JPH0511290B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a light scanning optical system, and particularly to a cylindrical lens used in a scanning light optical system using a rotating polygon mirror or the like and a cylindrical lens for surface tilt correction.

(Prior Art) In cases where it is necessary to scan a light beam, such as in a laser printer, a rotating device such as a rotating polygon mirror or a hologram disk is often used as a scanning device. However, in these scanning devices, it is known that an angular error of the reflecting surface of the polygon mirror with respect to the rotation axis or an angular error of the hologram with respect to the disk surface results in uneven pitch of the scanning line. In order to correct this pitch unevenness, a cylindrical lens that has refractive power only in the plane perpendicular to the plane containing the scanning line is placed near the scanning plane, and within this plane, the reflective surface etc. and the scanning plane are geometrically conjugated. It is being placed at a location where

This scanning optical system using a cylindrical lens has a large correctable range and is widely used, but it has been found that problems arise when attempting to reduce the spot diameter in order to improve image quality. That is, near the end of the scanning line, the beam enters the cylindrical lens obliquely, the effective focal length of the cylindrical lens becomes small, the focal spot diameter increases due to defocusing, and the reflection surface etc. and the scanning surface become Since the geometrical optical conjugate relationship is violated, the pitch unevenness of the scanning line cannot be completely corrected.

On the other hand, the cylindrical lens for surface tilt correction is arranged so that it curves into a concave arc toward the scanning surface so that as the incident angle of the beam increases, the cylindrical lens approaches the scanning surface. is proposed. (For example, Showa 58
However, this method requires the plastic cylindrical lens to be compressed and curved between lens frames with curved surfaces. Processing a lens frame with a curved surface increases costs, and since the cylindrical lens also needs to be bent, there are restrictions on the plastic material and its thickness. Furthermore, if the cylindrical lens is placed away from the scanning surface, the amount of defocus will increase, and the radius of curvature of the lens will become smaller, resulting in a problem of deterioration of imaging performance.

(Objective of the Invention) The present invention aims to obtain a cylindrical lens that can constitute a scanning optical system that does not have the above-mentioned drawbacks by gradually weakening the refractive power of the cylindrical lens from the center of the scanning line toward the ends. It is something.

(Structure of the Invention) The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a conceptual diagram showing the overall configuration of a scanning optical system using a cylindrical lens according to the present invention.

Normally, an incident beam 1, which is a parallel light beam, is reflected by a rotating polygon mirror 2 for scanning, and focused and scanned onto a photoreceptor surface 4 by a θ lens 3 formed in a rectangular shape.
In order to prevent pitch unevenness due to the tilting of the reflective surface of the rotating polygon mirror 2, a cylindrical lens 5 having refractive power only within the plane perpendicular to the plane including the scanning line shown in FIG. 1b is arranged. The scanning line on the photoreceptor surface 4 and the reflecting mirror surface are kept in a geometrically optically conjugate relationship. As a result, even if the reflected light of the incident light 1 swings up and down in the figure due to an angular error of the reflecting surface, the incident position on the photoreceptor surface 4 will not be affected.

However, due to a change in the angle of incidence of the scanning light 6 on the cylindrical lens 5, the effective focal length of the cylindrical lens 5 becomes as follows: where the focal length of the cylindrical lens 5 is the refractive index of the lens material and n is -(sin/n) ² } ^1/2 (1), and the focusing position of the scanning beam shifts to the front of the photoreceptor surface 4, as shown by the dotted line 7 in FIG. As mentioned above, this Tef focus causes spot enlargement and scanning line pitch unevenness.

In this invention, the focal length of the cylindrical lens 5 is changed from the center to the edge according to the incident angle of the scanning light 6 so that = ₀ / {1-(sin/n) ² } ^1/2 ... (2) It is designed to increase in size according to the following. However, is the focal length of the part where the incident angle is , and ₀ is the focal length of the part on the optical axis, that is, =0.

In this way, the effective focal length ′ for the scanning beam at the incident angle becomes ′={1−(sin/n ² } ^1/2 = ₀ according to equation (1), and the focusing position of the scanning beam 6 is set to the photoreceptor surface 4. can be made to match.

A cylindrical lens with the above-mentioned refractive power distribution has a height h from the optical axis = 0 of the lens depending on the incident angle, as shown in the cross-sectional shape in Figure 2.
The curvature R at R=R ₀ /{1-(sin/n) ² } ^1/2 , where R ₀ may be the radius of curvature on the optical axis.

(Effects of the Invention) With the above-described configuration, the present invention eliminates the need to curve the 1-cylindrical lens, thereby reducing the cost of the lens frame.

2. Since there is no need to curve the cylindrical lens, it is easy to arrange it at a position away from the scanning surface, and there are no restrictions on lens arrangement.

3. It is easy to handle because it can be a rod-shaped cylindrical lens.

4. If the lens is made of plastic mold, it is easy to prevent the cost increase of the lens itself.

It has remarkable effects such as

[Brief explanation of the drawing]

Fig. 1 is a conceptual diagram of a scanning optical system using the cylindrical lens of this invention, and Fig. 2 is a top view and a sectional view of each part of an embodiment of the cylindrical lens of this invention. 1...Incoming beam, 2... Rotating polygon mirror , 3...
θ lens, 4... Photoreceptor scanning surface, 5... Cylindrical lens, 6... Scanning beam, 7... Beam focusing position.

Claims (1)

[Scope of Claims] 1. A scanning/rotating device such as a rotating polygon mirror, a focusing lens, and a cylindrical lens having refractive power only in a plane perpendicular to the scanning line, in which the rotating scanning unit and the scanning line are connected to each other by geometric optics. The above-mentioned cylindrical lens is used in a scanning optical system arranged at a conjugate position, and the optical axis of the lens is adjusted from 0 according to the angle of incidence so that the effective focal length depending on the angle of incidence on the lens is constant. The curvature R at the height h is R=R ₀ /{1-(sin/n) ² } ^1/2 , where R ₀ : radius of curvature on the optical axis, n : refractive index of the lens material. Cylindrical lens for surface tilt correction scanning optical system.