JPH01200222A - Light beam scanning optical system - Google Patents

Abstract

PURPOSE:To condense light on the surface of a photosensitive body without passing a light-semitransmissive means by providing a toroidal lens between a deflector and a spherical mirror and inclining and arranging the toroidal lens and the spherical mirror in a plane perpendicular to a deflecting face. CONSTITUTION:In a light beam scanning optical system, a spherical mirror 20 is used instead of an ftheta lens and the scanning speed in the main scanning direction is so corrected that it is uniform throughout from the center to both end parts of the scanning area. A toroidal lens 13 is provided between a polygonal mirror 10 and the spherical mirror 20, and the toroidal lens 13 and the spherical mirror 20 are inclined and arranged in a plane perpendicular to the deflecting face. Consequently, the luminous flux is reflected in a direction different from the incidence direction by the spherical mirror 20 and is condensed on the surface of a photosensitive body 30 without using a light-semitransmissive means like a beam splitter. Curvature of a scanning line on the surface of the photosensitive body 30 due to inclination of the spherical mirror 20 is corrected because the toroidal lens 13 is inclined similarly.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is a light beam scanning optical system, particularly a light beam that is incorporated into a laser beam printer, facsimile, etc., and focuses a light beam carrying image information onto a photoreceptor. Regarding the structure of a scanning optical system.

Conventional technology and its problems In general, light beam scanning optical systems used in laser beam printers and facsimile machines basically consist of a semiconductor laser as a light source, a polygon mirror, and a galvano mirror.
It is composed of a deflector such as, and an fθ lens. A deflector scans the light beam emitted from a semiconductor laser at a constant angular velocity, and if left as it is, there will be a difference in scanning speed from the center in the main scanning direction to both ends on the condensing surface, making it impossible to obtain a uniform image. . The fθ lens is installed to correct such a difference in scanning speed.

By the way, the fθ lens is a combination of various concave lenses, convex lenses, etc., and the lens design is extremely complicated.
The number of surfaces to be polished is large, making it difficult to improve machining accuracy, and it is also expensive. Moreover, there are also constraints from the material standpoint, such as the need to select a material with good translucency.

Therefore, conventional methods have been to use an ellipsoidal mirror (Japanese Patent Publication No. 54-123040) or a parabolic mirror (Japanese Patent Publication No. 55-361) in place of the fθ lens.
No. 27), using a concave reflecting mirror (Japanese Patent Laid-Open No. 61
-173212) has been proposed. However, ellipsoidal mirrors and parabolic mirrors have a problem in that it is difficult to process them and to improve their processing accuracy.

Therefore, an object of the present invention is to adopt a scanning speed correction means that is easier to process and can improve processing accuracy, instead of an expensive and highly restricted f-theta lens or a parabolic mirror that has been proposed in the past. The object of the present invention is to make the system more compact, reduce the curvature of the image plane perpendicular to the main scanning direction at the condensing point, and effectively correct the surface tilt error of the deflector. In other words, when using a rotating polygon mirror such as a polygon mirror as a deflector, if there is a perpendicularity error (surface inclination error) between each surface, the scanning line on the photoreceptor surface will shift in the sub-scanning direction. becomes. The present invention is intended to correct pitch irregularities caused by such surface tilt errors.

Means for Solving the Problems In order to solve the above problems, a light beam scanning optical system according to the present invention includes: (a) a light source that generates an intensity-modulated light beam; and (b) a light beam emitted from the light source. means for converging the light into a straight line on the same plane as the scanning direction, and (e) placed near the condensing line;
a deflector that scans the convergent light beam at a constant angular velocity; (d) a spherical mirror that returns the light beam scanned by the deflector and focuses it on the photoreceptor surface; and (e) between the deflector and the spherical mirror. (f) The toroidal lens and the spherical mirror are each arranged to be inclined in a plane perpendicular to the deflection plane.

