Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-04-19
2001-07-24
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S208100, C359S212100, C359S216100, C359S900000, C347S243000
Reexamination Certificate
active
06266174
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to multi-beam scanning apparatuses and, more particularly, to a multi-beam scanning apparatus in which high-quality image is assured by controlling the scan line bow.
2. Description of the Related Art
Conventionally, a single-beam optical scanning apparatus for writing an image by optically scanning a scanned surface with a single deflected beam is widely known as an image forming apparatus for optical printers and the like. Recently, multi-beam optical scanning apparatuses for simultaneously scanning a scanned surface with a plurality of beams for the purpose of improving image writing speed are being developed.
In the single-beam scanning method, a beam emitted by a light source is guided toward a beam deflector such that a chief ray coincides with an optical axis of the optical system. The chief ray of the beam is deflected by the beam deflector on a plane that includes an optical axis of a scanning image-forming optic such as an f&thgr; lens. In the single-beam scanning apparatus, a scan line (a track of a beam spot is hardly curved. That is, no scan line bow is created.
In the multi-beam scanning method, however, an optical path of some of a plurality of beams traveling from respective light sources to the scanned surface is displaced with respect to an optical axis of the optical system in a direction (hereinafter, referred to as an unfolded sub-scanning direction) corresponding to a direction that matches the sub-scanning direction when the optical path from the light source to the scanned surface is unfolded. The scan line described by the beam displaced with respect to the optical axis is not straight. That is, a scan line bow is created.
FIGS. 1A and 1B
show how four beams from respective light sources produce four beam spots providing four scan lines S
1
-S
4
on a scanned surface. The dashed line A indicates a reference scan line produced when a chief ray of a deflected beam is deflected on a plane which includes an optical axis of a scanning image-forming optic and which is parallel with a direction (hereinafter, referred to as an unfolded main-scanning direction) corresponding to a direction that matches the main-scanning direction when the optical path from the light source to the scanned surface is unfolded. The reference scan line A corresponds to the scan line produced in the single-beam scanning arrangement and is therefore free of bow.
In the example shown in
FIG. 1A
, the scan lines S
1
-S
4
lie side by side in the sub-scanning direction (vertical direction on the paper) such that the scan lines S
1
and S
2
are symmetrical with the scan lines S
4
and S
3
, respectively, with respect to the reference scan line A. The scan lines S
1
and S
2
are bowed toward the top of the paper and the scan lines S
3
and S
4
are bowed toward the bottom of the paper.
FIG. 1A
shows how two scanning processes B
1
and B
2
each using four beam spots simultaneously are successively performed. Optical writing is performed such that the scanning processes B
1
and B
2
are alternately repeated.
Referring to the area I, a small interval (scan line pitch) is created between the scan line S
4
in the scanning process B
1
and the scan line S
1
in the scanning process B
2
. In the area II, a large scan line pitch is created between the scan lines S
2
and S
3
. As a result, there is produced a periodical variation in density of the optically-written image in the sub-scanning direction, near the center of the extent of main-scanning. Thereby, image quality is degraded.
FIG. 1B
shows how a scanning process C
1
using four beam spots is followed by a scanning process C
2
. The scan lines S
1
-S
4
lie side by side in the sub-scanning direction such that the scan lines S
1
and S
2
are symmetrical with the scan lines S
4
and S
3
, respectively, with respect to the reference scan line A. The scan lines S
1
-S
4
are slightly bowed away from the reference scan line A at two locations along the extent of main-scanning (left-to-right direction on the paper) and are bowed toward the reference line A at the center of the extent of main-scanning. The scan line S
1
is bowed with a larger radius of curvature than the scan line S
2
. The scan line S
4
is bowed with a larger radius of curvature than the scan line S
3
.
Since the scan line S
4
of the scanning process C
1
and the scan line S
1
of the scanning process C
2
are bowed in opposite directions, a large scan line pitch is created in the area III and a small scan line pitch is created in the area IV. As a result, there is produced a variation in density of an optical-written image in the main-scanning direction, at a boundary between two successive scanning processes C
1
and C
2
. Thereby, image quality is degraded.
A variation in scan line pitches occurring in a multi-beam scanning apparatus due to curvature of the scan lines, will be referred to as a differential pitch. As described above, differential pitch causes the quality of the optically-written image to be degraded.
One approach to reduce the differential pitch is disclosed in Japanese Laid-Open Patent Application No. 7-199109. In this approach, a scanning image-forming optic is designed to produce curvature of field. While producing curvature of field reduces differential pitch, it also causes a large variation in beam spot size on a scanned surface depending on image height. The beam spot size becomes relatively large where the image height is relatively large. Thereby, image quality is again degraded.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a multi-beam scanning apparatus in which the aforementioned problem is eliminated.
Another and more specific object is to provide a multi-beam scanning apparatus in which differential pitch is successfully reduced without causing a large variation in beam spot size.
In order to attain the aforementioned objects, the present invention provides a multi-beam scanning apparatus comprising: light source means for emitting a plurality of beams; beam deflector means for deflecting the plurality of beams from the light source means; and common scanning image-forming optic for converging the plurality of deflected beams so as to form a plurality of beam spots for simultaneously scanning a scanned surface, a plurality of scan lines respectively described by the plurality of beam spots being isolated from each other in a sub-scanning direction, wherein the plurality of scan lines are bowed in the same direction.
For example, the scan lines S
1
and S
2
in
FIG. 1A
are bowed in the same direction. The scan lines S
1
and S
2
in
FIG. 1B
are also described as being bowed in the same direction in that they are curved in an identical direction at an identical image height. By this definition, the scan lines S
2
and S
3
in
FIG. 1A
are described as being bowed in opposite directions. In the example shown in
FIG. 1A
, the scan lines S
1
and S
2
of the four scan lines S
1
-S
4
are bowed in the same direction, and the scan lines S
3
and S
4
are bowed in the same direction. The direction of bow of the scan lines S
1
and S
2
is opposite to the direction of bow of the scan lines S
3
and S
4
.
Assuming that the number of scan lines resulting from simultaneous scanning is n (>1), the present invention ensures that all of the n scan lines are bowed in the same direction.
As has been described with reference to
FIGS. 1A and 1B
, scan line pitch deviation occurs as a result of the adjacent scan lines being bowed to opposite directions. According to the present invention, scan line pitch deviation can be successfully reduced by ensuring that the n scan lines described by simultaneous scanning are bowed in the same direction.
Since it is not necessary to introduce a curvature of field for the purpose of reducing scan line pitch deviation, the beam spot size does not vary significantly depending on the image height.
The common image-forming optic could be a lens system such as an f&thgr; lens.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Phan James
Ricoh & Company, Ltd.
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