JP4172191B2 - Optical scanning device, method for manufacturing folding mirror used in optical scanning device, and color image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical scanning device used in an electrophotographic color printer, a color copying machine, and the like, a method of manufacturing a folding mirror used in the optical scanning device, and a color image forming apparatus, and more specifically, a tandem color image. The present invention relates to an optical scanning device that scans a plurality of laser beams with a single deflector in an optical scanning device used in the forming apparatus, a method of manufacturing a folding mirror used in the optical scanning device, and a color image forming apparatus.
[0002]
[Prior art]
A color image forming apparatus such as a color printer or a color copying machine that forms a full color image by electrophotography is four times with yellow (Y), magenta (M), cyan (C), and black (K) toners. In many cases, a method called “4-cycle” is used to form a full-color image in the image forming cycle. This is because the configuration can be shared with the widely used monochrome (single color) image forming apparatus, but the same cycle is repeated four times, so that the image output speed of the apparatus is ¼ that of the monochrome image forming apparatus. There is a drawback that it is reduced.
[0003]
In contrast, four image forming units with charging, exposure, development, and transfer functions are arranged in parallel, and Y, M, C, and K image formation is performed continuously, and a full-color image is equivalent to a monochrome image. An image forming apparatus called a “tandem system” that forms images at a high speed has been developed. However, since the tandem color image forming apparatus developed in the early stage is large in size and expensive, it is widely used only in some specific markets where priority is given to speed.
[0004]
However, with the spread of the Internet and digital cameras in recent years, color manuscripts and color images have been handled normally in general offices, and today's document colorization has become a reality. The output speed of this type of color image forming apparatus has become a stress in daily office work. However, since the conventional tandem color image forming apparatus is large and expensive, it is a color image forming apparatus that can be used in a general office, that is, a full-color image that can be installed in various places in the office as well as an installation space equivalent to a monochrome image forming apparatus Forming devices have become highly desirable.
[0005]
As an apparatus configuration that meets such requirements, there is an image forming apparatus described in Japanese Patent Application Laid-Open No. 2001-264655 by the present applicant. FIG. 9 is a configuration diagram of a color image forming apparatus that is miniaturized. In the illustrated color image forming apparatus 100, a single optical scanning device 104 is arranged below the four photoconductors 102Y, 102M, 102C, and 102K arranged in parallel.
[0006]
The point of downsizing the optical scanning device is to unify the deflector (shared with four beams) and to share a plurality of beams in the optical scanning device. Two laser beams are scanned in both directions by a single optical scanning device, and each two laser beams are incident on the polygon mirror at different angles in the sub-scanning direction. With such a configuration, four laser beams can be scanned by a thin single polygon mirror and two sets of imaging lenses mounted on a single deflector. In addition, since each two laser beams are deflected with an angle difference, the gap between the two beams gradually widens, and the laser beam is reflected from the same deflection surface of the polygon mirror by selectively folding back the laser beam with a plane mirror. Laser beams can be directed onto different photoreceptors.
[0007]
Other examples of using a single deflector to reduce the size of the optical scanning device include Japanese Patent Application Laid-Open Nos. 9-179046, 2001-4948, and 2001-108923. However, although the optical scanning device described in Japanese Patent Laid-Open No. 9-179046 uses a single deflector and one set of imaging lens, there is a problem that the lens becomes large. In addition, since the optical scanning devices described in Japanese Patent Application Laid-Open Nos. 2001-4948 and 2001-108923 make a plurality of parallel laser beams incident on the polygon mirror, the polygon mirror becomes thicker or the imaging lens is used. There is a problem that it becomes necessary or thick for each beam.
[0008]
On the other hand, the method in which a plurality of laser beams are incident on a single deflection surface of a polygon mirror with an angle difference has a feature that multiple beam scanning can be realized without increasing the size of the components. There is a problem of bending. This is because when the polygon mirror rotates with respect to the laser beam incident at an angle from the fixed direction to the sub-scanning direction, the incident angle changes continuously. This is for drawing a scanning line locus that warps in a direction away from the deflection point.
[0009]
Since the amount of bending of the scanning line increases according to the angle formed by the deflected beam in the sub-scanning direction with respect to the plane orthogonal to the rotation axis of the deflector, the amount of bending of the two laser beams on one side having an angle difference differs. It will be a thing.
[0010]
FIG. 10 is a diagram illustrating the internal configuration of the optical scanning device and the scanning line curve for realizing downsizing of the image forming apparatus. In the case of the configuration shown in FIG. 10, the laser beams 106Y, 106M, 106C, and 106K that are scanned in both directions are curved in the opposite directions across the deflector, so that the four laser beams are scanned. The scanning lines 110Y, 110M, 110C, and 110K according to the above all have different curved shapes including directions and amounts.
[0011]
In full-color images, the color registration, called color registration, greatly affects the image quality. Therefore, even if the image signal that should be overlapped as the same line is sent from the contrast roller to the image forming device, the scanning line is based on the optical characteristics of the optical scanning device. If there is a curvature, a registration error occurs and a significant deterioration in image quality occurs. In order to solve this problem, it is necessary to implement a function of adjusting the curvature of the scanning line by some method. As a method of adjusting the scanning line curve with a mechanical mechanism, a method of adjusting the posture and shape of the optical component to generate a curve in the direction opposite to the incident scanning line is conventionally used. For example, Japanese Patent Laid-Open No. 2001-228427 Japanese Patent Laid-Open No. 2000-180778.
[0012]
FIG. 11 is a diagram illustrating an example of a mirror curvature adjustment mechanism described in Japanese Patent Laid-Open No. 2001-228427. An adjusting mechanism for bending the mirror is incorporated in the support units 114A and 114B that support the vicinity of both ends of the flat mirror 112. When the adjusting screw 116 is screwed, the flat mirror 112 is bent along the main scanning direction, and the scanning line is scanned. Can be adjusted from the curved state 118A to the uncurved state 118B.
[0013]
When applied to the configuration shown in FIG. 10, the scanning line curvature is adjusted by independently adjusting the folding mirrors 120Y, 120M, 120C, and 120K corresponding to only a single laser beam after each laser beam is divided into optical paths. Thus, the four scanning lines 110Y, 110M, 110C, and 110K can be overlapped.
