JPH0411006B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD OF THE INVENTION The present invention relates to microscope objectives, in particular objectives with medium magnification and long working distance. (Background of the Invention) Recently, microscopes have been increasingly used to perform various operations on specimens while observing them. For example, in the industrial and metal industries, a magnetic field is applied to a specimen or the temperature is changed. In addition, in the biological field, things such as passing electricity through tissues and cells and puncturing them with needles are also carried out. In these cases, various devices, circuits, stylus, etc. are often placed around the specimen, and the working distance of the objective lens must be adjusted to facilitate specimen insertion, removal, and exchange. is preferably large. Conventionally, stereoscopic microscopes with long working distances and low-magnification objective lenses of 5 ^× to 10 ^× have been used for these purposes.
However, recently, especially in industrial fields, objective lenses with high resolving power and magnification are often used, and these medium-magnification objective lenses are now required to have a long working distance comparable to low-magnification objective lenses. I'm getting old. Until now, objective lenses with medium or higher magnifications had a working distance of around 5 mm at most, and these requirements could not be completely met. Furthermore, one of the reasons why it has been difficult to design medium-magnification objectives with a large working distance is that it has been difficult to correct chromatic aberration. That is, when the working distance is increased, the focal length of the front group must be increased, which increases chromatic aberrations and the like. In particular, the amount of residual secondary spectrum becomes large, and the high-order spherical aberration at short wavelengths becomes too excessive, making it extremely difficult to correct it. On the other hand, in the microscope objective lens disclosed in Japanese Patent Publication No. 47-37455, the rear group with negative refractive power is
The lens is constructed as a cemented lens, which ensures a relatively long working distance while effectively correcting spherical aberration, especially at short wavelengths. However, even with such an objective lens, the magnification is about 30 times, and the working distance is only about 80% of the focal length, so it has not been possible to fully meet the recent demands as described above. (Object of the Invention) The object of the present invention is to provide a microscope objective lens that has an extremely long working distance despite having a medium magnification of about 40 times, and has excellent imaging performance with various aberrations well corrected. It's about doing. (Summary of the Invention) The long working distance objective lens according to the present invention includes, in order from the object side, a positive meniscus lens with a concave surface facing the object side and a bonded positive lens, and has a convergence property that converts a light beam from an object into a convergent light beam. a first lens group, a second diverging lens group having a positive lens and a negative lens cemented to each other, and a third diverging lens group having a positive lens and a negative lens cemented to each other. . Further, the focal length of the entire system is F, the focal length of the first lens group is f _{1 , the air distance between the second lens group and the third lens group is D, and the distance between the second lens group and the third lens group is F 1 .} Combined focal length with lens group
When F', the following conditions are satisfied: 1.5F<f ₁ <3.0F 1.5F<D<2.5F 1.9F<|F'|<2.8F. Prior to this invention, the above-mentioned
As a result of studying the configuration disclosed in Publication No. 37455, the working distance can be increased by increasing the focal length of the front group with positive refractive power, but in this case, the chromatic aberration generated in the front group becomes too large. 3
It is difficult to make sufficient correction using only the rear group of laminated sheets, and if the working distance is forcibly increased without increasing the focal length of the front group, high-order spherical aberrations, especially at short wavelengths, will occur in the front group. It has become clear that correction using only one negative refractive power group is difficult in this case as well. Therefore, in the present invention, as described above, two diverging lens groups, the second and third, are provided behind the first convergent lens group, and optimal conditions for correcting the aberrations as described above have been found. It is. Each conditional expression will be explained in detail below. Equation (1) relates to the power of the first lens group. When the lower limit of this condition is exceeded, that is, when f ₁ becomes small, the Petzvaer sum becomes large and the curvature of field becomes insufficiently corrected. Furthermore, higher-order spherical aberrations for short wavelengths are undercorrected too much, resulting in negative asymmetric aberrations. This is particularly noticeable at short wavelengths. Furthermore, since the focal length of the first lens group becomes small, the working distance cannot be increased. On the other hand, when the upper limit of this condition is exceeded, that is, when f ₁ becomes large, the diameter of the light beam passing through the first lens group becomes large, and high-order spherical aberration occurs. Also,
Since the focal length of the first lens group is large, the negative power of the second and third lens groups must also be weak. Therefore, the power of the axial chromatic aberration correction of the second lens group and the power of the magnification chromatic aberration correction of the third lens group becomes weak, and in particular, the magnification chromatic aberration remains. Also,
High-order spherical aberrations at short wavelengths end up being overcorrected. Equation (2) relates to the distance between the second lens group and the third lens group, and is a condition for balancing the meridional image plane, lateral chromatic aberration, and coma aberration. If the lower limit of this condition is exceeded, that is, if the distance between the second lens group and the third lens group becomes small, the chromatic aberration of magnification will be undercorrected and the meridional image surface will also be undersized. In order to correct this chromatic aberration of magnification, if the powers of the second and third lens groups are increased, the higher-order spherical aberration at short wavelengths will be too underdeveloped. Furthermore, if the convergence effect of the object-side surface of the third lens group is strengthened in order to correct the meridional image surface, the balance of coma aberration will be lost and outer coma aberration will occur. On the other hand, if the upper limit of this condition is exceeded, that is, if the distance between the second lens group and the third lens group becomes large, the meridional image plane will go too far. If the convergence effect of the object-side surface of the third lens group is weakened in order to correct this, internal coma aberration will occur. In addition, by increasing this group spacing, the total length of the lens becomes longer than the object-image distance.
