JPH05288985A - Objective for endoscope - Google Patents

Abstract

PURPOSE:To suppress an irregularity in color tone and reduce the cost by a decrease in the number of components by constituting a lens right behind a stop by using an infrared-ray cutting filter. CONSTITUTION:The piano-convex lens L formed of a color temperature compensating and absorbing filter is arranged right behind the brightness stop S. Namely, this objective consists of a 1st group which has negative refracting power, the brightness stop S, a 2nd group with positive refracting power in which a surface on an image side arranged right behind the stop S is convex, and a 3rd group with positive refracting power in order from the object side, and the 2nd group consists of the color temperature compensating and absorbing filter. Further, 0.85<=¦R¦/D<=1.15 holds. Here, R is the radius of curvature of the image-side surface of the positive lens in the 2nd group and D is the thickness of this lens. Consequently, even if the angle of incidence of a light beam made incident on the 2nd group becomes large, there is no difference between an on-axis light beam and an off-axis light beam.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens for an electronic endoscope which uses a solid-state image pickup device as an image pickup device.

[0002]

2. Description of the Related Art Generally, a solid-state image sensor has sensitivity to infrared rays, and therefore, in an electronic endoscope using a solid-state image sensor as an image sensor, it is necessary to insert an infrared cut filter into an optical system. .. Therefore, conventionally, for example, as described in Japanese Utility Model Laid-Open No. 61-114414, an infrared cut filter is provided in an objective lens in front of a solid-state image sensor, or a part of the lens in the objective lens is cut by infrared rays. It was formed of glass (infrared absorption filter).

[0003]

Problems to be Solved by the Invention For example, as shown in FIG.
Objective Lens Described in Japanese Patent Application No. 2-93937
Then, by increasing the composite focal length of the post-stop group
Temperature correction filter F ₁ The incident angle of the light
The axial and off-axis chief ray paths are almost equal
However, to prevent uneven color tone between the center and the periphery of the image.
I was there. However, in this case, the focal length of the rear group is inevitable
As it became longer, the total length could not be shortened.

Further, as technology advances, solid-state image pickup devices (hereinafter referred to as CCDs) have been miniaturized, but the spectral sensitivity of CCDs does not change so much. It cannot be thinned. Therefore, the hard portion at the tip of the endoscope cannot be shortened for the miniaturization of the CCD. Further, in order to eliminate this drawback, it has been proposed to form part of the objective lens with a color temperature correction filter. However, since the objective optical system as shown in FIG. 10 is a telecentric system and the field lens is an infrared cut lens, a difference in length between the axial principal ray path and the off-axis principal ray path in this infrared ray cut lens L ₂ . Occurs, the path becomes shorter as it goes off axis. As a result, the infrared cut amount decreases toward the periphery of the image, so that the smear cannot be properly removed, or a problem such as uneven color tone occurs.

It is an object of the present invention to provide an endoscope objective lens in which the lens in the optical system is composed of a color temperature correction absorption filter, and the total length of the optical system is short and uneven color tone does not occur. To do.

[0006]

The endoscope objective lens of the present invention comprises, in order from the object side, a first group having a negative refracting power, an aperture stop, and an image side surface arranged immediately after the stop. It is characterized in that it is composed of a second group having a positive refractive power, which is a convex surface, and a third group having a positive refractive power, and the second group is composed of a color temperature correction absorption filter.

In the endoscope objective lens of the present invention, a positive lens disposed immediately after the aperture stop as shown in FIG. 1 is formed by a color temperature correction absorption filter, whereby the axial main axis of this lens is adjusted. The ray path and the off-axis chief ray path can be made almost equal to each other so as to suppress uneven color tone between the center and the periphery of the image.

Further, it is desirable that the objective lens of the present invention is constructed so as to satisfy the following condition (1). (1) 0.85 ≦ | R | /D≦1.15 where R is the radius of curvature of the image side surface of the second lens group positive lens, and D is the thickness of this lens.

