JP5288242B2 - Microscope objective lens - Google Patents

Description

  The present invention relates to a microscope objective lens.

In a microscope used for inspecting a semiconductor, a printed circuit board, and the like, an objective lens capable of obtaining a clear image at a high magnification is demanded as a circuit such as a semiconductor to be inspected is highly integrated and miniaturized. This requires a large numerical aperture and advanced aberration correction including chromatic aberration. In addition, in consideration of prevention of contact with the inspection sample, work efficiency, etc., the objective lens must be a dry objective lens having a necessary and sufficient working distance. As objective lenses disclosed for such purposes, for example, those described in Patent Documents 1 and 2 are known.
JP 2000-241710 A JP 2003-75724 A

  However, although the objective lens described in Patent Document 1 has a sufficient working distance as a dry objective lens having a magnification of 100 and a numerical aperture of 0.9, the chromatic aberration is an achromatic class, and improvement is achieved. There was a problem that room was left. Further, in the example of the 100 × objective lens described in Patent Document 2, the correction level of chromatic aberration is high, but it cannot be said that the shape of the tip lens is excellent in workability and productivity. was there.

  The present invention has been made in view of such problems, and has a high magnification, a high numerical aperture, a sufficient working distance, excellent chromatic aberration correction, and high imaging performance. An object of the present invention is to provide an objective microscope objective lens.

In order to solve the above problems, a microscope objective lens according to the present invention is composed of two positive meniscus lenses having a concave surface facing the object side in order from the object side, and a first lens group having a positive refractive power; comprises at least three sets of three cemented lenses, the divergent light emitted from the first lens group is converted into convergent light, a second lens group having a positive refractive power includes a cemented lens, exits from the second lens group And a third lens group having a negative refractive power that converts the convergent light into substantially parallel light and guides it to the imaging lens . The radius of curvature of the lens surface on the object side of the positive meniscus lens disposed closest to the object side of the first lens group is r1, the radius of curvature of the lens surface on the image side is r2, and the combined focal length of the first lens group is When f1 is set and the combined focal length of the third lens group is f3, the following expression 1.05 <r2 / r1 <1.25.
0.85 <| f1 / f3 | <1.15
Satisfy the conditions.

In such a microscope objective lens, three sets of three cemented lenses included in the second lens group are a first three cemented lenses in which a positive lens, a negative lens, and a positive lens are cemented in order from the object side. Three pairs of three lenses, a second three-piece cemented lens in which a positive lens, a negative lens, and a positive lens are joined, and a third three-piece cemented lens in which a negative lens, a positive lens, and a negative lens are joined F is the focal length of the entire objective lens system that includes lenses and does not include the imaging lens, f2 is the focal length of the second lens group, ng is the refractive index for the g-line, and nF is the refractive index for the F-line, The refractive index with respect to the C line is nC, and the partial dispersion ratio θ is expressed by the following equation: θ = (ng−nF) / (nF−nC)
Where the Abbe number of the negative lens in the first three-piece cemented lens is ν21n, the Abbe number of the negative lens in the second three-piece cemented lens is ν22n, and θ is θ22n. 00 <f2 / f <10.00
ν21n <ν22n
θ22n <0.55
It is preferable to satisfy the following conditions.

  In such a microscope objective lens, it is preferable that the cemented lens component included in the third lens group is a three-lens cemented lens in which a negative lens, a positive lens, and a negative lens are cemented in order from the object side.

In such a microscope objective lens, the Abbe number of the negative lens disposed on the object side of the third lens group is ν3n ₁ , the Abbe number of the negative lens disposed on the image side is ν3n ₂ , and the Abbe number of the positive lens Where ν3p is
{(Ν3n ₁ + ν3n ₂ ) / 2} −ν3p> 25
It is preferable to satisfy the following conditions.

  Although not specifically described, the focal length and power, which are basic lens system specifications in the present invention, are values relative to the reference wavelength (d-line).

