JPH09281422A - Scanning optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a scanning optical device which is compact and is suitable for high-fineness printing by adequately setting the lens shape of one element of fθ lens consisting of a plastic material. SOLUTION: This device is so constituted that the luminous flux emitted from a light source means 1 is linearly imaged longitudinally in a main scanning direction on the deflection plane 5a of a deflecting element 5 via a first optical element 2 and a second optical element 4 and the luminous flux deflected by this deflecting element 5 is imaged like a spot on a surface 7 to be scanned via a third optical element 6. The surface 7 to be scanned is scanned by this spot. In such a case, the third optical element 6 consists of a single lens made of a plastic material. Both lens faces of this single lens consists of toric faces of an aspherical shape within the main scanning plane. The curvature in the sub-scanning direction of the deflecting element side lens face of the single lens is made negative on the optical axis and the negative curvature is made gradually stronger the furtherer from the optical axis in the main scanning direction up to the peripheral part of the first lens.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical device, and in particular, after a light beam (light beam) optically modulated and emitted from a light source means is deflected and reflected by an optical deflector (deflecting element) such as a rotating polygon mirror, A laser beam printer (LBP) having an electrophotographic process, a digital copying machine, or the like, which records image information by optically scanning the surface to be scanned through an imaging optical system (fθ lens) having fθ characteristics The present invention relates to a scanning optical device suitable for the above device.

[0002]

2. Description of the Related Art Conventionally, in a scanning optical device such as a laser beam printer or the like, a light beam which is light-modulated from a light source means in accordance with an image signal and is emitted periodically by an optical deflector comprising a rotating polygon mirror (polygon mirror). Deflect, f
The light is focused in a spot shape on a photosensitive recording medium (photosensitive drum) surface by an imaging optical system having θ characteristics, and the surface is optically scanned to record an image.

FIG. 9 is a schematic view of a main part of a conventional scanning optical device of this type.

In the figure, a divergent light beam emitted from a light source means 91 is made into a substantially parallel light beam by a collimator lens 92, and the light beam (amount of light) is limited by a diaphragm 93 to have a cylindrical power having a predetermined refracting power only in the sub-scanning direction. It is incident on the lens 94. Within the main scanning cross section, the parallel light flux that has entered the cylindrical lens 94 is emitted as it is in the state of parallel light flux. Further, in the sub-scanning cross section, they are converged and imaged as a substantially linear image on the deflecting surface (reflecting surface) 95a of the optical deflector 95 composed of a rotating polygon mirror (polygon mirror).

Then, the light beam deflected and reflected by the deflecting surface 95a of the optical deflector 95 is passed through the image forming optical system (fθ lens) 96 having the fθ characteristic, and the photosensitive drum surface 97 as the surface to be scanned.
By guiding the light upward and rotating the optical deflector 95 in the direction of arrow A, the photosensitive drum surface 97 is optically scanned to record image information.

[0006]

In order to record image information with high accuracy in this type of scanning optical apparatus, the field curvature should be well corrected over the entire surface to be scanned and the spot diameter should be uniform. It is necessary to have a distortion aberration (fθ characteristic) in which the angle of light and the image height are in a proportional relationship. Various types of scanning optical devices or correction optical systems (fθ lenses) satisfying such optical characteristics have been conventionally proposed.

On the other hand, with the downsizing and cost reduction of laser beam printers, digital copying machines and the like, the same is required for scanning optical devices.

To satisfy these requirements, fθ
A scanning optical device having one lens is disclosed in, for example, Japanese Patent Publication No. 61-48684 and Japanese Patent Application Laid-Open No. 63-157122.
And JP-A-4-104213 and JP-A-4-5213
Various proposals have been made in Japanese Patent No. 0908.

Of these publications, Japanese Examined Patent Publication No. 61-4868
No. 4, JP-A-63-157122, etc., fθ
A single lens having a concave surface is used as the lens on the optical deflector side to focus the parallel light flux from the collimator lens on the surface of the recording medium. In Japanese Patent Application Laid-Open No. 4-104213, a single lens having a concave surface on the optical deflector side and a toroidal surface on the image surface side is used as an fθ lens, and a light beam converted into convergent light by a collimator lens is incident on the fθ lens. I have. In JP-A-4-50908, a single lens having a higher order aspherical surface introduced as a fθ lens is used, and a light beam converted into convergent light by a collimator lens is made incident on the fθ lens.

