JPH01300217A - Light beam scanning optical system - Google Patents

Abstract

PURPOSE:To correct an error due to the surface tilt of each reflecting surface of a deflector and to obtain excellent distortion characteristics and image plane flatness by interposing a spherical mirror and a cylindrical lens in the optical path from the deflector to the surface of a photosensitive body under specific conditions. CONSTITUTION:Luminous flux emitted by a light source is scanned by the deflector at an equal angular velocity, reflected by the spherical mirror 20, and converged on the surface of the photosensitive body through the cylindrical lens 7. In this case, inequalities I hold. In the inequalities I, RM is the radius of curvature of the spherical mirror, (s) is the distance from the reflection point of the luminous flux toward the center of a scanning area by the deflector to the convergence point after deflector reflection, and (d) is the distance from said luminous flux reflection point to the spherical mirror. Further, d2 is the center thickness of the cylindrical lens 7 and d3 is the distance from the projection surface of the cylindrical lens 7 to the convergence surface. Consequently, the error due to the surface tilt of the deflector can be corrected and the excellent distortion characteristics and excellent image plane flatness are obtained.

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.

In general, the light beam scanning optical system used in laser beam printers and facsimile machines basically consists of a semiconductor laser as a light source, a polygon mirror, a galvano mirror, etc.
It is composed of a deflector such as, and an rθ 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 rθ 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, conventionally, an ellipsoidal mirror is used instead of the fθ lens (Japanese Patent Application Laid-open No. 123040/1983).
, using a parabolic mirror (Special Publication No. 55-3612)
7), using a concave reflecting mirror (Japanese Patent Application Laid-open No. 1983-
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 system has been made more compact, and the curvature of the image plane perpendicular to the main scanning direction at the condensing point has been reduced, enabling a higher angle of view and higher density, as well as effectively reducing deflector surface tilt errors. The goal is to correct it. 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. (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; It is characterized by comprising: a spherical mirror that is folded back to condense light onto the photoreceptor surface; (e) a cylindrical lens disposed between the spherical mirror and its converging point; and (f) satisfying the following formula: do.

|s/Rml>0.5 0.15<d/l RMl<0.45(ldg
l + lag l ) / IRMI <0.45 However, RM: Radius of curvature of the spherical mirror S: Distance from the point where the light beam is reflected toward the center of the scanning area by the deflector to the focal point after reflection by the deflector d: Deflector Distance d2 from the reflection point of the light beam toward the center of the scanning area to the spherical mirror: Core thickness d3 of the cylindrical lens: Distance from the exit surface to the condensing surface of the cylindrical lens. The light beam is scanned at a constant angular velocity by a deflector, this scanning light beam is reflected by a spherical mirror, and is focused onto the photoreceptor surface via a cylindrical lens. An image is formed by main scanning by the deflector and sub-scanning by moving the photoreceptor surface. The light flux 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 across the French field angle. This provides excellent image plane flatness.

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 cylindrical lens focuses the light beam reflected by the spherical mirror onto the photoreceptor surface, correcting errors caused by the tilt of the deflector surface, and reducing field curvature, allowing for a higher angle of view and higher density. Make it.

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 Example] In FIG. 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 beam splitter,
(20) is a spherical mirror, (25) is a cylindrical lens, 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. The polygon mirror (10) is driven to rotate at a constant speed in the direction of arrow (a) about a 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. After passing through the beam splitter (15), this scanning light beam passes through the spherical mirror (10). (20), and is further reflected by the beam splitter (15), and then condensed onto the photoreceptor (30) via the cylindrical lens (25).At this time, the condensed luminous flux is The photoreceptor (30) is scanned at a constant speed in the axial direction of the photoreceptor (30), and this is called main scanning.The photoreceptor (30) is also rotated at a constant speed in the direction of arrow (b), and the scanning caused by this rotation is called sub-scanning. It is called.

That is, in the above light beam scanning optical system, 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. As shown in FIG. 2, a spherical mirror (20) and a cylindrical lens (25) replace the conventional fθ lens so that the scanning speed in the main scanning direction is made equal from the center of the scanning area to both ends thereof. to correct.

Further, the cylindrical lens (25) installed in the optical path between the spherical mirror (20) and the photoreceptor (30) corrects surface tilt errors of the polygon mirror (10) and reduces field curvature. In other words, the purpose is to correct the luminous flux in a direction perpendicular to the main scanning direction to flatten the image plane near the focal point. In other words, if there is an error in perpendicularity between the reflecting surfaces of the polygon mirror (10), the photoreceptor (
30) The upper scanning line shifts 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) (7) 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, the cylindrical lens (7) condenses the light beam onto the polygon mirror (10), while the cylindrical lens (25) maintains a conjugate relationship between each reflective surface of the polygon mirror (10) and the condensing surface. I try to do it.

