Image formation apparatus

Abstract

To reduce variations in the image width in the laser scanning direction even when a lens with fθ characteristics is not used.SOLUTION: An image formation apparatus 9 includes a rotatable photosensitive drum 4 and an optical scanning device 400 that scans laser light based on image data in the scanning direction to form a latent image on the photosensitive drum 4. The scanning speed being the speed of the laser light scanned by the photosensitive drum 4 is slower at the center portion in the scanning direction than at the end portions. The apparatus further includes a control unit 1 configured to correct the image data (S103 YES to S106) according to the number of consecutive pixels being the number of pixels forming the latent image continuing in the scanning direction and the pixel position being the position in the continuous pixels being the continuous pixels (S103).SELECTED DRAWING: Figure 16

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, for example, an image forming apparatus such as a laser beam printer, a digital copier, or a digital facsimile, which performs optical writing using a laser beam. [Background technology]

[0002] The electrophotographic image forming apparatus has an optical scanning unit for exposing a photoconductor. The optical scanning unit emits laser light based on image data, reflects the laser light on a rotating polygon mirror, and transmits the laser light through a scanning lens to irradiate and expose the photoconductor. A latent image is formed on the photoconductor by scanning the spot of the laser light formed on the surface of the photoconductor by rotating the rotating polygon mirror, thereby moving the spot of the laser light. The scanning lens is a lens having a so-called fθ characteristic. The fθ characteristic is an optical characteristic that forms an image of the laser light on the surface of the photoconductor so that the spot of the laser light on the surface of the photoconductor moves at a constant speed on the surface of the photoconductor when the rotating polygon mirror rotates at a constant angular speed. By using a scanning lens having the fθ characteristic in this way, appropriate exposure can be performed.

[0003] A scanning lens having such an fθ characteristic is relatively large and expensive. Therefore, in order to reduce the size and cost of an image forming apparatus, it is considered to not use a scanning lens at all, or to use a scanning lens that does not have an fθ characteristic. For example, Patent Document 1 discloses that even if the spot of the laser light on the surface of the photoconductor does not move at a constant speed on the surface of the photoconductor, electrical correction is performed to change the image clock frequency during one scan so that the pixels formed on the surface of the photoconductor are arranged evenly. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 08-125064 Summary of the Invention [Problem to be solved by the invention]

[0005] However, even if the width of each pixel is made constant by the above-mentioned electrical correction without using a scanning lens having fθ characteristics, the speed at which the spot of the laser light moves on the surface of the photoconductor is different. Specifically, the speed at which the spot of the laser light moves on the surface of the photoconductor to form one pixel is different between one pixel at the end of the main scanning direction and one pixel at the center. Therefore, the exposure amount per unit area is different between the pixels at the end and the pixels at the center in the main scanning direction. A problem caused by the difference in the exposure amount between the center and the end is the difference in the width (image width) of the toner image formed on the photoconductor. Since the exposure amount at the center is higher than the exposure amount at the end, a thicker latent image is formed on the photoconductor at the center, and as a result, the image width is also thicker at the center. This tendency is particularly noticeable when trying to print thin vertical lines.

[0006] The present invention has been made under these circumstances, and has an object to reduce the difference in image width in the laser scanning direction even when a lens having fθ characteristics is not used. [Means for solving the problem]

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

[0008] (1) An image forming apparatus comprising: a rotatable photosensitive member; and a light irradiation means for forming a latent image on the photosensitive member by scanning a laser beam based on image data in a scanning direction, wherein a scanning speed, which is the speed of the laser beam scanned on the photosensitive member, is slower in the center in the scanning direction than at the ends, and further comprising a correction means for correcting the image data in accordance with a number of consecutive pixels, which is the number of pixels that form the latent image that are consecutive in the scanning direction, and a pixel position, which is a position within the consecutive pixels that are consecutive pixels. Effect of the Invention

[0009] According to the present invention, even when a lens having fθ characteristics is not used, the difference in image width in the laser scanning direction can be reduced. [Brief description of the drawings]

[0010] [Figure 1] Schematic diagram of the main configuration of an image forming apparatus according to embodiments 1 to 3 [Diagram 2] 1A and 1B are main-scanning sectional views and sub-scanning sectional views of the optical scanning device according to the first to third embodiments. [Diagram 3] Graph showing the partial magnification versus image height of the optical scanning device according to the first to third embodiments [Figure 4] Electrical block diagram showing the exposure control configuration of the first to third embodiments. [Diagram 5] FIG. 1 shows a time chart and dot images of each signal in Examples 1 to 3. [Figure 6] Block diagram showing an image modulation unit according to the first to third embodiments. [Figure 7] FIG. 1 is a diagram showing an example of a screen according to Examples 1 to 3, and is a diagram illustrating pixels and pixel pieces. [Figure 8] Time chart regarding the operation of the image modulation unit in the first to third embodiments [Figure 9] FIG. 1 is a diagram showing an example of an image signal input to a halftone processing unit in each of Examples 1 to 3; a diagram showing a screen; and a diagram showing an example of a halftone-processed screen. [Figure 10] FIG. 1 is a diagram for explaining insertion and excerpt of pixel pieces in Examples 1 to 3. [Figure 11] FIG. 1 is a diagram for explaining partial magnification correction by clock frequency correction in the first to third embodiments. [Figure 12] FIG. 1 is a diagram for explaining the image width formed at the center and end of the print area in Examples 1 to 3. [Figure 13] FIG. 1 is a diagram for explaining the effect of the amount of laser light in Examples 1 to 3 on the formation of an electrostatic latent image. [Figure 14] FIG. 1 is a diagram for explaining the shortening of the pixel width of the outermost pixels in Examples 1 to 3. [Figure 15] FIG. 13 is a diagram for explaining the influence of the image width correction process in the first to third embodiments on the image width. [Figure 16] Flowchart showing the image width correction process according to the first embodiment [Figure 17] FIG. 1 is a diagram for explaining the effects of the first embodiment. [Figure 18] FIG. 11 is a diagram for explaining a difference in sensitivity of a photosensitive drum in the second embodiment, and a diagram for explaining an effect on uniformity of an image width in the scanning direction. [Figure 19] Flowchart showing the image width correction process of the second embodiment [Figure 20] 11 is a flowchart showing a method of image width correction processing according to the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiment of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the embodiment may be changed as appropriate depending on the configuration and various conditions of the device to which the invention is applied. In other words, the scope of the present invention is not limited to the following embodiment. EXAMPLES

[0012] (1) Image forming device Hereinafter, the image forming apparatus of the first embodiment will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram of the configuration of an image forming apparatus 9. A laser driving unit 300 in an optical scanning device 400, which is a light irradiation means, emits a scanning light (laser light) 208 based on an image signal output from an image signal generating unit 100 and a control signal output from a control unit 1. A photosensitive drum (photoconductor) 4 charged by a charging means (not shown) is scanned with the laser light 208 to form a latent image on the surface of the photosensitive drum 4. Then, a developing means (not shown) attaches toner to the latent image to form a toner image corresponding to the latent image. The toner image is transferred to a recording medium such as paper fed from a paper feed unit 8 and conveyed to a position where it contacts the photosensitive drum 4 by a roller 5. The toner image transferred to the recording medium is thermally fixed to the recording medium by a fixing unit 6 and discharged to the outside of the machine via a discharge roller 7.

[0013] The memory 4a is a memory for the photosensitive drum 4, and stores information about the photosensitive drum 4. The control unit 1 can read the information about the photosensitive drum 4 from the memory 4a. Here, the information about the photosensitive drum 4 includes the sensitivity information (good or bad) of the photosensitive drum 4, usage information, information about the usage environment, etc. The usage information includes the usage history of the photosensitive drum 4, for example, the lifespan of the photosensitive drum 4. The information about the usage environment includes temperature, humidity, etc. The image forming apparatus 9 is equipped with an environmental sensor 200, and information about the usage environment, such as temperature and humidity, can be detected by the environmental sensor 200.

