JPH02266318A - Optical system for light beam scanning - Google Patents

Abstract

PURPOSE:To make a field angle large, as well while correcting surface inclination by arranging a toric lens for surface inclination correction which is nearly concentric concerning to a scanning surface and has low power in the vicinity of a spherical lens group. CONSTITUTION:The light beam which is emitted by a semiconductor laser 1 is collimated by a collimator lens 2 into nearly parallel light, which is made uniform in size by an aperture stop 3 and made incident on a cylindrical lens 4. The light beam is held parallel as it is concerning to the scanning surface and image-formed concerning to a subscanning surface to be made incident on a rotary polygon mirror 5 and the rotary polygon mirror is rotated at high- speed at the uniform speed as shown by an arrow and the incident light beam is reflected and deflected at the high speed to make a scan. The deflected scanning light beam passes through a 1st spherical lens 6 with negative power and a 2nd spherical lens 7 with positive power, and the toric lens 8 which is concentric is image-formed on a photosensitive drum 9. Consequently, curvature of field can be corrected while being balanced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical system of a light beam scanning device used in a laser beam printer, a digital copying machine, and the like.

C. Prior Art Conventionally, in this type of light beam scanning device, the
As described in No. 2-3621O, etc., it is common to use a one-rotation polygon mirror to deflect and scan a light beam. As described in the same paper, a rotating polygon mirror is
This troublesome method is necessary in view of the fact that if there is an angular error in the direction of inclination of each reflective surface (so-called surface tilt) with respect to the rotation axis, the scanning position of the scanned light beam will be displaced, which will adversely affect the final image output. In order to eliminate this influence, it has already been proposed to use a toric lens to place the rotating polygon mirror and the surface to be scanned (surface of the irradiated object) in an optical military relationship.

Furthermore, as described in US Pat. No. 4,639,072, it has been proposed to alleviate the effects of surface tilt by arranging a cylindrical lens near the surface to be scanned by the scanning beam.

[Problems to be Solved by the Invention] However, in the above conventional example using a toric lens, in order to correct aberrations to maintain optical performance, the scanning surface of the toric lens (where the scanned light beam is formed over time) The shape of the lens is similar to that of a plano-convex lens,
The shape of the spherical concave lens placed between the toric lens and the rotating polygon mirror becomes biconcave or plano-concave.

Therefore, it becomes difficult to create a concentric shape (a shape in which the centers of the curvature radii of the curved surfaces of each lens are placed at approximately the same position) to obtain a large angle of view or a large scanning angle, and the scanning angle should be around 90 degrees. It becomes a limit. Therefore, there is a limit to the miniaturization of the entire device.

On the other hand, in conventional examples in which a cylindrical lens is arranged near the scanned surface, for example, in a laser beam printer using an electrophotographic method, processing devices such as a developer and a cleaner are arranged in close contact with the photoreceptor drum. Therefore, it is undesirable to place an optical element such as a cylindrical lens near the photoreceptor drum, and if such a cylindrical lens is placed near the photoreceptor drum, it will be adversely affected by toner stains, heat, ozone, etc. Moreover, the direction of curvature of such a cylindrical lens tends to be opposite to that of a concentric shape for aberration correction, which is not preferable for increasing the angle of view.

Accordingly, one object of the present invention is to provide an optical system for a light beam scanning device having a configuration that solves the above-mentioned problems (1) that makes it possible to increase the angle of view while correcting the inclination of the surface.

[Means for Solving the Problems J] In the present invention for solving the above object, a toric lens for correcting surface inclination, which has a shape close to concentric with respect to the scanning surface and has low power, is arranged near one spherical lens group. ing.

More specifically, in order from the deflection means side that deflects and scans the light beam, a meniscus-shaped first spherical lens having a negative power and a concave surface facing the deflection means side, and a second spherical lens having a positive power. and a third meniscus-shaped toric lens with a concave surface facing the deflection means side. The toric lens No. 3
It is placed closer to the spherical lens side.

[Function] According to the optical system having the above configuration, the toric lens for surface tilt correction has a shape close to a concentric shape with respect to the scanning plane, and is arranged close to the spherical lens group, so that images can be captured over a wide angle of view. The image curvature (scanning line curvature) is reduced, and since the power is relatively weak, the balance with the power of the spherical lens group is improved, resulting in f/θ characteristics (the ideal image height is the same as the incident angle θ of the light beam and the focal point of the optical system). This property is given by the product of the distance fif, and can correct the beam spot uniformity on the scanned surface) while balancing the field curvature in the scanning line direction, that is, the meridional direction.

