Optical scanner and image forming apparatus

Abstract

To provide an optical scanner that is reduced in size and / or has improved optical performance compared to a conventional configuration.SOLUTION: An optical scanner according to the present invention comprises: a deflector that deflects a light beam from a light source and scans a scanning target surface in a main scanning direction; an image forming optical element that has first and second optical surfaces; and a reflection optical element that has a reflection surface. The light beam from the light source is guided to the scanning target surface through the second optical surface, first optical surface, deflector, first optical surface, second optical surface, reflection surface, second optical surface, and first optical surface in this order.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 that is suitable for use in image forming apparatuses such as laser beam printers (LBPs), digital copying machines, and multifunction printers. [Background technology]

[0002] Conventionally, there has been a demand for an optical scanning device that is sufficiently miniaturized while maintaining optical performance. Patent document 1 discloses an optical scanning device that is miniaturized by adopting a configuration in which a light beam deflected by a deflector passes through an imaging optical element, is reflected by a reflective optical element, and then passes through the imaging optical element again to be guided to the surface to be scanned. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-59558 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the optical scanning device disclosed in Patent Document 1, each optical element forming the incident optical system is positioned at a large distance from the deflector so that the light beam reflected by the reflective optical element does not interfere with each other. As a result, the area for arranging the incident optical system increases, and the size is not sufficiently reduced.

[0005] On the other hand, in the optical scanning device disclosed in Patent Document 1, the shape of the imaging optical element for maintaining optical performance when adopting the above configuration has not been fully considered. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical scanning device that is more compact and / or has improved optical performance than conventional configurations. [Means for solving the problem]

[0006] The optical scanning device of the present invention comprises a deflector that deflects a light beam from a light source to scan a surface to be scanned in a main scanning direction, an imaging optical element having first and second optical surfaces, and a reflective optical element having a reflective surface, and is characterized in that the light beam from the light source is guided to the surface to be scanned via the second optical surface, the first optical surface, the deflector, the first optical surface, the second optical surface, the reflective surface, the second optical surface, and the first optical surface in that order. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an optical scanning device that is more compact and / or has improved optical performance than conventional configurations. [Brief explanation of the drawings]

[0008] [Figure 1] 2A and 2B are main scanning sectional views and sub scanning sectional views of the optical scanning device according to the first embodiment. [Figure 2] 10A and 10B are a main scanning sectional view and a sub scanning sectional view of an optical scanning device according to a second embodiment. [Figure 3] 10A and 10B are main scanning sectional views and sub scanning sectional views of an optical scanning device according to a third embodiment. [Figure 4] FIG. 10 is a main scanning sectional view of an optical scanning device according to a third embodiment. [Figure 5] 10A and 10B are main-scanning sectional views and sub-scanning sectional views of an optical scanning device according to a fourth embodiment. [Figure 6] FIG. 10 is a main scanning sectional view of an optical scanning device according to a fourth embodiment. [Figure 7] 13A and 13B are a main scanning sectional view and a partial sub-scanning sectional view of an optical scanning device according to a fifth embodiment. [Figure 8] FIG. 13 is a main scanning sectional view of an optical scanning device according to a fifth embodiment. [Figure 9] 1A and 1B are sub-scanning cross-sectional views of main parts of a monochrome image forming apparatus and a color image forming apparatus according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] The optical scanning device according to the present embodiment will be described in detail below with reference to the accompanying drawings. Note that the drawings may be drawn to a scale different from the actual scale in order to facilitate understanding of the present embodiment. In the following description, the sub-scanning direction is the direction parallel to the rotation axis of the deflector 4, and the main scanning direction is the direction in which the light beam scans the scanned surface 6. The sub-scanning cross section is a cross section perpendicular to the main scanning direction, and the main scanning cross section is a cross section perpendicular to the sub-scanning direction. In the following description, the main scanning direction is defined as the Y direction, the sub-scanning direction as the Z direction, and the direction perpendicular to the main scanning direction and the sub-scanning direction as the X direction.

[0010] [First embodiment] 2. Description of the Related Art Conventionally, printers and copiers using electrophotographic technology use optical scanning devices to form latent images on photosensitive drums. In recent years, various types of optical scanning devices have been developed to increase the speed and reduce the size of printers and copiers.

[0011] For example, one known method for increasing the speed of an optical scanning device that uses a rotating polygon mirror is to increase the number of reflective surfaces on the rotating polygon mirror, thereby increasing the number of scans made by the rotating polygon mirror during one rotation. On the other hand, if the number of reflecting surfaces of the rotary polygon mirror increases and the outer diameter of the rotary polygon mirror increases, the load on the motor for rotating the rotary polygon mirror increases.

[0012] Furthermore, if an expensive motor is used to increase the power of the motor in response to an increase in the load on the motor, this is not preferable because it increases costs. Therefore, an overfilled scan (OFS) method is known as a method for increasing the number of reflecting surfaces of a rotating polygon mirror while suppressing an increase in the outer diameter of the rotating polygon mirror.

[0013] In the OFS system, the width of the light beam incident on the reflecting surface of the rotating polygon mirror in the main scanning cross section is larger than the width of the reflecting surface. On the other hand, in the commonly used under-field scanning (UFS) method, the width of the reflecting surface of the rotating polygon mirror is made sufficiently larger than the beam width of the beam incident on the reflecting surface in the main scanning cross section so that the beam incident on the reflecting surface is not blocked. In other words, the OFS method employs a configuration that actively blocks light beams, making it possible to reduce the outer diameter of the rotating polygon mirror.

[0014] In the OFS method, the number of scans per rotation of the rotating polygon mirror can be increased by increasing the number of reflective surfaces of the rotating polygon mirror, but the angle that can be scanned by one reflective surface becomes smaller. Since the scanning width on the surface to be scanned is determined by the specifications, it is necessary to increase the fθ coefficient in accordance with the reduction in the scanning angle of one reflecting surface.

[0015] For example, in order to scan a surface having a scanning width of 310 mm using a rotating polygonal mirror having ten reflecting surfaces, the fθ coefficient must be set to a value of 247 mm / rad or more. The fθ coefficient is often close to the focal length of the imaging optical system in the main scanning section.

[0016] In other words, an increase in the fθ coefficient leads to an increase in the focal length of the imaging optical system, which in turn increases the distance between the rotating polygon mirror and the surface to be scanned. That is, while the OFS method is advantageous in terms of speed, it has a disadvantage in terms of miniaturization.

[0017] A simple method for reducing the size of an optical scanning device is known in which the optical path is bent using a folding mirror. On the other hand, when the optical path is bent using a folding mirror, there is a risk that an optical element disposed upstream of the folding mirror and the light beam reflected by the folding mirror may interfere with each other.

[0018] Therefore, an optical scanning device has been proposed that uses the OFS method, in which the distance between the rotating polygon mirror and the surface to be scanned is 470 mm, and aims to be compact by using three folding mirrors to guide the light beam reflected by the rotating polygon mirror to the surface to be scanned. However, in such an optical scanning device, there is a risk that the cost will increase due to the increased number of reflecting mirrors. Furthermore, deviations in the irradiation positions are likely to occur depending on the assembly tolerances of the respective folding mirrors, and there is a risk that adjustment of the irradiation positions will result in increased costs.

[0019] In addition, an optical scanning device has been proposed that aims to reduce size by adopting a configuration in which a light beam reflected by a rotating polygonal mirror passes through an imaging optical element, is reflected by a reflective optical element, and then passes through the imaging optical element again to be guided to the surface to be scanned. However, in the proposed optical scanning device, each optical element forming the incident optical system is positioned at a large distance from the rotating polygon mirror so that the light beam reflected by the reflective optical element does not interfere with each other. As a result, the area for arranging the incident optical system increases, and the size is not sufficiently reduced.

[0020] Therefore, an object of this embodiment is to provide an optical scanning device that is sufficiently miniaturized. 1(a) and 1(b) are a schematic main-scanning cross-sectional view and a schematic sub-scanning cross-sectional view, respectively, of an optical scanning device 50 according to the first embodiment.

[0021] The optical scanning device 50 according to this embodiment includes a light source 1, an anamorphic lens 2 (optical element), a diaphragm 3, a deflector 4, an imaging optical element 5, and a reflecting optical element . The light source 1 may be, for example, a semiconductor laser, and may have one or more light emitting points. The anamorphic lens 2 has a positive refractive power in the main scanning cross section, and changes the degree of convergence of the light beam emitted from the light source 1 in the main scanning cross section, specifically converting it into a weakly convergent light beam.

[0022] In the optical scanning device 50 of this embodiment, the refractive power required for the imaging optical element 5 in the main scanning cross section is reduced by using a weakly convergent light beam converted by the anamorphic lens 2, thereby ensuring the edge thickness up to the end of the imaging optical element 5. The anamorphic lens 2 also has a positive refractive power in the sub-scanning cross section, and focuses the light beam emitted from the light source 1 near the deflection surface 4a of the deflector 4, thereby forming a long line image in the main scanning direction on the deflection surface 4a.

[0023] The diaphragm 3 has a rectangular opening and restricts the width of the light beam in the sub-scanning direction that has passed through the anamorphic lens 2. The diaphragm 3 also restricts the width of the light beam in the main scanning direction. In the optical scanning device 50 according to this embodiment, the light beam that has passed through the aperture 3 is incident on the imaging optical element 5, and by passing through the imaging optical element 5, the light beam is further converged in both the main scanning cross section and the sub-scanning cross section. The opening of the diaphragm 3 is not limited to a rectangular shape, but may be formed in an elliptical or track shape.

[0024] A rotating polygon mirror having eight deflecting surfaces 4a is used as the deflector 4. However, the number of deflecting surfaces 4a of the deflector 4 is not limited to eight. The light beam that has passed through the imaging optical element 5 is deflected by the deflecting surface 4 a of the deflector 4 .

[0025] In the optical scanning device 50 according to this embodiment, the width of the light beam when it is incident on the deflecting surface 4a in the main scanning cross section is larger than the width of the deflecting surface 4a. Therefore, the width of the light beam deflected by the deflecting surface 4a of the deflector 4 in the main scanning direction is restricted by the deflecting surface 4a.

[0026] The imaging optical element 5 has two optical surfaces (lens surfaces), namely, a first optical surface 5a on the deflector 4 side and a second optical surface 5b on the reflecting optical element 7 side in the X direction perpendicular to both the sub-scanning direction and the main scanning direction. The shapes of the first and second optical surfaces 5a and 5b in the main scanning cross section are formed so that the light beam deflected by the deflecting surface 4a passes through the imaging optical element 5 twice via the reflective optical element 7, thereby scanning the surface to be scanned 6 with desired scanning characteristics, as will be described later.

[0027] The shapes of the first and second optical surfaces 5a and 5b in the sub-scanning cross section are such that the vicinity of the deflecting surface 4a and the vicinity of the surface to be scanned 6 are optically conjugate with each other, thereby compensating for surface tilt. The compensation for surface tilt referred to here means reducing the deviation of the scanning position on the surface to be scanned 6 in the sub-scanning direction when the deflecting surface 4a tilts.

[0028] In the optical scanning device 50 according to this embodiment, a light beam emitted from the light source 1 passes through the anamorphic lens 2 and the diaphragm 3, and then enters the imaging optical element 5 from the second optical surface 5b. Next, the light beam emerging from the first optical surface 5a of the imaging optical element 5 is deflected by the deflector 4, and then enters the imaging optical element 5 again from the first optical surface 5a.

[0029] The light beam emitted from the second optical surface 5b of the imaging optical element 5 is reflected by the reflecting optical element 7, and then enters the imaging optical element 5 again from the second optical surface 5b. Finally, the light beam emitted from the first optical surface 5a of the imaging optical element 5 is guided onto the surface 6 to be scanned. In other words, in the optical scanning device 50 according to this embodiment, the light beam from the light source 1 is guided to the scanned surface 6 via the second optical surface 5b, the first optical surface 5a, the deflector 4, the first optical surface 5a, the second optical surface 5b, the reflective surface of the reflective optical element 7, the second optical surface 5b, and the first optical surface 5a, in that order.

[0030] In the optical scanning device 50 according to this embodiment, the light beam passes through the imaging optical element 5 multiple times, thereby being focused (guided) in both the main scanning section and the sub-scanning section, thereby forming a spot-shaped image near the scanned surface 6. The deflector 4 is then rotated at a constant speed in the direction of the arrow B11 in Figure 1(a) by a drive unit not shown, and the scanned surface 6 is optically scanned in the main scanning direction, that is, the direction of the arrow C11 in Figure 1(a), thereby forming an electrostatic latent image on the scanned surface 6.

[0031] In the optical scanning device 50 according to this embodiment, an incident optical system that causes the light beam emitted from the light source 1 to be incident on the deflecting surface 4a of the deflector 4 is formed by the anamorphic lens 2, the diaphragm 3, and the imaging optical element 5. The imaging optical element 5 forms an imaging optical system that guides the light beam deflected by the deflecting surface 4 a of the deflector 4 to the surface 6 to be scanned.

[0032] That is, in the optical scanning device 50 according to this embodiment, the imaging optical element 5 is shared by both the incident optical system and the imaging optical system. In the optical scanning device 50 according to this embodiment, the reflective optical element 7 forms a reflective optical system that reflects the light beam deflected by the deflecting surface 4a of the deflector 4 so as to bend the optical path of the light beam.

[0033] In the optical scanning device 50 according to this embodiment, instead of using the anamorphic lens 2, for example, a coupling lens and a cylindrical lens may be used so as to share the optical function of the anamorphic lens 2. In the optical scanning device 50 according to this embodiment, the optical function of the anamorphic lens 2 may be performed by the imaging optical element 5 .

[0034] However, when such a configuration is adopted, the power required in the area of ​​the imaging optical element 5 through which the light beam incident on the deflector 4 passes will be different from the power required in the area through which the light beam deflected by the deflector 4 passes. Therefore, the shape of the imaging optical element 5 in the sub-scan section becomes discontinuous, at least optically.

[0035] Furthermore, the anamorphic lens 2 and the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment are plastic molded lenses formed by injection molding, but are not limited to this and glass molded lenses may also be used. Furthermore, molded lenses can be easily formed to have an aspherical shape and are suitable for mass production. Therefore, by using molded lenses as the anamorphic lens 2 and the imaging optical element 5 in the optical scanning device 50 according to this embodiment, it is possible to improve productivity and optical performance.

[0036] In the optical scanning device 50 according to this embodiment, the imaging optical system is formed by a single imaging optical element 5 in order to achieve low costs. In other words, in the optical scanning device 50 according to this embodiment, no refractive optical element other than the imaging optical element 5 is provided on the optical path of the light beam deflected by the deflector 4.

[0037] In the optical scanning device 50 according to this embodiment, the reflective optical system is formed by a single reflective optical element 7 having a planar shape in order to achieve low costs. In other words, in the optical scanning device 50 according to this embodiment, no reflective optical element other than the reflective optical element 7 is provided on the optical path of the light beam deflected by the deflector 4.

