JP4470668B2 - Optical scanning apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning device and an image forming apparatus, and more particularly to an optical scanning device that scans a plurality of laser beams and an image forming apparatus that includes the optical scanning device.

  An optical scanning device that deflects a plurality of laser beams and simultaneously scans a scanned object such as a photosensitive member to scan a plurality of scanning lines with a single deflection, or such an optical scanning device is provided. Conventional image forming apparatuses are known. In addition, using a surface emitting laser that can be easily arrayed as a light source, increasing the number of laser beams that are scanned simultaneously (the number of scanning lines that are simultaneously scanned by laser beams) increases the image formation speed. The structure which performs is also proposed (for example, refer patent document 1 etc.).

  In this way, in a configuration in which a plurality of laser beams are scanned simultaneously, the photosensitive member is exposed a plurality of times at both ends of a plurality of scanning lines formed by one scanning, and therefore, due to reciprocity failure and multiple exposure. Changes in density characteristics occur.

  FIG. 11 shows how this phenomenon occurs. Note that FIG. 11A shows an array of imaging spots on an actual photoconductor. When the deflection angle is the same, a plurality of laser beams are shifted in the main scanning direction. For simplicity, this deviation is eliminated and shown schematically in FIG. FIG. 11C shows an exposure image finally obtained. In this example, the number of scanning lines scanned simultaneously in one scan is 24, and the photosensitive member moves in the sub-scanning direction by 24 scan lines between the Nth scan and the N + 1th scan. .

  In this case, in the toner image shown in FIG. 11C, the density of the boundary portion between the Nth and N + 1th scanning lines is different from the peripheral portion, and periodic density unevenness occurs in the entire toner image.

  FIG. 12 shows an exposure energy distribution at a position in the sub-scanning direction (direction orthogonal to the main scanning direction) in each scan. In each exposure energy distribution, the area where the total exposure energy is applied by one exposure has a substantially flat trapezoidal shape, but the boundary area of one exposure forms a gentle skirt and the exposure energy gradually decreases. Therefore, the exposure energy overlaps with the next scanning.

The total amount of exposure energy applied to the boundary area is eventually equal to the exposure energy of the flat portion, but the actual image density is higher in the boundary area. This is because the recombination rate of electrons and holes generated by exposing the photosensitive member becomes higher when the exposure is performed a plurality of times, so that the amount of charge remaining as an electrostatic latent image may increase in the boundary region. Responsible.

In contrast, Patent Document 2, configured to reduce the density change described above is proposed by overwriting the ends of the scan line formed by one scan.

  However, in the configuration described in Patent Document 2, since only one part of a plurality of scanning lines is formed by overwriting a plurality of spots, as shown in FIG. When there is a positional deviation of the image spot in the scanning direction, the exposure distribution has a lower peak as shown in FIG. 14B than in FIG. As a result, as shown in FIG. 13B, a portion P1 in which the density of the overlapping portion is lowered is generated, and becomes a streak with respect to the peripheral portion. In the case where a plurality of scanning lines are formed by one scanning, there is a problem that the line spacing becomes a plurality of pitches, so that it is easy to visually recognize (defects).

In FIG. 15A, adjacent scanning lines are formed on the scanning object by one deflection, and the distance that the scanning object moves until the next scanning is half of the maximum interval of each scanning line. The exposure spot when all the scanning lines are formed by two scans is shown. Even in this configuration, if there is a positional deviation in the main scanning direction for each scanning, as shown in FIG. 15B, density unevenness occurs in a plurality of light beam periods.
JP-A-5-294005 JP 2003-182139 A

  In view of the above facts, an object of the present invention is to obtain an optical scanning apparatus and an image forming apparatus that can reduce density unevenness due to an imaging position shift, reciprocity failure, or the like on a surface to be scanned.

According to the first aspect of the present invention, the light source unit having a plurality of light emitting points, the pre-deflection optical system on which the plurality of laser beams emitted from the plurality of light emitting points of the light source unit are incident, and the light source unit rotate at a constant cycle. A deflecting unit that includes a rotating polygon mirror and deflects a plurality of laser beams that have passed through the pre-deflection optical system in a main scanning direction; an image carrier that is movable in a sub-scanning direction orthogonal to the main scanning direction;
A scanning optical system that forms an image of the laser beam deflected by the deflecting unit on a surface to be scanned on the image carrier, and a scanning line formed on the surface to be scanned is an imaging spot formed by a plurality of deflections. And the plurality of laser beams are arranged on the surface to be scanned so as to have non-uniform intervals in the sub-scanning direction, and the scanning lines are formed on the surface to be scanned. By repeating the scanning of the imaging spot in the main scanning direction by the deflection of the deflecting means and the subsequent movement of the image carrier in the sub-scanning direction, different 3 wherein two or three of superposition of imaging spots by the deflection of the above surface is formed on the surface to be scanned, characterized in that.

According to the second aspect of the present invention, the light source unit having a plurality of light emitting points, the pre-deflection optical system on which the plurality of laser beams emitted from the plurality of light emitting points of the light source unit are incident, and a predetermined period rotate. A deflecting unit that includes a rotating polygon mirror and deflects a plurality of laser beams that have passed through the pre-deflection optical system in a main scanning direction; an image carrier that is movable in a sub-scanning direction orthogonal to the main scanning direction; A scanning optical system that forms an image of the laser beam deflected by the deflecting unit on a surface to be scanned on the image carrier, and a scanning line formed on the surface to be scanned is an imaging spot formed by a plurality of deflections. N, the number of multiple laser beams is n, the number of laser beams to be overlapped when forming the same scan line is m, and each scan from the head of the multiple scan lines formed by one deflection is performed. line number to line When the respective L2 to Ln, the remainder obtained by dividing the L2 to Ln (n / m) is a natural number less than n / m, and appear m times, it is characterized.

