Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-11-10
2001-08-21
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S201500
Reexamination Certificate
active
06278108
ABSTRACT:
RELATED APPLICATIONS
This application claims the priority of Japanese Patent Application No. 10-332737 filed on Nov. 24, 1998, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a synchronous light detector for attaining a scanning timing of a light beam in an optical scanning apparatus equipped with light-deflecting means constituted by a rotary polygon mirror or the like.
2. Description of the Prior Art
Optical scanning apparatus, which are apparatus for optically scanning a surface to be scanned by deflecting a light beam, have been known as apparatus for writing optical information onto the surface and reading out information therefrom.
Conventionally known as such an optical scanning apparatus is one employing a system in which light-deflecting means deflects a light beam, thus deflected light beam is focused like a spot by an imaging lens onto a surface to be scanned, and the surface is optically scanned therewith. Such an optical scanning apparatus uses a rotary polygon mirror, for example, as the light-deflecting means.
The rotary polygon mirror is made of a right-regular prism whose side faces form a mirror surface. When the rotary polygon mirror is rotated while a light beam is incident on the mirror surface along a fixed direction, the light beam reflected by the mirror surface is periodically deflected. Thus deflected light beam is focused like a spot by the imaging lens onto the surface to be scanned. As the light beam is deflected, the spot of the light beam moves on the surface to be scanned, thereby optically scanning the latter.
Meanwhile, the above-mentioned conventional optical scanning apparatus is equipped with a synchronous light detector for attaining a timing for optical scanning. This synchronous light detector is configured such that the light beam deflected by the rotary polygon mirror toward its optical scanning area by way of the imaging lens is detected as synchronous light by a light-receiving device, so as to attain synchronization of a scanning start timing of the light beam.
Also, a cylindrical lens having a refracting power only in the direction orthogonal to the scanning plane of the light beam is disposed between the imaging lens and the light-receiving device, while the deflecting point of the light beam caused by the rotary polygon mirror and the imaging point on the light-receiving surface of the light-receiving device are made optically conjugate with each other within a plane orthogonal to the scanning direction of the light beam upon attaining synchronization of the scanning start timing of the light beam (as disclosed in Japanese Patent Publication No. 7-43462, for example).
However, when a point on the light-receiving surface of the light-receiving device is disposed at the position optically conjugate with the deflecting point of the light beam caused by the rotary polygon mirror, while the light beam forms an image on the light-receiving surface of the light-receiving device; if there is a flaw on the light-receiving surface of the light-receiving device or dirt is attached thereto, and the image is formed at the location where this flaw or dirt exists, then the light beam is scattered on the light-receiving surface, whereby synchronization signals may not be detected correctly. As a consequence, there has been a possibility that it becomes hard to attain synchronization of the scanning start timing of the light beam.
SUMMARY OF THE INVENTION
In view of the circumstances mentioned above, it is an object of the present invention to provide a synchronous light detector for an optical scanning apparatus, which can correctly detect a synchronization signal for optical scanning, even when there is a flaw on the light-receiving surface of the light-receiving device or dirt is attached thereto, by enhancing its light spot area on the light-receiving device.
For achieving the above-mentioned object, the synchronous light detector in accordance with the present invention is a synchronous light detector for an optical scanning apparatus in which a light beam is deflected by light-deflecting means, thus deflected light beam is focused like a spot by an imaging lens onto a surface to be scanned, and the surface is scanned with the spot; the synchronous light detector attaining an optical scanning start timing by detecting as synchronous light the light beam deflected by the light-deflecting means toward an optical scanning area;
the synchronous light detector comprising:
a light-receiving device for receiving the light beam as the synchronous light by way of the imaging lens; and
a cylindrical lens disposed between the imaging lens and the light-receiving device;
the cylindrical lens being disposed so as to have a generator direction thereof parallel to a scanning direction of the light beam;
a light-receiving surface of the light-receiving device being disposed at a position shifted from a paraxial image point by a predetermined distance in an optical axis direction of the light-receiving device within a plane orthogonal to the scanning direction of the light beam when a deflecting point of the light beam caused by the light-deflecting means is taken as an object point.
In the synchronous light detector having such a configuration, the light beam is converged at least by the cylindrical lens along a direction orthogonal to its scanning plane. Since the light-receiving surface of the light-receiving device is disposed as being shifted from the paraxial image point of the light beam by the predetermined distance in the optical axis direction of the light-receiving device within the plane orthogonal to the scanning direction, a linear image elongated along the direction orthogonal to the scanning direction of the light beam can be obtained on the light-receiving surface. As a consequence, even when the light-receiving device has a flaw or dirt is attached thereto, the light beam can be detected at a location free of the flaw and dirt, whereby the synchronous signal can be detected correctly.
Preferably, the light-receiving surface of the light-receiving device is disposed so as to satisfy the following conditional expression:
0.1ƒ<|&Dgr;|<0.3ƒ
where
&Dgr; is the distance between the paraxial image point and the light-receiving surface of the light-receiving device; and
f is the focal length of the cylindrical lens.
If the value of the distance &Dgr; exceeds the upper limit of the above-mentioned conditional expression, then the length of the linear image on the light-receiving surface becomes equivalent to or greater than the diameter of the light-receiving surface, whereby the accuracy in detection of the synchronous signal may lower. If the value of the distance &Dgr; is less than the lower limit of the above-mentioned conditional expression, on the other hand, then the image of the light beam on the light-receiving surface has a size nearly identical to that of the paraxial image point. In this case, if there is a flaw on the light-receiving surface or dirt is attached thereto, and the image is positioned at the location where the flaw or dirt exists, the light beam may be scattered on the light-receiving surface, so that the synchronous signal may not be detected correctly.
Preferably, the imaging lens is an f&thgr; lens.
Preferably, a return lens for returning the light beam from the imaging lens so as to direct the light beam to the light-receiving device is disposed between the imaging lens and the surface to be scanned.
Preferably, the imaging lens has a refracting power in a main scanning direction of the light beam alone.
The synchronous light detector is used in a laser printer, a scanner, or a copier, for example.
REFERENCES:
patent: 5877883 (1999-03-01), Inoue et al.
patent: 61172113 A (1986-08-01), None
Fuji Photo Optical Co., Ltd.
Le Que T.
Snider Ronald R.
Snider & Associates
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