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 optical scanner includes a light source and a polygon mirror. 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 a predetermined interval in a main scanning direction and a sub scanning direction, the evaluation patches each having first dot rows arranged continuously and linearly in the main scanning direction and second dot rows adjacent to the first dot rows and arranged continuously and linearly in the main scanning direction. The second evaluation pattern has a plurality of second evaluation patches arranged at a predetermined interval in the main scanning direction and the sub scanning direction, the evaluation patches having a symmetrical shape to the first evaluation patches in a parallel direction of the first evaluation pattern and the second evaluation pattern.SELECTED DRAWING: Figure 9

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] The image forming apparatus in Patent Document 1 calculates dot misalignment based on changes in the image density of the evaluation patch (the ratio of the total area of ​​dots drawn within a rectangular area enclosed by a line overlapping both ends of the evaluation patch in the main scanning direction and a line overlapping both ends of the evaluation patch in the sub-scanning direction). However, in the case of a model that forms images with development settings that result in a relatively high image density, the first dot row and the second dot row interfere with each other when the evaluation chart is formed. As a result, even if dot misalignment occurs, the total amount of developer drawn in the area (development area) hardly changes. This makes it difficult to detect dot misalignment and to adjust it.

[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 provides 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 multiple 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 which is 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 or spaced from the first dot row in the sub-scanning direction and are arranged consecutively in a straight line in the main scanning direction, and the first evaluation pattern has a plurality of first patch rows arranged at equal intervals in the main scanning direction. The second evaluation pattern has a second patch row configured by arranging a plurality of second evaluation patches in which second evaluation patches, the second evaluation patches being 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 has a plurality of second patch rows arranged at equal intervals in the main scanning direction. [Effects of the Invention]

[0011] According to the first configuration of the present invention, a white background portion is formed between the first dot row and the second dot row in the main scanning direction. Therefore, even if an image is formed using development settings with a relatively high image density, the first dot row and the second dot row are less likely to interfere with each other. This makes it easier to calculate changes in image density of the first evaluation pattern. The same applies to the second evaluation pattern. Therefore, it is possible to provide an image forming apparatus that makes it easier to detect changes in image density of the evaluation chart and easily 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] FIG. 1 is an enlarged plan view of a portion 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 Figure 9. [Figure 11] FIG. 9 is an enlarged view of a portion 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] A plan view of the evaluation chart CT on which dots were actually drawn using development settings with a relatively high image density. [Figure 15] 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 16] 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 17] 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 18] FIG. 11 is a plan view showing an evaluation chart CT according to the second embodiment in a state where dot misalignment occurs. [Figure 19] 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 20] Graph showing the relationship between the amount of misregistration and the development density difference [Figure 21] 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 22] FIG. 10 is a plan view showing an evaluation chart CT of a comparative example according to an embodiment of the present invention; [Figure 23] Graph showing changes in development rate in Examples 1 and 2 of the present invention and Comparative Example [Figure 24] Graph showing changes in the difference values ​​of inventions 1 and 2 according to the examples and comparative example 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 scan 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 26 has one laser diode, one scan line SL is drawn per deflection surface 63. If the light source unit 26 has multiple laser diodes, multiple scan 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 scan 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 value of the development voltage, the rotation angle of the light source unit 26, 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 arranged downstream of the first dot row DL1 in the sub-scanning direction. The upstream end of the second dot row DL2 in the main scanning direction is shifted by a predetermined number of dots (here, 6 dots) from the upstream end of the first dot row DL1 in the main scanning direction. A white portion C1 is formed between the first dot row DL1 and the second dot row DL2.

[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 by laser diodes LD1 to LD4, which are selectively turned on or off dot by dot.

[0055] As shown in Fig. 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 DT1 to DT4 that are drawn consecutively in a straight line, each row having a length of two dots in the sub-scanning direction and four dots in the main scanning direction. A white background portion C2 is formed between the third dot row DL3 and the fourth dot row DL4. 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] If dot misalignment occurs downstream in the main scanning direction (to the right in the figure) in the light beams LB2 to LB8, the dots DT2 to DT4 of the first evaluation patch PC1 and the second evaluation patch PC2 will be misaligned downstream in the main scanning direction, as shown in Fig. 13. This will cause the first evaluation patch PC1 and the second evaluation patch PC2 to have asymmetric shapes with respect to the main scanning direction, resulting in a difference in density between the image density of the first evaluation patch PC1 (the proportion of the total area of ​​the dots DT1 to DT4 drawn within a rectangular region bounded by lines overlapping both ends of the first dot array DL1 and the second dot array DL2 in the main scanning direction and lines overlapping both ends of the first evaluation patch PC1 in the sub-scanning direction) and the image density of the second evaluation patch PC2.