Operation In the above configuration, the light beam emitted from the light source is scanned at a constant angular velocity by the deflector, and this scanning light beam is reflected by the spherical mirror and condensed onto the surface of the photoreceptor. An image is formed by main scanning by the deflector and sub-scanning by moving the photoreceptor surface. The light beam reflected by the spherical mirror is corrected so that the scanning speed in the main scanning direction is equalized from the center of the scanning area to both ends thereof, and the condensing surface has good distortion characteristics over a wide angle of view. Image plane flatness can be obtained.

Also, the light flux emitted from the light source is in the scanning direction (in the deflection plane)
is converged into a straight line and incident on the deflector. The toroidal lens condenses the light beam scanned by the deflector onto the surface of the photoreceptor, thereby correcting errors caused by tilting the surface of the deflector.

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

Furthermore, in the above configuration, since the spherical mirror is arranged in an inclined state in a plane perpendicular to the deflection surface, the light beam is reflected by the spherical mirror in a direction different from the direction of incidence, and the beam splitter etc. Light is focused on the photoreceptor surface directly or by interposing a folding mirror without requiring any optical means. Note that the bending of the scanning line on the photoreceptor surface due to the tilting of the spherical mirror is corrected by similarly tilting the toroidal lens.

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

In Figure 1, (1> is a semiconductor laser, (6) is a collimator lens, (7) is a cylindrical lens, and (10) is a cylindrical lens.
) is a polygon mirror, (13) is a toroidal lens, (
20) is a spherical mirror, (25) is a flat folding mirror, and (30) is a drum-shaped photoreceptor.

A semiconductor laser (1) emits a diverging light beam that is intensity-modulated by a control circuit (not shown) and carries image information. This diverging light flux is corrected into a convergent light flux by passing through a collimator lens (6). Further, this convergent light beam passes through the cylindrical lens (7) and is converged in the scanning direction, that is, in the vicinity of the reflecting surface of the polygon mirror (10) described below (in the deflection plane) in a straight line. Polygon mirror (
10) can be driven to rotate at a constant speed in the direction of arrow (8) about a support shaft (11) by a motor (not shown). Therefore, the convergent light flux emitted from the cylindrical lens 7) is
It is continuously reflected on the surface of a polygon mirror (10〉) and scanned at a constant angular velocity. This scanning light beam is passed through a toroidal lens (
13), the light is reflected by the concave side of the spherical mirror (20>), and is further reflected by the folding mirror (25) before being focused onto the photoreceptor (30). The light beam is scanned at a constant speed in the axial direction of the photoreceptor 30),
This is called main scanning. Also, the photoconductor 30) is indicated by the arrow (
b) Rotationally driven at a constant speed in the direction, and scanning by this rotation is called sub-scanning.

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

Further, the toroidal lens (13) installed in the reflected optical path from the polygon mirror (10) is connected to the polygon mirror (10).
The main purpose is to correct the surface tilt error of 0).

That is, if there is an error in perpendicularity between the reflective surfaces of the polygon mirror (10), the scanning lines on the photoreceptor (30) will shift in the sub-scanning direction, causing pitch unevenness in the image. This tilting error can be corrected by setting each reflective surface of the polygon mirror (10) and the condensing surface of the photoreceptor (30) in a conjugate relationship in a cross section perpendicular to the deflection surface of the polygon mirror (10). . In the present invention, a cylindrical lens (7) is used to condense a luminous flux onto a polygon mirror (10), while a toroidal lens (13) is used to maintain a conjugate relationship between each reflective surface and a condensing surface of the polygon mirror (10). ing.

Kirani, the toroidal lens (13) is a polygon mirror (
10) In order to flatten the image plane created by the light beam in the cross section perpendicular to the deflection plane, the radius of curvature in the deflection plane is set to an appropriate value [(Rta) in the following experimental example.

(Rta), and (yt) in the (Y) direction in FIG.
) is preferable. The radius of curvature (Rta), (Rta) in this deflection plane is the point (
(hereinafter referred to as the deflection point) is slightly larger than the distance (do) from (10a) to the toroidal lens (13>).

