Image forming apparatus

Abstract

To suppress an occurrence of image defect such as uneven density even in an image forming apparatus, in which an image clock frequency is changed in a scanning direction, without using a scanning lens having an fθ characteristic.SOLUTION: An image forming apparatus comprises a pulse width correcting unit 101c that corrects input image data by a correction coefficient depending on a position in the main scanning direction, and a PWM control unit 101d that modulates the image data corrected by the pulse width correcting unit 101c into a driving signal. The correction coefficient increases as it goes toward an end from a center in the main scanning direction and includes an inflection point Pa where an inclination, which is an increase amount of the correction coefficient to a unit increase amount of the position in the main scanning direction, is changed, and the inflection point Pa is positioned near the end in the main scanning direction.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus, and more particularly to an electrophotographic image forming apparatus such as a laser beam printer, a digital copying machine, or a digital facsimile machine, which performs writing using a laser beam. [Background technology]

[0002] An electrophotographic image forming apparatus has an optical scanning unit for exposing a photosensitive member to light. The optical scanning unit emits laser light based on image data, reflects the laser light off a rotating polygon mirror, and transmits the laser light through a scanning lens to expose the photosensitive member. By rotating the rotating polygon mirror, a spot of laser light formed on the surface of the photosensitive member is moved and scanned, forming a latent image on the photosensitive member.

[0003] The scanning lens has a so-called fθ characteristic. The fθ characteristic is an optical characteristic that focuses a laser beam on the surface of a photoconductor so that the laser beam spot moves at a constant speed across the surface of the photoconductor when a rotating polygon mirror rotates at a constant angular velocity. Using a scanning lens with such fθ characteristic enables appropriate exposure. A scanning lens with such fθ characteristic is large, which increases the size of the image forming apparatus. Therefore, image forming apparatuses that do not use a scanning lens or that use a scanning lens without fθ characteristic have been considered for the purpose of miniaturizing image forming apparatuses. For example, Patent Document 1 discloses a method of performing electrical correction by changing the image clock frequency during one scan so that dots formed on the surface of the photoconductor have a constant width even when the laser beam spot on the surface of the photoconductor does not move at a constant speed across the photoconductor. Furthermore, for example, Patent Document 2 discloses pixel width correction, which corrects the length of each pixel of image data in the main scanning direction according to the scanning speed at which the laser light moves on the photosensitive body, and density correction, which corrects the image density so that the slower the scanning speed, the lighter the image density. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 58-125064 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-000510 Summary of the Invention [Problem to be solved by the invention]

[0005] However, if the image clock frequency is changed in the main scanning direction without using a scanning lens with fθ characteristics and correction is performed using a pixel width correction device, the correction width may be small in areas where the clock frequency in the main scanning direction is high. The correction width becomes even smaller as the speed of the device increases, since the overall clock frequency itself must be increased. If the correction results in the laser off time being less than a few nanoseconds, the correction width may be below the limit of the laser's responsiveness. In this case, the amount of laser light actually irradiated onto the photosensitive drum surface may not be the intended amount, resulting in image defects such as uneven density.

[0006] The present invention has been made under these circumstances, and aims to suppress the occurrence of image defects such as uneven density even in an image forming apparatus that changes the image clock frequency in the scanning direction without using a scanning lens with fθ characteristics. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0008] (1) An image forming apparatus comprising: a photosensitive member on which a latent image is formed; exposure means for scanning a laser beam, the scanning speed of which varies depending on the position in a main scanning direction, onto the photosensitive member to form the latent image; generation means for generating a clock signal for controlling the timing of lighting of the laser beam; first correction means for correcting the frequency of the clock signal for each position in the main scanning direction; development means for developing the latent image on the photosensitive member with toner to form a toner image; and drive means for driving the laser beam based on a drive signal, and the image forming apparatus transfers the toner image to a recording material to form an image, the image forming apparatus further comprising: second correction means for correcting input image data with a correction coefficient according to the position in the main scanning direction; and modulation means for modulating the image data corrected by the second correction means into the drive signal, the correction coefficient increasing from the center to the end in the main scanning direction, and having an inflection point at which a slope, which is the increase in the correction coefficient per unit increase in position in the main scanning direction, changes, the inflection point being located near an end in the main scanning direction. [Effects of the Invention]

[0009] According to the present invention, even in an image forming apparatus in which the image clock frequency is changed in the scanning direction without using a scanning lens having fθ characteristics, it is possible to suppress the occurrence of image defects such as uneven density. [Brief explanation of the drawings]

