Scanning optical system

Abstract

A scanning optical system includes a deflector, a plurality of collimator lenses arranged in a direction of a rotation axis of the deflector, a first lens, a second lens, and an image forming optical system. And a scanning optical system that forms a scanning light beam by means of the imaging optical system, said scanning light beam passing through one of the collimating lenses, the first lens, and the second lens, and being deflected by the deflector, a cross-section perpendicular to the rotation axis and including a common optical axis of the first lens and the second lens is defined as a first cross-section, a cross-section parallel to the rotation axis and including the optical axis is defined as a second cross-section, and the first cross-section of one surface of the first lens is formed so as to diverge a light beam. The second cross-section of the other surface of the first lens is formed to converge the light beam, the first cross-section of the one surface of the second lens is formed to collimate or converge the light beam, when the light beam reaches the surface of the deflector, the width of the light beam is larger on the first cross-section than the width of the surface, and the light beam is focused on the surface on the second cross-section.

Description

Technical Field

[0001] The present invention relates to a scanning optical system. Background Art

[0002] A scanning optical system is used, which includes an incident optical system, a deflector, and an imaging optical system. The incident optical system includes a plurality of collimating lenses arranged in the direction of the rotation axis of the deflector and is configured to process light beams from a plurality of light sources arranged in the direction of the rotation axis of the deflector. In addition, in order to achieve high-speed printing, an interference scanning optical system has been developed in which a light beam having a width wider than the width of the reflection surface of the deflector in the main scanning direction is incident on the deflector in the above-described scanning optical system (for example, Patent Document 1).

[0003] In order to make the imaging optical system of the scanning optical system as described above compact, it is necessary to set the transverse magnification of the imaging optical system in a cross section parallel to the rotation axis of the deflector to a value equal to or less than a specified value. In order to ensure the beam diameter of the scanning light beam at a transverse magnification equal to or less than the specified value, it is necessary to shorten the focal length of the collimating lens. On the other hand, when the focal length of the collimating lens is reduced, the width of the light beam in a cross section in the main scanning direction perpendicular to the rotation axis of the deflector is reduced, so the size of the incident optical system is increased to increase the width of the light beam.

[0004] Thus, a compact scanning optical system configured to process light beams from a plurality of light sources arranged in the direction of the rotation axis of the deflector and employing an interference method has not been developed. Therefore, there is a need for a compact scanning optical system configured to process light beams from a plurality of light sources arranged in the direction of the rotation axis of the deflector and employing an interference method.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2010-61144 (Japanese Patent No. 4780228) Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] An object of the present invention is to provide a compact scanning optical system configured to process light beams from a plurality of light sources arranged in the direction of the rotation axis of the deflector and employing an interference method.

[0010] Means for Solving the Problems

[0011] The scanning optical system of the present invention includes a deflector, a plurality of collimating lenses arranged in the direction of the rotation axis of the deflector, a first lens, a second lens, and an imaging optical system. The scanning optical system is configured such that the light beam passing through one of the collimating lenses, the first lens, and the second lens is deflected by the deflector and a scanning light beam is formed using the imaging optical system. The scanning optical system is formed such that a cross-section perpendicular to the rotation axis and including the common optical axis of the first lens and the second lens is defined as the first cross-section, and a cross-section parallel to the rotation axis and including the optical axis is defined as the second cross-section. Among the two surfaces of the first lens and one surface of the second lens, the shape of the first cross-section is different from the shape of the second cross-section. The first cross-section of one surface of the first lens is formed to diverge the light beam, the second cross-section of the other surface of the first lens is formed to converge the light beam, and the first cross-section of one surface of the second lens is formed to collimate or converge the light beam. In the scanning optical system of the present invention, when the light beam reaches the surface of the deflector, in the first cross-section, the width of the light beam is greater than the width of the surface, and in the second cross-section, the light beam is focused onto the surface.

[0012] According to the present invention, by adopting the first lens and the second lens having the above characteristics and being configured to process the light beams from a plurality of light sources arranged in the direction of the rotation axis of the deflector, a compact scanning optical system adopting an interference fit method can be achieved.

[0013] In the scanning optical system of the first embodiment of the present invention, the material of the first lens is plastic. When the effective diameter is represented by D11 and the absolute value of the focal length in the first cross-section is represented by f11 for one surface of the first lens, and the effective diameter is represented by D12 and the absolute value of the focal length in the second cross-section is represented by f12 for the other surface of the first lens, the following formula is satisfied:

[0014] 0.04 ≤ D11 / f11 ≤ 0.07 (6)

[0015] 0.007 ≤ D12 / f12 ≤ 0.011 (7).