[0014] <Optical scanning device> FIG. 2 is a cross-sectional view of the optical scanning device 400 of the first embodiment, where FIG. 2(a) shows the main scanning cross section and FIG. 2(b) shows the sub-scanning cross section. In the first embodiment, the laser light (light 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 passing through the coupling lens 403 is converted into substantially parallel light and enters the anamorphic lens 404. Note that the substantially parallel light includes weakly convergent light and weakly divergent light. The anamorphic lens 404 has a positive refractive power in the main scanning cross section and converts the incident light beam into convergent light in the main scanning cross section. In addition, the anamorphic lens 404 focuses the light beam in the vicinity of the deflection surface 405a of the deflector 405 in the sub-scanning cross section, forming a long line image in the main scanning direction.

[0015] Then, the light beam that has passed through the anamorphic lens 404 is reflected by a deflection surface 405a of the deflector 405. The light beam reflected by the deflection surface 405a passes through the imaging lens 406 as the laser beam 208 (see FIG. 1) and is incident on the surface of the photosensitive drum 4. The imaging lens 406 is an imaging optical element. In the first embodiment, the imaging optical system is configured with only a single imaging optical element (imaging lens 406). The surface of the photosensitive drum 4 on which the light beam that has passed through (transmitted) the imaging lens 406 is incident is a scanned surface 407 that is scanned by the light beam. The light beam is imaged on the scanned surface 407 by the imaging lens 406, forming a predetermined spot-shaped image (spot). By rotating the deflector 405 at a constant angular velocity in the direction of the arrow A by a driving unit (not shown), the spot moves in the main scanning direction on the scanned surface 407, forming an electrostatic latent image on the scanned surface 407. The main scanning direction is a direction parallel to the surface of the photosensitive drum 4 and perpendicular to the direction of movement of the surface of the photosensitive drum 4. The sub-scanning direction is a direction perpendicular to the main scanning direction and the optical axis of the light beam.

[0016] A beam detect (hereinafter referred to as BD) 409 and a BD lens 408 constitute a synchronization optical system that determines the timing of writing an electrostatic latent image onto a surface to be scanned 407. A light beam that passes through the BD lens 408 is incident on and detected by the BD 409 that includes a photodiode. The writing timing is controlled based on the timing at which the BD 409 detects the light beam.

[0017] The light source 401 is a semiconductor laser chip. The light source 401 of the first embodiment is configured to include one light emitting unit 11 (see FIG. 4). However, the light source 401 may include a plurality of light emitting units capable of independently controlling light emission. Even when a plurality of light emitting units are included, a plurality of light beams emitted from the light emitting units reach the scanned surface 407 via the coupling lens 403, the anamorphic lens 404, the deflector 405, and the imaging lens 406. On the scanned surface 407, spots corresponding to the respective light beams are formed at positions shifted in the sub-scanning direction. Note that various optical members such as the light source 401, the coupling lens 403, the anamorphic lens 404, the imaging lens 406, and the deflector 405 described above are stored in a housing 400a (optical box) (see FIG. 1).

[0018] <Imaging lens> As shown in FIG. 2, the imaging lens 406 has two optical surfaces (lens surfaces), an incident surface 406a (first surface) and an exit surface 406b (second surface). The imaging lens 406 is configured to scan the scanned surface 407 with a desired scanning characteristic with a light beam deflected by the deflecting surface 405a in the main scanning cross section. The imaging lens 406 is also configured to make the spot of the laser light 208 on the scanned surface 407 into a desired shape. The imaging lens 406 also makes the vicinity of the deflecting surface 405a and the vicinity of the scanned surface 407 in the sub-scanning cross section conjugate with each other. This allows the configuration to compensate for the surface tilt. Here, the compensation for the surface tilt refers to reducing the scanning position deviation in the sub-scanning direction on the scanned surface 407 when the deflecting surface 405a is tilted.

[0019] Although the imaging lens 406 in the first embodiment is a plastic molded lens formed by injection molding, a glass molded lens may be used as the imaging lens 406. Since a molded lens is easy to mold into an aspheric shape and is suitable for mass production, by using a molded lens as the imaging lens 406, it is possible to improve the productivity and optical performance.

[0020] The imaging lens 406 does not have so-called fθ characteristics. In other words, it does not have a scanning characteristic that moves the spot of the light beam passing through the imaging lens 406 at a constant speed on the scanned surface 407 when the deflector 405 rotates at a constant angular velocity. In this way, by using the imaging lens 406 that does not have the fθ characteristics, it is possible to arrange the imaging lens 406 close to the deflector 405, that is, at a position where the distance D1 is small. In addition, the imaging lens 406 that does not have the fθ characteristics can be made smaller in the main scanning direction (width LW) and the optical axis direction (thickness LT) than the imaging lens that has the fθ characteristics. For this reason, the housing 400a (see FIG. 1) of the optical scanning device 400 is made smaller. In addition, in the case of a lens that has the fθ characteristics, there may be a sharp change in the shape of the entrance surface and the exit surface of the lens when viewed in the main scanning cross section, and if there is such a shape constraint, there is a possibility that good imaging performance cannot be obtained. In contrast, the imaging lens 406 does not have an fθ characteristic, and therefore there is little abrupt change in the shapes of the entrance surface and exit surface of the lens when viewed in the main scanning section, so that good imaging performance can be obtained.

[0021] The scanning characteristic of the imaging lens 406 of the first embodiment is expressed by the following formula (1).

number

[0022] In formula (1), the scanning angle (scanning angle of view) by the deflector 405 is θ, the light-converging position (image height) in the main scanning direction on the scanned surface 407 of the light beam is Y [mm], the imaging coefficient at the on-axis image height is K [mm], and the coefficient (scanning characteristic coefficient) that determines the scanning characteristic of the imaging lens 406 is B. In the first embodiment, the on-axis image height refers to the image height on the optical axis (Y=0=Ymin) and corresponds to the scanning angle θ=0. The off-axis image height refers to the image height (Y≠0) outside the central optical axis (when the scanning angle θ=0) and corresponds to the scanning angle θ≠0. Furthermore, the most off-axis image height refers to the image height (Y=+Ymax, -Ymax) when the scanning angle θ is maximum (maximum scanning angle of view). The scanning width W, which is the width in the main scanning direction of a predetermined area (scanning area) on the scanned surface 407 where a latent image can be formed, is expressed as W=|+Ymax|+|-Ymax|. The center of the predetermined area is the on-axis image height, and the ends are the most off-axis image height.

[0023] Here, the imaging coefficient K is a coefficient equivalent to f in the scanning characteristic (fθ characteristic) Y=fθ when parallel light is incident on the imaging lens 406. In other words, the imaging coefficient K is a coefficient for making the light collection position Y and the scanning angle θ proportional to each other, similar to the fθ characteristic, when a light beam other than parallel light is incident on the imaging lens 406.

[0024] To add a bit more about the scanning characteristic coefficient, when B=0, formula (1) gives Y=Kθ, which corresponds to the scanning characteristic Y=fθ of an imaging lens used in a conventional optical scanning device. When B=1, formula (1) gives Y=Ktanθ, which corresponds to the projection characteristic Y=ftanθ of a lens used in an imaging device (camera) or the like. In other words, by setting the scanning characteristic coefficient B in the range of 0≦B≦1 in formula (1), it is possible to obtain a scanning characteristic between the projection characteristic Y=ftanθ and the fθ characteristic Y=fθ.

[0025] Here, by differentiating equation (1) with respect to the scanning angle θ, the scanning speed of the light beam on the scanned surface 407 with respect to the scanning angle θ can be obtained as shown in the following equation (2).

number

[0026] Furthermore, when equation (2) is divided by the velocity dY / dθ=K at the axial image height, the result is as shown in the following equation (3).

number

[0027] FIG. 3 shows the relationship between image height and partial magnification when the scanning position on the scanned surface 407 in the first embodiment is fitted with the characteristic of Y=Kθ. In the graph in FIG. 3, the horizontal axis indicates the image height [mm], and the vertical axis indicates the partial magnification [%]. The black dots indicate the partial magnification for Y=Kθ. In the first embodiment, the scanning characteristic shown in the formula (1) is given to the imaging lens 406, so that the scanning speed gradually increases from the on-axis image height to the off-axis image height, as shown in FIG. 3, and the partial magnification increases. A partial magnification of 30% means that when light is irradiated for a unit time, the irradiation length in the main scanning direction on the scanned surface 407 becomes 1.3 times. Therefore, if the pixel width in the main scanning direction is determined at a constant time interval determined by the cycle of the image clock, the pixel density will differ between the on-axis image height and the off-axis image height.

