Optical scanner

Abstract

To provide a compact optical scanner that can suppress variations in a spot diameter and illuminance associated with an OFS system.SOLUTION: A optical scanner includes a deflector deflecting a light flux from a light source to scan a surface to be scanned in a main scanning direction, and an imaging optical system guiding the light flux deflected by the deflector the surface to be scanned. A width of the light flux when entering the deflecting surface of the deflector is larger than that of a deflecting surface in a main scanning cross section. When a light flux toward an on-axis image height, which is an image height on an optical axis of the imaging optical system on the surface to be scanned, is an on-axis light flux, and light flux toward an outermost off-axis image height on the surface to be scanned is an outermost off-axis light flux, a scanning speed of light flux on the surface to be scanned is different, between the on-axis image height and the outermost off-axis image height. A first region from the outermost off-axis image height to the on-axis image height at one side with respect to an optical axis is longer than a second region from the outermost off-axis image height to the on-axis image height at the other side on the surface to be scanned. The imaging optical system guides the light flux to the first and second regions.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Conventionally, in order to achieve high printing speed and high resolution, an optical scanning device of the so-called OFS (overfilled scan) type has been used, which is equipped with a deflector composed of a deflection surface whose width is smaller than the width of the incident light beam in the main scanning direction. In such an optical scanning device, the deflection surface moves while changing its angle within the incident light beam, so that the width and energy of the reflected and deflected light beam change depending on the angle of the deflection surface, and therefore the spot diameter and illuminance on the scanned surface change depending on the image height. Patent Document 1 discloses an optical scanning device that suppresses such fluctuations in spot diameter and illuminance according to image height by providing an optical element in an incident optical system that has both a phase shift function and a diffraction function and has characteristics in which both the phase shift amount and the diffraction efficiency are asymmetric with respect to the optical axis. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-202242 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the optical scanning device disclosed in Patent Document 1 is large in size because it is necessary to add an optical element having a complex shape including a phase shift surface and a diffraction grating surface to the incident optical system. SUMMARY OF THE PRESENT EMBODIMENTS Accordingly, an object of the present invention is to provide a compact optical scanning device capable of suppressing fluctuations in spot diameter and illuminance that accompany the OFS method. [Means for solving the problem]

[0005] The optical scanning device of the present invention comprises a deflector which deflects a light beam from a light source to scan a scanned surface in a main scanning direction, and an imaging optical system which guides the light beam deflected by the deflector to the scanned surface, and in the main scanning cross section, the width of the light beam when it is incident on the deflection surface of the deflector is greater than the width of the deflection surface, and when the light beam toward an axial image height, which is the image height on the optical axis of the imaging optical system on the scanned surface, is defined as an axial light beam, and the light beam toward the most off-axis image height on the scanned surface is defined as an off-axis light beam, the scanning speed of the light beam on the scanned surface differs between the axial image height and the most off-axis image height, and on the scanned surface, a first region from the most off-axis image height to the on-axis image height on one side of the optical axis is longer than a second region from the most off-axis image height to the on-axis image height on the other side, and the imaging optical system guides the light beam to the first and second regions. Effect of the Invention

[0006] According to the present invention, it is possible to provide a small-sized optical scanning device that can suppress fluctuations in spot diameter and illuminance that accompany the OFS method. [Brief description of the drawings]

[0007] [Figure 1] 1 is a main-scanning sectional view of an optical scanning device according to a first embodiment. [Diagram 2] 3A to 3C are diagrams for explaining the definition of each angle in the optical scanning device according to the first embodiment. [Diagram 3] 5A to 5C are diagrams showing changes in spot diameter ratio, spot moving speed, and spot diameter, and DIST characteristics in the optical scanning device according to the first embodiment. [Figure 4] FIG. 11 is a main-scanning sectional view of an optical scanning device according to a second embodiment. [Diagram 5] 13A to 13C are diagrams showing changes in spot diameter ratio, spot moving speed, and spot diameter, and DIST characteristics in the optical scanning device according to the second embodiment. [Figure 6] FIG. 11 is a main-scanning sectional view of an optical scanning device according to a third embodiment. [Figure 7] 13A to 13C are diagrams showing changes in spot diameter ratio, spot moving speed, and spot diameter, and DIST characteristics in the optical scanning device according to the third embodiment. [Figure 8] 2 is a sub-scanning sectional view of a main part of the image forming apparatus according to the embodiment. FIG. [Figure 9] 1 is a main-scanning sectional view of an optical scanning device disclosed in Patent Document 1. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the optical scanning device according to the present embodiment will be described in detail with reference to the accompanying drawings. Note that the drawings shown below may be drawn at a scale different from the actual scale in order to facilitate understanding of the present embodiment. In the following description, the direction parallel to the optical axis of the imaging optical system 85 is defined as the X-axis, the main scanning direction as the Y-axis, and the sub-scanning direction as the Z-axis.

[0009] [First embodiment] 2. Description of the Related Art Conventionally, optical scanning devices have been used as exposure devices for image forming apparatuses such as laser beam printers that use an electrophotographic process. In an optical scanning device, the light beam emitted from the light source is modulated according to the image signal from the computer, and is guided by the incident optical system to a deflector such as a polygon mirror (rotating polygonal mirror), and then deflected by the deflection reflection surface of the deflector. The deflected light beam is then focused into a spot on a photosensitive drum, which serves as the surface to be scanned, by an imaging optical system having Fθ characteristics, and the spot formed on the surface to be scanned is scanned as the deflector rotates, thereby exposing and recording image information.

[0010] Also, various color image forming apparatuses have been proposed that form color images by scanning a plurality of photosensitive drums with a plurality of optical scanning devices. In recent years, in response to demands for faster and more precise printing, the OFS (overfilled scan) method, which involves increasing the number of faces of a polygon mirror, has been proposed.

[0011] FIG. 9 shows a main scanning sectional view of an optical scanning device 500 disclosed in Patent Document 1.

[0012] As shown in FIG. 9, the optical scanning device 500 disclosed in Patent Document 1 is composed of a light source 10, an incident optical system composed of a coupling lens etc., a polygon mirror (rotating polygonal mirror) 16 as a deflector, and an imaging optical system (Fθ lens) 18. The polygon mirror 16 provided in the optical scanning device 500 is a polyhedron having 12 deflection reflection surfaces.

[0013] An incident light beam I emitted from the light source 10 is guided toward the polygon mirror 16 by an incident optical system constituted by an optical element 30 and the like. Here, the incident light beam I has a width greater than the width of the deflection reflection surface of the polygon mirror 16 in the main scanning direction.

