What are the Types of Illumination Modes for Metallographic Microscopes?
The optical path of a metallographic microscope is equipped with a special illumination system. Its light source is generally mounted on the side or rear of the microscope body. To guide light into 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 make the light beam travel vertically. If the light source is designed at the bottom of the metallographic microscope, the illuminating beam passes directly through the objective lens to irradiate the metallographic specimen surface. The surface of the specimen reflects light to form an image, which is then vertically redirected by the reflector. Since it provides vertical illumination, this is known as vertical illuminator.
Metallographic microscopes use different reflectors to deflect light beams (or images). There are two illumination modes: brightfield illumination and darkfield illumination.
1. Brightfield Illumination
Brightfield illumination is a commonly used mode for metallographic microscopes. The vertical illuminator directs light from the light source to the objective lens. The objective lens converts the light into nearly vertical rays that shine on the polished surface of the metallographic specimen. Light reflected from the specimen surface passes back through the objective lens for preliminary magnification, followed by secondary magnification via the eyepiece.In typical metallographic microscopes, a 45° inclined plane glass plate and a total reflection prism are often used as vertical illuminators. Most large bench-type metallographic microscopes are fitted with both illumination devices. Switching between them is achieved by moving a handle forward, backward, left or right. As vertical illuminators, the plane glass plate and total reflection prism can both reflect and transmit light.
2. Darkfield Illumination
Darkfield illumination differs from brightfield illumination mainly in optical path distribution and lighting effect. Parallel light from the source is blocked by a light stop; the central light cannot pass through, forming an annular hollow light beam that enters the vertical illuminator. The light passes through the outer region of the objective lens and hits a special reflecting ring, which reflects the light onto the metallographic specimen surface.The reflected light has a very large inclination angle. For a polished specimen surface, light reflects backward at a steep angle and cannot enter the objective lens, resulting in a dark black field of view. Only light reflected from the depressions and pits of the specimen can enter the objective lens. Therefore, the specimen image observed under darkfield illumination is the opposite of that seen under brightfield illumination.