The lens will no longer fit inside the objective barrel.
In other words, since the parfocal distance (distance from the object to the barrel surface at the rear end of the objective lens) is different from that of other objective lenses, extensive focus correction is required each time the magnification is changed, resulting in poor operability. It gets very bad. Equation (3) relates to the combined power of the second lens group and the third lens group, and is used to correct the spherical aberration and coma aberration that occur in the first lens group, and to balance the aberrations of the entire system well. belongs to. If the lower limit of this condition is exceeded, that is, if the negative power of the second and third lens groups as the rear group becomes strong, spherical aberration will be overcorrected, the balance of coma will be disrupted, and internal coma will occur. Resulting in. The second and third lens groups diverge the light beam converged in the first lens group by the action of negative power, thereby increasing the total length to a predetermined value. Therefore, if the negative power is too strong, the overall length will become too long, so it will be necessary to reduce the overall size, and as a result, the working distance will become small. On the other hand, when the upper limit of this condition is exceeded, that is, when the negative power of the rear lens group becomes weaker, the outer coma aberration generated in the first lens group is generated in the second and third lens groups in order to cancel it. The amount of inner coma aberration that had been present becomes smaller, and outer coma aberration remains.
Furthermore, the second and third lens groups have weaker divergent powers, so the overall length cannot be increased. For this reason, if the focal length of the entire system is increased to increase the total length, the total length of the lens will also increase at the same time, making it impossible for the lens to fit inside the objective lens barrel. In the basic configuration of the present invention as described above, the third
Let the refractive index of the positive lens and the negative lens forming the lens group be n _P , n _N and their respective Atsube numbers ν _P ,
When ν _N , it is further desirable to satisfy the following conditions: 0.05<n _P −n _N <0.17 (4) ν _P <ν _N (5). In the third lens group, each aberration is balanced so that short-wavelength high-order spherical aberration and lateral chromatic aberration are corrected as a whole system. In other words, the short-wavelength high-order spherical aberration that occurs in the first lens group is canceled out,
Further, the axial chromatic aberration that has been overcorrected by the first and second lens groups is corrected by the third lens group. At the same time, residual chromatic aberration of magnification that could not be completely corrected by the first and second lens groups is also corrected. When the lower limit of the condition of equation (4) is exceeded, that is, when the difference in refractive index becomes small, the spherical aberration becomes excessive. For this reason, if the curvature of the cemented surface of the third lens group is strengthened to correct the spherical aberration, the meridional image surface becomes significantly undersized, causing the balance of coma aberration to be disrupted and internal coma aberration to occur. When the upper limit of the condition in equation (4) is exceeded, that is, when the difference in refractive index becomes large, the spherical aberration becomes undervalued. Therefore, if the curvature of the cemented surface is weakened to correct the high-order spherical aberration, the low-order spherical aberration remains uncorrected. On the other hand, high-order spherical aberrations with short wavelengths are overcorrected. Furthermore, the meridional image plane is overlapping, causing outer coma aberration. If the condition of equation (5) is violated, that is, if the Abbe number of the positive lens in the third lens group is larger than the Abbe number of the negative lens, the axial chromatic aberration will be overcorrected, and conversely,
Lateral chromatic aberration ends up being under-corrected. Therefore, in order to correct the axial chromatic aberration, if the amount of correction of the axial chromatic aberration in the first and second lens groups is reduced, the chromatic aberration of magnification becomes even more under-corrected, and in the end, only a large chromatic aberration of magnification remains. Furthermore, when the radius of curvature of the lens surface closest to the object side is r ₁ , it is also desirable to satisfy the condition −3F<r ₁ (6). This condition relates to the curvature of the object-side surface of the positive meniscus lens closest to the object side of the first lens group. If the lower limit of this condition is exceeded, that is, if the diverging effect of the lens surface closest to the object side is too strong, the Petzval sum becomes too small and the curvature of field becomes overcorrected. Also, f of the first lens group increases,
Correction of spherical aberration at short wavelengths becomes excessive. Furthermore, since the principal point of the lens moves closer to the image side, the effective working distance becomes smaller. (Example) In an example according to the present invention, as shown in the lens configuration diagram in FIG. 1, the first convergent lens group G ₁ consists of a positive meniscus lens L ₁ having a concave surface facing the object side in order from the object O side. , a negative meniscus lens L ₂ with a convex surface facing the object side and a biconvex positive lens L ₃ cemented to this.
and a biconvex positive lens _L4 . The second diverging lens group _G2 consists of three lenses: a negative meniscus lens L with _its convex surface facing the object side, a biconvex positive lens _L6 , and a negative lens _L7 with its surface with a stronger curvature facing the object side. Consists of pasting. Further, the third diverging lens group _G3 is composed of a positive lens _L8 having a surface with a stronger curvature facing toward the image side, and a biconcave negative lens _L9 cemented to the positive lens L8. In FIG. 1, peripheral rays from an on-axis object point are shown to make it easier to understand the effects of each lens group. The specifications of the first to fourth embodiments according to the present invention are shown below. However, F is the overall focal length, β is the magnification,
NA is the numerical aperture, WD is the working distance (distance from the object surface to the vertex of the frontmost lens surface), P is the Petzval sum, r is the radius of curvature of each lens surface, d is the center thickness and air spacing of each lens, n represents the refractive index of each lens, ν represents the Abbe number of each lens, and the subscript represents the order from the object side.

