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 that has light emitting units, and a polygon mirror. The developing unit forms a toner image. The storage unit stores an evaluation chart. The evaluation chart has a first evaluation pattern, a second evaluation pattern, a third evaluation pattern, and a fourth evaluation pattern. The first evaluation pattern has a plurality of first evaluation patches arranged in a main scanning direction and a sub scanning direction. The first evaluation patch has a first dot row, and a second dot row that is adjacent to the first dot row in the sub scanning direction and arranged to be deviated from the first dot row in the main scanning direction. The second evaluation pattern has a second evaluation patch having a symmetrical shape to the first evaluation patch in the main scanning direction. The third evaluation pattern has a plurality of third evaluation patches. The fourth evaluation pattern has a plurality of fourth evaluation patches.SELECTED DRAWING: Figure 15

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.

[0007] Furthermore, Patent Document 2 discloses an image forming apparatus that detects the amount of dot misalignment by intentionally forming multiple evaluation charts of various types prepared in advance, each with a predetermined amount of intentionally shifted light emission timing. This image forming apparatus calculates the average image density for each evaluation chart of the same type with different light emission timing, and calculates an approximation curve that shows the change in image density versus the amount of dot misalignment. The optimal light emission timing is determined from the peak value of this approximation curve. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-137447 [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-135955 Summary of the Invention [Problem to be solved by the invention]

[0009] 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.

[0010] Furthermore, the image forming apparatus in Patent Document 2 creates multiple evaluation charts each with a different amount of dot misalignment and calculates an approximate curve, which requires creating a large number of evaluation charts, making dot misalignment adjustments cumbersome.

[0011] 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]

[0012] 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 second, third, and fourth evaluation patterns that are 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 evaluation patch having a first dot row in which one or more dots in the sub-scanning direction are arranged consecutively in a straight line in the main scanning direction, and a second dot row in which dots whose number is equal to or less 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 whose shapes are 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.The third evaluation pattern has a third patch row configured by arranging a plurality of third evaluation patches at equal intervals in the sub-scanning direction, each of which has a fifth dot row in which the same number of dots as the first dot row are arranged consecutively in the main scanning direction in the sub-scanning direction, and a sixth dot row in which the same number of dots as the second dot row are arranged consecutively in the sub-scanning direction, adjacent to the fifth dot row in the sub-scanning direction and arranged consecutively in a linear manner so as to be offset from the fifth dot row in the main scanning direction, and the third evaluation pattern has a fourth patch row configured by arranging a plurality of fourth evaluation patches at equal intervals in the sub-scanning direction, each of which has a seventh dot row in which the same number of dots as the first dot row are arranged consecutively in the main scanning direction in the sub-scanning direction, and an eighth dot row in which the same number of dots as the second dot row are arranged consecutively in the sub-scanning direction, adjacent to the seventh dot row in the sub-scanning direction and arranged consecutively in a linear manner so as to be offset from the seventh dot row in the main scanning direction, and the fourth ... patch rows at equal intervals in the main scanning direction, each of which has a seventh dot row in which the same number of dots The deviation of the sixth dot row from the fifth dot row in the main scanning direction is greater than or less than the deviation of the second dot row from the first dot row by a predetermined first number of dots in the main scanning direction. The deviation of the eighth dot row from the seventh dot row is greater than or less than the deviation of the fourth dot row from the third dot row by a predetermined first number of dots in the main scanning direction. [Effects of the Invention]

[0013] According to the first configuration of the present invention, it is possible to detect changes in image density of the third and fourth evaluation patterns in addition to changes in image density of the first and second evaluation patterns, thereby providing an image forming apparatus that can detect changes in multiple image densities using a single evaluation chart and easily and accurately adjust dot misalignment in the main scanning direction. [Brief explanation of the drawings]

[0014] [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] FIG. 9 is an enlarged plan view of a portion of the second evaluation pattern PT2 shown in FIG. 8. [Figure 11] 1 is an enlarged plan view of the first evaluation pattern PT1 and the second evaluation pattern PT2. [Figure 12] FIG. 9 is an enlarged plan view of a portion of the third evaluation pattern PT4 shown in FIG. 8. [Figure 13] FIG. 9 is an enlarged plan view of a portion of the fourth evaluation pattern PT4 shown in FIG. 8. [Figure 14] An enlarged plan view of the third evaluation pattern PT3 and the fourth evaluation pattern PT4 [Figure 15] FIG. 10 is a plan view showing the evaluation chart CT according to the first embodiment in a state where dot misalignment has occurred; [Figure 16] Graph showing the relationship between the amount of misregistration and the development density difference [Figure 17] FIG. 10 is a plan view showing a modified example of the evaluation chart CT according to the first embodiment of the present invention. [Figure 18]FIG. 10 is a perspective view showing an intermediate transfer belt 8 on which an evaluation chart CT according to a second embodiment of the present invention is formed. [Figure 19] FIG. 19 is an enlarged plan view of a portion of the fifth evaluation pattern PT5 shown in FIG. [Figure 20] FIG. 19 is an enlarged plan view of a portion of the sixth evaluation pattern PT6 shown in FIG. [Figure 21] Enlarged plan view of part of the fifth evaluation patch PC5 and sixth evaluation patch PC6 [Figure 22] FIG. 10 is an enlarged plan view of a first evaluation pattern PT1 and a second evaluation pattern PT2 in an evaluation chart CT according to a third embodiment of the present invention. [Figure 23] Graph showing the relationship between the amount of misregistration and the development density difference [Figure 24] FIG. 10 is a diagram showing a modified example of the evaluation chart CT according to the present invention. [Figure 25] Graph showing changes in the development rate of each of the evaluation patterns PT1 to PT4 when the evaluation chart CT of the first embodiment is used. [Figure 26] 10 is a graph showing a difference value 1 between the first evaluation pattern PT1 and the second evaluation pattern PT2, and a difference value 2 between the third evaluation pattern PT3 and the fourth evaluation pattern PT4 when the evaluation chart CT of the first embodiment is used. [Figure 27] A graph showing the actual change in jitter deviation and the jitter deviation calculated from the change in difference value 1 and difference value 2. [Figure 28] Graph showing changes in the development rates of the fifth evaluation pattern PT5 and the sixth evaluation pattern PT6 when the evaluation chart CT of the second embodiment is used. [Figure 29] Graph showing difference values ​​1, 2, and 3 when the evaluation chart CT of the second embodiment is used. [Figure 30] 10 is a graph showing calculated values ​​of deviations such as jitter corrected using density difference correction values ​​according to the third embodiment and calculated values ​​of deviations such as jitter not corrected using density difference correction values; DETAILED DESCRIPTION OF THE INVENTION

[0015] 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.

