Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent, with a simple method, a change in the density of a reference image due to a shift of a beam pitch in executing light quantity calibration.SOLUTION: An image forming apparatus comprises an image forming unit, an optical scanner, an image density sensor, and a control unit. The optical scanner has a plurality of light sources, and scans a surface of an image carrier electrified by an electrifying device with light emitted from the light sources to form an electrostatic latent image. The control unit can execute light quantity calibration for adjusting the amount of exposure of the optical scanner on the basis of a result of detection of the density of a reference image performed by the image density sensor. The reference image used for the light quantity calibration is configured such that a dot pattern in which independent dots in a number equal to or more than the number of light sources are arranged in a main scanning direction at a constant interval, is repeatedly formed in the main scanning direction and a sub-scanning direction. The dots constituting the dot pattern are arranged shifted pixel by pixel at positions different from each other in the sub-scanning direction.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates to an image forming apparatus using an electrophotographic process, and more particularly to image density correction for adjusting the density of halftone images.

Background Art

[0002] In an electrophotographic image forming apparatus, the density of the formed image may change due to changes over time in the photoreceptor and toner, and changes in temperature and humidity around the apparatus. Therefore, conventionally, a technique has been proposed in which image density correction (calibration) is performed at a predetermined timing to stabilize image formation against the above changes.

[0003] There are two types of image density correction: density adjustment (bias calibration) for adjusting the density of solid images and density adjustment (light amount calibration) for adjusting the density of halftone images. Specifically, patches (reference images) of solid images or halftone images are formed on the photoreceptor drum, and the density level is detected by an image density sensor (ID sensor) disposed near the photoreceptor drum or the intermediate transfer belt, and the deviation amount from the target density is calculated. Then, the density is adjusted to the target density by changing the developing voltage or the light amount of the optical scanning device according to the deviation amount. By performing the above two types of density adjustments, the density from the low density to the high density region can be stabilized.

[0004] Here, in a multi-beam type optical scanning device that scans a photoreceptor drum using a plurality of laser diodes, when a deviation in the beam pitch occurs in the sub-scanning direction, uneven density occurs in the dot pattern depending on the positional relationship between the image pattern and the laser diode that exposes the image pattern, and the image density changes.

[0005] As described in Patent Document 1, when there is a deviation in the beam pitch in the sub-scanning direction and furthermore the relationship of the sub-scanning positions of the dot patterns changes, the image density changes at each position. For example, when there is a gap near the end of the dot pattern, the influence on the image density is small, but when there is a gap in the center of the dot pattern, the decrease in the image density of the entire dot pattern becomes large. As a result, the density of the halftone image will be significantly different between the case where there is a gap near the end of the dot pattern and the case where there is a gap in the center.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] In Patent Document 1, screening processing is performed on the input image data, skew correction is performed on the image data subjected to the screening processing, and based on the periodic characteristics of printing by batch scanning and the screen period by screening processing, image shift processing is performed in the sub-scanning direction which is the moving direction of the image carrier, thereby suppressing image quality defects. However, in the method of Patent Document 1, since screening processing and skew correction are performed on the image data, data processing and control become complicated.

[0008] In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of suppressing the density change of a reference image due to the deviation of the beam pitch when performing light amount calibration by a simple method.

Means for Solving the Problems

[0009] To achieve the above object, a first configuration of the present invention is an image forming apparatus including an image forming unit, an optical scanning device, an image density sensor, and a control unit. The image forming unit has an image carrier on which a photosensitive layer is formed on its surface, a charging device for charging the surface of the image carrier, a developer carrier for carrying a developer containing toner, and a developing device for developing an electrostatic latent image formed on the image carrier into a toner image, and forms an image using toner. The optical scanning device has a plurality of light sources, and scans the surface of the image carrier charged by the charging device with light emitted from the light sources to form an electrostatic latent image with attenuated charge. The image density sensor detects the density of the toner image formed by the image forming unit. The control unit controls the image forming unit and the optical scanning device. The control unit detects the density of a reference image for image density correction formed by the image forming unit with the image density sensor, and adjusts the density of a solid image by adjusting the developing voltage applied to the developer carrier based on the detection result, and can execute image density correction including bias calibration for adjusting the density of a halftone image by adjusting the exposure amount of the optical scanning device. The reference image used for the light amount calibration has a configuration in which independent dots equal to or more than the number of light sources are scattered at regular intervals in the main scanning direction which is the scanning direction of the optical scanning device, and are repeatedly formed in the main scanning direction and the sub-scanning direction orthogonal to the main scanning direction. The dots constituting the dot pattern are arranged with a pixel unit shift at different positions in the sub-scanning direction.

