Optical scanner

Abstract

To provide an optical scanner capable of suppressing changes in diameter and illuminance of a spot on a surface to be scanned according to an image height, and suppressing the generation of ghost light.SOLUTION: In a main scanning cross section, a first scanning region on one side where a first incident optical system is disposed with respect to an optical axis of a first imaging optical system is longer than a second scanning region on the other side where the first incident optical system is not disposed, and when the number of deflection surfaces of the deflector is N, an angle formed by an optical axis of the first incident optical system and an optical axis of the first imaging optical system in the main scanning cross section is α1 (degrees), and a value of a ratio of widths of scanning angles corresponding to the first and second scanning regions to a maximum width of a scanning angle of the deflection surface is Duty1, the following condition is satisfied. (720 / N)×(2×Duty1+1) / 3<α1<720 / N.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical scanning device, and more particularly to an optical scanning device suitable for use in image forming apparatuses such as laser beam printers and multifunction printers having an electrophotographic process. [Background technology]

[0002] 2. Description of the Related Art Conventionally, optical scanning devices that employ an overfilled scan (OFS) system in which a deflector is made compact and multifaceted in order to achieve high speed and high definition printing are known. It is also known that in an optical scanning device employing such an OFS method, the beam width of the light beam deflected by the deflection surface of the deflector changes depending on the scanning angle of the deflection surface of the deflector, and this may cause the diameter and illuminance of the spot formed on the scanned surface to change depending on the image height.

[0003] Patent document 1 discloses an optical scanning device that suppresses changes in the spot diameter and illuminance on the scanned surface according to the image height by making a light beam enter a deflector from a direction parallel to the optical axis of an imaging optical system and suppressing changes in the width of the light beam by suppressing changes in the scanning angle of the deflection surface. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2007-171979 A Summary of the Invention [Problem to be solved by the invention]

[0005] The optical scanning device disclosed in Patent Document 1 uses a light beam having a light beam width wider than the width of the deflection surface in the main scanning cross section, so that when a given deflection surface deflects the light beam, a deflection surface adjacent to the given deflection surface may also deflect the light beam. In this case, for example, when a light beam deflected by an adjacent deflection surface is reflected by a reflective optical element, there is a risk that the light beam deflected by a specific deflection surface will be incident as ghost light on a scanned surface other than the scanned surface to which it is guided.

[0006] If ghost light is incident on the surface to be scanned in this way, there is a risk that the quality of the image formed in the image forming apparatus in which the optical scanning device is mounted will deteriorate. SUMMARY OF THE PRESENT DISCLOSURE An object of the present invention is to provide an optical scanning device capable of suppressing changes in the spot diameter and illuminance on the scanned surface depending on the image height, and suppressing the generation of ghost light. [Means for solving the problem]

[0007] The optical scanning device according to the present invention includes a deflector having a plurality of deflecting surfaces that deflects a first light beam from a first light source to scan a first scanned surface in a main scanning direction, a first incident optical system that makes the first light beam from the first light source incident on the plurality of deflecting surfaces, and a first imaging optical system that guides the first light beam deflected by the plurality of deflecting surfaces to the first scanned surface, wherein in a main scanning cross section, a width of the first light beam when incident on each of the plurality of deflecting surfaces is larger than a width of each of the plurality of deflecting surfaces, and in the first scanned surface, a first scanning region on one side where the first incident optical system is arranged with respect to an optical axis of the first imaging optical system is longer than a second scanning region on the other side where the first incident optical system is not arranged, the number of the plurality of deflecting surfaces is N, an angle between the optical axis of the first incident optical system and the optical axis of the first imaging optical system in the main scanning cross section is α 1 (degrees), and the ratio of the width of the scanning angle corresponding to the first and second scanning regions to the maximum width of each of the multiple deflection surfaces is called Duty 1 When (720 / N)×(2×Duty 1 +1) / 3<α 1 <720 / N The present invention is characterized in that it satisfies the following conditions. Effect of the Invention

[0008] According to the present invention, it is possible to provide an optical scanning device that can suppress changes in the spot diameter and illuminance on the scanned surface depending on the image height, and can also suppress the generation of ghost light. [Brief description of the drawings]

[0009] [Figure 1] 2 is a schematic development view in a main-scanning section of the optical scanning device according to the first embodiment. FIG. [Diagram 2] 3 is a schematic main-scanning sectional view of the vicinity of a polygon mirror of the optical scanning device according to the first embodiment. FIG. [Diagram 3] 5A to 5C are diagrams showing changes in spot diameter ratio, spot moving speed, and spot diameter, as well as scanning characteristics, in the optical scanning device according to the first embodiment. [Figure 4] FIG. 11 is a schematic development view in a main-scanning section of an optical scanning device according to a second embodiment. [Diagram 5] 13A and 13B are diagrams showing changes in spot diameter ratio and spot diameter in an optical scanning device according to a second embodiment. [Figure 6] FIG. 11 is a schematic development view in a main-scanning section of an optical scanning device according to a third embodiment. [Figure 7] 13A and 13B are diagrams showing changes in spot diameter ratio and spot diameter in an optical scanning device according to a third embodiment. [Figure 8] FIG. 13 is a schematic development view in a main-scanning section of an optical scanning device according to a fourth embodiment. [Figure 9] FIG. 13 is a schematic development view in a main-scanning section of an optical scanning device according to a fifth embodiment. [Figure 10] FIG. 13 is a schematic development view in a main-scanning section of an optical scanning device according to a sixth embodiment. [Figure 11] FIG. 13 is a schematic sub-scanning sectional view of an optical scanning device according to a sixth embodiment. [Figure 12] 2 is a sub-scanning sectional view of a main part of the image forming apparatus according to the embodiment. FIG. [Figure 13] FIG. 2 is a schematic partial development view in the main-scanning section of the optical scanning device disclosed in Patent Document 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an optical scanning device according to the present embodiment will be described in detail with reference to the accompanying drawings. Note that the drawings shown below may be drawn at a scale different from the actual scale in order to facilitate understanding of the present embodiment.

[0011] In the following description, the main scanning direction is the direction perpendicular to the rotation axis of the polygon mirror 5 and the optical axis of the imaging optical system 85 (the direction in which the light beam is deflected by the polygon mirror 5), and the sub-scanning direction is the direction parallel to the rotation axis of the polygon mirror 5. Moreover, the main scanning section is a section perpendicular to the sub-scanning direction, and the sub-scanning section is a section perpendicular to the main scanning direction. In the following description, the direction parallel to the optical axis of the imaging optical system 85 is defined as the X direction, the main scanning direction as the Y direction, and the sub-scanning direction as the Z direction.

[0012] [First embodiment] 2. Description of the Related Art Conventionally, an optical scanning device is used as an exposure device in an image forming apparatus such as a laser beam printer using an electrophotographic process. In an optical scanning device, a light beam from a light source means modulated in response to an image signal from an external device such as a personal computer is guided to a deflector such as a polygon mirror (rotating polygonal mirror) by an incident optical system. The light beam deflected by the deflection reflection surface of the deflector is guided by the imaging optical system onto the photosensitive surface of a photosensitive drum, which is the surface to be scanned, to form a spot, and the spot scans the photosensitive surface to expose and record image information.

[0013] Also, various color image forming apparatuses have been proposed that form color images by scanning the photosensitive surfaces of a plurality of photosensitive drums using a plurality of optical scanning devices. In response to the demand for faster and more precise printing by optical scanning devices, an overfilled scan (OFS) method has been proposed in which a polygon mirror is made smaller and has more faces.

[0014] FIG. 13 is a partial schematic development view in the main-scanning section of an optical scanning device 50 disclosed in Patent Document 1. In FIG. The optical scanning device 50 disclosed in Patent Document 1 employs a double-sided scanning method in which four scanned surfaces 32Y, 32M, 32C and 32K are provided on both sides of a polygon mirror 35 serving as a common deflector in a direction parallel to the optical axis of the imaging optical system 6.

[0015] That is, in the optical scanning device 50 disclosed in Patent Document 1, two imaging optical systems 6 (Fθ lenses, scanning lenses) are provided symmetrically on both sides of the polygon mirror 35. Moreover, the polygon mirror 35 provided in the optical scanning device 50 disclosed in Patent Document 1 is a small multifaceted rotating mirror having a plurality of deflecting reflective surfaces.

[0016] As shown in FIG. 13, a light beam emitted from a light source 12A is converted into a light beam A having a wide light beam width in the main scanning direction by an incident optical system formed of optical elements 14A and 16A. The light beam A is then reflected by the mirror 82 towards the polygon mirror 35 .

[0017] The light beam A has a width wider than the width of the deflection reflecting surface of the polygon mirror 35 in the main scanning cross section. The deflection reflecting surface moves and deflects the light beam A incident on the polygon mirror 35 so as to cut it out, and the cut-out light beam A is guided to the imaging optical system 6.

[0018] The method of making a light beam having a width wider than the width of the deflection reflection surface of the deflector in the main scanning cross section enter the deflector in this manner is called an overfilled scan (OFS) method. On the other hand, a method in which a light beam having a width narrower than the width of the deflection reflection surface of the deflector in the main scanning cross section is made incident on the deflector is called an under-filled scan (UFS) method.

[0019] The light beam A incident on the imaging optical system 6 is guided to the printing area on the photosensitive drum 32Y, which is the surface to be scanned, by the imaging performance of the imaging optical system 6, thereby forming a beam spot, and the beam spot performs printing while scanning over the printing area. As shown in FIG. 13, the light beam emitted from the light source 12B is converted into a light beam B having a wide light beam width in the main scanning direction by an incident optical system formed of optical elements 14B and 16B. The light beam B is then reflected by the mirrors 81 and 82 towards the polygon mirror 35 .

[0020] The light beam B has a width wider than the width of the deflection reflecting surface of the polygon mirror 35 in the main scanning cross section. The deflection reflecting surface moves and deflects the light beam B incident on the polygon mirror 35 so as to cut it out, and the cut-out light beam B is guided to the imaging optical system 6.

[0021] The light beam B incident on the imaging optical system 6 is guided to the printing area on the photosensitive drum 32M, which is the surface to be scanned, by the imaging performance of the imaging optical system 6, thereby forming a beam spot, and the beam spot performs printing while scanning over the printing area. The light beams emitted from the light sources 12C and 12D are also guided onto the surfaces to be scanned 32C and 32K in the same manner as above.

[0022] In the OFS system, the deflector can be made multifaceted while remaining small compared to the UFS system, so it is possible to achieve faster and more precise printing without increasing the size of the optical scanning device. On the other hand, in the OFS system, the deflecting reflection surface moves while changing the angle within the light beam incident on the deflector as described above, so that the width and energy of the deflected light beam change according to the angle.

[0023] As a result, the spot diameter of the beam spot formed on the surface to be scanned changes according to the image height, and the image surface illuminance on the surface to be scanned changes according to the image height. Furthermore, the light amount of the beam spot formed on the surface to be scanned also changes depending on the image height, so that the image surface illuminance on the surface to be scanned changes depending on the image height.

[0024] If the spot diameter and illuminance on the scanned surface vary depending on the image height in this way, a problem arises in that it becomes difficult to obtain a uniform printed image in an image forming apparatus that uses an optical scanning device as an exposure device. Therefore, in the optical scanning device 50 disclosed in Patent Document 1, in order to suppress such fluctuations in spot diameter and illuminance on the scanned surface depending on the image height, each light beam is incident on a polygon mirror 35 from a direction parallel to the optical axis of the imaging optical system 6, as shown in Figure 13.

[0025] In this case, the traveling direction of the light beam scanning the center of the print area on the surface to be scanned when it enters the polygon mirror 35 is parallel to the optical axis of the imaging optical system 6 . The entire print area can be scanned by rotating the polygon mirror 35 so that the angle of the normal to the deflection reflection surface changes by 1 / 4 of the scanning angle corresponding to each of the most off-axis image heights on both sides of the print area. That is, in the optical scanning device 50 disclosed in Patent Document 1, the change in the angle of the deflection reflection surface is reduced, thereby reducing the change in the beam width of the beam deflected by the deflection reflection surface, thereby reducing the change in the spot diameter on the scanned surface.

[0026] However, the optical scanning device 50 disclosed in Patent Document 1 has the following problems. The first problem is that the size of the device increases in order to suppress interference between the optical path of the incident optical system and the optical path of the imaging optical system 6.

[0027] Specifically, as shown in FIG. 13, the light beam A emitted from the light source 12A enters the polygon mirror 35 from the side where the mirror 82 is disposed in the direction parallel to the optical axis of the imaging optical system 6. Then, the light beam A deflected by the polygon mirror 35 travels through the imaging optical system 6 towards the side where the mirror 82 is disposed.

