Image forming apparatus

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 storage unit stores an evaluation chart. The evaluation chart has a first evaluation pattern and a second evaluation pattern. The first evaluation pattern has a plurality of first evaluation patches arranged at a predetermined interval in a main scanning direction and a sub scanning direction. The first evaluation patch has a first dot row, a second dot row that is formed to be deviated from the first dot row with respect to the main scanning direction, and an auxiliary dot row that is arranged on at least one of both sides in the sub scanning direction of the first dot row and the second dot row and formed linearly in the main scanning direction. The second evaluation pattern has a plurality of second evaluation patches having a symmetrical shape to the first evaluation patches in the main scanning direction or the sub scanning direction arranged in the main scanning direction and the sub scanning direction.SELECTED DRAWING: Figure 3

Description

Technical Field

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

Background Art

[0002] Conventionally, there have been demands for higher speed and higher resolution in image output for electrophotographic image forming apparatuses. In response to this demand, an image forming apparatus that employs a multi-beam method in which a photosensitive drum is scanned with light beams from a multi-beam laser having a plurality of light emitters is known (Patent Document 1).

[0003] Patent Document 1 discloses an image forming apparatus having a main body portion and a multi-beam laser having a plurality of light emitters linearly arranged at the tip of the main body portion. The image forming apparatus draws dots by light beams emitted from each light emitter and forms an image with a plurality of dots. The interval between each dot with respect to the main scanning direction of the light beam can be adjusted by changing the timing of emission of the light beam (the lighting timing of each light emitting portion). The interval between each dot with respect to the sub-scanning direction (the direction orthogonal to the main scanning direction) of the light beam can be adjusted by changing the rotation angle of the main body portion. The ideal emission timing is stored in advance in a storage unit provided in the image forming apparatus.

[0004] By the way, in a conventional general image forming apparatus, due to jitter or the like caused by development characteristics or vibrations of a conveyance system such as a transfer belt, there has been a risk that dots shift in the main scanning direction, resulting in density unevenness and density variations. In contrast, the image forming apparatus of Patent Document 1 can detect the above-described dot shift by forming a plurality of predetermined evaluation charts and comparing the density differences of each evaluation chart. The control unit of the image forming apparatus can eliminate the dot shift by changing the light emission timing of each light emitting portion with respect to the detected dot shift.

[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 patch is 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 number of dots in the sub-scanning direction of the first dot row and the second dot row is 1. The second dot row is connected to the downstream side of the first dot row in the sub-scanning direction and is arranged shifted to the downstream side in the main scanning direction with respect to the first dot row.

[0006] If dot misalignment occurs in the main scanning direction, the area of the overlapping portion between the first dot row and the second dot row with respect to the main scanning direction changes. For this reason, a change occurs in the image density of the evaluation chart. According to this change amount, the light emission timing of each light emitting unit is adjusted. The image data of the evaluation chart and the correction value of the light emission timing according to the magnitude of the dot misalignment are stored in advance in the storage unit of the control unit.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, in the first dot row and the second dot row according to the image forming apparatus of Patent Document 1, the number of dots in the sub-scanning direction is 1 dot each. For this reason, under conditions with relatively high resolution, the development amount of the dots in the evaluation chart becomes small. For this reason, even if dot misalignment occurs, the change amount of the image density of the evaluation chart is small, and it becomes difficult to detect the dot misalignment.

[0009] An object of the present invention is to provide an image forming apparatus capable of easily and accurately adjusting dot misalignment in the main scanning direction.

Means for Solving the Problems

[0010] In order to achieve the above object, a first configuration of the present invention is an image forming apparatus including an optical scanning device, a developing unit, a control unit, and a storage unit. The optical scanning device includes a light source having four or more light emitting units arranged in a row at regular intervals with a predetermined angle with respect to the main scanning direction, and a polygon mirror that deflects and scans the light beams emitted from each light emitting unit, and forms an electrostatic latent image on the image carrier by the light beams. The developing unit forms a toner image obtained by visualizing the electrostatic latent image. The control unit controls the optical scanning device so as to form an electrostatic latent image according to the image data by switching on and off each light emitting unit. The storage unit stores a predetermined evaluation chart that is composed of dots by the light beams for each light emitting unit and determines the writing timing of each light emitting unit. The evaluation chart has a first evaluation pattern and a second evaluation pattern that is parallel to the first evaluation pattern with respect to the main scanning direction or the sub-scanning direction orthogonal to the main scanning direction. The first evaluation pattern has a first dot row linearly formed in the main scanning direction, a second dot row linearly formed so as to be displaced from the first dot row with respect to the main scanning direction, and an auxiliary dot row linearly formed on at least one side of both sides in the sub-scanning direction of the first dot row and the second dot row, and a first patch row in which a plurality of first evaluation patches having the above are arranged at equal intervals in the sub-scanning direction, and the first patch row is configured by arranging a plurality of them at equal intervals with a predetermined interval in the main scanning direction. At least one of the first dot row or the second dot row has 1 dot number in the sub-scanning direction. The auxiliary dot row is arranged on the side where the number of dots in the sub-scanning direction of one of the first dot row and the second dot row is 1 with respect to the sub-scanning direction, and is not arranged on the side where the number of dots in the sub-scanning direction is 2 or more, and is formed over the entire area between both ends of the first evaluation patch with respect to the main scanning direction. The second evaluation pattern has a second patch row in which a plurality of second evaluation patches having a symmetrical shape with the first evaluation patch in the parallel direction between the first evaluation pattern and the second evaluation pattern are arranged at the same interval as the parallel interval of the plurality of first evaluation patches with respect to the sub-scanning direction, and the second patch row is configured by arranging a plurality of them at equal intervals with a predetermined interval in the main scanning direction.

Advantages of the Invention

[0011] According to the first configuration of the present invention, a first auxiliary dot column or a second auxiliary dot column is arranged on at least one side of the first dot column and the second dot column. That is, the overlapping portion of the first dot column and the second dot column with respect to the main scanning direction and the first auxiliary dot column or the second auxiliary dot column are arranged adjacent to each other in the sub-scanning direction. Therefore, the total amount (development area) of the developer in the portion where the first auxiliary dot column or the second auxiliary dot column is arranged increases, and when dot misalignment occurs, it becomes easier to confirm the change in image density.

[0012] Therefore, it becomes easier to detect a change in image density, and it is possible to provide an image forming apparatus capable of easily and accurately adjusting dot misalignment in the main scanning direction.

Brief Description of Drawings

[0013]

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Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments 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. Inside the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the conveyance 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 cyan, magenta, yellow, and black images are sequentially formed by the respective steps of charging, exposure, development, and transfer.

[0015] Photoconductor drums 1a, 1b, 1c, and 1d for carrying visible images (toner images) of each color are arranged in these image forming units Pa to Pd. Further, an intermediate transfer belt 8 that rotates in the clockwise direction in FIG. 1 is provided adjacent to each of the image forming units Pa to Pd. The toner images formed on these photoconductor drums 1a to 1d are sequentially primary-transferred and superimposed on the intermediate transfer belt 8 that moves while contacting each of the photoconductor drums 1a to 1d. Thereafter, the toner image primary-transferred onto the intermediate transfer belt 8 is secondary-transferred onto a sheet S (recording medium), which is an example of a recording medium, by a secondary transfer roller 9. Further, the sheet S onto which the toner image has been secondary-transferred is discharged from the main body of the image forming apparatus 100 after the toner image is fixed by a fixing device 13. While rotating the photoconductor drums 1a to 1d in the counterclockwise direction in FIG. 1 by a main motor 40 (see FIG. 4), an image forming process for each of the photoconductor drums 1a to 1d is executed.

[0016] The paper S onto which the toner image is secondarily transferred is housed in a paper cassette 16 arranged at the lower part of the main body of the image forming apparatus 100, and is conveyed to the nip portion between the secondary transfer roller 9 and the driving roller 11 of the intermediate transfer belt 8 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a seamless belt having no seam is mainly used. Further, a blade-shaped belt cleaner 19 for removing toner and the like remaining on the surface of the intermediate transfer belt 8 is arranged on the downstream side of the secondary transfer roller 9.

