JPH0567003B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to an objective lens for a microscope, and particularly to a so-called objective lens with a correction ring capable of correcting aberration fluctuations when the thickness of a cover glass changes. [Background of the Invention] Generally, in the biological field, when preparing a microscopic specimen, in most cases, except for smears such as blood, the specimen is placed on a slide glass and a cover glass is placed on top of it. and enclose it,
This is a so-called specimen with a cover glass. The thickness and refractive index of this cover glass are also specified by JIS, and a thickness of 0.17 mm is usually used as the standard thickness for design. It is already known that errors in the thickness and refractive index of this cover glass affect aberrations. error has a large influence. This effect becomes more pronounced as the numerical aperture (NA) increases, and NA is 0.8.
When exceeding
It is necessary to cancel fluctuations in aberrations. When the numerical aperture exceeds 0.9, the fluctuation is particularly significant, and it must be said that it is no longer usable without a correction ring mechanism. In particular, high-resolution lenses are desired these days, and lenses with good image plane flatness over an ultra-wide field of view are also required in order to improve the efficiency of photography and microscopy. As an objective lens with a correction ring, for example,
There is one disclosed in Publication No. 51-87056. The first example has a magnification of 60 ^× , the NA is 0.95, and the cover glass correction range is 0.14 to 0.19 mm, and the second example has a magnification of 60×.
NA is 0.85 at 40 ^× , cover glass correction range is 0.14
~0.19mm. In Example 1, the NA is 0.95 and the resolution is sufficient, but the Petzval sum is 0.6 (f =
1), it is extremely large and the flatness of the image is not good. With this, only the center of the field of view is in focus, making it impossible to observe or photograph the periphery at the same time.
In addition, in Example 2, the Petzvaar sum is 0.03 (f=
1), which is an improvement over Example 1, and the flatness is better, but the NA is 0.85, which is unsatisfactory in terms of resolution. In both cases, the correction range of the cover glass is 0.14 to 0.19, which is still not sufficient. This is because in addition to variations in the thickness of the cover glass, the thickness of the specimen itself and the thickness of the mounting medium are actually added, so when these are included, the thickness of the parallel plane layer will vary by at least 0.13 to 0.21. This is because it will be done. As described above, it has been difficult to obtain an objective lens with a correction ring that has high resolving power and a flat image plane at the same time, and the reasons for this are as follows. One is the correction of chromatic aberration, which is
This is because it has been difficult to eliminate color disturbances not only in axial chromatic aberrations but also in high-order spherical aberrations over a large aperture. Another reason is the problem of flatness of the image plane. The problem is to correct curvature of field, but as the Petzvaar sum decreases, chromatic aberration worsens, so it becomes dramatically more difficult to correct chromatic aberration and curvature of field at the same time as the numerical aperture increases. I'm getting old. The thickness of the cover glass is the standard
It is difficult to correct aberrations even at 0.17mm, but even when the thickness changes, it is even more difficult to have each group share the amount of aberration correction so as to cancel out the fluctuations in aberrations. It's easy to imagine. [Object of the Invention] The object of the present invention is to increase the numerical aperture to 0.95, the maximum for a dry system, and to maintain extremely good flatness of the image plane, while also being able to withstand large changes in the thickness of the cover glass. It is an object of the present invention to provide a plan apochromat objective lens that can maintain excellent imaging performance even when the temperature is low. [Summary of the Invention] As shown in FIG. 1, the present invention divides the entire body into three groups, and sequentially from the object O side, includes a positive meniscus lens with a concave surface facing the object side. The first lens group has a positive refractive power that converts the light into a convergent light beam.
G ₁ , a second lens group G ₂ having a composite positive refractive power, which includes a laminated lens component having a diverging cemented surface and a laminated lens component having a convergent cemented surface; a negative refracting lens having a cemented surface; It consists of a third lens group _G3 . Then, variations in aberration due to changes in the thickness of the parallel plane plate P (usually a cover glass) disposed between the object plane and the first lens group _G1 are adjusted to the second lens with respect to the first lens group _G1 . This is corrected by relatively moving the group _G2 and the third lens group _G3 . Specifically, when the thickness of the parallel plane plate P is smaller than the reference state, the first lens group
moving the second lens group G ₂ and the third lens group G ₃ so that the distance between G ₁ and the second lens group G ₂ becomes smaller;
When the thickness of the parallel plane plate P is larger than the reference state, the second lens group G ₂ and the third lens group G are adjusted so that the distance between the first lens group G ₁ and the second lens group G ₂ becomes larger. A good correction is achieved by moving 3 and ₃ . Here, the combined focal length of the entire system is F, the focal length of the first lens group _G1 is _f1 , the focal length of the second lens group _G2 is _f2 , and the first lens group _G1 and the second lens group G When _{the air distance between the} It is necessary to. Due to the configuration of the present invention, the numerical aperture (NA) is larger and the resolution is higher than that of conventional ones.
It is a plan apochromatic objective lens with excellent correction of chromatic aberration and excellent flatness of the image plane, and it has become possible to make the correction range of the cover glass even wider than before. In the present invention, the entire lens is divided into three groups as described above, and each group is responsible for correcting aberrations. Therefore, the aberration characteristics of each lens group will be explained. First, each lens group _G1 has a converging effect, converting a divergent light beam from the object surface into a convergent light beam, and makes the light beam incident on the second lens group _G2 have a certain angle. . If the angle of this ray is parallel to the optical axis, even if the first lens group G ₁ and the second lens group G ₂ move relative to each other, the height of the ray that enters the second lens group G ₂ will change. Since there is no aberration, there is almost no variation in aberrations, making it useless as an aberration correction mechanism. On the other hand, if the angle of the light ray is too strong, although aberrations can be corrected with a small amount of movement, the burden on the refractive power of the first lens group _G1 is too large, making it difficult to correct the aberrations themselves. More specifically, the first lens group
The appropriate magnification for G ₁ is 10 to 15 times, and the second
Applying an additional 4x magnification to lens group G ₂ and beyond,