[0038] However, the present invention is not limited to this, and an imaging optical system may be formed by a plurality of imaging optical elements in order to improve imaging performance, or a reflective optical system may be provided with a reflective optical element having a shape other than a flat surface. In the optical scanning device 50 according to this embodiment, no refractive optical element is provided between the imaging optical element 5 and the deflector 4 on the optical path of the incident light beam in the incident optical system.

[0039] In the optical scanning device 50 of this embodiment, the light beam passes through the imaging optical element 5 three times: when it travels to the deflecting surface 4a after being emitted from the light source 1, when it travels to the reflective optical element 7 after being deflected by the deflecting surface 4a, and when it travels to the scanned surface 6 after being reflected by the reflective optical element 7. By adopting such a configuration, it is possible to achieve sufficient miniaturization even when using the OFS method, which generally tends to increase the optical path length and the area occupied by each optical system. In other words, by adopting the OFS method, it is possible to achieve both high-speed printing and compactness.

[0040] In particular, by making the light beam emitted from the light source 1 pass through the imaging optical element 5 and then incident on the deflection surface 4a of the deflector 4, a configuration can be used in which no reflective optical element is provided in the optical path between the light source 1 and the deflection surface 4a. This eliminates the need to dispose the light source 1 far away from the deflector 4 as in the optical scanning device disclosed in Patent Document 1, thereby achieving a compact incident optical system.

[0041] Furthermore, when the OFS method uses a configuration in which the light beam reflected by a reflective optical element is incident on a deflection surface, if the angle of the reflective optical element is not adjusted sufficiently, there is a risk of density unevenness occurring between the scanning area on the scanning surface at the start of writing and the scanning area on the end of writing. Therefore, adjusting the angle of the reflecting optical element requires a great deal of time and effort, which results in increased costs. On the other hand, in the optical scanning device 50 according to this embodiment, it is possible to adopt a configuration in which the light beam emitted from the light source 1 is incident on the deflection surface 4a of the deflector 4 without being reflected by a reflective optical element, thereby eliminating the need for the above-mentioned adjustments and suppressing increases in costs.

[0042] Furthermore, in the optical scanning device 50 according to this embodiment, a configuration can be adopted in which the light beam emitted from the light source 1 is incident on the deflection surface 4a of the deflector 4 without being reflected by a reflective optical element, as described above, and therefore the light beam can be incident on the deflection surface 4a directly from the front. That is, as shown in FIG. 1(a), when projected onto the main scanning cross section, the optical axes of the incident optical system and the imaging optical system are parallel to each other. In other words, in the optical scanning device 50 according to this embodiment, when projected within the main scanning cross section, the traveling direction of the light beam when it enters the deflector 4 and the traveling direction of the light beam when it is deflected by the deflector 4 toward the axial image height are parallel to each other.

[0043] Here, the position in the main scanning direction of the intersection between the scanned surface 6 and the sub-scanning section perpendicular to the main scanning direction, which includes a straight line passing through the vertices of the first and second optical surfaces 5a and 5b of the imaging optical element 5, is defined as the on-axis image height. The optical axis of the imaging optical system is defined as an axis passing through the deflection point (hereinafter referred to as the on-axis deflection point) on the deflection surface 4a of the chief ray of the light beam (hereinafter referred to as the on-axis light beam) deflected by the deflector 4 so as to scan the on-axis image height on the scanned surface 6, and the on-axis image height. The optical axis of the incident optical system is defined as an axis passing through the center of the light emitting surface of the light source 1 and the on-axis deflection point. A straight line passing through the vertices of the first and second optical surfaces 5a and 5b of the imaging optical element 5 is defined as the optical axis of the imaging optical element 5.

[0044] As described above, in the optical scanning device 50 according to this embodiment, the light beam emitted from the light source 1 is made to be incident on the deflecting surface 4a from the front. In general, the reflectance of the deflecting surface 4a for the incident light beam changes depending on the incident angle of the light beam.

[0045] In addition, the transmittance of the light beam passing through the refractive optical element also changes depending on the angle of incidence of the light beam. Therefore, in the optical scanning device 50 of this embodiment, the light beam emitted from the light source 1 is made to be incident directly on the deflection surface 4a as described above, thereby suppressing the occurrence of density unevenness between the scanning area on the start side of writing and the scanning area on the end side of writing on the scanned surface 6.

[0046] In addition, in the optical scanning device 50 according to this embodiment, as shown in FIG. 1(b), when projected within the sub-scanning cross section, the light beam emitted from the light source 1 is incident obliquely, i.e., obliquely incident, on the deflection surface 4a. This makes it possible to suppress interference of the light source 1 and the anamorphic lens 2 with the imaging optical element 5 and the reflecting optical element 7.

[0047] Furthermore, in the optical scanning device 50 according to this embodiment, by adopting the OFS method, the number of deflection surfaces 4a of the deflector 4 is increased to eight, i.e., more than the number of deflection surfaces of the deflector provided in an optical scanning device adopting the UFS method, thereby achieving faster printing. However, the optical scanning device 50 according to this embodiment may also adopt the UFS method, i.e., even in such a case, miniaturization can be achieved by adopting a configuration in which the light beam passes through the imaging optical element 5 three times.

[0048] As described above, the optical scanning device 50 according to this embodiment can be made smaller while achieving higher speeds by adopting the OFS method. In this case, when the number of deflecting surfaces 4a of the deflector 4 is N, it is preferable that the following conditional expression (1) be satisfied. 6≦N≦10 (1)

[0049] If the lower limit of conditional expression (1) is exceeded, it becomes difficult to achieve a sufficient increase in speed, which is undesirable. On the other hand, exceeding the upper limit of conditional expression (1) is not preferable because the scanning angle assigned to each deflection surface becomes too small, which requires increasing the fθ coefficient of the imaging optical system, thereby increasing the optical path length.

[0050] In the optical scanning device 50 according to this embodiment, it is preferable that the following conditional expression (2) be satisfied in order to achieve miniaturization. L2≦L1 (2) Here, L1 is the spatial distance between the reflecting surface of the reflecting optical element 7 and the on-axis deflection point on the deflecting surface 4a, and L2 is the spatial distance between the on-axis deflection point on the deflecting surface 4a and the scanned surface 6.

[0051] When the distance L1 is increased by ΔL1, the optical path length between the deflecting surface 4a of the deflector 4 and the scanned surface 6 increases by approximately 2×ΔL1, while when the distance L2 is increased by ΔL2, the optical path length increases by approximately ΔL2. Therefore, the total optical path length can be increased more easily by increasing the distance L1 than by increasing the distance L2. Therefore, in the optical scanning device 50 according to this embodiment, by satisfying conditional expression (2), it is possible to achieve miniaturization while maintaining the total optical path length.

[0052] Furthermore, in the optical scanning device 50 according to this embodiment, the light beam emitted from the light source 1 is converted into a convergent light beam within the main scanning cross section and is made incident on the deflecting surface 4a of the deflector 4. This reduces the total optical path length, thereby achieving miniaturization. This also reduces the power required for the imaging optical element 5 in the main scanning cross section, thereby reducing the thickness of the imaging optical element 5, thereby shortening the manufacturing time when manufacturing the imaging optical element 5 by injection molding.

[0053] In the optical scanning device 50 according to this embodiment, the imaging optical element 5 has a meniscus shape that is concave toward the deflecting surface 4a so that the imaging optical element 5 has positive power as a whole in the sub-scanning cross section. By forming the imaging optical element 5 in such a shape, the principal plane of the imaging optical element 5 can be disposed on the anamorphic lens 2 side relative to the position of the imaging optical element 5.

[0054] In addition, in the optical scanning device 50 according to this embodiment, the optical paths in the incident optical system and the imaging optical system are formed on opposite sides of a cross section parallel to the main scanning direction, the cross section including a straight line passing through the vertices of the first and second optical surfaces 5a and 5b of the imaging optical element 5. Furthermore, the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment has a positive power in the sub-scanning cross section as described above.

[0055] Therefore, the angle that the direction of travel of the light beam heading towards the imaging optical element 5 after being reflected by the deflection surface 4a makes with respect to the cross section is larger than the angle that the direction of travel of the light beam heading towards the reflecting optical element 7 after passing through the imaging optical element 5 makes with respect to the cross section. Therefore, in the optical scanning device 50 according to this embodiment, the principal plane of the imaging optical element 5 is disposed on the anamorphic lens 2 side, thereby increasing the distance between the optical paths in the incident optical system and the imaging optical system. This allows the light source 1 and the anamorphic lens 2 to be disposed between the imaging optical element 5 and the reflecting optical element 7 in the X direction.

[0056] In the optical scanning device 50 of this embodiment, when projected within the sub-scanning section, the axial deflection point on the deflection surface 4a is located near a straight line passing through the surface vertices of the first and second optical surfaces 5a and 5b of the imaging optical element 5. When projected onto the sub-scanning cross section, the straight line is located between the optical path of the light beam from the aperture 3 as it passes through the imaging optical element 5 and the optical path of the light beam deflected by the deflecting surface 4a as it passes through the imaging optical element 5. In other words, in the optical scanning device 50 according to this embodiment, at each of the first and second optical surfaces 5a and 5b of the imaging optical element 5, the light beam from the light source 1 and the light beam deflected by the deflector 4 pass through opposite sides of the surface vertex in the sub-scanning direction.

[0057] In particular, in the optical scanning device 50 of this embodiment, when projected within the sub-scanning cross section, the straight line is located approximately halfway between the optical path of the light beam from the aperture 3 as it passes through the imaging optical element 5 and the optical path of the light beam deflected by the deflecting surface 4a as it passes through the imaging optical element 5. If one of the optical paths were to be moved closer to the straight line without using such a configuration, the direction of travel of the rays of the light beam traveling along that optical path that pass through an area far from the straight line would change significantly compared to the direction of travel of the rays that pass through an area close to the straight line, resulting in a decrease in imaging performance.

[0058] That is, in the optical scanning device 50 according to this embodiment, the deflecting surface 4a and the scanned surface 6 are set in a conjugate relationship with each other, so the imaging optical element 5 needs to have a fairly strong positive power in the sub-scanning cross section. Therefore, it is necessary to set the sagittal radius of curvature of the imaging optical element 5 to a relatively small value, and therefore, as the area through which the light ray passes on the first and second optical surfaces 5a and 5b of the imaging optical element 5 becomes farther from the straight line, the incident direction approaches horizontal from vertical. When the angle of incidence of the light beam on the first and second optical surfaces 5a and 5b of the imaging optical element 5 becomes large to a certain extent, the incidence causes the traveling direction of the light beam to change suddenly, resulting in a deterioration in imaging performance.

[0059] In addition, in the optical scanning device 50 according to this embodiment, the light beam emitted from the light source 1 and passing through the imaging optical element 5 enters the deflector 4 without passing through a refractive optical element having power or being reflected by a reflective optical element. In other words, since the optical path of the incident light beam and the optical path of the deflected light beam deflected by the deflecting surface 4a are very close to each other, it is difficult to arrange such refractive optical elements or reflective optical elements so as not to interfere with the deflected light beam.

[0060] On the other hand, when attempting to place such a refractive optical element or reflective optical element on the optical path of the incident light beam, it is necessary to increase the incident angle of the incident light beam with respect to the deflecting surface 4a, thereby greatly separating the incident light beam and the deflected light beam from each other. If the angle of incidence is increased, oblique astigmatism will increase, which is undesirable as it will degrade the imaging performance.

[0061] A flat element for blocking rotational noise generated in the deflector 4 may be provided on the optical path of the incident light beam. This is because, by passing the deflected light beam as well as the incident light beam through such a flat element, it is not necessary to separate the incident light beam and the deflected light beam by a large distance as described above.

[0062] In the optical scanning device 50 according to this embodiment, in addition to the imaging optical element 5, an anamorphic lens 2 is also provided in the optical path of the incident light beam between the light source 1 and the deflecting surface 4a. By adopting such a configuration, the imaging optical element 5 can be formed into an optically continuous shape across a straight line passing through the vertices of the first and second optical surfaces 5a and 5b in the sub-scanning cross section. Therefore, the imaging optical element 5 can be formed at low cost by injection molding.

[0063] Although it is not impossible to form the imaging optical element 5 in a discontinuous shape with steps by injection molding, it is difficult to form the shape near the steps as designed by injection molding. Therefore, it is preferable that the imaging optical element 5 has a continuous shape so that it can be easily and stably formed in a desired shape.

[0064] The anamorphic lens 2 provided in the optical scanning device 50 according to this embodiment is a lens having different powers in the main scanning cross section and the sub-scanning cross section. That is, in order to achieve a focused state on the deflecting surface 4a in the optical scanning device 50 according to this embodiment, the power required in the main scanning cross section and the sub-scanning cross section differ greatly from each other.

[0065] Therefore, even if the powers in the main scanning cross section and the sub-scanning cross section of the imaging optical element 5 are different from each other, it is difficult to achieve the above-mentioned light-condensing state if the optical surface of the anamorphic lens 2 is formed to be rotationally symmetric. In other words, if the optical surfaces of the anamorphic lens 2 are formed to be rotationally symmetrical, while the powers of the imaging optical element 5 in the main scanning cross section and the sub-scanning cross section are made to differ greatly from each other, the first and second optical surfaces 5a and 5b of the imaging optical element 5 will become optically discontinuous, which is not desirable.

[0066] In the optical scanning device 50 according to this embodiment, the light beam deflected by the deflecting surface 4a is guided to the surface 6 to be scanned by the imaging optical element 5 and the reflecting optical element . That is, in the optical scanning device 50 according to this embodiment, the number of optical elements that guide the light beam deflected by the deflecting surface 4a to the surface to be scanned 6 is sufficiently reduced, thereby achieving cost reduction.

[0067] Here, consider a case where the imaging optical system is formed by a plurality of imaging optical elements including the imaging optical element 5, and a predetermined imaging optical element is disposed between the deflector 4 and the surface to be scanned 6 in the X direction. In this case, the imaging optical element arranged in this manner is required to have a shape that is elongated in the main scanning direction.

[0068] Therefore, when forming such an imaging optical element by injection molding, the number that can be molded at one time is reduced, resulting in high costs. On the other hand, if all of the imaging optical elements are disposed between the deflector 4 and the reflecting optical element 7 in the X direction, there is a risk that adjacent imaging optical elements may interfere with each other. That is, such an arrangement is not preferable because it may be difficult to do.

[0069] Furthermore, in the optical scanning device 50 according to this embodiment, the light beam deflected by the deflecting surface 4a as described above is guided to the scanned surface 6 by the imaging optical element 5 and the reflective optical element 7, but depending on the arrangement and shape of the imaging optical element 5, there is a risk that it may easily interfere with other optical elements. Therefore, in the optical scanning device 50 according to this embodiment, it is preferable that the first and second optical surfaces 5a and 5b of the imaging optical element 5 each satisfy the following conditional expression (3). -3.00≦(Pb-Pa) / t≦2.00 (3)

[0070] In conditional expression (3), t is the thickness of the imaging optical element 5, Pa is the coordinate in the X direction of the passage position of the on-axis light beam, and Pb is the coordinate in the X direction of the passage position of the light beam heading toward the most off-axis image height (hereinafter referred to as the most off-axis light beam). If the lower limit of conditional expression (3) is not reached, there is a risk that the imaging optical element 5 and the reflecting optical element 7 may interfere with each other, which is undesirable.

