JP2643224B2 - Light beam scanning optical system - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a light beam scanning optical system, and
The present invention relates to a structure of a light beam scanning optical system that is incorporated in a printer, a facsimile, or the like, and condenses a light beam carrying image information on a photoconductor.

2. Description of the Related Art Generally, a light beam scanning optical system used in a laser beam printer or a facsimile basically includes a semiconductor laser as a light source, a deflector such as a polygon mirror and a galvano mirror, and an fθ lens. It is configured. The deflector scans the light beam emitted from the semiconductor laser at a constant angular velocity, and as it is, a difference in the scanning speed occurs from the central portion in the main scanning direction to both ends on the light condensing surface, and a uniform image cannot be obtained. . The fθ lens is provided to correct such a scanning speed difference.

By the way, the fθ lens is a combination of various concave lenses, convex lenses, and the like, the lens design is extremely complicated, the number of polished surfaces is large, it is difficult to improve the processing accuracy, and it is expensive. In addition, there is a restriction from the viewpoint of the material that a material having good translucency must be selected.

Therefore, conventionally, an elliptical mirror has been used instead of the fθ lens (Japanese Patent Application Laid-Open No. 54-123040), a parabolic mirror has been used (Japanese Patent Publication No. 55-36127),
Using a concave reflecting mirror (Japanese Patent Application Laid-Open No. 61-173212)
Has been proposed. However, an elliptical mirror or a parabolic mirror has a problem that it is difficult to increase the processing itself and the processing accuracy.

Therefore, an object of the present invention is to employ a scanning speed correction unit which can process more easily and increase the processing accuracy, instead of an expensive and restrictive fθ lens or a conventionally proposed parabolic mirror. It is an object of the present invention to make the system compact, to reduce the curvature of the image plane perpendicular to the main scanning direction at the focal point, and to effectively correct the tilt error of the deflector. That is, when a rotating polygon mirror such as a polygon mirror is used as a deflector, a perpendicularity error between each surface (plane tilt error).
When this occurs, the scanning line on the surface of the photoconductor shifts in the sub-scanning direction. The present invention is intended to correct pitch unevenness due to such a tilt error.

Means for Solving the Problems To solve the above problems, a light beam scanning optical system according to the present invention comprises: (a) a light source for generating a light beam; and (b) a light beam emitted from the light source in a scanning direction. (C) a deflector that is placed near the condensing line and scans the converged light beam at a constant angular velocity; and (d) a light beam scanned by the deflector is turned back on the photosensitive member surface. And (e) a toroidal lens disposed between the deflector and the spherical mirror.

In the above configuration, the light beam emitted from the light source is scanned at a constant angular velocity by the deflector, and the scanned light beam is reflected by the spherical mirror and condensed on the surface of the photoconductor. An image is formed by main scanning by the deflector and sub-scanning by movement of the photoconductor surface. The light flux reflected by the spherical mirror is corrected so that the scanning speed in the main scanning direction is uniform from the center of the scanning area to both ends thereof, and the light-collecting surface has good distortion characteristics over a wide angle of view, Image plane flatness is obtained.

The light beam emitted from the light source is converged linearly in the scanning direction (within the deflection plane) and is incident on the deflector. Then, the toroidal lens focuses the light beam scanned by the deflector on the surface of the photoreceptor, and corrects an error due to the tilt of the deflector.

Further, by appropriately selecting the curvature of the toroidal lens, the image plane flatness is improved.

Embodiments Hereinafter, embodiments of a light beam scanning optical system according to the present invention will be described with reference to the accompanying drawings.

In FIG. 1, (1) is a semiconductor laser, (6) is a collimator lens, (7) is a cylindrical lens, (1)
0) is a polygon mirror, (13) is a toroidal lens, (1)
5) is a beam splitter, (20) is a spherical mirror, (30)
Is a drum-shaped photoreceptor.

