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Important parameters of microscope nbsp; Microscope principle nbsp

microscope technical data

during microscopic examination, people always hope to have a clear and bright ideal image, which requires that the optical technical parameters of the microscope reach a certain standard, and it is required that the relationship between the parameters must be coordinated according to the purpose of microscopic examination and the actual situation. Only in this way can we give full play to the due performance of the microscope and get a satisfactory microscopic examination effect

The optical technical parameters ofinclude: numerical aperture, resolution, magnification, focal depth, field width, coverage difference, working distance, etc. These parameters are not always the higher the better. They are interrelated and mutually restrictive. When using them, the relationship between parameters should be coordinated according to the purpose of microscopic examination and the actual situation, but the resolution should be guaranteed

1. numerical aperture

numerical aperture is abbreviated as Na. Numerical aperture is the main technical parameter of objective lens and condenser lens, and it is an important sign to judge the performance of both (especially for objective lens). The value is marked on the shell of the objective lens and the condenser respectively

numerical aperture (NA) is the product of the refractive index (n) of the medium between the lens in front of the objective lens and the tested object and half of the sine of the aperture angle (U). The formula is as follows: na=nsinu/2

aperture angle, also known as "aperture angle", is the angle formed by the object point on the optical axis of the objective lens 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 lens and inversely proportional to the distance from the focus

during microscope observation, if you want to increase the Na value, the aperture angle cannot be increased, and the only way is to increase the refractive index n value of the medium. Based on this principle, water immersion objective and oil immersion objective are produced. Because the refractive index n of the medium is greater than 1, the Na value can be greater than 1

[url=] numerical aperture [/therefore, it is beneficial to the inward growth of blood vessels. The maximum value of url] is 1.4, which has reached the limit in theory and technology. At present, bromonaphthalene with high refractive index is used as the medium. The refractive index of bromonaphthalene is 1.66, so the Na value can be greater than 1.4

it must be pointed out here that in order to give full play to the role of the numerical aperture of the objective lens, the Na value of the condenser lens should be equal to or slightly greater than the Na value of the objective lens during observation

numerical aperture is closely related to other technical parameters, which almost determines and affects other technical parameters. It is directly proportional to the resolution, directly proportional to the magnification, and inversely proportional to the depth of focus. With the increase of Na value, the field width and working distance will decrease accordingly

2. Resolution

refers to the minimum distance between two object points that can be clearly distinguished by the microscope, also known as "discrimination rate". Its calculation formula is σ=λ/Na

where σ Is the minimum resolution distance; λ Is the wavelength of light; Na is the numerical aperture of the objective lens. The resolution of the visible objective is determined by the Na value of the objective and the wavelength of the illumination source. The larger the Na value, the shorter the wavelength of the illumination light, and σ The smaller the value, the higher the resolution

to improve the resolution, i.e. reduce σ Value, the following measures can be taken

(1) reduce the wavelength λ Value, using a short wavelength light source

(2) increase the medium n value to increase the Na value (na=nsinu/2)

(3) increase the aperture angle u to increase the Na value

(4) increase the contrast between light and shade

3. magnification and effective magnification

due to the two magnifications of the objective lens and eyepiece, the total magnification of the microscope Γ It should be the objective magnification β And eyepiece magnification Γ Product of 1:

Γ=βΓ 1

obviously, compared with a magnifying glass, a microscope can have a much higher magnification, and the magnification of the microscope can be easily changed by changing the objective lens and eyepiece with different magnification

is an important parameter, but we cannot blindly believe that the higher the magnification, the better. The limit of microscope magnification is

resolution and magnification are two different but interrelated concepts. Relational formula: 500na《 Γ 1000na

when the numerical aperture of the selected objective lens is not enough, the measurement system is actively zeroed; large, that is, when the resolution is not high enough, the microscope cannot distinguish the fine structure of the object. At this time, even if the magnification is excessively increased, only an image with large outline but unclear details can be obtained, which is called invalid magnification. On the contrary, if the resolution meets the requirements and the magnification is insufficient, the microscope has the ability to distinguish, but because The image is too small to be clearly seen by the human eye. Therefore, in order to give full play to the resolution of the microscope, the numerical aperture of laboratory testing instruments with nearly the same function and parameters should be reasonably matched with the total magnification of the microscope

4. Depth of focus

depth of focus is the abbreviation of depth of focus, that is, when the focus is on an object, not only the points on the plane of the point can be seen clearly, but also within a certain thickness above and below the plane. The thickness of this clear part is the depth of focus. If the depth of focus is large, you can see the whole layer of the tested object, while if the depth of focus is small, you can only see a thin layer of the tested object. The depth of focus has the following relationship with other technical parameters:

(1) the depth of focus is inversely proportional to the total magnification and the numerical aperture of the objective lens

(2) large focal depth and reduced resolution

due to the large depth of field of the low power objective, it is difficult to take pictures with the low power objective. It will be introduced in detail during micrograph

5. Fieldofview

when observing the microscope, the bright circular range seen is called the field of view, and its size is determined by the field aperture in the eyepiece

, refers to the actual range that can accommodate the tested object in the circular field of view seen under the microscope. The larger the field diameter, the easier it is to observe

there is a formula f=fn/β

where f: field diameter, FN: fieldnumber (abbreviated as FN, marked on the outside of the eyepiece tube), β: Objective magnification

it can be seen from the formula:

(1) the diameter of the field of view is directly proportional to the number of fields of view

(2) increasing the multiple of the objective lens will reduce the field diameter. Therefore, if you can see the whole picture of the tested object under the low-power lens, and change to the high-power objective lens, you can only see a small part of the tested object

6. Coverage difference

the optical system of the microscope also includes the cover glass. Because the thickness of the cover glass is not standard, the light path after the light enters the air from the cover glass and refracts has changed, resulting in a phase difference, which is the coverage difference. The generation of poor coverage affects the sound quality of the microscope

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