A 50x objective for a metallographic microscope

Abstract

The application discloses a 50 times objective lens for a metallographic microscope, which is composed of a first lens with positive focal power, a second lens with positive focal power, a first double cemented lens with positive focal power, a second double cemented lens with positive focal power and a third double cemented lens with negative focal power from an object side to an image side. The first lens is a meniscus lens with an absolute value of the curvature radius of the object side being smaller than that of the image side. The second lens is a meniscus lens with an absolute value of the curvature radius of the object side being larger than that of the image side. The absolute value of the curvature radius of the object side of the first double cemented lens is larger than that of the image side. The absolute value of the curvature radius of the object side of the second double cemented lens is smaller than that of the image side. The absolute value of the curvature radius of the object side of the third double cemented lens is larger than that of the image side. The objective lens has the advantages of low cost, good chromatic aberration correction ability and high imaging quality by adopting the focal power sequence of 'positive-positive-positive-positive-negative' and three double cemented groups.

Description

Technical Field

[0001] This invention relates to an optical objective lens, and more particularly to a 50x objective lens for a metallurgical microscope. Background Technology

[0002] Metallurgical microscopes are crucial instruments for observing microstructures in materials science and industrial inspection, and their imaging quality largely depends on the performance of the microscope objectives. Currently, 50x objectives suitable for metallurgical microscopes primarily employ two typical optical design approaches:

[0003] The first type is the ordinary achromatic objective lens: This type of objective lens uses a relatively simple optical structure and mainly corrects the chromatic aberration of D-ray (589.3nm) and F-ray (486.1nm). However, its chromatic aberration correction capability is limited. Under high magnification and large numerical aperture conditions, the residual secondary spectrum is still quite obvious, resulting in color halo in the image and failing to meet the requirements of high-precision color metallographic analysis. Although this technical solution has a simple structure and low cost, image quality has become its performance bottleneck.

[0004] The second category is semi-apochromatic and apochromatic objectives: In pursuit of ultimate image quality, these objectives extensively utilize expensive special dispersive glass such as calcium fluoride (CaF2) in their optical structure, and correct various aberrations through more complex lens groups and cemented structures. This results in a complex optical system structure and high manufacturing costs, and they are typically used in high-end scientific research fields where cost is not a major concern, making them difficult to widely adopt in industrial inspection.

[0005] Therefore, existing technologies offer a binary choice: either choose a simple, low-cost, but low-image-quality ordinary achromatic objective, or choose a semi-apochromatic / apochromatic objective with excellent image quality but a complex and expensive structure. In short, a good balance between image quality, cost, and applicability cannot be achieved. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a 50x objective lens for metallurgical microscopes that can achieve significantly better performance than ordinary achromatic lenses and approach the level of apochromatic lenses at a controllable cost.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: a 50x objective lens for a metallurgical microscope, comprising, from the object side to the image side, a first lens with positive optical power, a second lens with positive optical power, a first cemented doublet with positive optical power, a second cemented doublet with positive optical power, and a third cemented doublet with negative optical power. The first lens is a meniscus lens whose absolute value of the radius of curvature of the object plane is smaller than that of the image plane. The second lens is a meniscus lens whose absolute value of the radius of curvature of the object plane is larger than that of the image plane. The absolute value of the radius of curvature of the object plane of the first cemented doublet is larger than that of the image plane. The absolute value of the radius of curvature of the object plane of the second cemented doublet is smaller than that of the image plane. The absolute value of the radius of curvature of the object plane of the third cemented doublet is larger than that of the image plane.

[0008] Compared with existing technologies, the advantages of this invention lie in its meticulous allocation of optical power and lens combination, employing an innovative "positive-positive-positive-positive-negative" optical power sequence and a three-set doublet composite synergistic technology to construct an apochromatic structure. This provides significantly superior chromatic aberration correction capabilities and imaging quality compared to ordinary apochromatic objectives while strictly controlling manufacturing costs. When applied to industrial scenarios without cover glass, such as metallographic analysis, semiconductor inspection, and MEMS observation, this invention achieves optimal imaging performance without any optical compensation, completely eliminating image quality degradation caused by cover glass thickness tolerances or absence, demonstrating excellent environmental adaptability and ease of operation.

