JP2011167860A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of forming high-image quality and high-resolution image even in image formation by means of a plurality of beams. <P>SOLUTION: The image forming apparatus 100, in which a polygon mirror 102a is used, a plurality of laser beams simultaneously scanned, photoreceptors 104a, 106a, 108a, 110a are exposed and, thereby, the image formation is performed, includes a calculation means 356 for calculating writing times of respective pixels in a region where continuous writing is performed in a sub scanning direction in image patterns formed on the photoreceptors, wherein, when the writing times calculated by the calculation means 356 is the minimum writing times of the patterns which form respective pixels, pixels are added in the sub scanning direction and the image formation is performed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus including an exposure unit that forms an image by scanning a plurality of laser beams on a photoreceptor.

  2. Description of the Related Art Conventionally, a multi-beam recording head that emits a plurality of laser beams for exposing an image carrier and an image forming apparatus including the same are known in order to cope with higher speed and higher resolution of image formation. In recent digital electrophotographic apparatuses, the photosensitive member needs to be rotated at a high linear speed for high resolution and high-speed printing output. In response to this, the laser beam scanning exposure system also increases the rotation speed of the polygon mirror to perform sub-scanning. It is necessary to increase the scanning frequency in the direction. Currently, the rotational speed of the polygon mirror is around 40,000 rpm, but there are difficult issues such as improvement of the mirror bearing in order to obtain a higher rotational speed, so high resolution and high-speed printing output without increasing the rotational speed of the polygon mirror. In order to achieve the above, a multi-beam scanning exposure method is employed in which a plurality of beam light sources are arranged in parallel in the sub-scanning direction and a plurality of beams are scanned by one scan in the main scanning direction. According to this method, for example, when exposure is performed with two beams, the number of rotations of the polygon mirror required when only one beam is used is two beams, so that the number of rotations can be halved. Thus, exposure scanning that was impossible is possible. Furthermore, when there is a margin in the number of rotations, high-speed writing according to the number of rotations becomes possible.

  As a means for deflecting a plurality of laser beams and scanning them simultaneously on a scanned object such as a photosensitive member and scanning a plurality of scanning lines in one main scanning, a surface emitting laser (which can be easily arrayed) Using VCSEL (Vertical Cavity Status Emitting Laser) or the like as a light source and increasing the number of laser beams to be scanned simultaneously (the number of scanning lines simultaneously scanned by laser beams) realizes further increase in the image forming speed. be able to.

  A surface emitting laser capable of extracting light perpendicular to the surface of a semiconductor substrate has the following advantages over a conventional edge emitting laser. First, since the volume of the active layer can be reduced, it can be driven with a low threshold current and low power consumption. Further, since the mode volume of the resonator is small, modulation of several tens of GHz is possible, which is suitable for high-speed transmission. Further, since the spread angle of the emitted light is small, coupling to an optical fiber is easy, and further, cleavage is not required for fabrication and the element area is small, so that parallelization and two-dimensional high density array are possible.

  In a multi-beam optical scanning device, a technique for performing interlaced scanning using an LD array in which a plurality of light-emitting points are arranged at equal intervals and scanning line intervals are non-uniform is disclosed in, for example, “Patent Document 1”. Yes. According to the technique disclosed in “Patent Document 1”, even when the lateral magnification in the sub-scanning direction between the light source and the surface to be scanned varies depending on the deflection angle by interlaced scanning, the variation in the scanning line interval due to the main scanning position is reduced. Can do. In other words, by using such an optical system, it is possible to perform image formation by high-resolution and high-speed electrophotography, and image formation that can cover an area conventionally called light printing. An apparatus can be provided.

  However, when forming high-resolution images such as light printing, high-quality images are required. However, the reciprocity failure phenomenon that occurs when high-resolution writing is performed in electrophotographic image formation. This will be described below.

  In image forming means and image forming apparatuses such as electrophotographic copying machines, printers, facsimiles, and multifunctional machines, areas corresponding to image patterns on the surface of an image carrier charged by charging means such as a corona charger and a charging roller The toner is irradiated with light to form an electrostatic latent image, and toner is electrostatically attached to the electrostatic latent image from the developing means to form a toner image. The latent image formation on the image carrier is performed by exposing the charged photoconductor to light and generating carriers inside the image carrier, whereby the charged charge is attenuated and an electrostatic latent image is formed. Is used. An optical attenuation curve (PIDC: Photo Induced Decay Curve) indicating the amount of attenuation of the charged potential with respect to the exposure energy is caused by the characteristics of the photoconductor as an image carrier, and an example of this PIDC is shown in FIG.

