodiment will be assigned the same reference numerals and will not be described again.

[0062] The image forming apparatus 100 according to the second embodiment focuses some of the light beams LB1 to LB8 on a predetermined first deflection surface 63a, and the remaining light beams LB1 to LB8 on a second deflection surface 63b adjacent to the first deflection surface 63a (see FIG. 2). More specifically, of the light beams LB1 to LB8, the light beams LB (here, light beams LB7 and LB8) that render the first dot array DL1 and the third dot array DL3 are focused on the first deflection surface 63a, and the light beams LB (here, light beams LB1 and LB2) that render the second dot array DL2 and the fourth dot array DL4 are focused on the second deflection surface 63b. That is, the first dot array DL1 and the third dot array DL3 are composed of dots DT7 and DT8. The second dot array DL2 is composed of dots DT1 and DT2.

[0063] As described above, the dot misalignment amount of the dot DT8 is the largest compared to the other dots DT2 to DT7. In this embodiment, the first dot row DL1 and the third dot row DL3 are composed of the dots DT7 and DT8, and the second dot row DL2 and the fourth dot row DL4 are composed of the dots DT1 and DT2. Therefore, as shown in FIG. 17, if dot misalignment occurs downstream in the main scanning direction in the laser diodes LD2 to LD8, the amount of dot misalignment will be relatively large. This will result in a relatively large difference in image density between the first evaluation patch PC1 and the second evaluation patch PC2. This makes it possible to more clearly detect the occurrence of dot misalignment.

[0064] As in the first embodiment, the first dot row DL1 and the fourth dot row DL4 of this embodiment can be single dot rows. Alternatively, the first dot row DL1, the second dot row DL2, the third dot row DL3, and the fourth dot row DL4 can be three dot rows in the sub-scanning direction.

[0065] Next, an image forming apparatus 100 according to a third embodiment will be described. Fig. 18 is an enlarged view of a first evaluation pattern PT1 constituting an evaluation chart CT according to the third embodiment. Note that, below, differences from the first embodiment will be described, and the same components as those in the first embodiment will be assigned the same reference numerals and will not be described.

[0066] 18, the evaluation chart CT according to the third embodiment includes the first evaluation pattern PT1 and the second evaluation pattern PT2 (upper portion of the figure) according to the first embodiment, and the first evaluation pattern PT1 and the second evaluation pattern PT2 (lower portion of the figure) according to the second embodiment. That is, the first evaluation pattern PT1 (same-surface first evaluation pattern) and the second evaluation pattern (same-surface second evaluation pattern) PT2 shown at the top of the figure are composed of first evaluation patches PC1 and second evaluation patches PC2 that are drawn by focusing all of the light beams LB1 to LB8 on a predetermined first deflection surface 63a (see FIG. 2) (hereinafter referred to as "same-surface scanning"). The difference in image density between the first evaluation pattern PT1 and the second evaluation pattern PT2 is referred to as the first density difference.

[0067] Meanwhile, the first evaluation pattern PT1 (different-scanning-plane first evaluation pattern) and second evaluation pattern PT2 (different-scanning-plane second evaluation pattern) at the bottom of the figure are composed of first evaluation patches PC1 and second evaluation patches PC2 that are drawn by focusing some of the light beams LB1 to LB8 (here, light beams LB7 and LB8) on a predetermined first deflection surface 63a and the remaining beams of the light beams LB1 to LB8 (here, light beams LB1 to LB6) on a second deflection surface 63b (see FIG. 2) (hereinafter referred to as "different-scanning-plane scanning"). The difference in image density between this first evaluation pattern PT1 and the second evaluation pattern PT2 is referred to as the second density difference.

[0068] In the image forming apparatus 100 of this embodiment, an evaluation chart CT (first evaluation chart) formed under a first set value (described later) and an evaluation chart CT (second evaluation chart) formed under a second set value are formed, and the amount of dot misalignment can be calculated from the first density difference and the second density difference of each evaluation chart CT. FIG. 19 is a graph showing the relationship between the amount of misalignment and the development density difference (image density difference). The horizontal axis represents the misalignment ratio, and the vertical axis represents the density difference value. Below, a specific example will be described in which the amount of dot misalignment is calculated from an evaluation chart CT formed by setting the distance between dots DT1 and DT8 in the main scanning direction to a first set value (-13.125 μm) and an evaluation chart CT formed by setting the distance to a second set value, which shifts dot DT8 downstream (+ side) by 21 μm from the first set value.