  When the microscope objective lens according to the present invention is configured as described above, it is a dry objective lens having both a sufficient working distance at a high magnification of about 100 times and a high numerical aperture of about NA = 0.90. A microscope objective lens in which aberrations are corrected well can be provided.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. First, the configuration of the microscope objective lens according to the present embodiment will be described with reference to FIG. The microscope objective lens OL includes, in order from the object O side, a first lens group G1 having a positive refractive power, a second lens group G2 having a positive refractive power, and a third lens group G3 having a negative refractive power. Consists of

  In such a microscope objective lens OL, the first lens group G1 is a lens group that guides the second lens group G2 while relaxing the angle with respect to the optical axis of the divergent light emitted from the object, and has a concave surface facing the object side. The positive meniscus lens is configured (in the case of FIG. 1, two positive meniscus lenses L1 and L2 are provided). The second lens group G2 is a lens group that converges the light beam emitted from the first lens group G1, and is configured to include at least three sets of three cemented lenses (in the case of FIG. 3 sets of three-piece cemented lens CL21, second three-piece cemented lens CL22, and third three-piece cemented lens CL23). The third lens group G3 is a lens group that converts the convergent light converged by the second lens group G2 into parallel light, and includes three lenses in which a negative lens, a positive lens, and a negative lens are cemented in order from the object side. It has a cemented lens (in the case of FIG. 11, three cemented lenses CL31). The second lens group G2 and the third lens group G3 are preferably all composed of three cemented lenses.

  In general, correction of chromatic aberration (secondary spectrum) is the most difficult problem for an objective lens having a high magnification and a long working distance. For correction of chromatic aberration, lens power arrangement, selection of glass material, and the like are important. It is also an effective means to use a sheet cemented lens. As shown in the present embodiment, by using a three-piece cemented lens for the second lens group G2 and the third lens group G3, the correction of chromatic aberration is improved in combination with means based on the lens power arrangement, glass material selection, and the like. Can be done. When a microscope is used for inspection of industrial products, inspection samples are often non-transmissive, and observation by epi-illumination becomes mainstream. In epi-illumination, the reflection of illumination light on the objective lens surface returns directly to the image plane, which may cause flare. This is effective in reducing flare.

  Further, the cemented lens disposed in the second lens group G2 closest to the object O (in the case of FIG. 1, the first three-lens cemented lens CL21) has a relatively strong positive refractive power, and gradually emits the divergent light beam. It has the effect of converting into a convergent luminous flux. Since the first lens group G1 includes only a positive lens, spherical aberration and chromatic aberration are not corrected, and the light beam emitted from the first lens group G1 includes these aberrations. The second lens group G2 is reached, and aberration correction is performed by the second lens group G2. The aberration correction state is an overcorrection state in the second lens group G2 alone, and the convergent light beam in which the aberration is almost canceled by the first and second lens groups G1 and G2 has the following negative refractive power. The third lens group G3 has a parallel light beam and is guided to an imaging lens (not shown).

  The conditions for configuring such a microscope objective lens OL will be described below. First, in the microscope objective lens OL, the first lens group G1 does not have a role of correcting spherical aberration and chromatic aberration, but it is necessary to suppress these generated aberrations as much as possible, and further, the first lens group G1 is arranged on the most object side. The lens surface of the positive meniscus lens (positive meniscus lens L1 in the case of FIG. 1) has a role of reducing the Petzval sum and keeping the image surface flat, and thus its shape and refractive power distribution are important. Therefore, in the first lens group G1, the radius of curvature of the lens surface on the object O side (the first surface in the case of FIG. 1) of the positive meniscus lens disposed closest to the object O is r1, and the lens surface on the image side (FIG. In the case of 1, when the radius of curvature of the second surface) is r2, the following conditional expression (1) is satisfied.

1.05 <r2 / r1 <1.25 (1)

  Conditional expression (1) is a condition for defining an appropriate shape of the positive meniscus lens disposed on the most object O side of the first lens group G1. If the lower limit of conditional expression (1) is not reached, the Petzval sum increases, and the flatness of the image plane cannot be maintained, which is not preferable. On the other hand, if the upper limit value of conditional expression (1) is exceeded, the luminous flux will spread too much, making it difficult to correct chromatic aberration and the like, which is not preferable.

  The microscope objective lens OL satisfies the following conditional expression (2) when the combined focal length of the first lens group G1 is f1 and the combined focal length of the third lens group G3 is f3. Composed.

