JPH10260362A - Liquid-dip type objective - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the liquid-dip type objective which has various aberrations compensated excellently by composing the lens of lens groups including a specific gradient index lens in order from the object side. SOLUTION: This lens consists of at least three groups, i.e., a 1st lens group which includes a a piano-convex single lens or piano-convex lens having a cemented surface greatly convex to the image side and has positive refracting power, a 2nd lens group which includes a gradient index lens with positive refracting power, and a 3rd lens group which includes a gradient index lens with negative refracting power in order from the object side. The refractive index N(r) of the gradient index lens used for the objective is represented as N(r)=N00 +N10 r<2> +N20r <4> +..., where (r) is the radial distance from the optical axis, N00 is the refractive index on the optical axis, N10 , N20 ... are secondary, biquadratic ... coefficients. Here, the gradient index lens can be reduced in the quantity of aberrations generated on its lens surfaces and the aberrations of the whole lens system can be compensated excellently even when the number of lenses is small.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an immersion microscope objective lens using a gradient index lens, particularly a so-called radial type gradient index lens whose refractive index changes in the radial direction.

[0002]

2. Description of the Related Art In an objective lens, in order to sufficiently correct various aberrations such as spherical aberration, coma, field curvature, and chromatic aberration, the number of lenses must be increased, which is a major problem in cost. was there. In order to solve this problem, it is known to use a refractive index distribution lens having a greater degree of freedom in correcting aberrations than homogeneous glass.

As a conventional example using a gradient index lens as a microscope objective, there is a lens system described in Japanese Patent Application Laid-Open No. Sho 62-31816. However, this microscope objective uses three refractive index distribution lenses, which is not desirable in terms of cost, and has a small NA of 0.46.

The lens system described in Japanese Patent Application Laid-Open No. Sho 62-34117 uses one refractive index distribution lens, but does not consider chromatic aberration correction. .93, and no consideration is given to an immersion type objective lens whose NA exceeds 1.0.

Further, the lens systems described in JP-A-6-3598 and JP-A-7-159697 are described as follows.
An immersion type objective lens in which the number of refractive index distribution lenses is small in a few lenses but NA is only up to 0.8 and NA exceeds 1.0 is not considered.

[0006]

SUMMARY OF THE INVENTION In the present invention, the NA is 1.
It is an object of the present invention to provide an immersion type objective lens exceeding 0, in which various aberrations are corrected well.

[0007]

The immersion type objective lens of the present invention has, in order from the object side, a plano-convex single lens or a plano-convex lens having a cemented surface with a strong convex surface facing the image side. At least three groups including a first lens group, a second lens group including a gradient index lens having a positive refractive power, and a third lens group including a gradient index lens having a negative refractive power. It is characterized by.

The refractive index distribution N (r) of the gradient index lens used in the objective lens of the present invention is represented by the following equation (a). N (r) = N ₀₀ + N ₁₀ r ² + N ₂₀ r ⁴ +... (A) In this equation (a), r is the distance in the radial direction from the optical axis, and N
₀₀ is the refractive index on the optical axis, N ₁₀ , N ₂₀ ,.
Fourth,... Coefficients.

Generally, a microscope objective lens has a working distance,
Since it is necessary to sufficiently correct various aberrations while satisfying various requirements such as NA and magnification, the number of lenses increases, the optical system becomes complicated, and there is a problem in cost.

In a lens made of homogeneous glass, aberration occurs due to refraction of light rays on the lens surface. However, in the case of a lens system with a small number of lenses, the refraction angle of light rays on each surface becomes large, and Causes a large amount of aberration. for that reason,
By increasing the number of lenses constituting the lens system and reducing the angle of refraction of light rays on each surface, aberrations occurring on each surface are reduced to correct the aberration of the entire lens system.

On the other hand, the refractive index distribution type glass can correct the aberration by giving the medium a refractive power. Therefore, the refractive index distribution lens can reduce the amount of aberration generated on the lens surface. Aberrations in the entire system can be corrected well.

As described above, the number of objective lenses can be reduced by using a refractive index distribution lens having a higher aberration correcting ability than a homogeneous lens.

