How much do you know about microscope terms used in metallographic analyzers?
Numerical aperture
The numerical aperture is the main technical parameter of the objective lens and the condenser lens of the metallographic microscope. The numerical aperture is abbreviated as NA, which is an important symbol to judge the performance of the two (especially for the objective lens). The size of its numerical value is respectively marked on the casing of the objective lens and condenser lens.
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 to be inspected and the sine of the half of the aperture angle (u). The formula is 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. Its calculation 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 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 σ value, 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 the magnifying glass, the microscope can have a much higher magnification, and the magnification of the microscope can be easily changed by exchanging the objective lens and eyepiece with different magnifications.
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. This is especially important in video microscopy.
Field diameter
When observing a microscope, the bright circular 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.
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 metallographic microscope.
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 the adjustment working distance of the metallographic microscope.