[0014] As the polygon mirror 16 rotates, a predetermined deflection reflection surface moves through the incident light beam I so as to cut it out, thereby reflecting and deflecting the incident light beam I towards the imaging optical system 18 . Thereafter, the reflected and deflected light beam scans a print area on the photosensitive drum 20, which is the surface to be scanned, while forming a beam spot due to the imaging performance of the imaging optical system 18, thereby performing printing.

[0015] A method of scanning with an incident light beam having a light beam width wider than the width of the deflection reflection surface in the main scanning direction, as in the optical scanning device 500, is called an OFS method. On the other hand, a method of scanning with an incident light beam having a light beam width sufficiently smaller than the width of the deflection reflection surface in the main scanning direction is called a UFS (under-field scanning) method.

[0016] Optical scanning devices using the OFS system are easier to make smaller when the polygon mirror is made multifaceted than those using the UFS system, making it possible to achieve faster and more precise printing without increasing the size of the optical scanning device. On the other hand, in an optical scanning device using the OFS method, the deflection reflection surface moves while changing its angle within the incident light beam I, so the width and energy of the reflected and deflected light beam change depending on the angle of the deflection reflection surface.

[0017] As a result, the spot diameter and illuminance change according to the image height on the surface to be scanned. If the spot diameter and illuminance vary at each image height on the scanned surface, a problem occurs in that a uniform printed image cannot be obtained when the device is used in an image forming apparatus.

[0018] In response to this, the optical scanning device 500 disclosed in Patent Document 1 aims to solve the above problem by providing an optical element 30 in the incident optical system, which has both a phase shift function and a diffraction function, and has characteristics in which both the phase shift amount and the diffraction efficiency are asymmetric with respect to the optical axis.

[0019] However, the optical scanning device 500 using the OFS method disclosed in Patent Document 1 has the following problems. First, it is necessary to add the optical element 30, which has a complex shape having both a phase shift surface and a diffraction grating surface, making it difficult to reduce the size and cost of the device.

[0020] Furthermore, since the difficulty of manufacturing the optical element 30 that achieves a predetermined phase shift amount increases, it is difficult to manufacture such an optical element 30 at low cost. Specifically, what is required is a surface shape in which the phase changes by 2π asymmetrically across the optical axis within the effective portion of the optical element 30 . It is difficult to manufacture optical element 30 having such a complex and precise optical surface shape with a high degree of accuracy by injection molding using a mold, and even when manufacturing it by cutting and polishing, it is difficult to reduce the cost.

[0021] Furthermore, providing the optical element 30 having a plurality of regions with different diffraction efficiencies also causes significant problems. Specifically, it is difficult to manufacture an optical element 30 having two regions in which the diffraction efficiencies of the zeroth-order light are 100% and 75%, respectively.

[0022] As described in paragraph

[0053] of Patent Document 1, the intensity distribution of the incident light beam I incident on the optical element 30 is a Gaussian distribution according to the FFP of the laser used in the light source 10. Here, it is known that the FFP of a laser has a tolerance, and the tolerance is generally large, ranging from 9° to 16°, or from 23° to 36°.

[0023] Therefore, the energy density in the light beam that spans the region in optical element 30 where the diffraction efficiency is 100% and the region where the diffraction efficiency is 75% varies depending on the laser, making it difficult to perform strict energy control in the incident light beam I. Furthermore, if the energy density changes within the incident light beam I, the shape of the spot on the scanned surface, that is, the photosensitive drum 20, also becomes distorted.

[0024] Furthermore, if we ignore the higher-order diffracted light in the region of the optical element 30 where the diffraction efficiency of the zero-order light is 75%, the ±1st-order diffracted light has 25% of the energy and reaches the photosensitive drum 20 as flare or ghost, thereby degrading the printing performance. In order to prevent such a deterioration in printing performance, it is necessary to provide a light-shielding wall for appropriately blocking such flare and ghost.

[0025] In consideration of the above problems, the present embodiment aims to provide an optical scanning device that can reduce the fluctuation in spot diameter and non-uniformity of image plane illuminance that occurs when using the OFS method, which achieves high speed and high resolution, without increasing the number of parts.

[0026] FIG. 1 shows a main scanning sectional view of an optical scanning device 100 according to the first embodiment. FIG. 2 is a diagram for explaining the definition of each angle in the optical scanning device 100 according to this embodiment.

[0027] The optical scanning device 100 according to this embodiment includes a light source 1, a diaphragm 2, a coupling lens 3, a deflector 5, a first imaging lens 61, and a second imaging lens 62.

[0028] The diaphragm 2 regulates the width of the light beam emitted from the light source 1 in both the main scanning direction and the sub-scanning direction. Furthermore, the width of the aperture 2 in the main scanning direction is set to be sufficiently wide so that the width of the light beam incident on the deflector 5 in the main scanning direction is wider than the width of the deflecting surface (deflection reflection surface) 51 of the deflector 5 in the main scanning cross section.

[0029] The coupling lens 3 is an anamorphic lens having different powers in the sub-scanning cross section and the main-scanning cross section. The light beam that has passed through the diaphragm 2 is converted by the coupling lens 3 into a parallel light beam in the main scanning cross section, and is condensed in the sub-scanning cross section. Note that the parallel light beam here includes not only a strictly parallel light beam, but also an approximately parallel light beam such as a weakly divergent light beam or a weakly convergent light beam.

[0030] The deflector 5 is a ten-facet polygon mirror, and is rotationally driven at a predetermined constant angular velocity by a drive motor (not shown), thereby deflecting and scanning the incident light beam. In the optical scanning device 100 according to this embodiment, a light beam whose width in the main scanning direction is wider than the width of the deflecting surface 51 in the main scanning cross section is incident on the deflector 5 as described above. In other words, the width of the light beam when it is incident on the deflecting surface 51 of the deflector 5 in the main scanning cross section is larger than the width of the deflecting surface 51 of the deflector 5 . Therefore, the width in the main scanning direction of the light beam deflected by the deflector 5 is determined by the width in the main scanning cross section of the deflecting surface 51. This type of method is called an OFS (overfilled scan) method.

[0031] The first imaging lens 61 and the second imaging lens 62 guide (concentrate) the light beam deflected by the deflector 5 onto the surface to be scanned .

