What Do You Know About Microscope Terms for Metallographic Analyzers?
Numerical Aperture
Numerical aperture (abbreviated as NA) is a key technical parameter for the objectives and condensers of metallographic microscopes. It serves as an important indicator for evaluating their performance, especially for objectives. The numerical aperture value is marked on the outer casing of objectives and condensers respectively.
Numerical aperture (NA) equals the product of the refractive index (n) of the medium between the front lens of the objective and the specimen, and the sine of half the aperture angle (u). The formula is expressed as follows:
\(NA=n\sin\frac{u}{2}\)
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 resolving power. Its calculation formula is:
\(\sigma=\lambda/NA\) Where \(\sigma\) is the minimum resolving distance; \(\lambda\) is the wavelength of light; NA stands for the numerical aperture of the objective. It can be seen that the resolution of an objective is determined by two factors: the NA value of the objective and the wavelength of the illumination light source. The larger the NA value and the shorter the wavelength of illumination light, the smaller the value of \(\sigma\), and the higher the resolution.
Magnification and Useful Magnification
Since magnification occurs twice through the objective and eyepiece, the total magnification \(\Gamma\) of the microscope is the product of the objective magnification \(\beta\) and eyepiece magnification \(\Gamma_1\):
\(\Gamma=\beta\Gamma_1\)
Obviously, compared with magnifying glasses, microscopes achieve much higher magnification. The magnification can be conveniently adjusted by replacing objectives and eyepieces of different magnifications.
Depth of Field
Depth of field is the abbreviation for focal depth. When focusing on an object under the microscope, not only points on the focal plane can be seen clearly, but also features within a certain thickness above and below this plane. The thickness of this clear zone is defined as depth of field. This characteristic is particularly important for video microscopes.
Field Diameter
The bright circular area visible during microscopic observation is called the field of view. Its size is determined by the field diaphragm inside the eyepiece.
Field diameter, also called field width, refers to the actual observable range of the specimen within the circular field of view of the microscope. A larger field diameter facilitates observation.
