Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-07-17
2003-09-30
Cherry, Euncha (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S216100, C347S243000
Reexamination Certificate
active
06628444
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-beam type of scanning optical device used in an image forming apparatus such as a laser beam printer or a digital copying machine.
2. Description of the Related Art
In recent years, multi-beam-type scanning optical devices capable of simultaneously writing a plurality of lines by using a laser light source having a plurality of light emitting points have recently been developed for use in electrophotograhpic apparatuses, e.g., laser beam printers.
This type of scanning optical device enables scanning with a plurality of scanning laser beams simultaneously used, as described below. For example, as shown in
FIG. 6A
, two laser beams P
1
and P
2
as light beams are emitted from two light-emitting points
111
and
112
of a multi-beam laser unit, are each collimated into a parallel beam by a collimator lens
102
, pass through a cylindrical lens
103
and an optical stop
104
, irradiate to a reflecting surface
105
a
of a rotary polygon mirror
105
, and travel through an f&thgr; lens system
106
to have an imaging point on a photosensitive member (photoconductor)
107
on a rotary drum.
Each of the two laser beams P
1
and P
2
incident upon the reflecting surface
105
a
of the rotary polygon mirror
105
is deflected by the mirror
105
to be scanned in a main scanning direction. Each beam moving in the main scanning direction by the rotation of the rotary polygon mirror
105
and moving in a sub-scanning direction by the rotation of the rotary drum forms an electrostatic latent image on the photosensitive member
107
.
The cylindrical lens
103
condenses each of the laser beams P
1
and P
2
so that the beam is condensed into a linear shape on the reflecting surface
105
a
of the rotary polygon mirror
105
. The cylindrical lens
103
and the f&thgr; lens system
106
form an optical face tangle error correction system to perform the function of preventing occurrence of an error in positioning of the above-mentioned imaging point in the sub-scanning direction on the photosensitive member
107
due to a face tangle error of the rotary polygon mirror
105
. Also, the f&thgr; lens system
106
has the function of correcting the scanning movement of each beam so that the imaging point moves at a constant speed in the main scanning direction on the photosensitive member
107
.
Writing with a plurality of beams P
1
and P
2
is thus performed to achieve high-speed, high-definition printing.
To reduce the spacing between the lines formed on the photosensitive member by the laser beams from the two light-emitting points of the laser unit, a method has been practiced in which the line connecting the two light-emitting points is set at an angle from a direction corresponding to the sub-scanning direction, that is, the two light-emitting points are shifted from each other in a direction corresponding to the main scanning direction, because there is a limit to the reduction between the distance between the two light-emitting points.
If the light-emitting points are positioned as described above, the necessary length of the reflecting surface
105
a
of the rotary polygon mirror
105
for simultaneously reflecting the plurality of beams P
1
and P
2
to perform scanning is increased, resulting in an increase in overall size of the rotary polygon mirror
105
. As a solution of this problem, means for reducing the distance between the points at which the laser beams P
1
and P
2
are incident upon the rotary polygon mirror
105
has been devised. That is, the distance by which the laser beams P
1
and P
2
is reduced by reducing the distance between the rotary polygon mirror
105
and the optical stop
104
on the upstream side of the rotary polygon mirror
105
. This arrangement is also effective in limiting a deterioration in image quality due to instability of focusing.
This arrangement will be described in more detail. The laser beam P
1
emitted from the light-emitting point
111
is deflected by the rotary polygon mirror
105
, passes through the f&thgr; lens system
106
and travels along a path L
1
to have an imaging point at a position D on the photosensitive member
107
. At this time, the laser beam P
2
emitted from the light-emitting point
112
has an imaging point located just behind (or on the upstream side of) the position D in the main scanning direction indicated by an arrow B.
Thereafter, with rotation of the rotary polygon mirror
105
in the direction indicated by an arrow A (
FIG. 6A
shows the states of rotation of the rotary polygon mirror although the rotary polygon mirror is illustrated as if it is not rotated because the amount of rotation is extremely small), the laser beam P
2
emitted from the light-emitting point
112
travels along a path L
2
to reach the position D.
It is assumed here that the photosensitive member
107
moves to the position indicated by the broken line in
FIG. 6A
due to a reduction in the accuracy with which the photosensitive member
107
and the optical box incorporating the optical device are positioned. Since the laser beams P
1
and P
2
respectively emitted from the light-emitting points
111
and
112
reach the position D on the photosensitive member
107
with an angular difference of an angle “&agr;” from each other, the positions of the imaging spots on the photosensitive member
107
of the laser beams P
1
and P
2
shown by a broken line traveling along the paths L
1
and L
2
are spaced apart from each other by a distance “r”.
FIG. 6B
is a diagram showing details of the encircled portion VI B of FIG.
6
A.
If the position of the optical stop
104
is brought closer to the rotary polygon mirror
105
to reduce the angle “&agr;”, the distance “r” between the imaging spots of the laser beams P
1
and P
2
on the photosensitive member
107
, resulting from an error in positioning of the photosensitive member
107
as indicated by the broken line, is reduced. Thus, an increase in the length of the reflecting surface of the rotary polygon mirror and a deterioration in image quality resulting from an error in the imaging position due to instability of focusing can be suppressed.
However, if the number of scanning laser beams is increased, even the above-described technique is not a sufficiently effective solution of the problem of an increase in size of the reflecting surface for reflecting and scanning a plurality of laser beams, resulting in an increase in overall size of the polygon mirror and the problem of deterioration in image quality due to instability of focusing.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a scanning optical device designed so as to prevent an increase in size of a deflecting scanning means due to an increase in length of a reflecting surface, and an image forming apparatus incorporating the scanning optical device.
Another object of the present invention is to provide a scanning optical device designed so as to prevent deterioration in image quality due to instability of focusing, and an image forming apparatus incorporating the scanning optical device.
Still another object of the present invention is to provide a scanning optical device and an image forming apparatus using the optical scanning device, the scanning device including a first light source unit for generating a plurality of light beams, a second light source unit for generating at least one light beam, and deflecting scanning means for deflecting by a reflecting surface the light beams generated by the first light source unit and the second light source unit to scan a member to be scanned, wherein the positions of the plurality of light beams generated by the first light source unit are different from each other in a direction corresponding to the direction of scanning performed by the deflecting scanning means on the reflecting surface of the deflecting scanning means, and the at least one light beam generated by the second light source unit is positioned between the plurality of light beams generated by
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