[0032] In the optical scanning device 100 according to this embodiment, with the above-described configuration, the light beam emitted from the light source 1 passes through the aperture 2 and the coupling lens 3, whereby it is converted into a parallel light beam (or a weakly convergent light beam) in the main scanning cross section, and is condensed to form a line image (focal line) on the deflection surface 51 of the deflector 5 in the sub-scanning cross section. The light beam deflected by the deflector 5 passes through a first imaging lens 61 and a second imaging lens 62 and is focused onto the surface to be scanned 7, and the surface to be scanned 7 is scanned by the deflector 5 rotating at a constant speed.

[0033] In the optical scanning device 100 of this embodiment, the aperture 2 and the coupling lens 3 form an incident optical system 75 (first optical system), and the first imaging lens 61 and the second imaging lens 62 form an imaging optical system 85 (second optical system). In the optical scanning device 100 according to this embodiment, an anamorphic lens is used as the coupling lens 3, but a rotationally symmetric coupling lens and a cylindrical lens having power in the sub-scanning cross section may be used instead.

[0034] As described above, the optical scanning device 100 of this embodiment employs an OFS method in which the incident light beam is converted by the incident optical system 75 so that the light beam width in the main scanning direction of the light beam incident on the deflector 5 is wider than the width of the deflection surface 51 in the main scanning cross section. As shown in FIG. 2, the optical axis (X i The optical axis (X axis) of the imaging optical system 85 and the optical axis (X axis) of the imaging optical system 85 are both disposed within the main scanning cross section. i The axis is at an incidence angle of θ i It is formed. In other words, in the optical scanning device 100 according to this embodiment, the traveling direction LP1 of the light beam when it enters the deflector 5 and the optical axis (X-axis) of the imaging optical system 85 are non-parallel to each other in the main scanning cross section. However, in the optical scanning device 100 according to this embodiment, X i Both the X-axis and the X-axis do not have to be arranged in the main scanning cross section.

[0035] In addition, in the optical scanning device 100 of this embodiment, the scanned surface 7 has a first scanning area 71 and a second scanning area 72 on each side of the axial image height 711 (Y=0) on the side where the incident optical system 75 is located (light source side) and on the side where it is not located (opposite the light source side). That is, the coordinates in the Y direction of each of the most off-axis image height 710 on the +Y side, the on-axis image height 711, and the most off-axis image height 712 on the -Y side are max+ , 0 and -Y max- In this case, the first scanning region 71 has a length Y from the most off-axis image height 710 on the +Y side to the on-axis image height 711. max+ It is an area having: The second scanning region 72 has a length Y from the most off-axis image height 712 on the −Y side to the on-axis image height 711. max- It is an area having: In the optical scanning device 100 of this embodiment, the light beam deflected by the deflector 5 is focused on the scanned surface 7 by the imaging optical system 85 to form a beam spot, and is scanned from the most off-axis image height 710 on the +Y side to the most off-axis image height 712 on the -Y side as the deflector 5 rotates.

[0036] Moreover, in the optical scanning device 100 according to this embodiment, the following conditional expression (1) is satisfied. Y max+ >Y max- (1) In this way, the distance Y from the on-axis image height 711 to the most off-axis image height 710 on the +Y side max+ is the distance Y from the on-axis image height 711 to the most off-axis image height 712 on the -Y side. max- By setting it larger, it is possible to monotonically change the power of the imaging optical system 85 from on the optical axis to off the optical axis. As will be described in detail later, by monotonically changing the power of the imaging optical system 85, the monotonically changing beam width that accompanies the monotonically changing angle of reflection of the beam by the deflection surface 51 can be offset, thereby suppressing fluctuations in the spot diameter.

[0037] In the optical scanning device 100 according to this embodiment, the second scanning region 72 is not provided. max- = 0, and only the first scanning region 71 may be set as the scanning range of the surface 7 to be scanned. By setting the scanning range of the scanned surface 7 in this manner, it is no longer necessary to design the first imaging lens 61 and the second imaging lens 62 that constitute the imaging optical system 85 asymmetrically with respect to the optical axis, and it is possible to design the printing position with high precision.

[0038] Next, the specifications of the optical scanning device 100 according to this embodiment are shown in Tables 1-1 to 1-3 below. In Table 1-3, "Ex" stands for "×10 -x " It means.

[0039] [Table 1-1] [Table 1-2] [Table 1-3]

[0040] In the optical scanning device 100 of this embodiment, the generatrix shapes (shapes in the main scanning cross section) of the exit surface of the coupling lens 3 and the entrance and exit surfaces of each of the first imaging lens 61 and the second imaging lens 62 are expressed by the following equation (2).

number

[0041] In addition, in formula (2), R is the radius of curvature (generatrix radius of curvature) in the main scanning section, and K, B 4 , B 6 , B 8 and B. 10 are the aspheric coefficients. In addition, the aspheric coefficient B 4 , B 6 , B 8 and B. 10 Regarding the above, the values ​​may be different between the +Y side and the -Y side. This allows the generatrix shape of the optical surface to be asymmetric across the optical axis in the main scanning direction. In Table 1-3 above, the aspheric coefficient on the +Y side (light source side) is B 4u , B 6u , B 8u and B. 10u and the aspheric coefficient on the -Y side (opposite the light source side) is B 4l , B 6l , B 8l and B. 10l It is expressed as follows.

[0042] Moreover, the sagittal shape (shape in the sub-scanning section) of the exit surface of the coupling lens 3 and the entrance surface and exit surface of each of the first imaging lens 61 and the second imaging lens 62 is expressed by the following formula (3).

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[0043] The sagittal shape referred to here includes a surface normal on the generatrix at each position in the main scanning direction and indicates a surface shape in a cross section perpendicular to the main scanning cross section. In addition, M in formula (3) jk In the optical scanning device 100 according to this embodiment, all of the aspherical coefficients M jk However, it is not limited to this, and in order to reduce various aberrations, at least one aspheric coefficient M jk may be set to a value other than 0. Specifically, the linear term of Z in equation (3) is a term that contributes to the amount of tilt (sagittal tilt amount) in the sub-scanning cross section of the optical surface. Therefore, the aspheric coefficient M on the +Y side (light source side) 0_1u ~M 16_1u and the aspheric coefficient M on the -Y side (opposite the light source) 0_1l ~M 16_1l By making the values ​​of and different from each other, the amount of sagittal tilt can be changed asymmetrically in the main scanning direction.

[0044] Furthermore, r' in equation (3) indicates the radius of curvature (sagittal radius of curvature) in the sub-scanning cross section at a position Y away from the optical axis in the main scanning direction, and is expressed by the following equation (4).