According to a third aspect of the present invention, a light source unit having a plurality of light emitting points, a pre-deflection optical system on which a plurality of laser beams emitted from the plurality of light emitting points of the light source unit are incident, and a predetermined period rotate. A deflecting unit having a rotating polygon mirror and deflecting a plurality of laser beams that have passed through the pre-deflection optical system in a main scanning direction; and a scanning optical system that forms an image of the laser beam deflected by the deflecting unit on a surface to be scanned; And an image carrier that is movable in the sub-scanning direction orthogonal to the main scanning direction and is exposed by the laser beam, and the scanning line formed on the scanned surface is an imaging spot formed by a plurality of deflections together are formed by superposition of the scanning lines, one of r the number of spot group forming a adjacent scan lines among the plurality scanning lines formed by the deflection, the number of spots in the spot group was c When next to In the spot group, the distance L between the head imaging spots is a number obtained by dividing the number r of spot spots and the natural number q having the greatest common divisor by 1 and the number of imaging spots c in the spot group divided by the number of overlaps m. And a distance obtained by multiplying the scanning line interval p, and the quotient of the number c of imaging spots in the spot group and the number m of overwritten lasers is a natural number, and is formed on the surface to be scanned. To do.

In any of the first to third aspects of the present invention, a plurality of laser beams emitted from a plurality of light emitting points of the light source section are deflected by the deflecting means via the pre-deflection optical system. Further, the plurality of laser beams are imaged on the surface to be scanned on the image carrier by the scanning optical system. Then, the image carrier moves in the sub-scanning direction, whereby an exposure image is obtained on the image carrier. As described above, since a plurality of scanning lines can be formed by simultaneously scanning a plurality of laser beams with one deflection, scanning at a high speed is possible.

In addition, since the scanning line formed on the surface to be scanned is formed by overlapping the imaging spots by a plurality of deflections, density unevenness due to multiple exposure can be suppressed.

According to the first aspect of the present invention, the scanning lines formed on the surface to be scanned are formed by overlapping the imaging spots by a plurality of deflections, and the plurality of laser beams are formed on the surface to be scanned. The scanning lines are arranged so as to have unequal intervals in the sub-scanning direction, and the scanning lines are moved in the main scanning direction by deflection of the imaging spot formed on the surface to be scanned. a scanning movement and to subsequent sub scanning direction of said image bearing member, by repeating the said object in the superposition of two or three imaging spots with different 3 or more sides of the deflection of the rotary polygonal mirror It is formed on the scanning surface.

  For this reason, even if there is a scanning position shift of the imaging spot due to an error of the rotary polygon mirror, it is possible to suppress the occurrence of a streak defect caused by this.

According to the second aspect of the present invention, the number of the plurality of laser beams is n, the number of the laser beams to be superimposed when forming the same scanning line is m, and each scanning is performed from the head of the plurality of scanning lines formed by one deflection. When the line numbers to the lines are L2 to Ln, the remainder obtained by dividing L2 to Ln by (n / m) is a natural number less than n / m and appears m times.

  For this reason, all the scanning lines can be scanned with a plurality of imaging spots without missing in accordance with all the scanning line pitches on the surface to be scanned.

In the invention described in claim 3, wherein the scanning lines, the number of spot group forming a scanning line adjacent among the plurality scanning lines formed by the deflection of one to r, the number of spots during the spot group and c In the adjacent spot group, the distance L between the head imaging spots is divided by the natural number q where the number r of spot groups and the greatest common divisor are 1, and the number c of imaging spots in the spot group is divided by the superposition number m. the number was, the distance obtained by multiplying the scanning line spacing p, and the quotient of the image spot number c and overlaid laser number m in the group of spots are formed on the surface to be scanned as a natural number, way ing.

  For this reason, scanning by a spot group is possible, and all scanning lines can be overwritten by a plurality of spots.

  According to a fourth aspect of the present invention, in the first aspect of the present invention, the light source unit is a surface emitting laser in which a plurality of light emitting points are two-dimensionally arranged. And

  Thereby, the freedom degree of arrangement | sequence of several light emitting points becomes high.

  According to a fifth aspect of the present invention, in the fourth aspect of the present invention, at least one of the light emitting points arranged in the main scanning corresponding direction corresponding to the main scanning direction of the laser beam among the plurality of light emitting points of the surface emitting laser. A part of the laser beam from the light emitting point is arranged so as to form a scanning line adjacent in the sub scanning direction on the surface to be scanned.

  In the configuration in which one scanning line is scanned a plurality of times to obtain a final exposure image, it is necessary to buffer the scanning line information as image data until the last scanning is completed. This buffer amount can be reduced.

  According to a sixth aspect of the present invention, in the first aspect of the present invention, the amount of movement of the image carrier in the sub-scanning direction per scanning period of the laser beam is a scanning line interval. It is an amount obtained by multiplying p by the number (n / m) obtained by dividing the number n of a plurality of laser beams by the number m of laser beams to be overlapped when forming the same scanning line.

  As a result, the entire surface to be scanned can be exposed by the scanning lines formed by the plurality of imaging spots.

  According to a seventh aspect of the present invention, in the first aspect of the present invention, the laser beams from the plurality of light emitting points are predetermined in the sub-scanning direction on the surface to be scanned. Scanning is performed at intervals, and an exposure image having a constant pitch is formed by a plurality of scans.

  That is, since so-called interlaced scanning is performed, it is possible to give a high degree of freedom to the interval between the light emitting points and the imaging magnification.

  Further, in this configuration, as described in claim 5, if the emission point array is an array in which the laser beam forms adjacent scanning lines on the surface to be scanned, interlaced scanning can be performed with the minimum use of a buffer memory. Can be done.

  According to an eighth aspect of the invention, the optical scanning device according to any one of the first to seventh aspects, a charging means for charging the image carrier, and exposure of a laser beam from the optical scanning device. The electrostatic latent image formed on the image carrier is developed to form a toner image, a transfer means for transferring the toner image to a recording medium, and the transfer means transferred to the recording medium. And a fixing unit for fixing the toner image.

  Therefore, the image carrier charged by the charging means is exposed with the laser beam from the optical scanning device, and an electrostatic latent image is formed on the image carrier. This electrostatic latent image is developed by the developing means, and a toner image is obtained. The toner image is transferred to a recording medium by a transfer unit, and further fixed by a fixing unit.

