JPS6350810A - Optical scanner - Google Patents

Abstract

PURPOSE:To constitute an optical system which has a surface tilt correcting function at low cost and to make a scan with high accuracy by providing three lenses, i.e. a spherical lens which has positive power, a cylindrical lens, and a spherical lens with negative power in order from the reflecting surface of a rotary polygon mirror. CONSTITUTION:The cylindrical lens 2 is arranged closely to the rotary polygon mirror 4. The positive lens 1 and negative lens 3 are arranged in front of and behind the cylindrical lens 2 to deflect luminous flux only in front of and behind the cylindrical lens and made it incident on the cylindrical lens 2 from the front, thereby eliminating a sagittal image surface curvature. Namely, the luminous flux L which is deflected by the reflecting surface of the rotary polygon mirror 4 is refracted inward by the positive lens 1 and made incident on the cylindrical lens 2 almost from the front. Further, luminous flux which is projected from the cylindrical lens 3 almost in parallel to the optical axis is expanded again outward by the negative lens 3 to obtain sufficient scanning width. Further, the curvature and surface distance of each lens surface are set to specific values to compensate the sagittal image surface curvature, tangential image plane curvature, velocity equality, etc.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a rotating polygonal mirror type optical scanning device used in a laser beam printer, etc., and in particular, to a rotating polygonal mirror for reducing errors in parallelism between each tii plane of the rotating polygonal mirror and the rotation axis. The present invention relates to the configuration of a scanning optical system having a correction function (correction of surface tilt error).

[Conventional technology]

The scanning optical system of a rotating polygon mirror optical scanning device used in laser beam printers, etc. generally corrects the curvature of the field,
It is designed for the purpose of correcting the surface tilt error of the rotating polygon mirror that causes scanning pitch unevenness, and applying distortion so that the scanning speed becomes constant on the scanning surface.

Various types of tilt correction optical systems have been proposed in the past, but basically, an anamorphic quick optical system with different curvatures in the direction of the deflection plane and in the direction perpendicular to it is used. The principle used is that by arranging the reflection point of the polygon mirror and the scanning surface at a conjugate image point or close to the conjugate image point in terms of direction, fluctuations in the scanning point position are suppressed even if the output angle changes due to the inclination of the polygon mirror surface.

As a configuration for realizing the above-mentioned principle, Japanese Patent Application Laid-Open No. 57-144
As disclosed in No. 514, a method using a lens having a toric surface having a major axis and a minor axis having refractive power in two orthogonal directions is known. Figure 4 (α), (b
) shows a tilt correction optical system using a toric lens. Figure 4 (α) is a diagram showing the light flux in the direction perpendicular to the deflection plane.
(b) is a diagram showing the light flux in the direction of the deflection surface, and the surface Sα is a toric surface. The light beam is reflected by the rotating polygon mirror 14 and passes through the spherical lens 11. An image is formed on a scanning plane 15 by a toric lens 12 . As shown by the broken line in FIG. 4 (α), since the reflection point of the polygon mirror and the scanning surface are conjugate points, even if a tilting error occurs, the scanning point position does not fluctuate. Furthermore, since the spherical lens 11 has negative power and the toric lens 12 has positive power,
This scanning lens system has a distortion such that the imaging spot on the scanning surface moves at a constant speed, and is a so-called 6/θ lens.

A method using a long cylindrical lens is also known, as disclosed in Japanese Patent Application Laid-open No. 153416/1983. Fifth
Figures C(1) and Cb) show a tilt correction optical system using a long cylindrical lens. FIG. 5(α) is a diagram showing the light flux in the direction perpendicular to the deflection surface, and FIG. 5(b) is a diagram showing the light flux in the direction of the deflection surface. Similarly, face tilt correction is performed. Also, the spherical lens 21
is a meniscus lens, and becomes an fθ lens by setting the inter-plane distance and curvature of each refractive surface to predetermined values.

[Problem that the invention seeks to solve]

Now, both of the above-mentioned two conventional optical systems have the problem that the lenses are expensive. The reason will be explained below.

In the method of using an increasingly long cylindrical lens, the cylindrical lens must be placed close to the scanning plane in order to suppress the spherical field curvature generated by the cylindrical lens to below an allowable value, which is equivalent to the scanning width. A cylindrical lens with a length of is required. This is because the spherical curvature of field caused by a cylindrical lens is approximately proportional to the distance between the cylindrical lens surface and the scanning surface, so as the distance becomes shorter, the absolute curvature of field becomes This is because M decreases. Such a long cylindrical lens is extremely expensive compared to a normal lens with a small diameter.

Now, the spherical curvature of field that occurs in the above-mentioned cylindrical lens is caused by the light beam obliquely entering the cylindrical lens when the scanning angle is large.

This is because when the light beam cuts the cylinder 32 obliquely as shown in the light beam B in FIG. 6, the curvature acts more strongly than when it cuts the cylinder 32 from the front like the light beam A. The aforementioned toric surface can be thought of as a curved surface created by bending what should be a cylindrical surface so that the light beam always enters from the front, in order to avoid this effect. If a toric surface is used in this way, spherical curvature of field can be suppressed, so the lens surface can be placed closer to the polygon mirror, and the lens aperture can be made smaller. However, since a toric lens is a special lens that is not used for normal purposes, its manufacturing cost is high, and the effect of eliminating the long cylindrical lens is lost.

As described above, the conventional surface tilt correction optical system has the drawback that the manufacturing cost of the lens is high.

