Scanning optical device

Abstract

To use a lens surface that is symmetrical in a main scanning direction and restrain an image surface curvature to a small level.SOLUTION: A first incidence surface 21 as an incidence surface of a first scanning lens 20 is symmetrical with respect to a main scanning direction. A first emission surface 22 as an emission surface of the first scanning lens 20 is symmetrical with respect to the main scanning direction. A second incidence surface 31 as an incidence surface of a second scanning lens 30 is symmetrical with respect to the main scanning direction. A second emission surface 32 as an emission surface of the second scanning lens 30 is symmetrical with respect to the main scanning direction. A center line 32C of the second emission surface 32 is shifted in the main scanning direction relative to a center line 31C of the second incidence surface 31. The center line 32C of the second emission surface 32 is tilted around an axis orthogonal to both of the main scanning direction and the center line 32C of the second emission surface 32 relative to the center line 31C of the second incidence surface 31.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a scanning optical device used in an electrophotographic image forming apparatus or the like. [Background technology]

[0002] In a scanning optical device that deflects a beam emitted from a light source by a polygon mirror to scan a surface to be scanned, the beam reflected by the polygon mirror is imaged on the surface of a photosensitive drum by a scanning lens. In such a scanning optical device, it is known that if the lens surface of the scanning lens is made symmetrical in the main scanning direction, the image formation position tends to shift in the beam traveling direction depending on the main scanning position, resulting in large field curvature.

[0003] In order to deal with such problems, a conventional technique employs a lens surface of a scanning lens that is asymmetric in the main scanning direction (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2002-048993 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, when a surface that is asymmetric in the main scanning direction is used, the manufacturing and precision control of the lens becomes difficult, resulting in a problem of reduced productivity.

[0006] SUMMARY OF THE PRESENT EMBODIMENTS An object of the present invention is to provide a scanning optical device capable of suppressing the curvature of field to a small value even when a lens surface symmetrical with respect to the main scanning direction is used. [Means for solving the problem]

[0007] A scanning optical device that solves the above-mentioned problems includes a light source that emits a beam, a polygon mirror that deflects the beam emitted from the light source in a main scanning direction, and a scanning optical system that images the beam deflected by the polygon mirror on a scanned surface, the scanning optical system having a first scanning lens and a second scanning lens arranged downstream of the first scanning lens in the beam traveling direction. The first entrance surface, which is the entrance surface of the first scanning lens, is symmetrical in the main scanning direction. The first exit surface, which is the exit surface of the first scanning lens, is symmetrical in the main scanning direction. The second entrance surface, which is the entrance surface of the second scanning lens, is symmetrical in the main scanning direction. The second exit surface, which is the exit surface of the second scanning lens, is symmetrical in the main scanning direction. The center line of the second exit surface is shifted in the main scanning direction relative to the center line of the second entrance surface. The center line of the second exit surface is tilted relative to the center line of the second entrance surface about an axis perpendicular to both the main scanning direction and the center line of the second exit surface.

[0008] The center line of the second exit surface is shifted in the main scanning direction with respect to the center line of the second entrance surface, and tilted about an axis perpendicular to both the main scanning direction and the center line of the second exit surface, so that even if the second entrance surface and the second exit surface are symmetrical in the main scanning direction, it is possible to suppress the field curvature to a small value. In addition, because the second entrance surface and the second exit surface are symmetrical in the main scanning direction, the lens can be manufactured and its precision controlled easily, and productivity can be improved.

[0009] The center line of the second exit surface may be shifted in the main scanning direction away from the rotation axis of the polygon mirror with respect to the center line of the second entrance surface.

[0010] The second scanning lens may have, as ends in the main scanning direction, a first end farther from the rotation axis of the polygon mirror and a second end closer to the rotation axis of the polygon mirror. The center line of the second exit surface may be tilted with respect to the center line of the second entrance surface such that the first end is away from the rotation axis of the polygon mirror.

[0011] The center line of the second entrance surface may be shifted in the main scanning direction toward the rotation axis of the polygon mirror with respect to the center line of the first exit surface.

[0012] The first entrance surface may be an axisymmetric aspheric surface, and the first exit surface may be an axisymmetric aspheric surface.

[0013] The first entrance surface has an axisymmetric shape, which makes it easy to manufacture the first entrance surface. The first exit surface has an axisymmetric shape, which makes it easy to manufacture the first exit surface.

