Endoscope objective lens
View Patent ↗An endoscope objective lens with a three-group, three lens element construction, is formed of, in order from the object side, a first lens element having either a negative meniscus shape or a plano-concave shape with its concave surface on the image side, a stop, a positive second lens element having a plano-convex shape with its convex surface on the image side, and a positive third lens element having a plano-convex shape with its convex surface on the object side. The stop is positioned on or in contact with the object-side surface of the second lens element so that the endoscope objective lens is substantially telecentric. Specified conditions are satisfied in order to reduce aberrations and make the manufacture of the endoscope objective lens easier and less expensive.
1. An endoscope objective lens having a three-group, three-lens element construction, comprising, in order from the object side:
a first lens element of negative refractive power and either a meniscus or a plano-concave shape with its concave surface on the image side;
a stop;
a second lens element of positive refractive power and a plano-convex shape with its convex surface on the image side;
a third lens element of positive refractive power and a plano-convex shape with its convex surface on the object side;
wherein
the image-side surface of the third lens element makes contact with an end surface of an optical fiber bundle, a surface of an image detector, or a cover glass for an image detector;
the stop is positioned on or in contact with the object-side surface of the second lens element; and
the following conditions are satisfied
2.00<| f 1/ f|< 3.00
2.50<| f 1/ D 2|<7.50
| D 3/ R 4|<1.00
where,
f 1 is the focal length of the first lens element,
f is the focal length of the endoscope objective lens,
D 2 is the on-axis spacing between the first lens element and the second lens element,
D 3 is the center thickness of the second lens element, and
R 4 is the radius of curvature of the image-side surface of the second lens element.
2. The endoscope objective lens according to claim 1 , wherein the following condition is satisfied:
nd 1>1.80
where
nd 1 is the refractive index at the d-line of the first lens element.
3. The endoscope objective lens according to claim 1 , wherein the first lens element is formed by a molding process.
4. The endoscope objective lens according to claim 2 , wherein the first lens element is formed by a molding process.
5. The endoscope objective lens according to claim 1 , wherein the second and third lens elements are formed by a grinding process.
6. The endoscope objective lens according to claim 2 , wherein the second and third lens elements are formed by a grinding process.
7. The endoscope objective lens according to claim 3 , wherein the second and third lens elements are formed by a grinding process.
8. The endoscope objective lens according to claim 4 , wherein the second and third lens elements are formed by a grinding process.