How much do you know about microscopy terms used in metallographic analyzers?
numerical aperture
Numerical aperture is the main technical parameter of the objective lens and condenser lens of a metallographic microscope. Numerical aperture, abbreviated as NA, is an important indicator for judging the performance of both, especially for the objective lens. The magnitude of its value is marked on the casing of the objective lens and the condenser lens, respectively.
Numerical aperture (NA) is the product of the refractive index (n) of the medium between the front lens of the objective lens and the object being inspected, and half of the sine of the aperture angle (u). The formula is as follows: NA=nsinu/2
resolving power
The resolution of a metallographic microscope refers to the small distance between two objects 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 * small 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 two factors: the NA value of the objective lens and the wavelength of the illumination light source. The larger the NA value and 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 eyepiece, the total magnification of the microscope should be the product of the objective magnification β and eyepiece magnification Gamma 1: Γ=βΓ1
Obviously, compared to a magnifying glass, a microscope can have much higher magnification, and by swapping the objective and eyepiece lenses with different magnifications, it is easy to change the magnification of the microscope.
Depth of Focus
Depth of focus, also known as focal depth, refers to the ability to see clearly not only all points located on the plane of a microscope when the focus is on an object, but also within a certain thickness above and below this plane. The thickness of this clear part is called depth of focus. This is particularly important in video microscopes.
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 of view aperture in the eyepiece.
Field diameter, also known as field width, refers to the actual range of objects that can be accommodated within a circular field of view seen under a microscope. The larger the field of view diameter, the easier it is to observe.
Poor coverage
The optical system of a microscope also includes a cover glass. Due to the non-standard thickness of the cover glass, the refractive path of light entering the air from the cover glass changes, resulting in a phase difference, which is called coverage difference. The poor coverage affects the quality of the sound produced by the metallographic microscope.
Working distance
Working distance, also known as object distance, refers to the distance between the surface of the lens in front of the objective lens and the object being inspected. During mirror inspection, the object being inspected should be between one to two times the focal length of the objective lens. Therefore, it and focal length are two different concepts, and what is commonly referred to as focusing is actually the adjustment of the working distance of a metallographic microscope.