[0071] On the other hand, if the upper limit of conditional expression (3) is exceeded, the length of the imaging optical element 5 in the main scanning direction becomes large, and the number that can be molded at one time when forming it by injection molding becomes small, resulting in high costs. Furthermore, if the shape of the imaging optical element 5 changes so that the lower limit of conditional expression (3) is exceeded or the upper limit is exceeded, this is not preferable because it will cause field curvature that is difficult to tolerate.

[0072] In the optical scanning device 50 according to this embodiment, it is more preferable that the following conditional expression (3a) be satisfied instead of the conditional expression (3). -2.50≦(Pb-Pa) / t≦-0.25 (3a) That is, it is preferable that the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment has a meniscus shape that is convex toward the deflector 4 when projected in the main scanning cross section so as to satisfy conditional formula (3a).

[0073] Next, the specifications of the optical scanning device 50 according to this embodiment, the surface spacing, refractive index and decentering arrangement information of each optical element, and the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 are shown in Tables 1, 2 and 3 below, respectively.

[0074] [Table 1]

[0075] [Table 2]

[0076] [Table 3]

[0077] The generating lines of the first optical surface 5a on the deflection surface 4a side and the second optical surface 5b on the reflecting optical element 7 side of the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment have aspherical shapes expressed by a 16th-order polynomial function shown in the following equation (4). Moreover, the generatrix of each of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 50 according to this embodiment also has an aspherical shape expressed by the following formula (4).

[0078]

number

[0079] In equation (4) and equations (5) and (6) shown below, the vertex of each optical surface is taken as the origin, the main scanning direction is taken as the Y axis, the sub-scanning direction is taken as the Z axis, and the directions perpendicular to both the main scanning direction and the sub-scanning direction are taken as the X axis. In addition, the direction from the reflective optical element 7 to the scanned surface 6, i.e., the direction from left to right in FIG. 1(b), is defined as positive on the X axis, and the direction from bottom to top in FIG. 1(b) is defined as positive on the Z axis.

[0080] In addition, in equation (4), R is the radius of curvature (generatrix radius of curvature) in the main scanning section, K, B4, B6, B8, B 10 , B 12 , B 14 and B 16 is the aspherical coefficient. In the optical scanning device 50 according to this embodiment, the aspherical coefficients B4 to B5 are set to the optical surfaces of the anamorphic lens 2 and the imaging optical element 5.16 have the same values ​​on the positive and negative sides of the Y direction. That is, the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment have generatrices that are symmetrical with each other about the optical axis in the main scanning direction.

[0081] Furthermore, the sagittal lines of the first optical surface 5a on the deflection surface 4a side and the second optical surface 5b on the reflecting optical element 7 side of the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment have shapes expressed by the following formula (5). Moreover, the sagittal lines of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 50 according to this embodiment also have shapes expressed by the following formula (5).

[0082]

number

[0083] In equation (5), S represents the sagittal shape defined in a cross section perpendicular to the main scanning cross section and including the normal to the generatrix at a predetermined position on the generatrix. Furthermore, the radius of curvature (sagittal radius of curvature) r' in the sub-scanning cross section at a position Y away from the optical axis in the main scanning direction shown in equation (5) is expressed by the following equation (6).

[0084]

number

[0085] In equation (6), r is the radius of curvature of the sagittal line on the optical axis, E1, E2, E4, E6, E8, E 10 and E 12 is the sagittal variation coefficient. That is, the sagittal curvature radius r' of each optical surface of the imaging optical element 5 changes depending on the position Y in the main scanning direction.

[0086] In the optical scanning device 50 according to this embodiment, the sagittal line change coefficients E1 to E2 are set to the respective optical surfaces of the imaging optical element 5. 12 have the same values ​​on the positive and negative sides of the Y direction. That is, the optical surfaces of the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment have sagittal shapes that are symmetrical with each other with respect to the optical axis in the main scanning direction.

[0087] Although the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment are defined by the functions expressed in the above equations (4) to (6), the definition of the shapes is not limited to this. The shapes of the optical surfaces of the imaging optical element 5 shown in Table 3 are identical to one another in the region where the light beam passes three times.

[0088] That is, the effective area of ​​each optical surface of the imaging optical element 5 provided in the optical scanning device 50 according to this embodiment has an optically continuous shape. In other words, the generatrix and sagittal line of the effective area of ​​each optical surface of the imaging optical element 5 are expressed by the single equation (4) and the single equations (5) and (6), respectively.

[0089] Here, the effective area of ​​each optical surface of the imaging optical element 5 is defined as an area between positions on the scanned surface 6 in the main scanning direction through which the most off-axis light beams traveling toward the most off-axis image heights on both sides pass. The effective area of ​​each optical surface of the imaging optical element 5 is defined in the sub-scanning direction as the area between the position where the light beam from the light source 1 passes and the position where the light beam reflected by the reflecting optical element 7 passes.

[0090] In the optical scanning device 50 according to this embodiment, the distance L1 in the X direction between the reflective optical element 7 and the deflecting surface 4a is 60 mm, and the distance L2 in the X direction between the deflecting surface 4a and the surface to be scanned 6 is 37 mm. That is, the overall size of the optical scanning device 50 according to this embodiment in the X direction is 97 mm.

[0091] Specifically, in the optical scanning device 50 according to this embodiment, the distance L1, which roughly corresponds to the distance of the optical path between the reflective optical element 7 and the deflecting surface 4a, through which the light beam travels back and forth, i.e., twice, is made longer than the distance L2, which roughly corresponds to the optical path between the deflecting surface 4a and the scanned surface 6, through which the light beam travels once. This allows the size of the entire optical scanning device 50 in the X direction to be reduced to 100 mm or less while ensuring a sufficient optical path length between the deflecting surface 4a and the surface to be scanned 6 required in the OFS method.

[0092] Furthermore, in the optical scanning device 50 according to this embodiment, the light beam emitted from the light source 1 passes through the imaging optical element 5 and then enters the deflector 4, so that the light source 1 and the anamorphic lens 2 can be arranged without providing a reflective optical element on the optical path of the light beam between the light source 1 and the deflector 4. In addition, in the optical scanning device 50 according to this embodiment, the optical axis of the anamorphic lens 2 is tilted by 2.00° in the sub-scanning section relative to the main scanning section, while the optical axis of the imaging optical element 5 is parallel to the main scanning section.

[0093] Here, a case will be considered in which the imaging optical element 5 is disposed at an angle with respect to the main scanning cross section so that the optical axis of the imaging optical element 5 is parallel to the optical axis of the anamorphic lens 2. In this case, the light beam deflected by the deflecting surface 4 a ends up entering the imaging optical element 5 while traveling in a direction that forms a large angle with respect to the optical axis of the imaging optical element 5 . Furthermore, if the imaging optical element 5 is positioned at such an angle, there is a risk that the first and second optical surfaces 5a and 5b will each have optically discontinuous shapes on either side of the optical axis in the sub-scanning direction.

[0094] In addition, in the optical scanning device 50 according to this embodiment, the light beam emitted from the light source 1 and passing through the imaging optical element 5 is refracted by the imaging optical element 5 and enters the deflection surface 4a at an angle of 5.70° in the sub-scanning cross section relative to the main-scanning cross section. That is, in the optical scanning device 50 according to this embodiment, the incident angle of the light beam to the deflecting surface 4a in the sub-scanning section, 5.70°, is larger than the incident angle of the light beam to the imaging optical element 5 in the sub-scanning section, 2.00°.

[0095] This is because the imaging optical element 5 has a positive power in the sub-scanning cross section, and the light beam emitted from the light source 1 passes below the imaging optical element 5 in the sub-scanning direction. In the optical scanning device 50 according to this embodiment, the light source 1 and the anamorphic lens 2 are disposed between the reflecting optical element 7 and the imaging optical element 5 in the X direction, thereby achieving miniaturization.

[0096] Therefore, in order to prevent the light beam deflected by the deflecting surface 4a from interfering with the light source 1 and the anamorphic lens 2, the optical path of the light beam and the optical path of the incident optical system must be sufficiently separated from each other. To achieve such a separation, it is advantageous to increase the separation of the imaging optics 5 from the deflecting surface 4a.

[0097] However, if the imaging optical element 5 is disposed away from the deflecting surface 4a in the optical scanning device 50 according to this embodiment, there is a risk that the imaging optical element 5 and the anamorphic lens 2 may interfere with each other. If the anamorphic lens 2 is disposed at a distance from the imaging optical element 5 in order to suppress such interference, the distance of the light source 1 from the deflecting surface 4a will also increase further.

[0098] In this case, the distance between the deflecting surface 4a and the light source 1 becomes larger than the distance between the deflecting surface 4a and the reflecting optical element 7, making it difficult to achieve sufficient miniaturization. Therefore, in the optical scanning device 50 according to this embodiment, the imaging optical element 5 is formed to have a meniscus shape that is concave toward the deflecting surface 4a in the sub-scanning cross section.

[0099] By forming the imaging optical element 5 in this manner, the position of the principal plane can be shifted to the side where the anamorphic lens 2 is disposed relative to the position where the imaging optical element 5 is disposed. This increases the distance between the optical path of the light beam emitted from the light source 1 and incident on the deflecting surface 4a and the optical path of the light beam deflected by the deflecting surface 4a, allowing the light source 1 and the anamorphic lens 2 to be positioned between the imaging optical element 5 and the reflective optical element 7 in the X direction.

[0100] In the optical scanning device 50 of this embodiment, the position of the chief ray of the axial light beam when deflected by the deflecting surface 4a and emitted from the second optical surface 5b of the imaging optical element 5 is +3.30 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. Furthermore, the position of the chief ray of the light beam when it is incident on the second optical surface 5b of the imaging optical element 5 after passing through the diaphragm 3 is separated from the optical axis of the imaging optical element 5 by −3.21 mm in the sub-scanning direction.

[0101] That is, the two positions are spaced apart by 3.30-(-3.21)=6.51 mm. The above two positions are approximately symmetrical to each other with respect to the optical axis of the imaging optical element 5, which makes it possible to suppress the occurrence of large sub-scanning coma aberration even if the imaging optical element 5 is formed in a shape that is symmetrical to each other with respect to the optical axis in the sub-scanning cross section.

[0102] In addition, when the axial light beam is deflected by the deflecting surface 4a and emerges from the second optical surface 5b of the imaging optical element 5, the position of the marginal ray on the optical axis side of the imaging optical element 5 is +1.80 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. Furthermore, when the light beam passes through the aperture 3 and then enters the second optical surface 5b of the imaging optical element 5, the position of the marginal ray on the optical axis side of the imaging optical element 5 is -1.78 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. In other words, since the two positions are separated from each other by at least 3 mm, the light source 1 and the anamorphic lens 2 can be positioned so as not to interfere with the light beam deflected by the deflecting surface 4a and emitted from the second optical surface 5b of the imaging optical element 5.

[0103] Furthermore, when the axial light beam is reflected by the reflecting surface of the reflecting optical element 7 after passing through the imaging optical element 5, the position of the marginal ray on the optical axis side of the imaging optical element 5 on the reflecting surface is +2.27 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. The position of the light emitting point of the light source 1 is −4.09 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction, i.e., it is sufficiently far away from the above position, so that a semiconductor laser diode (LD) can be arranged as the light source 1.

[0104] The position of the chief ray of the axial light beam when it is reflected by the reflecting surface of the reflecting optical element 7 and then incident on the second optical surface 5b of the imaging optical element 5 is +4.99 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. In other words, this position and the above-mentioned position of the chief ray of the axial light beam when it is deflected by the deflecting surface 4a and emerges from the second optical surface 5b of the imaging optical element 5 are only 4.99-3.30=1.69 mm apart from each other, which is smaller than the above-mentioned distance of 6.51 mm.

[0105] As described above, the optical scanning device 50 according to this embodiment is configured so that the position of the chief ray of the axial light beam when it is reflected by the reflecting surface of the reflecting optical element 7 and then incident on the second optical surface 5b of the imaging optical element 5 is not significantly separated from the optical axis of the imaging optical element 5. This makes it possible to reduce sub-scanning coma aberration. In the optical scanning device 50 according to this embodiment, the reflective optical element 7 is not disposed at an angle.

[0106] As described above, in the optical scanning device 50 according to this embodiment, the imaging optical element 5 is formed to have a meniscus shape that is concave toward the deflecting surface 4a in the sub-scanning cross section. In this case, the smaller the absolute value of the radius of curvature of each optical surface of the imaging optical element 5 in the sub-scan section, the greater the distance between the deflecting surface 4a and the position of the principal plane.

[0107] This makes it easier to separate the light beam emitted from the light source 1 and incident on the deflecting surface 4a from the light beam deflected by the deflecting surface 4a. On the other hand, since the two light beams each pass through an area of ​​the imaging optical element 5 that is separated from the optical axis, if the absolute value of the radius of curvature of each optical surface of the imaging optical element 5 in the sub-scanning cross section is made too small, sub-scanning coma aberration becomes more likely to occur.

[0108] Therefore, in the optical scanning device 50 according to this embodiment, the imaging optical element 5 is formed so that all light beams pass through an area within the sub-scanning cross section that is 0.35 times or less of the smaller absolute value of the radii of curvature on the optical axis of each of the first and second optical surfaces 5a and 5b. In other words, in the optical scanning device 50 according to this embodiment, it is preferable that the following conditional expression (7) be satisfied. |P s |≦0.35×|R s | (7)

[0109] In conditional expression (7), |R s | is a relatively small value among the absolute values ​​of the radii of curvature of the first and second optical surfaces 5a and 5b of the imaging optical element 5 on the optical axis. Also |P s | is the absolute value of the radius of curvature |R s | is the distance between the optical axis and the passage position of all the light beams on one optical surface of the imaging optical element 5 that is the furthest from the optical axis of the imaging optical element 5 in the sub-scanning section.

[0110] Specifically, in the optical scanning device 50 according to this embodiment, |R s | is 22.452 mm at the second optical surface 5b as shown in Table 3. Furthermore, on the second optical surface 5b of the imaging optical element 5, among the passing positions of the rays of all the light beams, the passing position of the marginal ray of the axial light beam reflected by the reflecting optical element 7 on the opposite side to the optical axis of the imaging optical element 5 is the furthest from the optical axis in the sub-scanning cross section.

[0111] Specifically, the passing position is separated from the optical axis by +7.263 mm in the sub-scanning cross section, so |P s | is 7.263mm. Therefore, since 7.263 mm / 22.452 mm=0.323, the above condition is satisfied in the optical scanning device 50 according to this embodiment.

[0112] As described above, in the optical scanning device 50 of this embodiment, the light beam from the light source 1 passes through the imaging optical element 5 and then enters the deflector 4, the light beam deflected by the deflector 4 passes through the imaging optical element 5, is reflected by the reflective optical element 7, and then passes through the imaging optical element 5 again. By adopting such a configuration, it is possible to achieve even greater miniaturization than before.