[0009] This invention features a compact structure and balanced performance, achieving top-tier optical performance while maintaining a compact overall system length and structural stability. Each lens group has a clearly defined function and a rational distribution of optical power, forming an efficient aberration correction network. This optimized structure not only reduces alignment sensitivity but also minimizes over-reliance on extremely expensive optical materials, exhibiting excellent manufacturability and laying a solid foundation for the mass production and cost control of high-performance objectives.

[0010] Preferably, the object plane of the first lens has a radius of curvature of -5mm to -1mm, the image plane has a radius of curvature of -10mm to -1mm, and a center thickness of 4mm to 6mm; the distance between the first lens and the second lens is 0.05mm to 1mm. The object plane of the second lens has a radius of curvature of -30mm to -10mm, the image plane has a radius of curvature of -15mm to -5mm, and a center thickness of 2mm to 4mm; the distance between the second lens and the first cemented doublet lens is 3mm to 4mm. The object plane of the first cemented doublet lens has a radius of curvature of 90mm to 110mm, the image plane has a radius of curvature of -30mm to -15mm, and the cemented surface has a radius of curvature of 10mm to 14mm. The first and second cemented doublet lenses have a center thickness of 4mm to 7mm, and the distance between them is 3mm to 5mm. The object plane of the second cemented doublet lens has a radius of curvature of 15mm to 30mm, the image plane has a radius of curvature of -110mm to -90mm, and the cemented surface has a radius of curvature of -14mm to -10mm. The center thickness is 4mm to 7mm, and the distance between them is 4mm to 10mm. The object plane of the third cemented doublet lens has a radius of curvature of 60mm to 70mm, the image plane has a radius of curvature of 3mm to 10mm, and the cemented surface has a radius of curvature of -13mm to -11mm. The center thickness is 10mm to 15mm.

[0011] A further preferred embodiment is that the object plane of the first lens has a radius of curvature of -3mm, the image plane has a radius of curvature of -4.6mm, and a center thickness of 4.7mm; the distance between the first lens and the second lens is 0.25mm. The object plane of the second lens has a radius of curvature of -23mm, the image plane has a radius of curvature of -10mm, and a center thickness of 3.04mm; the distance between the second lens and the first cemented doublet lens is 3.26mm. The object plane of the first cemented doublet lens has a radius of curvature of 100mm, the image plane has a radius of curvature of -19.5mm, and the cemented surface has a radius of curvature of 1... The first and second cemented doublet lenses have a center thickness of 5.7 mm and a distance of 3.14 mm between them. The object plane of the second cemented doublet lens has a radius of curvature of 19.5 mm, the image plane has a radius of curvature of -100 mm, the cemented surface has a radius of curvature of -12 mm, and a center thickness of 5.7 mm. The distance between the second and third cemented doublet lenses is 7.5 mm. The object plane of the third cemented doublet lens has a radius of curvature of 68 mm, the image plane has a radius of curvature of 6.5 mm, the cemented surface has a radius of curvature of -12.4 mm, and a center thickness of 11.9 mm.

[0012] Preferably, the first lens has a refractive index of 1.65-1.85 and an Abbe number of 40-50, the second lens has a refractive index of 1.75-1.95 and an Abbe number of 20-35.

[0013] Furthermore, the first lens has a refractive index of 1.76 and an Abbe number of 47.7, the second lens has a refractive index of 1.85 and an Abbe number of 23.8.

[0014] Preferably, the first cemented doublet is composed of a third lens and a fourth lens, the second cemented doublet is composed of a fifth lens and a sixth lens, and the third cemented doublet is composed of a seventh lens and an eighth lens. The refractive index of the third lens is 1.8-2, and the Abbe number of the third lens is 10-20. The refractive index of the fourth lens is 1.4-1.7, and the Abbe number of the fourth lens is 80-90. The refractive index of the fifth lens is 1.3-1.6, and the Abbe number of the fifth lens is 70-90. The refractive index of the sixth lens is 1.8-2, and the Abbe number of the sixth lens is 15-35. The refractive index of the seventh lens is 1.8-2, and the Abbe number of the seventh lens is 15-35. The refractive index of the eighth lens is 1.5-1.7, and the Abbe number of the eighth lens is 55-60.