[0010] [Figure 1] Schematic diagram of an image forming apparatus according to an embodiment of the present invention. [Figure 2] 1 is a cross-sectional view of an optical scanning device according to an embodiment of the present invention; [Figure 3] Relationship between image height and partial magnification in the embodiment [Figure 4] Electrical block diagram of the exposure control configuration of the embodiment [Figure 5] Timing relationship diagram of various synchronization signals and image signals in the embodiment [Figure 6] Functional block diagram of the image processing flow of the embodiment [Figure 7] FIG. 10 is a diagram showing an example of a table of pulse signals according to an embodiment. [Figure 8] FIG. 10 is a diagram showing a profile of a pulse width correction table according to an embodiment of the present invention. [Figure 9] FIG. 10 is a diagram showing a profile of a pulse width correction table of a comparative example for comparison with the embodiment; and FIG. 11 is a diagram showing a light amount distribution in the main scanning direction of the comparative example. [Figure 10] FIG. 10 is a diagram showing the light amount distribution in the main scanning direction in the embodiment. [Figure 11] FIG. 10 is a diagram showing a modified example of the profile of the pulse width correction table of the embodiment. DETAILED DESCRIPTION OF THE INVENTION [Example]

[0011] <Image forming device> Figure 1 is a schematic diagram of a laser printer 9 (hereinafter simply referred to as printer 9) as an image forming apparatus according to the embodiment. Printer 9 is a printer that can output up to letter-size paper (215.9 mm x 279.4 mm) in the width direction. The printer has a resolution of 600 dpi and a throughput of 24 ppm.

[0012] As the photosensitive drum (photoconductor) 4 rotates in the R direction, it is uniformly charged to a predetermined polarity and potential by a charging roller (charging means) (not shown). A laser driving unit (driving means) 300 in an optical scanning device (exposure means) 400 emits a laser beam (scanning beam) 208 based on an image signal (driving signal) output from an image signal generating unit 100. The optical scanning device 400 scans the laser beam 208 over the photosensitive drum 4 to form a latent image on the surface of the photosensitive drum 4. The printer 9 then applies toner to the latent image using a developing means (not shown), forming a toner image corresponding to the latent image. Paper P as a recording medium (recording material) is fed from a paper feed unit 8 and transported by a transport roller 5 to a transfer nip formed by the photosensitive drum 4 and a transfer roller 41. The toner image on the photosensitive drum 4 is transferred to the paper P using the transfer roller 41. The unfixed toner image transferred onto the paper P is thermally fixed to the paper P by a fixing unit 6, and the paper P is discharged outside the apparatus via a discharge roller 7.

[0013] The printer 9 of this embodiment uses a negatively charged photosensitive drum 4 and negatively charged toner, with the photosensitive drum 4 charging potential (Vd) set to -500V, the development potential (Vdc) set to -300V, and the exposure potential (Vl) set to -100V when the laser emits light over the entire area of ​​one pixel. The laser spot diameter is approximately 60 μm at the on-axis image height on the photosensitive drum 4 and approximately 80 μm at the most off-axis image height. The on-axis image height and the most off-axis image height will be described later. The pixel size is 42.3 μm x 42.3 μm.

[0014] <Optical scanning device> 2A and 2B are cross-sectional views of the optical scanning device 400 according to this embodiment, with Fig. 2A showing the main scanning cross section and Fig. 2B showing the sub-scanning cross section. The direction in which the laser spot moves on the photosensitive drum 4 due to the rotation of a rotary polygon mirror (described later) is called the main scanning direction, and the direction perpendicular to the main scanning direction is called the sub-scanning direction.

[0015] In this embodiment, the laser beam 208 emitted from the light source 401 is shaped into an elliptical shape by the aperture stop 402 and enters the coupling lens 403. The light beam that passes through the coupling lens 403 is converted into approximately parallel light and enters the anamorphic lens 404. The anamorphic lens 404 has positive refractive power in the main scanning cross section and converts the incident light beam into convergent light in the main scanning cross section. Furthermore, the anamorphic lens 404 focuses the light beam near a deflection surface (reflection surface) 405a of a rotating polygonal mirror (deflector) 405 in the sub scanning cross section, forming a linear image that is long in the main scanning direction.