[0017] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photosensitive drums 1a to 1d, charging devices 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d, an optical scanning device 5 for exposing the photosensitive drums 1a to 1d with image information, developing devices 3a, 3b, 3c, and 3d for forming a toner image on the photosensitive drums 1a to 1d, cleaning devices 7a, 7b, 7c, and 7d for removing the developer (toner) and the like remaining on the photosensitive drums 1a to 1d, and an image density sensor 50 (density detection mechanism) capable of detecting the density of the toner image primarily transferred to the intermediate transfer belt 8 are provided.

[0018] When image data is input from a host device such as a personal computer, first, the charging devices 2a to 2d uniformly charge the surfaces of the photosensitive drums 1a to 1d. Next, light irradiation is performed according to the image data by the optical scanning device 5 to form an electrostatic latent image corresponding to the image data on each of the photosensitive drums 1a to 1d. The developing devices 3a to 3d are filled with a prescribed amount of a two-component developer containing cyan, magenta, yellow, and black toners of each color. When the ratio of the toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a prescribed value due to the formation of the toner image described later, toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in the developer is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and adheres electrostatically. Thereby, a toner image corresponding to the electrostatic latent image formed by the exposure from the optical scanning device 5 is formed.

[0019] Then, with the primary transfer rollers 6a to 6d, an electric field is applied at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photoreceptor drums 1a to 1d, and the cyan, magenta, yellow, and black toner images on the photoreceptor drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. These four-color images are formed with a predetermined positional relationship determined in advance for predetermined full-color image formation. Thereafter, in preparation for the subsequent formation of a new electrostatic latent image, the toner and the like remaining on the surfaces of the photoreceptor drums 1a to 1d after the primary transfer are removed by the cleaning devices 7a to 7d.

[0020] The intermediate transfer belt 8 is stretched between the upstream driven roller 10 and the downstream driving roller 11. When the intermediate transfer belt 8 starts to rotate in the clockwise direction as the driving roller 11 rotates by the belt driving motor 51 (see FIG. 4), the sheet S is conveyed from the registration roller pair 12b to the nip portion (secondary transfer nip portion) between the driving roller 11 and the 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 sheet S. The sheet S onto which the toner image has been secondarily transferred is conveyed to the fixing device 13.

[0021] The image density sensor 50 is arranged so as to face the driven roller 10 with the intermediate transfer belt 8 interposed therebetween. The image density sensor 50 is, for example, a specular reflection type sensor that detects reflected light. The image density sensor 50 is composed of an LED light source arranged 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). Then, light is irradiated from the LED light source onto the toner image on the intermediate transfer belt 8, and the phototransistor detects the amount of the 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 a control unit 90 described later. Note that the image density sensor 50 may be any sensor that can detect the density information of the toner image. For example, it may be a sensor that can detect the density from an image obtained by imaging the toner image.

[0022] The sheet S conveyed to the fixing device 13 is heated and pressed by a fixing belt 21 (first fixing member) and a pressure roller 22 (second fixing member), and the toner image is fixed on the surface of the sheet S, forming a predetermined full-color image. The sheet S on which the full-color image is formed has its conveyance direction distributed by a branching portion 30 branched in a plurality of directions, and is then discharged to a discharge tray 17 by a pair of discharge rollers 15 (either as it is or after being sent to a duplex conveyance path 18 and having images formed on both sides).

[0023] 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 schematically showing the configuration of the optical scanning device 5. FIG. 3 is a perspective view showing a light source unit 26. The optical scanning device 5 performs optical scanning on each of the photosensitive drums 1a to 1d. Here, only the optical scanning on the photosensitive drum 1a will be described, and other descriptions will be omitted.

[0024] 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, and a polygon mirror 45, a scanning lens 49.

[0025] The light source unit 26 (light source) has a tip surface 27, laser diodes LD1 to LD8 (light emitters), and a beam generation 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 rotated in the circumferential direction with an axis (central axis L1) passing through the center of the tip surface 27 among the normals to the tip surface 27 as the rotation axis, thereby adjusting and fixing the intervals in the sub-scanning direction of the laser diodes LD1 to LD8.

[0026] The laser diodes LD1 to LD8 are linearly arranged at equal intervals along the radial direction of the light source unit 26. The beam generation unit 20 generates light beams LB (hereinafter also individually referred to as light beams LB1 to LB8) emitted separately from the laser diodes LD1 to LD8 based on the image information transmitted from a control unit 90 described later.

[0027] When the light source unit 26 is rotationally adjusted to adjust the interval in the sub-scanning direction among the laser diodes LD1 to LD8, the interval in the main scanning direction among the laser diodes LD1 to LD8 changes. In a state where the laser diodes LD1 to LD8 are arranged in a straight line parallel to the sub-scanning direction (the vertical direction in the drawing), the interval in the main scanning direction among the laser diodes LD1 to LD8 is minimized. On the contrary, in a state where the laser diodes LD1 to LD8 are arranged in a straight line parallel to the main scanning direction (the left-right direction in the drawing), the interval in the main scanning direction among the laser diodes LD1 to LD8 is maximized (refer to the broken line portions shown in FIG. 3 in both cases).

[0028] Returning to FIG. 2, the collimator lens 41 makes 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 linearly.

[0029] The polygon mirror 45 is a regular prism (here, a regular hexagonal prism) having deflection surfaces 63 formed on each side surface. Each deflection surface 63 is a mirror surface and can deflect the light beam LB emitted from the light source unit 26 by reflection. The polygon mirror 45 is rotatably supported about a central axis (not shown) extending along the vertical direction (the paper surface direction of FIG. 2). The polygon mirror 45 is connected to a polygon motor (not shown) and rotates by the rotational driving force of the polygon motor.

[0030] The scanning lens 49 is a lens having fθ characteristics. The scanning lens 49 is arranged between the photosensitive drum 1a and the polygon mirror 45. The light beam LB emitted from the light source unit 26 is incident in the order of the collimator lens 41 and the cylindrical lens 42 and forms an image as a diagram on the deflection surface 63. The light beam LB formed as an image on the deflection surface 63 is deflected and passes through the scanning lens 49, and forms an image on the photosensitive drum 1a with a spot diameter of a predetermined size.

[0031] The polygon mirror 45 rotates at a constant speed in the clockwise direction as shown by the polygon motor. For this reason, the light beam LB is scanned at a constant speed in the main scanning direction (the direction of arrow X' in the figure) on the scanned surface of the photosensitive drum 1a. Thereby, a scanning line SL extending linearly in the main scanning direction is formed on the scanned surface of the photosensitive drum 1a. When there is one laser diode in the light source unit 26, one scanning line SL is drawn for one deflection surface 63. When there are a plurality of laser diodes in the light source unit 26, a plurality of scanning lines SL are drawn for one deflection surface 63. The light beam LB is sequentially imaged on adjacent deflection surfaces 63 due to the rotation of the polygon mirror 45. Since the photosensitive drum 1a rotates, a plurality of scanning lines SL are formed in the sub-scanning direction and an electrostatic latent image is formed.

[0032] Figure 4 is a block diagram showing an example of the control path of the image forming apparatus 100 of the present embodiment. Note that since various controls of each part of the apparatus are performed when using the image forming apparatus 100, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, here, the parts necessary for the implementation of the present invention in the control path will be mainly described.

[0033] 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 plays a role 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 sheets. 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. A plurality (here, two) of I / Fs 96 are provided. The color shift correction unit 97 corrects the shift of the electrostatic latent image drawn on the photosensitive drum 1a to correct the color shift of the output image. Note that the control unit 90 can be disposed at an arbitrary location inside the main body of the image forming apparatus 100.