[0016] 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).

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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).

[0029] 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.

[0030] 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.

[0031] 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.

[0032] The polygon mirror 45 is rotated at a constant speed in the clockwise direction in the figure by a polygon motor 46. 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. One scanning line SL is drawn per deflection surface 63. As the polygon mirror 45 rotates, the light beam LB is imaged sequentially 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.

[0033] 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 section 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] The image density sensor 50 emits measurement light from a 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.

[0038] 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).

[0039] 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."

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] FIG. 8 is a diagram showing the intermediate transfer belt 8 on which the evaluation chart CT is formed. In addition, in the enlarged views of the evaluation chart CT including FIG. 8, 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.

[0048] 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.

[0049] The evaluation chart CT is composed of a first evaluation pattern PT1, a second evaluation pattern PT2, a third evaluation pattern PT3, and a fourth evaluation pattern PT4, each 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. The third evaluation pattern PT3 and the fourth evaluation pattern PT4 are also arranged adjacent to each other in the main scanning direction. The third evaluation pattern PT3 is adjacent to the first evaluation pattern PT1 on the downstream side in the sub-scanning direction. The fourth evaluation pattern PT4 is adjacent to the second evaluation pattern PT2 on the downstream side in the sub-scanning direction.

[0050] The first to fourth evaluation patterns PT1 to PT4 are formed by arranging a plurality of first to fourth evaluation patches PC1 to PC4, each with a different shape, in the main scanning direction and the sub-scanning direction. Below, the first evaluation pattern PT1 will be described in detail, and the other evaluation patterns PT3 to PT4 will be described only in terms of the differences from the first evaluation pattern PT1.

[0051] 9 is an enlarged plan view of a portion of the first evaluation pattern PT1 shown in FIG. 8. 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.

[0052] 10 is an enlarged plan view of a portion of the second evaluation pattern PT2 shown in FIG. 8. As shown in FIG. 10, 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.

[0053] Fig. 11 is an enlarged plan view of the first evaluation pattern PT1 and the second evaluation pattern PT2. As shown in Fig. 11, the first evaluation patch PC1 is drawn with light beams LB1 to LB4 emitted from laser diodes LD1 to LD4 by selectively turning on or off one dot at a time. The first evaluation patch PC1 is composed 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, DT2, and DT3, DT4 that are 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.

[0054] 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.

[0055] 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.

[0056] The second dot row DL2 is adjacent to the first dot row DL1 on the downstream side 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 relative to the first dot row DL1.

[0057] As shown in FIG. 11, the second evaluation patch PC2, like the first evaluation patch PC1, is drawn by light beams LB1 to LB4 emitted from laser diodes LD1 to LD4 by selectively turning on or off each dot.

[0058] 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.

[0059] Fig. 12 is an enlarged view of a portion of the third evaluation pattern PT3 shown in Fig. 8. As shown in Fig. 12, the third evaluation pattern PT3 is composed of a plurality of third evaluation patches PC3. The third evaluation patches PC3 are arranged at predetermined intervals in the sub-scanning direction to form a third patch row PL3.

[0060] Fig. 13 is an enlarged view of a portion of the fourth evaluation pattern PT4 shown in Fig. 8. As shown in Fig. 13, the fourth evaluation pattern PT4 is made up of a plurality of fourth evaluation patches PC4. The fourth evaluation patches PC4 are arranged at predetermined intervals in the sub-scanning direction to form a fourth patch row PL4.

[0061] Fig. 14 is an enlarged plan view of the third evaluation pattern PT3 and the fourth evaluation pattern PT4. As shown in Fig. 14, the third evaluation patch PC3 is composed of a fifth dot row DL5 and a sixth dot row DL6. The fifth dot row DL5 and the sixth dot row DL6 are rows of dots DT1, DT2, and DT3, DT4 that are 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.

[0062] The sixth dot row DL6 is adjacent to the fifth dot row DL5 downstream in the sub-scanning direction. The sixth dot row DL6 is shifted downstream in the main scanning direction relative to the fifth dot row DL5 by a predetermined number of dots. The dot shift amount of the sixth dot row DL6 relative to the fifth dot row DL5 in the main scanning direction is one dot larger or one dot smaller downstream in the main scanning direction than the dot shift amount of the first dot row DL1 relative to the second dot row DL2 in the main scanning direction (two dots, see FIG. 11). Here, we will explain an example where the dot shift amount is one dot larger downstream (the sixth dot row DL6 is shifted three dots downstream in the main scanning direction relative to the fifth dot row DL5).

[0063] The fourth evaluation patch PC4 is composed of a seventh dot row DL7 and an eighth dot row DL8. The seventh dot row DL7 and the eighth dot row DL8 are rows of dots DT1, DT2 and DT3, DT4 that are drawn consecutively in a straight line, with a length of two dots in the sub-scanning direction and a length of four dots in the main scanning direction, respectively.