Furthermore, since the first spherical lens has a meniscus shape with its concave surface facing the deflection means side, and the third meniscus-shaped toric lens also has a concentric shape with its concave surface facing the deflection means side, a large angle of view, that is, scanning can be achieved. The light beams are obediently incident on each lens surface at opposite corners, which is favorable for correcting aberrations.

[Example] 1 and 2 show an embodiment of the invention.

FIG. 1 shows the situation in the scanning plane, and FIG. 2 shows the situation in the sub-scanning plane, which is perpendicular to the scanning plane and includes the optical axis.

In FIG. 1, 1 is a semiconductor laser which is a light source,
A light beam emitted from a semiconductor laser 1 is made into substantially parallel light by a collimator lens 2, and the size of the light beam is adjusted by an aperture stop 3, and then enters a cylindrical lens 4. The cylindrical lens 4 has power on the sub-scanning surface but not on the scanning surface, so
The light beam remains as parallel light on the scanning surface 5, and is almost imaged on the rotating polygon mirror 5 on the sub-scanning surface. The light beam incident on the beam is reflected here and deflected and scanned at high speed.

The deflected and scanned light beam passes through a first spherical lens 6 with negative power, a second spherical lens 7 with positive power, and a toric lens 8, and is imaged on a photosensitive drum 9.

In FIG. 2, P indicates the position of the reflecting surface of the rotating polygon mirror 5, and the light beam is focused approximately at this position on the sub-scanning surface as described above. Here, since the reflective surface of P and the image writing position on the photosensitive drum 9 are set in an almost optically conjugate relationship, even if the reflective surface of The image is formed on the same scanning line above, and a so-called surface tilt correction system for the rotating polygon mirror 5 is configured.

Here, in order to obtain a good curvature of field with a large angle of view in the scanning plane, the first spherical concave lens 6 has a meniscus shape with its concave surface facing the polygon mirror 5 side that is the deflecting means. . For the same reason, the toric lens 8 also has a meniscus shape with a concave surface facing the polygon mirror 5 in the scanning plane.

That is, except for the surface on the polygon 8I5 side of the second spherical convex lens 7, the other five surfaces are concave toward the polygon 1c 5 Ill,
Furthermore, since the centers of their curvature radii are close to each other, which is similar to a concentric system, light beams with large scanning angles also enter each surface directly (without making the incident angle excessively large), correcting aberrations. Preferably, the second spherical lens 7 has a polygon i? ii 'ti 5 Regarding the R aspect, f.
Regarding the toric lens 8, which preferably has a shape close to a flat surface that has the effect of producing negative distortion in order to obtain the θ characteristic, it is closer to the spherical lens group 6.7 than the photosensitive drum 9 while maintaining a concentric shape. In order to make the toric lens small and to reduce the curvature of field over a wide angle of view, it is preferable to arrange the toric lens at a certain position.

Furthermore, if the power related to the scanning surface of the toric lens 8 is made too strong, it will not be possible to correct the curvature of field while balancing the fo characteristic and the scanning direction, that is, the meridional direction. When f is the focal length of the trick lens 8 on the scanning plane, it is preferable to make the focal length of the lens 21 large enough to satisfy the relationship -0,1<f'/fl<0°1. The focal length Hf of the toric lens 8 in the sub-scanning plane is defined as the focal length Hf when the toric lens 8 has a concentric form in the scanning plane and the image plane in the sagittal direction (the direction perpendicular to the optical axis in the sub-scanning plane). 0.l<f,/ so that the curvature is corrected.
It is preferable to make it small (that is, increase the power) to the extent that the relationship f<0.6 is satisfied.

With the above configuration, the angle of view, that is, the scanning angle is 100
It is possible to realize a light beam scanning device that can perform more than 300 Hz.

An aberration diagram showing field curvature in this embodiment is shown in FIG. 3, and a diagram showing f/θ characteristics is shown in FIG. 4.

A specific example of design values is shown below as Example 1.

In this design value example, as shown in FIGS. 1 and 2, the radius of curvature on the scanning surface of each surface is R, -R from the polygon mirror sm.
,, The radius of curvature on the sub-scanning surface is R,', R,', and the distance between each surface is expressed as D I'=D s. In addition, the distance between the deflection point of the rotating polygon mirror 5 and the R8 surface is set to 9 mm, and the refractive index of each lens is 115 (N from II!).
, , N□, N.

