Imaging Principles of the Metallographic Microscope
1. Bright field, dark field
Brightfield is the most basic observation method for observing samples under a microscope, which presents a bright background in the field of view of the microscope. The basic principle is that when the light source is vertical or nearly vertical and illuminates the sample surface through the objective lens, it is reflected back to the objective lens through the sample surface to form an image.
The difference between the dark field illumination method and the bright field illumination method is that a dark background appears in the microscope field area. The bright field illumination method is vertical or perpendicular incidence, while the dark field illumination method is through surrounding oblique illumination outside the objective lens. The sample will scatter or reflect the irradiation light, and the light scattered or reflected by the sample enters the objective lens to image the sample. Dark field observation can clearly observe colorless, tiny crystals or light-colored tiny fibers that are difficult to observe in bright field.
2. Polarized light, interference
Light is an electromagnetic wave, and electromagnetic wave is a transverse wave. Only transverse waves have polarization. It is defined as light whose electric vector vibrates in a fixed manner relative to the direction of propagation.
The polarization phenomenon of light can be detected with the help of experimental equipment. Take two identical polarizers A and B, and pass the natural light through the first polarizer A. At this time, the natural light also becomes polarized light, but because the human eye cannot distinguish it, a second polarizer B is needed. Fix polarizer A and place polarizer B on the same horizontal plane as A. Rotate polarizer B. You can find that the intensity of the transmitted light changes periodically as B rotates. The light intensity will gradually increase from maximum to maximum for each 90° rotation. It weakens to the darkest, and then rotates 90° and the light intensity gradually increases from the darkest to the brightest. Therefore, polarizer A is called a polarizer, and polarizer B is called an analyzer.
Interference is the phenomenon in which the light intensity is strengthened or weakened by the superposition of two columns of coherent waves (light) in the interaction zone. Light interference is mainly divided into double slit interference and thin film interference. Double-slit interference means that the light emitted by two independent light sources is not coherent light. The double-slit interference device makes one beam of light pass through the double slits and become two beams of coherent light, which communicate on the light screen to form stable interference fringes. In the double-slit interference experiment, when the distance difference between a certain point on the light screen and the double slits is an even number of half-wavelengths, bright stripes will appear at that point; when the distance difference between a certain point on the light screen and the double slits is an odd number of half-wavelengths , the dark stripes appearing at this point are Young's double-slit interference. Thin film interference is a phenomenon in which two beams of reflected light are formed after a beam of light is reflected by two surfaces of the film. This phenomenon is called thin film interference. In thin film interference, the path difference of reflected light from the front and back surfaces is determined by the thickness of the film, so in thin film interference the same bright fringe (dark fringe) should appear where the thickness of the film is equal. Since the wavelength of light waves is extremely short, when thin films interfere, the dielectric film should be thin enough to observe interference fringes.
3. Differential interference contrast DIC
The metallographic microscope DIC uses the principle of polarized light. The transmission DIC microscope mainly has four special optical components: polarizer, DIC prism I, DIC prism II and analyzer. The polarizer is installed directly in front of the condensing system to linearly polarize the light. A DIC prism is installed in the condenser. This prism can decompose a beam of light into two beams of light (x and y) with different polarization directions, and the two beams form a small angle. The condenser aligns the two beams of light parallel to the optical axis of the microscope. Initially, the two beams of light have the same phase. After passing through the adjacent areas of the specimen, the optical path difference between the two beams of light occurs due to the different thickness and refractive index of the specimen. A DIC prism II is installed at the back focal plane of the objective lens, which combines the two light waves into one. At this time, the polarization planes (x and y) of the two beams of light still exist. Finally the beam passes through the first polarizing device, the analyzer. Before the beam forms a DIC image in the eyepiece, the analyzer is oriented at right angles to the polarizer. The analyzer combines two perpendicular light waves into two beams with the same polarization plane, causing them to interfere. The optical path difference between x and y waves determines how much light is transmitted. When the optical path difference is 0, no light passes through the analyzer; when the optical path difference is equal to half the wavelength, the light that passes through reaches the maximum value. So on the gray background, the specimen structure shows a difference in light and dark. In order to achieve the best contrast of the image, the optical path difference can be changed by adjusting the longitudinal fine adjustment of DIC prism II. The optical path difference can change the brightness of the image. Adjusting DIC Prism II can make the fine structure of the specimen show a positive or negative projection image, usually one side is bright and the other side is dark, which creates an artificial three-dimensional sense of the specimen.