[0064] The eighth dot row DL8 is adjacent to the seventh dot row DL7 downstream in the sub-scanning direction. The eighth dot row DL8 is shifted upstream in the main scanning direction relative to the seventh dot row DL7 by a predetermined number of dots (the first number of dots). The amount of dot shift in the main scanning direction of the eighth dot row DL8 relative to the seventh dot row DL7 is either one dot smaller or one dot larger upstream in the main scanning direction than the amount of dot shift in the main scanning direction of the fourth dot row DL4 relative to the third dot row DL3 (two dots, see FIG. 12). Here, an example in which the eighth dot row DL8 is shifted one dot smaller upstream (the eighth dot row DL8 is shifted one dot upstream in the main scanning direction relative to the seventh dot row DL7) will be described.

[0065] In this example, the third evaluation patch PC3 and the fourth evaluation patch PC4 have shapes that are asymmetric in the main scanning direction. The shapes of the third evaluation patch PC3 and the fourth evaluation patch PC4 are the same as the shapes of the first evaluation patch PC and the second evaluation patch PC2, with the second dot array DL2 and the fourth dot array DL4 shifted by one dot downstream in the main scanning direction.

[0066] In other words, the sixth dot row DL6 and the eighth dot row DL8 are pre-positioned one dot downstream in the main scanning direction relative to the second dot row DL2 and the fourth dot row DL4. Hereinafter, to distinguish between this pre-positioned dot misalignment and unintentional dot misalignment caused by jitter or the like, the pre-positioned dot misalignment will be referred to as the "prescribed misalignment." Furthermore, dot misalignment caused by jitter or the like will be referred to as the "jitter or other misalignment."

[0067] 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 to fourth evaluation patches PC1 to PC4 will be misaligned downstream in the main scanning direction, as shown in Fig. 15. This causes 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.

[0068] More specifically, the first evaluation patch PC1 is deformed such that both ends of the first dot array DL1 and the second dot array DL2 in the main scanning direction are stretched as the dot DT4, which is located at the most downstream position in the second dot array DL2 in the main scanning direction, moves away from the dot DT1, which is located at the most upstream position in the first dot array DL1 in the main scanning direction. As a result, the area of ​​the white background portion where no dots are rendered increases in the first evaluation patch PC1. In contrast, the second evaluation patch PC2 is deformed such that both ends of the third dot array DL3 and the fourth dot array DL4 in the main scanning direction are compressed as the dot DT4, which is located at the most upstream position in the fourth dot array DL4 in the main scanning direction, moves closer to the dot DT2, which is located at the most downstream position in the third dot array DL3 in the main scanning direction. As a result, the area of ​​the white background portion decreases in the second evaluation patch PC2. 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.

[0069] A density difference occurs between the image density of the first evaluation patch PC1 and the image density of the second evaluation patch PC2, which also causes a density difference between the image density of the first evaluation pattern PT1 and the image density of the second evaluation pattern PT2. This difference in image density between the first evaluation pattern PT1 and the second evaluation pattern PT2 is referred to as the first density difference.

[0070] On the other hand, for the third evaluation patch PC3 and the fourth evaluation patch PC4, as described above, the sixth dot row DL6 and the eighth dot row DL8 are disposed so as to be shifted by one dot (prescribed shift) downstream in the main scanning direction. Therefore, a difference in image density between the two patches occurs even before unintended dot shift such as jitter occurs (this difference in image density is referred to as the second density difference). When dot shift occurs in the dots DT2 to DT4 as shown in FIG. 15, the third evaluation patch PC3 has a lighter image density, similar to the first evaluation patch PC1. Conversely, the fourth evaluation patch PC4 has a darker development density, similar to the second evaluation patch PC2. This causes a change in the second density difference.

[0071] The color shift correction unit 97 calculates a first density difference and a second density difference from the image density of the evaluation chart CT detected by the image density sensor 50, and calculates the amount of shift due to shift such as jitter using the first density difference and the second density difference.

[0072] Next, regarding the method for calculating the amount of dot misalignment when a predetermined jitter or other misalignment occurs, 2 ), the second density difference is -0.0148 (g / m 2 ) will be used as an example to explain the case with reference to FIG. 16. FIG. 16 is a graph showing the relationship between the magnitude of the specified deviation and the development density difference. The horizontal axis represents the magnitude of the specified deviation (μm), and the vertical axis represents the value of the development density difference between evaluation patterns adjacent in the main scanning direction.

[0073] As mentioned above, the specified deviation is defined based on the second dot row DL2 and the fourth dot row DL4, so there is no specified deviation for the second dot row DL2 and the fourth dot row DL4. In other words, the specified deviation in the main scanning direction is 0. If this is plotted as point P1, the coordinates (x, y) of point P1 become (0, 0.0072).

[0074] On the other hand, as described above, the sixth dot row DL6 and the eighth dot row DL8 are offset by one dot (+10.5 μm) downstream in the main scanning direction. When this is plotted as point P2, the coordinates (x, y) of point P2 are (10.5, -0.0148).

[0075] Here, the development density difference becomes zero when the jitter or other deviation and the specified deviation become equal and cancel each other out, and adjacent evaluation patterns in the main scanning direction become symmetrical, eliminating the development density difference between them. Therefore, the magnitude of the jitter or other deviation can be calculated by determining the value of the specified deviation when the development density difference is zero. In the above specific example, when points P1 and P2 are connected by a straight line, the x-coordinate of point 3, where the x-intercept of the line (the development density difference is zero) is 3.4188 μm. The specified deviation and the jitter or other deviation cancel each other out, i.e., the direction of the dot deviation is reversed (positive and negative). Therefore, the jitter or other deviation in this case is -3.4188 μm (3.4188 μm in the main scanning direction).

[0076] 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.

[0077] 17, the third evaluation patch PC3 and the fourth evaluation patch PC4 can have shapes other than those described above. For example, as shown in Fig. 17, the third evaluation patch PC3 can have the shape described above (where the sixth dot row DL6 is shifted three dots downstream in the main scanning direction relative to the fifth dot row DL5), and the fourth evaluation patch PC can have a dot shift amount in the main scanning direction of the eighth dot row DL8 relative to the seventh dot row DL7 that is one dot larger upstream in the main scanning direction than the dot shift amount in the main scanning direction of the fourth dot row DL4 relative to the third dot row DL3 (where the eighth dot row DL8 is shifted three dots upstream in the main scanning direction relative to the seventh dot row DL7).