  Since the image forming apparatus includes the optical scanning device according to any one of claims 1 to 7, density unevenness due to an imaging position shift or reciprocity failure on the surface to be scanned is reduced. it can

  Since the present invention has the above configuration, it is possible to reduce density unevenness due to an imaging position shift on the surface to be scanned, reciprocity failure, or the like.

  FIG. 1 shows an image forming apparatus 10 according to the first embodiment of the present invention. The image forming apparatus 10 includes a photosensitive drum 12 that is rotated in a direction of an arrow A at a predetermined rotational speed by a driving unit (not shown), and the outer peripheral surface of the photosensitive drum 12 is charged above the photosensitive drum 12. A charger 14 is provided.

  An optical scanning device 16 is disposed above the charger 14. The optical scanning device 16 modulates a plurality of laser beams emitted from a light source in accordance with an image to be formed and deflects the laser beams in the main scanning direction so that the outer surface of the photosensitive drum 12 is on the outer surface of the photosensitive drum 12. Scan parallel to the axis. As a result, an electrostatic latent image is formed on the outer peripheral surface of the photosensitive drum 12. The outer peripheral surface of the photosensitive drum 12 is a scanned surface according to the present invention.

  A developing device 18 is disposed on the side of the photosensitive drum 12. The developing device 18 includes a roller-shaped container that is rotatably arranged. Four containers are formed inside the container, and developing units 18Y, 18M, 18C, and 18K are provided in each container. Each of the developing devices 18Y, 18M, 18C, and 18K includes a developing roller 20, and stores toners of Y, M, C, and K colors therein. Further, on the opposite side of the developing device 18 across the photosensitive drum 12, a static eliminator / cleaner having a function of removing the outer peripheral surface of the photosensitive drum 12 and a function of removing unnecessary toner remaining on the outer peripheral surface. 22 is arranged.

  An endless intermediate transfer belt 24 is disposed below the photosensitive drum 12. The intermediate transfer belt 24 is wound around rollers 26, 28, and 30, and is disposed so that the outer peripheral surface is in contact with the outer peripheral surface of the photosensitive drum 12. Any one of the rollers 26, 28, and 30 is rotated by a driving force of a motor (not shown) and rotates the intermediate transfer belt 24 in the direction of arrow B in FIG.

  A transfer device 32 is disposed on the opposite side of the photosensitive drum 12 with the intermediate transfer belt 24 interposed therebetween. The toner image formed on the outer peripheral surface of the photosensitive drum 12 is transferred to the image forming surface of the intermediate transfer belt 24 by the transfer device 32. When the toner image formed on the outer peripheral surface of the photoconductive drum 12 is transferred to the intermediate transfer belt 24, the area carrying the transferred toner image on the outer peripheral surface of the photoconductive drum 12 is eliminated / cleaned. The container 22 is cleaned.

  The full-color image is formed by the image forming apparatus 10 according to the present embodiment while the intermediate transfer belt 24 rotates four times. In other words, while the photosensitive drum 12 rotates, the charger 14 charges the outer peripheral surface of the photosensitive drum 12, and the charge removal / cleaning device 22 continues to discharge the outer peripheral surface. The optical scanning device 16 scans the outer peripheral surface of the photosensitive drum 12 with a laser beam modulated according to any of Y, M, C, and K image data representing a color image to be formed. Further, the developing device 18 operates the developing unit corresponding to the outer peripheral surface in a state where any of the developing rollers 20 of the developing units 18Y, 18M, 18C, and 18K corresponds to the outer peripheral surface of the photosensitive drum 12. The electrostatic latent image formed on the outer peripheral surface of the photosensitive drum 12 is developed into a specific color, and a toner image of a specific color is formed on the outer peripheral surface of the photosensitive drum 12, and then the toner image on the intermediate transfer belt 24. Transcript. Each time the intermediate transfer belt 24 makes one rotation, this operation is repeated while switching the image data used for modulation of the laser beam and rotating the container so that the developing device used for developing the electrostatic latent image is switched.

  Thus, every time the intermediate transfer belt 24 makes one rotation, toner images of Y, M, C, and K are sequentially formed on the outer peripheral surface of the intermediate transfer belt 24 so as to overlap each other. When the belt 24 rotates four times, a full color toner image is formed on the outer peripheral surface of the intermediate transfer belt 24.

  A tray 34 is disposed below the intermediate transfer belt 24, and a large number of sheets P as recording materials are accommodated in the tray 34 in a stacked state. In FIG. 1, a take-out roller 36 is disposed obliquely above and to the left of the tray 34, and a roller pair 38 and a roller 40 are sequentially arranged on the downstream side in the take-out direction of the paper P by the take-out roller 36. The uppermost recording paper in the stacked state is taken out of the tray 34 by the rotation of the take-out roller 36 and is conveyed by the roller pair 38 and the roller 40.

  A transfer device 42 is disposed on the opposite side of the roller 30 with the intermediate transfer belt 24 in between. The paper P conveyed by the roller pair 38 and the roller 40 is sent between the intermediate transfer belt 24 and the transfer device 42, and the toner image formed on the image forming surface of the intermediate transfer belt 24 is transferred by the transfer device 42. . A fixing device 44 having a pair of fixing rollers is arranged downstream of the transfer device 42 in the conveyance direction of the paper P. The paper P on which the toner image is transferred is transferred to the paper P by the fixing device 44. After being fused and fixed, the sheet is discharged out of the image forming apparatus 10 and placed on a discharge tray (not shown).

  FIG. 2 shows an optical scanning device 16 according to the first embodiment. The optical scanning device 16 includes a surface emitting laser array 50 that emits m (m is at least 2) laser beams. 2 shows only three laser beams for simplification, the surface emitting laser array 50 formed by arraying surface emitting lasers should be configured to emit several tens of laser beams. Further, the arrangement of the surface emitting lasers (arrangement of the laser beams emitted from the surface emitting laser array 50) may be arranged two-dimensionally (for example, in a matrix) in addition to the arrangement in one column. Is possible. The light source unit according to the present invention capable of emitting a plurality of laser beams is not limited to such a surface emitting laser array, but in a surface emitting laser array, since the degree of freedom of arrangement of a plurality of light emitting points is high, preferable.

