JPH07270678A - Scanning optical system - Google Patents

Abstract

PURPOSE:To obtain an inexpensive scanning optical system capable of excellently correcting an image surface curvature and reduced in focus optical system scanning a surface to be scanned with a luminous flux for scanning in a main scanning direction by a light deflector via a scanning lens system. CONSTITUTION:In the scanning optical system constituted of a glass toric lens 22 and two plastic lenses 21 and 23, the plastic lens mainly correcting the image surface curvature in the subscanning direction is constituted so, that the surface of the side of the deflector 12 is a toric plane having a rotary axis nearly parallel with the main scanning direction, while the surface of the side of the surface to be scanned is the toric plane having the rotary axis nearly parallel with the subscanning direction and formed into a meniscus shape in both of main scanning and subscanning cross-sections and the shape of a double-faced toric meniscus projecting to the side of the surface to be scanned in the main scanning cross-section.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system, and more particularly to a scanning optical system capable of high-definition drawing while including a plastic lens.

[0002]

2. Description of the Related Art A scanning optical system is indispensable in a laser beam printer, a laser scanner, a bar code reader, etc., and a polygon mirror or a hologram disk is used as an optical deflector. The laser light emitted from the semiconductor laser enters the optical deflector and is scanned,
The scanned light beam is scanned on a surface to be scanned, for example, a photoconductor, via a scanning lens system such as an fθ optical system.

A high-definition fθ lens system of such a scanning optical system is conventionally composed of three or more glass lenses including an anamorphic lens, but there is a problem of high cost. Japanese Patent Laid-Open No. 4-110817 discloses fθ.
F composed of two toric lenses, mainly for the purpose of converting the entire lens system into a plastic lens.
A θ lens is proposed. However, the plastic lens has a problem that a focus shift occurs due to temperature change and humidity change, and this fθ lens system is not an exception when the entire lens system is made of a plastic lens. Further, Japanese Patent Laid-Open No. 4-277715 proposes an fθ lens system in which the focal point movement is hard to occur with respect to temperature change while making the entire fθ lens system plastic, but since f _B cannot be taken long, the device May impose restrictions on the configuration of.

[0004]

OBJECT OF THE INVENTION The present invention is of low cost, high performance,
An object is to obtain a scanning optical system having a long f _{B and a} small focus fluctuation with respect to changes in temperature and humidity. Another object of the present invention is to provide a scanning optical system which can correct field curvature particularly in the sub-scanning direction, has a relatively simple lens shape, and can be easily processed or molded.

[0005]

SUMMARY OF THE INVENTION According to the present invention, a glass toric lens has a main power, and the glass toric lens causes a field curvature in the main scanning direction and a field curvature in the sub-scanning direction to be a first plastic lens. In the scanning optical system having a three-lens structure that is mainly corrected by the first and second plastic lenses, in particular, the second plastic lens for correcting the field curvature in the sub-scanning direction has a deflector-side surface as a main component. It consists of a toric surface that has a rotation axis that is substantially parallel to the scanning direction, and the surface to be scanned has a toric surface that has a rotation axis that is substantially parallel to the sub-scanning direction, and it has a meniscus shape in both the main scanning section and the sub-scanning section. And has a shape which is convex toward the surface to be scanned in the main scanning section.

The glass toric lens can be constructed, for example, of a toric surface whose surface on the scanned surface side has a rotation axis substantially parallel to the sub-scanning direction and whose convex surface faces the scanned surface side. If the surface on the optical deflector side is a flat surface, it is easy to process.

The first plastic lens can correct the field curvature in the main scanning direction and the fθ characteristic by providing, for example, a rotationally symmetric aspherical surface.

By making the second plastic lens eccentric in the main scanning direction, it is possible to reduce the asymmetry of the image plane in the sub scanning direction with respect to the center in the main scanning direction.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below with reference to illustrated embodiments. [First Embodiment] FIGS. 1 and 2 show a first embodiment of the present invention. FIG. 1 shows a polygon mirror 12 that rotates about a rotary shaft 11 as an optical deflector. As is well known, a laser beam emitted from a semiconductor laser is made into a parallel light flux by a collimator lens, enters a polygon mirror 12 through a cylindrical lens 13, is reflected by each reflection surface 12R of the mirror peripheral surface, and is scanned. Then, the surface to be scanned 14 is scanned through the scanning lens system 20. Surface to be scanned 1
In the case of a laser beam printer, for example, 4 is a photosensitive drum.

The present invention is based on, for example, a scanning lens system 20 of a scanning optical system having the above-described structure.
In FIG. 2, a three-lens structure including a first plastic lens 21, a glass toric lens 22, and a second plastic lens 23 is arranged from the polygon mirror 12 side.

The glass toric lens 22 bears the main power of the scanning lens system in the main scanning direction and the sub-scanning direction, and the surface 22a on the surface to be scanned has a rotation axis substantially parallel to the sub-scanning direction. It is a toric surface having a convex surface facing the surface to be scanned, and the surface 22b on the polygon mirror 12 side is a flat surface.