[0078] In this case, the third evaluation patch PC3 and the fourth evaluation patch PC4 are symmetrical in the main scanning direction. Therefore, if there is no misalignment in the main scanning direction between the dots DT1 to DT8, the second density difference does not occur, and the jitter or other misalignment cannot be calculated from the first and second density differences. Therefore, in this case, the difference in image density between the first evaluation pattern PT1 and the fourth evaluation pattern PT4 is defined as the second' density difference, and the second' density difference and the first density are used instead of the second density difference to calculate the jitter or other misalignment using the method described above. In this case, the difference in the amount of misalignment in the main scanning direction between the eighth dot row DL8 and the fourth dot row DL4 (one dot in the main scanning direction) is synonymous with the magnitude of the specified misalignment. The second' density difference may also be the difference in image density between the second evaluation pattern PT2 and the third evaluation pattern PT3.

[0079] The third evaluation patch PC3 may be one in which the dot shift amount in the main scanning direction of the sixth dot row DL6 relative to the fifth dot row DL5 is one dot smaller downstream in the main scanning direction than the dot shift amount in the main scanning direction of the first dot row DL1 relative to the second dot row DL2 (the sixth dot row DL6 is shifted three dots downstream in the main scanning direction relative to the fifth dot row DL5), and the fourth evaluation patch PC may be one with the shape shown in Figure 17 (the eighth dot row DL8 is shifted three dots upstream in the main scanning direction relative to the seventh dot row DL7).In this case as well, deviations such as jitter can be calculated from the first density difference and the second' density difference.

[0080] Next, an image forming apparatus 100 according to a second embodiment will be described. In the following, 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 descriptions thereof will be omitted.

[0081] As shown in FIG. 18, the evaluation chart CT of the second embodiment is configured to include a fifth evaluation pattern PT5 and a sixth evaluation pattern PT6 in addition to the first to fourth evaluation patterns PT1 to PT4.

[0082] In the third evaluation patch PC3 of this embodiment, the dot misalignment amount of the sixth dot array DL6 relative to the fifth dot array DL5 in the main scanning direction is one dot larger downstream in the main scanning direction than the dot misalignment amount of the first dot array DL1 relative to the second dot array DL2 in the main scanning direction (the sixth dot array DL6 is shifted three dots downstream in the main scanning direction relative to the fifth dot array DL5).Furthermore, in the fourth evaluation patch PC4, the dot misalignment amount of the eighth dot array DL8 relative to the seventh dot array DL7 in the main scanning direction is one dot smaller upstream in the main scanning direction than the dot misalignment amount of the fourth dot array DL4 relative to the third dot array DL3 in the main scanning direction (the eighth dot array DL8 is shifted one dot upstream in the main scanning direction relative to the seventh dot array DL7).

[0083] 18, the fifth evaluation pattern PT5 is formed adjacent to and downstream of the first evaluation pattern PT1 in the sub-scanning direction. The sixth evaluation pattern PT6 is arranged adjacent to and downstream of the second evaluation pattern PT2 in the sub-scanning direction. The sixth evaluation pattern PT6 is adjacent to and downstream of the fifth evaluation pattern PT5 in the main scanning direction.

[0084] Fig. 19 is a plan view showing an enlarged portion of the fifth evaluation pattern PT5 shown in Fig. 18. Fig. 20 is a plan view showing an enlarged portion of the sixth evaluation pattern PT6. Fig. 21 is a plan view showing an enlarged portion of the fifth evaluation patch PC5 and the sixth evaluation patch PC6. As shown in Figs. 19 and 21, the fifth evaluation patch PC5 is composed of a ninth dot row DL9 and a tenth dot row DL10. The ninth dot row DL9 and the tenth dot row DL10 are rows of dots DT1, DT2 and DT3, DT4 that are 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.

[0085] The tenth dot row DL10 is adjacent to the ninth dot row DL9 downstream in the sub-scanning direction. The tenth dot row DL10 is shifted downstream in the main scanning direction relative to the ninth dot row DL9 by a predetermined number of dots (here, one dot). In other words, the dot shift amount in the main scanning direction of the tenth dot row DL10 relative to the ninth dot row DL9 is one dot (first dot number) smaller than the dot shift amount in the main scanning direction of the second dot row DL2 relative to the first dot row DL1 (two dots, see FIG. 11).

[0086] 20 and 21, the sixth evaluation patch PC6 is composed of an eleventh dot row DL11 and a twelfth dot row DL12. The sixth evaluation patch PC6 is composed of an eleventh dot row DL11 and a twelfth dot row DL12. The eleventh dot row DL11 and the twelfth dot row DL12 are rows of dots DT1, DT2, and DT3, DT4 that are drawn consecutively in a straight line, with a length of two dots in the sub-scanning direction and a length of four dots in the main scanning direction, respectively.

[0087] The twelfth dot row DL12 is adjacent to the eleventh dot row DL11 downstream in the sub-scanning direction. The twelfth dot row DL12 is shifted upstream in the main scanning direction relative to the eleventh dot row DL11 by a predetermined number of dots (here, one dot). In other words, the amount of dot shift in the main scanning direction of the twelfth dot row DL12 relative to the eleventh dot row DL11 is one dot (first dot number) larger upstream in the main scanning direction than the amount of dot shift in the main scanning direction of the fourth dot row DL4 relative to the third dot row DL3 (two dots, see FIG. 11).

[0088] As described above, the fifth evaluation pattern PT5 and the sixth evaluation pattern PT6 are adjacent to each other in the main scanning direction. Therefore, the laser diodes LD3 and LD4 sequentially form the tenth dot row DL10 and the twelfth dot row DL12 in the same scan. The control unit 90 adjusts the light emission timing (the timing of emitting the light beams LB3 and LB4) of the laser diodes LD3 and LD4 so that the light emission timing when drawing the tenth dot row DL10 and the twelfth dot row DL12 is one dot earlier in the main scanning direction than the light emission timing when drawing the first dot row DL1 and the third dot row DL3.