A total magnification of 40x to 60x is desirable. For this reason as well, it is necessary that the power of the first lens group _G1 be as shown in the above equation (1). By changing the height at which the convergent light flux having a certain inclination that comes out of the first lens group _G1 enters the second group, by moving the second and third groups relative to the first group, By changing the amount of various aberrations including spherical aberration occurring in the second and third groups, aberration fluctuations due to thickness errors of the plane parallel plates are canceled out. These facts can also be understood from Seidel's aberration coefficients, which will be described later. According to this, while the spherical aberration generated in a plane-parallel plate varies greatly depending on its thickness, the amount of spherical aberration generated in the first group itself hardly changes even if the thickness of the plane-parallel plate changes. Recognize. Next, regarding the second lens group _G2 and the third lens group _G3 , in these groups, if the thicknesses of the parallel plane plates are different, the amount of aberration generated will be different. This is because the height of the light beam incident on the second lens group _G2 (strictly speaking, its slope also changes slightly) changes by moving the second and third groups. The amount of variation in these aberrations and the amount of variation in aberration due to the plane-parallel plate cancel each other out, thereby correcting the aberration variation as a whole system. Each of the above conditional expressions will be explained below. The condition of equation (1) determines the appropriate range of the refractive power of the first lens group _G1 , and in terms of aberrations, it mainly affects the correction of spherical aberration, and also affects the WD (working distance). Related. If the lower limit of this condition is exceeded, the focal length f ₁ of the first lens group G ₁ becomes relatively small, and the convergence effect of the light flux of the first lens group becomes too strong, resulting in a small WD, making it impractical. . In addition, spherical aberration is insufficiently corrected, and in particular, a large amount of spherical aberration remains at high-order short wavelengths. Furthermore, negative asymmetric aberration also occurs, and this similarly remains particularly large on the short wavelength side. Regarding the field curvature, as f ₁ becomes smaller, the Petzvaar sum becomes larger, so the field curvature also becomes larger and the flatness deteriorates. Conversely, when the upper limit of this condition is exceeded, f ₁ increases and the convergence effect of the light beam exiting the first lens group G ₁ weakens. Therefore, the light beam spreads greatly, and spherical aberration increases, making it difficult to correct it with the second and third rear groups. Conditional expression (2) is the first lens group G ₁ and the second lens group G ₂
It specifies the appropriate value for the air spacing between the
When the thickness of the parallel plane plate becomes smaller than the standard thickness,
Negative spherical aberration occurs. For this reason, the second and third groups are moved toward the object side, that is, the air gap between the first group and the second group is reduced, and the second group is , the second lens group G ₂ and the third lens group G ₃
It is possible to generate positive spherical aberration and cancel out fluctuations in spherical aberration as a whole system. However, the first lens group G ₁ and the second lens group G ₂
If the air distance D ₁ becomes smaller and exceeds the lower limit of condition (2), even if the aberrations can be canceled, physically contact with the lens support hardware. Also,
In order to increase the effect of this air distance D ₁ on aberrations, the focal length f ₁ of the first lens group G ₁ is made smaller and the inclination of the light flux from the first group to the second group is increased (1) As described with respect to the formula, it becomes difficult to correct higher-order spherical aberration on the short wavelength side. When the upper limit of condition (2) is exceeded, if the air distance D ₁ between the first lens group G ₁ and the second lens group G ₂ is increased while the convergence effect of the first lens group remains strong, the second lens group G ₂
The height of incidence of the light beam on the lens becomes smaller, and the total length becomes shorter. When the total length is shortened, if the entire system is expanded proportionally, the composite center thickness of the lens increases, making it impossible to maintain parfocality. Therefore, if the convergence effect of the first lens group is weakened in order to increase the overall length, the air distance between the first lens group G ₁ and the second lens group G ₂ must be greatly changed in order to cancel out the fluctuations in spherical aberration. is required, making it difficult to make sufficient correction. In this case, even if axial aberrations can be canceled, off-axis aberrations cannot be canceled, and mainly asymmetric aberrations cannot be completely corrected. Condition (3), which defines the appropriate distribution of refractive power of the second lens group _G2 , is to maintain the aberration correction effect in the second lens group _G2 in an appropriate state in relation to other groups. The cemented surface in the second lens group _G2 is provided mainly to correct chromatic aberration. In other words, the second lens group G ₂ largely corrects the chromatic aberration of magnification that could not be completely corrected by the first lens group G ₁ , and compensates for the excessive high-order spherical aberration on the short wavelength side in the first lens group. is corrected by the second group. If the refractive power of the second group is strengthened beyond the lower limit of this condition (3), the convergent light beam from the first group will be converged even more strongly. The negative refractive power of the lens must be strengthened. However, if the positive refractive power of the second group becomes too strong, the height of incidence from the second group to the third group becomes too low, resulting in a short overall length. In addition, the negative spherical aberration becomes large, and together with the negative spherical aberration that occurs in the first lens group, the negative amount becomes too large and the third lens group becomes larger.
It is too large to generate and correct positive spherical aberration in the group, making it difficult to maintain a good correction state, and in particular, high-order spherical aberration cannot be corrected completely. Conversely, if the upper limit of condition (3) is exceeded and the refractive power of the second group becomes weak, the convergence effect will also become weak, the height of incidence on the third group will increase, and the total length will become longer. If you reduce the overall length proportionally and make it shorter, the WD will become smaller and the operability will deteriorate. Now, in the basic configuration according to the present invention as described above, the specific configuration of the second lens group _G2 is to provide a bonded lens component having a diverging cemented surface with a convex surface facing the object side on the object side, Subsequently, it is desirable to provide a laminated lens component having a convergent cemented surface with a convex surface facing the image side. Then, the radius of curvature of the diverging cemented surface with the convex surface facing the object side is R ₁ , the refractive power and Atsube number of the negative lens on the object side forming this diverging cemented surface are N _1o , ν _1o , and the positive lens on the image side is The refractive power and Atsube number are N _1p and ν _1p , the radius of curvature of the convergent cemented surface with the convex surface facing the image side is R ₂ , and the refractive power and Atsube number of the object-side positive lens forming this convergent cemented surface are When N _2p , ν _2p , and the refractive power and Atsube number of the image side negative lens are N _2o , ν _2o , 0<(|Φ ₁ |−|Φ ₂ |)×F<0.15 (4) However, Φ ₁ =N _1o −N _1p /R ₁ Φ ₂ =N _2o −N _2p /R ₂ 40<ν _1p −ν _1o (5) It is desirable to satisfy the following conditions: 15<ν _2o −ν _2p (6). The above conditional expression (4) relates to the power of the two cemented surfaces in the second lens group _G2 . There are at least two joint surfaces in the second group,
