The main components of the microscope optical system are the microscope objective lens and the eyepiece. The task is to enlarge and obtain clear images. There are many types of microscope objectives on the market. How do you judge the pros and cons of the objective lens? First of all, let's first come to know the objective lens.
1. Type of objective lens <br>The quality of the microscope objective directly affects the quality of the microscope image, which is related to the correction of the aberration. Therefore, the objective lens is classified according to the degree of aberration correction. It is known in the difference: the spherical aberration, chromatic aberration and curvature of field are the most influential to the quality of the image. The sharpness of the central part of the first two pairs has a great influence, while the bending of the curved field has a great effect on the edge of the image. influences.
Here, several kinds of objective lens characteristics that are commonly used are explained below.
â—The achromatic and planar achromatic objectives are only yellow, green, and two wave regions. The spherical aberration and chromatic aberration of other wave regions still exist, so the image does not have real color between the colors. Relationship, you can see the residual color difference when the focus changes. However, the general effect of low magnification is small. In view of the better correction of the yellow-green wave region, yellow-green light should be used as the illumination source or in the color-increasing red filter to avoid significant exposure to uncorrected chromatic aberration.
The achromatic left objective is often used in conjunction with a Fugen eyepiece or a correction eyepiece for low magnification and medium magnification. Because of its simple structure, the aberrations in the central part of the image can be roughly corrected, and the price is low, so it is applied more. Generally, benchtop microscope objectives are mostly of this type.
The curvature of the object of the planar achromatic objective is further corrected, so the projection is straight, so that the edge of the field of view and the center can be clearly imaged at the same time. Therefore, it is suitable for metallographic microscope photography.
â— Apochromatic objective lens and planar apochromatic objective lens apochromat objective lens are composed of multiple sets of lenses. The correction of the chromatic aberration is actually equal to the entire wave region of the wind, but the partial chromatic aberration still exists. When used with a Fugen eyepiece or other simple combination eyepiece, these residual chromatic aberrations will make the image edges slightly colored. Therefore, it is necessary to use it with a compensating eyepiece. The apochromatic objective lens has no limitation on the light source, and white light illumination can also obtain good effects. It is better to add a blue or yellow filter. It is the finest objective in a microscope.
The planar achromatic objective lens has the same degree of aberration correction as the apochromatic objective lens except for the correction of further image curvature. The apochromatic objective lens is not as flat as the achromatic objective lens, and the planar apochromatic objective lens makes the image clear and flat, further improving the image quality.
â— Semi-apochromatic objective microscope In terms of the degree of aberration correction, the semi-apochromatic objective lens is between the achromatic and apochromatic objective lenses, but other optical properties are close to the apochromat objective. Its low price, commonly used to replace the apochromatic objective lens, it is best to match the compensating eyepiece when using.
In addition, special-purpose high-temperature reflective objectives, reversing objective lenses, and ultraviolet objective lenses are all described in each special microscope.
The nature of the microscope objective
â— Numerical aperture of the objective lens The numerical aperture of the objective lens characterizes the concentrating ability of the objective lens, which is one of the important properties of the objective lens. Enhancing the concentrating ability of the objective lens can improve the discrimination rate of the objective lens.
The numerical aperture is usually expressed by the symbol "NA" (ie Numerical Aperture). According to the theoretical derivation:
NA=n.sinu
Where n is the refractive index of the medium between the objective lens and the observation;
u──The aperture half angle of the objective lens
Therefore, there are two ways to increase the numerical aperture:
Increase the diameter of the lens or reduce the focal length of the objective lens to increase the aperture half angle u. This method causes the aberration to increase and the manufacturing difficulty. In fact, the maximum value of sinu can only reach 0.95.
• Increase the refractive index n between the objective lens and the observation. It is a schematic diagram of the effect of the medium on the numerical aperture of the objective lens. When the light is directed toward the object in the direction of the optical axis, the reflected light emitted from the object S is diffracted by (S1 S1') (S2, S2') in addition to being reflected in the direction of the SO. (a) In the case of air as a medium (also called a dry objective lens), only the diffracted light in (S1 S1') can pass through the objective lens, and the diffracted light other than (S1 S1') such as (S2, S2') cannot pass. Objective lens. (b) When the conifer or other oil is used as the medium between the objective lens and the observation (also referred to as the oil immersion objective lens), the angle of the diffracted light is narrowed due to the increase in the refractive index n, resulting in (S2, S2') or even (S3). The diffracted light in S'3) can pass through the objective lens. Thus, the objective lens is passed through as many diffracted beams as possible to facilitate the identification of tissue details.
