JPH09145993A - Scanning lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the degree of freedom of design for aberration correction and compensate aberrations with good balance by forming three lenses as anamorphic lenses. SOLUTION: The scanning lens 5 is constituted by arranging a 1st lens 5a as a negative anamorphic lens which has its negative power larger in a vertical scanning direction than in a horizontal scanning direction, a 2nd lens 5b as an anamorphic lens which has its positive power set larger in the horizontal scanning direction than in the vertical scanning directions, and a 3rd lens 5c as an anamorphic lens which has its positive power set larger in the vertical scanning direction than in the horizontal scanning direction in order from the side of a polygon mirror 4 to the side of a scanned surface 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning lens used as an imaging lens of a scanning optical system such as a laser printer.

[0002]

2. Description of the Related Art A conventional scanning lens is disclosed in, for example, Japanese Patent Laid-Open No. 62-62.
No. 172317. The scanning lens disclosed in this publication is composed of first, second, and third lenses arranged in order from the polygon mirror side, which is a deflector.

The first lens is a negative anamorphic lens in which the negative power in the sub scanning direction is stronger in the main scanning direction. The second lens is a positive lens whose both surfaces are spherical.
The third lens is a positive anamorphic lens whose positive power in the sub scanning direction is stronger in the main scanning direction.

The first lens is a spherical surface on the mirror side, the surface to be scanned is a cylindrical surface having a negative power in the sub scanning direction, and the third lens is a cylindrical surface having a negative power on the mirror side in the sub scanning direction. The surface to be scanned is a toric surface whose positive power in the sub-scanning direction is larger than that in the main scanning direction.

[0005]

However, in the scanning lens described in the above publication, the field curvature in the sub-scanning direction is 5 mm or more with respect to the scanning width of 600 mm, so that the sub-scanning occurs. It is not suitable for systems that require high drawing accuracy because the spot diameter changes greatly in the direction.

The present invention has been made in view of the above-described problems of the prior art, and it is an object of the present invention to provide a scanning lens capable of suppressing the ratio of the amount of field curvature in the sub-scanning direction to the scanning width. To aim.

[0007]

A scanning lens according to the present invention comprises a first lens which is a negative anamorphic lens in which the negative power in the sub-scanning direction is set stronger than the negative power in the main scanning direction, and the main scanning A second lens which is a positive anamorphic lens whose positive power in the sub-scanning direction is larger than the positive power in both sub-scanning directions, and a second lens which is a positive anamorphic lens whose positive power in the sub-scanning direction is larger than the positive power in the main scanning direction The three lenses are arranged in this order from the deflector side to the scanning target surface side.

The difference from the scanning lens disclosed in Japanese Unexamined Patent Publication No. 62-172317 is that the lens of the publication uses a spherical lens as the second lens, whereas the scanning lens of the present invention uses the second lens. The point is that a positive anamorphic lens whose positive power in the main scanning direction is larger than that in the sub scanning direction is used. By setting the power of the second lens in this way, it is possible to suppress the curvature of field in the sub-scanning direction to be small.

That is, in the main scanning direction, by setting the positive power of the second lens high, the positive power in the main scanning direction that the third lens bears is reduced, and at the same time the main scanning direction with respect to the third lens is reduced. Since it is possible to reduce the incident height of the light, it is possible to reduce the curvature of field in the sub-scanning direction that is generated by the anamorphic third lens.

Further, in the sub-scanning direction, by suppressing the positive power of the second lens to be low, the negative power of the first lens and the positive power of the second and third lenses can be balanced. As a result, the field curvature in the sub-scanning direction can be suppressed. When the power balance between the first lens and the second and third lenses is lost, field curvature in the sub-scanning direction occurs.

