JP2790845B2 - Fθ lens system in optical scanning device - Google Patents

Description

The present invention relates to an fθ lens system in an optical scanning device.

2. Description of the Related Art An optical scanning device is known as a device for writing and reading information by scanning a light beam, and is used for a laser printer, a facsimile, and the like.

In such an optical scanning device, a substantially parallel light flux from a light source is formed into a long line image in the main scanning corresponding direction, and the above-mentioned light is scanned by a rotating polygon mirror having a reflecting surface near an image forming position of the line image. There is an apparatus that deflects a light beam at a constant angular velocity, forms an image of the deflected light beam in a spot shape on a scanning surface by an imaging lens system, and optically scans the scanning surface.

An optical scanning device using a rotating polygon mirror has a problem of surface tilt, and the angular velocity of a deflected light beam is constant. Therefore, the scanning speed of the scanning surface is not constant using a normal f · tan θ lens. Therefore, it is necessary to devise such that the scanning speed is constant.

The fθ lens system is a lens system that optically realizes constant-speed scanning of a scanning surface, and has an fθ function such that the image height of a light beam incident at an incident angle θ becomes fθ with a focal length f. Having.

As a method for solving the problem of surface tilt, a lens system provided between the rotating polygon mirror and the scanning surface is an anamorphic system, and the reflection position of the rotating polygon mirror and the scanning surface are geometrically optically defined in the sub-scanning direction. There is known a method of associating with various conjugate relationships. Even in the apparatus of the above-described method, in order to implement this method, a substantially parallel light beam from a light source is formed as a long line image in the main scanning corresponding direction near the reflecting surface of the rotary polygon mirror.

[Problems to be Solved by the Invention] The fθ lens system itself is made anamorphic to achieve constant-speed scanning and to solve the problem of tilting. What is disclosed in the publication of -78120 is known. However, in the lens system disclosed in Japanese Patent Application Laid-Open No. 60-100118, no sufficient study has been made on the correction of the curvature of field in the main and sub scanning directions.

Japanese Patent Application Laid-Open No. 63-78120 discloses an optical scanning device in which the correction of the curvature of field in the sub-scanning direction is insufficient and high-density optical scanning is performed. Cannot be realized.

Further, in the fθ lens system, a change in the position of the entrance pupil due to the rotation of the rotary polygon mirror must be considered.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described circumstances, and has been made in view of the above-described circumstances. It is an object of the present invention to provide a novel fθ lens system which can solve the above problem and has good linearity.

[Means for Solving the Problems] The fθ lens system according to the present invention is based on the principle that “a substantially parallel light flux from a light source forms a long line image in the main scanning corresponding direction, and a reflection surface is formed near the image forming position of this line image. An optical scanning device that deflects the light beam at a constant angular velocity by a rotating polygon mirror having a uniform angular velocity on the scanning surface by an imaging lens system and optically scans the scanning surface. Lens system for forming an image of the light beam deflected by the scanning surface on the scanning surface, having a function of making the reflection position of the rotating polygon mirror and the scanning surface substantially geometrically conjugate with respect to the sub-scanning direction, It has an fθ function in the main scanning direction.

The fθ lens system has a three-group, three-element structure in which first to third lenses are sequentially arranged from the rotating polygon mirror side toward the scanning surface side.

The "first lens" is a negative single lens in which the lens surface on the side of the rotary polygon mirror is a concave spherical surface, and the lens surface on the scanning surface side is a concave cylinder surface having power only in the sub-scanning corresponding direction, The “second lens” is a positive meniscus lens facing the concave surface on the side of the rotary polygon mirror, and the “third lens” is a lens surface on the side of the rotary polygon mirror that is flat, and the lens surface on the scanning surface side has a flat surface. The positive single lens is a toric surface having a strong curvature in the sub-scanning direction.

The combined focal length of the fθ lens system in the main scanning direction is f, the focal lengths of the first and third lenses are f _1X and f _{3X in} the main scanning direction, and f _1Y and f in the sub-scanning direction. _3Y , the focal length of the second lens is f ₂ , the radii of curvature of the surfaces of the first and second lenses on the rotating polygon mirror side are r ₁ and r ₃ , respectively, the reflection position of the rotating polygon mirror and the first lens The distance on the optical axis from the lens surface on the rotating polygon mirror side is d ₀ , and the scanning surface side lens surface of the second lens is
The distance between the lens surfaces on the polygonal mirror side of the lens is d ₄ ,
When the refractive indices of the third lens and the third lens are n ₁ and n ₃ , respectively, they are (I) −0.83 <f _1X / f _3X <−0.67 (II) −1.15 <f _1Y / f _3Y <−0.9 ( _{III) 0.75 <f 2 /f<1.17 (} IV) -0.22 <r 1 /f<-0.17 (V) -0.80 <r 3 /f<-0.62 (VI) -1.19 <r 1 / d 0 <-0.92 (VII) 0 <d ₄ /f<0.0041 (VIII) The condition 0.80 ≦ n ₁ / n ₃ ≦ 0.95 is satisfied.

