Categorization of Illuminators for Microscopes
The illumination system of a microscope is particularly important for obtaining high-quality images. Most of its illuminators are mounted on the side or rear of the microscope body. In order to allow the illumination light to enter the objective lens and form an image, it must intersect the optical axis at an appropriate angle-for example, using a reflector (plane mirror or prism) to deflect the optical path by 90°.
If the light source is integrated into the base of the microscope and the illumination is directed vertically through the objective lens onto the specimen surface, the image is formed by the reflected light from the specimen surface. This is called "vertical illumination."
In a microscope, vertical illumination is commonly used for epi-illumination observation. It utilizes the light from the vertical illuminator, which is directed toward the objective lens and deflected by a plane mirror at 45° or a beam splitter toward the specimen surface. The reflected light from the specimen then passes back through the objective lens and forms an image through the eyepiece.
Classification of Microscope Illuminators
Microscope illuminators are classified into vertical illuminators and side illuminators according to the different illumination methods, that is, whether the illumination is coaxial with the objective lens or incident from the side.
Vertical Illuminators
Vertical illumination is a widely used illumination method in microscopes. It relies on the vertical illuminator to direct light toward the objective lens, which then reflects or deflects the light at 90° onto the specimen surface. The reflected light from the specimen passes back through the objective lens and is imaged through the eyepiece.
In general microscopes, a 45° inclined plane mirror and a total reflection prism are often used as vertical illuminators. In large metallurgical microscopes, most of the vertical illumination systems with regular observation functions are equipped with these two types of illumination devices. Their switching is usually controlled by a lever that moves forward, backward, left, or right. Both the plane mirror and the total reflection prism can reflect light into the objective lens or transmit light through it, but each has its own characteristics, which should be noted in use.
Side Illuminators
The main difference between side illumination and vertical illumination lies in the optical path and the illumination effect. In side illumination, the parallel light from the light source passes through a ring-shaped aperture diaphragm, which blocks the central part of the beam, forming a hollow annular light beam. This beam is directed into the vertical illuminator and deflected by the objective lens onto the specimen surface at a very large oblique angle.
If the specimen is a polished surface, the specularly reflected light exits at an angle opposite to the incident direction and cannot enter the objective lens, so the field of view appears completely black. Only the scattered light from the depressed or textured areas of the specimen enters the objective lens, enabling clear observation of fine surface details.
