Image forming apparatus and control method thereof

Abstract

In an image forming apparatus using an optical scanning device with a different spot diameter in the main scanning direction, non-uniformity in line width at different positions in the main scanning direction is suppressed and prevented. The brightness is adjusted so that the line width is approximately equal at the center and at the ends, and the resolution in the sub-scanning direction is controlled to be lower than that in the main scanning direction.

Description

[Technical Field]

[0001] The present invention relates to an electrophotographic image forming apparatus and a control method thereof. [Background technology]

[0002] One exposure method used in the exposure unit of an electrophotographic image forming apparatus is a laser exposure method. The laser exposure method includes a lens that guides laser light from a light source to a scanning unit and focuses the laser light deflected and scanned by the scanning unit onto a photoconductor. It is desirable that the scanning speed of the laser light scanning the surface of the photoconductor be constant regardless of the position on the surface of the photoconductor. It is also desirable that the size of the spot shape (hereinafter referred to as spot diameter) focused on the surface of the photoconductor be uniform regardless of the position on the surface of the photoconductor. Therefore, a lens with fθ characteristics is generally used as the imaging lens. By using a lens with fθ characteristics as the imaging lens, the scanning speed of the laser light scanning the surface of the photoconductor is constant regardless of the position on the surface of the photoconductor, and the size of the spot shape (hereinafter referred to as spot diameter) focused on the surface of the photoconductor is uniform regardless of the position on the surface of the photoconductor.

[0003] On the other hand, there are design examples that use an imaging lens without fθ characteristics for the purpose of miniaturization and cost reduction. When an imaging lens without fθ characteristics is used, the scanning speed is not constant and the spot diameter is not uniform. Patent Document 1 discloses a method for correcting the emission brightness of laser light so that the exposure amount per unit area on the drum surface is constant without using a lens with fθ characteristics. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-000511 Summary of the Invention [Problem to be solved by the invention]

[0005] However, even if the emission brightness of the laser light is adjusted so that the amount of exposure per unit area on the drum surface is constant, the spot diameter varies depending on the position in the main scanning direction, resulting in uneven line widths.

[0006] The present invention aims to suppress or prevent non-uniform line widths across positions in the main scanning direction in an image forming apparatus that uses an optical scanning device in which the spot diameter varies depending on the position in the main scanning direction. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention has the following configuration: That is, an image forming apparatus includes a photoconductor, a charging unit that charges the photoconductor, an exposure unit that exposes the photoconductor to laser light to form a latent image, where the laser light passes through an imaging lens that does not have fθ characteristics and scans the photoconductor, so that the spot diameter at edge positions in the main scanning direction of the photoconductor is larger than the spot diameter at the center position, a developing unit that causes toner to adhere to the photoconductor on which the latent image has been formed, and develops the latent image into an image, and a control unit that controls the exposure of the photoconductor so that the amount of exposure per unit area of ​​the photoconductor is constant. The luminance of the laser light is such that the development width at the axial image height and the development width at the most off-axis image height are equal to each other. The control means receives designation of at least one of a first resolution and a second resolution lower than the first resolution as a resolution in a sub-scanning direction of the image, and when the first resolution is designated, rotates the photosensitive member at a first speed, and when the second resolution is designated, rotates the photosensitive member at a second speed higher than the first speed, and the control means To suppress fluctuations in line width depending on the position in the main scanning direction regardless of the resolution in the sub-scanning direction and to make the line width appropriate, The time ratio between the emission and non-emission of the laser light is controlled. [Effects of the Invention]

[0008] According to the present invention, in an image forming apparatus using an optical scanning device in which the spot diameter varies depending on the position in the main scanning direction, it is possible to suppress or prevent the line width from becoming non-uniform depending on the position in the main scanning direction. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus. [Figure 2] FIG. 1 is a diagram showing an example of the configuration of an optical scanning device. [Figure 3] FIG. 10 is a diagram showing the relationship between image height and partial magnification. [Figure 4] FIG. 2 is an electrical block diagram of the exposure control configuration. [Figure 5] FIG. 2 is a diagram showing the timing relationship between various synchronization signals and an image signal. [Figure 6] FIG. 2 is a functional block diagram showing the flow of image processing. [Figure 7] FIG. 3 is a diagram showing an example of a table of pulse signals according to the first embodiment. [Figure 8] FIG. 4 is a diagram showing an example of a light intensity profile according to the first embodiment. [Figure 9] FIG. 4 is a diagram showing an example of an integrated light intensity profile according to the first embodiment. [Figure 10] FIG. 1 is a diagram for explaining an EV curve. [Figure 11] FIG. 4 is a diagram showing an example of a potential profile according to the first embodiment. [Figure 12] FIG. 4 is a diagram showing a line width measurement result according to the first embodiment. [Figure 13] Diagram showing 1x1 dot and 1x3 dot images. [Figure 14] FIG. 10 is a diagram showing an example of an integrated light intensity profile according to the second embodiment. [Figure 15] FIG. 10 is a diagram showing an example of a potential profile according to the second embodiment. [Figure 16] FIG. 10 is a diagram showing a line width measurement result according to the second embodiment. [Figure 17] FIG. 11 is a flowchart showing a process according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. Note that the following embodiments are merely examples, and the present invention is not limited to the contents of the embodiments. In addition, in the following drawings, components that are not necessary for explaining the embodiments are omitted from the drawings.

