JPH041713A - Endscope objective optical system for intratube observation - Google Patents

Abstract

PURPOSE:To obtain the bright optical system which focuses from near the center of an image plane to the periphery and is small in F-number by constituting the optical system of a front group divergent lens system, a bright stop and a rear group convergent lens system successively from an object side and constituting the optical system so as to satisfy specific conditions. CONSTITUTION:The optical system at the front end of the endoscope formed by using a solid state image pickup element or image guide is the retrofocus type lens system consisting, successively from the object side, of the front group divergent lens system, the bright stop and the rear group convergent lens system and satisfies the conditions of equations I to IV. In the equations I to IV, f is the combined focal distance of the objective optical system; I is the max. image height; P is the Petzval's sum; fF is the front side focal distance of the objective optical system; LO is the nearest object distance at the time of intratube object observation; f2 is the focal distance of the rear group convergent lens system. The bright optical system which focuses from near the center of the image plane to the periphery when the inside of the tubular object is observed and is small in the F-number is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an endoscopic objective optical system for observing inside a tube, which is mainly used for industrial purposes.

[Prior Art] Conventionally, it has been assumed that the shape of an object to be observed by an endoscope optical system is a plane. Further, the imaging surface is also a flat surface. Therefore, the objective optical system of the endoscope is corrected so that the imaging plane conjugate to the object plane is approximately flat. In other words, in order to improve off-axis imaging performance, optical designs have been made to reduce the astigmatism difference and to have no field curvature. Therefore, the object side is close to a flat surface, that is, near the center of the imaging surface A good image could be obtained if the distances to the object plane corresponding to the periphery were approximately equal.

In addition, mainly in the field of industrial endoscopy, an endoscope objective optical system is used to inspect the inner surface of tubular objects such as water pipes and clay pipes.6 When observing tubular objects.

Since it is important to obtain more information with a larger angle and to be able to observe as close to perpendicular to the inner surface of the tube as possible, it is desirable to have a wide-angle objective optical system.

However, if the object plane is tubular, there will be a difference in the distance between the center of the image plane and the object plane corresponding to the periphery.

For example, if the distance from the object plane to the tip of the scope is φ, the inner diameter of the tube is φ, and the half angle of view of the objective optical system is ω, then it can be expressed as follows.

tanω tanω where E is the entrance pupil distance of the objective optical system. From the above equation, it can be seen that the object distance is proportional to the inner diameter φ of the tube and inversely proportional to the tangent of the half angle of view ω of the objective optical system. 12th
The figure is a schematic diagram showing the relationship between the angle of view and the object distance ff1L. As the angle of view of the objective optical system widens from ω1 to ω2, the object distance around the image plane moves closer to the periapsis from Ll to L2. . For this reason, with conventional objective optical systems, especially when observing small-diameter tubes, when focusing near the center of the image plane to infinity, the near point side deviates from the depth of field, and the focus is adjusted around the image plane. There was a problem that it wasn't there. Another possible solution to this problem would be to deepen the depth of field, but in that case, the lens would have to be stopped down to a large F-number, making the optical system dark.

[Problem to be Solved by the Invention 1] The present invention is an endoscope optical system using a solid-state image pickup device or an image guide, and when observing a tubular object, the focus is out of focus from the center of the image plane to the periphery. The purpose is to provide a bright optical system.

[Means for Solving the Problems] The present invention is an endoscope tip optical system using a solid-state image sensor or an image guide, which includes, in order from the object side, a front group diverging lens system, an aperture diaphragm, and a rear group converging lens. This is a retrofocus lens system consisting of a lens system that satisfies the following conditions.

[210,2<P−f (3) o,8<I/f (410,5≦f2/f≦3 f: Composite focal length of objective optical system ■=Maximum image height φ Niscope outer diameter P: Objective Petzval sum ω of the optical system: Half angle of view fF at maximum image height: Front focal length Lo of the objective optical system: Closest object distance when the endoscope is placed at the center of the tubular object when observing a tubular object (object distance is (measured along the optical axis of the objective lens) r+: i-th radius of curvature n, : i-th refractive index f2: composite focal length of the rear group convergent lens system In the lens system with the above configuration, the distance from the object plane to the tip of the scope If the rear focal length of the objective optical system is fIl, and the distance from the final surface to the Gaussian image plane is Sk, the following relationship holds in paraxial terms.