  Even if PIDC irradiates a light beam with the same energy amount, the potential on the surface of the photoreceptor after irradiation with the light beam may differ depending on the irradiation method. For example, when the charged photoconductor surface is irradiated only once with a light beam having a certain amount of light energy, the potential of the photoconductor surface is lowered, and half of the light beam is applied to the same portion of the photoconductor surface after charging. The degree of potential decrease on the surface of the photoreceptor when the light beam having the light energy is irradiated in two portions is different from each other, and the absolute value of the potential on the surface of the photoreceptor is greatly decreased in the latter case. This is a phenomenon generally known as “reciprocity failure” (see “Patent Document 2”).

  The reciprocity failure phenomenon described above can also be seen in an image forming method and an image forming apparatus using a multi-beam scanning method using a plurality of beams for the purpose of improving image quality and speeding up. In the multi-beam scanning exposure method, when a plurality of LD light sources are arranged and the number is N, when one main scanning direction exposure is performed on one surface of a rotating polygon mirror, N multi-beam lines are simultaneously formed on the photoconductor. Exposed above. One beam is usually an elliptical beam, and since the beams partially overlap each other, a stronger power than usual is irradiated at a time. When N multi-beam lines are scanned and exposed on the next surface of the polygon mirror, the beam of the previous last line (Nth) and the current first line (first) partially overlap. Thus, the scanning exposure is performed, and this means that the strong power is exposed in two portions. That is, even if the exposure energy given to the photoconductor is the same, in a multi-beam optical system, one point on the photoconductor may receive the same exposure energy once or twice. At this time, depending on the photoconductor, even if the same exposure energy is received, a so-called reciprocity failure phenomenon, in which the effect is different, appears. FIG. 21 shows the PIDC at this time. In FIG. 21, the solid line indicates PIDC when the same exposure energy is divided into two exposures (hereinafter referred to as sequential scanning exposure), and the broken line indicates PIDC when the exposure is performed once (hereinafter referred to as simultaneous scanning exposure). Respectively.

  In this manner, when a dot or line image is formed with a plurality of beams, a dot or line formed depending on whether the plurality of beams are a simultaneous scanning exposure or a sequential scanning exposure when the photoreceptor is a strong reciprocity failure. It has been found that there is a problem that an image defect called image unevenness occurs due to changes in the thickness and thickness of the image. An example is shown below.

  In a 1200 dpi, 4LD multi-beam image forming apparatus, when a pattern of 2 by 5 horizontal lines (repetitive horizontal lines of 2 dots + 5 dots blank) was formed, density unevenness occurred at a repetition pitch of 4 beams as shown in FIG. . In FIG. 22, the dark line is the line at the time of sequential scanning exposure when writing with LD4ch, 1ch (LD4, 1), and when writing with other LD1ch, 2ch (LD1, 2), LD2ch, 3ch. When writing is performed (LD2, 3) When writing is performed with LD3ch and 4ch (LD3, 4), the line is a line at the time of simultaneous scanning exposure. When the line is exposed by sequential scanning and simultaneous scanning, PIDC differs as shown in FIG. 21 depending on the scanning method. Therefore, even if exposure is performed with the same LD light amount, the surface potential of the photoreceptor after exposure is different. In this example, images with different densities have been formed. Such density unevenness has a problem that it is recognized as an abnormal image as banding or moire.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems and to provide an image forming apparatus capable of forming a high-quality and high-resolution image even in image formation using a plurality of beams.

  According to the first aspect of the present invention, in an image forming apparatus for performing image formation by simultaneously scanning a plurality of laser beams using a polygon mirror and exposing a photosensitive member, an image pattern formed on the photosensitive member in a sub-scanning direction. A calculation unit that calculates the number of times of writing of each pixel in a region where continuous writing is performed, and when the number of times of writing calculated by the calculating unit is the minimum number of times of writing of the pattern forming each pixel, An image is formed by adding pixels to the image.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, each of the laser beams exposes the photosensitive member by interlaced scanning to form an image.

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, when the data resolution in the sub-scanning direction in the region is 2 pixels or less, an image is formed by adding pixels in the sub-scanning direction. It is characterized by performing.

  According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, when the data resolution in the main scanning direction in the region is 2 pixels or less, the pixels in the sub-scanning direction Is added to form an image.