Further, it is desirable that the surface of the third lens on the deflector side is a spherical surface having a positive power. Since the third lens is an anamorphic lens, when the surface on the deflector side is a spherical surface, the surface on the scanning target surface side is a toric surface. By making the surface of the third lens on the deflector side have a positive power, the incident height on the toric surface, which is the other surface of the third lens, becomes low, and the surface of the third lens on the deflector side is reduced. Since the curvature of the toric surface in the main scanning direction can be set more gently than in the case where is a flat surface or a cylindrical surface, it is possible to appropriately set the balance of the toric surface in the main and sub-scanning directions. The radius of curvature ry of the third lens on the surface to be scanned in the main scanning direction and the radius of curvature rz in the sub-scanning direction preferably satisfy the relationship of 4 <ry / rz <5.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a scanning lens according to the present invention will be described below. 1A and 1B show a configuration of a scanning optical system including a scanning lens of the present invention, FIG. 1A is an explanatory diagram in a main scanning direction, and FIG. 1B is an explanatory diagram in a sub scanning direction.

The scanning optical system includes a semiconductor laser 1 which is a light source, a collimator lens 2 which makes a divergent light beam emitted from the semiconductor laser 1 into a parallel light beam, a cylindrical lens 3 having a positive power only in the sub-scanning direction, and a cylindrical lens. It is composed of a polygon mirror 4 that reflects and deflects a light beam incident through a lens, and a three-element fθ lens (scanning lens) 5 that converges the deflected light beam on a scan target surface 6 to form a spot. It

The collimator lens 2 is a rotationally symmetric lens having positive power in both the main and sub scanning directions. The cylindrical lens 3 temporarily forms an image of the light beam in the sub-scanning direction in the vicinity of the reflecting surface of the polygon mirror in order to avoid the influence of the surface tilt error of the polygon mirror 4.

The fθ lens 5 includes, in order from the polygon mirror 4 side, a first lens 5a which is a negative anamorphic lens in which the negative power in the sub-scanning direction is set stronger than the negative power in the main scanning direction, and the main scanning direction. The second lens 5b, which is a positive anamorphic lens whose positive power is larger than the positive power in the sub-scanning direction, and the positive anamorphic lens whose positive power in the sub-scanning direction is larger than the positive power in the main scanning direction. The three lenses 5c are arranged in this order.

In the first lens 5a, the surface on the polygon mirror 4 side is a spherical surface having negative power, and the surface on the scanning target surface 6 side is a cylindrical surface having negative power in the sub scanning direction, or in the sub scanning direction. It is formed as a negative toric surface in which the negative power is larger than the negative power in the main scanning direction.

The second lens 5b is a spherical surface having a negative power on the polygon mirror 4 side, or a toric surface having a negative power in the sub-scanning direction larger than the negative power in the main scanning direction, and the surface to be scanned side. Is formed as a toric surface whose positive power in the main scanning direction is larger than that in the sub scanning direction.

The third lens 5c is a spherical surface having a positive power on the polygon mirror 4 side, or a toric surface having a positive power in the sub-scanning direction larger than the positive power in the main scanning direction, and the surface to be scanned side. Is formed as a toric surface in which the positive power in the sub-scanning direction is larger than the positive power in the main-scanning direction.

[0019]

EXAMPLES Next, three concrete examples of numerical constitution of the reflection type scanning optical system according to the present invention will be explained.

[0020]

Example 1 FIG. 1 shows a scanning optical system according to Example 1. Table 1 shows specific numerical configurations of the first embodiment. In the table, K is the scanning coefficient, ry is the radius of curvature in the main scanning direction, rz is the radius of curvature in the sub-scanning direction (omitted for the surface to be rotated), d is the lens thickness on the optical axis or the air gap, and n780 is the wavelength. 780nm
Is the refractive index at.

Surface numbers 1 and 2 are cylindrical lenses 3,
The surface number 3 is the polygon mirror 4, the surface numbers 4 and 5 are the first lens 5a of the fθ lens 5, and the surface numbers 6 and 7 are the second lens 5.
b, surface numbers 8 and 9 indicate the third lens 5c.

[0022]

[Table 1]

FIG. 2 shows aberrations of the optical system of Example 1,
(A) is a linearity error which is an error between an ideal image height and an actual image point with respect to a deflection angle θ obtained by Kθ, and (B) is a field curvature (focus shift) in the main scanning plane, and M is the main scanning direction. Imaging position of
S indicates the image forming position in the sub-scanning direction. The vertical axis of each figure is the image height,
The horizontal axis represents the amount of aberration. All units are mm.

