JPH10260347A - Objective for endoscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To insert and arrange an optical path converting prism between a solid-state image pickup element and the objective by composing a front-group diverging lens system of a 1st lens and a rear-group converging lens system of a 2nd lens and a cemented lens of a 3rd and a 4th lens. SOLUTION: The 1st lens L1 which has a surface with a small radius of curvature on its image plane side and also has negative refracting power, the 2nd lens L2 which has a surface with a small radius of curvature on its image plane side and also has positive refracting power, the 3rd lens L3 consisting of a convex lens, and the 4th lens L4 consisting of a concave lens are arranged in order from the object side, and the 3rd and 4th lenses L5 and L4 are constituted as the cemented lens. The 1st lens L1 constitutes the front-group diverging lens and the 2nd to 4th lenses L2 to L4 constitute the rear-group converging lens. Then the objective satisfies dx>=3.0×|f1 |. Here, f1 is the focal length of the 1st lens L1 and dx is the interval between the rear-side principal point of the 1st lens L1 and the front-side principal point of the rear-group converging lens system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens provided at the distal end of an endoscope, and more particularly to an objective lens for an endoscope in which an optical path conversion prism is disposed on the image plane side.

[0002]

2. Description of the Related Art Many direct-viewing type endoscopes using a solid-state imaging device have the above-mentioned solid-state imaging device inserted in the axial direction at the distal end of the endoscope. In such an endoscope, generally, an optical path conversion prism is inserted and arranged between the objective lens and the solid-state imaging device.
Since the size of the optical path conversion prism is determined by the image size, it is necessary to ensure a sufficient distance from the final surface of the objective lens to the image forming position where the optical path conversion prism is inserted, that is, a sufficient back focus.

Further, although the image size has been reduced by downsizing the solid-state image pickup device, flare and ghost may occur unless a sufficient margin is provided between the prism wall surface and the effective light beam. In consideration of the processing accuracy and assembly accuracy of parts, the distance cannot be extremely reduced, and it is difficult to reduce the prism size in proportion to the image size. Therefore, an objective lens having a longer back focus than the focal length is required.

[0004]

However, with the demand for wide-angle objective lenses for endoscopes, the focal length tends to be short even if the image size is the same, and it is difficult to obtain a sufficient back focus. It was difficult. In this regard, the present applicant also proposes a lens system having a long back focus in Japanese Patent Application No. 7-117204, but the back focus is twice or more and three times as large as the total focal length of the entire system. It was up to the extent.

The present invention has been made in view of such circumstances, and an optical path changing prism is inserted and arranged between a solid-state imaging device and an objective lens arranged in an axial direction at a distal end portion of an endoscope. It is an object of the present invention to provide an endoscope objective lens having a sufficient length of back focus.

[0006]

SUMMARY OF THE INVENTION An objective lens for an endoscope according to the present invention is an objective lens for an endoscope comprising a front group diverging lens system and a rear group focusing lens system sandwiching a brightness stop. Wherein the front group diverging lens system includes a first lens having a negative refractive power with a surface having a small radius of curvature facing the image plane side, and the rear group converging lens system includes, in order from the object side, a lens having a radius of curvature. A second lens having a positive refractive power with the small surface facing the image plane side
A third lens and a fourth lens each including a convex lens and one including a concave lens;
It is characterized in that a cemented lens with a lens is provided and is configured to satisfy the following conditional expression. dx ≧ 3.0 × | f ₁ | where f ₁ : focal length of the first lens dx: distance between the rear principal point of the first lens and the front principal point of the rear group focusing lens system

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to Examples 1 to 5 and a conventional example. <Embodiment 1> FIG. 1 is a view showing a configuration of an endoscope objective lens according to the present embodiment. As shown in the figure, this objective lens has a three-group, four-lens configuration. That is, in order from the object side, the first lens L ₁ having a negative refractive power with the surface having a small radius of curvature facing the image surface side, and the first lens L ₁ having the positive refractive power with the surface having a small radius of curvature facing the image surface. 2 lenses L ₂ ,
The third lens L ₃ composed of a convex lens, and becomes a fourth lens L ₄ is arranged consisting of a concave lens, the third lens L ₃
The fourth lens L ₄ is configured as a cemented lens.

A brightness stop 1 (only a position on the optical axis X is shown in the figure) is disposed in the vicinity of the object side of the second lens L ₂ . lens L ₁ is constitutes a front-group concave lens system, the second, third,
The fourth lenses L ₂ , L ₃ and L ₄ constitute a rear group focusing lens system. Then, on the image side of the group focusing lens system,
An optical path conversion prism 2 is provided, and a solid-state image sensor (CC) is provided near the image plane side of the optical path conversion prism 2.
D) 3 is arranged.