However, in the conventional scanning optical device described above, in Japanese Patent Publication No. 61-48684, the field curvature in the sub-scanning direction remains and the parallel light flux is imaged on the surface to be scanned. The distance from the fθ lens to the surface to be scanned is the focal length f, which is long, which makes it difficult to construct a compact scanning optical device. In Japanese Unexamined Patent Publication No. 63-157122, there is a problem that the fθ lens has a large thickness, which makes it difficult to manufacture it by molding and causes a cost increase. Japanese Patent Laid-Open No. 4-10
The 4213 publication has a problem that distortion aberration remains. In Japanese Patent Laid-Open No. 4-50908, although a high-order aspherical f.theta. Lens is used and the aberration is corrected well, there is a problem that jitter of the polygon surface number cycle occurs due to an attachment error of a polygon mirror which is an optical deflector. there were.

Further, a problem common to the fθ lens constructed by one of these is that the spot diameter in the sub-scanning direction depends on the image height due to the non-uniformity of the lateral magnification in the sub-scanning direction between the optical deflector and the surface to be scanned. There is a problem that the spot diameter in the sub-scanning direction changes depending on the image height due to an increase in the spot diameter due to birefringence inside the lens when the fθ lens is made of a plastic material and molded.

According to the present invention, when the light flux from the light source means converted by the collimator lens or the like is imaged on the surface to be scanned by one fθ lens made of a plastic material through the optical deflector, the fθ lens By optimizing the lens shape, the change of the spot diameter due to the nonuniformity of the lateral magnification in the sub-scanning direction is added to the change of the spot diameter due to the birefringence to suppress the change of the spot diameter. An object of the present invention is to provide a scanning optical device that is compact and suitable for high-definition printing.

[0013]

In the scanning optical device of the present invention, (1-1) the luminous flux emitted from the light source means is mainly passed through the first optical element and the second optical element on the deflecting surface of the deflecting element. Scanning optical for forming an image in a linear shape longitudinal in the scanning direction, forming an image of the light beam deflected by the deflection element on the surface to be scanned in a spot shape through the third optical element, and scanning the surface to be scanned. In the apparatus, the third optical element is composed of a single lens made of a plastic material, both lens surfaces of the single lens are both composed of an aspherical toric surface in the main scanning plane, and the third lens is provided on the deflection element side of the single lens. The curvature of the lens surface in the sub-scanning direction is negative on the optical axis, and the negative curvature is gradually increased as the distance from the optical axis to the peripheral portion of the first lens in the main scanning direction increases.

Particularly, (1-1-1) the negative curvature is gradually weakened as the distance from the first lens peripheral portion to the second lens peripheral portion of the single lens is further increased, and (1-1- 2) The negative curvature is gradually weakened as the distance from the second lens peripheral portion of the single lens to the third lens peripheral portion is further increased to the peripheral portion, and the curvature is set to approximately 0 at the third lens peripheral portion, (1-1-
3) It is characterized in that the curvature is made positive as the distance from the third lens peripheral portion of the single lens to the peripheral portion further increases.

(1-2) A light beam emitted from the light source means is imaged in a linear shape elongated in the main scanning direction on the deflection surface of the deflection element via the first optical element and the second optical element, In a scanning optical device for forming an image of a light beam deflected by a deflecting element on the surface to be scanned in a spot shape through the third optical element and scanning the surface to be scanned, the third optical element is a plastic material. Of the single lens, both lens surfaces of which are aspherical toric surfaces in the main scanning plane, and the curvature of the lens surface of the single lens on the scanned surface side in the sub-scanning direction is the optical axis. It is characterized in that the upper part is negative, the outermost axis is made weaker than the curvature on the optical axis, and each element is set so as to satisfy the following conditional expression.

[0016]

[Equation 2] However, R _2a , R _2b , R _2c , and R _2d are the radii of curvature in the sub-scanning direction of the lens surface on the scanned surface side of the single lens, respectively, and are the outermost axes from the optical axis of the lens effective portion in the main scanning direction. It is at the positions of 0%, 30%, 70%, and 100% in the order of the height of the lens through which the light rays pass.