The object-side cemented surface functions to correct chromatic aberration, and the image-side cemented surface functions to reverse chromatic aberration correction. This is mainly for correcting lateral chromatic aberration. That is, the front component of the second group greatly corrects the longitudinal chromatic aberration and the chromatic chromatic aberration of magnification, and the rear component of the second group returns the axial chromatic aberration to its original state and slightly returns the chromatic aberration of magnification. Finally, the balance between spherical aberration, longitudinal chromatic aberration, and lateral chromatic aberration as a whole system is corrected by reverse achromatic aberration in the third group. When the lower limit of condition (4) is exceeded, the power of the cemented surface of the front component becomes weaker, causing spherical aberration and axial chromatic aberration.
The amount of correction for lateral chromatic aberration becomes smaller. For this reason, the second
The amount of spherical aberration correction for the group as a whole becomes smaller,
The burden on the third group becomes too great. As a result, the power of the cemented surface of the third group becomes strong, and the longitudinal chromatic aberration becomes large and undercorrected. Conversely, if the upper limit is exceeded, the power of the cemented surface of the front component becomes too strong, resulting in over-correction of spherical aberration, and as a result of weakening the power of the cemented surface of the third group, axial chromatic aberration becomes over-corrected. Put it away. The above condition (5) relates to second group achromatization. As stated in equation (4), the cemented surface of the front group of the second group greatly contributes to achromatization and correction of spherical aberration.
The amount of correction for spherical aberration depends largely on the radius of curvature R of the cemented surface according to equation (4) above, but the amount of correction for chromatic aberration is
Furthermore, in addition to this, the Abbe numbers of the positive lens and negative lens that make up this cemented lens are greatly involved. This conditional expression (5) is the second condition for correcting chromatic aberration.
This gives the relationship between the Abbe number ν _1p of the positive lens constituting the cemented lens of the front component of the group and the Abbe number ν _1o of the negative lens. When this conditional expression is not satisfied, the difference in Abbe's numbers becomes small and the amount of color correction becomes small. As a result, the amount of reverse achromatization in the rear group of the second group and the third group has to be reduced, and as a result, large lateral chromatic aberration remains. This is because the correction of lateral chromatic aberration is largely dependent on achromatization in the front components of the first and second groups, and the overcorrected longitudinal chromatic aberration is corrected by the rear components of the second group and the third group. This is because it is returned with In addition, if we try to compensate for the small difference in Atsube number by increasing the curvature of the cemented surface, the spherical aberration will be overcorrected, and the higher-order spherical aberration will be excessively excessive, especially on the short wavelength side. Put it away. The condition of equation (6) above is a condition regarding reverse achromatization in the rear component in the second lens group _G2 . As mentioned above, the second group is composed of a front component and a rear component, and the front component is achromatic and the rear component is reverse achromatic, so that the longitudinal chromatic aberrations almost cancel each other out. On the other hand, although the lateral chromatic aberration is largely corrected in the front component and slightly restored in the rear component, it is still insufficiently corrected, and the third group corrects that amount. Therefore, if condition (6) is violated and the difference in Abbe numbers between the positive and negative lenses that make up the rear component of the second group becomes small, this means that the amount of reverse achromatization in the rear component of the second group becomes small. This burden is placed on the third lens, which has a greater effect of correcting chromatic aberration of magnification, resulting in over-correction of chromatic aberration of magnification.
Furthermore, if the amount of reverse achromatization is increased by increasing the curvature of the cemented surface of the rear component, the spherical aberration here will be large and under-corrected, and the third group will have to compensate even more, which will also result in lateral chromatic aberration. This results in over-correction. [Example] Examples according to the present invention will be described below. The first embodiment according to the present invention shown in FIG.
It is a plan apochromat objective lens with a magnification of 40x and an NA of 0.95. In the figure, the state of the light ray from the on-axis object point is shown by a solid line. In this first embodiment, the first lens group G ₁ includes, in order from the object side, a positive meniscus lens L ₁ with a concave surface facing the object side, a positive meniscus lens L ₂ with a concave surface facing the object side,
Consists of a positive lens _L3 with a surface with a strong curvature facing the image side and a bonded positive lens _L4 with a surface with a strong curvature facing the image side, and the second lens group _G2 has a convex surface facing the object side. The front component _L5 consists of a negative meniscus lens and a positive lens with a surface with a stronger curvature facing the object side, and a positive lens with a surface with a stronger curvature facing the image side with a negative lens with a surface with a stronger curvature facing the object side. It consists of a posterior component _L6 consisting of a junction with a lens. The third lens group _G3 is composed of a bonded negative lens component _L7 with the concave surface facing the image side, and this negative lens component _L7 is formed by bonding a biconvex positive lens and a biconcave negative lens. has been done. Table 1 below shows the specifications of the first embodiment. In the table, the leftmost number represents the order from the object side, and the refractive index and Abbe number are values for the d-line (λ=587.6 nm). Also, d ₀ is a parallel plane plate (cover glass)
This is the distance from the objective lens side surface to the vertex of the first surface of the objective lens. In addition, in the following examples, the object plane and the first
When the thickness of the parallel plane plate P disposed between the lens group _G1 is t, and the refractive index and Abbe number for the d-line are nd and νd, respectively, t=0.17 nd=1.522 νd=58.8. It is designed based on this state as a standard, and is designed to perform correction when the thickness t of the parallel plane plate P changes.