0.85 <| f1 / f3 | <1.15 (2)

  Conditional expression (2) is a condition for defining the refractive power arrangement of the first lens group G1 and the third lens group G3 in the microscope objective lens OL. If the lower limit of conditional expression (2) is not reached, a sufficient working distance cannot be obtained, which is not preferable. On the contrary, if the upper limit value of the conditional expression (2) is exceeded, it is difficult to ensure a high numerical aperture, which is not preferable.

  In addition, the three sets of three cemented lenses included in the second lens group G2 are first three cemented lenses in which a positive lens, a negative lens, and a positive lens are cemented in order from the object side (in FIG. 1, a biconvex lens). L3, a biconcave lens L4, and a biconvex lens L5 are cemented to each other, a first three-lens cemented lens CL21), and a positive, negative, and positive lens are cemented to each other (a biconvex lens in FIG. 1). L6, a biconcave lens L7, and a biconvex lens L8 are cemented to a second three-lens cemented lens CL22), and a negative lens, a positive lens, and a negative lens are cemented to a third three-lens cemented lens (in FIG. 1). It is preferable to include a third three-lens cemented lens CL23 in which a negative meniscus lens L9, a biconvex lens L10, and a biconcave lens L11 are cemented. With such a configuration, it is possible to satisfactorily correct spherical aberration, coma aberration, and chromatic aberration generated in the first lens group G1.

  In order to satisfactorily correct other aberrations while greatly reducing chromatic aberration, particularly the secondary spectrum, which is the most difficult problem in the present invention, a three-piece cemented lens is frequently used in the second lens group G2. Not only is it important to select the appropriate glass material for these lenses. Since chromatic aberration occurs in proportion to the focal length of the lens, first, the focal length of the second lens group G2 is given within the range defined by conditional expression (3) below. Outside this condition range, even if a glass material is selected within the range of conditional expressions (4) and (5) described later, effective correction of chromatic aberration cannot be performed. In conditional expression (3), f represents the focal length of the entire objective lens OL that does not include the imaging lens, and f2 represents the focal length of the second lens group G2.

7.00 <f2 / f <10.00 (3)

  Under the conditional expression (3), the second lens group G2 has an Abbe number of ν21n of the negative lens (biconcave lens L4 in FIG. 1) in the first three-piece cemented lens arranged closest to the object side, When the Abbe number of the negative lens (biconcave lens L7 in FIG. 1) in the arranged second cemented lens is ν22n, it is preferable that the following conditional expression (4) is satisfied. .

ν21n <ν22n (4)

  Conditional expression (4) is a condition for defining the dispersion of the negative lens used in the second lens group G2. Since the first three-piece cemented lens closest to the object side in the second lens group G2 is still in the middle of the spreading of the luminous flux, the negative lens therein cannot increase its refractive power so much. However, if the chromatic aberration generated in the first lens group G1 is not corrected at an early stage, the principal point position shifts due to color, and the lateral chromatic aberration cannot be corrected. Therefore, it is necessary to use a glass material having a large dispersion to some extent, that is, a glass material having a small Abbe number. On the other hand, the second three-piece cemented lens that follows is disposed near the position where the light beam is fully spread and has the highest light beam height. Therefore, if the achromaticity is increased too much, higher-order spherical aberration will occur. A short wave tends to be overcorrected, and a long wave tends to be undercorrected, resulting in a large variation in spherical aberration due to color. Therefore, in the negative lens in the second three-piece cemented lens, it is desirable to use a glass material having a large Abbe number as compared with the negative lens in the first three-piece cemented lens.

  In addition, if the Abbe number of the glass material of the negative lens in the second three-piece cemented lens is large, it is possible to select a glass material that is advantageous for correcting the secondary spectrum. The second lens group G2 further satisfies the following conditional expression (5) when the partial dispersion ratio θ of the negative lens of the second three-piece cemented lens among the three sets of three-piece cemented lenses is θ22n. It is preferably configured to satisfy. By satisfying this conditional expression (5), it is possible to further reduce the difference in partial dispersion ratio θ with the positive lens (in the case of FIG. 1, biconvex lenses L6 and L8) that are the mating counterpart.

θ22n <0.55 (5)

  However, the partial dispersion ratio θ is a numerical value defined by the following formula (a), and ng, nF, and nC represent refractive indexes with respect to g-line, F-line, and C-line of the glass material of the negative lens, respectively. .