[0014] The second entrance surface and the second exit surface have a conic coefficient of K vt , the central curvature is C y , the 4th to 10th order aspheric coefficients are A 4 ,A 6 ,···,A 10 Then, the sag amount z at the coordinate y in the main scanning direction is

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[0015] The light source may be arranged shifted to one side in the sub-scanning direction with respect to the center of the mirror surface of the polygon mirror in the axial direction of the polygon mirror. In this case, the beam from the light source is incident on the mirror surface of the polygon mirror at an angle with respect to a plane perpendicular to the axial direction of the polygon mirror, and the center line of the second entrance surface may be shifted to the other side in the sub-scanning direction with respect to the center line of the first exit surface.

[0016] The center line of the second entrance surface may be tilted relative to a plane perpendicular to the axial direction of the polygon mirror so that the end of the second scanning lens on the other side in the sub-scanning direction approaches the polygon mirror. Effect of the Invention

[0017] According to the scanning optical device of the present invention, even if the lens surface is symmetrical in the main scanning direction, it is possible to suppress the curvature of field to a small value. [Brief description of the drawings]

[0018] [Figure 1] 1 is a main-scanning sectional view of a scanning optical device according to an embodiment. [Diagram 2] FIG. 2 is a sub-scanning sectional view of the scanning optical device. [Diagram 3] 5A and 5B are diagrams illustrating the shift and tilt of the lens surface in the main scanning cross section. [Figure 4] 11A and 11B are diagrams illustrating the shift and tilt of the lens surface in the sub-scanning section. [Diagram 5] 1 is a table showing specifications of an optical system according to an embodiment. [Figure 6] 11 is a table showing coefficients of a first scanning lens according to an example. [Figure 7] 11 is a table showing coefficients of a second scanning lens according to an embodiment. [Figure 8] 5 is a calculation result of a beam diameter on a surface to be scanned in the embodiment. [Figure 9] 4 is a diagram showing the calculation results of the field curvature in the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Next, an embodiment of the present invention will be described in detail with reference to the drawings as appropriate. As shown in FIG. 1, a scanning optical device 1 according to one embodiment has an incident optical system 10, a polygon mirror 5, and a scanning optical system 7, which are configured to focus the laser light emitted from the incident optical system 10 into a spot on a scanned surface 9A of a photosensitive drum 9 and perform scanning.

[0020] The incident optical system 10 emits a beam and is configured to include a semiconductor laser 11, a coupling lens 12, an aperture stop 13, and a cylindrical lens 14.

[0021] The semiconductor laser 11 is an example of a light source. The semiconductor laser 11 may have a plurality of light emitting points. For example, the semiconductor laser 11 may have a plurality of light emitting elements arranged in the sub-scanning direction (not shown).

[0022] The coupling lens 12 is a collimating lens that converts the laser light emitted from the semiconductor laser 11 into a parallel beam. The coupling lens 12 may be a lens that converts the laser light into a converging beam that forms an image at a finite focal position. Alternatively, the coupling lens 12 may be a lens that converts the laser light into a diverging beam.

[0023] The aperture stop 13 is a member having an aperture 13A that determines the diameter of the beam produced by the coupling lens 12.

[0024] The cylindrical lens 14 is a lens that forms an image of the beam that has passed through the coupling lens 12 and the aperture stop 13 on the mirror surface 5A of the polygon mirror 5, in a linear shape that is elongated in the main scanning direction.

[0025] The polygon mirror 5 is a member in which a plurality of mirror surfaces 5A are arranged at equal distances from the rotation axis 5B. In FIG. 1, an example of the polygon mirror 5 is illustrated as having five mirror surfaces 5A. The number of mirror surfaces 5A of the polygon mirror 5 is not limited to five, and may be, for example, four, six or more. The polygon mirror 5 rotates at a constant speed around the rotation axis 5B. The polygon mirror 5 deflects the beam that is emitted from the incidence optical system 10 and passes through the coupling lens 12, the aperture stop 13, and the cylindrical lens 14 in the main scanning direction. The direction in which the beam is deflected is the main scanning direction. The polygon mirror 5 rotates counterclockwise in FIG. 1.

[0026] The scanning optical system 7 is an optical system that forms an image on a surface 9A to be scanned with the beam deflected by the polygon mirror 5. The scanning optical system 7 has a first scanning lens 20 and a second scanning lens 30.

