JP2840354B2 - 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.

When deflecting a light beam for optical scanning at a constant angular velocity using a deflecting device such as a rotary polygon mirror, an fθ lens system is generally used for uniformity of optical scanning. In addition, a deflecting device such as a rotary polygon mirror has a problem of a so-called "surface tilt" of the deflecting reflection surface.

As a measure to solve the problem of the surface tilt by the fθ lens system itself, it has been proposed to configure the fθ lens system anamorphically by including a special lens surface called a toric surface in the lens surface of the fθ lens system (for example, JP-A-57-35825 and JP-A-59-147316).

[Problems to be Solved by the Invention] The anamorphic fθ lens system as described above includes a toric surface, which is a special surface that is not rotationally symmetric.

The present invention has been made in view of the above-described circumstances, and can be configured by a combination of spherical lenses that can be easily manufactured, and can be effectively corrected by combining with a cylinder lens for correcting surface tilt. The objective is to provide an fθ lens system.

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

The fθ lens system according to the present invention is configured such that “a substantially parallel light beam from the light source device is made incident on the deflecting / reflecting surface of the deflecting device via a spot diameter correcting cylinder lens having a positive power in the sub-scanning direction, and An image forming apparatus used in an optical scanning device which deflects at an angular velocity, forms an image of the deflected light beam as a light spot on a scanning surface by an imaging lens system and a cylinder lens for correcting surface tilt, and optically scans the scanning surface. This lens system has an fθ function in the main scanning direction.

The fθ lens system has a two-group, two-element configuration including first and second lenses arranged in the first and second order from the side of the deflection device toward the scanning surface.

The “first lens” is a positive meniscus lens having a concave surface facing the deflecting device, and the “second lens” has a refractive power on the scanning surface side which is stronger than that on the deflecting device side. It is a biconvex lens.

Tell me the combined focal length of the whole system from the deflecting device side, f
When the radius of curvature of the second lens surface is R _i , the surface interval is D _i , and the distance from the origin of deflection of the deflected light beam by the deflecting device to the first lens surface is D ₀ , R ₁ , R ₂ , D ₀ , D ₂ , and f are (I) −0.4 <R ₁ /f<−0.2 (II) 1.0 <R ₁ / R ₂ <1.3 (III) −0.6 <D ₀ </ R ₁ <−0.2 (IV ) 0.15 satisfies <D ₂ /f<0.3 following condition.

[Operation] The above conditions (I), (II), (III) and (IV) have the following meanings.

That is, the condition (I) is a condition for favorably correcting the amount of curvature of field in the main scanning direction. If the upper limit of the condition (I) is exceeded, the curvature of field in the main scanning direction will be under. Be over.

Condition (I) is also a condition for favorably correcting the amount of curvature of field in the main scanning direction. If the upper limit of condition (II) is exceeded, the curvature of field in the main scanning direction will be over, and if it exceeds the lower limit, it will be under. Become.

The condition (III) is a condition for maintaining the field curvature in the main scanning direction and the fθ characteristic, that is, the linearity in good condition. The field curvature in the main scanning direction exceeds the upper limit of the condition (III) and becomes lower. If it exceeds, it will be under. Also, the linearity becomes over when the value goes out of the range of the condition (III).

The condition (IV) is also a condition for maintaining good fθ characteristics. The linearity is under when the upper limit of the condition (IV) is exceeded and is over when the lower limit of the condition (IV) is exceeded, deteriorating the uniformity of optical scanning.

As is well known, even if the field curvature of the fθ lens system itself in the sub-scanning direction is considerably large, there is no problem because it can be easily removed by using a cylinder lens for correcting surface tilt.

Referring to FIG. 1, this figure schematically illustrates an example of an optical scanning apparatus using the fθ lens system of the present invention.
FIG. 2A shows a cross section including the main scanning direction and the optical axis of the optical arrangement of the optical scanning device.

A substantially parallel light beam from a light source or a light source device 1 including a light source and a collimating lens is incident on a deflecting reflecting surface 4 of a rotary polygon mirror 3 as a deflecting device via a cylinder lens 2 and is reflected by the deflecting reflecting surface 4. . The light beam reflected by the deflecting / reflecting surface 4 is deflected by the rotation of the rotary polygon mirror 3 and becomes a deflected light beam, which passes through the lenses 5, 6, 7;
Is formed as an optical spot on the scanning surface 8 by the image forming operation, and the scanning surface 8 is optically scanned at a constant speed.

The fθ lens system includes a first lens 5 provided immediately on the deflecting device and a second lens 6 provided immediately on the scanning surface.

The lens 7 is a cylinder lens for correcting surface tilt, and has a positive refractive power only in the sub-scanning direction.

As seen from the sub-scanning direction, as shown in FIG.
Is linked to a geometrically optically conjugate relationship.

FIG. 1 (B) shows the optical arrangement of the optical scanning device developed along the optical path and the vertical direction is the sub-scanning direction.