[0015] Ideally, the third lens has a refractive index of 1.92 and an Abbe number of 18.9; the fourth lens has a refractive index of 1.5 and an Abbe number of 81.6; the fifth lens has a refractive index of 1.5 and an Abbe number of 81.6; the sixth lens has a refractive index of 1.92 and an Abbe number of 18.9; the seventh lens has a refractive index of 1.92 and an Abbe number of 18.9; and the eighth lens has a refractive index of 1.58 and an Abbe number of 59.5.

[0016] Preferably, the object plane and image plane of the first lens, the second lens, the first cemented doublet lens, the second cemented doublet lens, and the third cemented doublet lens are all coated with a multilayer antireflective coating with a reflectivity of less than 0.5%. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the optical structure of a 50x objective lens for a metallurgical microscope according to an embodiment of the present invention;

[0018] Figure 2 This is an axial spherical aberration curve of a 50x objective lens for a metallurgical microscope according to an embodiment of the present invention;

[0019] Figure 3 This is a field curvature curve of a 50x objective lens for a metallurgical microscope according to an embodiment of the present invention.

[0020] Figure 4 This is a distortion curve diagram of a 50x objective lens for a metallurgical microscope according to an embodiment of the present invention.

[0021] Figure 5 This is an optical transfer function curve of a 50x objective lens for a metallurgical microscope according to an embodiment of the present invention.

[0022] Explanation of reference numerals in the attached figures:

[0023] L1, First lens; L2, Second lens; GL1, First cemented doublet lens; L3, Third lens; L4, Fourth lens; GL2, Second cemented doublet lens; L5, Fifth lens; L6, Sixth lens; GL3, Third cemented doublet lens; L7, Seventh lens; L8, Eighth lens. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0025] Example: An optical structure diagram of a 50x objective lens for a metallurgical microscope according to the present invention is shown below. Figure 1 As shown, along the direction of light propagation from the object plane to the image plane, it consists of a first lens L1, a second lens L2, a first cemented doublet GL1, a second cemented doublet GL2, and a third cemented doublet GL3.

[0026] The first lens L1 has positive optical power, and its object-facing surface is more curved. The refractive index of the first lens L1 is 1.65-1.85, and its Abbe number is 40-50. As the light-collecting front end of the system, the first lens L1's specific positive optical power design enables it to efficiently collect large-angle light rays at a considerable distance from the object surface to support a large numerical aperture, which is the primary optical basis for achieving long working distances.

[0027] The first lens L1 is a meniscus positive lens with an object plane radius of curvature of -5mm to -1mm and an image plane radius of curvature of -10mm to -1mm. However, the absolute value of the object plane radius of curvature must be smaller than the absolute value of the image plane radius of curvature. The center thickness of the first lens L1 is 4mm to 6mm. The distance between the first lens L1 and the second lens L2 is 0.05mm to 1mm.

[0028] The second lens L2 has positive optical power, a refractive index of 1.75-1.95, and an Abbe number of 20-35. As a transitional and initial converging element in the optical path, the second lens L2 receives the light from the first lens L1 and alters the optical path direction, preparing for subsequent aberration correction. Its positive optical power inherits and continues the converging trend of the first lens L1, making it a crucial element in maintaining the system's total optical power to ensure magnification while allowing the first lens L1 to "retreat." Its single-lens configuration, while transitioning the optical path, also lays the necessary light incident conditions for precise chromatic aberration correction by the subsequent cemented lens.

[0029] In the optical design of high-magnification microscope objectives, allowing the front lens group to be moved back is a protective mechanism. Usually, the rear lens group is allowed to take on part of the aberration correction task. This ensures that even if the front lens group undergoes a slight change in axis / tilt, the aberrations generated by the front lens group will have a minimal impact on the image quality of the entire optical system of the objective.

[0030] Specifically, in this embodiment, allowing the first lens L1 to retract from the second lens L2 means that during the design of the objective lens mechanical structure, the structural components that place the second lens L2 and the first lens L1 must ensure that a certain distance is reserved between the first lens L1 and the second lens L2, so that the first lens L1 can move independently of the second lens L2 and the lenses in the rear group and the imaging quality remains almost unchanged.