The objective lens is configured to satisfy the following conditional expression. dx ≧ 3.0 × | f ₁ |, however, f _1: the first lens L ₁ of focal length dx: distance between the front principal point of the first rear principal point of the lens L ₁ and the rear crowd bundle lens system

The basis for setting the above conditional expression is as follows. That is, in the two thin lenses, if one focal length is f ₁ , the other is f ₂ , and the distance between the two lenses is dx, the composite focal length f of the two lenses is 1 / f = 1 / f ₁ + 1 / f ₂ -dx / (f ₁ · f ₂ ). The back focus Bf is represented by Bf = f (1−dx / f ₁ ). Here, assuming that the back focus Bf is three times the combined focal length f, Bf = 3f, dx = −2f ₁ is obtained from 3f = f (1−dx / f ₁ ).

In this embodiment, since f ₁ <0 and f ₂ > 0, when dx ≧ 2 | f ₁ |, Bf ≧ 3f is realized.

The above is the case of a thin lens. In an actual optical system, the rear principal point position of the combination of the rear group focusing lens system is
Since the lens is almost as much as the combined focal length of the entire system, the value considering the principal point position and the back focus is (Bf) ', and (Bf)' = 4f is replaced with the above equation Bf = 3f. , A conditional expression of dx ≧ 3 | f ₁ | is obtained.

By satisfying this conditional expression, it is possible to secure a long back focus which is at least three times the combined focal length of the entire objective lens system. Thereby, the optical path conversion prism 2 is placed between the solid-state imaging device 3 and the objective lens arranged in the axial direction at the distal end of the endoscope.
, The subject image can be formed on the solid-state imaging device 3.

Next, in this embodiment, the radius of curvature r (mm) of each lens surface, the distance between the axial top surfaces of each lens (the center thickness of each lens and the air distance between each lens) d (mm), the d of each lens Table 1 shows the refractive index n _d and Abbe number ν _d of the line. However, in Table 1, the numbers corresponding to the respective symbols are sequentially increased from the object side, and the numerical values of r and d are standardized when the focal length is 1 mm.

[0015]

TABLE 1 _{r d n d ν d 1 ∞} 0.4117 1.88300 41.0 2 0.7959 1.5342 3 ( stop) 0.0549 4 ∞ 1.1155 1.71301 53.9 5 -2. 1182 0.1372 6 17.8650 1.2350 1.71301 53.9 7 -1.0676 0.4803 1.84666 23.8 8 -2.3608 0.6885 9 ４ 4.2628 1.51680 64.2 10 ∞

Next, the image size, subject distance, angle of view, back focus B obtained by the objective lens
f, the focal length f ₁ of the first lens L _1, rear principal point position of the first lens L _1, the front principal point position of the rear crowd bundle lens system, and a rear principal point position of the rear crowd bundle lens system, It is shown in Table 2.

[0017]

TABLE 2 Image size φ1.800mm object distance 8.2333mm angle 120 ° 50 'Bf = 3.386f focal length of the first lens L ₁ f _1: -0.9013 rear principal point of the first lens L ₁ Position: 0.0 Front principal point position of rear group focusing lens system: 1.2576 Rear principal point position of rear group focusing lens system: -0.7717

When dx is calculated from the above, dx = 0.0000 + 1.5342 + 0.0549 + 1.2576 = 2.8467 = 3.158 | f ₁ |, which satisfies the conditional expression dx ≧ 3.0 × | f ₁ | It is clear that

FIG. 2 is an aberration diagram showing various aberrations of the endoscope objective lens according to the present embodiment. Note that this aberration diagram is
Effective Fno. Object height 60%, 80%, 10 at 5.60
It shows a comparative aberration amount. As is clear from this figure, according to the present embodiment, it is possible to obtain an endoscope objective lens having good imaging performance up to the periphery of the visual field.

<Embodiment 2> FIG. 3 is a view showing a configuration of an endoscope objective lens according to this embodiment. As shown,
This objective lens has a lens configuration substantially similar to that of the first embodiment. Radius of curvature r of each lens surface in the present embodiment
(Mm), the axial distance d (mm) of each lens, the refractive index n _d of each lens at d-line, and the Abbe number ν _d are shown in Table 3.
Shown in The display method is the same as in the first embodiment.

[0021]

TABLE 3 _{r d n d ν d 1 -13.4697} 0.4041 1.88300 41.0 2 0.7770 1.4816 3 ( stop) 0.0539 4 -10.7757 1.0512 1.71301 53 0.95 -1.7152 0.1347 6 11.8525 1.2123 1.71301 53.9 7 -1.2266 0.4714 1.86666 23.8 8 -2.9232 0.81139． 4.3171 1.55920 53.9 10}

The image size, subject distance, angle of view, back focus B obtained by the objective lens
f, the focal length f ₁ of the first lens L _1, rear principal point position of the first lens L _1, the front principal point position of the rear crowd bundle lens system, and a rear principal point position of the rear crowd bundle lens system, It is shown in Table 4.