[0011] First Embodiment FIG. 1 is a schematic diagram of an image forming apparatus 9 according to this embodiment. A laser driver 300 of an optical scanning device (scanning means) 400 emits a laser beam (light) 208 based on image data output from an image signal generator 100. The laser beam 208 scans and exposes a photoconductor 4, which has been charged by a charging unit (not shown), to form a latent image on the surface of the photoconductor 4. A developing unit (not shown) develops the latent image by applying toner (developer) to form a toner image. A recording medium (e.g., paper) fed from a paper feed unit 8 is transported by roller 5 to a nip region between the photoconductor 4 and a transfer roller 41. The transfer roller 41 transfers the toner image formed on the photoconductor 4 to the conveyed recording medium. The recording medium is then transported to a fixing unit 6. The fixing unit 6 applies heat and pressure to the recording medium to fix the toner image to the recording medium. The recording medium with the fixed toner image is then ejected from the image forming apparatus 9 by a paper ejection roller 7.

[0012] FIG. 2 shows an example of the configuration of an optical scanning device 400 according to this embodiment, with FIG. 2(A) showing a cross section in the main scanning direction and FIG. 2(B) showing a cross section in the sub-scanning direction. Laser light 208 emitted by a light source 401 is shaped into an elliptical shape by an aperture stop 402 and enters a coupling lens 403. After passing through the coupling lens 403, the laser light 208 is converted into substantially parallel light and enters an anamorphic lens 404. Note that the term "substantially parallel light" includes weakly convergent light and weakly divergent light. 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 reflecting surface 405a of a deflector 405 in the sub-scanning cross section, forming a long linear image in the main scanning direction.

[0013] The light beam passing through the anamorphic lens 404 is reflected by a reflecting surface 405a of a deflector (polygon mirror) 405. Here, the deflector 405 is described as having four reflecting surfaces, but the number of reflecting surfaces is not limited to this. The laser beam 208 reflected by the reflecting surface 405a passes through an imaging lens 406 and is focused on the surface of the photoconductor 4, forming a predetermined spot-shaped image (hereinafter referred to as a "spot"). By rotating the deflector 405 at a constant angular velocity in the direction of arrow Ao (clockwise in FIG. 2) using a driving unit (not shown), the spot moves in the main scanning direction on the scanned surface 407 of the photoconductor 4, forming an electrostatic latent image on the scanned surface 407. The main scanning direction is a direction parallel to the surface of the photoconductor 4 and perpendicular to the direction of movement of the surface of the photoconductor 4. In the example of FIG. 2A, it corresponds to the width direction W of the photoconductor 4. The sub-scanning direction is the direction in which the surface of the photosensitive member 4 moves.

[0014] A beam detect (hereinafter referred to as BD) sensor 409 and a 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 light 208 that passes through the BD lens 408 is incident on and detected by the BD sensor 409, which includes a photodiode. A BD signal is output each time the reflecting surface of the deflector 405 is switched. The writing timing is controlled based on the timing at which the BD sensor 409 detects the laser light 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.

[0015] 2, the imaging lens 406 has two optical surfaces (lens surfaces): an incident surface 406a and an exit surface 406b. The imaging lens 406 is configured so that, in the main scanning cross section, the light beam deflected by the reflecting surface 405a scans the surface to be scanned 407 with desired scanning characteristics. The imaging lens 406 is also configured to shape the spot of the laser light 208 on the surface to be scanned 407 into a desired shape.

[0016] The imaging lens 406 according to this embodiment 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 deflector 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.

[0017] The imaging lens 406 according to this embodiment does not have an fθ characteristic, and therefore, when the deflector 405 rotates at a constant angular velocity, the spot does not move at a constant speed on the scanned surface 407. Furthermore, the spot diameter on the scanned surface 407 is not uniform. In particular, the shorter the optical path length (D2) from the deflector 405 to the photosensitive element 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 most off-axis image height described above. The objective of this embodiment is to maintain image quality in such an optical configuration.