(1-3) In (1-1) and (1-2), when the maximum effective scanning width on the surface to be scanned is w and the thickness of the single lens in the optical axis direction is t, 0. It is characterized in that the condition 035 <t / w <0.065 (3) is satisfied.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view of a main part in a main scanning direction according to a first embodiment of the present invention.

In the figure, reference numeral 1 denotes a light source means (light source section), which is composed of, for example, a semiconductor laser. A collimator lens 2 as a first optical element converts a divergent light beam (light beam) emitted from the light source means 1 into a convergent light beam. Reference numeral 3 denotes an aperture stop for adjusting the diameter of a passing light beam.

Reference numeral 4 denotes a cylindrical lens as a second optical element, which has a predetermined refracting power only in the sub-scanning direction, and a light beam which has passed through the aperture stop 3 and which will be described later in the sub-scanning section. The image is formed on the deflecting surface 5a of the (deflecting element) 5 as a substantially linear image. Reference numeral 5 is an optical deflector which is composed of, for example, a polygon mirror (rotating polygon mirror) as a deflecting element,
It is rotated at a constant speed in the direction of arrow A in the figure by a driving means (not shown) such as a motor.

Reference numeral 6 denotes an fθ lens (imaging optical system) which is a third optical element and is composed of a single lens made of a plastic material having fθ characteristics, and includes an optical deflector 5 and a photosensitive drum surface 7 as a surface to be scanned. And a light beam based on image information deflected and reflected by the light deflector 5 is imaged on the photosensitive drum surface 7, and the deflection surface of the light deflector 5 The trouble of is corrected.

In the f.theta. Lens 6 of this embodiment, both lens surfaces R1 and R2 of the f.theta. Lens 6 are aspherical toric surfaces in the main scanning plane, and the lens surface of the f.theta. The curvature in the scanning direction is negative on the optical axis, the negative curvature is gradually strengthened as the distance from the optical axis on the optical axis to the peripheral portion of the first lens in the main scanning direction is increased, and the curvature from the peripheral portion of the first lens to the second portion is increased. The negative curvature is gradually weakened as the distance from the lens to the peripheral portion is further decreased, and the negative curvature is gradually weakened as the distance from the second lens peripheral portion to the third lens peripheral portion is further reduced to the third peripheral portion. Curvature is almost zero at the lens periphery
(Refractive power is 0), and the curvature is made positive as the distance from the peripheral portion of the third lens is further increased. In this embodiment, the sub-scanning surface (f
The curvature within a plane (including the optical axis of the θ lens and orthogonal to the main scanning plane) is continuously changed from on-axis to off-axis in the effective portion of the lens.

In this embodiment, the divergent light beam emitted from the semiconductor laser 1 is converted into a convergent light beam by the collimator lens 2, and the light beam (amount of light) is limited by the aperture stop 3 and is incident on the cylindrical lens 4. The light beam incident on the cylindrical lens 4 is emitted as it is in the main scanning section. In the sub-scan section, the light is converged and formed as an almost linear image (a linear image elongated in the main scanning direction) on the deflection surface 5a of the optical deflector 5. The light beam deflected and reflected by the deflecting surface 5a of the optical deflector 5 has an fθ lens 6 having different refracting powers in the main scanning direction and the sub-scanning direction.
Light is guided onto the photosensitive drum surface 7 via the optical deflector 5 by rotating the optical deflector 5 in the direction of arrow A.
The upper part is scanned in the direction of arrow B. As a result, an image is recorded on the photosensitive drum surface 7, which is the recording medium.

In the present embodiment, as described above, the curvature of the lens surface R1 of the fθ lens 6 on the optical deflector 5 side in the sub-scanning direction is made negative on the optical axis, so that the main plane in the sub-scanning direction is further improved. The fθ lens 6 can be located on the image plane side, whereby the fθ lens 6 can be arranged closer to the optical deflector 5 side, and the fθ lens 6 can be made compact. Also, by gradually increasing the negative curvature in the sub-scanning direction from the optical axis in the main scanning direction to the first peripheral portion from the optical axis, an image of the main plane in the sub-scanning direction in the periphery of the optical axis can be obtained. It can be moved to the surface side, whereby the absolute value of the lateral magnification is reduced and the peripheral spot diameter is reduced.

