A water immersion objective lens with a 20x large numerical aperture

Abstract

The invention discloses a water immersion objective lens with a numerical aperture of 20 times. The water immersion objective lens is composed of a front lens group with positive focal power, a middle lens group with positive focal power and a rear lens group with negative focal power which are arranged in sequence on the same optical axis from the object side to the image side. The front lens group is composed of a first cemented component, a third lens and a second cemented component. The middle lens group is composed of a seventh lens, a third cemented component and a fourth cemented component. Corresponding to a change in cover glass thickness of 0.11 to 0.23 mm, a change range of the interval d1 between the seventh lens and the third cemented component is 0.3 to 0.7 mm. By designing a cemented lens with a specific shape and focal power, the numerical aperture of the water immersion objective lens reaches 0.95, which is adapted to a change in cover glass thickness of 0.11 to 0.23 mm. The system aberration caused by the change in cover glass thickness can be adjusted, and the lens has both a large field of view and wide spectrum chromatic aberration correction.

Description

Technical Field

[0001] The invention relates to a water immersion objective lens, in particular to a water immersion objective lens with a 20-fold large numerical aperture. Background Art

[0002] A water immersion objective is a microscope objective that increases the numerical aperture by filling liquid water between the cover glass and the objective. This design is intended to improve the imaging quality and resolution of the microscope. Although water and oil are both commonly used immersion media, water is easier to obtain and wipe than oil, making water immersion objectives more convenient to handle when used; at the same time, water immersion objectives are more suitable for biological research, especially in living cell and tissue imaging. In living cell research, living cells are often encapsulated in nutrient physiological solutions, which usually have a refractive index similar to that of water. Using water immersion objectives can reduce imaging problems caused by refractive index differences.

[0003] However, since the refractive index of water is lower than that of oil, the numerical aperture of water immersion objectives is usually lower than that of oil immersion objectives. The numerical aperture is a measure of the ability of the objective to collect light and is directly related to the resolution of the microscope. At present, the numerical aperture of the conventional 20x apochromatic objective in China can only reach 0.8 at most, which limits its use in some applications that require higher resolution. Some water immersion objectives with relatively high numerical apertures, such as Nikon's 40x water immersion objective, have a numerical aperture of 1.15, but their image field of view is only 22mm, and the chromatic aberration correction range is only in the visible light band, which cannot take into account both the larger clear field of view and the wide spectrum chromatic aberration correction range. This results in researchers being unable to observe more cells at the same time when using water immersion objectives for cell observation. At the same time, since the chromatic aberration correction range does not cover the entire fluorescence band, the fluorescence signal in some bands is weak, and it is impossible to capture high-quality fluorescence images of the entire band. As the numerical aperture of the objective lens increases, the depth of field of the objective lens becomes shallower and shallower, which makes the large numerical aperture water immersion objective lens very sensitive to the change of cover glass thickness when in use. However, in actual biological experiments, the cover glass as an experimental consumable cannot be very precise due to the cost and processing tolerance, and there are different thickness specifications for different experimental samples. The thickness of the cover glass usually varies in the range of 0.11~0.23mm. These tiny differences will bring certain aberrations to the objective lens, thereby affecting the observation effect of the sample. Some researchers have customized objectives with different cover glass thicknesses for different applications, which greatly increases the cost of the experiment. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a water immersion objective lens with a 20-fold large numerical aperture, which has a high numerical aperture, a large clear field of view and a wide spectrum chromatic aberration correction range and can perform aberration correction for changes in cover glass thickness.

[0005] The technical solution adopted by the present invention to solve the above technical problems is: a water immersion objective lens with a large numerical aperture of 20 times, which is composed of a front lens group with positive focal power, a middle lens group with positive focal power and a rear lens group with negative focal power arranged in sequence on the same optical axis from the object side to the image side, wherein the focal length fobj of the water immersion objective lens is 9.5-10.5 mm, the focal length fm1 of the front lens group satisfies: 1≤|fm1 / fobj|≤1.2, and the focal length fm2 of the middle lens group satisfies: 5≤|fm2 / fobj|≤ 16. The focal length fm3 of the rear lens group satisfies: |fm3 / fobj|>30. The front lens group is composed of a first cemented component, a third lens and a second cemented component. The first cemented component is composed of a first lens and a second lens. The first lens is a hemispherical lens with a flat object side, and the second lens is a meniscus lens with a concave object side. The middle lens group is composed of a seventh lens, a third cemented component and a fourth cemented component. The interval d1 between the seventh lens and the third cemented component is 0.3-0.7 mm.

