Objective lens

Abstract

To provide an objective lens which satisfies specifications of a long WD (working distance) and a wide field of view and in which aberration performance down to the periphery of the field of view is well corrected.SOLUTION: An objective lens 1 is provided, comprised of a positive first lens group G1 and a negative second lens group G2. The first lens group G1 includes a positive meniscus lens with a concave object-side surface disposed on the most object side. The second lens goop G2 includes a pair of meniscus lens components with concave surfaces facing each other. The objective lens 1 has three or more cemented lenses located on the object side of the pair of meniscus lens components. The objective lens 1 satisfies the following conditional expressions: 2.6≤φL1 / DL1≤16 ...(1), 0.1≤|R212| / f≤3.5 ...(2), where φL1 represents an outer diameter of the positive meniscus lens, L1 represents an optical axial thickness of the positive meniscus lens, R212 represents a curvature radius of a most image-side surface of an object-side meniscus lens component of the pair of meniscus lens components, and f represents a focal length of the objective lens 1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The disclosure herein relates to an objective lens. [Background technology]

[0002] Objective lenses used in industrial applications such as wafer inspection require a high numerical aperture (NA) to achieve high resolution. To achieve high throughput, they also require a wide field of view and a long working distance (WD) to improve transport speed while avoiding the risk of collision between the specimen and the objective lens. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 60-241009 Summary of the Invention [Problem to be solved by the invention]

[0004] For example, Patent Document 1 discloses an objective lens with a magnification of 100x and an NA of 0.8 or more. However, the actual field of view of such an objective lens is too small, making it difficult to obtain sufficient throughput. Furthermore, when attempting to achieve a wide field of view with this objective lens configuration, it becomes difficult to effectively correct the field curvature. As a result, it is difficult to achieve good resolution even at the periphery of the wide field of view.

[0005] An object according to one aspect of the present invention is to provide an objective lens that satisfies the specifications of a long working distance and a wide field of view, and in which aberration performance is well corrected even at the periphery of the field of view. [Means for solving the problem]

[0006] An objective lens according to one aspect of the present invention comprises a first lens group having positive refractive power and converting divergent light from an object point into convergent light, and a second lens group having negative refractive power and arranged closer to the image side than the first lens group. The first lens group includes a first lens, located closest to the object side, which is a meniscus lens having positive refractive power and whose concave surface faces the object side. The second lens group includes: A triplet of positive-negative-positive cemented lenses, with one negative lens flanked by positive lenses, The objective lens includes a pair of meniscus lens components with their concave surfaces facing each other. The objective lens includes three or more cemented lenses located closer to the object than the pair of meniscus lens components. The objective lens satisfies the following conditional expression: 2.6≦φ L1 / D L1 ≦16 (1) 0.1≦|R 212 | / f≦3.5 (2) However, φ L1 is the outer diameter of the first lens. D L1 is the thickness of the first lens on the optical axis. 212 is the radius of curvature of the surface closest to the image of the first meniscus lens component that is the meniscus lens component on the object side of the pair of meniscus lens components, and f is the focal length of the objective lens. [Effects of the Invention]

[0007] According to the above aspect, it is possible to provide an objective lens that satisfies the specifications of long working distance and wide field of view, and in which aberration performance is well corrected even at the periphery of the field of view. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view of an objective lens 1 according to Example 1 of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of the imaging lens 10. [Figure 3] 1 is an aberration diagram of an optical system consisting of an objective lens 1 and an imaging lens 10. FIG. [Figure 4] FIG. 3 is a cross-sectional view of an objective lens 2 according to Example 2 of the present invention. [Figure 5]2 is an aberration diagram of an optical system consisting of an objective lens 2 and an imaging lens 10. FIG. [Figure 6] FIG. 10 is a cross-sectional view of an objective lens 3 according to Example 3 of the present invention. [Figure 7] 10 is an aberration diagram of an optical system consisting of an objective lens 3 and an imaging lens 10. FIG. [Figure 8] FIG. 10 is a cross-sectional view of an objective lens 4 according to Example 4 of the present invention. [Figure 9] 10 is an aberration diagram of an optical system consisting of an objective lens 4 and an imaging lens 10. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] An objective lens according to an embodiment of the present invention will be described. The objective lens according to this embodiment (hereinafter simply referred to as the objective lens) is an infinity-corrected microscope objective lens used in combination with an imaging lens.

[0010] The objective lens is composed of a first lens group with positive refractive power that converts divergent light from an object point into convergent light, and a second lens group with negative refractive power that is positioned closer to the image than the first lens group. The lens component closest to the image in the first lens group is the lens component closest to the object that converts divergent light from the object point into convergent light and emits the convergent light. In other words, if the objective lens has multiple lens surfaces that emit convergent light, the lens surface closest to the object among those lens surfaces is the lens surface closest to the image in the first lens group. The boundary between the first and second lens groups can be identified by the above characteristics.

[0011] In this specification, a lens component refers to a single lens block, whether a single lens or a cemented lens, in which only two lens surfaces, the object-side surface and the image-side surface through which light rays from an object point pass, are in contact with air. In other words, one single lens is one lens component, and one cemented lens is also one lens component. On the other hand, multiple single lenses or multiple cemented lenses arranged with air between them are not called one lens component.