[0018] [Partial magnification correction] FIG. 3 shows the relationship between image height and partial magnification according to this embodiment. In FIG. 3, the horizontal axis represents image height [mm], and the vertical axis represents partial magnification [%]. An image height of 0 refers to a case where the spot is on the optical axis of the imaging lens 406, and is hereinafter referred to as the "axial image height." Image heights other than the axial image height are hereinafter referred to as the "off-axial image height." Furthermore, the maximum absolute value of the image height is referred to as the "most off-axial image height." As shown in FIG. 2A, the position of the most off-axial image height on the scanned surface 407 is W / 2 from the center. In FIG. 3, for example, a partial magnification of 30% at an image height means that the scanning speed at that image height is 1.3 times the scanning speed at an image height where the partial magnification is 0%. In the example of FIG. 3, the scanning speed at the axial image height is lowest, 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 clock cycle, the pixel density will differ between on-axis image height and off-axis image height. Therefore, in this embodiment, partial magnification correction is performed. Specifically, the clock frequency is adjusted according to the image height so that the pixel width is approximately constant regardless of the image height. Note that the method of partial magnification correction is not limited to a method that targets the clock frequency. For example, a method of adjusting the pixel width by inserting or removing a pixel piece, each having a size less than one pixel, at any position in the main scanning direction may also be used.

[0019] FIG. 5 is a diagram showing an example of the partial magnification correction described above. FIG. 5 illustrates an example in which a change in scanning speed is 35%, and a partial magnification correction of 135% occurs at the most off-axis image height when the on-axis image height is 100%. 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 to control the clock frequency. That is, 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%. At this time, the period during which the spot of the laser light 208 moves by one 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 light 208 at a pixel position corresponding to one pixel, the pixel width can be corrected, and latent images corresponding to each pixel can be formed at substantially equal intervals and with equal sizes in the main scanning direction. Note that even when performing partial magnification correction by inserting or removing pixel pieces as described above, the size of the pixel pieces to be inserted or removed is switched according to the ratio shown in Figure 3.

[0020] 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, luminance correction is performed to correct the total exposure amount per unit length in addition to the partial magnification correction described above.

[0021] [Brightness Correction] Next, the luminance correction will be described with reference to FIGS.

[0022] 4 is a diagram showing an outline of the configuration of each component used in image formation. Shown here are a control unit 1, an image signal generating unit 100, and a laser driving unit 300. The control unit 1 has a CPU 2, a ROM 3, a DA converter (not shown), and a regulator (not shown), and together with the laser driving unit 300, they constitute a brightness correction means. The laser driving unit 300 has a VI conversion circuit 306 that converts voltage to current, and a laser driver IC 307, and supplies a driving current to the light-emitting unit 11, which is a laser diode of the light source 401. The ROM 3 stores partial magnification characteristic information as well as information on the correction current to be supplied to the light-emitting unit 11.

[0023] Next, the operation of the laser driving unit 300 will be described. Based on the information on the correction current for the light emitting unit 11 stored in ROM 3, the control unit 1 outputs a brightness correction analog voltage 312 that increases or decreases in the main scanning direction of the photosensitive member 4 in synchronization with the BD signal 111. The brightness correction analog voltage 312 is then converted into a current value by the subsequent VI conversion circuit 306 and output to the laser driver IC 307.

[0024] The laser driver IC 307 automatically adjusts the brightness detected by a photodetector (not shown) provided in the light source 401 as a light intensity monitor for the light-emitting unit 11 by feedback control using a circuit inside the laser driver IC 307 so that the brightness is the desired brightness. This is called APC (Auto Power Control). The automatic adjustment of the brightness of the light-emitting unit 11 is performed while the light-emitting unit 11 is emitting light to detect the BD signal outside the printing area for each main scan line, as shown in FIG. 5.

[0025] The luminance of the light-emitting unit 11 is corrected by automatically adjusting the current required to obtain luminance at the most off-axis image height using APC, and then controlling the luminance correction analog voltage 312 based on the correction current information for the light-emitting unit 11 stored in ROM 3. Furthermore, by subtracting a predetermined current from the drive current of the light-emitting unit 11, correction is performed so that the luminance increases as the absolute value of the image height increases. In other words, the luminance of the laser beam 208 is controlled to decrease as the scanning position moves toward the center (axial image height) in the main scanning direction of the photosensitive member 4. As a result, when the luminance of the light source 401 is 100% at the most off-axis image height, it becomes 74% (≒100% / 135%) at the axial image height, and correction is performed so that the total exposure amount (integral light amount) per pixel is constant at each image height.

[0026] The method of brightness correction is not limited to the above method. For example, it is also possible to perform density correction on input image data, which is the original data, according to the drawing position (main scanning position) on the photoconductor 4, and then form an image based on the image data that has undergone this density correction.

[0027] [Image Processing] Next, the flow of image processing in the image forming apparatus according to this embodiment will be described. Fig. 6 is a functional block diagram for explaining image processing during printing. The image processing unit 101 has a density correction processing unit 101z, a halftone processing unit 101a, a position control unit 101b, and a PWM control unit 101c shown in Fig. 6, and performs the image processing described below.