(L + frl (Sm-fa) = f” where S
A f represents the difference between the imaging position of an object point at infinity and the imaging position of a nearby object point, and in the present invention, even when observing a single tubular object, the object distance differs depending on the angle of view. When ω=0, it is considered to be an object point at infinity, so 51fm has a different value for each angle of view, and represents the deviation between the Gaussian image plane and the real image plane at each angle of view. This amount of deviation (Sk-f
, ) obviously changes depending on the inner diameter φ of the tube being observed and the angle of view ω of the objective optical system. For a tube of arbitrary diameter φ, each image corresponds to the distance from the in-focus position at the center of the objective lens (object point at infinity) to the object plane at each angle of view at the periphery to the tip of the scope. FIG. 11 shows a schematic diagram of the shape of the image plane conjugate to the inner surface of the tube, which connects the distance Sk from the final surface to the Gaussian image plane. Here fF, f,
=O, f=1.

Next, on the image plane of the actual objective optical system, field curvature occurs, in which the image of a flat object is curved. In the region of third-order aberration, the image surface becomes a spherical surface, and its curvature is expressed by the following equation as a Petzpar sum.

When the value of P is positive, the image surface curves concavely toward the lens system G. Furthermore, the amount of deviation ΔS from the in-focus position at the center of the objective lens (object point at infinity) to the in-focus position at the periphery due to curvature is calculated as follows.

I2・P ΔS,=-- From the above, in order for (S, -f, l and ΔS to cancel each other out), both values must have opposite signs and almost equal absolute values. be. In other words, by correcting the field curvature so that the following equation (ratio of -ΔS8 and fslfs) approaches 1, the diameter of the observation tube ψ is reduced) (S - f, ) 2 f2 where: The closest distance when observing a tubular object is defined by the thickness of the tube that can be observed, that is, the outer diameter of the scope ψ, and the maximum angle of view ω, and is the object distance at that time. is the following formula% formula% This object distance is. Letting Smo be the distance from the final image plane to the Gaussian image plane corresponding to , the above ratio, that is, ΔS8 and (
The Sino-fBt ratio is as follows.

-ΔS, I"(LO+fF+P (S,, -fll) 2f2 If the above ratio is close to 1, that is, if the field curvature is corrected so that condition (1) is satisfied and condition (2) is also satisfied, the observation Good images can be obtained from near the center of the screen to the periphery, even for tubes with the smallest possible diameter.

The objective optical system for a scope with an extremely narrow outer diameter meets the condition f1.
The value of the formula shown in + is 0.1 or more, and the focal length of the entire system is f.
By setting the Petzval sum P・f, which is standardized by good images can be obtained. This means that in a tube with a small diameter at the observable limit (equivalent to the outside diameter of the scope), the value of △SA is (3
Two. -f, ), so the image plane tilts toward the imaging plane.

However, condition (1), (if the lower limit of 21 is not exceeded,
The amount of image plane tilt is not very large. Therefore, the imaging position from the maximum image height to the imaging position at approximately 70% of the image height can be considered to be almost a plane, and aligning the imaging plane to that position is considered to be most necessary when actually observing. Good images can be obtained within this range.

However, when the range of condition (2) is exceeded, the Petzpal image plane can be considered to be substantially flat, and the amount of inclination of the image plane on the imaging plane becomes large. For this reason, it becomes impossible to regard the imaging position from the imaging position at the maximum image height to approximately 70% of the image height as a substantially flat surface, making it impossible to obtain a good image up to the periphery.

On the other hand, if the upper limit of condition t1) is exceeded, the image plane will be too tilted, making it impossible to observe a large-diameter tube up to its periphery with a good image.