  According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the light amount of the additional pixel is lower than the light amount of the pixel that is normally written. To do.

  According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the calculation unit further minimizes the laser beam when the laser beam is reflected by only one surface of the polygon mirror. The number of times of writing is calculated.

  According to the present invention, when the number of scans is determined and writing is performed with the minimum number of times of writing, the image is formed by adding pixels in the sub-scanning direction so that the lower edge of the image is enlarged and compared with the time of sequential scanning. Thus, it is possible to prevent the image during the simultaneous scanning from becoming thin and small, and it is possible to form a high-quality image by reducing the occurrence of banding due to this.

1 is a schematic front view of an image forming apparatus to which an embodiment of the present invention can be applied. It is the schematic explaining the surface emitting laser used for one Embodiment of this invention. It is the schematic of the optical apparatus used for one Embodiment of this invention. FIG. 3 is a diagram illustrating a scanning position on a photosensitive drum that is scanned by a light beam emitted from a surface emitting laser 200 in the first embodiment of the present invention. 2 is a block diagram of a control unit of the image forming apparatus used in an embodiment of the present invention. FIG. It is a block diagram of GAVD used for one Embodiment of this invention. It is a block diagram of the image processing part used for one Embodiment of this invention. The photosensitive drum in which 1 pixel of data resolution is (A) simultaneously scanned on the photosensitive drum by the light beam emitted from the surface emitting laser in the first embodiment of the present invention (B) and (C) sequentially scanned It is the schematic explaining the upper writing position. It is the schematic explaining the pixel added in the subscanning direction in the 1st Embodiment of this invention. The writing position (B), (C), and (D) on the photoconductive drum on which the data resolution of 2 pixels is simultaneously scanned (A) are sequentially scanned by the light beam emitted from the surface emitting laser in the first embodiment of the present invention. FIG. 3 is a schematic diagram illustrating a writing position on a photosensitive drum. It is the schematic explaining the pixel added in the subscanning direction in the 1st Embodiment of this invention. It is a flowchart explaining the pixel addition determination in the 1st Embodiment of this invention. It is a figure explaining the scanning position on the photoconductive drum scanned by the light beam emitted from the surface emitting laser 200 in the second embodiment of the present invention. The writing position (D) 3 on the photosensitive drum where one pixel of data resolution is interlaced and scanned by (A), (B), and (C) two scans by the light beam emitted from the surface emitting laser in the second embodiment of the present invention. FIG. 4 is a schematic diagram illustrating a writing position on a photosensitive drum that is scanned in a scanning manner. It is the schematic explaining the pixel added in the subscanning direction in the 2nd Embodiment of this invention. In the second embodiment of the present invention, a data resolution of 2 pixels is interlaced and scanned by (A) 2 scans by a light beam emitted from a surface emitting laser in the second embodiment of the present invention. FIG. 3 is a schematic diagram illustrating a writing position on a photosensitive drum. It is the schematic explaining the pixel added in the subscanning direction in the 2nd Embodiment of this invention. It is a flowchart explaining the pixel addition determination in the 2nd Embodiment of this invention. It is a block diagram of the image processing part used for the 3rd Embodiment of this invention. It is a diagram showing the light attenuation curve which shows the attenuation amount of a charging potential. It is a diagram showing the light attenuation curve at the time of reciprocity failure phenomenon expression. It is the schematic explaining the conventional problem.

  FIG. 1 shows an image forming apparatus to which an embodiment of the present invention can be applied. In FIG. 1, an image forming apparatus 100 includes an optical device 102 including optical elements such as a surface emitting laser 200 (see FIGS. 2 and 3) and a polygon mirror 102a, photosensitive drums 104a, 106a, 108a, 110a, a charging device 104b, 106 b, 108 b, 110 b and developing devices 104 c, 106 c, 108 c, 110 c, and the like, an image forming unit 112 including the intermediate transfer belt 114, and the like. The optical device 102 includes a surface emitting laser 200 as a semiconductor laser. A laser beam emitted from the surface emitting laser 200 is collected by a first cylindrical lens 202 (see FIG. 3), and reflected by a polygon mirror 102a to a reflecting mirror 102b. Is deflected to.