<Example 1) Focal length of the entire system Maximum scanning angle deflection point ~ R1 plane R, = -30,68 N, = 1.57129 Rt = -35,55 Rs = ω N, = 1.63552 R, = - 78,06 R,=-59,69 N,=1.48595 R,=-4,968 Ra=-5Q,84 R,=-5,284 f/f,=0.007(5) f*/ f = 0, 355 Next, FIG. 5 shows a second embodiment. In the second embodiment,
The power of the toric lens 18 is spherical lens group 16.1
7, the power is larger than that of the first embodiment.

D,=1 ps=t.

D, = 40 D, = 5 15mm 105' mm D, = 11°Da = 88. 7 A specific design value example of this example is shown below as Example 2, and the code conventions are the same as in Example 1.

(Example 2) Focal length of the entire system 115 mm Maximum scanning angle 105° Deflection point ~ R1 plane 9 mm Rj=-27,3590,=11.4 N+ ''1.51072 R,=-32,3650,=1 Rh= ao Ds=ION*=1
, 63552 R,=-78,060,=40 Rs ''-111,050s =5 Nx=1,48595 R, knee 5.315 R,=-107,51D@=89.0SR-' =-5
, 504 f/f,=Q, 024 fl/f=0.361 FIG. 6 shows the third embodiment. In the third embodiment, the toric lens 28 is arranged closer to the spherical lens group. , even a small toric lens 28 can accommodate a wide scanning angle.

A specific example of design values is shown below as a third embodiment, and the conventions of symbols are the same as in the first embodiment.

(Example 3) Focal path 9 of the entire system 115 mm Maximum scanning angle lO5° Deflection point to n1 plane 9 mm R1 = -27,359DI = 11.4Nl = 1.5
1072 R,=-32,365D,="l R,=OO[),:IQ Nm=1.63552 R,=-78,0604=5 Rs=-45,82Ds=5 Ns=1,48592 R ,' =4.42 R,=-46,140,=127.7Re =4
．． 31 f/f,=0. 035 f, /f=0. 223 [Effects] According to the present invention having the above configuration, since it has a concentric configuration, the scanning angle can be 100 degrees or more,
Since the toric lens is arranged near the spherical lens group, the entire optical beam scanning device can be compactly assembled and can also be integrated into one unit.

Additionally, since the power on the scanning surface of toric lenses is weak, toric lenses can also be formed using plastic materials whose refractive index tends to change due to environmental changes or whose refractive index tends to fluctuate greatly during molding. can. Therefore, it is effective in reducing costs, and if it is made of plastic, it is possible to make the toric surface aspherical, so the performance can be further improved.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the state in the scanning plane of the first embodiment of the present invention, FIG. 2 is a diagram showing the state in the sub-scanning plane of the first embodiment, and FIG. 3 is a diagram showing the image curvature of the first embodiment. FIG. 4 is an aberration diagram explaining the f/θ characteristics of the first embodiment.
FIG. 5 is a diagram for explaining the second embodiment, and FIG. 6 is a diagram for explaining the third embodiment. 1... Semiconductor laser, 2... Collimator lens, 3... Aperture stop, 4... Cylindrical lens, 5... Rotating polygon mirror, 6... ...First spherical lens %7...Second spherical lens, 8...
...Toric lens, 9...Photosensitive drum

Claims (1)

[Scope of Claims] (1) In a scanning optical system disposed between the deflection means and the surface to be scanned in an optical beam scanning device that scans a surface to be scanned by deflecting a beam by a deflection means, the deflection means A spherical lens group and a toric lens are arranged in order from the side,
A scanning optical system characterized in that the toric lens has a shape close to concentric with respect to a scanning surface and its power is relatively weak. (2) The spherical lens group includes a meniscus-shaped first spherical lens having a negative power and having a concave surface facing the deflection means side, and a second spherical lens having a positive power, which are arranged in order from the deflection means side. The toric lens is a meniscus-shaped toric lens with a concave surface facing the deflecting means, and the toric lens is disposed closer to the second spherical lens than an intermediate position between the second spherical lens and the surface to be scanned. 2. The scanning optical system according to claim 1. (3) The surface of the second spherical lens on the deflection means side has a shape close to a flat surface, and the five surfaces of the first and second spherical lenses and the toric lens other than this surface have concave surfaces facing the deflection means side. 3. The scanning optical system according to claim 2, having a form close to concentric. (4) When the focal length of the toric lens in the scanning plane is f_1, the focal length in the sub-scanning plane is f_2, and the combined focal length of the first and second spherical lenses is f, -0.1<f/f_1 The scanning optical system according to claim 2, which satisfies <0.1 0.1<f_2/f<0.6. (5) The scanning optical system according to claim 1, wherein the toric lens is made of a plastic material.