[0024]

Second Embodiment Table 2 shows a specific numerical configuration of the second embodiment. The relationship between the surface number and the optical element is the same as that in the first embodiment. FIG. 3 shows aberrations of the optical system according to the second embodiment.

[0025]

[Table 2]

[0026]

Third Embodiment Table 3 shows a specific numerical configuration of the third embodiment. The relationship between the surface number and the optical element is the same as that in the first embodiment. FIG. 4 shows aberrations of the optical system according to the third embodiment.

[0027]

[Table 3]

In the first embodiment, all surfaces other than the surface of the first lens 5a on the polygon mirror 4 side are spherical surfaces are formed as toric surfaces. In addition, Examples 2 and 3
Of the first, second, and third lenses are spherical surfaces on the polygon mirror side, the surface of the first lens 5a on the scanning target surface 6 side is a cylindrical surface, and the remaining two surfaces, that is, the second and third lenses. The surface on the scanning target surface 6 side is a toric surface.

As shown in the aberration diagrams, in each of the examples,
Although the scanning coefficient K = 660 covers a wide scanning range of ± 320 mm, the maximum amount of field curvature is suppressed to about 1 mm. Therefore, according to each of the embodiments, Japanese Patent Laid-Open No. 62-17231 shown as a conventional example is shown.
As compared with the embodiment of Japanese Patent Laid-Open No. 7-75, the amount of field curvature in the sub-scanning direction can be suppressed to about 1/5 of that of the conventional example.

[0030]

As described above, according to the present invention, by forming the three lenses as anamorphic lenses, the degree of freedom in design for aberration correction can be increased, and aberrations can be corrected in a well-balanced manner. It becomes possible to do.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a scanning optical system provided with a scanning lens according to a first embodiment of the present invention, (A) being a main scanning direction, and (B).
Indicates the arrangement in the sub-scanning direction.

FIG. 2 shows aberrations of the scanning lens of Example 1, where (A) is linearity error and (B) is field curvature.

FIG. 3 shows aberrations of the scanning lens of Example 2, where (A) is linearity error and (B) is field curvature.

FIG. 4 shows aberrations of the scanning lens of Example 3, where (A) is linearity error and (B) is field curvature.

[Explanation of symbols]

 1 semiconductor laser 2 collimating lens 3 cylindrical lens 4 polygon mirror 5 fθ lens (scanning lens) 5a first lens 5b second lens 5c third lens 6 scanning surface

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location G02B 26/10 103 B41J 3/00 D

Claims (6)

[Claims] 1. A scanning lens for forming an image of a light beam emitted from a light source and deflected by a deflector on a scanning target surface, the scanning lenses being arranged adjacent to each other in order from the deflector side to the scanning target surface side. It is equipped with the first, second and third lenses,
The first lens has a negative power in both the main scanning direction and the sub-scanning direction, the negative power in the sub-scanning direction is set to be stronger in the main scanning direction, and the second lens has a negative power in the main scanning direction and the sub-scanning direction. The third lens has a positive power in both the main scanning direction and the sub-scanning direction, and a positive power in the main scanning direction is set stronger than that in the sub-scanning direction. A scanning lens characterized in that the power is set stronger than in the main scanning direction. 2. The scanning lens according to claim 1, wherein the second lens includes a toric surface having a larger positive power in the main scanning direction in the sub scanning direction. 3. The scanning lens according to claim 2, wherein the lens surface on the deflector side of the third lens is a spherical surface which is convex on the deflector side. 4. The scanning lens according to claim 2, wherein the lens surface of the first lens on the scanning target surface side has a negative power substantially only in the sub-scanning direction. 5. The scanning lens according to claim 2, wherein a lens surface of the first lens on the deflector side is a spherical surface having negative power. 6. The lens surface on the scanning target surface side of the third lens is a positive toric surface having a larger positive power in the sub scanning direction in the main scanning direction. The scanning lens according to any one of 1.