[0016] The light beam passing through the anamorphic lens 404 is reflected by a deflecting surface 405a of the rotating polygon mirror 405. Here, the rotating polygon mirror 405 is described as a deflector having four reflecting surfaces, but the number of reflecting surfaces is not limited to this. The laser light 208 reflected by the deflecting surface 405a passes through an imaging lens 406 and is focused on the surface of the photosensitive drum 4, forming a predetermined spot-shaped image (hereinafter referred to as a "spot"). Note that 406a is the incident surface of the imaging lens 406, and 406b is the exit surface of the imaging lens 406. By rotating the rotating polygon mirror 405 at a constant angular velocity in the direction of arrow Ao (clockwise in FIG. 2) by a drive unit (not shown), the spot moves in the main scanning direction on the scanned surface 407 of the photosensitive drum 4, forming an electrostatic latent image on the scanned surface 407.

[0017] The beam detector (hereinafter referred to as BD) 409 and BD lens 408 constitute a synchronization optical system that determines the timing of writing an electrostatic latent image on the scanned surface 407. The laser beam 208 that passes through the BD lens 408 is incident on the BD 409, which includes a photodiode, and detected. A BD signal is output each time the deflection surface 405a of the rotating polygon mirror 405 is switched. The writing timing is controlled based on the timing at which the BD 409 detects the laser beam 208. Although the light source 401 in this embodiment has one light-emitting element, the light source 401 may also have multiple light-emitting elements whose light emission can be controlled independently.

[0018] The light source 401 is a semiconductor laser chip. The imaging lens 406 does not have so-called fθ characteristics. By using the imaging lens 406 without fθ characteristics, the optical scanning device 400 can be made smaller. That is, the imaging lens 406 can be disposed close to the rotating polygon mirror 405 (at a position where the distance D1 is small). Furthermore, the imaging lens 406 without fθ characteristics can have a smaller length in the main scanning direction (width LW) and a smaller length in the optical axis direction (thickness LT) than an imaging lens with fθ characteristics.

[0019] Because the imaging lens 406 of this embodiment does not have an fθ characteristic, when the rotating polygon mirror 405 rotates at a constant angular velocity, the spot does not move at a constant speed on the scanned surface 407. In addition, the spot diameter on the scanned surface 407 is not uniform. In particular, the shorter the optical path length (D2) from the rotating polygon mirror 405 to the photosensitive drum 4, the larger the angle of view, and therefore the larger the difference in scanning speed and spot diameter between the on-axis image height and the extreme off-axis image height. In this embodiment, the objective is to maintain image quality with such an optical configuration.

[0020] <Partial magnification correction> FIG. 3 shows the relationship between image height and partial magnification as a characteristic of the optical scanning device 400 of this embodiment. In FIG. 3, the horizontal axis represents image height [mm], and the vertical axis represents partial magnification [%]. Note that an image height of 0 means that the spot is on the optical axis of the imaging lens 406, and will be referred to as the "axial image height" below. Image heights other than the axial image height will be referred to as the "off-axial image height" below. Furthermore, the maximum absolute value of the image height will be referred to as the "most off-axial image height."

[0021] 2(a), the length (width) in the main scanning direction of the scanned surface 407 of the photosensitive drum 4 is set to W. Then, the position of the most off-axis image height on the scanned surface 407 is W / 2 from the center. In FIG. 3, for example, a partial magnification of 130% at an image height means that the scanning speed at that image height is approximately 1.3 times the scanning speed at an image height where the partial magnification is 100%.

[0022] In the example of Figure 3, the scanning speed is lowest at the on-axis image height, and the scanning speed increases as the absolute value of the image height increases. Therefore, if the pixel width in the main scanning direction is determined at a constant time interval determined by the cycle of the clock signal used to time the laser light on, the pixel density will differ between the on-axis image height and the off-axis image height. For this reason, in this embodiment, partial magnification correction is performed. Specifically, the frequency of the clock signal (hereinafter referred to as the clock frequency) is adjusted according to the image height so that the pixel width is approximately constant regardless of the image height.

