Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2002-03-05
2003-11-18
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S216100, C347S243000
Reexamination Certificate
active
06650454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using the multi-beam scanning optical system. The present invention can be suitably applied to, for example, a laser beam printer or a digital copy machine with which high-speed, high-quality printing can be performed by simultaneously forming (recording) a plurality of scanning lines (dots) on the surface of a photosensitive member.
2. Description of the Related Art
In image forming apparatuses such as laser beam printers and digital copy machines, multi-beam scanning optical systems, with which a plurality of scanning lines can be simultaneously formed on the surface of a photosensitive member, are commonly used in order to perform high-speed printing.
In multi-beam scanning optical systems, there is a problem in that if the oscillation wavelengths of light sources are different from each other, dots in scanning lines formed by laser beams emitted from the light sources are displaced in a main-scanning direction due to the chromatic aberration of scanning lenses.
Accordingly, in Japanese Unexamined Patent Application Publication No. 2000-111820, a system is disclosed in which relative differences between the oscillation wavelengths of light sources are limited so that the dot displacement is reduced to within one-half of the size of a single pixel.
However, in color-image forming apparatuses which output halftone images such as color laser printers, etc., the dot displacement is not small enough when the allowable value thereof is one-half of the size of a single pixel.
FIGS. 12 and 13
are diagrams showing an example of a halftone image pattern which is generally used for forming a color image, where
FIG. 12
shows a case in which dots are arranged without displacements in the main-scanning direction and
FIG. 13
shows a case in which the dots are displaced in the main-scanning direction.
FIGS. 12 and 13
show a case in which two light beams are used. The solid lines extending in the horizontal direction are formed by one of the two light beams, and the dashed lines extending in the horizontal direction are formed by the other one of the light beams. In addition, the circles shown on the horizontal lines are dots formed by the scanning lines.
In
FIG. 13
, a region in which the gaps between the dots are wide (Wa) and a region in which the gaps between the dots are narrow (Wb) are alternately formed in an inclined manner. Since the regions in which the gaps between the dots are narrow appear dark and the regions in which the gaps between the dots are wide appear light in an actual image, this image looks like a striped pattern over the entire image area. In color laser printers, etc., many kinds of patterns like that shown in
FIGS. 12 and 13
are used, and a small dot displacement may greatly affect the image depending on the pattern. Therefore, it is necessary to set an allowable value of the dot displacement (Wa-Wb) to an extremely small value such as one-fourth of the size of a single pixel.
The above-described dot displacement in the main-scanning direction is caused not only by the difference in oscillation wavelengths but also by a difference in light path lengths.
Next, the dot displacement due to a difference in light path lengths will be described below with reference to
FIGS. 14 and 15
.
FIG. 14
is a sectional view of the main part of a multi-beam scanning optical system cut along the main-scanning direction (main-scanning sectional view), and
FIG. 15
is a sectional view of the main part of the multi-beam scanning optical system shown in
FIG. 14
cut along the sub-scanning direction (sub-scanning sectional view).
In
FIGS. 14 and 15
, a light source unit
100
includes two light sources (laser beam sources)
101
and
102
formed of, for example, semiconductor lasers. A collimator lens
103
collimates two laser beams emitted by the light source unit
100
, and a cylindrical lens
104
has a predetermined refractive power only in the sub-scanning direction. In addition, an aperture diaphragm
108
forms the laser beams emitted from the cylindrical lens
104
into optimal shapes. The collimator lens
103
, the cylindrical lens
104
, and the aperture diaphragm
108
form one element of an incident optical unit
114
.
A deflector
105
serves as a deflecting unit, and is formed of, for example, a rotating polygon mirror. The deflector
105
is rotated in the direction shown by the arrow A at a constant speed by a driving unit (not shown) such as a motor, etc. A scanning optical unit
106
has f&thgr; characteristics and includes first and second f&thgr; lenses
106
a
and
106
b
. The two laser beams deflected by the deflector
105
are focused onto the surface of a photosensitive member (recording medium)
107
by the scanning optical unit
106
in the shape of spots, so that two scanning lines S
101
and S
102
are formed. The scanning optical unit
106
is constructed such that a deflecting surface
105
a
of the deflector
105
and the surface of the photosensitive member
107
are conjugate to each other in the sub-scanning cross section, so that surface tilting is corrected.
The photosensitive member (photosensitive drum)
107
has an approximately cylindrical shape, and serves as a recording medium.
In
FIGS. 14 and 15
, two laser beams B
101
and B
102
, which are optically modulated in accordance with image information, are emitted from the light source unit
100
, collimated by the collimator lens
103
, and incident on the cylindrical lens
104
. The two laser beams B
101
and B
102
incident on the cylindrical lens
104
leave the cylindrical lens
104
without a change in the main-scanning cross section, and pass through the aperture diaphragm
108
(a part of each laser beam is blocked). In the sub-scanning cross section, the two laser beams B
101
and B
102
converge before they pass through the aperture diaphragm
108
(a part of each laser beam is blocked). Accordingly, the two laser beams B
101
and B
102
are focused onto the deflecting surface
105
a
of the deflector
105
in the shape of lines (lines that extend in the main-scanning direction). Then, the laser beams B
101
and B
102
are deflected by the deflecting surface
105
a
of the deflector
105
and focused onto the surface of the photosensitive member
107
by the scanning optical unit
106
in the shape of spots. By rotating the deflector
105
in the direction shown by the arrow A, the laser beams B
101
and B
102
scan over the surface of the photosensitive member
107
in the direction shown by the arrow B (in the main-scanning direction) at a constant speed. Accordingly, an image is recorded on the surface of the photosensitive member
107
, which serves as the recording medium.
In
FIGS. 14 and 15
, the laser beams B
101
and B
102
are emitted from the light sources
101
and
102
, travel along light paths L
101
and L
102
, and form the scanning lines S
101
and S
102
, respectively.
As shown in
FIG. 15
, the laser beams B
101
and B
102
must be incident on the surface of the photosensitive member
107
at positions displaced from the end point T of the photosensitive member
107
in the sub-scanning direction. In the case in which the laser beams B
101
and B
102
are incident on the surface of the photosensitive member
107
at the end point T, the following problem occurs. That is, when the laser beams B
101
and B
102
are at positions close to the central point in the main-scanning direction, they are reflected by the surface of the photosensitive member
107
, travel along the same light paths along which they have traveled in the reverse direction, and return to the light sources
101
and
102
. Accordingly, the optical outputs of the semiconductor lasers vary due to noise caused by the laser beams returning from the photosensitive member
107
, and the density of a printed image also varies.
However, when the laser beams B
101
and B
102
are incident on the surface of the photosensitive member
107
at po
Azami Junya
Ishikawa Yutaka
Kato Manabu
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Phan James
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