In the following description, a surface formed by sweeping the principal ray of the light beam ideally deflected by the rotating polygon mirror is referred to as a “deflection surface”. Further, a plane that includes the lens optical axis and is orthogonal to the deflection surface is hereinafter referred to as a “deflection orthogonal surface”.

The lens surface on the scanning surface side of the third lens is “a toric surface having a strong curvature in the sub-scanning direction” when the “radius of curvature in a plane orthogonal to deflection” of this toric surface is “curvature in the deflection plane”. Radius ". As is clear from this description, the “deflection surface” is parallel to the main scanning direction,
The “deflection reflection surface” is parallel to the sub-scanning direction.

[Operation] The above conditions (I) to (VIII) have the following meanings.

That is, the conditions (I), (II), (III), (V), and (VII)
I) is a condition for suppressing the amount of curvature of field in the main and sub-scanning directions. If the conditions deviate from the ranges of these conditions, the remaining amount of curvature of field in the main and sub-scanning directions becomes large and correction cannot be performed.

Conditions (IV) and (VI) are conditions for correcting coma aberration. If the conditions are out of the range, coma aberration occurs, and a light spot suitable for high-density scanning cannot be obtained.

The condition (VII) is a condition for making the fθ lens system compact. If the upper limit is exceeded, the outside diameter of the third lens becomes large, so that it becomes impossible to compact the fθ lens diameter.

 This will be described below with reference to the drawings.

FIG. 1 schematically illustrates an example of an optical scanning device using the fθ lens system of the present invention. FIG. 2 shows a state of the optical arrangement of FIG. 1 viewed from the sub-scanning direction, that is, a state in a deflection plane.

A substantially parallel light beam from a light source or a light source device 1 including a light source and a light condensing device is imaged by a cylinder lens 2 serving as a line image forming optical system in the vicinity of a reflection surface 4 of a rotary polygon mirror 3 in parallel with a deflection surface. As an image.

The light beam reflected by the reflecting surface 4 is imaged into a spot on the scanning surface 8 by the fθ lens system of the present invention, and the scanning surface 8 is moved at a constant speed according to the constant speed rotation of the rotary polygon mirror 3 in the arrow direction. Scanning.

The fθ lens system includes a first lens 5, a second lens 6, and a third lens 7, the lens 5 is disposed on the side of the rotary polygon mirror 3, the lens 7 is disposed on the side of the scanning surface 8, and the lens 6
Is disposed between the lenses 5 and 7. As viewed in the deflection plane, as shown in FIG. 2, the fθ lens system including the lenses 5 to 7 links the infinity on the light source side and the position of the scanning plane 7 to a geometric optic conjugate relationship.

On the other hand, when viewed in a plane orthogonal to the polarization, that is, in the sub-scanning direction, the fθ lens system links the reflection position of the rotary polygon mirror 3 and the scanning plane 8 to a geometrically optically conjugate relationship.
Therefore, even if the reflecting surface 4 is tilted as shown by reference numeral 4 'as shown in FIG.
The upper imaging position hardly moves in the sub-scanning direction (vertical direction in FIG. 3). Therefore, the tilting is corrected.

When the rotary polygon mirror 3 rotates, the reflecting surface 4 rotates about the axis 3A. Therefore, as shown in FIG. 4, the reflecting surface 4 rotates between the image forming position P and the reflecting surface 4 as the reflecting surface rotates. A position shift ΔX occurs, and the position P ′ of the conjugate image of the line image by the fθ lens system is shifted from the scanning plane 8 by ΔX ′.

The shift amount ΔX ′ is given by ΔX ′ = β ² ΔX, as is well known, where β is the lateral magnification of the fθ lens system in the sub-scanning direction.

The relative displacement between the position of the line image and the reflection surface due to the rotation of the reflection surface occurs two-dimensionally in the polarization plane, and moves asymmetrically with respect to the lens optical axis.

