Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-06-22
2001-11-13
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S206100, C359S216100, C347S232000, C347S233000, C347S235000, C347S243000, C347S244000
Reexamination Certificate
active
06317245
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a multi-beam scanning optical system in which a plurality of light beams emitted by a plurality of light source are deflected to scan on a surface to be scanned such as a circumferential surface of a photoconductive drum.
A scanning optical system to be employed in a laser printer for forming a monochrome (e.g., a black-and-white) image is provided with a laser diode, which is driven in accordance with image data. The laser beam emitted by the laser diode is collimated by a collimating lens, and is directed to a deflecting member such as a polygonal mirror. The laser beam, which is incident on light reflecting surfaces of the rotating polygonal mirror, is deflected to scan within a predetermined angular range. The scanning laser beam is incident on an f&thgr; lens, refracted and converged thereby, and then is incident on an evenly charged photoconductive surface of a photoconductive drum to form a beam spot which moves along the rotational axis of the photoconductive drum (i.e., along a main scanning direction). Since the laser diode is driven in accordance with the image data, the surface of the photoconductive drum is exposed to light corresponding to the image data. While the light beam scans on the surface of the photoconductive drum, it is rotated (i.e., an auxiliary scanning is performed). Thus, a two-dimensional latent image is formed on the photoconductive surface of the photoconductive drum.
Then, toner is applied to the latent image to form a developed image, which is transferred onto a recording sheet and fixed thereon.
Recently, color laser beam printers, which is capable of forming a color image, has been developed. In the color laser printer, generally, a plurality of laser diodes are provided (which will be referred to as a multi-beam laser printer). Further, the corresponding number of f&thgr; lenses, and the corresponding number of photoconductive drums are provided for forming images of respective color components (e.g., yellow, magenta, cyan and black components). The above-described, exposing and developing processes are performed for each color component, and thus formed color image (developed image) consisting of the four color components is transferred on a recording sheet and fixed.
In the multi-beam laser printer as described above, since a plurality of f&thgr; lenses are employed for respective color components, the problems indicated below occur.
Firstly, if temperature differences are caused among the f&thgr; lenses for respective color components, the optical characteristics of the f&thgr; lenses may vary among the f&thgr; lenses. In such a case, positional errors (i.e., displacements) occur among positions on the photoconductive drums, on which the beams emitted from the f&thgr; lenses are incident, in the scanning direction, and therefore the formed color image may contain a color shift.
Secondly, since a plurality of f&thgr; lenses are provided for the respective color components, a structure of the optical system tends to be complicated.
SUMMARY OF THE INVENTION
In view of the above problems, it is an object of the present invention to provide an improved multi-beam scanning optical system which has a simplified structure of the optical system and can prevent the color shift.
For the above object, according to the present invention, there is provided a multi-beam scanning optical system, which is provided with a light source unit emitting a plurality of beams, the plurality of beams being parallel to each other, a polygonal mirror having a plurality of reflection surfaces arranged along a rotational direction of the polygonal mirror, the plurality of beams emitted by the light source unit being incident on the polygonal mirror and reflected by the reflection surfaces of the polygonal mirror, and an f&thgr; lens system including a plurality of lenses, the plurality of beams reflected by the reflection surface of the polygonal mirror passing through the f&thgr; lens system and proceeding toward surfaces to be scanned. The plurality of beams reflected by the reflection surfaces of the polygonal mirror pass through at least one of the plurality of lenses, which is a single lens element formed of single material.
Since all the beams pass through at least one common lens, the structure of the scanning optical system can be simplified. Further, even if the optical characteristics of the lens, through which all the beams pass, are changed, the change affects by the same amount for all the beams, and therefore, the color shift may be well suppressed.
Preferably, the at-least-one of the plurality of lenses mainly contributes to convergence of the plurality of beams in a main scanning direction. In this case, the color shift can be suppressed effectively.
Optionally, the f&thgr; lens system includes a first lens, a second lens and a third lens, the plurality of beams passing through the first, second and third lenses in this order, and the at-least-one of the plurality of lenses may include the second lens.
Optionally, the second lens converges beams passed therethrough in the main scanning direction.
Further optionally, the first lens converges beams passed therethrough in the auxiliary scanning direction.
Still optionally, the third lens is provided for each of the plurality of beams, the third lens mainly converges a beam passed therethrough in the auxiliary scanning direction.
Furthermore, a plurality of third lenses are provided at positions facing a plurality of surfaces to be scanned, respectively.
Still optionally, each of the reflection surfaces of the polygonal mirror consists of a single reflection surface that reflects all of the plurality of beams.
In this case, the multi-beam scanning optical system may further include a horizontal synchronizing signal generating system provided with a light receiving element. In this case, the light source unit may include a plurality of light sources respectively emitting the plurality of beams, and one of the plurality of beams deflected by the polygonal mirror is directed to the light receiving element. The plurality of light sources being driven in accordance with an output of the horizontal synchronizing signal generating system.
According to this configuration, all the beams are controlled in accordance with the horizontal synchronizing signal generated based on a single beam, optical and electrical structure of the scanning optical system can be simplified.
Optionally, the light source emits four light beams for forming four different color images. In this case, the four different colors may include, for example, yellow, magenta, cyan and black. Of course, the invention is not limited to the four color system. Any number of beams and corresponding number of colors may be employed.
Still optionally, the surfaces to be scanned include surfaces of a plurality of photoconductive drums for forming different color images, rotational axes of the plurality of photoconductive drums extending in the main scanning direction, the rotational axes of the plurality of photoconductive drums being parallel and apart from each other in the auxiliary scanning direction that is perpendicular to the main scanning direction.
According to another aspect of the invention, there is provided a multi-beam laser printer for forming an image consisting of a plurality of color components in accordance with an electrophotographic imaging process. The printer is provided with a scanning optical system, which includes a light source unit emitting a plurality of beams, the plurality of beams being parallel to each other, a polygonal mirror having a plurality of reflection surfaces arranged along a rotational direction of the polygonal mirror, the plurality of beams emitted by the light source unit being incident on the polygonal mirror and reflected by the reflection surfaces of the polygonal mirror, and an f&thgr; lens system including a plurality of lenses, the plurality of beams reflected by the reflection surface of the polygonal mirror passing through the f&thgr; lens
Hama Yoshihiro
Mikajiri Susumu
Suzuki Yasushi
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Phan James
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