  The surface emitting laser 200 is a semiconductor laser, which is a plurality of light sources, arranged in a grid on the same chip. As shown in FIG. 2, 40 light emitting points from channel ch1 to channel ch40 are arranged in a diagonal grid. ing. The optical device 102 is a post-object type that does not use an fθ lens, and the number of light beams corresponding to each color of cyan (C), magenta (M), yellow (Y), and black (K) from each channel ch1-40. The light beam L that is emitted and deflected by the polygon mirror 102a is reflected by the reflection mirror 102b and condensed again by the second cylindrical lens 102c, and then the photosensitive drums 104a, 106a, 108a, and 110a are exposed. Since the irradiation of the light beam L is performed through a plurality of optical elements as described above, the scanning timing is synchronized in the main scanning direction and the sub-scanning direction. Hereinafter, the scanning direction of the light beam L is referred to as a main scanning direction, and the circumferential direction of the photosensitive drum orthogonal thereto is referred to as a sub-scanning direction.

  The photosensitive drums 104a, 106a, 108a, and 110a have a photoconductive layer including at least a charge generation layer and a charge transport layer on a conductive drum such as aluminum. Corresponding to the photosensitive drums 104a, 106a, 108a, 110a, charging devices 104b, 106b, 108b, 110b having corotrons, scorotrons, charging rollers or the like are arranged, respectively, from the charging devices 104b, 106b, 108b, 110b. A surface charge is applied to the photoconductive layers of the photosensitive drums 104a, 106a, 108a, and 110a. The applied electrostatic charge is imagewise exposed by the light beam L to form an electrostatic latent image. The formed electrostatic latent image is developed and visualized by developing devices 104c, 106c, 108c, 110c having a developing roller, a developer supply roller, a regulating blade, and the like.

  The toner images formed on the photosensitive drums 104a, 106a, 108a, and 110a are respectively transferred to a roller 114a by primary transfer rollers 104d, 106d, 108d, and 110d arranged to face the photosensitive drums 104a, 106a, 108a, and 110a, respectively. , 114b, 114c and are sequentially superimposed and transferred onto the intermediate transfer belt 114 that travels in the direction indicated by the arrow A in FIG. 1, and a full-color toner image is formed on the intermediate transfer belt 114. An intermediate transfer belt 114 carrying a full-color toner image is stretched around rollers 118a and 118b and is in contact with the secondary transfer belt 118 that travels in the direction indicated by arrow B in FIG. The full-color toner image carried by the secondary transfer bias applied to the roller 114b is collectively collected on the transfer sheet 124 such as high-quality paper or plastic sheet fed from the transfer sheet storage unit 128 by the conveying roller 126. Transcript.

  The transfer sheet 124 to which the toner image is transferred is sent to the fixing device 120 by the secondary transfer belt 118. The fixing device 120 includes a fixing roller 130 containing silicone rubber, fluorine rubber, or the like, and pressurizes and heats the transfer sheet 124 and toner to fix the toner image on the transfer sheet 124, thereby forming an image as a printed matter 132. Discharge to the outside of the apparatus 100. After the toner transfer, the transfer belt 114 is subjected to the next image forming process after the transfer residual toner is removed by a belt cleaning device 116 having a cleaning blade.

  FIG. 3 shows a case where the optical device 102 including the surface emitting laser 200 exposes the photosensitive drum 104a. A plurality of light beams emitted from the surface emitting laser 200 are collected by the first cylindrical lens 202 that shapes the light beam bundle, and are deflected by the polygon mirror 102 a via the reflection mirror 204 and the imaging lens 206. The polygon mirror 102a is rotationally driven by a spindle motor or the like that rotates several thousand to several tens of thousands per minute, and the light beam L reflected by the polygon mirror 102a is reflected by the reflection mirror 102b and then regenerated by the second cylindrical lens 102c. After being shaped, the photosensitive drum 104a is exposed.

  A reflection mirror 208 is provided to synchronize the scanning start timing of the light beam L in the sub-scanning direction. The reflection mirror 208 reflects the light beam L to the synchronization detection device 210 having a photodiode or the like before starting scanning in the sub-scanning direction. When detecting the light beam L, the synchronization detection device 210 generates a synchronization signal to start optical scanning in the sub-scanning direction, and synchronizes processes such as a drive control signal generation process for the surface emitting laser 200. The surface emitting laser 200 is driven by a pulse signal sent from a GAVD 310 (see FIG. 5), which will be described later, and a light beam L is exposed to a position corresponding to a predetermined image bit of the image data, as will be described later, on the photosensitive drum 104a. An electrostatic latent image is formed on the surface.