[0113] That is, according to this embodiment, even when the OFS method is adopted to increase speed, it is possible to provide an optical scanning device 50 that reduces the area in which the incident optical system and the imaging optical system are arranged and does not require adjustment of the position of the reflective optical element.

[0114] [Second embodiment] 2(a) and 2(b) are a schematic main-scanning cross-sectional view and a schematic sub-scanning cross-sectional view, respectively, of an optical scanning device 60 according to the second embodiment. The optical scanning device 60 of this embodiment has the same configuration as the optical scanning device 50 of the first embodiment except for different specification values, so the same components are given the same reference numerals and their explanations are omitted.

[0115] The specifications of the optical scanning device 60 according to this embodiment, the surface spacing, refractive index and decentering arrangement information of each optical element, and the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 are shown in Tables 4, 5 and 6 below, respectively.

[0116] [Table 4]

[0117] [Table 5]

[0118] [Table 6]

[0119] The generating lines of the first optical surface 5a on the deflecting surface 4a side and the second optical surface 5b on the reflecting optical element 7 side of the imaging optical element 5 provided in the optical scanning device 60 according to this embodiment have aspherical shapes expressed by the 16th-order polynomial function shown in the above equation (4). Moreover, the generatrix of each of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 60 according to this embodiment also has an aspherical shape expressed by the above formula (4).

[0120] Furthermore, the sagittal lines of the first optical surface 5a on the deflection surface 4a side of the imaging optical element 5 provided in the optical scanning device 60 according to this embodiment and the second optical surface 5b on the reflecting optical element 7 side have shapes expressed by the above formulas (5) and (6). Moreover, the sagittal lines of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 60 according to this embodiment also have shapes expressed by the above formulas (5) and (6). Although the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 provided in the optical scanning device 60 according to this embodiment are defined by the functions expressed in the above equations (4) to (6), the definition of the shapes is not limited to this.

[0121] In the optical scanning device 60 according to this embodiment, the distance L1 in the X direction between the reflective optical element 7 and the deflecting surface 4a is 60 mm, and the distance L2 in the X direction between the deflecting surface 4a and the scanned surface 6 is 34 mm. That is, the overall size of the optical scanning device 60 according to this embodiment in the X direction is 94 mm.

[0122] In the optical scanning device 60 of this embodiment, the distance L1, which roughly corresponds to the distance of the optical path between the reflective optical element 7 and the deflecting surface 4a, through which the light beam travels back and forth, i.e., twice, is made longer than the distance L2, which roughly corresponds to the optical path between the deflecting surface 4a and the scanned surface 6, through which the light beam travels once. This allows the size of the entire optical scanning device 60 in the X direction to be reduced to 100 mm or less while ensuring a sufficient optical path length between the deflecting surface 4a and the surface to be scanned 6 required in the OFS method. In particular, even if the reflective optical element 7 has a thickness of 5 mm, the size of the entire optical scanning device 60 in the X direction can be reduced to 100 mm or less.

[0123] In the optical scanning device 60 of this embodiment, the position of the chief ray of the axial light beam when it is deflected by the deflecting surface 4a and emerges from the second optical surface 5b of the imaging optical element 5 is +3.40 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. Furthermore, the position of the chief ray of the light beam when it is incident on the second optical surface 5b of the imaging optical element 5 after passing through the diaphragm 3 is separated from the optical axis of the imaging optical element 5 by −3.21 mm in the sub-scanning direction.

[0124] That is, the two positions are spaced apart by 3.40-(-3.21)=6.61 mm. The above two positions are approximately symmetrical to each other with respect to the optical axis of the imaging optical element 5, which makes it possible to suppress the occurrence of large sub-scanning coma aberration even if the imaging optical element 5 is formed in a shape that is symmetrical to each other with respect to the optical axis in the sub-scanning cross section.

[0125] In addition, when the axial light beam is deflected by the deflecting surface 4a and emerges from the second optical surface 5b of the imaging optical element 5, the position of the marginal ray on the optical axis side of the imaging optical element 5 is +1.98 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. Furthermore, when the light beam passes through the aperture 3 and then enters the second optical surface 5b of the imaging optical element 5, the position of the marginal ray on the optical axis side of the imaging optical element 5 is -1.78 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. In other words, since the two positions are separated from each other by at least 3 mm, the light source 1 and the anamorphic lens 2 can be positioned so as not to interfere with the light beam deflected by the deflecting surface 4a and emitted from the second optical surface 5b of the imaging optical element 5.

[0126] Furthermore, when the axial light beam is reflected by the reflecting surface of the reflecting optical element 7 after passing through the imaging optical element 5, the position of the marginal ray on the optical axis side of the imaging optical element 5 on the reflecting surface is +2.40 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. The position of the light emitting point of the light source 1 is −4.09 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction, i.e., it is sufficiently far away from the above position, so that a semiconductor laser diode (LD) can be arranged as the light source 1.

[0127] The position of the chief ray of the axial light beam when it is reflected by the reflecting surface of the reflecting optical element 7 and then incident on the second optical surface 5b of the imaging optical element 5 is +4.99 mm away from the optical axis of the imaging optical element 5 in the sub-scanning direction. In other words, this position and the above-mentioned position of the chief ray of the axial light beam when it is deflected by the deflecting surface 4a and emerges from the second optical surface 5b of the imaging optical element 5 are only 4.99-3.40=1.59 mm apart from each other, which is smaller than the above-mentioned distance of 6.61 mm.

[0128] As described above, the optical scanning device 60 of this embodiment is configured so that the position of the chief ray of the axial light beam when it is reflected by the reflective surface of the reflective optical element 7 and then incident on the second optical surface 5b of the imaging optical element 5 is not significantly separated from the optical axis of the imaging optical element 5. This makes it possible to reduce sub-scanning coma aberration.

[0129] Furthermore, in the optical scanning device 60 according to this embodiment, the imaging optical element 5 is formed so that all light beams pass through an area within the sub-scanning cross section that is 0.35 times or less of the smaller absolute value of the radii of curvature on the optical axis of each of the first and second optical surfaces 5a and 5b. In other words, in the optical scanning device 60 according to this embodiment, it is preferable that the above conditional expression (7) be satisfied.

[0130] Specifically, in the optical scanning device 60 according to this embodiment, |R s | is 21.212 mm at the second optical surface 5b as shown in Table 6. Furthermore, on the second optical surface 5b of the imaging optical element 5, among the passing positions of the rays of all the light beams, the passing position of the marginal ray of the axial light beam reflected by the reflecting optical element 7 on the opposite side to the optical axis of the imaging optical element 5 is the furthest from the optical axis in the sub-scanning cross section.

[0131] Specifically, the passing position is separated from the optical axis by +7.171 mm in the sub-scanning cross section, so |P s | is 7.171mm. Therefore, 7.171 mm / 21.212 mm=0.338, and therefore the optical scanning device 60 according to this embodiment satisfies the above conditional expression (7).

[0132] In the optical scanning device 60 according to this embodiment, it is preferable that conditional expression (3) be satisfied. It is preferable that the imaging optical element 5 provided in the optical scanning device 60 according to this embodiment has a meniscus shape that is convex toward the deflector 4 when projected in the main scanning cross section so as to satisfy conditional formula (3a).

[0133] As described above, in the optical scanning device 60 of this embodiment, the light beam from the light source 1 passes through the imaging optical element 5 and then enters the deflector 4, the light beam deflected by the deflector 4 passes through the imaging optical element 5, is reflected by the reflective optical element 7, and then passes through the imaging optical element 5 again. By adopting such a configuration, it is possible to achieve even greater miniaturization than before.

[0134] [Third embodiment] Conventionally, an optical scanning device has been proposed that aims to reduce size by adopting a configuration in which a light beam reflected by a rotating polygonal mirror passes through an imaging optical element, is reflected by a reflective optical element, and then passes through the imaging optical element again to be guided to the surface to be scanned. However, in the proposed optical scanning device, the imaging optical element through which the light beam passes multiple times is not formed so as to maintain sufficient optical performance.

[0135] Therefore, the present embodiment and the fourth and fifth embodiments described below aim to provide an optical scanning device that achieves both compactness and optical performance by easily forming an imaging optical element with sufficient optical performance. 3A and 3B are a schematic main-scanning cross-sectional view and a schematic sub-scanning cross-sectional view, respectively, of an optical scanning device 70 according to the third embodiment. FIG. 4 is a schematic main scanning cross-sectional view of an optical scanning device 70 according to a third embodiment. The optical scanning device 70 of this embodiment has the same configuration as the optical scanning device 50 of the first embodiment except for different specification values, so the same components are given the same reference numerals and their explanations are omitted.

[0136] The diaphragm 3 has a rectangular opening and regulates the width of the light beam that has passed through the anamorphic lens 2 in both the main scanning direction and the sub-scanning direction. A rotating polygon mirror having four deflecting surfaces 4a is used as the deflector 4. However, the number of deflecting surfaces 4a of the deflector 4 is not limited to four.

[0137] In the optical scanning device 70 according to this embodiment, the width of the light beam incident on the deflecting surface 4a in the main scanning cross section is sufficiently smaller than the width of the deflecting surface 4a, that is, the UFS method is adopted. Therefore, when the deflector 4 rotates and the scanning area on the surface 6 is scanned with the light beam, the light beam is not blocked.

[0138] In the optical scanning device 70 of this embodiment, the light beam passes through the imaging optical element 5 three times: when it travels to the deflecting surface 4a after being emitted from the light source 1, when it travels to the reflective optical element 7 after being deflected by the deflecting surface 4a, and when it travels to the scanned surface 6 after being reflected by the reflective optical element 7. The optical scanning device 70 according to this embodiment is made smaller by adopting such a configuration.

[0139] In particular, in the optical scanning device 70 of this embodiment, the light beam emitted from the light source 1 passes through an imaging optical element 5 (first imaging optical element) before being incident on the deflection surface 4a, so that no folding mirror is provided on the optical path between the light source 1 and the deflection surface 4a. This eliminates the need to place the light source 1 at a position far away from the deflector 4, thereby enabling miniaturization. In addition, the number of parts can be reduced, which also contributes to cost reduction.

[0140] In addition, in the optical scanning device 70 according to this embodiment, as shown in FIG. 3(a), the imaging optical element 5 is formed so as to have a meniscus shape that is convex toward the deflector 4 when projected in the main scanning cross section. This makes it possible to reduce aberrations, particularly main scanning coma aberration.

[0141] Generally, in order to reduce coma aberration, an imaging optical element arranged close to a deflector is formed in a concave meniscus shape facing the deflector, thereby allowing light rays to be incident at a substantially normal angle. On the other hand, in the optical scanning device 70 according to this embodiment, an area through which the light beam reflected by the reflecting optical element 7 passes is provided on the end side of the imaging optical element 5 in the main scanning direction.

[0142] Therefore, in the optical scanning device 70 according to this embodiment, the imaging optical element 5 is formed so that the light beam is incident substantially perpendicularly, particularly in this region, thereby reducing the coma aberration. However, in the optical scanning device 70 according to this embodiment, the imaging optical element 5 is formed so as not to be curved significantly. This is because if the imaging optical element 5 is curved significantly, a concave curvature of field occurs when viewed from the deflector 4 .

[0143] On the second optical surface 5b of the imaging optical element 5 provided in the optical scanning device 70 of this embodiment, the width in the main scanning direction of the area through which both the light beam deflected by the deflecting surface 4a of the deflector 4 and the light beam reflected by the reflecting surface of the reflecting optical element 7 pass is defined as A. Furthermore, on the second optical surface 5b of the imaging optical element 5, the width in the main scanning direction of the area through which the light beam deflected by the deflecting surface 4a of the deflector 4 does not pass, while the light beam reflected by the reflecting surface of the reflecting optical element 7 passes, is defined as B. That is, the width B is expressed as the sum of the width B1 and the width B2 shown in FIG. 3(a).

[0144] In this case, it is preferable that the optical scanning device 70 according to this embodiment fulfills the following conditional expression (8). 0.9≦B / A≦2.0 (8)

[0145] In the case of the above-mentioned region having width A, when a light beam deflected by deflecting surface 4a toward a predetermined image height and a light beam reflected by the reflecting surface toward a different image height pass through the same position, it is difficult to determine the shape of imaging optical element 5 to match both image heights. Therefore, in the optical scanning device 70 of this embodiment, as shown in conditional formula (8), the light beam deflected by the deflecting surface 4a of the deflector 4 does not pass through, while the width B of the area through which the light beam reflected by the reflecting surface of the reflecting optical element 7 passes is set to be approximately the same as or greater than the width A.

[0146] This allows the shape of the region having width A of the imaging optical element 5 to be determined in accordance with each image height near the on-axis image height, and by appropriately determining the shape of the region having width B, it becomes possible to suppress deterioration in imaging performance near the most off-axis image height. That is, if the lower limit of conditional expression (8) is not satisfied, the optical performance will be degraded when attempting to reduce the size of the optical scanning device 70 according to this embodiment, which is not preferable.

[0147] On the other hand, if the area having width B becomes so large that it exceeds the upper limit value of conditional expression (8), the distance between the imaging optical element 5 and the reflecting optical element 7 increases, which is undesirable because it increases the size of the optical scanning device 70. Furthermore, if the region having width B becomes so large as to exceed the upper limit of conditional expression (8), the length of the imaging optical element 5 in the main scanning direction increases, which is undesirable as it increases costs.

[0148] As shown in FIG. 4, on the second optical surface 5b of the imaging optical element 5, the distance between the optical axis and the position through which the chief ray of the light beam that passes through the position farthest from the optical axis of the imaging optical element 5 in the main scanning direction passes among the light beams deflected by the deflecting surface 4a is defined as h. Furthermore, the angle between the direction of travel of the principal ray when it emerges from the second optical surface 5b in the main scanning cross section and the normal to the reflecting surface of the reflecting optical element 7 is defined as θ, and the distance between the vertex of the second optical surface 5b and the reflecting surface of the reflecting optical element 7 is defined as L3. In this case, in the optical scanning device 70 according to this embodiment, it is preferable that the following conditional expression (9) be satisfied.

[0149]

number

[0150] In the area defined by the distance h, a light beam deflected by the deflecting surface 4a toward a predetermined image height and a light beam reflected by the reflecting surface toward a different image height may pass through the same position. In this case, it is difficult to determine the shape of the imaging optical element 5 in accordance with both image heights as described above.

[0151] On the other hand, consider the position where the chief ray of the light beam that is deflected by the deflecting surface 4a and then emitted from the position on the second optical surface 5b of the imaging optical element 5 that is farthest from the optical axis of the imaging optical element 5 is reflected by the reflecting optical element 7 and then re-enters the second optical surface 5b. In this case, the distance between the two positions can be approximately expressed as 2×L3×tan θ.

[0152] Therefore, in the optical scanning device 70 according to this embodiment, the distance is set to be approximately the same as the distance h or equal to or greater than the distance h, as indicated in conditional expression (9). This allows the shape of the imaging optical element 5 in the area defined by the distance h to be determined according to each image height near the on-axis image height, and by appropriately determining the shape in the area defined by 2×L3×tanθ, it is possible to suppress degradation of imaging performance near the most off-axis image height.