[0031] The second lens L2 is a meniscus positive lens with an object plane radius of curvature of -30mm to -10mm and an image plane radius of curvature of -15mm to -5mm. The absolute value of the object plane radius of curvature is greater than the absolute value of the image plane radius of curvature. The center thickness of the second lens L2 is 2mm to 4mm. The distance between the second lens L2 and the first cemented doublet lens GL1 is 3mm to 4mm.

[0032] The first cemented doublet GL1 is composed of a third lens L3 and a fourth lens L4 cemented together, with a positive overall optical power. The refractive index of the third lens L3 is 1.8–2, and its Abbe number is 10–20. The refractive index of the fourth lens L4 is 1.4–1.7, and its Abbe number is 80–90. The radius of curvature of the object plane of the third lens L3 is 90 mm to 110 mm, and the radius of curvature of the image plane of the fourth lens L4 is -30 mm to -15 mm. The radius of curvature of the cemented surface between the third and fourth lenses L3 is 10 mm to 14 mm. The center thickness of the first cemented doublet GL1 is 4 mm to 7 mm. The absolute value of the radius of curvature of the object plane of the third lens L3 must be greater than the absolute value of the radius of curvature of the image plane of the fourth lens L4. The distance between the first cemented doublet GL1 and the second cemented doublet GL2 is 3 mm to 5 mm. The first cemented doublet lens GL1 is the core of the system's primary chromatic aberration (positional chromatic aberration) correction. By combining optical materials with different Abbe numbers, it effectively corrects the axial chromatic aberration introduced by the front group and simultaneously corrects spherical aberration, precisely controlling the light path.

[0033] The second cemented doublet GL2 is composed of a fifth lens L5 and a sixth lens L6 cemented together, with a positive overall optical power. The refractive index of the fifth lens L5 is 1.3-1.6, and its Abbe number is 70-90. The refractive index of the sixth lens L6 is 1.8-2, and its Abbe number is 15-35. The radius of curvature of the object plane of the fifth lens L5 is 15mm to 30mm, the radius of curvature of the image plane of the sixth lens L6 is -110mm to -90mm, the radius of curvature of the cemented surface of the fifth lens L5 and the sixth lens L6 is -14mm to -10mm, the center thickness of the second cemented doublet GL2 is 4mm to 7mm, and the distance between the second cemented doublet GL2 and the third cemented doublet GL3 is 4mm to 10mm. The second cemented doublet GL2 is key to further correcting the secondary spectrum and residual chromatic aberration of the system. Working together with the first cemented doublet GL1, it enhances the chromatic aberration correction effect for specific wavelengths through optimized material pairing and optical power distribution, ensuring high resolution and color fidelity of the system across the entire field of view.

[0034] The third cemented doublet GL3 is composed of a seventh lens L7 and an eighth lens L8 cemented together, with an overall negative optical power. The refractive index of the seventh lens L7 is 1.8–2, and its Abbe number is 15–35. The refractive index of the eighth lens L8 is 1.5–1.7, and its Abbe number is 55–60. The radius of curvature of the object plane of the seventh lens L7 is 60 mm to 70 mm, the radius of curvature of the image plane of the eighth lens L8 is 3 mm to 10 mm, the radius of curvature of the cemented surface of the seventh and eighth lenses L7 is -13 mm to -11 mm, and the center thickness of the third cemented doublet GL3 is 10 mm to 15 mm. As a flat field lens in the system, the core function of the third cemented doublet GL3 is to precisely correct field curvature and astigmatism, ensuring a flat and clear image across the entire φ22 mm image plane. Its negative optical power plays a crucial role in system balancing. It compensates for the excessive convergence effect of the positive optical power of each lens in front by diverging, which not only corrects aberrations but also liberates the design constraints of each lens in front. It is the decisive optical link to finally achieve the coexistence of "high magnification, large aperture" and "long working distance".

[0035] The following is a best example of an embodiment of the present invention.