  FIG. 4 is a diagram for explaining a scanning position on the photosensitive drum scanned by the light beam L composed of 40 beams of the channels ch1 to 40 emitted from the surface emitting laser 200 in the first embodiment of the present invention. In this embodiment, the exposure resolution on the photosensitive drum is 4800 dpi, and the interval between the pixels in FIG. 4 is about 5.2 μm. FIG. 4 shows the scan S1 and the next scan S2 on the polygon mirror surface at a certain point in time, and one surface of the polygon mirror scans the line indicated by S1 by scanning 40 beams at a time. Then, an image is formed by scanning the line indicated by S2. This is the same for each color, and thus a writing unit with a resolution of 4800 dpi is configured.

  FIG. 5 shows a control unit 300 of the image forming apparatus 100, and the control unit 300 includes a scanner unit 302, a printer unit 308, and a main control unit 330. The scanner unit 302 functions as a means for reading an image. The VPU 304 performs A / D conversion on a signal read by the scanner and performs black offset correction, shading correction, and pixel position correction. It has an IPU 306 that performs image processing for digital conversion from color systems to image data in the CMYK color system. The read image acquired by the scanner unit 302 is sent to the printer unit 308 as digital data.

  The printer unit 308 functions as a control unit that performs drive control of the surface emitting laser 200, and an LD driver 312 that supplies a current for driving the semiconductor laser element to the semiconductor laser element based on a drive control signal generated by the GAVD 310. It has a surface emitting laser 200 or the like on which a two-dimensionally arranged semiconductor laser element is mounted. The GAVD 310 shown in the supplemental embodiment divides pixel data so that the image data sent from the scanner unit 302 corresponds to the spatial size of the semiconductor laser element emitted by the surface emitting laser 200 to the pixel data, and has high resolution. Execute the conversion process.

  The scanner unit 302 and the printer unit 308 are connected to the main control unit 330 via the system bus 316, and an image reading operation and an image forming operation are controlled based on a command from the main control unit 330. The main control unit 330 has a CPU (Central Processing Unit) 320 and a RAM 322 that provides a processing space used by the CPU 320 for processing. The CPU 320 receives a command from the user via the interface 328, calls a program module that executes processing corresponding to the command, and executes processing such as copying, facsimile, scanner, and image storage. The main control unit 330 has a ROM 324 and stores initial setting data, control data, programs, and the like of the CPU 320 so that the CPU 320 can use them. The image storage 326 is configured as a fixed or detachable memory device such as a hard disk device, an SD card, or a USB memory, and stores image data acquired by the image forming apparatus 100 so that it can be used for various processes by the user.

  When the image data acquired by the scanner unit 302 is output as an electrostatic latent image on the photosensitive drum 104a by driving the printer unit 308, the CPU 320 controls the main scanning direction of a transfer sheet such as high-quality paper or plastic film. Sub-scanning direction control is executed. When starting scanning in the sub-scanning direction, the CPU 320 outputs a start signal to the GAVD 310, and when the GAVD 310 receives the signal, the IPU 306 starts the scanning process. Thereafter, the GAVD 310 receives and processes the image data stored in the buffer memory or the like, and outputs the processed image data to the LD driver 312. When the LD driver 312 receives image data from the GAVD 310, the LD driver 312 generates a drive control signal for the surface emitting laser 200. The surface emitting laser 200 is turned on when a drive control signal is sent from the LD driver 312 to the surface emitting laser 200. The LD driver 312 drives the semiconductor laser element using PWM control or the like. In the present embodiment, the surface emitting laser 200 including a 40-channel semiconductor laser element is shown, but the number of channels of the semiconductor laser element is not limited to this.

  As shown in FIG. 6, the GAVD 310 has a memory 340 such as a FIFO buffer that receives the synchronization signal and stores and stores the image data sent from the IPU 306, and pre-loads the image data sent from the IPU 306. The image is sent to the image processing unit 342 in a first-out manner. The image processing unit 342 reads image data from the memory 340, and executes resolution conversion of the image data, assignment of semiconductor laser element channels, and correction processing in the sub-scanning direction of the image data.

  In the image data, the exposure position on the photosensitive drum 104a is defined based on the main scanning line address value defined in the main scanning direction and the sub scanning line address value defined in the sub scanning direction. Hereinafter, the address coordinate in the present embodiment refers to each address value that gives a specific image bit when image data is designated by a main scanning line address value (R address value) and a sub scanning line address value (F address value). Define as a set of These address values are determined by an address generation unit 354 in an image processing unit, which will be described later, and address coordinates are determined for each pixel arranged in a line in the main scanning direction and the sub-scanning direction, that is, for each pixel column.

