JPH04245214A - Light beam scanning optical system - Google Patents

Abstract

PURPOSE:To make correction of the scan speed difference (distort abberation) and a correction of the image surface curvature in aux. scanning direction using a simple optical configuration without provision of ftheta lens, and obtain a good distortion characteristic and good image surface flatness over a wide picture angle. CONSTITUTION:An optical system according to the invention is composed of a semiconductor laser 1, a collimator lens 6, a polygonal mirror 10, No.1 spherical surface mirror 20 which is convex, and No.2 spherical surface mirror 25 which is concave. A light flux emitted from the semiconductor laser 1 is scanned over the reflex surface 10a of the polygonal mirror 10 rotating in the direction of arrow (a), reflected by the convex surface of the No.1 spherical surface mirror 20 and tone convex surface of the No.2 spherical surface mirror 25, and condensed on a photo-sensitive element 30. Because of sphericalness of the surfaces of the two mirrors 20, the image surface curvatures in the main scanning direction and aux. scanning direction are canceled, and the scan speed difference (distort abberation) is also corrected.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a light beam scanning optical system,
In particular, the present invention relates to the structure of a light beam scanning optical system that is incorporated into a laser beam printer, facsimile, etc., and focuses a light beam carrying image information onto a scanning medium.

0002

[Prior Art] Generally, a light beam scanning optical system used in a laser beam printer or facsimile machine basically consists of a semiconductor laser as a light source, a deflector such as a polygon mirror or a galvano mirror, and an fθ lens. has been done. 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 scanning speed difference (distortion aberration), to correct the curvature of field in the main scanning direction, and to correct the curvature of field in the sub-scanning direction. However, the fθ lens is a combination of various concave lenses, convex lenses, etc., and the lens design is extremely complicated, the number of polished surfaces is large, making it difficult to improve the precision of processing, and it is also expensive. Moreover, there is also a constraint from the perspective of the material, such as the need to select a material with good translucency.

[0003] Therefore, recently, it has been proposed to use an ellipsoidal mirror, a parabolic mirror, or a concave reflecting mirror in place of the fθ lens. However, this type of mirror has a problem in that it is difficult to improve the processing itself and the processing accuracy. In view of the above problems, the applicant has adopted a spherical mirror that is easier to process and can improve processing accuracy, instead of the expensive and restrictive f-theta lens and the conventionally proposed parabolic mirror. proposed an optical system (JP-A-1-200219, JP-A-1-200)
(See Publication No. 220). However, even with these optical systems, it is difficult to sufficiently correct all aberrations over a wide angle of view. This curvature of field has the problem of producing a concave curvature of field with respect to the traveling direction of the light beam.

0004

[Object, structure, and operation of the invention] Therefore, the object of the present invention is to
Light beam scanning that enables correction of scanning speed difference (distortion aberration) over a wide angle of view, correction of field curvature in the main scanning direction, and correction of field curvature in the sub-scanning direction with a simple optical configuration without using an fθ lens. The objective is to provide an optical system.

In order to achieve the above object, a light beam scanning optical system according to the present invention includes a light source that generates a light beam modulated based on image information, and scans the light beam emitted from the light source at a constant angular velocity. It includes a deflector, and a first convex spherical mirror and a second concave spherical mirror that return the light beam scanned by the deflector and condense it onto a scanning medium. In the above configuration, the light beam emitted from the light source is scanned by the deflector at a constant angular velocity, and this scanning light beam is reflected by the first and second spherical mirrors and condensed onto the scanning medium. An image is formed by main scanning by the deflector and sub-scanning by moving the scanning medium. The image formed here has convex field curvature in the main scanning direction and the sub-scanning direction, but these field curvatures are corrected by the two spherical mirrors. moreover,
The beam reflected by the two spherical mirrors is corrected so that the scanning speed in the main scanning direction is equalized from the center of the scanning area to both ends, and the condensing surface has good distortion characteristics over a wide angle of view and a good image plane. Flatness can be obtained.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a light beam scanning optical system according to the present invention will be described with reference to the accompanying drawings. [First embodiment, see FIGS. 1 to 6] In FIG. 1, 1 is a semiconductor laser, 6 is a collimator lens, 10 is a polygon mirror, 20 is a convex first spherical mirror with negative power, and 25 is a positive power A concave second spherical mirror 30 having power is a drum-shaped photoreceptor.

The semiconductor laser 1 is intensity-modulated (turned on and off) by a control circuit (not shown) and emits a diverging beam carrying image information. This diverging light flux is corrected into a convergent light flux by passing through the collimator lens 6. The polygon mirror 10 is rotated at a constant speed in the direction of arrow a around a support shaft 11 by a motor (not shown). Therefore, the convergent light beam emitted from the collimator lens 6 is continuously reflected by the reflecting surface 10a of the polygon mirror 10 and scanned at a constant angular velocity. This scanning light beam is reflected by the convex surface of the first spherical mirror 20 and the concave surface of the second spherical mirror 25, and is focused onto the photoreceptor 30. At this time, the condensed light beam is
is scanned at a constant speed in the axial direction of the image plane, and this is called main scanning. Further, the photoreceptor 30 is driven to rotate at a constant speed in the direction of arrow b, and scanning by this rotation is called sub-scanning.

