JPH04110820A - Light beam scanning optical system - Google Patents

Abstract

PURPOSE:To excellently compensate the image plane performance by providing a lens which is arranged between a deflector and a spherical mirror and has a difference between the center of radius of curvature of an incidence surface and the center of the radius of curvature of a projection surface in a scanning plane, and satisfying specific conditional inequalities. CONSTITUTION:The inequalities preferably hold for the relation among the distance (object point) (s) (not shown in figure) from the deflection point 10a to the position where a collimator lens converges a beam in the absence of a toric lens 15 and the spherical mirror 20, the radius (R1a) of curvature of the spherical mirror 20, and the radius (RM) of curvature of the lens projection surface in the main scanning direction, the relation between the distance (d) from the deflection point 10a of the polygon mirror 10 to the mirror 20 and the radius (RM) of curvature, and the relation between the core thickness (d1) of the toric lens 15 and the radius (R2a) of curvature of the lens incidence surface in the main scanning direction. When the inequalities are satisfied, an excellent distortion characteristic and excellent image plane flatness are obtained over a wide view angle range.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to 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 scanning medium. Regarding the structure of a scanning optical system.

Conventional Technologies and Issues 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 is, there will be a difference in scanning speed from the center in the main scanning direction to both ends of the light condensing surface, making it impossible to obtain a uniform image. . The fθ lens is installed to correct such a difference in scanning speed. However, the rθ lens is a combination of various concave lenses, convex lenses, etc.
The lens design is extremely complicated, and the number of polished surfaces is large, making it difficult to improve processing precision, 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, 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 points, the present 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. We have proposed an optical system that can be made more compact, reduce the curvature of the image plane perpendicular to the main scanning direction of the light beam at the condensing point, and correct the surface tilt error of the deflector. Publication No. 1-200219, 1
-Refer to Publication No. 200221, etc.). However, even with this optical system, the image plane properties in the main scanning direction and the correction of distortion aberration are not necessarily satisfactory. For example, if emphasis is placed on correcting distortion aberrations, the image plane properties in the main scanning direction deteriorate, resulting in Another problem is that aberration correction in the main scanning direction is insufficient.

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. (C) a deflector placed near the condensing line and scanning the converged light beam at a constant angular velocity; (d) a means for converging the light beam scanned by the deflector into a straight line on the same plane as the scanning direction; (e) a spherical mirror that can be folded back to condense light onto the scanning medium; It is equipped with a lens in which the center of the radius of curvature of the surface and the center of the radius of curvature of the exit surface do not coincide, and (f) the following formula is satisfied: 0.6<(lR1I+d)/IRzl<1.3However,
R2: Radius of curvature in the main scanning direction plane on the entrance side of the lens R1: Radius of curvature in the main scanning direction plane on the exit side of the lens d: Characterized by the distance from the point where the light beam is reflected toward the center of the scanning area by the deflector to the spherical mirror shall be.

Here, the toroidal surface refers to a surface whose two principal meridians have different centers of curvature. The other surfaces of the lens may be spherical, flat, or cylindrical.

Effect In the above configuration, the light beam emitted from the light source is scanned at a constant angular velocity by the deflector, and after passing through the lens, this scanning light beam is reflected by the spherical mirror and focused onto the scanning medium. An image is formed by main scanning by the deflector and sub-scanning by moving the scanning medium. 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)
The light is converged into a straight line and incident on the deflector. The toroidal surface of the lens allows the light beam scanned by the deflector to be focused onto the photoreceptor surface, correcting errors due to surface tilt of the deflector, and correcting field curvature in the sub-scanning direction.

Furthermore, by shifting the center of the radius of curvature of the entrance surface of the lens and the center of the radius of curvature of the exit surface within the scanning plane,
Asymmetry in the main scanning direction of field curvature and distortion is corrected.

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.