  The output data control unit 344 converts the output data, which is a write signal corresponding to the image data generated by the image processing unit 342, from the F address value and the sub-scanning speed into a time-series drive pulse, and further, a synchronization detection device A synchronization control signal for giving a synchronization signal to 210 is added and generated. The generated drive control signal is sent to the LD driver 312 to drive the surface emitting laser 200. The output data control unit 344 receives the synchronization signal from the synchronization detection device 210 and synchronizes transmission of the drive control signal to the LD driver 312. The processing of the memory 340, the image processing unit 342, and the output data control unit 344 is synchronized with the operation clock by the PLL 346.

  When continuous writing is performed in the sub-scanning direction in the above-described configuration, as described in the section “Problems to be solved by the invention”, density unevenness occurs when the number of times of writing is different, such as sequential scanning and simultaneous scanning. May occur. The present invention solves this problem, and its configuration will be described below.

  FIG. 7 is a block diagram of the image processing unit 342 shown in FIG. In the figure, the resolution conversion unit 350 assigns a laser element channel for irradiating the pixel corresponding to the exposure resolution by the surface emitting laser 200 to 1200 dpi image data acquired from the memory 340, so that the vertical and horizontal times are four times. Perform density processing. As a result, one pixel having a data resolution of 1200 dpi is converted into 4 × 4 pixels having a writing resolution of 4800 dpi.

  The sub-scan addition determination unit 352 includes an image path selector 358 and a scanning number counting unit 356 as a calculation unit. The sub-scan addition determination unit 352 receives an F address and an R address used for forming an image from the address generation unit 354 and allocates a laser element channel of the surface emitting laser 200. The scan number counting unit 356 that has acquired the channel information assigned from the image path selector 358 determines the number of scans from the channel number information and generates an additional flag addition command signal according to the number of scans.

  In the case of one pixel with a data resolution of 1200 dpi, since it has been converted to 4 × 4 pixels with a resolution of 4800 dpi, the number of scans, which is the number of writes, is scanned on one surface of the polygon mirror 102 and four consecutive channels are scanned simultaneously. As shown in FIG. 8 (B) or FIG. 8 (C), scanning is performed on two surfaces of the polygon mirror 102 including the end of the laser element channel. In such a case, writing is performed in two scans. When two scans exceeding the minimum writing count of 1, that is, sequential scans are performed as described above, the image becomes thick and density unevenness occurs as shown in FIG. Therefore, when writing is performed with the minimum number of times of writing, that is, one scan, as shown in FIG. 9, a channel that is not originally written (channel 33 in FIG. 9) continuous to the channel to be written is turned on, and pixels are added. Write.

  In the case of two pixels with a data resolution of 1200 dpi, the number of scans is one scan as shown in FIG. 10A in which scanning is performed on one surface of the polygon mirror 102 and continuous eight channels are simultaneously scanned. In the case shown in FIG. 10B, FIG. 10C, or FIG. 10D in which scanning is performed on two surfaces of the polygon mirror 102 including the end of the channel, writing is performed in two scans. When writing is performed in one scan, the channel 38 is turned on as shown in FIG. 11, and writing is performed by adding pixels.

  FIG. 12 shows a flowchart of the addition determination in the sub-scan addition determination unit 352. Here, as a method of determining the minimum number of times of writing, the assigned channel information (channel number) is a continuous value, that is, the written pixels are scanned on one surface of the polygon mirror 102 and aligned in one line in the sub-scanning direction. Is additionally determined as the minimum write count.

  The image trailing edge determination unit 353 determines the image trailing edge address by examining the arrangement of pixels in the sub-scanning direction. The image rear end determination unit 353 has buffers for 40 pixels before and after the target pixel in the sub-scanning direction, counts the continuous number of on pixels with the target pixel as black (on), Output R address. The sub-scanning correction unit 360 determines whether or not to add a pixel bit from the outputs of the image trailing edge determination unit 353 and the scanning number counting unit 356. If it is determined that the pixel bit is added, the sub-scanning correction unit 360 functions the AND combination circuit 362 with the one-line delay 361. Let As a result, when writing is performed with the minimum number of times of writing, the lower edge of the image is enlarged as shown in FIG.