[0014]
FIG. 12 is a diagram for explaining a mechanism for correcting the scanning line curve by bending the plane mirror in the main scanning direction (see Japanese Patent Application Laid-Open No. 2000-180778). When the plane mirror 122 is curved along the main scanning direction (in the direction of arrow D in (A)), the reflection position of the laser beam LBc reflected at the center of the mirror moves from P2 to P2 ′, and in response to this, The beam position on the scanning plane 124 is also moved from P4 to P3, and the curvature of the scanning line is corrected. FIG. 12 is an explanation of the case where extrusion is performed so that the central reflection surface of the flat mirror has a convex shape. In the case of FIG. 11, the central portion of the reflection surface is pulled so as to be concave. These are appropriately selected according to the necessary correction direction of the scanning line curvature.
[0015]
As a position on the optical path of the plane mirror used for adjustment, as disclosed in Japanese Patent Laid-Open No. 2001-228427, a plane tilt correction mirror (an optical element having a plane tilt correction function) and a photosensitive member are disposed. It is desirable to arrange. This is because even if the plane mirror on the upstream side of the optical path with respect to the surface tilt correction mirror is curved and adjusted, the surface tilt correction mirror acts to return the optical path to the original state, so that the curvature adjustment amount of the plane mirror becomes excessive. This is because there is a problem. In addition, it is desirable to use a plane mirror having the maximum incident angle as the adjustment mirror. This is because a plane mirror with a larger incident angle has a larger scanning line curve correction amount with respect to the curve adjustment amount, that is, it can be adjusted with high sensitivity, and the curve amount of the plane mirror can be suppressed. Since the folded plane mirror often uses a glass mirror from the viewpoint of accuracy and cost, suppression of the curvature adjustment amount is an important requirement for preventing mirror breakage.
[0016]
In particular, in the case of bidirectional scanning, in addition to the difference in bending amount depending on the angle as described above, the bending direction is reversed in both directions, so that the mirror bending amount necessary for color registration correction becomes large, and therefore high sensitivity adjustment. It is effective from the viewpoint of reliability to mount the adjusting mechanism on the possible plane mirror.
[0017]
In color registration correction, it is important to superimpose a plurality of scanning lines, and the superimposed scanning lines do not necessarily have a scanning line bending amount of zero. This is because human vision is sensitive to the shift of dots, but the degree of recognition of image distortion is low. A 200 μm dot shift is easily recognized, but a 200 μm curve for A3 paper width (297 mm) is not recognized first.
[0018]
In this way, a plurality of laser beams are incident on the same deflecting surface of a single deflector with an angular difference in the sub-scanning direction and bidirectional scanning is performed, thereby reducing the size of the optical scanning device and the components of the multiple laser beams. Cost reduction by sharing can be realized, and by mounting the mirror curvature adjustment mechanism on the folding mirror provided between the surface tilt correction mirror and the photoconductor, scanning line curve correction can be performed with high sensitivity. Color registration correction including the generated scanning line curve difference is possible.
[0019]
[Problems to be solved by the invention]
However, the above configuration has a problem that an image quality defect is likely to occur due to mirror vibration.
[0020]
In order to suppress the amount of adjustment of the plane mirror adjustment in order to prevent destruction, it is effective to mount the adjustment amount adjustment mechanism on the plane mirror that has a large incident angle and is downstream of the surface tilt correction mirror as described above. In the configuration shown in FIG. 10, the final folding mirrors 120Y, 120M, 120C, and 120K are suitable as the bending amount adjusting mirror. In the laser scanning method in which the scanning width gradually increases from the deflection point, the final folding mirror is usually a maximum length mirror. Such a long mirror has a low eigenvalue and resonates with vibrations entering from the outside of the optical scanning device or vibrations generated from the deflector, so that uneven shading, so-called banding, is likely to occur on the image. The banding pitch is determined in consideration of the process speed, but banding of 0.3 mm pitch or more is recognized visually. Furthermore, since the mirror suitable for the curvature correction is configured so that the scanning line can be corrected with high sensitivity, even if the mirror slightly vibrates, the swing amplitude on the photosensitive member becomes large and banding is visually recognized. In other words, the configuration suitable for the correction of the scanning line curvature simultaneously has a characteristic that it is weak against banding due to vibration.
[0021]
FIG. 13 is a diagram of a bidirectional scanning optical scanning device disclosed in Japanese Patent Laid-Open No. 2001-264655. In the example of the optical scanning device 126, four laser beams 128Y, 128M, 128C, and 128K are emitted parallel to the upper left direction. As shown in FIG. 9, the laser beam incident position on the photoconductor is determined by the size and arrangement of each unit and the timing of charging, exposure, development, and transfer, and therefore cannot be determined by the convenience of the optical scanning device. Further, the layout of the optical scanning device in the image forming apparatus needs to be arranged without interference. As a result of these restrictions, the laser beam emission method of the optical scanning device is not necessarily left-right symmetric with respect to the deflector, and the incident angle to the final folding mirror is often left-right asymmetric.
[0022]
In the configuration example of FIG. 13, the four laser beams reflected by the final folding mirrors 130Y, 130M, 130C, and 130K have an obtuse folding that greatly exceeds 90 ° in the two optical paths on the right side, and the two laser beams on the left side. In the optical path, the turning angle is in the vicinity of 90 °. In this case, the banding is easily visually recognized by the mirror resonance caused by the vibration of the image forming color by the two right optical paths.
[0023]
Next, problems that occur in the entire image forming apparatus will be described. The small tandem type full-color image forming apparatus shown in FIG. 9 has an intermediate transfer belt that is stretched over a plurality of stretching rolls on top of four photoconductors, and superimposes the developed images formed on the photoconductors. After transferring the images together, the images are collectively transferred to a sheet at the secondary transfer point, and the image is fixed by a fixing device to output a full color image.
[0024]
Each component becomes smaller due to the downsizing of the apparatus, and when the tension of the tension roll is insufficient, the tension roll is deformed, and the peripheral speed of the intermediate transfer belt changes in the roll axis direction. Japanese Patent Application Laid-Open No. 2001-228673 describes that the curvature distortion of the toner image on the intermediate transfer belt is reversely corrected with the exposure image of the optical scanning device. In order to realize this, it is necessary to add the amount of correction of the distortion of the toner image to the amount of correction of the scanning line curvature of the optical scanning device.
[0025]
When the reverse correction amount of the curvature distortion of the toner image is applied in the direction to increase the curve adjustment amount of the scanning line, there is a concern about mirror breakage due to the curve adjustment, and there is a problem that banding occurs when the mirror is thinned to avoid breakage. .