[0016] The temperature of the first lens changes significantly over time due to the influence of the nearby light source. However, according to the new insights of the inventors, if D11 / f11 in Equation (6) and D12 / f12 in Equation (7) are below the upper limit values, the deterioration of the optical performance caused by changes in the refractive index and the like of the plastic first lens due to temperature changes is within an allowable range, and a first lens having two surfaces with different shapes of the first cross-section and the second cross-section can be used. Compared with a glass lens, a plastic lens has a smaller change in refractive index caused by temperature changes, but it is difficult to manufacture a glass lens having two surfaces with different shapes of the first cross-section and the second cross-section, and even if it can be manufactured, the cost is extremely high. On the other hand, if D11 / f11 in Equation (6) and D12 / f12 in Equation (7) are above the lower limit values, a compact scanning optical system can be achieved. As a result, by using a plastic first lens that satisfies Equation (6) and Equation (7), a compact and easily manufacturable scanning optical system can be realized.

[0017] In the scanning optical system according to the second embodiment of the present invention, the two surfaces of the first lens and the one surface of the second lens are cylindrical surfaces or toroidal surfaces.

[0018] In the scanning optical system according to the third embodiment of the present invention, the one surface of the first lens is the surface facing the second lens, and the other surface of the first lens is the surface facing the collimating lens.

[0019] In the scanning optical system according to the third embodiment of the present invention, by performing divergence of the light beam in the first cross-section of the first lens on the surface facing the second lens, the influence of changes in the refractive index and the like caused by temperature changes on the optical path can be reduced.

[0020] In the scanning optical system according to the fourth embodiment of the present invention, the material of the first lens is plastic, and the material of the second lens is glass.

[0021] The second lens is disposed near the deflector whose temperature rises during operation, so glass with a small change in refractive index and linear expansion with respect to temperature change is used as the material.

[0022] In the scanning optical system according to the fifth embodiment of the present invention, when the focal length of each collimating lens is set to fcol [mm], the absolute value of the focal length in the first cross-section of the first lens is set to f11 [mm], the absolute value of the focal length in the second cross-section of the first lens is set to f12 [mm], and the absolute value of the focal length in the first cross-section of the second lens is set to f21 [mm], the following formula is satisfied:

[0023] fcol ≤ 13(1)

[0024] 120 ≤ f12 ≤ 160 (2)

[0025] 3.5 ≤ f21 / f11 ≤ 4.0 (3).

[0026] In order to make the imaging optical system of a scanning optical system that processes light beams from a plurality of light sources arranged in the direction of the rotation axis of a deflector compact, it is necessary to limit the transverse magnification of the imaging optical system. When the transverse magnification of the imaging optical system is limited, in order to obtain a specified diameter of the scanning light beam, it is necessary to reduce the diameter of the second cross-section of the aperture. Therefore, in order to maintain the light efficiency, it is necessary to make the focal length fco of the collimating lens below a specified value. In addition, regarding the focal length f12 in the second cross-section of the first lens, from the perspective of the distance from the first lens to the deflector, it is preferably below a specified value, and from the perspective of the size of the imaging optical system in the direction of the rotation axis of the deflector, it is preferably above a specified value. In addition, by making the ratio of the absolute value f21 of the focal length in the first cross-section of the second lens to the absolute value f11 of the focal length in the first cross-section of the first lens within an appropriate range, it is possible to irradiate a deflector of an appropriate size with a light beam of an appropriate width.

[0027] In the scanning optical system according to the sixth embodiment of the present invention, when a first straight line obtained by projecting the path of the principal ray of the deflected light beam onto a plane perpendicular to the rotation axis and a second straight line obtained by projecting the scanning direction onto the plane are perpendicular, with the reflection point of the principal ray as a reference point, the distance from the reference point to the scanning plane, which is a plane that includes the position where the scanning light beam is focused and is perpendicular to the first straight line, is represented by L8, the distance from the vertex of the lens surface of the imaging optical system closest to the scanning plane to the scanning plane is represented by BF, a plane that includes the reference point and is parallel to the rotation axis and the first straight line is used as the third cross-section, and the transverse magnification in the third cross-section of the imaging optical system is represented by β, the following formula is satisfied:

[0028] 0.15 ≤ BF / L8 ≤ 0.2 (4)

[0029] 0.35 ≤ β ≤ 0.45 (5).