  On the laser beam emission side of the surface emitting laser array 50, a collimator lens 52 and a half mirror 54 are arranged in this order. The laser beam emitted from the surface emitting laser array 50 is made into a substantially parallel light beam by the collimator lens 52 and then incident on the half mirror 54, and a part of the laser beam is separated and reflected by the half mirror 54. A lens 56 and a light amount sensor 58 are arranged in this order on the laser beam reflecting side of the half mirror 54, and a part of the laser beam separated and reflected from the main laser beam (laser beam used for exposure) by the half mirror 54 is The light passes through the lens 56 and enters the light quantity sensor 58, and the light quantity is detected by the light quantity sensor 58.

  In addition, since the surface emitting laser does not emit a laser beam from the side opposite to the side from which the laser beam used for exposure is emitted (in the case of an edge emitting laser, it is emitted from both sides). Therefore, it is necessary to separate a part of the laser beam used for exposure as described above and use it for light quantity detection.

  An aperture 60, a cylinder lens 62 having power only in the sub-scanning direction, and a folding mirror 64 are arranged in this order on the main laser beam emission side of the half mirror 54, and the main laser beam emitted from the half mirror 54 is emitted from the aperture 60. Then, the light is refracted by the cylinder lens 62 so as to form a long linear image in the main scanning direction in the vicinity of the reflection surface of the rotary polygon mirror 66, and reflected by the folding mirror 64 to the rotary polygon mirror 66 side. Note that the aperture 60 is desirably disposed in the vicinity of the focal position of the collimating lens 52 in order to uniformly shape a plurality of laser beams.

  The rotating polygonal mirror 66 is rotated in the direction of arrow C in FIG. 4 by transmitting a driving force of a motor (not shown), and deflects and reflects the incident laser beam reflected by the folding mirror 64 along the main scanning direction. Fθ lenses 68 and 70 having power only in the main scanning direction are arranged on the laser beam emission side of the rotary polygon mirror 66, and the laser beam deflected and reflected by the rotary polygon mirror 66 is the outer periphery of the photosensitive drum 12. It is refracted by the Fθ lenses 68 and 70 so as to move on the surface at a substantially constant speed and so that the image forming position in the main scanning direction coincides with the outer peripheral surface of the photosensitive drum 12.

  Cylinder mirrors 72 and 74 having power only in the sub-scanning direction are sequentially arranged on the laser beam emission side of the Fθ lenses 68 and 70, and the laser beams transmitted through the Fθ lenses 68 and 70 are connected in the sub-scanning direction. The image position is reflected by the cylinder mirrors 72 and 74 so as to coincide with the outer peripheral surface of the photosensitive drum 12, and is irradiated on the outer peripheral surface of the photosensitive drum 12. The cylinder mirrors 72 and 74 also have a surface tilt correction function that conjugates the outer peripheral surfaces of the rotary polygon mirror 66 and the photosensitive drum 12 in the sub-scanning direction.

  A pickup mirror 76 is disposed on the laser beam emission side of the cylinder mirror 72 at a position corresponding to an end (SOS: Start Of Scan) of the scanning range of the laser beam. A beam position detection sensor 78 is arranged on the laser beam emission side. When the laser beam emitted from the surface emitting laser array 50 reflects the incident beam in a direction corresponding to the SOS, the reflecting surface of the rotary polygon mirror 66 reflects the laser beam. Then, the light is reflected by the pickup mirror 76 and enters the beam position detection sensor 78.

  The signal output from the beam position detection sensor 78 modulates the laser beam scanned on the outer peripheral surface of the photosensitive drum 12 as the rotary polygon mirror 66 rotates to form an electrostatic latent image each time. Used to synchronize the modulation start timing in main scanning.

  Further, in the optical scanning device 16 according to the present embodiment, the collimating lens 52, the cylinder lens 62, and the two cylinder mirrors 72, 74 are arranged so as to be afocal in the sub-scanning direction. As described in Japanese Patent Application No. 2000-28462 already filed by the applicant of the present application, the difference in the scanning line curvature (BOW) of the plurality of laser beams and the variation in the scanning line interval due to the plurality of laser beams are described. It is for suppressing.

 Here, as in the present embodiment, when a plurality of laser beams are simultaneously scanned and exposed on the outer peripheral surface of the photosensitive drum 12 to form a latent image, as shown in FIG. Since the laser beam is irradiated in the vicinity of the boundary of the scanning region in each scanning at a time, density unevenness on the stripe may occur near the boundary.

  In order to reduce density unevenness due to this phenomenon, for example, as shown in FIG. 4, the movement amount of the photosensitive drum 12 in the sub-scanning direction is set to half of the scanning area by one scanning (that is, the scanning lines of 24 lines). A scanning method for reducing density unevenness in the boundary region by superimposing half of the scanning lines in the Nth scanning and N + 1th scanning can be considered. However, since each scanning line is formed by a plurality of laser beams by the adjacent surface of the rotary polygon mirror 66, when an image formation position shift occurs due to jitter, the entire simultaneously scanned region is slightly different from the scanning region by the adjacent surface, The density is lowered only in the scanning region where the exposure distribution is broken and the imaging position is shifted. This density drop occurs easily in one rotation cycle of the rotary polygon mirror, so it is easy to see. For example, when six scanning surfaces of a rotating polygon mirror are used at a scanning density of 1200 dpi and 24 beams are scanned simultaneously, a density variation of 0.5 mm width occurs at a cycle of 1.5 mm. In the example shown in FIG. 4, since the image formation position shift due to jitter occurs in the exposure image obtained by scanning the second surface, the finally obtained toner image has an area corresponding to this (having a width that is easily visible). Area) is uneven density.