The present invention has been made to solve the above-mentioned problems, and its purpose is to provide an optical scanning device that can perform highly accurate scanning by inexpensively using an optical system that has a surface tilt correction function. The aim is to provide it at a low price.

[Means for solving problems]

The present invention includes a light beam generating means, a rotating polygon mirror deflecting device for deflecting and scanning the light beam, and a rotating polygon mirror deflector for deflecting the light beam to form a linear light beam in a direction parallel to the scanning direction on the reflecting surface of the rotating polygon mirror. The first condensing optical system disposed and the second condensing optical system disposed so as to form a point-like image of the linear light beam on the scanned plane without field curvature are cleaned, and the second condensing optical system is cleaned. The light optical system consists of three lenses in order from the reflecting surface of the rotating polygon mirror: a spherical lens with positive power, a cylindrical lens with positive power only in the direction perpendicular to the scanning direction, and a spherical lens with negative power. [Example] The optical system according to the present invention will be explained using an example. 1st
Figures (1) and (b) are optical path diagrams showing the light beams in the direction perpendicular to the deflection plane and in the direction of the deflection plane, respectively, in the scanning optical system of the present invention.

The main objective of the present invention is to correct surface tilt using a small cylindrical lens without increasing the length by arranging cylindrical lenses in a polygonal mirror band. In this case, the problem was the spherical curvature of field as described in the conventional example. It was also stated that in order to prevent the occurrence of spherical curvature of field, it is desirable that the light beam be incident from the front onto a surface that converges the light beam in a direction perpendicular to the left of the cylindrical deflection surface. A conventional toric lens can be considered to be a cylindrical lens bent for this purpose.

On the contrary, the invention has a structure in which a positive lens and a negative lens are arranged in front and behind a cylindrical lens, so that the light beam is bent only in front and behind the cylindrical lens and enters the cylindrical lens from the front, creating a spherical image surface. This prevents curvature. In FIG. 1Cb), the light beam deflected by the reflecting surface S1 of the rotating polygon mirror 40 is refracted inward by the positive lens 1 and enters the cylindrical lens 2 almost from the front. Furthermore, the light beam emitted from the cylindrical lens substantially parallel to the optical axis is expanded outward again by the negative lens 3, making it possible to obtain a sufficient scanning width. The broken line in Fig. 1 (α) shows the surface tilt correction effect in the scanning optical system of the present invention. Even if the direction of light/bundle emission changes due to the tilted polygonal mirror surface, the scanning point position changes. The above explanation is qualitative, but quantitatively, as shown in the numerical examples in Tables 1 and 2 and the aberration diagrams in Figures 2 and 3, the curvature and By setting the inter-plane distance to a predetermined value, spherical field curvature correction, meridional field curvature correction, and scanning uniformity correction are performed.

However, the initial imaging distance Sm=-261 Note in Table 2) oRm is the radius of curvature in the direction of the deflection surface.R8 is the radius of curvature in the direction perpendicular to the deflection surface. Point between the surface of

The O first surface is a reflecting surface of a rotating polygon mirror.

The initial imaging distance Sm is the degree of convergence or divergence of the light beam incident on the rotating polygon mirror, and is expressed as the distance of the imaging point measured from the reflection point of the polygon mirror.

Figure 2 (α”), Cb>, (c), @Figure 3 (α), C
b) and CC) are aberration diagrams showing spherical curvature of field, meridional curvature of field, and constant velocity scanning properties in the numerical examples of Tables 1 and 2, respectively. From this, it can be seen that the image plane curvature aberration and scanning uniformity are suppressed to sufficiently good values.

[Effects of the Invention] As described above, according to the present invention, the scanning optical system that meets the rotational polygonal mirror and the tilt correction optical system is configured such that the spherical lens having positive power is sequentially formed from the reflection point of the polygonal mirror. , a cylindrical lens that has positive power only in the direction perpendicular to the deflection plane, and a spherical lens that has negative power only in the direction perpendicular to the deflection plane. Since it is configured to have an image point conjugate with It has this effect.

[Brief explanation of drawings]

Fig. 1 Ca), Cb) is an optical path diagram showing an embodiment of the present invention, Fig. 2 (α), (b), (C) and Fig. 3 (α),
(b) and (C) are aberration diagrams according to numerical examples of the present invention, Figures 4 (α) and (b) and Figures 5 (α) and (b) are optical path diagrams showing conventional scanning optical systems; Figure '6 is a diagram for explaining the occurrence of spherical curvature of field aberration. 1... Positive lens 2... Cylindrical lens 3... Negative lens 4... Rotating polygon mirror 5... Scanning surface 6. ...Luminous flux or more 1 Positive O lens (Q) Fig. 1 Fig. 2

Claims (1)

[Claims] a light beam generating means, a rotating polygon mirror deflecting device for deflecting and scanning the light beam, and the light beam is arranged to form a linear light beam in a direction parallel to the scanning direction on the reflecting surface of the rotating polygon mirror. a first condensing optical system; a second condensing optical system arranged so that the linear light beam forms a dot-like image on the scanned plane without field curvature, and the imaged spot scans at a constant speed; Equipped with
The second condensing optical system includes three lenses, in order from the reflecting surface of the rotating polygon mirror: a ball lens having positive power, a cylindrical lens having positive power only in a direction perpendicular to the scanning direction, and a ball lens having negative power. An optical scanning device characterized by being composed of a single lens.