[0006] Compared with the prior art, the advantage of the present invention is that by designing a cemented lens of a specific shape and optical focal length, the problems of the conventional 20x apochromatic objective lens having a numerical aperture that is not high enough, a field of view that is not clear enough, a spectral chromatic aberration correction range that is not wide enough, and aberration correction for changes in cover glass thickness cannot be performed, etc., are solved. The numerical aperture of the water immersion objective lens reaches 0.95, and the system aberration caused by the change in the thickness of the cover glass within the range of 0.11-0.23 mm can be adjusted, while having both a large field of view and wide spectral chromatic aberration correction.

[0007] Preferably, the seventh lens has positive focal power, a convex surface on the object side, a concave surface on the image side, and a focal length fl7 of the seventh lens and a clear aperture d7 of the seventh lens satisfy: d7 / fl7>0.4. A higher clear aperture can collect light at a wider angle, thereby further improving the numerical aperture.

[0008] Preferably, the Abbe number vd1 of the first lens and the Abbe number vd2 of the second lens satisfy: |vd1-vd2|≥30, the second cemented member is composed of a fourth lens with positive focal power, a fifth lens with negative focal power and a sixth lens with positive focal power, and simultaneously satisfies: |vd4-vd5|≥35 and |vd5-vd6|≥35, wherein vd4 is the Abbe number of the fourth lens, vd5 is the Abbe number of the fifth lens, and vd6 is the Abbe number of the sixth lens, the refractive index nd1 of the first lens satisfies: 1.45<nd1<1.5, and the refractive index nd2 of the second lens satisfies: 1.9<nd2<1.95. The first cemented member and the second cemented member both use a combination of adjacent lenses with a large difference in Abbe numbers, and the first lens and the second lens use a combination of high and low dispersion materials, which can effectively reduce the chromatic aberration of the front lens group.

[0009] Preferably, the third cemented component is composed of an eighth lens with negative power and a ninth lens with positive power, and the Abbe number vd8 of the eighth lens and the Abbe number vd9 of the ninth lens satisfy: |vd8-vd9|≥35, and the fourth cemented component is composed of a tenth lens with positive power and an eleventh lens with negative power, and the Abbe number vd10 of the tenth lens and the Abbe number vd11 of the eleventh lens satisfy: |vd10-vd11|≥40. The third cemented component and the fourth cemented component also use a structural combination with a large difference in the Abbe numbers of adjacent lenses to correct the secondary spectrum and realize the function of apochromatism.

[0010] Preferably, the rear lens group consists of a fifth cemented element with negative optical power and a fourteenth lens with positive optical power.

[0011] Preferably, the thickness of the fifth cemented member is 5-8 mm. The fifth cemented member lens is thicker, which is beneficial to correcting field curvature.

[0012] Preferably, the rear lens group is composed of a fifth cemented component with negative optical power and a fourteenth lens with positive optical power, the fifth cemented component is composed of a twelfth lens with negative optical power and a thirteenth lens with positive optical power, the object side of the twelfth lens is concave, the image side is concave, the object side of the thirteenth lens is convex, and the image side is concave. The fifth cemented component has negative optical power, which can increase the working distance of the objective lens and correct vertical axis chromatic aberration. The fourteenth lens is a meniscus lens with a concave object side, and the refractive index nd14 of the fourteenth lens satisfies: 1.85<nd14<1.95, and a high refractive index material is used to avoid the introduction of high-order aberrations. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a schematic diagram of the structure of a water immersion objective lens according to Example 1 of an embodiment of the present invention:

[0014] Figure 2 This is a field curvature curve diagram of Example 1 of the embodiment of the present invention when the cover glass thickness is 0.17 mm, where the ordinate is the angle of the incident light and the abscissa is the magnitude of the field curvature;

[0015] Figure 3 This is a field curvature curve diagram of Example 1 in the embodiment of the present invention when the cover glass thickness is 0.11 mm;

[0016] Figure 4 This is a field curvature curve diagram of Example 1 in the embodiment of the present invention when the cover glass thickness is 0.23 mm;

[0017] Figure 5 This is a distortion curve diagram of Example 1 of the embodiment of the present invention when the cover glass thickness is 0.17 mm, where the ordinate is the angle of the incident light and the abscissa is the magnitude of the distortion;

[0018] Figure 6 This is a distortion curve diagram of Example 1 of the embodiment of the present invention when the cover glass thickness is 0.11 mm.

[0019] Figure 7 This is a distortion curve diagram of Example 1 of the embodiment of the present invention when the cover glass thickness is 0.23 mm.

