JPH03221910A - Ftheta lens system in optical scanner - Google Patents

Abstract

PURPOSE:To obtain the ftheta lens system which can correct effectively a surface tilt by constituting it of combination of spherical lenses which can be manufactured easily, and also, combining it with a cylinder lens for correcting a surface tilt. CONSTITUTION:The ftheta lens system is an image forming lens system which has an ftheta function with regard to the main scanning direction, and is constituted of two groups and two pieces in which a first lens 5 and a second lens 6 are placed in order from the side of a deflecting device to the scanning face side. A first lens 5 is a positive meniscus lens whose concave face is turned to the deflecting device side, and a second lens 6 is a biconvex lens in which refracting force of the lens face of the scanning face side is stronger than refracting force of the lens face of the deflecting device side. When a composite focal distance of the whole system, a radius of curvature of an i-th lens face counted from the deflecting device side, a face interval, and a distance extending from an origin of deflection of a deflecting luminous flux by the deflecting device to a first lens face are denoted as (f), Ri, Di, and Do, respectively, it is a feature that R1, R2, Do, and (f) satisfy the conditions of an expression I, an expression II, an expression III, and an expression IV. As for this lens system, the curvature of image in the main scanning direction is small, and the ftheta characteristic becomes satisfactory.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an FE lens system in an optical scanning device.

[Prior Art] Optical scanning devices are known as devices that write and read information by scanning a beam of light, and are used in laser printers, facsimile machines, and the like.

When deflecting a light beam for optical scanning at a constant angular velocity using a deflection device such as a rotating polygon mirror, an FE lens system is generally used to ensure constant velocity of optical scanning.

Furthermore, deflection devices such as rotating polygon mirrors have the problem of so-called "face tilt" of the deflection reflection surface.

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

[Problems to be Solved by the Invention] Since the anamorphic fθ lens system as described above includes a special surface called a toric surface that is not rotationally symmetrical, it is not necessarily easy to manufacture, and there is a problem that the cost is high.

The present invention was made in view of the above-mentioned circumstances, and
The purpose of the present invention is to provide a novel fθ lens system that can be configured by a combination of easily manufactured spherical lenses and can effectively correct surface tilt by combining it with a cylinder lens for surface tilt correction.

[Means to solve the problem] The present invention will be explained below.

The f-theta lens system of the present invention is characterized in that a substantially parallel light beam from a light source device is made incident on a deflection reflection surface of a deflection device through a cylinder lens for spot diameter correction having positive power in the sub-scanning direction, and the deflection device An imaging device used in an optical scanning device that deflects light at an angular velocity and images this deflected light beam as a light spot on the scanning surface using an imaging lens system and a cylinder lens for surface tilt correction to optically scan the scanning surface. It is a lens system and has an f8 function in the main scanning direction.

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

The "first lens" is a positive meniscus lens with a concave surface facing the deflection device side, and the "second lens" has a refractive power of the lens surface on the scanning surface side that is greater than the refractive power of the first lens surface on the deflection device side. It is a strong biconvex lens.

The combined focal length of the entire system is f1, counting from the deflection device side, the radius of curvature of the first lens surface is Ro, the distance between the surfaces is Dl, and it reaches the first lens surface from the starting point of deflection of the deflected light beam by the deflection device. Take the distance. When R,, R2, D0, D
2. f is (1) −0,4< R,/f
< -0,2(II) 1.0 < R,
/R2<1.3(III) -0,6<D
o/R+ < -0,2(IV) 0.15
< D2/f < 0.3 is satisfied.

[Action under the above conditions (I), (II), (III), (IV
) has the following meanings:

In other words, condition (I) is a condition for properly correcting the amount of curvature of field in the main scanning direction; if the upper limit of condition (I) is exceeded, the curvature of field in the main scanning direction will be under, and if the lower limit is exceeded, the curvature of field in the main scanning direction will be under. It becomes over.

Condition (1) is also a condition for properly 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 the lower limit is exceeded, it will be under. Become.

Condition (III) is a condition for maintaining good curvature of field in the main scanning direction and FE characteristics, that is, linearity.If the curvature of field in the main scanning direction exceeds the upper limit of condition (II), it will exceed the lower limit. If it exceeds, it becomes under. Furthermore, the linearity becomes excessive when it is out of the range of condition (III).

Condition (F) is also a condition for maintaining good fe characteristics, and the linearity is under when it exceeds the second limit of condition (IV), and over when it exceeds the lower limit, which impairs the uniformity of optical scanning.

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

Referring to FIG. 1, this figure illustratively suggests an example of an optical scanning device 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 beam of light from a light source device 1 consisting of a light source or a light source and a collimating lens enters a deflection reflection surface 4 of a rotating polygon mirror 3 as a deflection device via a cylinder lens 2, and is reflected by the deflection reflection surface 4. . The light beam reflected by the deflection reflection surface 4 is deflected by the rotation of the rotating polygon mirror 3, becomes a deflected light beam, and passes through the lenses 5, 8.7.
, 6°7, an image is formed as a light spot on the scanning surface 8, and the scanning surface 8 is optically scanned at a uniform speed.