[0069] If dots DT1 and DT8 are shifted 1 μm upstream (negative side) in the main scanning direction during same-surface scanning, dots DT1 and DT8 are shifted 7 μm downstream (positive side) in the main scanning direction during different-surface scanning. Therefore, for same-surface scanning, point P1 is plotted with -1 on the horizontal axis and the first density difference on the vertical axis, and for different-surface scanning, point P2 is plotted with 7 on the horizontal axis and the second density difference on the vertical axis.

[0070] Here, the first density difference in the first evaluation chart CT is 0.0072 (g / m 2 ), the second density difference is -0.0238 (g / m 2 ). Therefore, the coordinates (X, Y) of point P1 are (-1, 0.0072), and the coordinates (X, Y) of point P2 are (7, -0.0238). In addition, the first density difference in the second evaluation chart CT is -0.0042 (g / m 2 ), the second density difference is 0.0145 (g / m 2 ) For the second evaluation chart CT, points P1' (-1, -0.0042) and P2' (7, 0.0145) are similarly plotted. The y-intercept P3 (first noise value) of the line connecting points P1 and P2, and the y-intercept P3' (second noise value) of the line connecting points P1' and P2' are calculated.

[0071] The y-intercept P3 is 0.00328. In contrast, the y-intercept P3' is -0.00189. Because the ratio of P3 to P3' is 1:-0.57566, the aforementioned 21 μm is divided into two so that the ratio is 1:-0.57566. This results in -13.328 μm for the first evaluation chart CT and +7.672 μm for the second evaluation chart CT. The dot misalignment amount between adjacent laser diodes LD1-LD8 can be calculated by dividing these values ​​by 8, the number of laser diodes LD1-LD8, minus 1 (the number of gaps between adjacent laser diodes LD1-LD8). Therefore, the dot misalignment amount for the first evaluation chart CT is calculated as -1.904 μm, and the dot misalignment amount for the second evaluation chart CT is calculated as 1.096 μm. The dot misalignment correction value described above is calculated based on this dot misalignment amount.

[0072] As described above, the first evaluation chart CT is set to the first set value (-13.125 μm). Therefore, the actual dot misalignment amount of the first evaluation chart CT is -1.875 μm, which is the first set value (-13.125 μm) divided by the number of gaps between adjacent laser diodes LD1 to LD8 (7 in this case). The dot misalignment amount calculated using the above-mentioned method is -1.904 μm, which can be confirmed to be a value close to the actual dot misalignment amount (-1.875 μm). Similarly, the actual dot misalignment amount of the second evaluation chart CT is 1.125 μm, which is a value close to the dot misalignment amount of the second evaluation chart CT calculated using the above-mentioned method (1.096 μm).

[0073] In the conventional image forming apparatus 100, changes in image density were detected using an evaluation patch PC consisting of a first dot row DL1 to a fourth dot row DL4, each of which has one dot in the sub-scanning direction. Therefore, the overlapping portions of the first dot row DL1 and the second dot row DL2, and the overlapping portions of the third dot row DL3 and the fourth dot row DL4, are relatively short in the sub-scanning direction. In other words, the areas of these portions are relatively small. Even if dot misalignment occurs and changes occur in the areas of these portions, the amount of change is small, making it difficult to accurately detect dot misalignment.

[0074] On the other hand, the image forming apparatus 100 of the present invention employs the evaluation chart CT of each of the above embodiments, so that at least one of the first dot row DL1 and the second dot row DL2 has two or more dots in the sub-scanning direction. Therefore, when dot misalignment occurs in the main scanning direction, the image density of the evaluation chart CT changes relatively significantly. This makes it easier to detect changes in image density, and it is possible to provide an image forming apparatus that can easily adjust dot misalignment in the main scanning direction.

