Optical: systems and elements – Lens – Single component with multiple elements
Reexamination Certificate
1999-09-17
2001-05-01
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Single component with multiple elements
C359S784000
Reexamination Certificate
active
06226132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an achromatic lens system which is used in objective lens elements of an astronomical telescope, in collimator lens elements, and the like.
2. Description of the Related Art
An achromatic lens system in which a positive lens element is provided between two negative lens elements has been well known. Further, in order to decrease the secondary spectrum of axial chromatic aberration, an achromatic lens system which employs a positive lens element made of fluorite has been widely used. However, fluorite is known as a material which is difficult to be machined, and is expensive. Furthermore, recent CCD cameras have sensitivity in the near-infrared range, while achromatic lens systems which use fluorite cause the over-correcting in the near-infrared range. As a result, suitable chromatic correction in the visible range and the near-infrared range cannot be simultaneously performed.
For example, in the prior art, when astronomical photography is performed with a CCD camera, an infrared-cut filter has to be used in order to cut light in the near-infrared range because chromatic correction by an objective lens system (achromatic lens system) of the CCD camera is insufficient in the near-infrared range though the CCD camera has sensitivity in the near-infrared range. If sensitivity of CCD cameras in the near-infrared range is utilized, clear celestial images can be photographed through image processing technologies. The same can be applied to surveillance cameras which utilize near-infrared light.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an achromatic lens system in which no fluorite is used, in particular, to provide a three-lens-element achromatic lens system which is constituted by a negative lens element, a positive lens element and a negative lens element, and can suitably correct chromatic aberration in the near-infrared range. Furthermore, it is another object of the present invention to provide an achromatic lens system which can perform chromatic correction in both the near-infrared range and the visible range in a well-balanced manner.
In order to attain the above mentioned objects, there is provided an achromatic lens system which is constituted by a negative single lens element, a positive single lens element, and a negative single lens element, in this order from the object, and satisfies the following condition:
|&thgr;
pt
−&thgr;
nt
|<0.02 (1)
wherein
&thgr;
pt
designates the partial dispersion ratio at the t-line, defined by (n
C
−n
t
)/(n
F
−n
C
) with respect to the positive single lens element;
&thgr;
nt
designates the average of the partial dispersion ratios at the t-line, defined by (n
C
−n
t
)/(n
F
−n
C
) with respect to the negative single lens elements;
n
C
designates the refractive index at the C-line;
n
t
designates the refractive index at the t-line; and
n
F
designates the refractive index at the F-line.
Furthermore, it is preferable that the achromatic lens system satisfy the following condition:
|&thgr;
pg
−&thgr;
ng
|<0.02 (2)
wherein
&thgr;
pg
designates the partial dispersion ratio at the g-line, defined by (n
g
−n
F
)/(n
F
−n
C
) with respect to the positive single lens element;
&thgr;
ng
designates the average of the partial dispersion ratios at the g-line, defined by (n
g
−n
F
)/(n
p
−n
C
) with respect to the negative single lens elements; and
n
g
designates the refractive index at the g-line.
Preferably, the positive single lens element is not made of fluorite, but of an optical glass material. In addition, from another aspect, it is preferable that the positive single lens element be made of a glass material which satisfies the following condition:
&ngr;
P
>80.0 (3)
wherein
&ngr;
P
designates the Abbe number of the positive single lens element.
More preferably, the positive single lens element is made of a glass material which satisfies the following condition:
&ngr;
P
>91.0 (4)
The present disclosure relates to subject matter contained in Japanese Patent Application No.10-265305 (filed on Sep. 18, 1998) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 3572902 (1971-03-01), Uetake
patent: 4190324 (1980-02-01), Arai
patent: 4761064 (1988-08-01), Mercado et al.
patent: 5568325 (1996-10-01), Hirano et al.
patent: 5587841 (1996-12-01), Ito
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Lester Evelyn A.
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