[0028] Furthermore, as the image height Y moves away from the on-axis image height and approaches the most off-axis image height (as the absolute value of the image height Y increases), the scanning speed gradually increases. As a result, the time required to scan a unit length when the image height is near the most off-axis image height is shorter than the time required to scan a unit length when the image height on the scanned surface 407 is near the on-axis image height. This means that, when the emission luminance of the light source 401 is constant, the total exposure amount per unit length when the image height is near the most off-axis image height is less than the total exposure amount per unit length when the image height is near the on-axis image height.

[0029] In this way, when the optical configuration is as described above, the variation in the partial magnification and the total exposure amount per unit length in the main scanning direction may not be appropriate for maintaining good image quality. Therefore, in the first embodiment, in order to obtain good image quality, the partial magnification correction and the brightness correction for correcting the total exposure amount per unit length are performed.

[0030] In particular, as the optical path length from the deflector 405 to the photosensitive drum 4 becomes shorter, the angle of view becomes larger, and therefore the difference in scanning speed between the on-axis image height and the most off-axis image height described above becomes larger. According to the inventor's study, the optical configuration is such that the scanning speed at the most off-axis image height is 120% or more of the scanning speed at the on-axis image height, and the rate of change in scanning speed is 20% or more. With such an optical configuration, it may become difficult to maintain good image quality due to the influence of variations in partial magnification in the main scanning direction and total exposure amount per unit length.

[0031] The rate of change C (%) of the scanning speed is expressed as C = ((Vmax - Vmin) / Vmin) x 100, where Vmin is the slowest scanning speed and Vmax is the fastest scanning speed. In the optical configuration of the first embodiment, the scanning speed is the slowest at the on-axis image height (the center of the scanning area) and the fastest at the most off-axis image height (the end of the scanning area).

[0032] According to the inventor's study, it has been found that in the case of an optical configuration in which the angle of view is 52° or more, the rate of change in the scanning speed is 30% or more. The conditions for the angle of view being 52° or more are as follows. For example, in the case of an optical configuration in which a latent image having the width of the short side of an A4 sheet is formed in the main scanning direction, the scanning width W is 214 mm, and the optical path length D2 (see FIG. 2) from the deflection surface 405a to the scanned surface 407 when the scanning angle of view is 0° is 125 mm or less. In the case of an optical configuration in which a latent image having the width of the short side of an A3 sheet is formed in the main scanning direction, the scanning width W is 300 mm, and the optical path length D2 (see FIG. 2) from the deflection surface 405a to the scanned surface 407 when the scanning angle of view is 0° is 247 mm or less. In an image forming apparatus having such an optical configuration, by using the configuration of the embodiment 1 described below, it is possible to obtain good image quality even if an imaging lens that does not have an fθ characteristic is used.

[0033] <Exposure control configuration> FIG. 4 is an electrical block diagram showing an exposure control configuration in the image forming apparatus 9. The image signal generating unit 100 has an image modulation unit 101, a CPU 102, and a ROM 102a. The image signal generating unit 100 receives print information from a host computer (not shown) and generates a VDO signal 110 corresponding to image data (image signal). The image signal generating unit 100 also has a function as a pixel width correction unit and a function as a density correction unit that corrects image density. The control unit 1 controls the image forming apparatus 9. The laser driving unit 300 is equipped with a memory 304, a laser driver IC 90, and a light emitting unit 11 of a light source 401. The laser driver IC 90 controls ON / OFF of the light emission of the light source 401 based on the VDO signal 110 by switching between passing a current IL through the light emitting unit 11 to emit light or passing a current IL through a dummy resistor 10 to turn off the light emitting unit 11 in response to the VDO signal 110. The photodetector 12 detects the amount of light of the light emitting unit 11. The CPU core 2 reads information from the memory 304 by performing serial communication 307 with the memory 304 .

[0034] When the image signal generating unit 100 is ready to output an image signal for image formation, it instructs the control unit 1 to start printing via serial communication 113. The control unit 1 has a CPU core 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 generating unit 100. Upon receiving the synchronization signal, the image signal generating unit 100 outputs a VDO signal 110, which is an image signal, to the laser driving unit 300 at a predetermined timing.

[0035] 5(a) is a timing chart of various synchronization signals and image signals when performing an image forming operation equivalent to one page of a recording medium. Time passes from left to right in the figure. A "HIGH" TOP signal 112 indicates that the leading edge of the recording medium 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, a light source 401 emits light to form a latent image on the photosensitive drum 4.

[0036] 5(a), for the sake of simplicity, the VDO signal 110 is depicted as being continuously output across a plurality of 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.

[0037] <Partial magnification correction method> (How to insert and remove pixel pieces) Next, the partial magnification correction method will be explained. Prior to the explanation, the cause of partial magnification and the correction principle will be explained with reference to FIG. 5(b). FIG. 5(b) is a diagram showing the timing of the BD signal 111 and the VDO signal 110, and the dot image formed by the latent image on the scanned surface 407. Time progresses from left to right in the figure.

[0038] When the image signal generating unit 100 receives the rising edge of the BD signal 111, it transmits a VDO signal 110 after a predetermined timing so that a latent image can be formed at a position a desired distance away from the left end of the photosensitive drum 4. Then, the light source 401 emits light based on the VDO signal 110, and a latent image corresponding to the VDO signal 110 is formed on the scanned surface 407.

[0039] Here, a case will be described in which a dot-shaped latent image is formed by emitting light from the light source 401 for the same period at the on-axis image height and the most off-axis image height based on the VDO signal 110. The size of this dot corresponds to one dot at 600 dpi (width of 42.3 um in the main scanning direction). As described above, the optical scanning device 400 has an optical configuration in which the scanning speed is faster at the end (the most off-axis image height) than at the center (the on-axis image height) on the scanned surface 407. As shown in latent image A, the latent image dot1 at the most off-axis image height is enlarged (stretched) in the main scanning direction compared to the latent image dot2 at the on-axis image height. Therefore, in the first embodiment, the period and time width of the VDO signal 110 are corrected according to the position in the main scanning direction as partial magnification correction. That is, the partial magnification correction shortens the interval of the emission time at the most off-axis image height compared to the interval of the emission time at the on-axis image height, and makes the latent image dot3 at the most off-axis image height and the latent image dot4 at the on-axis image height the same size as shown in latent image B. By such a correction, it becomes possible to form dot-shaped latent images corresponding to each pixel at substantially equal intervals in the main scanning direction.

[0040] Next, specific processing of partial magnification correction that shortens the irradiation time of the light source 401 by an amount corresponding to an increase in the partial magnification as the image height moves from the on-axis image height to the off-axis image height will be described with reference to Fig. 6 to Fig. 10. Fig. 6 is a block diagram showing an example of the image modulation unit 101. The halftone processing unit 122 performs a screen (dither) process on the multi-value parallel 8-bit image signal input from the density correction processing unit 121, and performs a conversion process for expressing density in the image forming device 9.

[0041] 7(a) shows an example of a screen, which expresses density with a 200-line matrix 153 of 3 pixels in the main scan direction and 3 pixels in the sub-scan direction. The white parts in the figure are parts where the light source 401 is not illuminated (OFF), and the black parts are parts where the light source 401 is illuminated (ON). The matrix 153 is provided for each gradation, and the gradation increases (the density becomes darker) in the order shown by the arrows. In the first embodiment, one pixel 157 is a unit that divides image data to form one dot of 600 dpi on the scanned surface 407.