In addition, the appropriate shift amount (Y) of the toroidal lens (13)
A) varies depending on the size of the polygon mirror (10), the angle of view, the direction of incidence of the light beam on the polygon mirror (10), etc. Specific examples are shown in Experimental Examples (I) to (IV) below.

Further, as shown in FIG. 3, the spherical mirror (20) is arranged to be inclined at an angle (θ, ) in a plane perpendicular to the deflection surface. This is because the light beam is reflected in a direction different from the direction in which it is incident. When the tilt angle (θ, ) is zero, the light beam is reflected in the same direction as the incident light, and a semi-transparent means such as a beam splitter is required to guide this reflected light to the photoreceptor (30). However, if a semi-transparent means is used, the amount of light will be attenuated. Therefore, in this embodiment, the spherical mirror (20) is tilted to turn back the optical path, and the light is focused onto the photoreceptor (30) via the mirror (25>).

However, if only the spherical mirror (20〉) is tilted by an angle (θ,), the scanning line at the condensing surface will be bent.To correct this, the toroidal lens (13〉) will also tilt ( It was decided to tilt it at an angle of θ, ).

Further, in this embodiment, the collimator lens (6) corrects the divergent light beam into a convergent light beam. This is to correct the curvature at the focal point (imaging surface) on the photoreceptor (30) by converging the light beam. In other words, polygon mirror ke 0
When a converging or diverging beam is incident on the polygon mirror (10) (the same is true for other rotating deflectors),
Assuming that there are no optical components after the polygon mirror (10), the focal point after reflection at the polygon mirror (10) will be approximately arc-shaped with the reflection point as the center, and if this point is received in a straight line, curvature of field will occur. . When a convergent light beam is incident on the polygon mirror 5-(10), a concave curvature of field occurs in the direction of the light beam incidence.

Also, depending on the degree of convergence of the incident light, the spherical mirror (20)
The distance between the image plane and the image plane also changes. The curvature of field also changes with this change in distance. That is, the curvature of field caused by the converging light beam corrects the curvature caused by the concave surface of the spherical mirror (20), and as a result, the curvature of field at the converging surface is reduced, and the flatness of the image surface is improved. do.

When the curvature of field becomes smaller, fluctuations in the diameter of the condensed beam due to differences in scanning position (image height) become smaller, allowing the optical system to be used at a wide angle of view, and because the diameter of the condensed beam can be made smaller, the diameter of the condensed beam becomes smaller. This has the advantage of allowing higher density.

In detail, as shown in Figure 2, the polygon mirror (10)
from the deflection point (10a) to the vertex (2) of the spherical mirror (20)
0a) and a spherical mirror (20
) (7) Relationship with the radius of curvature (RM), and the relationship between this radius of curvature (RM) and the deflection point (10a) to the polygon mirror (10
), the relationship with the distance *<Es> (not shown) to the focal point after reflection at
, 1<(d/IRMI)<0.7
It is desirable to satisfy the following formula.

In addition, in Fig. 2, (d') is a spherical mirror (20).
It is the distance from the vertex (20a) to the photoreceptor (30).

When the above formulas (1) and (2) are satisfied, good distortion characteristics and good image plane flatness can be obtained over a wide angle of view. The lower and upper limits in each equation are values set as empirically acceptable ranges depending on the degree of image distortion on the photoreceptor (30). If the lower limit of the above formula 0 is exceeded, the image surface approaches the spherical mirror (20), making arrangement difficult, and the distortion characteristics also deteriorate.

On the other hand, when the lower limit of the above formula 0 is exceeded, the positive distortion increases as the scanning angle increases, and the image becomes elongated at both ends in the main scanning direction (near the start of scanning and near the end of scanning). Also,
When the upper limit is exceeded, negative distortion increases as the scanning angle increases, the image shrinks at both ends in the main scanning direction, and the curvature of field increases.