In Figure 1, (1) is a semiconductor laser, (6) is a collimator lens, (7) is a cylindrical lens', and (1) is a collimator lens.
0) is a polygon mirror, (15) is a toric lens,
(25) is a beam splitter, (20) is a spherical mirror (
30) 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 divergent beam carrying image information. This divergent light beam can be corrected into a convergent light beam by passing through a collimator lens (6). Furthermore, this convergent light beam passes through a cylindrical lens (7) in the scanning direction, that is, the following polygon mirror (10).
It can be converged in a straight line (within the deflection plane) near the reflecting surface. The polygon mirror (10) is rotated by a spindle (1) by a motor (not shown).
1) is rotated at a constant speed in the direction of arrow (8). Therefore, the convergent light beam completely emitted from the cylindrical lens (7) is continuously reflected on the surface of the polygon mirror (10) and can be scanned at a constant angular velocity. After this scanning light beam passes through the toric lens (15) and the beam splitter (25), it is reflected on the concave side of the spherical mirror (20).
Furthermore, after being reflected by a beam splitter (25), the light is focused onto a photoreceptor (30). The condensed light beam at this time is scanned at a constant speed in the axial direction of the photoreceptor (30), and this is called main scanning. Further, the photoreceptor (30) can be rotated at a constant speed in the direction of arrow (b), and scanning by this rotation is called sub-scanning.

Here, the toric lens (15) refers to a lens in which either the entrance side or the exit side is a toroidal surface and the other surface is a spherical, flat, or cylindrical surface. A toroidal surface is a surface whose two principal meridians have different centers of curvature. FIGS. 2 and 3 show a toric lens (15) having a spherical surface on the entrance side and a toroidal surface on the exit side. The structure of the toric lens (15) is arbitrary, and FIG. 4 shows an optical system incorporating a toric lens (15) having a toroidal surface on the entrance side and a spherical surface on the exit side. FIG. 5 shows an optical system incorporating a toric lens (15) having a toroidal surface on the entrance side and a cylindrical surface on the exit side.

FIG. 6 shows an optical system incorporating a toric lens (15) having a cylindrical surface on the entrance side and a toroidal surface on the exit side.

In the light beam scanning optical system having the above configuration, an image (electrostatic latent image) is formed on the photoreceptor (30) by the intensity modulation of the semiconductor laser (1) and the main scanning and sub-scanning. The spherical mirror (20) replaces the conventional f-theta lens and works with the toric lens (15) to correct (distortion aberration) so that the scanning speed in the main scanning direction is equal from the center of the scanning area to both ends thereof. , the curvature of field in the main scanning direction on the photoreceptor 30) is corrected.

In addition, the toroidal surface of the toric lens (15) installed in the reflected optical path from the polygon mirror (10) corrects the surface tilt error of the polygon mirror (10), and also Corrects the field curvature of

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 surface tilt 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 this embodiment, the cylindrical lens (7) focuses the light beam onto the polygon mirror (10), while the toroidal surface of the toric lens (15) creates a conjugate relationship between each reflective surface of the polygon mirror (10) and the condensing surface. I try to keep it. On the other hand, the other surface (spherical surface, cylindrical surface) of the toric lens (15) mainly corrects field curvature in the main scanning direction, and also corrects distortion aberration.

Furthermore, the toric lens (15) is a polygon mirror (
10) In order to flatten the image plane (correct the field curvature in the sub-scanning direction) due to 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 [in the following experimental example ( Rla), (Rlb), (R2a).

(R2b)], and the entrance side surface and the exit side surface are eccentric in the main scanning direction [(yt) in the (Y) direction in FIG.

It is preferable to make (Y, ) eccentric. This eccentricity corrects the left-right balance when field curvature and distortion are not symmetrical left and right from the center point in the main scanning direction. As a result, overall curvature and aberration are reduced. A similar effect is
The spherical mirror (20) is also decentered in the main scanning direction [see Figure 2 (
This can also be achieved by eccentricity of YM). Specific examples of the eccentricity (y+), (yz), and (yM) are shown in Experimental Examples (I) to (IX) below.

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. That is, polygon mirror (1
When a convergent beam or a diverging beam is incident on the polygon mirror 10 (the same applies to other rotary deflectors), the polygon mirror 10
), 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 it is received in a straight line, curvature of field will occur. Become. When a convergent light beam is incident on the polygon mirror (10), a concave curvature of field occurs in the direction of the light beam incidence. Furthermore, the distance between the spherical mirror (20) and the image plane changes depending on the degree of convergence of the incident light. The curvature of field also changes with this change in distance. In other words, due to the curvature of field due to the convergent light beam,
The curvature due to the concave surface of the spherical mirror (20) is corrected, and as a result, the curvature of field at the condensing surface is reduced, and the flatness of the image surface is improved.

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 can be reduced. This has the advantage of allowing higher density.