[0057] More specifically, in the first evaluation patch PC1, as the dots DT4 of the second dot array DL2 move away from the first dot array DL1, both ends of the first dot array DL1 and the second dot array DL2 in the main scanning direction are stretched, and the area of ​​the white background portion C1 increases. In contrast, in the second evaluation patch PC2, as the dots DT4 of the fourth dot array DL4 move closer to the third dot array DL3, both ends of the third dot array DL3 and the fourth dot array DL4 in the main scanning direction are compressed, and the area of ​​the white background portion C2 decreases. As a result, the image density of the first evaluation patch PC1 becomes lighter, while the image density of the second evaluation patch PC2 becomes darker. This results in a difference in image density between the first evaluation patch PC1 and the second evaluation patch PC2.

[0058] The control unit 90 can detect that dot misalignment in the main scanning direction has occurred in the dots DT1 to DT8 by detecting this density difference with the image density sensor 50. 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, and can therefore confirm that dot misalignment has occurred in the dots DT1 to DT8.

[0059] FIG. 14 is a plan view showing an evaluation chart CT on which dots are actually drawn using development settings that provide a relatively high image density. As shown in FIG. 14, when the evaluation chart CT is formed using development settings that provide a relatively high image density, the dots DT1 to DT8 interfere with (overlap) each other in the main scanning direction and the sub-scanning direction. In the evaluation chart CT according to this embodiment, the first dot row DL1 and the second dot row DL2 (the third dot row DL3 and the fourth dot row DL4) are spaced apart in the main scanning direction. Therefore, the dot DT2 downstream of the first dot row DL1 (the third dot row DL3) in the sub-scanning direction and the dot DT3 upstream of the second dot row DL2 (the fourth dot row DL4) in the sub-scanning direction are spaced apart without overlapping.

[0060] In the evaluation chart for the conventional image forming apparatus 100, the first dot row DL1 and the second dot row DL2 (and the third dot row DL3 and the fourth dot row DL4) are formed to overlap in the main scanning direction. Therefore, even if dot misalignment occurs between the first dot row DL1 and the second dot row DL2 (or the third dot row DL3 and the fourth dot row DL4) in the main scanning direction, the change in the development area of ​​the first evaluation patch PC1 (second evaluation patch PC2) is relatively small. In contrast, when the evaluation chart CT according to the present invention is used, the change in the development area of ​​the first evaluation patch PC1 (second evaluation patch PC2) is relatively large. This makes it easier to detect the density difference using the image density sensor 50, and makes it easier to calculate the dot misalignment correction value described above more accurately. Furthermore, the difference in image density between the first evaluation pattern PT1 and the second evaluation pattern PT2 is easily confirmed visually.

[0061] 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 dot accumulates sequentially up to dots DT2 to DT7. Furthermore, it is preferable that the interval (number of dots) in the main scanning direction between the white background portions C1 and C2 be equal to or greater than one dot and less than three dots. Furthermore, it is preferable that the size of each of the dots DT1 to DT8 described above is one pixel.

[0062] 15, the first evaluation patch PC1 of this embodiment can be composed of a plurality of (here, two) first dot rows DL1 arranged in parallel at a predetermined interval in the main scanning direction, and a plurality of (here, two) second dot rows DL2 arranged in parallel at a predetermined interval in the main scanning direction. The second dot row DL2 located upstream in the main scanning direction is shifted by a predetermined number of dots (here, two dots) in the main scanning direction relative to the first dot row DL1 located upstream in the main scanning direction. The downstream second dot row DL2 is also shifted by the same number of dots (here, two dots) in the main scanning direction relative to the downstream first dot row DL1. In this case, the second evaluation patch PC2 is also symmetrical to the first evaluation patch PC1 in the main scanning direction (not shown).

