JP3364558B2 - Single lens fθ lens and optical scanning device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single-lens fθ lens and an optical scanning device.

[0002]

2. Description of the Related Art An optical scanning device of a type in which an optical deflector having a deflecting / reflecting surface deflects a light beam at a constant angular velocity and a f.theta. It is widely known in connection with various image forming apparatuses such as a digital copying machine and the like.

In such an optical scanning device, in order to prevent the position of the light spot for each optical scanning from fluctuating in the sub-scanning direction due to "face tilt" in the optical deflector, the f.theta. At any position on the optical path from the light source to the surface to be scanned, which corresponds to the direction parallel to the sub-scanning direction), the position of the deflecting / reflecting surface and the position of the surface to be scanned are in a geometrical-optical conjugate relationship, and the optical deflection is performed. A long line image in the direction corresponding to the main scanning direction (a direction that corresponds to the main scanning direction at an arbitrary position on the optical path from the light source to the surface to be scanned) near the deflective reflection surface. Imaging is performed.

In order to effectively perform such "surface tilt correction", the curvature of field in the sub-scanning direction of the fθ lens must be corrected well.

That is, the point on the field curvature in the sub-scanning direction is the "image point in the sub-scanning corresponding direction" of the line image,
Although this image point does not fluctuate in the sub-scanning corresponding direction, if the field curvature in the sub-scanning direction is large, the distance from the image point to the surface to be scanned also increases, and the deflected light beam is deflected by the surface tilt. When "blurred", the position of the light spot on the surface to be scanned also changes in the sub-scanning direction, and the effect of surface tilt correction is halved.

Therefore, when an fθ lens in which the curvature of field in the sub-scanning direction is not sufficiently corrected is used, the tolerance for tilting of the deflective reflection surface of the optical deflector becomes small, and the deflective reflection surface of the optical deflector is reduced. There is no choice but to use an expensive one that realizes the parallelism with respect to the rotation axis with high accuracy.

The field curvature of the f.theta. Lens in the sub-scanning direction also causes the spot diameter of the light spot in the sub-scanning direction to fluctuate depending on the image height in the main scanning direction, which causes deterioration of the image quality of a recorded image due to optical scanning. Becomes

From the above viewpoints, it is desirable that the fθ lens has its field curvature in the main and sub-scanning directions corrected, and particularly the field curvature in the sub-scanning direction corrected. It is desirable that the “fθ characteristic” of the fθ lens is well corrected.

On the other hand, with the intention of reducing the size and cost of the optical scanning device and increasing the speed by improving the light transmission efficiency,
It has been proposed to use an fθ lens as a single lens (for example, JP-A-7-174998).

[0010]

SUMMARY OF THE INVENTION In view of the above-mentioned circumstances, it is an object of the present invention to reduce the cost in an optical scanning device by increasing the tolerance of surface tilt for the optical deflector.

According to the present invention, in a single-lens fθ lens for cost reduction, miniaturization, and speedup in an optical scanning device, the field curvature in the main / sub scanning directions and the fθ characteristic can be corrected well. Is an issue.

[0012]

The fθ lens of the present invention is an optical deflector having a deflecting / reflecting surface in the vicinity of an image forming position of a line image of a light beam formed into a line image long in the main scanning corresponding direction. The fθ lens is a “single-lens configuration”, which is used in an “optical scanning device that deflects light at a constant angular velocity by using the fθ lens and collects it as a light spot on the surface to be scanned by the fθ lens to perform uniform optical scanning. .

In the fθ lens of the present invention, the surface on the optical deflector side is the "first surface" and the surface on the scanned surface side is the "second surface".

Further, an "ideally deflected light beam deflected by the optical deflector", that is, a virtual plane on which the principal ray of the deflected light beam deflected without being affected by the surface tilt of the deflection reflection surface is swept is defined. It is called a "deflection scanning plane".

The single-lens fθ lens described in claim 1 has the following features.

That is, the first surface has a "surface shape obtained by rotating a non-circular arc curve around the rotation axis".

