JP2927846B2 - Fθ lens system in optical scanning device - Google Patents

Description

Description: TECHNICAL FIELD 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.

Among such optical scanning devices, a substantially parallel light flux from the light source device is formed into a long linear image in the main scanning corresponding direction, and a rotating polygon mirror having a reflecting surface near an image forming position of the linear image is used. There is an apparatus of a system that deflects the 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 so-called tilting. In addition, since a deflected light beam has a constant angular velocity of the rotating polygon mirror, scanning is performed at a constant speed using a normal f-tan θ lens. Is not done. The fθ lens system is a lens system that optically realizes constant-speed scanning of the scanning surface,
An fθ function is provided so that the image height of a light beam incident at an angle θ with respect to the lens optical axis is fθ where f is the focal length.

As a method for solving the problem of surface tilt, the lens system provided between the rotating polygon mirror and the scanning plane is an anamorphic system, and the reflection position of the rotating polygon mirror and the scanning plane are conjugated in the sub-scanning direction. There are known ways to tie.

[Problems to be Solved by the Invention] There are various known fθ lens systems that use an anamorphic lens to perform constant-speed scanning and solve the problem of surface tilt. For example, Japanese Patent Application Laid-Open No. 63-19617 discloses a two-piece f
A θ lens system is disclosed.

However, this fθ lens system does not always sufficiently correct the curvature of field, and the diameter of the image spot on the scanning surface varies considerably depending on the scanning position, so that it is difficult to realize high-density optical scanning.

The present invention has been made in view of the above-described circumstances, and is a novel fθ lens capable of sufficiently correcting the field curvature in the main and sub-scanning directions and solving the problem of surface tilt in a rotating polygon mirror. The purpose is to provide a system.

[Means for Solving the Problems] Hereinafter, the present invention will be described.

As shown in FIG. 1, the fθ lens system according to the present invention is configured such that “a substantially parallel light beam from the light source device 1 is formed into a long line image in the main scanning corresponding direction, and reflected near the image forming position of the line image. The light beam is deflected at a constant angular velocity by the rotary polygon mirror 3 having the surface 4, and the deflected light beam is focused on the scanning surface 7 in the form of a spot by the imaging lens system, so that the scanning surface 7 is substantially uniformly illuminated. In the optical scanning device for scanning, an image forming lens system for forming an image of the light beam deflected by the rotating polygon mirror 3 on the scanning surface 7 "
It has a “function of linking the reflection position of the rotary polygon mirror and the scanning surface to a substantially optically conjugate relationship in the sub-scanning direction”, and has an “fθ function in the main scanning direction”.

This fθ lens system is provided in a first and second order from the side of the rotary polygon mirror toward the scanning surface, and is composed of a first lens 5 and a second lens 6 in two groups and two lenses. The first and second lenses 5 and 6 are anamorphic.

That is, the first lens 5 is an anamorphic meniscus lens having a concave surface facing the object side in the main and sub scanning directions, and at least one surface is a toric surface. The second lens 6 is an anamorphic lens having a positive refracting power along with the main and sub-scanning directions.
The image side is constituted by a toric surface.

The focal lengths of the first and second lenses 5 and 6 in the main scanning direction are f _1m and f _2m , respectively, and the radii of curvature of the object-side and image-side lens surfaces of the first lens 5 in the sub-scanning direction are r _1Y , respectively. when r _2Y, the combined focal length in the main scanning direction in the entire system and f _{m,
They, (I) f 1m / f} m> 2 (II) f 2m / f m> 1.2 (III) -0.05 (100 / f m) <{(1 / r 1Y) - (1 / r 2Y)} <satisfies 0.0 (100 / f _m) following condition.

In FIG. 1, reference numeral 2 denotes a cylinder lens for forming a line image.

[Operation] The above conditions will be described.

The condition (I) is a condition for favorably correcting the field curvature in the main scanning direction and the fθ characteristic.

Condition (II) is a condition for favorably correcting the curvature of field in the sub-scanning direction.

Condition (III) is a condition for correcting the imaging performance between image planes in the sub-scanning direction. When the upper limit is exceeded, the spot diagram becomes outward at a high image height position, and when the lower limit is exceeded, the spot diagram is reversed. Turn inward.

Here, an outline of the optical scanning will be described with reference to FIGS.