[0023]

Table 4 Image size φ1.740mm object distance 8.0818mm angle 119 ° 15 'Bf = 3.465f focal length of the first lens L ₁ f _1: -0.8211 rear principal point of the first lens L ₁ Position: 0.0116 Front principal point position of rear group focusing lens system: 1.1024 Rear principal point position of rear group focusing lens system: -0.7620

When dx is calculated from the above, dx = 0.116 + 1.4816 + 0.0539 + 1.1024 = 2.6495 = 3.227 | f ₁ |, which satisfies the conditional expression dx ≧ 3.0 × | f ₁ | It is clear that

FIG. 4 is an aberration diagram showing various aberrations of the endoscope objective lens according to the present embodiment. The display method is the same as in the first embodiment. As is clear from this figure, according to the present embodiment, it is possible to obtain an endoscope objective lens having good imaging performance up to the periphery of the visual field.

<Embodiment 3> FIG. 5 is a view showing the configuration of an endoscope objective lens according to this embodiment. As shown,
The objective lens is are substantially the same lens configuration as in Example 1, the third lens L ₃ constituting the cemented lens of the rear crowd bundle lens system becomes a concave lens, the fourth lens L ₄ is a convex lens This is different from the first embodiment in the point. Table 5 shows the radius of curvature r (mm) of each lens surface, the axial distance d (mm) of each lens, the refractive index n _{d at} the d-line of each lens, and the Abbe number ν _d in this example. The display method is the same as in the first embodiment.

[0027]

TABLE 5 _{1 r d n d ν d -3.6588} 0.3805 1.88300 41.0 2 0.9112 1.4318 3 ( stop) 0.0507 4 ∞ 1.1673 1.71301 53.9 5 -1.9667 0.1268 6 3.7126 0.4439 1.84666 23.8 7 1.3323 1.1414 1.714301 53.9 8 -5.7317 0.6365 9 4.2178 1.62041 60 .3 10∞

The image size, subject distance, angle of view, back focus B obtained by the objective lens
f, the focal length f ₁ of the first lens L _1, rear principal point position of the first lens L _1, the front principal point position of the rear crowd bundle lens system, and a rear principal point position of the rear crowd bundle lens system, It is shown in Table 6.

[0029]

TABLE 6 Image size φ1.620mm object distance 7.6090mm angle 118 ° 19 'Bf = 3.116f focal length of the first lens L ₁ f _1: -0.7951 rear principal point of the first lens L ₁ Position: 0.0388 Front principal point position of rear group focusing lens system: 0.8982 Rear principal point position of rear group focusing lens system: -0.9267

When dx is calculated from the above, dx = 0.0388 + 1.4318 + 0.0507 + 0.8982 = 2.4195 = 3.043 | f ₁ |, which satisfies the conditional expression dx ≧ 3.0 × | f ₁ | It is clear that

FIG. 6 is an aberration diagram showing various aberrations of the objective lens for an endoscope according to the present embodiment. The display method is the same as in the first embodiment. As is clear from this figure, according to the present embodiment, it is possible to obtain an endoscope objective lens having good imaging performance up to the periphery of the visual field.

<Embodiment 4> FIG. 7 is a view showing a configuration of an endoscope objective lens according to the present embodiment. As shown,
The objective lens is are substantially the same lens configuration as in Example 1, the third lens L ₃ constituting the cemented lens of the rear crowd bundle lens system becomes a concave lens, the fourth lens L ₄ is a convex lens This is different from the first embodiment in the point. Table 7 shows the radius of curvature r (mm) of each lens surface, the distance d (mm) between the axial top surfaces of each lens, the refractive index n _d of each lens at the d-line, and the Abbe number ν _d in this example. The display method is the same as in the first embodiment.

[0033]

TABLE 7 _{r d n d ν d 1 ∞} 0.4110 1.88300 41.0 2 0.6769 1.7605 3 ( stop) 0.0548 4 -10.9604 0.9118 1.71301 53.9 5 -1.7955 0.1370 6 4.5972 0.4795 1.84666 23.8 7 1.5513 1.2330 1.71301 53.9 8 -5.7561 0.8258 9 ４ 4.43781 1.559920 53 .9 10∞

The image size, subject distance, angle of view, back focus B obtained by the objective lens
f, the focal length f ₁ of the first lens L _1, rear principal point position of the first lens L _1, the front principal point position of the rear crowd bundle lens system, and a rear principal point position of the rear crowd bundle lens system, It is shown in Table 8.