[0028] The image forming apparatus according to this embodiment performs image processing to obtain continuous halftone images by performing dither-based tone conversion. 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 processed by a density correction processor 101z (described later). The print data is then sent to halftone processor 101a. Halftone processor 101a performs multilevel dithering on the 8-bit (256 gradations) print data to convert it into 5-bit (32 gradations) image data. Position controller 101b adds 2-bit position control data, representing the dot growth direction, to the image data output by halftone processor 101a, using a position control matrix corresponding to the dither matrix used by halftone processor 101a for the multilevel dithering. PWM controller 101c performs PWM control on the 7-bit image data with the added position control data to convert it into a pulse signal, a VDO signal 110, and outputs it to laser driver 300.

[0029] By image processing using such a dither method, the print data is converted into a VDO signal 110 for exposure that has undergone halftone processing for appropriate gradation expression in the image forming device 9 .

[0030] [PWM processing] The PWM (Pulse Width Modulation) processing by the PWM control unit 101c will now be described. Fig. 7 shows an example of a table showing the relationship between data (7 bits) assigned to each pixel by the position control unit 101b and the pulse signal generated by PWM processing. This table contains information on the width of the pulse signal (PWM value) and the position of the pulse. The PWM control unit 101c performs PWM processing on the input image data by dividing the 7-bit data assigned to each pixel into the lower 5 bits of data (level value: 0 to 31) and the upper 2 bits of data (position control data: C, L, R), and generating a pulse signal.

[0031] The PWM value is assigned an integer value between 0 and 255 for each level 0 to 31. The pulse position is information corresponding to the delay of the pulse rising position from the reference position (e.g., the starting point of one pixel) of the image clock, which defines the pixel interval for synchronizing the pulse signal. In the table shown in FIG. 7, as the level increases from level 0 (non-light emission), the pulse width is set to increase in the pulse position and growth direction corresponding to the position control data. When the position control data is C, the pulse width grows uniformly from the reference position at the center of one pixel to the left and right. When the position control data is L, the pulse width grows from the reference position at the left edge of one pixel to the right. When the position control data is R, the pulse width grows from the reference position at the right edge of one pixel to the right. When level 31 is reached, the PWM value becomes 255, and light is emitted across the entire pixel width. By performing this processing, 7-bit image data is converted into a video signal (VDO signal 110), which is a pulse signal. Note that the degree of growth of the pulse width according to the level is not limited to that shown in FIG. 7, and any degree of growth may be set.

[0032] [Light Profile] The laser spot diameter on the scanned surface 407 of the optical scanning device 400 according to this embodiment is 60 μm at the on-axis image height and 80 μm at the most off-axis image height. As described above, the distance between the deflector 405 and the scanned surface 407 of the photosensitive element 4 is greater at the end (the most off-axis image height) of the deflector 405 in the main scanning direction, and therefore the spot diameter increases toward the end. FIG. 8 shows an example of a stationary spot light intensity profile, with the vertical axis representing the light intensity [arb] and the horizontal axis representing the main scanning direction position [μm]. In addition, the solid line in FIG. 8 represents the stationary spot light intensity profile at the on-axis image height, and the dotted line represents the stationary spot light intensity profile at the most off-axis image height.

[0033] Next, we will explain the integrated light intensity profile in the main scanning direction corresponding to axis a of the 1x1 dot image shown in Figure 13(a). In Figure 13, the axis corresponds to the main scanning direction, and the direction perpendicular to this is the sub-scanning direction. The integrated light intensity profile of a 1x1 dot in the main scanning direction is calculated by adding the stationary spot light intensity profile shown in Figure 8 for one dot (one pixel width: 42.3 μm) in the main scanning direction. In other words, since there are no other adjacent dots in a 1x1 dot, it is not affected by the integrated light intensity profiles of other dots.

[0034] Figure 13(b) shows a 1x3 dot vertical line image with a resolution of 600 dpi in both the main scanning direction and the sub-scanning direction, while Figure 13(c) shows a 1x3 dot vertical line image with a resolution of 600 dpi in the main scanning direction and 400 dpi in the sub-scanning direction.

[0035] The integrated light intensity profile of a 1x3 dot vertical line is calculated by adding together three integrated light intensity profiles of a 1x1 dot image in the sub-scanning direction. The integrated light intensity profile on axis b is also affected by the integrated light intensity profiles of dots other than the central dot, i.e., the dots located above and below. When the sub-scanning resolution is 400 dpi, the amount of overlap between the integrated light intensity profiles of the 1x1 dots is smaller than when the sub-scanning resolution is 600 dpi. Therefore, the integrated light intensity profile in the main scanning direction on axis b when the sub-scanning resolution is 400 dpi has a lower peak value and a narrower base than the integrated light intensity profile in the main scanning direction on axis b when the sub-scanning resolution is 600 dpi.