[0153] That is, if the lower limit of conditional expression (9) is not satisfied, the optical performance will be degraded when attempting to reduce the size of the optical scanning device 70 according to this embodiment, which is not preferable. On the other hand, if the distance L3 is so large as to exceed the upper limit of the conditional expression (9), the distance between the imaging optical element 5 and the reflecting optical element 7 increases, which undesirably increases the size of the optical scanning device 70. Furthermore, if the angle θ becomes so large as to exceed the upper limit of the conditional expression (9), the length of the imaging optical element 5 in the main scanning direction increases, which is undesirable as it increases the cost.

[0154] Also, as shown in Figure 4, the distance between the vertex of the first optical surface 5a of the imaging optical element 5, which is the optical surface closest to the deflector 4 among the optical surfaces through which the light beam deflected by the deflector 4 passes, and the center of the rotation axis of the deflector 4 is defined as L4. In addition, if the center of the rotation axis of the deflector 4 is shifted from the optical axis of the imaging optical element 5, the distance between the intersection between the perpendicular line from the center to the optical axis and the optical axis and the vertex of the first optical surface 5a is defined as L4. Furthermore, the radius of the inscribed circle of the rotary polygonal mirror that forms the deflector 4 is set to L5. In this case, in the optical scanning device 70 according to this embodiment, it is preferable that the following conditional expression (10) be satisfied.

[0155]

number

[0156] In other words, the distance between the vertex of the second optical surface 5b of the imaging optical element 5, which is the imaging optical element closest to the deflector 4 among at least one imaging optical element included in the imaging optical system, and the reflecting surface of the reflecting optical element 7 is L3. The distance between the surface vertex of the first optical surface 5a of the imaging optical element 5 and the on-axis deflection point on the deflecting surface 4a of the deflector 4 is defined as L6. In this case, in the optical scanning device 70 according to this embodiment, it is preferable that the following conditional expression (10a) be satisfied. 0.8≦L3 / L6≦1.2 (10a)

[0157] If the imaging optical element 5 and the reflecting optical element 7 become closer to each other as the distance L3 becomes smaller as it falls below the lower limit of the conditional expression (10), the region having the width B described above becomes narrower. In this case, when miniaturization is attempted as in the optical scanning device 70 according to this embodiment, the optical performance is degraded, which is not preferable.

[0158] On the other hand, if the difference between the distance L4 and the radius L5 becomes so small that it exceeds the upper limit of the conditional expression (10), and the distance between the imaging optical element 5 and the deflector 4 becomes small, the following inconvenience occurs. Specifically, since the imaging optical element 5 has positive power in the sub-scanning cross section, the angle at which the light beam exits the imaging optical element 5 is larger than the angle at which the light beam enters the imaging optical element 5 after passing through the aperture 3 in the sub-scanning cross section.

[0159] Therefore, if the distance between the imaging optical element 5 and the deflector 4 becomes so small that it exceeds the upper limit value of conditional expression (10), it becomes difficult to arrange the light source 1 and the anamorphic lens 2 so as not to interfere with the light beam that exits the imaging optical element 5 and then travels to the reflecting optical element 7, which is not preferable. In this case, the distance between the deflecting surface 4a and the light source 1 or the anamorphic lens 2 becomes greater than the distance between the deflecting surface 4a and the reflective optical element 7, which is not desirable because it makes it difficult to achieve sufficient miniaturization.

[0160] Next, the specifications of the optical scanning device 70 according to this embodiment, the surface spacing, refractive index and decentering arrangement information of each optical element, and the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 are shown in Tables 7, 8 and 9 below, respectively.

[0161] [Table 7]

[0162] [Table 8]

[0163] [Table 9]

[0164] The generating lines of the first optical surface 5a on the deflection surface 4a side and the second optical surface 5b on the reflecting optical element 7 side of the imaging optical element 5 provided in the optical scanning device 70 according to this embodiment have aspherical shapes expressed by the 16th-order polynomial function shown in the above equation (4). Moreover, the generatrix of each of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 70 according to this embodiment also has an aspherical shape expressed by the above formula (4).

[0165] Furthermore, the sagittal lines of the first optical surface 5a on the deflection surface 4a side of the imaging optical element 5 provided in the optical scanning device 70 according to this embodiment and the second optical surface 5b on the reflecting optical element 7 side have shapes expressed by the above formulas (5) and (6). Moreover, the sagittal lines of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 70 according to this embodiment also have shapes expressed by the above formulas (5) and (6). Although the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 provided in the optical scanning device 70 according to this embodiment are defined by the functions expressed in the above equations (4) to (6), the definition of the shapes is not limited to this.

[0166] In the optical scanning device 70 according to this embodiment, the distance L1 in the X direction between the reflective optical element 7 and the deflecting surface 4a is 60 mm, and the distance L2 in the X direction between the deflecting surface 4a and the scanned surface 6 is 37 mm. That is, the overall size of the optical scanning device 70 according to this embodiment in the X direction is 97 mm.

[0167] Specifically, in the optical scanning device 70 according to this embodiment, the distance L1, which roughly corresponds to the distance of the optical path between the reflective optical element 7 and the deflecting surface 4a, through which the light beam travels back and forth, i.e., twice, is made longer than the distance L2, which roughly corresponds to the optical path between the deflecting surface 4a and the scanned surface 6, through which the light beam travels once. This allows the size of the entire optical scanning device 70 in the X direction to be reduced to 100 mm or less while ensuring a sufficient optical path length between the deflecting surface 4a and the surface 6 to be scanned.

[0168] In the optical scanning device 70 according to this embodiment, the radii of curvature of the reference spherical surfaces of the first and second optical surfaces 5a and 5b of the imaging optical element 5 in the main scanning cross section are set to -254 mm and -369 mm, respectively. The reference spherical surface here is a spherical surface that approximately represents only the area through which the light beam guided to the scanning area on the surface 6 to be scanned passes, that is, the effective area.

[0169] That is, the imaging optical element 5 is formed so that both the first and second optical surfaces 5a and 5b have a convex shape facing the deflector 4, in other words, a convex meniscus shape. This achieves a reduction in aberrations, particularly main scanning coma aberration.

[0170] In the optical scanning device 70 according to this embodiment, the absolute values ​​of the radii of curvature of the first and second optical surfaces 5a and 5b are set to large values ​​as described above in order to suppress the occurrence of concave curvature of field as seen from the deflector 4. That is, in the optical scanning device 70 according to this embodiment, the absolute values ​​of the radii of curvature of the first and second optical surfaces 5a and 5b are set to the large values ​​described above, taking into consideration the balance between the main-scanning coma aberration and other aberrations.

[0171] Furthermore, after being deflected by the deflecting surface 4a, on the second optical surface 5b, the exit position of the chief ray of the light beam that emerges from the position farthest from the optical axis of the imaging optical element 5 in the main scanning cross section is a distance of 26.203 mm from the optical axis. In addition, the exit position of the marginal ray of the light beam on the opposite side to the optical axis is separated from the optical axis by a distance of 27.846 mm.

[0172] On the other hand, the position of incidence on the second optical surface 5b of the chief ray of the light beam reflected by the reflecting surface of the reflecting optical element 7 is spaced apart from the optical axis by a distance of 62.658 mm. The position of incidence on second optical surface 5b of the marginal ray of the light beam reflected by the reflecting surface of reflecting optical element 7 on the opposite side to the optical axis is separated from the optical axis by a distance of 63.468 mm.

[0173] Therefore, the width A in the main scanning direction of the area on the second optical surface 5b through which both the light beam deflected by the deflecting surface 4a of the deflector 4 and the light beam reflected by the reflecting surface of the reflecting optical element 7 pass is calculated to be 2 x 27.846 = 55.692 mm. Furthermore, while the light beam deflected by the deflecting surface 4a does not pass through the second optical surface 5b, the width B in the main scanning direction of the area through which the light beam reflected by the reflecting surface passes is calculated to be 2×(63.468−27.846)=71.244 mm.

[0174] Therefore, in the optical scanning device 70 according to this embodiment, the calculation yields B / A=71.244 / 55.692=1.28, which indicates that conditional expression (8) is satisfied. In this way, in the optical scanning device 70 according to this embodiment, a region having the width B is sufficiently secured, thereby suppressing a decrease in optical performance.

[0175] Furthermore, after being deflected by the deflecting surface 4a, on the second optical surface 5b, the distance h between the exit position of the chief ray of the light beam that emerges from the position farthest from the optical axis of the imaging optical element 5 in the main scanning cross section and the optical axis is 26.203 mm. Furthermore, the angle θ between the traveling direction of the chief ray when it is emitted from the second optical surface 5b in the main-scan cross section and the normal to the reflecting surface of the reflecting optical element 7 is 39.55°. The distance L3 between the vertex of the second optical surface 5b and the reflecting surface of the reflecting optical element 7 is 25.00 mm.

[0176] Therefore, in the optical scanning device 70 according to this embodiment, it is calculated that (2×L3×tan θ) / h=(2×25.00×0.826) / 26.203=1.58, which shows that conditional expression (9) is satisfied. In this way, in the optical scanning device 70 of this embodiment, the light beam deflected by the deflecting surface 4a does not pass through the second optical surface 5b, while a sufficient area is secured through which the light beam reflected by the reflecting surface of the reflecting optical element 7 passes, thereby suppressing a deterioration in optical performance.

[0177] In addition, in the optical scanning device 70 of this embodiment, the distance L4 between the vertex of the first optical surface 5a of the imaging optical element 5, which is the optical surface closest to the deflector 4 among the optical surfaces through which the light beam deflected by the deflector 4 passes, and the center of the rotation axis of the deflector 4, is 34.071 mm. The radius L5 of the inscribed circle of the rotating polygonal mirror forming the deflector 4 is 7.071 mm.

[0178] Therefore, in the optical scanning device 70 according to this embodiment, it is calculated that L3 / (L4-L5)=25.00 / (34.071-7.071 mm)=0.93, and it is therefore clear that conditional expression (10) is satisfied. That is, in the optical scanning device 70 of this embodiment, in the area between the reflective optical element 7 and the deflector 4 that corresponds approximately to the sum of the distances L3 and L4, a sufficient area is secured between the reflective optical element 7 and the imaging optical element 5 that corresponds to the distance L3.

[0179] This prevents the light beam deflected by the deflecting surface 4a from passing through the second optical surface 5b, while ensuring a sufficient area through which the light beam reflected by the reflecting surface of the reflecting optical element 7 passes, thereby preventing a deterioration in optical performance. In addition, the light source 1 and the anamorphic lens 2 can be disposed in the region between the reflecting optical element 7 and the imaging optical element 5 in the X direction, which also contributes to miniaturization.

[0180] In the optical scanning device 70 according to this embodiment, the light beam emitted from the light source 1 as described above is converted into a weakly convergent light beam in the main scanning cross section by the anamorphic lens 2. Therefore, the light beam deflected by the deflector 4 is focused at a position 238.882 mm away from the deflecting surface 4a, assuming that no optical element is provided between the deflector 4 and the surface to be scanned 6.

[0181] Therefore, in the optical scanning device 70 according to this embodiment, the imaging optical element 5 does not need to have a large power, and can be formed to have a thickness of 8 mm. In addition, the optical path length between the deflector 4 and the surface to be scanned 6 can also be shortened.

[0182] As described above, in the optical scanning device 70 of this embodiment, the light beam from the light source 1 passes through the imaging optical element 5 and then enters the deflector 4, the light beam deflected by the deflector 4 passes through the imaging optical element 5, is reflected by the reflective optical element 7, and then passes through the imaging optical element 5 again. By adopting such a configuration, it is possible to achieve even greater miniaturization than before.

[0183] In the optical scanning device 70 according to this embodiment, the imaging optical element 5 has a meniscus shape that is convex toward the deflector 4 in the main scanning cross section. By adopting such a configuration, it is possible to improve optical performance compared to the prior art.

[0184] [Fourth embodiment] 5(a) and 5(b) are a schematic main-scanning cross-sectional view and a schematic sub-scanning cross-sectional view, respectively, of an optical scanning device 80 according to the fourth embodiment. FIG. 6 is a schematic main scanning cross-sectional view of an optical scanning device 80 according to a fourth embodiment. The optical scanning device 80 of this embodiment has the same configuration as the optical scanning device 50 of the first embodiment except for different specification values, so the same components are given the same reference numerals and their explanations are omitted.

[0185] In the optical scanning device 80 according to this embodiment, as shown in FIG. 5(a), the imaging optical element 5 is formed so as to have a meniscus shape that is convex toward the deflector 4 when projected in the main scanning cross section. This makes it possible to reduce aberrations, particularly main scanning coma aberration.

[0186] Generally, in order to reduce coma aberration, an imaging optical element arranged close to a deflector is formed in a concave meniscus shape facing the deflector, thereby allowing light rays to be incident at a substantially normal angle. On the other hand, in the optical scanning device 80 according to this embodiment, an area through which the light beam reflected by the reflecting optical element 7 passes is provided on the end side of the imaging optical element 5 in the main scanning direction.

[0187] Therefore, in the optical scanning device 80 according to this embodiment, the imaging optical element 5 is formed so that the light beam is incident substantially perpendicularly, particularly in this region, thereby reducing the coma aberration. However, in the optical scanning device 80 according to this embodiment, the imaging optical element 5 is formed so as not to be curved significantly. This is because if the imaging optical element 5 is curved significantly, a concave curvature of field occurs when viewed from the deflector 4 .

[0188] In the optical scanning device 80 according to this embodiment, it is preferable that at least one of the above conditional expressions (8), (9) and (10) be satisfied. Next, the specifications of the optical scanning device 80 according to this embodiment, the surface spacing, refractive index and decentering arrangement information of each optical element, and the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 are shown in Tables 10, 11 and 12 below, respectively.

[0189] [Table 10]

[0190] [Table 11]

[0191] [Table 12]

[0192] The generating lines of the first optical surface 5a on the deflection surface 4a side and the second optical surface 5b on the reflecting optical element 7 side of the imaging optical element 5 provided in the optical scanning device 80 according to this embodiment have aspherical shapes expressed by the 16th-order polynomial function shown in the above equation (4). Moreover, the generatrix of each of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 80 according to this embodiment also has an aspherical shape expressed by the above formula (4).

[0193] Furthermore, the sagittal lines of the first optical surface 5a on the deflection surface 4a side and the second optical surface 5b on the reflecting optical element 7 side of the imaging optical element 5 provided in the optical scanning device 80 according to this embodiment have shapes expressed by the above formulas (5) and (6). Moreover, the sagittal lines of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 80 according to this embodiment also have shapes expressed by the above formulas (5) and (6). Although the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 provided in the optical scanning device 80 according to this embodiment are defined by the functions expressed in the above equations (4) to (6), the definition of the shapes is not limited to this.