[0063] In this case, a white portion C3 is formed between the closest points of the downstream end of the second dot row DL2 located upstream in the main scanning direction and the upstream end of the first dot row DL1 located downstream. Similarly, a white portion (not shown) is formed between the closest points of the downstream end of the third dot row DL3 located downstream in the main scanning direction and the upstream end of the fourth dot row DL4 located upstream. Similarly, when this evaluation chart CT is used, the control unit 90 uses the image density sensor 50 to detect the image density of the first evaluation patch PC1 (the proportion of the total area of ​​the dots DT1 to DT4 drawn within a rectangular area surrounded by lines overlapping both ends of the first evaluation patch PC1 in the main scanning direction (the upstream end of the first dot row DL1 in the main scanning direction, located upstream in the main scanning direction, and the downstream end of the first dot row DL1 in the main scanning direction, located downstream in the main scanning direction) and lines overlapping both ends of the first evaluation patch PC1 in the sub-scanning direction (the upstream end of the first dot row DL1 in the sub-scanning direction and the downstream end of the second dot row DL2 in the sub-scanning direction). The control unit 90 also detects the image density of the second evaluation patch PC in the same way and calculates the difference in image density between the first evaluation patch PC1 and the second evaluation patch PC2. Based on this calculation result, the direction and magnitude of the dot misalignment are calculated. Note that the spacing (number of dots) in the main scanning direction of the white background portion C3 is preferably at least one dot but less than three dots.

[0064] 16, the evaluation patches PC1 and PC2 of this embodiment can employ a configuration in which an auxiliary dot array DLA is disposed downstream of the second dot array DL2 and the fourth dot array DL4 in the sub-scanning direction. The auxiliary dot array DLA is composed of dots DT6 drawn by a laser diode LD6. The auxiliary dot array DLA is formed across the entire area of ​​the first evaluation patch PC1 and the entire area of ​​the second evaluation patch PC2 in the main scanning direction. The auxiliary dot arrays DLA of the first evaluation patch PC1 and the auxiliary dot arrays DLA of the second evaluation patch PC2 that are adjacent in the main scanning direction are adjacent to each other. In other words, the auxiliary dot array DLA is a dot array of dots DT6 that is linearly arranged across the entire area of ​​the first evaluation pattern PT1 and the entire area of ​​the second evaluation pattern PT2 in the main scanning direction.

[0065] By doing this, the white background portions C1 of the first evaluation patches PC that are adjacent in the sub-scanning direction are separated by the auxiliary dot array DLA. This separates the white background portions C1 into fixed ranges, making it easier to detect changes in image density. The same is true for the second evaluation patch PC2. The auxiliary dot array DLA may also be positioned upstream of the first dot array DL1 and the third dot array DL3 in the sub-scanning direction. This makes it easier to detect changes in image density.

[0066] Next, an image forming apparatus 100 according to a second embodiment will be described. Fig. 17 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.

[0067] 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 beam LB (here, light beams LB7 and LB8) that draws the first dot row DL1 is focused on the first deflection surface 63a, and the light beam LB (here, light beams LB1 and LB2) that draws the second dot row DL2 is focused on the second deflection surface 63b.

[0068] 17, the first dot row DL1 is composed of dots DT7 and DT8 drawn by light beams LB7 and LB8 emitted from laser diodes LD7 and LD8. The second dot row DL2 is composed of dots DT1 and DT2 drawn by light beams LB1 and LB2 emitted from laser diodes LD1 and LD2. As in the first embodiment, the second evaluation patch PC2 of this embodiment has a symmetrical shape in the main scanning direction to the first evaluation patch PC1, and the relationship between the second evaluation patch PC2 and the first evaluation patch PC1 is the same as in the first embodiment, so a description thereof will be omitted.

[0069] 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 dots DT7 and DT8, and the second dot row DL2 and the fourth dot row DL4 are composed of dots DT1 and DT2. Therefore, as shown in FIG. 18, if dot misalignment occurs downstream in the main scanning direction in the laser diodes LD2 to LD8, the dot misalignment amount of the second dot row DL2 relative to the first dot row DL1 and the dot misalignment amount of the fourth dot row DL4 relative to the third dot row DL3 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.

