JPH01282514A - Endoscope objective optical system - Google Patents

Abstract

PURPOSE:To facilitate the assembly to a thin diameter and a wide angle by representing the refractive index N(r) at a position of radial distance (r) from the center as N(r)=N0(1-1/2Ar<2>), and satisfying specific conditions. CONSTITUTION:This optical system is formed by joining a negative lens 11 which has a concave surface on its object side and a plane surface on its image side and a distributed index lens 12 whose surfaces are both plane in order from the object side, providing a stop 13 between the both, and joining them with an image guide 14 in one body. In this case, the length Z of the distributed index lens 12 needs to satisfy a conditional expression I of the angle of projection to the image guide 14. Further, the length Z satisfies conditional expressions II, III, and IV so as to form an image on the incidence surface of the image guide 14. Consequently, the thin diameter, wide angle, and easy assembly are realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an endoscope objective optical system using a gradient index lens.

[Conventional technology]

Endoscope objective optical systems have become increasingly thinner in recent years.

A wider angle is required.

For example, as an endoscope objective optical system,
A retrofocus type lens as described in Japanese Patent No. 121547 is widely known. Also, JP-A-58-594
As described in Japanese Patent No. 20, an image guide in which a gradient index lens is bonded to an image guide has also been proposed.

[Problems that the invention solves]

However, since the above-mentioned retrofocus type requires a lens spacing tube, it is extremely difficult to mount a lens with a small outer diameter (1.5 mm or less in diameter) due to its structure.

Furthermore, in the case where a gradient index lens is cemented to an image guide, the viewing angle of the gradient index lens is determined by its refractive index distribution, so it is very difficult to widen the viewing angle.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide an endoscope objective optical system that has a small diameter, a wide angle, and is easy to assemble.

[Means and effects for solving the problems] The endoscope objective optical system of the present invention has the configuration shown in FIG. That is,
Negative lens 1 has a concave surface on the object side and a flat surface on the image side, and the refractive index of the center is No. 1. When the radial distance from the optical axis is ``, and the second-order coefficient of the refractive index distribution is A, the refractive index N (r ) is represented by N (r) = No (1--Ar"), and a gradient index lens 2 having a very small outer diameter and having both end surfaces polished to a flat surface is cemented in order from the object side, and both A diaphragm 3 is provided between the diaphragm 3 and an image guide 4, a relay lens, or a solid-state imaging element, and the diaphragm 3 is bonded to the image guide 4, a relay lens, or a solid-state imaging element, and the following conditions are satisfied.

7・π n-3-f+ N o L, J￥ Cog (g Z) +sin
(J”r Z) = 0・−1−−−−−・(iv
) In a fiber optic or rigid scope, the chief ray must be perpendicular to the image plane.

That is, as the angle of the chief ray to the image plane increases,
This is because a problem arises in that the amount of light is attenuated inside the image guide 4 or the relay lens, and the periphery of the field of vision becomes dark. Therefore, the angle of the chief ray to prevent the periphery from becoming dark is approximately 7° in a lens medium, and 10' in air. In order to satisfy this condition, the length Z of the gradient index lens 2 must satisfy the following equation.

・－・・・・・・－(1) However, M is a natural number and h is the image height.

On the other hand, in order to form an image of the light that has passed through the homogeneous concave plano negative lens l cemented to the front surface of the gradient index lens 2, the length 2 of the gradient index lens 2 must be equal to or larger than the pitch. Therefore, from formula (11, (21) 7
π 11−−−−−−−− (i) is derived.

In addition, if a solid-state single-image element is used instead of an image guide or relay lens, there is no need to worry about the amount of peripheral light being insufficient. Color crosstalk occurs when the light passes through a filter.

Furthermore, if the exit angle of the principal ray is large, the height of the ray becomes high, making it difficult to reduce the diameter. Therefore, the above conditional expression -
If the requirements are met, the surrounding area will be bright and the height of the light beam will be low, allowing for a smaller diameter. Furthermore, by using a retrofocus type lens, it is possible to obtain a wider-angle optical system than when using a single gradient index lens.