[0026]
In view of the above circumstances, it is a first object of the present invention to provide an optical scanning device that scans a plurality of beams with a single deflector and that has a scanning line curvature correction function and is less likely to cause banding. Objective. It is a second object of the present invention to provide an image forming apparatus in which banding is unlikely to occur even when the toner image is curved and deformed due to deformation of the tension roll of the intermediate transfer belt.
[0027]
[Means for Solving the Problems]
  In order to achieve the above object, an invention according to claim 1 is an optical scanning device that exposes a plurality of photosensitive members by scanning a plurality of laser beams with a single deflector. A final folding mirror in each of a plurality of optical paths between the element and the plurality of photoconductors;Each of the final folding mirrors includes an adjustment mechanism that adjusts the curvature of the reflecting surface to correct the scanning line curvature,Comparing the laser beam folding angle between the final folding mirrors that make the laser beam incident on the photoconductor, the final folding mirror having a large folding angle is made thicker than the final folding mirror having a small folding angle,The target value of the curvature correction by the adjusting mechanism is set near the initial scanning line curve of the optical path where the mirror is thick among the final folding mirrors.It is characterized by.
[0028]
  According to the first aspect of the present invention, with respect to the fluctuation of the reflected beam caused by the resonance caused by the vibration of the folding mirror, the folding angle is increased by utilizing the fact that the amplitude of the fluctuation depends on the folding angle of the laser beam. The swing amplitude tends to increaseFinalThe folding mirror has a small folding angle.Last turnThicker than the mirror.
[0029]
As a result, when a laser beam is guided from a single deflector to a plurality of photoconductors, mirror resonance occurs due to different optical path layouts of the plurality of laser beams and different folding angles of a plurality of folding mirrors having the same function. The difference in fluctuation sensitivity of the reflected beam with respect to is offset, and banding is suppressed.
[0031]
  Claim1In the invention described in (4), in the configuration in which a plurality of laser beams are incident on the deflecting surface of the deflector at an angle in the sub-scanning direction, the deflector is thinned and the plurality of deflected beams are imaged on each photoconductor. There is an advantage that the image lens can be shared. However, since the scanning line is curved due to oblique incidence, a final folding mirror having an adjustment mechanism that corrects the scanning line curvature by curving the reflection surface of the mirror (planar mirror) is provided on the plurality of optical paths.
[0032]
    Equipped with this adjustment mechanismFinalThe folding mirror is desirably thin in order to secure a certain amount of curvature adjustment while preventing breakage. However, if the folding mirror is preferentially made thin in consideration of adjustability (curvature), banding due to mirror resonance increases. Therefore, a plurality of adjustment mechanismsFinalThe difference in the fluctuation sensitivity of the reflected beam with respect to mirror resonance, which occurs when the folding angle of the folding mirror is different, is offset by providing a difference in the mirror plate thickness. As a result, it is possible to achieve both suppression of banding and curve line adjustability.
[0034]
  Claim1In the invention described in (4), the curvature of the scanning line can be corrected with high sensitivity by curving the final folding mirror (planar mirror) on the optical path between the optical element having the surface tilt correction function and the photosensitive member. The destruction of the folding mirror is suppressed.
  For this reason, the target value of the curve correction by the adjusting mechanism is set to a position close to the initial scanning line curve of the optical path where the mirror is thick among the final folding mirrors.
[0035]
  And a plurality of this adjustment mechanismFinalLarge folding angle with respect to folding mirrorFinalFolding mirror has a small folding angleLast turnBy making it thicker than the mirror, the banding is suppressed by canceling the sensitivity difference of the oscillation amplitude of the reflected beam due to mirror resonance. Furthermore, by utilizing the fact that image distortion is less visible than dot displacement, the image is slightly distorted, but both mirror breakage avoidance and banding suppression are more reliably achieved. In other words, the folding angle that increases banding sensitivity is large.Last turnBy using a thick mirror to suppress banding, and at the same time, slightly leave the scan line curve on the scanned surface after adjustment.Last turnReduce the amount of mirror curvature adjustment.
[0036]
  In this way, the adjustment amount for correcting the color registrationFinalBy setting according to the thickness of the folding mirror and making it smaller as the mirror thickness is thicker, mirror breakage due to curvature adjustment is suppressed, and compatibility with banding suppression is achieved.
[0037]
  Claim2In the optical scanning apparatus that scans a plurality of laser beams incident on a single deflector at an angle in the sub-scanning direction to expose a plurality of photosensitive members, an optical device having a surface tilt correction function is provided. Each of a plurality of optical paths between the element and the plurality of photoconductors includes an adjustment mechanism that adjusts the curvature of the reflecting surface to correct the scanning line curve, and has a final folding mirror that causes the laser beam to enter the photoconductor. The laser beam folding angles between the plurality of final folding mirrors are compared, and the final folding mirror having the larger folding angle is thicker than the final folding mirror having the smaller folding angle, and the final thickness is increased. The folding mirror is characterized in that, in an unadjusted state, the reflecting surface has a curved shape along the main scanning direction in a direction for correcting the scanning line curvature.
[0038]
  Claim2In the invention described in, a mirror that makes it possible to correct the scanning line curve with high sensitivity by curving the final folding mirror (planar mirror) on the optical path between the optical element having the surface tilt correction function and the photosensitive member. The destruction of is suppressed.
[0039]
  And for a plurality of folding mirrors equipped with this adjustment mechanism, the folding angle is large.FinalFolding mirror has a small folding angleLast turnBy making it thicker than the mirror, banding is suppressed by offsetting the sensitivity difference of the oscillation amplitude of the reflected laser beam due to mirror resonance. Furthermore, it was set thickFinalThe reflection surface of the folding mirror is already curved in the unadjusted state (natural state) of the mirror, and its shape is a curved shape along the main scanning direction in the direction of correcting the scanning line curvature. The amount of adjustment required has been reduced and thickened to suppress resonance.Last turnThe amount of bending of the mirror is reduced. This makes it possible to adjust the scanning line curvature while avoiding the risk of destruction.
[0040]
  Claim3The optical scanning device according to claim 3, wherein an adjustment amount of each of the plurality of final folding mirrors to adjust the color registration by adjusting the curvature of the reflecting surface is the thickness of the plurality of final folding mirrors. It is characterized by being set accordingly.
