Objective optical system for an endoscope

• US 7,466,490 B2
• Filed: 09/13/2007
• Issued: 12/16/2008
• Est. Priority Date: 10/23/2006
• Status: Active Grant

• Current Assignee: Olympus Corporation
• Original Assignee: Olympus Medical Systems Corporation (Olympus Corporation)
• Inventors: Igarashi, Tsutomu
• Primary Examiner(s): Lester; Evelyn A.
• Application Number:
  US11/898,524
• Publication Number:
  US 20080180809A1
• Time in Patent Office: 460 Days
• Field of Search: 359643-645, 359656-661, 359/740, 359/781, 359/784
• US Class Current: 359/651
• CPC Class Codes: A61B 1/00188 with focusing or zooming features
• CPC Class Codes: G02B 23/2438 Zoom objectives

Abstract

An objective optical system is disclosed that is suitable for a high-precision endoscope that performs focusing by moving a single lens component. The objective optical system includes, in order from the object side, a front lens group, a middle lens group, and a rear lens group. The middle lens group is composed of two or more lens components, at least two of which have positive refractive power, and the lens component that is moved for focusing is in the middle lens group. Additionally, the objective optical system satisfies two specified conditions, and other conditions may apply.

Claims

1. An objective optical system for an endoscope having an object side and an image side and comprising, arranged along an optical axis in order from the object side, as follows:

a front lens group having negative refractive power and consisting of a first lens component;

a middle lens group having positive refractive power and including at least two lens components, each of said at least two lens components having positive refractive power; and

a rear lens group having positive refractive power and consisting of one lens component;

wherein

only one lens component of said middle lens group moves in the direction of the optical axis during focusing;

a stop for controlling image brightness is positioned within said middle lens group or between said front lens group and said middle lens group; and

the following conditions are satisfied;

2<

|f_UM/f_TF|<

10

−

1.25<

f_U1/f_TF<

−

0.6

where

f_UM is the focal length of said only one lens component;

f_TF is the focal length of the entire objective optical system for an endoscope focused at the far point on the object side; and

f_U1 is the focal length of said first lens component.

2. The objective optical system for an endoscope of claim 1, wherein said middle lens group is formed of at least three lens components.

3. The objective optical system for an endoscope of claim 2, wherein said middle lens group consists of, arranged in the following order from the object side, a second lens component having positive refractive power, a third lens component, and a fourth lens component having positive refractive power.

4. The objective optical system for an endoscope of claim 2, wherein said middle lens group consists of, arranged in the following order from the object side, a second lens component having positive refractive power, a third lens component, a fourth lens component having positive refractive power, and a fifth lens component having positive refractive power.

5. The objective optical system for an endoscope of claim 1, wherein said middle lens group consists of, arranged in the following order from the object side, a second lens component having positive refractive power, said stop, and a third lens component having positive refractive power.

6. The objective optical system for an endoscope of claim 1, wherein the following condition is satisfied:

0.85<

f_TN/f_TF<

1.15

where

f_TN is the focal length of the entire objective optical system for an endoscope focused at the near point on the object side.

7. The objective optical system for an endoscope of claim 1, wherein the following condition is satisfied:

0.85<

f_TN/f_TF<

1

where

f_TN is the focal length of the entire objective optical system for an endoscope focused at the near point on the object side.

8. The objective optical system for an endoscope of claim 1, wherein the following conditions are satisfied:

n_U1>

1.7

v_U1>

38

where

n_U1 is the refractive index at the d-line of said first lens component; and

v_U1 is the Abbe number at the d-line of said first lens component.

9. The objective optical system for an endoscope of claim 1, wherein said first lens component consists of a plano-concave lens element formed of sapphire material.

10. The objective optical system for an endoscope of claim 1, wherein the following condition is satisfied:

|f_U(GM,GR)|≧

1.4f_TF

where

f_U(GM,GR) is the focal length of each of the lens components of each of said middle lens group and of said rear lens group.

11. The objective optical system for an endoscope of claim 1, wherein the following condition is satisfied:

|R(GM,GR)|>

0.8f_TF

where

R(GM,GR) is the radius of curvature of each lens surface of each lens element of each of the lens components of each of said middle lens group and of said rear lens group.

12. The objective optical system for an endoscope of claim 1, wherein said rear lens group consists of a cemented lens component formed of a lens element having negative refractive power and a lens element having positive refractive power that satisfy the following conditions:

n(GR_n)>

1.82

v(GR_n)<

26

n(GR_p)<

1.78

v(GR_p)>

49

where

n(GR_n) is the refractive index at the d-line of said lens element having negative refractive power;

v(GR_n) is the Abbe number at the d-line of said lens element having negative refractive power;

n(GR_p) is the refractive index at the d-line of said lens element having positive refractive power; and

v(GR_p) is the Abbe number at the d-line of said lens element having positive refractive power.

13. The objective optical system for an endoscope of claim 1, wherein the following condition is satisfied:

2<

f_UR/f_TF<

12

where

f_UR is the focal length of the lens component of said rear lens group.

14. The objective optical system for an endoscope of claim 1, wherein each of at least two lens components consists of a lens element having negative refractive power that is cemented to a lens element having positive refractive power and having a refractive index that is at least 0.1 less than the refractive index of said lens element having negative refractive power.

15. The objective optical system for an endoscope of claim 1, wherein said only one lens component consists of a single lens element having positive refractive power and a refractive index of 1.75 or less.

16. The objective optical system for an endoscope of claim 1, wherein said only one lens component has positive refractive power and consists of a lens element having negative refractive power and a lens element having positive refractive power, with the lens element having positive refractive power having a refractive index that is at least 0.1 less than the refractive index of said lens element having negative refractive power.

17. The objective optical system for an endoscope of claim 1, wherein said only one lens component is adjacent said stop.

18. The objective optical system for an endoscope of claim 1, wherein the one lens component of the middle lens group that moves in the direction of the optical axis during focusing is other than the object-side lens component or the image-side lens component of said middle lens group.

19. The objective optical system for an endoscope of claim 1, wherein each of the object-side lens component of said middle lens group and the image-side lens component of said middle lens group has positive refractive power.