Advantages of the Invention

[0010] According to the first configuration of the present invention, even when the beam scanning positions in the sub-scanning direction and the main scanning direction are shifted, the difference in the image density of the dot pattern for each adjustment during light amount calibration can be reduced. Therefore, light amount calibration can be performed accurately, and a halftone image can be adjusted to an appropriate image density to form an image. In addition, since it is not necessary to perform complicated processing on the image data, the control for executing the light amount calibration can be simplified.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Mode for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of the image forming apparatus 100 according to an embodiment of the present invention, and FIG. 2 is an enlarged view of the vicinity of the image forming unit Pa in FIG. 1

[0013] The image forming apparatus 100 shown in FIG. 1 is a so-called tandem type color printer and has the following configuration. Inside the main body of the image forming apparatus 100, four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side (left side in FIG. 1) in the conveyance direction. These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and yellow, cyan, magenta, and black images are sequentially formed by respective processes of charging, exposure, development, and transfer.

[0014] Photoconductor drums 1a, 1b, 1c, and 1d for carrying visible images (toner images) of respective colors are arranged in these image forming units Pa to Pd. Further, an intermediate transfer belt 8 that rotates in the counterclockwise direction in FIG. 1 is provided adjacent to each of the image forming units Pa to Pd. After the toner images formed on these photoconductor drums 1a to 1d are sequentially transferred onto the intermediate transfer belt 8 that moves while contacting each of the photoconductor drums 1a to 1d, they are transferred onto a sheet S as an example of a recording medium at once by a secondary transfer roller 9. Further, after being fixed on the sheet S in a fixing unit 13, it is discharged from the main body of the image forming apparatus 100. While rotating the photoconductor drums 1a to 1d in the clockwise direction in FIG. 1, an image forming process for each of the photoconductor drums 1a to 1d is executed.

[0015] The sheet S onto which the toner image is transferred is accommodated in a sheet cassette 16 at the lower part of the main body of the image forming apparatus 100, and is conveyed to the secondary transfer roller 9 via a paper feed roller 12a and a registration roller pair 12b. A belt having no seam (seamless) is mainly used for the intermediate transfer belt 8.

[0016] Next, the image forming units Pa to Pd will be described. Hereinafter, the image forming unit Pa will be described in detail. However, since the image forming units Pb to Pd basically have the same configuration, the description thereof will be omitted. As shown in FIG. 2, around the photosensitive drum 1a, a charging device 2a, a developing device 3a, and a cleaning device 7a are arranged along the drum rotation direction (clockwise direction in FIG. 2), and a primary transfer roller 6a is arranged with the intermediate transfer belt 8 interposed therebetween. Further, on the upstream side in the rotation direction of the intermediate transfer belt 8 with respect to the photosensitive drum 1a, a belt cleaning unit 19 facing the tension roller 11 with the intermediate transfer belt 8 interposed therebetween is arranged.

[0017] Next, the image forming procedure in the image forming apparatus 100 will be described. When the start of image formation is input by the user, first, the rotation of the photosensitive drums 1a to 1d is started by the main motor 61 (see FIG. 5), and the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging rollers 20 of the charging devices 2a to 2d. Next, the surfaces of the photosensitive drums 1a to 1d are irradiated with light by the beam light (laser light) emitted from the optical scanning device 5, and an electrostatic latent image corresponding to the image signal is formed on each of the photosensitive drums 1a to 1d.

[0018] The developing devices 3a to 3d are each filled with a predetermined amount of toner of each color: yellow, cyan, magenta, and black. When the ratio of the toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a specified value due to the formation of the toner image described later, toner is supplied from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in this developer is supplied onto the photosensitive drums 1a to 1d by the developing rollers 21 of 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, a transfer electric field is applied at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d by the primary transfer rollers 6a to 6d, and the yellow, cyan, magenta, and black toner images on the photosensitive drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. These four-color images are formed with a predetermined positional relationship predetermined for a predetermined full-color image formation. Thereafter, in preparation for the subsequent formation of a new electrostatic latent image, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning blades 22 and rubbing rollers 23 of the cleaning devices 7a to 7d.