By satisfying this condition (1), the second
Even if the incident angle of the light rays entering the group becomes large, the optical path lengths of the on-axis light rays and the off-axis light rays hardly change. That is, the combined focal length of the lens groups after the diaphragm can be shortened, and the total length of the objective lens can be shortened.

Further, it is desirable to satisfy the following condition (2) in order to improve the optical performance. (2) 2 <f ₂ / f <10 where f is the focal length of the entire system and f ₂ is the focal length of the second group.

If the value exceeds the upper limit of the condition (2), that is, if the focal length of the second lens unit becomes long, the curvatures of other lenses become tight, and coma aberration, astigmatism, etc. worsen. Since the number of lenses must be increased, the total length becomes long, which is unsuitable as an endoscope objective lens. When the value goes below the lower limit of the condition (2), the focal length of the second lens unit becomes short and the lens has a strong curvature, and since the second lens unit is arranged immediately after the stop, the influence on the spherical aberration is large and this aberration is deteriorated. Not preferable. In this way, condition (2)
It is preferable in order to improve the optical performance by satisfying

However, when a thick color temperature correction filter is required and it is difficult to satisfy the conditions (1) and (2) at the same time, the correction filter can be divided into a lens and a plane parallel plate. .. In this case, it is desirable to satisfy the following condition (3). (3) 0.5 <| f ₁ /f|<2.0 where f ₁ is the focal length of the first group negative lens.

The condition (3) is for providing a space for arranging the filter immediately before the aperture stop and defining the total length.

When | f ₁ / f | becomes less than the lower limit of the condition (3), the distance between the negative lens group and the positive lens group becomes short, and it becomes difficult to provide a filter immediately before the aperture stop. Also, if | f ₁ / f | exceeds the upper limit of condition (3),
The distance between the negative lens group and the positive lens group becomes too large,
It is not preferable because the total length of the lens system becomes long. Therefore, it is desirable to satisfy the condition (3) in order to provide the filter in the optical system and shorten the total length of the optical system.

As described above, by disposing the parallel plane plate, which is a divided filter so as to satisfy the condition (3), immediately before the diaphragm, the total thickness of the filter is changed without changing the total length of the objective lens. It becomes possible to thicken. Conversely, the total length of the objective lens can be shortened without changing the total thickness of the filter.

[0016]