The non-circular arc curve is "symmetrical with respect to the optical axis" set in the deflecting reflection surface, and when the Z coordinate is taken in conformity with the optical axis and the Y coordinate is taken orthogonal to the optical axis, the non-circular curve is on the optical axis. Let R _{m be} the radius of curvature of R, K, A, B, C and D be constants, and Z = Y ² / [R _m + √ {R _m ² − (1 + K) ² }] + A · Y ⁴ + B · Y ⁶ + C ^{· Y 8 + D · Y 10} +. ． ． It is a curve represented by (1). The right side of the equation (1) can further add a term of 12th power of Y or more, if necessary.

The "non-arc curve" is a "convex arc" with the paraxial portion facing the optical deflector side.

This non-circular curve is rotated about the axis of rotation to form the shape of the first surface, but the "axis of rotation" is "the optical axis is orthogonal to the optical axis in the deflecting and reflecting surface, and the light is deflected from the first surface. It is a certain distance away from the container side. "

The second surface is "a circular arc whose shape in the deflection scanning surface is convex toward the surface to be scanned".

A plane which passes through the center of this arc (which is a point on the optical axis) and is orthogonal to the deflecting / reflecting surface is called a "deflection orthogonal surface".

When the angle between the orthogonal plane of deflection and the optical axis is ξ, the radius of curvature of the second surface in the orthogonal plane of deflection is “angle: ξ so as to satisfactorily correct the field curvature in the sub-scanning direction. Is changing with the changes in.

That is, the single-lens fθ lens according to the first aspect is
The second surface is "the function to correct the field curvature in the sub-scanning direction"
have.

The single-lens fθ lens described in claim 2 is the single-lens fθ lens described in claim 1, wherein paraxial curvature radii of the first and second surfaces in the deflection scanning plane are R _m1 and R, respectively.
_{When m2} , these are characterized by satisfying the condition: (I) −0.7 <R _m2 / R _m1 <0.

A single-lens fθ lens described in claim 3 is the single-lens fθ lens described in claim 1 or 2, wherein the “curvature radius in the plane orthogonal to the deflection” of the second surface is at an angle ξ = 0. and R _s2, angle ranging from xi] = 0 in the periphery: when the maximum value and the minimum value of the radius of curvature of the deflecting perpendicular plane respectively variable region of xi] and R _s2 max, R _s2 min, these R _s2 and R
_s2 max and R _s2 min satisfy the condition: (II) | (R _s2 max-R _s2 min) / R _s2 | <0.1.

From the above description, "R _s2 " is "the radius of curvature of the second surface in the plane including the optical axis and orthogonal to the deflection scanning surface".
It will be easily understood that

The single-lens fθ lens described in claim 4 has the following features.

That is, the first surface has a "non-arcuate curved shape whose paraxial portion is a convex arc whose paraxial portion faces the optical deflector side". The non-circular arc shape is a curve represented by the above formula (1).

A plane parallel to the optical axis and orthogonal to the deflection scanning plane is called an "orthogonal deflection plane".

Assuming that the distance from the optical axis of the orthogonal deflection surface is η, the "radius of curvature of the first surface in the orthogonal deflection surface" at each position of η is "to make the field curvature in the sub-scanning direction favorable. It changes according to the change of distance: η so as to correct it. "

The second surface has "a toroidal surface formed by a rotation axis orthogonal to the deflection scanning surface and having a convex surface shape on the surface to be scanned".

That is, the single-lens fθ lens described in claim 4 is
The first surface is "a function to correct field curvature in the sub-scanning direction"
have.

The single-lens fθ lens described in claim 5 is the single-lens fθ lens described in claim 4, wherein paraxial radii of curvature of the first and second surfaces in the deflection scanning plane are R _m1 and R, respectively.
_{When m2} , these are characterized by satisfying the condition: (III) −0.7 <R _m2 / R _m1 <0.

A single-lens fθ lens according to a sixth aspect is the single-lens fθ lens according to the fourth or fifth aspect, wherein R _{s1 is the} radius of curvature of the first surface in the orthogonal deflection plane at the distance η = 0. (Curvature radius of the first surface in a plane including the optical axis and orthogonal to the deflection scanning plane), the distance from η = 0 to the peripheral portion: the maximum value of the radius of curvature in the deflection orthogonal plane within the range of η and When the minimum values are R _s1 max and R _s1 min, respectively,
R _s1 and R _s1 max, R _s1 min satisfy the condition: (IV) | (R _s1 max-R _s1 min) / R _s1 | <0.1.