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[0045] Here, r is the radius of curvature of the sagittal on the optical axis, and E 2 , E 4 , E 6 , E 8 and E 10 are the aspheric coefficients (sagittal change coefficients). In addition, the aspheric coefficient E 2 ~E 10 Regarding the above, the values ​​may be different between the +Y side and the -Y side. This makes it possible to set the aspheric amount of the secondary line shape asymmetrically across the optical axis in the main scanning direction. In Table 1-3 above, the aspheric coefficient on the +Y side (light source side) is E 2u , E 4u , E 6u , E 8u and E 10u The aspheric coefficient on the -Y side (opposite the light source side) is E 2l , E 4l , E 6l , E 8l and E 10l It is expressed as follows. Furthermore, although equation (4) includes only the even-numbered terms of Y, odd-numbered terms of Y may be added.

[0046] Next, a characteristic configuration of the optical scanning device 100 according to this embodiment will be described. As shown in FIG. 2, the optical axis (Xi axis) of the incident optical system 75 is at an angle θ i and intersect at the origin O. The angle that the normal LP2 of the deflection surface 51 of the deflector 5 makes with the optical axis of the imaging optical system 85 is defined as θ p Then, the angle θ p varies with the rotation of the deflector 5.

[0047] In addition, when the angle (angle of view) that the traveling direction of the principal ray of the light beam deflected by the deflecting surface 51 makes with respect to the optical axis of the imaging optical system 85 is θ, the angle of view θ is θ max+ From θ max- It changes up to. Here, θ max+ is the angle that the traveling direction of the chief ray of the light beam L10 when it is deflected by the deflecting surface 51 so as to proceed toward the most off-axis image height 710 forms with the optical axis of the imaging optical system 85. max- is the angle that the traveling direction of the chief ray of the light beam L12 when it is deflected by the deflecting surface 51 so as to proceed toward the most off-axis image height 712 forms with the optical axis of the imaging optical system 85.

[0048] At this time, the angle θ i , θ p and θ satisfy the relationship expressed by the following equation (5).

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[0049] As shown in Table 1-1, in the optical scanning device 100 according to this embodiment, θ max- and θ i are set to −8.712° (−0.152 radians) and 63° (1.100 radians), respectively, and the number N of deflection surfaces 51 of the deflector 5 is ten. Therefore, in the optical scanning device 100 according to this embodiment, the following conditional expression (6) is satisfied.

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[0050] If conditional expression (6) is not satisfied, when the scanned surface 7 is scanned with a light beam reflected by a specific deflection surface 51 of the deflector 5, the light beam reflected by an adjacent deflection surface 51 will reach the scanning area on the scanned surface 7 as a ghost, which is not preferable. In this manner, in the optical scanning device 100 according to this embodiment, it is preferable to set the outermost off-axis image heights 712 and 710 asymmetrically with respect to the optical axis so that conditional expression (6) is satisfied.

[0051] As described above, the optical scanning device 100 according to this embodiment uses the OFS method in which the deflecting surface 51 reflects and deflects an incident light beam having a wide light beam width in the main scanning direction while moving. Therefore, the width of the light beam in the main scanning direction directed toward each image height on the scanned surface 7 changes according to the reflection angle θ by the deflecting surface 51 . Specifically, the beam width in the main scanning direction of the light beam incident on each main scanning direction position of the imaging optical system 85 monotonically increases from the light beam incident on an on-axis position of the imaging optical system 85 toward the on-axis image height 711 to the light beam incident on a predetermined off-axis position (the most off-axis position) of the imaging optical system 85 toward the most off-axis image height 710.

[0052] On the other hand, the first imaging lens 61 and the second imaging lens 62 constituting the imaging optical system 85 each have an aspheric shape, and are designed so that the focal length varies depending on the position in the main scanning direction. Specifically, the focal length at each main scanning direction position of the imaging optical system 85 monotonically increases (i.e., the power (refractive power) monotonically decreases) from the position (on-axial position, first position) where the chief ray (on-axial ray) of the light beam (on-axial light beam) heading toward the axial image height 711 passes through to the position (most off-axial position, second position) where the chief ray (most off-axial ray) of the light beam (most off-axial light beam) heading toward the most off-axial image height 710 passes through.

[0053] For this reason, the distance that the spot moves on the scanned surface 7 as the deflector 5 rotates and scans in unit time increases monotonically from the on-axis image height 711 to the most off-axis image height 710 . That is, in the optical scanning device 100 according to this embodiment, the imaging optical system 85 has non-uniform speed scanning characteristics.

[0054] In the optical scanning device 100 according to this embodiment, by adopting the above-mentioned configuration, it is possible to suppress a change in spot diameter in the main scanning direction at each image height on the scanned surface 7, as will be described in detail below.

[0055] As described above, since the optical scanning device 100 according to this embodiment employs the OFS method, the width of the light beam deflected by the deflecting surface 51 in the main scanning direction (in the main scanning cross section) changes depending on the angle of view θ. Here, when the circumscribing circle diameter of the deflector 5 is φ and the number of deflection surfaces 51 of the deflector 5 is N, the width W of the deflection surface 51 in the main scanning section is 0 is expressed as the following equation (7).

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[0056] The angle between the optical axis of the incident optical system 75 and the normal LP2 of the deflection surface 51 is θ i -θ p Therefore, the width W of the light beam deflected by the deflecting surface 51 in the main scanning direction is expressed by the following formula (8).

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[0057] Then, by substituting equation (5) into equation (8), we obtain the following equation (9).

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[0058] That is, from formula (8), the width of the light beam in the main scanning direction of the light beam incident on each main scanning direction position of the imaging optical system 85 is determined by the distance from the light beam deflected toward the on-axis image height 711 to the light beam deflected toward the most off-axis image height 710, that is, the angle θ p (In equation (9), it increases monotonically as angle θ increases.

[0059] Further, the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 is expressed by the following formula (10).

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[0060] Here, the optical scanning device 100 according to this embodiment adopts the OFS method, and the light beam width W is expressed as in equation (9). Therefore, by substituting equation (9) into equation (10), the following equation (11) is obtained.

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[0061] Next, in an optical scanning device using a conventional OFS method, the image height Y has a distortion characteristic (hereinafter referred to as a DIST characteristic) as expressed by the following equation (12). Y = Fθ (12) Here, F is called the Fθ coefficient, and is equal to the focal length f(θ=0) of the imaging optical system 85 at the on-axis position.

[0062] That is, in an optical scanning device using a conventional OFS method, there is a constant-speed scanning relationship between the amount of change dθ in the angle of view θ caused by the deflector 5 rotating at a constant speed per unit time and the distance dY that the spot moves while scanning the scanned surface 7, as expressed by the following equation (13).