[0036] Example:

[0037] The radius of curvature of the first surface of the first lens L1 is -3mm, the radius of curvature of the second surface of the first lens L1 is -4.6mm, the center thickness of the first lens L1 is 4.7mm, the refractive index of the optical glass used in the first lens L1 is 1.76, the Abbe number is 47.7, and the distance between the first lens L1 and the second lens L2 is 0.25mm.

[0038] The radius of curvature of the first surface of the second lens L2 is -23mm, the radius of curvature of the second surface of the second lens L2 is -10mm, the center thickness of the second lens L2 is 3.04mm, the refractive index of the optical glass used in the second lens L2 is 1.85, the Abbe number is 23.8, and the distance between the second lens L2 and the third lens L3 is 3.26mm.

[0039] The object plane of the third lens L3 has a radius of curvature of 100 mm, the image plane of the fourth lens L4 has a radius of curvature of -19.5 mm, the cemented surface of the third lens L3 and the fourth lens L4 has a radius of curvature of 12 mm, the center thickness of the first cemented doublet GL1 is 5.7 mm, the optical glass used in the third lens L3 has a refractive index of 1.92 and an Abbe number of 18.9, the optical glass used in the fourth lens L4 has a refractive index of 1.5 and an Abbe number of 81.6, and the distance between the fourth lens L4 and the fifth lens L5 is 3.14 mm.

[0040] The object plane radius of curvature of the fifth lens L5 is 19.5 mm, the image plane radius of curvature of the sixth lens L6 is -100 mm, the cemented surface radius of curvature of the fifth lens L5 and the sixth lens L6 is -12 mm, the center thickness of the second cemented doublet GL2 is 5.7 mm, the distance between the sixth lens L6 and the seventh lens L7 is 7.5 mm, the optical glass used for the fifth lens L5 has a refractive index of 1.5 and an Abbe number of 81.6, and the optical glass used for the sixth lens L6 has a refractive index of 1.92 and an Abbe number of 18.9.

[0041] The object plane radius of curvature of the seventh lens L7 is 68 mm, the image plane radius of curvature of the eighth lens L8 is 6.5 mm, the cemented surface radius of curvature of the seventh lens L7 and the eighth lens L8 is -12.4 mm, the center thickness of the third cemented doublet GL3 is 11.9 mm, the refractive index of the optical glass used in the seventh lens L7 is 1.92, and the Abbe number is 18.9; the refractive index of the optical glass used in the eighth lens L8 is 1.58, and the Abbe number is 59.5.

[0042] The entire objective lens has a focal length of f = 2 mm, a numerical aperture of NA = 0.8, and a working distance of ≥ 1.8 mm.

[0043] All lenses are made of optical glass produced by Chengdu Guangming Company. The object plane and image plane of all lenses are coated with multi-layer anti-reflection film, and the reflectivity is less than 0.5%.

[0044] Figure 2 In the diagram, "T" corresponds to meridional astigmatism, and "S" corresponds to sagittal astigmatism. Figure 2 As can be seen from the axial spherical aberration curve, the second-order spectrum is less than 2μm, and the chromatic aberration is strictly corrected within the large aperture range. The F, d, and C rays are almost overlapping, achieving imaging performance close to the apochromatic level.

[0045] from Figure 3 Field curve diagram and Figure 4 As can be seen from the distortion curve, the field curvature and distortion are well corrected, the deviations of the sagittal astigmatism and meridional astigmatism curves are small, the field curvature at the edge of the field of view is less than 2μm, and the maximum distortion of the optical system is within 1%. The optical system achieves a flat field effect.

[0046] Optical transfer function (MTF) curve as shown Figure 5 As shown, where:

[0047] The solid curve at point A is the meridional diffraction limit line, and the dashed curve at point A is the sagittal diffraction limit line.

[0048] The solid curve at point B is the 0-field-of-view meridional MTF curve, and the dashed curve at point B is the 0-field-of-view sagittal MTF curve.

[0049] The solid curve at point C is the 0.3 field-of-view meridional MTF curve, and the dashed curve at point C is the 0.3 field-of-view sagittal MTF curve.

[0050] The solid curve at point D is the 0.5 field-of-view meridional MTF curve, and the dashed curve at point D is the 0.5 field-of-view sagittal MTF curve.

