The objective lens is composed of severallensA lens group formed by combination.The purpose of combined use is to overcome the imaging defects of single lens and improve the optical quality of objective lens.The magnification of the microscope mainly depends on the objective lens. The quality of the objective lens directly affects the image quality of the microscope. It is the main component that determines the resolution and imaging clarity of the microscope, so the correction of the objective lens is very important.
The objective lens is the most important optical component of the microscope. It uses light to make the first image of the object to be examined, so it directly affects and affects the quality of image formation and various optical technical parameters, and is a measure of the quality of a microscopePrimary standards。[1]
The objective lens is complex in structure and precision in manufacture. It is usually composed of lens groups. Each lens is separated from each other by a certain distance to reduceaberration。Each group of lenses consists of one or several lenses with different materials and parametersGluingAnd become.Objective lens has many specific requirements, such asShaft coupling, in focus.
Modern microscopic objective has reached a high level of perfectionnumerical aperture Is close to the limit,fieldThe difference between the resolution of the center and the theoretical value is negligible.However, it is still possible to continue to increase the field of view of the microscopic objective and improve the image quality of the field edge. This kind of research work is still in progress.
AutofocusThat is, when the image is clear when observed with an objective lens of one magnification, the image should be basically clear when the objective lens of another magnification is converted, and the deviation of the image center should also be within a certain range, that isShaft couplingDegree.The quality of the focusing performance and the degree of axial alignment is an important sign of the quality of the microscopeObjective lens converterThe precision of the.
There is a difference between microscope objective and eyepiece in terms of participating in imaging. Objective is the most complex and important part of microscope. It works in wide beam (large aperture), but the inclination between these beams and optical axis is small (small field of view);The eyepiece works in a narrow beam, but its inclination angle is large (large field of view). When calculating the objective and eyepiece, there is a big difference in eliminating aberration.
Microscopic objective is one cancellationSpherical aberrationSystem, which means: a pair ofConjugate pointFor example, when spherical aberration is eliminated and the sine condition is realized, each objective lens has only two such spherical aberration. Therefore, any change in the calculated positions of objects and images will lead to larger aberration.Installed at the lower end of the lens barrelRotatorGenerally, there are 3-4 objective lenses, among which the shortest one with "10 ×" is low magnification lens, the longer one with "40 ×" is high magnification lens, and the longest one with "100 ×" isOil mirrorIn addition, a circle of lines of different colors is often added on the high-power mirror and oil mirror to show the difference.
type
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objective lense
(1) According to the length of microscope tube (in mm):Transmitted light160 mirror tube with 0.17mm thick or thickerCover slide；reflected light190 lens cylinder without cover glass;The transmitted light and reflected light use a mirror cylinder with an infinite length.
(2) According to the characteristics of immersion method: non immersion (dry), immersion (oil immersion, water immersionglycerolImmersion and other immersion methods).
⑶ According to the optical device:transmissionType, reflective type and catadioptric type.
⑷ According to numerical aperture andMagnification: Low power（NA≤ 0.2 and β ≤ 10X), medium power (NA ≤ 0.65 and β ≤ 40X), high power (NA ＞ 0.65 and β ＞ 40X).
(5) According to the condition of corrected aberration, it is generally divided intoAchromatic objective, semi apochromatic objective, apochromatic objective, head up field apochromatic objective, head up field apochromatic objective andMonochromatic objective。
1. Achromatic objective lens is a common objective lens (Table 1-1), which is composed of several groups of positive and negative gluing lenses with different curved radii, and can only be correctedSpectral lineMedium red and blueaxialColor difference.At the same time, the point on the axis is correctedSpherical aberrationAnd paraxial pointComa The objective lens can not eliminate the secondary spectrum, only correcting the spherical aberration and chromatic aberration in the yellow and green wave areas, but not eliminating the residual chromatic aberration and spherical aberration and chromatic aberration in other wave areas, andImage field curvatureIt is still large, that is, only clear images in the middle of the field of view can be obtained.Use yellow green lightLighting source, or onOptical pathInsert yellow green infilter。This kind of objective lens is simple in structure, economical and practical, and often works with FugeneyepieceThe correction eyepiece is used together, and is widely used on medium and low power microscopes.Green can be used for black and white photographyColor filterReduce residual axial chromatic aberration to obtaincontrast ratioGood photos.
