Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-07-26
2004-02-24
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S235000, C359S206100, C359S662000
Reexamination Certificate
active
06696681
ABSTRACT:
BACKGROUND ARTS
The present invention relates to an f-&thgr; lens that focuses light beams on a plane and scans the light beams across the scanning plane at substantially constant speed. The present invention relates also to an beam scanning device using the f-&thgr; lens and a color imaging apparatus using the beam scanning device.
The beam scanning device is provided with a light source generating light beams, and an optical deflection device, like a polygonal mirror, that deflects the light beams in a constant direction. The deflected light beams are projected onto a plane through the f-&thgr; lens, to sweep or scan the plane in a constant direction. The f-&thgr; lens is a lens that has a property to make the light beams move on the scanning plane proportionally to a deflection angle &thgr; of the optical deflection device. In the beam scanning device, the f-&thgr; lens is used for making the scanning speed of the light beams on the scanning plane proportional to the deflection speed (the speed per deflection angle of the optical deflection device). Because the scanning plane is held flat in the deflecting or scanning direction, the scanning speed would not be proportional to the deflection speed without the f-&thgr; lens.
The f-&thgr; lens has usually been composed of two or three spherical lens elements, or one or two aspherical lens elements. In the f-&thgr; lens, the light beams pass through each spherical lenses merely in a narrow central zone extending in the deflecting direction, so other peripheral portions of the spherical lenses are cut away, to shape the spherical lenses into strips when used as the components of the f-&thgr; lens.
However, cutting the spherical lens into a strip requires is labor-consuming, because it is necessary to locate the cutting positions very strictly. In addition, since the peripheral portions of the spherical lens are thrown way uselessly, it is a waste of material, and the material cost are pretty large. It is possible to use the whole spherical lenses in the f-&thgr; lens, in order to omit the labor-consuming cutting process. But it is still a waste of lens material, as there remain unused portions in those spherical lenses. Moreover, as compared to the strip-shaped spherical lenses, the whole ones obviously require a larger space, and thus enlarges the apparatus size.
Meanwhile, in the color imaging apparatus where a plurality of light beams having different wavelengths are scanned across a photosensitive material at the same time by the beam scanning device, the scanning lengths of the light beams on the photosensitive material can differ from each other because of chromatic aberration of the f-&thgr; lens. In that case, recorded images will suffer color failures. As well-known in the art, the chromatic aberration is resulted from the fact that the refractive index through a glass or a lens varies depending upon the wavelength.
To prevent the color failure, optical correction of the chromatic aberration of the f-&thgr; lens has usually been made. For correction of the chromatic aberration, selection of glass materials for the lens elements of the f-&thgr; lens is important. Usually, lenses made of crown glass and lenses made of flint glass are usually combined in order to correct the chromatic aberration. For better correction, however, the sorts of glass materials for the f-&thgr; lens are limited to expensive ones, and also the number of lens elements must be increased. Thus, the f-&thgr; lens becomes expensive.
Indeed Japanese Laid-open Patent Application No. 6-18803 discloses an f-&thgr; lens using a cylindrical lens as one component, but the other component of this prior art is a spherical lens, so it needs a relatively high manufacture cost. Although there has been disclosed an f-&thgr; lens system whose components are all cylindrical lenses in Japanese Laid-open Patent Application No. 3-130717, the f-&thgr; lens of this prior art has a problem that sufficient lens performance cannot always be achieved because of tolerance in manufacture. To avoid this problem, this prior art suggests replacing one of the cylindrical lenses of the f-&thgr; lens with a spherical lens. However, this solution results in rising the manufacture cost.
SUMMARY OF THE INVENTION
In view of the foregoing, the present invention has an object to provide an f-&thgr; lens that has superior f-&thgr; characteristics but may be produced at a low cost, on the assumption that the f-&thgr; lens is placed between an optical deflection device that deflects light beams at an approximately constant angular speed in a deflecting direction, and a scanning plane, for converging the deflected light beams on the scanning plane such that the light beams scan the scanning plane at an approximately constant speed in a scanning direction, wherein the scanning plane is flat in the scanning direction of the light beams.
Another object of the present invention is to provide a beam scanning device and a color imaging apparatus, using the f-&thgr; lens of the present invention.
According to the present invention, the f-&thgr; lens is composed of a first lens element having a negative power of refraction, a second lens element having a positive power of refraction and a third lens element having a positive power of refraction, arranged in this order from the side of the optical deflection device, wherein at least one surface of the lens elements is a cylindrical surface that has a refractive power only in the deflecting direction.
The f-&thgr; lens of the present invention uses a cylindrical lens having a curvature only in the deflecting direction of the light beams. Such a cylindrical lens may be manufactured by forming a primary cylindrical lens whose cylindrical surface has a pretty large length in a perpendicular direction to the direction of curvature, and then cutting the primary cylindrical lens along the direction of curvature into a number of lens pieces of a constant width or height. Every one of subsequent cut pieces is usable as a component of the f-&thgr; lens, so any waste is not produced from the primary cylindrical lens. Therefore, the cylindrical lenses used in the f-&thgr; lens of the present invention may be manufactured at a low cost, so the manufacture cost of the f-&thgr; lens of the present invention is low in comparison with the conventional f-&thgr; lens using those lens elements which are manufactured by cutting round lenses into strips. It is to be noted that cutting process of the primary cylindrical lens may be done at any stage of manufacturing the individual lens elements, e.g. before finishing the surface of the primary cylindrical lens.
According to a preferred embodiment, the f-&thgr; lens satisfies the conditions: N
1
>N
2
=N
3
, v
1
<v
2
=v
3
, and 4 mm≦|f
23
/f
1
·DO|≦16 mm, wherein N
1
, N
2
and N
3
represent refractive indexes, v
1
, v
2
and v
3
Abbe represent constants of the first to third lens elements respectively, fi represents the focal length of the first lens element, f
23
represents the composite focal length of the second and third lens elements, and DO a distance from the optical deflection device to the lens surface of the first lens element.
This configuration is effective to prevent curvature of the field of the f-&thgr; lens from being worsened, and maintain good speed f-&thgr; characteristics. It is to be noted that the speed f-&thgr; characteristics indicate the proportionality of the scanning speed of the light beams on the scanning plane to the angular speed of deflection. That is, with good f-&thgr; characteristics, the scanning speed of the light beams is maintained constant through the entire scanning length.
According to another preferred embodiment, at least one of the three lens elements is inclined about a parallel axis to the deflecting direction. This configuration is effective to prevent interference caused by multiple reflection of light beams between the lens surfaces. Especially by inclining the first lens element, those light beams multiple-reflected between the op
Fuji Photo Film Co. , Ltd.
Lee Patrick J.
Porta David
LandOfFree
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