  According to the above-described configuration, when writing is performed with the minimum number of times by determining the number of scans, the image is formed by adding pixels in the sub-scanning direction, so that the lower edge of the image is enlarged and sequentially scanned. On the other hand, it is possible to prevent the image during the simultaneous scanning from becoming thinner and smaller, and to reduce the occurrence of banding resulting from this, it is possible to form a high-quality image.

  FIG. 13 is a diagram for explaining a scanning position on the photosensitive drum that is interlaced and scanned by the light beam L composed of 40 beams of the channels ch1 to 40 emitted from the surface emitting laser 200 in the second embodiment of the present invention. . The exposure resolution on the photosensitive drum in this embodiment is 4800 dpi, and the interval between each pixel in FIG. 11 is about 5.2 μm. FIG. 11 shows a scan S1, a next scan S2, and a next scan S3 on the polygon mirror surface at a certain point in time. The beam is 40 channels, and the channel ch20 and the channel ch21 in the center are close to each other. The other channels are configured to have a positional relationship of scanning in the main scanning direction by forming a gap of one scanning line.

  A position a which is an intermediate point between the channel ch20 and the channel ch21 is the center of the lens group in the sub-scanning direction, and the lens group is positioned symmetrically in the sub-scanning direction. When horizontal scanning is performed on one surface of the polygon mirror 102, the line S1 is scanned, and on the next surface, the line S2 is scanned. At this time, the channel ch1 of the line S2 is configured to scan between the channel ch21 and the channel ch22 of the line S1. Similarly, the line S3 is scanned on the next surface of the polygon mirror 102. At this time, the channel ch1 of the line S3 scans between the channel ch21 and the channel ch22 of the line S2. This is the same for each color, and thus a writing unit with a resolution of 4800 dpi is configured.

  When such interlaced scanning is performed, it is possible to reduce variations in the amount of light by setting the distance between the channels wide. Further, since the central channel ch20 and the channel ch21 are close to each other, it is possible to skip over multiples of 8, which is advantageous in configuring a circuit for image processing. Further, when a surface emitting laser is used as a light source, there is an advantage that the degree of freedom of light source arrangement is high. In addition, the sub-scanning beam interval between channels ch1 to 40 is narrower than in normal interlaced scanning, and the distance that the outermost beam moves away from the optical axis is reduced. Therefore, variations in sub-scanning beam pitch and beam spot diameters are reduced. Can be reduced. Furthermore, the effective range of the scanning lens can be reduced, and the optical element can be miniaturized.

  In the case of this interlaced scanning, similarly to the first embodiment, one pixel with a data resolution of 1200 dpi is converted into 4 × 4 pixels with a writing resolution of 4800 dpi by the resolution conversion unit 350, and the image path selector 358 and the scanning number counting unit 356 are converted. The additional determination is performed in the sub-scan addition determination unit 352 having the same.

  In the case of one pixel with a data resolution of 1200 dpi, the number of scans is two scans as shown in FIG. 14 (A), FIG. 14 (B) or FIG. 14 (C) in which four channels are scanned on the two surfaces of the polygon mirror 102. However, in the case shown in FIG. 14D where scanning is performed on three surfaces of the polygon mirror 102 including the end of the laser element channel, writing is performed by three scannings. When writing is performed with two scans, channel 38 is set as shown in FIG. 15A, channel 21 is set as shown in FIG. 15B, or channel 1 is set as shown in FIG. 15C. Light up and write by adding pixels.

  In the case of two pixels with a data resolution of 1200 dpi, the number of scans is two scans as shown in FIG. 16A in which eight channels are scanned on two surfaces of the polygon mirror 102. In the case as shown in FIG. 16B where scanning is performed on three surfaces of the included polygon mirror 102, writing is performed in three scans. When writing is performed by two scans, the channel 30 is turned on as shown in FIG. 17A or the channel 1 is turned on as shown in FIG. 17B, and writing is performed by adding pixels.

  In the case of interlaced scanning, 1200 dpi is written by two or three scans, so that the number of scans can be determined based on the presence or absence of channel assignment used at the time of three-scan writing. FIG. 18 shows an example of an additional determination flowchart during interlaced scanning. In the second embodiment, the same effect as that of the first embodiment can be obtained.

