The role of color filters in metallographic microscopes
The metallographic microscope is an important testing equipment for metallographic analysis of metal materials. In order to conduct accurate metallographic observation of metal materials, color filters are usually required. What is the role of the color filter of a metallographic microscope?
Near the aperture diaphragm of the metallographic microscope illumination system, there is a place for placing a color filter. The color filter is a type of optical filter. Its functions are mainly reflected in the following aspects:
1. Improve the resolving power of the objective lens
Using color filters to obtain short-wavelength illumination can improve the resolving power of metallographic microscope objectives. When the numerical aperture is constant, it is inversely proportional to the wavelength of the illumination light used. Therefore, when using a semi-apochromatic or apochromatic objective lens, since almost all have good achromatic achromatic in the visible light region, all kinds of color filters can be used, and even white light can be used as the light source without using color filters. However, in order to improve the resolving power of the objective lens, using a blue (λ = 0.44 micron) color filter can improve the resolving power by about 25% compared to using a green (λ = 0.55 micron) color filter.
2. Increase the contrast of multiphase alloys in metallographic photos
After the multi-phase alloy sample is processed by film dyeing or heat dyeing, each component phase will have different characteristic colors. The effect is very good when observed with eyes or recorded with color photography. But if you use black and white negatives to take pictures, you will feel blurred. This is mainly because although there are color differences between the colors, the brightness of different colors may be equal. If you use black and white negatives to take pictures, it will be unclear. If a color filter is added that is exactly complementary to the color of a certain phase, the color of that phase will be absorbed by the color filter and become dark, thus making each phase appear on the black and white film.
3. Helps identify subtle parts with colored tissue
If the purpose of the inspection is to identify the subtle parts of a certain component phase of a multiphase alloy, the contrast between the component phases is not important in this case. The color filter selected at this time should be consistent with the color of the phase to be identified. same. For example, after heat dyeing of quenched high carbon steel, the austenite appears brown and the martensite appears green. In order to study the details inside the martensite, a metallographic microscope can be photographed after adding a green color filter. In this way, the inside of the martensite The details will be clearer.