The present invention relates to an immersion type objective lens having an NA of 1.
The objective is to obtain an objective lens as large as 0 or more. For this purpose, the first lens group is composed of a plano-convex positive lens or a plano-convex cemented lens having a cemented surface with a strong convex surface facing the image side. The plano-convex single lens or the plano-convex cemented lens has a convex surface which satisfies the aplanation condition to suppress the occurrence of spherical aberration.

The first lens group has a positive refractive power and suppresses the divergence of light rays to guide the light rays to the second lens group.
The second lens group has a positive refractive power as a whole, corrects chromatic aberration, and guides the incident light beam as a convergent light beam to the third lens group. The third lens group must have a principal point position on the object point side for making the lens system a high-power objective lens with a short focal length, and is configured to have a negative refractive power.

The objective lens of the present invention has a short focal length and a high power to make it a high-magnification objective lens. By using a refractive index distribution lens, various aberrations that cannot be completely corrected on the lens surface are optimally corrected. I have. That is, as described above, the second
By disposing a refractive index distribution lens having a positive refractive power in the lens group and a refractive index distribution lens having a negative refractive power in the third lens group, various aberrations can be satisfactorily corrected in the high-power objective lens having the above-described configuration. I have.

In the objective lens according to the present invention, the refractive index distribution lens used for the second lens group in the lens system having the above-mentioned configuration is provided.
The thickness is made thickest on the axis and becomes thinner as it goes off the optical axis, and the refractive power of the medium is made positive to better correct various aberrations.

In order to favorably correct various aberrations of the entire objective lens system, the second lens group needs to have a positive refractive power. If the second lens group has a negative refractive power, the combined refractive power of the second lens group and the third lens group becomes a strong negative refractive power, and accordingly, the first lens group also has a strong positive refractive power. helpful. In the case of a high NA objective lens having an NA of 1.0 or more, such as an immersion objective lens, if the positive refractive power of the first lens group becomes too large, it becomes impossible to correct spherical aberration of the entire system. Therefore, the second lens group needs to have a positive refractive power. When only the power of the lens surface has the positive power of the refractive index distribution lens used for the second lens group, the power of this surface becomes strong, the spherical aberration becomes large, and the correction cannot be completed.

Therefore, in the lens system of the present invention, it is desirable that the shape of the refractive index distribution lens in the second lens unit having a positive power be as described above, and that the refractive power of the medium be positive. It is desirable that the medium be given a positive refractive power so as to reduce the positive power of the surface to suppress the occurrence of various aberrations and to optimally correct various aberrations, especially spherical aberration.

Next, in the lens system of the present invention, the refractive index distribution lens in the third lens group is formed to have a shape such that its thickness is thin on the axis and becomes thicker as it goes off the optical axis. It is desirable that the medium has a negative refractive power.

Since the objective lens of the present invention has a high magnification and a short focal length, the first lens unit and the second lens unit have a positive refractive power, and spherical aberration which cannot be completely corrected by these lens units.
It is necessary to correct various aberrations such as curvature of field and coma,
The third lens group needs to have a negative refractive power.

A liquid immersion type objective lens such as the objective lens of the present invention does not allow the immersion liquid to enter the lens surface closest to the object (the surface that comes into contact with the immersion liquid) or does not deteriorate the performance due to the entry of air. The surface closest to the object must be flat, and the front lens is a plano-convex lens. When the front lens is a plano-convex lens, the field curvature cannot be sufficiently corrected.
In order to correct this field curvature, in the lens system of the present invention, the third lens group has a negative refractive power.

However, if the negative power of the lens surface of the third lens group is increased in order to satisfactorily correct the curvature of field of the lens system, spherical aberration and coma cannot be optimally corrected. Therefore, in the lens system of the present invention, the refractive index distribution lens in the third lens group is formed as described above, and the power of the negative surface in the third lens group is reduced by setting the refractive power of the medium to a negative value. In addition, it is preferable that spherical aberration and coma are optimally corrected together with the correction of the field curvature.

In the microscope objective lens of the present invention, the second
The thickness d of the refractive index distribution lens of the lens group is the following condition (1).
It is desirable to satisfy (1) d> 30 / f where f is the focal length of the entire objective lens system.