[0042] As shown in FIG. 7B, before the pixel width is corrected, one pixel is composed of 16 pixel pieces each having a width 1 / 16 of the pixel width, and the light source 401 can be switched on and off for each pixel piece. In other words, one pixel can express 16 steps of gradation. The PS conversion unit 123 is a parallel-serial conversion unit, and converts the parallel 16-bit signal 129 input from the halftone processing unit 122 into a serial signal 130. The FIFO 124 receives the serial signal 130, accumulates it in a line buffer (not shown), and outputs it as a serial signal to the laser driving unit 300 at the downstream stage as a VDO signal 110 after a predetermined time. The writing and reading of the FIFO 124 are controlled by the pixel piece insertion / removal control unit 128, which controls the write enable signal WE 131 and the read enable signal RE 132 based on the partial magnification characteristic information received from the CPU 102 via the CPU bus 103. The PLL unit 127 supplies a clock (VCLK×16) 126 , which is a clock (VCLK) 125 corresponding to one pixel and has a frequency multiplied by 16, to the PS conversion unit 123 and the FIFO 124 .

[0043] Next, the operation after the forced OFF process in the block diagram of Fig. 6 will be described using the time chart relating to the operation of the image modulation unit 101 in Fig. 8. As described above, the PS conversion unit 123 takes in the multi-value 16-bit signal 129 from the halftone processing unit 122 (forced OFF processing unit) in synchronization with the clock 125, and sends the serial signal 130 to the FIFO 124 in synchronization with the clock 126.

[0044] The FIFO 124 takes in the signal 130 only when the WE signal 131 is valid and "HIGH." When shortening the image in the main scanning direction to correct the partial magnification, the pixel piece insertion / removal control unit 128 controls the FIFO 124 not to take in the serial signal 130 by partially disabling the WE signal and setting it to "LOW." In other words, a pixel piece is extracted. FIG. 8 shows an example in which one pixel is normally composed of 16 pixel pieces, but one pixel piece is extracted from the 1st pixel, resulting in a composition of 15 pixel pieces.

[0045] Also, the FIFO 124 reads out the accumulated data in synchronization with the clock 126 (VCLKx16) only when the RE signal 132 is enabled "HIGH" and outputs the VDO signal 110. When the image is lengthened in the main scanning direction for partial magnification correction, the pixel piece insertion / removal control unit 128 partially disables the RE signal 132 "LOW" so that the FIFO 124 does not update the read data and continues to output the data from one clock before the clock 126. In other words, a pixel piece of the same data as the data of the adjacent pixel piece on the upstream side in the main scanning direction processed immediately before is inserted. FIG. 8 shows an example in which, in a configuration in which one pixel is normally composed of 16 pixel pieces, two pixel pieces are inserted into the 2nd pixel, resulting in a configuration of 18 pixel pieces. Note that the FIFO 124 used in the first embodiment has been described as a circuit configured to continue the previous output rather than going into a Hi-Z state when the RE signal is disabled "LOW".

[0046] 9 and 10 are diagrams illustrating the parallel 16-bit signal 129, which is the input image of the halftone processing unit 122, to the VDO signal 110, which is the output of the FIFO 124, using image images. FIG. 9(a) is an example of a multi-value parallel 8-bit image signal input to the halftone processing unit 122. Each pixel has 8-bit density information. The pixel 150 has density information of F0h, the pixel 151 has density information of 80h, the pixel 152 has density information of 60h, and the white part has density information of 00h. FIG. 9(b) is a screen, which is a 200-line screen that grows from the center as described in FIG. 7. FIG. 9(c) is an image image of the image signal, which is the parallel 16-bit signal 129 after halftone processing, and as described above, each pixel 157 is composed of 16 pixel pieces. 154 indicates a pixel piece that is ON, and 155 indicates a pixel piece that is OFF.

[0047] FIG. 10 shows an example of inserting pixel pieces to extend an image and an example of extracting image pieces to shorten an image, focusing on an area 158 of 7 pixels in the main scanning direction in FIG. 9(c) for the serial signal 130. 156 indicates a pixel piece. FIG. 10(a) shows an example of increasing the partial magnification by 8%. By inserting a total of 8 pixel pieces at equal or approximately equal intervals into a group of 100 consecutive pixel pieces, the pixel width can be changed to increase the partial magnification by 8%, and the latent image can be extended in the main scanning direction. FIG. 10(b) shows an example of decreasing the partial magnification by 7%. By extracting a total of 7 pixel pieces at equal or approximately equal intervals from a group of 100 consecutive pixel pieces, the pixel width can be changed to decrease the partial magnification by 7%, and the latent image can be shortened in the main scanning direction.

[0048] In this way, in the partial magnification correction, the pixel width whose length in the main scanning direction is less than one pixel is changed so that the dot-shaped latent images corresponding to each pixel of the image data can be formed at substantially equal intervals in the main scanning direction. Note that substantially equal intervals in the main scanning direction also include pixels that are not completely evenly spaced. In other words, as a result of the partial magnification correction, the pixel intervals may vary to some extent, as long as the pixel intervals are equal on average within a predetermined image height range. As described above, when pixel pieces are inserted or extracted at equal or approximately equal intervals, when the number of pixel pieces constituting a pixel is compared between two adjacent pixels, the difference in the number of pixel pieces constituting a pixel is 0 or 1. Therefore, the variation in image density in the main scanning direction compared to the original image data can be suppressed, and good image quality can be obtained. In addition, the position where the pixel pieces are inserted or extracted may be the same position for each scanning line (line) in the main scanning direction, or the position may be shifted.

[0049] As described above, the larger the absolute value of the image height Y, the faster the scanning speed. For this reason, in partial magnification correction, the above-mentioned pixel pieces are inserted and / or extracted so that the image becomes shorter (the length of one pixel becomes shorter) as the absolute value of the image height Y becomes larger. In this way, latent images corresponding to each pixel are formed at substantially equal intervals in the main scanning direction, and the partial magnification can be appropriately corrected.

[0050] (Clock frequency correction method) A specific method for performing partial magnification correction is not limited to the pixel piece insertion / removal method described above. Here, a method for adjusting the clock frequency according to the image height so that the pixel width is approximately constant regardless of the image height will be described. FIG. 11 is a diagram showing an example of partial magnification correction described in FIG. 3. (i) shows the BD signal 111 output from the BD 409, and (ii) shows the ratio of the clock frequency (clock frequency ratio) (%). For example, the clock frequency ratio is 100% at the on-axis image height and 135% at the most off-axis image height. (iii) shows the current applied to the light source 401, and (iv) shows the light emission amount of the light source 401. (v) shows the gradation value after correction. For example, the gradation value after correction is 171 at the on-axis image height and 255 at the most off-axis image height. (vi) shows the gradation value of the print data before density correction. In this example, it is 255 over all image heights. (vii) shows the gradation value of the print data after density correction, which is divided into seven regions ((A) to (G)) in the main scanning direction, and for example, in region (G) which includes the on-axis image height, the value is 171, and in regions (A) and (G) which include the extreme off-axis image height, the value is 255. (viii) shows the density of the print image, which is constant in the main scanning direction due to correction of the clock frequency.

[0051] In FIG. 11, a case is described in which the change in scanning speed is 35%, and a partial magnification correction of 135% occurs at the most off-axis image height when the axial image height is 100%. The ROM 102a in FIG. 4 stores the clock frequency ratio related to the optical scanning device 400, and the CPU core 2 transmits the video clock signal VCLK 114 to the image modulation unit 101 based on this information to control the clock frequency. That is, the clock frequency ratio of the VDO signal 110 transmitted from the image modulation unit 101 is set to 135% at the most off-axis image height when the axial image height is 100%. At this time, the period during which the spot of the laser light 208 moves by the width of one pixel (for example, 42.3 um) on the scanned surface 407 is 0.74 times the axial image height at the most off-axis image height. In this way, the pixel width is corrected by controlling the exposure time of the laser light 208 at the pixel position corresponding to one pixel, and latent images corresponding to each pixel can be formed at substantially equal intervals and with equal sizes in the main scanning direction.