[0041]
  Claim3In the invention described in, in correcting the scanning line curve difference caused by the oblique incidence by adjusting the curvature of the mirror (planar mirror), the final folding mirror having a large folding angle with high banding sensitivity is changed to the final folding mirror having a small folding angle. Banding is suppressed by making it comparatively thick. At this time, in order to prevent destruction of the thick mirror due to the adjustment curve, a mirror whose reflection surface is curved in advance is used as the thick mirror, thereby reducing the difference in curve between scanning lines before adjustment and reducing the adjustment amount. In addition, for the color registration correction in which the mirror reflecting surface is curved and adjusted, the adjustment target value is offset from the scanning line curve zero in the direction in which the adjustment amount by the thick mirror is reduced.
[0042]
  In this way, the adjustment amount for correcting the color registrationFinalBy setting according to the thickness of the folding mirror and making it smaller as the mirror thickness is thicker, the adjustment amount of the thick mirror is further reduced and mirror breakage is suppressed.
[0043]
  Claim4The invention described in claim2Or claims3In the optical scanning device described above, the final folding mirror in which the reflecting surface is curved along the main scanning direction in a direction in which the scanning line is corrected in an unadjusted state corrects the scanning line curvature of the mirror material. It is characterized in that it is bent and held in the opposite direction to the direction in which the reflecting surface is made, and the reflecting surface side is flattened and manufactured in this holding state.
[0044]
  Claim4In the invention described in, when manufacturing the final folding mirror that corrects the scanning line curvature by curving the reflecting surface, the mirror material is curved and held in a direction opposite to the direction in which the scanning line curvature is corrected, and the state is maintained. By flattening the reflective surface side, the reflective surface becomes a curved shape along the main scanning direction in the direction of correcting the scanning line curvature in an unadjusted state of the mirror. As a result, a final folding mirror having a curved shape necessary for scanning line curvature correction is obtained, and its manufacture is facilitated.
[0045]
  Claim5The invention described in claim 1 to claim 14The optical scanning device according to any one of the above, wherein the plurality of laser beams are scanned two by two by the single deflector.
[0046]
  Claim5In the invention described in (2), two laser beams obliquely incident in the sub-scanning direction are scanned in both directions by two opposing surfaces of the polygon mirror. As a result, in addition to thinning the optical deflector and sharing the imaging lens by oblique incidence of a plurality of laser beams in the sub-scanning direction, the incident angle difference in the sub-scanning direction between the two laser beams distributed in both directions is reduced. This minimizes the imaging characteristics among a plurality of laser beams.
[0047]
  Claim6The invention described in claim1~ Claim55. The optical scanning device according to claim 1, wherein at least one of the plurality of final folding mirrors does not include the adjustment mechanism, and a final folding mirror that does not include the adjustment mechanism is based on a laser beam provided in the optical path. The final folding mirror having the adjusting mechanism corrects the scanning line curvature of the laser beam provided in the optical path.
[0048]
  Claim6In the invention described in the above, a plurality ofFinalBy not providing an adjustment mechanism in at least one of the folding mirrors, the cost can be reduced, the apparatus configuration can be simplified, and the color registration maintainability can be improved and restrictions on the optical path layout can be eliminated. Further, the adjustment mechanism can be deleted without substantially affecting the image quality by targeting the laser beam whose distance from the adjustment target value is the minimum.
[0051]
  Claim7The image forming apparatus according to claim 1.6The optical scanning device according to claim 1, a plurality of photosensitive members irradiated with a plurality of laser beams scanned by the optical scanning device, and a plurality of stretching rolls are stretched and configured endlessly. A color image forming apparatus, comprising: an intermediate transfer body that, after the electrostatic latent image development images formed on each of the plurality of photoconductors are sequentially superimposed and primarily transferred, and then secondarily transferred to an image forming medium. In the exposure image formed by the plurality of laser beams on the plurality of photoconductors in the uncorrected state of the scanning line curve, the scanning line curve direction is reversed, and adjacent exposure images are located between the scanning centers. The distance is larger than the distance between the scanning ends.
[0052]
  Claim7In the invention described in the above, the color registration performance is high by mounting the optical scanning device in consideration of unevenness in the roll axis direction of the peripheral speed of the transfer body due to the deformation of the stretch roll over which the endless intermediate transfer body is stretched, An image forming apparatus that does not generate banding is realized. The intermediate transfer member has a peripheral speed that is faster than the center of the transfer member due to deformation due to insufficient rigidity of the stretching roll due to downsizing of the device. As a result, when the development image transferred first reaches the transfer position of the development image transferred last, both ends of the development image have a development image shape curved in a direction advanced with respect to the moving direction of the transfer body.
[0053]
Therefore, if an exposure image that reversely corrects this curvature is formed, an image output without color registration deviation can be obtained from the image forming apparatus. Then, by aligning the exposure image curving direction in the reverse correction direction with the scanning line curving direction generated by the oblique incidence by the optical scanning device, that is, in the uncorrected state of the scanning line curvature, In the exposure images formed on the body, the scanning line curve direction is reversed, and between adjacent exposure images, the amount of adjustment of the scan line curve is increased by making the distance between the scanning centers larger than the distance between the scanning ends. Reduced, high-accuracy color registration and high-quality images without banding can be obtained.
[0054]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0055]
[First Embodiment]
FIG. 1 shows an optical scanning device according to a first embodiment of the present invention, and FIG. 2 shows a tandem color image forming apparatus equipped with the optical scanning device.
[0056]
As shown in FIG. 1, the optical scanning device 10 includes a single housing 12, and a single polygon mirror 14 mounted on a motor (not shown) is disposed at the center of the device. Are scanned with laser beams (AY and AM, AC and AK). The laser beams deflected by the polygon mirror 14 pass through a common fθ lens 16 (16A, 16B and 16A ′, 16B ′), respectively, and return mirrors 18Y, 18M disposed near both ends of the housing 12; After being folded back to the polygon mirror 14 side by 18C and 18K, it is folded back approximately in the reverse direction by the cylindrical mirrors 20Y, 20M, 20C, and 20K, and finally folded back by the final folding mirrors 22Y, 22M, 22C, and 22K. And reach the photoreceptors 24Y, 24M, 24C, and 24K, respectively, to form images.
[0057]
The direction of scanning on the photosensitive member 24 is reversed by two with the polygon mirror 14 in between because the deflection is performed on the two opposing surfaces of the single polygon mirror 14. The laser beam is modulated by image signals corresponding to yellow (Y), magenta (M), cyan (C), and black (K) in order from the right.