[0012] The first lens group converts divergent light from an object point into convergent light and then makes it incident on the second lens group. The second lens group converts converging light from the first lens group into parallel light. By converting divergent light from an object point into convergent light in the first lens group and then making it incident on the second lens group, the height of marginal rays within the second lens group can be made lower than the height of marginal rays within the first lens group. This makes it possible to effectively correct Petzval sum with the second lens group, which has negative refractive power, and as a result, it is possible to effectively correct field curvature over a wide field of view.

[0013] The first lens group includes a meniscus lens (hereinafter referred to as the first lens) with positive refractive power and a concave surface facing the object side, located closest to the object. To achieve an objective lens with a high NA and a long WD, the divergence of light before it enters the objective lens inevitably increases the height of the marginal ray at the time of incidence. Therefore, it is necessary to place a lens with positive refractive power closest to the object side to suppress the divergence of the ray bundle. In this case, if the lens with positive refractive power located closest to the object side is a meniscus lens with a concave surface facing the object side, it is possible to minimize mainly spherical aberration and coma aberration. In the case of an objective lens with a particularly long WD, as will be shown in the examples below, the height of the marginal ray at the time of incidence is very high. Therefore, without the above-mentioned lens, it is difficult to achieve good aberration correction throughout the entire optical system. Therefore, a meniscus lens with positive refractive power and a concave surface facing the object side is located closest to the object side of the objective lens.

[0014] The second lens group includes a pair of meniscus lens elements with their concave surfaces facing each other. The inclusion of a pair of meniscus lens elements, which are Gaussian groups with their convex surfaces facing outward and their concave surfaces facing inward, within the second lens group onto which convergent light is incident, reduces the height of marginal rays at the opposing concave surfaces. As a result, it becomes possible to effectively correct the Petzval sum with a concave surface having negative refractive power, and to sufficiently reduce the curvature of field.

[0015] The objective lens includes three or more cemented lenses located closer to the object than the pair of meniscus lens components described above. By including three or more cemented lenses formed by cementing together lenses with different optical characteristics, chromatic aberration can be sufficiently corrected. In particular, a cemented lens consisting of a low-dispersion positive lens and a high-dispersion negative lens has the effect of correcting axial chromatic aberration, which is generally known as achromatism. By arranging three or more cemented lenses with achromatism in a region closer to the object than the pair of meniscus lens components, which is a region where the marginal ray height is large, good correction of axial chromatic aberration can be achieved.

[0016] Moreover, the objective lens is configured to satisfy the following conditional expressions (1) and (2). 2.6≦φ L1 / D L1 ≦16 (1) 0.1≦|R 212 | / f≦3.5 (2) However, φ L1 is the outer diameter of the first lens. D L1 is the thickness of the first lens on the optical axis. 212 is the radius of curvature of the surface closest to the image of the object-side meniscus lens component (hereinafter also referred to as the first meniscus lens component) of the pair of meniscus lens components. f is the focal length of the objective lens. Note that the outer diameter of the first lens is usually about 0.5 mm larger than the effective diameter (diameter) of the image-side surface of the first lens.

[0017] Conditional formula (1) is a conditional formula for effectively correcting primarily spherical aberration and coma in an objective lens with a long working distance. Divergent light that enters at a large marginal ray height due to the long working distance cannot be significantly suppressed by the object-side concave surface of the first lens element, but satisfying conditional formula (1) makes it possible to effectively correct primarily spherical aberration and coma.

[0018] φ L1 / D L1If is below the lower limit (2.6), the thickness of the first lens becomes too large, and the height of the marginal ray inside the first lens becomes excessively large. As a result, the height of the marginal ray incident on the image-side surface of the first lens and on the subsequent optical system becomes too large, making it difficult to suppress the amount of spherical aberration and coma aberration. As a result, it becomes difficult to achieve good aberration correction in the entire optical system. On the other hand, φ L1 / D L1 If exceeds the upper limit (16), the thickness of the first lens becomes too thin compared to its outer diameter, making it difficult to ensure the rigidity of the first lens. As a result, manufacturing errors in the surface shape increase, making it difficult to achieve the desired aberration correction.

[0019] Conditional expression (2) is a conditional expression that is primarily intended to provide excellent correction for curvature of field. By satisfying conditional expression (2), Petzval sum can be appropriately corrected by the first meniscus lens component, which has a concave surface facing the image side, and therefore curvature of field can be effectively corrected for throughout the entire optical system.

[0020] |R 212 If | / f exceeds the upper limit (3.5), the radius of curvature of the concave surface of the first meniscus lens becomes too large, making it impossible to sufficiently correct the Petzval sum and making it difficult to satisfactorily correct the field curvature throughout the entire optical system. In particular, in an optical system with a long WD, the height of the marginal ray at the point of incidence inevitably becomes large, so in areas relatively close to the object, such as the first lens group included in the optical system, it is difficult to arrange a strong concave surface that acts in the direction of increasing the ray height, making it difficult to correct the Petzval sum. For this reason, in order to have a long WD and to achieve sufficient aberration correction over a wide field of view, it is necessary to sufficiently correct the Petzval sum with the first meniscus lens. On the other hand, if |R 212 If | / f falls below the lower limit (0.1), the Petzval sum will be overcorrected by the first meniscus lens, making it difficult to effectively correct the field curvature of the entire optical system.