[0030] By satisfying the conditions of this embodiment, it is possible to make the imaging optical system of a scanning optical system that processes light beams from a plurality of light sources arranged in the direction of the rotation axis of a deflector compact. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a diagram showing a scanning optical system 100 according to an embodiment of the present invention.

[0032] Figure 2 is a diagram showing a cross-section of an incident optical system that includes the z-axis of the incident optical system and is parallel to the x-axis.

[0033] Figure 3 It is a diagram showing a cross-section of an imaging optical system parallel to the x-axis and perpendicular to the y-axis.

[0034] Figure 4 It is a diagram for explaining the incident angle (main).

[0035] Figure 5 It is a diagram for explaining the incident angle (sub).

[0036] Figure 6 It is a diagram for explaining the surface-to-surface distance L2 - L8 shown in Table 2.

[0037] Figure 7 It is a diagram for explaining the "sub-shift amount" of the second scanning lens in Table 2.

[0038] Figure 8 It is a diagram for explaining the "main-shift amount" of the second scanning lens in Table 2. Detailed implementation mode

[0039] Figure 1 It is a diagram showing the scanning optical system 100 of an embodiment of the present invention. The scanning optical system 100 includes an incident optical system, a deflector 109, and an imaging optical system. The incident optical system includes a collimating lens 101, an aperture 103, a first lens 105, and a second lens 107. The imaging optical system includes a first scanning lens 111 and a second scanning lens 113. The light beam emitted from the light source 200 is collimated by the collimating lens 101, passes through the aperture 103, and then reaches the first lens 105. The first lens 105 diverges the light beam in the Figure 1 shown cross-section. The light beam passing through the first lens 105 reaches the second lens 107. The second lens 107 collimates or converges the light beam in the Figure 1 shown cross-section. The light beam passing through the second lens 107 reaches the deflector 109. The deflector 109 deflects the light beam by rotating around a rotation axis perpendicular to the Figure 1 shown cross-section. The deflected light beam is converged by the first scanning lens 111 and the second scanning lens 113 to form a scanning light beam.

[0040] The x-axis that determines the direction of the rotation axis of the deflector 109 and the y-axis in the scanning direction. In addition, the common optical axis (central axis) of the first lens 105 and the second lens 107 is set as the z-axis of the incident optical system. Figure 1 It is a diagram showing a cross-section of the incident optical system perpendicular to the x-axis and including the z-axis of the incident optical system.

[0041] Figure 2 It is a diagram showing a cross-section of the incident optical system including the z-axis of the incident optical system and parallel to the x-axis. In Figure 2In the cross section shown, the light beam emitted from one of the plurality of light sources 200 arranged in the direction of the rotation axis of the deflector 109 is collimated by one of the plurality of collimating lenses 101 arranged in the direction of the rotation axis of the deflector 109, passes through one of the plurality of apertures 103 arranged in the direction of the rotation axis of the deflector 109, and then reaches the first lens 105. In Figure 2 In the cross section shown, the first lens 105 and the second lens 107 focus the light beam on one surface of the deflector 109.

[0042] Figure 3 is a diagram showing a cross section of the imaging optical system parallel to the x-axis and perpendicular to the y-axis. When the principal ray of the deflected light beam travels in a direction perpendicular to the y-axis direction in the Figure 1 cross section shown, the reflection point in the deflection plane of the principal ray is referred to as the reference point P. Figure 1 The cross section shown and Figure 3 the cross section shown are cross sections including the reference point P. In Figure 1 the cross section shown, the straight line passing through the reference point P and perpendicular to the y-axis direction is set as the z-axis of the imaging optical system. Figure 1 The cross section shown is a cross section including the z-axis of the imaging optical system and perpendicular to the x-axis. Figure 3 The cross section shown is a cross section including the z-axis of the imaging optical system and parallel to the x-axis. In Figure 3 the cross section shown, the light beam reflected at the reference point P diverges and reaches the first scanning lens 111. In Figure 3 the cross section shown, the first scanning lens 111 converges the light beam so that the light beam passing through the second lens 107 forms a scanning light beam.

[0043] An embodiment of the present invention will be described below.

[0044] Embodiment

[0045] Table 1 is a table showing the specifications of the scanning optical system of the embodiment.