[0036] Figure 9 shows the integrated light intensity profiles for sub-scanning direction resolutions of 600 dpi and 400 dpi. In each integrated light intensity profile shown in Figure 9, the vertical axis represents the integrated light intensity [arb], and the horizontal axis represents the main scanning direction position [μm]. Figures 9(a), (b), and (c) show the integrated light intensity profiles for a 1 × 3 dot image when the sub-scanning direction resolution is 600 dpi. Figures 9(d), (e), and (f) show the integrated light intensity profiles for a sub-scanning direction resolution of 400 dpi. Figures 9(a) and (d) show the integrated light intensity profiles in the main scanning direction on axis b when the luminance is P, Figures 9(b) and (e) show the luminance when P × 1.5, and Figures 9(c) and (f) show the luminance when P × 2.0.

[0037] The solid line in Figure 9 shows the integrated light intensity profile at the on-axis image height, and the dotted line shows the integrated light intensity profile at the extreme off-axis image height. As shown in Figure 9, regardless of luminance, the integrated light intensity profile at the extreme off-axis image height has a lower peak and a wider base than the integrated light intensity profile at the on-axis image height. The reason why the integrated light intensity profiles differ between the on-axis image height and the extreme off-axis image height is because, as shown in Figure 8, the light intensity profile of a stationary spot has a lower peak and a wider base at the extreme off-axis image height compared to the on-axis image height. Furthermore, when comparing cases where the resolution in the sub-scanning direction is different at the same luminance, the peak value is lower and the base is narrower when the resolution in the sub-scanning direction is lower.

[0038] [EV curve] Figure 10 shows the relationship between the drum surface exposure amount per unit area of ​​the photosensitive drum 4 (photosensitive drum) according to this embodiment and the drum potential (EV curve). In Figure 10, the vertical axis represents the drum potential [-V], and the horizontal axis represents the drum surface light amount [μJ / cm2]. As shown in Figure 10, when the exposure amount is 0, i.e., when the light source 401 is not emitting light, the surface potential of the photosensitive drum 4 is approximately -540 V, and the potential of the photosensitive drum 4 tends to decrease (the absolute value becomes smaller) as the exposure amount increases.

[0039] [Potential profile] Figure 11 shows potential profiles calculated based on the integrated light intensity profile shown in Figure 9 and the EV curve shown in Figure 10. In each potential profile shown in Figure 11, the vertical axis represents the drum potential [-V], and the horizontal axis represents the main scanning direction position [μm]. Figures 11(a), (b), and (c) show the surface potential profiles of the photoconductor 4 at axis b for a 1 × 3 dot image when the sub-scanning direction resolution is 600 dpi. Figures 11(d), (e), and (f) show the potential profiles when the sub-scanning direction resolution is 400 dpi. Figures 11(a) and (d) show the main scanning direction potential profiles on each axis when the luminance is P, Figures 11(b) and (e) show the luminance when P × 1.5, and Figures 11(c) and (f) show the luminance when P × 2.0.

[0040] The solid line in Fig. 11 indicates the potential profile at the on-axial image height, and the dotted line indicates the potential profile at the extreme off-axial image height. As shown in Fig. 11, regardless of luminance, the potential profile at the extreme off-axial image height has a lower peak value and a wider base than the potential profile at the on-axial image height. The reason the potential profiles differ between the on-axial image height and the extreme off-axial image height is because, as shown in Fig. 9, the peak value of the integrated light intensity profile is lower and the base is wider at the extreme off-axial image height than at the on-axial image height.

[0041] The dashed line Vdc in Fig. 11 indicates the development potential (here, -470 V) according to this embodiment, and the width of the arrow in Fig. 11 indicates the width of the portion where the drum potential is equal to or less than the development potential (hereinafter referred to as the "development width"). Experiments have proven that this development width correlates with the line width, which will be described later.

[0042] As shown in Figures 11(a) and 11(d), when the luminance is P, the potential profile at the most off-axis image height is shallower than the potential profile at the on-axis image height, resulting in a smaller development width. On the other hand, when the luminance is P × 1.5, as shown in Figures 11(b) and 11(e), the potential profile is deeper, resulting in a larger development width. The development width is particularly pronounced at the most off-axis image height. This is because the base of the potential profile at the most off-axis image height is wider, so increasing the luminance has a greater effect on the development width. When the luminance is further increased to P × 2.0, as shown in Figures 11(c) and 11(f), the magnitude relationship is reversed from when the luminance is P, and the development width is larger at the most off-axis image height than at the on-axis image height.