[0021] The objective lens configured as described above satisfies the specifications of long working distance and wide field of view, and can correct aberrations well even at the periphery of the field of view.

[0022] The objective lens may be configured to satisfy the following conditional formula (1-1) or (1-2) instead of conditional formula (1). The objective lens may be configured to satisfy the following conditional formula (2-1) or (2-2) instead of conditional formula (2). 3.3≦φ L1 / D L1 ≦12 (1-1) 3.8≦φ L1 / D L1 ≦8 (1-2) 0.2≦|R 212 | / f≦1.7 (2-1) 0.3≦|R 212 | / f≦1.2 (2-2)

[0023] A desirable configuration of the objective lens will be described below. The second lens group desirably includes a positive-negative-positive triplet cemented lens, with positive lenses located on either side of a single negative lens. Having an achromatic lens component consisting of a triplet cemented lens makes it possible to effectively correct axial chromatic aberration while efficiently using the space within the objective lens. Furthermore, in order for achromatic lens components to exert their full effect, they are desirably located in an area where the marginal ray height is large; however, such areas inevitably result in larger lens diameters. As described above, using an achromatic lens component consisting of a triplet cemented lens makes it possible to maintain high rigidity of the lens component even when the lens diameter is large.

[0024] It is desirable that each of the pair of meniscus lens components be a cemented lens. As described above, the pair of meniscus lens components mainly function to suppress the Petzval sum and correct curvature of field, but by forming them into cemented lenses, they can be given the function of correcting chromatic aberration in addition to curvature of field. This makes it possible to effectively correct primarily axial chromatic aberration.

[0025] It is also desirable that the objective lens satisfy at least one of the following conditional expressions (3) to (5). 0.5≦|R 211 | / f≦7 (3) 18≦νdL≦31 (4) 1.51≦ndH≦1.75 (5) However, R 211 is the radius of curvature of the surface of the first meniscus lens component closest to the object. νdL is the minimum Abbe number of at least one positive lens element located closer to the image than the surface of the first meniscus lens component closest to the image. ndH is the maximum refractive index of at least one negative lens element included in the objective lens.

[0026] Conditional formula (3) is a conditional formula primarily for further improving correction of field curvature. As mentioned above, the first meniscus lens component with its convex surface facing the object side has a corrective effect for Petzval sum, but to obtain sufficient correction, it is desirable to make the height of the marginal ray at the image-side concave surface of the first meniscus lens component sufficiently small.

[0027] |R 211 By ensuring that | / f does not exceed the upper limit (7), it is possible to prevent the radius of curvature of the convex surface on the object side of the first meniscus lens component from becoming too large. This allows the incident light to be converged at the convex surface, and the height of the marginal ray at the concave surface on the image side to be kept sufficiently small, thereby enabling good correction of field curvature. In addition, |R 211 By ensuring that | / f does not fall below the lower limit (0.5), it is possible to prevent the radius of curvature of the object-side convex surface of the first meniscus lens component from becoming too small. This prevents the occurrence of various aberrations, such as coma, on that convex surface from becoming too large, enabling good aberration correction.

[0028] Conditional formula (4) is a conditional formula for effectively correcting lateral chromatic aberration as well as axial chromatic aberration. Because the chief ray of off-axial light from the objective lens intersects with the optical axis inside the objective lens, the sign of the height of the off-axial chief ray is reversed between the area on the object side of the intersection and the area on the image side. In this configuration, by using a high-dispersion glass material for the positive lens element located in the area on the image side of the intersection, it becomes possible to effectively correct lateral chromatic aberration occurring in the area on the object side.

[0029] When vdL does not exceed the upper limit (31), the above-mentioned action enables the objective lens to effectively correct the chromatic aberration of magnification. Furthermore, when vdL does not fall below the lower limit (18), the amount of axial chromatic aberration generated is prevented from becoming too large, and the objective lens as a whole can effectively correct the axial chromatic aberration.

[0030] Conditional formula (5) is a conditional formula for effectively correcting wavefront aberrations such as spherical aberration. To give a lens negative refractive power, at least one surface must be concave, which generally results in the lens being thinner at the center than at the periphery. Such a lens shape is prone to surface shape errors during manufacturing.

[0031] By ensuring that ndH does not exceed the upper limit (1.75), the refractive index of the negative lens does not become too large, and the effect of lens surface shape errors on wavefront aberration can be minimized. This allows for good correction of wavefront aberrations such as spherical aberration. Furthermore, glass materials with low refractive indices generally tend to have low dispersion. Therefore, by ensuring that ndH does not fall below the lower limit (1.51), the dispersion of the negative lens does not become too small, and chromatic aberration can be well corrected.