[0075] Furthermore, in the conventional image forming apparatus 100, multiple evaluation charts CT were formed in which the amount of dot misalignment in the main scanning direction of each evaluation patch PC was intentionally changed by a predetermined amount, and the amount of dot misalignment was estimated from the progression of changes in image density of each evaluation chart CT. Therefore, in order to calculate the amount of dot misalignment precisely, a huge number of evaluation charts CT had to be formed, making adjusting the dot misalignment in the main scanning direction a cumbersome task. In contrast, by adopting the image forming apparatus 100 of the second embodiment of the present invention, it is possible to calculate the amount of dot misalignment more precisely by simply forming a pair of evaluation charts CT. This makes it possible to precisely and easily adjust the dot misalignment in the main scanning direction.

[0076] Furthermore, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention. For example, in the above-described embodiments, the first evaluation pattern PT1 and the second evaluation pattern PT2 are arranged adjacent to each other in the sub-scanning direction, but they may also be arranged adjacent to each other in the sub-scanning direction.

[0077] 20, multiple evaluation charts CT of each of the above embodiments may be arranged at equal intervals in the main scanning direction of the intermediate transfer belt 8. This makes it possible to detect changes in image density at multiple locations in the main scanning direction. Therefore, even if dot misalignment occurs at different amounts at different positions in the main scanning direction, it is possible to appropriately correct the dot misalignment at each position in the main scanning direction. In this case, the color misalignment correction unit 97 detects the image density of each evaluation chart CT using the boundary (center) between the adjacent first evaluation pattern PT1 and second evaluation pattern PT2 as the reference position.

[0078] In this case, instead of the image density sensor 50 described above, a scanner (not shown) of the image forming apparatus 100 can be used to simultaneously scan multiple evaluation charts CT on the intermediate transfer belt 8 to detect image density. In this case, dot misalignment for each position in the main scanning direction can be simultaneously detected, making it easier to correct the dot misalignment. In this case, the first evaluation pattern PT1 and the second evaluation pattern PT2 that make up the evaluation chart CT can be disposed adjacent to each other in the main scanning direction.

[0079] The effects of the present invention will be described in more detail below with reference to examples. [Example]

[0080] The change in image density for the evaluation chart CT for each aspect of the evaluation pattern was investigated using an analytical method. The conditions were as follows: the image forming apparatus 100 shown in FIG. 1 was equipped with the optical scanning device 5 shown in FIG. 2, and the evaluation chart CT according to the first and second embodiments of the present invention was printed on printing paper (recording medium). The image density (%) was calculated using an analytical method, and the results were compared by sequentially shifting the dot positions in the main scanning direction. As a comparative example, the image density of the evaluation chart CT in which the number of dots in the sub-scanning direction for the first dot row DL1, second dot row DL2, third dot row DL3, and fourth dot row DL4 was one was also calculated.

[0081] Three types of evaluation charts CT according to the first embodiment, three types of evaluation charts CT according to the second embodiment, and two types of evaluation charts as comparative examples were prepared, and their image densities were compared. The analysis results for the evaluation charts according to the first embodiment are designated as Inventions 1, 2, and 3, respectively. The analysis results for the evaluation charts according to the second embodiment are designated as Inventions 4, 5, and 6, respectively.

[0082] In the evaluation chart CT of present invention 1, the number of dots in the sub-scanning direction of the first dot row DL1, the second dot row DL2, the third dot row DL3, and the fourth dot row DL4 is each 2 (see FIGS. 9 and 11). In the evaluation chart CT of present invention 2, the number of dots in the sub-scanning direction of the first dot row DL1 and the third dot row DL3 is 1, and the number of dots in the sub-scanning direction of the second dot row DL2 and the fourth dot row DL4 is 2 (see FIG. 14). In the evaluation chart CT of present invention 3, the number of dots in the sub-scanning direction of the first dot row DL1, the second dot row DL2, the third dot row DL3, and the fourth dot row DL4 is each 3 (see FIG. 15).