[0194] In the optical scanning device 80 according to this embodiment, the distance L1 in the X direction between the reflective optical element 7 and the deflecting surface 4a is 60 mm, and the distance L2 in the X direction between the deflecting surface 4a and the surface to be scanned 6 is 37 mm. That is, the overall size of the optical scanning device 80 according to this embodiment in the X direction is 97 mm.

[0195] Specifically, in the optical scanning device 80 according to this embodiment, the distance L1, which roughly corresponds to the distance of the optical path between the reflective optical element 7 and the deflecting surface 4a, through which the light beam travels back and forth, i.e., twice, is made longer than the distance L2, which roughly corresponds to the optical path between the deflecting surface 4a and the scanned surface 6, through which the light beam travels once. This allows the size of the entire optical scanning device 80 in the X direction to be reduced to 100 mm or less while ensuring a sufficient optical path length between the deflecting surface 4a and the surface to be scanned 6 required in the OFS method.

[0196] In the optical scanning device 80 according to this embodiment, the radii of curvature of the reference spherical surfaces of the first and second optical surfaces 5a and 5b of the imaging optical element 5 in the main scanning cross section are set to −266 mm and −229 mm, respectively. That is, the imaging optical element 5 is formed so that both the first and second optical surfaces 5a and 5b have a convex shape facing the deflector 4, in other words, a convex meniscus shape. This achieves a reduction in aberrations, particularly main scanning coma aberration.

[0197] In the optical scanning device 80 according to this embodiment, the absolute values ​​of the radii of curvature of the first and second optical surfaces 5a and 5b are set to large values ​​as described above in order to suppress the occurrence of concave curvature of field as seen from the deflector 4. That is, in the optical scanning device 80 according to this embodiment, the absolute values ​​of the radii of curvature of the first and second optical surfaces 5a and 5b are set to the large values ​​described above, taking into consideration the balance between the main scanning coma aberration and other aberrations.

[0198] Furthermore, after being deflected by the deflecting surface 4a, on the second optical surface 5b, the exit position of the chief ray of the light beam that emerges from the position farthest from the optical axis of the imaging optical element 5 in the main scanning cross section is a distance of 26.457 mm from the optical axis. In addition, the exit position of the marginal ray of the light beam on the opposite side to the optical axis is separated from the optical axis by a distance of 27.945 mm.

[0199] On the other hand, the position of incidence on the second optical surface 5b of the chief ray of the light beam reflected by the reflecting surface of the reflecting optical element 7 is spaced apart from the optical axis by a distance of 60.393 mm. The position of incidence on second optical surface 5b of the marginal ray of the light beam reflected by the reflecting surface of reflecting optical element 7 on the opposite side to the optical axis is separated from the optical axis by a distance of 61.160 mm.

[0200] Therefore, the width A in the main scanning direction of the area on the second optical surface 5b through which both the light beam deflected by the deflecting surface 4a of the deflector 4 and the light beam reflected by the reflecting surface of the reflecting optical element 7 pass is calculated to be 2 x 27.945 = 55.890 mm. Furthermore, while the light beam deflected by the deflecting surface 4a does not pass through the second optical surface 5b, the width B in the main scanning direction of the area through which the light beam reflected by the reflecting surface passes is calculated to be 2×(61.160−27.945)=66.430 mm.

[0201] Therefore, in the optical scanning device 80 according to this embodiment, the calculation yields B / A=66.430 / 55.890=1.19, which indicates that conditional expression (8) is satisfied. In this way, in the optical scanning device 80 according to this embodiment, a region having the width B is sufficiently secured, thereby suppressing a decrease in optical performance.

[0202] Furthermore, after being deflected by the deflecting surface 4a, on the second optical surface 5b, the distance h between the exit position of the chief ray of the light beam that emerges from the position farthest from the optical axis of the imaging optical element 5 in the main scanning cross section and the optical axis is 26.457 mm. Furthermore, the angle θ between the traveling direction of the chief ray when it is emitted from the second optical surface 5b in the main-scan cross section and the normal to the reflecting surface of the reflecting optical element 7 is 38.71°. The distance L3 between the vertex of the second optical surface 5b and the reflecting surface of the reflecting optical element 7 is 25.00 mm.

[0203] Therefore, in the optical scanning device 80 according to this embodiment, it is calculated that (2×L3×tan θ) / h=(2×25.00×0.801) / 26.457=1.51, and it is therefore clear that conditional expression (9) is satisfied. In this way, in the optical scanning device 80 of this embodiment, the light beam deflected by the deflecting surface 4a does not pass through the second optical surface 5b, while a sufficient area is secured through which the light beam reflected by the reflecting surface of the reflecting optical element 7 passes, thereby suppressing a deterioration in optical performance.

[0204] In addition, in the optical scanning device 80 of this embodiment, the distance L4 between the vertex of the first optical surface 5a of the imaging optical element 5, which is the optical surface closest to the deflector 4 among the optical surfaces through which the light beam deflected by the deflector 4 passes, and the center of the rotation axis of the deflector 4, is 34.071 mm. The radius L5 of the inscribed circle of the rotating polygonal mirror forming the deflector 4 is 7.071 mm.

[0205] Therefore, in the optical scanning device 80 according to this embodiment, it is calculated that L3 / (L4-L5)=25.00 / (34.071-7.071 mm)=0.93, and it is therefore clear that conditional expression (10) is satisfied. That is, in the optical scanning device 80 according to this embodiment, in the area between the reflective optical element 7 and the deflector 4 that corresponds approximately to the sum of the distances L3 and L4, a sufficient area is secured between the reflective optical element 7 and the imaging optical element 5 that corresponds to the distance L3.

[0206] This prevents the light beam deflected by the deflecting surface 4a from passing through the second optical surface 5b, while ensuring a sufficient area through which the light beam reflected by the reflecting surface of the reflecting optical element 7 passes, thereby preventing a deterioration in optical performance. In addition, the light source 1 and the anamorphic lens 2 can be disposed in the region between the reflecting optical element 7 and the imaging optical element 5 in the X direction, which also contributes to miniaturization.

[0207] In the optical scanning device 80 according to this embodiment, the light beam emitted from the light source 1 as described above is converted into a weakly convergent light beam in the main scanning cross section by the anamorphic lens 2. Therefore, the light beam deflected by the deflector 4 is focused at a position 225.1 mm away from the deflecting surface 4a, assuming that no optical element is provided between the deflector 4 and the surface to be scanned 6.

[0208] Therefore, in the optical scanning device 80 according to this embodiment, the imaging optical element 5 does not need to have a large power, and can be formed to have a thickness of 8 mm. In addition, the optical path length between the deflector 4 and the surface to be scanned 6 can also be shortened.

[0209] As described above, in the optical scanning device 80 of this embodiment, the light beam from the light source 1 passes through the imaging optical element 5 and then enters the deflector 4, the light beam deflected by the deflector 4 passes through the imaging optical element 5, is reflected by the reflective optical element 7, and then passes through the imaging optical element 5 again. By adopting such a configuration, it is possible to achieve even greater miniaturization than before.

[0210] In the optical scanning device 80 according to this embodiment, the imaging optical element 5 has a meniscus shape that is convex toward the deflector 4 in the main scanning cross section. By adopting such a configuration, it is possible to improve optical performance compared to the prior art.

[0211] [Fifth embodiment] 7A and 7B are a schematic main-scanning cross-sectional view and a partial schematic sub-scanning cross-sectional view, respectively, of an optical scanning device 90 according to the fifth embodiment. FIG. 8 is a schematic main scanning cross-sectional view of an optical scanning device 90 according to a fifth embodiment.

[0212] The optical scanning device 90 according to this embodiment includes a light source 1, an anamorphic lens 2, a diaphragm 3, a deflector 4, an imaging optical element 5, a first reflecting optical element 7, and a second reflecting optical element 9. The light source 1 may be, for example, a semiconductor laser, and may have one or more light emitting points. The anamorphic lens 2 has a positive refractive power in the main scanning cross section, and converts the light beam emitted from the light source 1 into a weakly convergent light beam in the main scanning cross section.

[0213] In the optical scanning device 90 of this embodiment, the refractive power required for the imaging optical element 5 in the main scanning cross section is reduced by using a weakly convergent light beam converted by the anamorphic lens 2, thereby ensuring the edge thickness up to the end of the imaging optical element 5. The anamorphic lens 2 also has a positive refractive power in the sub-scanning cross section, and focuses the light beam emitted from the light source 1 near the deflection surface 4a of the deflector 4, thereby forming a long line image in the main scanning direction on the deflection surface 4a.

[0214] The diaphragm 3 has a rectangular opening and regulates the width of the light beam that has passed through the anamorphic lens 2 in both the main scanning direction and the sub-scanning direction. The opening of the diaphragm 3 is not limited to a rectangular shape, but may be formed in an elliptical or track shape. The second reflecting optical element 9 reflects the light beam that has passed through the diaphragm 3 toward the deflecting surface 4 a of the deflector 4 .

[0215] A rotating polygon mirror having four deflecting surfaces 4a is used as the deflector 4. However, the number of deflecting surfaces 4a of the deflector 4 is not limited to four. In the optical scanning device 90 according to this embodiment, the width of the light beam incident on the deflecting surface 4a in the main scanning cross section is sufficiently smaller than the width of the deflecting surface 4a, that is, the UFS method is adopted. Therefore, when the deflector 4 rotates and the scanning area on the surface 6 is scanned with the light beam, the light beam is not blocked.

[0216] The imaging optical element 5 has two optical surfaces (lens surfaces), namely, a first optical surface 5a on the deflector 4 side and a second optical surface 5b on the reflecting optical element 7 side in directions perpendicular to the sub-scanning direction and the main scanning direction, respectively. The shapes of the first and second optical surfaces 5a and 5b in the main scanning cross section are formed so that the light beam deflected by the deflecting surface 4a passes through the imaging optical element 5 twice via the reflective optical element 7, thereby scanning the surface to be scanned 6 with desired scanning characteristics, as will be described later. The shapes of the first and second optical surfaces 5a and 5b in the sub-scanning cross section are such that the vicinity of the deflecting surface 4a and the vicinity of the surface to be scanned 6 are optically conjugate with each other, thereby compensating for surface tilt.

[0217] In the optical scanning device 90 according to this embodiment, a light beam emitted from a light source 1 passes through an anamorphic lens 2 and a diaphragm 3, and is reflected by a second reflecting optical element 9 to enter a deflector 4. The light beam deflected by the deflector 4 is incident on the first optical surface 5 a of the imaging optical element 5 .

[0218] The degree of convergence changes as the light passes through the imaging optical element 5, and the light beam emitted from the second optical surface 5b of the imaging optical element 5 is reflected by the first reflecting optical element 7 and then re-enters the imaging optical element 5 from the second optical surface 5b. Finally, the light beam emitted from the first optical surface 5a of the imaging optical element 5 is guided onto the surface 6 to be scanned. In other words, in the optical scanning device 90 of this embodiment, the light beam deflected by the deflector 4 is guided to the scanned surface 6 via the first optical surface 5a, the second optical surface 5b, the reflective surface of the first reflective optical element 7, the second optical surface 5b, and the first optical surface 5a, in that order.

[0219] In the optical scanning device 90 according to this embodiment, the light beam passes through the imaging optical element 5 multiple times, and is thereby focused in both the main scanning section and the sub-scanning section, thereby forming a spot-shaped image near the scanned surface 6. Then, by rotating the deflector 4 at a constant speed in the direction of arrow B11 in Figure 7(a) by a drive unit not shown, the scanned surface 6 is optically scanned in the main scanning direction, that is, the direction of arrow C11 in Figure 7(a), thereby forming an electrostatic latent image on the scanned surface 6.

[0220] In the optical scanning device 90 according to this embodiment, an incident optical system that causes the light beam emitted from the light source 1 to be incident on the deflecting surface 4a of the deflector 4 is formed by the anamorphic lens 2, the diaphragm 3, and the second reflecting optical element 9. The imaging optical element 5 forms an imaging optical system that guides the light beam deflected by the deflecting surface 4 a of the deflector 4 to the surface 6 to be scanned. In the optical scanning device 90 according to this embodiment, the first reflecting optical element 7 forms a reflecting optical system that reflects the light beam deflected by the deflecting surface 4a of the deflector 4 so as to bend the optical path of the light beam.

[0221] In the optical scanning device 90 according to this embodiment, instead of using the anamorphic lens 2, for example, a coupling lens and a cylindrical lens may be used so as to share the optical function of the anamorphic lens 2. Furthermore, the anamorphic lens 2 and the imaging optical element 5 provided in the optical scanning device 90 according to this embodiment are plastic molded lenses formed by injection molding, but are not limited to this and glass molded lenses may also be used.

[0222] Furthermore, molded lenses can be easily formed to have an aspherical shape and are suitable for mass production. Therefore, by using molded lenses as the anamorphic lens 2 and the imaging optical element 5 in the optical scanning device 90 according to this embodiment, it is possible to improve productivity and optical performance.

[0223] In addition, in the optical scanning device 90 according to this embodiment, in order to achieve low costs, the imaging optical system is formed by a single imaging optical element 5, and the reflective optical system is formed by a first reflective optical element 7 having a single planar shape. However, the present invention is not limited to this, and an imaging optical system may be formed by a plurality of imaging optical elements in order to improve imaging performance, or a reflective optical system may be provided with a reflective optical element having a shape other than a flat surface.

[0224] In the optical scanning device 90 of this embodiment, the light beam passes through the imaging optical element 5 twice: once when it proceeds to the first reflective optical element 7 after being deflected by the deflecting surface 4a, and once when it proceeds to the scanned surface 6 after being reflected by the first reflective optical element 7. By adopting such a configuration, miniaturization can be achieved.

[0225] In addition, in the optical scanning device 90 according to this embodiment, the light source 1, the anamorphic lens 2, and the aperture 3 are each positioned away from the second reflective optical element 9 so as not to interfere with the light beam deflected by the deflecting surface 4a. In addition, by adopting a configuration in which the light beam is obliquely incident on the deflection surface 4a so that the angle in the sub-scanning section relative to the main-scanning section is large, and by greatly separating the light source 1, anamorphic lens 2, and diaphragm 3 from the light beam, it is possible to suppress interference between them. However, in this case, the angle of oblique incidence becomes large, which may result in a deterioration in optical performance, and therefore is not preferable.

[0226] In addition, in the optical scanning device 90 according to this embodiment, as shown in Figure 7(a), the imaging optical element 5 is formed so as to have a meniscus shape that is convex toward the deflector 4 when projected in the main scanning cross section. This makes it possible to reduce aberrations, particularly main scanning coma aberration.

[0227] Generally, in order to reduce coma aberration, an imaging optical element arranged close to a deflector is formed in a concave meniscus shape facing the deflector, thereby allowing light rays to be incident at a substantially normal angle. On the other hand, in the optical scanning device 90 according to this embodiment, an area through which the light beam reflected by the reflecting optical element 7 passes is provided on the end side of the imaging optical element 5 in the main scanning direction.