[0028] In the optical scanning device 50 disclosed in Patent Document 1, the optical path in the incident optical system and the optical path in the imaging optical system 6 are on the same side in the direction parallel to the optical axis of the imaging optical system 6 as described above. For this reason, it is necessary to suppress interference between the optical path in the incident optical system and the optical path in the imaging optical system 6.

[0029] In the optical scanning device 50 disclosed in Patent Document 1, in order to solve the above problem, a light beam is obliquely incident on the polygon mirror 35 in the sub-scanning cross section. Thereby, the optical path in the incident optical system and the optical path in the imaging optical system 6 are provided on opposite sides to each other with respect to a cross section that includes the center of the deflective reflecting surface and is parallel to the main scanning cross section, thereby suppressing interference therebetween.

[0030] In this case, however, in order to prevent the light beam deflected by the polygon mirror 35 from being blocked by the mirrors 81 and 82, it is necessary to increase the angle of oblique incidence of the light beam with respect to the polygon mirror 35. Furthermore, on one side of the polygon mirror 35 in a direction parallel to the optical axis of the imaging optical system 6, specifically on the right side of Figure 13, it is necessary to suppress interference between the optical paths in the incident optical systems of each of the light beams A and B and the optical paths in each of the imaging optical systems 6. In order to increase the angle of oblique incidence and suppress the interference, the optical scanning device 50 must be increased in size in the sub-scanning direction.

[0031] A second problem is that in the optical scanning device 50 disclosed in Patent Document 1, ghost light and flare light generated in the imaging optical system 6 provided on one side in a direction parallel to the optical axis of the imaging optical system 6 are likely to be incident on the opposing imaging optical system 6 on the other side. In the optical scanning device 50 disclosed in Patent Document 1, the light beam A has a width wider than the width of the deflection reflection surface of the polygon mirror 35 in the main scanning cross section as described above.

[0032] In other words, a portion of the light beam having such a wide beam width that is guided by the incident optical system provided on one side passes outside the polygon mirror 35 according to the tolerance, making it easier for the portion to enter the imaging optical system 6 provided on the other side. In addition, the light beam reflected on the surface of a specified lens of the imaging optical system 6 provided on one side also passes outside the polygon mirror 35, making it easier for it to enter the imaging optical system 6 provided on the other side.

[0033] The third problem is that in the optical scanning device 50 disclosed in Patent Document 1, adjacent surface ghosts are likely to occur, and therefore flare light is likely to occur unless the device is appropriately disposed. In the OFS method, in which the polygon mirror 35 is made smaller and more multifaceted, a light beam having a wide beam width is used as described above, so that the deflection-reflection surfaces adjacent to a given deflection-reflection surface in the polygon mirror 35 that deflects the light beam also reflect the light beam. Such ghosts that are generated by the reflection of a light beam by an adjacent deflection reflection surface are called adjacent surface ghosts.

[0034] For example, if the polygon mirror 35 has ten deflection-reflecting surfaces, an adjacent surface ghost will be generated that travels along a direction that forms an angle of ±72 degrees in the main-scan cross section with respect to the direction of travel of the light beam deflected by a predetermined deflection-reflecting surface. Furthermore, if the polygon mirror 35 has eight deflection-reflecting surfaces, an adjacent surface ghost will be generated that travels along a direction that forms an angle of ±90 degrees in the main-scan cross section with respect to the traveling direction of the light beam deflected by a specified deflection-reflecting surface.

[0035] If a reflective optical element such as a mirror is placed on the optical path of the adjacent surface ghost generated in this manner, the adjacent surface ghost will be reflected by the end face of the reflective optical element, resulting in flare light or ghost light. In the optical scanning device 50 disclosed in Patent Document 1, for example, as shown in Figure 13, consider a case in which light beam A prints near the most off-axis image height on the light source 12A side (lower side in Figure 13) in the main scanning direction on the photosensitive drum 32Y.

[0036] At this time, adjacent surface ghosts propagate in directions forming angles of 72 degrees or 90 degrees with respect to the traveling direction of light beam A on the opposite side to light source 12A in the main scanning direction (upper side in FIG. 13). In this case, the adjacent surface ghost light is reflected by the mirrors 30B and 30A, resulting in flare light and ghost light.

[0037] As described above, in the optical scanning device 50 disclosed in Patent Document 1, when the OFS method is adopted to achieve high speed and high definition, the fluctuation of the spot diameter and illuminance on the scanned surface according to the image height is suppressed, but the device becomes large and flare light and ghost light are generated. Therefore, the objective of this embodiment is to provide a compact OFS type optical scanning device that can suppress the variation in spot diameter and illuminance on the scanned surface depending on the image height, and also suppress the occurrence of flare light and ghost light.

[0038] FIG. 1 shows a schematic development view in the main-scanning section of an optical scanning device 100 according to the first embodiment. FIG. 2(a) shows a schematic main-scanning cross-sectional view in the vicinity of the polygon mirror 5 for illustrating the definition of each angle in the optical scanning device 100 according to this embodiment. FIG. 2(b) is a schematic main-scanning sectional view in the vicinity of the polygon mirror 5 for illustrating the definition of each angle in an optical scanning device according to a modified example of this embodiment.

[0039] The optical scanning device 100 according to this embodiment includes a light source 1 (first light source), a diaphragm 2, an anamorphic collimator lens 3, a polygon mirror 5, a first imaging optical element 61, and a second imaging optical element 62. The light source 1 is configured to emit a light beam (first light beam) having a predetermined wavelength, specifically, a wavelength of 790 nanometers in the optical scanning device 100 according to this embodiment. The diaphragm 2 is configured to limit the width of the light beam emitted from the light source 1 in both the main scanning direction and the sub-scanning direction.

[0040] The anamorphic collimator lens 3 is a coupling element having different refractive powers in the main scanning section and the sub-scanning section. The anamorphic collimator lens 3 converts the light beam that has passed through the diaphragm 2 into a parallel light beam in the main scanning direction. Note that the parallel light beam here includes not only a strictly parallel light beam, but also an approximately parallel light beam such as a weakly divergent light beam or a weakly convergent light beam. Moreover, the anamorphic collimator lens 3 condenses the light beam that has passed through the diaphragm 2 toward the polygon mirror 5 in the sub-scanning direction.

[0041] In the optical scanning device 100 according to this embodiment, the diaphragm 2 and the anamorphic collimator lens 3 form an incident optical system 75 (first incident optical system) that causes the light beam from the light source 1 to be incident on the polygon mirror 5. That is, in the incident optical system 75 in the optical scanning device 100 according to this embodiment, a reflecting optical element that reflects the light beam from the light source 1 is not provided.

[0042] The polygon mirror 5 is a deflector having multiple deflection reflecting surfaces (deflection surfaces) 51, specifically ten deflection reflecting surfaces 51 in the optical scanning device 100 of this embodiment, and is configured to rotate by a driving means such as a motor not shown. In the optical scanning device 100 according to this embodiment, a so-called overfilled scan (OFS) method is adopted in which the width of the incident light beam generated by the incident optical system 75 in the main scanning direction is larger than the width of the deflection reflection surface 51 of the polygon mirror 5 in the main scanning cross section.

[0043] Next, the light beam deflected by the polygon mirror 5 is focused (guided) onto the scanned surface 7 (first scanned surface) by the first imaging optical element 61 and the second imaging optical element 62, thereby forming a beam spot on the scanned surface 7. As the polygon mirror 5 rotates, the beam spot scans the scanned surface 7 from the most off-axis image height 711 (Y=Ymax+) on the positive side of the Y direction to the most off-axis image height 712 (Y=Ymax-) on the negative side of the Y direction.

[0044] In the following, the most off-axis image height 711 on the positive side in the Y direction and the most off-axis image height 712 on the negative side in the Y direction are referred to as the most positive off-axis image height 711 and the most negative off-axis image height 712, respectively. In the optical scanning device 100 of this embodiment, the first imaging optical element 61 and the second imaging optical element 62 form an imaging optical system 85 (first imaging optical system) that guides the light beam deflected by the polygon mirror 5 to the scanned surface 7.

[0045] As shown in FIG. 1, the first imaging optical element 61 is closest to the polygon mirror 5 in the imaging optical system 85 along the X direction parallel to the optical axis of the imaging optical system 85 . In the optical scanning device 100 according to this embodiment, the optical axis of the incident optical system 75 and the optical axis of the imaging optical system 85 are both parallel to the main scanning cross section.

[0046] As described above, on the scanned surface 7, a positive side scanning area (first scanning area) is formed between the on-axis image height 710 (Y=0) and the positive side most off-axis image height 711, and a negative side scanning area (second scanning area) is formed between the on-axis image height 710 and the negative side most off-axis image height 712. In the optical scanning device 100 according to this embodiment, the plus side scanning region is wider in the Y direction than the minus side scanning region. As shown in Figure 1, in the optical scanning device 100 of this embodiment, the incident optical system 75 is provided on the positive side of the Y direction, so the positive side of the Y direction and the negative side of the Y direction can also be referred to as the incident optical system side and the anti-incident optical system side, respectively.

[0047] In addition, the power (refractive power) of the imaging optical system 85 in the main scanning cross section is set so as to monotonically decrease from the position where the light beam guided to the on-axis image height 710 (hereinafter referred to as the on-axis light beam) passes through to the position where the light beam guided to the off-axis image height (hereinafter referred to as the off-axis light beam) passes through. This makes it possible to suppress fluctuations in the diameter of the beam spot formed on the scanned surface 7 by offsetting the change in the beam width of the light beam that accompanies a change in the deflection angle of the light beam by the deflection reflection surface 51 of the polygon mirror 5 in the optical scanning device 100 of this embodiment.

[0048] As a modification of the optical scanning device 100 according to this embodiment, the Y coordinate Ymax- of the minus side most off-axis image height 712 may be set to zero. That is, the scanning range of the surface to be scanned 7 may be set using only the plus side scanning area without using the minus side scanning area. In this case, it is possible to accurately design the printing position on the scanned surface 7 without designing the shapes of the first imaging optical element 61 and the second imaging optical element 62 to be asymmetric with respect to the optical axis of the imaging optical system 85.

[0049] In the optical scanning device 100 according to this embodiment, an OFS method is adopted in which the deflecting reflection surface 51 moves through a wide light beam incident on the polygon mirror 5 to deflect the light beam, so that the light beam widths in the main scanning direction of the light beams guided to each image height on the scanned surface 7 are different from one another. Specifically, consider the beam width W(θ) in the main scanning direction when the beam is deflected at a scanning angle (deflection angle, angle of view) θ by the polygon mirror 5 so as to be guided to a predetermined off-axis image height and enters the imaging optical system 85.

[0050] Also, consider the beam width W(0) in the main scanning direction when the beam is deflected at a scanning angle θ=0 by the polygon mirror 5 so as to be guided to the axial image height and enters the imaging optical system 85. At this time, in the optical scanning device 100 according to the present embodiment, in the positive scanning region, the beam width in the main scanning direction of the light beam is monotonically increasing so that W(0) < W(θ).

[0051] On the other hand, in the optical scanning device 100 according to the present embodiment, the imaging optical system 85 has an aspherical shape that is set so that the focal length varies according to the passing position. Specifically, consider the focal length f(θ) in the main scanning cross-section at the position where the light beam deflected at the scanning angle θ is incident so as to be guided to a predetermined off-axis image height by the polygon mirror 5 in the imaging optical system 85.

[0052] Also, consider the focal length f(0) in the main scanning cross-section at the position where the light beam deflected at the scanning angle θ = 0 is incident so as to be guided to the on-axis image height by the polygon mirror 5 in the imaging optical system 85. At this time, in the optical scanning device 100 according to the present embodiment, in the positive scanning region, the focal length in the main scanning cross-section at each position of the imaging optical system 85 is monotonically increasing (that is, the power is monotonically decreasing) so that f(0) < f(θ).

[0053] By adopting the above configuration in the optical scanning device 100 according to the present embodiment, the distance that the beam spot moves while the polygon mirror 5 rotates for only one unit time at a predetermined off-axis image height in the positive scanning region on the scanned surface 7 is monotonically longer than the distance at the on-axis image height. That is, the imaging optical system 85 provided in the optical scanning device 100 according to the present embodiment has non-uniform scanning characteristics.

[0054] Then, define the ratio W(θ) / W(0) of W(θ) to W(0) as dW(θ), and the ratio f(θ) / f(0) of f(θ) to f(0) as df(θ). At this time, in the optical scanning device 100 according to the present embodiment, it is preferably set so that 0.90 < df(θ) / dW(θ) < 1.10 at any scanning angle θ corresponding to the positive scanning region.