[0089] The control unit 90 calculates the difference in image density between the first evaluation pattern PT1 and the second evaluation pattern PT2 (first density difference), the difference in image density between the third evaluation pattern PT3 and the fourth evaluation pattern PT4 (second density difference), and the difference in image density between the fifth evaluation pattern PT5 and the sixth evaluation pattern PT6 (third density difference). Using the first density difference and the second or third density difference, the amount of dot misalignment due to misalignment such as jitter can be calculated using the same method as the method of the first embodiment described above.

[0090] Here, the direction of the specified deviation for the fifth evaluation pattern PT5 and the sixth evaluation pattern PT6 is opposite to that for the third evaluation pattern PT3 and the fourth evaluation pattern PT4. That is, the specified deviation for the tenth dot row DL10 and the twelfth dot row DL12 is one dot upstream (negative side) in the main scanning direction. When the deviation due to jitter or the like is downstream (positive side) in the main scanning direction, the first density difference and the third development density difference are used to calculate the amount of dot deviation due to jitter or the like. When the deviation due to jitter or the like is upstream (negative side) in the main scanning direction, the first density difference and the second density difference are used to calculate the amount of dot deviation due to jitter or the like. This allows for more accurate calculation of the amount of dot deviation due to jitter or the like. The control unit 90 can simultaneously calculate the first to third density differences from one evaluation chart CT and select the optimal amount of dot deviation by comparing the amount of dot deviation calculated from the first density difference and the second density difference with the amount of dot deviation calculated from the first density and the third density difference.

[0091] Next, an image forming apparatus 100 according to a third embodiment will be described. Fig. 22 is an enlarged plan view of a first evaluation pattern PT1 and a second evaluation pattern PT2 in an evaluation chart CT according to the third embodiment. The evaluation chart CT according to this embodiment includes first evaluation patterns PT1 to PT6 (upper part of the figure) formed by focusing all of the light beams LB1 to LB8 on a predetermined first deflection surface 63a (hereinafter referred to as "same-surface scanning"), and first evaluation patterns PT1 to PT6 (lower part of the figure) formed by focusing some of the light beams LB1 to LB8 on the predetermined first deflection surface 63a and the remaining beams of the light beams LB1 to LB8 on a second deflection surface 63b adjacent to the first deflection surface 63a (hereinafter referred to as "different-scanning-surface scanning").

[0092] To explain the different scanning plane scanning in more detail, of the light beams LB1 to LB8, the light beams LB (here, light beams LB7 and LB8) that draw the first dot row DL1, the third dot row DL3, the fifth dot row DL5, the seventh dot row DL7, the ninth dot row DL9, and the eleventh dot row DL11 are focused on the first deflection surface 63a, and the light beams LB (here, light beams LB1 and LB2) that draw the second dot row DL2, the fourth dot row DL4, the sixth dot row DL6, the eighth dot row DL8, the tenth dot row DL10, and the twelfth dot row DL12 are focused on the second deflection surface 63b (the third evaluation pattern PT3 to the sixth evaluation pattern PT6 are not shown).

[0093] The image density difference between the first evaluation pattern PT1 (same-surface first evaluation pattern) and the second evaluation pattern PT2 (same-surface second evaluation pattern) formed by the same-surface scanning at this time is referred to as the same-surface first density difference. Also, the image density difference between the first evaluation pattern PT1 (different-scanning-surface first evaluation pattern) and the second evaluation pattern PT2 (different-scanning-surface second evaluation pattern) formed by the different-scanning-surface scanning is referred to as the different-scanning-surface first density difference.

[0094] In the image forming apparatus 100 of this embodiment, a density difference correction value is calculated from the same-surface first density difference and the different-scanning surface first density difference, and by subtracting the density difference correction value from the first density difference to the third density difference calculated in each of the above embodiments, it is possible to calculate a more accurate amount of dot shift due to jitter, etc.

[0095] The method for calculating the density difference correction value will be explained below with reference to the graph in Fig. 23. Fig. 23 is a graph showing the relationship between the magnitude of the specified deviation and the development density difference. The horizontal axis represents the magnitude of the specified deviation (μm), and the vertical axis represents the value of the development density difference between evaluation patterns adjacent in the main scanning direction.

[0096] For example, when the distance in the main scanning direction between dot DT1 and dot DT8 is set to -13.125 μm to simulate a shift such as jitter, the first density difference on the same surface is 0.0072 (g / m 2 ), the first density difference on the different scanning surface is -0.0238 (g / m 2 ) Here, 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, on the graph of FIG. 23, point P1 is plotted for same-surface scanning with the horizontal axis set to -1 and the vertical axis set to the same-surface first density difference, and point P2 is plotted for different-scanning surface scanning with the horizontal axis set to 7 and the vertical axis set to the different-scanning surface first density difference. That is, the coordinates (X, Y) of point P1 are (-1, 0.0072), and the coordinates (X, Y) of point P2 are (7, -0.0238). In this case, the y-intercept P3 is 0.00328. This y-intercept P3 is the density difference correction value when the distance in the main scanning direction between dots DT1 and DT8 is set to -13.125 μm. This density difference correction value is subtracted from the first density difference, the second density difference, and the third density difference.

[0097] If the amount of dot misalignment due to jitter, etc., were calculated using the same-surface first density difference and same-surface second density difference (second density difference during same-surface scanning) without using the density difference correction value described above and the method using the graph in FIG. 16, the result would be -3.4188 μm. If the density difference correction value described above were subtracted from the first to third density differences and the result were used to calculate the amount of dot misalignment due to jitter, etc., using the method described above (the method of calculating the y-intercept using the graph in FIG. 16), the result would be -1.8514 μm. In this case, the distance in the main scanning direction between dot DT1 and dot DT8 is -13.125 μm, and if the dot misalignment amount is calculated by subtracting 1 from the number of laser diodes LD1 to LD8 (8 minus 1) (the number of gaps between adjacent laser diodes LD1 to LD8), the result would be -1.875 μm. Therefore, the calculated value after correction (-1.8514 μm) is closer to the amount of dot misalignment than the calculated value before correction (-3.4188 μm).