_Assuming that the refractive power of the medium of the gradient index lens is φ _M , φ _M can be approximated by the following equation (b). φ _M = −2 · N ₁₀ d (b)

[0025] From this equation (b), the refractive index distribution lens of the second lens, to satisfy the condition (1), it is possible to reduce the value of having and N ₁₀ sufficient power in the medium. Therefore, the refractive index difference between the optical axis and the vicinity of the outer diameter of the gradient index lens can be reduced, and the manufacture of the gradient index lens becomes easy.

[0026] Conversely to the refractive index difference between the optical axis and the outside diameter periphery of the condition (1) is not satisfied when the value of N ₁₀ is increased refractive index lens becomes large production becomes difficult.

[0027] Further, in the objective lens of the present invention, the second lens group, the point of view of one of the fractional coefficients V ₁₀ of the refractive index distribution lens of the third lens unit to satisfactorily correct the chromatic aberration that is negative Desirable.

The axial chromatic aberration PAC of the radial type gradient index lens can be expressed by the following equation (c). _{PAC = K (φ S / V} 00 + φ M / V 10) (c) where phi _S, phi _M is the refractive power of the surface and the medium of the respective gradient index lens, also V _00, V ₁₀ in the following formula Is a value given by V ₀₀ = (N _00d -1) / (N _00F -N _00C ) V ₁₀ = N _10d / (N _10F -N _10C ) where N _00d , N _00F , and N _00C are d-line, F-line, and C-line, respectively.
The refractive indices on the optical axis of the line, N _10d N _10F and N _10C are d
It represents linear, F line, the refractive index distribution coefficient N ₁₀ for the C line.

[0029] from the formula (c), by changing the value of V _10, it is possible to vary the axial chromatic aberration generated in the medium.

In equation (c), φ _S representing the refractive power of the surface of the refractive index distribution lens of the second lens group is a positive value, and φ _M is required to reduce the axial chromatic aberration. The term must be negative. However, as described above, the refractive power φ _M of the medium of the refractive index distribution lens of the second lens group is
Must have a positive refractive power in order to correct the aberration in good balance, in order to optimally correct the axial chromatic aberration of the lens system, V ₁₀ must satisfy the condition that a negative value .

[0031] Also, the refractive index distribution lens in the third lens group, in order to phi _S representing the refractive power of the surface in the formula (c) is a negative value, in order to reduce the axial chromatic aberration, phi _M
Must take a positive value. However, as described above, in the refractive index distribution lens of the third lens group, the refractive power φ _{M of the} medium is a negative value in order to correct aberrations in a well-balanced manner. Therefore in order to optimally correct the axial chromatic aberration, it is desirable that the value of the gradient index lens V ₁₀ in the third lens group satisfies the condition that it is a negative value.

In the lens system according to the present invention, it is desirable that the gradient index lens in the third lens group satisfies the following condition (2). (2) φ _M (3) / V ₁₀ (3)> 0.001 where φ _M (3) is the refractive power of the medium of the refractive index distribution lens in the third lens unit, and V ₁₀ (3) is the third power. is the value of V ₁₀ representing the dispersion of the refractive index distribution lens in the lens group.

If the above condition (2) is satisfied, it is possible to correct various aberrations and chromatic aberration caused by the refractive power of the medium of the refractive index distribution lens in the third lens group in an optimal balance. If the condition (2) is not satisfied, the above-mentioned balance is lost and it becomes difficult to correct various aberrations in a well-balanced manner.

[0034]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the immersion objective lens according to the present invention will be described based on respective examples.

1 to 3 are cross-sectional views of Examples 1 to 3 of the lens system according to the present invention. Data of these Examples is as follows.