[0023] The ROM 3 in FIG. 4 stores the clock frequency ratio for the optical scanning device 400. Based on this information, the CPU 2 transmits the video clock signal VCLK 113 to the image processing unit 101 and controls the clock frequency. In other words, the clock frequency ratio of the VDO signal 110 transmitted from the image processing unit 101 is set to 135% at the most off-axis image height when the on-axis image height is 100%. The CPU 2 functions as a clock signal generating unit and a first correcting unit that corrects the frequency of the video clock signal (clock signal) for each position in the main scanning direction based on the clock frequency ratio in FIG. 3. In this case, the time it takes for the spot of the laser beam 208 to move a single pixel width (e.g., 42.3 μm) on the scanned surface 407 is 0.74 times the on-axis image height at the most off-axis image height. In this way, by controlling the exposure time of the laser beam 208 at pixel positions corresponding to one pixel, the pixel width is corrected, allowing latent images corresponding to each pixel to be formed at substantially equal intervals and with equal sizes in the main scanning direction.

[0024] However, when the luminance of the light source 401 is constant, the total exposure amount per unit length near the most off-axis image height is less than the total exposure amount per unit length near the on-axis image height. Therefore, in this embodiment, in order to obtain good image quality, pulse width correction is performed to correct the total exposure amount per unit length in addition to the partial magnification correction described above. The pulse width correction will be described in detail later.

[0025] <Exposure control configuration> FIG. 4 is a block diagram showing the exposure control configuration of the printer 9. The image signal generation unit 100 includes an image processing unit 101 and a ROM 102. The image signal generation unit 100 receives print data from a host computer (not shown) and generates a corresponding VDO signal 110. The image signal generation unit 100 also functions as a density correction unit that corrects image density. The control unit 1 controls the image signal generation unit 100 and the entire printer 9. The laser drive unit 300 is equipped with a laser driver IC (not shown). The laser driver IC controls the ON (light emission) / OFF (exit) of the light source 401 based on the VDO signal 110. The laser driver IC automatically adjusts the brightness by performing feedback control using a circuit within the laser driver IC so that the brightness detected by a photodetector (not shown) provided in the light source 401 as a light intensity monitor becomes the desired brightness. This is called APC (Auto Power Control).

[0026] When the image signal generation unit 100 is ready to output an image signal for image formation, it instructs the control unit 1 to start printing. The control unit 1 has a CPU 2, and when it is ready to print, it sends a TOP signal 112, which is a sub-scanning synchronization signal, and a BD signal 111, which is a main-scanning synchronization signal, to the image signal generation unit 100. Upon receiving the synchronization signals, the image signal generation unit 100 outputs a VDO signal 110, which is an image signal (drive signal), to the laser drive unit 300 at a predetermined timing.

[0027] FIG. 5 is a diagram showing the timing relationship between various synchronization signals and image signals along the position in the sub-scanning direction when an image forming operation equivalent to one page of recording medium is performed. Time passes from left to right in the diagram. A "HIGH" TOP signal 112 indicates that the leading edge of the paper P has reached a predetermined position. When the image signal generating unit 100 receives a "HIGH" TOP signal 112, it transmits a VDO signal 110 in synchronization with a BD signal 111. Based on this VDO signal 110, the light source 401 emits light, and a latent image is formed on the photosensitive drum 4.

[0028] 5, for the sake of simplicity, the VDO signal 110 is depicted as being continuously output across multiple BD signals 111. However, in reality, the VDO signal 110 is output during a predetermined period between the output of a BD signal 111 and the output of the next BD signal 111. The automatic adjustment of the brightness of the light source 401 is performed while the light source 401 is emitting light to detect the BD signal 111 outside the printing area for each main scan line.

[0029] <Image processing flow> Next, the image processing flow of the printer 9 of this embodiment will be described. Figure 6 is a functional block diagram illustrating the image processing flow during printing. The image processing unit 101 has a density correction processing unit 101z, a halftone processing unit 101a, a position control unit 101b, a pulse width correction unit 101c, and a PWM control unit 101d shown in Figure 6, and executes the image processing flow described below.

[0030] The printer 9 of this embodiment performs image processing to convert gradations based on a dithering method and obtain continuous halftone images. Print data input from a host computer (not shown) is temporarily stored in memory 103. The print data is then read from memory 103, and the halftone print data is corrected using a correction table by a density correction processor 101z. The print data is then sent to a halftone processor 101a, which serves as a halftone processing unit. The halftone processor 101a performs multilevel dithering on the 8-bit (256 gradations) print data to convert it into 8-bit image data. The position controller 101b adds 2-bit position control data, representing the dot growth direction, to the image data output by the halftone processor 101a, using a position control matrix corresponding to the dither matrix used by the halftone processor 101a for the multilevel dithering. The pulse width corrector 101c and PWM controller 101d will be described in detail later. The pulse width correction unit 101c performs pulse width correction on the 8-bit image data to which position control data has been added according to the position in the main scanning direction, and the PWM control unit 101d performs PWM control to convert it into a VDO signal 110, which is a pulse signal, and outputs it to the laser driving unit 300. The image processing unit 101 converts the print data into a VDO signal 110 for exposure that has undergone halftone processing for appropriate gradation expression in the printer 9 through image processing using such a dither method.

