What are the performance parameters of an optical microscope
Numerical Aperture
Numerical aperture abbreviation NA, numerical aperture is the main technical parameters of the objective lens and condenser lens, is to judge the two (especially for the objective lens) the performance of the important signs of high and low. The size of the numerical aperture is marked on the housing of the objective lens and the condenser lens respectively. Numerical aperture (NA) is the object before the objective lens and the object to be examined between the refractive index of the medium (n) and the aperture angle (u) half of the sine of the product. Expressed in the following formula: NA = nsinu / 2
Aperture angle, also known as the "mirror mouth angle", is the objective lens on the optical axis of the object point and the effective diameter of the lens before the objective lens formed by the angle. The larger the aperture angle, the greater the light flux into the objective lens, it is proportional to the effective diameter of the objective lens, and the focal point of the distance is inversely proportional.
Microscope observation, if you want to increase the NA value, the aperture angle can not be increased, the only way is to increase the refractive index of the medium n value. Based on this principle, it produces a water immersion objective and oil immersion objective, because the refractive index of the medium n value is greater than 1, NA value can be greater than 1.
The maximum value of numerical aperture is 1.4, a value that has reached its theoretical and technical limits. Currently, there is a high refractive index of bromonaphthalene 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, in the observation, the NA value of the focusing lens should be equal to or slightly larger than the NA value of the objective lens.
Numerical aperture has a close relationship with other technical parameters, it almost determines and affects all other technical parameters. It is proportional to the resolution, proportional to the magnification, and inversely proportional to the depth of focus, NA value increases, the field of view width and working distance will be correspondingly smaller.
Resolution
Microscope resolution refers to the microscope can be clearly distinguished by the minimum distance between the two object points, also known as the "discrimination rate". The 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 objective lens is determined by the NA value of the objective lens and the wavelength of the illumination light source, the larger the NA value, the shorter the wavelength of the illumination light, the smaller the σ value, the higher the resolution.
To improve the resolution, that is, to reduce the σ value, the following measures can be taken:
1, reduce the wavelength λ value, the use of short wavelength light source.
2, increase the medium n value to improve the NA value (NA = nsinu/2).
3, increase the aperture angle u value to improve the NA value.
4, increase the contrast between light and dark.
Magnification and effective magnification
As a result of two magnifications through the objective and eyepiece, the total microscope magnification Γ should be the product of the objective magnification β and the eyepiece magnification Γ1:
Γ=βΓ1
Obviously, a microscope can have a much higher magnification compared to a loupe, and the magnification of a microscope can be easily changed by switching the objective and eyepiece with different magnifications.
Magnification is also an important parameter of a microscope, but one should not blindly believe that the higher the magnification the better. The limit of microscope magnification is the effective magnification.
Resolution and magnification are two different but related concepts. There is a relationship between: 500NA<Γ<1000NA
Elected objective lens numerical aperture is not large enough, that is, the resolution is not high enough, the microscope can not distinguish between the microstructure of the object, even if the magnification is increased excessively, can only get a large outline but the details of the image is not clear, known as the invalid magnification. Conversely, if the resolution has to meet the requirements of the magnification is not enough, the microscope has the ability to distinguish, but because the image is too small and still can not be clearly seen by the human eye. So in order to give full play to the resolving power of the microscope, should make the numerical aperture and the total magnification of the microscope reasonable match.
