The Darkfield Illumination Method for Observing Particles with a Microscope
1. Translucent lighting
Biological microscopes are often used to observe transparent specimens and require illumination with transmitted light. There are two types of lighting methods
(1) The critical illumination light source is imaged on the object plane after passing through a condenser lens, as shown in Figure 5. If the loss of light energy is ignored, the brightness of the light source image is the same as that of the light source itself. Therefore, this method is equivalent to placing the light source on the object plane. Obviously, in critical illumination, if the surface brightness of the light source is uneven or shows obvious small structures such as filaments, it will seriously affect the microscopic observation effect, which is the disadvantage of critical illumination. The remedy is to place milky white and heat absorbing color filters in front of the light source to make the illumination more uniform and avoid long-term exposure to the light source that may damage the object being inspected. When illuminated with transmitted light, the aperture angle of the imaging beam of the objective lens is determined by the aperture angle of the focused mirror square beam. In order to fully utilize the numerical aperture of the objective lens, the focusing lens should have the same or slightly larger numerical aperture as the objective lens.
(2) The disadvantage of uneven surface illumination in critical lighting of Kola lighting can be eliminated in Kola lighting. Add an auxiliary spotlight 2 between light source 1 and spotlight 5, as shown in Figure 6. It can be seen that since the auxiliary condenser lens 2 (also known as a colander) uniformly illuminated by the light source is not directly used to image the specimen 6, the field of view (specimen) of the objective lens is uniformly illuminated.
2. Falling light illumination
When observing opaque objects, such as metal grinding plates through a metallographic microscope, illumination is often applied from the side or top. At this point, there is no glass cover on the surface of the observed object, and the formation of the specimen image relies on reflected or scattered light entering the objective lens. As shown in Figure 7.
3. Illumination method for observing particles using dark field of view
The dark field method can be used to observe ultrafine particles. The so-called ultrafine particles refer to those tiny particles that are smaller than the resolution limit of a microscope. The principle of dark field illumination is to prevent the main illumination light from entering the objective lens, and only the light scattered by particles can enter the objective lens for imaging. Therefore, the image of bright particles is given on a dark background, and although the field of view background is dark, the contrast is good, which can improve the resolution.