[0027] The first scanning lens 20 is an fθ lens. That is, the first scanning lens 20 has an fθ characteristic such that the beam deflected at a constant angular velocity by the polygon mirror 5 is scanned at a constant velocity on the scanned surface 9A. The first scanning lens 20 also converges the beam in the main scanning direction.

[0028] The second scanning lens 30 is disposed downstream of the first scanning lens 20 in the traveling direction of the beam. The second scanning lens 30 is a lens that focuses the beam that has passed through the first scanning lens 20 into a spot shape on the scanned surface 9A. The second scanning lens 30 mainly converges the beam in the sub-scanning direction. In addition, the second scanning lens 30 corrects the deviation of the imaging position in the sub-scanning direction caused by the tilt of the mirror surface 5A of the polygon mirror 5.

[0029] The scanning optical device 1 further includes a BD sensor 40. The BD sensor 40 is disposed upstream of the second scanning lens 30 in the rotation direction of the polygon mirror 5 and downstream of the incident optical system 10 in the rotation direction of the polygon mirror 5. The BD sensor 40 is an optical sensor that detects a beam. For example, when the scanning optical device 1 is used in a laser printer, the BD sensor is a sensor for determining the start timing of laser exposure. In FIG. 1, the BD sensor 40 is disposed so as to detect a beam that passes through the first scanning lens 20 but does not pass through the second scanning lens 30, but may be disposed so as to detect a beam that passes through the second scanning lens 30, or may be disposed so as to detect a beam that does not pass through the first scanning lens 20.

[0030] In Fig. 1, each component is shown to be arranged in a plane, but in the actual scanning optical device 1, the laser light is reflected by a mirror, and the positions of each component are shifted in the direction of the rotation axis 5B of the polygon mirror 5. Fig. 1 shows the optical path expanded in a plane with the mirror omitted. The positional relationship of each component defined in the present invention is defined based on the state in which the optical path is expanded in a plane with the mirror excluded.

[0031] The scanning optical device 1 of this embodiment is, as an example, a device for a color laser printer. As shown in FIG. 2, the scanning optical device 1 has a plurality of semiconductor lasers 11. For example, the two semiconductor lasers 11 are arranged on both sides of a center plane 5C in the axial direction (sub-scanning direction) of the polygon mirror 5. One coupling lens 12 is provided corresponding to each of the two semiconductor lasers 11. One opening 13A of the aperture stop 13 is provided corresponding to each of the two semiconductor lasers 11. Only one cylindrical lens 14 is provided corresponding to each of the two semiconductor lasers 11.

[0032] Semiconductor laser 11A is disposed shifted to one side in the sub-scanning direction, for example, to the lower side in Fig. 2, with respect to the center of mirror surface 5A of polygon mirror 5 in the axial direction of polygon mirror 5. Semiconductor laser 11B is disposed shifted to the other side in the sub-scanning direction, for example, to the upper side in Fig. 2, with respect to the center of mirror surface 5A of polygon mirror 5 in the axial direction of polygon mirror 5. In Fig. 2, a center plane 5C that is perpendicular to the axial direction of polygon mirror 5 and passes through the center of mirror surface 5A of polygon mirror 5 is shown to indicate the center of mirror surface 5A in the axial direction of polygon mirror 5.

[0033] In FIG. 2, the laser light emitted from the semiconductor laser 11A arranged on one side of the central plane 5C is refracted upward by the coupling lens 12 as shown by the solid line in FIG. 2, and enters the mirror surface 5A of the polygon mirror. The beam reflected by the mirror surface 5A proceeds to the other side of the central plane 5C, that is, the upper side. The laser light emitted from the semiconductor laser 11B arranged on the other side of the central plane 5C, that is, the upper side, is refracted downward by the coupling lens 12 as shown by the two-dot chain line in FIG. 2, and enters the mirror surface 5A of the polygon mirror 5. The beam reflected by the mirror surface 5A proceeds to one side of the central plane 5C, that is, the lower side. That is, the beam from the semiconductor laser 11A enters the mirror surface 5A of the polygon mirror 5 at an angle θ_sub (minor cross-sectional ray angle) with respect to the central plane 5C perpendicular to the axial direction of the polygon mirror 5.