⒉Apochromatic objective(Apochronomatic)optical glassAnd advanced lens group made of fluorspar.Red, blueYellow lightThe axial chromatic aberration is corrected and the secondary spectrum is eliminated, so the image quality is good, but there are many lenses and it is difficult to process and assemble.Correction of color differencevisible lightAll wave areas of.If blue or yellow is addedfilterBetter results.It is the best objective in the microscope, and has good correction for spherical aberration and chromatic aberration. It is suitable for high magnification.But still need toCompensating eyepieceUsed together to eliminate residual color difference.
3. Plana chromatic objective adopts complex optical structure of multiple lens combination, which can be well correctedAstigmatismandImage field curvature, make the wholefieldAll can be displayed clearly, suitable forPhotomicrography。The objective pairSpherical aberrationThe correction of sum color difference is still limited to the yellow green wave area, and there is still residual color difference.
4. Plane apochromatic objective (PF, Planapochromat), in addition to further image field bending correctionaberrationThe correction degree is the same as that of apochromatic objective lens, so that the image is clear and flat;But the structure is complex and difficult to manufacture.
⒌Semi apochromatic objective(Halfapochromatic) Some lenses are made of fluorspar, so they are also called fluorspar objectives. Their performance is better than achromatic objectives and their price is cheaper than apochromatic objectives.The degree of corrected aberration is between achromatic and apochromatic objectives, but other optical properties are similar to the latter;The price is low, and it is best to use with compensating eyepiece.
sixSpecial objectiveThe so-called special objective lens is designed to achieve certain effects on the basis of the above objective lens.According to the use, there are mainly the following types:
(1) Phase contrast objectivePhase contrast microscopeOf course, it can also be used regularly.It is characterized byBack focusA phase plate is installed at the plane to delay the light wave.
(2)Correction Collar objective (correction collar objective) An annular adjustment ring is installed in the objective lens.When the adjusting ring is rotated, the distance between the lens groups (generally the second and third groups of lenses) in the objective lens can be adjusted to correct the non-standard thickness of the cover glassPoor coverage。
(3) IridescentDiaphragmThe objective lens (iris diaphragm objective) is equipped with an iris diaphragm at the upper part of the objective lens barrel. The external side also has a rotatable adjustment ring, which can be adjusted when rotatingApertureSize of.This structure is the most advanced oil immersed objective.
(4) Strainless objective lens overcomes the existence of stress in the assembly of lens group and is specially designed for transmissionPolarizerThe objective lens used for inspection can achieve better inspection effect of polarizer.
(5) Non fluorescent objective is specially designed for fallingfluorescence microscopeObjective lens on.The objective lens does not emit fluorescence even when it is strongly excited by a light source.
(6) No cover objective Some objects to be examined, especially smeared films, cannot be covered with a cover glass. In this way, no cover objective should be used for microscopic examination, otherwiseimage qualityIt will decrease obviously, especially in high power microscopic examination.
① The magnification refers to the ratio of the size of the image seen by the eye to the size of the corresponding specimen.It refers to the ratio of length rather than area.Example: The magnification is 100 ×, which means the length is 1μmSpecimen of, zoom inAfterimageThe length of is 100 μ m. If it is calculated by area, it will be magnified 10000 times.The total magnification of the microscope is equal to the objective andeyepieceThe product of magnification.
② Numerical aperture is also calledMirror aperture ratio, abbreviated as NA or A, is the objective lens andCondenserThe main parameters ofResolutionIs proportional.The numerical aperture of the drying objective is 0.05-0.95,Oil immersion objective（Cedar oil）The numerical aperture of is 1.25.
③、Working distanceIt refers to the distance from the front lens of the objective lens to the cover glass of the specimen when the specimen observed is the clearest.The working distance of the objective lens is related to the focal length of the objective lens. The longer the focal length of the objective lens is, the lower the magnification is, and the longer the working distance is.For example, 10/0.25 and 160/0.17 are marked on the 10 times objective lens, where 10 is the magnification of the objective lens;0.25 is the numerical aperture;160 is the length of the lens barrel (unit: mm);0.17 is the standard thickness of cover glass (unit: mm).The effective working distance of 10 times objective lens is 6.5mm, and the effective working distance of 40 times objective lens is 0.48mm.
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Magnification
The magnification of the objective lens refers to the ability index of the objective lens to magnify the real object in the online length.There are two kinds of representation methods. One is to directly scale 8 ×, 10 ×, 45 ×, etc. on the objective lens;The other is to scale the focal length f of the objective lens on the objective lens. The shorter the focal length, the higher the magnification.The former objective magnification formula is M object=L/f object, and L is the length of the optical lens barrel. The value of L is very accurate in design, but in practical application, the length of the mechanical lens barrel is commonly used because it is difficult to measure.The length of mechanical lens barrel refers to the length from the interface of microscope eyepieceLinear distance。The length of the mechanical lens barrel is marked with numbers on each objective lens.