[0058]
The polygon mirror 14 includes twelve reflecting surfaces, and the fθ lens 16 has refractive power only in the main scanning direction, and has a concave cylindrical surface and a flat surface (16A, 16A ′), and a flat surface and a convex cylindrical surface (16B, 16B ′). ). The cylindrical mirror 20 realizes a surface tilt correction function by making the reflecting surface of the polygon mirror 14 and the surface of the photoconductor 24 have a substantially conjugate imaging relationship on each optical path. In this configuration, the cylindrical mirror 20 corresponds to an optical element having a surface tilt correction function.
[0059]
The housing 12 has a non-hole structure with an opening on the upper side, and distortion due to attachment to the image forming apparatus and temperature change is minimized. A cover 28 equipped with four window glasses 26Y, 26M, 26C, and 26K is attached to the upper side of the casing 12, and the interior of the casing 12 is sealed.
[0060]
The laser beams AY, AM, AC, and AK are deflected with an angle in a direction away from the plane perpendicular to the rotation axis of the polygon mirror 14. The deflected beam has a scanning line curve in which both ends of the scanning are located above the scanning center, and the amount of the bending is maximized on the reflection surface of the cylindrical mirror 20, and the curve gradually decreases and the photosensitive member 24 is bent. However, the four scanning lines scanned on the photosensitive member 24 remain curved.
[0061]
FIG. 3 is a diagram showing the scanning line curvature on the surface to be scanned (surface position of the photosensitive member 24) scanned by the optical scanning device 10. As shown in FIG. The four scanning lines (exposure images) LY, LM, LC, and LK are curved outwardly with a polygon mirror 14 in between, and the distance between the scanning lines is scanned with respect to the scanning center distance L1. The end-to-end distance L2 is large. Although the amount of bending is exaggerated for the sake of explanation, it is about 130 μm at the maximum. This amount of bending is a size that is hardly visible in the case of monochromatic image formation, but in the case of full-color image formation in which the four colors are superimposed, it is an unacceptable amount, and it is necessary to correct the curvature of the scanning line. Further, the laser beam that scans the outer photoconductors 24Y and 24K has an angle formed by the deflection beam with respect to a plane perpendicular to the rotation axis of the polygon mirror 14, as compared with the laser beam that scans the inner photoconductors 24M and 24C. Is large, the amount of bending becomes large. The angles that the laser beam makes with the orthogonal plane of the polygon mirror 14 are 1.2 ° (M, C) and 2.4 ° (Y, K).
[0062]
Next, the incident laser beam on the polygon mirror will be described. Laser light sources 30Y (not shown), 30M, 30C, and 30K are disposed on one side of housing 12 (upper side in FIG. 1B). Two of the laser beams AY, AM, AC, and AK emitted from the four independent light sources (AM, AC) are folded back by the folding mirrors 32M (not shown) and 32C, and then the common mirrors 34YM, 34CK. Are folded two by two and enter the polygon mirror 14. The incidence to the polygon mirror 14 is performed through the fθ lens 16, and the double-pass incidence method is employed in which the deflected beam is transmitted again through the fθ lens 16. Incidence to the polygon mirror 14 is performed at an angle from both sides of the optical axis of the fθ lens 16 in the main scanning direction and upward from the bottom surface side of the housing 12 in the sub-scanning direction. Further, the incident method to the polygon mirror 14 is an overfilled method in which the incident beam is wider than the width of the deflection surface in the main scanning direction. As a result, a beam diameter of 60 μm is generated by a 12-sided polygon mirror having an inscribed circle diameter of 25 mm, and high-speed and high-resolution scanning is realized.
[0063]
In the configuration shown in FIG. 1A, laser beams AY, AM, AC, and AK directed toward the photoconductors 24Y, 24M, 24C, and 24K are emitted from the optical scanning device 10 in a direction inclined to the left. This is determined by the configuration of the image forming apparatus, but due to the configuration of the small tandem type image forming apparatus, it is rare that the optical scanning device can be configured symmetrically. For this reason, in the final folding mirror 22, the folding angles of the folding mirrors 22Y and 22M are small, and the folding angles of the folding mirrors 22C and 22K are large.
[0064]
The final folding mirror requires a length close to the scanning width on the photosensitive member, as is apparent from FIG. The final folding mirror 22 in the present embodiment is 280 mm in distance between the support points at both ends. Such a long mirror has a low eigenvalue and is difficult to be set to 400 Hz or more. Therefore, it tends to resonate with vibration propagated from the outside of the optical scanning device or vibration generated from the polygon motor in the device. The resonance of the mirror causes the reflected beam to oscillate, but the final mirror is provided after the cylindrical mirror with a surface tilt correction function, so that the oscillation is not reduced by the optical system on the photoconductor. It is easy to see as banding.
[0065]
FIG. 4 is a graph showing the relationship between the laser beam folding angle of the mirror and the oscillation amplitude of the beam on the photosensitive member. The vibration mode is determined by the parameters of the beam, except for the minute difference in support conditions of the mirror. However, even if the same mirror is used and the same resonance occurs, the laser beam oscillation width (amplitude) on the photosensitive member differs if the folding angle of the laser beam is different.
[0066]
As shown in the figure, when the folding angle is α> β, when the two mirrors 36A and 36B cause the same vibration (the amplitude A is the same amount), the beam swing width B on the photoconductor (scanned surface 38) is B1> B2. Accordingly, in the case of the optical path layout shown in FIG. 1, the laser beam swing on the photoconductors 24C and 24K is larger than that of the photoconductors 24Y and 24M, and banding on cyan and black images is easily recognized.
[0067]
In order to solve this problem, in the first embodiment, the thicknesses of the final folding mirrors 22C and 22K on the cyan and black optical paths with large folding angles are relatively on the yellow and magenta optical paths. It is thicker than the final folding mirrors 22Y and 22M having a small folding angle. As a numerical example of the plate thickness, the folding mirrors 22Y and 22M can be set to 8 mm, and the folding mirrors 22C and 22K can be set to 10 mm. The folding angle is slightly different between the folding mirror 22Y and the folding mirror 22M, and between the folding mirror 22C and the folding mirror 22K, but here, two types of mirror thickness mirrors are arranged due to common parts and differences in effect. . Further, the scanning line curvature due to the oblique incidence is corrected by reversely correcting the image data in advance.
