Several important optical technical parameters of the microscope
The microscope has the following important optical technical parameters: numerical aperture, resolution, magnification, focal depth, field of view diameter, working distance, etc. These parameters are not always as high as possible, and they are interrelated and restrict each other. It is necessary to select matching parameters according to the actual needs of the inspection, so as to achieve the best results.
1. Numerical aperture (N.A.)
Numerical aperture is a key element in judging the performance (resolution, depth of focus, and brightness) of an objective lens.
The numerical aperture (N.A.) was calculated by the following formula.
N.A.=n×sinx
n = the refractive index of the medium between the sample and the objective lens (air: n=1, oil: n=1.515)
X: The angle formed by the optical axis and the refracted light farthest from the center of the objective lens.
When observing with a microscope, if you want to increase the NA value, the aperture angle cannot be increased. The best way is to increase the refractive index n value of the medium. Based on this principle, water immersion objective lenses and oil immersion objective lenses are produced. Because the refractive index n value of the medium is greater than one, the NA value can be greater than one.
The maximum numerical aperture is 1.4, which has reached the limit both theoretically and technically. At present, bromonaphthalene with a high refractive index is used as a 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, and it almost determines and influences other technical parameters. It is proportional to the resolution, proportional to the magnification, and inversely proportional to the depth of focus. As the NA value increases, the width of the field of view and the working distance will decrease accordingly.
2. Resolution
Resolution is also known as "discrimination rate" and "resolution". It is another important technical parameter to measure the performance of the 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; NA is the numerical aperture of the objective lens. The resolution of the visible objective lens is determined by two factors: the NA value of the objective lens and the wavelength of the illumination source. The larger the NA value, the shorter the wavelength of the illumination light, and the smaller the d value, the higher the resolution.
To increase the resolution, i.e. reduce the d value, the following measures can be taken
1. Reduce the wavelength l value and use a short wavelength light source.
2. Increase the n value of the medium and increase the NA value (NA=nsinu/2).
3. Increase the aperture angle.
4. Increase the contrast between light and dark.
3. Magnification
The magnification is the magnification, which refers to the ratio of the size of the final image seen by the human eye to the size of the original object after the object being inspected is magnified by the objective lens and then magnified by the eyepiece, which is the product of the magnification of the objective lens and the eyepiece.
The magnification is also an important parameter of the microscope, but we should not blindly believe that the higher the magnification, the better. The numerical aperture of the objective lens should be considered first when choosing.
4. Depth of focus
Depth of focus is the abbreviation of depth of focus, that is, when using a microscope, when the focus is on a certain object, not only all points on the plane of this point can be seen clearly, but also within a certain thickness above and below the plane, To be clear, the thickness of this clear part is the depth of focus. Depth of focus,
You can see the whole layer of the object under inspection, but with a small depth of focus, you can only see a thin layer of the object under inspection. 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. The depth of focus is large, and the resolution is reduced.
Due to the large depth of field of the low magnification objective lens, it is difficult to take pictures with the low magnification objective lens. This will be described in more detail in photomicrographs. 5. Field of view diameter
When observing a microscope, the bright original area seen is called the field of view, and its size is determined by the field diaphragm in the eyepiece.
The diameter of the field of view is also called the width of the field of view, which refers to the actual range of the inspected object that can be accommodated in the circular field of view seen under the microscope. The larger the diameter of the field of view, the easier it is to observe.
It can be seen from the formula:
1. The diameter of the field of view is proportional to the number of fields of view.
2. Increasing the multiple of the objective lens reduces the diameter of the field of view. Therefore, if you can see the whole picture of the inspected object under the low power lens, and change to a high power objective lens, you can only see a small part of the inspected object.
6. Working distance
The working distance is also called the object distance, which refers to the distance from the surface of the front lens of the objective lens to the object to be inspected. During microscope inspection, the object to be inspected should be between one and two times the focal length of the objective lens. Therefore, it and the focal length are two concepts. What is usually called focusing is actually adjusting the working distance.
In the case of a certain numerical aperture of the objective lens, the working distance is short and the aperture angle is large.
A high-power objective lens with a large numerical aperture has a small working distance.
7. Poor coverage
The optical system of the microscope also includes the coverslip. Due to the non-standard thickness of the cover glass, the optical path of the light after entering the air from the cover glass is changed, resulting in a phase difference, which is poor coverage. The generation of poor coverage affects the sound quality of the microscope.
According to international regulations, the standard thickness of the cover glass is 0.17mm,
The allowable range is 0.16-0.18mm. The phase difference in this thickness range has been calculated in the manufacture of the objective lens. The standard on the objective lens shell is indeed 0.17, which means that the objective lens requires the thickness of the cover glass.