The single-lens fθ lens of the present invention can be made of glass, but any of the single-lens fθ lenses described in claims 1 to 6 can be manufactured as a “molded product made of plastic” ( Claim 7).

The above conditions (I) and (III) are conditions for satisfactorily correcting the "field curvature and fθ characteristic in the main scanning direction". If the upper limit is exceeded, the fθ characteristic tends to be undercorrected, and the lower limit is exceeded. If it exceeds, the field curvature in the main scanning direction tends to be overcorrected, and it becomes difficult to realize good fθ characteristics and good field curvature in the main scanning direction.

The above conditions (II) and (IV) relate to the "shape of the single-lens fθ lens". If the condition (II) is out of the range, the “change in power in the sub-scanning corresponding direction” on the second surface of the single lens fθ lens according to claim 1 is large, and the condition (I
If it deviates from the range of V), the “change in power in the sub-scanning corresponding direction” on the first surface of the single-lens fθ lens according to claim 2 is large, and a positional error in the main-scanning direction occurs when the single-lens fθ lens is assembled. More susceptible.

When the single-lens fθ lens is formed as a "molded product made of plastic" (claim 7), the above conditions are satisfied.
In the range of (II) and (IV), shape errors such as “sink” and “waviness” are likely to occur in the lens during injection molding, and these shape errors have an adverse effect on the curvature of field particularly in the sub-scanning direction. give.

In each of the single-lens fθ lenses described in claims 1 to 6, the first surface has negative power in the sub-scanning corresponding direction,
Since the second surface has a positive power, when the single-lens fθ lens is made of plastic, when the lens is deformed due to temperature change, the power change on the first surface is effectively offset by the power change on the second surface. To be done.

An optical scanning device according to an eighth aspect of the present invention is that "a light beam imaged in a long line image in the main scanning direction is made to have an equal angular velocity by an optical deflector having a deflection reflection surface near the image forming position of the line image. An optical scanning device for performing constant-speed optical scanning by deflecting the light beam and converging it as a light spot on the surface to be scanned by the fθ lens, wherein the fθ lens is any one of claims 1 to 7. The single-lens fθ lens is used.

This optical scanning device includes a light source (LD or L
ED, etc.), a coupling lens for coupling the light flux from this light source, and a line image imaging in which the coupled light flux is converged in the sub-scanning corresponding medical direction to form a long line image in the main scanning corresponding direction. It has an optical system (convex cylinder lens and concave cylinder mirror).

The optical deflector is a polygon mirror, a rotary dihedral mirror,
Rotating single-sided mirror (causing tilting of the rotating shaft to cause surface tilt)
Can be used.

In the optical scanning device of the eighth aspect, the "light beam incident on the optical deflector" may be a "parallel light beam in the main scanning corresponding direction" (claim 9) or "weak convergence in the main scanning corresponding direction". Or weakly divergent light flux "(claim 10).

[0044]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically shows an embodiment of an optical scanning device according to the present invention.

The divergent light beam from the light source LD1 is coupled by the coupling lens 2 to be converted into a parallel light beam or a weak divergent or convergent light beam, and by the cylinder lens 3 which is a line image forming optical system. The light is condensed in the sub-scanning corresponding direction (direction orthogonal to the drawing), the optical path is bent by the mirror 4, enters the deflecting reflection surface 6 of the polygon mirror 5 which is an optical deflector, and is near the deflection reflection surface 6. The image is formed as a "long line image in the main scanning corresponding direction".

The mirror 4 can be omitted depending on the layout of the optical system.

The light flux reflected by the deflecting / reflecting surface 6 is deflected at a constant angular velocity as the polygon mirror 5 rotates at a constant velocity, and a single lens f
The θ lens 7 collects a light spot on the surface 8 to be scanned and scans the surface 8 to be scanned at a constant speed.

FIG. 2 is a diagram for explaining the “shape of the first surface” in the single-lens fθ lens described in claim 1.

The "YZ plane" determined by the coordinate axes X, Y, Z shown in the figure is the "deflection scanning plane", and the left side of the figure is the optical deflector side. The Z axis coincides with the lens optical axis.

Non-circular curve: Z (Y) is set in the deflection scanning plane, and its specific shape is specified by the above equation (1), but the paraxial portion, that is, the portion near the Z axis is "light". It is a convex arc (curvature radius: R _m1 ) facing the deflector side (left side in the figure).

