Transmission type of microscope accompanying metallographic analysis instruments
Metallographic analytical instruments supporting microscope illumination is generally divided into "transmitted illumination", and "falling illumination" two categories. The former applies to transparent or semi-transparent objects to be examined, the vast majority of biological microscopes belong to this type of illumination; the latter applies to non-transparent objects to be examined, the light source from above, also known as "" reflective illumination ". Mainly used with metallurgical microscopy or fluorescence microscopy.
1. Transmissive illumination
Biological microscope is used to observe transparent specimens, the need for transmitted light to illuminate. There are two types of illumination
(1) Critical illumination (Critical illumination) The light source passes through the condenser mirror and is imaged on the object plane. If the loss of light energy is ignored, the brightness of the light source image is the same as 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 not uniform, or obviously show small structures, such as filaments, etc., then it is necessary to seriously affect the microscope observation effect, which is the disadvantage of critical illumination. The remedy is to place opalescent and heat-absorbing filters in front of the light source, so that the illumination becomes more uniform and to avoid prolonged irradiation of the light source and damage to the object being examined. Illumination with transmitted light, the objective lens imaging beam of the aperture angle, the aperture angle of the beam is determined by the spotlight mirror like a square, in order to make full use of the numerical aperture of the objective lens, the spotlight mirror should be the same as the objective lens or a slightly larger numerical aperture.
(2) Cora Illumination The disadvantage of uneven illumination of the object surface in critical illumination can be eliminated in Cora illumination. In the light source 1 and the focusing lens 5 between the addition of an auxiliary focusing lens 2. visible, because not directly to the light source, but the light source is uniformly illuminated by the auxiliary focusing lens 2 (also known as the Cora mirror) imaging in the specimen 6, so the field of view of the objective lens (the specimen) to get a uniform illumination.
2. Falling illumination
When observing an opaque object, such as a metal abrasive disc through a metallurgical microscope, illumination is often applied from the side or from above. At this time, the surface of the object to be observed on the surface of the glass is not covered, the specimen image is generated by entering the objective lens of the reflected or scattered light. As shown in Figure 7.
3. Illumination method of observing particles with dark field of view
The dark-field method can be used to observe the supermicroscopic plasmas. The so-called supermicroscopic plasmas are those tiny plasmas that are smaller than the resolution limit of the microscope. The principle of dark-field illumination is that the main illumination rays do not enter the objective lens, but only the rays scattered by the particles are able to enter the image of the objective lens. As a result, the image of bright particles is given on a dark background, and the field of view is dark, but the contrast is good, allowing for improved resolution.
Dark-field illumination is divided into unidirectional and bidirectional.
(1) Unidirectional dark-field illumination A diagram of unidirectional dark-field illumination. Visible from the figure, the light from the illuminator 2, by the opaque specimen piece 1 after reflection, the main light are not into the objective lens 3, into the objective lens is mainly by the particles or the unevenness of the fine part of the scattered light. Obviously, this unidirectional dark field illumination, the presence and movement of particles is effective for observation, but not effective for the reproduction of object details, that is, there is a "distortion" phenomenon.
(2) Two-way dark field illumination Two-way dark field illumination can eliminate the shortcomings of one-way distortion. In front of the ordinary three-lens condenser, the placement of a ring diaphragm, you can achieve two-way dark field illumination. Between the last piece of the condenser and the carrier glass is immersed in liquid, and between the cover glass and the objective lens is dry. Thus, metallographic analytical instruments supporting microscope transmission and falling illumination, through the ring beam of the condenser lens, the cover glass in the total reflection and can not enter the objective lens, the formation of the circuit as shown in the figure. What enters the objective lens is only the light scattered by the particles on the specimen, forming a two-way dark field illumination. For other related instruments, such as iron analyser, carbon and silicon analyser, etc., please consult the technical department of Tong Pu.