The fθ1 non-lens system is composed of a first lens 5 disposed on the deflection device side and a second lens 6 disposed on the scanning surface side.

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

As shown in FIG. 1(A), when viewed from the sub-scanning direction, the fθ lens system consisting of lenses 5 and 6 is located at infinity on the light source side and at the scanning surface 8.
and the position of , in a geometrically optically approximately conjugate relationship.

In FIG. 1(B), the optical arrangement of the optical scanning device is 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 direction corresponding to the sub-scanning.

As seen in FIG. 1(B), most of the imaging in the sub-scanning direction is based on the refractive power of the cylinder lens 7. Therefore, as shown in the figure, when the deflection-reflection surface 4 is tilted as indicated by the symbol 4', the light ray passing through the cylinder lens 7 changes as shown by the broken line, but the image formation position on the scanning surface 8 is in the sub-scanning direction. It hardly moves in the vertical direction (FIG. 1(B)). Therefore, the surface inclination is corrected.

[Example] Four specific examples are listed below.

In each example, f represents the composite focal length of the fθ lens system, and this value is normalized to 100. Further, 2θ indicates the deflection angle (unit: twice). As shown in FIG. 1(A), R4 is the radius of curvature of the i-th lens surface counting from the deflection device side;
D represents the i-th surface spacing, Do represents the distance from the reflecting surface of the rotating polygon mirror to the first lens surface, and 1.1 represents the refractive index of the j-th lens.

Furthermore, K, = R1/f, 2 = RI/R2, 3 = D
O/R□ represents =D2/f.

Example 1 f=100.2θ=60.0, +=-0,248
, 2"1.162°K3=-0,544, R4=0.
247. Do”13.513i R,d,
j nl -24,8422,424
11,486012-21,37924,707 3204,9868,89421,511184-12
1,839 Example 2 f=100.2θ=72.0, =-0,279
, 2=1.124°R3”-0,417, Ka=0.
242. Do”11.636i R,d,
j nil -27,9052,90
911,486012-24,82624,193 3386,83711,01221,511184-8
1,107 Example 3 f=100.2θ=80.0, Kl”-0,263
, 2"1.120°R3"-0,447, K4=0.
200. Do=11.749” Ri
d i jn Hl -26,29
63,23211,486012-23,47419,
969 3304, 41911, 85821, 511184-9
0464 Example 4 f=100, 2θ=90.0, =-0,263
, to,,=1.081°K,=-0,350,K,=0
．． 175. Do=9.2091R+dt
j nl -26,2963,6361
1,486012-24,32617,484 3441,60513,60521,511184-7
7,110 The specific value of f is f=2 in Example 1 above.
06.262, f=171.887 in Example 2,
In Example 3, f=154.699, and in Example 4, f=137.509.

2 to 5 show field curvature diagrams and fθ characteristic diagrams for Examples 1 to 4. 0 of curvature of field The solid line in the figure indicates the imaging position in the sub-scanning direction, and the broken line indicates the imaging position in the main scanning direction.

Note that 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 act only in the direction corresponding to the sub-scanning direction, their presence has no effect on the curvature of field in the main-scanning direction and the fθ characteristics. cylinder lens 7
Since the use of the cylinder lens 7 is omitted, the curvature of field in the sub-scanning direction is quite large in each embodiment, but this curvature of field 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 lens system for an optical scanning device can be provided. As described above, this lens system has a small curvature of field in the main scanning direction and has good fθ characteristics, so it is possible to perform extremely good optical scanning.

Furthermore, since it is used together with a cylinder lens for correcting surface tilt, it can be constructed using only a spherical lens that is easy to manufacture, and since it is possible to use a material with a low refractive index, it is also possible to use inexpensive plastic.

1

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining one 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 for each embodiment. It is. 2) Circle K)

Claims (1)

[Claims] A substantially parallel light beam from a light source device is made incident on a deflection reflecting surface of a deflection device through a cylinder lens for spot diameter correction having a positive power in the sub-scanning direction, and is deflected at a constant angular velocity by the deflection device. An imaging lens system used in an optical scanning device that optically scans the scanning surface by focusing this deflected light beam as a light spot on the scanning surface using an imaging lens system and a cylinder lens for surface tilt correction. Two groups, each consisting of a first lens and a second lens, have an fθ function in the main scanning direction and are arranged in the order of first and second from the deflection device side toward the scanning surface side. The first lens is a positive meniscus lens with a concave surface facing the deflection device, and the second lens has a refractive power of the lens surface on the scanning surface side that is greater than the refractive power of the lens surface on the deflection device side. It is a strong biconvex lens, and the combined focal length of the entire system is f, the radius of curvature of the i-th lens surface counting from the deflection device side is R_i, the distance between the surfaces is D_i, and the first distance from the origin of deflection of the deflected light beam by the deflection device is When the distance to the th lens surface is D_0, R_1, R_2,
D_0, D_2, 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
．． An fθ lens system characterized by satisfying the following condition: 15<D_2/f<0.3.