Furthermore, in this embodiment, the collimator lens (6) corrects the divergent light beam into a convergent light beam. This is to correct the curvature of the image plane near the photoreceptor (30) by creating a convergent light beam. That is, when a converging light beam or a diverging light beam is incident on the polygon mirror (10) (the same applies to other rotary deflectors), the focal point after reflection on the polygon mirror (10) is the polygon mirror (10). Assuming that there are no optical components after the reflection point, the reflection point will form a substantially circular arc shape, and if it is received by a straight line, curvature of field will occur. When a convergent light beam is incident on the BoIJ conversion mirror (10), a concave curvature of field occurs in the light beam incident direction. 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. That is, the curvature of field caused by the convergent 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 condensing surface is reduced, and the flatness of the image surface is improved.

In this respect, the cylindrical lens (25) also has the effect of reducing the field curvature, and when the field curvature is reduced,
Fluctuations in the diameter of the condensed beam due to differences in scanning position (image height) are reduced, allowing the optical system to be used at a wide angle of view, and the diameter of the condensed beam can be made smaller, making it possible to increase the density of images. has advantages.

For details, as shown in Figure 2, polygon mirror <10)
from the deflection point (10g) to the vertex of the spherical mirror (20) <2
0a) to the distance 11!1(d) and the spherical mirror (20)
The relationship between the radius of curvature (R&l) and the distance (S) (not shown) from the deflection point (10a) to the focal point after reflection at the polygon mirror (10) relationship, as well as the radius of curvature (RM) and the cylindrical lens (2
5) Core thickness (aS) and cylindrical lens <25)
Regarding the relationship between the distance m (dm) from the exit surface to the photoreceptor (30), I s/RM l > 0.5
, , , , ■0, 15< d/l R
M l < 0.45 ・・・・・・■(
Id*l+1dal)/1Ryl<o, 4s...
It is desirable to satisfy the following formula.

In addition, in Fig. 2, (dl) is a spherical mirror (20)
It is the distance from the vertex (20a) of

When the above formulas 0, 2, 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). When the lower limit of the above equation 0 is exceeded, the image surface becomes a spherical mirror (2
0), arrangement becomes difficult and distortion characteristics deteriorate.

On the other hand, when the lower limit of the formula (2) 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). 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 or the distortion characteristics deteriorate.

Furthermore, when the upper limit of the formula (2) is exceeded, the curvature of field becomes large.

Here, experimental examples (I) and (I) in the first embodiment.

The configuration data for (III>, (mV), and (V) are shown in Table 1. The facing distance of the polygon mirror (10) was 23.5 mm.

[The following experimental examples (I), (IF), (I [I), (I
Aberrations at the photoreceptor condensing surface in V) and (V) are shown in FIGS. 4, 5, 6, 7, and 8, 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. 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.

[Second Embodiment] As is clear from FIGS. 9 and 11, in this second embodiment, the spherical mirror (20) is set at an angle (θ1) in a plane perpendicular to the deflection plane.
It differs from the first embodiment in that it is arranged at an angle.

By tilting the spherical mirror (20) in this way, the light beam from the polygon mirror (lO) is directed to the spherical mirror (20) in a direction different from the direction of incidence [angle (θ, ),
twice the inclination angle (θ, )], and can be reflected directly or by interposing a folding mirror (21) without requiring a semi-transparent means such as a beam splitter (15) as in the first embodiment. , cylindrical lens (25)
The light can be focused on the photoreceptor surface through the .

Incidentally, when the spherical mirror (20) is tilted, the scanning line bends. This generated M prisoner will be explained with reference to FIG. 15. Figure 15 shows the optical axis in a plane perpendicular to the deflection plane, where point (P) is the principal ray reflection point when the deflection angle is (0°), and point (Q) is the deflection angle (θ). This is the principal ray reflection point when .