[0020] Figure 8 The optical transfer function MTF curve of Example 1 in the embodiment of the present invention with a cover glass thickness of 0.17 mm, the ordinate represents the magnitude of the MTF value, and the abscissa represents the spatial frequency;

[0021] Fig. 9 The optical transfer function MTF curve of Example 1 in the embodiment of the present invention with a cover glass thickness of 0.11 mm;

[0022] Fig.10 The optical transfer function MTF curve of Example 1 in the embodiment of the present invention with a cover glass thickness of 0.23 mm;

[0023] Fig.11 1 is an axial aberration curve diagram of Example 1 in the embodiment of the present invention when the cover glass thickness is 0.17 mm, the ordinate represents the normalized pupil, and the abscissa represents the spherical aberration value of the on-axis field of view of different color lights;

[0024] Fig.12 This is an axial aberration curve diagram of Example 1 in the embodiment of the present invention when the cover glass thickness is 0.11 mm;

[0025] Fig.13 This is an axial aberration curve diagram of Example 1 in the embodiment of the present invention when the cover glass thickness is 0.23 mm;

[0026] Fig.14This is a schematic diagram of the structure of the water immersion objective lens of Example 2 in the embodiment of the present invention:

[0027] Fig.15 This is a field curvature curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.17 mm, where the ordinate is the angle of the incident light and the abscissa is the magnitude of the field curvature;

[0028] Fig.16 This is a field curvature curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.11 mm;

[0029] Fig.17 This is a field curvature curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.23 mm;

[0030] Fig.18 This is a distortion curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.17 mm, where the ordinate is the angle of the incident light and the abscissa is the magnitude of the distortion;

[0031] Fig.19 This is a distortion curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.11 mm.

[0032] Fig. 20 This is a distortion curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.23 mm.

[0033] Fig.21 The optical transfer function MTF curve of Example 2 of the embodiment of the present invention with a cover glass thickness of 0.17 mm, the ordinate represents the magnitude of the MTF value, and the abscissa represents the spatial frequency;

[0034] Fig. 22 The optical transfer function MTF curve of Example 2 in the embodiment of the present invention with a cover glass thickness of 0.11 mm;

[0035] Fig.23 The optical transfer function MTF curve of Example 2 in the embodiment of the present invention with a cover glass thickness of 0.23 mm;

[0036] Fig.24 : is an axial aberration curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.17 mm, the ordinate represents the normalized pupil, and the abscissa represents the spherical aberration value of the on-axis field of view of different color lights;

[0037] Fig.25 This is an axial aberration curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.11 mm;

[0038] Fig.26 This is an axial aberration curve diagram of Example 2 of the embodiment of the present invention when the cover glass thickness is 0.23 mm;

[0039] Fig. 27 This is a schematic diagram of the structure of the water immersion objective lens of Example 3 in the embodiment of the present invention:

[0040] Fig.28 This is a field curvature curve diagram of Example 3 of the embodiment of the present invention when the cover glass thickness is 0.17 mm, where the ordinate is the angle of the incident light and the abscissa is the magnitude of the field curvature;

[0041] Fig.29 This is a field curvature curve diagram of Example 3 of the embodiment of the present invention when the cover glass thickness is 0.11 mm;

[0042] Fig.30 This is a field curvature curve diagram of Example 3 of the embodiment of the present invention when the cover glass thickness is 0.23 mm;

[0043] Fig.31 This is a distortion curve diagram of Example 3 of the embodiment of the present invention when the cover glass thickness is 0.17 mm, where the ordinate is the angle of the incident light and the abscissa is the magnitude of the distortion;

[0044] Fig.32 This is a distortion curve diagram of Example 3 of the embodiment of the present invention when the cover glass thickness is 0.11 mm;

[0045] Fig.33 This is a distortion curve diagram of Example 3 of the embodiment of the present invention when the cover glass thickness is 0.23 mm;

[0046] Fig.34 The optical transfer function MTF curve of Example 3 of the embodiment of the present invention with a cover glass thickness of 0.17 mm, the ordinate represents the magnitude of the MTF value, and the abscissa represents the spatial frequency;

[0047] Fig.35 The optical transfer function MTF curve of Example 3 in the embodiment of the present invention with a cover glass thickness of 0.11 mm;

[0048] Fig.36 The optical transfer function MTF curve of Example 3 in the embodiment of the present invention with a cover glass thickness of 0.23 mm;

[0049] Fig.37 : is an axial aberration curve diagram of Example 3 of the embodiment of the present invention when the cover glass thickness is 0.17 mm, the ordinate represents the normalized pupil, and the abscissa represents the spherical aberration value of the on-axis field of view of different color lights;

[0050] Fig.38 This is an axial aberration curve diagram of Example 3 in the embodiment of the present invention when the cover glass thickness is 0.11 mm;

[0051] Fig.39This is an axial aberration curve diagram of Example 3 in the embodiment of the present invention when the cover glass thickness is 0.23 mm. DETAILED DESCRIPTION

[0052] The present invention is further described in detail below with reference to the accompanying drawings.

