Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2002-11-15
2004-04-06
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S204200, C359S216100
Reexamination Certificate
active
06717705
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a scanning optical system employed in a scanning optical device such as a laser beam printer or the like.
In a scanning optical system for a laser beam printer, a laser beam emitted by a laser diode is deflected by a polygonal mirror to scan within a predetermined angular range. The scanning beam passes through an f&thgr; lens, which converges the beam to form a scanning beam spot on a photoconductive surface. As the polygonal mirror rotates, the beam spot moves on the photoconductive surface. By ON/OFF modulating the beam spot as it moves, an electrostatic latent image is formed on the photoconductive surface. Hereinafter, a direction, on the photoconductive surface, in which the beam spot moves as the polygonal mirror rotates is referred to as a main scanning direction, while a direction perpendicular to the main scanning direction, on the photoconductive surface, is referred to as an auxiliary scanning direction.
Further, shape and direction of power of each optical element is described with reference to directions on the photoconductive surface. Further, a plane perpendicular to a rotation axis of the polygonal mirror and including an optical axis of a scanning lens is defined as a main scanning plane.
Sometimes, a multi-beam scanning optical system is configured such that a plurality of beams are deflected simultaneously by a single polygonal mirror. With such a configuration, since the single polygonal mirror is used as a deflector for each of the plurality of beams, the number of optical elements can be decreased, and a room for such elements can be reduced. If the plurality of beams are respectively inclined in the auxiliary scanning direction, and are incident on substantially the same point on the polygonal mirror, the thickness of the polygonal mirror can be reduced, which reduces a manufacturing cost of the polygonal mirror.
However, if a laser beam is incident on the polygonal mirror as inclined in the auxiliary scanning direction, a bow occurs, that is, a scanning line, which is defined as a locus of a beam on a surface to be scanned, curves.
In addition, the degree of a curve of the scanning line varies according to an incident angle of the laser beam impinging on the polygonal mirror in the auxiliary scanning direction. Therefore, if the multi-beam scanning optical system is configured to form more than one scanning line at predetermined intervals on one photoconductive surface simultaneously, a distance between adjacent scanning lines varies according to positions in the main scanning direction. Such a change of the distance between adjacent scanning lines is called a differential bow.
Since both of the bow and the differential bow deteriorate an imaging accuracy, these should be suppressed particularly for a high-resolution scanning system.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved scanning optical system that is configured to compensate for the bow and the differential bow while satisfying essential characteristics for a scanning optical system such as a f&thgr; characteristic, correction of a curvature of field and the like.
For the object, according to the invention, there is provided a scanning optical system for emitting at least one beam scanning in a main scanning direction. The scanning optical system is provided with a light source that emits at least one beam, an anamorphic optical element that converges the at least one beam emitted by the light source in an auxiliary scanning direction, a polygonal mirror that rotates and deflects the at least one beam emerged from the anamorphic optical element to scan in the main scanning direction within a predetermined angular range, and an imaging optical system that converges the at least one beam deflected by the polygonal mirror to form at least one beam spot on a surface to be scanned, the at least one beam spot scanning in the main scanning direction on the surface to be scanned.
In the above configuration, the imaging optical system has a scanning lens, and at least one compensation lens provided on the surface side with respect to said scanning lens, the at least one compensation lens compensating for curvature of field. Further, one surface of the scanning lens has an anamorphic aspherical surface, said anamorphic aspherical surface being defined as a surface whose curvature in the auxiliary scanning direction at a point spaced from an optical axis thereof in the main scanning direction is determined independently from a cross-sectional shape thereof along the main scanning direction.
Further, one surface of the at least one compensation lens has an aspherical surface, the aspherical surface being defined as a surface in which a tilt angle of a cross-sectional shape in the auxiliary scanning direction changes with a position in the main scanning direction, the aspherical surface being asymmetrical with respect to a plane perpendicular to the auxiliary scanning direction and including a central point thereof.
With this configuration, it becomes possible to determine a power of the imaging optical system in the auxiliary scanning direction independently from a power in the main scanning direction. Accordingly, it becomes possible to compensate for a bow and a differential bow which are aberrations in the auxiliary scanning direction.
Since, with the above configuration, a curve of a scanning line (i.e., a bow) is well suppressed, it is not necessary to use a toric surface in the imaging optical system. Therefore, all surfaces except for the anamorphic aspherical surface of the scanning lens and the aspherical surface of the compensation lens can be formed as rotational symmetrical surfaces.
In a particular case, the anamorphic aspherical surface of the scanning lens may be configured such that a cross-sectional shape thereof in the auxiliary scanning direction is formed as an arc.
Optionally, the curvature of the anamorphic aspherical surface in the auxiliary scanning direction may decrease as a distance from the optical axis increases.
In a particular case, the aspherical surface of the at least one compensation lens may be defined by a two-dimensional polynomial expression in which a SAG amount between a point on the aspherical surface and a plane tangential to the aspherical surface at the central point is defined by coordinates along the main scanning direction and the auxiliary scanning direction.
Optionally, the tilt angle of the cross-sectional shape of the aspherical surface in the auxiliary scanning direction may increase as a distance from the central point of said aspherical surface increases. By using the aspherical surface defined by the two-dimensional polynomial expression, it becomes possible to prevent a widening of the beam spot due to a fluctuation of a wavefront.
In another case, the light source may emit a plurality of beams, the plurality of beams including first beams whose incident angles with respect to the polygonal mirror in the auxiliary scanning direction are different from each other. Further, the scanning lens may have a single lens through which the plurality of beams deflected by said polygonal mirror pass, and the at least one compensation lens may, include a plurality of compensation lenses through which the first beams pass, respectively.
Optionally, the plurality of beams may include second beams whose incident angles with respect to the polygonal mirror in the auxiliary scanning direction are substantially the same, and the second beams emerged from the scanning lens enter the same compensation lens.
Further optionally, the anamorphic aspherical surface of the scanning lens may be symmetrical with respect to a line intersecting the optical axis and parallel with the main scanning direction, and the aspherical surface of each of the plurality of compensation lenses may be symmetrical with respect to a line intersecting the central point thereof and parallel with the auxiliary scanning direction.
Further optionally, the first beams may include two beams, inci
Greenblum & Bernstein P.L.C.
PENTAX Corporation
Phan James
LandOfFree
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