[0228] Therefore, in the optical scanning device 90 according to this embodiment, the imaging optical element 5 is formed so that the light beam is incident substantially perpendicularly, particularly in this region, thereby reducing the coma aberration. However, in the optical scanning device 90 according to this embodiment, the imaging optical element 5 is formed so as not to be curved significantly. This is because if the imaging optical element 5 is curved significantly, a concave curvature of field occurs when viewed from the deflector 4 .

[0229] In the optical scanning device 90 according to this embodiment, it is preferable that the above conditional expressions (8) and (9) be satisfied. Also, as shown in Figure 8, the distance between the vertex of the first optical surface 5a of the imaging optical element 5, which is the optical surface closest to the deflector 4 among the optical surfaces through which the light beam deflected by the deflector 4 passes, and the center of the rotation axis of the deflector 4 is defined as L4.

[0230] In addition, if the center of the rotation axis of the deflector 4 is shifted from the optical axis of the imaging optical element 5, the distance between the intersection between the perpendicular line from the center to the optical axis and the optical axis and the vertex of the first optical surface 5a is defined as L4. Furthermore, the radius of the inscribed circle of the rotary polygonal mirror that forms the deflector 4 is set to L5. In this case, in the optical scanning device 90 according to this embodiment, it is preferable that the following conditional expression (11) be satisfied.

[0231]

number

[0232] If the imaging optical element 5 and the reflecting optical element 7 become closer to each other as the distance L3 becomes smaller as it falls below the lower limit of the conditional expression (11), the region having the width B described above becomes narrower. In this case, when miniaturization is attempted as in the optical scanning device 90 according to this embodiment, the optical performance is degraded, which is not preferable.

[0233] On the other hand, if the difference between the distance L4 and the radius L5 becomes so small that it exceeds the upper limit of the conditional expression (11), and the distance between the imaging optical element 5 and the deflector 4 becomes small, the following inconvenience occurs. That is, it is not preferable because it is difficult to arrange the incident optical system and there is a risk that the imaging optical element 5 may interfere with the motor that drives the deflector 4 and the substrate that drives the motor.

[0234] Furthermore, in the optical scanning device 90 according to this embodiment, the light beam emitted from the light source 1 is converted into a convergent light beam in the main scanning cross section and is made incident on the deflecting surface 4a of the deflector 4. This reduces the total optical path length, thereby achieving miniaturization. This also reduces the power required for the imaging optical element 5 in the main scanning cross section, thereby reducing the thickness of the imaging optical element 5, thereby shortening the manufacturing time when manufacturing the imaging optical element 5 by injection molding.

[0235] In the optical scanning device 90 according to this embodiment, the light beam emitted from the light source 1 is made to be incident on the deflecting surface 4a from the front. In general, the reflectance of the deflecting surface 4a for the incident light beam changes depending on the incident angle of the light beam.

[0236] In addition, the transmittance of the light beam passing through the refractive optical element also changes depending on the angle of incidence of the light beam. Therefore, in the optical scanning device 90 of this embodiment, the light beam emitted from the light source 1 is made to be incident directly on the deflection surface 4a as described above, thereby suppressing the occurrence of density unevenness between the scanning area on the start side of writing and the scanning area on the end side of writing on the scanned surface 6.

[0237] In the optical scanning device 90 according to this embodiment, as shown in FIG. 7(b), the light beam emitted from the light source 1 is made obliquely incident on the deflecting surface 4a when projected onto the sub-scanning cross section. This makes it possible to suppress interference between the second reflecting optical element 9 and the light beam deflected by the deflecting surface 4a.

[0238] Furthermore, in the optical scanning device 90 according to this embodiment, the imaging optical system is formed by a single imaging optical element 5, and the reflective optical system is formed by a single first reflective optical element 7, thereby reducing costs. In addition, when the imaging optical system is formed by a plurality of imaging optical elements, if a certain imaging optical element is placed between the deflector 4 and the scanned surface 6 in the X direction, the length of the certain imaging optical element in the main scanning direction increases.

[0239] Furthermore, when the predetermined imaging optical element that is long in the main scanning direction is formed by injection molding, the number that can be molded at one time is reduced, resulting in high costs. On the other hand, if all of the multiple imaging optical elements are arranged between the deflector 4 and the first reflecting optical element 7 in the X direction, the distance between the multiple imaging optical elements will be reduced, making it difficult to arrange them, which is not preferable.

[0240] Next, the specifications of the optical scanning device 90 according to this embodiment, the surface spacing, refractive index, and decentering arrangement information of each optical element, and the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 are shown in Tables 13, 14, and 15 below, respectively.

[0241] [Table 13]

[0242] [Table 14]

[0243] [Table 15]

[0244] The generating lines of the first optical surface 5a on the deflection surface 4a side and the second optical surface 5b on the reflecting optical element 7 side of the imaging optical element 5 provided in the optical scanning device 90 according to this embodiment have aspherical shapes expressed by the 16th-order polynomial function shown in the above equation (4). Moreover, the generatrix of each of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 90 according to this embodiment also has an aspherical shape expressed by the above formula (4).

[0245] Furthermore, the sagittal lines of the first optical surface 5a on the deflection surface 4a side of the imaging optical element 5 provided in the optical scanning device 90 according to this embodiment and the second optical surface 5b on the reflecting optical element 7 side have shapes expressed by the above formulas (5) and (6). Moreover, the sagittal lines of the entrance surface and exit surface of the anamorphic lens 2 provided in the optical scanning device 90 according to this embodiment also have shapes expressed by the above formulas (5) and (6). Although the shapes of the optical surfaces of the anamorphic lens 2 and the imaging optical element 5 provided in the optical scanning device 90 according to this embodiment are defined by the functions expressed in the above equations (4) to (6), the definition of the shapes is not limited to this.

[0246] In the optical scanning device 90 according to this embodiment, the distance L1 in the X direction between the reflective optical element 7 and the deflecting surface 4a is 60 mm, and the distance L2 in the X direction between the deflecting surface 4a and the surface to be scanned 6 is 37 mm. That is, the overall size of the optical scanning device 90 according to this embodiment in the X direction is 97 mm.

[0247] Specifically, in the optical scanning device 90 according to this embodiment, the distance L1, which roughly corresponds to the distance of the optical path between the reflective optical element 7 and the deflecting surface 4a, through which the light beam travels back and forth, i.e., twice, is made longer than the distance L2, which roughly corresponds to the optical path between the deflecting surface 4a and the scanned surface 6, through which the light beam travels once. This allows the size of the entire optical scanning device 90 in the X direction to be reduced to 100 mm or less while ensuring a sufficient optical path length between the deflecting surface 4a and the surface 6 to be scanned.

[0248] In the optical scanning device 90 according to this embodiment, the optical axis of the anamorphic lens 2 is tilted by 2° in the sub-scanning cross section with respect to the main-scanning cross section. This allows the light beam incident on the second reflective optical element 9 and the light beam deflected by the deflecting surface 4a to be separated from each other, i.e., interference between the second reflective optical element 9 and the light beam deflected by the deflecting surface 4a can be suppressed. Furthermore, the light source 1, the anamorphic lens 2, and the diaphragm 3 are also arranged outside the area through which the light beam deflected by the deflecting surface 4a travels, so as not to interfere with the light beam.

[0249] In the optical scanning device 90 according to this embodiment, the radii of curvature of the reference spherical surfaces of the first and second optical surfaces 5a and 5b of the imaging optical element 5 in the main scanning cross section are set to −458 mm and −986 mm, respectively. That is, the imaging optical element 5 is formed so that both the first and second optical surfaces 5a and 5b have a convex shape facing the deflector 4, in other words, a convex meniscus shape. This achieves a reduction in aberrations, particularly main scanning coma aberration.

[0250] In the optical scanning device 90 according to this embodiment, the absolute values ​​of the radii of curvature of the first and second optical surfaces 5a and 5b are set to large values ​​as described above in order to suppress the occurrence of concave curvature of field as seen from the deflector 4. That is, in the optical scanning device 90 according to this embodiment, the absolute values ​​of the radii of curvature of the first and second optical surfaces 5a and 5b are set to the large values ​​described above, taking into consideration the balance between the main-scanning coma aberration and other aberrations.

[0251] Furthermore, after being deflected by the deflecting surface 4a, on the second optical surface 5b, the emission position of the chief ray of the light beam that emerges from the position farthest from the optical axis of the imaging optical element 5 in the main scanning cross section is a distance of 25.201 mm from the optical axis. In addition, the exit position of the marginal ray of the light beam on the opposite side to the optical axis is separated from the optical axis by a distance of 26.412 mm.

[0252] On the other hand, the position of incidence on the second optical surface 5b of the chief ray of the light beam reflected by the reflecting surface of the reflecting optical element 7 is spaced apart from the optical axis by a distance of 66.273 mm. The position of incidence on second optical surface 5b of the marginal ray of the light beam reflected by the reflecting surface of reflecting optical element 7 on the opposite side to the optical axis is separated from the optical axis by a distance of 67.784 mm.

[0253] Therefore, the width A in the main scanning direction of the area on the second optical surface 5b through which both the light beam deflected by the deflecting surface 4a of the deflector 4 and the light beam reflected by the reflecting surface of the reflecting optical element 7 pass is calculated to be 2 x 26.412 = 52.824 mm. Furthermore, while the light beam deflected by the deflecting surface 4a does not pass through the second optical surface 5b, the width B in the main scanning direction of the area through which the light beam reflected by the reflecting surface passes is calculated to be 2×(67.784−26.412)=82.744 mm.

[0254] Therefore, in the optical scanning device 90 according to this embodiment, the calculation yields B / A=82.744 / 52.824=1.57, which indicates that conditional expression (8) is satisfied. In this way, in the optical scanning device 90 according to this embodiment, a region having the width B is sufficiently secured, thereby suppressing a decrease in optical performance.

[0255] Furthermore, after being deflected by the deflecting surface 4a, on the second optical surface 5b, the distance h between the exit position of the chief ray of the light beam that emerges from the position farthest from the optical axis of the imaging optical element 5 in the main scanning cross section and the optical axis is 25.201 mm. Furthermore, the angle θ between the traveling direction of the chief ray when it is emitted from the second optical surface 5b in the main-scan cross section and the normal to the reflecting surface of the reflecting optical element 7 is 40.40°. The distance L3 between the vertex of the second optical surface 5b and the reflecting surface of the reflecting optical element 7 is 25.00 mm.

[0256] Therefore, in the optical scanning device 90 according to this embodiment, it is calculated that (2×L3×tan θ) / h=(2×25.00×0.851) / 25.201=1.69, and it is therefore clear that conditional expression (9) is satisfied. In this way, in the optical scanning device 90 of this embodiment, the light beam deflected by the deflecting surface 4a does not pass through the second optical surface 5b, while a sufficient area is secured through which the light beam reflected by the reflecting surface of the reflecting optical element 7 passes, thereby suppressing a deterioration in optical performance.

[0257] In addition, in the optical scanning device 90 of this embodiment, the distance L4 between the vertex of the first optical surface 5a of the imaging optical element 5, which is the optical surface closest to the deflector 4 among the optical surfaces through which the light beam deflected by the deflector 4 passes, and the center of the rotation axis of the deflector 4, is 34.071 mm. The radius L5 of the inscribed circle of the rotating polygonal mirror forming the deflector 4 is 7.071 mm.

[0258] Therefore, in the optical scanning device 90 according to this embodiment, it is calculated that L3 / (L4-L5)=25.00 / (34.071-7.071 mm)=0.93, and it is therefore clear that conditional expression (11) is satisfied. That is, in the optical scanning device 90 of this embodiment, in the area between the reflective optical element 7 and the deflector 4 that corresponds approximately to the sum of the distances L3 and L4, a sufficient area is secured between the reflective optical element 7 and the imaging optical element 5 that corresponds to the distance L3. This prevents the light beam deflected by the deflecting surface 4a from passing through the second optical surface 5b, while ensuring a sufficient area through which the light beam reflected by the reflecting surface of the reflecting optical element 7 passes, thereby preventing a deterioration in optical performance.

[0259] In the optical scanning device 90 according to this embodiment, the light beam emitted from the light source 1 as described above is converted into a weakly convergent light beam in the main scanning cross section by the anamorphic lens 2. Therefore, the light beam deflected by the deflector 4 is focused at a position 159.7 mm away from the deflecting surface 4a, assuming that no optical element is provided between the deflector 4 and the surface to be scanned 6.

[0260] Therefore, in the optical scanning device 90 according to this embodiment, the imaging optical element 5 does not need to have a large power, and can be formed to have a thickness of 8 mm. In addition, the optical path length between the deflector 4 and the surface to be scanned 6 can also be shortened.

[0261] As described above, in the optical scanning device 90 according to this embodiment, the light beam deflected by the deflector 4 passes through the imaging optical element 5, is reflected by the reflecting optical element 7, and then passes through the imaging optical element 5 again. The imaging optical element 5 has a meniscus shape that is convex toward the deflector 4 in the main scanning cross section. By adopting such a configuration, it is possible to improve optical performance compared to the prior art.

[0262] [Monochrome image forming device] FIG. 9A is a sub-scanning cross-sectional view of a main part of a monochrome image forming apparatus 104 including an optical scanning device according to any one of the first to fifth embodiments.

[0263] As shown in FIG. 9(a), the monochrome image forming apparatus 104 receives code data Dc output from an external device 117 such as a personal computer. The input code data Dc is converted into image data (dot data) Di by a printer controller 111 provided in the monochrome image forming apparatus 104.

[0264] The converted image data Di is then input to the optical scanning unit 100, which is an optical scanning device according to any one of the first to fifth embodiments. Next, the optical scanning unit 100 emits a light beam 103 modulated in accordance with the input image data Di, and the emitted light beam 103 scans the photosensitive surface of the photosensitive drum 101 in the main scanning direction.

[0265] A photosensitive drum 101, which is an electrostatic latent image carrier (photosensitive member), is rotated clockwise by a motor 115. As the photosensitive drum 101 rotates, the photosensitive surface of the photosensitive drum 101 moves in a sub-scanning direction perpendicular to the main scanning direction relative to the light beam 103 .

[0266] Above the photosensitive drum 101, a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided so as to come into contact with the surface. The surface of the photosensitive drum 101 charged by the charging roller 102 is then irradiated with a light beam 103 scanned by the optical scanning unit 100 .

[0267] As described above, the light beam 103 is modulated based on the image data Di, and an electrostatic latent image is formed on the surface of the photosensitive drum 101 by irradiating the surface with the light beam 103. The formed electrostatic latent image is then developed into a toner image by a developing device 107 that is disposed so as to contact the photosensitive drum 101 further downstream in the rotation direction of the photosensitive drum 101 than the irradiation position of the light beam 103.

[0268] Next, the toner image developed by the developing unit 107 is transferred onto a sheet of paper 112 as a transfer material by a transfer roller 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101 . The paper 112 is stored in a paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 9(a)), but can also be fed manually. A paper feed roller 110 is disposed at the end of the paper cassette 109, and feeds paper 112 in the paper cassette 109 into the transport path.

[0269] The paper 112 onto which the unfixed toner image has been transferred as described above is further transported to a fixing device 150 disposed behind the photosensitive drum 101 (on the left side in FIG. 9(a)). The fixing unit 150 is formed by a fixing roller 113 having a fixing heater (not shown) therein, and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113 .