[0034] The ROM 92 stores data such as the control program of the image forming apparatus 100 and numerical values necessary for control, which will not be changed during the use of the image forming apparatus 100. The ROM 92 stores an evaluation chart CT (image data used for calibration) for performing color shift correction (calibration). The RAM 93 stores necessary data generated during the control of the image forming apparatus 100 and data temporarily required for the control of the image forming apparatus 100. Also, a density correction table used for color shift correction and the like is stored in the RAM 93 (or the ROM 92).

[0035] Also, the control unit 90 transmits a control signal to each part and device in the image forming apparatus 100 from the CPU 91 through the I / F 96. Also, a signal indicating the state thereof and an input signal are transmitted from each part and device to the CPU 91 through the I / F 96. Examples of the parts and devices controlled by the control unit 90 include the image forming units Pa to Pd, the optical scanning device 5, the primary transfer rollers 6a to 6d, the secondary transfer roller 9, the main motor 40, the image density sensor 50, the belt drive motor 51, the transfer roller drive motor 64, the image input unit 70, the voltage control circuit 71, the operation unit 80, and the like.

[0036] The image density sensor 50 emits measurement light to the evaluation chart CT formed on the intermediate transfer belt 8 from a light emitting element, and measures the photometric intensity and the like of the measurement light (including the light reflected by the toner and the light reflected by the belt surface) that is reflected and incident on the light receiving element.

[0037] The reflected light from the toner and the belt surface includes specularly reflected light and diffusely reflected light. After being separated by a polarization separation prism, the specularly reflected light and the diffusely reflected light are 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 shift correction unit 97).

[0038] The color misregistration 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 (the characteristic changes in the output signals of the specularly reflected light and the diffusely reflected light). The color misregistration correction unit 97 compares this determination result with the reference density and reference position stored in the ROM 92 in advance, and adjusts the characteristic value of the developing 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, and perform density correction and color misregistration correction for each color. Hereinafter, the dots drawn by each of the light beams LB1 to LB8 are referred to as "dots DT1 to DT8". Note that one dot here is the minimum drawing unit at a resolution of 2400 dpi. When the resolution is higher, the one dot described below can be read as a larger number of dots according to the difference in resolution. For example, in the case of 4800 dpi, the one dot described below can be read as two dots.

[0039] Also, the color misregistration correction unit 97 determines whether there is any misregistration (dot misregistration) in the dots DT1 to DT8 from the determination result of the image density of the evaluation chart CT. When dot misregistration occurs, 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 (the amount of shift in the emission timing of the light beams LB1 to LB8) of the laser diodes LD1 to LD8 based on this dot misregistration correction value. Thereby, the positions of the dots DT1 to DT8 in the main scanning direction can be adjusted to correct the dot misregistration.

[0040] 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.

[0041] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 indicating various states. The user operates the stop / clear button of the operation unit 80 to cancel image formation, and operates the reset button to set various settings of the image forming apparatus 100 to the default state. The liquid crystal display unit 81 is configured to indicate the state of the image forming apparatus 100, display the image forming status and the number of printed sheets. Various settings of the image forming apparatus 100 are made from the printer driver of the personal computer.

[0042] FIG. 5 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 simultaneously lit and all of the light beams LB1 to LB8 are emitted. FIG. 6 is a diagram showing a state in which the light beams LB1 to LB8 are scanned in the main scanning direction from the state of FIG. 5. FIG. 7 is a diagram showing an electrostatic latent image in a state where the emission timings of the laser diodes LD1 to LD8 are adjusted so that the writing positions of the dots DT1 to DT8 are at the same position with respect to the main scanning direction.

[0043] The light beams LB1 to LB8 draw the dots DT1 to DT8 on the scanned surface of the photosensitive drum 1a (see FIG. 5). When all of the laser diodes LD1 to LD8 are caused to emit light at the same timing from the light source unit 26 at a predetermined rotation angle to emit the light beams LB1 to LB8, as shown in FIG. 5, a straight line (the column of the dots DT1 to DT8) inclined with respect to the sub-scanning direction is drawn on the scanned surface. 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, as shown in FIG. 6, an electrostatic latent image in a state where the written portion is obliquely inclined in the sub-scanning direction is drawn.

[0044] In order to align the writing positions in the main scanning direction of the electrostatic latent image, the color shift correction unit 97 controls the light emission timings of the laser diodes LD1 to LD8. For example, in order to draw an electrostatic latent image such that the writing positions of the light beams LB1 to LB8 are at the same position with respect to the main scanning direction, the light emission timings of the laser diodes LD1 to LD8 are set in the order of laser diodes LD8, LD7, LD6, LD5, LD4, LD3, LD2, LD1 so as to start writing in order from the one that is most shifted downstream in the main scanning direction among the dots DT1 to DT8, and the light beams LB1 to LB8 are emitted.

[0045] Conversely, when aligning the end portion of writing in the main scanning direction, the turning-off timings of the laser diodes LD1 to LD8 are set in the same order as above (here, in the order of laser diodes LD8, LD7, LD6, LD5, LD4, LD3, LD2, LD1) (not shown). The color shift correction unit 97 calculates the on and off timings 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.

[0046] FIG. 8 is a diagram showing the intermediate transfer belt 8 on which the evaluation chart CT is formed. FIG. 9 is an enlarged plan view of a part of the first evaluation pattern PT1 of the evaluation chart CT. For the enlarged view of the evaluation chart CT including FIG. 9, the horizontal direction of the paper surface (in the direction of the arrow X-X' in the drawing) is taken as the main scanning direction, and the vertical direction of the paper surface (in the direction of the arrow Y-Y' in the drawing) is taken as the sub-scanning direction. As shown in FIG. 8, the evaluation chart CT visualized by the developing devices 3a to 3d is formed on the intermediate transfer belt 8. The evaluation chart CT is drawn in each of magenta, cyan, yellow, and black.

[0047] The evaluation chart CT is drawn in a plurality of straight lines at predetermined intervals in the circumferential direction of the intermediate transfer belt 8 (in the sub-scanning direction (the direction of arrow Y-Y' in FIG. 8)). The evaluation chart CT is arranged at a position overlapping with the image density sensor 50 with respect to the width direction of the intermediate transfer belt 8 (the main scanning direction (the direction of arrow X-X' in FIG. 8)). As the intermediate transfer belt 8 rotates, the image density sensor 50 can measure the evaluation chart CT a plurality of times, and by calculating the deviation (dot deviation) of dots DT1 to DT8 from the average value of the plurality of measurement results, uneven detection is reduced.

[0048] The evaluation chart CT is composed of a first evaluation pattern PT1 and a second evaluation pattern PT2 drawn 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.

[0049] As shown in FIG. 9, the first evaluation pattern PT1 is composed of a plurality of first evaluation patches PC1. Each first evaluation patch PC1 is arranged at a predetermined interval in the main scanning direction (the direction of arrow X-X' in the figure) and the sub-scanning direction (the direction of arrow Y-Y' in the figure). A plurality of first evaluation patches PC1 are arranged at a predetermined interval in the sub-scanning direction to form a first patch row PL1. A plurality of first patch rows PL1 are arranged so as to be connected to each other in the main scanning direction to form the first evaluation pattern PT1.

[0050] FIG. 10 is an enlarged view of a part of the first evaluation patch PC1 shown in FIG. 9. The first evaluation patch PC1 is drawn by light beams LB1 to LB4 emitted by selectively turning on and off one dot each from laser diodes LD1 to LD4. As shown in FIG. 10, the first evaluation patch PC1 is composed of a first dot row DL1, a second dot row DL2, a first auxiliary dot row DLA1, and a second auxiliary dot row DLA2.

[0051] The first dot column DL1 is drawn by the light beam LB2 emitted from the laser diode LD2 (see FIG. 3). That is, the first dot column DL1 is composed of a plurality of dots DT2 arranged linearly and continuously in the main scanning direction.

[0052] The second dot column DL2 is drawn by the light beam LB3 emitted from the laser diode LD3 (see FIG. 3). That is, the second dot column DL2 is composed of a plurality of dots DT3 arranged linearly and continuously in the main scanning direction.