θ = (ng−nF) / (nF−nC) (a)

Further, in the microscope objective lens OL, the third lens group G3 has a strong negative refractive power, and the third lens group G3 also has an effect of reducing the Petzval sum. In order to correct lateral chromatic aberration, chromatic aberration must be corrected almost independently in this group. Therefore, in the third lens group G3, the Abbe number of the negative lens L12 disposed on the object O side is ν3n ₁ , the Abbe number of the negative lens L14 disposed on the image side is ν3n ₂ , and the Abbe number of the positive lens L13 is It is preferable that ν3p be configured so as to satisfy the following conditional expression (6).

{(Ν3n ₁ + ν3n ₂ ) / 2} −ν3p> 25 (6)

  Conditional expression (6) is a condition for defining an appropriate difference in dispersion of the glass material in the third lens group G3. Exceeding the lower limit or upper limit of conditional expression (6) is not preferable because the radius of curvature of the joint surface becomes too tight and it becomes difficult to correct coma aberration and the like satisfactorily.

  In the following, two examples of the microscope objective lens OL according to the present invention are shown. Since the microscope objective lens OL according to this example is of an infinity correction type, the configuration shown in FIG. Are used together with an imaging lens IL having the specifications shown in Table 1. In Table 1, f indicates the focal length, and the first column m is the number of each optical surface from the object side, and corresponds to the surface numbers 1 to 6 shown in FIG. The second column r is the radius of curvature of each optical surface, the third column d is the distance on the optical axis from each optical surface to the next optical surface, the fourth column nd is the refractive index with respect to the d-line, and the fifth column A column νd indicates the Abbe number. The description of the specification table is the same in the following embodiments.

(Table 1)
f = 200mm
m r d nd νd
1 75.043 5.1 1.62801 57.03
2 -75.043 2.0 1.74950 35.19
3 1600.580 7.5
4 50.256 5.1 1.66755 41.96
5 -84.541 1.8 1.61266 44.41
6 36.911

  The imaging lens IL includes a cemented lens in which a biconvex lens L21 and a biconcave lens L22 are cemented in order from the object side, and a cemented lens in which a biconvex lens L23 and a biconcave lens L24 are cemented.

[First embodiment]
FIG. 1 used in the above description shows a microscope objective lens OL1 according to the first embodiment of the present invention. As described above, the microscope objective lens OL1 includes, in order from the object O side, a first lens group G1 having a positive refractive power, a second lens group G2 having a positive refractive power, and a third lens having a negative refractive power. Consists of a lens group G3. The first lens group G1 includes, in order from the object side, a positive meniscus lens L1 having a concave surface facing the object side, and a positive meniscus lens L2 having a concave surface facing the object side. The second lens group G2 includes, in order from the object side, a first three-piece cemented lens CL21 in which a biconvex lens (positive lens) L3, a biconcave lens (negative lens) L4, and a biconvex lens (positive lens) L5 are cemented. A second three-piece cemented lens CL22 in which a biconvex lens (positive lens) L6, a biconcave lens (negative lens) L7, and a biconvex lens (positive lens) L8 are cemented, and a negative meniscus lens L9 (with a convex surface facing the object side) A negative lens), a biconvex lens (positive lens) L10, and a biconcave lens (negative lens) L11 are cemented to form a third three-piece cemented lens CL23. Further, the third lens group G3 includes a three-piece cemented lens CL31 obtained by cementing a biconcave lens L12 (negative lens), a biconvex lens (positive lens) L13, and a biconcave lens (negative lens) L14.

  Table 2 shows the specifications of the microscope objective lens OL1 according to the first example shown in FIG. In Table 2, f is the focal length of the entire system, NA is the numerical aperture, β is the magnification, and d0 is a surface on the object O side of the first lens (positive meniscus lens L1) from the object O (object surface) ( The distance on the optical axis to the apex of the (first surface) is shown. In addition, the numbers 1-20 of each optical surface shown in the 1st column m respond | correspond to the surface numbers 1-20 shown in FIG. Table 2 also shows values corresponding to the conditional expressions (1) to (5), that is, the condition corresponding values. The description of the above table is the same in other embodiments.

  In addition, the focal length f, the radius of curvature r, the surface interval d and other length units published in all the following specifications are generally “mm” unless otherwise specified, but the optical system is proportionally enlarged or proportional. Since the same optical performance can be obtained even if the size is reduced, the unit is not limited to “mm”, and other appropriate units can be used.