[0098] In the conventional image forming apparatus 100, several types of evaluation charts were sequentially formed with the dots shifted by a predetermined amount in the main scanning direction, and the change in image density of each evaluation chart was detected to detect dot misalignment. As a result, multiple evaluation charts with different dot misalignment amounts were formed for each evaluation chart CT, which required the formation of a huge number of evaluation charts, making image density detection cumbersome.

[0099] On the other hand, by adopting the evaluation chart CT of each of the above-mentioned embodiments, the image forming apparatus 100 of the present invention makes it possible to compare the image densities of evaluation patterns PT having multiple types of dot misalignment amounts using a single evaluation chart, and makes it possible to accurately adjust dot misalignment using a small number of evaluation charts.

[0100] Furthermore, by adopting the evaluation chart CT of the third embodiment, it is possible to obtain a density difference correction value that allows the magnitude of dot misalignment to be detected more precisely, thereby making it possible to adjust dot misalignment more accurately.

[0101] 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, as shown in FIG. 24, multiple evaluation charts CT 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 positions in the main scanning direction, it is possible to appropriately correct the dot misalignment for each position in the main scanning direction. Furthermore, in this case, as shown in FIG. 24, the evaluation patterns PT1 to PT6 can be arranged linearly in the sub-scanning direction. In this case, the color misalignment correction unit 97 detects the image density of each evaluation chart CT using the center of the evaluation chart CT in the main scanning direction as the reference position.

[0102] 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, it becomes possible to simultaneously detect dot misalignment for each position in the main scanning direction, making it easier to correct the dot misalignment.

[0103] The effects of the present invention will be described in more detail below with reference to examples. [Example]

[0104] The change in image density for the evaluation chart CT for each 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 charts CT according to the first and third embodiments of the present invention were printed on printing paper (recording medium). The image density (%) was calculated analytically, and the results were compared by sequentially shifting the dot positions in the main scanning direction. Note that, among the evaluation charts CT according to the first embodiment, the one shown in FIG. 14 (in which the sixth dot row DL6 is shifted three dots downstream in the main scanning direction relative to the fifth dot row DL5, and the eighth dot row DL8 is shifted one dot upstream in the main scanning direction relative to the seventh dot row DL7) was used.

[0105] In the test, the dot misalignment amount (the distance between dot DT1 and dot DT8 in the main scanning direction) was changed in stages between -21 μm and 21 μm or less to simulate a state in which misalignment such as jitter occurred. At this time, the change in the development rate (%) (the ratio of the image density of the evaluation pattern PT when the image density (the ratio of the area occupied by the black background to the entire area of ​​the evaluation pattern PT) when the evaluation pattern PT is drawn entirely in black is set to 1) was calculated (see Figure 25). The dot misalignment amount was adjusted by changing the light emission timing of the laser diodes LD1 to LD8. The dot misalignment amount is defined as a positive value for misalignment in the main scanning direction and a negative value for misalignment in the direction opposite to the main scanning direction. In addition, the change in the difference in development rate between the evaluation patterns PT1 to PT6 adjacent in the main scanning direction was calculated as a function of the change in the dot misalignment amount (see Figure 26).

[0106] Fig. 25 is a graph showing the change in development rate of each of the evaluation patterns PT1 to PT4 when using the evaluation chart CT of the first embodiment. In Fig. 25, the first evaluation pattern PT1 is shown as a black circle, the second evaluation pattern PT2 as a circle, the third evaluation pattern PT3 as a square, and the fourth evaluation pattern PT4 as a square.

[0107] 25, the first evaluation pattern PT1 and the third evaluation pattern PT are graphs that slope downward to the right, whereas the second evaluation pattern PT2 and the fourth evaluation pattern PT4 are graphs that slope upward to the right. This is because the first evaluation pattern PT1 and the second evaluation pattern PT2 are symmetrical in shape in the left-right direction (see FIGS. 11 and 14), and therefore the first evaluation pattern PT1 and the second evaluation pattern PT2 have an inverse relationship in terms of how the development rate changes with respect to changes in the amount of misalignment in the main scanning direction.

[0108] That is, in the first evaluation pattern PT1, the second dot row DL2 is pre-shifted toward the downstream side (+ side) of the main scanning direction relative to the first dot row DL1 (see FIG. 11). Therefore, as the dot misalignment amount increases, the dot misalignment between the first dot row DL1 and the second dot row DL2 gradually increases. This reduces the image density of the first evaluation patch PC1 alone, and the development rate of the first evaluation pattern PT1 also decreases. Meanwhile, in the second evaluation pattern PT2, the fourth dot row DL4 is pre-shifted toward the upstream side (- side) of the main scanning direction relative to the third dot row DL3 (see FIG. 14). Therefore, as the dot misalignment amount increases, the dot misalignment between the second dot row DL2 and the third dot row DL3 gradually decreases. This reduces the image density of the second evaluation patch PC2 alone, and the development rate of the second evaluation pattern PT2 also increases. A similar relationship holds for the third evaluation pattern PT3 and the fourth evaluation pattern PT4.

[0109] 26 is a graph showing the difference value 1 (▲) in the development rates between the first evaluation pattern PT1 and the second evaluation pattern PT2, and the difference value 2 (△) in the development rates between the third evaluation pattern PT3 and the fourth evaluation pattern PT4 when the evaluation chart CT of the first embodiment is used. As shown in FIG. 26, as the dot shift amount increases downstream in the main scanning direction, the difference value 1 and the difference value 2 gradually decrease. Because the first evaluation pattern PT1 and the second evaluation pattern PT2 are symmetrical in the main scanning direction, when the dot shift amount in the main scanning direction is zero, the difference value also becomes zero. On the other hand, because the third evaluation pattern PT3 and the fourth evaluation pattern PT4 are asymmetrical in shape with respect to the main scanning direction, the difference value does not become zero even when the dot shift amount in the main scanning direction is zero.