If the lower limit of condition (3) is exceeded, the angle of view will become narrower, making it impossible to obtain more information at once when observing a tubular object or to observe it as close to perpendicular to the inner surface of the tube as possible. This makes it difficult to use as an objective optical system for observing inside a tube. Furthermore, since the focal length becomes longer, the depth of field becomes shallower.

When the optical system of the present invention is an endoscope tip optical system using a solid-state image sensor, it is necessary to arrange an optical low-pass filter for removing moiré in front of the solid-state image sensor.

To achieve this, the back focus of the optical system must be made longer. Here, if the focal length of the rear group of the objective optical system is I2, the rear focal length of the entire objective optical system is fll, and the imaging magnification of the rear group is β2, the following relational expression holds true.

flI:I2(1-β2) From the above formula, it can be seen that in order to increase I8, it is necessary to increase I2.

In condition (4), if fa/f exceeds the lower limit, sufficient back focus cannot be obtained. Also, condition (4)
If the upper limit of is exceeded, the negative power of the front group becomes strong, and the outer diameter of the front group becomes large in order to satisfy other conditions, which is not preferable.

As described above, the present invention has the above-mentioned lens configuration under the condition +1).
By satisfying (4) to (4), an objective optical system that is in focus from near the center of the image plane to the periphery can be obtained even if the F number is small when observing a tubular object.

[Examples] Next, examples of the objective optical system for an endoscope for intraluminal observation of the present invention will be shown.

Example 1 f = 1.000, fF = 0.537, f, =
-0,074I H=0.85080, 2ω=12
0”r1=■ d, = 0.2321 n, = 1.88300
Vl= 40.78ra"1.5702 d, = 0.6704 r3=■(aperture) ds" 0.3287 r4=-4,7591 d4= 0.4126 r, =-o, 7634 d, = 0.0516 nz=1.51633 shi, = 64.15 ra= ■ d6= 0.7736 n, = 1.52000 shi3=74.00 r, :Oo d,= 0.0516 ra=3.0923 d,= 1 .0315 rs=-1,0356 n, = 1.69680 si4=56.49 d*= o, 1547 ns" 1.83350 ν5==21.0O rl. =-2,3175 d, 0== 0 .2579 rth =00 d,, =0.9644 n, =1.54814
shi,=45.78r+z °00 d,,=0.2063 n7”=1.51633
shi,=64.15r1. =■ (I2・(Lo1tr)・P)/+2・f”) =0
．． 373 (ψ = 8■)P-f =0.362
, I/f =0.851, f2/f=
1.303 Example 2 f = 1.000, fF = 0.610
, fa=-〇, 116I H=0.8486
1, 2ω = 120'r, = ■ d, = 0.2315 n+ = 1.88300
ν + = 40.78ra = 1.5682 d, = 0.6687 rs = oo (aperture) d, = 0.2418 r, = -2,2522 d4 = (), 3601 n, = 1.51633
172 = 64.15r@= -0,6923 d, = 0.0514 r6:cIO d, = 0.7716 na = 1.5200
0 173= 74.00r7=oO d7= 0.0514 ra=11.4513 da" 0.8745 n4= 1.69680
y-= 56.49re"-0,9894 ds" 0.1543 ns= 1.8335[
11) s=21.0Or,. = −1,8066 d,. = 0.3601 r++ =3.5020 d++=0.4205 n5=1.51633
si, = 64.15rI2 :0O d1*= 0.9619 n, = 1.5481
4 vt = 45.78r13 =■ d+*=0.2058 n, = 1.5163
3 v, = 64.15r14: 00 (1”・(Lo+fy) 4)/12・f2) =
0.402 (+ = 8 mm) P-f = 0
．． 382, 1/f =0.849, f2/f
=1.378 Example 3 f = 1.000, f, =0.231, f,
=-0,076I H=0.85521, 2
ω = 1406r1; ■ dl = 0.2333 nt = 1.8830
Ovl = 40.18r, = 1.3210 d laziness = 0.2286 r3 = c .15 r5=-Q, 48D1 d5=0.0518 rs=■ d,=0.7776 r7=■ d,=mouth, 0518 ra=8. o120 d, = 0.1555 ri+=1.5851 ns” 1.52000 fi4= 1.68893 si, =74.00 ν, =31.08 d, = 0.9331 ns= 1.77250 ν, =49. 66 r+o =-1,8143 d, .= 0.0985 rth=Oa dl, = 0.7776 r122″l d1□=0.2074 ns” 1.54814 sig=45.78 n, = 1.51633 v, = 64.15
! −+i= ■ (I” (Lo+fr) ・P)/ f2・f”l
= 0.297 (ψ=81III)P-f =0
．． 482, 1/f = 0.855, f
t/f=1.077 Example 4 f=1.000, f,=0.827, f,
=-0,035I H=0.95321, 2ω=14
0゜r=■ d1= 0.2600 n1= 1.51633
v+= 64.15rz=1.5337 d2= 0.6970 r3=oo (aperture) d, = 0.0420 f4=-2,4797 d, = 0.4622 jl, = 1.5163
3 v, = 64.15rs = -0,7052 ds = 0.0578 ra = ■ d, = 0J667 11. = 1.52000
v, = 74.00rr = ■ d, = 0.0578 ra = 8.6149 d, = o, 1155 n, = 1.72825
v, =28.46r9=1.3287 d-=1-0399 n5=1.72916
375=54.68r1o=-1,216
6 (aspherical surface) dl. =0.08 rll” ■ d,,=0.8667 n6=1.54814
d, z= 0.2311 ny= 1.5163
3 Schiff = 64.15r, :CIO Aspheric coefficient P = 0.8541, E = 0.86247
x 10-'F = 0.12932 x 10-
'G = 0.31100 x 10-''H=
0.10222
．． 538 (φ = 8■嘗)! ”f=0.51
9, I/f=0.953, fg/f=1.2
83 However, rll rR9-...- is the radius of curvature of each lens surface, d,, d,,... is the thickness and lens spacing of each lens, n+, nz,... is the refractive index of each lens, and ．． C2. ... is the Atsube number of each lens.

