Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-05-22
2003-07-08
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S204200, C359S207110, C359S212100, C359S216100, C347S244000, C347S259000, C347S261000
Reexamination Certificate
active
06590688
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical scanning apparatus and an image forming apparatus using the same, and particularly is suitable for an apparatus such as a laser beam printer or a digital copier having, for example, the electrophotographic process adapted to reflect and deflect one or more beams emitted from light source means by a polygon mirror as a light deflector, and optically scan a surface to be scanned through an f&thgr; lens system having the f&thgr; characteristic to thereby record image information.
Particularly, the present invention relates to an optical scanning apparatus in which the shapes of a plurality of lenses constituting scanning optical means are appropriately set to thereby correct the curvature of image field in the main scanning direction and the sub scanning direction, the f&thgr; characteristic and the fluctuation of the magnification in the sub scanning direction so that good images are always obtained, and an image forming apparatus using the same.
2. Related Background Art
Heretofore, in an optical scanning apparatus such as a laser beam printer, a beam light-modulated and emitted from light source means in conformity with an image signal is periodically deflected by a light deflector comprising, for example, a rotatable polygon mirror, and is converged into a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by an f&thgr; lens system having the f&thgr; characteristic, and optically scans the surface of the recording medium to thereby effect image recording.
FIG. 18
of the accompanying drawings is a schematic view of the essential portions of an optical scanning optical system according to the prior art. In this figure, a divergent beam emitted from light source means
91
is made into a substantially parallel beam by a collimator lens
92
, and this beam (the quantity of light) is shaped by an aperture stop
93
and enters a cylindrical lens
94
having refractive power only in the sub scanning direction. Of the beam having entered the cylindrical lens
94
, that part in the main scanning section intactly emerges and that part in the sub scanning section converges and is imaged as a substantially linear image near the deflecting surface
95
a
of a light deflector
95
comprising a rotatable polygon mirror.
The beam reflected and deflected by the deflecting surface
95
a
of the light deflector
95
is directed onto the surface of a photosensitive drum as a surface
97
to be scanned through an f&thgr; lens system
96
having the f&thgr; characteristic, and the light deflector
95
is rotated in the direction of arrow A to thereby optically scan the surface
97
of the photosensitive drum in the direction of arrow B (the main scanning direction) and effect the recording of image information.
To effect highly accurate recording of image information in an optical scanning apparatus of this kind, it is important that curvature of image field is well corrected over the entire surface to be scanned and the spot diameter is uniform, that an equal speed is kept when the surface of the photosensitive drum is light-scanned (f&thgr; characteristic), that the lateral magnification in the sub scanning direction is uniformly corrected over the entire effective scanning area and the spot diameter in the sub scanning direction is uniform, and that in a multibeam scanning apparatus using light source means emitting a plurality of beams, the lateral magnification in the sub scanning direction is uniformly corrected over the entire effective scanning area and the pitch interval between scanning lines is made constant. Various optical scanning apparatuses or f&thgr; lens systems satisfying such optical characteristics have heretofore been proposed.
For example, Japanese Patent Application Laid-Open No. 7-318796 discloses an f&thgr; lens system comprising a combination of a glass toric lens and a plastic toric lens each having a cylindrical lens surface on the incidence surface side thereof and a toric surface on the emergence surface side thereof. In this publication, however, one surface is a cylindrical surface and therefore, there has been the problem that the degree of freedom is small with regard to the above-mentioned aberration correction and the above-mentioned aberration correction is difficult. So, in the present invention, as will be described later, all f&thgr; lenses constituting an f&thgr; lens system are made into toric lenses having toric surfaces on both surfaces thereof. Further, each of the aforementioned f&thgr; lenses has its both surfaces made into a non-arcuate main scanning sectional shape and has its radius of curvature in the sub scanning direction continuously varied, whereby the above-mentioned aberrations are corrected well. Also, the above-mentioned publication does not bear the description of sub scanning magnification, and has not taken it into consideration to reduce the degree of sensitivity of focus fluctuation in the sub scanning direction and to uniformly correct sub scanning magnification in an effective scanning area on a surface to be scanned. The present invention takes these into consideration and can construct an optical scanning apparatus suited for the highly accurate recording of image information.
Also, in Embodiment 1 of Japanese Patent Application Laid-Open No. 7-318796, the power of a glass toric lens
22
on the scanned surface
14
side in the main scanning direction is greater than the power of a plastic toric lens
21
on the polygon mirror
12
side in the main scanning direction and therefore, a problem is left in achieving compactness. In Embodiment 2 of Japanese Patent Application Laid-Open No. 7-318796, both of the power of the plastic toric lens
21
in the sub scanning direction and the power of the glass toric lens
22
in the sub scanning direction are positive and therefore, there is left the problem that when the two lenses
21
and
22
are brought close to the polygon mirror
12
side, sub scanning magnification increases.
On the other hand, with the compactness and lower cost of laser beam printers, digital copiers, etc., similar conditions have also been required for image forming apparatuses.
What makes these requirements compatible is proposed, for example, in Japanese Patent Application Laid-Open No. 10-232346. In this publication, curvature of image field and distortion are corrected well and the influence of a change or the like in the spot diameter by image height is reduced.
However, to achieve further compactness of the optical scanning apparatus, it is necessary to shorten the focal length of the f&thgr; lens system, widen the angle of field thereof and bring the f&thgr; lens close to the polygon mirror which is deflecting means. All these are factors which make aberration correction difficult, and there has been the problem that when compactness has been made, the curvature of image field in a wide field angle area, the f&thgr; characteristic, and the fluctuation of the magnification in the sub scanning direction are not corrected well.
Also, another problem arises with the widening of the angle of field. Heretofore, a beam emitted from light source means has been incident on the deflecting surface of the polygon mirror obliquely with respect to the optical axis of the f&thgr; lens system, and at this time, the reflecting position at which the beam is reflected by the deflecting surface changes continuously and asymmetrically with respect to the center of scanning. This asymmetrical change in the reflecting position affects particularly the imaging position and it becomes difficult to obtain flat curvature of image field.
The above-mentioned asymmetrical change in the reflecting position is caused by making the beam from the light source means incident obliquely with respect to the optical axis of the f&thgr; lens system and therefore, it can be eliminated by making the beam from the light source means incident from the direction of the optical axis of the f&thgr; lens system, but the dispositi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Phan James
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