What are the methods of illumination used in metallographic microscopes?
The optical path of a metallographic microscope has a special illumination system. Most of its illumination sources are installed on the side or back and bottom of the mirror body. In order to achieve the purpose of allowing light to enter the objective lens and then into the eyepiece, a reflector (plane mirror or prism) must be installed at the intersection of the two optical axes to allow the light to pass through. Vertical steering. If the light source is designed at the bottom of a metallographic microscope, its illumination beam will directly hit the surface of the metallographic sample through the objective lens, and will be reflected from the specimen surface to the objective lens to form an image. Finally, it will be vertically steered by the reflector. Due to its The function of vertical lighting, so it is called "vertical illuminator".
Metallurgical microscopes use different forms of reflectors to turn light beams (or images). There are two types of illumination: brightfield and darkfield illumination.
1. Brightfield lighting
Brightfield illumination is a commonly used lighting method in metallographic microscopes. It relies on a vertical illuminator to emit light from the light source to the objective lens, which then irradiates vertical or nearly vertical light onto the grinding surface of the 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 microscopes, 45-degree tilted flat glass and total reflection prisms are often used as vertical illuminators. Most of the brightfield illumination systems of large horizontal metallurgical microscopes are equipped with these two illumination devices, and their changes are achieved by moving the handle back and forth or left and right. Flat glass and total reflection prisms serve as vertical illuminators, although both can reflect and transmit light.
2. Dark field lighting
The difference between dark field and bright field is mainly the distribution of light path and lighting effect. The parallel light rays of the light source are blocked by the annular light barrier, and the light in the center cannot pass through, forming a hollow annular light beam that is injected into the vertical illuminator, so that the light passes through the periphery of the objective lens and is transmitted to the special reflective condenser, which reflects the light on the metal. The ground surface of the sample has a very large tilt angle for the reflected light. If the sample is a polished mirror surface, the light on the sample will still be reflected in the opposite direction at a very large tilt and cannot enter the objective lens, so The field of view is dark, and only the recesses of the sample emit light into the objective lens. Therefore, observing the sample under the dark field of the microscope is exactly the opposite of that under the bright field.