[0070] Next, an image forming apparatus 100 according to a third embodiment will be described. Fig. 19 is an enlarged view of a first evaluation pattern PT1 constituting an evaluation chart CT according to the third embodiment. Note that the following will focus on differences from the first embodiment, and the same components as those in the first embodiment will be assigned the same reference numerals and will not be described.

[0071] The evaluation chart CT according to the third embodiment includes the first evaluation pattern PT1 and the second evaluation pattern PT2 (shown at the bottom in the figure) according to the first embodiment, and the first evaluation pattern PT1 and the second evaluation pattern PT2 (shown at the top in 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 bottom in the figure are made up 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.

[0072] 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 top 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.

[0073] 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. 20 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.

[0074] 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-scanning surface scanning. Therefore, for same-surface scanning, point P1 is plotted on the graph with -1 on the horizontal axis and the first density difference on the vertical axis, and for different-scanning surface scanning, point P2 is plotted on the graph with 7 on the horizontal axis and the second density difference on the vertical axis.

[0075] Here, the first density difference in the first evaluation chart CT is -0.0087 (g / m 2 ), the second density difference is 0.0292 (g / m 2). Therefore, the coordinates (X, Y) of point P1 are (-1, -0.0087), and the coordinates (X, Y) of point P2 are (7, 0.0292). In addition, the first density difference in the second evaluation chart CT is 0.0054 (g / m 2 ), the second density difference is -0.0192 (g / m 2 ) For the second evaluation chart CT, points P1' (-1, 0.0054) and P2' (7, -0.0192) 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.

[0076] The y-intercept P3 is -0.004. In contrast, the y-intercept P3' is 0.0023. Because the ratio of P3 to P3' is 1:-0.5744, the aforementioned 21 μm is divided into two so that the ratio is 1:-0.5744. This results in -13.338 μm for the first evaluation chart CT and +7.662 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 -1.905 μm, and the dot misalignment amount for the second evaluation chart CT is calculated as 1.095 μm. The dot misalignment correction value described above is calculated based on this dot misalignment amount.

[0077] 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 -1.905 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 (1.095 μm).

[0078] In a conventional image forming apparatus 100, dot misalignment is calculated based on changes in the image density of the evaluation patch (the ratio of the total area of ​​dots drawn within a rectangular area enclosed by a line overlapping both ends of the evaluation patch in the main scanning direction and a line overlapping both ends of the evaluation patch in the sub-scanning direction). However, in the case of a model that forms images using development settings with a relatively high image density, the first dot row DL1 and the second dot row DL2 interfere with each other when the evaluation chart CT is formed. As a result, even if dot misalignment occurs, the total amount of developer drawn in the above area (development area) hardly changes. This makes it difficult to detect dot misalignment and to adjust it.

[0079] On the other hand, by adopting the evaluation chart CT of each of the above embodiments, the image forming apparatus 100 of the present invention forms a white background portion C1 between the first dot row DL1 and the second dot row DL2 in the main scanning direction, and the image density sensor 50 detects changes in the image density of the first evaluation pattern PT1 from changes in the area of ​​this white background portion C1. Therefore, even if an image is formed using development settings with a relatively high image density, the first dot row DL1 and the second dot row DL2 are less likely to interfere with each other, making it easier to calculate changes in the image density of the first evaluation pattern PT1. The same is true for the second evaluation pattern PT2. Therefore, it is easier to detect changes in the image density of the evaluation chart CT, making it possible to provide an image forming apparatus that can easily adjust dot misalignment in the main scanning direction.

[0080] Furthermore, in the conventional image forming apparatus 100, changes in image density were detected using an evaluation patch PC consisting of the first dot row DL1 to the fourth dot row DL4, each of which has one dot in the sub-scanning direction. As a result, the area of ​​the white background portions located between the first dot row DL1 and the second dot row DL2, and between the third dot row DL3 and the fourth dot row DL4, becomes smaller. As a result, even if dot misalignment occurs and the area of ​​these white background portions changes, it becomes difficult to accurately detect the amount of dot misalignment.

[0081] On the other hand, the image forming apparatus 100 of the present invention employs the evaluation chart CT of each of the above-described embodiments, so that the number of dots in the sub-scanning direction of the first dot row DL1 and the second dot row DL2 is two or more. Therefore, when dot misalignment occurs in the main scanning direction, the image density of the first evaluation pattern PT1 and the second evaluation pattern PT2 (image density of the evaluation chart CT) changes relatively significantly. Therefore, dot misalignment can be detected more accurately.