[0032] The objective lens may be configured to satisfy the following conditional formula (3-1) or (3-2) instead of conditional formula (3). The objective lens may be configured to satisfy the following conditional formula (4-1) or (4-2) instead of conditional formula (4). The objective lens may be configured to satisfy the following conditional formula (5-1) or (5-2) instead of conditional formula (5). 0.8≦|R 211 | / f≦4 (3-1) 1.2≦|R 211 | / f≦2.5 (3-2) 20≦νdL≦30 (4-1) 24≦νdL≦29 (4-2) 1.55≦ndH≦1.71 (5-1) 1.61≦ndH≦1.66 (5-2)

[0033] The objective lens having the above-described configuration has a medium magnification, more specifically, a magnification of 60 or less. That is, if the focal length of this objective lens is f and the focal length of the imaging lens used in combination with this objective lens is ft, then the relationship ft / f≦60 holds.

[0034] Furthermore, the objective lens with the above-described configuration achieves a high NA and a long WD despite its compact configuration. More specifically, it satisfies the following conditional expressions: 0.065≦d0 / L≦0.3 (6) NA≧0.75 (7) 40mm≦L≦75mm (8) where d0 is the distance on the optical axis from the specimen surface to the surface of the objective lens closest to the object. L is the distance on the optical axis from the specimen surface to the surface of the objective lens closest to the image. NA is the numerical aperture of the objective lens on the object side. In other words, d0 is approximately equal to WD, and L is approximately equal to the sum of WD and the total length of the objective lens (more strictly, the total length of the optical system from the first lens group to the second lens group, which will be described later).

[0035] In particular, satisfying conditional expression (6) achieves both a long working distance and a compact configuration. If d0 / L is below the lower limit, the working distance becomes too short or the objective lens becomes too large. On the other hand, if d0 / L is above the upper limit, the number of lenses and their shapes become too limited, making it difficult to correct aberrations.

[0036] Examples of the above-mentioned objective lens will now be described in detail. [Example 1] 1 is a cross-sectional view of an objective lens 1 according to this embodiment. The objective lens 1 is a microscope objective lens, and is composed of a first lens group G1 with positive refractive power that converts divergent light from an object point into convergent light, and a second lens group G2 with negative refractive power that is disposed closer to the image side than the first lens group G1.

[0037] The first lens group G1 is composed of, arranged in order from the object side, a lens L1 which is a meniscus lens having positive refractive power with its concave surface facing the object side, a lens L2 which is also a meniscus lens with its concave surface facing the object side, a cemented lens CL1, and a cemented lens CL2.

[0038] The cemented lens CL1 is a two-element cemented lens consisting of, arranged in order from the object side, lens L3 which is a biconvex lens and lens L4 which is a meniscus lens with its concave surface facing the object side. The cemented lens CL2 is a two-element cemented lens consisting of, arranged in order from the object side, lens L5 which is a meniscus lens with its concave surface facing the image side and lens L6 which is a biconvex lens.

[0039] The second lens group G2 is composed of, arranged in order from the object side, a cemented lens CL3, a cemented lens CL4, and a cemented lens CL5. The cemented lenses CL4 and CL5 are a pair of meniscus lens components with their concave surfaces facing each other. The objective lens 1 includes three cemented lenses (cemented lenses CL1, CL2, and CL3) on the object side of the pair of meniscus lens components.

[0040] The cemented lens CL3 is a triplet of lenses, and is made up of, in order from the object side, a biconvex lens L7, a biconcave lens L8, and a biconvex lens L9. That is, the cemented lens CL3 is a positive-negative-positive triplet of lenses, with positive lenses (lenses L7 and L9) located on either side of a single negative lens (lens L8).

[0041] The cemented lens CL4 is a two-element cemented lens consisting of, arranged in order from the object side, a biconvex lens L10 and a biconcave lens L11. The cemented lens CL5 is a two-element cemented lens consisting of, arranged in order from the object side, a biconcave lens L12 and a biconvex lens L13.

[0042] The various data of the objective lens 1 are as follows. Note that β is the magnification when the objective lens 1 is combined with the imaging lens 10. NA ob is the object-side numerical aperture of the objective lens 1. f, f1, and f2 are the focal length of the objective lens, the focal length of the first lens group G1, and the focal length of the second lens group G2, respectively. The other parameters are as described above. NA ob =0.77, β=50, f=3.6mm, f1=8.461mm, f2=-17.921mm, L=48.7mm, d0=4.04mm, φ L1 = 9.44 mm, D L1 =2.262mm, R 211 =6.0569mm, R 212 =2.8444mm, νdL=28.43, ndH=1.65412

[0043] The lens data of the objective lens 1 is as follows: Note that INF in the lens data represents infinity (∞). Objective lens 1 srd nd νd 1 INF 4.040 2 -9.4060 2.262 1.88300 40.76 3 -5.4648 0.200 4 -27.1526 2.125 1.56907 71.30 5 -10.2529 0.150 6 44.3339 5.641 1.43875 94.66 7 -7.7306 1.550 1.61340 44.27 8 -20.8088 0.200 9 39.7597 2.398 1.65412 39.68 10 14.1869 5.904 1.43875 94.66 11 -12.1348 0.250 12 22.5186 4.207 1.43875 94.66 13 -10.2578 1.010 1.61340 44.27 14 6.4582 5.189 1.43875 94.66 15 -65.9501 0.200 16 6.0569 5.033 1.56907 71.30 17 -26.5773 2.570 1.65412 39.68 18 2.8444 2.437 19 -4.4665 1.210 1.48749 70.23 20 7.1260 2.122 1.78880 28.43 21 -13.6986 110.000