Example 1 f = 1.8, NA = 1.25, Image Height 10.5, WD = 0.168 r ₁ = ∞ d ₁ = 0.375 n ₁ = 1.51742 ν ₁ = 52.41 r ₂ = -0.9 d ₂ = 2.99 n ₂ = 1.883 ν ₂ = 40.78 r ₃ = −2.65 d ₃ = 0.1 r ₄ = 61.22 d ₄ = 30 Refractive index distributed lens 1 r ₅ = -19.57 d ₅ = 8.61 r ₆ = -7.119 d ₆ = 6.65 Refractive index distributed lens 2 r ₇ = 17.646 Refractive index distribution lens 1 N ₀₀ N ₁₀ N ₂₀ N ₃₀ d-line 1.7 -1.9538 × 10 ^-3 1.8434 × 10 ^-6 4.4850 × 10 ^-9 V ₀₀ = 50 V ₁₀ = -77.334 Refractive index distribution lens 2 N ₀₀ N ₁₀ N ₂₀ N ₃₀ d line ^{1.7 1.4085 × 10 -3 -6.3656 × 10} -5 1.1066 × 10 -6 V 00 = 55 V 10 = -4.3066 f 2 = 12.1875, f 3 = -5.634, φ M (3 ) = ^-4 × 10 ^-4

[0037] Example 2 f = 1.8, NA = 1.25 , the image height _{10.5, WD = 0.168 r 1 =} ∞ d 1 = 3.367 n 1 = 1.883 ν 1 = 40.78 r 2 = -3.9643 d 2 = 0.1 r 3 = 146.449 d ₃ = 26.83 Refractive index distributed lens 1 r ₄ = -31.6762 d ₄ = 8.5 r ₅ = -9.1565 d ₅ = 9.93 Refractive index distributed lens 2 r ₆ = 10.179 Refractive index distributed lens 1 N ₀₀ N ₁₀ N ₂₀ N ₃₀ d line ^{1.7 -2.9563 × 10 -3 3.6355 × 10} -6 2.5749 × 10 -9 V 00 = 50 V 10 = -5.2481 × 10 2 gradient index lens _{2 N 00 N 10 N 20 N} 30 d line 1.7 3.0107 × 10 ^{- 4} -5.573 × 10 ^-5 5.7221 × 10 ^-7 V ₀₀ = 55 V ₁₀ = -1.5448 f ₂ = 10.1124, f ₃ = -5.4159, φ _M (3) =-2.8 × 10 ^-2

Example 3 f = 3, NA = 1.25, Image Height 10.5, WD = 0.168 r ₁ = ∞ d ₁ = 0.375 n ₁ = 1.51742 ν ₁ = 52.41 r ₂ = -0.9 d ₂ = 2.99 n ₂ = 1.883 ν ₂ = 40.78 r ₃ = −2.65 d ₃ = 0.1 r ₄ = 46.791 d ₄ = 33.9 refractive index distribution lens 1 r ₅ = -19.657 d ₅ = 6.97 r ₆ = -9.876 d ₆ = 4.44 refractive index distribution lens 2 r ₇ = -30 Refractive index distribution lens 1 N ₀₀ N ₁₀ N ₂₀ N ₃₀ d line 1.7 -1.5914 × 10 ^-3 6.7574 × 10 ^-7 8.6123 × 10 ^-9 V ₀₀ = 50 V ₁₀ = -64.37 Refractive index distribution lens 2 N ₀₀ N ₁₀ N ₂₀ N ₃₀ d-line 1.7 6.6418 × 10 ^-3 -4.964 × 10 ^-5 2.5591 × 10 ^-7 V ₀₀ = 55 V ₁₀ = -25.93 f ₂ = 12.1875, f ₃ = -5.634, φ _M ( 3) = − 5.9 × 10 ^{−2 where} r ₁ , r ₂ ,... Are the radii of curvature of the respective surfaces of the lens, d
_.. , D ₂ ,...
₁ , n ₂ ,... Are the refractive indices of each lens, ν ₁ , ν ₂ ,.
Is the Abbe number of each lens.

In the first embodiment, the first lens unit is composed of a cemented lens having a cemented surface with a strong convex surface facing the image side, and the second lens unit is of a biconvex shape, that is, the optical axis. The third lens group is a biconcave shape, that is, on the optical axis, which has a shape in which the upper thickness is large and the thickness decreases with distance from the optical axis, and the medium has a positive refractive power. Is an objective lens comprising a refractive index distribution lens in which the thickness is small and the thickness increases with distance from the optical axis, and the medium has a negative refractive power.

