Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-01-25
2003-11-04
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S217200
Reexamination Certificate
active
06643043
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning optical device and image forming apparatus and, more particularly, to a compact image forming apparatus suitable for high-resolution printing with, e.g., a color electrophotographic process, such as a laser beam printer (LBP) or color digital copying machine, in which an imaging position variation caused by the wavelength fluctuation of a light source means, an aberration variation upon an environmental fluctuation, and the like in each scanning optical device are compensated by a compensation means in a color image forming apparatus having a plurality of scanning optical devices, thereby suppressing an imaging position deviation in especially the main scanning direction, and reducing color misregistration, and image density nonuniformity at low cost with a simple arrangement.
2. Related Background Art
Conventionally in a scanning optical device used in an image forming apparatus such as a laser beam printer or digital copying machine, a beam modulated and emitted by a light source means in accordance with an image signal is periodically deflected by an optical deflector formed from, e.g., a rotary polyhedral mirror (polygon mirror), and converged into a spot on a photosensitive recording medium (photosensitive drum) surface by an imaging optical system (scanning optical system) having an f-&thgr; characteristic. This surface is optically scanned to record an image.
FIG. 24
is a schematic view showing the main part of a conventional scanning optical device.
In
FIG. 24
, a divergent beam emitted by a light source means
91
is converted into a nearly collimated beam by a collimator lens
92
. The beam is limited by a stop
93
, and enters a cylindrical lens
94
having a predetermined refracting power in only the sub scanning direction. Of the nearly collimated beam that enters the cylindrical lens
94
, light components in the main scanning plane directly emerge as a nearly collimated beam. In the sub scanning plane, light components are converged to form an almost line image on a deflection surface (reflection surface)
95
a
of an optical deflector
95
formed from a rotary polyhedral mirror (polygon mirror).
The beam deflected and reflected by the deflection surface
95
a
of the optical deflector
95
is guided onto a photosensitive drum surface
98
as a surface to be scanned via an imaging optical element (f-&thgr; lens)
96
having f-&thgr; characteristics. By rotating the optical deflector
95
in a direction indicated by an arrow A, the beam scans the photosensitive drum surface
98
in a direction indicated by an arrow B to record image information.
FIG. 25
is a schematic view showing the main part of a conventional color image forming apparatus. In
FIG. 25
, the color image forming apparatus simultaneously uses a plurality of scanning optical devices
111
to
114
like the above-described one to record pieces of image information of respective colors on corresponding photosensitive drums
121
to
124
, thereby forming a color image. In this color image forming apparatus, it is important to reduce not only an imaging position deviation represented by jitter of a single color but also scanning line deviations (to be referred to as “misregistration” hereinafter) between colors, and image density nonuniformity between colors in order to align a plurality of scanning lines and form an image. For this purpose, the scanning optical device must satisfy the following conditions.
(1) An imaging position variation (spot position deviation) in the main scanning direction along with the wavelength fluctuation of a beam emitted by a semiconductor laser serving as a light source (to be simply referred to as the “wavelength fluctuation of the semiconductor laser” hereinafter) is compensated.
(2) A focus variation in the sub scanning direction along with an environmental fluctuation such as influential temperature elevation is compensated (a focus variation in the main scanning direction is originally small and does not pose any problem in many cases).
(3) An imaging position deviation in the main scanning direction along with an environmental fluctuation such as temperature elevation is small.
(4) An imaging position deviation in the main scanning direction caused by a wavelength variation (not accompanied by any environmental fluctuation) is small.
The image forming apparatus requires not only stable optical performance in one scanning optical device (single color) but also an arrangement which can prevent misregistration or image nonuniformity between colors even when the light source wavelength (wavelength of a beam emitted by the light source), initial wavelength, or use environment (particularly ambient temperature) of the scanning optical device fluctuates, or even when the light source wavelengths, initial wavelengths, or use environments of a plurality of scanning optical devices differ.
Further, when such a color image forming apparatus uses a light source having a plurality of light-emitting portions represented by multi-beam lasers (e.g., multi-semiconductor lasers), jitter by a plurality of beams (scanning interval fluctuation in the main scanning direction on the photosensitive drum surface) occurs owing to the wavelength difference between the light-emitting portions, greatly degrading the image quality. To prevent this, (1) compensation of a spot position deviation and (4) compensation of a spot imaging position deviation not accompanied by any environmental fluctuation become more important.
Such a scanning optical device has conventionally used a glass lens or glass mirror which hardly exhibits a characteristic fluctuation caused by an environmental fluctuation. However, an aberration variation caused by the wavelength fluctuation of a semiconductor laser remains, advanced aberration correction by an aspherical surface cannot be performed, and the cost is high. Thus, demands have arisen for compensation of a wavelength fluctuation, environmental fluctuation, or the like by a scanning optical device using a plastic material.
Japanese Patent Application Laid-Open No. 3-231218 discloses a scanning optical device in which a scanning optical system is constituted by a glass spherical lens and plastic toric lens. In this reference, an imaging position deviation in the main scanning direction caused by a wavelength variation of 5 nm is 64.6 &mgr;m, and a focus deviation in the sub scanning direction at a temperature elevated by 25° C. is +1.7 mm. For example, a color image forming apparatus using a plurality of such scanning optical devices suffers misregistration or image nonuniformity between colors.
Japanese Patent Application Laid-Open No. 7-128603 discloses an example in which a glass lens and glass cylinder mirror are used as the scanning optical system of a scanning optical device used in a color image forming apparatus. In this reference, all the scanning optical systems are made of a glass material. An aberration variation caused by the wavelength fluctuation of a semiconductor laser remains. Aberration correction by an aspherical surface cannot be performed, so the optical path is long. In addition, the cost is high.
Japanese Patent Application Laid-Open No. 10-232347 discloses an example in which a scanning optical system is formed from two toric lenses. In this reference, all the scanning optical systems are made of a plastic, which is advantageous for aberration correction. However, the laser wavelength increases as the temperature is elevated. When the refractive index of the scanning optical system decreases, the laser wavelength varies in a direction in which the focal length of a scanning lens increases. As a result, the imaging position in the main scanning direction greatly deviates.
It is an object of the present invention to provide a compact scanning optical device and color image forming apparatus suitable for high-resolution printing in which an imaging position variation caused by the wavelength fluctuation of a light source means, an aber
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Robinson Mark A.
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