[0068]
Next, a color image forming apparatus equipped with the optical scanning device having the above configuration will be described. As shown in FIG. 2, in the color image forming apparatus 40, the photoconductors 24Y, 24M, 24C, and 24K are arranged above the optical scanning device 10 and are juxtaposed in the horizontal direction. The intermediate transfer belt 42 is in contact with the tension rolls 41A, 41B, and 41C. When the photosensitive members 24Y, 24M, 24C, and 24K are scanned and exposed by the light beams AY, AM, AC, and AK from the optical scanning device 10 to form electrostatic latent images, each electrostatic latent image is developed by a developing device (not shown). And is transferred onto the intermediate transfer belt 42 at the primary transfer point 44. On the intermediate transfer belt 42, the developed image is superimposed in the order of yellow (Y), magenta (M), cyan (C), and black (K) by driving the belt in the direction of arrow C in the figure. Further, the image is collectively transferred onto the paper 50 sent out from the paper feed cassette 48 at the secondary transfer point 46, and the paper 50 is sent to the upper fixing device 52 so that the transferred image becomes a fixed image. Through the above process, a color image is printed on the paper 50 by the color image forming apparatus 40, and the printed paper 50 is discharged onto the paper discharge tray 54 at the top of the apparatus.
[0069]
With the above configuration, in the optical scanning device 10 of the present embodiment, the folding mirrors 22C and 22K having a large folding angle in which the oscillation width of the reflected beam due to resonance tends to be large are relatively small. Banding is suppressed by being thicker than 22Y and 22M.
[0070]
In this embodiment, the case where the laser beam is incident obliquely has been described as an example, but the same applies to a configuration in which a parallel beam is incident on the polygon mirror. In this case, scanning line bending due to optical characteristics does not occur. Therefore, by determining the mirror plate thickness corresponding to the folding angle, a high-quality image without banding can be obtained without reverse correction of image data in advance. .
[0071]
[Second Embodiment]
Next, a second embodiment of the present invention will be described. However, since the optical layout is the same as that of the first embodiment, description will be made using the optical scanning device 10 of FIG. The difference from the first embodiment is that the final folding mirror is provided with a mirror curvature adjusting mechanism.
[0072]
In the present embodiment, the four final folding mirrors 22Y, 22M, 22C, and 22K are provided with a mirror bending adjustment mechanism that corrects the scanning line curvature by curving the reflecting surface of the mirror. For example, a configuration as shown in FIG. 11 is used as the adjustment mechanism. As in the first embodiment, the thickness of each mirror is such that the folding mirrors 22C and 22K with a large folding angle are thicker than the folding mirrors 22Y and 22M with a small folding angle.
[0073]
As a result, the scanning line curvature (scanning lines LY, LM, LC, and LK in FIG. 3) generated by the configuration in which the laser beams AY, AM, AC, and AK having an angle difference in the sub-scanning direction are incident are reflected mirror 22Y Correction is possible by adjusting the curvature of 22M, 22C, and 22K, and a complicated configuration that reversely corrects image data in advance is not required, and both color registration correction and banding suppression can be achieved.
[0074]
[Third Embodiment]
Next, a third embodiment of the present invention will be described. In this embodiment, the configuration of the optical scanning device is the same as that of the second embodiment. That is, the four final folding mirrors 22Y, 22M, 22C and 22K are equipped with a mirror bending adjustment mechanism as shown in FIG. 11, and the folding mirrors 22C and 22K having a large folding angle are connected to the folding mirrors 22Y and 22M. It is structured thickly. The difference from the second embodiment is that the adjustment target value in the correction of the scanning line curvature is set as an offset.
[0075]
FIG. 5 is a diagram showing the relationship between the scanning line curve generated in the configuration of FIG. 1 and the adjustment target value. As described above, the risk of banding is avoided by increasing the thickness of the mirror for C and K where the folding angle is large and banding is likely to occur. However, increasing the thickness of the mirror having the mirror curvature adjusting mechanism has a risk of mirror breakage due to the adjustment. To avoid this problem, the color registration adjustment target value is offset.
[0076]
Specifically, as shown in FIG. 5, a target value for curve correction is set near the initial scan line curve of the thick optical path of the mirror (adjustment target value scan line LS). Since the maximum amount of the initial scanning line curve is about 130 μm in this embodiment, even if an intermediate area between C and K is aimed at, the image distortion in the adjusted state is 100 μm or less, and there is no practical problem. Become a level.
[0077]
As a result, the adjustment amount is reduced with respect to the thick optical path of the mirror, the risk of mirror breakage can be avoided, and both color registration correction and banding suppression can be achieved.
[0078]
[Fourth Embodiment]
Next, a fourth embodiment of the present invention will be described. The configuration of the optical scanning device is substantially the same as in the second embodiment. The difference is that the thick mirror is preliminarily curved along the main scanning direction in the direction of correcting the scanning line curvature.
[0079]
FIG. 6 is a schematic diagram for explaining a manufacturing process of a mirror whose reflecting surface is curved along the main scanning direction. First, as shown to (A), the mirror material 56 for a process created by cleaving the float glass is mounted so as to follow the processing jig 60 formed with the convex curved shape as the copying surface 58. (B) is a state in which the mirror material 56 is mounted on the processing jig 60. Next, a portion 62 of the mirror material 56 that protrudes from the processing jig 60 into a convex shape is subjected to planar processing by grinding, polishing, or the like. (C) is a state after processing, and the processing surface 64 of the mirror material 56 is flush with the upper surface 66 of the processing jig 60. Then, when the mirror material 56 is removed from the processing jig 60, a mirror material 56 having a shape in which the processing surface 64 is concavely curved is completed as shown in FIG.
[0080]
By depositing aluminum or the like on the concave curved surface, it is possible to manufacture a folding mirror whose reflecting surface is previously curved along the main scanning direction. By mounting the folding mirrors manufactured by such a manufacturing method as the final folding mirrors 22C and 22K, the amount of unadjusted scanning line bending on the photoconductors 24C and 24K is reduced, and the scanning line bending adjustment is performed. A high-quality image free from image distortion can be obtained while more reliably avoiding the risk of mirror breakage.
[0081]
Although the method of manufacturing the concave curved surface has been described with reference to FIG. 6, the convex curved surface may be necessary depending on the optical path design of the optical scanning device. In that case, it can be similarly manufactured by making the copying surface 58 of the processing jig 60 into a concave shape. In addition, the method of offsetting the adjustment target value of the color registration described as the third embodiment may be combined with the optical scanning device in which the folding mirror previously curved along the main scanning direction is mounted. Is possible. In this case, banding suppression and mirror destruction risk can be avoided more reliably. In addition, it is possible to use a margin that has been secured as a degree of design freedom and to expand the selection range of the incident angle in the sub-scanning direction and the optical path folding angle, thereby laying out a more compact and high-performance optical scanning device.