[0055] In the optical scanning device 100 according to this embodiment, by using the imaging optical system 85 formed as described above, it is possible to suppress fluctuations in the spot diameter on the scanned surface 7 that tend to occur in the OFS method. Furthermore, in the optical scanning device 100 according to this embodiment, unlike the optical scanning device 50 disclosed in Patent Document 1 shown in FIG. 13, the above-mentioned configuration eliminates the need to make the optical axis of the imaging optical system 85 and the optical axis of the incident optical system 75 parallel to each other.

[0056] In the optical scanning device 100 according to this embodiment, the aperture 2 and the anamorphic collimator lens 3, i.e., the incident optical system 75, are disposed between the exit surface of the first imaging optical element 61 and the scanned surface 7 in the X direction parallel to the optical axis of the imaging optical system 85. In other words, the incident optical system 75 is arranged in the X direction between the exit surface of the first imaging optical element 61, which is arranged closest to the polygon mirror 5 among at least one imaging optical element included in the imaging optical system 85, and the scanned surface 7.

[0057] In other words, the input optical system 75 is disposed on the opposite side of the polygon mirror 5 with respect to the exit surface of the first imaging optical element 61 in the X direction. In addition, in the optical scanning device 100 of this embodiment, the aperture 2 and the anamorphic collimator lens 3, i.e., the incident optical system 75, are each arranged between the positive side end E1 of the imaging optical system 85 and the positive side most off-axis image height 711 in the main scanning direction. In other words, in the optical scanning device 100 of this embodiment, the incident optical system 75 is arranged in the main scanning direction between the first imaging optical element 61 and the second imaging optical element 62 included in the imaging optical system 85 and the positive side most off-axis image height 711.

[0058] In the optical scanning device 100 according to this embodiment, the plus side end E1 of the imaging optical system 85 in the main scanning direction is the plus side end E1 of the second imaging optical element 62. Furthermore, in the optical scanning device 100 of this embodiment, unlike the optical scanning device 50 disclosed in Patent Document 1 shown in FIG. 13, no reflective optical element is provided to reflect the light beam emitted from the light source 1 toward the polygon mirror 5.

[0059] Furthermore, in the optical scanning device 100 of this embodiment, as shown in FIG. 1, the light source 1 is disposed in the main scanning direction between the positive-side most off-axis image height 711 and the positive-side end E1 of the imaging optical system 85, i.e., the positive-side end E1 of the second imaging optical element 62. In other words, in the optical scanning device 100 of this embodiment, the light source 1 is arranged in the main scanning direction between the first imaging optical element 61 and the second imaging optical element 62 included in the imaging optical system 85 and the positive side most off-axis image height 711. Furthermore, the light source 1 is disposed between the emission surface of the first imaging optical element 61 and the surface to be scanned 7 in the X direction parallel to the optical axis of the imaging optical system 85 .

[0060] In the optical scanning device 100 according to this embodiment, by arranging the light source 1 and the incident optical system 75 as described above, interference between the optical paths in the incident optical system 75 and the imaging optical system 85 can be suppressed. Furthermore, in the optical scanning device 100 according to this embodiment, miniaturization can be achieved by arranging the light source 1 and the incident optical system 75 on the relatively wider Y direction positive side of the Y direction positive side and the Y direction negative side which are asymmetric with each other in the main scanning direction.

[0061] Next, the conditional expressions for the traveling direction of the light beam in the optical scanning device 100 according to this embodiment will be described with reference to FIGS. 2(a) and 2(b). In FIG. 2A, a principal ray Lp of a light beam (hereinafter referred to as a positive-side most off-axis light beam) that is deflected by the deflection reflecting surface 51 of the polygon mirror 5 and travels to a positive-side most off-axis image height 711 is max+ is shown. In FIG. 2A, the principal ray Lp of the light beam (hereinafter referred to as the minus-side most off-axis light beam) that is deflected by the deflection reflecting surface 51 of the polygon mirror 5 and travels to the minus-side most off-axis image height 712 ismax- is shown.

[0062] As shown in FIG. 2(a), when projected onto the main scanning section, the principal ray Lp max+ and the chief ray Lp max- The angle that the traveling direction of each of the beams makes with respect to the optical axis of the imaging optical system 85 is θ max+ and θ max- It is defined as: At this time, the width θ2 of the scanning angle (printing angle of view) for the plus side scanning area and the minus side scanning area on the scanned surface 7 in the optical scanning device 100 according to this embodiment is defined by the following formula (1). θ2=θ max+ -θ max- (1)

[0063] In addition, in the polygon mirror 5 having N deflection reflecting surfaces 51, the angle between the normals of adjacent deflection reflecting surfaces 51 is 360 / N (degrees). That is, the light beam deflected by the deflection reflection surface 51 can scan the surface to be scanned 7 by a scanning angle of 720 / N (degrees).

[0064] In other words, the maximum width θ1 (degrees) of the scanning angle (printing angle of view) in the optical scanning device 100 according to this embodiment is defined by the following formula (2). θ1=720 / N (2)

[0065] Here, if the ratio of the scanning angle width θ2 to the maximum scanning angle width θ1 is defined as the duty, the duty in the optical scanning device 100 according to this embodiment can be expressed by the following formula (3).

number

[0066] Next, the traveling direction of ghost light generated by adjacent deflection reflection surfaces 51 in the optical scanning device 100 according to this embodiment will be described. In the polygon mirror 5 having N deflection reflecting surfaces 51, the angle between the normal of a given deflection reflecting surface 51 and each of the adjacent deflection reflecting surfaces 51 is +360 / N (degrees).

[0067] Moreover, the angle between the normal lines of a given deflection reflection surface 51 and the other adjacent deflection reflection surface 51 is −360 / N (degrees). Therefore, in the optical scanning device 100 of this embodiment, the direction of travel of the ghost light generated by a deflection reflection surface 51 adjacent to a predetermined deflection reflection surface 51 forms an angle of ±720 / N (degrees) with respect to the direction of travel of the light beam deflected by the predetermined deflection reflection surface 51.

[0068] In other words, in the optical scanning device 100 of this embodiment which employs the OFS method, the ghost light scans a position on the scanned surface 7 which is spaced a distance corresponding to the scanning angle ±720 / N (degrees) from the position scanned by the light beam deflected by a predetermined deflection reflection surface 51. In addition, the angle ±720 / N (degrees) between the direction of travel of the ghost light generated by a deflection-reflection surface 51 adjacent to a given deflection-reflection surface 51 and the direction of travel of the light beam deflected by the given deflection-reflection surface 51 is equal to the maximum width θ1 of the above-mentioned scanning angle.

[0069] Here, when the optical axis of the incident optical system 75 is represented as the Xi axis and the optical axis of the imaging optical system 85 is represented as the X axis, consider the angle between the Xi axis and the X axis when projected onto the main scanning cross section, that is, the incident angle α (degrees) of the light beam with respect to the polygon mirror 5. In the optical scanning device 100 according to this embodiment, the incident angle α is maximum when the Y coordinate Ymax- of the minus-side most off-axis image height 712 is 0 as shown in FIG. 2(b).

[0070] At this time, when the light beam deflected by a given deflection reflection surface 51 travels in a direction of scanning angle θ = 0 (degrees) so as to scan the minus-side most off-axis image height 712, ghost light generated by an adjacent deflection reflection surface 51 travels in a direction of scanning angle θ = ±720 / N (degrees). In this case, if the incident angle α is set to 720 / N (degrees) or more, the ghost light will travel along the optical axis of the incident optical system 75 when scanning a specified image height between the negative side most off-axis image height 712 and the positive side most off-axis image height 711 on the scanned surface 7.

[0071] In the optical scanning device 100 according to this embodiment, if the ghost light travels along the optical axis of the incident optical system 75 and returns to the light source 1, there is a risk that the automatic light intensity control (auto power control: APC) will malfunction. Therefore, in the optical scanning device 100 according to this embodiment, the incident angle α (degrees) is required to satisfy the following conditional expression (4). α<720 / N ···(4)

[0072] On the other hand, as described above, the optical scanning device 100 according to this embodiment can scan only a scanning area on the surface to be scanned 7 that corresponds to the width θ2 of the scanning angle within the maximum width θ1 of the scanning angle. That is, in the optical scanning device 100 according to this embodiment, it is required to perform synchronous detection within the maximum scan angle width θ1 other than the scan angle width θ2, in other words, within the range of θ1-θ2.

[0073] Here, in the optical scanning device 100 according to this embodiment, when the width of the scanning angle required for synchronous detection is represented as θ3, the width θ3 is set to a width other than the scanning angle width θ2, that is, 1 / 3 of θ1-θ2. That is, in the optical scanning device 100 according to this embodiment, the width θ3 (degrees) is required to be set as shown in the following formula (5) from formulas (2) and (3).

number

[0074] Here, in order for the optical path in the incident optical system 75 and the optical path in the synchronous detection optical system (not shown) to not interfere with each other, i.e., to not overlap with each other, it is required that the incident angle α be set to be greater than the sum θ2 + θ3 of the widths θ2 and θ3. Therefore, in the optical scanning device 100 according to this embodiment, the incident angle α (degrees) is required to satisfy the following conditional expression (6) using expressions (3) and (5).

number

[0075] For the above reasons, the optical scanning device 100 according to this embodiment is required to satisfy the following conditional expression (7).

number

[0076] As described above, if the lower limit of conditional expression (7) is not reached, the optical path in the incident optical system 75 will interfere with the optical path in the imaging optical system 85 and the optical path in the synchronous detection optical system (not shown). On the other hand, if the upper limit of condition (7) is exceeded, ghost light generated by the adjacent deflection reflection surface 51 will be incident on the light source 1 .

[0077] In the optical scanning device 100 according to this embodiment, the width θ3 of the scanning angle required for synchronous detection is preferably set to a width other than the width θ2 of the scanning angle, that is, to ½ of θ1-θ2. In other words, in the optical scanning device 100 according to this embodiment, it is preferable to set the width θ3 as expressed by the following formula (5a).

number

[0078] That is, in the optical scanning device 100 according to this embodiment, it is preferable that the incident angle α (degrees) is set so as to satisfy the following conditional expression (6a).

number

[0079] Therefore, in the optical scanning device 100 according to this embodiment, it is preferable that the following conditional formula (7a) be satisfied.

number

[0080] Next, the specifications of the optical scanning device 100 according to this embodiment are shown in Tables 1 and 2 below. Unless otherwise specified in Table 1, the unit of distance is millimeters, and in Table 2, "Ex" stands for "×10 -x " It means.

[0081] [Table 1]

[0082] [Table 2]

[0083] In the optical scanning device 100 of this embodiment, the generatrix shapes (shapes in the main scanning cross section) of the exit surface of the anamorphic collimator lens 3 and the entrance and exit surfaces of each of the first imaging optical element 61 and the second imaging optical element 62 are expressed by the following equation (8).

number

[0084] Note that equation (8) uses a local coordinate system whose origin is the surface vertex, which is the intersection point of each lens surface (optical surface) with the optical axis. The direction in which the light beam travels (i.e., the optical axis) is the X-axis, the axis perpendicular to the X-axis in the main-scanning cross section is the Y-axis, and the axis perpendicular to the X-axis in the sub-scanning cross section is the Z-axis. In particular, each of the lens surfaces of the first imaging optical element 61 and the second imaging optical element 62 has an aspheric shape that is expressed by a function of Y up to the tenth order.

[0085] In addition, in formula (8), R is the radius of curvature (generatrix radius of curvature) in the main scanning section, and K, B 2 , B 4 , B 6 , B 8and B. 10 are the aspheric coefficients. In addition, the aspheric coefficient B 2 , B 4 , B 6 , B 8 and B. 10 Regarding the above, the values ​​may be different between the +Y side and the -Y side.

[0086] This allows the generatrix shape of the optical surface to be asymmetric across the optical axis in the main scanning direction. In Table 2 above, the aspheric coefficients on the +Y side (light source side) are B 2u , B 4u , B 6u , B 8u and B. 10u and the aspheric coefficient on the -Y side (opposite the light source side) is B 2l , B 4l , B 6l , B 8l and B. 10l It is expressed as follows.

[0087] Moreover, the sagittal shapes (shapes in the sub-scanning cross section) of the exit surface of the anamorphic collimator lens 3, and the entrance and exit surfaces of each of the first and second imaging optical elements 61 and 62 are expressed by the following formula (9).

[0088]

number

[0089] The sagittal shape referred to here includes a surface normal on the generatrix at each position in the main scanning direction and indicates a surface shape in a cross section perpendicular to the main scanning cross section. In addition, M in formula (9) jk In the optical scanning device 100 according to the present embodiment, all the additional aspherical coefficients M jk However, it is not limited to this, and at least one additional aspheric coefficient M jk may be set to a value other than 0.