[0051] The solid curve at point E is the 0.7 field-of-view meridional MTF curve, and the dashed curve at point E is the 0.7 field-of-view sagittal MTF curve.

[0052] The solid curve at point F is the full-field meridional MTF curve, and the dashed curve at point F is the full-field sagittal MTF curve.

[0053] As can be seen from the optical transfer function (MTF) curves, the meridional and sagittal MTF curves represented by points B to F are close to the meridional and sagittal diffraction limits represented by point A, indicating that the optical system designed in this example can well guarantee imaging quality and has high contrast and resolution.

[0054] from Figure 5 As can be seen from the optical transfer function (MTF) curves, the MTF curves for each field of view are close to the diffraction limit, which can ensure good imaging quality and have high contrast and resolution.

Claims

1. A 50x objective lens for a metallurgical microscope, characterized in that, From object to image, the lens consists of a first lens with positive optical power, a second lens with positive optical power, a first cemented doublet with positive optical power, a second cemented doublet with positive optical power, and a third cemented doublet with negative optical power. The first lens is a meniscus lens whose absolute value of the object plane's radius of curvature is smaller than that of the image plane. The second lens is a meniscus lens whose absolute value of the object plane's radius of curvature is larger than that of the image plane. The absolute value of the object plane's radius of curvature of the first cemented doublet is larger than that of the image plane. The absolute value of the object plane's radius of curvature of the second cemented doublet is smaller than that of the image plane. The absolute value of the object plane's radius of curvature of the third cemented doublet is larger than that of the image plane. The object plane's radius of curvature of the first lens is -3mm, the image plane's radius of curvature is -4.6mm, and the center thickness is 4.7mm. The distance between the first and second lenses is 0.25mm. The object plane's radius of curvature of the second lens is... The diameter is -23mm, the radius of curvature of the image plane is -10mm, the center thickness is 3.04mm, and the distance between the second lens and the first cemented doublet is 3.26mm; the object plane of the first cemented doublet has a radius of curvature of 100mm, the image plane has a radius of curvature of -19.5mm, the cemented surface has a radius of curvature of 12mm, the center thickness is 5.7mm, and the distance between the first cemented doublet and the second cemented doublet is 3.14mm; The second cemented doublet has an object plane radius of curvature of 19.5 mm, an image plane radius of curvature of -100 mm, a cemented surface radius of curvature of -12 mm, and a center thickness of 5.7 mm. The distance between the second and third cemented doublets is 7.5 mm. The third cemented doublet has an object plane radius of curvature of 68 mm, an image plane radius of curvature of 6.5 mm, a cemented surface radius of curvature of -12.4 mm, and a center thickness of 11.9 mm.

2. The 50x objective lens for a metallurgical microscope as described in claim 1, characterized in that, The first lens has a refractive index of 1.76 and an Abbe number of 47.

7. The second lens has a refractive index of 1.85 and an Abbe number of 23.

8.

3. A 50x objective lens for a metallurgical microscope as described in claim 1 or 2, characterized in that, The first cemented doublet is composed of a third lens and a fourth lens; the second cemented doublet is composed of a fifth lens and a sixth lens; and the third cemented doublet is composed of a seventh lens and an eighth lens. The refractive index of the third lens is 1.92, and the Abbe number of the third lens is 18.

9. The refractive index of the fourth lens is 1.5, and the Abbe number of the fourth lens is 81.

6. The refractive index of the fifth lens is 1.5, and the Abbe number of the fifth lens is 81.

6. The refractive index of the sixth lens is 1.92, and the Abbe number of the sixth lens is 18.

9. The refractive index of the seventh lens is 1.92, and the Abbe number of the seventh lens is 18.

9. The refractive index of the eighth lens is 1.58, and the Abbe number of the eighth lens is 59.

5.

4. A 50x objective lens for a metallurgical microscope as described in claim 1, characterized in that, The object plane and image plane of the first lens, the second lens, the first cemented doublet lens, the second cemented doublet lens, and the third cemented doublet lens are all coated with a multilayer antireflective coating with a reflectivity of less than 0.5%.

Images