As will be described later, when the fθ lens 6 is manufactured by molding a plastic material, the enlargement of the spot diameter due to the birefringence inside the lens is caused by the peripheral image height from the optical axis as shown in FIG. Since it becomes larger, the spot diameter eventually becomes uniform on the optical axis and at the peripheral image height.

Further, the enlargement of the spot diameter due to the birefringence becomes maximum at the intermediate image height, and the enlargement of the spot diameter is reduced from that point on the outermost axis, so that the peripheral portion of the first lens to the peripheral portion of the second lens as described above. The negative curvature is gradually weakened as the distance from the second lens increases to the third,
The negative curvature is gradually weakened as the distance from the lens to the peripheral part is further decreased so that the curvature becomes approximately 0 at the third lens peripheral part, and as the distance from the third lens peripheral part to the peripheral part further increases. By making the curvature positive, the absolute value of the lateral magnification is increased by further separating the main plane in the sub-scanning direction from the image plane in the peripheral portion. As a result, the spot diameter is made uniform.

Further, in this embodiment, both lens surfaces R1 and R2 of one fθ lens 6 made of a plastic material are aspherical toric surfaces in the main scanning plane, and the f
The curvature of the lens surface R2 of the θ lens 6 on the scanned surface 7 side in the sub-scanning direction is made negative on the optical axis, and is made weaker than the curvature on the optical axis at the outermost axis, whereby the main plane in the sub-scanning direction is formed. Is more off-axis than on-axis and further away from the image plane to suppress changes in lateral magnification on-axis and off-axis, and by setting each element to satisfy the following conditional expressions, the spot diameter Is being made uniform.

[0028]

(Equation 3) However, R _2a , R _2b , R _2c , and R _2d are the radii of curvature in the sub-scanning direction of the lens surface R2 on the scanned surface 7 side of the fθ lens 6, respectively, and are the maximum from the optical axis of the lens effective portion in the main scanning direction. 0%, 30%, 70% in order of the height to the lens effective part through which the off-axis rays pass,
It is at the position of 100%.

Next, the technical meanings of the above conditional expressions (1) and (2) will be described.

Conditional expression (1) relates to the ratio of the change in the curvature R near the axis to the change in the curvature R near the off-axis. If the lower limit of conditional expression (1) is exceeded, the negative curvature R from the optical axis to the vicinity of the optical axis becomes too tight (strong), and the spot diameter becomes too small at the intermediate image height near the axis rather than on the axis. It's not good because it ends up. If the upper limit of conditional expression (1) is exceeded, the negative curvature R from the optical axis to the vicinity of the optical axis becomes too loose (weak), and the spot diameter becomes too large at the intermediate image height near the axis rather than on the axis. It is not good because it will end up. More preferably, conditional expression (1)
It is preferable to set the lower limit value of -0.01 and the upper limit value of 0.1.

Conditional expression (2) is for loosening (weakening) the negative curvature R near the outermost axis. If conditional expression (2) is not satisfied, it becomes difficult to loosen the negative curvature R near the outermost axis, which is not preferable.

In the present embodiment, more preferably, assuming that the maximum effective scanning width on the surface to be scanned 7 is w and the thickness of the fθ lens 6 in the optical axis direction is t, 0.035 <t / w <0.065. (3) To satisfy the following condition.

Conditional expression (3) relates to the ratio between the thickness of the fθ lens 6 and the maximum effective scanning width w on the surface 7 to be scanned,
When the wall thickness is made smaller than the lower limit of conditional expression (3), the birefringence becomes small, and it becomes necessary to make the lateral magnification in the sub-scanning direction more uniform in order to make the spot diameter uniform. This is not good because the change in the curvature of becomes significant and causes a problem in processing accuracy. On the other hand, if the upper limit of conditional expression (3) is exceeded, birefringence becomes large, and it becomes difficult to make a small spot due to an enlarged spot diameter.