[0043] [Line width measurement results] FIG. 12 shows the line width measurement results when a 1 x 200 dot vertical line image was printed at each main scanning direction position. Here, line width corresponds to the length of one dot (one pixel) in the main scanning direction. In FIG. 12, the vertical axis represents the line width measurement result [μm], and the horizontal axis represents the main scanning direction position [mm]. The measuring device used was a ScanMate F10. Along with Result 1 according to this embodiment, the line width measurement results of Comparative Examples 1 and 2 are also shown as comparative examples. In FIG. 12, (a) shows the line width measurement result of Result 1 according to this embodiment. In FIG. 12, (b) shows the line width measurement result of Comparative Example 1. In FIG. 12, (c) shows the line width measurement result of Comparative Example 2.

[0044] Table 1 below shows each configuration and the line width evaluation results for each configuration. Each configuration has a different combination of drum surface light quantity and brightness per unit area, and resolution in the sub-scanning direction. The drum surface light quantity per unit area for Result 1 of this embodiment and Comparative Example 1 is 0.3 μJ / cm2. On the other hand, in Comparative Example 2, the drum surface light quantity per unit area is 0.45 μJ / cm2.

[0045] Furthermore, if the brightness of Comparative Example 1 is P, the brightness of Result 1 according to this embodiment and Comparative Example 2 is 1.5 times that, or P×1.5. Furthermore, Comparative Examples 1 and 2 have a sub-scanning direction resolution of 600 dpi, whereas Result 1 according to this embodiment has a sub-scanning direction resolution of 400 dpi. In the following explanation, resolutions of 600 dpi and 400 dpi are used as examples, but the present invention is not limited to these.

[0046] Here, the rotation speed of the deflector 405 is the same when the resolution in the sub-scanning direction is 600 dpi and when it is 400 dpi. On the other hand, the process speed, i.e., the rotation speed of the photosensitive drum 4, is 1.5 times faster when the resolution in the sub-scanning direction is 400 dpi than when it is 600 dpi. Therefore, the amount of light on the drum surface per unit area is the same for Result 1 according to this embodiment and Comparative Example 1. [Table 1]

[0047] As shown in (b) of Figure 12, the line width at the most off-axis image height is narrower than the line width at the on-axis image height in the configuration of Comparative Example 1. This is because the development width at the most off-axis image height is narrower than the development width at the on-axis image height, as shown in the potential profile of the 1 × 3 dot image in (a) of Figure 11.

[0048] On the other hand, as shown in FIG. 12(c), in Comparative Example 2, the line width is almost uniform at each image height. This is because, as shown in the potential profile of the 1 × 3 dot image in FIG. 11(b), the development width is almost the same at the on-axis image height and the most off-axis image height. However, in the case of Comparative Example 2, although the line width is uniform regardless of image height, the line width itself is thicker than the appropriate value, making it unsuitable for line images. With this configuration, for example, when printing a one-dot white character, the white portion becomes unclear.

[0049] On the other hand, with the configuration according to this embodiment, it is possible to set the line width to an appropriate value while maintaining line uniformity, as shown in (a) of Fig. 12. This is because, by setting the luminance to P x 1.5 and the resolution in the sub-scanning direction to 400 dpi, the line width is made uniform regardless of the image height, and the line width itself is prevented from becoming thicker than the appropriate value.

[0050] As described above, even when an optical scanning device with a different spot diameter depending on the image height is used, in this embodiment, the brightness is adjusted so that the line width is approximately equal at the center and the edges, and the resolution in the sub-scanning direction is lower than that in the main scanning direction. This prevents the line width from changing depending on the position in the main scanning direction, and ensures an appropriate line width.

[0051] Second Embodiment In the first embodiment, the PWM value was controlled according to the input image data as shown in Fig. 7. In contrast to this, in the second embodiment, if the PWM value used in the first embodiment is PWM, for example, the PWM value is controlled to PWM x 400 dpi / 600 dpi = PWM x 2 / 3. For example, the PWM value at the highest image gradation (i.e., level 31) in the pulse signal table of the first embodiment is "255," whereas the PWM value at level 31 in this embodiment is "170."

[0052] In this embodiment, light emission and non-emission are repeated at a fixed ratio for each pixel. Specifically, based on the above control content, Lighting time: Non-lighting time = 170:(255-170) = 2:1 Light emission and non-light emission are repeated for each pixel at a ratio of 170. As a result, for example, the amount of light per pixel at the highest image gradation will be a value equivalent to "170". Note that for each pixel, either the non-light emission operation or the light emission operation can be performed first.

[0053] Another difference is that the image resolution in the sub-scanning direction in the first embodiment is 400 dpi, whereas the image resolution in the sub-scanning direction in this embodiment is 600 dpi. Also, the image resolution in the main scanning direction in this embodiment is 600 dpi. The laser brightness in this embodiment is P×1.5, just like in the first embodiment. Other configurations are the same as in the first embodiment, so detailed explanations will be omitted.