[0090] Specifically, the linear term of Z in equation (9) is a term that contributes to the amount of tilt (sagittal tilt amount) in the sub-scanning cross section of the optical surface. Therefore, the aspheric coefficient M on the +Y side (light source side) 0_1u ~M 16_1u and the aspheric coefficient M on the -Y side (opposite the light source) 0_1l ~M 16_1l By making the values ​​of and different from each other, the amount of sagittal tilt can be changed asymmetrically in the main scanning direction.

[0091] Furthermore, r' shown in equation (9) indicates the radius of curvature (sagittal radius of curvature) in the sub-scanning cross section at a position Y away from the optical axis in the main scanning direction, and is expressed by the following equation (10).

number

[0092] Here, r is the radius of curvature of the sagittal on the optical axis, and E 2 , E 4 , E 6 , E 8 and E 10 are the aspheric coefficients (sagittal change coefficients). In addition, the aspheric coefficient E 2 ~E 10 Regarding the above, the values ​​may be different between the +Y side and the -Y side. This makes it possible to set the aspheric amount of the secondary line shape asymmetrically across the optical axis in the main scanning direction.

[0093] In Table 2 above, the aspheric coefficient on the +Y side (light source side) is E 2u , E 4u , E 6u , E 8u and E 10u The aspheric coefficient on the -Y side (opposite the light source side) is E 2l , E 4l , E 6l , E 8l and E 10l It is expressed as follows. Furthermore, although equation (10) includes only even-numbered terms of Y, odd-numbered terms of Y may be added. In addition, in the optical scanning device 100 according to this embodiment, the generatrix shape and sagittal shape of each optical element are defined using the functions expressed in equations (8) and (9), respectively, but they may be defined using other functions without being limited thereto.

[0094] As shown in Table 1, in the optical scanning device 100 according to this embodiment, N and Duty are 10 and 0.609, respectively. Therefore, the upper and lower limits of condition (7) are 72 degrees and 53.2 degrees, respectively, and the upper and lower limits of condition (7a) are 72 degrees and 57.9 degrees, respectively.

[0095] As shown in Table 1, in the optical scanning device 100 according to this embodiment, the incident angle α is 65 degrees, and therefore conditional expressions (7) and (7a) are satisfied. In this manner, in the optical scanning device 100 according to this embodiment, the plus-side most off-axis image height 711 and the minus-side most off-axis image height 712 are set asymmetrically to each other so that the conditional expressions (7) and (7a) are satisfied. In addition, in the optical scanning device 100 according to this embodiment, N is 10, and therefore the condition N≧9 is also satisfied.

[0096] Next, the optical characteristics of the optical scanning device 100 according to this embodiment will be described in detail. In the optical scanning device 100 according to this embodiment, the size of the light beam emitted from the light source 1 is determined by the diaphragm 2 . The width of the diaphragm 2 in the main scanning direction is made sufficiently wide so that the width of the incident light beam in the main scanning direction is greater than the width of the deflection reflection surface 51 of the polygon mirror 5 in the main scanning cross section.

[0097] The light beam that has passed through the diaphragm 2 is converted into a parallel light beam or a weakly convergent light beam in the main scanning cross section by an anamorphic collimator lens 3, which is a coupling lens having anamorphic power. The light beam that has passed through the diaphragm 2 is converted by the anamorphic collimator lens 3 into converging light so as to form a line image on the deflection reflecting surface 51 of the polygon mirror 5 in the sub-scanning cross section. In the optical scanning device 100 according to this embodiment that employs the OFS method, the beam width of the beam deflected by the polygon mirror 5 is determined by the width of the deflection reflection surface 51 of the polygon mirror 5 .

[0098] The polygon mirror 5 provided in the optical scanning device 100 according to this embodiment is a deflector having ten deflection reflecting surfaces 51, and is rotated at a predetermined constant angular velocity by a drive motor (not shown). The light beam deflected by the polygon mirror 5 is guided onto the surface to be scanned by the imaging optical system 85, and as the polygon mirror 5 rotates at a constant speed, the scanning range on the surface to be scanned is scanned and printed by the light beam.

[0099] As described above, since the optical scanning device 100 according to this embodiment employs the OFS method, the width of the light beam deflected by the deflection reflection surface 51 in the main scanning direction changes according to the scanning angle θ. Here, when the circumscribing circle diameter of the polygon mirror 5 is φ and the number of deflection reflection surfaces 51 of the polygon mirror 5 is N, the width W of the deflection reflection surface 51 in the main scanning section is 0 is expressed as the following equation (11).

number

[0100] The angle that the normal to the deflection reflection surface 51 of the polygon mirror 5 makes with the optical axis of the imaging optical system 85 is defined as θ p Then, the angle θ p changes with the rotation of the polygon mirror 5. At this time, the angle that the optical axis of the incident optical system 75 makes with the normal to the deflection reflection surface 51 is α-θ p Therefore, the width W of the light beam in the main scanning direction of the light beam deflected by the deflection reflection surface 51 is expressed by the following formula (12).

number

[0101] Also, angles α and θ p and θ satisfy the relationship expressed by the following equation (13).

number

[0102] Therefore, by substituting equation (13) into equation (12), we obtain the following equation (14).

number

[0103] That is, from equation (14), from the light beam deflected toward the on-axis image height 710 to the light beam deflected toward the positive-side most off-axis image height 711, that is, as the scanning angle θ increases, the light beam width in the main scanning direction increases monotonically.

[0104] Further, the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 is expressed by the following formula (15).

number

[0105] Here, K is a constant, λ is the wavelength of the light beam emitted from the light source 1, f is the focal length of the imaging optical system 85, and W is the width of the light beam incident on the imaging optical system 85 in the main scanning direction. The constant K is estimated to be about 1.42 when the cross-sectional shape of the light beam entering the imaging optical system 85 is rectangular.

[0106] Here, the optical scanning device 100 according to this embodiment adopts the OFS method, and the light beam width W is expressed as in equation (14). Therefore, by substituting equation (14) into equation (15), the following equation (16) is obtained.

number

[0107] In a conventional optical scanning device using the OFS method, the image height Y has a scanning characteristic expressed by the following formula (17). Y = Fθ (17) Here, F is called the Fθ coefficient, and is equal to the focal length f(θ=0) of the imaging optical system 85 at the on-axis position.

[0108] That is, in an optical scanning device using a conventional OFS method, there is a constant-speed scanning relationship between the amount of change dθ in the scanning angle θ caused by the polygon mirror 5 rotating at a constant speed per unit time and the distance dY that the spot moves while scanning the scanned surface 7, as expressed by the following equation (18).

number

[0109] Here, dY / dθ can also be called the moving distance of the spot per unit time, that is, the moving speed of the spot, since the polygon mirror 5 rotates at a constant speed. In other words, dY / dθ can be called the distance that the light beam scans on the surface 7 to be scanned per unit time, that is, the scanning speed of the light beam on the surface 7 to be scanned.

[0110] Here, the ratio of the beam width in the main scanning direction of the beam incident on each main scanning direction position of the imaging optical system 85 to the beam width in the main scanning direction of the beam incident on the axial position of the imaging optical system 85 is defined as dW(θ). Also, the ratio of the spot diameter in the main scanning direction at each image height on the scanned surface 7 to the spot diameter in the main scanning direction at the on-axis image height on the scanned surface 7 is denoted as dSPOT(θ).

[0111] At this time, the following equation (19) is obtained based on equation (16).

number

[0112] In this way, in an optical scanning device using the conventional OFS method, the spot diameter ratio dSPOT(θ) at each scanning angle depends on the light beam width ratio dW(θ) at each scanning angle according to equation (19).

[0113] Based on equation (19), the dependency of the spot diameter ratio dSPOT on the scanning angle θ is shown by the dashed line in FIG. As shown in FIG. 3(a), dSPOT varies by approximately 17.8% over the entire range of the scanning angle θ. In an optical scanning device using the conventional OFS method, the variation in the light beam width W was the cause of the variation in the spot diameter SPOT in the main scanning direction.

[0114] On the other hand, the optical scanning device 100 according to this embodiment has a scanning characteristic as expressed by the following formula (20).

number

[0115] Here, F is called the imaging coefficient, and is equal to the focal length f(θ=0) of the imaging optical system 85 at the on-axis position. Specifically, the imaging coefficient F corresponds to the Fθ coefficient F in Y=Fθ shown in the above equation (17) which represents the scanning characteristic (Fθ characteristic) when a parallel light beam is incident on the imaging optical system 85.

[0116] That is, the imaging coefficient F is a coefficient for making the light collection position Y and the scanning angle θ proportional to each other, similar to the Fθ characteristic, when a light beam other than a parallel light beam is incident on the imaging optical system 85. In the optical scanning device 100 according to this embodiment, the imaging coefficient F is 270, as shown in Table 1.

[0117] In addition, αi (i=2, 3, 4, . . . ) in the formula (20) is a scanning characteristic coefficient for determining the scanning characteristic of the imaging optical system 85. For example, when αi = 0 for all values ​​of i, equation (20) can be expressed as Y = Fθ, and therefore equation (20) corresponds to the scanning characteristic Y = Fθ of an imaging optical system provided in a conventional optical scanning device that performs constant speed scanning.

[0118] That is, in the optical scanning device 100 according to this embodiment, there is a non-uniform scanning relationship between the amount of change dθ in the scanning angle θ caused by the polygon mirror 5 rotating at a uniform speed per unit time and the distance dY that the spot moves while scanning the scanned surface 7, as expressed in the following equation (21). In other words, in the optical scanning device 100 according to this embodiment, the scanning speed of the light beam on the scanned surface 7 varies depending on the image height.

number

[0119] FIG. 3(b) shows the scanning characteristics of a conventional optical scanning device and the scanning characteristics of the optical scanning device 100 according to this embodiment. As shown in FIG. 3(b), in a conventional optical scanning device, the image height Y changes with respect to the scanning angle θ as a linear function shown in equation (17), whereas in the optical scanning device 100 according to this embodiment, the image height Y changes with respect to the polynomial function shown in equation (20). As the scanning angle θ increases, the difference between the respective scanning characteristics increases. i is set.

[0120] Figure 3(c) shows the dependence of the movement speed (scanning speed) dY / dθ on the scanning angle θ in a conventional optical scanning device shown in equation (18), and the dependence of the movement speed dY / dθ on the scanning angle θ in the optical scanning device 100 of this embodiment shown in equation (21). As shown in FIG. 3(c), in a conventional optical scanning device, the moving speed dY / dθ is a constant value F regardless of the scanning angle θ, whereas in the optical scanning device 100 of this embodiment, the moving speed dY / dθ monotonically increases as the scanning angle θ increases.

[0121] In this manner, in the optical scanning device 100 according to this embodiment, the imaging optical system 85 has a non-uniform scanning scanning characteristic in which the moving speed dY / dθ monotonically increases as the scanning angle θ increases from the on-axis image height 710 to the positive-side most off-axis image height 711. When the ratio of the focal length f(θ) at each main scanning direction position of the imaging optical system 85 to the focal length f(0) at the on-axis position of the imaging optical system 85 is df(θ), the following equation (22) is satisfied.

number

[0122] In equation (22), A is a constant. From equation (22), the focal length f(θ) of the imaging optical system 85 at each position in the main scanning direction can be expressed as in equation (23) below.

number

[0123] Thus, in the optical scanning device 100 according to this embodiment, as the scanning angle θ increases from the on-axis image height 710 to the positive-side most off-axis image height 711, the power of the imaging optical system 85 in the main scanning cross section decreases, i.e., the focal length f(θ) in the main scanning cross section becomes longer. That is, in the optical scanning device 100 according to this embodiment, equations (16) and (19) can be rewritten as the following equations (24) and (25).

number

number

[0124] In this case, the beam width does not change depending on the scanning angle θ, that is, when dW(θ) is set to 1, the spot diameter ratio dSPOT(θ) at each scanning angle depends on the focal length ratio df(θ) at each scanning angle according to equation (25).

[0125] The dotted line in FIG. 3(a) shows the dependency of the spot diameter ratio dSPOT on the scanning angle θ when dW(θ) is set to 1 based on equation (25). The dependence of dSPOT on the scan angle θ when both dW(θ) and df(θ) are changed based on equation (25) is shown by the solid line in FIG. 3(a). As shown in Figure 3(a), it can be seen that the change in the spot diameter ratio dSPOT(θ) when the light beam width ratio dW(θ) changes and the change in the spot diameter ratio dSPOT(θ) when the focal length ratio df(θ) changes cancel each other out.