In the same medium, a light source with a short wavelength will have a larger refractive index. Similarly, there will be more diffracted beams entering the objective lens.
Generally, high power microscope objectives are often designed as oil mirrors. Oil mirrors are specially designed for a particular medium and should be used as specified. The most commonly used medium is pine oil (n=1.515) with a maximum numerical aperture of NA=1.40; a-deuterated bromine as a medium, n=1.658, with a maximum numerical aperture of up to 1.60.
â— The discrimination rate of the objective lens and the effective magnification of the microscope The discrimination rate of the objective lens of the microscope means that the objective lens has the maximum ability to clearly distinguish the two object points, and is represented by the reciprocal of the minimum distance d which can be clearly distinguished by the two object points. The smaller d is, the higher the discrimination rate of the objective lens.
It is necessary to understand that the discrimination rate can be limited, which is explained by the diffraction phenomenon caused by the light passing through the lens. When an object is imaged by an optical instrument, each object point corresponds to an image point, but due to the diffraction of light, the image of the object point is no longer a geometric point, but a diffraction spot with a certain size. If two or two bright spots, which are close to each other, overlap each other, the two object points are unclear, which limits the resolution of the optical system. Obviously, the larger the radius of the central spot of the diffraction image on the image surface, the smaller the resolution of the system.
How to determine the ultimate resolution of the objective lens, which can be analyzed by diffraction of the object points A1, A2 through the lens. A1' and A2' are diffraction images of object points A1 and A2, which are concentric rings. The light intensity at the center is the largest, and the light intensity of the diffraction ring gradually decreases as the diameter of the ring increases.
Rayleigh proposed a speculation (also known as the Rayleigh criterion): that A1 and A2 are distinguishable when the first minimum of the A1' diffraction pattern falls exactly at the maximum of the A2' diffraction pattern. The two object point distances A1 and A2 determined at this time are used as the resolution limit of the optical system. Θ0 is called the limit resolution angle. It goes without saying that when θ>θ0, it is completely resolvable, and when θ<θ0, it is indistinguishable.
Obtained by the theory of circular aperture diffraction: θ0=1.22λ / D
Where λ is the wavelength of the incident light;
D—The maximum allowable aperture (lens diameter) of the incident light.
Since θ0 is small, it is obtained from Figure 2-4:
D'≈θ0=1.22λS / D
When the microscope objective is designed, it always satisfies the Abe sine condition, ie ndsinu=n'd'sinu'
Where n and n' are the refractive indices of the space in which the object and image are located, and the imaging is always in the air medium, so n'=1; uu' are the aperture angles of the light at the conjugate point of the object and the space, respectively. ;d and d' are the spacing of the object point and the center point of the image point, respectively.
Considering that the incident light in the microscope is not all parallel light, there is oblique light, and the above formula is corrected appropriately. Therefore, nSinu is the numerical aperture of the objective lens. Therefore, the above formula or write: d=0.5λ /NA
Therefore, it shows that the larger the numerical aperture of the objective lens, the shorter the wavelength of the incident light, the higher the resolution of the objective lens. In visible light, yellow-green light (λ ≈ 4400A) is often used for observation, which can increase the resolution by about 25%.
For an optical metallographic microscope, if a blue light is used and an oil immersion objective is used, the maximum resolution of the objective lens is about 1600A.
The discrimination rate of the metallographic microscope can only reach the discrimination rate of the objective lens, so the discrimination rate of the objective lens can be called the discrimination rate of the microscope. Because the eyepiece cannot further increase the discrimination rate of the entire optical system, it is at best used to ensure the full utilization of the objective discriminating rate. If you want to further improve the discrimination rate of the microscope, you should use shorter wavelength electron waves. This is the electron microscope.