[0053] The second dot column DL2 is connected downstream of the first dot column DL1 in the sub-scanning direction. The second dot column DL2 is shifted downstream in the main scanning direction from the first dot column DL1 by a predetermined number of dots (here, 2 dots).

[0054] The first auxiliary dot column DLA1 is drawn by the light beam LB1 emitted from the laser diode LD1 (see FIG. 3). That is, the first auxiliary dot column DLA1 is composed of a plurality of dots DT1 arranged linearly and continuously in the main scanning direction. The first auxiliary dot column DLA1 is drawn over the entire area of the first evaluation patch PC1 in the main scanning direction. The first auxiliary dot column DLA1 is connected upstream of the first dot column DL1 in the sub-scanning direction.

[0055] The second auxiliary dot column DLA2 is drawn by the light beam LB4 emitted from the laser diode LD4 (see FIG. 3). That is, the second auxiliary dot column DLA2 is composed of a plurality of dots DT2 arranged linearly and continuously in the main scanning direction. The second auxiliary dot column DLA2 is drawn over the entire area of the first evaluation patch PC1 in the main scanning direction. The second auxiliary dot column DLA2 is connected downstream of the second dot column DL2 in the sub-scanning direction.

[0056] The first auxiliary dot arrays DLA1 of the first evaluation patches PC adjacent to each other in the main scanning direction are connected to each other in the main scanning direction and are each arranged in a straight line (see FIG. 9). The same applies to the second auxiliary dot array DLA2. That is, the first auxiliary dot array DLA1 and the second auxiliary dot array DLA2 extend linearly over the entire area of the first evaluation pattern PT1 with respect to the main scanning direction.

[0057] FIG. 11 is an enlarged view of a part of the second evaluation pattern PT2 shown in FIG. 8. The second evaluation pattern PT2 is composed of a plurality of second evaluation patches PC2. Each second evaluation patch PC2 is arranged at a predetermined interval in the main scanning direction and the sub-scanning direction. A plurality of second evaluation patches PC2 are arranged at a predetermined interval in the sub-scanning direction to form a second patch row PL2. A plurality of second patch rows PL2 are arranged so as to be connected to each other in the main scanning direction, and the second evaluation pattern PT2 is formed.

[0058] FIG. 12 is an enlarged view of a part of the second evaluation patch PC2 shown in FIG. 11. Similar to the first evaluation patch PC1, the second evaluation patch PC2 is drawn by the light beams LB1 to LB4 emitted by selectively turning on and off one dot each from the laser diodes LD1 to LD4. As shown in FIG. 12, the second evaluation patch PC2 is composed of a third dot array DL3, a fourth dot array DL4, a third auxiliary dot array DLA3, and a fourth auxiliary dot array DLA4.

[0059] The third dot array DL3 and the fourth dot array DL4 are columns of dots DT2 and DT3 that are continuously drawn linearly with a length of one dot in the sub-scanning direction and four dots in the main scanning direction. Corners C3 and C4 are formed between the third dot array DL3 and the fourth dot array DL4. The third auxiliary dot array DLA3 and the fourth auxiliary dot array DLA4 are columns of dots DT1 and DT4 that are continuously drawn linearly over the entire area of the second evaluation patch PC2 in the main scanning direction. Since the arrangement of the dots DT1 to DT4 of the second evaluation patch PC2 is symmetric with respect to the first evaluation patch PC1 in the main scanning direction, the description thereof is omitted.

[0060] If, as shown in FIG. 13, dot misalignment occurs in the downstream direction (right side in the figure) of the main scanning direction in the light beams LB2 to LB8, the dots DT2 to DT4 of the first evaluation patch PC1 and the second evaluation patch PC2 are misaligned in the downstream direction of the main scanning direction. Then, the dot misalignment amount of the second dot column DL2 with respect to the first dot column DL1 increases, while the dot misalignment amount of the fourth dot column DL4 with respect to the third dot column DL3 decreases. For this reason, the first evaluation patch PC1 is deformed so as to be stretched in the main scanning direction, and the second evaluation patch PC2 is deformed so as to be reduced in the main scanning direction. That is, the area of the overlapping portion between the first dot column DL1 and the second dot column DL2 with respect to the main scanning direction becomes smaller, and conversely, the area of the overlapping portion between the third dot column DL3 and the fourth dot column DL4 becomes larger.

[0061] Therefore, in such a case, the first evaluation patch PC1 and the second evaluation patch PC2 have an asymmetric shape with respect to the main scanning direction, and there is a density difference between the image density of the first evaluation patch PC1 (the ratio of the total area of the dots DT1 to DT4 drawn in the rectangular region surrounded by the straight line overlapping both ends in the main scanning direction of the first dot column DL1 and the second dot column DL2 and the straight line overlapping both ends in the sub-scanning direction of the first evaluation patch PC1) and the image density of the second evaluation patch PC2. As a result, there is a difference in the image density between the first evaluation pattern PT1 and the second evaluation pattern PT2. In the case shown in FIG. 13, the first evaluation patch PC1 has a lower density than the second evaluation patch PC2. By detecting this density difference with the image density sensor 50, the control unit 90 can detect that dot misalignment has occurred in the main scanning direction in the dots DT1 to DT8. In addition, since the user can visually confirm the density difference by the breakdown of the symmetry between the first evaluation pattern PT1 and the second evaluation pattern PT2, the user can confirm that dot misalignment has occurred in the dots DT1 to DT8.

[0062] By arranging the first auxiliary dot column DLA1 and the second auxiliary dot column DLA2 at both ends of the first dot column DL1 and the second dot column DL2 in the sub-scanning direction, the number of dots arranged in the evaluation patches PC1 and PC2 increases. That is, the evaluation chart CT according to the image forming apparatus 100 of the present embodiment has a developer amount (development area) of the first evaluation patch PC1 and the second evaluation patch PC2 increased as compared with the evaluation chart CT in which the first auxiliary dot column DLA1 and the second auxiliary dot column DLA2 are not arranged. For this reason, if dot misregistration occurs in the main scanning direction, the density difference between the image densities of the first evaluation pattern PT1 and the second evaluation pattern PT2 becomes large. Then, it becomes easier to detect the density difference by the image density sensor 50, and it becomes easier to calculate the above-described dot misregistration correction value more accurately. Also, visually, it becomes easier to confirm the density difference between the image densities of the first evaluation pattern PT1 and the second evaluation pattern PT2.

[0063] Note that the misregistration amount of each of the dots DT2 to DT8 in the state where dot misregistration has occurred increases in the order of the dots DT2 to DT8 (in the order away from the dot DT1). That is, the dot misregistration amount of the dot DT8 is the largest compared to the other dots DT2 to DT7. This is because the misregistration amounts of the respective dots are sequentially accumulated up to the dots DT2 to DT7.

[0064] The first evaluation patch PC1 and the second evaluation patch PC2 of the present embodiment can be configured to provide a predetermined gap (here, a gap corresponding to one dot) between the first dot column DL1 and the first auxiliary dot column DLA1, and between the second dot column DL2 and the second auxiliary dot column DLA2 in the sub-scanning direction as shown in FIG. 14, for example. In this case, the second evaluation patch PC2 is also configured to provide a similar gap between the third dot column DL3 and the third auxiliary dot column DLA3, and between the fourth dot column DL4 and the fourth auxiliary dot column DLA4. Note that these gaps preferably have two dots or less in the sub-scanning direction.

[0065] Also, as shown in FIG. 15, the first evaluation patch PC1 and the second evaluation patch PC2 can adopt a configuration in which the number of dots in the sub-scanning direction of the first auxiliary dot columns DLA1 to the fourth auxiliary dot columns DLA4 is 2 or more. Also in this case, as shown in FIG. 16, between the first dot column DL1 and the first auxiliary dot column DLA1, between the second dot column DL2 and the second auxiliary dot column DLA2, between the third dot column DL3 and the third auxiliary dot column DLA3, and between the fourth dot column DL4 and the fourth auxiliary dot column DLA4, a predetermined gap (here, a gap of one dot) can be provided. These gaps are preferably 2 dots or less in the sub-scanning direction.