[0083] In the evaluation chart CT of invention 4, the number of dots in the sub-scanning direction of the first dot row DL1, the second dot row DL2, the third dot row DL3, and the fourth dot row DL4 is each 2 (see FIG. 16). In the evaluation chart CT of invention 5, the number of dots in the sub-scanning direction of the first dot row DL1 and the third dot row DL3 is 1, and the number of dots in the sub-scanning direction of the second dot row DL2 and the fourth dot row DL4 is 2. In the evaluation chart CT of invention 5, the number of dots in the sub-scanning direction of the first dot row DL1, the second dot row DL2, the third dot row DL3, and the fourth dot row DL4 is each 3.

[0084] In the evaluation charts CT of Comparative Examples 1 and 2, the number of dots in the sub-scanning direction of the first dot row DL1, the second dot row DL2, the third dot row DL3, and the fourth dot row DL4 is two, respectively.

[0085] The first dot row DL1, the second dot row DL2, the third dot row DL3, and the fourth dot row DL4 of Comparative Example 1 are all drawn by light beams LB1 and LB2 deflected by the same deflection surface 63a. That is, the first dot row DL1, the second dot row DL2, the third dot row DL3, and the fourth dot row DL4 of Comparative Example 1 are each composed of dots DT1 and DT2.

[0086] The first dot row DL1 and the third dot row DL3 of Comparative Example 2 are drawn by the light beam LB8 deflected by the first deflection surface 63a. The second dot row DL2 and the fourth dot row DL4 of Comparative Example 2 are drawn by the light beam LB1 deflected by the second deflection surface 63b. That is, the first dot row DL1 and the third dot row DL3 are composed of dots DT8, and the second dot row DL2 and the fourth dot row DL4 are composed of dots DT1.

[0087] In the test, the amount of dot misalignment was gradually changed between -21 μm and 21 μm or less, and the change in the development rate (%) (the ratio of the image density of the evaluation chart CT when the image density (the ratio of the area occupied by the black background to the entire area of ​​the evaluation chart CT) when the entire evaluation chart CT is drawn in solid black is set to 1) was calculated (see Figures 21 and 23). The amount of dot misalignment was adjusted by changing the light emission timing of the laser diodes LD1 to LD8. The amount of dot misalignment was defined as + for misalignment in the main scanning direction and - for misalignment in the direction opposite to the main scanning direction. In addition, the development rate when no dot misalignment occurred (when the amount of dot misalignment was 0 μm) was used as the reference value, and the change in the difference between the development rate and the reference value when the amount of dot misalignment was changed was calculated (see Figures 22 and 24).

[0088] FIG. 21 is a graph showing the changes in development rate for Inventions 1 to 3 and Comparative Example 1. FIG. 22 is a graph showing the changes in the difference value for Inventions 1 to 3 and Comparative Example 1. FIG. 23 is a graph showing the changes in development rate for Inventions 4 to 6 and Comparative Example 2. FIG. 24 is a graph showing the changes in the difference value for Inventions 4 to 6 and Comparative Example 2. In FIGS. 21 to 23, Invention 1 is represented by a ● graph, Invention 2 is represented by a ◆ graph, Invention 3 is represented by a ■ graph, and Comparative Example 1 is represented by a ▲ graph. In addition, Invention 4 is represented by a ◯ graph, Invention 5 is represented by a ◇ graph, Invention 6 is represented by a □ graph, and Comparative Example 2 is represented by a △ graph.

[0089] As shown in Fig. 21, the development rates of present inventions 1 to 3 remain at higher values ​​than those of comparative example 1. Also, as shown in Fig. 22, the change rate of the difference value (the magnitude of the slope of the graph in Fig. 22) of present inventions 1 to 3 is larger than that of comparative example 1. In other words, the change rate of the development rate when the amount of deviation changes is larger for the evaluation charts CT of present inventions 1 to 3 than for the evaluation chart of comparative example 1.

[0090] 23, the development rates of Inventions 4 to 6 remain at values ​​greater than those of Comparative Example 2. Also, as shown in Fig. 24, the change rate of the difference value (the magnitude of the slope of the graph in Fig. 24) of Inventions 4 to 6 is greater than that of Comparative Example 2. That is, the change rate of the development rate when the amount of deviation changes is greater for the evaluation charts CT of Inventions 4 to 6 than for the evaluation chart of Comparative Example 2.