The first surface has a shape obtained by rotating the non-circular curve: Z (Y) around the rotation axis RAX. The rotation axis RAX is the optical axis (Z
A predetermined distance from the first surface to the optical deflector side: R _s1
Just away.

FIG. 3 is a diagram for explaining the “shape of the second surface” in the single-lens fθ lens described in claim 1. As in FIG. 2, the "YZ plane" is a deflection scanning plane,
The right side of the figure is the scanned surface side. The Z axis coincides with the lens optical axis.

The second surface has a shape of "an arc (curvature radius: R _m2 ), which is convex on the surface to be scanned, in the deflection scanning surface".

When the angle formed by the optical axis (Z axis) and the "deflection orthogonal plane" which is a plane orthogonal to the center of this arc and passing through Q and orthogonal to the deflection reflection surface is ξ, the second plane in the plane orthogonal to the deflection. Radius of curvature: R _s2 (ξ) changes in accordance with the change in angle: ξ so as to “correctly correct the field curvature in the sub-scanning direction”.

FIG. 4 is a diagram for explaining the shape of the “first surface” of the single-lens fθ lens described in claim 4.

The "YZ plane" is the "deflection scanning plane", and the left side of the drawing is the optical deflector side. The Z axis coincides with the lens optical axis.

The first surface has a shape in the deflection scanning plane which is "a non-arcuate curve shape in which a paraxial portion is a convex arc whose direction is toward the optical deflector: Z (Y)".

When the distance from the optical axis of the "orthogonal deflection surface" which is parallel to the optical axis (Z axis) and is orthogonal to the deflection scanning plane is Y (η in the above description), it is within the orthogonal deflection surface of the first surface. Curvature radius: R
_s1 (Y) "changes in accordance with the change in the distance: Y so as to satisfactorily correct the field curvature in the sub-scanning direction".

FIG. 5 is a diagram for explaining the shape of the second surface of the single-lens fθ lens described in claim 4.

The "YZ plane" is a "deflection scanning plane", and the right side of the drawing is the scanned surface side. The Z axis coincides with the lens optical axis.

The second surface is a "toroidal surface" formed by a rotation axis orthogonal to the deflection scanning surface, and has a surface shape which is convex toward the surface to be scanned. That is, the shape in the deflection scanning plane (YZ plane) is an arc shape (curvature radius:
R _m2 ), and in the "ZX plane", the radius of curvature: R
_A surface obtained by rotating a circle having _s2 around a rotation axis (rotation axis orthogonal to the deflecting reflection surface) passing through the center of the arc shape: P and parallel to the X direction (radius “R _m2 −R _s2 ”). The shape.

[0062]

[Examples] Four specific examples will be given below.

In specifying the shape of the single-lens fθ lens, as shown in FIG. 1, the radius of curvature on the optical axis of the first surface is set to "R _m1 " in the main scanning corresponding direction and "R _m1 " in the sub scanning corresponding direction.
_s1 ", he occupied a radius of curvature on the optical axis of the second surface in the main scanning corresponding direction" R _{m @ 2} ", per sub-scanning direction is" R _s2 ".

Regarding the "non-arc shape", the above "R _m1 " or "R _m2 " is given as the value of "R _m " in the expression (1) according to the expression (1), and the constant: K, A,
The shape is specified by giving B, C, and D.

Regarding the shape of the second surface in the single-lens fθ lens of claim 1, the above-mentioned “R _m2 ”, which is the radius of curvature of the arc in the deflection scanning plane, is given, and the “R” explained with reference to FIG. _s2 (ξ) ”, the angle of view: θ instead of the variable: ξ
Multiple values are given to "repeatedly".

[0066] With respect to the first surface shape of the single lens fθ lens according to claim 4, with the "R _m1" is an arc of a radius of curvature of the paraxial portion of the deflection scanning plane constant: K, A, B,
C and D are given, the non-circular arc shape according to the equation (1) is specified, and “R _s1 (Y)” described with reference to FIG.
Multiple values for angle of view: θ instead of Y
Give to.

The thickness of the single-lens fθ lens on the optical axis is D ₁ , the distance from the origin of deflection by the deflecting and reflecting surface (image forming position of the line image) to the first surface is D ₀ , and the distance from the second surface is The distance on the optical axis to reach the scanning plane is D ₂ . The refractive index (for wavelength: 780 nm) of the material of the single lens fθ lens is represented by N.