The second embodiment is as shown in FIG.
The third embodiment differs from the first embodiment in that the first lens group is formed of a plano-convex single lens.

The third embodiment has the same configuration as that shown in FIG. 3 and is similar to the objective lens of the first embodiment.

Each of the objective lenses of Examples 1 to 3 is an infinity-correction type in which a light beam emitted from the objective lens is a parallel light beam, and does not form an image by itself. Therefore, it is used in combination with an imaging lens such as the lens system shown in FIG. 4 and having the following data. _{R 1 = 68.7541 D 1 = 7.7321} N 1 = 1.48749 V 1 = 70.20 R 2 = -37.5679 D 2 = 3.4742 N 2 = 1.80610 V 2 = 40.95 R 3 = -102.8477 D 3 = 0.6973 R 4 = 84.3099 D 4 = 6.0238 aberration of _{N 3 = 1.83400 V 3 = 37.16} R 5 = -50.7100 D 5 = 3.0298 N 4 = 1.64450 V 4 = 40.82 R 6 = 40.6619 examples 1-3 are shown respectively Figures 5-7. Each of the aberration curve diagrams is obtained when the lens is combined with the imaging lens shown in FIG.

In the above data, the unit of r, d, R, D, etc. is
mm.

Although the embodiment is designed with an infinity objective lens, a finite system objective lens is of course easy to design, and the present invention is not limited to an infinity correction type objective lens.

In addition to the objective lens described in the claims, the objective lens described in each of the following items can also achieve the object of the present invention.

(1) In the lens system according to any one of claims 1, 2 and 3, the refractive index distribution lens of the second lens group satisfies the following condition (1). System objective lens. (1) d> 30 / f

(2) The refractive index distribution of one or both of the second lens unit and the third lens unit in the lens system according to claim 1, 2 or 3, or the lens system according to the above (1). immersion objective lens, wherein the dispersion coefficient V ₁₀ of the lens is negative both.

(3) In the lens system according to claim 1, 2, or 3, or (1) or (2), the dispersion coefficient V of the refractive index distribution lens in the third lens group is set.
₁₀ An immersion objective lens characterized in that (3) satisfies the following condition (2). (2) φ _M (3) / V ₁₀ (3)> 0.001

[0049]

The objective lens of the present invention is a liquid immersion type objective lens having an NA exceeding 1.0, and various aberrations are well corrected by a small lens system having a small number of lenses.

[Brief description of the 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 diagram showing an example of an imaging lens used in combination with the objective lens of the embodiment.

FIG. 5 is an aberration curve diagram according to the first embodiment of the present invention.

FIG. 6 is an aberration curve diagram according to the second embodiment of the present invention.

FIG. 7 is an aberration curve diagram according to the third embodiment of the present invention.

Claims (3)

[Claims] 1. A first lens group having a positive refractive power including a plano-convex single lens or a plano-convex lens having a cemented surface with a strong convex surface facing the image side, and a refractive lens having a positive refractive power in order from the object side. A second lens group including a refractive index distribution lens whose refractive index distribution is represented by the following equation (a), and a refractive index distribution lens having a negative refractive power and a refractive index distribution represented by the following equation (a): Third
An immersion type objective lens having at least three groups including a lens group. N (r) = N ₀₀ + N ₁₀ r ² + N ₂₀ r ⁴ +... (A) where r is the distance in the radial direction from the optical axis, N ₀₀ is the refractive index on the optical axis, N ₁₀ , N ₂₀ , ... are the second, fourth, etc. respectively
The coefficient of 2. The refractive index distribution lens of the second lens group has a shape in which the thickness is the largest on the axis and the thickness is reduced toward the outside of the optical axis, and the refractive power of the medium is positive. 2. The immersion microscope objective lens according to claim 1, wherein: 3. The refractive index distribution lens of the third lens group,
3. The immersion microscope objective lens according to claim 1, wherein the thickness of the medium is thinnest on the axis, and the thickness increases toward the outside of the optical axis, and the refractive power of the medium is negative.