[0046] [Table 1]

[0047]

[0048] In Table 1, the incident angle (main) refers to the angle formed by the straight line obtained by projecting the principal ray incident on the deflector onto the Figure 1 cross section shown and the z-axis of the imaging optical system.

[0049] Figure 4 is a diagram for explaining the incident angle (main).

[0050] In Table 1, the incident angle (sub) refers to the projection of the principal ray incident on the deflector ontoFigure 2 The angle formed by the straight line obtained from the cross section shown and the direction of the z-axis of the incident optical system.

[0051] Figure 5 It is a diagram for explaining the incident angle (sub).

[0052] In Table 1, the system focal length is represented by f that satisfies y = f·θ, where θ (in radians) is the angle formed by the straight line obtained by projecting the path of the light beam that reaches the maximum image height y after being deflected by the deflector onto the Figure 1 cross section shown and the z-axis of the imaging optical system. The above-mentioned θ is shown in Figure 4 .

[0053] The light source 200 is a semiconductor laser light source.

[0054] In Table 1, "θ⊥" of the light source represents Figure 1 the divergence angle of the semiconductor laser light source in the cross section shown, and "θ / / " of the light source represents Figure 2 the divergence angle of the semiconductor laser light source in the cross section shown.

[0055] The core thickness of the lens refers to the thickness of the lens along the z-axis of the incident optical system or the z-axis of the imaging optical system.

[0056] The material of the collimating lens 101 is glass, and the refractive index is 1.576. The material of the first lens 105 is a polycycloolefin resin, and the material of the second lens 107 is borosilicate crown glass. The material of the first scanning lens 111 is a polycycloolefin resin, and the material of the second scanning lens 113 is a polymethyl methacrylate resin.

[0057] The length of the "main" of the aperture represents Figure 1 the length in the cross section shown, and the length of the "sub" of the aperture represents Figure 2 the length in the cross section shown.

[0058] Table 2 is a table showing the distances between surfaces, etc. of the scanning optical system of the embodiment.

[0059] [Table 2]

[0060]

[0061] In Table 2, the distance between surfaces represents the distance along the z-axis of the incident optical system or the z-axis of the imaging optical system.

[0062] Figure 6 It is a diagram for explaining the distances between surfaces L1 - L8 shown in Table 2. Figure 6 The cross section shown is the same as Figure 1 the cross section shown.

[0063] In Table 2, the secondary displacement amounts of the light source, the collimating lens, and the aperture represent the distances between the centers of the light source, the collimating lens, and the aperture and the z-axis of the incident optical system in the cross-section shown in Figure 2 The cross-section shown in

[0064] In Table 2, the "scanning plane" represents an imaginary plane perpendicular to the z-axis of the imaging optical system that includes the position where the beam for scanning is focused. In Figure 6 300 is used to represent the scanning plane. Scanning in the y-axis direction is performed on the scanning plane.

[0065] Figure 7 is a diagram for explaining the "secondary displacement amount" of the second scanning lens in Table 2. Figure 7 The cross-section shown in Figure 3 is defined in the same manner as the cross-section shown in Figure 7 In the cross-section shown in Figure 7 Only two lenses with positive secondary displacement amounts are shown. In Figure 7 SX1 represents the absolute value of the secondary displacement amount of the inner lens, and SX2 represents the absolute value of the secondary displacement amount of the outer lens.

[0066] Figure 8 is a diagram for explaining the "primary displacement amount" of the second scanning lens in Table 2. Figure 8 The cross-section shown in Figure 1 is defined in the same manner as the cross-section shown in Figure 8 In the cross-section shown in Figure 8 the "primary displacement amount" of the second scanning lens represents the distance between the surface defining center (the vertex of the lens surface) of the incident surface or the exit surface of the second scanning lens and the z-axis of the imaging optical system. In Figure 8 SY is used to represent the primary displacement amount. In

[0067] The collimating lens 101 of the incident optical system collimates the divergent light emitted from the light source 200. The focal length of the collimating lens 101 is 10 mm, satisfying Equation (1).

[0068] The surface of the collimating lens 101 is described. The surface of the collimating lens is represented by the following formula.

[0069]

[0070] z is the sag of the lens surface, representing the coordinate in the z-axis direction of the incident optical system of the points on the lens surface with respect to the vertex of the lens surface. r represents the distance of the point on the surface from the z-axis of the incident optical system. R represents the radius of curvature, k represents the conic constant, and A i represents the aspherical coefficient. In the above formula and the following content, the radius of curvature R of the surface is defined as positive when the surface bulges toward the object side and negative when it bulges toward the image side.