  On the other hand, in the present embodiment, in order to suppress such density unevenness, a plurality of laser beams are arranged at unequal intervals in the sub-scanning direction on the surface to be scanned. That is, in the laser beam scanning according to the present embodiment, a plurality of (three or more) deflection scans are performed on the scanning area to be scanned simultaneously, and this scanning area is overlapped with a plurality of imaging spots having equal intervals. It fills with the scanning line by.

  FIG. 3 schematically shows an imaging spot formed by one deflection scanning in one row. 3 and the subsequent drawings, the numerical value on the left side of each imaging spot indicates (the number of deflections, the imaging spot number). For example, in the first scan, the third imaging in the sub-scanning direction is performed. The spot is expressed as (1,3). In addition, the number of each scanning line is displayed as Ln for the nth as necessary.

  As shown here, in this embodiment, one imaging spot group is constituted by six imaging spots that are continuous in the sub-scanning direction on the surface to be scanned. Four groups are arranged at intervals of the scanning line. For example, when focusing on the imaging spot on the first surface, a series of imaging spots are arranged from (1, 1) to (1, 6) to form one imaging spot group, from (1, 6) to (1 , 7), three scanning line intervals (L7 to L9) are provided. The same interval is repeated with the same period from (1, 7) to (1, 12). Further, the second and subsequent imaging spots are arranged in the same manner. However, it is not always necessary to arrange them without deviation in the main scanning direction, and even when each imaging spot is imaged at a different position in the main scanning direction, the writing timing from the synchronization signal is set for each spot. As a result, a latent image matching the main scanning direction is obtained on the image.

  At this time, the amount of movement of the surface to be scanned in the sub-scanning direction is ½ of the number of imaging spots, that is, an interval of 12 lines. Due to the arrangement of the imaging spots and the amount of movement of the surface to be scanned, each scanning line by superimposing two imaging spots is equidistant from a rotating polygonal mirror having six deflection surfaces by deflection scanning for five surfaces. Will be formed.

  As a result, when each main scanning region is viewed in the sub-scanning direction, it is divided into a plurality of regions, so that an imaging position shift due to jitter or the like occurs in the imaging spot in a specific main scanning. However, the density variation of the toner image due to this deviation can be set to a period that is difficult to visually recognize. Also, this can be suppressed with respect to density unevenness that occurs at the boundary of the scanning region. Specifically, in the present embodiment, assuming that there is an imaging position shift due to jitter in the scanning line on the third surface as shown in FIG. As a result, the density fluctuation period is 0.19 mm pitch for 9 lines in a 1200 dpi period, and 5 line / mm in the spatial frequency, and the contrast sensitivity is lowered, so that it is difficult to visually recognize and high-quality image formation is possible. Become.

  In the present embodiment, each of the plurality of scanning lines formed by one deflection scanning is finally overlapped with scanning lines obtained by a plurality of different deflection scannings to form a desired scanning line. For example, in the example shown in FIG. 3, focusing on the scanning line deflected and scanned on the third surface, (3, 1) (3, 2) (3, 3) (3, 7) (3, 8) (3, 9 ) (3, 13), (3, 14) and (3, 15) are image spots that are deflected and scanned on the second surface (2, 10) (2, 11) (2, 12). It overlaps with the imaging spots of (2, 16) (2, 17) (2, 18) (2, 22) (2, 23) (2, 24). Hereinafter, similarly, the imaging spots (3, 4) (3, 5) (3, 6) are deflected and scanned on the first surface (1, 19) (1, 20) (1, 21) imaging spots. (3, 10) (3, 11) (3, 12) (3, 16) (3, 17) (3, 18) (3, 22) (3, 23) (3, 24) The image spots are each deflected and scanned on the fourth surface (4,1) (4,2) (4,3) (4,7) (4,8) (4,9) (4,13) (4 , 14) (4, 15) and the imaging spots (3, 19) (3, 20) (3, 21) are deflected and scanned on the fifth surface (5, 4) ( 5, 5) It overlaps with the imaging spot of (5, 6). Therefore, the imaging spot deflected on the third surface overlaps with the imaging spot deflected and scanned on the first surface, the second surface, the fourth surface, or the fifth surface.

  As a result, not only the above-mentioned jitter but also characteristic variation (for example, focus difference due to surface accuracy) due to the rotary polygon mirror 66 occurs on multiple surfaces, density fluctuation due to the error is dispersed, and high-quality image formation is possible It is.

  Thus, in order for all scanning lines to be scanned at a predetermined number of times of overwriting, the remainder obtained by dividing the line numbers L1 to Ln by the number of imaging spots is a natural number less than n / m, And it is necessary and sufficient condition to appear m times. That is, in this embodiment, for example, the number of overwriting is 2 and the number of imaging spots is 24, and the quotient obtained by dividing the number of imaging spots by the number of overwriting lasers is 12. The remainder obtained by dividing the line numbers L1 to Ln by the quotient is 1, 2, 3, 4, 5, 6, 10, 11, 0, 1, 2, 3, 7, 8, 9 for each imaging spot number. , 10, 11, 0, 4, 5, 6, 7, 8, 9 and the natural number of less than 12 appears twice, so that the scanning region can be scanned with the same scanning line interval.

  In addition, the amount of light per laser beam can be reduced by forming scanning lines by overwriting a plurality of spots as in the present embodiment. For example, even a surface emitting laser that is difficult to increase in output can be used within the output range to form a desired scanning line.

  In addition, as described above, in order for each of the plurality of scanning lines formed by one deflection scanning to be finally overlapped with a scanning line by a plurality of different deflection scannings, each spot is not necessarily provided. The group spot images do not have to be adjacent in the sub-scanning direction on the surface to be scanned, but the adjacent configuration as in this embodiment reduces the amount of buffer memory for obtaining the final exposure image. Thus, interlaced scanning can be performed.

  Next, a second embodiment of the present invention will be described. In the second embodiment, the basic configurations of the optical scanning device and the image forming apparatus are substantially the same as those of the first embodiment, and thus detailed description thereof is omitted.