In FIG. 1, a parallel light beam from a light source or a light source device 1 comprising a light source and a light condensing device is converted into a linear image near a reflecting surface 4 of a rotary polygon mirror 3 by a cylinder lens 2 as a line image forming optical system. As an image. The longitudinal direction of this line image is a main scanning corresponding direction (a direction parallel to the drawing in FIG. 1).

The light beam reflected by the rotary polygon mirror 3 is imaged into a spot on the scanning surface 7 by the fθ lens system, and scans the scanning surface 7 at a constant speed according to the constant rotation of the rotary polygon mirror 3 in the arrow direction. .

Among the first lens 5 and the second lens 6 constituting the fθ lens system, the lens 5 is disposed on the rotating polygon mirror 3 side, and the lens 6 is disposed on the scanning plane 7 side.

In the plane of FIG. 1, that is, in the main scanning corresponding direction, a lens
The fθ lens system according to 5, 6 links the infinity on the light source device side and the position of the scanning plane 7 to a geometrical conjugate relationship.

On the other hand, in the sub-scanning direction, that is, in the direction orthogonal to the drawing of FIG. 1, the fθ lens system links the reflection position of the rotary polygon mirror 3 and the scanning surface 7 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. 2, the imaging position on the scanning surface 7 by the fθ lens system is in the sub-scanning direction (vertical direction in FIG. 2). Hardly move. Therefore, the tilting is corrected.

When the rotating polygon mirror 3 rotates, the reflecting surface 4 is separated from the rotation axis, so that the position is shifted between the image forming position P of the line image and the reflecting surface 4 with the rotation of the reflecting surface 4 as shown in FIG. 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 7 by ΔX ′. As is well known, the deviation amount ΔX ′ is expressed by Δ assuming that the lateral magnification of the fθ lens system in the
X ′ = β ² ΔX.

FIG. 4 and FIG. 5 show the relationship between θ and ΔX when the angle between the optical axis b of the fθ lens system and the principal ray of the deflected light beam is θ in the plane of FIG. FIG. FIG. 4 illustrates a specific incident angle α described later as 90 degrees and the radius R of the inscribed circle of the rotary polygon mirror 3 as a parameter. In FIG. 5, the radius R of the inscribed circle is set to 40 mm, and the specific incident angle α is used as a parameter.

As can be seen from FIGS. 4 and 5, ΔX increases as the radius R of the inscribed circle increases and as the specific incident angle α decreases.

Here, the specific incident angle α will be described. In FIG. 1, reference symbol a indicates a principal ray of a light beam incident on the rotary polygon mirror 3, and reference symbol b indicates an optical axis of the fθ lens system. The specific incident angle α is defined as the intersection angle between the principal ray a and the optical axis b as shown in the figure. The X and Y axes are determined as shown in the figure with the position of the intersection of the principal ray a and the optical axis b as the origin, and the coordinates of the rotation axis position of the rotary polygon mirror 3 are Xc and Yc.

In order to minimize the variation of the displacement ΔX between the line image position and the reflecting surface, as known, the radius of the inscribed circle of the rotary polygon mirror is defined as R 0 <Xc <Rcos (α / 2) 0 The condition of <Yp <Rsin (α / 2) may be imposed on Xc and Yc.

In order to prevent the principal ray a of the incident light beam from existing outside the effective main scanning area and prevent the return light from the scanning surface 6 from re-entering the main scanning area on the scanning surface as ghost light, the rotating polygon mirror 3 must be used. Assuming that the number of surfaces is N and the deflection angle is θ, a condition of θ <α <(4π / N) −θ may be imposed on the above α.

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 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 fθ characteristic must be well corrected in the main scanning direction.

[Examples] Hereinafter, three specific examples will be given.

F _m in each example represents a combined focal length in the main scanning direction of the fθ lens system, this value is normalized to 100. The f _S is the composite focal length in the sub-scanning direction, 2 [Theta] is the deflection angle (in degrees), alpha-specific incidence angle (in degrees), R 'represents a radius of an inscribed circle of the rotating polygon mirror.

As shown in FIG. 1, r _jx (i = 1 to 4) is the radius of curvature of the i-th lens surface in the main scanning direction counted from the rotating polygon mirror, and r _iY (i = 1 to 4) is i. The radius of curvature of the i-th lens surface in the sub-scanning direction, d _i (i = 1 to 3) indicates the distance between the i-th lens surfaces. D ₀ is the first value from the reflection surface of the rotating polygon mirror.
The distance to the lens surface, n _j (j = 1 to 2), represents the refractive index of the j-th lens.