[0020] When the intermediate transfer belt 8 starts to rotate counterclockwise with the rotation of the drive roller 10 by the belt drive motor 63 (see FIG. 5), the sheet S is conveyed from the registration roller pair 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the full-color image is transferred. The sheet S onto which the toner image has been transferred is conveyed to the fixing unit 13. The toner remaining on the surface of the intermediate transfer belt 8 is removed by the belt cleaning unit 19.

[0021] The sheet S conveyed to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a so that the toner image is fixed on the surface of the sheet S, and a predetermined full-color image is formed. The sheet S on which the full-color image has been formed has its conveyance direction sorted by the branching unit 14 branched in a plurality of directions, and is then discharged to the discharge tray 17 by the discharge roller pair 15 (either as it is or after being sent to the duplex conveyance path 18 for duplex printing).

[0022] An image density sensor 25 is disposed at a position facing the driving roller 10 with the intermediate transfer belt 8 interposed therebetween. As the image density sensor 25, an optical sensor including a light emitting element generally composed of an LED or the like and a light receiving element composed of a photodiode or the like is generally used. When measuring the toner adhesion amount on the intermediate transfer belt 8, when measurement light is irradiated from the light emitting element onto each patch image (reference image) formed on the intermediate transfer belt 8, the measurement light enters the light receiving element as light reflected by the toner and light reflected by the belt surface.

[0023] 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 enter 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 a control unit 90 (see FIG. 5).

[0024] FIG. 3 is a side cross-sectional view showing the internal configuration of an optical scanning device 5 according to an embodiment of the present invention. As shown in FIG. 3, the optical scanning device 5 includes a housing 48. The housing 48 has a main body portion 48a and a lid portion 48b. A polygon mirror 45 is disposed on the bottom surface of the main body portion 48a. In the present embodiment, the polygon mirror 45 is composed of a rotating polygon mirror having a plurality of deflection surfaces (reflective surfaces) on its side surface and rotates at a predetermined speed by a polygon motor 38. The polygon motor 38 is fixed to a motor support plate 39, and the motor support plate 39 is fixed to the bottom surface of the main body portion 48a.

[0025] A light source unit 26 (see FIG. 4), a collimator lens, an aperture, a cylindrical lens (none of which are shown), a first scanning lens 46a, second scanning lenses 47a to 47d, and plane mirrors 49a to 49c are disposed in the housing 48. The first scanning lens 46a and the second scanning lenses 47a to 47d have fθ characteristics and form images of the laser lights D1 to D4 deflected and reflected by the polygon mirror 45 on the photosensitive drums 1a to 1d. Further, plane mirrors 49a to 49c are disposed on the optical paths of the respective laser lights D1 to D4 from the polygon mirror 45 to the photosensitive drums 1a to 1d.

[0026] The scanning operations of the laser beams D1 and D2 by the optical scanning device 5 configured as described above will be described. First, the laser beams D1 and D2 emitted from the light source unit 26 are made into substantially parallel light beams by the collimator lens and are given a predetermined optical path width by the aperture. Next, the laser beams D1 and D2 that have become substantially parallel light beams are incident on the cylindrical lens. The laser beams D1 and D2 incident on the cylindrical lens are emitted in a converged state in the sub-scanning direction while remaining in the state of parallel light beams in the main scanning cross-section, and are imaged as line images on the deflection surface of the polygon mirror 45. At this time, in order to facilitate the optical path separation of the two laser beams D1 and D2 deflected by the polygon mirror 45, these laser beams D1 and D2 are configured to be incident at different angles in the sub-scanning direction with respect to the deflection surface.

[0027] The laser beams D1 and D2 incident on the polygon mirror 45 are deflected at a constant angular velocity by the polygon mirror 45 and then deflected at a constant velocity by the first scanning lens 46a. The laser beams D1 and D2 that have passed through the first scanning lens 46a are reflected by the plane mirrors 49a arranged in their respective optical paths. The laser beam D1 is incident on the second scanning lens 47a, and the laser beam D2 is incident on the second scanning lens 47b, and are deflected at a constant velocity by the second scanning lenses 47a and 47b. Then, the laser beams D1 and D2 that have been deflected at a constant velocity are reflected by the final plane mirrors 49c arranged in their respective optical paths, pass through the window portions 70a and 70b formed in the lid portion 48b that covers the opening of the main body portion 48a, and are distributed to the photoreceptor drums 1a and 1b.