  As shown in FIG. 5, in this embodiment, the number of deflection surfaces of the rotary polygon mirror is 6, the number of imaging spots is 4, and 4 scans with a scan line interval of 3 lines by one deflection scan. A line is formed. The amount of movement of the photosensitive drum 12 in the sub-scanning direction is two lines of 4/2 = 2 per scan.

  FIG. 5 shows scanning by imaging spots that are also separated in the main scanning direction. In this respect, unlike the first embodiment, the imaging spot group consisting of imaging spots that are continuous in the main scanning direction is configured. It has not been. This is an example of an imaging spot array when a surface emitting laser in which light emitting points are two-dimensionally arranged is used for a semiconductor laser array. Since the surface emitting laser emits the laser in the direction perpendicular to the bonding direction of the semiconductor elements, it is easy to make a multi-beam, and it can be arranged two-dimensionally as shown. By arranging in two dimensions, the interval in the sub-scanning direction can be made narrower than the actual interval between light emitting points, and the degree of freedom of the optical system increases.

  In the present embodiment, focusing on the scanning line that performs deflection scanning on the fifth surface, the imaging spot of (5, 1) is the imaging spot of (1, 4) and the imaging spot of (5, 2) is (4 , 3), (5, 3) image spot, (6, 2) image spot, (5, 4) image spot, (3, 1) image spot. It is superimposed. That is, a scanning line is formed by superimposing all the imaging spots formed by the deflection scanning on the fifth surface with the imaging spot spots formed by the four deflection surfaces different from the first surface, the fourth surface, the sixth surface, and the third surface. ing. With this configuration, each scanning line is formed by an imaging spot formed by deflection scanning on two different surfaces. That is, L1 is 2 and 3, L2 is 1 and 5; L3 is 3 and 4; L4 is 2 and 6; L5 is 4 and 5; L6 is 3 and 1; L7 is deflected by 5 and 6 surfaces, L8 is deflected by 4 and 2 surfaces, and L9 is deflected by 6 and 1 surfaces. Accordingly, since the scanning lines of the same surface combination do not adjoin and the combinations change discretely, even if there is a characteristic error due to variations in the reflecting surface of the rotary polygon mirror 12, it can be reduced by overlapping the imaging spots. Become.

  Next, a third embodiment of the present invention will be described. Also in the third embodiment, the basic configurations of the optical scanning device and the image forming apparatus are substantially the same as those of the first embodiment, and thus detailed description thereof is omitted.

  In the third embodiment, the number of deflection surfaces of the rotary polygon mirror 66 is six. In addition, as shown in FIG. 6, one imaging spot group is constituted by three imaging spots, and a total of 15 imaging is performed for five spot groups having a uniform interval (for four lines) in the sub-scanning direction. It consists of the number of spots. The number of imaging spots (number of overwriting) forming one scanning line is 3, and the amount of movement of the photosensitive drum 12 in the sub-scanning direction is 5 lines per scan.

  Here, in the adjacent spot group, the distance between the head imaging spots is L, and L is the number of spot groups r and the natural number q having the greatest common divisor of 1, and the number of imaging spots c in the spot group is overlapped. It is an amount obtained by multiplying the number divided by the total number m and the scanning line interval p. By setting such a distance, it is possible to scan the scanning region with the same scanning line interval. In the present embodiment, the quotient of the imaging spot number c and the overwriting number m in one spot group is a natural number, which is a condition for all the scanning lines to be overwritten and exposed.

  In the present embodiment, the natural number q is set to 7. Then, the number of spots in the spot group is 3, the number of overwriting is 3, and the L between the first imaging spots of adjacent spot groups is 7 lines, so that the three imaging spots are overlapped in the entire scanning region. A plurality of scanning lines having the same scanning line interval are formed.

  In addition, by setting the number of overwriting to 3 or more as in the present embodiment, it is possible to eliminate an allowable value relating to the scanning position shift of the imaging spot, and it is possible to relax the required accuracy for parts.

  Next, a fourth embodiment of the present invention will be described. Also in the fourth embodiment, the basic configurations of the optical scanning device and the image forming apparatus are substantially the same as those of the first embodiment, and thus detailed description thereof is omitted.

  FIG. 8 shows an example of an imaging spot and a scanning line in the conventional configuration as a comparative example. In this comparative example, the number of imaging spots is 4, and the amount of movement of the photosensitive drum per scan is 4 lines. In this case, consider a case where there is an error in the imaging magnification in the sub-scanning direction. In the surface emitting laser array 50, the intervals in the sub-scanning direction of the light emitting points are 32 μm for S1 and S3, and the interval in the sub-scanning direction is 3.2 μm by arranging S1 and S2 in two dimensions. When scanning with a scanning density of 600 dpi using this light source (surface emitting laser array), a scanning line interval of 42.3 μm can be realized on the scanning surface by setting the imaging magnification M to 13.23 times. . Here, if there is an error of 1% in the imaging magnification of the optical system, the scanning line interval between the imaging spot S1 and the imaging spot S2 on the scanning surface is 42.75 μm, which is 0 with respect to the ideal interval. Whereas only .4 um is different, the interval between the imaging spot S1 and the imaging spot S3 on the surface to be scanned has a deviation (interval error) of 427.57 um and 4.23 um. A shift of about 10% occurs with respect to the scanning line interval of 42.3 um, and the influence of the number of scanning lines separated from each other occurs.

  When a scanning line is formed in such a state with a gap error, as shown in FIG. 8, exposure unevenness occurs between adjacent scanning lines in the exposure image, and the final toner image has a high density and a low density. So-called banding in which the density occurs periodically occurs.

  With respect to such a problem, as in the example shown in FIG. 7, in this embodiment, the number of laser beams is set to 8, and the amount of movement of the photosensitive drum 12 per scan is ½ of the number of spots. As a line, two overwriting exposures are performed. When there is an imaging magnification error, the spacing error between the imaging spot S1 and the imaging spot S5 is 4.23 um as in the conventional example, but the r2 spot group having a large spot spacing error is the r1 spot due to a different deflection surface. The spot images constituting the group are overwritten and exposed. For example, focusing on the second spot group on the second surface, the imaging spots (2, 5) (2, 6) are the imaging spots (4, 3) (4, 4) on the fourth surface, and the imaging spots (2 , 7) (2, 8) is overwritten with the imaging spot (5, 1) (5, 2) on the fifth surface. These superimposed image spots (4, 3), (4, 4) and image spots (5, 1), (5, 2) constitute the first spot group scanned simultaneously. Yes.