[0052] <Image width correction processing> Next, an image width correction process will be described, which corrects the difference in width (image width) of the toner image formed at the center of the printing area and at the end of the printing area in the first embodiment by image processing. Even if the width of each pixel is made constant by partial magnification correction without using a scanning lens having fθ characteristics, the speed at which the spot of the laser light moves on the surface of the photosensitive drum 4 to form one pixel is different between one pixel at the end of the main scanning direction and one pixel at the center. Therefore, the amount of exposure per unit area is different between the pixels at the end and the pixels at the center in the main scanning direction. One problem that arises due to the difference in the amount of exposure between the center and the end is the difference in the width (image width) of the toner image formed on the photosensitive drum 4.

[0053] Fig. 12 is a diagram for explaining the difference in image width between a toner image formed in the center of a print area and a toner image formed at the edge of the print area. The horizontal axis of Fig. 12(a) indicates the number of continuous pixels when forming a toner image, and the vertical axis indicates the ratio of image width between the center and the edge (center ÷ edge). The number of continuous pixels indicates the number of pixels continuously exposed in the main scanning direction, and as an example, Fig. 12(b) shows image data for two continuous pixels, four continuous pixels, and six continuous pixels.

[0054] As shown in FIG. 12(a), when the number of consecutive pixels is 2 or 4, the image width in the center is wider than the image width at the ends. On the other hand, when the number of consecutive pixels is 6 or 8, the image width does not differ that much between the center and the ends. In other words, the relationship between the image width in the center and the image width at the ends changes depending on the number of consecutive pixels, and this difference is larger when the number of consecutive pixels is small. Here, we will explain the reason why a difference in image width occurs between the center and the ends when the number of consecutive pixels is small.

[0055] Figure 13 is a diagram explaining the effect of the laser exposure amount on the formation of an electrostatic latent image. The horizontal axis of Figure 13(a) indicates the position in the scanning direction, and the vertical axis of Figure 13(a) indicates the magnitude of the exposure amount, showing an example of the laser light amount distribution when light is emitted to one pixel. As shown in Figure 13(a), the laser light amount distribution is not constant in the scanning direction, and generally has a Gaussian distribution.

[0056] FIG. 13(b) is a diagram showing the difference in the light amount distribution of the laser at the center and at the end when light is emitted to one pixel. As described above, the amount of exposure of the laser differs between the center and the end, and the exposure amount of the center shown by the dashed line is higher than that of the end shown by the solid line, and the light amount distribution also changes. This light amount distribution determines the width of the latent image formed on the photosensitive drum 4, and also determines the width of the developed toner image, i.e., the image width. As shown in the lower part of FIG. 13(b), when light is emitted to one pixel, the image width at the center is wider than the image width at the end.

[0057] FIG. 13(c) shows the difference in image width between the center and the edges when multiple pixels are exposed continuously. Due to the difference in light distribution between the center and the edges, the image width is wider in the center than at the edges. However, compared to when only one pixel is illuminated, when multiple pixels are exposed continuously, the image width itself is wider, so the rate of change in image width between the center and the edges due to the difference in light distribution is minor. As a result, as shown in FIG. 12(a), when the number of continuous pixels is small, the image width in the center is wider, and when the number of continuous images is large, the difference in image width between the center and the edges is smaller.

[0058] Here, a method for uniformizing the image width at the center and ends by performing image processing will be described. As described above, the image width is determined by the width of the electrostatic latent image formed on the photosensitive drum 4. Therefore, by narrowing the exposure width on the photosensitive drum 4, the width of the electrostatic latent image becomes narrower, making it possible to narrow the image width.

[0059] As a means for narrowing the image width of the continuous pixels, narrowing the exposure width of the outermost pixels located at both ends of the continuous pixels can be mentioned. By narrowing the exposure width of the outermost pixels, it is possible to narrow the width of the electrostatic latent image of the entire continuous pixels. In the first embodiment, as shown in FIG. 7(b), one pixel is composed of pixel pieces in which one dot of 600 dpi is divided into 16, and the light source is turned off for each image piece according to the processing value read from the memory 304, and the electrostatic latent image width can be narrowed. In addition, even if the exposure width of the inner pixels of the continuous pixels is changed, it has almost no effect on the width of the electrostatic latent image of the entire continuous pixels.

[0060] Fig. 14 is a diagram explaining how the light source is turned off for each pixel and the pixel width of the outermost pixel is shortened when the number of consecutive pixels is 2. Fig. 14(a) is an example in which the pixel width is shortened with the number of pixel pieces extracted per pixel set to 1. Fig. 14(b) is an example in which the pixel width is shortened with the number of pixel pieces extracted per pixel set to 2.

[0061] The influence of the image width correction process of the first embodiment on the image width will be described with reference to Fig. 15. The horizontal axis of Fig. 15 indicates the number of pixel pieces extracted per pixel in the center. The vertical axis of Fig. 15 indicates the ratio of image width between the center and the end (center / end). Fig. 15 shows the difference in image width in three cases, with the thin solid line indicating the result when there are 2 consecutive pixels, the thick solid line indicating the result when there are 4 consecutive pixels, and the dashed line indicating the result when there are 6 consecutive pixels.

[0062] When the number of consecutive pixels is 2 or 4, the image width in the center is wider when the number of excerpts is 0, as mentioned above. As the number of excerpts in the center increases, the image width in the center becomes narrower, and the excerpt number at which the widths of the toner images in the center and at the edges become closer is 4 when the number of consecutive pixels is 2, and 2 when the number of consecutive pixels is 4. On the other hand, when the number of consecutive pixels is 6, the images in the center and at the edges are closer when the number of excerpts is 0. Also, when the number of consecutive pixels is 6, as the number of excerpts increases, the width of the toner image in the center becomes narrower than the edges.

[0063] Table 1 shows the optimal number of excerpts to make the image widths of the center and edges similar. The first line of Table 1 shows the number of contiguous pixels, and the second line shows the optimal number of excerpted pixel pieces for that number of contiguous pixels. The optimal number of excerpts varies depending on the number of contiguous pixels, and the fewer the contiguous pixels, the greater the optimal number of excerpts. [Table 1] Based on this result, in the first embodiment, the number of pixel pieces to be extracted from the outermost pixel is changed according to the number of consecutive pixels for forming the toner image, thereby making the image width uniform between the center and the edges. The control unit 1 functions as a determination unit that determines the number of pixel pieces to be extracted from the outermost pixel according to the number of consecutive pixels.

[0064] The relationship between the number of continuous pixels and the optimal number of extracted pixel pieces (Figure 15, Table 1) varies depending on the difference in the exposure between the center and the edges. For example, if the exposure of the center is higher than that of the edges, the image width of the center will be wider, and the optimal number of extracted pixel pieces will be higher.

[0065] <Control related to image width correction processing settings> In the first embodiment, in consideration of the relationship between the number of pixel pieces extracted in the image width correction process and the image width as described above, the method of extracting pixel pieces in the image width correction process is changed based on the number of continuous pixels. Fig. 16 is a flowchart showing the method of the image width correction process in the first embodiment. The control unit 1 functions as a correction means for performing the image width correction process.

[0066] In step (hereinafter, referred to as S) 101, the control unit 1 receives print job information and starts image formation. In S102, in the first embodiment, the control unit 1 reads a processing value for image width correction (the number of pixel pieces extracted) from the memory 304. The read processing value is stored in the image modulation unit 101. In the first embodiment, the processing value for image width correction is saved in the memory 304 in the table format shown in Table 1.

[0067] In S103, the control unit 1 determines whether the pixel to be exposed (hereinafter, referred to as the target pixel) is the outermost pixel of consecutive pixels. If the control unit 1 determines in S103 that the target pixel is not the outermost pixel of consecutive pixels, the process proceeds to S107. If the control unit 1 determines in S103 that the target pixel is the outermost pixel of consecutive pixels, the process proceeds to S104.

[0068] In S104, the control unit 1 counts the number of consecutive pixels including the target pixel (consecutive number). In S105, the control unit 1 refers to a table for image width correction (e.g., Table 1) and determines a processing value. For example, if the target pixel is the outermost pixel of the consecutive pixels and the consecutive number is 4, the control unit 1 determines the processing value, i.e., the optimal number of pixel pieces to be extracted, from Table 1 to be "2".