EXAMPLE Examples of the endoscope objective lens of the present invention will be described below. Example 1 f = 1.000, F number = 3.968, image height = 0.88590, object distance = -19.93267 r ₁ = ∞ d ₁ = 0.7752 n ₁ = 1.88300 ν ₁ = 40.78 r ₂ = 1.4324 d ₂ = 2.3476 r ₃ = ∞ _{(stop) d 3 = 2.4362 n 2 =} 1.52000 ν 2 = 74.00 r 4 = -2.1951 d 4 = 0.1107 r 5 = 5.0013 d 5 = 1.6611 n 3 = 1.69680 ν 3 = 55.52 r 6 = -1.5243 d 6 = 0.5315 n ₄ = 1.84666 ν ₄ ＝ 23.78 r ₇ ＝ -3.7368 d ₇ ＝ 0.8859 r ₈ ＝ ∞ d ₈ ＝ 1.8825 n ₅ ＝ 1.54814 ν ₅ ＝ 45.78 r ₉ ＝ ∞ d ₉ ＝ 0.8859 n ₆ ＝ 1.51633 ν ₆ ＝ 64.15 r ₁₀ = ∞ | R | /D=0.901, f 2 /f=4.221 example 2 f = 1.000, F-number = 3.996, image height = 0.95532, object distance _{= -21.49459 r 1 = ∞ d 1} = 0.8359 n 1 = 1.88300 v ₁ = 40.78 r ₂ = 1.6878 d ₂ = 3.3506 r ₃ = ∞ (aperture) d ₃ = 2.6271 n ₂ = 1.52000 v ₂ = 47.00 r ₄ = -2.8898 d ₄ = 0.1194 r ₅ = 5.0366 d ₅ = 1.7912 n ₃ = 1.69680 ν ₃ = 55.52 _{6 = -1.6275 d 6 = 0.5732 n} 4 = 1.84666 ν 4 = 23.78 r 7 = -3.8073 d 7 = 0.9553 r 8 = ∞ d 8 = 2.0300 n 5 = 1.54814 ν 5 = 45.78 r 9 = ∞ d 9 = 0.9553 n _{6 = 1.51633 ν 6 = 64.15 r} 10 = ∞ | R | /D=1.100, f 2 /f=5.557 example 3 f = 1.000, F-number = 3.975, image height = 0.86427, object distance = -19.44600 r ₁ = ∞ d ₁ = 0.7562 n ₁ = 1.88300 ν ₁ ＝ 40.78 r ₂ ＝ 1.0323 d ₂ ＝ 0.8962 r ₃ ＝ ∞ d ₃ ＝ 0.8643 n ₂ ＝ 1.52000 ν ₂ ＝ 74.00 r ₄ ＝ ∞ d ₄ ＝ 0.0648 r ₅ ＝ ∞ ( _{stop) d 5 = 1.5454 n 3 =} 1.52000 ν 3 = 74.00 r 6 = -1.5768 d 6 = 0.1080 r 7 = 6.1717 d 7 = 1.6205 n 4 = 1.69680 ν 4 = 55.52 r 8 = -1.1865 d 8 = 0.5186 n 5 = 1.84666 ν ₅ = 23.78 r ₉ ＝ -2.7780 d ₉ ＝ 0.8643 r ₁₀ ＝ ∞ d ₁₀ ＝ 1.8366 n ₆ ＝ 1.54814 ν ₆ ＝ 45.78 r ₁₁ ＝ ∞ d ₁₁ ＝ 0.8643 n ₇ ＝ 1.51633 ν ₇ ＝ 64.15 r ₁₂ ＝ _{∞ | R | /D=1.020, f 2} / = 3.034, | f ₁ /f|=1.169 Example 4 f = 1.000, F-number = 3.971, image height = 0.85955, object distance _{= -19.33987 r 1 = ∞ d 1} = 0.5372 n 1 = 1.88300 ν 1 = 40.78 r _{2 = 0.9008 d 2 = 0.5275 r} 3 = ∞ d 3 = 1.0744 n 2 = 1.52000 ν 2 = 74.00 r 4 = ∞ d 4 = 0.0645 r 5 = ∞ ( _{stop) d 5 = 1.2893 n 3 =} 1.52000 ν 3 = 74.00 r ₆ = -1.3573 d ₆ = 0.1074 r ₇ = ∞ d ₇ = 1.0744 n ₄ = 1.52000 ν ₄ = 74.00 r ₈ = ∞ d ₈ = 0.1074 r ₉ = 3.7903 d ₉ = 1.2893 n ₅ = 1.9680 ν ₅ = 55.52 r _{10 = -1.3316 d 10 = 0.4298 n} 6 = 1.84666 ν 6 = 23.78 r 11 = -3.6407 d 11 = 0.5372 r 12 = ∞ d 12 = 1.8265 n 7 = 1.54814 ν 7 = 45.78 r 13 = ∞ d 13 = 0.8595 n ₈ = 1.51633 ν ₈ = 64.15 r ₁₄ = ∞ | R | /D=1.053, f ₂ /f=2.610, | f ₁ /f|=1.020 where r ₁ , r ₂ , ... Are the curvatures of each lens surface. Radius, d
₁ , d ₂ , ... Is the thickness of each lens and the lens interval, n
₁ , n ₂ , ... Are the refractive indices of the respective lenses, ν ₁ , ν ₂ ,.
Is the Abbe number of each lens.

The first embodiment has the configuration shown in FIG. 1, and a plano-convex lens L formed by a color temperature correction absorption filter is arranged immediately after the aperture stop S. F ₃ is a crystal filter for removing moire, and C is a CCD cover glass.

The second embodiment is shown in FIG. 2 and is aimed at further widening the angle in order to widen the observation range.