[0126] Specifically, it can be seen that the fluctuation range of the spot diameter ratio dSPOT(θ), which was approximately 17.8% over the entire range of the scanning angle θ due to the change in the light beam width ratio dW(θ), can be reduced to approximately 0% by changing the focal length ratio df(θ). That is, in an optical scanning device using the conventional OFS method, the spot diameter ratio dSPOT(θ) changes by approximately 17.8% over the entire range of the scanning angle θ. On the other hand, in the optical scanning device 100 according to this embodiment, the change in the spot diameter ratio dSPOT(θ) can be reduced to approximately 0% over the entire range of the scanning angle θ.

[0127] FIG. 3(d) shows the size of the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 calculated using the specification values ​​shown in Tables 1 and 2 in the optical scanning device 100 of this embodiment. As shown in FIG. 3(d), in the optical scanning device 100 according to this embodiment, the spot diameter SPOT has a value of 70.2 micrometers to 72.5 micrometers over the entire image height from the negative side most off-axis image height 712 to the positive side most off-axis image height 711. That is, the spot diameter SPOT has a fluctuation range of about 3.3% over the entire image height.

[0128] Here, the fluctuation range of the spot diameter SPOT shown in FIG. 3(d) is approximately 3.3%, which is greater than the reduced fluctuation range of the spot diameter ratio dSPOT(θ) shown in FIG. 3(a) which is approximately 0%. This is because the calculation shown in FIG. 3(d) includes the effects of residual aberrations such as wavefront aberration and partial magnification.

[0129] Moreover, the optical scanning device 100 according to this embodiment is preferably designed to satisfy the following conditional expression (26) at any scanning angle θ from the on-axis image height 710 to the plus-side most off-axis image height 711.

number

[0130] As described above, in the optical scanning device 100 according to this embodiment, when the OFS method is adopted to achieve high speed and high resolution while suppressing an increase in the number of parts, it is possible to suppress the change in spot diameter and illuminance on the scanned surface 7 according to the image height, and also to suppress the generation of ghost light.

[0131] In the optical scanning device 100 according to this embodiment, the anamorphic collimator lens 3 is used in the incident optical system 75, but the present invention is not limited to this. That is, instead of the anamorphic collimator lens 3, a combination of a rotationally symmetric coupling lens and a cylindrical lens having power in the sub-scanning cross section may be used.

[0132] [Second embodiment] FIG. 4 shows a schematic development view in the main-scanning section of an optical scanning device 200 according to the second embodiment. The optical scanning device 200 according to this embodiment has the same configuration as the optical scanning device 100 according to the first embodiment except for different specification values, so the same components are given the same reference numerals and descriptions thereof are omitted.

[0133] Specifically, in the optical scanning device 200 according to this embodiment, the angle between the optical axis of the incident optical system 75 and the optical axis of the imaging optical system 85, ie, the incident angle α of the light beam on the polygon mirror 5, is 59 degrees. In the optical scanning device 200 according to this embodiment, the incident optical system 75 causes the light beam to be obliquely incident on the deflection reflection surface 51 of the polygon mirror 5 at an angle αs of 2 degrees in the sub-scanning section with respect to the main-scanning section.

[0134] Next, the specifications of the optical scanning device 200 according to this embodiment are shown in Tables 3 and 4 below. Unless otherwise specified in Table 3, the unit of distance is millimeters, and in Table 4, "Ex" stands for "×10 -x " It means.

[0135] [Table 3]

[0136] [Table 4]

[0137] As shown in Table 3, in the optical scanning device 200 according to this embodiment, N and Duty are 10 and 0.615, respectively. Therefore, the upper and lower limits of condition (7) are 72 degrees and 53.5 degrees, respectively, and the upper and lower limits of condition (7a) are 72 degrees and 58.1 degrees, respectively. As shown in Table 3, in the optical scanning device 200 according to this embodiment, the incident angle α is 59 degrees, and therefore conditional expressions (7) and (7a) are satisfied.

[0138] FIG. 5(a) shows, by a solid line, the dependency of dSPOT on the scanning angle θ in the optical scanning device 200 according to this embodiment. As shown by the dashed line in FIG. 5(a), in an optical scanning device using the conventional OFS method, the spot diameter ratio dSPOT(θ) changed by approximately 15.1% over the entire range of the scanning angle θ.

[0139] On the other hand, in the optical scanning device 200 according to this embodiment, the spot diameter is also changed by changing the power in the main scanning cross section of the imaging optical system 85 according to the magnitude of the scanning angle θ, as shown by the dotted line in FIG. 5(a). This makes it possible to reduce the change in the spot diameter ratio dSPOT(θ) to approximately 0% over the entire range of the scanning angle θ.

[0140] FIG. 5(b) shows the size of the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 calculated using the specification values ​​shown in Tables 3 and 4 in the optical scanning device 200 of this embodiment. As shown in FIG. 5(b), in the optical scanning device 200 according to this embodiment, the spot diameter SPOT has a value between 70.4 micrometers and 75.2 micrometers over the entire image height from the negative side most off-axis image height 712 to the positive side most off-axis image height 711. That is, the spot diameter SPOT has a fluctuation range of about 6.8% over the entire image height.

[0141] Here, the fluctuation range of the spot diameter SPOT shown in FIG. 5(b) is approximately 6.8%, which is greater than the reduced fluctuation range of the spot diameter ratio dSPOT(θ) shown in FIG. 5(a) which is approximately 0%. This is because the calculation shown in FIG. 5(b) includes the effects of residual aberrations such as wavefront aberration and partial magnification.

[0142] As described above, in the optical scanning device 200 according to this embodiment, when the OFS method is adopted to achieve high speed and high definition while suppressing an increase in the number of parts, it is possible to suppress the change in spot diameter and illuminance on the scanned surface 7 according to the image height, and also to suppress the generation of ghost light.

[0143] [Third embodiment] FIG. 6 shows a schematic development view in the main-scanning section of an optical scanning device 300 according to the third embodiment. The optical scanning device 300 according to this embodiment has the same configuration as the optical scanning device 200 according to the second embodiment except for different specification values, so the same components are given the same reference numerals and descriptions thereof are omitted.

[0144] Specifically, in the optical scanning device 300 of this embodiment, the positive side most off-axis image height 711 and the negative side most off-axis image height 712 are each shifted by approximately 20 mm to the negative side in the Y direction compared to the optical scanning device 200 of the second embodiment.

[0145] Next, the specifications of the optical scanning device 300 according to this embodiment are shown in Tables 5 and 6 below. Unless otherwise specified in Table 5, the unit of distance is millimeters, and in Table 6, "Ex" stands for "×10 -x " It means.

[0146] [Table 5]

[0147] [Table 6]

[0148] As shown in Table 5, in the optical scanning device 300 according to this embodiment, N and Duty are 10 and 0.623, respectively. Therefore, the upper and lower limits of condition (7) are 72 degrees and 53.9 degrees, respectively, and the upper and lower limits of condition (7a) are 72 degrees and 58.4 degrees, respectively. As shown in Table 5, in the optical scanning device 300 according to this embodiment, the incident angle α is 59 degrees, and therefore conditional expressions (7) and (7a) are satisfied.

[0149] FIG. 7(a) shows, by a solid line, the dependency of dSPOT on the scanning angle θ in the optical scanning device 300 according to this embodiment. As shown by the dashed line in FIG. 7(a), in an optical scanning device using the conventional OFS method, the spot diameter ratio dSPOT(θ) changed by approximately 15.1% over the entire range of the scanning angle θ.

[0150] On the other hand, in the optical scanning device 300 according to this embodiment, the spot diameter is also changed by changing the power in the main scanning cross section of the imaging optical system 85 according to the magnitude of the scanning angle θ, as shown by the dotted line in FIG. 7(a). This makes it possible to reduce the change in the spot diameter ratio dSPOT(θ) to approximately 0% over the entire range of the scanning angle θ.

[0151] FIG. 7(b) shows the size of the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 calculated using the specification values ​​shown in Tables 5 and 6 in the optical scanning device 300 of this embodiment. As shown in FIG. 7(b), in the optical scanning device 300 according to this embodiment, the spot diameter SPOT has a value between 69.6 micrometers and 75.9 micrometers over the entire image height from the negative side most off-axis image height 712 to the positive side most off-axis image height 711. That is, the spot diameter SPOT has a fluctuation range of about 9.1% over the entire image height.

[0152] Here, the fluctuation range of the spot diameter SPOT shown in FIG. 7(b) is approximately 9.1%, which is greater than the reduced fluctuation range of the spot diameter ratio dSPOT(θ) shown in FIG. 7(a) which is approximately 0%. This is because the calculation shown in FIG. 7(b) includes the effects of residual aberrations such as wavefront aberration and partial magnification.

[0153] As described above, in the optical scanning device 300 according to this embodiment, when the OFS method is adopted to achieve high speed and high definition while suppressing an increase in the number of parts, it is possible to suppress the change in spot diameter and illuminance on the scanned surface 7 according to the image height, and also to suppress the generation of ghost light.

[0154] [Fourth embodiment] FIG. 8 shows a schematic development view in the main-scanning section of an optical scanning device 400 according to the fourth embodiment. The optical scanning device 400 according to this embodiment has the same configuration as the optical scanning device 300 according to the third embodiment except for different specification values, so the same components are given the same reference numerals and descriptions thereof are omitted.

[0155] Specifically, in the optical scanning device 400 according to this embodiment, the second imaging optical element 62 is shifted by approximately 20 mm toward the scanned surface 7 side compared to the optical scanning device 300 according to the third embodiment.

[0156] Next, the specifications of the optical scanning device 400 according to this embodiment are shown in Tables 7 and 8 below. Unless otherwise specified in Table 7, the unit of distance is millimeters, and in Table 8, "Ex" stands for "×10 -x " It means.

[0157] [Table 7]

[0158] [Table 8]

[0159] As shown in Table 7, in the optical scanning device 400 according to this embodiment, N and Duty are 10 and 0.623, respectively. Therefore, the upper and lower limits of condition (7) are 72 degrees and 53.9 degrees, respectively, and the upper and lower limits of condition (7a) are 72 degrees and 58.4 degrees, respectively. As shown in Table 7, in the optical scanning device 400 according to this embodiment, the incident angle α is 59 degrees, and therefore conditional expressions (7) and (7a) are satisfied.

[0160] As described above, in the optical scanning device 400 according to this embodiment, when the OFS method is adopted to achieve high speed and high definition while suppressing an increase in the number of parts, it is possible to suppress the change in spot diameter and illuminance on the scanned surface 7 according to the image height, and also to suppress the generation of ghost light.

[0161] [Fifth embodiment] FIG. 9 shows a schematic development view in the main-scanning section of an optical scanning device 500 according to the fifth embodiment. The optical scanning device 500 according to this embodiment has the same configuration as the optical scanning device 200 according to the second embodiment except for different specification values, so the same components are given the same reference numerals and descriptions thereof are omitted.

[0162] Specifically, in the optical scanning device 500 according to this embodiment, the angle between the optical axis of the incident optical system 75 and the optical axis of the imaging optical system 85, ie, the incident angle α of the light beam on the polygon mirror 5, is 62 degrees. Furthermore, the width θ2 of the scanning angle is smaller than that of the optical scanning device 200 according to the second embodiment.

[0163] Next, the specifications of the optical scanning device 500 according to this embodiment are shown in Tables 9 and 10 below. Unless otherwise specified in Table 9, the unit of distance is millimeters, and in Table 10, "Ex" stands for "×10 -x " It means.

[0164] [Table 9]

[0165] [Table 10]

[0166] As shown in Table 9, in the optical scanning device 500 according to this embodiment, N and Duty are 10 and 0.554, respectively. Therefore, the upper and lower limits of condition (7) are 72 degrees and 50.6 degrees, respectively, and the upper and lower limits of condition (7a) are 72 degrees and 55.9 degrees, respectively. As shown in Table 9, in the optical scanning device 500 according to this embodiment, the incident angle α is 62 degrees, and therefore conditional expressions (7) and (7a) are satisfied.

[0167] As described above, in the optical scanning device 500 according to this embodiment, when the OFS method is adopted to achieve high speed and high definition while suppressing an increase in the number of parts, it is possible to suppress the change in spot diameter and illuminance on the scanned surface 7 according to the image height, and also to suppress the generation of ghost light.

[0168] The numerical values ​​in the conditional expressions (7) and (7a) for each of the optical scanning devices according to the first to fifth embodiments are shown in Table 11 below.

[0169] [Table 11]

[0170] [Sixth embodiment] 10 and 11 are a schematic projection view in a main-scanning section and a schematic partial sub-scanning section, respectively, of an optical scanning device 600 according to the sixth embodiment. In addition, in FIG. 10, the optical paths of the light beams guided to each of the four photosensitive drums YD, MD, CD and BkD are also shown in a developed form.