[0066] Also, the first evaluation patch PC1 and the second evaluation patch PC2 can adopt a configuration in which the number of dots in the sub-scanning direction of the first dot columns DL1 to the fourth dot columns DL4 is 2 or more. In this case, a part of the first auxiliary dot columns DLA1 to the fourth auxiliary dot columns DLA4 can be omitted. Specifically, the first auxiliary dot column DLA1 and the second auxiliary dot column DLA2 are not arranged on the side with a larger number of dots in the sub-scanning direction among the first dot column DL1 and the second dot column DL2 with respect to the sub-scanning direction, and can adopt a configuration in which they are arranged only on the side with a smaller number of dots in the sub-scanning direction. For example, as shown in FIG. 17, when the number of dots in the sub-scanning direction of the first dot column DL1 and the third dot column DL3 is 2, and the number of dots in the sub-scanning direction of the second dot column DL2 and the fourth dot column DL4 is 1, only the second auxiliary dot column DLA2 and the fourth auxiliary dot column DLA4 can be arranged, and the first auxiliary dot column DLA1 and the third auxiliary dot column DLA3 can be omitted.

[0067] Next, the image forming apparatus 100 of the second embodiment will be described. FIG. 18 is an enlarged view of the first evaluation patch PC1 that constitutes the evaluation chart CT according to the second embodiment. In the following, the differences from the first embodiment will be described, and the same components as those in the first embodiment are denoted by the same reference numerals and the description thereof is omitted.

[0068] The image forming apparatus 100 according to the second embodiment forms an image of a part of the light beams LB1 to LB8 on a predetermined first deflection plane 63a and an image of the remaining beams of the light beams LB1 to LB8 on a second deflection plane 63b adjacent to the first deflection plane 63a (see FIG. 2).

[0069] For example, among the light beams LB1 to LB8, the light beams LB7 and LB8 that draw the first auxiliary dot rows DLA1 and the first dot row DL1 are imaged on the first deflection plane 63a, and the light beams LB1 and LB2 that draw the second auxiliary dot rows DLA2 and the second dot row DL2 are imaged on the second deflection plane 63b.

[0070] In this case, as shown in FIG. 18, the first auxiliary dot row DLA1 and the first dot row DL1 are composed of dots DT7 and DT8 drawn by the light beams LB7 and LB8 emitted from the laser diodes LD7 and LD8. The second auxiliary dot row DLA2 and the second dot row DL2 are composed of dots DT1 and DT2 drawn by the light beams LB1 and LB2 emitted from the laser diodes LD1 and LD2.

[0071] Similar to the first embodiment, the evaluation chart CT according to the present embodiment has two or more dots in the sub-scanning direction of the first dot rows DL1 to DL4 dot rows, and a part of the first auxiliary dot rows DLA1 to DLA4 dot rows can be omitted. For example, as shown in FIG. 19, the number of dots in the sub-scanning direction of the first dot row DL1 and the third dot row DL3 is two, and the number of dots in the sub-scanning direction of the second dot row DL2 and the fourth dot row DL4 is one. The first auxiliary dot row DLA1 and the fourth auxiliary dot row DLA4 are omitted, and only the second auxiliary dot row DLA2 and the fourth auxiliary dot row DLA4 are arranged.

[0072] In this case, the first dot column DL1 (the third dot column DL3) is imaged on the same deflection plane 64a. That is, the first dot column DL1 is composed of dots DT7 and DT8 drawn by the light beams LB7 and LB8 emitted from the laser diodes LD7 and LD8, and is imaged on the first deflection plane 64a. The second dot column DL2 and the second auxiliary dot column DLA2 are composed of dots DT1 and DT3 drawn by the light beams LB1 and LB3 emitted from the laser diodes LD1 and LD3, and are imaged on the second deflection plane 64b. Here, the evaluation patch PC1 in which a gap of one dot (dot DT2) is formed between the second dot column DL2 and the second auxiliary dot column DLA2 in the sub-scanning direction has been described. However, a configuration in which the second dot column DL2 and the second auxiliary dot column DLA2 are connected without providing this gap can also be adopted. Further, since the second evaluation patch PC has a symmetrical shape with respect to the first evaluation patch PC in the main scanning direction, the description thereof is omitted.

[0073] Here, as described above, the dot deviation amount of the dot DT8 is the largest compared to the other dots DT2 to DT7. And the first dot column DL1 and the third dot column DL3 of the present embodiment are composed of the dots DT7 and DT8, and the second dot column DL2 and the fourth dot column DL4 are composed of the dots DT1 and DT2. Therefore, as shown in FIG. 20, if dot deviation occurs in the downstream direction in the main scanning direction in the laser diodes LD2 to LD8, the dot deviation amount of the second dot column DL2 with respect to the first dot column DL1 and the dot deviation amount of the fourth dot column DL4 with respect to the third dot column DL3 become relatively large. Then, the density difference in the image density between the first evaluation patch PC1 and the second evaluation patch PC2 becomes relatively large. Therefore, it becomes possible to more clearly detect the occurrence of dot deviation.

[0074] Next, the image forming apparatus 100 of the third embodiment will be described. FIG. 21 is an enlarged view of the first evaluation pattern PT1 constituting the evaluation chart CT according to the third embodiment. Hereinafter, the differences from the first embodiment will be described, and the same components as those in the first embodiment will be denoted by the same reference numerals and the description thereof will be omitted.

[0075] As shown in FIG. 21, the evaluation chart CT according to the third embodiment includes the first evaluation pattern PT1 and the second evaluation pattern PT2 (shown below in the figure) according to the first embodiment, and the first evaluation pattern PT1 and the second evaluation pattern PT2 (shown above in the figure) according to the second embodiment. That is, the first evaluation pattern PT1 (the first evaluation pattern on the same surface) and the second evaluation pattern PT2 (the second evaluation pattern on the same surface) shown below in the figure are composed of the first evaluation patch PC1 and the second evaluation patch PC2 formed by imaging all of the light beams LB1 to LB8 on a predetermined first deflection surface 63a (see FIG. 2) and drawing (hereinafter referred to as "scanning on the same surface"). The difference between the image density of the first evaluation pattern PT1 and the image density of the second evaluation pattern PT2 is defined as the first density difference.

[0076] On the other hand, the first evaluation pattern PT1 (the first evaluation pattern on a different scanning surface) and the second evaluation pattern PT2 (the second evaluation pattern on a different scanning surface) shown above in the figure are composed of the first evaluation patch PC1 and the second evaluation patch PC2 formed by imaging a part of the light beams LB1 to LB8 (here, the light beams LB7, LB8) on a predetermined first deflection surface 63a and imaging the remaining beams of the light beams LB1 to LB8 (here, the light beams LB1 to LB6) on a second deflection surface 63b (see FIG. 2) and drawing (hereinafter referred to as "scanning on a different scanning surface"). The difference between the image density of the first evaluation pattern PT1 and the image density of the second evaluation pattern PT2 is defined as the second density difference.

[0077] In the image forming apparatus 100 of the present embodiment, an evaluation chart CT (first evaluation chart) formed under a first set value described later and an evaluation chart CT (second evaluation chart) formed under a second set value are formed, and the dot deviation amount can be calculated from the first density difference and the second density difference of each evaluation chart CT. FIG. 22 is a graph showing the relationship between the positional deviation amount and the development density difference (difference in image density). The horizontal axis represents the ratio of positional deviation, and the vertical axis represents the value of the density difference. Hereinafter, a specific example of calculating the dot deviation amount will be described using an evaluation chart CT formed by setting the distance in the main scanning direction between dot DT1 and dot DT8 to a first set value (-13.125 μm), and an evaluation chart CT formed by setting a second set value in which dot DT8 is shifted 21 μm downstream (+ side) from the first set value.