[0031] <Pulse width modulation control section (PWM control section)> The PWM (Pulse Width Modulation) process performed by the PWM control unit 101d, which is a modulation means, will now be described. Fig. 7 shows an example of a table showing the relationship between data assigned to each pixel by the position control unit 101b and pulse signals generated by PWM process. This table includes information on the width (PWM value) of the pulse signal and the position of the pulse signal. The PWM control unit 101d performs PWM process on input image data by dividing it into 8-bit data (PWM value: 0 to 255) assigned to each pixel and 2-bit data (position control data: C, L, R), and generates pulse signals.

[0032] The PWM value is assigned an integer value between 0 and 255. The pulse position is information corresponding to the delay of the rising edge of the pulse signal from the reference position (e.g., the starting point of one pixel) of the image clock signal, which defines the pixel interval for synchronizing the pulse signal. In the table shown in FIG. 7, the pulse signal width is set to increase in width from the reference position at the center of one pixel to the left and right as the PWM value increases from 0 (non-emission). When the position control data is C, the pulse signal width increases in a substantially uniform manner from the reference position at the center of one pixel to the left and right as the PWM value increases. When the position control data is L, the pulse signal width increases from the reference position at the left edge of one pixel to the right as the PWM value increases. When the position control data is R, the pulse signal width increases from the reference position at the right edge of one pixel to the right as the PWM value increases. When the PWM value reaches 255, the entire pixel width is illuminated. By performing this processing, the PWM control unit 101d converts the 8-bit image data into a video signal (VDO signal 110), which is a pulse signal.

[0033] <Pulse width correction section> The pulse width correction unit 101c, which serves as second correction means and is a feature of this embodiment, will now be described. In this embodiment, as described above, the scanning speed in the main scanning direction is not constant, and clock frequency control is performed. Therefore, if the amount of light emitted by the laser is kept constant, a constant amount of light cannot be obtained in the main scanning direction on the photosensitive drum 4. Therefore, the pulse width correction unit 101c corrects the pulse width processed by the PWM control unit 101d.

[0034] FIG. 8 shows a graph of the pulse width correction table generated by the pulse width correction unit 101c for each main scanning position. The pulse width correction table is a table that associates the position in the main scanning direction with the pulse width correction coefficient. In the pulse width correction table graph of FIG. 8, the horizontal axis represents the position in the main scanning direction [mm], and the vertical axis represents the pulse width correction coefficient. A feature of this embodiment is that the shape of the pulse width correction table is such that the center is concave compared to the ends in the main scanning direction, and further has inflection points Pa near both ends. In this embodiment, the inflection points Pa are located at positions ±87 mm from the position in the main scanning direction.

[0035] The slope of the pulse width correction table in this embodiment is 0.00326 at positions 81 to 86 in the main scanning direction, which are closer to the center than inflection point Pa, and 0.000362 at positions 88 to 93 in the main scanning direction, which are closer to the end than inflection point Pa. In this way, the slope of the pulse width correction table changes significantly before and after inflection point Pa. Specifically, the slope closer to the center than inflection point Pa is greater than the slope closer to the end than inflection point Pa.

[0036] The pulse width correction unit 101c applies the correction table of Fig. 8 to the image data of each pixel input from the position control unit 101b according to the position of each pixel in the main scanning direction. Specifically, the image data before correction is multiplied by the value (correction coefficient) of the correction table shown in Fig. 8 according to each position in the main scanning direction, and the value is rounded off to the nearest integer to obtain the corrected image data.

[0037] For example, image data of 255 at position 0 in the main scanning direction (center in the longitudinal direction (axial image height)) is multiplied by a correction coefficient of 0.698 to obtain 255 × 0.698 = 177.99, which is rounded off to the nearest integer and equals 178. Similarly, image data of 128 at position 0 in the main scanning direction is multiplied by a correction coefficient of 0.698 to obtain 128 × 0.698 = 89.344, which is rounded off to the nearest integer and equals 89. Data of 255 at position 100 in the main scanning direction is multiplied by a correction coefficient of 0.897 to obtain 255 × 0.897 = 228.735, which is rounded off to the nearest integer and equals 229. The data of 128 at position 100 in the main scanning direction is multiplied by the correction coefficient 0.897 to obtain 128×0.897=114.816, which is rounded off to the nearest integer to obtain 115.