The semiconductor laser (1) emits a divergent light beam which is intensity-modulated by a control circuit (not shown) and carries image information. This divergent light beam is corrected into a convergent light beam by passing through the collimator lens (6). Further, this convergent light beam is converged in the scanning direction, that is, linearly (in the deflecting surface) near the reflecting surface of the following polygon mirror (10) by passing through the cylindrical lens (7). The polygon mirror (10) is rotationally driven at a constant speed in the direction of the arrow (a) around the support shaft (11) by a motor (not shown). Therefore, the convergent light beam emitted from the cylindrical lens (7) is continuously reflected on the surface of the polygon mirror (10) and scanned at a constant angular velocity. This scanning light beam is a toroidal lens (1
3) After being transmitted through the beam splitter (15), the light is reflected on the concave surface side of the spherical mirror (20) and further condensed on the photoconductor (30) after being reflected by the beam splitter (15). The condensed light beam at this time is scanned at a constant speed in the axial direction of the photoconductor (30), and this is referred to as main scanning. The photoconductor (3
0) is driven to rotate at a constant speed in the direction of the arrow (b), and scanning by this rotation is referred to as sub-scanning.

That is, in the above light beam scanning optical system, an image (electrostatic latent image) is formed on the photoconductor (30) by the intensity modulation of the semiconductor laser (1) and the main scanning and the sub-scanning. Then, as shown in FIG. 2, the spherical mirror (20) is
Instead of the lens, the scanning speed in the main scanning direction is corrected so as to be uniform from the center of the scanning area to both ends thereof.

The toroidal lens (13) set in the optical path reflected from the polygon mirror (10) is a polygon mirror (10).
The main purpose is to correct the surface tilt error. That is, if there is an error in the perpendicularity between the respective reflection surfaces of the polygon mirror (10), the scanning lines on the photoconductor (30) are displaced in the sub-scanning direction, and pitch unevenness occurs in the image. This surface tilt error can be corrected by setting each reflection surface of the polygon mirror (10) and the condensing surface of the photoconductor (30) in a conjugate relationship in a section perpendicular to the deflection surface of the polygon mirror (10). . In the present invention, the light beam is condensed on the polygon mirror (10) by the cylindrical lens (7), and each reflection surface and the light condensing surface of the polygon mirror (10) are kept conjugate by the toroidal lens (13). ing.

Further, the toroidal lens (13) sets the radius of curvature in the deflection surface to an appropriate value in order to flatten the image surface due to the light flux having a cross section perpendicular to the deflection surface by the polygon mirror (10) [[R _{1 a), (R 2 a} ) Browse, and it is preferably arranged by shifting in FIG. 2 (Y) in a direction (Y _T). Radius of curvature at the deflection plane _{(R 1 a), (R} 2 a) , the light beam reflecting point of the scanning area center by the polygon mirror (10) (hereinafter, referred to as deflection point) (10a)
Is slightly larger than the distance (d ₀ ) from to the toroidal lens (13). The appropriate shift amount (Y _T ) of the toroidal lens (13) is determined by the size of the polygon mirror (10),
It depends on the angle of view, the direction of incidence of the light beam on the polygon mirror (10), and the like. Specific examples are shown in the following experimental examples (I) to (V).

In this embodiment, the divergent light beam is corrected to a convergent light beam by the collimator lens (6). This is to correct the curvature at the converging point (imaging plane) on the photoconductor (30) by using a convergent light beam. That is, the polygon mirror (1
When a convergent light beam or a divergent light beam is incident on 0) (as is the case with other rotary deflectors), a polygon mirror (10)
If there is no optical component after the polygon mirror (10), the condensing point after the reflection at the point becomes a substantially arc shape centering on the reflection point, and if this is received in a straight line, field curvature will occur. . When a convergent light beam is incident on the polygon mirror (10), a concave curvature of field occurs in the light incident direction. Also, the distance between the spherical mirror (20) and the image plane changes depending on the degree of convergence of the incident light. The change in the distance changes the field curvature. That is, the curvature due to the concave surface of the spherical mirror (20) is corrected by the curvature of field due to the convergent light beam, and as a result, the curvature of field on the light-collecting surface is reduced, and the flatness of the image plane is improved.

When the curvature of field is reduced, the fluctuation of the condensed light beam diameter due to the difference in the scanning position (image height) is reduced, and the optical system can be used at a wide angle of view, and the condensed light beam diameter can be reduced. This has the advantage that the density can be increased.

Specifically, as shown in Fig. 2, a polygon mirror (10)
(D) from the deflection point (10a) to the vertex (20a) of the spherical mirror (20) and the radius of curvature (R _M ) of the spherical mirror (20)
And the relationship between the radius of curvature (R _M ) and the distance (s) (not shown) from the deflecting point (10a) to the converging point after reflection by the polygon mirror (10), are as follows: | s / R _M |)> 0.4... 0.1 <(d / | R _M |) <0.7.