[0078] When dot DT1 and dot DT8 are shifted 1 μm upstream (- side) in the main scanning direction during the same surface scanning, dot DT1 and dot DT8 are shifted 7 μm downstream (+ side) in the main scanning direction during the different surface scanning. Therefore, for the same surface scanning, point P1 is plotted on the graph with the horizontal axis being -1 and the vertical axis being the first density difference, and for the different surface scanning, point P2 is plotted on the graph with the horizontal axis being 7 and the vertical axis being the second density difference.

[0079] Here, the first density difference in the first evaluation chart CT is 0.0044 (g / m 2 ), and the second density difference is -0.0196 (g / m 2 ). Therefore, the coordinates (X, Y) of point P1 in the graph of FIG. 22 are (-1, 0.0044), and the coordinates (X, Y) of point P2 are (7, -0.0196). Also, the first density difference in the second evaluation chart CT is -0.0027 (g / m 2 ), and the second density difference is 0.012 (g / m 2 ). Similarly, in the case of the second evaluation chart CT, points P1' (-1, -0.0027) and P2' (7, 0.012) are plotted (see FIG. 22). The y-intercept P3 (first noise value) of the straight line connecting point P1 and point P2 and the y-intercept P3' (second noise value) of the straight line connecting point P1' and point P2' are calculated.

[0080] The y-intercept P3 is 0.00143. In contrast, the y-intercept P3' is -0.00082. Since the ratio of P3 to P3' is 1:-0.57509, the above-mentioned 21 μm is divided into two parts in the ratio of 1:-0.57509. Then, it becomes -13.333 μm on the first evaluation chart CT and +7.667 μm on the second evaluation chart CT. By dividing these values by the value obtained by subtracting 1 from the number of laser diodes LD1 to LD8 (the number of gaps between adjacent laser diodes LD1 to LD8), the dot misalignment amount between adjacent laser diodes LD1 to 8 can be calculated. Therefore, the dot misalignment amount of the first evaluation chart CT is calculated to be -1.905 μm, and the dot misalignment amount of the second evaluation chart CT is calculated to be 1.095 μm. The above-mentioned dot misalignment correction value is calculated based on this dot misalignment amount.

[0081] Here, as described above, on the first evaluation chart CT, it is set to the first set value (-13.125 μm). For this reason, the actual dot misalignment amount of the first evaluation chart CT is -1.875 μm, which is obtained by dividing the first set value (-13.125 μm) by the number of gaps between adjacent laser diodes LD1 to LD8 (here, 7). Since the dot misalignment amount calculated by the above-mentioned method is -1.905 μm, it can be confirmed that the value is close to the actually occurred dot misalignment amount (-1.875 μm). Similarly, the actual dot misalignment amount of the second evaluation chart CT is 1.125 μm, which is close to the dot misalignment amount (1.095 μm) of the second evaluation chart CT calculated by the above-mentioned method.

[0082] Here, in the conventional image forming apparatus 100, the detection of the change in image density was performed using an evaluation patch PC composed of first to fourth dot columns DL1 to DL4 in which the number of dots in the sub-scanning direction is 1. For this reason, in the portions where the first dot column DL1 and the second dot column DL2 overlap with respect to the main scanning direction, and in the portions where the third dot column DL3 and the fourth dot column DL4 overlap, the lengths in the sub-scanning direction are relatively short. That is, the areas of these portions are relatively small. Then, even if dot misregistration occurs and the area of the above-described portions changes, the amount of change is small, making it difficult to accurately detect the dot misregistration.

[0083] On the other hand, the image forming apparatus 100 of the present invention adopts the evaluation chart CT of each of the above embodiments, so that the first auxiliary dot column DLA1 or the second auxiliary dot column DLA2 is arranged on at least one side of the first dot column DL1 and the second dot column DL2. That is, the portion where the first dot column DL1 and the second dot column DL2 overlap with respect to the main scanning direction and the first auxiliary dot column DLA1 or the second auxiliary dot column DLA2 are arranged adjacent to each other in the sub-scanning direction (see FIG. 13). Then, the total amount of developer (development area) of this portion increases by the amount of developer for drawing the first auxiliary dot column DLA1 and the second auxiliary dot column DLA2. Therefore, when dot misregistration occurs in the main scanning direction, the image density of the evaluation chart CT changes relatively greatly. Accordingly, it is possible to provide an image forming apparatus in which the change in image density can be easily detected and the dot misregistration in the main scanning direction can be easily adjusted.

[0084] In the conventional image forming apparatus 100, a plurality of evaluation charts CT in which the dot shift amount in the main scanning direction of each evaluation patch PC is intentionally changed by a predetermined amount is formed, and the dot shift amount is estimated from the change in the image density of each evaluation chart CT. Therefore, in order to calculate a precise dot shift amount, it is necessary to form a huge amount of evaluation charts CT, and the adjustment of the dot shift in the main scanning direction has been a complicated operation. On the other hand, by adopting the image forming apparatus 100 according to the second embodiment of the present invention, it is possible to calculate a more precise dot shift amount by forming only a pair of evaluation charts CT. Therefore, it is possible to adjust the dot shift in the main scanning direction precisely and simply.

[0085] In addition, 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. 23, a plurality of the evaluation charts CT of the above-described embodiments may be arranged at equal intervals at a predetermined interval in the main scanning direction of the intermediate transfer belt 8. In this way, it becomes possible to detect changes in the image density at a plurality of locations in the main scanning direction. Therefore, even when dot shifts with different shift amounts occur at each position in the main scanning direction, it becomes possible to appropriately correct the dot shift for each position in the main scanning direction. Further, in this case, the color shift correction unit 97 detects the image density of each evaluation chart CT with the boundary portion (central portion) between the adjacent first evaluation pattern PT1 and the second evaluation pattern PT2 as a reference position.

[0086] Further, in this case, instead of the above-described image density sensor 50, a configuration may be adopted in which a plurality of evaluation charts CT on the intermediate transfer belt 8 are scanned at once using a scanner (not shown) provided in the image forming apparatus 100 to detect the image density. In this case, it becomes possible to detect the dot shift for each position in the main scanning direction at once, and the dot shift can be corrected more simply. Further, the configuration for detecting the image density using the scanner can also be adopted in a configuration in which the first evaluation pattern PT1 and the second evaluation pattern PT2 constituting the evaluation chart CT are arranged adjacent to each other in the main scanning direction.

[0087] Hereinafter, the effects of the present invention will be described in more detail with reference to examples.

Example

[0088] Regarding the evaluation chart CT, the change in image density for each aspect of the evaluation pattern was investigated by an analytical method. As conditions, when the optical scanning device 5 shown in FIG. 2 was mounted on the image forming apparatus 100 shown in FIG. 1 and the evaluation chart CT according to the first and second embodiments of the present invention was drawn on printing paper (recording medium), the image density (%) was calculated by an analytical method, and the results of sequentially shifting the dot positions in the main scanning direction were compared. Further, as comparative examples, the image densities of the evaluation charts CT in which the number of dots in the sub-scanning direction of the first dot column DL1, the second dot column DL2, the third dot column DL3, and the fourth dot column DL4 is 1 were also calculated.

[0089] Five types (Inventions 1 to 5) of the evaluation chart CT according to the first embodiment, five types (Inventions 6 to 10) of the evaluation chart CT according to the second embodiment, and two types (Comparative Example 1 and Comparative Example 2) of the evaluation chart CT as comparative examples were prepared, and their image densities were compared.

[0090] The evaluation chart CT of Invention 1 is the evaluation chart CT shown in FIGS. 10 and 12 described above. The evaluation chart CT of Invention 2 is the evaluation chart CT shown in FIG. 14 described above. The evaluation chart CT of Invention 3 is the evaluation chart CT shown in FIG. 15 described above. The evaluation chart CT of Invention 4 is the evaluation chart CT shown in FIG. 16 described above. The evaluation chart CT of Invention 5 is the evaluation chart CT shown in FIG. 17 described above.