[0035]

Table 8 Image size φ1.780mm object distance 8.2203mm angle 120 ° 00 'Bf = 3.520f focal length of the first lens L ₁ f _1: -0.7666 rear principal point of the first lens L ₁ Position: -0.00000 Front principal point position of rear group focusing lens system: 0.8207 Rear principal point position of rear group focusing lens system: -0.9184

When dx is calculated from the above, dx = 0.00000 + 1.7605 + 0.0548 + 0.8207 = 2.6360 = 3.439 | f ₁ |, which satisfies the conditional expression dx ≧ 3.0 × | f ₁ | It is clear that

FIG. 8 is an aberration diagram showing various aberrations of the endoscope objective lens according to the present embodiment. The display method is the same as in the first embodiment. As is clear from this figure, according to the present embodiment, it is possible to obtain an endoscope objective lens having good imaging performance up to the periphery of the visual field.

<Conventional Example> FIG. 9 is a structural diagram showing a conventional four-piece endoscope objective lens as a comparative example with respect to the above-described Examples 1-4. In this conventional example, the radius of curvature r (mm) of each lens surface and the distance d (m)
Table 9 shows m), the refractive index n _d of each lens at d-line, and Abbe number ν _d . The display method is the same as in the first embodiment.

[0039]

Table 9 _{r d n d ν d 1 ∞} 0.2486 1.83500 42.6 2 0.3816 0.1422 3 3.3087 1.0044 1.80518 25.4 4 ∞ 0.0355 5 ( squeeze) 0.00000 6 ０．４ 0.4263 1.69680 55.6 7 -0.7028 0.0711 8 2.3647 0.4263 1.69680 55.6 9 -9.3143

The image size, subject distance, angle of view, back focus B
f, the focal length f ₁ of the first lens L _1, rear principal point position of the first lens L _1, the front principal point position of the rear crowd bundle lens system, and a rear principal point position of the rear crowd bundle lens system, It is shown in Table 10.

[0041]

TABLE 10 Image size φ1.833mm object distance 8.5250mm angle 117 ° 310 'Bf = 2.358f focal length of the first lens L ₁ f _1: -0.9140 rear principal point of the first lens L ₁ Position: -0.00000 Front principal point position of rear group focusing lens system: 1.8048 Rear principal point position of rear group focusing lens system: -0.9274

When dx is calculated from the above, dx = 0.00000 + 0.2845 + 1.04848 = 2.0893 = 2.286 | f ₁ |, which satisfies the conditional expression dx ≧ 3.0 × | f ₁ | Clearly not.

[0043]

As described above, the objective lens for an endoscope according to the present invention is configured so as to satisfy the following conditional expression: dx ≧ 3 | f ₁ | Back focal length that is at least three times the combined focal length of For this reason, even when an optical path conversion prism is inserted between the solid-state imaging device axially arranged at the distal end of the endoscope and the objective lens, the subject image is formed on the solid-state imaging device. Can be done. In addition, the objective lens for an endoscope according to the present invention can achieve the above-mentioned effects with a small number of lens elements, that is, four lenses in three groups, whereby the size and cost of the objective lens can be reduced.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram illustrating an endoscope objective lens according to a first embodiment of the present invention.

FIG. 2 is an aberration diagram illustrating various aberrations of the endoscope objective lens according to the first embodiment.

FIG. 3 is a lens configuration diagram showing an endoscope objective lens according to a second embodiment of the present invention;

FIG. 4 is an aberration diagram showing various aberrations of the endoscope objective lens according to Example 2.

FIG. 5 is a lens configuration diagram illustrating an endoscope objective lens according to a third embodiment of the present invention.

FIG. 6 is an aberration diagram showing various aberrations of the endoscope objective lens according to Example 3;

FIG. 7 is a lens configuration diagram illustrating an endoscope objective lens according to a fourth embodiment of the present invention.

FIG. 8 is an aberration diagram showing various aberrations of the endoscope objective lens according to Example 4;

FIG. 9 is a lens configuration diagram showing a conventional endoscope objective lens.

[Explanation of symbols]

Reference Signs List 1 aperture stop 2 optical path conversion prism 3 solid-state imaging device (CCD) d-axis upper surface interval L ₁ first lens L ₂ second lens L ₃ third lens L ₄ fourth lens r radius of curvature X optical axis

Claims (1)

[Claims] 1. An endoscope objective lens comprising a front lens group diverging lens system and a rear lens group converging lens system sandwiching a brightness stop, wherein the front lens group diverging lens system defines a surface having a small radius of curvature. A first lens having a negative refractive power toward the image surface side, wherein the rear group focusing lens system has, in order from the object side, a positive refractive power having a surface having a small radius of curvature facing the image surface side; Two lenses, and a cemented lens of a third lens and a fourth lens, one of which is formed of a convex lens and the other is formed of a concave lens, are arranged to satisfy the following conditional expression. An endoscope objective lens characterized in that: dx ≧ 3.0 × | f ₁ | where f ₁ : focal length of the first lens dx: distance between the rear principal point of the first lens and the front principal point of the rear group focusing lens system