Embodiment 1 has the configuration shown in FIG. 1, which includes, in order from the object side, a negative lens with a concave surface on the image side, an aperture, a positive lens with a convex surface on the image side, and a near-infrared region on the solid-state image sensor (CCD). a color temperature correction filter F1 to prevent light from entering, a positive lens made of a material with a large Abbe number, and a negative meniscus lens made of a material with a small Abbe number, in order to correct off-axis chromatic aberration of magnification. A cemented achromatic lens that has a positive refractive power as a whole, an optical low-pass filter F2 that cuts high frequency components to prevent moiré and false color, and a CCD cover glass C are arranged. It is. In Example 1, the angle of view is approximately 120°.
This is a wide-angle objective optical system.

In this embodiment, when the color temperature correction filter F3 in the convergent lens system in the rear group than the aperture stop is an absorption filter, if the incident angle of off-axis incident light is large (approximately 40 degrees or more), The optical path difference with the upper light beam is large (the spectral transmittance of light with wavelengths on the red side at the periphery is lower than on the axis. This causes problems such as a slight bluish tinge at the periphery of the screen. Also, CCD light reception Simultaneous CCDs in which mosaic color filters such as R, G, and B are provided in front of the element have the disadvantage that color unevenness is likely to occur if the incident angle to the light receiving element is large. The principal ray at each image height must be incident on the CCD almost perpendicularly, that is, the objective optical system is preferably a telecentric optical system in which the pupil position is approximately at infinity.

In order to solve the above problem, in this first embodiment, the composite focal length fl+ of the lens behind the filter F1 with respect to the image height is set as shown below, and f□ is lengthened to adjust the incident angle to the filter F1. is made smaller.

I/ f * l< tan40@= 0184 Also, by shortening the focal path 111f of the rear group converging lens system, the light rays can be sufficiently bent behind the front group diverging lens system, and the refractive power of the concave lens can be reduced. I tried to get it. In other words, if the focal length of the front group diverging lens system is f, then 1ft
Since l>ri, 1f11 can be made long and the ray intersects at a high point with the concave surface of the concave lens, so the radius of curvature r21 of the concave surface can be made large, thereby pushing the Petzval sum P in the positive direction.