[0091] The evaluation chart CT of the present invention 6 is the evaluation chart CT shown in FIG. 18 described above. The evaluation chart CT of the present invention 7 has the same shape as the evaluation chart CT of the present invention 2. That is, the number of dots in the sub-scanning direction of the first dot column DL1 to the fourth dot column DL4 and the first auxiliary dot column DLA1 to the fourth auxiliary dot column DLA4 is 1 respectively, and there is a gap of one dot in the sub-scanning direction between the first dot column DL1 and the first auxiliary dot column DLA1, between the second dot column DL2 and the second auxiliary dot column DLA2, between the third dot column DL3 and the third auxiliary dot column DLA3, and between the fourth dot column DL4 and the fourth auxiliary dot column DLA4. The first auxiliary dot column DLA1 and the third auxiliary dot column are composed of dots DT8 drawn by the light beam LB8 emitted from the laser diode LD8. The first dot column DL1 and the third dot column DL3 are composed of dots DT3. The second dot column DL2 and the fourth dot column DL4 are composed of dots DT4. The second auxiliary dot column DLA2 and the fourth auxiliary dot column DLA4 are composed of dots DT5.

[0092] The evaluation chart CT of the present invention 8 has the same shape as the evaluation chart CT of the present invention 3. That is, the number of dots in the sub-scanning direction of the first dot column DL1 to the fourth dot column DL4 is 1, and the number of dots in the sub-scanning direction of the first auxiliary dot column DLA1 to the fourth auxiliary dot column DLA4 is 2. The first dot column DL1 and the first auxiliary dot column DLA1, the second dot column DL2 and the second auxiliary dot column DLA2, the third dot column DL3 and the third auxiliary dot column DLA3, and the fourth dot column DL4 and the fourth auxiliary dot column DLA4 are connected without a gap therebetween. The first auxiliary dot column DLA1 and the third auxiliary dot column are composed of dots DT8 and DT1. The first dot column DL1 and the third dot column DL3 are composed of dots DT2. The second dot column DL2 and the fourth dot column DL4 are composed of dots DT3, and the second auxiliary dot column DLA2 and the fourth auxiliary dot column DLA4 are composed of dots DT4 and DT5.

[0093] The evaluation chart CT of the present invention 9 has the same shape as the evaluation chart CT of the present invention 4. That is, the number of dots in the sub-scanning direction of the first dot column DL1 to the fourth dot column DL4 is 1, and the number of dots in the sub-scanning direction of the first auxiliary dot column DLA1 to the fourth auxiliary dot column DLA4 is 2. A gap of one dot is formed between the first dot column DL1 and the first auxiliary dot column DLA1, between the second dot column DL2 and the second auxiliary dot column DLA2, between the third dot column DL3 and the third auxiliary dot column DLA3, and between the fourth dot column DL4 and the fourth auxiliary dot column DLA4. The first auxiliary dot column DLA1 and the third auxiliary dot column are composed of dots DT7 and DT8. The first dot column DL1 and the third dot column DL3 are composed of dot DT2. The second dot column DL2 and the fourth dot column DL4 are composed of dot DT3, and the second auxiliary dot column DLA2 and the fourth auxiliary dot column DLA4 are composed of dots DT5 and DT6.

[0094] The evaluation chart CT of the present invention 10 is the evaluation chart CT shown in FIG. 19 described above.

[0095] In the evaluation charts CT of Comparative Example 1 and Comparative Example 2, the number of dots in the sub-scanning direction of the first dot column DL1, the second dot column DL2, the third dot column DL3, and the fourth dot column DL4 is 2, respectively. The first auxiliary dot column DLA1 to the fourth auxiliary dot column DLA4 are not arranged.

[0096] The first dot column DL1, the second dot column DL2, the third dot column DL3, and the fourth dot column DL4 of Comparative Example 1 are all drawn by the light beams LB1 and LB2 deflected by the same deflection plane 63a. That is, the first dot column DL1 to the fourth dot column DL4 of Comparative Example 1 are composed of dots DT1 and DT2.

[0097] The first dot column DL1 and the third dot column DL3 of Comparative Example 2 are drawn by the light beam LB8 deflected by the first deflection surface 63a. The second dot column DL2 and the fourth dot column DL4 of Comparative Example 2 are drawn by the light beam LB1 deflected by the second deflection surface 63b. That is, the first dot column DL1 and the third dot column DL3 are composed of dots DT8, and the second dot column DL2 and the fourth dot column DL4 are composed of DT1.

[0098] The test was conducted by sequentially changing the dot misalignment amount from -21 μm to 21 μm or less, and calculating the change in the development rate (%) (the ratio of the image density of the evaluation chart CT when the evaluation chart CT is drawn entirely in black (the ratio of the area occupied by the black background part in the entire evaluation chart CT) to 1) (see FIGS. 24 and 26). The dot misalignment amount is adjusted by changing the emission timing of the laser diodes LD1 to LD8. The dot misalignment amount is defined as + for the shift to the downstream side in the main scanning direction and - for the shift to the upstream side in the main scanning direction. Also, with the development rate when there is no dot misalignment (when the dot misalignment amount is 0 μm) as the reference value, the change in the difference value between the development rate when the dot misalignment amount changes and the reference value was calculated (see FIGS. 25 and 27).

[0099] FIG. 24 is a graph showing the change in the development rate of the first to fifth inventions and Comparative Example 1. FIG. 25 is a graph showing the change in the above difference value of the first to fifth inventions and Comparative Example 1. FIG. 26 is a graph showing the change in the development rate of the sixth to tenth inventions and Comparative Example 2. FIG. 27 is a graph showing the change in the above difference value of the sixth to tenth inventions and Comparative Example 2. In FIGS. 19 to 24, the first invention is shown as a ● graph, the second invention as a ◆ graph, the third invention as a ■ graph, the fourth invention as a × graph, the fifth invention as a * graph, and Comparative Example 1 as a ▲ graph. Also, the sixth invention is shown as a 〇 graph, the seventh invention as a ◇ graph, the eighth invention as a □ graph, the ninth invention as a × (a mark with a white × superimposed on a black background) graph, the tenth invention as a * (a mark with a white * superimposed on a black background) graph, and Comparative Example 2 as a △ graph.

[0100] As shown in Fig. 24, the development rates of the first to fifth inventions of the present invention change with values larger than those of Comparative Example 1. Also, as shown in Fig. 26, the change rate of the difference value (the magnitude of the slope of the graph in Fig. 26) of the first to fifth inventions of the present invention is larger than that of Comparative Example 1. That is, the evaluation chart CT of the first to fifth inventions of the present invention has a larger change rate of the development rate when the deviation amount changes compared to the evaluation chart of Comparative Example 1.

[0101] Also, as shown in Fig. 25, the development rates of the sixth to tenth inventions of the present invention change with values larger than those of Comparative Example 2. Also, as shown in Fig. 27, the change rate of the difference value (the magnitude of the slope of the graph in Fig. 27) of the sixth to tenth inventions of the present invention is larger than that of Comparative Example 2. That is, the evaluation chart CT of the sixth to tenth inventions of the present invention has a larger change rate of the development rate when the deviation amount changes compared to the evaluation chart of Comparative Example 2.

[0102] Therefore, the evaluation chart CT of the first to tenth inventions of the present invention makes it easier to detect the image density by the image density sensor 50 compared to the evaluation charts of Comparative Examples 1 and 2. Also, when visually confirmed by a user or the like, the evaluation chart CT of the first to tenth inventions of the present invention makes it easier to understand the change in image density compared to the evaluation charts CT of Comparative Examples 1 and 2.

Industrial Applicability

[0103] The present invention is applicable to an image forming apparatus that employs a multi-beam method in which a photosensitive drum is scanned with light beams from a multi-beam laser having a plurality of light emitters. By using the present invention, it is possible to provide an image forming apparatus in which the change rate of the image density of the evaluation chart for color misregistration correction increases, and the main scanning dot position misregistration can be corrected more accurately.