[0028] Similarly, the laser beams D3 and D4 emitted from the light source unit 26 pass through the collimator lens, the aperture, and the cylindrical lens, and then are deflected at a constant angular velocity by the polygon mirror 45 and deflected at a constant linear velocity by the first scanning lens 46a. The laser beam D3 that has passed through the first scanning lens 46a is reflected twice by the plane mirrors 49a and 49b disposed in the optical path, and then the laser beam D3 is incident on the second scanning lens 47c, and the laser beam D4 that has passed through the first scanning lens 46a is incident on the second scanning lens 47d, and both are deflected at a constant linear velocity. Further, the laser beam D3 is reflected by the final plane mirror 49c, and the laser beam D4 is reflected by the plane mirror 49a, and then the laser beams pass through the window portions 70c and 70d formed in the lid portion 48b and are distributed to the photosensitive drums 1c and 1d.

[0029] The polygon mirror 45, the first scanning lens 46a, the second scanning lenses 47a to 47d, and the plane mirrors 49a to 49c constitute a scanning optical system that scans the laser beams D1 to D4 and guides them onto the photosensitive drums 1a to 1d.

[0030] FIG. 4 is a perspective view showing the light source unit 26. The optical scanning device 5 includes four light source units 26 in order to irradiate the photosensitive drums 1a to 1d with the laser beams D1 to D4, respectively.

[0031] The light source unit 26 has a front end face 27, laser diodes LD1 to LD8 (light sources), and a beam generation unit 28 (see FIG. 5). As shown in FIG. 4, the front end face 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 about an axis (central axis L1) passing through the center of the front end face 27 among the normals to the front end face 27, thereby adjusting and fixing the intervals in the sub-scanning direction of the laser diodes LD1 to LD8.

[0032] 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 28 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 the control unit 90 (see FIG. 5).

[0033] When the light source unit 26 is rotationally adjusted to adjust the intervals of the laser diodes LD1 to LD8 in the sub-scanning direction, the intervals of the laser diodes LD1 to LD8 in the main scanning direction change. 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 figure), the interval of the laser diodes LD1 to LD8 in the main scanning direction is minimized. Conversely, 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 figure), the interval of the laser diodes LD1 to LD8 in the main scanning direction is maximized (see the dashed line portions shown in FIG. 4 in both cases).

[0034] FIG. 5 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 image forming apparatus 100 are performed when using the image forming apparatus 100, the control path of the entire image forming apparatus 100 becomes complex. Therefore, here, the parts necessary for the implementation of the present invention in the control path will be mainly described.

[0035] The control unit 90 includes at least a CPU (Central Processing Unit) 91 as a central arithmetic processing unit, a ROM (Read Only Memory) 92 as a read-only storage unit, a RAM (Random Access Memory) 93 as a readable and writable storage unit, a temporary storage unit 94 that temporarily stores image data and the like, a counter 95, and a plurality (here, two) of I / F (interfaces) 96 that transmit control signals to each device in the image forming apparatus 100 and receive input signals from the operation unit 80. Further, the control unit 90 can be arranged at an arbitrary location inside the main body of the image forming apparatus 100.

[0036] The ROM 92 stores data such as the control program for the image forming apparatus 100, numerical values necessary for control, etc., which are not to be changed during the use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for the control of the image forming apparatus 100, etc. Also, the RAM 93 (or ROM 92) stores an image density correction table, a look-up table, etc. used for calibration. The counter 95 accumulates and counts the number of printed sheets.

[0037] 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 its state 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 image density sensor 25, the beam generation unit 28, the main motor 61, the belt drive motor 63, the image input unit 70, the voltage control circuit 71, the operation unit 80, etc.

[0038] 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.

[0039] The voltage control circuit 71 is connected to the charging voltage power supply 72, the developing voltage power supply 73, and the transfer voltage power supply 74, and operates these power supplies with an output signal from the control unit 90. These power supplies, according to the control signal from the voltage control circuit 71, the charging voltage power supply 72 applies a predetermined charging voltage to the charging rollers 20 in the charging devices 2a to 2d. The developing voltage power supply 73 applies a predetermined developing voltage, which is a DC voltage with an AC voltage superimposed, to the developing rollers 21 in the developing devices 3a to 3d. The transfer voltage power supply 74 applies a predetermined transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9, respectively.

