JPH0664252B2 - Rotating polygon mirror - Google Patents

Description

The present invention relates to a rotating polygon mirror used as an optical deflector.

An optical deflector using a rotating polygon mirror is used as a device for deflecting and scanning a light beam on a predetermined screen as a light beam scanning device for various devices such as a laser beam printer. FIG. 1 shows an example of a light beam scanning device using a rotating polygon mirror, and FIG. 2 shows an example of a light beam scanning device using the rotating polygon mirror as an optical deflector. The operation will be briefly described. A light beam L is incident from a laser generator 2 which is a light source on a mirror surface 1a of a rotary polygon mirror 1 which is attached to a rotating shaft of a motor and rotates in a direction of an arrow. The light beam reflected by the mirror surface 1a is deflected by an angle θ from L ₁ to L ₂ by the rotation of the rotary polygon mirror 1. The deflected light beam is converged by the imaging lens 3, forms an image on the surface 4 to be scanned, and scans from the point A to the point B in the arrow direction. Reference numeral 5 denotes a beam detector, which detects the light beam reflected by the mirror 6 immediately before the light beam scans point A, and a control circuit connected to the beam detector 5 starts generation of signal light after a fixed time after detection. It is to let me. This is for eliminating the influence of the division error of each mirror surface and always aligning the writing start position of the signal light with the fixed point A on the surface to be scanned. If each mirror surface is a perfect plane, the writing end position of the signal light, which ends after a fixed time from the start of generation, is always the fixed point B by this control.

Now, if each mirror surface of the rotary polygon mirror 1 is a perfect plane, the writing end position of the signal light always gathers at a fixed point as described above.
Not really because it's not a perfect plane. Each mirror surface of the rotary polygon mirror 1 has a minute convex shape (dashed line) or a concave shape (dashed line) as shown in FIG. 3 due to problems in processing accuracy during mirror surface processing. And has a complicated shape and various shapes. When the mirror surface is curved like this, the writing start point A of the signal light is constant due to the action of the beam detector, but the writing end point B is shown by points B'and B "in FIG. As shown in Fig. 4, Fig. 5 and Fig. 2, Fig. 4 shows the mirror surface of the rotary polygon mirror 1 of Fig. 1 in a plane orthogonal to the axis of rotation. In the case of the rotary polygonal mirror whose cross-sectional shape of the mirror surface is a convex arc when cut, when the light beam L is incident on the mirror surface 1a ', its deflected beam is caused by the rotation of the rotary polygonal mirror. It is seen at the time when the deflection is started and when the deflection is ended that the direction of the beam changes from the rotating polygonal mirror 1 to the position P1 and the light beam L is the mirror surface 1a'-1.
Incident on point S. The light beam reflected at the point S is reflected in the direction of L4. If the mirror surface is a perfect plane like 1b-1, the light beam L is reflected at the point S'on the mirror surface 1b-1, and the reflected light is
Head toward L3. The angle of deviation between L3 and L4 is mirror surface 1a′-1
If the angle between the tangent line at point S and the 1b-1 plane is Δφ, then 2Δφ.

Next, when the deflection is completed, the rotary polygon mirror 1 is at the position P2, and the light beam L is incident on the point E on the mirror surface 1a'-2. The light beam reflected at point E is reflected in the direction of L5. If the mirror surface
If it is a perfect plane like 1b-2, the light beam L is reflected by E'on the mirror surface 1b-2, and the reflected light goes in the direction of L6. The deviation angle between L5 and L6 is 2ΔΨ, where ΔΨ is the angle between the tangent at point E on the mirror surface 1a′-2 and the 1b-2 surface.
When beam scanning is performed using the rotating polygon mirror in the beam scanning device shown in FIG. 2, the scanning start point is point A and the scanning end point is point B '. That is, point B is closer to point A. When a rotating polygon mirror is processed normally, each mirror surface does not bend in a constant shape, but generally bends in a random shape, convex or concave. FIG. 5 shows the trajectory of each scanning line when the surface to be scanned 4 in FIG. 2 moves at a constant speed in the direction perpendicular to the scanning beam. When each mirror surface is curved in various shapes, the position of the scanning end point B varies as shown in the figure. As a result, in the case of a laser beam printer, the recorded image near the scanning end point fluctuates left and right. Therefore, the image quality is deteriorated.

For example, if the mirror surface is curved in a convex arc shape as shown by the broken line in FIG. 6, the length H of the mirror surface is set to 30 mm,
If the maximum height δ of the circular arc is 0.3 μm, Δψ + ΔΨ will be about 11 seconds, and if the surface to be scanned is scanned through a lens having a focal length of 500 m, the deviation BB ′ at the scanning end point will be 55 μm. Become. Actually, as described above, the curvature of the mirror surface is not only convex but also concave (6th curve).
(Dashed line in the figure), so the total width B'of the misalignment on all mirror surfaces
-B "is 110 .mu.m (corresponding to .DELTA.l in FIG. 5). If the scanning line pitch is 100 .mu.m, this deviation corresponds to the scanning line pitch, which is a serious problem in image quality. In order to make B′−B ″ as small as possible, the height difference 2δ between the apex of the convex surface and the bottom point of the concave surface must be kept as small as possible. In the above example, 2δ
= 0.3 μm, that is, δ = 0.15 μm or less. In the conventional mirror surface processing, the flatness of the mirror surface is strictly suppressed (that is, it is as close as possible to a perfect flat surface) in order to obtain such high accuracy. In the above example, the flatness is kept to 0.15 μm. However, it is difficult to suppress the flatness severely, and the yield of parts is bad.

