Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-07-07
2002-05-14
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S206100, C359S204200, C347S223000, C347S244000, C347S259000
Reexamination Certificate
active
06388792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Optical scanning devices are widely used in ‘image forming apparatus’ such as a digital copier, an optical printer, an optical plate-making machine, a facsimile machine and so forth. A writing density of optical scanning devices has been increased to 1200 dpi, 1600 dpi, and is intended to be increased, further higher.
In order to achieve such high-density writing, it is necessary to form a beam spot having a small diameter, and, also, quality and stability of a beam spot is needed to be improved. Stability of a beam spot is determined from determining whether or not ‘a variation in a beam-spot diameter on a surface to be scanned due to a variation in an image height’ is very small and stable. Quality of a beam spot is determined from determining whether or not ‘the light-intensity distribution of a beam spot has a simple mountain shape and does not have a complicated lower slope shape’.
In order to achieve a beam spot having high-quality and stability, it is necessary for a scanning and image-forming optical system of an optical scanning device to have a high performance for forming a beam spot on a surface to be scanned using a deflected light flux. A factor causing a beam-spot diameter to fluctuate is, as is well known, ‘curvature of field in a scanning and image-forming optical system’, and many scanning and image-forming optical systems in which curvature of field is well corrected have been proposed. Further, it is important for an optical magnification in a scanning and image-forming optical system to be fixed when an image height of a beam spot changes.
However, in order to form a beam spot having stability and high quality, not only it is necessary to correct an optical performance such as curvature of field and an optical magnification but also it is important to ‘set a wave-optical wavefront aberration to be fixed between respective image heights’.
SUMMARY OF THE INVENTION
An object of the present invention is to achieve high-density, satisfactory optical scanning with a stable and high-quality beam spot, by well correcting not only curvature of field and optical magnification but also ‘wavefront aberration on pupil’ in a scanning and image-forming optical system.
An optical scanning device according to the present invention is ‘a device which deflects one or a plurality of light flux(es) originating from a light source by an optical deflecting unit, gathers the deflected light flux(es) to cause it (them) to form a beam spot(s) on a surface to be scanned by a scanning and image-forming optical system, and, thus, performs optical scanning of the surface to be scanned’.
As one or a plurality of light flux(es) is (are) emitted from the light source, the optical scanning device according to the present invention can be put in to practice either as an optical scanning device in a single-beam-scanning system in which optical scanning is performed using a single beam spot or an optical scanning device in a multi-beam-scanning system in which a plurality of scan lines are scanned simultaneously by a plurality of beam spots.
The scanning and image-forming optical system includes one or a plurality of optical component(s) including a lens. Accordingly, the scanning and image-forming optical system may include, other than the lens, ‘a reflecting-surface component having a mirror surface having an image-forming function’.
Further, at least one surface of the lens included in the scanning and image-forming optical system is a sub-non-arc surface.
The ‘sub-non-arc surface’ is a surface having an arc or non-arc shape in a main scanning plane, and having a non-arc shape in a sub-scanning plane.
The ‘main scanning plane’ is a plane including the optical axis of the lens and parallel to main scanning directions in the lens or in the vicinity thereof.
The ‘sub-scanning plane’ is a plane perpendicular to the main scanning directions in the lens or in the vicinity thereof.
The optical scanning device according to the present invention is characterized in that the sub-non-arc surface is formed in ‘a lens in which a diameter of a light flux passing through the scanning and image-forming optical system is largest in the sub-scanning plane’.
That is, when the scanning and image-forming optical system has ‘one sub-non-arc surface’, this sub-non-arc surface is formed in the above-mentioned ‘lens in which the diameter of the light flux passing through the scanning and image-forming optical system is largest in the sub-scanning plane’, and, when the scanning and image-forming optical system has two or more sub-non-arc surfaces, at least one thereof is provided in the above-mentioned ‘lens in which the diameter of the light flux passing through the scanning and image-forming optical system is largest in the sub-scanning plane’. In this case, both the surfaces of this lens may be sub-non-arc surfaces.
In this optical scanning device, the sub-non-arc surface may be a surface of the ‘lens in which the diameter of the light flux passing through the scanning and image-forming optical system is largest in the sub-scanning plane’, ‘in which surface a diameter of the light flux passing through the scanning and image-forming optical system is largest in the sub-scanning plane’.
The optical scanning device according to another aspect of the present invention is characterized in that a sub-non-arc surface is formed in ‘a lens having the largest effective diameter in the main scanning plane’.
In this case, the sub-non-arc surface may be a surface of the lens, which surface has the largest effective diameter in the main scanning plane’.
Degradation in wavefront aberration is likely to occur as a wave surface is large. Accordingly, a surface of a lens through which correction of wavefront aberration can be effectively made is a portion at which a wave surface is large, and, therefore, a surface of each of the above-mentioned lenses or each of the above-mentioned surfaces of the lenses is suitable for having a sub-non-arc surface through which wavefront aberration is corrected. Further, in such a portion as that in which a wave surface is large, a sub-non-arc surface itself is large, and, thereby, it is easy to form the sub-non-arc surface.
The optical scanning device according to another aspect of the present invention is characterized in that the sub-non-arc is formed in ‘a lens of a scanning and image-forming optical system having a surface in which, throughout an effective range of the lens, the incidence angle of the chief ray of a deflected light flux incident on the respective surfaces of the lens is equal to or less than 25 degrees’.
In this case, a ‘surface of the lens, in which surface, in which, throughout the effective range of the lens, the incidence angle of the chief ray of a deflected light flux incident on the respective surfaces of the lens is equal to or less than 25 degrees’ may be formed to be the sub-non-arc surface.
In a surface in which the incidence angle is larger than 25 degrees, a refractive index of this surface in a main scanning plane is large. When such a surface is formed to be the sub-non-arc surface, it is not necessarily easy to achieve both an effect of correction of wavefront aberration in the sub-scanning directions and an effect of correction of characteristics in the main scanning directions. Accordingly, it is preferable for a ‘surface, in which the incidence angle of the chief ray of a deflected light flux is equal to or less than 25 degrees throughout an effective range of the lens’, to be the sub-non-arc surface.
The optical scanning device according to another aspect of the present invention is characterized in that the sub-non-arc surface expressed by a coordinate X(Y, Z) in the optical-axis direction is expressed by the following equation:
X
(
Y, Z
)=
CmY
2
/[1+{square root over ({1+L −(
1+
K
+L )
Cm
2
Y
2
+L })}]+&Sgr;
A
n
Y
n
&
Aoki Magane
Atsuumi Hiromichi
Sakai Kohji
Suzuki Seizo
Phan James
Ricoh & Company, Ltd.
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