Next, the power of the negative lens l and the length of the gradient index lens 2 will be explained.

In FIG. 2, R is the radius of curvature of the first surface of the negative lens l;
n is the refractive index of the negative lens 1, d is the axial thickness of the negative lens 1,
f, (<Q) is the focal length of the negative lens 1, S (<O) is the distance from the first surface of the negative lens 1 to the object, and Ll is the distance from the image formation point of the negative lens 1 to the gradient index lens 2. The air-equivalent optical path length to the incident surface, L2, is the air-equivalent optical path length from the exit surface of the gradient index lens 2 to the imaging surface. And 1
'+, I-+, and Lx can be expressed as follows using other constituent factors.

f, ” −(f, < O) −−−−(
ii) d f+! S f+ N o L+J'Tcos (J'X Z) +
sin (ni':' Z)N o L, J7k si
n (J”r Z) −cos (,IT Z)−−
----I force Here, since the gradient index lens 2 and the image guide 4 are joined, it is necessary that Lx = 0 at 20). Therefore, it is necessary to satisfy the following formula.

NoL+ Acos(AZ)+ 5in(AZ)-0
−−−−−−−−−(iv) That is, formulas (ii), (iii), (iv)
If the length Z of the gradient index lens 2 is selected so as to simultaneously satisfy the following conditions, an image can be formed on the end surface of the image guide 4.

Note that in practice, due to the influence of aberrations, etc., it may be better to leave a certain distance rather than L2=0.

In this way, the objective optical system and the image guide can all be joined together, eliminating the need for a spacing tube, and as a result, assembly becomes significantly easier.

As mentioned above, conditional expressions (i), (ii),
(iii).

When all conditions (iv) are satisfied, it is possible to obtain an endoscope objective optical system that has a small diameter, a wide angle, a bright peripheral area, and is easy to assemble.

〔Example〕

The present invention will be described in detail below based on the illustrated embodiments.

FIG. 3 shows a first embodiment, in which a negative lens 11 having a concave surface on the object side and a flat surface on the image side and a gradient index lens 12 having flat surfaces on both surfaces are joined in order from the object side. A diaphragm 13 is provided between the two, which is joined to an image guide 14 to form an integral body. In this case, the length Z of the gradient index lens 12 is the same as that of the image guide 14.
It is necessary to satisfy conditional expression (i) of the exit angle to In addition, in order to form an image on the incident surface of the image guide 14, the length Z is determined by conditional expressions (ii), (iii), (iv
) must be satisfied.

The data of the first example is shown below.

r+ =-4,1037 d, =2.6738 0+ -1,883y,
=40.78j, =OO (aperture) d2 = 4.7059 N'o = 2.0
r ko eo.

f-1, F/3.0, object point = -26,738.

Z = 4.7059-0.28P, 5'''-0
, 374.

h −0,5348 However, 'l,'! ,' is the radius of curvature of each surface, dl, d
! is the distance between each surface, n is the refractive index of the homogeneous lens, No is the axial refractive index of the gradient index lens, ν is the Abbe number of the homogeneous lens, f is the focal length of the entire system, F is the F number, and Z is The length of the gradient index lens, A is the second-order coefficient of the gradient index, P
is the pitch of the gradient index lens (length to form images twice),
h is the image height.

The aberration curve of the first embodiment is as shown in FIG.

Now, let's check whether the above data satisfies each conditional expression.

From formula (i), when π M=0, -4,993, -, 4,20< Z < 4.993 Z in this example
-4,7059 satisfies this.

From Take (11) to Ni (iii) 1.883 26.73797 + 4.647 formula (i
v ) from N o L r B cos (仄Z) + sin
(JT Z) = 0 left side - 2X5.379 Xo, 3
74cos(1,76)+5in(1,76)=0.2
254 =0 Therefore, the data of the first embodiment are expressed by formulas (i)' and (ii).

(iii) and (iv) are satisfied.