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[Table] Various aberration diagrams of the first to fourth embodiments are shown in FIGS. 2 to 5, respectively. Each aberration diagram shows spherical aberration (Sph), astigmatism (Ast), distortion aberration (Dis),
Comatic aberration (Coma) is shown, and in the spherical aberration diagram, in addition to the d-line (λ = 587.6 nm) as a reference ray, C
The state of the line (λ = 656.3 nm), F line (λ = 486.1 nm), and g line (λ = 435.8 nm) is also shown. From each aberration diagram, it is clear that all examples maintain excellent imaging performance despite having a magnification of 40x and a long working distance of more than twice the focal length. it is obvious. (Effects of the Invention) As described above, in order to correct large aberrations that occur in the front group when the working distance is increased, the present invention configures the rear group with two negative lens groups, thereby reducing the focal length by 2. This is an ultra-long working distance medium magnification objective lens that has a working distance of more than twice that. And for a 40x objective lens, the working distance was previously 2 to 3 mm at most, but now it is 10 mm.
As described above, it has a working distance equivalent to a conventional 10x objective lens, which further improves operability when handling specimens. Moreover, it is clear that both the magnification and numerical aperture (NA) are larger than the 10x objective lens, and therefore the resolution is of course excellent, and it fully meets the demands of recent users who require both resolution and operability. It is something that satisfies.

[Brief explanation of drawings]

FIG. 1 is a lens configuration diagram showing an example of an objective lens according to the present invention, and FIGS. 2 to 5 are various aberration diagrams of first to fourth embodiments according to the present invention. (Explanation of symbols of main parts), O...object, G ₁ ...
...Convergent first lens group, G ₂ ...Divergent second lens group
Lens group, G ₃ ...Divergent third lens group.

Claims (1)

[Claims] 1. In order from the object side, a convergent first lens group that has a positive meniscus lens with a concave surface facing the object side and a bonded positive lens and converts a light beam from an object into a convergent light beam, which are bonded to each other. A second divergent lens group has a positive lens and a negative lens, and a third divergent lens group has a positive lens and a negative lens cemented to each other, and the focal length of the entire system is F, Let f ₁ be the focal length of the first lens group, D be the spatial distance between the second lens group and the third lens group, and F' be the combined focal length of the second lens group and the third lens group. A long working distance objective characterized by satisfying the following conditions: 1.5F<f ₁ <3.0F (1) 1.5F<D<2.5F (2) 1.9F<|F'|<2.8F (3) lens.