[0069] In S106, the control unit 1 extracts pixel pieces for the target pixel based on the processing value (optimum number of pixel pieces to be extracted) determined in S105. Extracting pixel pieces for the target pixel means, in more detail, dividing the pixel data of one pixel into a predetermined number (for example, 16) of pixel pieces, performing the extraction process, and then generating an image signal corresponding to the pixel data of one pixel after the pixel pieces are extracted. That is, after correcting the data of one pixel to the optimum number of pixel pieces, an image signal for one pixel corresponding to the number of pixel pieces after the correction is generated. In S107, the control unit 1 judges whether or not the image formation has been completed. If the control unit 1 judges in S107 that the image formation has not been completed, the control unit 1 returns the process to S103, and if the control unit 1 judges that the image formation has been completed, the control unit 1 ends the image formation.

[0070] <Effects> Next, the effect of the first embodiment will be further explained using a specific example of the image width correction method. Here, an image was formed in a normal temperature and normal humidity environment (23°C, 50% RH) using the image forming apparatus 9 having the configuration of the first embodiment, and the difference between the width of the toner image at the center and the width of the toner image at the edge of the print area was evaluated. In addition, the recording medium used was a sheet having a basis weight of 75 g / m2. 2 The LTR size used was

[0071] The effect of Example 1 will be described with reference to FIG. 17. The horizontal axis of FIG. 17 indicates the number of continuous pixels that form a toner image. The vertical axis of FIG. 17 indicates the ratio of image width at the center to the edge (center ÷ edge). FIG. 17 shows the difference in image width in three cases, with the thin solid line indicating the result for Example 1, the thick solid line indicating the result for Comparative Example 1, and the dashed line indicating the result for Comparative Example 2. The configurations and operations of the image forming apparatuses of Comparative Examples 1 and 2 are substantially the same as those of image forming apparatus 9 of Example 1, except that the image width correction method described below is different.

[0072] Comparative Example 1 shows the conventional results when no pixel pieces outside the outermost pixel are extracted. When the number of consecutive pixels is 6, the widths of the toner images at the center and the edges are close, but the difference becomes larger as the number of consecutive pixels decreases. Comparative Example 2 shows the results when the number of pixel pieces extracted outside the outermost pixel is set to a fixed value of "2" regardless of the number of consecutive pixels. When the number of consecutive pixels is 4, the widths of the toner images at the center and the edges are close, but in other cases, a difference occurs.

[0073] In the first embodiment, the number of pixel pieces outside the outermost pixel is changed according to the number of consecutive pixels. Specifically, when the number of consecutive pixels is 2, the number of pixels is set to 4, when the number of consecutive pixels is 4, the number of pixels is set to 2, and when the number of consecutive pixels is 6, the number of pixels is set to 0. In this case, the difference in width between the toner image at the center and the edge can be reduced regardless of the number of consecutive pixels.

[0074] As described above, in the first embodiment, the number of pixel pieces extracted from the outermost pixel is changed based on the number of consecutive pixels in the image data. This makes it possible to reduce the difference in image width between the center and the edges, regardless of the number of consecutive pixels in the image data.

[0075] In the first embodiment, the number of pixel pieces to be extracted is determined based on a representative value of the light amount distribution in the scanning direction for the scanner laser, but the number of pixel pieces to be extracted may be determined by measuring the characteristics for each scanner. In addition, in the first embodiment, the number of pixel pieces to be extracted was determined in consideration of the image width results at the center and the ends. The number of pixel pieces to be extracted at positions between the center and the ends may also be set arbitrarily, and the number of pixel pieces to be extracted may be corrected according to the distance from the center. In addition, in the first embodiment, pixel pieces are extracted from the outermost pixels of consecutive pixels in the center based on the light amount distribution of the scanner laser, thereby achieving uniformity in the main scanning direction of the image width, but the present invention is not limited to this embodiment. It is also possible to achieve uniformity in the main scanning direction of the image width by adding pixel pieces to the outermost pixels of consecutive pixels at the edge or pixels further outward, and changing the number of pixel pieces added depending on the number of consecutive pixels. In addition, in the first embodiment, pixel pieces are extracted from the outermost pixels of the continuous pixels in the center based on the light amount distribution of the scanner laser, thereby achieving uniformity in the main scanning direction of the image width, but the present invention is not limited to this embodiment. It is also possible to achieve uniformity in the main scanning direction of the image width by extracting pixel pieces from pixels inside the outermost pixel of the continuous pixels and changing the number of pixel pieces extracted depending on the number of continuous pixels.

[0076] Furthermore, in the first embodiment, the processing value for image width correction (the number of pixel pieces extracted), the partial magnification characteristic information, and the correction value for adjusting the light amount of the light emitting unit 11 (hereinafter referred to as the correction value, etc.) are stored in the memory 304 provided in the laser driving unit 300, but are not limited thereto. For example, the information on the correction value, etc. may be stored in the ROM 102a provided in the image signal generating unit 100, and the location where the information on the correction value, etc. is stored is not particularly limited. In addition, these correction values, etc. may be appropriately acquired via the CPU bus 103 and the serial communication 113, etc., when various correction processes are performed. Furthermore, the information on the correction value, etc. stored in the memory 304, etc. may be appropriately rewritable. The same applies to the following embodiments.

[0077] As described above, according to the first embodiment, even if a lens having fθ characteristics is not used, the difference in image width in the laser scanning direction can be reduced. EXAMPLES

[0078] In the first embodiment, the difference in image width between the center and the end portions caused by the difference in the amount of laser light is addressed by changing the number of pixel pieces on the outside of the outermost pixel based on the number of continuous pixels in the image data, thereby making it possible to achieve uniformity of the image width in the longitudinal direction. Here, as described above, the image width is determined by the electrostatic latent image formed on the photosensitive drum 4. Therefore, the image width is influenced not only by the laser characteristics but also by the latent image formation characteristics (sensitivity) of the photosensitive drum 4. In the second embodiment, an example will be described in which uniformity of the width of the toner image in the longitudinal direction between the center and the end portions is achieved even when the sensitivity of the photosensitive drum 4 is changed.

[0079] The basic configuration and operation of the image forming apparatus of the second embodiment are the same as those of the first embodiment. Therefore, elements having the same or equivalent functions and configurations as those of the first embodiment are given the same reference numerals and detailed explanations are omitted. Items in the second embodiment that are not specifically explained here are the same as those in the first embodiment.

[0080] <Sensitivity of photosensitive drum 4> FIG. 18(a) is a diagram for explaining differences in sensitivity of the photosensitive drum 4. The horizontal axis indicates the amount of laser light (exposure amount), and the vertical axis indicates the surface potential (V) of the photosensitive drum 4. The solid line in FIG. 18(a) indicates the transition of the surface potential when the sensitivity of the photosensitive drum 4 is good, and the dashed line in FIG. 18(a) indicates the transition of the surface potential when the sensitivity of the photosensitive drum 4 is poor. When exposure is performed with the same amount of laser light, the photosensitive drum 4 with good sensitivity has a lower surface potential than the photosensitive drum 4 with poor sensitivity, and a wider electrostatic latent image is formed. As a result, the image width is wider on the photosensitive drum 4 with good sensitivity.

[0081] 18(b) is a diagram for explaining the effect of the image width correction process on the uniformity of the image width in the scanning direction when photosensitive drums 4 with different sensitivities are used. The horizontal axis shows the number of pixel pieces extracted in the center of the image width correction process. The vertical axis shows the ratio of the image width at the center to the edge (center ÷ edge) when the number of continuous pixels is 2.

[0082] The solid line indicates the result of the image width ratio when a photosensitive drum 4 with good sensitivity is used, and the dashed line indicates the result of the image width ratio when a photosensitive drum 4 with poor sensitivity is used. When the number of pixel pieces extracted is 4, the difference in image width between the center and the end is small for the photosensitive drum 4 with good sensitivity, as shown in Example 1. On the other hand, when a photosensitive drum 4 with poor sensitivity is used, when the number of pixel pieces extracted is 4, the image width at the center is narrower than the image width at the end. This is because when pixel pieces are extracted from a photosensitive drum 4 with poor sensitivity, the amount of light at the end of the light-emitting area decreases, making it difficult to form an electrostatic latent image on a photosensitive drum 4 with poor latent image formation characteristics. In this case, if the number of pixel pieces to be extracted is set to 3, the difference in image width between the center and the end can be reduced.