In each of the examples, the "oscillation wavelength" of the LD, which is the light source, is 780 nm, and the "maximum half angle of view" of optical scanning is 42 degrees.

Example 1 Example 1 is an example of the single-lens fθ lens described in claims 1, 2, 3 and 7, and an optical scanning device as a form of its use is the optical scanning device described in claim 9. An example.

That is, in this embodiment, the light beam incident on the deflecting / reflecting surface of the optical deflector (polygon mirror) is a "parallel light beam" in the main scanning corresponding direction, and the image height of the light spot is small in the sub scanning corresponding direction. An image is formed as a long line image in the main scanning corresponding direction at the position of the deflecting reflection surface when it becomes zero.

I R _mi R _si Di N 0 51.871 1 461.302 −73.845 21.754 1.53664 2 −97.244 R _s2 (ξ) 149.0070.

First surface: non-circular curve K = 35.9369, A = −2.20457 × 10 to ⁷ , B = 2.73241 × 10 to ¹¹ , C = −4.995866 × 10 to ¹⁵ , D = 1.60481 × 10 to ¹⁹ Second surface: R _s2 (ξ) Half angle of view: θ (degree) 0 8.4 16.8 25.2 33.6 42 R _s2 (ξ) -20.645 -20.669 -20.723- 20.784 -20.776 -20.572 The value of the parameter of the conditional expression R _m2 / R _m1 = 0.211, | (R _s2 max-R _s2 min) / R _s2 | = 0.010.

Example 2 Example 2 is an example of the single-lens fθ lens described in claims 1, 2, 3 and 7, and an optical scanning device as a form of its use is the optical scanning device described in claim 10. An example.

That is, in this embodiment, the light beam incident on the deflecting / reflecting surface of the optical deflector (polygon mirror) is a "converging light beam" in the main scanning corresponding direction, and the image height of the light spot is small in the sub scanning corresponding direction. An image is formed as a long line image in the main scanning corresponding direction at the position of the deflecting reflection surface when it becomes zero.

The convergent light flux in the main scanning corresponding direction is
"Natural condensing point" that naturally condenses in the main scanning direction
, The distance from the origin of deflection by the polygon mirror is "2
530.9 ".

I R _mi R _si Di N 0 50.802 1 449.713 −66.236 30.0 1.536664 2 −100.929 R _s2 (ξ) 143.465.

First surface: non-circular curve K = 33.7540, A = −2.20457 × 10 to ⁷ , B = 2.73241 × 10 to ¹¹ , C = −4.995866 × 10 to ¹⁵ , D = 1.60481 × 10 to ¹⁹ Second surface: R _s2 (ξ) Half angle of view: θ (degree) 0 8.4 16.8 25.2 33.6 42 R _s2 (ξ) -21.484 -21.508 -21.562- 21.624 -21.614 -21.408 value of conditional equation parameters _{R m2 / R m1 = 0.224,} | (R s2 max-R s2 min) / R s2 | = 0.010.

In the first and second embodiments, R _s2 (ξ) is smoothly continuous between the respective half angle-of-view values.

Example 3 Example 3 is an example of the single-lens fθ lens described in claims 4, 5, 6 and 7, and an optical scanning device as a form of its use is the optical scanning device described in claim 9. An example.

That is, in this embodiment, the light beam incident on the deflecting / reflecting surface of the optical deflector (polygon mirror) is a "parallel light beam" in the main scanning corresponding direction, and the image height of the light spot is small in the sub scanning corresponding direction. An image is formed as a long line image in the main scanning corresponding direction at the position of the deflecting reflection surface when it becomes zero.

I R _mi R _si Di N 0 51.871 1 461.302 R _s1 (Y) 21.754 1.53664 2 -97.244 -20.645 149.070.

First surface: non-circular curve K = 35.9369, A = −2.20457 × 10 to ⁷ , B = 2.73241 × 10 to ¹¹ , C = −4.995866 × 10 to ¹⁵ , D = 1.60481 × 10 to ¹⁹ First surface: R _s1 (Y) Half angle of view: θ (degree) 0 8.4 16.8 25.2 33.6 42 R _s1 (Y) -73.845 -73.446 -72.160- 72.160 -71.722 -74.811 Value of parameter of conditional expression R _m2 / R _m1 = 0.211, | (R _s2 max-R _s2 min) / R _s2 | = 0.036.