[0082] 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.

[0083] 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.

[0084] 21, 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.

[0085] 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.

[0086] The effects of the present invention will be described in more detail below with reference to examples. [Example]

[0087] 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.

[0088] Two types of evaluation charts CT according to the present invention and one type of evaluation chart as a comparative example were prepared, and their image densities were compared. Hereinafter, the analysis results of the evaluation charts CT according to the present invention will be referred to as Inventions 1 and 2, respectively.

[0089] 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 two (see FIGS. 9 and 11). A gap of two dots (white portion C1) is formed between the first dot row DL1 and the second dot row DL2 in the main scanning direction. Similarly, a gap of two dots (white portion C2) is formed between the third dot row DL3 and the fourth dot row DL4.

[0090] The first evaluation patch PC1 of the second aspect of the invention is composed of a plurality (two in this case) of first dot rows DL1 arranged in parallel at a predetermined interval in the main scanning direction, and a plurality (two in this case) of second dot rows DL2 arranged in parallel at a predetermined interval in the main scanning direction (see FIG. 15). The second evaluation patch PC2 is symmetrical to the first evaluation patch PC1 in the main scanning direction. The first dot row DL1 and the third dot row DL3 each have two dots in the sub-scanning direction, and the second dot row DL2 and the fourth dot row DL4 each have two dots in the sub-scanning direction.

[0091] The second dot row DL2 located on the upstream side in the main scanning direction is shifted by a predetermined number of dots (here, two dots) in the main scanning direction relative to the first dot row DL1 located on the upstream side in the main scanning direction. The second dot row DL2 located on the downstream side is also shifted by the same number of dots (here, two dots) in the main scanning direction relative to the first dot row DL1 located on the downstream side. In this case, too, the second evaluation patch PC2 has a symmetrical shape to the first evaluation patch PC1 in the main scanning direction (not shown).

[0092] 22, in the evaluation chart CT of the comparative example, the first dot row DL1 to the fourth dot row DL4 each have two dots in the sub-scanning direction. The second dot row DL2 is shifted two dots downstream in the main scanning direction relative to the first dot row DL1, and the fourth dot row DL4 is shifted two dots downstream in the main scanning direction relative to the third dot row DL3 (not shown). Adjacent first evaluation patches PC1 and second evaluation patches PC2 are spaced three dots or more apart in the main scanning direction.

[0093] The test was performed with the developing conditions of the developing devices 3a to 3d set to a relatively high density, and the dot misalignment amount was gradually changed from -21 μm to 21 μm or less to calculate the change in the development rate (%) (the ratio of the image density of the evaluation chart CT when the entire evaluation chart CT is drawn in solid black (the ratio of the area occupied by the black background to the entire area of ​​the evaluation chart CT) is set to 1) (see Figure 23). In addition, the development rate when there is no dot misalignment (when the dot misalignment amount is 0 μm) was used as the reference value, and the change in the difference between the development rate and the reference value when the dot misalignment amount is changed was calculated (see Figure 24). The dot misalignment amount is adjusted by changing the light emission timing of the laser diodes LD1 to LD8. The dot misalignment amount is defined as a positive value when the misalignment amount is downstream in the main scanning direction and a negative value when the misalignment amount is upstream in the main scanning direction.

[0094] In Fig. 23 and Fig. 24, the present invention 1 is shown by the graph ●, the present invention 2 is shown by the graph ■, and the comparative example is shown by the graph ▲.

[0095] As shown in FIG. 23, the development rate of Invention 2 remains higher than that of the Comparative Example. Furthermore, as shown in FIG. 24, the rate of change in the difference values ​​(the magnitude of the slope of the graph in FIG. 24) for Inventions 1 and 2 is larger than that of the Comparative Example. That is, the rate of change in development rate when the amount of dot misalignment changes is larger for the evaluation chart CT of Inventions 1 and 2 than for the evaluation chart of Comparative Example 1. Therefore, when the development conditions are set to a relatively high density, it is clear that the development rate is less likely to change and it is difficult to detect dot misalignment with the evaluation chart CT of the Comparative Example. This is thought to be because the first dot row DL1 and the second dot row DL2 are adjacent in the sub-scanning direction, causing interference between the first dot row DL1 and the second dot row DL2, reducing the rate of change in image density of the first evaluation pattern PT1. The same is true for the second evaluation pattern PT2. Furthermore, the relatively wide white area (C4 shown) between adjacent first evaluation patches PC1 in the main scanning direction is thought to reduce the impact of dot misalignment on image density.