[0270] Then, the paper 112 conveyed from the transfer section is heated and pressed at the pressure contact portion between the fixing roller 113 and the pressure roller 114, whereby the unfixed toner image on the paper 112 is fixed. Further, a paper discharge roller 116 is disposed behind the fixing roller 113 , and the paper 112 on which the toner has been fixed is discharged to the outside of the monochrome image forming apparatus 104 .

[0271] Although not shown in FIG. 9(a), the printer controller 111 not only converts the data described above, but also controls the components within the monochrome image forming device 104, including the motor 115, and the polygon motor within the optical scanning unit 100. Furthermore, the recording density of the monochrome image forming device 104 is not particularly limited.

[0272] [Color image forming device] FIG. 9B is a sub-scanning cross-sectional view of a main part of a color image forming apparatus 260 including an optical scanning device according to any one of the first to fifth embodiments.

[0273] The color image forming apparatus 260 employs a tandem type in which optical scanning devices 211, 212, 213, and 214 according to any one of the first to fifth embodiments operate in parallel to record image information on the surface of a photosensitive drum, which is an image carrier. The color image forming apparatus 260 includes optical scanning devices 211 to 214 according to any one of the first to fifth embodiments, and photosensitive drums 221, 222, 223, and 224 as image carriers. The color image forming apparatus 260 also includes developing units 231 , 232 , 233 and 234 , a conveyor belt 251 , a printer controller 253 and a fixing unit 254 .

[0274] As shown in FIG. 9(b), color signals of R (red), G (green), and B (blue) are input to a color image forming apparatus 260 from an external device 252 such as a personal computer. The input color signals are then converted into image data (dot data) of C (cyan), M (magenta), Y (yellow), and K (black) by a printer controller 253 provided in the color image forming device 260.

[0275] Next, the converted image data is input to the optical scanning devices 211 to 214, respectively. Light beams 241, 242, 243 and 244 modulated in accordance with the image data are emitted from the optical scanning devices 211 to 214, respectively, and the emitted light beams 241 to 244 scan the photosensitive surfaces of the photosensitive drums 221 to 224 in the main scanning direction, respectively.

[0276] A charging roller (not shown) for uniformly charging the surfaces of the photosensitive drums 221 to 224 is provided in contact with the surfaces. Then, the surfaces of the photosensitive drums 221 to 224 charged by the charging rollers are irradiated with light beams 241 to 244 from the optical scanning devices 211 to 214, respectively.

[0277] As described above, the light beams 241 to 244 are modulated based on image data for each color, and electrostatic latent images are formed on the surfaces of the photosensitive drums 221 to 224 by irradiating the light beams 241 to 244, respectively. The formed electrostatic latent images are developed into toner images by developing devices 231 to 234 that are disposed so as to contact the photosensitive drums 221 to 224 .

[0278] The toner images developed by the developing devices 231 to 234 are transferred in multiple layers onto a sheet of paper (transfer material) (not shown) conveyed on a conveyor belt 251 by transfer rollers (transfer devices) (not shown) arranged opposite the photosensitive drums 221 to 224. In this way, one full-color image is formed. The paper onto which the unfixed toner image has been transferred as described above is further transported to a fixing device 254 provided behind the photosensitive drums 221 to 224 (on the left side in FIG. 9(b)).

[0279] The fixing unit 254 is formed by a fixing roller having a fixing heater (not shown) therein, and a pressure roller disposed so as to be in pressure contact with the fixing roller. The paper conveyed from the transfer section is heated and pressed by the pressure contact portion between the fixing roller and the pressure roller, thereby fixing the unfixed toner image on the paper. Furthermore, a paper discharge roller (not shown) is disposed behind the fixing roller, and the paper discharge roller discharges the paper on which the fixed image has been formed to the outside of the color image forming apparatus 260 .

[0280] In the color image forming device 260, optical scanning devices 211 to 214 correspond to the colors C (cyan), M (magenta), Y (yellow), and K (black), respectively, and each record an image signal (image information) on the photosensitive surface of photosensitive drums 221 to 224 in parallel. This allows color images to be printed at high speed.

[0281] The external device 252 may be, for example, a color image reading device equipped with a CCD sensor. In this case, the color image reading device and the color image forming device 260 form a color digital copying machine.

[0282] 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.

[0283] The disclosure of this embodiment includes the following configuration. (Configuration 1) An optical scanning device comprising: a deflector that deflects a light beam from a light source to scan a surface to be scanned in the main scanning direction; an imaging optical element having first and second optical surfaces; and a reflective optical element having a reflective surface, wherein the light beam from the light source is guided to the surface to be scanned via the second optical surface, the first optical surface, the deflector, the first optical surface, the second optical surface, the reflective surface, the second optical surface, and the first optical surface in that order. (Configuration 2) An optical scanning device according to configuration 1, comprising an incident optical system that causes a light beam from a light source to be incident on a deflector, the incident optical system not including a reflective optical element that reflects the light beam. (Configuration 3) An optical scanning device according to configuration 1 or 2, characterized in that when the distance between the on-axis deflection point of the deflector and the reflecting surface is L1 and the distance between the on-axis deflection point and the surface to be scanned is L2, the condition L2≦L1 is satisfied. (Configuration 4) The optical scanning device according to any one of configurations 1 to 3, wherein the imaging optical element has a meniscus shape that is concave toward the deflector in the sub-scanning cross section. (Configuration 5) The optical scanning device according to any one of configurations 1 to 4, wherein the imaging optical element has a meniscus shape that is convex toward the deflector in the main scanning cross section. (Configuration 6) An optical scanning device described in any one of configurations 1 to 5, characterized in that on the second optical surface, the light beam from the light source and the light beam deflected by the deflector pass on opposite sides of the vertex of the second optical surface in the sub-scanning direction. (Configuration 7) The optical scanning device according to any one of configurations 1 to 6, wherein the effective areas of the first and second optical surfaces each have a continuous shape in the sub-scan cross section. (Configuration 8) An optical scanning device described in any one of configurations 1 to 7, comprising an incident optical system that causes a light beam from a light source to be incident on a deflector, the incident optical system having an optical element that changes the convergence of the light beam from the light source in the main scanning cross section and focuses the light beam from the light source in the sub-scanning cross section. (Configuration 9) The optical scanning device according to any one of configurations 1 to 8, wherein no refractive optical element other than the imaging optical element is provided on the optical path of the light beam deflected by the deflector. (Configuration 10) The optical scanning device according to any one of configurations 1 to 9, wherein no reflective optical element other than the reflective optical element is provided on the optical path of the light beam deflected by the deflector. (Configuration 11) An optical scanning device described in any one of configurations 1 to 10, comprising an incident optical system that causes a light beam from a light source to be incident on a deflector, wherein in the incident optical system, no refractive optical element is provided between the imaging optical element and the deflector on the optical path of the light beam. (Configuration 12) An optical scanning device comprising: a deflector that deflects a light beam from a light source to scan a surface to be scanned in a main scanning direction; a first imaging optical element having first and second optical surfaces; and a first reflecting optical element having a reflecting surface, wherein the light beam deflected by the deflector is guided to the surface to be scanned via the first optical surface, the second optical surface, the reflecting surface, the second optical surface, and the first optical surface in that order; and wherein the first imaging optical element has a meniscus shape that is convex toward the deflector in the main scanning cross section. (Configuration 13) An optical scanning device according to configuration 12, characterized in that, when the width in the main scanning direction of an area on the second optical surface through which both the light beam deflected by the deflector and the light beam reflected by the reflecting surface pass is A, and the width in the main scanning direction of an area through which the light beam deflected by the deflector does not pass but the light beam reflected by the reflecting surface passes is B, the condition 0.9≦B / A≦2.0 is satisfied. (Structure 14) An optical scanning device according to structure 12 or 13, characterized in that, on the second optical surface, the distance between the optical axis and the passing position of the principal ray of the light beam deflected by the deflector that passes through the position farthest from the optical axis of the first imaging optical element in the main scanning direction is h, the angle between the traveling direction of the principal ray when it emerges from the second optical surface and the normal to the reflecting surface is θ, and the distance between the vertex of the second optical surface and the reflecting surface is L3, satisfies the condition 0.9≦2×L3×tanθ / h≦2.0. (Structure 15) An optical scanning device described in any one of structures 12 to 14, characterized in that it has at least one imaging optical element including a first imaging optical element closest to the deflector on the optical path of the light beam deflected by the deflector, and is equipped with an imaging optical system that guides the light beam deflected by the deflector to the surface to be scanned, and when the distance between the vertex of the second optical surface and the reflecting surface is L3 and the distance between the vertex of the first optical surface and the axial deflection point of the deflector is L4, it satisfies the condition 0.8≦L3 / L4≦3.0. (Configuration 16) An optical scanning device according to any one of configurations 12 to 15, wherein the light beam from the light source is incident on the deflector via the second optical surface and the first optical surface in this order. (Configuration 17) The optical scanning device according to any one of configurations 12 to 16, further comprising a second reflecting optical element that reflects the light beam from the light source and guides it to the deflector. (Structure 18) An optical scanning device described in any one of structures 1 to 17, characterized in that when projected onto a main scanning cross section, the traveling direction of the light beam when it enters the deflector and the traveling direction of the light beam when it is deflected by the deflector toward the axial image height are parallel to each other. (Configuration 19) An optical scanning device according to any one of configurations 1 to 17, wherein in the main scanning cross section, the width of the light beam when it is incident on the deflection surface of the deflector is larger than the width of the deflection surface. (Configuration 20) An image forming apparatus comprising an optical scanning device according to any one of configurations 1 to 17, a developing unit that develops an electrostatic latent image formed on a surface to be scanned by the optical scanning device into a toner image, a transfer unit that transfers the developed toner image to a transfer material, and a fixing unit that fixes the transferred toner image to the transfer material. (Configuration 21) An image forming apparatus comprising: an optical scanning device according to any one of configurations 1 to 17; 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]

[0284] 1 light source 4 Deflector 5 Imaging optics 5a First Optical Surface 5b Second Optical Surface 6 Scanned surface 7 Reflective Optical Elements 50 Optical scanning device

Claims

1. a deflector that deflects a light beam from a light source to scan the surface to be scanned in a main scanning direction; an imaging optical element having first and second optical surfaces; a reflective optical element having a reflective surface, an optical scanning device characterized in that the light beam from the light source is guided to the scanned surface via the second optical surface, the first optical surface, the deflector, the first optical surface, the second optical surface, the reflecting surface, the second optical surface, and the first optical surface in that order.

2. an incident optical system that causes the light beam from the light source to be incident on the deflector; 2. The optical scanning device according to claim 1, wherein the incident optical system does not include a reflective optical element that reflects the light beam.

3. The distance between the axial deflection point of the deflector and the reflecting surface is L １ , the distance between the axial deflection point and the scanned surface is L ２ When L ２ ≦L １ 2. The optical scanning device according to claim 1, wherein the following conditions are satisfied:

4. 2. The optical scanning device according to claim 1, wherein the imaging optical element has a meniscus shape that is concave toward the deflector in the sub-scan section.

5. 2. The optical scanning device according to claim 1, wherein the imaging optical element has a meniscus shape that is convex toward the deflector in the main scanning cross section.

6. 2. The optical scanning device according to claim 1, wherein, on the second optical surface, the light beam from the light source and the light beam deflected by the deflector pass on opposite sides of the vertex of the second optical surface in the sub-scanning direction.

7. 2. The optical scanning device according to claim 1, wherein the effective areas of the first and second optical surfaces each have a continuous shape in the sub-scan cross section.

8. an incident optical system that causes the light beam from the light source to be incident on the deflector; 2. The optical scanning device according to claim 1, wherein the incident optical system has an optical element that changes the convergence of the light beam from the light source in the main scanning cross section and that condenses the light beam from the light source in the sub-scanning cross section.

9. 2. The optical scanning device according to claim 1, wherein no refractive optical element other than said imaging optical element is provided on the optical path of said light beam deflected by said deflector.

10. 2. The optical scanning device according to claim 1, wherein no reflective optical element other than the reflective optical element is provided on the optical path of the light beam deflected by the deflector.

11. an incident optical system that causes the light beam from the light source to be incident on the deflector; 2. The optical scanning device according to claim 1, wherein in said incident optical system, no refractive optical element is provided between said imaging optical element and said deflector on the optical path of said light beam.

12. a deflector that deflects a light beam from a light source to scan the surface to be scanned in a main scanning direction; a first imaging optical element having first and second optical surfaces; a first reflective optical element having a reflective surface; the light beam deflected by the deflector is guided to the scanned surface via the first optical surface, the second optical surface, the reflecting surface, the second optical surface, and the first optical surface in this order; 10. An optical scanning device, wherein the first imaging optical element has a meniscus shape that is convex toward the deflector in a main scanning cross section.

13. On the second optical surface, when the width in the main scanning direction of an area through which both the light beam deflected by the deflector and the light beam reflected by the reflecting surface pass is defined as A, and the width in the main scanning direction of an area through which the light beam deflected by the deflector does not pass but the light beam reflected by the reflecting surface passes is defined as B, 0.9≦B / A≦2.0 13. The optical scanning device according to claim 12, wherein the following conditions are satisfied:

14. On the second optical surface, the distance between the optical axis and a passing position of a principal ray of the light beam deflected by the deflector that passes through a position farthest from the optical axis of the first imaging optical element in the main scanning direction is defined as h, the angle between the traveling direction of the principal ray when it is emitted from the second optical surface in the main scanning cross section and the normal to the reflecting surface is defined as θ, and the distance between the surface vertex of the second optical surface and the reflecting surface is defined as L. ３ When 0.9≦2×L ３ ×tanθ / h≦2.0 13. The optical scanning device according to claim 12, wherein the following conditions are satisfied:

15. an imaging optical system including at least one imaging optical element including the first imaging optical element closest to the deflector on an optical path of the light beam deflected by the deflector, and guiding the light beam deflected by the deflector to the scanned surface; The distance between the vertex of the second optical surface and the reflecting surface is L ３ , the distance between the vertex of the first optical surface and the on-axis deflection point of the deflector is L ４ When 0.8≦L ３ / L ４ ≦3.0 13. The optical scanning device according to claim 12, wherein the following conditions are satisfied:

16. 13. The optical scanning device according to claim 12, wherein the light beam from the light source is incident on the deflector via the second optical surface and the first optical surface in this order.

17. 13. The optical scanning device according to claim 12, further comprising a second reflecting optical element that reflects the light beam from the light source and guides it to the deflector.

18. 18. An optical scanning device according to claim 1, wherein, when projected onto a main scanning cross section, the traveling direction of the light beam when it enters the deflector and the traveling direction of the light beam when it is deflected by the deflector toward an axial image height are parallel to each other.

19. 18. The optical scanning device according to claim 1, wherein, in a main scanning cross section, a width of the light beam when it is incident on a deflecting surface of the deflector is larger than a width of the deflecting surface.

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

21. 18. An image forming apparatus comprising: the optical scanning device according to claim 1; 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.

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