Illumination Method for Observing Particles Using Dark Field
Ultramicroscopic particles can be observed with the dark field method. The so-called ultramicroscopic particles refer to those tiny particles that are smaller than the resolution limit of the microscope. The principle of dark field illumination is: do not let the main illumination light enter the objective lens, and only the light scattered by the particles can enter the objective lens for imaging.
Therefore, the image of bright particles is given on the dark background. Although the background of the field of view is dark, the contrast (contrast) is very good, which can improve the resolution.
Dark field illumination can be divided into one-way and two-way
(1) One-way dark field illumination Figure 8 is a schematic diagram of one-way dark field illumination. It can be seen from the figure that after the light emitted by the illuminator 2 is reflected by the opaque specimen sheet 1, the main light does not enter the objective lens 3, and the light entering the objective lens is mainly scattered by particles or uneven details. Obviously, this one-way dark field illumination is effective for observing the existence and movement of particles, but it is not effective for reproducing the details of objects, that is, there is a phenomenon of "distortion".
(2) Two-way dark field illumination Two-way dark field illumination can eliminate the distortion defect caused by one-way. In front of the common three-lens condenser, place an annular diaphragm, as shown in Figure 9, to realize two-way dark field illumination. The liquid is immersed between the last piece of the condenser and the objective glass, while the space between the cover glass and the objective lens is dry. Therefore, the ring-shaped beam passing through the condenser is totally reflected in the cover glass and cannot enter the objective lens, forming a loop as shown in the figure. Only the light scattered by the particles on the specimen enters the objective lens, forming a two-way darkfield illumination