[0096] Therefore, it was confirmed that by adjusting the dot misalignment using the evaluation charts CT of present inventions 1 and 2, it becomes easier to detect changes in image density of the evaluation chart CT. Also, as shown in Figure 23, the development rate of present invention 2 remains higher than the development rates of present invention 1 and the comparative example. From this, it was confirmed that by adopting the evaluation chart of present invention 2, it is possible to more clearly confirm changes in image density due to dot misalignment. [Industrial Applicability]

[0097] 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]

[0098] 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 deflection surface 92 ROM (memory section) 100 Image forming device C1~C4 White area 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 a plurality of 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; 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 patch row 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 sections, are successively arranged 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 the light-emitting sections minus the number of dots in the first dot row in the sub-scanning direction, are adjacent to or spaced from the first dot row in the sub-scanning direction and are successively arranged in a straight line in the main scanning direction, the first evaluation patches having a first dot row configured by arranging a plurality of first evaluation patches at equal intervals in the main scanning direction, the first evaluation patches having a second dot row in which dots, the number of which is equal to or smaller than the number of the light-emitting sections minus the number of dots in the first dot row in the sub-scanning direction, are successively arranged in a straight line in the main scanning direction, the second evaluation patches having a first dot row arranged at equal intervals in the sub-scanning direction, the the second dot row is located downstream of the first dot row in the same first evaluation patch in the main scanning direction, a white background portion of at least one dot is formed between the first dot row and the second dot row in the same first evaluation patch; The second evaluation pattern is an image forming apparatus having a second patch row configured by arranging a plurality of second evaluation patches, each second evaluation patch having a symmetrical shape to the first evaluation patch in the parallel direction of the first evaluation pattern and the second evaluation pattern, at intervals equal to the parallel spacing of the plurality of first evaluation patches in the sub-scanning direction; and

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. 4. An image forming apparatus according to claim 1, wherein the first evaluation patch has auxiliary dot rows arranged on both sides of the first dot row and the second dot row in the sub-scanning direction, the auxiliary dot rows being formed in a straight line across the entire area between both ends of the first evaluation patch in the main scanning direction.

5. a density detection mechanism capable of detecting the image density of the toner image visualized by the evaluation chart; 5. An image forming apparatus according to claim 1, wherein the control unit shifts the timing of emission of the light beam from each of the light-emitting units based on the image density between the closest points of the first evaluation patches that are overlapping in the sub-scanning direction and adjacent in the main scanning direction.

6. 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; 6. The image forming apparatus according to claim 5, 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.

7. 7. The image forming apparatus according to claim 6, wherein the control unit determines the shift amount of the emission timing 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.

8. 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; 8. The image forming apparatus according to claim 5, wherein the density detection mechanism detects the image density of the evaluation chart that has been primarily transferred onto the intermediate transfer belt.

9. a fixing device that fixes the toner image on the image carrier, which has been visualized by the developing unit, onto a recording medium; 8. The image forming apparatus according to claim 5, wherein the density detection mechanism detects the image density of the evaluation chart fixed on the recording medium.

10. An image forming apparatus as described in any one of claims 1 to 9, characterized in that all of the light beams emitted from each light-emitting element are reflected by one of the deflection surfaces of the polygon mirror, thereby forming the first evaluation patch and the second evaluation patch.

11. An image forming apparatus as described in any one of claims 1 to 9, 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.

12. The evaluation chart is a same-surface 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 deflecting surfaces of the polygon mirror; a different-scanning-surface 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 of the deflecting surfaces and other light beams being reflected by a second deflecting surface adjacent to the first deflecting surface; a same-surface 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 deflecting surfaces; and a different-scanning-surface 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 of the deflecting surfaces and other light beams being reflected by a second deflecting surface adjacent to the first deflecting 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