[0054] With the configuration of this embodiment, when the image resolution in the sub-scanning direction is the same as the image resolution in the main scanning direction, it is possible to prevent the line width from changing depending on the position in the main scanning direction, as in the first embodiment, and to ensure an appropriate line width. Specifically, with the configuration of this embodiment, it is possible to prevent the line itself from becoming thicker than the appropriate value, which was a problem in Comparative Example 2 described in Figure 12 of the first embodiment.

[0055] The reason for this is explained below. FIG. 14 shows an integrated light intensity profile according to this embodiment, taken along axis b of the 1×3 dot image shown in FIG. 13(b). In FIG. 14, the vertical axis represents the integrated light intensity [arb], and the horizontal axis represents the main scanning direction position [μm]. Meanwhile, the integrated light intensity profile shown in FIG. 9(b) is that of Comparative Example 2. Both the integrated light intensity profile of FIG. 14 according to this embodiment and the integrated light intensity profile of Comparative Example 2 shown in FIG. 9(b) are for a sub-scanning resolution of 600 dpi and a brightness of P×1.5. Compared to the integrated light intensity profile of FIG. 9(b), the integrated light intensity profile of FIG. 14 has a lower peak value and a narrower base. In FIG. 9(b), the PWM value is "255," and a static spot light intensity profile of approximately 42.3 μm (one pixel) is integrated in the main scanning direction. In contrast to this, in the case of this embodiment, the PWM value is "170", and the stationary spot light amount profile is integrated only for about 28.2 μm in the main scanning direction, resulting in the configuration shown in FIG.

[0056] FIG. 15 shows a potential profile according to this embodiment on axis b of the 1×3 dot image shown in FIG. 13(b). In FIG. 15, the vertical axis represents the drum potential [−V], and the horizontal axis represents the main scanning direction position [μm]. On the other hand, the potential profile shown in FIG. 11(b) is a potential profile of Comparative Example 2. Both the potential profile of FIG. 15 according to this embodiment and the potential profile of Comparative Example 2 shown in FIG. 11(b) are for cases where the sub-scanning resolution is 600 dpi and the brightness is P×1.5. Compared to the potential profile of FIG. 11(b), the potential profile of FIG. 15 has a lower peak value and a narrower base.

[0057] Fig. 16 shows the line width measurement results according to this embodiment. In Fig. 16, (a) shows the line width measurement results as Result 2 of this embodiment. In Fig. 16, (b) shows the line measurement results as Comparative Example 2. As described above, both Result 2 of this embodiment and Comparative Example 2 show the cases where the main scanning resolution and sub-scanning resolution are both 600 dpi and the brightness is P × 1.5.

[0058] 16, the configuration of this embodiment can prevent the line width from varying depending on the position in the main scanning direction, and can also ensure an appropriate line width, as is clear from the potential profile in FIG.

[0059] Third Embodiment As a third embodiment, an embodiment having a function of switching the PWM value according to the image resolution in the sub-scanning direction will be described. That is, the image forming apparatus can perform image formation in multiple modes by switching the image resolution in the sub-scanning direction, and when switching, the PWM value is also switched. Note that detailed description of the same configuration as the first and second embodiments will be omitted.

[0060] [Processing flow] 17 is a flowchart showing the PWM value switching process according to this embodiment. This process flow is controlled by a plurality of processing units working together, so the image forming apparatus 9 will be described here as the main processing unit.

[0061] When the image forming apparatus 9 receives image resolution information in the sub-scanning direction from the user via a printer driver (not shown), it sets a process speed according to the image resolution information in the sub-scanning direction and operates the image forming apparatus. Here, the explanation will be given assuming that either 400 dpi or 600 dpi is specified as the image resolution in the sub-scanning direction. It is assumed that the process speed PS, the number of rotations F of the deflector 405, and the brightness P described below are predetermined and stored in the image forming apparatus 9.

[0062] In S1701, the image forming apparatus 9 determines whether the image resolution in the sub-scanning direction specified is 400 dpi. If 400 dpi is specified (YES in S1701), the process proceeds to S1702, and if 600 dpi is specified (NO in S1701), the process proceeds to S1706.

[0063] In S1702, the image forming apparatus 9 sets the process speed to PS. Specifically, the rotation speed of the photosensitive member 4 is set to 120 mm / s.

[0064] In S1703, the image forming apparatus 9 controls the driving unit (not shown) so that the rotation speed of the deflector 405 converges to a constant number of rotations F regardless of the image resolution information in the sub-scanning direction.

[0065] In S1704, the image forming apparatus 9 controls the light emission luminance of the light source 401 to be luminance P×1.5 (=600 / 400) regardless of the image resolution information in the sub-scanning direction.

[0066] In S1705, the image forming apparatus 9 sets the PWM value of the highest image gradation to 255. In accordance with this, the PWM values ​​of each gradation of the image are set. In other words, the setting is made to achieve the configuration using FIG. 7 in the first embodiment. Then, this processing flow ends.