[0038] As described above, the pulse width correction unit 101c corrects the image data (0 to 255) before correction according to the position of each pixel in the main scanning direction, and obtains the corrected image data (0 to 255). In this embodiment, the maximum value in the pulse width correction table is actually 0.905, so the corrected image data takes values ​​from 0 to 231.

[0039] After the pulse width correction unit 101c performs the above correction, the 8-bit image data is converted into a video signal (VDO signal 110), which is a pulse signal, by the PWM control unit 101d described in FIG. 7 and sent to the laser driving unit 300. As a result of the pulse width correction unit 101c performing the above correction, the pulse width of the PWM signal sent from the PWM control unit 101d as the VDO signal 110 is thinned. The correction table is stored in the memory 103.

[0040] <Effects of the invention> To explain the effects of the present invention, a comparative example will be described, along with a case where a correction table with a monotonous concave shape is applied. Figure 9(a) shows an example of a comparative correction table with a monotonous concave shape proportional to the clock frequency. The horizontal and vertical axes of the graph in Figure 9(a) are the same as those in Figure 8. For an ideal light source 401 that exhibits response characteristics consistent with the VDO signal 110, such correction should result in a uniform light quantity distribution on the photosensitive drum 4 in the main scanning direction. However, this may not be the case depending on the response characteristics of the laser element. As mentioned above, the clock frequency is higher near the edges in the main scanning direction. Furthermore, the amount of thinning near the edges in the main scanning direction is small, and the laser OFF time, calculated by multiplying the pulse width thinning rate by the time for one pixel, which is the reciprocal of the clock frequency, is very short. According to research by the inventors of the present invention, when the laser OFF time is 5 nsec or less, especially 3.5 nsec or less, the light source 401 may not respond in time, resulting in the light emission from the light source 401 not actually being turned OFF. This response characteristic includes the characteristics of the laser element itself as well as the characteristics of the electrical circuit on the laser substrate.

[0041] When a pulse width correction table like the comparative example in Figure 9(a) is used, the distribution of light quantity on the photosensitive drum 4 in the main scanning direction will have a partially upwardly convex shape near the end in the main scanning direction (dashed circle α), as shown in Figure 9(b). In Figure 9(b), the horizontal axis represents position in the main scanning direction [mm], and the vertical axis represents light quantity [au]. Note that the light quantity is expressed as 1 at 0 mm (center) in the main scanning direction.

[0042] The amount of light on the surface of the photosensitive drum 4 can be confirmed by placing a light receiving element at a position corresponding to the surface of the photosensitive drum 4 and measuring the irradiated laser light 208. When an image is printed in a state like the comparative example, the amount of toner carried increases in areas where the amount of light is strong (dashed circle α), resulting in a high image density near the edges of the image in the main scanning direction.

[0043] FIG. 10 shows the distribution of light intensity in the main scanning direction on the surface of the photosensitive drum 4 when the pulse width correction table of this embodiment is used. The horizontal and vertical axes in FIG. 10 are the same as those in FIG. 9(b). In this embodiment, as shown in FIG. 10, a nearly flat light intensity distribution in the main scanning direction is obtained. At the ends in the main scanning direction, the pulse width thinning using the correction table of FIG. 8 is increased compared to the comparative example. In this way, by increasing the thinning in the time region where the laser is less responsive, a flat light intensity distribution can be obtained. In this embodiment, the shape of the pulse width correction table has an inflection point Pa in the region near ±87 mm in the main scanning direction, where the laser OFF time is approximately 3.5 nsec.

[0044] In this embodiment, even if the image data before correction is data corresponding to solid black, such as 255, correction is performed by the pulse width correction unit 101c, so that there are no pixels that substantially emit light. In other words, in the correction table of Fig. 8, the correction value is less than 1 throughout the longitudinal direction. By performing the pulse width correction described above, a nearly flat light intensity distribution in the main scanning direction can be obtained, resulting in a uniform image.

