Introduction to the application areas of metallurgical microscopes and the principles of imaging
Each imaging principle of metallographic microscope
1, bright field, dark field
Bright field of view is the most basic way of observing samples in the microscope, presenting a bright background in the field of view area. The basic principle is that when the light source is perpendicular or nearly perpendicular through the objective lens irradiation to the surface of the sample, the surface of the sample reflected back to the objective lens to make its image.
Dark-field illumination and bright field of view is different in that, in the microscope field of view area presents a dark background, bright field of view of the irradiation method for the vertical or vertical incidence, while the dark-field irradiation method for the illumination of the sample through the objective lens outside of the surrounding oblique illumination sample, the sample will play a role in the illumination of the light scattering or reflecting the role of the light scattered or reflected by the sample into the objective lens to make the sample imaging. Dark-field observation, the bright field of view is not easy to observe the colourless, small crystals or light-coloured small fibres, in the dark field of view clearly observed.
2, polarised light, interference
Light is a kind of electromagnetic wave, and electromagnetic wave is a transverse wave, only transverse waves have polarisation. It is defined as the electric vector relative to the direction of propagation in a fixed way vibration of light.
The phenomenon of polarisation of light can be detected with the help of an experimental setup. Take two pieces of the same polarizer A, B, the natural light first through the ** piece of polarizer A, at this time the natural light also becomes polarized light, but because the human eye can not be identified, so you need to ** piece of polarizer B. Polarizer A fixed, polarizer B placed in the same level with A, rotate the polarizer B, you can find that the intensity of the transmitted light with the rotation of the B and the emergence of cyclic changes in the intensity of the light every 90 ° turn will be gradually weakened from * large to * dark, the intensity of light will be gradually reduced to * dark, the intensity of light will be reduced to * dark, the intensity of light will be reduced to * dark, the intensity of light will be reduced to * dark. Large light intensity will gradually weaken to * dark, and then turn 90 ° light intensity will be gradually enhanced from * dark to * bright, so the polarizer A is called the starting polariser, polarizer B is called the bias detector.
Interference is the superposition of two coherent waves (light) in the interaction zone is produced by the phenomenon of strengthening or weakening the intensity of light. The interference of light is mainly divided into double-slit interference and thin-film interference. Double-slit interference for two independent light sources are not coherent light, double-slit interference device so that a beam of light through the double-slit into two beams of coherent light, in the light screen passes through the formation of stable interference fringes. In the double-slit interference experiment, a point on the light screen to the double slit distance difference for an even number of times the half-wavelength, the point of the bright fringe; light screen to a point on the double slit distance difference for an odd number of times the half-wavelength, the point of the dark fringes for the Young's double-slit interference. Thin-film interference for a beam of light reflected by the two surfaces of the film, the formation of two beams of reflected light interference phenomenon called thin-film interference. In thin-film interference, before and after the surface of the reflected light by the thickness of the film to determine the distance difference, so thin-film interference in the same bright fringes (dark fringes) should appear in the thickness of the film in the same place. Because the wavelength of light waves is extremely short, so when thin film interference, the dielectric film should be thin enough to observe the interference fringes.
3,Differential Interference Lining DIC
Metallographic microscope DIC uses the principle of polarised light. Transmission DIC microscopes have four main special optical components: the starting polariser, DIC prism Ⅰ, DIC prism Ⅱ, and the detecting polariser. The starting polariser is mounted directly in front of the concentrator system to linearly polarise the light. A DIC prism is mounted in the concentrator, which breaks a beam of light into two beams of light (x and y) with different polarisation directions, both at a small angle. The concentrator aligns the two beams of light in a direction parallel to the optical axis of the microscope. Initially the two beams of light are in phase, and after passing through an adjacent area of the specimen, the difference in thickness and refractive index of the specimen causes the two beams of light to undergo an optical range difference. A DIC prism II is mounted at the back focal plane of the objective lens, which combines the two beams into a single beam. At this point the polarisation planes (x and y) of the two beams of light remain. Finally the beam passes through the first polarising device, the detector polariser. Before the beams form the eyepiece DIC image, the detector polariser is oriented at right angles to the polariser. The check polariser interferes with two perpendicular beams of light by combining them into two beams of light with the same plane of polarisation. the difference in optical range between the x and y waves determines how much light is transmitted. When the optical range difference is 0, no light passes through the check polariser; when the optical range difference is equal to half of the wavelength, the light that passes through reaches its maximum value. As a result, the structure of the specimen appears bright and dark on a grey background. In order to achieve the best contrast of the image, the optical range difference can be changed by adjusting the longitudinal fine adjustment of the DIC prism II, which changes the brightness of the image. Adjusting the DIC prism Ⅱ can make the fine structure of the specimen present a positive or negative projection image, usually one side is bright, while the other side is dark, which creates an artificial three-dimensional sense of the specimen.