[0067] In S1706, the image forming apparatus 9 sets the process speed to (PS×400 / 600). Specifically, the rotation speed of the photosensitive member 4 is set to 80 mm / s.

[0068] In S1707, the image forming apparatus 9 controls the driving unit (not shown) so that the rotation speed of the deflector 405 converges to a constant number of rotations F regardless of the image resolution information in the sub-scanning direction.

[0069] In S1708, the image forming apparatus 9 controls the light emission luminance of the light source 401 to be luminance P×1.5 (=600 / 400) regardless of the image resolution information in the sub-scanning direction.

[0070] In S1709, the image forming apparatus 9 controls the PWM value of the highest image gradation to be 170 (=255×400 / 600). In accordance with this, the PWM value of each gradation of the image is controlled to be a value weighted by 2 / 3. In other words, the configuration described in the second embodiment is set. Then, this processing flow ends.

[0071] As described above, the configuration of this embodiment makes it possible to prevent the line width from changing depending on the position in the main scanning direction, and to provide an appropriate line width, regardless of the image resolution in the sub-scanning direction.

[0072] In the above configuration, the resolutions in the main scanning direction and sub-scanning direction are described as 600 dpi and 400 dpi, respectively. However, this combination is not limiting and other configurations are also possible. The relationship between image height and partial magnification shown in FIG. 3 is merely an example, and the information used for various controls may be defined according to variations in this relationship. For example, consider the cases where the resolution in the sub-scanning direction is Adpi and B (>A) dpi. In this case, if the PWM value for A is set to "255," the PWM value for B is controlled as "255 × (A / B)." The ratio of laser light emission to non-emission at a certain scanning position is then 255 × (A / B):{255 - (255 × (A / B))} = (A / B):{1 - (A / B)}. Furthermore, the brightness P of the laser light is controlled to be (B / A).

[0073] <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. [Explanation of symbols]

[0074] 4...photosensitive member, 9...image forming device, 100...image signal generating unit, 101...image processing unit, 101a...halftone processing unit (halftone processing means), 101z...density correction processing unit, 208...laser light (light), 400...optical scanning device (scanning means)

Claims

1. A photoreceptor; a charging means for charging the photosensitive member; an exposure unit that exposes the photosensitive member to laser light to form a latent image, the laser light passing through an imaging lens that does not have fθ characteristics and scanning the photosensitive member, so that the spot diameter at an end position of the photosensitive member in the main scanning direction is larger than the spot diameter at a central position of the photosensitive member; a developing means for developing the latent image into an image by attaching toner to the photosensitive member on which the latent image is formed; a control means for controlling the exposure of the photosensitive member so that the exposure amount per unit area of ​​the photosensitive member is constant; Equipped with the luminance of the laser light is such that the development width at an on-axis image height is equal to the development width at an extreme off-axis image height, The control means accepts designation of at least one of a first resolution and a second resolution lower than the first resolution as a resolution in a sub-scanning direction of the image; When the first resolution is specified, the photoconductor is rotated at a first speed; When the second resolution is specified, the photoconductor is rotated at a second speed higher than the first speed; the control means controls a time ratio between light emission and non-light emission of the laser light so as to suppress fluctuations in line width depending on positions in the main scanning direction regardless of the resolution in the sub-scanning direction and to make the line width appropriate. Image forming device.

2. 2. The image forming apparatus according to claim 1, wherein when the second resolution is specified, the control means increases the pulse width when driving the laser light by pulse width modulation compared to when the first resolution is specified.

3. A photoreceptor; a charging means for charging the photosensitive member; an exposure unit that exposes the photosensitive member to laser light to form a latent image, the laser light passing through an imaging lens that does not have fθ characteristics and scanning the photosensitive member, so that the spot diameter at an end position of the photosensitive member in the main scanning direction is larger than the spot diameter at a central position of the photosensitive member; a developing means for developing the latent image into an image by attaching toner to the photosensitive member on which the latent image is formed; a control means for controlling the exposure of the photosensitive member so that the exposure amount per unit area of ​​the photosensitive member is constant; A control method for an image forming apparatus having the luminance of the laser light is such that the development width at an on-axis image height is equal to the development width at an extreme off-axis image height, Accepting designation of at least one of a first resolution and a second resolution lower than the first resolution as a resolution in a sub-scanning direction of the image; rotating the photoconductor at a first speed when the first resolution is specified; When the second resolution is specified, rotating the photoconductor at a second speed higher than the first speed; controlling a time ratio between light emission and non-light emission of the laser light so as to suppress fluctuations in line width depending on positions in the main scanning direction regardless of the resolution in the sub-scanning direction and to make the line width appropriate; Including, Control method.

Images