In FIG. 2, (d ') is a spherical mirror (20)
Is the distance from the vertex (20a) to the photoconductor (30).

When the above expressions are satisfied, good distortion characteristics over a wide angle of view and good image plane flatness can be obtained. The lower limit and the upper limit in each expression are values set as ranges that are empirically acceptable depending on the degree of image distortion on the photoconductor (30). If the lower limit of the above expression is exceeded, the image plane will approach the spherical mirror (20), and it will be difficult to arrange the mirror, and the distortion characteristics will also deteriorate.

On the other hand, if the lower limit of the above expression is exceeded, the positive distortion increases with an increase in the scanning angle, and the image is elongated at both ends (near the start of scanning and near the end of scanning) in the main scanning direction. If the upper limit is exceeded, the negative distortion increases with an increase in the scanning angle, the image shrinks at both ends in the main scanning direction, and the field curvature further increases.

Here, experimental examples (I), (II), and
The configuration data in (III), (IV), and (V) are shown. The facing distance of the polygon mirror (10) was 23.5 mm.

Each of the above experimental examples (I), (II), (III), (IV),
The aberration at the photoconductor condensing surface in (V) is
This is shown in FIGS. 5, 5, 6, 7, and 8. In each figure, (a) is a graph in which the horizontal axis is the scanning angle and the vertical axis is the degree of distortion. In each figure, (b) is a graph in which the horizontal axis represents the scanning angle and the vertical axis represents the degree of curvature. Shows surface curvature.

The light beam scanning optical system according to the present invention is not limited to the above embodiment, but can be variously modified within the scope of the invention.

For example, as the deflector, the polygon mirror (10)
In addition, various types can be used as long as they can scan a light beam on one plane at a constant angular velocity. Further, other than a semiconductor laser, other laser generating means or a point light source may be used as the light source.

On the other hand, the above embodiment does not refer to the shift of the spherical mirror in the main scanning direction (the (Y) direction in FIG. 2, the shift amount (Y _M )). However, in consideration of aberration correction and ease of arrangement, it is conceivable to shift the spherical mirror in the above direction. For example, when the distortion is not bilaterally symmetric as in the above-mentioned experimental example (I) (see FIG. 4), the distortion can be further reduced by such a shift of the spherical mirror.

Further, in the above embodiment, the divergent light beam emitted from the semiconductor laser by the collimator lens is corrected to a convergent light beam, but may be simply corrected to a substantially parallel light beam.

Advantageous Effects of the Invention As is clear from the above description, according to the present invention, since the spherical mirror is interposed in the optical path from the deflector to the photoreceptor surface, the scanning speed in the main scanning direction can of course be corrected uniformly. Good distortion characteristics and good image plane flatness can be obtained over a wide angle of view on the light-collecting surface. Furthermore, spherical mirrors are easier to process than conventional fθ lenses and have improved processing accuracy. Since they do not need to be transparent, they can be selected from a wide range of materials, making it a cheap and high-performance scanning optical system as a whole. it can. In addition, the optical path is folded by the spherical mirror itself, and the entire optical system becomes compact. Further, it is advantageous in processing and accuracy as compared with a parabolic mirror or an elliptical mirror, and can be downsized as compared with a conventional concave reflecting mirror.

Further, since the toroidal lens is disposed between the deflector and the spherical mirror, the toroidal lens can correct an error due to the inclination of each reflecting surface of the deflector, and can correct the uneven pitch in the sub-scanning direction of the image. .

[Brief description of the drawings]

1 shows an embodiment of the present invention, FIG. 1 is a perspective view showing a schematic configuration, FIGS. 2 and 3 are diagrams for schematically explaining an optical path, and FIGS. 4 is a graph showing image distortion on a light surface. (1) Semiconductor laser, (6) Collimator lens, (7) Cylindrical lens, (10) Polygon mirror, (13) Toroidal lens, (20)
Spherical mirror, (30) ... Photoconductor.

Claims (1)

(57) [Claims] A light source for generating a light beam; a means for converging a light beam emitted from the light source in a straight line on the same plane as a scanning direction; and a light source disposed near a converging line for scanning the converged light beam at a constant angular velocity. A deflector, a spherical mirror that folds a light beam scanned by the deflector and focuses the light beam on a photoconductor surface, and a toroidal lens disposed between the deflector and the spherical mirror. Light beam scanning optical system.