[0044] Here, s represents the surface number, r represents the radius of curvature (mm), d represents the surface spacing (mm), nd represents the refractive index for the d-line, and vd represents the Abbe number. These symbols are the same in the following examples. The surface indicated by surface number s1 is the specimen surface. The surfaces indicated by surface numbers s2 and s21 are the lens surface closest to the object and the lens surface closest to the image of the objective lens 1, respectively. For example, the surface spacing d1 represents the distance on the optical axis from the surface indicated by surface number s1 to the surface indicated by surface number s2. The surface spacing d21 represents the distance (110 mm) on the optical axis from the surface indicated by surface number s21 to the imaging lens.

[0045] The objective lens 1 satisfies the following conditional expressions (1) to (8). (1)φ L1 / D L1 =4.173 (2)|R 212 | / f=0.790 (3)|R 211 | / f=1.682 (4) νdL=28.430 (lens L13) (5) ndH=1.654 (Lens L5, Lens L11) (6) d0 / L=0.083 (7)NA=0.77 (8) L = 48.7 mm

[0046] FIG. 2 is a cross-sectional view of an imaging lens 10 used in combination with objective lens 1. The imaging lens 10 is a microscope imaging lens that forms a magnified image of an object in combination with an infinity-corrected objective lens. The imaging lens 10 is a cemented lens CTL1 consisting of a biconvex lens TL1 and a meniscus lens TL2, which is placed on the image side of the biconvex lens and has its concave surface facing the object. The imaging lens 10 is positioned so that the distance on the optical axis from the lens surface (surface number s21) of objective lens 1 closest to the image to the lens surface (surface number s1) of imaging lens 10 closest to the object is 110 mm. The focal length of the imaging lens 10 is 180 mm.

[0047] The lens data of the imaging lens 10 is as follows: Imaging lens 10 srd nd νd 1 193.123 5.5 1.48749 70.23 2 -61.238 4.6 1.72047 34.71 3 -105.391

[0048] FIG. 3 is a diagram of aberrations of an optical system consisting of the objective lens 1 and the imaging lens 10, showing the aberrations at the image plane where the objective lens 1 and the imaging lens 10 form an optical image. FIG. 3(a) is a diagram of spherical aberration. FIG. 3(b) is a diagram showing the amount of violation of the sine condition. FIG. 3(c) is a diagram of astigmatism. FIG. 3(d) is a diagram of coma aberration at an image height ratio of 70% (image height 9.27 mm). In the diagram, "M" indicates the meridional component, and "S" indicates the sagittal component. As shown in FIG. 3, in this embodiment, aberrations are well corrected over a wide field of view.

[0049] [Example 2] 4 is a cross-sectional view of the objective lens 2 according to this embodiment. The objective lens 2 is a microscope objective lens, and is composed of a first lens group G1 having a positive refractive power that converts divergent light from an object point into convergent light, and a second lens group G2 having a negative refractive power that is disposed closer to the image side than the first lens group G1.

[0050] The first lens group G1 is composed of, arranged in order from the object side, a lens L1 which is a meniscus lens having positive refractive power with its concave surface facing the object side, a lens L2 which is also a meniscus lens with its concave surface facing the object side, a cemented lens CL1, and a cemented lens CL2.

[0051] The cemented lens CL1 is a two-element cemented lens consisting of, arranged in order from the object side, lens L3 which is a biconvex lens and lens L4 which is a meniscus lens with its concave surface facing the object side. The cemented lens CL2 is a two-element cemented lens consisting of, arranged in order from the object side, lens L5 which is a meniscus lens with its concave surface facing the image side and lens L6 which is a biconvex lens.

[0052] The second lens group G2 is composed of, arranged in order from the object side, a cemented lens CL3, a cemented lens CL4, and a cemented lens CL5. The cemented lenses CL4 and CL5 are a pair of meniscus lens components with their concave surfaces facing each other. The objective lens 2 includes three cemented lenses (cemented lenses CL1, CL2, and CL3) on the object side of the pair of meniscus lens components.

[0053] The cemented lens CL3 is a triplet of lenses, and is made up of, in order from the object side, a biconvex lens L7, a biconcave lens L8, and a biconvex lens L9. That is, the cemented lens CL3 is a positive-negative-positive triplet of lenses, with positive lenses (lenses L7 and L9) located on either side of a single negative lens (lens L8).

[0054] The cemented lens CL4 is a two-element cemented lens consisting of, arranged in order from the object side, a biconvex lens L10 and a biconcave lens L11. The cemented lens CL5 is a two-element cemented lens consisting of, arranged in order from the object side, a biconcave lens L12 and a biconvex lens L13.