  As a result, the exposure images have the same exposure distribution regardless of the position, and a uniform density distribution without density unevenness can be obtained even in the toner image.

  In this embodiment, the amount of movement of the scanned surface per scan is n / m, and the above-described condition, that is, the remainder obtained by dividing each line number L1 to Ln by the number of imaging spots is a natural number less than n / m. And the condition of appearing m times.

  In adjacent spot groups, L is the distance between the first imaging spots, and L is the number of spot groups r and the natural number q with the greatest common divisor being 1, and the number of imaging spots c in the spot group is the number of laser overlays. It is an amount obtained by multiplying the number divided by m and the scanning line interval p, and the quotient of the imaging spot number c in the spot group and the overwriting number m satisfies the condition of a natural number.

Next, reference examples of the present invention will be described. Also in the reference example , the basic configurations of the optical scanning device and the image forming apparatus are substantially the same as those in the first embodiment, and thus detailed description thereof is omitted.

In the reference example , an example in which a rotating polygon mirror 66 having six deflection surfaces is used and scanning is performed with 24 beams is given. That is, as shown in FIG. 9, 24 scanning lines are formed by one deflection scanning, and by moving the surface to be scanned, the image forming spot is sub-scanned by 6 scanning lines in the next scanning of the deflection surface. Scanned to the position moved in the direction. In the reference example , a plurality of scanning lines (24 in the present embodiment) formed by one deflection scanning are adjacent to each other without being separated in the sub-scanning direction. This is different from the fourth embodiment.

  By repeating such scanning, one scanning line is formed by four times of superposition by deflection scanning for the four deflection surfaces of the rotary polygon mirror 66. For example, paying attention to the (2,19) line in the scanning line formed by the deflection scanning on the second surface, (3,13) on the third surface, (4,7) on the fourth surface, (5,1) on the fifth surface. ), That is, three different scanning lines overlap.

  By configuring in this way, even if there is a positional deviation accompanying scanning (for example, jitter due to surface accuracy, pitch unevenness due to tilt correction error, pitch error of multiple beam intervals), image quality deterioration due to the influence can be suppressed. it can.

  FIG. 9 shows an example in which there is a scanning position shift due to jitter due to the surface accuracy of the deflection surface during the deflection scanning of the second surface. The effect of this embodiment will be described with reference to FIG. 10 in comparison with the conventional example.

  FIG. 10 shows the beam intensity distribution on the surface to be scanned, FIG. 10 (A) shows the intensity distribution of single dots without overlay, and FIG. 10 (B) shows one dot. This is a case where it is formed by two overwrites of b1 and b2. FIG. 10C shows the case of this embodiment in which one dot is formed by four times overwriting of c1 to c4.

  In the case of the two-time overwriting shown in FIG. 10B, b1 and b2 are originally drawn at the same position, and the combined intensity distribution is the same as a1. However, assuming that the beam is shifted in the main scanning direction due to jitter when scanning b2, for example, the combined intensity distribution spreads in the main scanning direction, the peak intensity decreases, and the center of gravity position is indicated by a thick one-dot chain line B Shift to position. In the case of this conventional example, if b2 shifts by 1 dot with jitter equivalent to 1 dot, the center of gravity shifts by 1/2 dot, and the image quality deteriorates such as the wobbling of fine lines and the disturbance of the screen structure.

  On the other hand, in this embodiment, since one dot is formed by overwriting four times, even if jitter occurs in one scan, the influence is suppressed to ¼. That is, as shown in FIG. 10 (C), for example, even if there is a jitter equivalent to 1 dot in c2, the influence is reduced to 1/4 because of the overwriting of 4 times. It is only necessary to change the position of the center of gravity. In addition, the reduction in the peak intensity of the combined intensity distribution is reduced, and the latent image potential intensity by the exposed image can be maintained large. For this reason, the electric field strength is increased, and a decrease in toner developability can be suppressed. For example, even if irregular exposure position shifts due to vibration or the like overlap, the influence can be reduced.

  In this embodiment, the case where a positional deviation occurs in the main scanning direction has been described as an example. However, the same applies to a positional deviation in the sub-scanning direction, that is, pitch unevenness due to a tilt correction error and a plurality of beam interval errors. Has an effect.

  In this way, a plurality of scanning lines formed by one deflection scan are overlapped by deflection scanning of three or more different surfaces of the rotary polygon mirror, thereby suppressing the influence due to the error of the scanning device and reducing the influence of disturbance. It becomes difficult to receive and it is possible to obtain a good print.