In order to overcome such a problem in the processing accuracy of the rotary polygon mirror, the present invention provides a scanning end point for each scanning line as long as the curved shapes of the respective mirror surfaces are all the same even if the flatness is poor. Paying attention to the fact that the end points do not vary, and even if this causes the absolute position of the scan end point to shift, it does not pose a problem in practical use. The degree of tolerance of the degree is greatly relaxed to facilitate the processing of the rotary polygon mirror.

The present invention, in a rotary polygon mirror having a plurality of reflecting plane mirrors, when performing the mirror surface processing of each of the plane mirrors, the curved shape of each of the plane mirrors, all the convex surface or all the concave surface to the same shape, the amount of curvature Is the same, so that the flatness tolerance of each of the plane mirrors is set to 1 μm.

FIG. 7 shows an embodiment of the present invention, and two typical examples are shown in one drawing. The point is that the curved shapes of the mirror surfaces are all the same and the amount of curvature is also the same. The shape of the broken line is the case of a convex shape, and the dashed-dotted line is the case of a concave shape. In each case, the curved shape of each mirror surface is a circular arc with almost the same shape,
The amount of bending δ is almost the same. Even if the flatness of the mirror surface is poor, if the curved shape of each mirror surface is the same, the scanning end points of each mirror surface are all near the B'point or near the B "point shown in FIG. There are no gatherings and variations.
When the mirror surface is curved in this way, each scanning point on the scanning line of the surface to be scanned deviates from the normal position as it moves away from the scanning start point, but this deviation is practically a problem except for some special applications. do not become. In the case of a laser beam printer, when characters are simply printed, there is no problem even if point B is misaligned by 1 mm at the scanning end point as shown in FIG. It is easier to bend the mirror surface to process it than to give it flatness.In this way, if the shapes of the mirror surfaces are intentionally curved and aligned, the flatness of the mirror surface can be considerably loosened. It will be much easier. In the case of the above example, the variation in the curvature of each mirror surface, that is, the curvature of each mirror surface when all the cross-sectional shapes when the mirror surface of the rotary polygon mirror is cut along a plane orthogonal to the rotation axis is overlapped Variation 0.3
Within the range of .mu.m, even if the flatness is .delta. = 1 .mu.m, the variation of the scanning points by each mirror surface at the scanning end point is 55.
Since it falls within μm, it can be used without any problem.

In order to process the mirror surface to almost the same distortion shape, when processing the mirror surface, the rotary polygon mirror should be held so that mechanical deformation and thermal deformation applied to the mirror surface are always constant on each mirror surface. This can be achieved by improving the accuracy of repeating the movement of each mirror surface of the blade for processing the mirror surface. To give a specific example,
When mirror-finishing a material made of hard material such as glass by lapping, it is easy to sag in a convex shape near both ends, but this sagging amount is equal, and the curved shape of the central part is also convex so that the whole is almost the same. In order to form a convex arc shape, the auxiliary materials that sandwich each mirror surface from both sides during mirror surface processing are made uniform, and when both the mirror surface and the auxiliary material are at the same temperature, they are joined by lapping. The load may be adjusted so that the polishing pressures at the time become equal.

The distorted shape of the mirror surface is not only a simple one like an arc,
It may have various complicated shapes, for example,
It may have a wavy uneven shape as shown in the figure, or it may have a complicated shape.

As described above, according to the present invention, it is possible to supply a rotary polygon mirror which is very easy to manufacture and which has a very low mirror surface processing accuracy while maintaining the image quality.

[Brief description of drawings]

FIG. 1 is a perspective view of a rotary polygon mirror, FIG. 2 is a view showing an example of a light beam scanning device, FIG. 3 is a perspective view of one mirror surface of the rotary polygon mirror, and FIG. 4 is a rotary polygon mirror showing a light beam. FIG. 5 is a diagram for explaining the state of deflection, FIG. 5 is a diagram showing scanning lines written on the surface to be scanned by the conventional rotary polygon mirror, and FIG. 6 is one mirror surface of the rotary polygon mirror orthogonal to the rotation axis. FIG. 7 is a sectional view taken along a plane, FIG. 7 is a view showing a rotary polygon mirror of the present invention, FIG. 8 is a perspective view of a mirror surface of another embodiment of the rotary polygon mirror of the present invention, and FIG. 9 is a view showing the present invention. The figure which showed an example of the scanning line written on the to-be-scanned surface with a rotating polygon mirror. 1,1 ', 1 "... Rotating polygon mirror 1a, 1a'-1,1a'-2 ... Mirror surface 2 ... Laser generator 3 ... Imaging lens 4 ... Scanned surface L ... Light beam

Claims (1)

[Claims] 1. In a rotary polygon mirror having a plurality of reflecting plane mirrors, when performing the mirror surface processing of each of the plane mirrors, the curved shape of each of the plane mirrors is made to be the same shape of all convex surfaces or all concave surfaces, and the curved shape. A rotary polygon mirror characterized in that the flatness tolerance of each plane mirror is set to 1 μm by arranging the amounts to be substantially the same.