Microscopy is used for the inspection of specimens by using optical or digital magnification in order to enhance the appearance of small features. This is in contrast to macroscopy, which involves inspection of a specimen using the naked eye.
Optical microscopy is used for the inspection at magnifications of up to 1,000 times microstructures. Electron microscopy with magnifications of up to 500,000 times is typically used for failure analysis in R&D labs and in educational institutions.
Four types of microscopy are used in materialographic testing, depending on the nature of the workpiece and the object of investigation, and they are described below.
Optical Microscopy In optical microscopy different filters are used to improve contrast and emphasize specific features based on material properties. This can be achieved with magnifications typically ranging from 2.5 times up to 1,000 times. In materialography, reflected light is the most commonly used type of light optical microscopy. Transmitted optical microscopy is also used, but mainly for mineralogy specimens.
Stereo Optical Microscopy The stereo microscope is an optical microscope variant, designed for low magnification observation of a specimen, using the light reflected from the specimen surface.
Scanning Electron Microscopy A scanning electron microscope (SEM) is a type of electron microscope that produces images of a specimen by scanning the specimen surface with a focused beam of electrons. The electrons interact with the atoms in the specimen, producing various signals that can be translated into information about the surface topography and the composition of the specimen.
Transmission Electron Microscopy Transmission electron microscopy (TEM) uses a beam of electrons transmitted through an ultra-thin specimen and that interacts with the specimen as it passes through it. Generated signals can be translated into various types of information, including information on the type and orientation of individual crystals.
How to Do Light Optical Microscopy
Brightfield (BF) contrast is the most common contrasting technique. Only details with a difference in reflectivity will be distinguished from each other.
Contrasting techniques like DarkField (DF), Differential Interference Contrast (DIC), and Polarized Light (POL) make it possible to observe details which differ from those seen in BF.
The choice of filters in optical microscopy depends on the nature of the surface and the characteristics and details to be investigated.
4. Image acquisition
In addition to the above, capturing the true microstructure depends on factors which are important during image acquisition. The two most predominant factors are exposure and white balance.
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