Further, K ₁ , K ₂ , and K ₃ are used for the above conditions (I) to
Each parameter in (III), that is, (f _1m / f _m ), (f
_2m / f _m), - it represents the value of _{{(1 / r 1Y) (} 1 / r 2Y)}.

Example 1 f _m = 100, f _S = 24.973, α = 60, 2θ = 64.0, K ₁ = 2.664, K ₂ = 1.404, K ₃ = −0.018, R ′ = 13.963, d ₀ = 21.812 ir _iX r _iY d _i j n _i 1 −74.467 −14.893 3.838 1 1.76605 2 −55.778 −55.778 33.831 3 ∞ −70.230 8.609 2 1.76605 4 −107.567 −19.857 Example 2 f _m = 100, f _S = 28.287, α = 60,2θ _{= 64.0, K 1 = 2.664,} K 2 = 1.404, K 3 = -0.013, R '= 13.963, d 0 = 21.812 i r iX r iY d i j n i 1 -74.467 -11.170 3.838 1 1.76605 2 -55.778 - 18.617 33.831 3 ∞ −76.604 8.609 2 1.76605 4 −107.567 −20.990 Example 3 f _m = 100, f _S = 30.403, α = 60, 2θ = 64.0, K ₁ = 2.664, K ₂ = 1.404, K ₃ = −0.012 _{, R '= 13.963, d 0} = 21.812 i r iX r iY d i j n i 1 -74.467 -8.191 3.838 1 1.76605 2 -55.778 -11.170 33.831 3 ∞ -90.820 8.609 2 1.76605 4 -107.567 -22.692 Figure 6 to FIG. 8 shows aberration diagrams of Examples 1 to 3 and fθ.
9 to 11 show characteristic diagrams, and spot diagrams in the effective scanning area of the first to third embodiments.

In the field curvature diagram, the solid line indicates the sub-scanning direction, and the broken line indicates the main scanning direction. The field curvature is shown over the entire effective deflection angle. Note specific numerical values of f _m are Examples 1-3 both f _m = 26
8.574.

Each of the embodiments has good aberration.
The correction is well performed in both the sub-scanning direction. Also, the fθ characteristics are good.

[Effects of the Invention] As described above, according to the present invention, a novel fθ lens system can be provided. Since this fθ lens system has the above-described configuration, it is possible to satisfactorily correct the surface tilt of the rotating polygon mirror and satisfactorily correct the field curvature in the main and sub-scanning directions to realize optical scanning. Optical scanning becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an optical scanning device using the fθ lens system of the present invention, FIGS. 2 to 5 are diagrams for explaining optical scanning, and FIGS. 9 to 11 are aberration diagrams and fθ characteristic diagrams for the examples, and are spot diagrams for the respective examples. DESCRIPTION OF SYMBOLS 1 ... Light source device, 2 ... Cylinder lens, 3 ... Rotating polygon mirror, 5, 6 ... 1st and 2nd lens which comprises ftheta lens system

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁶ , DB name) G02B 26/10 G02B 13/00

Claims (1)

(57) [Claims] 1. A method according to claim 1, further comprising the step of: forming a substantially parallel light beam from the light source device into a long linear image in the direction corresponding to the main scanning; In a light scanning apparatus that optically scans the scanning surface at a substantially constant speed 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 rotating polygon mirror is deflected. An imaging lens system for forming an image on a scanning surface, having a function of connecting a reflection position of a rotary polygonal mirror and a scanning surface to a substantially optically conjugate relationship with respect to the sub-scanning direction, and fθ with respect to the main scanning direction. A first lens group, a second lens group, and a second lens group having a first lens and a second lens arranged in the first and second order from the side of the rotary polygon mirror toward the scanning surface. Lens has a concave surface on the object side in the main and sub-scanning directions. The second lens is an anamorphic lens having a positive refractive power in both the main and sub-scanning directions, a concave cylinder surface on the object side and an image side on the image side. The focal length of the first and second lenses in the main scanning direction is f
_{1 m,} f _2m, first lens on the object side and the image side lens surface in the sub-scanning direction of the radius of curvature of each r _1Y, r _2Y, a combined focal length of the entire system in the main scanning direction is f _m when they _{are, (I) f 1m / f} m> 2 (II) f 2m / f m> 1.2 (III) -0.05 (100 / f m) <{(1 / r 1Y) - (1 / r 2Y )} <0.0 (fθ lens system that satisfies the 100 / f _m) following condition.