(Table 2)
f = 2mm
NA = 0.9
β = -100
d0 = 2.22

m r d nd νd
1 -3.264 2.70 1.816000 46.62
2 -3.682 0.10 1.000000
3 -11.084 3.10 1.569070 71.31
4 -6.804 0.15 1.000000
5 31.697 5.20 1.569070 71.31
6 -9.397 1.10 1.613397 44.27
7 17.247 6.50 1.433852 95.25
8 -12.327 0.15 1.000000
9 23.145 4.90 1.434250 95.02
10 -15.000 1.00 1.640000 60.09
11 16.912 4.50 1.497820 82.56
12 -23.021 0.15 1.000000
13 13.724 1.20 1.654115 39.68
14 6.867 5.80 1.433852 95.25
15 -18.351 1.00 1.516800 64.12
16 51.885 16.10 1.000000
17 -11.441 1.00 1.696800 55.52
18 12.084 1.90 1.738000 32.27
19 -4.998 1.20 1.487490 70.45
20 5.235

(Conditional value)
(1) r2 / r1 = 1.13
(2) | f1 / f3 | = 1.04
(3) f2 / f = 8.75
(4) ν21n = 44.27, ν22n = 60.09
(5) θ22n = 0.537
(6) {(ν3n ₁ + ν3n ₂ ) / 2} −ν3p = 30.72

  Thus, it can be seen that all the conditional expressions (1) to (6) are satisfied in the first embodiment. FIG. 2 shows spherical aberration and astigmatism with respect to rays of the d-line (587.6 nm), C-line (656.3 nm), F-line (486.1 nm) and g-line (435.8 nm) in the first embodiment. FIG. 5 shows various aberration diagrams of coma and distortion. In the spherical aberration diagram, the vertical axis shows the value of the numerical aperture NA, the astigmatism diagram and the distortion diagram show the value of the image height Y, and the coma aberration diagram shows 9 mm when the image height Y is 12.5 mm. , When 6 mm and when 0 mm. In the spherical aberration diagram and the coma aberration diagram, the solid line indicates the d line, the broken line indicates the C line, the alternate long and short dash line indicates the F line, and the alternate long and two short dashes line indicates the g line. In the figure, the broken line at each wavelength indicates the meridional image plane, and the solid line indicates the sagittal image plane. The description of the various aberration diagrams is the same in the following examples. As is apparent from the respective aberration diagrams shown in FIG. 2, it can be seen that in the first embodiment, various aberrations are satisfactorily corrected and excellent imaging performance is ensured.

[Second Embodiment]
Next, a microscope objective lens OL2 shown in FIG. 3 will be described as a second embodiment. The microscope objective lens OL2 shown in FIG. 3 also has, in order from the object O side, a first lens group G1 having a positive refractive power, a second lens group G2 having a positive refractive power, and a first lens group G2 having a negative refractive power. It is composed of three lens groups G3. The first lens group G1 includes a positive meniscus lens L1 having a concave surface facing the object side, and a positive meniscus lens L2 having a concave surface facing the object side. The second lens group G2 includes a first three-piece cemented lens CL21 formed by cementing a biconvex lens (positive lens) L3, a biconcave lens (negative lens) L4, and a biconvex lens (positive lens) L5, and a biconvex lens (positive lens). ) A second three-lens cemented lens CL22 in which L6, a biconcave lens (negative lens) L7 and a biconvex lens (positive lens) L8 are cemented, and a negative meniscus lens L9 (negative lens) having both convex surfaces facing the object side. A third three-piece cemented lens CL23 is formed by cementing a convex lens (positive lens) L10 and a biconcave lens (negative lens) L11. Further, the third lens group G3 includes a three-piece cemented lens CL31 obtained by cementing a biconcave lens L12 (negative lens), a biconvex lens (positive lens) L13, and a biconcave lens (negative lens) L14.

  Table 3 shows the specifications of the microscope objective lens OL2 according to the second example shown in FIG. In addition, the numbers 1-20 of each optical surface shown in the 1st column m respond | correspond to the surface numbers 1-20 shown in FIG.