Tube length
The length of the lens barrel refers to the distance from the bottom surface of the objective lens to the top surface of the eyepiece.Since the aberration of the objective lens is corrected according to the image at a certain position, the objective lens must be used on the specified length of the mechanical lens barrel. Generally, the length of the mechanical lens barrel of a microscope is 160mm, 170mm[2]190mm。metallurgical microscopeDuring photography, due to different magnification, the image projection distance varies greatly. Therefore, the aberration of excellent objective lens is corrected according to the length of any lens tube, that is, within an infinite range, the object image differenceAll havecorrecting.
numerical aperture
Numerical Aperture Characterization of Objective LensLight gathering ability, is one of the important properties of objective lens, usually expressed as "NA".The numerical aperture size of the objective determines theResolution(Identification) and effective magnification.According to theoretical derivation, there are two ways to increase the numerical aperture of objective lens when NA=nsin θ:
(1) Increase the diameter of the lens or reduce the focal length of the objective lens, that is, design the objective lens with short focal length to increase the half angle θ of the aperture.But this method will lead toaberrationIt is difficult to increase and manufacture, and is generally not used.In fact, sin θMaximumIt can only reach 0.95.
(2) Increase the distance between the objective lens and the observation objectRefractive indexn。The dry objective takes air as the medium, with the refractive index n=1, and is generally used forLow power objective。Oil system objective lens often uses pine and cypress oil (n=1.515, NA=1.4) and α - first generation bromonaphthalene (n=1.658, NA=1.60) as the medium for high power objective lens.The numerical aperture of the oil lens can reach 1.30~1.40 at this time, and its magnification can reach 100~140 times.However, oil cannot be used as the medium for the dry objective lens.
Minimum numerical aperture series, parameters, color circles and marks of objective lens
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Different marks, immersion marks, objective categoryMagnification、numerical aperture , mechanical cylinder lengthCover slideThickness.Oil: indicates that the immersion solution isPine and cypressOil;100 ×/1.25: it means the objective magnification is 100 times, and the numerical aperture is 1.25;160/0: it indicates that the length of the mechanical lens tube is 160mm;"0" means uncovered glass slide.Some objective lenses are engraved with 160/-: indicating that the length of the mechanical lens barrel is 160mm."-" indicates that there may be or may not be cover slides.The objective lens is engraved with a color circle to indicate the magnification of the objective lens.High power objectiveUsuallyOil immersionThe oil lens is represented by "oil" (or OiI, Ö L, HL) or a black circle painted on the shell.
Discriminative ability
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The discrimination ability of microscope mainly depends on the objective lens.The identification ability of objective lens can be divided into plane and vertical identification ability.Objective lensoptical microscopeThe most important optical unit for basic performance and function.Therefore, in order to meet various needs and applications, we have developed objective lenses with the best optical performance and functions (which are also the most important performance and functions for optical microscopes), and launched a variety of objective lens products that can meet different purposes.Basically, objective lenses are classified according to use, observation method, magnification, performance (aberration correction), etc.Where, according toaberrationIt is unique to microscope objectiveclassification method 。
classification
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Classification by use
objective lense
The use of optical microscope can be roughly divided into two categories: "biological use" and "industrial use".Objectives can also be divided into "biological" and "industrial" objectives according to these two purposes.In biological use, generallyBiological specimenPlace onSlideAnd use it from aboveCover slideCover and fix.Since the biological objective lens needs to observe the sample through the cover glass, the optics considering the thickness of the cover glass (generally 0.17 mm) is adoptedsystem design。In industrial applicationsMetallic mineralsSlice, semiconductorwafer And electronic parts and components are not covered.Therefore, the industrial objective lens adopts the best optical system design without cover glass between the front end of the objective lens and the specimen.
Classification according to observation method
Various observation methods have been developed according to the use of optical microscope, and special objective lenses corresponding to these observation methods have also been developed.The objective lens can be divided according to the observation method.For example, "ReflectionDark fieldUse objective lens (there is a ring lighting light path around the inner lens) "," differential interference objective lens (reduce the internal distortion of the lens, optimize the optical characteristics combination with the differential interference prism) "," fluorescent objective lens (improve theultraviolet raysDomaintransmissivity）”、“polarized lightUse objective lens (which greatly reduces the internal distortion of the lens) "and“phase differenceUse objective lens (built-inPhase plate）”Etc.
