Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-12-22
2003-02-11
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S204200, C359S205100, C359S212100, C359S216100, C347S235000, C347S250000, C347S243000, C347S244000, C347S258000, C347S261000
Reexamination Certificate
active
06519070
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanning apparatus, a multi-beam optical scanning apparatus, and an image forming apparatus using the same and, more particularly, to an apparatus suitable for an image forming apparatus such as a laser beam printer or digital copying machine, which can obtain a satisfactory image on a scanned surface (photosensitive surface) by appropriately setting the elements such that the amount of jitter that occurs in the main scanning direction due to a tilt of a deflecting surface of an optical deflector falls within an allowable range.
2. Related Background Art
In an optical scanning apparatus conventionally used for an image forming apparatus such as a laser beam printer or digital copying machine, a light beam emitted from a light source means and optically modulated in accordance with an image signal is periodically deflected by an optical deflector formed by, e.g., a rotary polyhedral mirror (polygon mirror), and focused to a spot on the photosensitive recording medium (photosensitive drum) surface through an imaging optical system having f-&thgr; characteristics, and the surface is optically scanned to record an image.
FIG. 20
is a schematic view showing the main part of a conventional optical scanning apparatus.
Referring to
FIG. 20
, an almost parallel light beam emitted from a laser unit
201
is incident on a cylindrical lens
202
having a predetermined refracting power only in the sub-scanning direction. Of the almost parallel light beam incident on the cylindrical lens
202
, a light component in a main scanning section emerges without any change. In a sub-scanning cross-section, the light beam converges and forms an almost linear image on a deflecting surface
203
a
of an optical deflector
203
made of a rotary polyhedral mirror. The light beam reflected/deflected by the deflecting surface
203
a
of the optical deflector
203
is focused onto a photosensitive drum
206
serving as a scanned surface through an imaging optical system (f-&thgr; lens system)
207
having f-&thgr; characteristics, and the optical deflector
203
is rotated in the direction indicated by an arrow A to optically scan the photosensitive drum
206
in the direction indicated by an arrow B (main scanning direction), thereby recording image information.
In recent years, along with size reduction of a printer and the like, an optical scanning apparatus is also required to be compact and have high performance. To meet this requirement, various optical scanning apparatuses have been proposed for which size reduction of the entire apparatus is achieved by bending the optical path using, e.g., a return mirror. Accordingly, sometimes the incident optical system may have to be inclined with respect to a plane perpendicular to the rotational axis of the optical deflector (this system will also be referred to as an oblique incident optical system hereinafter).
In an optical scanning apparatus using such an oblique incident optical system, for example, the light beam incident position on the scanned surface shifts in the main scanning direction due to a tilt of the deflecting surface of the optical deflector, resulting in a shift in drawing position in the main scanning direction, i.e., so-called jitter.
In a recent image forming apparatus having high resolution of 1,200 dpi or more, a jitter amount in the main scanning direction conspicuously appears as degradation in image quality, and the allowable range for the jitter amount in the main scanning direction is narrowing.
A problem posed here is a variation in tilt angle between the plurality of deflecting surfaces.
If the tilt angle varies between the plurality of deflecting surfaces, the drawing position expands/contracts to both sides of the optical axis. For this reason, even when BD detection can be done at the center to minimize the jitter at the center, the drawing position does shift by the above amount.
This will be described in detail. If the tilt angles of adjacent deflecting surfaces are different, the length of a scanning line formed by a light beam reflected/deflected by one deflecting surface is different from that of a scanning line formed by a light beam reflected/deflected by the other deflecting surface. That is, the magnification changes in the main scanning direction.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical scanning apparatus using an oblique incident optical system, which can obtain a satisfactory image by appropriately setting the elements such that the amount of jitter that occurs in the main scanning direction due to a tilt of a deflecting surface of an optical deflector falls within an allowable range, and an image forming apparatus using the optical scanning apparatus.
It is another object of the present invention to provide a multi-beam optical scanning apparatus using an oblique incident optical system, which can obtain a satisfactory image by appropriately setting the elements such that the sum of the amount of jitter that occurs in the main scanning direction due to a tilt of a deflecting surface of an optical deflector and the amount of jitter that occurs when a plurality of light sources are used falls within an allowable range, and an image forming apparatus using the multi-beam optical scanning apparatus.
In one aspect of the invention, an optical scanning apparatus comprises an incident optical system for causing a light beam emitted from light source means to be obliquely incident in a sub-scanning cross-section on a deflecting surface of an optical deflector having a plurality of deflecting surfaces, and an imaging optical system for forming, on a scanned surface, an image of the light beam reflected/deflected by the deflecting surface of the optical deflector, wherein letting K (dpi) be a recording density in a main scanning direction on the scanned surface, &ggr; be an absolute value of a maximum difference in a tilt angle between the deflecting surfaces in the sub-scanning cross-section, &agr; be an incident angle of the light beam incident on the deflecting surface in the sub-scanning cross-section, &thgr; be a scanning angle of an outermost off-axis light beam with respect to an optical axis of the imaging optical system in a main scanning cross-section, f be a focal length of the imaging optical system, and &phgr; be an angle a scanning central axis makes with an optical axis when an optical axis of the incident optical system opposing the deflecting surface is projected to the main scanning cross-section, a condition given by
&LeftBracketingBar;
sin
θ
2
×
sin
γ
(
cos
α
×
sin
γ
×
cos
φ
×
cos
θ
2
+
cos
α
×
sin
γ
×
sin
φ
×
sin
θ
2
+
sin
α
×
cos
γ
)
&RightBracketingBar;
<
6.35
f
×
K
is satisfied.
In another aspect of the invention, at least some optical elements of the imaging optical system form some elements of the incident optical system.
In another aspect of the invention, the optical axis is substantially parallel to the scanning central axis when the optical axis of the incident optical system opposing the deflecting surface is projected to the main scanning cross-section.
In another aspect of the invention, the further comprises synchronization detection means, arranged near the scanned surface and including a slit, for controlling a timing at a scanning start position on the scanned surface, wherein the slit has an aperture portion inclined in a sub-scanning direction.
In another aspect of the invention, the light beam emitted from the light source means becomes incident on the deflecting surface while having a width larger than a main scanning width of the deflecting surface of the optical deflector.
In another aspect of the invention, in the
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Phan James
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