Therefore, in the optical scanning device as shown in FIG. 1, it is necessary to satisfactorily correct the field curvature of the fθ lens system in the main and sub scanning directions. Needless to say, the linearity, that is, the fθ characteristic, must be properly corrected in the main scanning direction.

[Examples] Eleven examples will be specifically described below.

In each embodiment, f represents the combined focal length of the fθ lens system in the main scanning direction, that is, the combined focal length in a plane parallel to the polarization plane. 2θ indicates a polarization angle (unit: degree).

r _ix curvature of the polarization plane of the i-th lens surface counted from the side of the rotating polygon mirror radius, r _iY the i-th lens surface curvature radius of polarization orthogonal plane of, d _i is between i-th lens surface the distance, d _o is the distance from the reflecting surface of the rotary polygon mirror to the first lens surface, n _j denotes the refractive index of the j-th lens from the side of the rotating polygon mirror.

Further, with K _m (m = 1 to 8), conditions (I) to
This represents each condition parameter in (VIII).

Incidentally, for i = 1, 3, 4, 5 is _{r iX = r iY, r 1X} = r 1Y are particularly regard to _{i = 1,3, r 3X = r} 3Y the conditional expressions (I
These are described as r ₁ and r ₃ in relation to (V), (V) and (VI).

The radius of the inscribed circle of the rotating polygon mirror is R, and the number of reflecting surfaces is N. The angle between the principal ray of the light beam incident on the reflecting surface and the optical axis of the fθ lens system is 60 degrees.

Example 1 f = 903.3,2θ = 58, K 1 = -0.687, K 2 = -1.051, K 3 = 0.815, K 4 = -0.183, K 5 = -0.646, K 6 = -0.988, K 7 = 0.0011 _{, K 8 = 0.938, R =} 35, n = 6, d o = 167.00 i r iX r iY d i j n i 1 -165.00 -165.00 12.18 1 1.67500 2 ∞ 362.00 12.93 3 -583.23 -583.23 29.70 2 1.51118 4 - 232.72 −232.72 1.00 5 ∞ 57.75 3 1.78571 6 −279.49 −124.40 FIG. 5 shows a diagram of the spherical aberration SA, a diagram of the coma aberration COMA, a diagram of the fθ characteristic DIST, and a diagram of the field curvature DS and DM according to the first embodiment. Is shown. The fθ characteristic is defined as (h−fθ) · 100 / (fθ) using the actual image height h with respect to the ideal image height fθ, and the unit is%. In the figures of the field curvature, the solid line indicates the field curvature in the sub-scanning direction, and the broken line indicates the field curvature in the main scanning direction. Since the field curvature is asymmetric due to the change in the position of the entrance pupil due to the rotation of the rotating polygon mirror, it is shown over the entire polarization range.

Example 2 f = 903.3, 2θ = 58, K ₁ = −0.684, K ₂ = −0.965, K ₃ = 0.864, K ₄ = −0.183, K ₅ = −0.755, K ₆ = −0.988, K ₇ = 0.0011 _{, K 8 = 0.897, R =} 35, n = 6, d o = 167.00 i r iX r iY d i j n i 1 -165.00 -165.00 11.41 1 1.51118 2 ∞ 188.20 28.24 3 -682.23 -682.23 32.98 2 1.51118 4 - 255.87 −255.87 1.00 5 ∞ 55.54 3 1.68439 6 −322.94 −120.70 FIG. 6 shows various aberrations and fθ characteristics of the second embodiment.

Example 3 f = 903.3, 2θ = 58, K ₁ = −0.715, K ₂ = −1.089, K ₃ = 0.865, K ₄ = −0.194, K ₅ = −0.653, K ₆ = −1.048, K ₇ = 0.0011 _{, K 8 = 0.933, R =} 35, n = 6, d o = 167.00 i r iX r iY d i j n i 1 -175.00 -175.00 13.36 1 1.70217 2 ∞ 367.00 12.33 3 -590.09 -590.09 27.69 2 1.51118 4 - 242.04 -242.04 1.00 5 ∞ 55.19 3 1.82485 6 -287.66 -126.59 FIG. 7 shows various aberrations and fθ characteristics of the third embodiment.

