What are the lighting methods of metallographic microscope?
The optical path of metallographic microscope has a special illumination system. Most of its lighting sources are installed at the side or the lower back of the mirror body. In order to achieve the purpose that light can be absorbed into the objective lens and then into the eyepiece, it is necessary to install a reflector (plane mirror or prism) at the intersection of the two optical axes to make the light turn vertically. If the light source is designed at the bottom of the metallographic microscope, its lighting beam directly hits the surface of the metallographic sample through the objective lens, is reflected by the surface of the sample to be imaged, and finally is turned vertically by the reflector. Because it plays the role of vertical lighting, it is called ".
Metallographic microscope uses different forms of mirrors to turn the light beam (or image), and there are two illumination methods: bright field illumination and dark field illumination.
1, bright field lighting
Bright-field illumination is a commonly used illumination method for metallographic microscope. It relies on a vertical illuminator to emit light from a light source to an objective lens, and then the objective lens irradiates vertical or nearly vertical light on the grinding surface of a metallographic sample. Then the light reflected from the grinding surface of the sample is amplified vertically through the objective lens, and finally amplified again by the eyepiece. In general metallographic microscope, 45-degree inclined plane glass and total reflection prism are often used as vertical illuminators. Most of the bright field illumination systems of large horizontal metallographic microscope have these two kinds of illumination devices, and its change is realized by moving the handle back and forth or left and right. As vertical illuminators, plane glass and total reflection prism can both reflect and transmit light.
2, dark field lighting
The difference between dark field and bright field is mainly the distribution of optical path and lighting effect. The parallel light of the light source is blocked by the annular diaphragm, and the central light cannot pass through, so that a hollow annular light beam is injected into the vertical illuminator, so that the light passes through the periphery of the objective lens and is transmitted on the special reflective condenser, which reflects the light on the polished surface of the metallographic sample. Because the reflected light has a great inclination angle, if the sample is a polished mirror, the light on the sample still reflects in the opposite direction with a great inclination, and it is impossible to enter the objective lens, so the field of view is dark, and only the concave of the sample is left.