1 is a schematic front view illustrating an overall configuration of an image forming apparatus according to a first embodiment of the present invention. 1 is a schematic perspective view illustrating an optical scanning device of an image forming apparatus according to a first embodiment of the present invention. It is explanatory drawing which shows the effect | action of the optical scanning device of 1st Embodiment of this invention. It is explanatory drawing which shows the principle which a density nonuniformity produces in the past for the comparison with the optical scanning device of 1st Embodiment of this invention. It is explanatory drawing which shows the effect | action of the optical scanning device of 2nd Embodiment of this invention. It is explanatory drawing which shows the effect | action of the optical scanning device of 3rd Embodiment of this invention. It is explanatory drawing which shows the effect | action of the optical scanning device of 4th Embodiment of this invention. For comparison to the optical scanning device of the fourth embodiment of the present invention, is an explanatory diagram showing the principle of density unevenness occurs in the prior art. It is explanatory drawing which shows the effect | action of the optical scanning device of the reference example of this invention. It is a graph which shows intensity distribution of the laser beam on a to-be-scanned surface, (A) is a case where there is no superposition, (B) is a case where two times of overwriting are performed, and (C) is a case where four times of overwriting are performed. This is a case of a reference example to be performed. It is explanatory drawing which shows the principle which density characteristic change generate | occur | produces in the conventional optical scanning device. It is a graph which shows the relationship between the position of the subscanning direction in a conventional optical scanning device, and exposure energy. It is explanatory drawing which shows the principle which a stripe-shaped density nonuniformity generate | occur | produces in the conventional optical scanning device. It is a graph which shows the relationship between the position of the main scanning direction and the beam intensity in the conventional optical scanning device. It is explanatory drawing which shows the principle which a stripe-shaped density nonuniformity generate | occur | produces in the conventional optical scanning device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Photosensitive drum 14 Charging device 16 Optical scanning device 18 Developing device 18Y, 18M, 18C, 18K Developing device 20 Developing roller 22 Static elimination / cleaning device 24 Intermediate transfer belt 26 Roller 26, 28, 30 Roller 30 Roller 32 Transfer device 34 Tray 36 Roller 38 Roller pair 40 Roller 42 Transfer device 44 Fixing device 50 Surface emitting laser array 52 Collimating lens 54 Half mirror 56 Lens 58 Light quantity sensor 60 Aperture 62 Cylinder lens 64 Mirror 66 Rotating polygon mirror 68, 70 Fθ lens 72 Cylinder mirrors 72 and 74 Cylinder mirror 76 Pickup mirror 78 Beam position detection sensor P Paper
c Number of imaging spots L Distance between imaging spots c Number of imaging spots m Number of superposed laser beams p Scanning line spacing r Number of spot groups

Claims (8)

A light source unit having a plurality of light emitting points;
A pre-deflection optical system on which a plurality of laser beams emitted from a plurality of light emitting points of the light source unit are incident;
A deflecting unit that has a rotating polygon mirror that rotates at a constant period and deflects a plurality of laser beams that have passed through the pre-deflection optical system in a main scanning direction;
An image carrier that is movable in a sub-scanning direction orthogonal to the main scanning direction;
A scanning optical system that forms an image of the laser beam deflected by the deflecting unit on a surface to be scanned on the image carrier;
With
A scanning line formed on the surface to be scanned is formed by overlapping imaging spots by a plurality of deflections, and
The plurality of laser beams are arranged so as to have unequal intervals in the sub-scanning direction on the scanned surface,
The scanning line is scanned in the main scanning direction by the deflection of the deflection means of the imaging spot formed on the scanned surface, and then the image carrier is moved in the sub-scanning direction, by repeating the above two or three imaging superposition of spots by the rotating polygon mirror of different three sides or more deflection is formed on the surface to be scanned, the optical scanning device, characterized in that. A light source unit having a plurality of light emitting points;
A pre-deflection optical system on which a plurality of laser beams emitted from a plurality of light emitting points of the light source unit are incident;
A deflecting unit that has a rotating polygon mirror that rotates at a constant period and deflects a plurality of laser beams that have passed through the pre-deflection optical system in a main scanning direction;
An image carrier that is movable in a sub-scanning direction orthogonal to the main scanning direction;
A scanning optical system that forms an image of the laser beam deflected by the deflecting unit on a surface to be scanned on the image carrier;
With
The scanning line formed on the surface to be scanned is formed by overlapping imaging spots by a plurality of deflections, and
The number of laser beams is n, the number of laser beams to be overlapped when forming the same scanning line is m, and the line numbers from the head of the plurality of scanning lines formed by one deflection to each scanning line are L2 to An optical scanning device, wherein when Ln is taken, a remainder obtained by dividing L2 to Ln by (n / m) is a natural number less than n / m and appears m times. A light source unit having a plurality of light emitting points;
A pre-deflection optical system on which a plurality of laser beams emitted from a plurality of light emitting points of the light source unit are incident;
A deflecting unit that has a rotating polygon mirror that rotates at a constant period and deflects a plurality of laser beams that have passed through the pre-deflection optical system in a main scanning direction;
A scanning optical system that forms an image of the laser beam deflected by the deflecting unit on a surface to be scanned;
An image carrier that is movable in a sub-scanning direction orthogonal to the main scanning direction and is exposed by the laser beam;
With
The scanning line formed on the surface to be scanned is formed by overlapping imaging spots by a plurality of deflections, and
When the number of spot groups forming adjacent scanning lines among a plurality of scanning lines formed by one deflection is r and the number of spots in the spot group is c, The distance L between the first imaging spots is obtained by dividing the number of spot groups r and the natural number q having the greatest common divisor of 1 by the number of imaging spots c in the spot group divided by the number of superpositions m, and the scanning line interval p. An optical scanning device characterized by being formed on the surface to be scanned so as to be a multiplied distance and the quotient of the number c of imaging spots in the spot group and the number m of overwritten lasers is a natural number.   4. The optical scanning device according to claim 1, wherein the light source unit is a surface emitting laser in which a plurality of light emitting points are two-dimensionally arranged. 5.   Among the plurality of light emitting points of the surface emitting laser, at least a part of the light emitting points arranged in the main scanning corresponding direction corresponding to the main scanning direction of the laser beam has a laser beam from the light emitting point on the surface to be scanned. The optical scanning device according to claim 4, wherein the optical scanning device is arranged so as to form scanning lines adjacent in the scanning direction.   The amount of movement of the image carrier in the sub-scanning direction per laser beam scanning period is divided by the number m of laser beams to be overlapped when forming the same scanning line at the scanning line interval p. 6. The optical scanning device according to claim 1, which is an amount obtained by multiplying the calculated number (n / m).   The laser beams from the plurality of light emitting points are scanned at predetermined intervals in the sub-scanning direction on the surface to be scanned, and an exposure image having a constant pitch is formed by a plurality of scans. The optical scanning device according to claim 1. The optical scanning device according to any one of claims 1 to 7,
Charging means for charging the image carrier;
Developing means for developing a latent electrostatic image formed on the image carrier by exposure of a laser beam from the optical scanning device to form a toner image;
Transfer means for transferring the toner image to a recording medium;
Fixing means for fixing the toner image transferred to the recording medium by the transfer means;
An image forming apparatus comprising:

Images