[0053] like Figure 1 As shown, a 20x large numerical aperture water immersion objective lens is composed of a front lens group M1 with positive focal power, a middle lens group M2 with positive focal power, and a rear lens group M3 with negative focal power, which are arranged in sequence on the same optical axis from the object side to the image side. The focal length fobj of the water immersion objective lens is 9.5-10.5 mm, the focal length fm1 of the front lens group M1 satisfies: 1≤|fm1 / fobj|≤1.2, and the focal length fm2 of the middle lens group M2 satisfies : 5≤|fm2 / fobj|≤16, the focal length fm3 of the rear lens group M3 satisfies: |fm3 / fobj|>30, the front lens group M1 bears the main optical focal length, which can greatly reduce the numerical aperture of the rear lens group M3, and the middle lens group M2 is a weak light focal length group with positive optical focal length, which mainly corrects the complex aberrations caused by the wide spectrum and large numerical aperture. At the same time, the weak light focal length property of the middle lens group M2 can significantly relax the processing requirements of the glass. The front lens group M1 is composed of the first cemented component GL1, the third lens L3 and the second cemented component GL2. The air interval between the first cemented component GL1 and the third lens L3 is 0.1~0.2mm, and the air interval between the third lens L3 and the second cemented component GL2 is 0.1~0.2mm. The first cemented component GL1 is composed of the first lens L1 and the second lens L2. The first lens L1 is a hemispherical lens with a flat surface on the object side, and the second lens L2 is a meniscus lens with a concave surface on the object side. This combination can share the focal length while collecting light and improving the numerical aperture. The middle lens group M2 is composed of the seventh lens L7, the third cemented component GL3 and the fourth cemented component GL4. Corresponding to the change of the cover glass thickness of 0.11~0.23mm, the interval d1 between the seventh lens L7 and the third cemented component GL3 is controlled in the range of 0.3~0.7mm, which can adjust the system aberration caused by the change of the cover glass thickness.

[0054] The seventh lens L7 has positive power, a convex surface on the object side, and a concave surface on the image side. The focal length fl7 of the seventh lens L7 and the clear aperture d7 of the seventh lens L7 satisfy: d7 / fl7>0.4. A higher clear aperture can collect light at a wider angle, thereby further improving the numerical aperture.

[0055] The Abbe number vd1 of the first lens L1 and the Abbe number vd2 of the second lens L2 satisfy: |vd1-vd2|≥30, the focal length of the second lens L2 is -10 to -5 mm, the third lens L3 is a plano-convex lens with a plane on the object side, the second cemented member GL2 is composed of a fourth lens L4 with positive focal power, a fifth lens L5 with negative focal power and a sixth lens L6 with positive focal power, and simultaneously satisfies: |vd4-vd5|≥35 and |vd5-vd6|≥35, wherein vd4 is the Abbe number of the fourth lens L4, vd5 is the Abbe number of the fifth lens L5, and vd6 is the Abbe number of the sixth lens L6. The refractive index nd1 of the first lens L1 satisfies: 1.4<nd1<1.5, which is close to the refractive index of immersion water, and the refractive index nd2 of the second lens L2 satisfies: 1.85<nd2<1.95. The first cemented member GL1 and the second cemented member GL2 both use a combination of adjacent lenses with a large difference in Abbe numbers, and the first lens L1 and the second lens L2 use a combination of high-dispersion and low-dispersion materials, which can effectively reduce the chromatic aberration of the front lens group M1.

[0056] The third cemented member GL3 is composed of an eighth lens L8 having negative power and a ninth lens L9 having positive power, the eighth lens L8 having a convex surface on the object side and a concave surface on the image side, the ninth lens L9 having a convex surface on the object side and a convex surface on the image side. The Abbe number vd8 of the eighth lens L8 and the Abbe number vd9 of the ninth lens L9 satisfy: |vd8-vd9|≥35, and the fourth cemented member GL4 is composed of a tenth lens L10 having positive power and an eleventh lens L11 having negative power, the tenth lens L10 having a convex surface on the object side and a convex surface on the image side, the eleventh lens L11 having a concave surface on the object side and a concave surface on the image side. The Abbe number vd10 of the tenth lens L10 and the Abbe number vd11 of the eleventh lens L11 satisfy: |vd10-vd11|≥40. The third glued component GL3 and the fourth glued component GL4 also use a structural combination with a large difference in Abbe numbers of adjacent lenses to correct the secondary spectrum and achieve the function of apochromatism.