[0055] The various data of the objective lens 2 are as follows: NA ob =0.8, β=50, f=3.6mm, f1=8.529mm, f2=-17.839mm, L=48.699mm, d0=4.04mm, φ L1 = 9.9 mm, D L1 =2.306mm, R 211 = 6.0577 mm, R 212 =2.8933mm, νdL=28.43, ndH=1.65412

[0056] The lens data of the objective lens 2 is as follows: Objective Lens 2 srd nd νd 1 INF 4.040 2 -9.4059 2.306 1.88300 40.76 3 -5.5903 0.200 4 -28.3283 2.230 1.56907 71.30 5 -10.0330 0.150 6 39.2527 5.456 1.43875 94.66 7 -7.8854 1.587 1.61340 44.27 8 -21.2762 0.200 9 42.9739 2.616 1.65412 39.68 10 14.1151 6.128 1.43875 94.66 11 -12.1339 0.250 12 20.2765 4.284 1.43875 94.66 13 -10.3822 0.692 1.61340 44.27 14 6.2850 5.015 1.43875 94.66 15 -93.6689 0.200 16 6.0577 4.804 1.56907 71.30 17 -105.9012 2.570 1.65412 39.68 18 2.8933 2.585 19 -4.5205 1.191 1.48749 70.23 20 7.2372 2.194 1.78880 28.43 21 -14.3850 110.000

[0057] The objective lens 2 satisfies the following conditional expressions (1) to (8). (1)φ L1 / D L1 =4.293 (2)|R 212 | / f=0.804 (3)|R 211 | / f=1.683 (4) νdL=28.430 (lens L13) (5) ndH=1.654 (Lens L5, Lens L11) (6) d0 / L=0.083 (7)NA=0.80 (8) L = 48.699 mm

[0058] FIG. 5 is a diagram of aberrations of an optical system consisting of the objective lens 2 and the imaging lens 10, showing the aberrations at the image plane where the objective lens 2 and the imaging lens 10 form an optical image. FIG. 5(a) is a diagram of spherical aberration. FIG. 5(b) is a diagram showing the amount of violation of the sine condition. FIG. 5(c) is a diagram of astigmatism. FIG. 5(d) is a diagram of coma aberration at an image height ratio of 70% (image height 9.27 mm). As shown in FIG. 5, in this embodiment, aberrations are well corrected over a wide field of view.

[0059] [Example 3] 6 is a cross-sectional view of the objective lens 3 according to this embodiment. The objective lens 3 is a microscope objective lens, and is composed of a first lens group G1 having a positive refractive power that converts divergent light from an object point into convergent light, and a second lens group G2 having a negative refractive power that is disposed closer to the image side than the first lens group G1.

[0060] The first lens group G1 is composed of, arranged in order from the object side, a lens L1 which is a meniscus lens having positive refractive power with its concave surface facing the object side, a lens L2 which is also a meniscus lens with its concave surface facing the object side, a cemented lens CL1, and a cemented lens CL2.

[0061] The cemented lens CL1 is a two-element cemented lens consisting of, arranged in order from the object side, lens L3 which is a biconvex lens and lens L4 which is a meniscus lens with its concave surface facing the object side. The cemented lens CL2 is a two-element cemented lens consisting of, arranged in order from the object side, lens L5 which is a meniscus lens with its concave surface facing the image side and lens L6 which is a biconvex lens.

[0062] The second lens group G2 is composed of, arranged in order from the object side, a cemented lens CL3, a cemented lens CL4, and a cemented lens CL5. The cemented lenses CL4 and CL5 are a pair of meniscus lens components with their concave surfaces facing each other. The objective lens 3 includes three cemented lenses (cemented lenses CL1, CL2, and CL3) on the object side of the pair of meniscus lens components.

[0063] The cemented lens CL3 is a triplet of lenses, and is made up of, in order from the object side, a biconvex lens L7, a biconcave lens L8, and a biconvex lens L9. That is, the cemented lens CL3 is a positive-negative-positive triplet of lenses, with positive lenses (lenses L7 and L9) located on either side of a single negative lens (lens L8).

[0064] The cemented lens CL4 is a two-element cemented lens consisting of, arranged in order from the object side, lens L10 which is a meniscus lens with its concave surface facing the image side, and lens L11 which is a meniscus lens with its concave surface facing the image side. The cemented lens CL5 is a two-element cemented lens consisting of, arranged in order from the object side, lens L12 which is a biconcave lens, and lens L13 which is a biconvex lens.

[0065] The various data of the objective lens 3 are as follows: NA ob =0.82, β=50, f=3.6mm, f1=8.643mm, f2=-17.573mm, L=49.299mm, d0=3.733mm, φ L1 = 9.76 mm, D L1 =2.306mm, R 211 = 6.337 mm, R 212 =3.0206mm, νdL=29.84, ndH=1.65412