[0083] In the second embodiment, the number of pixel pieces to be extracted is corrected based on the sensitivity information of the photosensitive drum 4. Specifically, when the sensitivity characteristics of the photosensitive drum 4 are worse than a specified value, a correction is performed to decrease the number of pixel pieces to be extracted by one. Note that in the above example, an example has been described in which the image widths of the center and ends are made uniform when the sensitivity of the photosensitive drum 4 is poor. Conversely to the example described, when the sensitivity characteristics of the photosensitive drum 4 are even better, the number of pixel pieces to be extracted may be increased.

[0084] In the second embodiment, in consideration of the relationship between the number of pixel pieces extracted in the image width correction process and the image width described above, the method of extracting pixel pieces in the image width correction process is changed based on the number of continuous pixels. Note that in the second embodiment, information on the sensitivity characteristics of the photosensitive drum 4 is written in the memory 4a for the photosensitive drum 4, and correction is performed based on that information.

[0085] <Control related to image width correction processing settings> Fig. 19 is a flowchart showing a method of image width correction processing in the second embodiment. Note that the processing of S201 and S202 is similar to the processing of S101 and S102 in Fig. 16, and therefore a description thereof will be omitted. Here, the processing value read in S202 is stored in the image modulation unit 101. In the second embodiment, the processing value for image width correction is saved in the memory 304 in the table format shown in Table 1, similar to the first embodiment.

[0086] In S203, the control unit 1 reads the sensitivity information of the photosensitive drum 4 from the memory 4a for the photosensitive drum 4. Note that the processes of S204 to S206 are similar to the processes of S103 to S105 in FIG. 16, and therefore the description thereof will be omitted. In S207, the control unit 1 judges whether or not the sensitivity of the photosensitive drum 4 is poor based on the sensitivity information of the photosensitive drum 4 read in S203. If the control unit 1 does not judge that the sensitivity of the photosensitive drum 4 is poor in S207, the process proceeds to S209. That is, the control unit 1 uses the processing value determined in S206 without correcting it. If the control unit 1 judges that the sensitivity of the photosensitive drum 4 is poor in S207, the process proceeds to S208. In S208, the control unit 1 corrects the processing value determined in S206. The processes of S209 and S210 are similar to the processes of S106 and S107 in FIG. 16, and therefore the description thereof will be omitted.

[0087] According to the above flowchart, the width of the toner image at the center and the end can be made uniform by changing (correcting) the number of pixel pieces extracted from the outermost pixels of the continuous pixels based on the sensitivity characteristics of the photosensitive drum 4. In the above-mentioned second embodiment, the sensitivity characteristics of the photosensitive drum 4 are classified into two types, and the number of pixel pieces extracted is changed, but this configuration is not limited. For example, the sensitivity characteristics of the photosensitive drum 4 may be classified in more detail to change the number of pixel pieces extracted in more detail, thereby further uniforming the width of the toner image at the center and the end.

[0088] As described above, according to the second embodiment, even if a lens having fθ characteristics is not used, the difference in image width in the laser scanning direction can be reduced. EXAMPLES

[0089] In the second embodiment, the difference in image width between the center and the end portions caused by the difference in sensitivity of the photosensitive drum 4 is made uniform by changing the number of pixel pieces of the outermost pixel of the continuous pixels based on the sensitivity information of the photosensitive drum 4 stored in the memory 4a. Here, the sensitivity characteristic of the photosensitive drum 4 may change depending on the usage history of the photosensitive drum 4 and the usage environment of the main body. In the third embodiment, an example is described in which the number of pixel pieces is changed based on the usage history of the photosensitive drum 4 and the usage environment information of the main body. Note that the usage environment information is, for example, temperature and humidity, and can be detected by the environmental sensor 200. The basic configuration and operation of the image forming apparatus of the third embodiment are the same as those of the first embodiment. Therefore, the elements having the same or equivalent functions and configurations as those of the first embodiment are given the same reference numerals and detailed description is omitted.

[0090] <Photosensitive drum sensitivity, usage history, and usage environment> It is known that the sensitivity of the photosensitive drum 4 varies depending on the usage history and the usage environment. In the case of the photosensitive drum 4 used in the embodiment 3, when the photosensitive drum 4 had a long usage history, the surface of the photosensitive drum 4 was scraped off and the film thickness became thin, so that the sensitivity of the photosensitive drum 4 deteriorated. In addition, when the temperature of the usage environment of the image forming apparatus 9 was high, the sensitivity of the photosensitive drum 4 improved.

[0091] As shown in the second embodiment, when the sensitivity of the photosensitive drum 4 changes, the relationship between the width of the toner image at the center and the edge changes. Therefore, in order to make the width of the toner image at the center and the edge uniform, information on the possible change in the sensitivity of the photosensitive drum 4 is obtained and the number of extracted pixel pieces is corrected.

[0092] In the photosensitive drum 4 used in the third embodiment, when the photosensitive drum 4 was used for more than half of its expected life (longer than the specified life), the amount of correction for the number of extracted pixel pieces was increased, so that the width of the toner image at the center and the ends became uniform. Also, when the temperature of the environment in which the image forming apparatus 9 was used was 28° C. or higher (above the specified temperature), the amount of correction for the number of extracted pixel pieces was decreased, so that the width of the toner image at the center and the ends became uniform.

[0093] In the third embodiment, the usage correction table shown in Table 2 was used to perform correction for the number of extracted pixel pieces set in the first embodiment when the usage history of the photosensitive drum 4 or the temperature of the usage environment was different from the assumption. [Table 2] Here, the first line of Table 2 indicates the correction conditions (usage history and temperature of the usage environment), and the second line indicates the correction amount of the extracted pixel piece count (-1 (subtract a specified value), +1 (add a specified value)).

[0094] In the third embodiment, in consideration of the relationship between the number of pixel pieces extracted in the image width correction process and the image width described above, the method of extracting pixel pieces in the image width correction process is changed based on the number of continuous pixels. Note that in the third embodiment, the use history of the photosensitive drum 4 and temperature information of the usage environment are referenced, and control is performed based on that information.

[0095] <Control related to image width correction processing settings> FIG. 20 is a flowchart showing a method of image width correction processing in the third embodiment. Note that S301 and S302 are similar to the processing of S101 and S102 in FIG. 16, and therefore a description thereof will be omitted. In the third embodiment, the processing values ​​for image width correction are stored in the memory 304 in the table format shown in Table 1, as in the first embodiment. In S303, the control unit 1 reads the used correction table (Table 2) from the memory 304. In S304, the control unit 1 reads information on the use history and the use temperature of the photosensitive drum 4 from the memory 4a. In the third embodiment, the used correction table is stored in the memory 304 in the table format shown in Table 2. The processing of S305 to S307 is similar to the processing of S103 to S105 in FIG. 16, and therefore a description thereof will be omitted.

[0096] In S308, the control unit 1 refers to the usage correction table read in S303 and corrects the processing value. For example, if the usage history of the photosensitive drum 4 is more than half of the expected life (life is more than 50%) based on the information read in S304, the control unit 1 subtracts 1 from the processing value determined in S307 (-1). Also, for example, if the temperature of the usage environment of the photosensitive drum 4 is 28° C. or higher based on the information read in S304, the control unit 1 adds 1 to the processing value determined in S307 (+1). The processes of S309 and S310 are similar to the processes of S106 and S107 in FIG. 5, and therefore will not be described.

[0097] According to the above flowchart, the width of the toner image at the center and ends can be made uniform by changing the number of pixel pieces extracted from the outermost pixels of consecutive pixels based on information on the usage history of the photosensitive drum 4 and the temperature of the usage environment.