[0171] The optical scanning device 600 according to this embodiment includes first and third light sources 101 and 201, first and third diaphragms 102 and 202, and first and third anamorphic collimator lenses 103 and 203. Moreover, the optical scanning device 600 according to this embodiment includes second and fourth light sources, second and fourth diaphragms, and second and fourth anamorphic collimator lenses, all of which are not shown.

[0172] The optical scanning device 600 according to this embodiment also includes a polygon mirror 5, first imaging optical elements 161 and 261, and second imaging optical elements 162, 262, 163 and 263. The optical scanning device 600 according to this embodiment also includes folding mirrors 181, 182, 183, 281, 282, and 283.

[0173] In the optical scanning device 600 according to this embodiment, the first diaphragm 102 and the first anamorphic collimator lens 103 form a first incident optical system. Furthermore, a second input optical system is formed by the second diaphragm and the second anamorphic collimator lens. Furthermore, the third aperture 202 and the third anamorphic collimator lens 203 form a third incident optical system, and the fourth aperture and the fourth anamorphic collimator lens form a fourth incident optical system.

[0174] In the optical scanning device 600 according to this embodiment, a first imaging optical system is formed by the lower portion of the first imaging optical element 161 in the sub-scanning direction and the second imaging optical element 162. Further, the upper portion of the first imaging optical element 161 in the sub-scanning direction and the second imaging optical element 163 form a second imaging optical system. In addition, a third imaging optical system is formed by the lower portion of the first imaging optical element 261 in the sub-scanning direction and the second imaging optical element 262, and a fourth imaging optical system is formed by the upper portion of the first imaging optical element 261 in the sub-scanning direction and the second imaging optical element 263.

[0175] In the optical scanning device 600 according to this embodiment, the return mirrors 181 and 182 form a first reflective optical system, and the return mirror 183 forms a second reflective optical system. Moreover, the return mirrors 281 and 282 form a third reflective optical system, and the return mirror 283 forms a fourth reflective optical system.

[0176] In the optical scanning device 600 according to this embodiment, a first light flux emitted from a first light source 101 is guided to a polygon mirror 5 by a first incident optical system. The first light beam deflected by the polygon mirror 5 is guided to the first scanned surface, the photosensitive drum CD, by the first imaging optical system and the first reflecting optical system, and the photosensitive drum CD is scanned as the polygon mirror 5 rotates at a constant speed.

[0177] Furthermore, in the optical scanning device 600 according to this embodiment, the second light flux emitted from the second light source is guided to the polygon mirror 5 by the second incident optical system. The second light beam deflected by the polygon mirror 5 is guided to the second scanned surface, the photosensitive drum BkD, by the second imaging optical system and the second reflecting optical system, and the photosensitive drum BkD is scanned as the polygon mirror 5 rotates at a constant speed.

[0178] In the optical scanning device 600 according to this embodiment, the third light flux emitted from the third light source 201 is guided to the polygon mirror 5 by the third incident optical system. The third light beam deflected by the polygon mirror 5 is guided to the photosensitive drum MD, which is the third scanned surface, by a third imaging optical system and a third reflecting optical system, and the photosensitive drum MD is scanned as the polygon mirror 5 rotates at a constant speed.

[0179] In the optical scanning device 600 according to this embodiment, the fourth light flux emitted from the fourth light source is guided to the polygon mirror 5 by the fourth incident optical system. The fourth light beam deflected by the polygon mirror 5 is guided to the photosensitive drum YD, which is the fourth scanned surface, by a fourth imaging optical system and a fourth reflecting optical system, and the photosensitive drum YD is scanned as the polygon mirror 5 rotates at a constant speed.

[0180] As shown in Figures 10 and 11, in the optical scanning device 600 of this embodiment, four incident optical systems and four imaging optical systems are provided to correspond to the four photosensitive drums YD, MD, CD and BkD, respectively. The first and second incident optical systems and the first and second imaging optical systems are provided on one side of the polygon mirror 5 in a direction parallel to the optical axis of each imaging optical system. Further, on the other side of the polygon mirror 5, a third and a fourth incident optical system and a third and a fourth imaging optical system are provided. That is, the optical scanning device 600 according to this embodiment employs a double-sided scanning method.

[0181] In addition, the first, second, third and fourth incident optical systems and the first, second, third and fourth imaging optical systems provided in the optical scanning device 600 of this embodiment have the same specification values ​​as those of the optical scanning device 300 of the third embodiment.

[0182] That is, in the optical scanning device 600 according to this embodiment, the first and second incident optical systems cause the first and second light beams from the first and second light sources to be obliquely incident on the first deflection reflecting surface of the polygon mirror 5 at angles different from each other relative to the main scanning cross section within the sub-scanning cross section. The third and fourth incident optical systems cause the third and fourth light beams from the third and fourth light sources to be obliquely incident on the second deflection reflecting surface of the polygon mirror 5 at angles different from each other with respect to the main scanning section in the sub-scanning section.

[0183] The first and second imaging optical systems guide the first and second light beams deflected by the first deflection reflecting surface (first deflection surface) of the polygon mirror 5 to the first and second surfaces to be scanned. The third and fourth imaging optical systems guide the third and fourth light beams deflected by the second deflection reflecting surface (second deflection surface) of the polygon mirror 5 to the third and fourth surfaces to be scanned.

[0184] In the main scanning cross section, the width of the first and second light beams when they are incident on the first deflection-reflection surface of the polygon mirror 5 is larger than the width of the first deflection-reflection surface. In addition, in the main scanning cross section, the widths of the third and fourth light beams when they are incident on the second deflection-reflection surface of the polygon mirror 5 are larger than the width of the second deflection-reflection surface.

[0185] Furthermore, within the first scanned surface, the Y-direction positive side scanning area (first scanning area) on one side where the first incident optical system is arranged relative to the optical axis of the first imaging optical system is longer than the Y-direction negative side scanning area (second scanning area) on the other side where the first incident optical system is not arranged. In addition, within the second scanned surface, the Y-direction positive side scanning area (third scanning area) on one side where the second incident optical system is arranged relative to the optical axis of the second imaging optical system is longer than the Y-direction negative side scanning area (fourth scanning area) on the other side where the second incident optical system is not arranged.

[0186] In addition, within the third scanned surface, the Y-direction positive side scanning area (fifth scanning area) on one side where the third incident optical system is arranged relative to the optical axis of the third imaging optical system is longer than the Y-direction negative side scanning area (sixth scanning area) on the other side where the third incident optical system is not arranged. In addition, within the fourth scanned surface, the Y-direction positive side scanning area (seventh scanning area) on one side where the fourth incident optical system is arranged relative to the optical axis of the fourth imaging optical system is longer than the Y-direction negative side scanning area (eighth scanning area) on the other side where the fourth incident optical system is not arranged.

[0187] The number of deflecting surfaces of the polygon mirror 5 is N, and the angle that the optical axis of the first incident optical system makes with the optical axis of the first imaging optical system in the main scanning section is α 1 (degrees), and the angle that the optical axis of the second incident optical system makes with the optical axis of the second imaging optical system in the main scanning section is α 2 (degrees). The angle that the optical axis of the third incident optical system makes with the optical axis of the third imaging optical system in the main scanning section is α 3 (degrees), and the angle that the optical axis of the fourth incident optical system makes with the optical axis of the fourth imaging optical system in the main scanning section is α 4 (degrees).

[0188] The ratio of the width of the scanning angle for the scanning area on the first scanned surface to the maximum width of the scanning angle by the first deflection reflection surface is defined as Duty. 1 Let us assume that. The ratio of the width of the scanning angle for the scanning area on the second surface to the maximum width of the scanning angle by the first deflection reflection surface is defined as Duty. 2 Let us assume that. The ratio of the width of the scanning angle for the scanning area on the third surface to the maximum width of the scanning angle by the second deflection reflection surface is defined as Duty. 3 Let us assume that. The ratio of the width of the scanning angle for the scanning area on the fourth surface to the maximum width of the scanning angle by the second deflection reflection surface is defined as Duty. 4 Let us assume that.

[0189] At this time, in the optical scanning device 600 according to this embodiment, the following conditional expressions (27), (28), (29) and (30) are satisfied.

number

number

number

number

[0190] In the optical scanning device 600 according to this embodiment, the following conditional expressions (31) and (32) are satisfied. |α 1 |=|α 2 |=|α 3 |=|α 4 | (31) Duty 1 =Duty 2 =Duty 3 =Duty 4 (32) Here, |α 1 |, |α2 |, |α 3 | and |α 4 | are the angles α 1 , α 2 , α 3 and α 4 The absolute value of

[0191] In the optical scanning device 600 according to this embodiment, it is preferable that the following conditional expressions (27a), (28a), (29a) and (30a) be satisfied.

number

number

number

number

[0192] As described above, the four incident optical systems and the four imaging optical systems provided in the optical scanning device 600 of this embodiment have the same specification values ​​as those of the optical scanning device 300 of the third embodiment, but are not limited to this. In other words, they may have the same specification values ​​as those of the optical scanning device according to the first, second, fourth or fifth embodiment.

[0193] As shown in FIG. 10, in the optical scanning device 600 according to this embodiment, the first light source 101 and the first incident optical system are arranged in the Y direction between the positive side end E1 of the first reflective optical system and the positive side most off-axis image height E2 of the first scanned surface. In other words, in the optical scanning device 600 of this embodiment, the first light source 101 and the first incident optical system are arranged in the Y direction between the folding mirrors 181 and 182 included in the first reflective optical system and the positive-side most off-axis image height E2 of the first scanned surface.

[0194] The second light source and the second incident optical system are disposed between the positive side end E1 of the second reflecting optical system and the positive side most off-axis image height E2 of the second scanned surface in the Y direction. In other words, in the optical scanning device 600 of this embodiment, the second light source and the second incident optical system are arranged in the Y direction between the folding mirror 183 included in the second reflective optical system and the positive-side most off-axis image height E2 of the second scanned surface.

[0195] The third light source 201 and the third incident optical system are disposed in the Y direction between the positive side end E1 of the third reflecting optical system and the positive side most off-axis image height E2 of the third scanned surface. In other words, in the optical scanning device 600 of this embodiment, the third light source 201 and the third incident optical system are arranged in the Y direction between the folding mirrors 281 and 282 included in the third reflective optical system and the positive-side most off-axis image height E2 of the third scanned surface.

[0196] The fourth light source and the fourth incident optical system are disposed between the positive side end E1 of the fourth reflecting optical system and the positive side most off-axis image height E2 of the fourth scanned surface in the Y direction. In other words, in the optical scanning device 600 of this embodiment, the fourth light source and the fourth incident optical system are arranged in the Y direction between the folding mirror 283 included in the fourth reflective optical system and the positive-side most off-axis image height E2 of the fourth scanned surface.

[0197] In the optical scanning device 600 according to this embodiment, the polygon mirror 5 is disposed between the photosensitive drum CD, which is the first surface to be scanned, and the photosensitive drum MD, which is the third surface to be scanned, in the X direction. In other words, in the optical scanning device 600 according to this embodiment, the polygon mirror 5 is disposed between the photosensitive drums CD and MD in the X direction when projected onto the main scanning cross section.

[0198] On the other hand, the first light source 101, the second light source, and the first and second incident optical systems are arranged in the X direction between the photosensitive drum CD, which is the first scanned surface, and the photosensitive drum BkD, which is the second scanned surface. In other words, the first light source 101, the second light source, and the first and second incident optical systems are disposed between the photosensitive drum CD and the photosensitive drum BkD in the X direction when projected onto the main scanning cross section.

[0199] In addition, the third light source 201, the fourth light source, and the third and fourth incident optical systems are arranged in the X direction between the photosensitive drum MD, which is the third scanned surface, and the photosensitive drum YD, which is the fourth scanned surface. In other words, the third light source 201, the fourth light source, and the third and fourth incident optical systems are disposed between the photosensitive drums MD and YD in the X direction when projected onto the main scanning cross section.

[0200] In the optical scanning device 600 of this embodiment, by arranging the first to fourth light sources and the first to fourth incident optical systems as described above, it is possible to suppress interference between the optical paths in each of the first to fourth incident optical systems and the optical paths in each of the first to fourth imaging optical systems. In addition, the area between the photosensitive drum BkD, which is the second surface to be scanned, and the photosensitive drum YD, which is the fourth surface to be scanned, in the X direction, and on the positive side in the Y direction, can be effectively utilized.

[0201] As described above, in the optical scanning device 600 according to this embodiment, the scanning area on the positive side in the Y direction is larger than the scanning area on the negative side in the Y direction on each of the first to fourth surfaces to be scanned. As a result, the area on the positive side in the Y direction in which the first to fourth light sources and the first to fourth incident optical systems are arranged is formed to be sufficiently wide.

