Endoscopic objective lens

Details Endoscopic objective lens Endoscopic objective lens

1999-10-07

2001-06-26

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With graded refractive index

C359S719000

Reexamination Certificate

active

06252722

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an objective lens to be fitted at the distal end of an endoscope, more particularly, to a wide-angle endoscopic objective lens that can provide a view angle (a range in which an image can be focussed by the lens) greater than 50°.
Endoscopes are so constructed that a real image created with an objective lens fitted at the distal end is transmitted through image transmission optics to a position convenient for observation. A suitable image transmission optics is selected depending on use from among various types of fiber bundle and relay optics consisting of an ordinary lens and a gradient index lens.
From the viewpoint of use in connection with the endoscope, the objective lens at the distal end of the endoscope is required to be small in diameter and yet have an ability of forming an in-focused image over a wide view angle. An optics comprising a plurality of spherical lenses is capable of providing a wide visual field while effectively correcting various aberrations. However, the use of a plurality of spherical lenses having an outer diameter smaller than 1 mm results in extremely increased cost associated with difficulty in lens polishing, assembling and adjusting operations.
To overcome this problem, a rod lens having a gradient refractive index in a radial direction is proposed as a low-cost, small-diameter objective lens. The rod lens can be manufactured easily and at low cost by ion-exchange and other techniques to have a diameter not larger than 1 mm. Further, since the rod lens is planar on both sides, the rod lens offers the advantage of great simplicity in polishing both end faces, assembling into an endoscope and achieving alignment of optical axes.
The refractive index of a rod-shaped, gradient index lens is expressed as
n
(
r
)
2
=n
o
2
·{1−(
g·r
)
2
+h
4
(
g·r
)
4
+h
6
(
g·r)
6
+h
8
(
g·r
)
8
+ . . . }
where
r: the distance from the optical axis
n(r): the refractive index at distance r from the optical axis
n
o
: the refractive index on the optical axis
r
0
: the effective radius of the gradient index lens
g: the gradient index coefficient (2nd order)
h
4
, h
6
, h
8
, . . . : the gradient index coefficient (higher order).
The peripheral portion of the gradient index lens usually has a great departure from the design value of gradient refractive index, and thus cannot be substantively used as a lens. In some cases, the peripheral portion of the lens is made opaque in order to prevent the stray light due to the reflection from the lateral surfaces of the lens. Hence, the range of the gradient index lens that has a sufficiently good gradient reflective index to contribute to the formation of an in-focused image is defined as the effective radius r
o
. The effective radius r
o
of a rod lens is not necessarily the same as its apparent radius.
If the view angle of a rod-shaped, gradient index lens used as an objective lens is written as &thgr;, the following relation holds
&thgr;=
n
o
·g·r
o
(rad)
Note that the objective lens designated later as Comparative Example (see
FIG. 17
) has an view angle &thgr; of 38.7°.
If an objective lens having a wide visual field is used with an endoscope, the subject can be seen through a correspondingly wide range, facilitating various operations with the endoscope. In order to increase the view angle &thgr; of the rod-shaped, gradient index lens, the value of n
o
·g·r
o
, hence, the difference in refractive index between the center and the periphery of the lens, must be increased. However, it is generally held that the maximum value of n
o
g·r
o
for the rod-shaped, gradient index lens that can be easily attained by the ordinary ion-exchange technology in current use is no more than about 0.70. In other words, a wide-angle rod-shaped, gradient index lens having a view angle &thgr; in excess of 0.7 rad (i.e. about 40°) is considerably difficult or troublesome to manufacture.
SUMMARY OF THE INVENTION
An object, therefore, of the present invention is to solve the aforementioned problem of the prior art by providing a wide-angle endoscopic objective lens that can be manufactured easily at low cost.
The endoscopic objective lens of the invention comprises the combination of a single homogeneous planoconvex lens having a homogeneous refractive index and a single gradient index lens having a gradient refractive index in a radial direction thereof, wherein
(1) an object plane, the planoconvex lens, the gradient index lens and an image plane are arranged in that order along an optical axis;
(2) both surfaces of the gradient index lens are planar and its gradient refractive index is expressed as
n
(
r
)
2
=n
o
2
{1−(
g·r
)
2
+h
4
(
g·r
)
4
+h
6
(
g·r
)
6
+h
8
(
g·r
)
8
+ . . . }
where
1.45≦n
o
≦1.90
0.45≦n
o
·g·r
o
≦0.90
provided that
r: a distance from the optical axis
n(r) a refractive index at distance r from the optical axis
n
o
: a refractive index on the optical axis
r
o
: an effective radius of the gradient index lens
g: a gradient index coefficient (2nd order)
h
4
, h
6
, h
8
, . . . : a gradient index coefficient (higher order)
(3) the planoconvex lens has its convex surface positioned closer to the gradient index lens and its planar surface closer to an object, with its refractive index n and radius of curvature R of the convex surface satisfying
1.45≦n≦4.00
0.8r
o
≦R≦3.0r
o
and the image plane being positioned either at or near an end face of the gradient index lens.
As used in the specification, the “effective radius r
o
of the gradient index lens” means the radius of that portion of the lens which effectively works as a focusing lens. This may be defined as the “range in which the RMS wave front aberration on the optical axis is no more than 0.07&lgr;”. The effective radius r
o
of the gradient index lens affects its refractive power and hence serves as a numerical reference in design. In fact, there are many cases that the peripheral portion of a manufactured gradient index lens is widely offset from the design value of gradient refractive index and fails to provide the intended lens action. Take, for example, a gradient index lens has a nominal outside diameter of 1 mm, but the portion of the lens, which provides the intended lens action, is 0.8 mm in diameter. In this case, the effective radius r
o
is 0.4 mm. According to the present invention, a rod-shaped, gradient index lens is combined with a planoconvex lens having a homogeneous refractive index to provide a view angle greater than 50°.
FIG. 1
shows the optical path of the endoscopic objective lens of the invention. The planoconvex lens indicated by
10
is provided in front of the rod-shaped, gradient index lens
12
to reduce the angle of oblique incident light before said light is launched into the rod-shaped, gradient index lens
12
. An aperture diaphragm
14
is provided ahead of the planoconvex lens
10
to eliminate the peripheral light apart from the optical axis. The gradient index lens
12
forms a real image at an end face
12
a
and the real image thus formed is transmitted through a fiber bundle
16
or other image transmitting optics to a point convenient for observation.
FIG. 2
shows the optical path of a comparative case solely comprised of the gradient index lens
12
. The objective lens of the invention uses the gradient index lens
12
having the same value of n
o
·g·r
o
as the comparative case, and yet it provides a far wider view angle.
The gradient index lens used in the invention is so designed that the its on-axis refractive index n
o
is between 1.45 (inclusive) and 1.90 (inclusive) and this defines the range over which the lens can be practically manufactured by ion-exchange and other techniques. The value of n
o
·g·r
o
which corresponds to refractive power is adjusted to lie between 0.45 (inclusive) and 0.90 (inclusive). Below the lower limit 0.45, the obtained view angle is too narrow. As already mentioned, the upper limit that can be readily attained by

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