Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-04-06
2003-04-29
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S207110, C359S217200
Reexamination Certificate
active
06556332
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus using the optical scanner. More particularly, the present invention relates to an optical scanner which is suitable for use in an apparatus, such as a laser beam printer or a digital copying machine, making use of an electrophotographic process, and which is used to record image information by deflecting (reflecting) light which has exited from a light source means using a polygon mirror serving as a light deflector, and by scanning a scan surface with the light through a scanning optical means (that is, an f&thgr; lens system). Even more particularly, the present invention relates to an optical scanner which makes it possible to always provide a proper image by correcting the curvature of field and the f&thgr; characteristic in a main scanning direction as a result of forming a plurality of f&thgr; lenses which make up the scanning optical means with proper shapes.
2. Description of the Related Art
Hitherto, in a light scanner of, for example, a laser beam printer, light from a light source means which has been modulated in accordance with an image signal is periodically deflected by a deflecting means, such as a rotating polygon mirror. Then, the deflected light is focused in the form of a spot on the surface of a photosensitive recording medium (that is, a photosensitive drum) by a scanning optical means having an f&thgr; characteristic in order to scan the surface of the recording medium with the light, whereby an image is recorded.
FIG. 22
is a schematic view of the main portion of a conventional light scanner. In
FIG. 22
, divergent light beams which have exited from a light source means
91
are converted into substantially parallel light beams by a collimator lens
92
. These substantially parallel light beams travel to a diaphragm
93
which limits the amount of light incident upon a cylindrical lens
94
having a refractive power only in a subscanning direction. Of the substantially parallel light beams incident upon the cylindrical lens
94
, those within a main scanning cross-sectional plane exit from the cylindrical lens
94
as substantially parallel light beams, while those within the subscanning cross-sectional plane are focused in order to form an image which is a substantially linear image on a deflecting surface
95
a
of a deflecting means
95
, which is a rotating polygon mirror.
The light beams which have been deflected (reflected) at the deflecting surface
95
a
of the deflecting means
95
are led to a photosensitive drum surface
98
serving as a scan surface through a scanning optical means
99
having an f&thgr; characteristic. By rotating the deflecting means
95
in the direction of arrow A, the photosensitive drum surface
98
is scanned with the light beams in order to record image information.
In order to record image information with high precision using this type of light scanner, it is important that the diameter of the spot be the same by properly correcting the curvature of field throughout the entire scan surface, and that the scanning speed on the scan surface be kept the same (f&thgr; characteristic). Conventionally, various light scanners and f&thgr; lens systems (that is, scanning optical means) which satisfy such optical characteristics have been proposed. Since laser beam printers, digital copying machines, and the like are becoming more compact and lower in cost, there has been a demand for making light scanners more compact and lower in cost.
A light scanner which is both more compact and lower in cost is disclosed in, for example, Japanese Patent Laid-Open No. 10-23246. According to this document, curvature of field and distortion are properly corrected, and the influences of, for example, changes in spot diameters due to image heights are made small.
In general, in order to make a light scanner more compact, it is necessary to decrease the focal length of the f&thgr; system, to increase the angle of view, and to bring f&thgr; lenses of the f&thgr; lens system closer to the polygon mirror serving as a deflecting means. All of these make it difficult to correct aberrations, resulting in the problem that the curvature of field and the f&thgr; characteristic in a wide angle-of-view area are not properly corrected when the light scanner is made more compact by the aforementioned ways.
When the angle of view is increased, another problem arises. In conventional light scanners, light which has exited from the light source means is incident upon a deflecting surface of the polygon mirror obliquely from the optical axis of the f&thgr; lens system. The location where the light is deflected (reflected) at the deflecting surface changes continuously and asymmetrically with respect to the center of scanning. This asymmetrical change in the location of deflection influences the location of image formation, making it difficult to obtain a flat curvature of field.
In the first place, the asymmetrical change in the location of reflection occurs because light from the light source means is incident upon the polygon mirror obliquely from the optical axis of the f&thgr; lens system. Therefore, the asymmetrical change can be eliminated by causing the light from the light source means to be incident upon the polygon mirror from the direction of the optical axis of the f&thgr; lens system. However, this is not possible because of the lens arrangement. Since the light must be made incident upon the polygon mirror from outside the f&thgr; lens system, it is impossible to eliminate the asymmetry of the curvature of field caused by asymmetrical changes in the location of reflection.
Examples that make the upper and lower portions of the generating-line shapes of the f&thgr; lenses asymmetrical are proposed in, for example, Japanese Patent Laid-Open Nos. 4-60608 and 9-265041.
In order to make the f&thgr; lens system more compact, it is necessary to properly correct the curvature of field and the f&thgr; characteristic even in an area having a wide angle of view of ±47 degrees. Therefore, past proposals have not necessarily made it possible to satisfactorily correct the curvature of field and the f&thgr; characteristic.
In order to make the light scanner capable of using multiple beams, it is necessary to convert the light beams which have exited from the collimator lens into substantially parallel light beams to decrease jitters in the main scanning direction.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an optical scanner which can properly correct curvature of field and distortion in a main scanning direction and curvature of field and changes in magnification in a subscanning direction by forming a plurality of f&thgr; lenses of a scanning optical means with proper shapes, and which is suitable for high-definition printing using a compact structure. It is also an object of the present invention to provide an image forming apparatus using the same.
To these ends, according to a first aspect of the present invention, there is provided a light scanner comprising light-incident optical means that guides light which has exited from light source means to deflecting means, and scanning optical means for forming an image on a scan surface using the light reflected and deflected by the deflecting means. In the light scanner, the scanning optical means comprises at least a first lens and a second lens. The first lens is a meniscus positive lens whose concave surface faces the deflecting means within a main scanning cross-sectional plane, and the second lens is a meniscus lens whose convex surface faces the deflecting means within the main scanning cross-sectional plane. The shape of a light-incident surface of the second lens within the main scanning cross-sectional plane is such that the curvature changes from convex to concave from the optical axis of the second lens to either end of an effective portion of the second lens. Portions of a surface of the second lens at eithe
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Robinson Mark A.
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