[0066] The lens data of the objective lens 3 is as follows: Objective Lens 3 srd nd νd 1 INF 3.733 2 -8.2556 2.306 1.88300 40.76 3 -5.5117 0.200 4 -23.7453 2.143 1.56907 71.30 5 -8.7229 0.150 6 38.6247 6.006 1.43875 94.66 7 -7.8819 2.525 1.61340 44.27 8 -17.7012 0.200 9 52.3455 1.560 1.65412 39.68 10 15.0020 6.140 1.43875 94.66 11 -12.6408 0.250 12 18.8709 4.205 1.43875 94.66 13 -10.6324 0.500 1.61340 44.27 14 6.4238 4.954 1.43875 94.66 15 -117.9018 0.200 16 6.3370 4.768 1.56907 71.30 17 9.8501 2.570 1.65412 39.68 18 3.0206 2.522 19 -4.7575 2.814 1.48749 70.23 20 8.9801 1.552 1.80000 29.84 21 -21.1397 110.000

[0067] The objective lens 3 satisfies the following conditional expressions (1) to (8). (1)φ L1 / D L1 =4.232 (2)|R 212 | / f=0.839 (3)|R 211 | / f=1.760 (4) νdL=29.840 (lens L13) (5) ndH=1.654 (Lens L5, Lens L11) (6) d0 / L=0.076 (7)NA=0.82 (8) L = 49.299 mm

[0068] FIG. 7 is a diagram of aberrations of an optical system consisting of the objective lens 3 and the imaging lens 10, showing the aberrations at the image plane where the objective lens 3 and the imaging lens 10 form an optical image. FIG. 7(a) is a diagram of spherical aberration. FIG. 7(b) is a diagram showing the amount of violation of the sine condition. FIG. 7(c) is a diagram of astigmatism. FIG. 7(d) is a diagram of coma aberration at an image height ratio of 70% (image height 9.27 mm). As shown in FIG. 7, in this embodiment, aberrations are well corrected over a wide field of view.

[0069] [Example 4] 8 is a cross-sectional view of the objective lens 4 according to this embodiment. The objective lens 4 is a microscope objective lens, and is composed of a first lens group G1 having a positive refractive power that converts divergent light from an object point into convergent light, and a second lens group G2 having a negative refractive power that is disposed closer to the image side than the first lens group G1.

[0070] The first lens group G1 is composed of, arranged in order from the object side, a lens L1 which is a meniscus lens having positive refractive power with its concave surface facing the object side, a lens L2 which is also a meniscus lens with its concave surface facing the object side, a cemented lens CL1, and a cemented lens CL2.

[0071] The cemented lens CL1 is a two-element cemented lens consisting of, arranged in order from the object side, lens L3 which is a biconvex lens and lens L4 which is a meniscus lens with its concave surface facing the object side. The cemented lens CL2 is a two-element cemented lens consisting of, arranged in order from the object side, lens L5 which is a meniscus lens with its concave surface facing the image side and lens L6 which is a biconvex lens.

[0072] The second lens group G2 is composed of, arranged in order from the object side, a cemented lens CL3, a cemented lens CL4, a cemented lens CL5, and a biconvex lens L14. The cemented lenses CL4 and CL5 are a pair of meniscus lens components with their concave surfaces facing each other. The objective lens 4 includes three cemented lenses (cemented lenses CL1, CL2, and CL3) closer to the object than the pair of meniscus lens components.

[0073] The cemented lens CL3 is a triplet of lenses, and is made up of, in order from the object side, a biconvex lens L7, a biconcave lens L8, and a biconvex lens L9. That is, the cemented lens CL3 is a positive-negative-positive triplet of lenses, with positive lenses (lenses L7 and L9) located on either side of a single negative lens (lens L8).

[0074] The cemented lens CL4 is a two-element cemented lens consisting of, arranged in order from the object side, lens L10 which is a meniscus lens with its concave surface facing the image side, and lens L11 which is a meniscus lens with its concave surface facing the image side. The cemented lens CL5 is a two-element cemented lens consisting of, arranged in order from the object side, lens L12 which is a biconcave lens, and lens L13 which is a biconvex lens.

[0075] The various data of the objective lens 4 are as follows: NA ob =0.8, β=50, f=3.6mm, f1=8.824mm, f2=-17.901mm, L=53.001mm, d0=3.905mm, φ L1 = 9.74 mm, D L1 =2.359mm, R 211 = 6.1484 mm, R 212 =3.0222mm, νdL=25.42, ndH=1.673

[0076] The lens data of the objective lens 4 is as follows: Objective Lens 4 srd nd νd 1 INF 3.906 2 -8.3496 2.359 1.88300 40.76 3 -5.5048 0.200 4 -30.7266 2.031 1.56907 71.30 5 -9.4401 0.150 6 37.1063 5.361 1.43875 94.66 7 -8.0669 1.659 1.61340 44.27 8 -19.4171 2.208 9 49.6209 2.651 1.67300 38.26 10 14.6608 5.647 1.43875 94.66 11 -12.3950 0.250 12 16.9791 4.017 1.43875 94.66 13 -11.4067 1.000 1.61340 44.27 14 6.0053 4.506 1.43875 94.66 15 -241.4394 0.200 16 6.1484 4.711 1.56907 71.30 17 31.8644 2.570 1.65412 39.68 18 3.0222 2.290 19 -4.1357 1.000 1.48749 70.23 20 10.8194 0.941 1.76182 26.52 21 -39.7297 4.548 22 33.3089 0.796 1.80518 25.42 23 -73.8152 110.000