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. The first and second spherical mirrors 20 and 25, in place of the conventional fθ lens, correct the scanning speed in the main scanning direction so that it is uniform from the center of the scanning area to both ends thereof (distortion aberration correction), and , field curvature in the main scanning direction and the sub-scanning direction is corrected.

That is, the reflective surface 10a of the polygon mirror 10
If is a flat surface, and there is no optical component after the polygon mirror 10, the light beam deflected and scanned by the polygon mirror 10 will have a substantially circular arc shape centered on the reflection point, and if this is scanned in a straight line, it will be in the main scanning direction and This results in convex curvature of field in both the sub-scanning direction. Furthermore, the scanning speed of the light beam is faster in both end regions in the main scanning direction than in the central region, resulting in positive distortion.

On the other hand, if a convex spherical mirror is placed after the polygon mirror 10, a convex curvature of field and positive distortion will occur, but this curvature of field is larger in the main scanning direction than in the sub-scanning direction. . On the other hand, if a concave spherical mirror is disposed, a concave curvature of field and negative distortion will occur. Concave curvature of field and negative distortion can also be generated by making the incident light beam convergent.

In other words, the power of the convex spherical mirror,
By adjusting the balance of three powers: the power of the concave spherical mirror and the degree of convergence of the incident light beam, good curvature of field characteristics in the main scanning direction and sub-scanning direction and good distortion characteristics can be obtained. To explain this example in detail,
As shown in FIGS. 2 and 3, the relationship between the distance D2 from the second spherical mirror 25 to the photoreceptor 30 and the radius of curvature Rm2 of the second spherical mirror 25, and the distance from the deflection point (reflection point) 10a of the polygon mirror 10 to Distance D1 (d1
+|d2|) and the distance D2, and the relationship between the radius of curvature Rm1 of the first spherical mirror 20 and the radius of curvature Rm2 of the second spherical mirror 25, preferably satisfy the following equations.

0.2<(D2/Rm2)<0.5
...■ 0
．． 8<(D1/D2)<1.3
……■ 0.3<(
Rm2/Rm1)<0.5
...■If the above formulas ■, ■, and ■ are satisfied, good distortion characteristics and good image plane flatness can be obtained over a wide angle of view. The lower limit and upper limit in each formula are photoreceptor 30
This value is set as an allowable range based on experience based on the degree of image distortion described above.

When the lower limit of the equation (2) is exceeded (when the radius of curvature of the second spherical mirror 25 becomes large), as the scanning angle increases, positive distortion increases and convex curvature of field increases. Also, if the upper limit of formula (2) is exceeded (second spherical mirror 25
(as the radius of curvature becomes smaller), the negative distortion increases as the scan angle increases, and the concave curvature of field increases. When the lower limit of the above formula (2) is exceeded, as the scanning angle increases, negative distortion increases and concave curvature of field increases. Also,
When the upper limit of equation (2) is exceeded, positive distortion increases as the scanning angle increases, and convex curvature of field increases.

When the upper limit of the equation (2) is exceeded (when the radius of curvature of the first spherical mirror 20 is increased), the negative distortion increases as the scanning angle increases, and the convex curvature of field in the main scanning direction increases. , the concave curvature of field in the sub-scanning direction increases. Furthermore, when the lower limit of equation (2) is exceeded (when the radius of curvature of the first spherical mirror 20 is decreased), the positive distortion increases as the scanning angle increases, and the concave curvature of field in the main scanning direction increases, causing The convex curvature of field in the scanning direction increases.

Regarding the angle of incidence of the light beam on the polygon mirror 10, in the first embodiment, the angle of incidence θy on the plane parallel to the sub-scanning direction is 0° (therefore, this angle of incidence does not appear in FIG. 3). The incident angle θz (see FIG. 2) on a plane parallel to the main scanning direction is 90°. Next, the inclination of the spherical mirrors 20 and 25 will be explained.

After scanning the light beam with the polygon mirror 10,
In order for the light to be returned by the two spherical mirrors 20 and 25 to reach the photoreceptor 30, it is necessary to tilt the spherical mirrors 20 and 25 in a plane parallel to the sub-scanning direction. However, when the spherical mirrors 20 and 25 are tilted in this way, the angle formed by the normal line of the spherical mirror and the incident light flux changes depending on the scanning angle of view, so the reflection angle of the light flux changes depending on the scanning angle of view, which increases the exposure to light. The scanning line on the body 30 becomes an arc-shaped bow. In order to make the scanning line linear, the two spherical mirrors 20 and 25 may generate bows in opposite directions so that they cancel each other out. In FIG. 3, the eccentric angle of the first spherical mirror 20 is θy1, the second
The eccentric angle of the spherical mirror 25 is indicated by θy2.