Explanation of Reference Numerals

[0104] 1a - 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 First deflection plane 63b Second deflection plane 92 ROM (memory unit) 100 Image forming apparatus CT Evaluation chart DL1 First dot column DL2 Second dot column DL3 Third dot column DL4 Fourth dot column DLA1 First auxiliary dot column DLA2 Second auxiliary dot column DLA3 Third auxiliary dot column DLA4 Fourth auxiliary dot column DT1~DT8 Dots LB1~LB8 Light beams LD1~LD8 Laser diodes PC1 First evaluation patch PC2 Second evaluation patch PL1 First patch column PL2 Second patch column PT1 First evaluation pattern PT2 Second evaluation pattern P1 First density difference at the first set value P1´ First density difference at the second set value P2 Second density difference at the first set value P2´ Second density difference at the second set value P3 y-intercept (first noise value) P3´ y-intercept (second noise value) Pa~Pd Image forming section S Paper (recording medium)

Claims

1. A light source having four or more light emitting units arranged in a row at regular intervals with a predetermined angle with respect to the main scanning direction, and a polygon mirror that deflects and scans the light beams emitted from the respective light emitting units, and an optical scanning device that forms an electrostatic latent image on an image carrier by the light beams, A developing unit that forms a toner image obtained by visualizing the electrostatic latent image, A control unit that controls the optical scanning device so as to form the electrostatic latent image according to image data by switching on and off each of the light emitting units, A storage unit that stores a predetermined evaluation chart configured in dot units by the light beams for each of the light emitting units and for determining the writing timing of each of the light emitting units, Comprising, The evaluation chart is, A first evaluation pattern, A second evaluation pattern parallel to the first evaluation pattern with respect to the main scanning direction or a sub-scanning direction orthogonal to the main scanning direction, The first evaluation pattern is, A first dot row linearly formed in the main scanning direction, a second dot row linearly formed so as to be displaced from the first dot row with respect to the main scanning direction, and at least one of both sides in the sub-scanning direction of the first dot row and the second dot row. An auxiliary dot row linearly formed in the main scanning direction, which is arranged on one side, has a first patch row in which a plurality of first evaluation patches are arranged at equal intervals in the sub-scanning direction, and the first patch row is configured by arranging a plurality of them at equal intervals with a predetermined interval in the main scanning direction. At least one of the first dot row or the second dot row has 1 dot number in the sub-scanning direction, The auxiliary dot row is arranged on the side where the number of dots in the sub-scanning direction of the first dot row and the second dot row is 1, and is not arranged on the side where the number of dots in the sub-scanning direction is 2 or more, and is formed over the entire area between both ends of the first evaluation patch with respect to the main scanning direction. The second evaluation pattern is, An image forming apparatus characterized in that a second patch row in which a plurality of second evaluation patches having a symmetrical shape with respect to the first evaluation patch in the parallel direction between the first evaluation pattern and the second evaluation pattern are arranged at the same interval as the parallel interval of the plurality of first evaluation patches with respect to the sub-scanning direction, and the second patch row is configured by arranging a plurality of them at equal intervals with a predetermined interval in the main scanning direction.

2. The first dot column and the second dot column each have one dot in the sub-scanning direction. The auxiliary dot column includes a first auxiliary dot column disposed at a predetermined interval from the second dot column with the first dot column interposed therebetween in the sub-scanning direction, and a second auxiliary dot column disposed at a predetermined interval from the first dot column with the second dot column interposed therebetween in the sub-scanning direction. The image forming apparatus according to claim 1, characterized in that it is composed of the above.

3. In the sub-scanning direction, the first auxiliary dot column is disposed at a predetermined interval from the first dot column, and the second auxiliary dot column is disposed at a predetermined interval from the second dot column. The image forming apparatus according to claim 2, characterized in that it is arranged.

4. The auxiliary dot columns of the evaluation patches adjacent to each other in the main scanning direction are in overlapping positions in the sub-scanning direction and are connected in the main scanning direction. The image forming apparatus according to any one of claims 1 to 3, characterized in that it is.

5. The first patch column is configured with a predetermined interval between the first evaluation patches adjacent to each other in the sub-scanning direction. The image forming apparatus according to any one of claims 1 to 4, characterized in that it is.

6. A plurality of the evaluation charts are arranged at predetermined intervals in the main scanning direction or the sub-scanning direction. The image forming apparatus according to any one of claims 1 to 5, characterized in that it is.

7. It is provided with a density detection mechanism capable of detecting the image density of the toner image obtained by visualizing the evaluation chart. Based on the image density difference between the first dot column and the second dot column detected by the density detection mechanism, the control unit shifts the timing of the emission of the light beam from each light emitting unit. The image forming apparatus according to any one of claims 1 to 6, characterized in that it is.

8. A plurality of the evaluation charts are arranged at predetermined intervals in the main scanning direction. The density detection mechanism detects the image density for each evaluation chart. Based on the detection result of the density detection mechanism, the control unit adjusts the amount of shift in the emission timing of the light beam of each light emitting unit for each position of the evaluation chart. The image forming apparatus according to claim 7, characterized in that it is.

9. The control unit determines the amount of deviation of the emission timing of the light beam of each light emitting unit with reference to the center portion between the adjacent first evaluation pattern and the second evaluation pattern of each evaluation chart. The image forming apparatus according to claim 8, characterized in that.

10. An intermediate transfer belt that is disposed opposite to the image carrier and that primary transfers the toner image on the image carrier visualized by the developing unit. The density detection mechanism detects the image density of the evaluation chart primarily transferred onto the intermediate transfer belt. The image forming apparatus according to any one of claims 7 to 9, characterized in that.

11. A fixing device that fixes the toner image on the image carrier visualized by the developing unit onto a recording medium. The density detection mechanism detects the image density of the evaluation chart fixed onto the recording medium. The image forming apparatus according to any one of claims 7 to 9, characterized in that.

12. All of the light beams emitted from each light emitting unit are reflected by one of the plurality of deflection surfaces of the polygon mirror, and the first evaluation patch and the second evaluation patch are formed. The image forming apparatus according to any one of claims 1 to 11, characterized in that.

13. A part of the light beams emitted from each light emitting unit is reflected by a first deflection surface of the plurality of deflection surfaces of the polygon mirror, and the other light beams are reflected by a second deflection surface adjacent to the first deflection surface, and the first evaluation patch and the second evaluation patch are formed. The image forming apparatus according to any one of claims 1 to 11, characterized in that.

14. The evaluation chart is The first identical-surface evaluation pattern composed of the first evaluation patches formed by reflecting all the light beams emitted from the respective light-emitting units on one of the plurality of deflection surfaces of the polygon mirror, the first different-scanning-surface evaluation pattern composed of the first evaluation patches formed by reflecting a part of the light beams emitted from the respective light-emitting units on the first deflection surface of the plurality of deflection surfaces and reflecting the other light beams on the second deflection surface adjacent to the first deflection surface, the second identical-surface evaluation pattern composed of the second evaluation patches formed by reflecting all the light beams emitted from the respective light-emitting units on one of the plurality of deflection surfaces, and the second different-scanning-surface evaluation pattern composed of the second evaluation patches formed by reflecting a part of the light beams emitted from the respective light-emitting units on the first deflection surface of the plurality of deflection surfaces and reflecting the other light beams on the second deflection surface adjacent to the first deflection surface. The control unit can control the formation of a pair of the evaluation charts by changing the emission timings of the light beams from the respective light-emitting units to a first set value and a second set value. The control unit A first noise value that can be calculated based on a first density difference, which is a difference in developed density between the first identical-surface evaluation pattern and the second identical-surface evaluation pattern of the evaluation chart formed with the first set value, and a second density difference, which is a difference in developed density between the first different-scanning-surface evaluation pattern and the second different-scanning-surface evaluation pattern. A second noise value that can be calculated based on the first density difference and the second density difference of the evaluation chart formed with the second set value. The image forming apparatus according to any one of claims 1 to 11, characterized in that the control unit can calculate the dot misregistration amount in the main scanning direction of the adjacent light-emitting units based on the first noise value and the second noise value.

Images