[0071] Table 3 is a table showing the constants and coefficients of the surfaces of the collimating lens 101.

[0072] [Table 3]

[0073]

[0074] The surfaces of the first lens 105 and the second lens 107 of the incident optical system will be described.

[0075] Table 4 is a table explaining the surfaces of the first lens 105 and the second lens 107. R represents the radius of curvature.

[0076] [Table 4]

[0077]

[0078] The Figure 1 cross-section of the incident surface of the first lens 105 has a curvature of 0, Figure 2 and the Figure 2 cross-section has a positive curvature. The incident surface of the first lens 105 is a cylindrical surface with a positive curvature (bulging toward the object side) having a Figure 1 cross-section. The Figure 2 cross-section of the exit surface of the first lens 105 has a positive curvature, Figure 1 and the

[0079] cross-section has a curvature of 0. The exit surface of the first lens 105 is a cylindrical surface with a positive curvature (bulging toward the object side) having a Figure 1 cross-section. Figure 2 The Figure 1 cross-section and the Figure 2 cross-section of the incident surface of the second lens 107 have a curvature of 0. The incident surface of the second lens 107 is a plane. The Figure 1 cross-section of the exit surface of the second lens 107 has a negative curvature,

[0080] and the

[0081] Table 5 shows theFigure 1 the cross-section of Figure 2 a table of the absolute value of the focal length of the cross-section of

[0082] [Table 5]

[0083]

[0084] Let the Figure 1 absolute value of the focal length f21 of the cross-section of Figure 1 the second lens and the absolute value of the focal length f11 of the cross-section of the first lens, when the ratio M = f21 / f11 = 3.77. Equations (2) and (3) are satisfied.

[0085] Table 6 shows the ratio of the effective diameter D11 of the deflector-side surface of the first lens to Figure 1 the absolute value of the focal length f11 of the cross-section of Figure 2 and the ratio of the effective diameter D12 of the light source-side surface of the first lens to

[0086] [Table 6]

[0087] Embodiment D11 [mm] 1.966 f11 [mm] 35.876 D12 [mm] 1.260 f12 [mm] 156.1 D11 / f11 0.055 D12 / f12 0.0081

[0088] Generally, regarding D11 / f11 and D12 / f12, it is preferable to satisfy the following conditions.

[0089] 0.04 ≤ D11 / f11 ≤ 0.07 (6)

[0090] 0.007 ≤ D12 / f12 ≤ 0.011 (7)

[0091] The temperature of the first lens 105 varies significantly over time due to the influence of the nearby light source 200. However, according to the inventors' new insights, if D11 / f11 in Equation (6) and D12 / f12 in Equation (7) are below the upper limit values, the deterioration of the optical performance caused by changes in the refractive index and the like of the plastic first lens due to temperature changes is within an allowable range, and a first lens having two surfaces with different shapes of the first cross-section and the second cross-section can be used. On the other hand, if D11 / f11 in Equation (6) or D12 / f12 in Equation (7) is less than the lower limit value, the size of the scanning optical system becomes too large.

[0092] Table 7 shows Figure 1 the beam diameter in the cross-section of

[0093] [Table 7]

[0094] Item Beam diameter before incidence of the first lens (beam diameter 1) 1.97 [mm] Beam diameter after exiting from the second lens (beam diameter 2) 8.94 [mm] Beam diameter 2 / Beam diameter 1 4.54

[0095] In Figure 1In the cross-section, the first lens 105 diverges the collimated light, and the second lens 107 collimates the diverging light.

[0096] On the other hand, for the Figure 1 The cross-section of the polygon mirror 109 is a regular dodecagon, and the diameter of its inscribed circle is 20 mm. Therefore, the length of the deflection surface is 2·10·tan(15°) = 5.36 [mm]. Thus, the diameter of the light beam that reaches the polygon mirror 109 after passing through the second lens 107 in the Figure 1 cross-section is larger than the length of the deflection surface.

[0097] The surfaces of the first scanning lens 111 and the second scanning lens 113 of the imaging optical system will be described. The surfaces of the first scanning lens 111 and the second scanning lens 113 are represented by the following equations.