[0034] The semiconductor laser 11B is used for forming an image of a different color from that of the semiconductor laser 11A. The scanning optical device 1 includes two pairs of the semiconductor laser 11B and the semiconductor laser 11A, thereby enabling exposure for forming an image of four colors.

[0035] The first scanning lens 20 is a single lens into which both the beam from the semiconductor laser 11A and the beam from the semiconductor laser 11B enter. The second scanning lens 30 is provided with a lens into which the beam from the semiconductor laser 11A enters and a lens into which the beam from the semiconductor laser 11B enters, respectively. Note that in FIG. 2, the two second scanning lenses 30 are shown overlapping each other, but in reality, the beam from the semiconductor laser 11A and the beam from the semiconductor laser 11B pass through positions separated from each other by a mirror (not shown), so the two second scanning lenses 30 do not interfere with each other. The beams that pass through the first scanning lens 20 and the second scanning lens 30 form an image on the scanned surface 9A at a position Dx away from the center plane 5C in the sub-scanning direction.

[0036] Next, the first scanning lens 20 and the second scanning lens 30 will be described in detail with reference to FIGS.

[0037] [First scanning lens 20] The first scanning lens 20 has a first entrance surface 21 as an entrance surface on which the beam is incident, and a first exit surface 22 as an exit surface from which the beam is emitted. The first entrance surface 21 has a shape symmetrical with respect to the main scanning direction. The first exit surface 22 has a shape symmetrical with respect to the main scanning direction. A center line 22C of the first exit surface 22 coincides with a center line 21C of the first entrance surface 21.

[0038] The first entrance surface 21 and the first exit surface 22 are even-order aspheric surfaces symmetrical with respect to the center lines 21C and 22C. Specifically, the first entrance surface 21 and the first exit surface 22 have a conic coefficient k es , the central curvature is C, and the aspheric coefficients of the 4th to 12th orders are α 4 ,α 6 ,...,α 12 Then, the sag amount z at the distance r from each of the center lines 21C and 22C is

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[0039] A center line 20C of the first scanning lens 20 is shifted in the main scanning direction with respect to the scanning center line 9C. The scanning center line 9C is the center of the beam reflected by the polygon mirror 5 and incident on the first scanning lens 20 at an incident angle of 0°. Specifically, the center line 20C of the first scanning lens 20 is shifted by Sy1 in the main scanning direction toward the side farther from the rotation axis 5B of the polygon mirror 5 with respect to the scanning center line 9C. The center line 20C of the first scanning lens 20 is also the center line of the first entrance surface 21 and the center line 22C of the first exit surface 22.

[0040] [Second scanning lens 30] The second scanning lens 30 has a second entrance surface 31 as an entrance surface on which the beam is incident and a second exit surface 32 as an exit surface from which the beam exits. The second scanning lens 30 also has, as ends in the main scanning direction, a first end 33 farther from the rotation axis 5B of the polygon mirror 5 and a second end 34 closer to the rotation axis 5B of the polygon mirror 5.

[0041] The second entrance surface 31 has a shape symmetrical in the main scanning direction. In particular, the second entrance surface 31 is symmetrical in the main scanning direction with respect to a sub-scanning plane including a center line 31C. The second exit surface 32 has a shape symmetrical in the main scanning direction. In particular, the second exit surface 32 is symmetrical in the main scanning direction with respect to a sub-scanning plane including a center line 32C. The second entrance surface 31 and the second exit surface 32 are aspheric in the main scanning direction. The curvatures of the second entrance surface 31 and the second exit surface 32 in the sub-scanning direction change continuously and symmetrically from the respective center lines 31C, 32C toward the outside in the main scanning direction. The second entrance surface 31 and the second exit surface 32 are modified toric surfaces. In particular, the second entrance surface 31 and the second exit surface 32 have a conic coefficient K vt , the central curvature is C y , the 4th to 10th order aspheric coefficients are A 4 ,A 6 ,···,A 10 Then, the sag amount z at the coordinate y in the main scanning direction is

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[0042] In addition, the second entrance surface 31 and the second exit surface 32 have a sag amount s with respect to the generatrix at the coordinate x in the sub-scanning direction in the sagittal direction, which is

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[0043] The center line 31C of the second entrance surface 31 is shifted in the main scanning direction with respect to the center line 22C of the first exit surface 22. Specifically, the center line 31C of the second entrance surface 31 is shifted by Sy2 in the main scanning direction with respect to the center line 22C of the first exit surface 22 toward the side closer to the rotation axis 5B of the polygon mirror 5. The center line 31C of the second entrance surface 31 is on the side closer to the rotation axis 5B of the polygon mirror 5 with respect to the scanning center line 9C.