  In each of the embodiments described above, the configuration in which pixels in the sub-scanning direction are added according to the number of scans of each pixel and the rear end of the image to prevent banding due to reciprocity failure is described. Adding pixels in the direction is also effective, and this will be described as a third embodiment. Banding tends to stand out as the image length in the sub-scanning direction is shorter. This is because banding due to reciprocity failure occurs due to a difference in image width in the sub-scanning direction, and variation in image width due to reciprocity failure is about several μm to several tens μm, and this ratio is an image in the sub-scanning direction. This is because the longer the width, the smaller. Therefore, in the case of an image having a resolution of 1200 dpi, it is desirable to perform the sub-scan addition determination only for an image formed with a width of 1 to 2 dots in the sub-scan direction. FIG. 19 shows a block diagram of the image processing unit 343 for an image formed with a width of 1 to 2 dots in the sub-scanning direction.

  In FIG. 19, the resolution conversion unit 350 converts 1200 dpi image data acquired from the memory 340 according to the exposure resolution of the surface emitting laser 200. Further, the front and rear image determination unit 355 that determines whether or not there is an image before and after the target pixel based on the address information obtained from the address generation unit 354 and the image data acquired from the memory 340, outputs the flags of the previous image and the subsequent image. To do. The sub-scan addition determination unit 352 includes an image path selector 358 and a scan number counting unit 356. The sub-scan addition determination unit 352 receives an F address and an R address used to form an image sent from the address generation unit 354 and receives a surface emitting laser. Allocate 200 laser element channels. The scan number counting unit 356 that has acquired the channel information assigned from the image path selector 358 determines the number of scans from the channel number information and generates an additional flag addition command signal according to the number of scans.

  In the sub-scanning correction unit 360, the addition determination unit 364 outputs an address at the time of line addition and addition based on the addition flag from the scanning number counting unit 356 and the front and rear image flag from the front and rear image determination unit 355. When it is determined that the addition is performed, the AND synthesis circuit 362 with the one-line delay 363 is caused to function. As a result, only the image that is likely to be affected by the reciprocity failure is enlarged at the lower edge of the image when writing is performed with the minimum number of times of writing, and banding due to the reciprocity failure can be prevented.

  As described above, the image width in the sub-scanning direction is directly attributable to the banding image. However, it has been confirmed that this tendency becomes more remarkable when the image width in the main scanning direction is short. Therefore, the front and rear image determination unit 355 not only outputs the flag of the previous image and the rear image, but also adds and outputs a flag for determining the presence / absence of image data of the F address and the previous R address value. It is possible to determine the presence or absence of image processing according to the size of the image in the main scanning direction as well as the previous and subsequent images, which is more effective.

  In each of the above-described embodiments, an image processing method for preventing the occurrence of banding due to reciprocity failure is disclosed, but it is possible not only to add pixels but also to achieve higher image quality by controlling the light quantity of the pixels. it can. As a pixel light amount control method, it is desirable to make the light amount of the added pixel lower than the light amount of the pixel at the time of normal writing. This is because the addition of one pixel can improve an image that has become thinner or thinner by simultaneous exposure, but may be too thick or too dark. Therefore, it is possible to achieve higher image quality by adjusting the light amount of the additional pixels to be weak.

100 Image forming apparatus 102a Polygon mirrors 104a, 106a, 108a, 110a Photosensitive drum 356 calculating means (scanning count unit)

JP 2006-215270 A JP 2003-205642 A

Claims (6)

In an image forming apparatus that simultaneously scans a plurality of laser beams using a polygon mirror and exposes a photosensitive member to form an image,
A calculating unit that calculates the number of times of writing of each pixel in an area in which continuous writing is performed in the sub-scanning direction of the image pattern formed on the photosensitive member, and the number of times of writing calculated by the calculating unit is An image forming apparatus that performs image formation by adding pixels in the sub-scanning direction when the number of times of writing of the pattern for forming the image is the minimum. The image forming apparatus according to claim 1.
An image forming apparatus, wherein each laser beam exposes the photosensitive member by interlaced scanning to form an image. The image forming apparatus according to claim 1 or 2,
An image forming apparatus, wherein an image is formed by adding pixels in the sub-scanning direction when the data resolution in the sub-scanning direction in the region is 2 pixels or less. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus for performing image formation by adding pixels in the sub-scanning direction when the data resolution in the main scanning direction in the region is 2 pixels or less. The image forming apparatus according to any one of claims 1 to 4,
An image forming apparatus, wherein a pixel to be added has a light amount lower than a light amount of a pixel that is normally written. The image forming apparatus according to any one of claims 1 to 5,
The image forming apparatus characterized in that the calculating means calculates the minimum number of times of writing when the laser beam is reflected by only one surface of the polygon mirror.

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