[Table] Various aberration diagrams for the first example above are shown in the second example.
Shown in Figures A to 2C. Figure 2A shows a state in which the cover glass has the standard thickness (t = 0.17 mm), Figure 2B shows a state in which the cover glass is thinner than the standard (t = 0.11 mm),
FIG. 2C shows a state in which the cover glass is thicker than the standard (t=0.23 mm). Further, the second embodiment, whose lens configuration diagram is shown in FIG. 3, is a high-magnification objective lens having a magnification of 60 times,
It has almost the same configuration as the first embodiment described above except for the configuration of the third lens group _G3 . 3rd lens group
G ₃ is a combination of three lenses: a negative lens with a concave surface facing the object side, a positive lens with a surface with a stronger curvature facing the image side, and a biconcave negative lens with a surface with a stronger curvature facing the object side. It is made up of Table 2 below shows the second example as well as the first example above.
The specifications of the example are shown.

[Table] Various aberration diagrams for the second example above are shown in the fourth example.
Shown in FIGS. A to 4C. Figure 4A shows a state in which the cover glass has the standard thickness (t = 0.17 mm), Figure 4B shows a state in which the cover glass is thinner than the standard (t = 0.13 mm),
FIG. 4C shows a state in which the cover glass is thicker than the standard (t=0.21 mm). Furthermore, Tables 3 and 4 below show the third-order aberration coefficients of spherical aberration for the first and second examples, respectively, when the thickness of the cover glass changes. In each table, the numbers at the left end represent the order from the object side, and indicate the values on the cover glass P, as well as the subtotals for each lens group and the total value for the entire system.