Here, Experimental Examples 1 and 2 in the first embodiment
, 3 are shown in Tables 1a and 1b, and characteristic data are shown in FIGS. 4, 5, and 6.

[0018]

[Table 1]

[0019]

[Table 2]

The aberrations on the photoreceptor condensing surface in each of the above experimental examples 1, 2, and 3 are shown in FIGS. 4, 5, and 6, respectively. 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 (distortion aberration). (b) in each figure is a graph in which the horizontal axis is the scanning angle and the vertical axis is the degree of curvature.
The solid line represents the curvature of field (
curvature of field in the sub-scanning direction). (c) in each figure is a graph in which the horizontal axis is the scanning angle and the vertical axis is the focus position in the sub-scanning direction. Shows the bending of the scan line.

[Second embodiment, see FIGS. 7 to 11] This second embodiment
In the embodiment, the beam emission position, that is, the semiconductor laser 1 and the collimator lens 6 are provided between the polygon mirror 10 and the first spherical mirror 20 to make the optical system more compact. This is similar to the example. Compared to the first embodiment, the incident angle θy (see FIG. 9) on the plane parallel to the sub-scanning direction is set to 1.5° in order to set an angle between the incident angle and the outgoing angle at the polygon mirror 10. , the incident angle θz on the plane parallel to the main scanning direction is 0° (therefore,
This angle of incidence does not appear in FIG. 8).

In the second embodiment as well, it is preferable that formulas (1), (2), and (2) shown in the first embodiment are satisfied. In addition, the configuration data for Experimental Examples 4 and 5 in this second embodiment are shown in Tables 2a and 2b, and the characteristic data is shown in FIGS. 10 and 1.
Shown in 1. Note that FIGS. 10 and 11 correspond to FIGS. 4, 5, and 6 described above.

[0023]

[Table 3]

[0024]

[Table 4]

[Other Embodiments] Note that the light beam scanning optical system according to the present invention is not limited to the above embodiments.
Various modifications can be made within the scope of the gist. For example, 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.

Furthermore, 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 is also possible to simply correct the substantially parallel light beam.

[0027]

As is clear from the above description, according to the present invention, the light beam deflected and scanned by the deflector is focused onto the scanning medium via the convex first spherical mirror and the concave second spherical mirror. Because the light is emitted, the scanning speed in the main scanning direction can be corrected evenly (distortion aberration correction) without using an fθ lens.
At the same time, field curvature in the main scanning direction and the sub-scanning direction can be corrected, and good distortion characteristics and good field flatness can be obtained over a wide angle of view. Moreover, spherical mirrors are easier to process and have higher processing accuracy than conventional fθ lenses.
Since it does not need to be transparent, a wide range of materials can be selected, and even if plastic is used, it is less affected by changes in performance due to changes in temperature and humidity, eliminating restrictions from the material aspect. Furthermore, 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.

[Brief explanation of the drawing]

1 to 6 show a first embodiment of a light beam scanning optical system according to the present invention.

FIG. 1 is a perspective view showing a schematic configuration of a light beam scanning optical system.

FIG. 2 is an optical path diagram on a plane parallel to the main scanning direction.

FIG. 3 is an optical path diagram on a plane parallel to the sub-scanning direction.

FIG. 4 is an aberration diagram at the condensing surface in Experimental Example 1.

FIG. 5 is an aberration diagram at the condensing surface in Experimental Example 2.

FIG. 6 is an aberration diagram at the condensing surface in Experimental Example 3. 7 to 11 show a second embodiment of the light beam scanning optical system according to the present invention.

FIG. 7 is a perspective view showing a schematic configuration of a light beam scanning optical system.

FIG. 8 is an optical path diagram on a plane parallel to the main scanning direction.

FIG. 9 is an optical path diagram on a plane parallel to the sub-scanning direction.

FIG. 10 is an aberration diagram at the condensing surface in Experimental Example 4.

FIG. 11 is an aberration diagram at the condensing surface in Experimental Example 5.

[Explanation of symbols]

1...Semiconductor laser 6...Collimator lens 10...Polygon mirror 10a...Reflective surface 20...first spherical mirror 25...Second spherical mirror 30...Photoreceptor

Claims (1)

[Claims] 1. A light source that generates a light beam modulated based on image information; a deflector that scans the light beam emitted from the light source at a constant angular velocity; A light beam scanning optical system comprising a first convex spherical mirror and a second concave spherical mirror that converge light upward.