[0057] The rear lens group M3 is composed of a fifth cemented member GL5 with negative optical power and a fourteenth lens L14 with positive optical power. The fifth cemented member GL5 is composed of a twelfth lens L12 with negative optical power and a thirteenth lens L13 with positive optical power. The object side of the twelfth lens L12 is concave and the image side is concave. The object side of the thirteenth lens L13 is convex and the image side is concave. The fifth cemented member GL5 has a negative optical power, which can increase the working distance of the objective lens and correct vertical axis chromatic aberration.

[0058] The thickness of the fifth cemented part GL5 is 5-8 mm. The fifth cemented part GL5 is thicker, which is conducive to correcting the field curvature. The fourteenth lens L14 is a meniscus lens with a concave surface on the object side, and its positive optical power can balance the overall optical power. The refractive index nd14 of the fourteenth lens L14 satisfies: 1.85<nd14<1.95, and a high refractive index material is used to avoid the introduction of high-order aberrations.

[0059] The present embodiment is further described below through several examples.

[0060] Example 1:

[0061] The structure of the water immersion objective lens in this example is as follows Figure 1 As shown, it consists of a front lens group M1, a middle lens group M2 and a rear lens group M3. The focal length of the water immersion objective lens is fobj = 9.85mm, NA = 0.95, magnification β = -20 times, immersion layer thickness is 1.1mm, cover glass thickness is 0.17mm, spectral range is 405~656nm, field of view range is 25mm, and the medium is water. The water immersion objective lens has the following characteristics:

[0062] The focal length of the front lens group M1 is fm1=10.44mm, the focal length of the middle lens group M2 is fm2=53.95mm, the focal length of the rear lens group M3 is fm3=-332.74mm, |fm1 / fobj|=1.06, |fm2 / fobj|=5.48, |fm3 / fobj|=33.78.

[0063] The refractive index of the first lens L1 is nd1=1.46, the Abbe number of the first lens L1 is vd1=67.8, the Abbe number of the second lens L2 is vd2=35.3, and |vd1-vd2|=32.5; the focal length of the second lens L2 is fl2=-9.04mm, the Abbe number of the fourth lens L4 is vd4=81.6, the Abbe number of the fifth lens L5 is vd5=44.3, and the Abbe number of the sixth lens L6 is vd6=95, |vd4-vd5|=37.3, and |vd5-vd6|=50.7.

[0064] The focal length fl7 of the seventh lens L7 is 37.59 mm, the lens aperture d is 20.6 mm, and d / fl7 is 0.55. The Abbe number vd8 of the eighth lens L8 is 33.3, the Abbe number vd9 of the ninth lens L9 is 95, and |vd8-vd9| is 61.7; the Abbe number vd10 of the tenth lens L10 is 81.6, and the Abbe number vd11 of the eleventh lens L11 is 28.3, and |vd10-vd11| is 53.3.

[0065] The focal length of the fifth cemented member GL5 is fgl5=-12.92 mm, and the lens thickness is 5.4 mm. The focal length of the fourteenth lens L14 is fl14=25.47 mm, and the refractive index nd14=1.9.

[0066] Table 1: Parameters of the water immersion objective lens of Example 1

[0067] Face number Radius (mm) Thickness (mm) Refractive index Nd Abbe number Vd L1 Object Plane unlimited 1.95 1.46 67.8 L2 Object Plane -2.06 4.67 1.91 35.3 L2 Image Plane -5.72 0.15 L3 Object Plane unlimited 4.6 1.43 95 L3 Image plane -12.52 0.1 L4 Object Plane 28.05 7 1.5 81.6 L5 Object Plane -14.23 1.52 1.61 44.3 L6 Object Plane 24.18 6.8 1.43 95 L6 Image plane -22.97 0.12 L7 Object Plane 30.27 3.5 1.92 20.9 L7 Image plane 208.21 0.5 L8 Object Plane 35.95 1.6 1.8 33.3 L9 Object Plane 11.86 7.2 1.43 95 L9 Image plane -30.96 0.5 L10 Object plane 8.81 5.5 1.5 81.6 L11 Object plane -99.05 1.1 1.74 28.3 L11 Image plane 6.23 5.5 L12 Object plane -7.46 2.7 1.65 33.8 L13 Object plane 18.14 2.7 1.83 42.7 L13 Image plane 574.42 3.14 L14 Object plane -45.45 6.99 1.9 31.3 L14 Image plane -16.48

[0068] Among them, the radius refers to the radius of curvature of the surface, and the thickness refers to the on-axis distance from the current surface to the next surface. For example, the thickness of the object plane of surface L1 is the distance from the object plane L1 to the object plane L2, which may be the on-axis thickness of the medium or lens, or the on-axis air gap between them.