[0040] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 indicating various states. The user can operate the stop / clear button of the operation unit 80 to cancel image formation, and operate 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, and 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.

[0041] In the image forming apparatus 100, even if the developing voltages of the developing devices 3a to 3d and the light amount setting of the optical scanning device 5 are appropriately adjusted at the start of use to obtain a desired halftone density, when printing (durable printing) is performed over a long period, the density of the halftone image may change due to the change over time of the developing devices 3a to 3d and the optical scanning device 5.

[0042] Therefore, in the image forming apparatus 100, the image density can be made constant by performing image density adjustment (calibration) at a predetermined timing (for example, the timing when the cumulative number of printed sheets reaches a predetermined number). When performing calibration, a plurality of reference images with the toner adhesion amount changed stepwise are formed on the intermediate transfer belt 8. Then, the formed reference images are read by the image density sensor 25, and the image forming conditions such as the charging voltage, developing voltage, and exposure amount of the optical scanning device 5 are set for each color so that the image density becomes the target density compared with a predetermined reference density.

[0043] There are two types of calibration: density adjustment (bias calibration) for adjusting the density of solid images and density adjustment (light amount calibration) for adjusting the density of halftone images. By performing both bias calibration and light amount calibration, the image density from the low density region to the high density region can be stabilized.

[0044] FIG. 6 is a diagram showing an example of a conventional dot pattern DP used for light quantity calibration. Here, a black dot pattern DP will be described as an example, but the dot pattern DPs of yellow, cyan, and magenta have exactly the same configuration. FIG. 6 shows a case where image formation is performed by the optical scanning device 5 including the light source unit 26 having eight laser diodes LD1 to LD8 as shown in FIG. 4, in an ideal state where the beam (optical beams LB1 to LB8) scanning positions are not shifted.

[0045] As shown in FIG. 6, in the dot pattern DP, dots P of 4 pixels × 4 pixels are arranged at intervals (pitches) of 4 pixels in the main scanning direction (XX' direction) and the sub-scanning direction (YY' direction). That is, when there is no shift in the beam scanning position, writing (exposure) of the dot pattern DP is performed using the optical beams LB1 to LB4 emitted from the laser diodes LD1 to LD4, and the laser diodes LD5 to LD8 (optical beams LB5 to LB8) are not used for writing the dot pattern DP.

[0046] FIG. 7 is a diagram showing a state where the beam scanning positions of the laser diodes LD1 to LD8 are shifted in the main scanning direction and the sub-scanning direction from the state of FIG. 6. Due to durability printing or the like, as shown at the left end of FIG. 7, a shift in the beam scanning position occurs in the sub-scanning direction and the main scanning direction. When there is a shift in the beam scanning position, the image density of the dot P changes greatly depending on the relationship between the image position of the dot P and the beam scanning position to be exposed.

[0047] FIG. 8 is a diagram showing the density change of the dot pattern DP when the dot pattern DP is shifted by one pixel at a time in the sub-scanning direction in the state of FIG. 7. As shown in FIG. 8, in the initial state (dot pattern DP1) where there is no shift in the beam scanning position, each dot P is scanned by the beams (optical beams LB1 to LB4) emitted from the laser diodes LD1 to LD4 corresponding to each pixel. Therefore, there is no influence of the beam scanning position shift, and the image density becomes an ideal state.

[0048] When the dot pattern DP is shifted by one pixel at a time in the sub-scanning direction, up to +4 pixels (dot patterns DP2 to DP5) are scanned by the beams (light beams LB1 to LB8) within the same scan. Therefore, the influence of the beam scanning position shift is small, and the image density is not significantly different from the ideal state. However, when it reaches +5 pixels (dot pattern DP6), the portion where the beam scanning position shift occurs (the gap between light beams LB8 and LB1) is located inside dot P. As a result, dot P becomes separated in the sub-scanning direction, and the image density becomes thinner.

[0049] When it reaches +6 pixels (dot pattern DP7), the influence of the beam scanning position shift becomes most significant, and a large difference occurs in the image density from the ideal state. When it reaches +7 pixels (dot pattern DP8), since the portion where the beam scanning position shift occurs (the gap between light beams LB8 and LB1) shifts in the direction of moving outside dot P, the influence of the beam scanning position shift becomes smaller. When it reaches +8 pixels (dot pattern DP9), the influence of the beam scanning position shift disappears, and the image density returns to the ideal state.

