Optical: systems and elements – Lens – High distortion lens
Reexamination Certificate
2001-03-14
2002-05-14
Mack, Ricky (Department: 2873)
Optical: systems and elements
Lens
High distortion lens
C359S787000
Reexamination Certificate
active
06388817
ABSTRACT:
RELATED APPLICATIONS
This application claims the priority of Japanese Patent Application No. 2000-090803 filed on Mar. 29, 2000, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an f&thgr; lens used for scanning a light beam such as laser beam on an optical scanning surface at a constant speed in a laser printer or the like; and, in particular, to an f&thgr; lens for a scanning optical system using three laser beams having wavelengths different from one another.
2. Description of the Prior Art
Conventionally known is an optical beam scanning apparatus which records a color image onto a silver halide film by using three laser beams having wavelengths different from one another, e.g., wavelengths corresponding to R, G, and B for the silver halide film.
In such an optical beam scanning apparatus, an f&thgr; lens is disposed upstream its optical scanning surface such that the laser beams can be scanned on the film surface at a constant speed. In particular, those used for recording color images are required to correct chromatic aberrations concerning the three colors of laser beams.
Among the chromatic aberrations, axial chromatic aberration does not become a serious problem as long as it is not greater than a predetermined value, since it can be corrected upon adjusting at least one of pairs of collimating lenses
2
a
,
2
b
,
2
c
and cylindrical lenses
3
a
,
3
b
,
3
c
disposed between respective light sources
1
a
,
1
b
,
1
c
and a deflector
4
in an optical beam scanning apparatus shown in
FIGS. 9A and 9B
, for example.
As for lateral chromatic aberration, on the other hand, there has been known not only a technique in which respective light emission timings for different colors of laser beams are shifted from each other so as to correct the aberration, but also a technique in which an f&thgr; lens itself is used for correction.
Known as an example of technique in which an f&thgr; lens itself is used as such is one disclosed in Japanese Unexamined Patent Publication No. 7-191261 using a two-group, three-element lens.
In the technique disclosed in the above-mentioned publication, however, though lateral chromatic aberration is favorably corrected, no particular attention is paid to other aberrations, whereby the latter are not always made favorable. For example, letting the wavelength in use &lgr; be 680 nm, the spot diameter will be about 120 &mgr;m if the F-number of f&thgr; lens is 50. In this case, coma is 40 &mgr;m, thus reaching ⅓ of this spot diameter.
When correcting lateral chromatic aberration by employing the above-mentioned technique of shifting light emission timings of different colors of laser beams, the scanning position of a laser beam can substantially align with a given scanning position of a reference laser beam upon changing the light emission period. However, it requires complicated control and may be problematic in terms of cost.
Therefore, it is desirable that lateral chromatic aberration be initially made favorable to a practically permissible level by use of an f&thgr; lens, and then the above-mentioned electric correction technique be used when a finer image is required.
SUMMARY OF THE INVENTION
In view of such circumstances, it is an object of the present invention to provide a compact f&thgr; lens which can make lateral chromatic aberration favorable and suppress various aberrations such as coma to a low level.
The present invention provides an f&thgr; lens comprising, successively from a luminous flux entrance side, a first lens made of a negative lens having a flat surface on one side, a second lens made of a positive lens having a surface with a stronger curvature directed onto an image side, and a third lens made of a negative meniscus lens, the second and third lenses being cemented together;
the f&thgr; lens satisfying the following conditional expressions (1) to (3):
−2.2
<f
1
/f<−
1.7 (1)
0.35
<f
2
/f
(2)
2.21
<N
3
+0.0176&ngr;
3
<2.27 (3)
where
f is the composite focal length of the whole lens system;
f
1
is the focal length of first lens;
f
2
is the focal length of second lens;
N
3
is the refractive index of third lens at d-line; and
&ngr;
3
is the Abbe number of third lens at d-line.
Preferably, the first lens is a cylindrical lens.
Preferably, the cylindrical lens is configured such as to have a power only in a main scanning direction.
Preferably, the first lens is configured such that the flat surface is directed onto the luminous flux entrance side.
Due to the configuration mentioned above, lateral chromatic aberration and coma can be ameliorated by the lens having a relatively simple configuration.
Operations and effects obtained when the above-mentioned conditional expressions (1) to (3) are satisfied will now be explained.
The above-mentioned conditional expression (1) defines the focal length f
1
of first lens. Coma becomes greater if f
1
/f exceeds the upper limit of conditional expression (1), whereas the f&thgr; lens becomes greater if f
1
/f is less than the lower limit.
The above-mentioned conditional expression (2) defines the focal length f
2
of second lens. Coma becomes greater if f
2
/f is less than the lower limit of conditional expression (2).
The above-mentioned conditional expression (3) defines the refractive index N
3
and Abbe number &ngr;
3
of third lens at d-line. Lateral chromatic aberration becomes greater if the upper limit of conditional expression (3) is exceeded, whereas curvature of field becomes greater if the lower limit is not satisfied.
Since the f&thgr; lens of the present invention satisfies each of the above-mentioned conditional expressions (1), (2), and (3), lateral chromatic aberration, coma, and curvature of field become favorable at the same time, and the lens system can be made smaller.
Since one surface of the first lens is made flat, the f&thgr; lens can be made at a lower cost.
The first lens may be made as a cylindrical lens and may have a power only in a main scanning direction, whereby the f&thgr; lens can be made at a further lower cost.
In this case, if the surface of first lens on the luminous flux entrance side is made flat, it will be preferable in that it can restrain the light reflected by the surface of first lens on the luminous flux entrance side from returning to a polygon mirror and being reflected again so as to pass through the f&thgr; lens and form a ghost image on the image surface.
REFERENCES:
patent: 5694251 (1997-12-01), Shimada et al.
Japanese Patent Office, Japanese Unexamined Patent Publication No. 7-191261, Date of Publication: Jul. 28, 1995, Application No.: 6-11573, Filing Date: Feb. 3, 1994, pp. 1-27 (English Language Abstract).
Lester Evelyn A.
Mack Ricky
Snider Ronald R.
Snider & Associates
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