[0069] In this example 1, when the thickness of the cover glass is 0.11 mm, the distance d1 between the seventh lens L7 and the third cementing member GL3 is 0.4 mm, and the distance d2 between the third cementing member GL3 and the fourth cementing member GL4 is 0.6 mm.

[0070] When the thickness of the cover glass is 0.23 mm, the distance d1 between the seventh lens L7 and the third cementing member GL3 is 0.6 mm, and the distance d2 between the third cementing member GL3 and the fourth cementing member GL4 is 0.4 mm.

[0071] from Figures 2~4 Field curvature plots and Figures 5 to 7 From the distortion curve, we can see that the field curvature and distortion under different cover glass thicknesses are well corrected, the maximum field curvature at the edge of the field of view is no more than 2um, and the maximum distortion of the optical system is within 1%.

[0072] from Figures 8-10 It can be seen from the optical transfer function MTF curve that the cutoff frequency of the optical system is 4000lp / mm, and the MTF curves of each field of view under different cover glass thicknesses are close to the diffraction limit, which can well ensure the imaging quality.

[0073] from Figures 11 to 13 From the axial aberration curve, it can be seen that spherical aberration has been strictly corrected within the full aperture range, and light of each wavelength is almost overlapping.

[0074] Example 2:

[0075] The structure of the water immersion objective lens in Example 2 is as follows: Fig.14As shown, it consists of a front lens group M1, a middle lens group M2 and a rear lens group M3. The focal length of the water immersion objective lens is fobj = 10.23mm, NA = 0.95, magnification β = -20 times, immersion layer thickness is 1.1mm, cover glass thickness is 0.17mm, spectral range is 405~656nm, field of view range is 25mm, and the medium is water. The water immersion objective lens has the following characteristics:

[0076] The focal length of the front lens group M1 is fm1=11.79mm, the focal length of the middle lens group M2 is fm2=59.48mm, the focal length of the rear lens group M3 is fm3=-2270.3mm, |fm1 / fobj|=1.15, |fm2 / fobj|=5.81, |fm3 / fobj|=221.9.

[0077] The refractive index of the first lens L1 is nd1=1.46, the Abbe number of the first lens L1 is vd1=67.8, the Abbe number of the second lens L2 is vd2=35.3, and |vd1-vd2|=32.5; the focal length of the second lens L2 is fl2=-7.92mm, the Abbe number of the fourth lens L4 is vd4=81.6, the Abbe number of the fifth lens L5 is vd5=44.3, and the Abbe number of the sixth lens L6 is vd6=95, |vd4-vd5|=37.3, and |vd5-vd6|=50.7.

[0078] The focal length of the seventh lens L7 is fl7=40.06mm, the lens aperture is d=20.7mm, and d / fl7=0.52. The Abbe number of the eighth lens L8 is vd8=33.3, the Abbe number of the ninth lens L9 is vd9=95, |vd8-vd9|=61.7; the Abbe number of the tenth lens L10 is vd10=81.6, and the Abbe number of the eleventh lens L11 is vd11=28.3, |vd10-vd11|=53.3.

[0079] The focal length of the fifth cemented member GL5 is fgl5=-13.57 mm, and the lens thickness is 5.48 mm. The focal length of the fourteenth lens L14 is fl14=22.91 mm, and the refractive index nd14=1.9.

[0080] Table 2: Parameters of the water immersion objective lens of Example 2

[0081] Face number Radius (mm) Thickness (mm) Refractive index Nd Abbe number Vd L1 Object Plane unlimited 1.95 1.46 67.8 L2 Object Plane -1.93 4.67 1.91 35.3 L2 Image plane -5.68 0.15 L3 Object Plane unlimited 4.57 1.43 95 L3 Image plane -11.04 0.11 L4 Object Plane 135.72 5.6 1.5 81.6 L5 Object Plane -11.45 1.5 1.61 44.3 L6 Object Plane 29.69 6.7 1.43 95 L6 Image plane -17.38 0.11 L7 Object Plane 21.06 2.99 1.92 20.9 L7 Image plane 44.94 0.5 L8 Object Plane 33.34 1.7 1.8 33.3 L9 Object Plane 12.33 7.3 1.43 95 L9 Image plane -38.88 0.5 L10 Object plane 9.58 5.5 1.5 81.6 L11 Object plane -588.58 1 1.74 28.3 L11 Image plane 6.73 5.6 L12 Object plane -8 2.68 1.65 33.8 L13 Object plane 18.62 2.8 1．83 42.7 L13 Image plane 293.45 3.2 L14 Object plane -57.45 4 1.9 31.3 L14 Image plane -15.81

[0082] In the second example, when the thickness of the cover glass becomes 0.11 mm, the distance d1 between the seventh lens L7 and the third cementing member GL3 is 0.41 mm, and the distance d2 between the third cementing member GL3 and the fourth cementing member GL4 is 0.59 mm.

