Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-12-05
2002-12-03
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S241000, C359S204200
Reexamination Certificate
active
06489982
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 same and, more particularly, to a multi-beam scanning optical system suitably used for an image forming apparatus applicable to any of monochrome image and color image, e.g., a laser beam printer or digital copying machine, which can attain a high operation speed and high recording density by using a light source means with a plurality of light sources.
2. Related Background Art
FIG. 7
is a sectional view (main scanning sectional view) of the main part of a conventional multi-beam scanning optical system in the main scanning direction.
Referring to
FIG. 7
, a plurality of light beams emitted from a multi-beam semiconductor laser (multiple light sources)
71
having a plurality of light sources (light-emitting units) are converted into substantially parallel light beams or convergent light beams by a collimator lens
72
, and the light beams strike a cylindrical lens
74
while the cross-sectional area of the light beams is limited by an aperture stop
73
. Of the light beams incident on the cylindrical lens
74
, light beams in a main scanning cross-section emerge without any change. In a sub-scanning cross-section, light beams converge and are formed into almost linear images (elongated in the main scanning direction) on a deflecting surface (reflecting surface)
75
a
of an optical deflector
75
. Note that each of the collimator lens
72
, aperture stop
73
, cylindrical lens
74
, and the like forms one element of an incidence optical system. A plurality of light beams reflected/deflected by the deflecting surface
75
a
of the optical deflector
75
are formed into spots on a photosensitive drum surface
77
by an imaging optical system (f−&thgr; lens system)
76
. By rotating the optical deflector
75
in the direction indicated by an arrow C, these light beams are scanned on the photosensitive drum surface
77
at a constant speed in the direction indicated by the arrow D (main scanning direction). With this operation, an image is recorded on the photosensitive drum surface
77
serving as a recording medium.
In such a multi-beam scanning optical system, to accurately control the write position of an image, a write position sync signal detection means is generally arranged immediately before an image signal write position.
Referring to
FIG. 7
, a bending mirror (BD mirror)
78
reflects, toward the BD sensor
81
side, a light beam (BD light beam) for detecting a write position sync signal for adjusting the timing at a scanning start position on the photosensitive drum surface
77
. A slit (BD slit)
79
is located at a position equivalent to the photosensitive drum surface
77
. A BD lens
80
serving as an imaging means makes the BD mirror
78
and BD sensor
81
optically conjugate to each other, thereby correcting a tilt of the surface of the BD mirror
78
. The optical sensor (BD sensor)
81
serves as a write position sync signal detection element. Note that each of the BD slit
79
, BD lens
80
, BD sensor
81
, and the like forms one element of a write position sync signal detection means
91
.
Referring to
FIG. 7
, the timing at a scanning start position for image recording on the photosensitive drum surface
77
is adjusted by using an output signal from the BD sensor
81
.
In this multi-beam scanning optical system, as shown in
FIG. 8
, if a plurality of light sources (light-emitting units) A and B are placed vertically in the sub-scanning direction, the distance between scanning lines on a scanned surface in the sub-scanning direction becomes much larger than the recording density. For this reason, in general, as shown in
FIG. 9
, the plurality of light sources A and B are placed obliquely in the sub-scanning direction with respect to the main scanning direction, and a tilt angle &dgr; is adjusted to accurately match the distance between the scanning lines on the scanned surface in the sub-scanning direction to the recording density.
If a relative wavelength error occurs in light beams emitted from the plurality of light sources, relative focus errors occur on the scanned surface in the main and sub-scanning directions in correspondence with the respective light-emitting units, resulting in a deterioration in image quality. For this reason, a collimator lens having undergone proper chromatic aberration correction is used to prevent relative focus errors in the main and sub-scanning directions from occurring even if a relative wavelength error occurs in light beams emitted from a plurality of light sources, thereby effectively preventing a deterioration in image quality.
In the multi-beam scanning optical system having the conventional arrangement described above, since a plurality of light sources are placed obliquely in the sub-scanning direction with respect to the main scanning direction, light beams emitted from the light sources A and B strike the deflecting surface of the optical deflector (polygon mirror) at positions spaced apart from each other in the main scanning direction, and are reflected/deflected by the optical deflector at different angles, as shown in FIG.
10
. As a consequence, spots are formed on the scanned surface
77
at positions spaced apart from each other in the main scanning direction (beams A
1
and B
1
).
In the multi-beam scanning optical system having this arrangement, therefore, image data are sent with a timing shift of a predetermined time &dgr;T such that a light from a given reference light source is formed into an image on the scanned surface, and then a light beam from another light source is formed into an image at the same imaging position.
Referring to
FIG. 10
, when the timing shifts by &dgr;T, the deflecting surface
75
a
is set at the angle of a deflecting surface
75
b
. At this time, a beam B
2
is reflected/deflected by the deflecting surface
75
b
in the direction indicated by an arrow B
2
′, i.e., in the same direction indicated by an arrow A
1
′ as that of the beam A
1
, thereby matching the imaging positions of the respective spots to each other.
Consider a case wherein a focus error occurs in the main scanning direction due to some cause, e.g., a positional error between the scanned surface and the optical unit holding the optical system or a mounting error in mounting an optical component in the optical unit. Assume, in this case, that a normal position
77
a
of the scanned surface
77
shifts to a position
77
b
. In this case, as is obvious from
FIG. 10
, the imaging position of each beam shifts in the main scanning direction by &dgr;Y.
In the prior art, when the imaging positions of light beams from a plurality of light sources (multi-beam semiconductor laser) shift in the main scanning direction, the printing precision and image quality deteriorate.
There are various factors that cause focus errors in the main scanning direction. It is very difficult to reduce all these factors to zero. Even if such adjustment is to be made, the adjustment process will cost much. In addition, recently, an optical system having an f−&thgr; lens made of a plastic material is often used in consideration of cost. A plastic lens is manufactured by injection molding. The surface precision of this lens is inferior to that obtained by polishing optical glass. In such plastic lens, in particular, errors tend to occur in some portions in convex forms with respect to design values, but errors tend to occur in other portions in concave forms with respect to design values. When focus errors are caused by such surface precision errors, it is impossible to correct the focus errors throughout the scanned surface. It is therefore difficult to correct a deterioration in image quality due to the occurrence of errors in the imaging positions of light beams from a plurality of light sources in the main scanning direction.
It is an object of the present invention to provide an multi-beam scanning optical system which can effectively reduce e
Canon Kabushiki Kaisha
Pendegrass Joan
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