How much do you know about microscopy terminology used for metallographic analyzers?
numerical aperture
Numerical aperture is the main technical parameter of the objective lens and condenser of a metallographic microscope. Numerical aperture, abbreviated as NA, is an important symbol for judging the performance of the two (especially the objective lens). The numerical values are marked on the housings of the objective lens and condenser respectively.
Numerical aperture (NA) is the product of the refractive index (n) of the medium between the objective front lens and the object being inspected and the sine of half the aperture angle (u). The formula is expressed as follows: NA=nsinu/2
resolution
The resolution of a metallographic microscope refers to the minimum distance between two object points that can be clearly distinguished by the microscope, also known as the discrimination rate. The calculation formula is σ=λ/NA
In the formula, σ is the minimum resolution distance; λ is the wavelength of light; NA is the numerical aperture of the objective lens. It can be seen that the resolution of the 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, the smaller the σ value, and the higher the resolution.
Magnification and effective magnification
Due to the two magnifications of the objective lens and the eyepiece, the total magnification Γ of the microscope should be the product of the objective lens magnification β and the eyepiece magnification Γ1: Γ=βΓ1
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 exchanging objective lenses and eyepieces with different magnifications.
Depth of focus
Focal depth is the abbreviation of depth of focus. That is, when using a microscope, when the focus is on an object, not only all points on the plane at that point can be seen clearly, but also all points within a certain thickness above and below the plane can be seen clearly. It must be clear that the thickness of this clear part is the depth of focus. This is particularly important in video microscopy.
Field of view diameter
When observing a microscope, the bright circular range 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, also called the width of the field of view, refers to the actual range of the object being inspected that can be accommodated in the circular field of view seen under the microscope. The larger the field of view diameter, the easier it is to observe.
poor coverage
The optical system of a microscope also includes the coverslip. Due to the non-standard thickness of the cover glass, the optical path of the light refracted from the cover glass into the air changes, resulting in a phase difference, which is the coverage difference. The occurrence of poor coverage affects the sound quality of metallographic microscopes.
working distance
The working distance is also called the object distance, which refers to the distance between the surface of the front lens of the objective lens and the object being inspected. During microscopic examination, the object to be inspected should be between one and two times the focal length of the objective lens. Therefore, it and focal length are two different concepts. What we usually call focusing is actually the adjustment of the working distance of a metallographic microscope.