[0098] In the above embodiment, a threshold value (e.g., 50% or 28°C) is set for the usage history of the photosensitive drum 4 and the temperature of the usage environment, and the number of extracted pixel pieces is corrected when the threshold value is exceeded, but the present invention is not limited to this configuration. For example, by classifying items related to the usage history and usage temperature of the photosensitive drum 4 in detail, the number of extracted pixel pieces may be finely changed, and the width of the toner image at the center and the ends may be further uniformed. Furthermore, the number of pixel pieces extracted, which is set based on the sensitivity information of the photosensitive drum 4 used in the second embodiment, may be further corrected by referring to the correction table used in the third embodiment. As described above, by correcting the number of extracted pixel pieces in response to the change in the sensitivity of the photosensitive drum 4, the width of the toner image at the center and the ends can be made more uniform.

[0099] (About humidity) The use environment of the photosensitive drum 4 may be humidity. When the humidity of the use environment becomes low, the surface potential of the photosensitive drum 4 after charging becomes high. As a result, the surface potential of the photosensitive drum 4 after exposure to laser light also becomes high. Therefore, when the humidity of the use environment is low, the image width formed on the photosensitive drum 4 becomes narrower than when the humidity of the use environment is high. That is, the difference in image width when the light amount is changed between the center and the end in the scanning direction becomes smaller. For this reason, when the humidity is low, the control unit 1 needs to make the number of pixel pieces of the outermost pixels of the continuous pixels smaller than when the humidity is high. In consideration of this situation, for example, a correction condition of the humidity of the use environment may be added to the use correction table of Table 2, and when the humidity of the use environment becomes equal to or lower than a threshold value (equal to or lower than a predetermined humidity), the correction amount may be set to, for example, "-1" (a predetermined value is subtracted).

[0100] As described above, according to the third embodiment, even if a lens having fθ characteristics is not used, the difference in image width in the laser scanning direction can be reduced.

[0101] <Other embodiments> The present invention can also be realized by a process in which a program for realizing one or more functions of the above-mentioned embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. Also, the present invention can be realized by a circuit (e.g., ASIC) for realizing one or more functions.

[0102] The disclosure of this embodiment includes the following configuration. (Configuration 1) A rotatable photoreceptor; a light irradiation unit that forms a latent image on the photosensitive member by scanning a laser beam based on image data in a scanning direction; Equipped with An image forming apparatus in which a scanning speed, which is a speed of a laser beam scanned on the photoconductor, is slower in a central portion than in an end portion in the scanning direction, an image forming apparatus comprising: a correction means for correcting the image data in accordance with a number of consecutive pixels, which is the number of pixels forming the latent image that are consecutive in the scanning direction, and a pixel position, which is a position within the consecutive pixels that are a plurality of consecutive pixels. (Configuration 2) 2. The image forming apparatus according to claim 1, wherein the correction means performs correction on a central portion of the image data in the scanning direction. (Configuration 3) The image forming apparatus according to configuration 1, wherein the correction means corrects the image data by removing pixel pieces smaller than one pixel from the image data or by inserting the pixel pieces into the image data. (Configuration 4) The image forming apparatus according to any one of configurations 1 to 3, wherein the correction means changes an amount of correction when correcting the image data depending on whether the pixel position is at an end or an interior of the continuous pixels. (Configuration 5) 5. The image forming apparatus according to any one of configurations 1 to 4, wherein the correction unit increases a correction amount when correcting the image data as the scanning speed decreases. (Configuration 6) 6. The image forming apparatus according to any one of configurations 1 to 5, wherein the correction unit increases a correction amount when correcting the image data as the number of continuous pixels decreases. (Configuration 7) The image forming apparatus according to any one of configurations 1 to 6, wherein the correction means changes a correction amount when correcting the image data based on a distribution of the light amount of the laser light in the scanning direction. (Configuration 8) 8. The image forming apparatus according to claim 7, wherein the correction means increases the amount of correction as the amount of light increases. (Configuration 9) 9. The image forming apparatus according to any one of configurations 1 to 8, wherein the correction means changes a correction amount when correcting the image data based on the sensitivity of the photoconductor. (Configuration 10) 10. The image forming apparatus according to claim 9, wherein the correction means increases the amount of correction when correcting the image data as the sensitivity of the photoconductor increases. (Configuration 11) 11. The image forming apparatus according to any one of configurations 1 to 10, wherein the correction means changes a correction amount when correcting the image data based on a usage history of the photoconductor. (Configuration 12) 12. The image forming apparatus according to claim 11, wherein the correction means subtracts a predetermined value from the amount of correction when correcting the image data when the life of the photoconductor is longer than a predetermined life. (Configuration 13) 13. The image forming apparatus according to any one of configurations 1 to 12, wherein the correction means changes a correction amount when correcting the image data, based on an environment in which the image forming apparatus is used. (Configuration 14) The usage environment is temperature, 14. The image forming apparatus according to claim 13, wherein the correction means adds a predetermined value to the amount of correction when correcting the image data when the temperature is equal to or higher than a predetermined temperature. (Configuration 15) The usage environment is humidity, 14. The image forming apparatus according to claim 13, wherein the correction means subtracts a predetermined value from the amount of correction when correcting the image data when the humidity is equal to or lower than a predetermined humidity. [Explanation of symbols]

[0103] 1. Control section 4 Photosensitive drum 400 Optical Scanner

Claims

1. A rotatable photoreceptor; a light irradiation unit that forms a latent image on the photosensitive member by scanning a laser beam based on image data in a scanning direction; Equipped with An image forming apparatus in which a scanning speed, which is a speed of a laser beam scanned on the photoconductor, is slower in a central portion than in an end portion in the scanning direction, an image forming apparatus comprising: a correction means for correcting the image data in accordance with a number of consecutive pixels, which is the number of pixels forming the latent image that are consecutive in the scanning direction, and a pixel position, which is a position within the consecutive pixels that are a plurality of consecutive pixels.

2. 2. The image forming apparatus according to claim 1, wherein the correction means performs correction on a central portion of the image data in the scanning direction.

3. 2. The image forming apparatus according to claim 1, wherein the correction unit corrects the image data by removing pixel pieces having a size smaller than one pixel from the image data or by inserting the pixel pieces into the image data.

4. 2. The image forming apparatus according to claim 1, wherein the correction means changes a correction amount when correcting the image data depending on whether the pixel position is at an end or inside of the continuous pixels.

5. 2. The image forming apparatus according to claim 1, wherein the correction unit increases the amount of correction applied to the image data as the scanning speed decreases.

6. 2. The image forming apparatus according to claim 1, wherein the correction unit increases the amount of correction applied to the image data as the number of continuous pixels decreases.

7. 2. The image forming apparatus according to claim 1, wherein the correction unit changes a correction amount when correcting the image data, based on a distribution of the amount of the laser light in the scanning direction.

8. 8. The image forming apparatus according to claim 7, wherein the correction unit increases the amount of correction as the amount of light increases.

9. 2. The image forming apparatus according to claim 1, wherein the correction unit changes a correction amount when correcting the image data based on the sensitivity of the photoconductor.

10. 10. The image forming apparatus according to claim 9, wherein the correction unit increases the amount of correction when correcting the image data as the sensitivity of the photoconductor increases.

11. 2. The image forming apparatus according to claim 1, wherein the correction unit changes a correction amount when correcting the image data based on a usage history of the photoconductor.

12. 12. The image forming apparatus according to claim 11, wherein the correction unit subtracts a predetermined value from the amount of correction when correcting the image data when the life of the photoconductor is longer than a predetermined life.

13. 2. The image forming apparatus according to claim 1, wherein the correction unit changes a correction amount when correcting the image data, based on an environment in which the image forming apparatus is used.

14. The usage environment is temperature, 14. The image forming apparatus according to claim 13, wherein the correction unit adds a predetermined value to the amount of correction when correcting the image data when the temperature is equal to or higher than a predetermined temperature.

15. The usage environment is humidity, 14. The image forming apparatus according to claim 13, wherein the correction unit subtracts a predetermined value from the amount of correction when correcting the image data when the humidity is equal to or lower than a predetermined humidity.

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