In this embodiment, the lens shape of the fθ lens 6 is an aspherical shape which can be expressed by a function up to the 10th order in the main scanning direction, and the subscanning direction is composed of a spherical surface which continuously changes in the image height direction. . The lens shape is, for example, the origin at the intersection of the fθ lens 6 and the optical axis, the optical axis direction is the X axis, the axis orthogonal to the optical axis in the main scanning plane is the Y axis, and the optical axis is orthogonal to the optical axis in the sub scanning plane. When the axis to be set is the Z axis, the bus line direction corresponding to the main scanning direction is

[0035]

(Equation 4) The sagittal direction corresponding to the sub-scanning direction (direction orthogonal to the main scanning direction including the optical axis) can be expressed by

[0036]

(Equation 5) However, when Y ≧ 0, the lens surface R1 is 1 / r ′ = 1 / r + D _2U Y ² + D _4U Y ⁴ + D _6U Y ⁶ + D
_8U Y ⁸ + D _10U Y ^{10 When} Y <0 1 / r '= 1 / r + D _2L Y ² + D _4L Y ⁴ + D _6L Y ⁶ + D
_8L Y ⁸ + D _10L Y ^{10 The} lens surface R2 is r ′ = r (1 + D ₂ Y ² + D ₄ Y ⁴ + D ₆ Y ⁶ + D ₈ Y
⁸ + D ₁₀ Y ¹⁰ ) (where r is the radius of curvature, D ₂ , D ₄ , D ₆ , D ₈ , D ₁₀
Is an aspherical surface coefficient).

2 and 3 respectively show fθ in this embodiment.
FIG. 9 is an explanatory diagram showing a change in curvature of a lens surface R1 of the lens 6 on the optical deflector 5 side and a lens surface R2 of the scanned surface 7 side in the sub-scanning direction. FIG. 4 is an explanatory diagram showing the spot diameter in the sub-scanning direction when the enlargement of the spot diameter due to birefringence is excluded in the present embodiment.

As can be seen from FIG. 4, when the image plane defocus is 0, each spot at the axial image height y = 0, the intermediate image height y = 78.5, and the most off-axis image height y = 107.3. Diameter SP0, SP
Comparing 1 and SP2, SP0>SP2> SP1.

In this embodiment, the measured values of the spot diameter in the sub-scanning direction at each image height due to birefringence are shown in FIG. As can be seen from FIG. 5, the on-axis image height y = 0, the intermediate image height y = ± 78.5, and the most off-axis image height y = ± 1.
Comparing the spot diameters SP0, SP1, SP2 at 07.3, SP0 <SP2 <SP1. As a result, if the variation of the spot diameter due to the non-uniformity of the lateral magnification in the sub-scanning direction (FIG. 4) is added to the non-uniformity of the enlargement of the spot diameter due to birefringence (FIG. 5),
The spot diameter can be made more uniform.

Next, Tables 1 and 2 show a list of each coefficient representing the lens surface shape of the fθ lens 6 and other various characteristics in the present embodiment.

[0041]

[Table 1]

[0042]

[Table 2] Next, a second embodiment of the present invention will be described.

The structure of the scanning optical device in this embodiment is the same as that of the first embodiment shown in FIG. Figure 6,
FIG. 7 is an explanatory view showing the curvature changes in the sub-scanning direction of the lens surface R1 of the f.theta. Lens 16 on the optical deflector 5 side and the lens surface R2 of the scanned surface 7 side in the present embodiment, and FIG. FIG. 7 is an explanatory diagram showing a spot diameter in the sub-scanning direction when the enlargement of the spot diameter due to birefringence is excluded in the embodiment.

As can be seen from FIG. 8, when the image plane defocus is 0, each spot at the axial image height y = 0, the intermediate image height y = 78.5, and the most off-axis image height y = 107.3. Diameter SP0, SP
Comparing 1 and SP2, SP0>SP2> SP1.

Further, in the present embodiment, the measured values of the spot diameter in the sub-scanning direction at each image height due to the birefringence are substantially the same as those in FIG. 5, so that the lateral magnification in the sub-scanning direction is not uniform. By adding the nonuniformity of enlargement of the spot diameter due to birefringence (FIG. 5) to the change of the spot diameter due to (FIG. 8), the spot diameter can be made more uniform at each image height.

Next, Tables 3 and 4 show a list of each coefficient representing the lens surface shape of the f.theta. Lens 16 and other characteristics according to the present embodiment.

[0047]

[Table 3]

[0048]

[Table 4] Table 5 shows the relationship between the conditional expressions (1) to (3) described above and various numerical values in the first and second embodiments.