[0044] The center line 32C of the second exit surface 32 is shifted in the main scanning direction with respect to the center line 31C of the second entrance surface 31. Specifically, the center line 32C of the second exit surface 32 is shifted by Sy2R in the main scanning direction with respect to the center line 31C of the second entrance surface 31 to the side farther from the rotation axis 5B of the polygon mirror 5. The center line 32C of the second exit surface 32 is on the side farther from the rotation axis 5B of the polygon mirror 5 with respect to the scanning center line 9C.

[0045] Moreover, center line 32C of second exit surface 32 is tilted with respect to center line 31C of second entrance surface 31 around an axis (around dot 32M in the figure) perpendicular to both the main scanning direction and center line 32C of second exit surface 32. Specifically, center line 32C of second exit surface 32 is tilted by Tα2R with respect to center line 31C of second entrance surface 31 such that first end 33 moves away from rotation axis 5B of polygon mirror 5.

[0046] 4, the center line 22C of the first exit surface 22 coincides with the center plane 5C of the mirror surface 5A in the sub-scanning direction. The center 31M of the second entrance surface 31 is shifted by Sx2 to the other side in the sub-scanning direction with respect to the center line 22C of the first exit surface 22. The other side in the sub-scanning direction here refers to the side opposite to the side where the semiconductor laser 11 (11A) that emits the light that passes through the second scanning lens 30 is disposed, with the center plane 5C sandwiched therebetween.

[0047] Further, the center line 31C of the second entrance surface 31 is tilted with respect to a center plane 5C perpendicular to the axial direction of the polygon mirror 5 so that the other end of the second scanning lens 30 in the sub-scanning direction approaches the polygon mirror 5. In Fig. 4, the other end of the second scanning lens 30 in the sub-scanning direction is the upper end 30T. The second scanning lens 30 is tilted in the negative direction of the rotation direction shown in Fig. 4 so that the upper end 30T approaches the polygon mirror 5.

[0048] In the scanning optical device 1 described above, the center line 32C of the second exit surface 32 is shifted in the main scanning direction relative to the center line 31C of the second entrance surface 31, and tilted around an axis perpendicular to both the main scanning direction and the center line 32C of the second exit surface 32. This makes it possible to suppress the field curvature small even if the second entrance surface 31 and the second exit surface 32 are symmetrical in the main scanning direction. In addition, since the second entrance surface 31 and the second exit surface 32 are symmetrical in the main scanning direction, the manufacturing and precision control of the lens are facilitated, and the productivity can be improved. In addition, since the first entrance surface 21 has an axially symmetric shape, it is easy to manufacture. Since the first exit surface 22 also has an axially symmetric shape, it is easy to manufacture. EXAMPLES

[0049] The inventors confirmed that the scanning optical device 1 described in the embodiment can achieve the desired optical performance. The scanning optical device 1 has the specifications shown in FIG. 5. The locations of the dimensions and angles in FIG. 5 are shown in FIG. 2 and FIG. 3. SOS means Start of Scan, and corresponds to the start end of the effective scanning area. COS means Center of Scan, and corresponds to the center of the effective scanning area. EOS means End of Scan, and corresponds to the end end of the effective scanning area. The first scanning lens 20 is a lens surface represented by the above-mentioned formula (1), and each coefficient has a value shown in FIG. 6. The amount of shift and tilt of the first entrance surface 21 and the first exit surface 22 is a value shown in FIG. 6. The positive and negative directions of the shift and tilt are shown in FIG. 3 and FIG. 4.

[0050] The second scanning lens 30 has a lens surface represented by the above-mentioned formulas (2) and (3), and the coefficients of formulas (1) and (2) are set to the values ​​shown in Fig. 7. The amounts of shift and tilt of the second entrance surface 31 and the second exit surface 32 are set to the values ​​shown in Fig. 7.

