What are the types of illumination methods for a metallurgical microscope?
A metallurgical microscope features an optical path equipped with a specialized illumination system. Its light source is mostly installed on the side or rear lower part of the microscope body. To guide light into the objective lens and then the eyepiece, a reflector (plane mirror or prism) must be mounted at the intersection of the two optical axes to vertically deflect the light. When the light source is designed at the bottom of the metallurgical microscope, the illumination beam passes directly through the objective lens to the surface of the metallurgical specimen. The light reflected from the specimen surface enters the objective lens to form an image, which is then vertically deflected by the reflector. Due to its function of vertical illumination, this component is named the vertical illuminator.
Metallurgical microscopes use different types of reflectors to deflect light beams (or images), and there are two main illumination modes: brightfield illumination and darkfield illumination.
1. Brightfield Illumination
Brightfield illumination is the most commonly used mode for metallurgical microscopes. The vertical illuminator directs light from the source to the objective lens, which projects vertical or nearly vertical light onto the polished surface of the metallurgical specimen. The light reflected from the specimen surface passes vertically back through the objective lens for primary magnification, followed by secondary magnification via the eyepiece.
Ordinary metallurgical microscopes typically use a 45° inclined flat glass or a total reflection prism as the vertical illuminator. Large horizontal metallurgical microscopes are generally equipped with both devices in their brightfield illumination systems; switching between them is controlled by moving a handle forward, backward, left or right. Both the flat glass and total reflection prism used as vertical illuminators can achieve light reflection and transmission simultaneously.
2. Darkfield Illumination
The core differences between darkfield and brightfield illumination lie in the optical path layout and observation effect. Parallel light from the source is blocked by an annular diaphragm, blocking central light and forming a hollow annular beam that enters the vertical illuminator. The light travels through the outer perimeter of the objective lens to a dedicated reflective condenser, which reflects the light onto the polished surface of the metallurgical specimen at a steep oblique angle.
If the specimen is a smooth polished mirror surface, the reflected light will bounce back at an equally large oblique angle and cannot enter the objective lens, resulting in an entirely dark field of view. Only light reflected from depressions and irregularities on the specimen can enter the objective lens. Therefore, the observation effect of specimens under darkfield illumination is completely opposite to that under brightfield illumination.