Example 4 Example 4 is an example of the single-lens fθ lens described in claims 4, 5, 6 and 7, and an optical scanning device as a form of its use is the optical scanning device described in claim 10. An example.

That is, in this embodiment, the light beam incident on the deflecting / reflecting surface of the optical deflector (polygon mirror) is a "converging light beam" in the main scanning corresponding direction, and the image height of the light spot is small in the sub scanning corresponding direction. An image is formed as a long line image in the main scanning corresponding direction at the position of the deflecting reflection surface when it becomes zero.

The convergent light flux in the main scanning corresponding direction is
"Natural condensing point" that naturally condenses in the main scanning direction
, The distance from the origin of deflection by the polygon mirror is "2
530.9 ".

I R _mi R _si Di N 0 50.808 1 449.713 R _s1 (Y) 30.0 1.53662 −100.929 −21.489 143.465.

First surface: non-circular curve K = 33.7540, A = -2.20457 × 10 to ⁷ , B = 2.73241 × 10 to ¹¹ , C = −4.995866 × 10 to ¹⁵ , D = 1.60481 × 10 to ¹⁹ First surface: R _s1 (Y) Half angle of view: θ (degree) 0 8.4 6.8 25.2 33.6 42 R _s1 (Y) -66.260 -65.886 -64.686- 63.773 64.285 -67.216 value of conditional equation parameters _{R m2 / R m1 = 0.224,} | (R s2 max-R s2 min) / R s2 | = 0.052.

In the third and fourth embodiments, R _s1 (Y) is smoothly continuous between the above half field angle values.

FIGS. 6 to 9 show, in order, the field curvature and f.theta. Characteristics of Examples 1 to 4 above. In the figure of field curvature, the broken line indicates "field curvature in the main scanning direction", and the solid line indicates "field curvature in the sub scanning direction".

The "f.theta. Characteristics" are good in Examples 1 to 4, and the field curvature in the main and sub-scanning directions is also corrected very well.

[0091]

As described above, according to the present invention, the fθ characteristic of the single-lens fθ lens and the field curvature in the main / sub scanning directions can be corrected very well.

By using the single-lens fθ lens having good characteristics as described above, the tolerance for surface tilt becomes large, so that the cost of the optical scanning device can be reduced by using an inexpensive optical deflector. , Miniaturization of optical scanning device,
It is possible to increase the speed of light scanning with the improvement of light transmission efficiency.

[Brief description of drawings]

FIG. 1 is a diagram for explaining one embodiment of an optical scanning device of the present invention.

FIG. 2 is a diagram for explaining the shape of the first surface of the single-lens fθ lens according to claim 1;

FIG. 3 is a diagram for explaining the shape of the second surface of the single-lens fθ lens according to claim 1;

FIG. 4 is a diagram for explaining the shape of the first surface of the single-lens fθ lens according to claim 4;

FIG. 5 is a diagram for explaining the shape of the second surface of the single-lens fθ lens according to claim 4;

FIG. 6 is a diagram of field curvature and fθ characteristics for the first embodiment.

FIG. 7 is a diagram of field curvature and fθ characteristics relating to Example 2;

FIG. 8 is a diagram of field curvature and fθ characteristics for the third embodiment.

FIG. 9 is a diagram of field curvature and fθ characteristic relating to Example 4;

[Explanation of symbols]

1 Light source (LD) 2 coupling lens 3 cylinder lens 4 mirror 5 polygon mirror 6 Deflection reflective surface 7 Single lens fθ lens 8 Scanned surface

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-174998 (JP, A) JP-A-8-190062 (JP, A) JP-A-8-248308 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G02B 13/00 G02B 26/10

Claims (10)