[0082]
Furthermore, in the example shown in FIG. 5, the scanning line curve adjustment target value is set between C and Y. For example, C is set as the target value, and scanning line curve adjustment is performed so that Y, M, and K overlap C. May be performed. In this case, since C does not perform scanning line adjustment, it is not necessary to provide an adjustment mechanism on the optical path, and the configuration of the apparatus can be simplified and cost reduction can be realized. In addition, since the structure is simplified, there are fewer fluctuation factors, leading to improvement in the maintenance of color registration.
[0083]
[Fifth Embodiment]
Finally, as a fifth embodiment of the present invention, a small tandem full-color image forming apparatus using the above optical scanning device will be described.
[0084]
FIG. 14 is a diagram for explaining the bending distortion of the toner image due to the stretching roll deformation of the intermediate transfer belt. As shown in (A), the belt center interval L3 of the intermediate transfer belt 70 is made shorter than the belt end interval L4 due to the deformation of the stretching rolls 68A and 68B (L3 <L4). A speed difference occurs. As a result, as shown in (B), the first color toner image 72A formed without the scanning line curve at the position of T1 reaches the position of the downstream side T4 where the fourth color is transferred in the belt traveling direction. It has a concave curve (toner image 72B).
[0085]
Further, as shown in (C) and (D), the second color toner image 74A formed without the scan line curve at the position T2, and the third color toner image formed without the scan line curve at the position T3. For 76A, as it reaches the position of T4, the concave curve is similarly formed in the belt traveling direction (toner images 74B and 76B). However, the amount of curvature of each toner image is 72B> 74B> 76B, and in particular, the first and fourth color misregistration that causes a large difference in the amount of curvature of the scanning line becomes a problem.
[0086]
In order to correct this, the first color may be curved in the reverse direction, or the fourth color may be curved in advance in accordance with the distorted first color. In any case, when the first color and the fourth color are compared, it is only necessary to form a correlation toner image in which the distance between the scanning centers is shorter than the distance between the scanning ends.
[0087]
FIG. 7 is a diagram showing the relationship between exposure, development, and transfer. The curved profile of the exposure latent image shown on the lower side is that of the optical scanning device 10 shown in FIG. 1 described as the first embodiment. Here, since the photosensitive members 24Y, 24M, 24C, and 24K rotate about a half turn from the exposure to the transfer to the intermediate transfer belt 42, the transferred toner images are transferred to the four scanning lines LY ′, LM ′, and LC ′. , LK ′ is inverted to a profile that is convex outward. Then, when the toner image curve due to the stretching roll deformation of the intermediate transfer belt 42 is added to the transfer toner image on the photoreceptor 24Y, both ends of the belt in the traveling direction greatly advance in the traveling direction at the timing of overlapping with the toner image on the photoreceptor 24K. The curved toner image becomes a warp in the opposite direction to that of the toner image on the photosensitive member 24K, causing a significant color registration shift.
[0088]
From this mechanism, if an optical scanning device having an exposure state opposite to that of the optical scanning device having the configuration shown in FIG. The combination of the toner image curvature and the canceling direction due to the deformation of the tension rolls reduces the amount of correction of the scanning line curvature, and a high-quality color registration and high-quality image without banding can be obtained.
[0089]
FIG. 8 is a diagram showing a specific example of an optical scanning device in which the initial scanning line curve is convex outward. The curve direction of the scanning line on the surface to be scanned (on the photosensitive member) is determined by the number of times of folding by the mirror. The optical scanning device 10 shown in FIG. 1 reflects three times (odd times), whereas the optical scanning device 80 shown in FIG. 8 has four folding mirrors 82 and 84 provided on each optical path. Since the reflection is performed four times (even times) by 86 and 88, the curve direction of the scanning line is reversed.
[0090]
Further, in the optical scanning device 80, the folding angles of Y and M among the final folding mirrors 88Y, 88M, 88C and 88K are larger than those of C and K. Therefore, if the optical scanning device 80 is mounted on the image forming apparatus with a mounting layout as shown in the figure and the folding mirrors 88Y and 88M are thicker than the folding mirrors 88C and 88K, the risk of mirror breakage is avoided. Thus, an image forming apparatus that outputs a good image without banding can be obtained. The contents described as the second to fourth embodiments can also be carried out similarly to the case of the optical scanning device shown in FIG.
[0091]
【The invention's effect】
As described above, according to the first aspect of the present invention, when a laser beam is guided from a single deflector to a plurality of photosensitive members, the optical path layout of the plurality of laser beams is different, and a plurality of the same functions are provided. The difference in oscillation sensitivity of the reflected beam with respect to mirror resonance that occurs when the folding angles of the folding mirrors are different cancels the banding.
[0092]
According to the second aspect of the present invention, the difference in the oscillation sensitivity of the reflected beam with respect to the mirror resonance, which is generated when the folding angles of the plurality of folding mirrors provided with the adjusting mechanism are different, is provided in the mirror plate thickness. By canceling out, it is possible to achieve both suppression of banding and curvature adjustment of the scanning line.
[0093]
According to the third aspect of the invention, it is possible to more reliably achieve mirror breakage avoidance and banding suppression by utilizing the fact that image distortion is less visible compared to dot displacement.
[0094]
According to the fourth aspect of the present invention, the reflecting surface of the folding mirror set to be thick is already curved in the natural state of the mirror, and the shape thereof is a curved shape along the main scanning direction in the direction of correcting the scanning line curvature. For this reason, the amount of adjustment necessary for correcting the scanning line curvature is reduced, the amount of bending of the mirror thickened to suppress the resonance is reduced, and the scanning line curvature can be adjusted in a state where the risk of destruction is avoided.
[0095]
According to the fifth aspect of the present invention, with respect to the color registration correction that adjusts by curving the mirror reflecting surface, the adjustment target value is offset from the scanning line curve zero in the direction in which the adjustment amount by the thick mirror decreases. Therefore, the adjustment amount of the thick mirror is further reduced and the mirror breakage is suppressed.
[0096]
According to the sixth aspect of the present invention, since a curved mirror that can be manufactured without using a special material and processing technique can be used, a high-performance optical scanning device can be provided while suppressing an increase in cost.