Among the aberrations produced by the glass toric lens 22, the plastic lens 21 corrects the field curvature in the main scanning direction (the direction orthogonal to the optical axis O in FIG. 1 on the paper surface) and the fθ characteristic. is there. At least one surface of the plastic lens 21 is preferably a rotationally symmetric aspherical surface.

In the plastic lens 23, the surface 23b on the polygon mirror 12 side is a toric surface having a rotation axis substantially parallel to the main scanning direction, and the surface 23a on the scanned surface has a rotation axis substantially parallel to the sub-scanning direction. It has a toric surface. Further, the plastic lens 23 has a meniscus shape in both the main scanning cross section and the sub scanning cross section, and is convex on the scanned surface side in the main scanning cross section.

FIG. 9 is a diagram for explaining the shape of the plastic lens 23, which is a feature of the present invention. Assuming that the left-right direction in FIG. 9 is the main scanning direction, the surface 23 on the polygon mirror 12 side
b is a toric surface having an axis X1 parallel to the main scanning direction as a rotation axis. The radius of curvature of this toric surface is the smallest at the center and gradually increases toward the periphery.
That is, the radius of curvature of the toric surface 23b in the sub-scanning cross-section direction gradually increases from the center to the periphery, and the power gradually decreases. Plastic lens 23
Has a meniscus shape that is convex toward the surface to be scanned 14 in the main scanning direction.

On the other hand, the surface 23a on the surface to be scanned 14 is a toric surface whose axis of rotation is an axis X2 parallel to the sub-scanning direction (direction perpendicular to the paper surface). That is, the power of the surface 23a in the sub-scanning direction is constant anywhere in the main scanning direction, and the change in the power of the surface 23b on the polygon mirror 12 side appears as it is on the image surface (scanned surface). Therefore, the field curvature in the sub-scanning direction, which tends to increase at the end in the main scanning direction, can be reduced. That is, the focal length can be extended at the end portion in the main scanning direction as compared with the central portion, and as a result, the field curvature in the sub scanning direction can be reduced. Further, if the radii of curvature in the central portions of the surfaces 23a and 23b are the same, the plastic lens 23 has no power in the sub-scanning direction in the central portion, and only the field curvature in the sub-scanning direction in the peripheral portion. Can be selectively reduced. Also, this plastic lens 23
Since its main purpose is to reduce the field curvature in the sub-scanning direction, its power can be small, and thus it is less likely to be affected by changes in temperature and humidity.

Further, the plastic lens 23 is
It also has a meniscus shape in the sub-scan section. With the meniscus shape, the power can be reduced and only the field curvature of the sub-scanning section can be effectively corrected. The meniscus shape in the sub-scan section is
Although it is convex on the polygon mirror 12 side in the illustrated example, it may be convex on the scanned surface 14 side.

The plastic lens 23 is eccentric to the semiconductor laser side by a distance e from the optical axis O of the fθ lens system 20. By decentering the plastic lens 23 in this way, it is possible to reduce the asymmetry of the image plane in the sub scanning direction with respect to the center in the main scanning direction (tilt with respect to the main scanning direction). This asymmetry becomes particularly large when the incident angle α of the light beam from the semiconductor laser exceeds a certain value.

Table 1 shows specific numerical data of Example 1, and FIG. 3 is a graph showing the fθ characteristic of the scanning lens system 20 of the specific numerical data shown in Table 1. Similarly, FIG. 4 is a graph showing field curvatures of meridional (main scanning direction) M and sagittal (sub scanning direction) S. The vertical axis in each of FIGS. 3 and 4 represents the position in the main scanning direction, the horizontal axis in FIG. 3 represents the deviation (mm) from the ideal image height, and the horizontal axis in FIG. 4 represents the relative focus position (mm). Shows.

In the table, f is the focal length, f _B is the back focus, K is the scanning coefficient, R is the radius of curvature in the main scanning section of each surface of the lens, R _Z is the radius of curvature in the same sub-scanning section, and D is the lens thickness. Alternatively, the lens interval, N is the refractive index for a wavelength of 780 nm. The cylindrical lens 13 is a lens arranged between the semiconductor laser and the polygon mirror 12.

[0020]

[Table 1] f = 135.37 f _B = 124.54 K = 135.5 Surface No. RR _Z DN Cylindrical ∞ 17.88 4.00 1.51072 (Glass) Lens ∞ 32.00 Polygon Mirror 29.94 1 * 1053.00 6.73 1.48617 (Plastic) 2 * -160.66 2.29 3 ∞ 14.50 1.51072 (glass) 4 -100.00 -18.46 3.00 5 -110.00 26.10 4.00 1.48617 (plastic) 6 -142.58 23.30 124.54 * is a rotationally symmetric aspherical surface, 1 surface; K = 5.32, A4 = -3.83 × 10 ^-6 , A6 = 2.10 × 10 ^-9 , A8 =-
4.29 × 10 ^-13 , 2 sides; K = 10.56, A4 = -2.25 × 10 ^-6 , A6 = 5.76 × 10 ^-10 ,
A8 = 1.18 × 10 ⁻¹³ , decentering amount e of plastic lens 23; −3.30 decentering of image plane (origin of aberration diagram); 0.45 However, the aspheric surface is defined by the following equation. x = cy ² / {1+ [1- (1 + K) c ² y ² ] ^1/2 } + A4y ⁴ + A6y ⁶ + A8y ⁸