(57) [Claims] 1. A light beam formed into a long line image in the main scanning direction is deflected at an equal angular velocity by an optical deflector having a deflecting / reflecting surface in the vicinity of the image forming position of the line image. An fθ lens used in an optical scanning device that converges a light spot on a surface to be scanned and performs optical scanning at a constant speed, has a single-lens configuration, and the first surface on the optical deflector side is , In the deflection scanning plane, a non-circular curve whose paraxial portion is a convex arc facing the optical deflector side is orthogonal to the optical axis in the deflection reflecting surface, and is separated from the first surface by a predetermined distance to the optical deflector side. The second surface, which is a surface on the scanned surface side, is a circular arc whose shape in the deflection scanning surface is convex on the scanned surface side. Let ξ be the angle formed by the optical axis and the deflection orthogonal plane that is a plane that passes through the center of the arc and is orthogonal to the deflection reflection surface. The single lens fθ is characterized in that the radius of curvature of the second surface in the plane orthogonal to the deflection is changed according to the change of the angle ξ so as to favorably correct the field curvature in the sub-scanning direction. lens. 2. The single-lens fθ lens according to claim 1, wherein paraxial radiuses of curvature of the first and second surfaces in the deflection scanning plane are R _m1 and R _m2 , respectively. I) A single-lens fθ lens that satisfies −0.7 <R _m2 / R _m1 <0. 3. The single-lens f.theta. Lens according to claim 1 or 2, wherein the radius of curvature of the second surface in the plane orthogonal to the deflection at the angle ξ = 0 is R _s2, and ξ = 0 to the peripheral portion. square: R _s2 maximum value of the radius of curvature of the deflecting perpendicular plane with the variable region of ξ and the minimum value respectively max, when the R _s2 min, R _s2 and R _s2
A single-lens fθ lens characterized by satisfying the conditions of max and R _s2 min: (II) | (R _s2 max-R _s2 min) / R _s2 | <0.1. 4. A light beam formed into a line image long in the main scanning direction is deflected at an equal angular velocity by an optical deflector having a deflecting / reflecting surface in the vicinity of the image forming position of the line image. An fθ lens used in an optical scanning device that converges a light spot on a surface to be scanned and performs uniform optical scanning, has a single-lens configuration, and the first surface on the optical deflector side is , The shape in the deflection scanning plane is a non-circular arc shape whose paraxial part is a convex arc facing the optical deflector side, and the shape of the orthogonal deflection plane parallel to the optical axis and orthogonal to the deflection scanning plane and the optical axis When the distance is η, the radius of curvature in the orthogonal deflection surface at each position of η is changed according to the change of the above distance: η so as to satisfactorily correct the field curvature in the sub-scanning direction. The second surface, which is the surface to be scanned, is a toroidal surface formed by a rotation axis orthogonal to the deflection scanning surface. Thus, the single-lens fθ lens has a surface shape that is convex on the surface to be scanned. 5. The fθ lens according to claim 4, wherein when paraxial radiuses of curvature of the first and second surfaces in the deflection scanning plane are R _m1 and R _m2 , respectively, these conditions are: (III) A single-lens fθ lens characterized by satisfying −0.7 <R _m2 / R _m1 <0. 6. The single-lens fθ lens according to claim 4 or 5, wherein the radius of curvature of the first surface in the orthogonal deflection plane at a distance η = 0 is R _s1, and η = 0 extends to the peripheral portion. Distance: η
When R _s1 max and R _s1 min are the maximum and minimum values of the radius of curvature in the plane orthogonal to the deflection, respectively, R _s1 and R _s1 max, R _s1 min _satisfy the condition: (IV) | (R _s1 max-R _s1 min) / R _s1 | <0.1, a single-lens fθ lens. 7. The single-lens fθ lens according to claim 1, 2 or 3 or 4 or 5 or 6, wherein the single-lens fθ lens is manufactured as a molded product of plastic. 8. A light beam imaged in a long line image in the main scanning direction is deflected at an equal angular velocity by an optical deflector having a deflecting / reflecting surface in the vicinity of the image forming position of the line image, and the fθ lens is used. An optical scanning device for converging a light spot on a surface to be scanned to perform optical scanning at a constant speed, wherein the single-lens fθ lens according to any one of claims 1 to 7 is used as an fθ lens. An optical scanning device. 9. The optical scanning device according to claim 8, wherein the light beam incident on the optical deflector is a parallel light beam in the main scanning corresponding direction. 10. The optical scanning device according to claim 8, wherein the light beam incident on the optical deflector is in the main scanning corresponding direction.
An optical scanning device, which is a light beam having a weak convergence or a weak divergence.