Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2002-10-01
2004-03-30
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S216100
Reexamination Certificate
active
06714332
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a scanning optical system employed in a laser beam printer or the like. More specifically, the present invention relates to a scanning optical system which includes a curved reflection surface to reflect a laser beam deflected by a polygonal mirror and to direct the reflected laser beam to a surface to be scanned.
Conventionally, a scanning optical system includes a polygonal mirror which deflects a beam emitted by a laser diode to scan a surface to be scanned (e.g., a photoconductive surface of a photoconductive drum), and an f&thgr; lens which converges the beam to form a scanning beam spot on the surface such that the scanning beam spot scans on the surface at a constant speed.
As the polygonal mirror rotates about its rotational axis, the beam spot moves on the surface to be scanned. Hereinafter, a direction, on the surface to be scanned, in which the beam spot moves as the polygonal mirror rotates is referred to as a main scanning direction. By ON/OFF modulating the beam spot as it moves in the main scanning direction, and by moving the surface to be scanned in a direction perpendicular to the main scanning direction, a two dimensional image can be formed on the surface. Hereinafter, the direction, on the surface to be scanned, perpendicular to the main scanning direction is referred to as an auxiliary scanning direction. Further, shape and direction of power of each optical element provided in the scanning optical system is described with reference to directions on the surface to be scanned.
A cylindrical lens which has a positive power only in the auxiliary scanning direction is also provided in the scanning optical system. The beam passed through the cylindrical lens is converged in the auxiliary scanning direction, and the converged beam is incident on a reflection surface of the polygonal mirror. Further, a power of the f&thgr; lens in the auxiliary scanning direction is determined so that the reflection surface of the polygonal mirror and the surface to be scanned have a conjugate relationship. With this configuration, a facet error of the polygonal mirror can be corrected.
Recently, an optical scanning system, which uses a reflector having a curved reflection surface (which will be referred to as an f&thgr; mirror hereinafter) in place of the f&thgr; lens, has been provided. With this type of scanning optical system, it is expected that chromatic aberration is reduced. It is also expected that the size of the optical scanning system is reduced because of the bent configuration of an optical path. In general, in such a scanning optical system, a polygonal mirror whose thickness is reduced in order to achieve weight reduction thereof is employed.
The f&thgr; mirror reflects the beam deflected by the polygonal mirror, and directs the beam to the surface to be scanned. The reflection surface of the f&thgr; mirror is typically an anamorphic surface, which is symmetrical with respect to an auxiliary scanning plane including the rotational axis of the polygonal mirror and being perpendicular to the surface to be scanned.
Similar to the f&thgr; lens, the f&thgr; mirror has functions of maintaining the constant scanning speed of the beam spot on the surface to be scanned, correcting curvature of field both in the main scanning direction and in the auxiliary scanning direction, and correcting the facet error of the polygonal mirror.
In the scanning optical system employing the f&thgr; mirror, the light source, the polygonal mirror, and the f&thgr; mirror are arranged such that the beam incident on the polygonal mirror travels in the auxiliary scanning plane, in order to prevent occurrence of aberration caused by the reflection surface of the f&thgr; mirror.
FIG. 1
shows a typical configuration of a conventional scanning optical system which uses an f&thgr; mirror
5
. As shown in
FIG. 1
, a laser beam emerging from a cylindrical lens
3
is reflected by a polygonal mirror
4
, and travels in the reverse direction. Then, the laser beam is reflected by the f&thgr; mirror
5
, and is directed to a photoconductive drum
11
.
Travel of the laser beam in the cylindrical lens
3
will be described in detail. As shown in
FIG. 1
, the laser beam emitted by a light source (not shown) enters the cylindrical lens
3
through a cylindrical surface
3
a
, exits from a planar surface
3
b
, and travels to the polygonal mirror
4
along an optical path Ax. Generally, a collimating lens is arranged between the light source and the cylindrical lens
3
. Accordingly, a collimated laser beam is incident on the cylindrical lens
3
.
When the laser beam emitted by the laser source is incident on the cylindrical surface
3
a
, most part of the laser beam passes through the cylindrical surface
3
a
and exits from the planar surface
3
b
. However, a remaining part of the laser beam is reflected by the planar surface
3
b
. Further, the part of the laser beam reflected by the planar surface
3
b
is partially reflected by the cylindrical surface
3
a
, thereby stray light G being generated and emerging from the planar surface
3
b.
Since the stray light G is converged twice by the cylindrical surface
3
a
, it converges on a point between the cylindrical lens
3
and the polygonal mirror
4
, and is incident on the polygonal mirror
4
as a diverging beam.
As can be seen in
FIG. 1
, a portion of the stray light G is deflected by the polygonal mirror
4
, and therefore, the portion of the stray light scans on the photoconductive drum
11
. It is understood that the portion of the stray light G scanning on the photoconductive drum
11
does not affect the quality of an image formed on a photoconductive surface of the photoconductive drum
11
substantially because it is very weak and does not stay on the same position on the photoconductive surface.
On the contrary, most part of the stray light G passes by the polygonal mirror
4
and is directly incident on the photoconductive drum
11
. That is because the thickness of the polygonal mirror
4
is reduced in order to achieve weight reduction thereof. The stray light G directly incident on the photoconductive drum
11
affects the quality of an image since, although it is very weak, the stray light G directly incident on the photoconductive drum
11
is not deflected by the polygonal mirror
4
and stays on the same position on the photoconductive surface.
SUMMARY OF THE INVENTION
The present invention is advantageous in that it provides a scanning optical system, which is capable of preventing deterioration of the quality of an image formed on a surface to be scanned due to the stray light generated by the cylindrical lens.
According to an aspect of the invention, there is provided a scanning optical system for emitting a beam scanning in a main scanning direction. The scanning optical system is provided with a light source that emits a beam, a cylindrical lens that converges the beam emitted by the light source in an auxiliary scanning direction which is perpendicular to the main scanning direction, a polygonal mirror that rotates and deflects the beam emerged from the cylindrical lens to scan in the main scanning direction, and an optical element that has a reflection surface to reflect the beam deflected by the polygonal mirror. The optical element is configured to converge the beam deflected by the polygonal mirror to form a beam spot on a surface to be scanned, and to enable the beam spot to scan on the surface to be scanned at a constant speed. In this case, the cylindrical lens is arranged such that a central axis of the beam, which enters into the cylindrical lens and exits from the cylindrical lens without being reflected by inner surfaces of the cylindrical lens, is deflected in the auxiliary scanning direction by the cylindrical lens and is incident on the polygonal mirror.
With this configuration, it becomes possible to prevent a portion of the light beam, which is reflected by the inner surfaces of the cylindrical lens a plurality of times and exits from the cylindrical
Greenblum & Bernstein P.L.C.
PENTAX Corporation
Phan James
LandOfFree
Scanning optical system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Scanning optical system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Scanning optical system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3288185