In the microscope, the magnification of the microscope corresponding to the resolution of the objective lens is fully utilized, which is called the effective magnification of the microscope. The effective magnification can be derived from the following relationship: the resolution of the human eye at the bright distance (250 mm) is 0.15 to 0.30. Therefore, the distance d to be discriminated by the objective lens must be magnified by the microscope to be 0.15 to 0.30 mm to be recognized by the human eye. . If M is the magnification of the microscope, then
Dm=0.15~0.30
M=0.15~0.30 / d=(0.15~0.30)(NA) / 0.5λ =0.3~0.6 NA / λ
The magnification at this time is the effective magnification of the microscope and is usually expressed in M. Therefore M is effective = 0.3 ~ 0.6NA / λ
It can be seen that the effective magnification of the microscope is determined by the numerical aperture of the objective lens and the wavelength of the incident light. Knowing the effective magnification can correctly select the cooperation of the objective lens and the eyepiece to fully exert the discriminating ability of the objective lens without causing virtual amplification.
For example: select 32× objective lens with NA=0.65, when λ=5500A, M is valid=(500～1000)NA=325～650×
Therefore, you should choose 10 to 20 times the eyepiece. If the magnification of the eyepiece is less than 10 times, the discriminating ability of the objective lens is not fully utilized; if the magnification of the eyepiece is higher than 20 times, the virtual magnification will be caused, and the fine structure of the objective discriminating rate cannot be displayed.
Vertical discrimination rate The vertical discrimination rate, also known as depth of field, is defined as the fact that, in the case of a fixed phase point, the image plane moves axially to maintain a clear image range. Characterizing the nature of the objective lens corresponding to whether the details of the object on different planes can be clearly imaged, the magnitude of the vertical discrimination is measured by the distance between the two extreme positions of the plane of satisfactory imaging (before and after the focal plane).
If the resolution of the person and the resolution is 0.15 to 0.30 mm, where n is the refractive index of the medium in which the object is located, (NA) is the numerical aperture of the objective lens, and M is the magnification of the microscope, the vertical discrimination rate h can be obtained by the following formula:
h=n / (NA).M ×(0.15～0.30)mm
It can be seen from the above formula that if a large vertical discrimination ratio is required, it is preferable to use an objective lens with a small numerical aperture or reduce the aperture stop to reduce the working aperture of the objective lens, which inevitably reduces the resolution of the microscope. These two contradictory factors can only be decided by specific circumstances.
â— The magnification of the objective lens is known from the principle of microscope amplification in Chapter 1. The magnification of the objective lens is M = â–³ / F, which indicates that after determining the focal length (F) of the objective lens, the magnification is longer with the optical lens barrel of the microscope. (â–³) changes. The magnification of the objective lens is meaningful only after the length of the optical microscope tube is determined. Therefore, the magnification of the objective lens is calibrated according to the length of the designed optical lens barrel and must be used on the specified barrel length, otherwise its magnification will change.
◠Microscope barrel length The aberration of the objective lens is corrected based on the image of a certain position. For example, when the length of the optical lens barrel is Δ, the lens imaging condition is corrected: G (green light) R (red light) is imaged at O', and there is no aberration: (a) (c) respectively, the lens barrel is shorter than And the situation is longer than the original design, so that the aberrations are repeated. Therefore, the objective lens must be at a specified length of the mechanical barrel length, which is fixed for a given set of optical instruments. Generally, the length of the mechanical barrel of the microscope is 160mm, 170mm, and 190mm. In addition, when the metallographic microscope is photographed, the image projection distance is changed by the magnification, and the aberration of the objective lens is corrected.
Performance Indicators of Microscope Objectives <br>The main performance of the objective lens has been marked on the outer casing of the objective lens, including:
â— Objective type The product of the national product is marked with the Chinese pinyin prefix of the objective lens type, such as the plane achromatic object lens "PC".
The product mirrors of Western European countries are marked with the English name or prefix of the objective lens category, such as the plane achromatic objective lens labeled "Plana-pchromatic or PI", and the achromatic objective lens labeled "Achromatic" apochromatic objective lens labeled "Apochromatic"
◠Magnification is indicated by 15×, 20×, 32×, 40×, etc., which indicate that the magnification is 15 times and 20 times. 32 times, 40 times.
â—The values ​​of the numerical aperture of the microscope numerical aperture objective lens are directly marked, such as 0.30, 0.65, 0.95, etc., respectively, indicating that the numerical aperture of the objective lens is 0.30, 0.65, 0.95.
â—Applicable mechanical barrel length is marked as 170mm, 190mm, ∞/0 respectively indicates that the length of the mechanical cylinder for the objective lens is 170mm, 190mm, infinite length.
â— Oil immersion objective lens All oil immersion objective lenses have special signs, engraved with HI, oil, national product mirror engraved with "oil" or Chinese pinyin prefix "y".
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