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[0063] Here, the ratio of the beam width in the main scanning direction of the beam incident on each main scanning direction position of the imaging optical system 85 to the beam width in the main scanning direction of the beam incident on the axial position of the imaging optical system 85 is defined as dW(θ). Also, the ratio of the spot diameter in the main scanning direction at each image height on the scanned surface 7 to the spot diameter in the main scanning direction at the on-axis image height on the scanned surface 7 is denoted as dSPOT(θ).

[0064] At this time, the following equation (14) is obtained based on equation (11).

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[0065] As described above, in an optical scanning device using the conventional OFS method, the spot diameter ratio dSPOT(θ) at each angle of view depends on the light flux width ratio dW(θ) at each angle of view according to equation (14).

[0066] Based on equation (14), the dependency of the spot diameter ratio dSPOT on the angle of view θ is shown by the dashed line in FIG. As shown in FIG. 3(a), dSPOT varies by approximately 23% over the entire range of the angle of view θ. In an optical scanning device using the conventional OFS method, the variation in the light beam width W was the cause of the variation in the spot diameter SPOT in the main scanning direction.

[0067] On the other hand, the optical scanning device 100 according to this embodiment has the DIST characteristic as expressed by the following formula (15).

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[0068] In other words, there is a non-uniform scanning relationship between the amount of change dθ in the angle of view θ caused by the deflector 5 rotating at a uniform speed per unit time and the distance dY that the spot moves while scanning the scanned surface 7, as expressed in the following equation (16), and the scanning speed of the light beam on the scanned surface 7 differs depending on the image height.

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[0069] FIG. 3B shows the DIST characteristics in a conventional optical scanning device and the DIST characteristics in the optical scanning device 100 according to this embodiment. As shown in FIG. 3(b), in a conventional optical scanning device, the image height Y changes with respect to the angle of view θ as a linear function shown in equation (12), whereas in the optical scanning device 100 according to this embodiment, the image height Y changes with respect to the polynomial function shown in equation (15). As the angle of view θ increases, the difference between the DIST characteristics increases. i is set.

[0070] Figure 3(c) also shows the dependence of the moving speed dY / dθ on the angle of view θ in a conventional optical scanning device as shown in equation (13), and the dependence of the moving speed dY / dθ on the angle of view θ in the optical scanning device 100 of this embodiment as shown in equation (16). As shown in FIG. 3(c), in a conventional optical scanning device, the moving speed dY / dθ is a constant value F regardless of the angle of view θ, whereas in the optical scanning device 100 of this embodiment, the moving speed dY / dθ monotonically increases as the angle of view θ increases.

[0071] In this manner, in the optical scanning device 100 according to this embodiment, the imaging optical system 85 has a non-uniform scanning DIST characteristic in which the moving speed dY / dθ monotonically increases as the angle of view θ increases from the on-axis image height 711 to the most off-axis image height 710. When the ratio of the focal length f(θ) at each main scanning direction position of the imaging optical system 85 to the focal length f(0) at the on-axis position of the imaging optical system 85 is df(θ), the following equation (17) is satisfied.

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[0072] From equation (17), the focal length f(θ) of the imaging optical system 85 at each position in the main scanning direction can be expressed as in equation (18) below.

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[0073] Thus, in the optical scanning device 100 of this embodiment, as the angle of view θ increases from the on-axis image height 711 to the most off-axis image height 710, the power of the imaging optical system 85 in the main scanning cross section decreases, that is, the focal length f(θ) in the main scanning cross section becomes longer.

[0074] That is, in the optical scanning device 100 according to this embodiment, equations (11) and (14) can be rewritten as the following equations (19) and (20).

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number

[0075] The dotted line in FIG. 3(a) shows the dependency of the spot diameter ratio dSPOT on the angle of view θ when dW(θ) is set to 1 based on equation (20).

[0076] The dependence of dSPOT on the angle of view θ when both dW(θ) and df(θ) change based on equation (20) is shown by the solid line in FIG. As shown in Figure 3(a), it can be seen that the change in the spot diameter ratio dSPOT(θ) when the light beam width ratio dW(θ) changes and the change in the spot diameter ratio dSPOT(θ) when the focal length ratio df(θ) changes cancel each other out.

[0077] Specifically, it can be seen that the fluctuation range of the spot diameter ratio dSPOT(θ), which was approximately 23% over the entire range of the field of view θ due to the change in the light beam width ratio dW(θ), can be reduced to approximately 11% by changing the focal length ratio df(θ). That is, in an optical scanning device using the conventional OFS method, the spot diameter ratio dSPOT(θ) changed by approximately 23% over the entire range of the angle of view θ, but in the optical scanning device 100 of this embodiment, the change in the spot diameter ratio dSPOT(θ) can be reduced to approximately 11% over the entire range of the angle of view θ.

[0078] It is preferable that the optical scanning device 100 according to this embodiment is designed to satisfy the following conditional expression (21) at any angle of view θ from the on-axis image height to the most off-axis image height.

number

number

[0079] In the optical scanning device 100 according to this embodiment, the constant A and the DIST coefficient α i are determined so as to satisfy the conditions (21) and (21a).

[0080] FIG. 3(d) shows the size of the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 calculated using the parameter values ​​shown in Tables 1-1 to 1-3 in the optical scanning device 100 of this embodiment.

[0081] As shown in FIG. 3(d), in the optical scanning device 100 according to this embodiment, the spot diameter SPOT has a value between 67.5 micrometers and 74.5 micrometers over the entire image height range from the most off-axis image height 712 to the most off-axis image height 710. That is, the spot diameter SPOT has a fluctuation range of about 10.3% over the entire image height. Here, the fluctuation range of approximately 10.3% shown in FIG. 3(d) is smaller than the reduced fluctuation range of approximately 11% of the spot diameter ratio dSPOT(θ) shown in FIG. 3(a) because the calculation shown in FIG. 3(d) includes the effects of residual aberrations such as wavefront aberration and partial magnification.

[0082] Furthermore, when considering the change in light amount at each image height on the scanned surface 7 in the optical scanning device 100 according to this embodiment, df(θ) / dW(θ) shown in equation (20) is equivalent to the F-number at the angle of view θ. In other words, in the optical scanning device 100 of this embodiment, the change in the light beam width ratio dW(θ) and the change in the focal length ratio df(θ) are offset by each other as described above, so that the change in the F-number of the spot scanning the scanned surface 7 can also be suppressed. This also suppresses the change in the amount of light at each image height on the scanned surface 7, thereby reducing non-uniformity in the image surface illuminance distribution.