[0098]

[0099] z is the sag of the lens surface, representing the coordinate in the z-axis direction of the incident optical system of a point on the lens surface with respect to the vertex of the lens surface. x represents the coordinate in the x-axis direction of a point on the lens surface with respect to the vertex of the lens surface, and y represents the coordinate in the y-axis direction of a point on the lens surface with respect to the vertex of the lens surface. r represents the distance of a point on the surface from the z-axis of the imaging optical system. R represents the radius of curvature, k represents the conic constant, and A i represents the aspheric coefficient. The following relationships hold.

[0100]

[0101] Table 8 is a table showing the constants and coefficients of the surfaces of the first scanning lens 111 and the second scanning lens 113.

[0102] [Table 8]

[0103]

[0104] Table 9 is a table showing the specifications of the imaging optical system.

[0105] [Table 9]

[0106] Unit BF mm 50 L8 mm 300 BF / L8 - 0.17 β - 0.39

[0107] BF is the distance from the vertex of the exit surface of the second scanning lens 113 to the scanning surface 300. L8 is the distance along the z-axis of the imaging optical system from the reference point to the scanning surface 300 as described above. β is the Figure 2 transverse magnification of the imaging optical system in the cross-section. Equations (4) and (5) are satisfied.

Claims

1. A scanning optical system comprising a deflector, a plurality of collimating lenses arranged in the direction of the rotation axis of the deflector, a first lens, a second lens, and an imaging optical system. The scanning optical system is configured such that a light beam that passes through one of the collimating lenses, the first lens, and the second lens and is deflected by the deflector forms a scanning light beam using the imaging optical system. Among them, the scanning optical system is configured such that a cross-section perpendicular to the rotation axis and including the common optical axis of the first lens and the second lens is defined as a first cross-section, and a cross-section parallel to the rotation axis and including the optical axis is defined as a second cross-section. Among the two surfaces of the first lens and one surface of the second lens, the shape of the first cross-section is different from the shape of the second cross-section. The first cross-section of one surface of the first lens is configured to diverge the light beam. The second cross-section of the other surface of the first lens is configured to converge the light beam. The first cross-section of one surface of the second lens is configured to collimate or converge the light beam. When the light beam reaches the surface of the deflector, in the first cross-section, the width of the light beam is greater than the width of the surface, and in the second cross-section, the light beam is condensed onto the surface.

2. The scanning optical system according to claim 1, wherein the material of the first lens is plastic. When, with respect to the one surface of the first lens, D11 represents the effective diameter and f11 represents the absolute value of the focal length in the first cross-section, and with respect to the other surface of the first lens, D12 represents the effective diameter and f12 represents the absolute value of the focal length in the second cross-section, the following formula is satisfied: 0.04 ≤ D11 / f11 ≤ 0.07(6) 0.007 ≤ D12 / f12 ≤ 0.011(7).

3. The scanning optical system according to claim 1, wherein the two surfaces of the first lens and the one surface of the second lens are cylindrical surfaces or toric surfaces.

4. The scanning optical system according to claim 1, wherein the one surface of the first lens is the surface facing the second lens. the other surface of the first lens is the surface facing the collimating lens.

5. The scanning optical system according to claim 1, wherein the material of the first lens is plastic. the material of the second lens is glass.

6. The scanning optical system according to claim 1, wherein When the focal length of each collimating lens is set to fcol [mm], the absolute value of the focal length in the first cross-section of the first lens is set to f11 [mm], the focal length in the second cross-section of the first lens is set to f12 [mm], and the absolute value of the focal length in the first cross-section of the second lens is set to f21 [mm], the following formula is satisfied: fcol ≤ 13(1) 120≤f12≤160(2) 3.5 ≤ f21 / f11 ≤ 4.0(3).

7. The scanning optical system according to claim 1, wherein When the first straight line obtained by projecting the path of the chief ray of the deflected light beam onto a plane perpendicular to the rotation axis is perpendicular to the second straight line obtained by projecting the scanning direction onto this plane, taking the reflection point of the chief ray as the reference point, representing the distance from this reference point to the scanning plane, which is a plane containing the position where the scanning light beam is focused and perpendicular to the first straight line, as L8, representing the distance from the vertex of the lens surface of the imaging optical system closest to the scanning plane to the scanning plane as BF, taking the plane containing this reference point and parallel to the rotation axis and the first straight line as the third cross-section, and representing the transverse magnification in this third cross-section of the imaging optical system as β, the following formula is satisfied: 0.15≤BF / L8≤0.2(4) 0.35≤β≤0.45(5)。

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