[0050] The positions of dot patterns DP1 to DP9 change every time the timing of the image writing position such as color misregistration is shifted. Therefore, even if the image density of dot P does not actually change, it is detected by the image density sensor 25 as if the image density has changed. As a result, accurate light amount calibration cannot be performed.

[0051] FIG. 9 is a diagram showing an example of the dot pattern DP used for light amount calibration in the image forming apparatus 100 of the present embodiment. In the present embodiment, a reference image (halftone image) for light amount calibration is formed using the dot pattern DP as shown in FIG. 9. The dot pattern DP in FIG. 9 is composed of dots P1 to P8 having different positions in the sub-scanning direction. More specifically, dots P1 to P8 are in a pattern that is shifted by one pixel at a time in the sub-scanning direction as going from the upstream side to the downstream side in the main scanning direction (from left to right in FIG. 9). FIG. 9 shows the case where the scanning position of the beam is in the ideal state, and the image densities of all dots P1 to P8 are in the ideal state.

[0052] FIG. 10 is a diagram showing a case where the beam scanning positions of the laser diodes LD1 to LD8 are shifted in the sub-scanning direction and the main scanning direction in the dot pattern DP of FIG. 9. As shown in FIG. 10, the portion where the beam scanning position shift occurs (the gap between the light beams LB8 and LB1) is located inside the dots P6 to P8. Therefore, the image density of the dots P6 to P8 is low.

[0053] FIG. 11 is a diagram showing the density change of the dot pattern DP when the dot pattern DP is shifted by one pixel at a time in the sub-scanning direction in the state of FIG. 10. As shown in FIG. 11, in the initial state (dot pattern DP1), the image density of the three dots P6 to P8 at the right end is low.

[0054] As the dot pattern DP is shifted by one pixel at a time in the sub-scanning direction, it can be seen that the number of dots with low image density is the same (three), but the positions of the dots with low image density are shifted one by one to the left. For example, at +1 pixel (dot pattern DP2), the image density of the dots P5 to P7 is low. At +2 pixels (dot pattern DP3), the image density of the dots P4 to P6 is low. And when it reaches +8 pixels (dot pattern DP9), it returns to the initial state (dot pattern DP1).

[0055] That is, even when the position of the dot pattern DP is shifted in the sub-scanning direction from the initial state (dot pattern DP1), the area ratio of the low-density portion and the high-density portion of the image density is always equal (here 5:3). Therefore, regardless of the position change of the dot pattern DP in the sub-scanning direction, the average image density of the entire dot pattern DP is always equal.

[0056] Although a density distribution occurs within the dot pattern DP, when forming a reference image (patch) for light quantity calibration, since a large number of the patterns shown in FIG. 9 are arranged without gaps in the vertical and horizontal directions, the density distribution is averaged as the entire reference image. Further, the spot size when detecting the image density by the image density sensor 25 is sufficiently larger compared to the size of the dot pattern DP in FIG. 11. Therefore, since the density distribution is averaged and detected, there is no risk of being affected by the difference in the density distribution within the dot pattern DP during the execution of the light quantity calibration.

[0057] By using the dot pattern DP as shown in FIG. 9, even when the beam scanning positions (beam pitches) in the sub-scanning direction and the main scanning direction are shifted, the difference in the image density of the dot pattern DP for each adjustment during the light quantity calibration can be reduced.

[0058] Therefore, the light quantity calibration can be performed accurately, and the halftone image can be adjusted to an appropriate image density to perform image formation. Further, since it is not necessary to perform complicated processing on the image data, the control during the execution of the light quantity calibration can be simplified.

[0059] FIG. 12 is a diagram showing another example of the dot pattern DP used for the light quantity calibration in the image forming apparatus 100 of the present embodiment. The dot pattern DP shown in FIG. 12 shows the case where the beam scanning positions of the laser diodes LD1 to LD8 are shifted in the sub-scanning direction and the main scanning direction.

[0060] In the dot pattern DP shown in FIG. 12, a dot P2 located on the downstream side of the dot P1 in the main scanning direction is a dot shifted 5 pixels in the sub-scanning direction from the dot P1 (corresponding to the dot P6 in FIG. 9). Dots P3 and P4 located on the downstream side of the dot P2 are dots shifted 1 pixel and 2 pixels in the sub-scanning direction from the dot P1 (corresponding to the dots P2 and P3 in FIG. 9), respectively.

