Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-06-21
2001-03-27
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S206100, C359S207110, C359S212100, C359S216100
Reexamination Certificate
active
06208450
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning optical device and, more particularly, to a scanning optical device suitably used for image forming apparatuses such as a laser beam printer and digital copying machine, in which a light beam emitted from a light source means is guided onto a scanned surface as a recording medium surface through a light deflector such as a rotating polygon mirror, and the light beam is scanned on the scanned surface, thereby recording characters, information, and the like.
2. Related Background Art
In a conventional scanning optical device used for a laser beam printer, a digital copying machine, and the like, a light beam emitted from a light source means is deflected by a deflection means, and the deflected light beam forms a spot on a photosensitive drum surface as a scanned surface via a scanning optical means, thereby scanning the light beam on the scanned surface.
FIG. 1
is a schematic view of the main part of a conventional scanning optical device. Referring to
FIG. 1
, a light beam emitted from a light source means
11
constituted by a semiconductor laser and the like is converted into a substantially collimated light beam by a collimator lens
12
. This substantially collimated light beam is shaped into an optimal beam shape by an aperture stop
13
and strikes a cylindrical lens
14
. The cylindrical lens
14
has a power in the sub-scanning direction and forms a light beam image elongated in the main scanning direction near a deflection surface
15
a
of the light deflector
15
constituted by a rotating polygon mirror and the like. In this case, the main scanning direction is a direction perpendicular to the deflection scanning direction, and the sub-scanning direction is a direction perpendicular to the deflection scanning direction. This applies to the following description. The light beam is reflected/deflected by the light deflector
15
at an equal angular velocity to form a spot on a photosensitive drum surface (recording medium surface)
18
as a scanned surface via an f&thgr; lens
16
which is a single element lens serving as a scanning optical means having an f&thgr; characteristic. This light beam is scanned on the photosensitive drum surface
18
at an equal velocity.
In a scanning optical device of this type, a rotating polygon mirror having many deflection surfaces (reflection surfaces) is generally used as a deflection means, and an optical axis is set at a given angle before and after deflection in the overall scanning optical system. That is, a light beam emitted from a light source means is obliquely incident on the deflection means within a scanning plane. For this reason, deflection surfaces asymmetrically enter and exit within one scanning operation with reference to on-axis scanning. The influences of this asymmetry are noticeable in a sub-scanning cross-section in which a light beam is temporarily imaged near the deflection surface to set the deflection surface and the scanned surface conjugate with each other. This makes it difficult to correct curvature of field.
To realize a scanning optical device with a higher resolution (smaller spot), the influences of asymmetry on curvature of field and f&thgr; characteristics in the main scanning direction must be considered. As a means for solving these problems, for example, the scanning optical devices disclosed in Japanese Laid-Open Patent Application Nos. 8-122635 and 8-248308 use a means for asymmetrically changing this radius of curvature in the sub-scanning direction with respect to the optical axis. This allows proper correction of curvature of field in the sub-scanning direction throughout the entire scanning width. A lens having such a shape is generally manufactured by plastic molding in consideration of technical and economical advantages.
As is known, however, plastic materials are susceptible to environmental variations. With a change in ambient temperature, in particular, the refractive index of a plastic material changes. As a consequence, the focal point of the plastic lens varies, and for example, the spot size on a scanned surface increases. This makes it difficult to obtain a high-quality printed image.
SUMMARY OF THE INVENTION
It is the first object of the present invention to provide a compact scanning optical device which can properly correct the optical characteristics (curvature of field and the like) by forming a diffraction grating on at least one surface of the optical surfaces of optical elements constituting a scanning optical means and asymmetrically changing the power of the diffraction grating without any symmetrical axis in at least the main scanning direction or sub-scanning direction, is resistant to focus changes due to environmental variations (temperature changes), and is capable of high-resolution printing with a simple arrangement.
It is the second object of the present invention to provide a compact scanning optical device which can properly correct the optical characteristics (curvature of field and the like) by forming a diffraction grating on at least one surface of the optical surfaces of optical elements constituting a scanning optical means, asymmetrically changing the pitch of the diffraction grating without any symmetrical axis in at least the main scanning direction or sub-scanning direction, and also asymmetrically changing the shape of at least one surface of the optical surfaces of the optical elements constituting the scanning optical means from an on-axis position to an off-axis position in at least the main scanning direction or the sub-scanning direction, is resistant to focus changes due to environmental variations (temperature changes), and is capable of high-resolution printing with a simple arrangement.
A scanning optical device of the present invention for guiding a light beam emitted from light source means to deflection means, causing scanning optical means to form a spot of the light beam deflected by the deflection means on a scanned surface, and scanning the light beam on a scanned surface, is characterized in that a diffraction grating having a power based on a change in pitch is formed on at least one of optical surfaces of optical elements constituting the scanning optical means, and the power of the diffraction grating asymmetrically changes within a plane of the optical element surface, on which the diffraction grating is formed, without any symmetrical axis, in at least one of a main scanning direction and sub-scanning direction.
The present invention is characterized in that the power of the diffraction grating in the main scanning direction asymmetrically changes from an on-axis position to an off-axis position with respect to a reference axis.
The present invention is characterized in that the power of the diffraction grating in the sub-scanning direction asymmetrically changes from an on-axis position to an off-axis position with respect to a reference axis in the main scanning direction, and a power Po on the on-axis position and a power Py on the off-axis position both with respect to the reference axis satisfy
Po>Py
The present invention is characterized in that both the powers of the diffraction grating in the main scanning direction and sub-scanning direction asymmetrically change from an on-axis position to an off-axis position with respect to a reference axis, and the power in the sub-scanning direction satisfies
Po>Py
where Po is a power on the on-axis position and Py is a power on the off-axis position both with respect to the reference axis.
The present invention is characterized in that the scanning optical means corrects a focus shift in at least one of the main scanning direction and sub-scanning direction, which is caused by a change in refractive index of a lens material due to an environmental variation in the device, by using a diffraction power change caused by a wavelength variation in the light source means.
The present invention is characterized in that the optical element on which the diffraction grating is fo
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Phan James
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