[0091] 22 and 24, the rate of change in the difference value for Inventions 4 to 6 (the slope of the graph in FIG. 24) is greater than the rate of change in the difference value for Inventions 1 to 3 (the slope of the graph in FIG. 22). In other words, for Inventions 4 to 6, as the amount of dot misalignment increases, the difference value also changes relatively greatly. This makes it easier to detect dot misalignment.

[0092] Therefore, the evaluation charts CT of the present inventions 1 to 6 are easier to detect image density with the image density sensor 50 than the evaluation charts of the comparative examples 1 and 2. Furthermore, when a user or the like visually checks the evaluation charts CT of the present inventions 1 to 6, the change in image density is easier to see than the evaluation charts CT of the comparative examples 1 and 2. [Industrial Applicability]

[0093] The present invention can be used in image forming apparatuses that employ a multi-beam system in which a photosensitive drum is scanned with a light beam from a multi-beam laser having multiple light emitters. By using the present invention, an image forming apparatus can be provided that increases the rate of change in image density of an evaluation chart for color misregistration correction and can more accurately correct dot misregistration in the main scanning direction. [Explanation of symbols]

[0094] 1a to 1d Photosensitive drum (image carrier) 5 Optical scanning device 8 Intermediate transfer belt 13 Fixing device 26 Light source unit (light source) 45 Polygon Mirror 50 Image density sensor 63a 1st deflection surface 63b Second polarization plane 92 ROM (memory section) 100 Image forming device CT Evaluation Chart DL1 First dot row DL2 Second dot row DL3 Third dot row DL4 4th dot row DT1~DT8 dots LB1~LB8 Light beams LD1~LD8 Laser diodes PC1 1st evaluation patch PC2 2nd Evaluation Patch PL1 1st patch row PL2 2nd patch row PT1 First evaluation pattern PT2 Second evaluation pattern P1 First density difference at first set value P1´ First density difference at the second set value P2 Second density difference at first set value P2´ Second concentration difference at second set value P3 y-intercept (first noise value) P3´ y-intercept (second noise value) Pa~Pd Image forming section S Paper (recording medium)

Claims

1. an optical scanning device including a light source having three or more light emitting units arranged in a row at regular intervals at a predetermined angle with respect to the main scanning direction, and a polygon mirror that deflects and scans the light beams emitted from the light emitting units, and that forms an electrostatic latent image on an image carrier by the light beams; a developing unit that forms a toner image by visualizing the electrostatic latent image; a control unit that controls the optical scanning device to switch on and off the light-emitting units to form the electrostatic latent image according to image data; a storage unit configured to store a predetermined evaluation chart configured in units of dots by the light beams of the light-emitting units and used to determine the write timing of each of the light-emitting units; a density detection mechanism capable of detecting the image density of the toner image visualized on the evaluation chart; Equipped with The evaluation chart is a first evaluation pattern; a second evaluation pattern that is parallel to the first evaluation pattern in the main scanning direction or a sub-scanning direction orthogonal to the main scanning direction, The first evaluation pattern is a first patch row configured by arranging a plurality of first evaluation patches at equal intervals in the sub-scanning direction, the first evaluation patches having a first dot row in which dots in the sub-scanning direction, the number of which is smaller than the number of the light-emitting units, are successively arranged in a straight line in the main scanning direction; and a second dot row in which dots equal to or smaller than the number of the light-emitting units minus the number of dots in the sub-scanning direction of the first dot row are adjacent to the first dot row in the sub-scanning direction and are successively arranged in a straight line so as to be shifted from the first dot row in the main scanning direction, and the first patch row is configured by arranging a plurality of the first patch rows at equal intervals in the main scanning direction, The second evaluation pattern is a second patch row configured by arranging a plurality of second evaluation patches, each of which has a symmetrical shape to the first evaluation patch in a direction parallel to the first evaluation pattern and the second evaluation pattern, at intervals equal to the parallel intervals of the plurality of first evaluation patches in the sub-scanning direction, and a plurality of the second patch rows configured by arranging a plurality of the second evaluation patches at equal intervals in the main scanning direction at predetermined intervals; The control unit shifts the timing of emission of the light beam from each light-emitting unit based on the difference in image density between the first dot row and the second dot row detected by the density detection mechanism.