[0077] The objective lens 4 satisfies the following conditional expressions (1) to (8). (1)φ L1 / D L1 =4.129 (2)|R 212 | / f=0.840 (3)|R 211 | / f=1.708 (4) νdL=25.420 (lens L14) (5) ndH=1.673 (lens L5) (6) d0 / L=0.074 (7)NA=0.80 (8) L = 53.001 mm

[0078] FIG. 9 is a diagram of aberrations of an optical system consisting of the objective lens 4 and the imaging lens 10, showing the aberrations at the image plane where the objective lens 4 and the imaging lens 10 form an optical image. FIG. 9(a) is a diagram of spherical aberration. FIG. 9(b) is a diagram showing the amount of violation of the sine condition. FIG. 9(c) is a diagram of astigmatism. FIG. 9(d) is a diagram of coma aberration at an image height ratio of 70% (image height 9.27 mm). As shown in FIG. 9, in this embodiment, aberrations are well corrected over a wide field of view. [Explanation of symbols]

[0079] 1, 2, 3, 4...Objective lens 10. Imaging lens G1: First lens group G2: Second lens group L1~L14, TL1~TL2 lenses CL1~CL5, CTL1... cemented lenses

Claims

1. An objective lens, a first lens group having a positive refractive power that converts divergent light from an object point into convergent light; a second lens group having negative refractive power, the second lens group being disposed closer to the image side than the first lens group; the first lens group includes a first lens, which is located closest to the object side and is a meniscus lens having positive refractive power and a concave surface facing the object side; the second lens group includes a positive-negative-positive triplet cemented lens in which positive lenses are disposed on both sides of one negative lens, and a pair of meniscus lens components with concave surfaces facing each other, the objective lens includes three or more cemented lenses located closer to the object than the pair of meniscus lens components, An objective lens characterized by satisfying the following conditional expression: 2.6≦φ Ｌ１ / D Ｌ１ ≦16 ・・・(1) 0.1≦|R ２１２ | / f≦3.5 ・・・(2) However, φ Ｌ１ is the outer diameter of the first lens. Ｌ１ is the thickness of the first lens on the optical axis. ２１２ is the radius of curvature of the surface closest to the image of the first meniscus lens component which is the meniscus lens component on the object side of the pair of meniscus lens components, and f is the focal length of the objective lens.

2. 2. The objective lens according to claim 1, An objective lens characterized by satisfying the following conditional expression: 0.5≦|R ２１１ | / f≦7 ・・・(3) However, R ２１１ is the radius of curvature of the surface of the first meniscus lens component closest to the object.

3. 3. The objective lens according to claim 1, Each of the pair of meniscus lens components is a cemented lens. An objective lens characterized by:

4. The objective lens according to any one of claims 1 to 3, An objective lens characterized by satisfying the following conditional expression: 18≦νdL≦31 (4) Here, νdL is the minimum value of the Abbe number of at least one positive lens arranged on the image side of the surface of the first meniscus lens component closest to the image side.

5. The objective lens according to any one of claims 1 to 4, An objective lens characterized by satisfying the following conditional expression: 1.51≦ndH≦1.75 (5) Here, ndH is the maximum value of the refractive index of at least one negative lens included in the objective lens.

6. An objective lens, a first lens group having a positive refractive power that converts divergent light from an object point into convergent light; a second lens group having negative refractive power, the second lens group being disposed closer to the image side than the first lens group; the first lens group includes a first lens, which is located closest to the object side and is a meniscus lens having positive refractive power and a concave surface facing the object side; the second lens group includes a pair of meniscus lens components with concave surfaces facing each other, the second lens group further includes a positive-negative-positive triplet cemented lens in which positive lenses are disposed on both sides of one negative lens, An objective lens characterized by satisfying the following conditional expression: 3.3≦φ Ｌ１ / D Ｌ１ ≦12 ・・・(1-1) 0.1≦|R ２１２ | / f≦3.5 ・・・(2) However, φ Ｌ１ is the outer diameter of the first lens. Ｌ１ is the thickness of the first lens on the optical axis. ２１２ is the radius of curvature of the surface closest to the image of the first meniscus lens component which is the meniscus lens component on the object side of the pair of meniscus lens components, and f is the focal length of the objective lens.

7. 7. The objective lens according to claim 6, An objective lens characterized by satisfying the following conditional expression: 0.5≦|R ２１１ | / f≦7 ・・・(3) However, R ２１１ is the radius of curvature of the surface of the first meniscus lens component closest to the object.

8. The objective lens according to claim 6 or 7, Each of the pair of meniscus lens components is a cemented lens. An objective lens characterized by:

9. The objective lens according to any one of claims 6 to 8, An objective lens characterized by satisfying the following conditional expression: 18≦νdL≦31 (4) Here, νdL is the minimum value of the Abbe number of at least one positive lens arranged on the image side of the surface of the first meniscus lens component closest to the image side.

10. The objective lens according to any one of claims 6 to 9, An objective lens characterized by satisfying the following conditional expression: 1.51≦ndH≦1.75 (5) Here, ndH is the maximum value of the refractive index of at least one negative lens included in the objective lens.

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