What are the illumination methods of metallographic microscopes?
The optical path of a metallographic microscope has a special illumination system. Its illumination light source is mostly installed on the side or lower back of the mirror body. In order to achieve the purpose of allowing light to enter the objective lens and then enter the eyepiece, a reflecting mirror (plane mirror or prism) must be installed at the intersection of the two optical axes to vertically turn the light. If the light source is designed at the bottom of the metallographic microscope, its illumination beam directly passes through the objective lens to the surface of the metallographic sample, is reflected by the sample surface to the objective lens for imaging, and finally is vertically turned by the reflecting mirror. Due to its vertical illumination function, it is called a "vertical illuminating device".
Metallographic microscope uses different forms of mirrors to turn the beam (or image), and there are two illumination methods: bright field illumination and dark field illumination.
1. Bright field illumination
Bright field illumination is a commonly used illumination method in metallographic microscopes. It relies on a vertical illumination device to emit light from the light source to the objective lens, which then illuminates the grinding surface of the metallographic specimen with vertical or nearly vertical light. Then, the light reflected from the sample grinding surface is vertically magnified through the objective lens, and then further magnified by the eyepiece lens. In general metallographic microscopes, a 45 degree inclined plane glass and a total reflection prism are often used as vertical illumination devices. In the bright field illumination system of large horizontal metallographic microscopes, most of them have these two types of illumination devices, and their changes are achieved by moving the handle back and forth or left and right. Flat glass and total reflection prisms, as vertical lighting fixtures, can both reflect and transmit light.
2. Dark field illumination
The difference between dark field and bright field mainly lies in the distribution of optical path length and lighting effect. The parallel rays of the light source are blocked by the annular light barrier, and the central ray cannot pass through, forming a hollow annular beam that enters the vertical illumination device. This allows the light to pass through the periphery of the objective lens and be transmitted through a specially designed reflective condenser, which reflects the light on the polished surface of the metallographic sample. Due to the extremely large tilt angle of the reflected light, if the sample is a polished mirror surface, the light on the sample still reflects in the opposite direction at a great tilt, and cannot enter the objective lens. Therefore, the field of view is dark, and only the concave and convex parts of the sample emit light into the objective lens. Therefore, when observing the sample in the dark field of the microscope, it is exactly opposite to that in the bright field.