Example 4 f = 903.3, 2θ = 58, K ₁ = −0.696, K ₂ = −1.039, K ₃ = 0.842, K ₄ = −0.193, K ₅ = −0.763, K ₆ = −1.044, K ₇ = 0.0011 _{, K 8 = 0.923, R =} 35, n = 6, d o = 167.00 i r iX r iY d i j n i 1 -174.35 -174.35 12.63 1 1.60909 2 ∞ 263.00 18.76 3 -688.88 -688.88 30.66 2 1.51118 4 - 252.20 -252.20 1.00 5 ∞ 54.05 3 1.74405 6 -305.93 -121.88 FIG. 8 shows various aberrations and fθ characteristics of the fourth embodiment.

Example 5 f = 903.3, 2θ = 58, K ₁ = −0.676, K ₂ = −1.038, K ₃ = 0.806, K ₄ = −0.172, K ₅ = −0.633, K ₆ = −0.932, K ₇ = 0.0011 _{, K 8 = 0.923, R =} 35, n = 6, d o = 167.00 i r iX r iY d i j n i 1 -155.64 -155.64 11.35 1 1.60909 2 ∞ 322.70 15.48 3 -572.10 -572.10 30.81 2 1.51118 4 - 229.61 −229.61 1.00 5 ∞ 58.89 3 1.74405 6 −281.25 −122.42 FIG. 9 shows various aberrations and fθ characteristics of the fifth embodiment.

Example 6 f = 903.3, 2θ = 58, K ₁ = −0.718, K ₂ = −0.980, K ₃ = 0.928, K ₄ = −0.199, K ₅ = −0.794, K ₆ = −1.078, K ₇ = 0.0028 _{, K 8 = 0.890, R =} 35, n = 6, d o = 167.00 i r iX r iY d i j n i 1 -180.00 -180.00 13.70 1 1.52447 2 ∞ 180.00 27.37 3 -717.24 -717.24 22.37 2 1.51118 4 - 272.33 -272.33 2.50 5 ∞ ∞ 53.04 3 1.71221 6 -340.28 -123.05 FIG. 10 shows various aberrations and fθ characteristics of the sixth embodiment.

Example 7 f = 306.9, 2θ = 56, K ₁ = −0.811, K ₂ = −1.143, K ₃ = 1.159, K ₄ = −0.210, K ₅ = −0.642, K ₆ = −1.143, K ₇ = 0.004 _{, K 8 = 0.828, R =} 37.5, n = 8, d o = 56.5 i r iX r iY d i j n i 1 -64.60 -64.60 5.10 1 1.51118 2 ∞ 74.12 7.03 3 -197.00 -197.00 11.00 2 1.51118 4 - 96.34 -96.34 1.23 5 ∞ 21.00 3 1.82485 6 -128.489 -48.125 FIG. 11 shows various aberrations and fθ characteristics of the seventh embodiment.

Example 8 f = 436.59, 2θ = 60, K ₁ = −0.742, K ₂ = −1.084, K ₃ = 0.957, K ₄ = −0.213, K ₅ = −0.761, K ₆ = −1.163, K ₇ = 0.0023 _{, K 8 = 0.901, R =} 42.0, n = 8, d o = 80.0 i r iX r iY d i j n i 1 -93.00 -93.00 7.10 1 1.60909 2 ∞ 127.50 8.76 3 -332.10 -332.10 15.00 2 1.51118 4 - 132.00 −132.00 1.00 5 ∞ ∞ 29.45 3 1.78571 6 −161.70 −63.18 FIG. 12 shows various aberrations and fθ characteristics of the eighth embodiment.

Example 9 f = 831.76, 2θ = 57.94, K ₁ = −0.716, K ₂ = −1.060, K ₃ = 0.887, K ₄ = −0.198, K ₅ = −0.745, K ₆ = −1.071, K ₇ = 0.0024 _{, K 8 = 0.911, R =} 32.5, n = 6, d o = 154.0 i r iX r iY d i j n i 1 -165.00 -165.00 13.00 1 1.60910 2 ∞ 243.50 16.59 3 -619.50 -619.50 27.75 2 1.51118 4 - 238.00 -238.00 2.00 5 ∞ ∞ 49.75 3 1.76605 6 -290.00 -115.30 FIG. 13 shows various aberrations and fθ characteristics of the ninth embodiment.