[0083] When the thickness of the cover glass becomes 0.23 mm, the distance d1 between the seventh lens L7 and the third cementing member GL3 is 0.58 mm, and the distance d2 between the third cementing member GL3 and the fourth cementing member GL4 is 0.42 mm.

[0084] from Figures 15-17 Field curvature plots and Figures 18 to 20 From the distortion curve, we can see that the field curvature and distortion under different cover glass thicknesses are well corrected, the maximum field curvature at the edge of the field of view is no more than 2um, and the maximum distortion of the optical system is within 0.5%.

[0085] from Figures 21 to 23 It can be seen from the optical transfer function MTF curve that the cutoff frequency of the optical system is 4000lp / mm, and the MTF curves of each field of view under different cover glass thicknesses are close to the diffraction limit, which can well ensure the imaging quality.

[0086] from Figures 24 to 26 From the axial aberration curve, it can be seen that spherical aberration has been strictly corrected within the full aperture range, and light of each wavelength is almost overlapping.

[0087] Example 3:

[0088] The structure of the water immersion objective lens in Example 3 is as follows Fig. 27 As shown, it consists of a front lens group M1, a middle lens group M2 and a rear lens group M3. The focal length of the water immersion objective lens is fobj = 9.81 mm, NA = 0.95, magnification β = -20 times, immersion layer thickness is 1.1 mm, cover glass thickness is 0.17 mm, spectral range is 405~656 nm, field of view range is 25 mm, and the medium is water. The water immersion objective lens has the following characteristics:

[0089] The focal length of the front lens group M1 is fm1=10.04mm, the focal length of the middle lens group M2 is fm2=156.77mm, the focal length of the rear lens group M3 is fm3=-3779.66mm, |fm1 / fobj|=1.02, |fm2 / fobj|=15.98, |fm3 / fobj|=385.3.

[0090] The refractive index of the first lens L1 is nd1=1.46, the Abbe number of the first lens L1 is vd1=67.8, the Abbe number of the second lens L2 is vd2=35.3, and |vd1-vd2|=32.5; the focal length of the second lens L2 is fl2=-7.82mm, the Abbe number of the fourth lens L4 is vd4=81.6, the Abbe number of the fifth lens L5 is vd5=44.3, and the Abbe number of the sixth lens L6 is vd6=95, |vd4-vd5|=37.3, and |vd5-vd6|=50.7.

[0091] The focal length of the seventh lens L7 is fl7=48.15mm, the lens aperture is d=20.1mm, and d / fl7=0.42. The Abbe number of the eighth lens L8 is vd8=30.1, the Abbe number of the ninth lens L9 is vd9=95, |vd8-vd9|=64.9; the Abbe number of the tenth lens L10 is vd10=81.6, and the Abbe number of the eleventh lens L11 is vd11=27.5, |vd10-vd11|=54.1.

[0092] The focal length of the fifth cemented member GL5 is fgl5=-11.83 mm, and the lens thickness is 7 mm. The focal length of the fourteenth lens L14 is fl14=21.2 mm, and the refractive index nd14=1.85.

[0093] Table 3: Parameters of the water immersion objective lens of Example 3

[0094] Face number Radius (mm) Thickness (mm) Refractive index Nd Abbe number Vd L1 Object Plane unlimited 1.95 1.46 67.8 L2 Object Plane -1.88 4.67 1.91 35.3 L2 Image plane -5.58 0.2 L3 Object Plane unlimited 4 1.43 95 L3 Image plane -10.44 0.2 L4 Object Plane 52.27 6 1.5 81.6 L5 Object Plane -10.52 2 1.61 44.3 L6 Object Plane 1009.45 6 1.43 95 L6 Image plane -19.02 0.1 L7 Object Plane 23.42 2.99 1.95 17.9 L7 Image plane 44.58 0.5 L8 Object Plane 29.93 1.7 1.85 30.1 L9 Object Plane 11.5 7 1.43 95 L9 Image plane -34.54 0.5 L10 Object plane 8.85 5 1.5 81.6 L11 Object plane -66.23 1 1.76 27.5 L11 Image plane 6.52 5 L12 Object plane -7.8 3 1.67 32.2 L13 Object plane 16.78 4 1.88 40.9 L13 Image plane 95.6 2.5 L14 Object plane -89.74 5 1.85 30.1 L14 Image plane -15.5

[0095] In this example three, when the thickness of the cover glass becomes 0.11 mm, the distance d1 between the seventh lens L7 and the third cementing member GL3 is 0.38 mm, and the distance d2 between the third cementing member GL3 and the fourth cementing member GL4 is 0.62 mm.