The cylinder lens 2 is for correcting the spot system of the light spot in the sub-scanning direction, and has a weak positive refractive power only in the sub-scanning corresponding direction. As can be seen from FIG. 1B, the image formed in the sub-scanning direction mostly depends on the refractive power of the cylinder lens 7. For this reason, as shown in the figure, when the deflecting / reflecting surface 4 is tilted as indicated by reference numeral 4 ', the light beam passing through the cylinder lens 7 changes as shown by a broken line, but the image forming position on the scanning surface 8 is in the sub-scanning direction. It hardly moves in the vertical direction (FIG. 1B). Therefore, the tilting is corrected.

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

In each embodiment, f represents the combined focal length of the fθ lens system, and this value is normalized to 100. 2θ indicates a deflection angle (unit: degree). As shown in FIG. 1 (A), R _i is the radius of curvature of the i-th lens surface counted from the side of the deflection device, D _i
Is the i-th surface interval, D ₀ is the distance from the reflecting surface of the rotating polygon mirror to the first lens surface, and _ni is the refractive index of the j-th lens.

Furthermore, K ₁ = R ₁ / f, K ₂ = R ₁ / R ₂ , K ₃ = D ₀ / R ₁ , and K ₄ = D ₂ / f.

Example 1 f = 100, 2θ = 60.0, K ₁ = −0.248, K ₂ = 1.162, K ₃ = −0.544, K ₄ = 0.247, D ₀ = 13,514 i R _i d _i j n _i 1 −24.842 2.424 1 1.48601 2 −21.379 24.707 3 204.986 8.894 2 1.51118 4 −121.839 Example 2 f = 100, 2θ = 72.0, K ₁ = −0.279, K ₂ = 1.124, K ₃ = −0.417, K ₄ = 0.242, D ₀ = 11,636 i R _i d _i j n _i 1 −27.905 2.909 1 1.48601 2 −24.826 24.193 3 386.837 11.012 2 1.51118 4 −81.107 Example 3 f = 100, 2θ = 80.0, K ₁ = −0.263, K ₂ = 1.120, K ₃ = −0.447, K ₄ = 0.200, D ₀ = 11,749 i R _i d _i j n _i 1 −26.296 3.232 1 1.48601 2 −23.474 19.969 3 304.419 11.858 2 1.51118 4 −90.864 Example 4 f = 100, 2θ = 90.0, K ₁ = -0.263, K ₂ = 1.081, K ₃ = -0.350, K ₄ = 0.175, D ₀ = 9,209 i R _i d _i j n _i 1 -26.296 3.636 1 1.48601 2 -24.326 17.484 3 441.605 13.605 2 1.51118 4 −77.110 Note that the specific numerical value of f is f =
206.262, f = 171.887 in the second embodiment, f = 154.699 in the third embodiment, and 137.509 in the fourth embodiment.

2 to 5 show field curvature diagrams and fθ characteristic diagrams for the first to fourth embodiments. The solid line in the figure of curvature of field indicates the image forming position in the sub-scanning direction, and the broken line indicates the image forming position in the main scanning direction.

In calculating these aberration diagrams, the use of the cylinder lenses 2 and 7 in FIG. 1 is omitted.
Since the cylinder lenses 2 and 7 both act only in the direction corresponding to the sub-scanning, their presence does not affect the curvature of field in the main scanning direction and the fθ characteristic at all. Since the use of the cylinder lens 7 is omitted, the field curvature in the sub-scanning direction is considerably large in each of the embodiments. However, the field curvature in the sub-scanning direction can be eliminated by using the cylinder lens 7.

[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 curvature of field in the main scanning direction, and has excellent fθ characteristics, so that extremely good optical scanning is possible. Further, since it is used together with a cylinder lens for correcting surface tilt, it can be constituted only by a spherical lens which is easy to manufacture, and a low refractive index material can be used, so that an inexpensive plastic can be used.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an example of an optical scanning device using the fθ lens system of the present invention, and FIGS. 2 to 5 are diagrams showing field curvature diagrams and fθ characteristics according to each embodiment. It is. 5 ... first lens, 6 ... second lens

Claims (1)

(57) [Claims] 1. A substantially parallel light beam from a light source device is made incident on a deflecting / reflecting surface of a deflecting device via a spot diameter correcting cylinder lens having a positive power in the sub-scanning direction, and the deflecting device makes the deflecting device have uniform angular velocity. This is an imaging lens system used in an optical scanning apparatus that optically scans a scanning surface by deflecting the deflected light beam to form an image as a light spot on the scanning surface with an imaging lens system and a cylinder lens for correcting surface tilt. A first group and a second lens having an fθ function with respect to the main scanning direction and arranged in a first and second order from the deflecting device side toward the scanning surface side. The first lens is a positive meniscus lens having a concave surface facing the deflecting device, and the second lens is higher in refractive power on the scanning surface than on the deflecting device. A biconvex lens The combined focal length of the entire system f, and the radius of curvature of the i-th lens surface as counted from the deflector side R _i, the surface distance D _i, first lens surface from the origin of the deflection of the deflected light beam by the deflection device when the D ₀ the leading _{distance, R 1, R 2, D} 0, D 2, f is _{(I) -0.4 <R 1 /f<-0.2} (II) 1.0 <R 1 / R 2 <1.3 (III) An fθ lens system characterized by satisfying a condition of −0.6 <D ₀ </ R ₁ <−0.2 (IV) 0.15 <D ₂ /f<0.3.