Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-06-25
2001-07-03
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S206100, C359S216100
Reexamination Certificate
active
06256133
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanning apparatus.
2. Discussion of Related Art
A currently used optical scanning apparatus is arranged such that a light beam from a light source is caused to be deflected at a constant velocity, via a light deflecting system having a deflective reflecting surface. The deflected light beam, effected by the scanning image formation lens, is converged into a beam spot on a surface to be scanned, thus causing a beam spot to scan the surface to be scanned at a constant speed. In general, such an optical scanning apparatus is commonly used in an information writing device of a laser printer, a facsimile machine, or a digital copy machine.
As a light deflecting system for deflecting a light beam introduced into a scanning image formation lens, a polygon mirror has been commonly used. Because the rotating center axis of the polygon mirror is generally located at a position that is not coincident with the optical axis of the scanning image formation lens, a rotation of the polygon mirror will cause a change in a relationship between a deflective reflecting point and a scanning lens, producing a phenomenon called “sag”.
When such a sag condition occurs, it is very likely that deterioration of the written image will occur, particularly in an image curvature in a sub-scanning direction (hereinafter referred to as “sub-scanning image curvature”).
More specifically, in an optical scanning apparatus using a polygon mirror, in order to correct a surface tilt of the polygon mirror, an incident light beam introduced into a scanning image formation lens is not a parallel light beam with respect to the sub-scanning direction. With a change of the deflective reflecting point, an image formation position in the sub-scanning direction will be changed, resulting in deterioration of the sub-scanning image curvature.
A deflected light beam introduced into the scanning image formation lens, with respect to the main scanning direction, is allowed to be a parallel light beam or is allowed to be a light beam having a certain divergence or a certain convergence, thereby providing some technical merits when being used in various conditions. On the other hand, if a deflected light beam to be introduced into the scanning image formation lens is a light beam having a certain convergence or a certain divergence in the main scanning direction, an influence possibly brought about by the sag is likely to cause a deterioration in the image curvature in the main scanning direction (hereinafter referred to as “main scanning image curvature”) and in a constant scanning speed property, as in the sub-scanning direction.
As a method for reducing an influence possibly caused due to the sag, it has been known that a scanning image formation lens may be shifted or tilted within a main scanning plane (ideally, in a plane where the main light ray of a deflected light beam is just sweeping). However, with this method, both the main scanning image curvature and the sub-scanning image curvature are difficult to be completely corrected at the same time. Therefore, this method is not sufficient for controlling a diameter change of a beam spot when such a beam spot diameter has already been made small in order to obtain a high writing density for information recording. As a result, the above method cannot satisfy the demand for recording information with a high writing density.
Further, as a method for correcting an influence which will possibly be caused by the sag, there has been suggested another method in which a sub-scanning radius of curvature of a scanning image formation lens is non-symmetrical with respect to an optical axis (Japanese Unexamined Patent Publication No. 2-23313; Japanese Examined Patent Publication No. 7-69521; Japanese Unexamined Patent Publication No. 7-113950; Japanese Unexamined Patent Publication No. 8-122635; Japanese Unexamined Patent Publication No. 8-297256).
With the use of the method as suggested in the above-noted Japanese Patent Publications, because an image formation position of a light beam is coincident with a scanned surface with respect to each image height, in principle, it has become possible to completely correct a sub-scanning image curvature irrespective of the existence of the sag.
Recently, with a remarkable development in obtaining a high density for information writing, a demand for making a diameter of a light beam spot smaller and improving stability in obtaining a light beam spot having a smaller diameter has increased. In order to satisfy such a demand, it has become important that not only the image curvature but also the optical magnification of a scanning optical system be uniformly set irrespective of what an image height is. If a beam spot has a change in its optical magnification according to an image height, the waist diameter of the beam spot will also change substantially in proportion to a lateral magnification. As a result, it is impossible to obtain “a stabilized beam spot” capable of avoiding a change in the diameter of a beam spot, which change will otherwise be caused according to an image height.
In any of the inventions described in the above-noted Japanese patent publications, a sub-scanning radius of curvature has a monotone change (having no inflection point). As a result, it is difficult to ensure a uniformity for an optical magnification while also correcting an image curvature. Further, if an optical system is constructed only of a lens surface which has a sub-scanning radius of curvature having a monotone change, the scanning image formation lens will require more than two lens elements. In addition, a change in the radius of curvature will become large between the vicinity of an optical axis and the peripheral areas, resulting in a problem that a lens will have a large difference in its thickness between the vicinity of the optical axis and the peripheral areas, and such a lens is difficult to make of a plastic material in an extrusion molding process.
In addition, another scanning image formation lens has a cross section in a sub-scanning direction in the vicinity of an optical axis having a biconvex shape and the sub-scanning radius of curvature has a plurality of extreme values (Japanese Unexamined Patent Publication No. 10-148755).
However, because such kind of a scanning image formation lens has a biconvex shape, it is impossible to freely set a principal point position for each image height, hence rendering it difficult to maintain a constant optical lateral magnification for each image height, resulting in a problem that the diameter of a beam spot changes.
SUMMARY OF THE INVENTION
In order to overcome the above-described and other problems, preferred embodiments of the present invention provide a scanning image formation lens constructed to correct an image curvature regardless of whether a sag condition occurs and also making it possible to form a beam spot having a uniform beam spot diameter regardless of image height.
The preferred embodiments of the present invention further provide an optical scanning apparatus capable of writing information with a beam spot having a uniform beam spot diameter regardless of image height so as to effectively perform an information writing (information recording) operation with a high recording density.
A scanning image formation lens according to preferred embodiments of the present invention includes a scanning image formation lens for use in an optical scanning apparatus in which a light beam from a light source is formed into an elongated linear image arranged in the main scanning direction and is caused to deflect at a constant velocity via a light deflecting system having a deflective reflecting surface which is located in the vicinity of an image formation position of the linear image. The deflected light beam is transmitted through the scanning image formation lens so as to be converged into a beam spot on a surface to be scanned, thereby causing the beam spot to scan the surface to be scanned at a co
Aoki Magane
Sakai Kohji
Suzuki Seizo
Phan James
Ricoh Co. Ltd.
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