Here, experimental example (1), (I[) in this example.

The configuration data for (I[I) and (IIV) is shown.

The facing distance of the polygon mirror (10) is 23.5m.
It was set as m.

[Margin below] Each of the above experimental examples (I), (If), (III), (I
Figures 4 and 4 show the aberrations at the photoreceptor condensing surface in V), respectively.
Figure 5.

It is shown in FIGS. 6 and 7. 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 is the scanning angle and the vertical axis is the degree of curvature.The dotted line shows the curvature of field due to the light beam in the deflection plane, and the solid line shows the image due to the light beam in the plane perpendicular to the deflection plane. Shows surface curvature. (c) in each figure is a graph in which the horizontal axis is the scanning angle and the vertical axis is the scanning line distortion degree.
That is, it shows the bending of the scanning line.

Note that the light beam scanning optical system according to the present invention is not limited to the above-described embodiments, and can be modified in various ways within the scope of the gist.

For example, instead of one folding mirror (25), a plurality of folding mirrors (25) may be provided, or the reflected light from the spherical mirror (20) may be directly focused onto the photoreceptor (30).

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

On the other hand, in the embodiment described above, there is no mention of the shift of the spherical mirror in the main scanning direction [(Y) direction in FIG. 2, shift amount (YM)]. 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 aberration is not bilaterally symmetrical, such as in Experimental Example (I) (see FIG. 4), the distortion can be further reduced by such a shift of the spherical mirror.

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

Effects of the Invention As is clear from the above explanation, according to the present invention, a spherical mirror is interposed in the optical path from the deflector to the photoreceptor surface so as to satisfy the above formulas (1) and (2). Not only can the scanning speed of the optical system be uniformly corrected, but also good distortion characteristics and good image plane flatness can be obtained over a wide angle of view at the condensing surface.

Furthermore, spherical mirrors are easier to process and have improved processing accuracy compared to conventional f-theta lenses, and since they do not need to be transparent, a wide range of materials can be selected, making it possible to create an overall inexpensive and high-performance scanning optical system. can.

Moreover, the optical path is folded back by the spherical mirror itself, making the entire optical system compact. Furthermore, it is advantageous in terms of processing and accuracy compared to parabolic mirrors and ellipsoidal mirrors, and can be made smaller than conventional concave reflecting mirrors.

Furthermore, since a toroidal lens is placed between the deflector and the spherical mirror, the toroidal lens can correct errors caused by surface inclination of each reflective surface of the deflector, and correct pitch unevenness in the sub-scanning direction of the image. . Moreover, since the toroidal lens and the spherical mirror are each tilted in a plane perpendicular to the deflection plane, the reflected light from the spherical mirror is given an appropriate angle, the bending of the scanning line is corrected, and then the light is transmitted through the semi-transparent means. light can be focused on the photoreceptor without
Arbitrarity in the arrangement of optical members is improved, and attenuation of the amount of light is also reduced.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention; FIG. 1 is a perspective view showing a schematic configuration, FIGS. 2 and 3 are diagrams for schematically explaining the optical path, and FIGS. 4 to 7 are a collection. It is a graph showing image distortion on a light plane. (1)...Semiconductor laser, (6)...Collimator lens, (7>...Cylindrical lens, (10)...
... Polygon mirror, (13) ... Toroidal lens, (20) ... Spherical mirror, (30) ... Photoreceptor.

Claims (1)

[Scope of Claims] 1. A light source that generates an intensity-modulated light beam; a means for converging the light beam emitted from the light source into a straight line on the same plane as the scanning direction; a deflector that scans a light beam at a constant angular velocity; a spherical mirror that returns the light beam scanned by the deflector and focuses it on a photoreceptor surface; and a toroidal lens disposed between the deflector and the spherical mirror. A light beam scanning optical system, characterized in that the toroidal lens and the spherical mirror are each arranged to be inclined in a plane perpendicular to the deflection plane.