[0110] FIG. 27 is a graph showing the change in actual jitter deviation, etc., and the change in jitter deviation, etc., calculated from the changes in difference value 1 and difference value 2. Difference value 1 is calculated using the method described above, with difference value 1 being the first density difference and difference value 2 being the second density difference. The actual jitter deviation, etc. (the distance between dot DT1 and dot DT8) is indicated by the circle, and the amount of jitter deviation, etc., calculated from difference value 1 and difference value 2 using the method of the first embodiment is indicated by the black circle. As shown in FIG. 27, the value of jitter deviation, etc., calculated from difference value 1 and difference value 2 also changes in line with the change in actual jitter deviation, etc. Therefore, it was confirmed that it is possible to calculate an approximate value of jitter deviation, etc., from difference value 1 (first density) and difference value 2 (second density) using the evaluation chart CT of the first embodiment.

[0111] Next, Fig. 28 is a graph showing the changes in the development rates of the fifth evaluation pattern PT5 and the sixth evaluation pattern PT6 when the evaluation chart CT of the third embodiment is used. In Fig. 28, the fifth evaluation pattern PT5 is indicated by a ◆ graph, and the sixth evaluation pattern PT6 is indicated by a ◇ graph. Note that the first evaluation pattern PT1 to the fourth evaluation pattern PT4 of the third embodiment have the same shapes as those of the first embodiment. Therefore, their development rates are the same as those shown in the graph of Fig. 26. Therefore, the graphs of the first evaluation pattern PT1 to the fourth evaluation pattern PT4 are omitted in Fig. 28.

[0112] As shown in FIG. 28, the graph of the fifth evaluation pattern PT5 (the graph marked with a diamond) slopes downward to the right, and conversely, the graph of the sixth evaluation pattern PT6 (the graph marked with a diamond) slopes upward to the right.

[0113] Figure 29 is a graph showing difference value 1 (graph with ▲), difference value 2 (graph with △), and difference value 3 (graph with ×) of the development rate between the fifth evaluation pattern PT5 and the sixth evaluation pattern PT6 when using the evaluation chart CT of the second embodiment described above.

[0114] 30 is a graph showing calculated values ​​of jitter and other deviations corrected using the density difference correction value according to the third embodiment, and calculated values ​​of jitter and other deviations not corrected using the density difference correction value. The actual values ​​of jitter and other deviations are indicated by circles. Furthermore, the amount of jitter and other deviations calculated using the method of the first embodiment from difference values ​​1 and 2 without correction using the density correction value is indicated by circles. Furthermore, the amount of jitter and other deviations calculated by calculating the density difference correction value using the method described above and correcting difference values ​​1 to 3 using this density difference correction value is indicated by *.

[0115] 30, the calculated values ​​of the jitter and other deviations after correction (values ​​indicated by the * graph) are closer to the actual jitter and other deviations (values ​​indicated by the ○ graph) than the calculated values ​​of the jitter and other deviations without correction (values ​​indicated by the ● graph). For this reason, by correcting each difference value (first to third density differences) by subtracting the density difference correction value from each difference value (first to third density differences), it is possible to calculate a more accurate amount of dot deviation due to jitter and other deviations. [Industrial Applicability]

[0116] 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]

[0117] 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 CT Evaluation Chart DL1 First dot row DL2 Second dot row DL3 Third dot row DL4 4th dot row DL5 5th dot row DL6 6th dot row DL7 7th dot row DL8 8th dot row DL9 9th dot row DL10 10th dot row DL11 11th dot row DL12 12th dot row DT1~DT8 dots LB1~LB8 Light beams LD1~LD8 Laser diodes PC1 1st evaluation patch PC2 2nd Evaluation Patch PC3 3rd Evaluation Patch PC4 4th Evaluation Patch PC5 5th Evaluation Patch PC6 6th Evaluation Patch PL1 1st patch row PL2 2nd patch row PL3 3rd patch row PL4 4th patch row PL5 5th patch row PL6 6th patch row PT1 First evaluation pattern PT2 Second evaluation pattern PT3 Third evaluation pattern PT4 4th evaluation pattern PT5 5th evaluation pattern PT6 6th evaluation pattern 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, a third evaluation pattern, and a fourth evaluation pattern that are 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 one or more dots in the sub-scanning direction are successively arranged in a straight line in the main scanning direction, and a second dot row in which dots whose number is equal to or less than the number of the light-emitting units 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 successively arranged in a straight line so as to be shifted from the first dot row in the main scanning direction; The second evaluation pattern is a second patch row configured by arranging a plurality of second evaluation patches, each having a symmetrical shape to the first evaluation patch in the parallel direction of the first evaluation pattern and the second evaluation pattern, at the same intervals as 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 the plurality of second patch rows at equal intervals at predetermined intervals in the main scanning direction; The second evaluation patch is a third dot row that is located at a position overlapping in the sub-scanning direction with the first dot row included in the first evaluation patch that is adjacent in the parallel direction; a fourth dot row that overlaps in the sub-scanning direction with the second dot row included in the first evaluation patch that is adjacent to the first evaluation patch in the parallel direction; and The third evaluation pattern is a third patch row configured by arranging a plurality of third evaluation patches at equal intervals in the sub-scanning direction, the third evaluation patches having a fifth dot row in which the same number of dots as the first dot row are arranged consecutively in the main scanning direction in the sub-scanning direction, and a sixth dot row in which the same number of dots as the second dot row are adjacent to the fifth dot row in the sub-scanning direction and are arranged consecutively in a straight line so as to be shifted from the fifth dot row in the main scanning direction, the third patch row being configured by arranging a plurality of the third patch rows at equal intervals in the main scanning direction, The fourth evaluation pattern is a fourth patch row configured by arranging a plurality of fourth evaluation patches at equal intervals in the sub-scanning direction, the fourth evaluation patches having: a seventh dot row in which the same number of dots as the first dot row are arranged consecutively in the main scanning direction in the sub-scanning direction; and an eighth dot row in which the same number of dots as the second dot row are adjacent to the seventh dot row in the sub-scanning direction and are arranged consecutively in a straight line so as to be shifted from the seventh dot row in the main scanning direction; and a deviation of the sixth dot row from the fifth dot row in the main scanning direction is greater than or smaller than a deviation of the second dot row from the first dot row by a predetermined first number of dots in the main scanning direction; An image forming apparatus characterized in that the deviation of the eighth dot row from the seventh dot row in the main scanning direction is greater by the first dot number than the deviation of the fourth dot row from the third dot row, or is smaller by the first dot number in the main scanning direction.