[Second Embodiment] FIGS. 5 and 6 show a second embodiment of the present invention. In this embodiment, the plastic lens 21
Or, the plastic lenses 23 are arranged at different positions, and the glass toric lens 22, the plastic lens 23, and the plastic lens 21 are arranged in this order from the polygon mirror 12 side. The numerical data of this example are shown in Table 2, the fθ characteristic is shown in FIG. 7, and the field curvature is shown in FIG.
Shown in.

[0022]

[Table 2] f = 135.48 f _B = 128.78 K = 135.5 No. RR _Z DN Cylindrical ∞ 25.54 4.00 1.51072 (glass) Lens ∞ 58.35 Hollow Mirror 35.67 1 ∞ 14.00 1.51072 (glass) 2 -85.00 -18.40 2.00 3 ** -90.00 44.33 4.00 1.48617 (Plastic) 4 -92.70 29.50 1.00 5 * 310.00 6.00 1.48617 (Plastic) 6 * ∞ 128.78 * is a rotationally symmetric aspherical surface, ** is a toric surface with an aspherical main-scanning cross section, 3 surfaces; K = 0, A4 = -2.63 × 10 ^-9 5 faces; K = -6.80, A4 = -1.46 × 10 ^-6 , A6 = 2.00 × 10 ^-10 ,
A8 = -1.51 × 10 ^-14 , 6 sides; K = -22.00, A4 = -1.32 × 10 ^-6 , A6 = 1.25 × 10 ^-10 ,
A8 = -7.00 × 10 ^-15 , Decentering amount of plastic lens 23; -2.84 Decentering of image plane (origin of aberration diagram); 0.07

[0023]

According to the present invention, in a scanning lens system having a three-lens structure consisting of one glass toric lens and two plastic lenses, a plastic lens that is particularly involved in correction of field curvature in the sub-scanning direction is a double-sided toric lens. Since the main-scanning direction and the sub-scanning direction are composed of meniscus lenses, it is possible to excellently correct the field curvature in the sub-scanning direction. Since the toric surface has the rotation axis, a precise shape can be obtained at low cost. Further, the two plastic lenses contribute to the field curvature correction and the fθ characteristic correction, and the glass toric lens bears the main power, so there is little focus fluctuation due to temperature and humidity changes, and a low-cost scanning optical system is obtained. You can Furthermore, according to the invention, a long f _B
Can be obtained.

[Brief description of drawings]

FIG. 1 is a plan view showing a first embodiment of a scanning optical system according to the present invention.

FIG. 2 is a front view of FIG.

FIG. 3 is a graph showing an fθ characteristic of the scanning optical system shown in FIGS. 1 and 2.

FIG. 4 is a graph showing field curvature in the scanning optical system of FIGS. 1 and 2.

FIG. 5 is a plan view showing a second embodiment of the scanning optical system according to the present invention.

FIG. 6 is a front view of FIG.

FIG. 7 is a graph showing the fθ characteristic of the scanning optical system shown in FIGS.

8 is a graph showing field curvature in the scanning optical system of FIGS. 5 and 6. FIG.

FIG. 9 is a diagram illustrating the shape of a plastic lens that is used for correction of field curvature in the sub-scanning direction according to the present invention.

[Explanation of symbols]

 12 polygon mirror (deflector) 13 cylindrical lens 14 surface to be scanned 20 scanning lens system 21 first plastic lens 22 glass toric lens 23 second plastic lens

Claims (4)

[Claims] 1. A scanning optical system for scanning a light beam scanned by an optical deflector onto a surface to be scanned through a scanning lens system, wherein the scanning lens system is mainly a glass toric lens having power; A first plastic lens that corrects the field curvature in the main scanning direction and the fθ characteristic by the glass toric lens; and a second plastic lens that mainly corrects the field curvature in the sub scanning direction by the glass toric lens. In the second plastic lens, the surface on the deflector side is a toric surface having a rotation axis substantially parallel to the main scanning direction, and the surface on the scanned surface side has a rotation axis substantially parallel to the sub-scanning direction. It is a toric surface and has a meniscus shape in both the main scanning section and the sub-scanning section, and a convex shape on the scanned surface side in the main scanning section. Scanning optical system to. 2. The glass toric lens according to claim 1, wherein the surface on the scanned surface side is a toric surface having a rotation axis substantially parallel to the sub-scanning direction and a convex surface facing the scanned surface side. Scanning optics. 3. The scanning optical system according to claim 2, wherein the glass toric lens has a flat surface on the optical deflector side. 4. The scanning optical system according to claim 1, wherein the first plastic lens has a rotationally symmetric aspherical surface.