Aperture angle, also known as "mirror mouth angle", isobjective lenseoptic axisThe angle formed by the object point on the and the effective diameter of the lens in front of the objective lens.The larger the aperture angle, the greater the luminous flux entering the objective lens, which is proportional to the effective diameter of the objective lensfocusIs inversely proportional to the distance.
Aperture angle, also known as "mirror mouth angle", isobjective lenseoptic axisThe angle formed by the object point on the and the effective diameter of the lens in front of the objective lens.The larger the aperture angle, the greater the luminous flux entering the objective lens, which is proportional to the effective diameter of the objective lensfocusIs inversely proportional to the distance.
numerical aperture
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numerical aperture Abbreviation NA, numerical aperture is the objective lens andCondenserThe main technical parameters of are important signs to judge the performance of both (especially for the objective lens).The size of its value is respectively marked on the shell of the objective lens and the condenser lens.
Numerical aperture (NA）Is the refractive index of the medium between the front lens of the objective lens and the inspected object (n[2]）And half of the aperture angle (u).The formula is as follows: NA=n · sin (u/2).[3]
When observing under microscope, if you want to increase NAThe aperture angle cannot be increased. The only way is to increase the refractive index h of the medium.Based on this principle, water immersion objective lens and oil immersion objective lens are produced, because the refractive index n value of the medium is greater than one[2]，NAThe value can be greater than one.
The maximum value of numerical aperture is 1.4, which has reached the theoretical and technical limit.At present, bromonaphthalene with high refractive index is used as the medium, and the refractive index of bromonaphthalene is 1.66, so NAThe value can be greater than 1.4.
It must be pointed out here that in order to give full play to the objective numerical aperture, when observing, the NAThe value should be equal to or slightly greater than N of the objective lensAValue.
Numerical aperture is closely related to other technical parameters, which almost determines and affects other technical parameters.It is proportional to the resolution, the magnification and the focal depth, NAAs the value increases, the field width and working distance will decrease accordingly.
Impact on resolution
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Resolution is also called "discrimination rate" and "resolution power".It is another important technical parameter to measure the performance of microscope.
The resolution of the microscope is expressed by the formula: d=l/NA
Where d is the minimum resolution distance;L is the wavelength of light;NAIs the numerical aperture of the objective lens.The resolution of the visible objective is determined by the N of the objectiveAThe value is determined by two factors, namely, the wavelength of the light source.NAThe higher the value, the shorter the wavelength of the illumination light, the smaller the d value, and the higher the resolution.
To improve the resolution, that is, to reduce the value of d, the following measures can be taken
1. Reduce the wavelength l value and use a short wavelength light source.
2. Zeng increased the medium h value and increased NAValue (NA=hsinu/2).
3. Increase the aperture angle.
4. Increase the contrast between light and shade
Impact on working distance
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Working distanceAlso called object distance, it refers to the distance between the surface of the front lens of the objective lens and the object to be tested.During microscopic inspection, the object to be inspected shall be between one and two times the focal length of the objective lens.Therefore, it and focus are two concepts. What is usually called focusing is actually adjusting the working distance.
When the numerical aperture of the objective lens is fixed, the aperture angle is large when the working distance is short.
High power objective with large numerical aperture has small working distance.
In aperture aperture
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definition
Aperture diaphragmIt is an aperture that limits the solid angle (cone angle) of the imaging beam of the object point on the axis.That is, the diaphragm that determines the light energy (i.e. the image plane illuminance) that can pass through the optical system.
Illustration
Place this stop QoneQQtwoIf the object space is imaged by the lens in front of it, the image PonePPtwoThis determines the object square aperture angle of the optical system (the light cone angle determined by the aperture diaphragm is called the aperture angle U).The aperture aperture is called the image formed by the light group in front of it in the object space of the optical systemEntrance pupil, short forEntrance pupil。
Aperture diaphragm QoneQQtwoThe image P formed by the lens (light group) behind it in the image spaceone'P'Ptwo'calledExit pupil, short forPupillary exodus, which determines the system image square aperture angle U '.
The aperture Q corresponding to the entrance pupil and the exit pupil that actually limits the beamoneQQtwoIt is the aperture diaphragm.
Role of aperture angle
The aperture angle of the beam is one of the important performance parameters to characterize the function of the actual optical system.It not only determines the illuminance of the image plane, but also determines the resolution of the optical system.
The object surface can be divided into three areas according to the different aperture angles of the imaging beam:
Illustration
The first area is marked with BoneA is a circular area of radius, in which each point is imaged with all beams full of the incident pupil.Edge point B of this areaoneFrom the lower edge of the incident pupil PtwoAnd lower edge point M of incident windowtwoIs determined by the connection of.
The second area is marked with BoneBtwoThe annular area formed by rotating around the optical axis. In this area, each point can no longer be imaged with a light beam filled with pupil. When viewing the light beam in the axial plane, BonePoint to BtwoPoint, which can pass through the light beam of the incident pupil and gradually change from 100% to 50%, this is the vignetting phenomenon of the off-axis point.Edge point B of this areatwoFrom the center P of the incident pupil and the lower edge M of the incident windowtwoConnection confirmation of
The third area is marked with BtwoBthreeThe annular area formed by rotating around the optical axis, in which the light beam vignetting at each point is more serioustwoThe vignetting coefficient decreases from 50% to 0 from point B3.BthreePoint is the most edge point of the visible field of view, which is defined by the upper edge point P of the incident pupiloneAnd lower edge point M of incident windowtwoIs determined by the connection of.
The above three areas are only roughly divided. In fact, on the object plane, BoneTo BthreeThe vignetting coefficient of a point is gradual from 100% to 0, with no obvious boundary.When the object surface is observed through a magnifying glass with eyes, the optical system composed of magnifying glass and eyes is like this.[1]