[0061] Dot P5 located on the downstream side of dot P4 has dots (corresponding to dot P7 in FIG. 9) that are shifted by 6 pixels in the sub-scanning direction from dot P1. Dots P6 and P7 located on the downstream side of dot P5 have dots (corresponding to dots P4 and P5 in FIG. 9) that are shifted by 3 pixels and 4 pixels respectively in the sub-scanning direction from dot P1.

[0062] That is, in the dot pattern DP shown in FIG. 12, when shifts occur in the scanning positions of the light beams LB1 to LB8 emitted from the laser diodes LD1 to LD8, the shift amounts in the sub-scanning direction of each dot are determined so that dots (dots P2, P5, P8) whose image density changes are evenly allocated in the main scanning direction within the dot pattern DP.

[0063] By using the dot pattern DP shown in FIG. 12, the density distribution within the dot pattern DP is averaged compared to the dot pattern DP shown in FIG. 9. Therefore, the density distribution of the reference image (patch) composed of a large number of dot patterns DP is further averaged. Accordingly, the influence of the difference in the density distribution within the dot pattern DP during the execution of the light amount calibration can be further reduced.

[0064] Furthermore, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention. For example, in the above-described embodiment, the optical scanning device 5 using the light source unit 26 including eight laser diodes LD1 to LD8 has been described, but the number of laser diodes may be plural and can be arbitrarily set.

[0065] Here, for the dot pattern DP, it is necessary that dots equal to or more than the number of laser diodes are scattered at regular intervals in the main scanning direction. For example, when using the light source unit 26 including four laser diodes, in the dot pattern DP, four or more dots arranged at regular intervals in the main scanning direction are arranged with pixel unit shifts at different positions in the sub-scanning direction.

[0066] Furthermore, the present invention is not limited to the tandem type color printer shown in FIG. 1, and can be applied to various image forming apparatuses using a multi-beam type optical scanning device 5, such as a color copier or a color multifunction peripheral.

Industrial Applicability

[0067] The present invention can be used in an image forming apparatus using a multi-beam type optical scanning device. By using the present invention, it is possible to provide an image forming apparatus capable of suppressing, by a simple method, a change in density of a reference image due to a deviation in beam pitch when performing light amount calibration.

Explanation of Signs

[0068] Pa to Pd Image forming unit 1a to 1d Photoconductor drum (image carrier) 2a to 2d Charging device 3a to 3d Developing device 5 Optical scanning device 6a to 6d Primary transfer roller 8 Intermediate transfer belt 9 Secondary transfer roller 25 Image density sensor 26 Light source unit 90 Control unit 100 Image forming apparatus S Paper (recording medium) LD1 to LD8 Laser diode (light source) LB1 to LB8 Light beam DP Dot pattern P1 to P8 Dot

Claims

1. An image carrier having a photosensitive layer formed on its surface, A charging device for charging the surface of the image carrier, A developing device having a developer carrier that carries a developer containing toner, and developing an electrostatic latent image formed on the image carrier into a toner image, An image forming unit that forms an image using the toner, A light scanning device having a plurality of light sources, scanning the surface of the image carrier charged by the charging device with light emitted from the light sources, and forming an electrostatic latent image with reduced charge, An image density sensor for detecting the density of the toner image formed in the image forming unit, A control unit for controlling the image forming unit and the light scanning device, In an image forming apparatus comprising: The control unit: Detects the density of a reference image for image density correction formed in the image forming unit by the image density sensor, and adjusts the developing voltage applied to the developer carrier based on the detection result to adjust the density of a solid image (bias calibration), and adjusts the exposure amount of the light scanning device to adjust the density of a halftone image (light amount calibration). In an image forming apparatus capable of performing image density correction including: The reference image used for the light amount calibration has a dot pattern in which independent dots equal to or more than the number of the light sources are scattered at regular intervals in the main scanning direction which is the scanning direction of the light scanning device, and is repeatedly formed in the main scanning direction and the sub-scanning direction orthogonal to the main scanning direction, The dots constituting the dot pattern are arranged with a shift in pixel units at different positions in the sub-scanning direction. An image forming apparatus characterized by this.

2. The dot pattern is such that the shift amount in the sub-scanning direction of each dot is determined so that dots whose image density changes when a shift occurs in the scanning position of the light emitted from a plurality of the light sources are evenly assigned in the main scanning direction within the dot pattern. The image forming apparatus according to claim 1, characterized by this.

Images