[0096] When the thickness of the cover glass becomes 0.23 mm, the interval d1 between the seventh lens L7 and the third cementing member GL3 is 0.62 mm, and the interval d2 between the third cementing member GL3 and the fourth cementing member GL4 is 0.38 mm.

[0097] from Figures 28 to 30 Field curvature plots and Figures 31-33 From the distortion curve, we can see that the field curvature and distortion under different cover glass thicknesses are well corrected, the maximum field curvature at the edge of the field of view is no more than 2um, and the maximum distortion of the optical system is within 1%.

[0098] from Figures 34-36 It can be seen from the optical transfer function MTF curve that the cutoff frequency of the optical system is 4000lp / mm, and the MTF curves of each field of view under different cover glass thicknesses are close to the diffraction limit, which can well ensure the imaging quality.

[0099] from Figures 37-39 From the axial aberration curve, it can be seen that spherical aberration has been strictly corrected within the full aperture range, and light of each wavelength is almost overlapping.

Claims

1. A water immersion objective lens with a large numerical aperture of 20 times, which is composed of a front lens group with positive focal power, a middle lens group with positive focal power and a rear lens group with negative focal power arranged in sequence on the same optical axis from the object side to the image side, wherein the focal length fobj of the water immersion objective lens is 9.5-10.5 mm, and is characterized in that: The focal length fm1 of the front lens group satisfies: 1≤|fm1 / fobj|≤1.2, the focal length fm2 of the middle lens group satisfies: 5≤|fm2 / fobj|≤16, the focal length fm3 of the rear lens group satisfies: |fm3 / fobj|>30, the front lens group is composed of a first cemented component, a third lens with positive focal power and a second cemented component, the first cemented component is composed of a first lens and a second lens, the first lens is a hemispherical lens with a plane on the object side, the second lens is a meniscus lens with a concave surface on the object side, the second lens has a negative focal power, the second cemented component is composed of a fourth lens with positive focal power, a fifth lens with negative focal power and a sixth lens with positive focal power, the middle lens group is composed of a seventh lens, The lens assembly is composed of a third cemented component and a fourth cemented component, the interval d1 between the seventh lens and the third cemented component is 0.3-0.7 mm, the seventh lens has positive focal power, the third cemented component is composed of an eighth lens with negative focal power and a ninth lens with positive focal power, the fourth cemented component is composed of a tenth lens with positive focal power and an eleventh lens with negative focal power, the rear lens group is composed of a fifth cemented component with negative focal power and a fourteenth lens with positive focal power, the fifth cemented component is composed of a twelfth lens with negative focal power and a thirteenth lens with positive focal power, the object side of the twelfth lens is concave and the image side is concave, the object side of the thirteenth lens is convex and the image side is concave, and the fourteenth lens is a meniscus lens with a concave object side.

2. A water immersion objective lens with a numerical aperture of 20 times as large as claimed in claim 1, characterized in that: The object side of the seventh lens is convex, and the image side is concave. The focal length fl7 of the seventh lens and the clear aperture d7 of the seventh lens satisfy: d7 / fl7>0.

4.

3. A water immersion objective lens with a 20-fold large numerical aperture as claimed in claim 1, characterized in that: The Abbe number vd1 of the first lens and the Abbe number vd2 of the second lens satisfy: |vd1-vd2|≥30, the Abbe number vd4 of the fourth lens, the Abbe number vd5 of the fifth lens and the Abbe number vd6 of the sixth lens satisfy: |vd4-vd5|≥35 and |vd5-vd6|≥35, the refractive index nd1 of the first lens satisfies: 1.45<nd1<1.5, and the refractive index nd2 of the second lens satisfies: 1.9<nd2<1.

95.

4. A water immersion objective lens with a 20-fold large numerical aperture as claimed in claim 1, characterized in that: The Abbe number vd8 of the eighth lens and the Abbe number vd9 of the ninth lens satisfy: |vd8-vd9|≥35, and the Abbe number vd10 of the tenth lens and the Abbe number vd11 of the eleventh lens satisfy: |vd10-vd11|≥40.

5. A water immersion objective lens with a 20-fold large numerical aperture as claimed in claim 1, characterized in that: The thickness of the fifth glued part is 5-8 mm.

6. A water immersion objective lens with a 20-fold large numerical aperture as claimed in claim 1, characterized in that: The refractive index nd14 of the fourteenth lens satisfies: 1.85<nd14<1.95.

Images