Transmission and Epi-illumination of Microscopes for Metallographic Analysis Instruments
The lighting methods of microscopes for metallographic analysis instruments are generally divided into two categories: "transmissive lighting" and "episcopic lighting". The former is suitable for transparent or translucent objects, and most biological microscopes belong to this type of lighting method; the latter is suitable for non-transparent objects, and the light source comes from above, also known as "reflective lighting". Main applications with metallographic microscopy or fluorescence microscopy.
1. Trans-illumination
Biological microscopes are mostly used to observe transparent specimens and need to be illuminated with transmitted light. There are two lighting methods
(1) Critical illumination After the light source passes through the condenser, it is imaged on the object plane, as shown in Figure 5. If the loss of light energy is ignored, the brightness of the light source image is the same as the light source itself, so this method is equivalent to placing the light source on the object plane. Obviously, in critical illumination, if the brightness of the surface of the light source is not uniform, or obviously shows small structures, such as filaments, etc., then the observation effect of the microscope will be seriously affected, 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 damage to the object to be inspected due to long-term irradiation of the light source. When illuminated with transmitted light, the aperture angle of the imaging beam of the objective lens is determined by the aperture angle of the square beam of the condenser mirror. In order to make full use of the numerical aperture of the objective lens, the condenser lens should have the same or slightly larger numerical aperture as the objective lens.
(2) Kola lighting The disadvantage of uneven illumination on the object surface in critical lighting can be eliminated in Kola lighting. An auxiliary condenser lens 2 is added between the light source 1 and the condenser lens 5, as shown in FIG. 6 . It can be seen that the field of view (specimen) of the objective lens is uniformly illuminated because the light source is not directly illuminated, but the auxiliary condenser 2 (also called a Kolar mirror) uniformly illuminated by the light source is imaged on the specimen 6 .
2. epi-illumination
When observing opaque objects, such as observing metal grinding discs through a metallographic microscope, it is often illuminated from the side or from above. At this time, there is no cover glass on the surface of the object to be observed, and the image of the specimen is generated by the reflected or scattered light entering the objective lens.
3. 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 metallographic analysis instrument is equipped with the transmission and epi-type illumination of the microscope, and the annular beam passing through the condenser is totally reflected in the cover glass and cannot enter the objective lens, forming a circuit 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. For other related instruments such as molten iron analyzer, carbon silicon analyzer, etc., please consult Tongpu Technology Department.