[0083] In the optical scanning device 100 according to this embodiment, as shown in formula (1), the angle of view θ max+and the angle of view θ to the most off-axis image height 712 max- have asymmetric values ​​with respect to the optical axis of the imaging optical system 85. On the other hand, in the optical scanning device 100 according to this embodiment, as shown in Table 1-3, the shapes of the optical surfaces of the first imaging lens 61 and the second imaging lens 62 are symmetrical about the optical axis of the imaging optical system 85.

[0084] As described above, in the optical scanning device 100 according to this embodiment, when the light beam is deflected by the deflecting surface 51 of the deflector 5 to scan the surface to be scanned 7, the angle of view θ is θ max+ From θ max- It changes up to. The width W of the light beam incident on the imaging optical system 85 in the main scanning direction is expressed as θ max+ From θ max- The signal changes monotonically over a range from

[0085] Therefore, in order to cancel out the change in the light beam width W and the change in the focal length f of the imaging optical system 85 as described above, the focal length f of the imaging optical system 85 also needs to be changed monotonically. Here, the angle of view θ max+ and θ max- If the focal length f and the focal length x are set to have symmetrical values ​​with respect to each other across the optical axis of the imaging optical system 85, then in order to monotonically change the focal length f, it is necessary to design the shapes of the optical surfaces of the first imaging lens 61 and the second imaging lens 62 to be largely asymmetric with respect to the optical axis of the imaging optical system 85.

[0086] Therefore, in the optical scanning device 100 according to the present embodiment, in order to reduce the burden involved in manufacturing such an asymmetric optical surface, the angle of view θ max+ and θ max- and are set to values ​​asymmetrical with respect to the optical axis of the imaging optical system 85.

[0087] In the optical scanning device 100 according to this embodiment, as shown in formula (1), the distance Y max+ is the distance Y from the on-axis image height 711 to the most off-axis image height 712. max-It is set larger. That is, the angle of view θ max+ While increasing the angle of view θ max- By reducing the size of the device, the device can be made more compact.

[0088] In the optical scanning device 100 according to this embodiment, in order to make the shapes of the optical surfaces of the first imaging lens 61 and the second imaging lens 62 symmetrical with respect to the optical axis of the imaging optical system 85, the DIST coefficient α of the even-order term in the polynomial representing the DIST characteristic expressed in Equation (15) is 2 , α 4 , , , , are set to 0, while the DIST coefficients of odd-order terms, α 3 , α 5 , may be set to predetermined constants.

[0089] As described above, the optical scanning device 100 according to this embodiment can suppress fluctuations in spot diameter that occur in the OFS method employed to achieve high-speed, high-resolution printing without increasing the number of parts, and can also reduce non-uniformity in the image plane illuminance distribution.

[0090] [Second embodiment] FIG. 4 shows a main scanning sectional view of an optical scanning device 200 according to the second embodiment. Since the optical scanning device 200 according to this embodiment has the same configuration as the optical scanning device 100 according to the first embodiment, the same members are given the same reference numerals and the description thereof will be omitted.

[0091] Moreover, the specifications of the optical scanning device 200 according to this embodiment are shown in the following Tables 2-1 to 2-3.

[0092] [Table 2-1] [Table 2-2] [Table 2-3]

[0093] Note that the definition of the local coordinate system of each optical surface and the definition of the surface shape of each optical surface are similar to those of the optical scanning device 100 according to the first embodiment.

[0094] FIG. 5A shows the dependency of the spot diameter ratio dSPOT in the main scanning direction on the angle of view θ in the optical scanning device 200 according to this embodiment. Specifically, the dependency of the spot diameter ratio dSPOT on the angle of view θ when the light flux width ratio dW(θ) changes based on equation (14) is shown by the dashed line in FIG. 5(a).

[0095] As shown in FIG. 5(a), the fluctuation range of dSPOT over the entire range of the angle of view θ is approximately 19%, which indicates that the fluctuation of the light beam width W is a factor in the fluctuation of the spot diameter SPOT in the main scanning direction.

[0096] In the optical scanning device 200 of this embodiment, similarly to the optical scanning device 100 of the first embodiment, as the angle of view θ increases from the on-axis image height 711 to the most off-axis image height 710, the power of the imaging optical system 85 in the main scanning cross section decreases, i.e., the focal length f(θ) in the main scanning cross section becomes longer. The dotted line in FIG. 5(a) shows the dependency of the spot diameter ratio dSPOT on the angle of view θ when the light beam width does not change at each angle of view based on equation (20), that is, when dW(θ) is 1.

[0097] The dependence of dSPOT on the angle of view θ when both dW(θ) and df(θ) change based on equation (20) is shown by the solid line in FIG. 5(a). As shown in Figure 5(a), it can be seen that the change in the spot diameter ratio dSPOT(θ) when the light beam width ratio dW(θ) changes and the change in the spot diameter ratio dSPOT(θ) when the focal length ratio df(θ) changes cancel each other out.

[0098] Specifically, it can be seen that the fluctuation range of the spot diameter ratio dSPOT(θ), which was approximately 19% over the entire range of the field of view θ due to changes in the light beam width ratio dW(θ), can be reduced to almost 0% by changing the focal length ratio df(θ). That is, in an optical scanning device using the conventional OFS method, the spot diameter ratio dSPOT(θ) changed by approximately 19% over the entire range of the angle of view θ, but in the optical scanning device 200 of this embodiment, the change in the spot diameter ratio dSPOT(θ) can be reduced to almost 0% over the entire range of the angle of view θ.

[0099] The optical scanning device 200 according to this embodiment is designed to satisfy the conditional expressions (21) and (21a) at any angle of view θ from the on-axis image height to the most off-axis image height.

[0100] FIG. 5B shows the DIST characteristics in a conventional optical scanning device and the DIST characteristics in the optical scanning device 200 according to this embodiment. As shown in FIG. 5(b), in a conventional optical scanning device, the image height Y changes with respect to the angle of view θ as a linear function as shown in equation (12), whereas in the optical scanning device 200 according to this embodiment, the image height Y changes with respect to the angle of view θ as a polynomial function as shown in equation (15). As the angle of view θ increases, the difference between the DIST characteristics increases. i is set.