2. 2. The image forming apparatus according to claim 1, wherein the first patch row is configured such that a predetermined interval is provided between the first evaluation patches adjacent to each other in the sub-scanning direction.

3. 3. The image forming apparatus according to claim 1, wherein a plurality of the evaluation charts are arranged at predetermined intervals in the main scanning direction or the sub-scanning direction.

4. a plurality of the evaluation charts are arranged at predetermined intervals in the main scanning direction; the density detection mechanism detects the image density for each of the evaluation charts; 2. The image forming apparatus according to claim 1, wherein the control unit adjusts the shift amount of the emission timing of the light beam of each light-emitting element for each position of the evaluation chart based on the detection result of the density detection mechanism.

5. 5. The image forming apparatus according to claim 4, wherein the control unit determines the shift amount of the timing of emission of the light beam of each light-emitting element using the center of the adjacent first evaluation pattern and the adjacent second evaluation pattern of each evaluation chart as a reference position.

6. an intermediate transfer belt disposed opposite the image carrier and onto which the toner image on the image carrier, which has been visualized by the developing unit, is primarily transferred; 6. The image forming apparatus according to claim 1, wherein the density detection mechanism detects the image density of the evaluation chart that has been primarily transferred onto the intermediate transfer belt.

7. a fixing device that fixes the toner image on the image carrier, which has been visualized by the developing unit, onto a recording medium; 6. The image forming apparatus according to claim 1, wherein the density detection mechanism detects the image density of the evaluation chart fixed on the recording medium.

8. An image forming apparatus as described in any one of claims 1 to 7, characterized in that all of the light beams emitted from each of the light-emitting elements are reflected by one of the deflection surfaces of the polygon mirror, thereby forming the first evaluation patch and the second evaluation patch.

9. An image forming apparatus as described in any one of claims 1 to 7, characterized in that a portion of the light beam emitted from each light-emitting element is reflected by a first deflection surface among the multiple deflection surfaces of the polygon mirror, and the other light beam is reflected by a second deflection surface adjacent to the first deflection surface, thereby forming the first evaluation patch and the second evaluation patch.

10. The evaluation chart is a same-plane first evaluation pattern which is the first evaluation pattern constituted by the first evaluation patches formed by all of the light beams emitted from the light-emitting units being reflected by one of the deflection surfaces of the polygon mirror; a different-scanning-plane first evaluation pattern which is the first evaluation pattern constituted by the first evaluation patches formed by some of the light beams emitted from the light-emitting units being reflected by a first deflection surface of the plurality of deflection surfaces and other light beams being reflected by a second deflection surface adjacent to the first deflection surface; a same-plane second evaluation pattern which is the second evaluation pattern constituted by the second evaluation patches formed by all of the light beams emitted from the light-emitting units being reflected by one of the deflection surfaces; and a different-scanning-plane second evaluation pattern which is the second evaluation pattern constituted by the second evaluation patches formed by some of the light beams emitted from the light-emitting units being reflected by a first deflection surface of the plurality of deflection surfaces and other light beams being reflected by a second deflection surface adjacent to the first deflection surface, the control unit is capable of controlling formation of the pair of evaluation charts by changing the timing of emission of the light beams from the light-emitting units between a first set value and a second set value; The control unit a first noise value that can be calculated based on a first density difference that is a difference in development density between the same-surface first evaluation pattern and the same-surface second evaluation pattern, and a second density difference that is a difference in development density between the different-scanning-surface first evaluation pattern and the different-scanning-surface second evaluation pattern, in the evaluation chart formed with the first set value; a second noise value that can be calculated based on the first density difference and the second density difference of the evaluation chart formed with the second set value; and capable of calculating the amount of dot misalignment in the main scanning direction of each of the adjacent light-emitting units based on the first noise value and the second noise value.

Images