Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2002-06-26
2004-01-27
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S217200, C347S256000
Reexamination Certificate
active
06683707
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus using the scanning optical apparatus. More particularly, the present invention relates to a scanning optical apparatus that is suitably used for an apparatus, such as a laser beam printer or a digital copying machine having an electrophotographic process, in which an optically modulated light flux emitted from a light source means is reflected and deflected by a polygon mirror functioning as an optical deflection means and then optically scans a surface to be scanned through an scanning optical system, thereby recording image information. In particular, the present invention relates to a scanning optical apparatus with which there is always obtained a favorable image where the sensitivity of scanning line bending to rotational decentration of a single lens constituting a scanning optical system is reduced. The present invention also relates to an image forming apparatus using the scanning optical apparatus.
Also, the present invention relates to a color image forming apparatus that uses a plurality of scanning optical apparatuses and is constructed from a plurality of image bearing members corresponding to respective colors.
2. Related Background Art
In a conventional scanning optical apparatus applied to a laser beam printer (LBP) or the like, a light flux optically modulated in accordance with an image signal and emitted from a light source means is periodically deflected by a light deflector composed of a rotary polygon mirror (polygon mirror) or the like. The deflected light flux is converged to form a spot on a surface of a photosensitive recording medium (photosensitive drum) by an imaging scanning optical system having an f&thgr; characteristic and optically scans the surface, thereby performing image recording.
FIG. 20
is a schematic diagram showing the main part of a conventional scanning optical apparatus.
In this drawing, a diverging light flux emitted from a light source means
171
is converted into a nearly parallel light flux by a collimator lens
172
, an aperture stop
173
limits the light flux, and the limited light flux strikes a cylindrical lens
174
having a predetermined refractive power only in the sub scanning direction. The nearly parallel light flux striking the cylindrical lens
174
is emitted as it is in a main scanning cross-section. Also, in a sub scanning cross-section, the light flux is converged and imaged as a nearly linear image on a deflecting surface (a reflecting surface)
175
a
of a light deflector (a deflection means)
175
composed of a polygon mirror.
Then, the light flux deflected by the deflecting surface
175
a
of the light deflector (the deflection means)
175
is guided by an imaging scanning optical system
76
having an f&thgr; characteristic onto a photosensitive drum surface
178
that is a surface to be scanned. By having the light deflector (the deflection means)
175
rotate in the direction of arrow A, the photosensitive drum surface
178
is optically scanned in the direction of arrow B, thereby performing the recording of image information.
In order to perform the recording of image information with high precision in a scanning optical apparatus like this, it is required that the following requirements are met. For instance, the curvature of field is favorably corrected across the entire of the surface to be scanned. Also, there exists a distortion characteristic (f&thgr; characteristic) having a uniform speed property between an angle of view &thgr; and an image height Y. Further, the spot diameter on an image surface remains uniform irrespective of differences in image height. There have conventionally been proposed various kinds of scanning optical apparatuses or imaging scanning optical systems that satisfy optical characteristics like these.
On the other hand, as the sizes and prices of apparatuses, such as laser beam printers and digital copying machines, are reduced, the same demand is made to scanning optical apparatuses.
As a construction satisfying the demand like this, JP 04-50908 A and JP 09-33850 A, for instance, propose a scanning optical apparatus whose imaging scanning optical system is constructed from a single f&thgr; lens.
In JP 04-50908 A, an aberration characteristic is relatively favorably corrected by using an aspheric surface of a high order in the main scanning direction of an f&thgr; lens. However, the magnification between a deflection means and a surface to be scanned in the sub scanning direction does not remain constant, so that there is a tendency for a spot diameter in the sub scanning direction to change in accordance with the differences in image height.
On the other hand, in JP 09-33850 A, there is proposed a method described below. In a scanning optical apparatus, on at least two surfaces of lens surfaces of an f&thgr; lens, the curvature in the sub scanning direction continuously changes along the main scanning direction within an effective region of the f&thgr; lens as well as independently of the curvature in the main scanning direction. With this method, the position of the principal plane in the sub scanning direction is controlled by the bending of two surfaces. In this manner, the sub scanning magnification is kept constant irrespective of the differences in image height, so that there is obtained a constant spot diameter.
With the proposed method described above, in order to obtain a constant sub scanning magnification, at least two surfaces are bent and the position of the principal plane is controlled so that the magnification is kept constant. Consequently, it becomes possible to set shapes in the main scanning direction and the sub scanning direction completely independently of each other. However, because of various demands such as a demand to reduce the thickness of a lens, the lens shape in the main scanning direction has a relatively large amount of an aspheric surface in many cases.
In a lens like this whose amount of an aspheric surface in the main scanning direction is large, optical performance is significantly degraded due to errors caused during the arrangement of each lens surface and the lens. Among the degradations of optical performance, as distinct from the aberration of the height of a scanning line, the inclination of the scanning line, and the like, the bending of the scanning line in the sub scanning direction in particular causes a significant problem because it is impossible to correct this bending by the adjustment of a mirror or the like arranged in an apparatus main body. Consequently, in order to suppress the scanning line bending to a minute amount, it is required to precisely arrange each lens surface and the lens in accordance with design values or to adjust the positions of each lens surface and the lens so as to coincide with design positions by providing the lens with an adjustment mechanism.
Further, in the case of a color image forming apparatus in which scanning optical apparatuses are arranged using four photosensitive members (photosensitive drums), latent images are formed by laser light, and images on an original in respective colors of Y (yellow), M (magenta), C (cyan), and Bk (black) are formed on the surfaces of their corresponding photosensitive members, images in four colors of Y, M, C, and Bk formed on the surfaces of respective photosensitive members are superimposed on each other on a transfer member such as paper. Consequently, if the scanning lines of the scanning optical apparatuses corresponding to respective photosensitive members are bent, there occur differences in shape between scanning lines for the four colors, which causes a problem that color drift occurs in an image on the transfer member and therefore image performance is significantly degraded.
SUMMARY OF THE INVENTION
The present invention has been made to solve these problems and an object of the present invention is to provide a scanning optical apparatus that is capable of suppressing scanning line bending ca
Ishihara Keiichiro
Kato Manabu
Sato Hiroshi
Shimomura Hidekazu
Canon Kabushiki Kaisha
Robinson Mark A.
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