[0045] The correction coefficients shown in FIG. 8 of this embodiment increase from the center toward the edges in the main scanning direction. That is, when viewed over the entire area in the main scanning direction, the correction coefficients change so as to form a concave shape at the center position in the main scanning direction. The correction coefficients also change to have an inflection point Pa near the edges in the main scanning direction. When the area from the center to the edges in the main scanning direction is divided into two (equal parts), the inflection point Pa is included in the area closest to the edges. That is, as shown in FIG. 8, the inflection point Pa is included in the area Rb that is closer to the edges of the two equal parts Ra and Rb. For example, if the paper P, which has the maximum size (maximum paper width) that the printer 9 can form an image on, is letter size, the area Ra is 0 mm to ±54 mm, and the area Rb is ±55 mm to ±108 mm. In this case, the inflection point Pa of the correction coefficients is included in the area Rb, which is ±55 mm to ±108 mm. In addition, in other printers with a different maximum paper width from printer 9, the range of areas Ra and Rb in the main scanning direction and the position of inflection point P will also be different, but inflection point Pa will be included in area Rb regardless of the maximum paper width.

[0046] In the above configuration, the resolution in the main scanning direction and the sub-scanning direction has been described using 600 dpi as an example, but this is not limited to this and other resolutions may be used. Furthermore, the relationship between the position in the main scanning direction and the partial magnification shown in this embodiment (FIG. 3) is merely an example, and information used for various controls may be defined according to variations in this relationship. In this embodiment, an example has been given in which halftones are expressed based on multi-level dithering, but this is not limited to this and other halftone expression methods may also be used.

[0047] <Cases where the amount of light is insufficient at the edges> In this embodiment, an example was given in which the slope of the pulse width correction table is smaller at the edge of the inflection point Pa than at the center in the main scanning direction. On the other hand, in the case of an image with a short light emission width, such as a one-dot image, in the region where the clock frequency is high at the edge of the main scanning direction, a response delay in turning on the light source 401 may result in an insufficient amount of light. In such a case, as shown in FIG. 11 , a pulse width correction table may be applied in which the slope is larger at the edge of the inflection point Pb. FIG. 11 is a graph showing a correction table for the case where, when control is performed to turn on the light source 401 in a short time at the edge, the laser cannot actually be turned on due to a response delay of the light source 401, resulting in an insufficient amount of light.

[0048] In the graph of Fig. 11, the horizontal axis represents the position in the main scanning direction [mm], and the vertical axis represents the pulse width correction coefficient. In the graph of Fig. 11, the slope of the graph in the end region of the inflection point Pb is greater than the slope of the graph in the central region. Note that the rest of the configuration of the correction table is the same as that described in Fig. 8.

[0049] The control unit 1 can distinguish the image from the image data, for example, by pattern matching, and apply a pulse width correction table such as that shown in Figure 11 to only images with one dot, and a pulse width correction table such as that shown in Figure 8 to other images.

[0050] As described above, the correction tables shown in FIGS. 8 and 11 in this embodiment associate the position in the main scanning direction with the pulse width correction coefficient. The correction coefficient is set to increase from the center toward the edge in the main scanning direction. In this embodiment, an inflection point is provided at which the slope, which represents the increase in the correction coefficient per unit increase in the main scanning direction position, changes. That is, between the center and the edge in the main scanning direction, the slope in the region closer to the center than the inflection point is different from the slope in the region closer to the edge than the inflection point. When the distance from the center to the edge in the main scanning direction is divided into two (equal parts), the inflection point is included in the edge region of the two regions. When controlling the light source 401 to turn off in a short time, a response delay of the light source 401 may prevent the light from actually being turned off, resulting in an excessive light intensity. To correct this type of image data, a correction coefficient is used that is set so that the slope closer to the edge than the inflection point is smaller than the slope closer to the center than the inflection point. When controlling the light source 401 to emit light in a short time, a response delay of the light source 401 may prevent the light from actually being emitted, resulting in an insufficient light intensity. In correcting such image data, a correction coefficient is used that is set so that the slope on the edge side of the inflection point is greater than the slope on the center side of the inflection point. The correction table may also be one in which a plurality of correction coefficients are set for a plurality of positions in the main scanning direction, or one in which the correction coefficients are given as a function of the position in the main scanning direction.

[0051] As described above, according to this embodiment, even in an image forming apparatus in which the image clock frequency is changed in the scanning direction without using a scanning lens having fθ characteristics, it is possible to suppress the occurrence of image defects such as uneven density.

