JPH0823628B2 - Objective lens for microscope - Google Patents

Description

TECHNICAL FIELD The present invention relates to a microscope objective lens having a smaller distortion than a general microscope objective lens.

[Conventional technology]

Distortion aberration in a conventional medium-high magnification microscope objective lens generally tends to be positive, and the magnitude thereof can be suppressed to some extent, but it is limited. The residual aberration amount of the distortion aberration is the most obstacle to the measurement of the object to be measured, and directly affects the measurement accuracy.

[Problems to be solved by the invention]

Regarding this distortion aberration amount, specifically, for example, when the image height y = f (f: composite focal length of the objective lens) at a magnification of 20 times, the distortion amount is corrected to about + 0.4% or less. It was difficult to balance with the aberration correction of.

Therefore, an object of the present invention is to provide an objective lens for a plan achromat class microscope, which has a medium magnification and the distortion is better corrected.

[Means for solving problems]

The microscope objective lens according to the present invention is composed of, in order from the object side, a first group consisting of a positive meniscus lens having a convex surface facing the object side, a second group having a positive refracting power, and a biconvex lens and a biconcave lens cemented together. Is based on a configuration having a third group having negative refracting power and a fourth group having a positive refracting power by being composed of a biconcave lens and a biconvex lens cemented together. And
The refractive index of the biconcave lens in the fourth group is n _n , the refractive index of the biconvex lens in the fourth group is n _p , the central thickness of the biconvex lens in the third group is D _p , and R _a , the radius of curvature of the object-side lens surface of the biconvex lens, the central thickness of the biconcave lens in the third group is D _n , and the d-line of the biconcave lens in the third group (λ = 587.6 nm)
Is the Abbe number for ν _n, and the combined focal length of the entire system is f
Then n _p −n _n > 0.2 (1) 1.4R _a <D _p + D _n (2) 0 <R _a <2.5f (3) υ _n <46 (4) is there.

Further, in the above configuration, when the radius of curvature of the image side lens surface of the biconvex lens in the fourth group is R _b , it is desirable that the condition of 0> R _b > −3f (5) is further satisfied.

[Action]

The distortion, which is a major feature of the present invention, is small,
Moreover, it is considered that the microscope objective lens capable of satisfactorily correcting the image plane curvature is limited to the above-mentioned configuration. The influence on the curvature of field and distortion is largely due to the lens shape of the group close to the image plane. However, the image surface curvature,
The distortion aberration changes in the same direction as the lens shape changes. As described above, since distortion aberration generally tends to remain positive in a microscope objective lens of medium to high magnification, when the distortion aberration is brought close to 0, the image plane deviation shifts in the negative direction, and the flatness of the image deteriorates. Therefore, even though the image surface curvature and the distortion aberration change in the same direction with respect to the change of the lens shape, there is a difference in the rate of change, and for the shape of the group close to the image surface, Focusing on the tendency that the distortion aberration is larger than the field curvature, the distortion aberration is greatly reduced in the fourth lens group, which contributes most to the distortion aberration,
It is considered that correcting the image surface curvature in the third lens group results in the smallest distortion aberration when the image surface curvature is properly corrected for the entire system.

Specifically, the distortion aberration is made negative in the fourth group G ₄ according to the above condition (1), and the image plane curvature is corrected in the third group G ₃ under the conditions (2) and (3).
(2) The Petzval sum is satisfactorily corrected on the lens surface of strong negative refractive power closest to the image surface of the third lens unit G ₃ which is inevitably generated from the conditions of (3). And axial chromatic aberration,
The condition of (4) above is necessary to satisfactorily correct the balance of chromatic aberration of magnification.

When the condition (1) is not satisfied, the distortion aberration remains positive. Then, for better correction of distortion, when the radius of curvature of the object-side lens surface of the biconvex lens in the fourth group is R _c , then n _p −n _n > 0.2R _c (6) It is desirable to satisfy the conditions.

When the condition (2) is not satisfied, the Petzval sum becomes too positive and the image plane curvature becomes negative, so that the flatness of the image plane deteriorates. If the condition (3) is not satisfied,
Achromatization in the third group becomes insufficient. Second to make up for this
If achromatization is performed on the object side of the group, spherical aberration in the short wavelength region such as the F line is overcorrected. Further, in the case of performing achromatization in the fourth lens group, chromatic aberration of magnification is insufficiently corrected.

By the way, when so-called coaxial epi-illumination in which the illumination light to the object to be inspected is supplied through the microscope objective lens, the flare generated by the reflected light on the lens surface when the illumination light passes through the objective lens is an image. May significantly deteriorate the contrast. When the microscope objective lens of the present invention is used with a coaxial epi-illuminator, the reflected light from the lens surface closest to the image plane may cause strong flare. Therefore, as in the condition (5), it is possible to minimize the flare that tends to concentrate on the image surface by making the most image-side lens surface of the fourth lens group a surface having a strong curvature.