(Table 3)
f = 2mm
NA = 0.9
β = -100
d0 = 2.24

m r d nd νd
1 -3.252 2.70 1.816000 46.62
2 -3.709 0.10 1.000000
3 -11.189 3.10 1.569070 71.31
4 -6.760 0.15 1.000000
5 39.720 5.10 1.569070 71.31
6 -9.202 1.10 1.613397 44.27
7 17.996 6.50 1.433852 95.25
8 -11.905 0.15 1.000000
9 20.284 5.30 1.434250 95.02
10 -13.457 1.00 1.603110 60.68
11 13.761 4.50 1.497820 82.56
12 -22.902 0.15 1.000000
13 13.820 1.20 1.613397 44.27
14 6.535 5.80 1.433852 95.25
15 -18.644 1.00 1.640000 60.09
16 46.280 16.00 1.000000
17 -12.170 1.00 1.696800 55.52
18 12.166 1.90 1.738000 32.27
19 -4.993 1.00 1.487490 70.45
20 5.306

(Conditional value)
(1) r2 / r1 = 1.14
(2) | f1 / f3 | = 0.99
(3) f2 / f = 8.55
(4) ν21n = 44.27, ν22n = 60.68
(5) θ22n = 0.542
(6) {(ν3n ₁ + ν3n ₂ ) / 2} −ν3p = 30.72

  Thus, it can be seen that all the conditional expressions (1) to (6) are satisfied in the second embodiment. FIG. 4 shows various aberration diagrams in the second embodiment. As is apparent from the respective aberration diagrams shown in FIG. 4, in the second example, it is understood that various aberrations are favorably corrected and excellent imaging performance is ensured.

[Third embodiment]
Next, as a third embodiment, a microscope objective lens OL3 shown in FIG. 5 will be described. The microscope objective lens OL3 shown in FIG. 5 also has, in order from the object O side, a first lens group G1 having a positive refractive power, a second lens group G2 having a positive refractive power, and a first lens group having a negative refractive power. It is composed of three lens groups G3. The first lens group G1 includes a positive meniscus lens L1 having a concave surface facing the object side, and a positive meniscus lens L2 having a concave surface facing the object side. The second lens group G2 includes a first three-piece cemented lens CL21 formed by cementing a biconvex lens (positive lens) L3, a biconcave lens (negative lens) L4, and a biconvex lens (positive lens) L5, and a biconvex lens (positive lens). ) A second three-lens cemented lens CL22 in which L6, a biconcave lens (negative lens) L7 and a biconvex lens (positive lens) L8 are cemented, and a negative meniscus lens L9 (negative lens) having both convex surfaces facing the object side. A third three-piece cemented lens CL23 is formed by cementing a convex lens (positive lens) L10 and a biconcave lens (negative lens) L11. Further, the third lens group G3 is composed of a two-piece cemented lens CL31 in which a positive meniscus lens L12 (positive lens) having a concave surface directed toward the object side and a biconcave lens (negative lens) L13 are cemented.

  Table 4 shows the specifications of the microscope objective lens OL3 according to the third example shown in FIG. The numbers 1 to 19 of the optical surfaces shown in the first column m correspond to the surface numbers 1 to 19 shown in FIG.

(Table 4)
f = 2mm
NA = 0.9
β = -100
d0 = 2.23

m r d nd νd
1 -3.119 2.70 1.816000 46.62
2 -3.615 0.10 1.000000
3 -11.721 3.10 1.569070 71.31
4 -6.780 0.15 1.000000
5 20.126 5.20 1.497820 82.56
6 -10.126 1.10 1.613397 44.27
7 15.656 6.40 1.433852 95.25
8 -11.988 0.15 1.000000
9 28.008 4.80 1.434250 95.02
10 -15.463 1.00 1.603110 60.68
11 13.098 4.50 1.497820 82.56
12 -32.286 0.15 1.000000
13 14.975 1.20 1.613397 44.27
14 6.630 5.80 1.433852 95.25
15 -30.864 1.00 1.516800 64.12
16 106.474 19.09 1.000000
17 -19.223 1.90 1.784720 25.68
18 -4.749 1.00 1.516800 64.12
19 4.769

(Conditional value)
(1) r2 / r1 = 1.16
(2) | f1 / f3 | = 0.91
(3) f2 / f = 9.52
(4) ν21n = 44.27, ν22n = 60.68
(5) θ22n = 0.542

  Thus, it can be seen that the conditional expressions (1) to (5) are satisfied in the third embodiment. In the third example, since the third lens group G3 is not a three-piece cemented lens, the conditional expression (6) is not applied. FIG. 6 shows various aberrations in the third example. As is apparent from the respective aberration diagrams shown in FIG. 6, in the third example, it is understood that various aberrations are favorably corrected and excellent imaging performance is ensured.

