Objective lens for endoscope

Details Objective lens for endoscope Objective lens for endoscope

1999-10-29

2001-01-30

Sugarman, Scott J. (Department: 2873)

Optical: systems and elements

Lens

Microscope objective

C359S558000, C359S660000

Reexamination Certificate

active

06181481

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens disposed at a front end portion of an endoscope; and, in particular, to an objective lens for an endoscope, which matches a high-resolution imaging device used for transmitting image information.
2. Description of the Prior Art
In general, a lens having a wide angle of view and a long back focus is required as an objective lens for an endoscope. Also, for carrying out highly accurate viewing and precise diagnosis, it is important to correct chromatic aberration. For example, an objective lens for an endoscope has already been disclosed in commonly-assigned Japanese Unexamined Patent Publication No. 2-188709 as one satisfying the requirements mentioned above.
Recently, however, as CCD devices and image fibers used for image transmission have been attaining higher resolutions, lenses with higher resolutions have been demanded. In view of a higher resolution in the device used for image transmission, the endoscope objective lens disclosed in Japanese Unexamined Patent Publication No. 2-188709 needs to further improve its chromatic aberration in magnification.
On the other hand, there has been known an objective lens using an optical system equipped with a diffraction optical surface (hereinafter referred to as “diffraction optical element”) as means for correcting chromatic aberration. The diffraction optical element has an Abbe number of &ngr;
d
=−3.45, whereby its dispersion is greater than that of a normal glass lens by one digit while having an opposite polarity. Hence, it is advantageous in that chromatic aberration can effectively be corrected to an extent which is unpredictable from the normal lens. An example of such an endoscope objective lens is disclosed in Japanese Unexamined Patent Publication No. 10-197806.
The endoscope objective lens disclosed in Japanese Unexamined Patent Publication No. 10-197806, however, may be disadvantageous in that diffracted light of different orders forming unnecessary light in terms of design generated due to the use of a diffraction optical element, flare light occurring due to errors in manufacture or the like, and so forth affect the achievement of a higher resolution.
SUMMARY OF THE INVENTION
In view of such circumstances, it is an object of the present invention to provide an endoscope objective lens which, while favorably correcting aberrations, such as chromatic aberration in magnification in particular, by using a diffraction optical element, reduces light which may affect the achievement of such a higher resolution.
In one aspect, the endoscope objective lens in accordance with the present invention comprises, successively from an object side, a first lens group having at least one lens with a negative refracting power, a stop, and a second lens group having a positive refracting power, the first lens group including at least one lens having a diffraction optical surface, the diffraction optical surface satisfying the following conditional expression (1):
E>0  (1)
where
E is the coefficient of the second-order term of the phase difference function of the diffraction optical surface.
Preferably, in the endoscope objective lens in accordance with the present invention, the first lens group includes at least one lens with a negative refracting power having a diffraction optical surface, whereas a lens having the strongest negative refracting power as the lens having the diffraction optical surface in the first lens group satisfies the following conditional expression (2):
−2.0<
f
N
/f<−
0.3  (2)
where
f
N
is the focal length of the lens having the diffraction optical surface and the strongest negative refracting power in the first lens group; and
f is the focal length of the whole lens system.
Preferably, in the endoscope objective lens in accordance with the present invention, the first lens group includes at least one lens with a negative refracting power having a diffraction optical surface, whereas a lens having the strongest negative refracting power as the lens having the diffraction optical surface in the first lens group satisfies the following conditional expression (3):
f
N
/f<−
100  (3)
where
f
N
is the focal length of the lens having the diffraction optical surface and the strongest negative refracting power in the first lens group; and
f is the focal length of the whole lens system.
In another aspect, the endoscope objective lens in accordance with the present invention comprises, successively from an object side, a first lens group having at least one lens with a negative refracting power, a stop, and a second lens group having a positive refracting power, the first lens group including at least one lens with a positive refracting power having a diffraction optical surface, a lens having the strongest positive refracting power as the lens having the diffraction optical surface in the first lens group satisfying the following conditional expressions (4) and (5):
E>0  (4)
1.0<
f
P
/f
  (5)
where
E is the coefficient of the second-order term of the phase difference function of the diffraction optical surface;
f
P
is the focal length of the lens having the diffraction optical surface and the strongest positive refracting power in the first lens group; and
f is the focal length of the whole lens system.
Here, “phase difference function of the diffraction optical surface” is one represented by following expression.
&PHgr;(
Y
)=
EY
2
+FY
4
+GY
6
+HY
8
+IY
10


REFERENCES:
patent: 5841586 (1998-11-01), Nagaoka et al.
patent: 5999327 (1999-12-01), Nagaoka
patent: 5999334 (1999-12-01), Kohno
patent: 02188709 (1990-07-01), None
patent: 10197806 (1998-07-01), None

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