Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-08-31
2001-03-06
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S207110, C347S256000
Reexamination Certificate
active
06198563
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optical scanning apparatuses and more particularly, to an optical scanning apparatus for converging a light beam carrying image information on a scanned medium, using an image-forming mirror. The optical scanning apparatus of the present invention is suitably used in a write optical system of laser printers, image forming apparatuses, measuring equipment and testing devices.
2. Description of the Related Art
Various optical apparatuses have been proposed for converging a light beam deflected by a deflector on a scanned medium, using an image forming mirror. Many of these optical scanning apparatuses are provided with facet-inclination-correction function for correcting shifts of points of beam convergence occurring as a result of an inclination of a facet of the deflector. For example, Japanese Laid-Open Patent Application No. 1-200221 provides for facet inclination correction by providing a facet-inclination-correction toroidal lens in an optical system. Japanese Laid-Open Patent Application No. 6-123844 provides for facet inclination correction by providing a facet-inclination-correction elongated cylindrical element in an optical system. These approaches are designed to eliminate the scan line shift due to the inclination of the polygon facet of the deflector by placing the polygon facet and the scanned surface in a conjugated arrangement.
In an alternative approach, Japanese Laid-Open Patent Application No. 4-194814 discloses an image-forming mirror provided with a facet-inclination-correction function.
However, providing a facet-inclination-correction elongated cylindrical element (No. 6-123844) or a facet-inclination-correction (elongated) toroidal lens (No. 1-200221) in an optical system increases the cost of the optical scanning apparatus and decreases the flexibility in designing the apparatus.
According to the approach disclosed in Japanese Laid-Open Patent Application No. 4-194814, it is difficult to provide a satisfactory separation of optical paths. If the separation of optical paths is small, the flexibility in designing the apparatus suffers greatly.
In order to ensure a large angle of separation, a half mirror must be used. However, use of a half mirror results in a relatively high cost and a relatively large space required for the apparatus. Another disadvantage in using a half mirror is that a beam intensity drops to approximately ¼ after passing through the half mirror.
Providing a large separation of optical paths without using a half mirror, however, results in a relatively large scan line bow, necessitating a reduction in the angle of field, as is well known in the art. If the angle of field is reduced, the beam has to travel a relatively long distance in order to scan an area of a desired size. In order to ensure a satisfactorily large angle of field, the image-forming mirror must be machined so that it is deflected in a sub-scanning direction.
As described above, the optical scanning apparatuses according to the related art have various inherent problems in terms of their cost and production.
Japanese Laid-Open Patent Application No. 64-78214 provides alternative optical scanning wherein a light beam is reflected multiple times in a reflective optical system comprising a spherical concave mirror and a reflecting mirror arranged opposite to each other. However, the optical scanning system disclosed in Japanese Laid-Open Patent Application No. 64-78214 does not provide for facet inclination correction for correcting a scan line shift due to an inclination of a facet of a deflector such as a polygon mirror. Accordingly, a variation in scanning pitches occurring as a result of an inclination of a deflector cannot be prevented.
SUMMARY OF THE INVENTION
Accordingly, a general object of the present invention is to provide an optical scanning apparatus in which the aforementioned problems are eliminated.
Another and more specific object of the present invention is to provide an optical scanning apparatus capable of separating optical paths with an advantageously small scan line bow without using a half mirror. The present invention achieves this by causing a light beam to be reflected multiple times by an image-forming mirror provided with a facet inclination correction function.
Still another object of the present invention is to provide an optical scanning apparatus capable of reducing the length of an optical path of a synchronization light beam for obtaining a synchronizing signal, so that the size of the optical scanning apparatus is reduced.
The aforementioned objects can be attained by an optical scanning apparatus comprising: a light source unit for emitting a light beam for optical scanning; an image-forming lens for producing a line image, elongated in a main-scanning direction, of the light beam from the light source unit; a beam deflector for causing the line image to be reflected by its deflecting and reflecting surface so as to produce a deflected light beam deflected at a constant angular velocity; a reflective optical system including a plurality of reflecting mirrors for reflecting the deflected light beam incident on the reflective optical system a plurality of times; wherein reflecting surfaces of the plurality of reflecting mirrors are tilted with respect to a system axis of the optical scanning apparatus; the reflective optical system includes an image-forming mirror for converging the deflected light beam to form a beam spot for scanning a scanned surface at a constant velocity; and the image-forming mirror has an anamorphic configuration obtained by rotating a curve drawn with a first radius on a main-scanning plane, around an axis residing on the main-scanning plane and parallel with the main-scanning direction, maintaining a second radius.
According to the optical scanning apparatus of the present invention, since the light beam is reflected by an anamorphic image-forming mirror multiple times, it is possible to provide a facet inclination correction and a satisfactory separation of optical paths without using a half mirror, while maintaining a small scan line bow at the same time. Since the apparent optical path length can also be reduced, the size of the optical scanning apparatus is reduced.
The image-forming mirror may have an aspherical configuration obtained by rotating an aspherical curve drawn on the main-scanning plane, around an axis residing on the main-scanning plane and parallel with the main-scanning direction, maintaining the second radius.
According to this aspect of the present invention, the curvature of field characteristic and the constant velocity characteristic (f&THgr; characteristic) are improved.
The reflective optical system may be constructed such that a plurality of reflecting mirrors are formed to be integral with each other and a transparent window for causing an exit ray to pass through is provided as part of the reflective optical system.
According to this aspect of the present invention, the cost of producing the optical scanning apparatus is reduced.
The plurality of reflecting mirrors formed to be integral with each other may be provided on a transparent window of the optical scanning apparatus by vapor deposition.
According to this aspect of the present invention, the transparent window may serve as a dust guard and the reflecting mirrors simultaneously, thus reducing the number of components constituting the optical scanning apparatus, and the cost of producing the same.
The deflected light beam entering the reflective optical system may first be incident on the image-forming mirror.
According to this aspect of the present invention, a large separation of optical paths is ensured and the flexibility in designing the optical scanning apparatus is improved.
The reflecting mirrors in the reflective optical system may be provided on a transparent window of the optical scanning apparatus by vapor deposition.
According to this aspect of the present invention, the transparent window may serve as a dust g
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
Schuberg Darren
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