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Metallurgical Microscope
Dr. Dmitri Kopeliovich
Metallurgical microscope is the optical microscope, differing from other microscopes in the method of the specimen illumination.
Since metals are opaque substances they must be illuminated by frontal lighting, therefore the source of light is located within the microscope tube.
This is achieved by plain glass reflector, installed in the tube.
The optical scheme of metallurgical microscope is shown in the picture.
The image quality and its resolving power are mainly determined by the quality of the objective.
The objective magnification depends on its focal length (the shorter focal length, the higher magnification).
The eyepiece is the lens nearest the eye. The image is magnified by eyepiece in x6, x8 or x10.
The total magnification of the microscope may be calculated by the formula:
M = L*E/ F
Where
L- the distance from back of objective to eyepiece;
F – the focal length of the objective;
E- the magnifying power of the eyepiece.
The common magnification of metallurgical microscope is in the range x50 – x1000.
- Bright field illumination is used for producing images with bright background and dark non-flat structure features (pores, edges, etched grain boundaries).
- Dark field illumination is used for producing images with dark background and bright non-flat structure features (pores, edges, etched grain boundaries).
- Polarized light is used for viewing metals with non-cubic crystalline structure (magnesium, alpha-titanium|, zinc and others), responding to cross-polarized light. Polarized light is created by polarizer, located before the illuminator and analyzer, placed before the eyepiece.
- Nomarsky prism is used for differential interference contrast system (DIC), enabling to observe features not visible in bright field.