[0202] In addition, in the optical scanning device 600 of this embodiment, the chief ray of the first light beam deflected by the polygon mirror 5 toward the photosensitive drum CD, which is the first scanned surface, travels on an optical path that follows a figure-four shape by the folding mirrors 181 and 182. Moreover, the principal ray of the third light flux deflected by the polygon mirror 5 toward the photosensitive drum MD, which is the third surface to be scanned, also travels on an optical path along a figure-four by the folding mirrors 281 and 282 .

[0203] In other words, an intersection occurs (is formed) within the optical paths of the principal rays of the first and third light beams deflected by the polygon mirror 5, thereby making it possible to reduce the height in the sub-scanning direction of each of the first and third imaging optical systems. Such optical paths in each of the first and third imaging optical systems are formed by arranging the first to fourth incident optical systems as described above. On the other hand, in the optical paths of the chief rays of the second and fourth light beams deflected by the polygon mirror 5, no intersection occurs (is formed).

[0204] In the optical scanning device 600 according to this embodiment, the polygon mirror 5 is disposed between the photosensitive drums CD and MD in the X direction (first direction) perpendicular to the main scanning direction and the sub-scanning direction as described above. In the optical scanning device 600 according to this embodiment, as described above, in the X direction perpendicular to the main scanning direction and the sub-scanning direction, the first and second light sources and the optical elements of the first and second incident optical systems are disposed between the photosensitive drum CD and the photosensitive drum BkD. This suppresses interference in the sub-scanning direction between each optical element of the first and second incident optical systems and the optical path along the figure-four of the first imaging optical system that proceeds to the photosensitive drum CD, which is the first scanned surface.

[0205] As described above, the third and fourth light sources and the optical elements of the third and fourth incident optical systems are disposed between the photosensitive drums MD and YD in the X direction perpendicular to the main scanning direction and the sub-scanning direction. This suppresses interference in the sub-scanning direction between each optical element of the third and fourth incident optical systems and the optical path along the figure-four of the third imaging optical system proceeding to the photosensitive drum MD, which is the third scanned surface.

[0206] In the optical scanning device 600 according to this embodiment, the optical axes of the first and second incident optical systems form an angle α with the optical axes of the first and second imaging optical systems in the main scanning cross section. This makes it possible to prevent the first and second light beams guided by the first and second incident optical systems according to the tolerance from entering the third and fourth imaging optical systems on the opposite side, even if they pass outside the polygon mirror 5.

[0207] Similarly, in the main scanning cross section, the optical axes of the third and fourth incident optical systems form an angle α with the optical axes of the third and fourth imaging optical systems. This makes it possible to prevent the third and fourth light beams guided by the third and fourth incident optical systems according to the tolerance from entering the first and second imaging optical systems on the opposite side, even if the third and fourth light beams pass outside the polygon mirror 5.

[0208] As described above, in the optical scanning device 600 according to this embodiment, when the OFS method is adopted to achieve high speed and high definition while suppressing an increase in the number of parts, it is possible to suppress the change in spot diameter and illuminance on each scanned surface according to the image height, and also to suppress the generation of ghost light.

[0209] Although the optical scanning device 600 that scans four photosensitive drums has been described above, the present embodiment is not limited to this. That is, the configuration of this embodiment can also be applied to an optical scanning device in which first and second light beams deflected by a predetermined deflection reflection surface of the polygon mirror 5 scan first and second photosensitive drums.

[0210] [Image forming device] FIG. 12 is a sub-scanning sectional view of a main part of a color image forming apparatus 90 equipped with an optical scanning device 600 according to the sixth embodiment.

[0211] The image forming apparatus 90 is a tandem type color image forming apparatus that records image information on the surfaces of the photosensitive drums serving as image carriers, using an optical scanning device 600 according to the sixth embodiment. The image forming apparatus 90 includes an optical scanning device 600 according to the sixth embodiment, photosensitive drums (photoconductors) 23, 24, 25, and 26 as image carriers, and developing units 15, 16, 17, and 18. The image forming apparatus 90 also includes a conveyor belt 91 , a printer controller 93 , and a fixing unit 94 .

[0212] To the image forming apparatus 90, R (red), G (green), and B (blue) color signals (code data) output from an external device 92 such as a personal computer are input. The input color signals are then converted by a printer controller 93 in the image forming apparatus 90 into image data (dot data) for each of C (cyan), M (magenta), Y (yellow), and Bk (black).

[0213] Each converted image data is input to the optical scanning device 600. Then, the optical scanning device 600 emits light beams 19, 20, 21, and 22 modulated in accordance with each image data. Then, the photosensitive surfaces of the photosensitive drums 23, 24, 25 and 26 are exposed to the light beams 19 to 22.

[0214] The image forming apparatus 90 is also provided with charging rollers (not shown) for uniformly charging the surfaces of the photosensitive drums 23 to 26 so as to come into contact with the surfaces. Then, the light beams 19 to 22 from the optical scanning device 600 are irradiated onto the surfaces of the photosensitive drums 23 to 26 charged by the charging rollers, respectively.

[0215] As described above, the light beams 19 to 22 are modulated based on image data for each color, and electrostatic latent images are formed on the surfaces of the photosensitive drums 23 to 26 by irradiating the light beams 19 to 22. The electrostatic latent images thus formed are developed into toner images by developing devices 15, 16, 17, and 18 disposed so as to contact the photosensitive drums 23 to 26.

[0216] The toner images developed by the developing units 15 to 18 are transferred in multiple layers onto a sheet of paper (transfer receiving material) (not shown) conveyed on a conveyor belt 91 by a transfer roller (transfer unit) (not shown) disposed opposite the photosensitive drums 23 to 26. As a result, a full-color image is formed on the sheet of paper. The paper onto which the unfixed toner image has been transferred in the above manner is further transported to a fixing device 94 behind the photosensitive drums 23 to 26 (on the left side in FIG. 12).

[0217] The fixing unit 94 includes a fixing roller having an internal fixing heater (not shown) and a pressure roller arranged to be in pressure contact with the fixing roller. The conveyed paper is then heated and pressed at the pressure contact portion between the fixing roller and the pressure roller, whereby the unfixed toner image on the paper is fixed. Furthermore, a paper discharge roller (not shown) is disposed behind the fixing unit 94, and the paper on which the fixed image has been fixed is discharged to the outside of the image forming apparatus 90 by the paper discharge roller.

[0218] The image forming apparatus 90 uses an optical scanning device 600 to record image signals (image information) on the photosensitive surfaces of the photosensitive drums 23, 24, 25, and 26 corresponding to each of the colors C, M, Y, and Bk, thereby printing color images at high speed. As the external device 92, for example, a color image reading device equipped with a CCD sensor can be used. In this case, the color image reading device and the image forming device 90 form a color digital copying machine. Moreover, in the image forming apparatus 90, the optical scanning device according to any one of the first to fifth embodiments may be used in place of the optical scanning device 600 according to the sixth embodiment.

[0219] Although the preferred embodiments have been described above, the present invention is not limited to these embodiments and various modifications and changes are possible within the scope of the gist thereof.

[0220] The disclosure of this embodiment includes the following configuration. (Configuration 1) A deflector having a plurality of deflecting surfaces that deflects a first light beam from a first light source to scan a first scanned surface in a main scanning direction, a first incident optical system that makes the first light beam from the first light source incident on the plurality of deflecting surfaces, and a first imaging optical system that guides the first light beam deflected by the plurality of deflecting surfaces to the first scanned surface, wherein in a main scanning cross section, a width of the first light beam when it is incident on each of the plurality of deflecting surfaces is larger than a width of each of the plurality of deflecting surfaces, and within the first scanned surface, a first scanning region on one side where the first incident optical system is arranged with respect to an optical axis of the first imaging optical system is longer than a second scanning region on the other side where the first incident optical system is not arranged, the number of the plurality of deflecting surfaces is N, and an angle between the optical axis of the first incident optical system and the optical axis of the first imaging optical system in the main scanning cross section is α 1 (degrees), and the ratio of the width of the scanning angle corresponding to the first and second scanning regions to the maximum width of each of the multiple deflection surfaces is called Duty 1 Then, (720 / N)×(2×Duty 1 +1) / 3<α 1 An optical scanning device characterized by satisfying the condition <720 / N. (Configuration 2) An optical scanning device described in configuration 1, characterized in that each optical element included in the first incident optical system is arranged on the opposite side of the deflector with respect to the exit surface of the imaging optical element that is arranged closest to the deflector among at least one imaging optical element included in the first imaging optical system in a direction parallel to the optical axis of the first imaging optical system. (Configuration 3) An optical scanning device described in configuration 1 or 2, characterized in that each optical element included in the first incident optical system is arranged in the main scanning direction between each imaging optical element included in the first imaging optical system and the most off-axis image height of the first scanning area. (Configuration 4) An optical scanning device described in any one of configurations 1 to 3, characterized in that the refractive power of the first imaging optical system in the main scanning cross section monotonically decreases from the position where the on-axis light beam passes toward the position where the most off-axis light beam passes. (Configuration 5)(720 / N)×(Duty 1 +1) / 2<α 1 5. The optical scanning device according to any one of configurations 1 to 4, wherein the optical scanning device satisfies the condition: <720 / N. (Structure 6) An optical scanning device described in any one of structures 1 to 5, characterized in that it comprises a first light source arranged in the main scanning direction between each optical element included in the first imaging optical system and the most off-axis image height of the first scanning area. (Configuration 7) In the optical scanning device according to any one of configurations 1 to 6, the first incident optical system does not have a reflective optical element that reflects the first light flux. (Configuration 8) The optical scanning device according to any one of configurations 1 to 7, wherein the condition N≧9 is satisfied. (Configuration 9) An optical scanning device described in any one of configurations 1 to 8, characterized in that the scanning speed of the first light beam on the first scanned surface increases monotonically from the on-axis image height to the most off-axis image height. (Configuration 10) In the optical scanning device according to any one of configurations 1 to 9, the optical axes of the first incident optical system and the first imaging optical system are parallel to the main scanning cross section. (Configuration 11) In the main scanning cross-section, when the traveling direction of the first light beam when deflected by a plurality of deflection surfaces forms an angle θ with the optical axis of the first imaging optical system, the ratio dW(θ) of the width of the first light beam deflected toward the axial image height of the first scanned surface when the first light beam enters the first imaging optical system to the width of the first light beam deflected by the angle θ, and the ratio df(θ) of the focal length at the position where the first light beam deflected toward the axial image height of the first imaging optical system passes to the focal length at the position where the first light beam deflected by the angle θ passes, the optical scanning device according to any one of Configurations 1 to 10, characterized in that the condition 0.90 < df(θ) / dW(θ) < 1.10 is satisfied. (Configuration 12) A second incident optical system that obliquely incident a second light beam from a second light source on a first deflection surface among a plurality of deflection surfaces at a predetermined angle with respect to the main scanning cross-section in the sub-scanning cross-section, and a second imaging optical system that guides the second light beam deflected by the first deflection surface to a second scanned surface. The first incident optical system obliquely incident a first light beam on the first deflection surface at an angle different from the predetermined angle with respect to the main scanning cross-section in the sub-scanning cross-section. The first imaging optical system guides the first light beam deflected by the first deflection surface to the first scanned surface. The deflector deflects the second light beam from the second light source to scan the second scanned surface in the main scanning direction. In the main scanning cross-section, the widths of the first and second light beams when entering the first deflection surface are larger than the width of the first deflection surface. In the second scanned surface, the third scanning region on one side where the second incident optical system is arranged with respect to the optical axis of the second imaging optical system is longer than the fourth scanning region on the other side where the second incident optical system is not arranged. The angle formed by the optical axis of the second incident optical system and the optical axis of the second imaging optical system in the main scanning cross-section is α 2 (degrees), the value of the ratio of the widths of the scanning angles corresponding to the third and fourth scanning regions to the maximum width of the scanning angle of the first deflection surface is Duty 2 When set as, (720 / N)×(2×Duty 2 +1) / 3 < α 2 <720 / N, the optical scanning device according to any one of Configurations 1 to 11, characterized in that the condition is satisfied. (Configuration 13) A scanning optical system includes third and fourth incident optical systems that obliquely incident third and fourth light beams from third and fourth light sources onto a second deflecting surface among a plurality of deflecting surfaces at angles different from each other with respect to the main scanning cross section in a sub-scanning cross section, and third and fourth imaging optical systems that guide the third and fourth light beams deflected by the second deflecting surface to a third and fourth scanned surface, and the deflector deflects the third and fourth light beams from the third and fourth light sources to scan the third and fourth scanned surfaces in the main scanning direction, and the widths of the third and fourth light beams when incident on the second deflecting surface in the main scanning cross section are a fifth scanning region on one side where the third incident optical system is arranged with respect to the optical axis of the third imaging optical system within the third scanned surface is longer than a sixth scanning region on the other side where the third incident optical system is not arranged, and a seventh scanning region on one side where the fourth incident optical system is arranged with respect to the optical axis of the fourth imaging optical system within the fourth scanned surface is longer than an eighth scanning region on the other side where the fourth incident optical system is not arranged, and an angle between the optical axis of the third incident optical system and the optical axis of the third imaging optical system within the main scanning cross section is defined as α 3 (degrees), and the angle between the optical axis of the fourth incident optical system and the optical axis of the fourth imaging optical system in the main scanning section is α 4 (degrees), and the ratio of the width of the scanning angle corresponding to the fifth and sixth scanning regions to the maximum width of the scanning angle of the second deflection surface is called Duty 3 The ratio of the width of the scanning angle corresponding to the seventh and eighth scanning regions to the maximum width of the scanning angle of the second deflection surface is defined as Duty 4 Then, (720 / N)×(2×Duty 3 +1) / 3<α 3 <720 / N, (720 / N)×(2×Duty 4 +1) / 3<α 4 13. The optical scanning device according to claim 12, wherein the optical scanning device satisfies the condition: <720 / N. (Configuration 14) An optical scanning device according to configuration 13, comprising first and second reflective optical systems which reflect the first and second light beams deflected by the first deflection surface, and third and fourth reflective optical systems which reflect the third and fourth light beams deflected by the second deflection surface, wherein each optical element included in the first incident optical system is arranged in the main scanning direction between each reflective optical element included in the first reflective optical system and the most off-axis image height of the first scanning region, each optical element included in the second incident optical system is arranged in the main scanning direction between each reflective optical element included in the second reflective optical system and the most off-axis image height of the third scanning region, each optical element included in the third incident optical system is arranged in the main scanning direction between each reflective optical element included in the third reflective optical system and the most off-axis image height of the fifth scanning region, and each optical element included in the fourth incident optical system is arranged in the main scanning direction between each reflective optical element included in the fourth reflective optical system and the most off-axis image height of the seventh scanning region. (Configuration 15) An optical scanning device according to configuration 13 or 14, characterized in that, in the sub-scanning cross section, an intersection is formed in the optical path in the first imaging optical system, and an intersection is formed in the optical path in the third imaging optical system. (Configuration 16) An optical scanning device according to configuration 15, characterized in that, in the sub-scanning cross section, no intersection is formed in the optical path in the second imaging optical system, and no intersection is formed in the optical path in the fourth imaging optical system. (Configuration 17) An optical scanning device described in any one of configurations 13 to 16, characterized in that the deflector is arranged between the first scanned surface and the third scanned surface in a first direction perpendicular to the main scanning direction and the sub-scanning direction. (Structure 18) An optical scanning device described in any one of structures 13 to 17, characterized in that the first and second incident optical systems are arranged between the first scanned surface and the second scanned surface in a first direction perpendicular to the main scanning direction and the sub-scanning direction, and the third and fourth incident optical systems are arranged between the third scanned surface and the fourth scanned surface in the first direction. (Configuration 19) An image forming apparatus comprising: an optical scanning device according to any one of configurations 1 to 18; a developing unit which develops an electrostatic latent image formed on a first scanned surface by the optical scanning device into a toner image; a transfer unit which transfers the developed toner image to a transfer material; and a fixing unit which fixes the transferred toner image to the transfer material. (Configuration 20) An image forming apparatus comprising: an optical scanning device according to any one of configurations 1 to 18; and a printer controller that converts a signal output from an external device into image data and inputs the image data to the optical scanning device. [Explanation of symbols]