Example 10 f = 399.36, 2θ = 50.8, K ₁ = −0.723, K ₂ = −1.076, K ₃ = 0.925, K ₄ = −0.202, K ₅ = −0.734, K ₆ = −1.072, K ₇ = 0.0025 _{, K 8 = 0.899, R =} 32.335, n = 8, d o = 75.10 i r iX r iY d i j n i 1 -80.50 -80.50 7.40 1 1.61420 2 ∞ 125.50 8.23 3 -293.00 -293.00 13.70 2 1.51390 4 - 117.00 -117.00 1.00 5 ∞ 22.30 3 1.79465 6 -144.00 -58.17 FIG. 14 shows various aberrations and fθ characteristics of the tenth embodiment.

Example 11 f = 489.4, 2θ = 44.6, K ₁ = −0.756, K ₂ = −1.066, K ₃ = 0.965, K ₄ = −0.211, K ₅ = −0.725, K ₆ = −1.161, K ₇ = 0.0026 _{, K 8 = 0.893, R =} 38.5, n = 12, d o = 89.0 i r iX r iY d i j n i 1 -103.30 -103.30 7.40 1 1.63180 2 ∞ 120.00 11.02 3 -355.00 -355.00 15.00 2 1.52223 4 - 147.60 -147.60 1.265 5 ∞ 21.30 3 1.82715 6 -179.00 -67.33 FIG. 15 shows various aberrations and fθ characteristics of the eleventh embodiment.

[Effects of the Invention] As described above, according to the present invention, a novel fθ in the optical scanning device
A lens system can be provided. As described above, this lens system has a small field curvature in both the main and sub-scanning directions, so that high-density writing is possible, and since a long cylinder lens is not required for correcting surface tilt, the optical scanning device can be made compact. It becomes possible to configure.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view showing an outline of an optical scanning device using the fθ lens system of the present invention. FIGS. 2 and 3 are diagrams for explaining the fθ lens system of the present invention. FIG. 5 is a diagram for explaining a change in the position of the entrance pupil due to the rotation of the rotating polygon mirror, and FIGS. 5 to 15 are aberration diagrams. 5 first lens, 6 second lens, 7 third
Lens

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Claims (1)

(57) [Claims] 1. A method according to claim 1, wherein a substantially parallel light beam from the light source is formed into a long line image in a direction corresponding to the main scanning, and the light beam is subjected to uniform angular velocity by a rotary polygon mirror having a reflecting surface near an image forming position of the line image. In a light scanning device that optically scans the scanning surface by forming an image of the deflected light beam in a spot shape on the scanning surface by the imaging lens system, the light beam deflected by the rotary polygon mirror is imaged on the scanning surface. A lens system having a function of making the reflection position of the rotary polygonal mirror and the scanning surface substantially geometrically conjugate with each other in the sub-scanning direction, and having an fθ function in the main scanning direction; A three-group, three-element configuration in which first to third lenses are sequentially arranged from the mirror side toward the scanning surface side, wherein the first lens has a concave spherical surface on the rotating polygon mirror side. And the lens surface on the scanning surface side has power only in the direction corresponding to the sub-scanning A negative single lens is a cylinder surface,
The second lens is a positive meniscus lens having a concave surface facing the rotary polygon mirror, and the third lens has a flat lens surface on the rotary polygon mirror side and a sub-scanning lens surface on the scanning surface side. A positive single lens which is a toric surface having a strong curvature in the direction. The combined focal length in the main scanning direction is f, and the focal lengths of the first and third lenses are f _1X and f _{3X in the} main scanning direction. , F _1Y and f _3Y in the sub-scanning direction, and the focal length of the second lens is f
_2. The radii of curvature of the surfaces of the first and second lenses on the rotary polygon mirror side are r ₁ and r ₃ , respectively, and the optical axes of the reflection position of the rotary polygon mirror and the lens surface of the first lens on the rotary polygon mirror side. The upper distance is d ₀ , the surface distance between the scanning surface side lens surface of the second lens and the lens surface of the third lens on the rotating polygon mirror side is d ₄ , and the refractive indexes of the first and third lenses are n _1. when the n _3, it is _{(I) -0.83 <f 1X /} f 3X <-0.67 (II) -1.15 <f 1Y / f 3Y <-0.9 (III) 0.75 <f 2 /f<1.17 (IV ) −0.22 <r ₁ /f<−0.17 (V) −0.80 <r ₃ /f<−0.62 (VI) −1.19 <r ₁ / d ₀ <−0.92 (VII) 0 <d ₄ /f<0.0041 ( VIII) An fθ lens system in an optical scanning device, characterized by satisfying the following condition: 0.80 ≦ n ₁ / n ₃ ≦ 0.95.