The third embodiment is as shown in FIG. 3, in which the color temperature correction filter lens L is replaced by a plane parallel plate filter F.
By dividing the lens into ₁ and the plano-convex lens L ₁ and arranging the filter F ₁ in front of the aperture stop S, the lens length of the rear group can be shortened and the overall lens length is shortened. is there. As a result, the length of the endoscope distal end hard portion is further shortened, and a great effect is obtained in reducing pain to the patient.

The fourth embodiment has the configuration shown in FIG. 4, in which the color temperature correction filter lens L is divided into a plano-convex lens L ₁ and parallel plane plate filters F ₁ and F ₂ , and the filters F ₁ and
In this configuration, F ₂ is divided into the front of the aperture stop S and the rear of the plano-convex lens L ₁ . With this configuration, a measure for preventing the total thickness of the correction filter from becoming thicker than the total combined focal length due to the reduction in size of the CCD and thus making the filter F ₁ divided into the lens thicker Here is an example. That is, as the filter F ₁ becomes thicker, the gap between the aperture stop and the first group negative lens becomes wider,
This is to prevent the outer diameter of the lens from increasing. By dividing into three as described above, the filter F ₂ is provided after the plano-convex lens L ₁ without increasing the thickness of the filter F ₁ arranged in front of the diaphragm S so much. This makes it possible to obtain the effect of reducing the outer diameter of the distal end while hardly changing the length of the rigid end portion of the endoscope. The filter F ₂ may be arranged after the third group.

Further, in the type in which the color temperature correction filter lens L is divided, it is not necessary to use the same filter, and different filters may be used in combination. In addition, YAG on the filter or plano-convex lens surface
It is also possible to provide a YAG cut interference film coat that reflects light (wavelength 1.06 μm).

In addition, it is possible to make the absorption filter thinner by using a combination of infrared cut interference films, which have substantially the same effect as the color temperature correction absorption filter.

[0023]

In the endoscope objective lens of the present invention, the lens immediately after the diaphragm is constructed by an infrared cut filter so as to suppress the uneven color tone, and the cost is reduced by eliminating the number of parts of the entire lens system and the lens. Facilitating assembly work of parts,
Also, by shortening the overall length, the hard portion at the distal end of the endoscope can be shortened and the insertability is good. It is a lens system suitable for an electronic endoscope.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a sectional view of a second embodiment of the present invention.

FIG. 3 is a sectional view of a third embodiment of the present invention.

FIG. 4 is a sectional view of a fourth embodiment of the present invention.

FIG. 5 is an aberration curve diagram of Example 1 of the present invention.

FIG. 6 is an aberration curve diagram of Example 2 of the present invention.

FIG. 7 is an aberration curve diagram of Example 3 of the present invention.

FIG. 8 is an aberration curve diagram of Example 4 of the present invention.

FIG. 9 is a cross-sectional view of a conventional objective lens for an electronic endoscope.

FIG. 10 is a sectional view of another conventional objective lens for an electronic endoscope.

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[Procedure amendment]

[Submission date] May 26, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0023

[Correction method] Change

[Correction content]

[0023]

According to the endoscope objective lens of the present invention, the lens immediately after the diaphragm is constructed by an infrared cut filter so as to suppress uneven color tone, and the number of parts of the entire lens system is reduced.
To reduce the cost and facilitate the assembly work of the lens assembly,
Further, by shortening the overall length, the rigid portion at the distal end portion of the endoscope is shortened and the insertability is good, and this lens system is suitable for an electronic endoscope .

Claims (2)

[Claims] 1. A first group having negative refractive power in order from the object side,
The brightness stop, the second group having a positive refractive power of which the image-side surface is convex immediately after the diaphragm and having a positive refractive power, and the third group having a positive refractive power, and the second group has a color temperature correction absorption filter. Objective lens for endoscopes whose lens is configured in. 2. The second group satisfies the following condition.
Objective lens for endoscope. 0.85 ≦ | R | /D≦1.15 where R is the radius of curvature of the image-side convex surface of the second lens unit, and D is the central thickness of the second lens unit.