【table】

【table】

【table】

〔Effect of the invention〕

As described above, according to the present invention, the numerical aperture is increased to 0.95, which is the maximum for a dry system, and the thickness of the cover glass is increased compared to the conventional one while maintaining extremely good flatness of the image plane. A plan apochromatic objective lens that can maintain excellent imaging performance even when changing over a range is realized.

[Brief explanation of the drawing]

FIG. 1 is a lens configuration diagram of the first embodiment according to the present invention, FIGS. 2A, 2B, and 2C are various aberration diagrams for the first embodiment when the cover glass thickness changes, and FIG. The lens configuration diagrams of the second embodiment, FIG. 4A, FIG. 4B, and FIG. 4C are diagrams of various aberrations when the cover glass thickness changes in the second embodiment. Explanation of symbols of main parts, G ₁ ... first lens group,
G ₂ ... 2nd lens group, G ₃ ... 3rd lens group, P
……cover glass.

Claims (1)

[Claims] 1. In order from the object side, a first lens group G ₁ having a positive meniscus lens with a concave surface facing the object side and having a positive refractive power that converts a light beam from the object surface into a convergent light beam; A second lens group G ₂ having a composite positive refractive power, which includes a laminated lens component having a cemented surface and a laminated lens component having a convergent cemented surface, and a third lens group having a negative refractive power and having a cemented surface. _G3 , and the first lens group is configured to correct aberration fluctuations caused by changes in the thickness of the parallel plane plate disposed on the object surface.
The _second lens group _G2 and the third lens group _G3 are movable relative to G1, and the combined focal length of the entire system is F, and the focal length of the first lens group _G1 is f. _1. When the focal length of the second lens group G ₂ is f ₂ and the air distance between the first lens group G ₁ and the second lens group G ₂ is D ₁ , 1.1<f ₁ /F<4. A microscope objective lens that satisfies the following conditions: (1) 0.2<D ₁ /F<1.5 (2) 5<f ₂ /F<20 (3). 2 The divergent cemented surface of the bonded lens component in the second lens group G ₂ faces the object side, and the convergent cemented surface of the bonded lens component in the second lens group G ₂ faces the image side. The radius of curvature of the diverging cemented surface with the convex surface facing the object side is R ₁ , and the refractive power and Atsube number of the negative lens on the object side forming the diverging cemented surface are N _1o , ν _1o , the refractive power and Abbe number of the image-side positive lens are N _1p and ν _1p , the radius of curvature of the convergent cemented surface with the convex surface facing the image side is R ₂ , and the object-side positive lens forming the convergent cemented surface When the refractive power and Atsube number of the lens are N _2p , ν _2p , and the refractive power and Atsube number of the image side negative lens are N _2o , ν _2o , 0<(|Φ ₁ |−|Φ ₂ |)×F<0.15 (4) However, Φ ₁ =N _1o −N _1p /R ₁ Φ ₂ =N _2o −N _2p /R ₂ 40＜ν _1p −ν _1o (5) 15＜ν _2o −ν _2p (6) Microscope objective according to claim 1, characterized in that it satisfies the requirements of claim 1.