Since the spherical mirror (20) has a curvature (the curvature within the deflection plane is considered here), the reflection point shifts in the X-axis direction between the deflection angles (0°) and (θ). Furthermore, the reflected light (n3) at a deflection angle (θ) with respect to the incident light (nl) is shifted in the Z-axis direction with respect to the reflected light (n2) at a deflection angle (0°). This shift is caused by the deflection angle 〈θ ), and the reflected light (n
2), (n3) are not included in the same plane. Therefore, the scanning line also bends in two axial directions within a plane perpendicular to the optical axis. However, this type of scanning line bending can be corrected with the cylindrical lens (25). That is, if the imaging relationship by the cylindrical lens (25) is set so as to form a reduced image in the sub-scanning direction, the bending of the scanning line here will also be reduced in the sub-scanning direction.

Furthermore, in order to correct the bending of the scanning line, it may be possible to shift the cylindrical lens (25) in the direction perpendicular to the optical path (the amount of shift is indicated by (Zc)) as shown in FIG.

The other configurations are the same as those of the first embodiment.

Also, in the second embodiment, the relationships of equation 0, equation (2), and equation (2) are valid. In particular, if the upper limit is exceeded in formula ■,
Not only the field curvature but also the bending of the scanning line increases.

Here, an experimental example (Vl>, (■)) in the second embodiment.

The configuration data for (■) is shown in Table 2, and the polygon mirror (
The facing distance of 10) is 23.5 mm as in the first embodiment.

[Margins below] The aberrations at the photoreceptor condensing surface in each of the above experimental examples (W), (■), and (■) are shown in Figures 12, 13, and 14, respectively.
(a) in each figure 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, where the - dotted line indicates the curvature of field due to the light flux within the deflection plane, and the solid line indicates the curvature of field due to the light flux within the plane perpendicular to the deflection plane. (e) in each figure showing the field curvature is
A graph with the horizontal axis as the scanning angle and the vertical axis as the scanning line distortion degree.
It shows a positional shift in the direction perpendicular to the deflection plane of the scanning line, that is, 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, 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.

On the other hand, in each of the above-mentioned embodiments, the shift of the spherical mirror in the main scanning direction J [the (Y) direction in FIGS. 2 and 10 is not mentioned. However, in consideration of aberration correction and ease of arrangement, it is conceivable to shift the spherical mirror in the above direction. For example, experimental example (I) in the first embodiment (fourth
If the distortion aberration is not symmetrical, such as in the experimental example (■) in the second embodiment (see Fig. 12), the distortion can be further reduced by shifting the spherical mirror like this. .

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.

As is clear from the above explanation, according to the present invention, a spherical mirror and a cylindrical lens are provided in the optical path from the deflector to the photoreceptor surface so as to satisfy the above formulas 0, 2, and 2. Because of the interposition, it is possible to uniformly correct the scanning speed in the main scanning direction, as well as to correct errors caused by the surface inclination of each reflective surface of the deflector, and to correct pitch unevenness in the vertical direction of the sub-scanning of the image.
Good distortion characteristics and good image plane flatness can be obtained over a wide angle of view on the light 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.

Moreover, as explained in the second embodiment, if the spherical mirror is tilted in a plane perpendicular to the deflection surface, the light can be focused on the photoreceptor without using a semi-transparent means, and the optical member can be Arbitrarity in arrangement is improved, and attenuation of the amount of light is also reduced.

[Brief explanation of the drawing]

1 to 8 show a first 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 FIG. Figures 8 through 8 are graphs showing image distortion on the light condensing surface. 9 to 15 [1 shows the second embodiment of the present invention, FIG. 9 is a perspective view showing the schematic structure,
Figures 0 and 11 are diagrams for schematically explaining the optical path, Figures 12 to 14 are graphs showing image distortion on the condensing plane,
FIG. 5 is a diagram for explaining the bending of the scanning line due to tilting the spherical mirror. (1)...Semiconductor laser, (6)...Collimator lens, (7)...Cylindrical lens, (10)...
... Polygon mirror, (15) ... Beam splitter, (20) ... Spherical mirror, (25) ... Cylindrical lens, (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 the light beam at a constant angular velocity; a spherical mirror that returns the light beam scanned by the deflector and focuses it on the surface of the photoreceptor; and a cylindrical lens disposed between the spherical mirror and its focusing point. |s/R_M|>0.5 0.15<d/|R_M|<0.45 (|d_2|+|d_3|)/|R_M|<0.45 However, R_M: of the spherical mirror Radius of curvature s: Distance from the point where the beam is reflected by the deflector toward the center of the scanning area to the focal point after reflection by the deflector d: Distance from the point where the beam is reflected by the deflector toward the center of the scanning area to the spherical mirror d_2: A light beam scanning optical system characterized by satisfying the following three equations: core thickness d_3 of cylindrical lens: distance from the exit surface to the condensing surface of the cylindrical lens.