[0101] Figure 5(c) also shows the dependence of the moving speed dY / dθ on the angle of view θ in a conventional optical scanning device as shown in equation (13), and the dependence of the moving speed dY / dθ on the angle of view θ in the optical scanning device 200 of this embodiment as shown in equation (16). As shown in FIG. 5(c), in a conventional optical scanning device, the moving speed dY / dθ is a constant value F regardless of the angle of view θ, whereas in the optical scanning device 200 of this embodiment, the moving speed dY / dθ monotonically increases as the angle of view θ increases.

[0102] FIG. 5(d) also shows the size of the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 calculated using the parameter values ​​shown in Tables 2-1 to 2-3 in the optical scanning device 200 of this embodiment.

[0103] As shown in FIG. 5(d), in the optical scanning device 200 according to this embodiment, the spot diameter SPOT has values ​​ranging from 69.7 micrometers to 72.5 micrometers over the entire image height range from the most off-axis image height 712 to the most off-axis image height 710. That is, the spot diameter SPOT has a fluctuation range of about 4.0% over the entire image height. Here, the fluctuation range of approximately 4.0% shown in FIG. 5(d) is larger than the reduced fluctuation range of approximately 0% of the spot diameter ratio dSPOT(θ) shown in FIG. 5(a) because the calculation shown in FIG. 5(d) includes residual aberrations such as wavefront aberration and partial magnification.

[0104] As described above, in the optical scanning device 200 according to this embodiment, it is possible to further suppress the fluctuation in spot diameter that occurs in the OFS method adopted to achieve high-speed, high-resolution printing without increasing the number of parts, and to further reduce the non-uniformity in the image plane illuminance distribution.

[0105] [Third embodiment] FIG. 6 shows a main scanning sectional view of an optical scanning device 300 according to the third embodiment. The optical scanning device 300 of this embodiment has the same configuration as the optical scanning device 100 of the first embodiment, except that a single imaging lens 63 is used instead of the first imaging lens 61 and the second imaging lens 62. Therefore, the same components are given the same reference numbers and their descriptions are omitted.

[0106] Moreover, the specifications of the optical scanning device 300 according to this embodiment are shown in the following Tables 3-1 to 3-3.

[0107] [Table 3-1] [Table 3-2] [Table 3-3]

[0108] Note that the definition of the local coordinate system of each optical surface and the definition of the surface shape of each optical surface are similar to those of the optical scanning device 100 according to the first embodiment.

[0109] FIG. 7A shows the dependency of the spot diameter ratio dSPOT in the main scanning direction on the angle of view θ in the optical scanning device 300 according to this embodiment. Specifically, the dependency of the spot diameter ratio dSPOT on the angle of view θ when the light flux width ratio dW(θ) changes based on equation (14) is shown by the dashed line in FIG. 7(a).

[0110] As shown in FIG. 7(a), the fluctuation range of dSPOT over the entire range of the angle of view θ is approximately 23%, which indicates that the fluctuation of the light beam width W is a factor in the fluctuation of the spot diameter SPOT in the main scanning direction.

[0111] In the optical scanning device 300 of this embodiment, similarly to the optical scanning device 100 of the first embodiment, as the angle of view θ increases from the on-axis image height 711 to the most off-axis image height 710, the power of the imaging optical system 85 in the main scanning cross section decreases, i.e., the focal length f(θ) in the main scanning cross section becomes longer. The dotted line in FIG. 7(a) shows the dependency of the spot diameter ratio dSPOT on the angle of view θ when the light beam width does not change at each angle of view based on equation (20), that is, when dW(θ) is 1.

[0112] The dependency of dSPOT on the angle of view θ when both dW(θ) and df(θ) change based on equation (20) is shown by the solid line in FIG. 7(a). As shown in FIG. 7(a), the spot diameter ratio dSPOT(θ) when the light beam width ratio dW(θ) changes and the spot diameter ratio dSPOT(θ) when the focal length ratio df(θ) changes cancel each other out.

[0113] Specifically, it can be seen that the fluctuation range of the spot diameter ratio dSPOT(θ), which was approximately 23% over the entire range of the field of view θ due to the change in the light beam width ratio dW(θ), can be reduced to approximately 11% by changing the focal length ratio df(θ). That is, in an optical scanning device using the conventional OFS method, the spot diameter ratio dSPOT(θ) changed by approximately 23% over the entire range of the angle of view θ, but in the optical scanning device 300 of this embodiment, the change in the spot diameter ratio dSPOT(θ) can be reduced to approximately 11% over the entire range of the angle of view θ.

[0114] The optical scanning device 300 according to this embodiment is designed to satisfy the conditional expressions (21) and (21a) at any angle of view θ from the on-axis image height to the most off-axis image height.

[0115] FIG. 7B shows the DIST characteristics in a conventional optical scanning device and the DIST characteristics in the optical scanning device 300 according to this embodiment. As shown in FIG. 7(b), in a conventional optical scanning device, the image height Y changes with respect to the angle of view θ as a linear function as shown in equation (12), whereas in the optical scanning device 300 according to this embodiment, the image height Y changes with respect to the angle of view θ as a polynomial function as shown in equation (15). As the angle of view θ increases, the difference between the DIST characteristics increases. i is set.

[0116] Figure 7(c) also shows the dependence of the moving speed dY / dθ on the angle of view θ in a conventional optical scanning device as shown in equation (13), and the dependence of the moving speed dY / dθ on the angle of view θ in the optical scanning device 300 of this embodiment as shown in equation (16). As shown in FIG. 7(c), in a conventional optical scanning device, the moving speed dY / dθ is a constant value F regardless of the angle of view θ, whereas in the optical scanning device 300 of this embodiment, the moving speed dY / dθ monotonically increases as the angle of view θ increases.

[0117] FIG. 7(d) also shows the size of the spot diameter SPOT in the main scanning direction at each image height on the scanned surface 7 calculated using the parameter values ​​shown in Tables 3-1 to 3-3 in the optical scanning device 300 of this embodiment.

[0118] As shown in FIG. 7(d), in the optical scanning device 300 according to this embodiment, the spot diameter SPOT has a value between 68.2 micrometers and 76.4 micrometers over the entire image height range from the most off-axis image height 712 to the most off-axis image height 710. That is, the spot diameter SPOT has a fluctuation range of about 12.0% over the entire image height. Here, the reason why the fluctuation range of approximately 12.0% shown in FIG. 7(d) is larger than the reduced change amount of the spot diameter ratio dSPOT(θ) of approximately 11% shown in FIG. 7(a) is because the calculation shown in FIG. 7(d) includes residual aberrations such as wavefront aberration and partial magnification.