[0052] <Other embodiments> The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

[0053] The disclosure of this embodiment includes the following configuration. (Configuration 1) a photoreceptor on which a latent image is formed; an exposure unit that scans the photosensitive member with a laser beam whose scanning speed changes depending on the position in the main scanning direction to form the latent image; a generating means for generating a clock signal for controlling the timing of turning on the laser light; a first correction means for correcting the frequency of the clock signal for each position in the main scanning direction; a developing means for developing the latent image on the photosensitive member with toner to form a toner image; a driving means for driving the laser light based on a driving signal; an image forming apparatus for transferring the toner image onto a recording material to form an image, a second correction means for correcting the input image data using a correction coefficient according to the position in the main scanning direction; a modulation means for modulating the image data corrected by the second correction means into the drive signal; Equipped with the correction coefficient increases from the center toward the end in the main scanning direction, and has an inflection point at which a slope, which is an increase in the correction coefficient per unit increase in position in the main scanning direction, changes; The image forming apparatus is characterized in that the inflection point is located near an end in the main scanning direction. (Configuration 2) The image forming apparatus according to configuration 1, wherein the inflection point is included in the end region of the two regions when the area from the central portion to the end region is divided into two equal parts. (Configuration 3) 3. The image forming apparatus according to claim 1, wherein the first correction unit corrects the clock signal so that the laser light is turned on or off within 5 nsec at a position on the edge side from the position in the main scanning direction corresponding to the inflection point. (Configuration 4) 4. The image forming apparatus according to any one of configurations 1 to 3, wherein the slope in the region closer to the edge than the inflection point is smaller than the slope in the region closer to the center than the inflection point. (Configuration 5) 4. The image forming apparatus according to any one of configurations 1 to 3, wherein the slope in the region closer to the edge than the inflection point is greater than the slope in the region closer to the center than the inflection point. (Configuration 6) the correction coefficient is less than 1 over the entire range of positions in the main scanning direction, 6. The image forming apparatus according to any one of configurations 1 to 5, wherein the second correction means multiplies the input image data by the correction coefficient to obtain corrected image data. (Configuration 7) 7. The image forming apparatus according to any one of configurations 1 to 6, further comprising a halftone processing unit that performs halftone processing using a dither matrix before the correction by the second correction unit. [Explanation of symbols]

[0054] 2 CPU 4 Photosensitive drum 100 Image signal generation unit 101c Pulse width correction section 101d PWM control unit 300 Laser driver 400 Optical Scanning Device

Claims

1. a photoreceptor on which a latent image is formed; an exposure unit that scans the photosensitive member with a laser beam whose scanning speed changes depending on the position in the main scanning direction to form the latent image; a generating means for generating a clock signal for controlling the timing of turning on the laser light; a first correction means for correcting the frequency of the clock signal for each position in the main scanning direction; a developing means for developing the latent image on the photosensitive member with toner to form a toner image; a driving means for driving the laser light based on a driving signal; an image forming apparatus for transferring the toner image onto a recording material to form an image, a second correction means for correcting the input image data using a correction coefficient according to the position in the main scanning direction; a modulation means for modulating the image data corrected by the second correction means into the drive signal; Equipped with the correction coefficient increases from the center toward the end in the main scanning direction, and has an inflection point at which a slope, which is an increase in the correction coefficient per unit increase in position in the main scanning direction, changes; The image forming apparatus is characterized in that the inflection point is located near an end in the main scanning direction.

2. 2. The image forming apparatus according to claim 1, wherein the inflection point is included in the end region of the two regions when the area from the central portion to the end region is divided into two equal parts.

3. 2. The image forming apparatus according to claim 1, wherein the first correction means corrects the clock signal so that the laser light is turned on or off in a time period of 5 nsec or less at a position toward the end of the main scanning direction from the position corresponding to the inflection point.

4. 2. The image forming apparatus according to claim 1, wherein the slope in the region closer to the end than the inflection point is smaller than the slope in the region closer to the center than the inflection point.

5. 2. The image forming apparatus according to claim 1, wherein the slope in the region closer to the end than the inflection point is greater than the slope in the region closer to the center than the inflection point.

6. the correction coefficient is less than 1 over the entire range of positions in the main scanning direction, 6. The image forming apparatus according to claim 1, wherein the second correction unit multiplies the input image data by the correction coefficient to obtain corrected image data.

7. 6. The image forming apparatus according to claim 1, further comprising a halftone processing unit that performs halftone processing using a dither matrix before the correction by the second correction unit.

Images