[0221] 1 Light source (first light source) 5 Polygon mirror (deflector) 7 Scanned surface (first scanned surface) 51 Deflection reflective surface (deflection surface) 75 Incident optical system (first incident optical system) 85 Imaging optical system (first imaging optical system) 100 Optical scanning device

Claims

1. a deflector having a plurality of deflection surfaces for deflecting a first light beam from a first light source to scan a first scanned surface in a main scanning direction; a first incident optical system that causes the first light flux from the first light source to be incident on the plurality of deflection surfaces; a first imaging optical system that guides the first light flux deflected by the plurality of deflection surfaces to the first scanned surface, a width of the first light beam incident on each of the plurality of deflection surfaces in a main scanning cross section is larger than a width of each of the plurality of deflection surfaces; a first scanning region on one side of the first scanning surface where the first incident optical system is disposed relative to an optical axis of the first imaging optical system is longer than a second scanning region on the other side where the first incident optical system is not disposed; The number of the plurality of deflecting surfaces is N, and the angle between the optical axis of the first incident optical system and the optical axis of the first imaging optical system in the main scanning section is α １ (degrees), the ratio of the width of the scanning angle corresponding to the first and second scanning regions to the maximum width of the scanning angle of each of the plurality of deflection surfaces is Duty １ When (720 / N)×(2×Duty １ +1) / 3<a １ <720 / N An optical scanning device characterized in that the following conditions are satisfied.

2. The optical scanning device described in claim 1, characterized in that each optical element included in the first incident optical system is arranged on the opposite side of the deflector with respect to the exit surface of the imaging optical element that is arranged closest to the deflector among at least one imaging optical element included in the first imaging optical system in a direction parallel to the optical axis of the first imaging optical system.

3. The optical scanning device according to claim 1, characterized in that each optical element included in the first incident optical system is arranged in the main scanning direction between each imaging optical element included in the first imaging optical system and the most off-axis image height of the first scanning area.

4. 2. The optical scanning device according to claim 1, wherein the refractive power of the first imaging optical system in the main scanning cross section decreases monotonically from a position where an on-axis light beam passes to a position where an off-axis light beam passes.

5. (720 / N)×(Duty １ +1) / 2<α １ <720 / N 2. The optical scanning device according to claim 1, wherein the following conditions are satisfied:

6. 2. The optical scanning device according to claim 1, further comprising the first light source arranged in the main scanning direction between each optical element included in the first imaging optical system and the most off-axis image height of the first scanning area.

7. 2. The optical scanning device according to claim 1, wherein the first incident optical system does not include a reflecting optical element that reflects the first light beam.

8. N≧9 2. The optical scanning device according to claim 1, wherein the following conditions are satisfied:

9. 2. The optical scanning device according to claim 1, wherein a scanning speed of the first light beam on the first scanned surface increases monotonically from an on-axis image height to an extreme off-axis image height.

10. 2. The optical scanning device according to claim 1, wherein the optical axis of each of the first incident optical system and the first imaging optical system is parallel to a main scanning cross section.

11. When an angle that the traveling direction of the first light flux when deflected by the multiple deflecting surfaces in a main scanning cross section forms with respect to the optical axis of the first imaging optical system is defined as θ, a ratio of a width of the first light flux deflected at the angle θ to a width of the first light flux deflected toward an on-axis image height of the first scanned surface when entering the first imaging optical system is defined as dW(θ), and a ratio of a focal length at a position through which the first light flux deflected at the angle θ passes to a focal length at a position through which the first light flux deflected toward the on-axis image height of the first imaging optical system passes is defined as df(θ), 0.90<df(θ) / dW(θ)<1.10 2. The optical scanning device according to claim 1, wherein the following conditions are satisfied:

12. a second incidence optical system that causes a second light flux from a second light source to be obliquely incident on a first deflection surface among the plurality of deflection surfaces at a predetermined angle in a sub-scanning cross section with respect to a main-scanning cross section; a second imaging optical system that guides the second light beam deflected by the first deflection surface to a second scanned surface, the first incident optical system makes the first light beam obliquely incident on the first deflection surface at an angle different from the predetermined angle with respect to the main scanning cross section in the sub-scanning cross section; the first imaging optical system guides the first light beam deflected by the first deflection surface to the first scanned surface; the deflector deflects the second light beam from the second light source to scan the second scanned surface in a main scanning direction; In the main scanning cross section, the widths of the first and second light beams when incident on the first deflection surface are larger than the width of the first deflection surface, a third scanning region on one side of the second scanning surface where the second incident optical system is disposed relative to an optical axis of the second imaging optical system is longer than a fourth scanning region on the other side where the second incident optical system is not disposed; The angle between the optical axis of the second incident optical system and the optical axis of the second imaging optical system in the main scanning section is defined as α ２ (degrees), the ratio of the width of the scanning angle corresponding to the third and fourth scanning regions to the maximum width of the scanning angle of the first deflection surface is defined as Duty ２ When (720 / N)×(2×Duty ２ +1) / 3<a ２ <720 / N 2. The optical scanning device according to claim 1, wherein the following conditions are satisfied:

13. third and fourth incident optical systems that obliquely incident third and fourth light beams from third and fourth light sources onto a second deflection surface among the plurality of deflection surfaces at angles different from each other with respect to the main scanning cross section in a sub-scanning cross section; a third and fourth imaging optical system that guides the third and fourth light beams deflected by the second deflection surface to the third and fourth scanned surfaces, the deflector deflects the third and fourth light beams from the third and fourth light sources to scan the third and fourth scanned surfaces in a main scanning direction; In the main scanning cross section, the widths of the third and fourth light beams when incident on the second deflection surface are larger than the width of the second deflection surface, a fifth scanning region on one side of the third scanned surface where the third incident optical system is disposed relative to an optical axis of the third imaging optical system is longer than a sixth scanning region on the other side where the third incident optical system is not disposed; a seventh scanning area on one side of the fourth scanned surface where the fourth incident optical system is disposed relative to an optical axis of the fourth imaging optical system is longer than an eighth scanning area on the other side where the fourth incident optical system is not disposed; The angle between the optical axis of the third incident optical system and the optical axis of the third imaging optical system in the main scanning section is defined as α ３ (degrees), and the angle between the optical axis of the fourth incident optical system and the optical axis of the fourth imaging optical system in the main scanning section is α ４ (degrees), the ratio of the width of the scanning angle corresponding to the fifth and sixth scanning regions to the maximum width of the scanning angle of the second deflection surface is Duty ３ , the ratio of the width of the scanning angle corresponding to the seventh and eighth scanning regions to the maximum width of the scanning angle of the second deflection surface is defined as Duty ４ When (720 / N)×(2×Duty ３ +1) / 3<a ３ <720 / N (720 / N)×(2×Duty ４ +1) / 3<a ４ <720 / N 13. The optical scanning device according to claim 12, wherein the following condition is satisfied:

14. a first reflecting optical system that reflects the first and second light beams deflected by the first deflecting surface; and a third and fourth reflecting optical system that reflects the third and fourth light fluxes deflected by the second deflecting surface, each optical element included in the first incident optical system is disposed between each reflective optical element included in the first reflective optical system and an outermost off-axis image height of the first scanning area in a main scanning direction; each optical element included in the second incident optical system is disposed between each reflective optical element included in the second reflective optical system and an outermost off-axis image height of the third scanning area in a main scanning direction; each optical element included in the third incident optical system is disposed between each reflective optical element included in the third reflective optical system and an outermost off-axis image height of the fifth scanning area in a main scanning direction; The optical scanning device according to claim 13, characterized in that each optical element included in the fourth incident optical system is arranged in the main scanning direction between each reflective optical element included in the fourth reflective optical system and the most off-axis image height of the seventh scanning area.

15. 14. The optical scanning device according to claim 13, wherein, in a sub-scanning cross section, an intersection is formed in an optical path in the first imaging optical system, and an intersection is formed in an optical path in the third imaging optical system.

16. 16. The optical scanning device according to claim 15, wherein, in the sub-scanning cross section, no intersection is formed in the optical path in the second imaging optical system, and no intersection is formed in the optical path in the fourth imaging optical system.

17. 14. The optical scanning device according to claim 13, wherein the deflector is disposed between the first scanned surface and the third scanned surface in a first direction perpendicular to a main scanning direction and a sub-scanning direction.

18. the first and second incident optical systems are disposed between the first scanned surface and the second scanned surface in a first direction perpendicular to a main scanning direction and a sub-scanning direction; 14. The optical scanning device according to claim 13, wherein the third and fourth incident optical systems are disposed between the third and fourth scanned surfaces in the first direction.

19. 19. An image forming apparatus comprising: an optical scanning device according to claim 1; a developing unit which develops an electrostatic latent image formed on the first scanned surface by the optical scanning device into a toner image; a transfer unit which transfers the developed toner image onto a transfer material; and a fixing unit which fixes the transferred toner image onto the transfer material.

20. 19. An image forming apparatus comprising: an optical scanning device according to claim 1; and a printer controller for converting a signal output from an external device into image data and inputting the image data to the optical scanning device.

Images