Classification according to magnification
The optical microscope is equipped with multiple objective lenses on a device called objective lens converter.In this way, as long as the objective lens converter is rotated, the low magnification can be switched to the high magnification, and the magnification conversion can be easily completed.Therefore, a group of objective lenses with different magnification are generally installed on the objective lens converter.For this reason, the product lineup of objective lenses consists of low magnification (5 ×, 10 ×), medium magnification (20 ×, 50 ×) and high magnification (100 ×) objectives.Among them, especially in high magnification products, in order to obtain high resolution imaging, we have introduced the filling between the front end of the objective lens and the specimenSynthetic oil, water and other special liquids with high refractive index.In addition, it also launched theSpecial purposeUltra low magnification (1.25 ×, 2.5 ×) and ultra-high magnification (150 ×) objective lenses.
Aberration correction and classification of objective lens
According to the classification (grade) of color difference correction, it can be divided into achromatic, semi achromaticApochromatic aberration3 grades.The product lineup is also sorted from ordinary level to high level, with different prices.
In the axial chromatic aberration correction, the objective lens that corrects the two colors of line C (red: 656.3 nm) and line F (blue: 486.1 nm) is called achromat.The light other than red and blue (usually purple g line: 435.8 nm) is leavingfocal planeThis g line is called the second order spectrum.chromatic aberrationCorrection rangeThe objective lens that reaches this level 2 spectrum is calledApochromatic lens（Apochromat）。That is to say, apochromatic lens is an objective lens for axial chromatic aberration correction of three colors (C line, F line, g line).Figure 1 usesWavefront aberrationThe difference between achromatic lens and apochromatic lens in chromatic aberration correction is shown.As can be seen from the figureAchromatic lensIn contrast, apochromatic lenses can correct chromatic aberration in a wider wavelength range.
Figure 1
Comparison of chromatic aberration correction (achromatic lens and apochromatic lens)
On the other hand, the chromatic aberration correction degree of the second order spectrum (g line) is set at the position of achromatic lens andApochromatic lensThe objective lens in the middle of is called semi achromatic lens (or Fluorite).
In the optical system design of microscope objective, generally speaking, NA. The larger, or the larger the magnification, the more difficult it is to correct the axial chromatic aberration of the secondary spectrum.Not only that, but also because of variousaberrationAnd sine condition must be corrected, so it is more difficult.For this reason, the higher the magnificationApochromatic objective, more aberration correction lenses are needed, and even objective lenses with more than 15 lenses are used.In order to accurately calibrate the secondary spectrum, the effective method is to convert the secondary spectrumdispersionLess "abnormal dispersion glass" is used in the lens group with stronger effectConvex lens。The abnormal dispersion glass is represented by fluorite（CaF2）Although fluorite is difficult to process, it has been used for a long timeApochromatic lens。The newly developed anomalous dispersion andfluoriteVery closeoptical glass，ProcessabilityIt has also been improved and gradually replaced fluorite as the mainstream.
According to the classification of field curve correction, in the use of microscope, photograph taking andTV cameraThe shooting of is becoming more and more commonfieldThere are more and more requirements for images.Therefore, the plane objective that can accurately correct the field curve has gradually become the mainstream.When correcting the field music, it is necessary tooptical system OfPittsburgh（Petzval）curvatureThe design is 0, and the higher the magnification, the more difficult the correction is (difficult to coexist with other aberration correction).On the corrected objective lens, the front lens group is strongly concave, while the rear lens group is also strongly concave, which is a feature of lens types.
effect
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objective lense
The objective lens is used to magnify the specimen for the first time. It is the most important part that determines the performance of the microscope - the level of resolution.Resolution is also called resolution orDiscriminative ability。The resolution is determined by the resolution distance (minimum distance）Is represented by the numerical value of.stayPhotopic distance(25cm), the normal human eye can see two object points 0.073mm apart, and the value of 0.073mm is the resolution distance of the normal human eye.The smaller the resolution distance of the microscope, the higher its resolution, and the better its performance.
The resolution of the microscope is determined by the resolution of the objective lens, which is determined by its numerical aperture and the wavelength of the illumination light.
When using the common central illumination method (the photopic illumination method that makes the light evenly penetrate the specimen), the resolution distance of the microscope is d=0.61 λ/NA, where d is the resolution distance of the objective lens, unit:nm。λ - wavelength of illumination light, in nm.NA - numerical aperture of objective lens, such asOil immersion objectiveThe numerical aperture of is 1.25,visible lightWavelength rangeIs 400-700nm, and the average wavelength is 550 nm, then d=270 nm,Approximately equal toHalf the wavelength of the illumination light.In general, the resolution limit of a microscope illuminated by visible light is 0.2 μ m.
