What are the lighting methods of metallographic microscopes?
The optical path of the metallographic microscope has a special illumination system. Most of its illumination sources are installed on the side or rear bottom of the mirror body. In order to achieve the purpose of allowing light to enter the objective lens and then enter the eyepiece, a reflector (plane mirror or prism) must be installed at the intersection of the two optical axes to make the light Vertical steering, if the light source is designed at the bottom of the metallographic microscope, its illuminating beam directly hits the surface of the metallographic sample through the objective lens, and is reflected from the surface of the sample to the objective lens to form an image, and finally the mirror is used for vertical steering. The function of vertical lighting, so it is called "vertical illuminator".
Metallographic microscopes use different forms of mirrors to steer the light beam (or image), and there are two types of illumination methods: bright field and dark field illumination.
1. Bright field illumination
Bright field illumination is a commonly used lighting method for metallographic microscopes. It relies on a vertical illuminator to emit the light from the light source to the objective lens, and then the objective lens irradiates the vertical or approximately vertical light on the grinding surface of the metallographic sample. . Then the light reflected from the grinding surface of the sample is magnified vertically through the objective lens, and finally magnified again by the eyepiece. In a general metallographic microscope, a 45-degree inclined plane glass and a total reflection prism are often used as the vertical illuminator. Most of the bright field illumination systems of large horizontal metallurgical microscopes have these two illumination devices, and its changes are realized by moving the handle back and forth or left and right. Flat glass and total reflection prisms act as vertical illuminators, although both reflect and transmit light.
2. Dark field illumination
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 diaphragm, and the central light cannot pass through, forming a hollow annular light beam and entering 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 gold On the grinding surface of the sample, because the reflected light has a very large inclination angle, if the sample is a polished mirror, the light on the sample is still reflected in the opposite direction with a large inclination, and it is impossible to enter the objective lens, so The field of view is a dark color, and only the light from the concave part of the sample is emitted into the objective lens, so the observation of the sample under the dark field of the microscope is just the opposite of that under the bright field.