[0119] As described above, the optical scanning device 300 according to this embodiment can suppress fluctuations in spot diameter that occur in the OFS method employed to achieve high-speed, high-resolution printing without increasing the number of parts, and can also reduce non-uniformity in the image plane illuminance distribution.

[0120] [Image forming device] FIG. 8 is a sub-scanning sectional view of a main part of an image forming apparatus (electrophotographic printer) 104 including an optical scanning device according to any one of the first to third embodiments.

[0121] As shown in FIG. 8, the image forming apparatus 104 receives code data Dc from an external device 117 such as a personal computer. The input code data Dc is then converted into image data (dot data) Di by a printer controller 111 within the device.

[0122] The converted image data Di is input to an optical scanning unit 400 which is an optical scanning device according to any one of the first to third embodiments. Then, a light beam (light flux) 103 modulated in accordance with image data Di is emitted from the optical scanning unit 400, and the photosensitive surface of the photosensitive drum 101 is scanned in the main scanning direction by the light beam 103.

[0123] 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 .

[0124] A charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided above the photosensitive drum 101 so as to come into contact with the surface. The surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with a light beam 103 scanned by an optical scanning unit 400 .

[0125] 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 developed into a toner image by a developing unit 107 disposed so as to come into contact with the photosensitive drum 101 downstream of the irradiation position of the light beam 103 in the cross section of the rotation of the photosensitive drum 101 .

[0126] The toner image developed by the developing unit 107 is transferred onto a sheet of paper 112, which is a transfer material, by a transfer roller (transfer unit) 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101. Incidentally, the paper 112 is stored in a paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 8), but it can also be fed manually. Then, a paper feed roller 110 disposed at an end of the paper cassette 109 feeds paper 112 in the paper cassette 109 into the transport path.

[0127] The paper 112 onto which the unfixed toner image has been transferred in the above manner is further conveyed to a fixing device behind the photosensitive drum 101 (on the left side in FIG. 8). The fixing unit is composed of a fixing roller 113 having an internal fixing heater (not shown) and a pressure roller 114 arranged so as to be in pressure contact with the fixing roller 113 .

[0128] Then, the paper 112 conveyed from the transfer roller 108 is heated while being pressed by the pressure contact portion between the fixing roller 113 and the pressure roller 114, so that the unfixed toner image on the paper 112 is fixed. Further, a paper discharge roller 116 is disposed behind the fixing unit, and the paper 112 on which the toner has been fixed is discharged to the outside of the image forming apparatus 104 .

[0129] Although not shown in FIG. 8, in addition to the above-mentioned data conversion, the printer controller 111 also controls each component within the image forming device 104, such as the motor 115, and components within the optical scanning unit 400, such as the polygon motor. Further, although the optical scanning device according to any one of the first to third embodiments is shown here as being applied to an image forming apparatus 104 that prints in monotone, it goes without saying that the optical scanning device can also be applied to a color image forming apparatus that scans and draws on multiple photosensitive elements with multiple light beams. In this case, for example, in a color image forming apparatus in which four color images are superimposed, four optical scanning devices according to any one of the first to third embodiments may be provided in parallel.

[0130] 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. [Explanation of symbols]

[0131] 1 light source 5 Deflector 51 Deflection surface 7 Scanned Surface 85 Imaging Optical System 100 Optical scanning device

Claims

1. 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 system that guides the light beam deflected by the deflector to the surface to be scanned, 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, When the light beam traveling toward an on-axis image height, which is an image height on the optical axis of the imaging optical system on the scanned surface, is defined as an on-axis light beam, and the light beam traveling toward the most off-axis image height on the scanned surface is defined as an most off-axis light beam, a scanning speed of the light beam on the scanned surface differs between the on-axis image height and the outermost off-axis image height, on the scanned surface, a first region from the most off-axis image height to the on-axis image height on one side with respect to the optical axis is longer than a second region from the most off-axis image height to the on-axis image height on the other side, The optical scanning device according to claim 1, wherein the imaging optical system guides the light beam to the first and second regions.

2. 2. The optical scanning device according to claim 1, wherein the refractive power of the imaging optical system in the main scanning cross section changes monotonically between a first position through which the on-axis light beam passes and a second position through which the most off-axis light beam passes.

3. 3. The optical scanning device according to claim 2, wherein the refractive power at the second position is smaller than the refractive power at the first position.

4. 4. The optical scanning device according to claim 1, wherein the scanning speed changes monotonically between the on-axis image height and the most off-axis image height.

5. 5. The optical scanning device according to claim 4, wherein the scanning speed at the most off-axis image height is greater than the scanning speed at the on-axis image height.

6. 6. The optical scanning device according to claim 1, wherein, in a main scanning cross section, a traveling direction of the light beam when it is incident on the deflecting surface and an optical axis of the imaging optical system are non-parallel to each other.

7. 7. An optical scanning device according to claim 1, further comprising an incident optical system for making the light beam from the light source incident on the deflection surface of the deflector, the incident optical system being arranged on the one side.

8. In the main scanning cross section, the angle that the traveling direction of the most off-axis light beam toward the most off-axis image height on the other side when deflected by the deflecting surface forms with respect to the optical axis of the imaging optical system is defined as θ. ｍａｘ－ θ is the angle that the optical axis of the incident optical system makes with the optical axis of the imaging optical system. ｉ , where N is the number of deflection surfaces of the deflector, i ｉ -θ ｍａｘ－ <4π / N 8. The optical scanning device according to claim 7, wherein the following condition is satisfied:

9. 9. The optical scanning device according to claim 7, wherein the optical axes of the incident optical system and the imaging optical system are parallel to a main scanning cross section.

10. In the main scanning cross section, when the traveling direction of the light beam deflected by the deflecting surface is defined as an angle θ with respect to the optical axis of the imaging optical system, when the ratio of the width of the off-axis light beam deflected at the angle θ to the width of the on-axis light beam when it enters the imaging optical system is defined as dW(θ), and when the ratio of the focal length at a position where the off-axis light beam passes to the focal length at a first position where the on-axis light beam passes of the imaging optical system is defined as df(θ), 0.90<dW(θ) / df(θ)<1.10 10. The optical scanning device according to claim 1, wherein the following condition is satisfied:

11. 11. The optical scanning device according to claim 1, wherein the first area is on the same side as the light source with respect to the optical axis of the imaging optical system.

12. An optical scanning device according to any one of claims 1 to 11, characterized in that in an optical element included in the imaging optical system, an area through which the light beam heading toward the first area passes is longer than an area through which the light beam heading toward the second area passes.

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

14. 13. 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.

Images