Furthermore, to explain this example in detail, as shown in FIG. 2, a toric lens (15) and a spherical mirror (20)
The distance (object point) from the deflection point (10a) to the position where the collimator lens (6) focuses the beam when there is no
s) (not shown) and the radius of curvature (R
M), the deflection point (10) of the polygon mirror (10)
The relationship between the distance 11i (d) from a) to the spherical mirror (20) and the radius of curvature (RM), and the toric lens (
15), the core thickness (dl), the radius of curvature of the lens entrance surface in the main scanning direction (Rla), and the radius of curvature of the lens exit surface in the main scanning direction (R2
Regarding the relationship with a), respectively, (1s/R21)
〉0.4 ・・・・・・■0.1< (d/
lRMI)<0.7...
・■0.6< (l Rla l +d+)/ l R
It is desirable to satisfy the following formula: 2a l <1.3 −■.

In addition, in Fig. 2, (d') is a spherical mirror (20).
The distance from to the photoreceptor (30), (do) is the deflection point (1
0a) to the entrance surface of the toric lens (15), and (d,) is the distance from the exit surface of the toric lens (15) to the spherical mirror (20).

When the above formulas 0, 0, and 0 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 becomes a spherical mirror (20)
, the arrangement becomes difficult and the distortion characteristics deteriorate.

When the lower limit of the above equation 0 is exceeded, 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). In addition, the above 0
When the upper limit of the equation 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.

When the lower and upper limits of the above equation 0 are exceeded, the curvature of field becomes large.

Here, experimental examples (I) and (I[) in this example.

(1), (It/ ), (v), (VI), (■), (
3) shows the configuration data and characteristic data of (IX).

[Left below] (1) Each experimental example (I), (I[), (III),
(IV), (V), (VI).

The aberrations at the photoreceptor condensing surface in (■), (■), and (IX) are shown in Figures 7, 8, 9, 10, and 1, respectively.
1, FIG. 12, FIG. 13, FIG. 14, and FIG. 15. 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). 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 indicates the curvature of field due to the light beam in the deflection plane (curvature of field in the main scanning direction), and the solid line indicates the deflection angle. This shows the curvature of field (curvature of field in the sub-scanning direction) due to a light beam in a plane perpendicular to the surface.

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, as a deflector, the polygon mirror (10) described above may be used.
In addition, 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.

Further, in the embodiment described above, the diverging 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 diverging light beam into a substantially parallel light beam.

Effects of the Invention As is clear from the above explanation, according to the present invention, each reflective surface of the deflector is It goes without saying that it is possible to correct errors caused by surface tilt of the lens, and evenly correct pitch irregularities in the sub-scanning direction of the image or scanning speed in the main scanning direction. Because it is arranged so that the center of the radius of curvature of the exit surface does not coincide with the center of the radius of curvature and satisfies the above equation 0, distortion aberration and image surface properties in the main scanning and sub-scanning directions are improved over a wide angle of view on the scanning medium. It can be corrected to something.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a schematic configuration of an embodiment of the optical system according to the present invention, and FIG. 2 is a diagram schematically illustrating the optical path on the deflection surface of the optical system shown in FIG. 1. , Figure 3, Figure 4,
Figures 5 and 6 are diagrams showing optical paths on a plane perpendicular to the deflection plane of the optical system when various toric lenses are used, Figures 7, 8, 9, and 10. Figure 11,
FIGS. 12, 13, 14, and 15 are graphs showing image distortion on the photoreceptor. (1)...Semiconductor laser, (6)...Collimator lens, (7)...Cylindrical lens, (10)...
... Polygon mirror, (15) ... Toric lens, (20) ... Spherical mirror, (25) ... Beam splitter, (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 folds back the light beam scanned by the deflector and focuses it on a scanning medium; and a spherical mirror that is disposed between the deflector and the spherical mirror, and that is arranged between an incident surface and an exit surface. A lens having one of its surfaces has a toroidal surface, and the center of the radius of curvature of the entrance surface and the center of the radius of curvature of the exit surface do not coincide in the scanning plane, 0.6<(|R_1|+d) /|R_2|<1.3 However, R_1: Radius of curvature in the main scanning direction plane on the entrance side of the lens R_2: Radius of curvature d in the main scanning direction plane on the exit side of the lens: Luminous flux toward the center of the scanning area by the deflector A light beam scanning optical system characterized by satisfying a conditional expression that is greater than or equal to the distance from a reflection point to a spherical mirror.