[0049]

[Table 5]

[0050]

According to the present invention, when the light flux converted by the collimator lens as described above is imaged on the surface to be scanned by the single fθ lens made of the plastic material through the optical deflector, the fθ is changed. By optimizing the lens shape of the lens, it is possible to add the unevenness of the spot diameter enlargement due to the birefringence to the change of the spot diameter due to the unevenness of the lateral magnification in the sub-scanning direction. It is possible to achieve a compact scanning optical device capable of suppressing a change in diameter and suitable for high-definition printing.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part in a main scanning direction and a sub scanning direction according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a change in curvature in a sub-scanning direction of a lens surface R1 of the fθ lens according to the first embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a curvature change in a sub-scanning direction of a lens surface R2 of the fθ lens according to the first embodiment of the present invention.

FIG. 4 is an explanatory diagram showing the spot diameter in the sub-scanning direction when the spot diameter enlargement according to the first embodiment of the present invention is excluded.

FIG. 5 is an explanatory diagram showing measured values of a large amount of spot diameter in the sub-scanning direction at each image height according to the first embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a curvature change in the sub-scanning direction of a lens surface R1 of the fθ lens according to the second embodiment of the present invention.

FIG. 7 is an explanatory diagram showing a curvature change in a sub-scanning direction of a lens surface R2 of the fθ lens according to the second embodiment of the present invention.

FIG. 8 is an explanatory diagram showing a spot diameter in the sub-scanning direction when the spot diameter is not enlarged according to the second embodiment of the present invention.

FIG. 9 is a schematic view of a main part of a conventional scanning optical device.

[Explanation of symbols]

 1 Light Source Means 2 First Optical Element (Collimator Lens) 3 Aperture Stop 4 Second Optical Element (Cylindrical Lens) 5 Deflection Element (Optical Deflector) 6 Third Optical Element (fθ Lens) 7 Scanned Surface ( (Photosensitive drum surface)

Claims (6)

[Claims] 1. A light beam emitted from a light source means is imaged through a first optical element and a second optical element on a deflecting surface of the deflecting element in a linear shape elongated in the main scanning direction, and the deflecting element In a scanning optical device for forming an image of the deflected light beam on the surface to be scanned in a spot shape through the third optical element and scanning the surface to be scanned, the third optical element is composed of a single lens made of a plastic material. Both of the lens surfaces of the single lens are made of an aspherical toric surface in the main scanning plane, and the curvature of the lens surface of the single lens on the deflecting element side in the sub-scanning direction is negative on the optical axis, A scanning optical device characterized in that a negative curvature is gradually increased as the distance from the optical axis to the peripheral portion of the first lens increases in the main scanning direction. 2. The scanning optical device according to claim 1, wherein the negative curvature is gradually weakened as the distance from the first lens peripheral portion to the second lens peripheral portion of the single lens is further increased to the periphery. 3. The negative curvature is gradually weakened as the distance from the second lens peripheral portion to the third lens peripheral portion of the single lens is further increased to the peripheral portion, and the curvature is substantially 0 at the third lens peripheral portion.
The scanning optical device according to claim 2, wherein 4. The scanning optical device according to claim 3, wherein the curvature becomes positive as the distance from the peripheral portion of the third lens of the single lens further increases. 5. The light beam emitted from the light source means is imaged through the first optical element and the second optical element on the deflecting surface of the deflecting element into a linear shape elongated in the main scanning direction. In a scanning optical device for forming a spot-like image of a deflected light beam on a surface to be scanned through a third optical element and scanning the surface to be scanned, the third optical element is a single lens made of a plastic material. Both of the lens surfaces of the single lens are aspherical toric surfaces in the main scanning plane, and the curvature of the lens surface of the single lens on the scanned surface side in the sub-scanning direction is negative on the optical axis. The scanning optical device is characterized in that each element is set to be weaker than the curvature on the optical axis outside the most axis and satisfy the following conditional expression. [Equation 1] However, R _2a , R _2b , R _2c , and R _2d are the radii of curvature in the sub-scanning direction of the lens surface on the scanned surface side of the single lens, respectively, and are the outermost axes from the optical axis of the lens effective portion in the main scanning direction. It is at the positions of 0%, 30%, 70%, and 100% in the order of the height of the lens through which the light rays pass. 6. The maximum effective scanning width on the surface to be scanned is w,
The scanning optical device according to claim 1 or 5, wherein a condition of 0.035 <t / w <0.065 is satisfied, where t is a thickness of the single lens in the optical axis direction.