It is a lens block diagram of the microscope objective lens which concerns on 1st Example. FIG. 6 is a diagram illustrating various aberrations of the microscope objective lens according to the first example. It is a lens block diagram of the microscope objective lens which concerns on 2nd Example. FIG. 6 is various aberration diagrams of the microscope objective lens according to the second example. It is a lens block diagram of the microscope objective lens which concerns on 3rd Example. FIG. 6 is various aberration diagrams of the microscope objective lens according to the third example. It is a lens block diagram of the imaging lens used with the said microscope objective lens.

Explanation of symbols

OL (OL1 to OL2) Microscope objective lens G1 First lens group L1 Positive meniscus lens L2 Positive meniscus lens G2 Second lens group CL21 First three-piece cemented lens L3 Biconvex lens (positive lens)
L4 Biconcave lens (negative lens) L5 Biconvex lens (positive lens)
CL22 Second three-piece cemented lens L6 Biconvex lens (positive lens)
L7 Biconcave lens (negative lens) L8 Biconvex lens (positive lens)
CL23 Third three-piece cemented lens L9 Negative meniscus lens (negative lens)
L10 Biconvex lens (positive lens) L11 Biconcave lens (negative lens)
G3 Third lens group CL31 Three-piece cemented lens L12 Biconcave lens (negative lens)
L13 Biconvex lens (positive lens) L14 Biconcave lens (negative lens)

Claims (4)

From the object side,
A first lens group comprising two positive meniscus lenses having a concave surface facing the object side, and having a positive refractive power;
A second lens group having a positive refractive power, including at least three sets of three cemented lenses, and converting divergent light emitted from the first lens group into convergent light ;
A third lens group having a negative refractive power that includes a cemented lens , converts the convergent light emitted from the second lens group into substantially parallel light, and guides it to the imaging lens ;
The radius of curvature of the object-side lens surface of the positive meniscus lens arranged closest to the object side of the first lens group is r1, the radius of curvature of the image-side lens surface is r2, and the combined focal length of the first lens group Is f1, and the combined focal length of the third lens group is f3, the following expression 1.05 <r2 / r1 <1.25.
0.85 <| f1 / f3 | <1.15
Microscope objective lens that satisfies the above conditions. Three sets of the three-piece cemented lenses included in the second lens group include, in order from the object side, a first three-piece cemented lens in which a positive lens, a negative lens, and a positive lens are cemented, and a positive lens and a negative lens. A second three-lens cemented lens in which a positive lens is cemented, and a third three-lens cemented lens in which a negative lens, a positive lens, and a negative lens are cemented,
The focal length of the entire objective lens system that does not include the imaging lens is f, and the focal length of the second lens group is f2.
The refractive index for the g line is ng, the refractive index for the F line is nF, the refractive index for the C line is nC, and the partial dispersion ratio θ is expressed by the following equation: θ = (ng−nF) / (nF−nC)
Defined in
When the Abbe number of the negative lens in the first three-piece cemented lens is ν21n, the Abbe number of the negative lens in the second three-piece cemented lens is ν22n, and θ is θ22n, the following formula 7 .00 <f2 / f <10.00
ν21n <ν22n
θ22n <0.55
The microscope objective lens according to claim 1, which satisfies the following condition.   3. The microscope according to claim 1, wherein the cemented lens component included in the third lens group includes a three-lens cemented lens in which a negative lens, a positive lens, and a negative lens are cemented in order from the object side. Objective lens. When the Abbe number of the negative lens disposed on the object side of the third lens group is ν3n ₁ , the Abbe number of the negative lens disposed on the image side is ν3n ₂ , and the Abbe number of the positive lens is ν3p, Next formula
{(Ν3n ₁ + ν3n ₂ ) / 2} −ν3p> 25
The microscope objective lens as described in any one of Claims 1-3 which satisfy | fills these conditions.

Images