〔Example〕

Examples of the present invention will be described below. In the first embodiment according to the present invention, as shown in FIG. 1, the first group G ₁ is composed of a single positive meniscus lens L ₁ having a concave surface facing the object side, and the second group G ₂ is directed toward the object side. A cemented positive lens L ₂ that is formed by cementing a negative meniscus lens having a convex surface and a biconvex lens, and a negative meniscus lens having a convex surface facing the object side, a biconvex lens, and a negative meniscus lens having a concave surface facing the object side. It is composed of a cemented doublet lens L ₃ having a weak negative refractive power and made of a cemented lens. The third group G ₃ is composed of a cemented negative lens L ₄ which is formed by cementing a biconvex lens having a surface having a strong curvature toward the object side and a biconcave lens having a surface having a strong curvature toward the image side,
The fourth group G ₄ is composed of a cemented positive lens L _{5 which} is formed by cementing a biconcave lens and a biconvex lens.

The second embodiment according to the present invention, as shown in FIG.
Except for the group G ₂ , the lens configuration is almost the same as that of the first embodiment. That is, the direction of the cemented surface of the cemented positive lens L ₂ on the object side of the second group G ₂ is reversed, and this cemented positive lens L ₂ is a biconvex lens and a negative meniscus whose concave surface faces the object side. The difference from the structure of the first embodiment is that it is made up of a cemented lens.

In the structure of the third embodiment, as shown in FIG. 3, the object-side positive lens L ₂ of the second group G ₂ is composed of a single positive lens, and its shape is the same as that of the first and second embodiments. As in the case of the cemented positive lens L ₂ in the example, it has a shape with a surface having a stronger curvature facing the image side. The lens configurations of the other groups are almost the same.

The following Table 1 to Table 3 show the specifications of the above-described embodiment. In the table, the numbers at the left end represent the order from the object side, and the refractive index and the Abbe number are values for the d-line (λ = 587.6 nm).
Note that d ₀ is the distance between the object surface and the vertex of the lens surface closest to the object side.

In each of the above embodiments, the second group is composed of a single or cemented positive lens and a cemented triplet lens,
Not limited to this configuration, it is also possible to arrange three or more components of a cemented lens, which is formed by cementing two positive and negative lenses.

Aberration diagrams of the first, second and third examples are shown in FIGS. 4, 5 and 6, respectively. In each of the examples, as a microscope objective lens with a practical magnification of 20 times,
It is the amount of various aberrations when the combined focal length f of the entire system is f = 11 mm.

As shown in each aberration diagram, in each of the examples, the chromatic spherical aberration and the image plane curvature are well corrected, and the residual amount of the distortion aberration is the image height y = f while sufficiently maintaining the performance as the plan achromat. At the position, it is clear that the correction is extremely good at about + 0.2%.

〔The invention's effect〕

As described above, according to the present invention, it is possible to achieve an objective lens having a medium magnification and better correcting distortion.
It is possible to provide an excellent microscope objective lens that can be used for measurement as a plan achromat class objective lens and contributes to improvement of inspection accuracy.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram of a first embodiment according to the present invention, and FIG.
FIG. 4 is a lens configuration diagram of the second embodiment, FIG. 3 is a configuration diagram of the third embodiment, FIG. 4 is a diagram of various aberrations of the first embodiment, and FIG.
FIG. 6 is a diagram of various types of aberration of the example, and FIG. 6 is a diagram of various types of aberration of the third example. [Explanation of Signs of Main Parts] O ... Object plane G ₁ ...... 1st group G ₂ ...... 2nd group G ₃ ...... 3rd group G ₄ ...... 4th group

Claims (3)

[Claims] 1. A first group consisting of a positive meniscus lens having a concave surface facing the object side, a second group having a positive refracting power, and a biconvex lens and a biconcave lens cemented together in order from the object side. And a fourth group consisting of a biconcave lens and a biconvex lens cemented together and having a positive refracting power, wherein the biconcave lens in the fourth group has a refractive index n _n , N _{p is} the refractive index of the biconvex lens, the center thickness of the biconvex lens in the third group is D _p , the radius of curvature of the object-side lens surface of the biconvex lens in the third group is R _a , and The central thickness of the biconcave lens is D _n ,
When the Abbe number for the d-line (λ = 587.6 nm) of the biconcave lens in the third group is ν _n and the combined focal length of the entire system is f, n _p −n _n > 0.2 (1) 1.4R _a <D _p + D _n (2) 0 <R _a <2.5f (3) υ _n <46 (4) The objective lens for a microscope is characterized by satisfying the respective conditions. 2. When the radius of curvature of the image-side lens surface of the biconvex lens in the fourth group is R _b , the condition 0> R _b > −3f (5) is further satisfied. Claim 1st
The objective lens for a microscope according to the item. 3. The second group has a single or cemented positive lens and a cemented lens having three weak cemented lenses arranged on the image side and having a weak negative refractive power. The objective lens for a microscope according to claim 2.