FIG. 5 shows a second embodiment, in which a negative lens 21 having a concave surface on the object side and a flat surface on the image side, and a gradient index lens 22 having flat end surfaces on both sides are joined in order from the object side. A diaphragm 23 is provided between the two, and the image guide 25 is joined to the image guide 25 via a parallel plane plate 24 to form an integrated structure.

When a parallel plane plate is sandwiched between the gradient index lens and the image guide as in this example, conditions (i) and (iv
) instead of the following condition (i゛).

(iv) must be satisfied.

−・・・・・・・・・ (i゛) 1□ l Nx Noσ N Ot, νsin (Jr Z) −cos (J
'r Z), j...
・・・・・・ (iv conditional expression (iv is Lx Nt-j
! This is derived from s.

However, N8 is the refractive index of the parallel plane plate 24, and l is the thickness of the parallel plane [24].

The data of the second example is shown below.

rI = 1.50 dr = 2.25 nl -1,883v+
=40.78'', =oo (aperture) d t = 4.8 No -1,644r
'j, x o.

ds ”1.4439 ng -1,51633V
z = 64.15r 4 ” ψ f = 1, F/4.3, object point = -25,
0.

Z-4,8=0.262 p, σ-0,34
28.

h' = 0.505 where h' is the height of the light beam at the exit end surface of the gradient index lens 22.

The aberration curve of the second embodiment is as shown in FIG.

Here, the above data satisfies each conditional expression. I'll try to find out.

From the formula '), when M=O, the left side -□・0.3428 7π -3,399 The right side=□・0.3428 -7π =5.765 ゛, 3.399 < Z < 5.765 of this example. Z-4,8 satisfies this requirement.

From formula (ii) From formula (iii) -3-f.

, 2.786 From formula (iv) o5 ■ Left side -□・0,564 0.564 X 2.786 X 5in1.645-
cosl, 645・0.952 Right side -1,4439/1.51633-0.952,・
, left side - right side Therefore, the data of the second example is equation (1').

(ii), (iii), (iv'
).

FIG. 7 shows a third embodiment, in which a negative lens 31 having a concave surface on the object side and a flat surface on the image side, and a gradient index lens 32 having flat end surfaces and an aperture 33 inside are used as an object. These parts are joined in order from the sides, and then joined to the image guide 34 to form an integrated structure. Here, the diaphragm may be provided at the position shown in the figure, but it is provided at a position optically conjugate to this, and its image is formed at the position shown (so-called virtual diaphragm).
In this case, the gradient index lens 32
Let Y be the length from the image side end surface to the diaphragm 33, and Z be the length between both end surfaces of the gradient index lens 32.The length Y is determined by the condition (1 )
Instead, it is necessary to satisfy the following formula (i'').

−・−・・・−(i') In addition, in order to form an image on the incident surface of the image guide 24, the length 2 is determined by conditional expressions (ii), (iii), (i
v) must be satisfied.

The data of the third example is shown below.

r + = -1,8 (103 d, =2.1443 n, -1,883v
, -40,78 "2 2 (to) d! = 1.2772 No -1,644
j, lICl0 (virtual aperture) d s -4,8899N o = 1.644r,
= (to) f=1, F/4.4, object point---25
, 1256.

2-6.1671-0.335 P, σ"-0
, 341°h -0,5025 The aberration curve of the third embodiment is as shown in FIG. Let's check whether the above data satisfies each conditional expression.

From equation (1''), when M-0, -3,406 -5,805,', 3.406 < Y < 5.805 Y -4,8899 of this example satisfies this.

From equation (11), equation (from il) 3.025 From equation (6), N o L + Acos (JX Z) + sin
(J'r Z) -0 left side -1.644 X3. Q
25 xO, 34109Xcos(2,1035)+5
in(2,1035) =4.013Xl0-'
= 0 Therefore, the data of the third embodiment is the formula (i''').

(ii), (iii), and (iv) are satisfied.