[0097]
According to the invention described in claim 7, in addition to the effects of the invention described in claims 2 to 6, the difference in incident angle in the sub-scanning direction between the two laser beams distributed in two directions is minimized. Therefore, it is possible to provide an optical scanning device that makes it easy to uniformize imaging characteristics between a plurality of laser beams.
[0098]
According to the eighth aspect of the present invention, it is possible to reduce the cost by deleting at least one of the adjustment mechanisms, simplify the apparatus configuration, improve the color registration maintainability, and restrict the optical path layout. Can be eliminated.
[0099]
According to the ninth aspect of the present invention, it is possible to realize a manufacturing method capable of easily and inexpensively manufacturing a bent mirror having a curved shape necessary for scanning line curve correction without using a special material or processing technique.
[0100]
According to the tenth aspect of the present invention, the scanning line is formed by a combination of canceling the toner image curving direction caused by the stretching roll deformation of the intermediate transfer member and the scanning line curving direction generated by the oblique incidence by the optical scanning device. The amount of curvature adjustment can be reduced, and an image forming apparatus that achieves both high-precision color registration and high-quality images without banding can be obtained.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an optical scanning device according to first to fourth embodiments of the present invention.
2 is a configuration diagram of a color image forming apparatus equipped with the optical scanning device of FIG. 1;
3 is a diagram showing scanning line curvature on a photosensitive member in the optical scanning device of FIG. 1; FIG.
FIG. 4 is a graph showing the relationship between the mirror folding angle and the beam swing amplitude on the photosensitive member.
FIG. 5 is a diagram showing a relationship between scanning line curvature and an adjustment target value.
FIG. 6 is a diagram for explaining a manufacturing process of a mirror that is curved in advance along the main scanning direction;
FIG. 7 is a diagram showing the relationship between exposure, development, and transfer according to a fifth embodiment of the present invention.
FIG. 8 is a configuration diagram of an optical scanning device according to a fifth embodiment of the present invention.
FIG. 9 is a configuration diagram of a conventional full-color image forming apparatus that achieves miniaturization.
10 is a diagram showing an internal configuration and scanning line curvature of the optical scanning device in FIG. 9;
FIG. 11 is a diagram illustrating an example of a mirror bending adjustment mechanism.
FIG. 12 is a diagram for explaining a mechanism for correcting a scanning line curvature by curving a plane mirror in the main scanning direction.
FIG. 13 is a diagram showing a configuration of a conventional bidirectional scanning optical scanning device.
FIG. 14 is a diagram for explaining the curvature of a toner image due to deformation of a tension roll of an intermediate transfer belt.
[Explanation of symbols]
10. . . Optical scanning device
14 . . Polygon mirror (single deflector)
16, 16 '. . . Scanning lens (optical element with surface tilt function)
22Y, 22M, 22C, 22K. . . Folding mirror
24Y, 24M, 24C, 24K. . . Photoconductor
40. . . Color image forming apparatus
41A, 41B. . . Tension roll
42. . . Intermediate transfer belt (intermediate transfer member)
50. . . Paper (image forming medium)
56. . . Mirror material
60. . . Processing jig (holding means)
64. . . Processing surface (reflection surface)
80. . . Optical scanning device
88Y, 88M, 88C, 88K. . . Folding mirror
AY, AM, AC, AK. . . Laser beam
α, β. . . Folding angle

Claims (7)

In an optical scanning apparatus that exposes a plurality of photosensitive members by scanning a plurality of laser beams with a single deflector,
In each of a plurality of optical paths between the optical element having a surface tilt correction function and the plurality of photosensitive members, a final folding mirror,
Each of the final folding mirrors includes an adjustment mechanism that adjusts the curvature of the reflecting surface to correct the scanning line curvature,
Comparing the laser beam folding angle between the final folding mirrors that make the laser beam incident on the photosensitive member, the final folding mirror having a large folding angle is made thicker than the final folding mirror having a small folding angle,
The optical scanning device according to claim 1, wherein the target value of the curvature correction by the adjusting mechanism is set to a position close to the initial scanning line curvature of the optical path where the mirror is thick among the final folding mirrors . In an optical scanning apparatus that exposes a plurality of photosensitive members by scanning a plurality of laser beams incident on a single deflector at an angle in the sub-scanning direction,
  An adjustment mechanism for correcting the curvature of the scanning line by correcting the curvature of the reflecting surface in each of a plurality of optical paths between the optical element having a surface tilt correction function and the plurality of photosensitive members, and for applying the laser beam to the photosensitive member. Comparing laser beam folding angles between the plurality of final folding mirrors, the final folding mirror having a large folding angle is made thicker than the final folding mirror having a small folding angle. The optical scanning device is characterized in that the thickened final folding mirror is in an unadjusted state and the reflecting surface is curved along the main scanning direction in the direction of correcting the scanning line curvature. The adjustment amount for correcting the color registration by adjusting the curvature of the reflecting surface of each of the plurality of final folding mirrors is set according to the thickness of the plurality of final folding mirrors. The optical scanning device described. The optical scanning device according to claim 2 or 3,
  The final folding mirror in which the reflecting surface is in a curved shape along the main scanning direction in a direction to correct the scanning line curvature in an unadjusted state,
  An optical scanning device characterized in that a mirror material is curved and held in a direction opposite to the direction in which the scanning line curve is corrected, and the reflecting surface side is planarly processed in the held state. 5. The optical scanning device according to claim 1, wherein the plurality of laser beams are scanned two by two in both directions by the single deflector. 6. At least one of the plurality of final folding mirrors does not include the adjusting mechanism, and the final folding mirror having the adjusting mechanism is provided in the optical path with reference to the laser beam provided with the final folding mirror not provided with the adjusting mechanism in the optical path. 6. The optical scanning device according to claim 1, wherein a scanning line curvature of the provided laser beam is corrected. The optical scanning device according to any one of claims 1 to 6, a plurality of photosensitive members irradiated with a plurality of laser beams scanned by the optical scanning device, and a plurality of stretching rolls. An intermediate transfer member that is endlessly configured and electrostatically developed on each of the plurality of photoconductors is sequentially superimposed and primarily transferred and then secondarily transferred to an image forming medium. In a color image forming apparatus comprising:
  In the exposure image formed by the plurality of laser beams on the plurality of photoconductors in the uncorrected state of the scanning line curve, the scanning line curve direction is reversed, and the adjacent exposure images have a scanning center distance. An image forming apparatus characterized in that it is larger than a distance between scanning ends.

Images