[0051] Under the above conditions, the imaging state on the scanned surface 9A was confirmed by optical analysis, and the beam diameter in the main scanning direction (main diameter) was 80 μm or less from one end of the scanning range to the other end as shown in Fig. 8. Also, the beam diameter in the sub-scanning direction (sub-diameter) was 100 μm or less from one end of the scanning range to the other end as shown in Fig. 9. Also, the field curvature was ±0.2 mm or less from one end of the scanning range to the other end in both the main scanning direction (main curvature) and the sub-scanning direction (sub-curvature).

[0052] Although the embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment as long as it is within the scope of the present invention. Furthermore, the elements described in the above-described embodiment and modified examples may be combined in any manner. [Explanation of symbols]

[0053] 1. Scanning optical device 5 Polygon Mirror 7 Scanning Optical System 9 Photoconductor drum 9A Scanned surface 10 Input optical system 11 (11A, 11B) Semiconductor laser 20 First scanning lens 20C center line 21 1st entrance plane 21C center line 22 First exit surface 22C center line 30 Second scanning lens 31 Second entrance plane 31C center line 32 Second exit surface 32C center line 33 First end 34 Second end

Claims

1. A light source that emits a beam; a polygon mirror that deflects the beam emitted from the light source in a main scanning direction; a scanning optical system that forms an image of the beam deflected by the polygon mirror on a scanned surface, the scanning optical system having a first scanning lens and a second scanning lens disposed downstream of the first scanning lens in a traveling direction of the beam; a first incidence surface which is an incidence surface of the first scanning lens is symmetrical with respect to the main scanning direction; a first exit surface which is an exit surface of the first scanning lens is symmetrical with respect to a main scanning direction; a second incidence surface which is an incidence surface of the second scanning lens is symmetrical with respect to the main scanning direction; a second exit surface which is an exit surface of the second scanning lens is symmetrical with respect to the main scanning direction; a center line of the second exit surface is shifted in a main scanning direction with respect to a center line of the second entrance surface, A scanning optical device, characterized in that a center line of the second exit surface is tilted with respect to a center line of the second entrance surface about an axis perpendicular to both a main scanning direction and the center line of the second exit surface.

2. 2. The scanning optical device according to claim 1, wherein a center line of the second exit surface is shifted in the main scanning direction with respect to a center line of the second entrance surface toward a side farther from a rotation axis of the polygon mirror.

3. the second scanning lens has, as ends in a main scanning direction, a first end farther from a rotation axis of the polygon mirror and a second end closer to the rotation axis of the polygon mirror; 2. The scanning optical device according to claim 1, wherein a center line of the second exit surface is tilted relative to a center line of the second entrance surface such that the first end is away from a rotation axis of the polygon mirror.

4. 2. The scanning optical device according to claim 1, wherein a center line of the second entrance surface is shifted in the main scanning direction toward a side closer to a rotation axis of the polygon mirror with respect to a center line of the first exit surface.

5. the first entrance surface is an axisymmetric aspheric surface; 2. The scanning optical device according to claim 1, wherein the first exit surface is an axisymmetric aspheric surface.

6. The second entrance surface and the second exit surface have a conic coefficient of K vt , the central curvature is C y , the 4th to 10th order aspheric coefficients are A 4 , A 6 , ..., A 10 Then, the sag amount z at the coordinate y in the main scanning direction is expressed as follows: [0010] The sag amount s with respect to the generatrix at the coordinate x in the sub-scanning direction is expressed by the following formula: [0025] In the above formula, the radius of curvature r′ in the sub-scanning direction at the coordinate y in the main scanning direction of the second entrance surface and the second exit surface is expressed by the central curvature in the sagittal direction as C x And B 2 , B 4 , ..., B 10 are coefficients determined for the second entrance surface and the second exit surface, respectively. [0030] 2. The scanning optical device according to claim 1,

7. the light source is disposed shifted to one side in a sub-scanning direction with respect to a center of the mirror surface of the polygon mirror in an axial direction of the polygon mirror, a beam from the light source is incident on a mirror surface of the polygon mirror at an angle with respect to a plane perpendicular to an axial direction of the polygon mirror; 2. The scanning optical device according to claim 1, wherein a center line of the second entrance surface is shifted to the other side in the sub-scanning direction with respect to a center line of the first exit surface.

8. 8. The scanning optical device according to claim 7, wherein a center line of the second incident surface is tilted relative to a plane perpendicular to the axial direction of the polygon mirror so that an end of the second scanning lens on the other side in the sub-scanning direction approaches the polygon mirror.

Images