Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-12-21
2001-05-15
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S196100, C359S205100, C359S206100
Reexamination Certificate
active
06233081
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical scanning device, an image forming apparatus including the optical scanning device and an optical scanning method.
2. Description of the Related Art
An optical scanning device has been widely used in a laser printer, a facsimile machine, a digital copier and other such image forming apparatuses. Recently, image forming density has been increased, and high-density image forming in which the image density exceeds 1200 dpi is being used.
In order to achieve such high-density image formation, it is necessary to sufficiently reduce a diameter of a beam spot which scans a surface to be scanned. In order to obtain a small-diameter beam spot, it is necessary to reduce the wavelength of the light emitted by a light source, and, also, to increase the NA of an optical system.
Further, in order to cause the diameter of the beam spot to be stable and uniform size, it is necessary to achieve the optical system in which the allowable degree for depth of focus is high, which system tolerates possible ‘component allowance (the curvature radiuses, thickness, refractive indexes) deviations’ of optical elements, mounting errors, and environment variation such as variation in temperature/humidity. For this purpose, it is necessary to optimize not only the optical system on and after the optical deflector but also the optical system on the light-source side of the optical deflector.
SUMMARY OF THE INVENTION
Preferred embodiments of the present invention provide an optical scanning device that achieves a desired small-diameter beam spot, and produces a stable and uniform diameter of the beam spot despite component allowance deviations of optical elements, mounting errors, and environment variations, in an optical scanning device.
An optical scanning device according to a preferred embodiment of the present invention includes a light source, a coupling lens, an optical deflector, an aperture and a scanning image-forming optical system.
The light source to be used is one which emits a light flux, the central wavelength of which is smaller than about 680 nm.
The coupling lens couples the divergent light flux emitted by the light source to form a parallel light flux, a weakly convergent light flux or a weakly divergent light flux.
The optical deflector has a deflection reflective surface, and deflects the light flux from the coupling lens at a uniform angular velocity. As the optical deflector, a polygon mirror, a rotary two-surface mirror, a rotary single-surface mirror or the like may be used.
The aperture is provided between the light source and optical deflector, and is arranged to cut off a peripheral portion of the light flux so as to control a beam spot diameter formed on a surface to be scanned.
The scanning image-forming optical system converges the light flux deflected by the optical deflector to a beam spot on the surface to be scanned so as to scan the surface to be scanned at a uniform velocity.
NAO, &kgr;, &bgr;
0
, satisfy the following condition:
&bgr;
0
/(NAO•&kgr;)<100, (1)
where:
NAO represents the light-source-side numerical aperture of the coupling lens;
&kgr; represents the ratio A/D of the larger one A of the diameters, in a main scanning direction and a sub-scanning direction, of the opening of the aperture, relative to the diameter D of the light flux emitted from the coupling lens; and
&bgr;
0
represents the larger one of the lateral magnifications, in a main scanning direction and a sub-scanning direction, of the entire optical system with respect to an optical path extending from the light source to the surface to be scanned, which optical path reaches a point of the image surface, at which point the image height is 0.
Further, NAO satisfies the following condition:
0.18<NAO<0.35. (2)
In an optical scanning device according to another preferred embodiment of the present invention,
NAO, &kgr;, &bgr;
0
satisfy the following condition:
&bgr;
0
/(NAO•&kgr;)<100, (1)
and, also, &bgr;
0
satisfies the following condition:
&bgr;
0
<30. (3)
In an optical scanning device according to another preferred embodiment of the present invention,
NAO, &kgr;, &bgr;
0
satisfy the following condition:
&bgr;
0
/(NAO·&kgr;)<100, (1)
and, also, r. satisfies the following condition:
0.2<&kgr;<0.6. (4)
It is possible that the above-mentioned conditions (1), (2) and (3) are satisfied.
Further, it is possible that the above-mentioned conditions (1), (2) and (4) or (1), (3) and (4) are satisfied.
Further, it is possible that, in any of the above-mentioned arrangements, a line-image image-forming optical system having positive power only in the sub-scanning cross section is provided on an optical path extending between the light source and the optical deflector, and the aperture is placed on an optical path extending between the coupling lens and the line-image image-forming optical system.
As the line-image image-forming optical system, a positive cylinder lens or a concave cylinder mirror may be used.
It is possible that the scanning image-forming optical system includes image-forming lenses or an image-forming lens. However, it is not necessary to be limited to this. Alternatively, it is possible that the scanning image-forming system includes an image-forming mirror, or a combination of an image-forming mirror and an image-forming lens.
Assuming that the beam spot diameter co on the image surface (coincident with the surface to be scanned, ideally) in the optical scanning device is ‘1/e
2
diameter’ in a light intensity distribution, the beam spot diameter &ohgr; is expressed approximately as follows:
&ohgr;=0.82·&lgr;/NA≈(0.82·&lgr;·&bgr;
0
)/(NAO·&kgr;) (5)
where:
&lgr;: the central wavelength of the light emitted by the light source;
NA: the numerical aperture, on the emitting side (the scanned-surface side), of the coupling lens;
NAO: the numerical aperture, on the light-source-side, of the coupling lens;
&kgr;: the ratio A/D of the larger one A of the diameters, in the main scanning direction and the sub-scanning direction, of the opening of the aperture, to the diameter D of the light flux emitted from the coupling lens;
&bgr;
0
: the larger lateral magnification of magnifications in the main scanning direction and sub-scanning direction, of the entire optical system with respect to an optical path extending from the light source to the surface to be scanned, which optical path reaches a point of the image surface, at which point the image height is 0.
In order to obtain a small-diameter beam spot which satisfies high-density writing exceeding 1600 dpi, it is necessary to shorten the wavelength &lgr; to less than about 680 nm, and, also, optimize the magnification of the optical system, the numerical aperture of the coupling lens and the ratio of beam-spot-diameter controlled by the aperture, it is possible to obtain an advantageous optical system for obtaining a small-diameter beam spot. That is, when &bgr;
0
/(NAO•&kgr;)=100, (the maximum value) in the condition (1), the beam spot diameter co on the image surface is 55.76 &mgr;m by using the above-mentioned equation (5). Therefore, it can be said that, by satisfy the condition (1), it is possible to achieve a beam spot diameter smaller than about 55.76 &mgr;m. When the parameter in the condition (1) exceeds the maximum value 100 (that is, when&bgr;
0
/(NAO•&kgr;)>100), it becomes difficult to obtain a small-diameter beam spot which is necessary for high-density image formation.
It is preferable that the numerical aperture NAO, on the incident side of the coupling lens satisfies the condition (2). When the NAO exceeds the maximum value about 0.35, it is likely that the wavefront aberration of the coupling lens is degraded, and, thereby achievement of a small-diameter beam spot is prevented. On the other hand, when the NAO is smaller than about 0.18, it is not possible to efficiently draw the light flux from
Kouchiwa Taira
Suzuki Seizo
Takanashi Kenichi
Greenberg & Traurig, LLP
Ricoh & Company, Ltd.
Schuberg Darren
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