Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-11-06
2002-12-31
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S216100, C347S258000, C347S261000
Reexamination Certificate
active
06501586
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanning apparatus and, more particularly, to prevention of flare from a scanning mirror in the optical scanning apparatus used in printers in which an image is drawn with laser light.
2. Related Background Art
FIG. 8
is an optical path diagram of a conventional optical scanning apparatus. Reference numeral
91
designates a laser unit,
92
a cylindrical lens,
93
a scanning mirror,
94
a spherical lens,
95
a toric lens, and
96
a photosensitive drum. Parallel laser light emitted from the laser unit
91
is converged only in the sub-scanning direction by the cylindrical lens
92
to impinge on a surface of the scanning mirror
93
. The scanning mirror
93
rotates at a constant speed and the light reflected by the scanning mirror
93
travels through the spherical lens
94
and the toric lens
95
to undergo correction for F&thgr;, whereby converging light scans the surface on the photosensitive drum
96
. The photosensitive drum
96
rotates at a constant speed in synchronism with a driving signal of the semiconductor laser and the scanning light forms an electrostatic latent image on the photosensitive drum
96
. An image is printed from this electrostatic latent image onto paper by the electrophotographic process.
Recently, there are strong desires for higher definition of image and higher speed of output and there are thus increasing needs to increase the width of the polygon mirror, i.e., to increase the size of the polygon mirror in order to make the F&thgr; optical system brighter and, in addition thereto, to rotate the polygon mirror at a higher speed. On the other hand, there is a limit to the performance of the motor for rotating the polygon mirror of the thus increased size at the higher speed and the cost for the polygon mirror and motor increases.
Under such circumstances, the scanning efficiency of the polygon mirror is increased by illuminating the polygon mirror with a beam greater than the main scanning width of the polygon mirror (an overfield optical system), for example, as disclosed in Japanese Patent Application Laid-Open No. 6-143677. The scanning efficiency of the polygon can be increased by this method, but there arises a problem that spot sizes are nonuniform depending upon image heights. An effective method for relaxing this nonuniformity is to make the beam incident to the polygon mirror from in a plane made by the rotation axis of the polygon and the optical axis of the f&thgr; lens. This can minimize the nonuniformity of the spot, as compared with that in the case of incidence from other positions, and the spot becomes symmetric with respect to the image heights.
However, if the width of the incident beam is made greater than the width of the reflective facets of the polygon, there will arise a problem that reflected light from an adjacent facet appears as flare at the image plane to deteriorate the image.
An object of the present invention is, therefore, to maintain sufficient rays in the portion of the polygon mirror associated with the formation of image and efficiently intercept the reflected light from the surface not associated with the image.
SUMMARY OF THE INVENTION
In the present invention for accomplishing the above object, in order to efficiently intercept the flare, a distal end of a shield member is located within a predetermined range determined by a diameter &phgr; of an inscribed circle to the polygon and a focal length f of the f&thgr; lens or the like.
An optical scanning apparatus according to an aspect of the invention is an optical scanning apparatus comprising a scanning mirror having a plurality of reflective facets for deflectively reflecting a first beam emitted from a laser light source and an f&thgr; lens for focusing a second beam deflectively reflected by a reflective facet of said scanning mirror, in a spot shape on an image plane,
wherein a width in a main scanning direction of the first beam incident to said scanning mirror is wider than a width of the reflective facets of said scanning mirror in the main scanning direction,
said optical scanning apparatus including a shield member for intercepting a third beam reflected by a reflective facet adjacent to the reflective facet deflectively reflecting the second beam.
In the optical scanning apparatus according to another aspect of the invention, the first beam travels through said f&thgr; lens to be incident to said scanning mirror and the first beam incident to the scanning mirror is present within a plane made by a rotation axis of the scanning mirror and the optical axis of the f&thgr; lens.
In the optical scanning apparatus according to another aspect of the invention, where the origin is set at the center of the rotation axis of said scanning mirror, an x-coordinate axis is taken along the optical axis of the first beam of incident rays, the positive direction of the x-coordinate axis is taken along a direction in which the second beam and third beam of reflected rays from the scanning mirror travel, and a y-coordinate axis along the main scanning direction, a distal end of said shield member is located in the range defined by the following equation between the reflective facet of the scanning mirror and a ray output surface of the f&thgr; lens:
&agr;
x+&bgr;≦|y|≦&agr;′x+&bgr;′
[Eq. 1]
&agr;=tan 2&thgr;
0
&bgr;=(sec 2&thgr;
0
)×(
a
cos &thgr;
0
−2&phgr; sin &thgr;
0
)/2
&agr;′=tan 2&thgr;
1
&bgr;′=(−sec 2&thgr;
1
)×(
a
cos &thgr;
1
+2&phgr; sin &thgr;
1
)/2
&thgr;
0
=y
0
/2
f
&thgr;
1
=2
&pgr;
−&thgr;
0
a
=&phgr; tan(&pgr;/
n
),
where n is the number of facets of a polygon being the scanning mirror, f a focal length of the f&thgr; lens, y
0
a maximum image height in the main scanning direction, and &phgr; a diameter of an inscribed circle to the polygon.
In the optical scanning apparatus according to another aspect of the invention, said shield member is means for positioning said f&thgr; lens.
In the optical scanning apparatus according to another aspect of the invention, said shield member is a portion of a lens surface in a noneffective image portion of said f&thgr; lens, said portion being treated by a light-intercepting treatment.
In the optical scanning apparatus according to another aspect of the invention, said shield member is formed so as to be integral with an optical box.
In the optical scanning apparatus according to another aspect of the invention, where the origin is set at the center of the rotation axis of said scanning mirror, an x-coordinate axis is taken along the optical axis of the first beam of incident rays, the positive direction of the x-coordinate axis along a direction in which the second beam and third beam of reflected rays from the scanning mirror travel, and a y-coordinate axis along the main scanning direction, a distal end of said shield member is located in the range defined by the following equation between the f&thgr; lens and said image plane:
&agr;
x+&bgr;≦|y|≦&agr;′x+&bgr;′
[Eq. 2]
&agr;=2(2
f&thgr;
0
cos 2&thgr;
0
+&phgr; sin &thgr;
0
−x
1
sin 2&thgr;
0
)/(cos &thgr;
0
+2
f
cos 2&thgr;
0
)
&bgr;=2
f&thgr;
0
−[2(
f+x
1
)(2
f&thgr;
0
cos 2&thgr;
0
+&phgr; sin &thgr;
0
−x
1
sin 2&thgr;
0
)/(cos &thgr;
0
+2
f
cos 2&thgr;
0
)]
&agr;′=2(2
f&thgr;
1
cos 2&thgr;
1
+&phgr; sin &thgr;
1
−x
1
sin 2&thgr;
0
)/(−cos &thgr;
1
+2
f
cos 2&thgr;
1
)
&bgr;′=2
f&thgr;
1
+[2(
f+x
1
)(2
f&thgr;
1
cos 2&thgr;
1
+&phgr; sin &thgr;
1
−x
1
sin 2&thgr;
1
)/(cos &thgr;
1
−2
f
cos 2&thgr;
1
)]
&thgr;
0
=y
0
/2
f
&thgr;
1
=2
&pgr;
−&thgr;
0
a
=&phgr; tan(&pgr;/
n
),
where n is the number of facets of a polygon being the scanning mirror
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