Also, by reducing the refractive index difference between the positive and negative lenses of the cemented lens and increasing the refractive index of each lens, it is possible to bring the Petzval sum to the positive side, increasing the overall Petzval sum. There is.

FIG. 5 shows astigmatism when observing a tubular object surface using this optical system. At this time, the outer diameter of the scope was set to 81-, which is a narrow scope with strict conditions.

For comparison, FIG. 10 shows an astigmatism diagram of Example 7 of JP-A-62-173415 shown in FIG. 9, in which the Petzval sum is approximately zero at the same angle of view.

As is clear from a comparison between FIG. 5 and FIG. 10, in Example 1, the inclination of the image plane is improved to about 173 points, especially in astigmatism in the direction of spherical breakout, compared to the conventional example.

Embodiment 2 has the configuration shown in FIG. 2, in which a positive field lens with a convex surface facing the object side is placed immediately before the optical low-pass filter F2. As mentioned in the explanation of Example 1, this is a simultaneous COD in which mosaic color filters such as R, G, and B are provided in front of the light receiving element.
When the angle of incidence on the light-receiving element is large, color unevenness tends to occur. Therefore, it is necessary to make the chief ray at each image height perpendicularly incident on the CCD. Moreover, it is desired that the tip of the endoscope be made as thin as possible.
The further the final lens is separated from the CCD, the larger the outer diameter of the final lens becomes. For that purpose, an optical low-pass filter F2
By providing a positive field lens with a convex surface facing the object side in front of the CCD, the diameter of the lens in front of this field lens can be made smaller, and the color is incident perpendicularly to the CCD. This makes it possible to obtain even images.

Embodiment 3 is an objective optical system having the configuration shown in FIG. 3 and having a wider angle of view 1401 to enable observation of a wider range.

Example 4 has the configuration shown in FIG. 5, and has an angle of view of 140°. In this embodiment, the image side surface of the positive lens in the cemented lens is an aspherical surface, and by using this aspherical lens, off-axis aberrations such as coma aberration are corrected. In this embodiment, an aspherical surface is used for the final lens surface, but the same effect can be obtained by using an aspherical surface for the front group. Furthermore, it is more effective to use aspheric surfaces for both the front group and the rear group.

The shape of the above aspherical surface is expressed by the following formula, where the optical axis direction is the X axis and the direction perpendicular to the optical axis is the y axis.6where r is the radius of curvature near the optical axis, E, F, G.

Hl... is an aspheric coefficient.

[Effects of the Invention] As described above, the endoscopic optical system for observing inside a tube according to the present invention is a bright optical system that is in focus from near the center of the image plane to the periphery and has a small F number when observing the inside of a tubular object. It is.

[Brief explanation of drawings]

1 to 4 are cross-sectional views of Examples 1 to 4 of the objective optical system of the present invention, FIGS. 5 to 8 are astigmatism curve diagrams of Examples 1 to 4, and FIG. 9 is a cross-sectional view of a conventional endoscope objective optical system, FIG. 1O is an astigmatism curve diagram of the conventional example,
FIG. 11 is a schematic diagram of the image surface shape conjugate to the inner surface of the tube, and FIG. 12 is a diagram showing the relationship between the angle of view and the object distance. Applicant Olympus Optical Industry Co., Ltd. Agent Hiroshi Mukai Figure 2 Figure 5 Figure 6 Figure 9wJ Figure 10 Procedural Amendment July 9, 1991

Claims (1)

[Claims] An endoscope optical system for intraductal observation, which comprises, in order from the object side, a front group diverging lens system, an aperture diaphragm, and a rear group converging lens system, and satisfies the following conditions. . (1) ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ (2) 0.2<P・f (3) 0.8<I/f (4) 0.5≦f_2/f≦3 where f is objective optical The combined focal length of the system, I is the maximum image height,
P is the Petzval sum of the objective optical system, f_F is the front focal length of the objective optical system, L_O is the closest object distance when observing a tube object, and f_2 is the focal length of the rear group converging lens system.