FIG. 9 shows a fourth embodiment, which includes a concave-plano negative lens 41, a plano-concave gradient index lens 42, and a convex-plano positive lens 43.
are joined in order from the object side, and a diaphragm 44 is provided between a concave-plano negative lens 41 and a plano-concave gradient index lens 42, which is joined to an image guide 45 and integrated.

FIG. 10 shows a fifth embodiment, which includes a concave plano negative lens 51, a planoconvex gradient index lens 52, and a concave plano negative lens 5.
3 are joined in order from the object side, and a diaphragm 54 is provided between a plano-concave negative lens 51 and a plano-convex gradient index lens 52, which is joined to an image guide 55 and integrated.

FIG. 11 shows a sixth embodiment, which includes a biconcave negative lens 61, a biconvex gradient index lens 62, and a concave plano negative lens 6.
3 are bonded in order from the object side, and a diaphragm 64 is provided between a biconcave negative lens 61 and a biconvex gradient index lens 62, which are bonded to an image guide 65 and integrated.

The fourth to sixth embodiments described above are advantageous in correcting aberrations due to the increased number of curved surfaces.

〔Effect of the invention〕

As described above, the endoscope objective optical system according to the present invention has important practical advantages in that it is small in diameter, has a wide angle, has a bright peripheral area, and is easy to assemble.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of an endoscope objective optical system according to the present invention, FIG. 2 is a diagram showing the constituent factors of the optical system, and FIG.
The figures show the first embodiment, FIG. 4 shows the aberration curve of the first embodiment, FIG. 5 shows the second embodiment, FIG. 6 shows the aberration curve of the second embodiment, and FIG. 7 shows the aberration curve of the second embodiment. The figure shows the third embodiment, FIG. 8 is an aberration curve diagram of the third embodiment, and FIGS. 9 to 11 show the fourth to sixth embodiments, respectively. 1.11, 21.31... Negative lens, 2.12゜2
2.32...gradient index lens, 3,13°23
．． 33.44, 54.64...Aperture, 4.14.2
5, 34.45, 55.65... image guide,
24...Parallel plane plate, 41,51.53゜63...
...Plano-concave negative lens, 42...Plano-concave gradient index lens, 43...Convex-plano positive lens, 52...Plano-convex gradient index lens, 61...Bi-concave negative Lens, 62...
...Biconvex gradient index lens. Figure 1 Figure 2 Figure 4 Figure O 8 Procedural amendment (voluntary) August IO day 1, 1988, indication of the case Patent application No. 112775-1988 2, issued
Name: Endoscope Objective Optical System 4, Agent: 196-5, Shinbashi, Minato-ku, Tokyo 105, Contents of Correction (No. 11, Specification Tube, page 8, line 9, NoL + A cos (A Z) + sin (A
Z) = 0 as N0t, +/r Co5(/X Z) +sin
(4Z) Correct it to -0. (2) "=0" on page 12, line 9 of the specification and page 21, line 2 are corrected to "'40j." (3) "Formula (iv) J on page 16, line 7 of the specification 1
Formula (iv')" is corrected. (4) “Formula (6)” on page 20, line 13 of the specification is 1
Formula (iv)" is corrected.

Claims (1)

[Claims] When a negative lens has a concave surface on the object side and a flat surface on the image side, and the refractive index at the center is N_o, the refractive index N(r) at a position at a radial distance r from the center is N(r). =N_o(1-1/2Ar
A gradient index lens represented by ^2) whose both end surfaces are flat are joined in order from the object side, and an aperture is provided between the two, and the following conditions (i) to (iv) are met. Satisfied endoscope objective optical system. (i) ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ (ii) f_1=R/n-1 (f_1<0) (iii) L_1=d/n-([f_1^2/-S-f
_1]+f_1) (iv)N_oL_1√Acos(√AZ)+sin(
√AZ)=0 However, A is the second-order coefficient of the refractive index distribution, f_1 is the focal length of the negative lens, R is the radius of curvature of the first surface of the negative lens, n is the refractive index of the negative lens, and S is the negative lens The distance from the first surface of
is the image height.