Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2001-09-05
2003-08-05
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S244000
Reexamination Certificate
active
06603500
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning optical apparatus and color image-forming apparatus using it and, particularly, is suitably applicable to apparatuses, for example, such as laser beam printers, digital copying machines, multi-function printers, and so on involving the electrophotographic process, which are constructed to deflect a beam emitted from a light source means, by a deflecting element consisting of a rotary polygon mirror and then guide the beam through a scanning lens system having the f-&thgr; characteristic, to optically scan an area on a surface to be scanned, thereby recording image information thereon.
2. Related Background Art
In the conventional scanning optical apparatus used in laser beam printers (LBPs), the digital copying machines, etc., the beam optically modulated according to an image signal and emitted from the light source means is periodically deflected by an optical deflector, for example, consisting of a rotary polygon mirror (polygon mirror), is focused in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by the scanning lens system having the f-&thgr; characteristic, and is moved to optically scan the area on the surface to record an image thereon.
FIG. 12
is a schematic view of major part of a conventional scanning optical apparatus, for example, proposed in U.S. Pat. No. 6,133,935 (correspondent of Japanese Patent Application Laid-Open No. 10-232347).
In the same figure a divergent beam emitted from light source means
91
is collimated into a nearly parallel beam by collimator lens
92
and the beam (light amount) is limited by stop
93
to enter a cylinder lens (cylindrical lens)
94
having a predetermined refracting power only in the sub scanning direction. In the main scanning section the nearly parallel beam entering the cylinder lens
94
emerges in the nearly parallel beam state as it is. In the sub scanning section the beam is converged to be focused as a nearly linear image on a deflective facet
95
a
of the optical deflector
95
consisting of the rotary polygon mirror (polygon mirror).
Then the beam deflected and reflected by the deflective facet
95
a
of the optical deflector
95
is guided through the scanning lens system
106
having the f-&thgr; characteristic, onto the photosensitive drum surface
98
as a surface to be scanned, and the optical deflector
95
is rotated in the direction of arrow A to optically scan the area on the photosensitive drum surface
98
in the direction of arrow B. This implements recording of an image on the photosensitive drum surface
98
being a recording medium.
In the above-stated scanning optical apparatus the scanning lens system
106
is composed of two toric lenses
96
,
97
and in the same example all the surfaces (four surfaces) of the two lenses
96
,
97
are toric surfaces, thereby correcting various aberrations well.
In general a lens having a large power in the sub scanning direction or an optical element disposed in the vicinity of the optical deflector demonstrates high sensitivity to optical decentration caused during production.
FIG. 13
is a chart showing movement of irradiated position on the surface to be scanned, for example, due to decentration (decentration amount of 0.05 mm) in the sub scanning direction of the optical surfaces of the first toric lens in Example 1 of U.S. Pat. No. 6,133,935. In the specification of the present application the “decentration” in the sub scanning direction means decentration in the direction normal to the surface to be scanned.
It is apparent from the same figure that the irradiated position moves largely due to the decentration in the sub scanning direction of the toric lens with the largest power in the sub scanning direction and it will pose a problem in recording of high-definition images. This decentration does not affect only the irradiated position but also affects imaging performance. In this optical system a large decentration amount will considerably degrade the spot shape on the surface to be scanned.
This problem may lead to degradation of imagery, because the movement of irradiated position due to decentration causes jitter, pitch unevenness, and color registration, particularly, in the multi-beam scanning optical apparatus for simultaneously scanning the surface with a plurality of beams from a multi-beam light source as a light source, the color image-forming apparatus for guiding beams emitted from a plurality of scanning optical devices, onto image carrier surfaces corresponding to respective colors, to record a color image, and so on.
Further, in the case of optical elements produced by plastic molding or glass molding, in addition to the total decentration of the optical surface, there is a possibility that partial (local) optical decentration can occur depending upon accuracy of mirror finish of a mold. For these reasons, the scanning optical systems with high sensitivity to decentration are not preferable in terms of achievement of higher image quality and improvement in productivity of the scanning optical apparatus. Therefore, there are desires for a scanning optical system permitting good aberration correction and reduced in the sensitivity to decentration.
In the scanning optical apparatus proposed in Japanese Patent Application Laid-Open No. 61-87123, the power in the sub scanning direction is concentrated in an optical element placed in the vicinity of the surface to be scanned, so as to decrease the power in the sub scanning direction of the scanning lens system, thereby reducing the sensitivity to decentration.
The scanning optical apparatus of this reducing system in the sub scanning direction, however, is not suitable for recording of high-definition imagery for the following reasons; a stop normally becomes oblate because of setting of its magnification, coupling efficiency of the collimator lens becomes lower, so as to decrease the light amount, the shape of the stop is close to a rectangle, so as to make the spot shape worse, and so on.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a scanning optical apparatus suitably applicable to formation of high-quality imagery with little pitch unevenness, little color registration, or the like by properly setting shapes, powers, etc. of plural optical elements constituting a third optical element for focusing a beam deflected by a deflecting element on a surface to be scanned and thereby reducing the problems of the movement of irradiated position and the degradation of the spot due to the deviation in the sub scanning direction of the third optical element at low cost and in a simple configuration, and also provide a color image-forming apparatus using the scanning optical apparatus.
In one aspect of the invention, there is provided a scanning optical apparatus comprising light source means, a deflecting element for deflecting and reflecting a beam emitted from the light source means, and a scanning optical element for focusing the beam deflected by the deflecting element, on a surface to be scanned,
wherein the scanning optical element comprises a plurality of optical elements,
wherein a shape in a main scanning direction of at least one surface out of optical surfaces of the plurality of optical elements is an aspheric shape,
wherein an optical element having a largest power in a sub scanning direction on the optical axis out of the plurality of optical elements is located on the deflecting element side with respect to a middle point in an optical-axis direction between the deflecting element and the surface to be scanned,
wherein the optical element having the largest power in the sub scanning direction has two optical surfaces,
wherein, where &phgr;S
1
represents a power in the sub scanning direction of the optical element having the largest power in the sub scanning direction and &phgr;S
1X
a power of an optical surface having a smaller power in the sub scanning direction out of the two optical surfaces of the optical element havi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pham Hai
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