2. 2. The image forming apparatus according to claim 1, wherein the fourth evaluation patch has a shape symmetrical to the third evaluation patch in the parallel direction.

3. 2. The image forming apparatus according to claim 1, wherein the fourth evaluation patch has an asymmetrical shape with respect to the third evaluation patch in the juxtaposition direction.

4. the evaluation chart has a fifth evaluation pattern and a sixth evaluation pattern that are parallel to the first evaluation pattern in the main scanning direction or the sub-scanning direction, the fifth evaluation pattern has a fifth patch row configured by arranging a plurality of fifth evaluation patches at equal intervals in the sub-scanning direction, the fifth evaluation patch row having: a ninth dot row in which the same number of dots as the first dot row are arranged consecutively in the main scanning direction in the sub-scanning direction; and a tenth dot row in which the same number of dots as the second dot row are adjacent to the ninth dot row in the sub-scanning direction and are arranged consecutively in a straight line so as to be shifted from the ninth dot row in the main scanning direction; and the fifth evaluation pattern is configured by arranging a plurality of the fifth patch rows at equal intervals in the main scanning direction, the fifth patch row having a same number of dots as the first dot row in the sub-scanning direction and being shifted from the ninth dot row in the main scanning direction; the sixth evaluation pattern has a sixth patch row configured by arranging a plurality of sixth evaluation patches at equal intervals in the sub-scanning direction, the sixth evaluation pattern having an eleventh dot row in which the same number of dots as the first dot row are arranged consecutively in the main scanning direction in the sub-scanning direction, and a twelfth dot row in which the same number of dots as the second dot row are adjacent to the eleventh dot row in the sub-scanning direction and arranged consecutively in a straight line so as to be shifted from the eleventh dot row in the main scanning direction, the sixth evaluation pattern having a sixth patch row configured by arranging a plurality of sixth evaluation patches at equal intervals in the main scanning direction at predetermined intervals, a deviation of the sixth dot row from the fifth dot row in the main scanning direction is greater than a deviation of the second dot row from the first dot row by the first dot number; a deviation of the eighth dot row from the seventh dot row is smaller by the first dot number in the main scanning direction than a deviation of the fourth dot row from the third dot row; a deviation of the tenth dot row from the ninth dot row is smaller by the first dot number in the main scanning direction than a deviation of the second dot row from the first dot row; 4. The image forming apparatus according to claim 3, wherein a deviation of the twelfth dot row from the eleventh dot row in the main scanning direction is greater by the first dot number than a deviation of the fourth dot row from the third dot row.

5. a density detection mechanism capable of detecting the image density of the toner image visualized by the evaluation chart; 4. The image forming apparatus according to claim 3, wherein the control unit shifts the timing of emission of the light beam from each of the light-emitting units based on a comparison result obtained by comparing a difference value between the image density of the first evaluation pattern and the image density of the second evaluation pattern detected by the density detection mechanism with a difference value between the image density of the third evaluation pattern and the image density of the fourth evaluation pattern detected by the density detection mechanism.

6. a density detection mechanism capable of detecting the image density of the toner image visualized by the evaluation chart; 5. The image forming apparatus according to claim 4, wherein the control unit shifts the timing of emission of the light beam from each of the light-emitting units based on a comparison result obtained by comparing a difference value between the image densities of the first evaluation pattern and the second evaluation pattern detected by the density detection mechanism, a difference value between the image densities of the third evaluation pattern and the fourth evaluation pattern detected by the density detection mechanism, and a difference value between the image densities of the fifth evaluation pattern and the sixth evaluation pattern detected by the density detection mechanism.

7. a plurality of the evaluation charts are arranged at predetermined intervals in the main scanning direction; The image forming apparatus according to claim 5 or 6, characterized in that the control unit adjusts the amount of shift in the timing of emission of the light beam of each light-emitting element for each position of the evaluation chart based on the comparison result for each evaluation chart.

8. 8. The image forming apparatus according to claim 7, wherein the control unit determines the shift amount of the timing of emission of the light beam from each of the light-emitting elements using the center of each of the evaluation charts as a reference position in the main scanning direction.

9. 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; 9. 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.

10. a fixing device that fixes the toner image on the image carrier, which has been visualized by the developing unit, onto a recording medium; 9. 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.

11. 11. An image forming apparatus according to claim 1, wherein all of the light beams emitted from the light-emitting elements are reflected by one of the deflection surfaces of the polygon mirror, thereby forming the evaluation chart.

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 a same-surface density difference which is a difference in development density between the same-surface first evaluation pattern and the same-surface second evaluation pattern; a different scanning surface density difference, which is a difference in development density between the first different scanning surface evaluation pattern and the second different scanning surface evaluation pattern; a density difference correction value calculated from the same surface density difference and the different scanning surface density difference; can be calculated, 11. The image forming apparatus according to claim 5, wherein the control unit is capable of correcting each difference value based on the density difference correction value and adjusting the timing of emission of the light beam from each of the light-emitting units.

Images