What is the working principle of metallographic microscope? Detailed explanation of the working principle of metallographic microscope
Metallographic microscope is a commonly used laboratory analysis instrument, which can combine optical microscope technology, photoelectric conversion technology, and computer image processing technology, and is widely used in laboratories. What is the working principle of metallographic microscope? The following editor will introduce it in detail, I hope it can help everyone.
Working principle of metallographic microscope
The magnification system is the key to the usefulness and quality of the microscope. It is mainly composed of objective lens and eyepiece.
The magnification of the microscope is:
M display = L/f object × 250/f eye = M object × M eye In the formula [m1] M display - represents the magnification of the microscope; [m2] M object, [m3] M object and [f2] f object, [f1]f eye represents the magnification and focal length of the objective lens and eyepiece, respectively; L is the length of the optical lens barrel; 250 is the photopic distance. The unit of length is mm.
Resolution and Aberrations The resolution of a lens and the degree of correction of aberration defects are important indicators of the quality of a microscope. In metallographic technology, resolution refers to the minimum resolution distance of the objective lens to the object. Due to the diffraction phenomenon of light, the minimum resolving distance of the objective lens is limited. The German Abb proposed the following formula for the minimum resolution distance d
d=λ/2nsinφ where λ is the wavelength of the light source; n is the refractive index of the medium between the sample and the objective lens (air; = 1; turpentine: = 1.5); φ is half of the aperture angle of the objective lens.
It can be seen from the above formula that the resolution increases with the increase of and . Because the wavelength of visible light [kg2][kg2] is between 4000 and 7000. In the most favorable case where the [kg2][kg2] angle is close to 90, the resolving distance will not be higher than [kg2]0.2m[kg2]. Therefore, the microstructure smaller than [kg2]0.2m[kg2] must be observed with the aid of an electron microscope (see), while the microstructure, distribution, and crystallinity of which the scale is between [kg2]0.2~500m[kg2] Changes in particle size, as well as the thickness and spacing of slip bands, can be observed with an optical microscope. This plays an important role in analyzing alloy properties, understanding metallurgical processes, performing quality control of metallurgical products, and analyzing component failures.
The degree of aberration correction is also an important factor affecting the image quality. In the case of low magnification, the aberration is mainly corrected by the objective lens, and in the case of high magnification, the eyepiece and the objective lens need to be corrected together. There are seven main aberrations of lenses, of which five are spherical aberration, coma, astigmatism, field curvature and distortion for monochromatic light. There are two types of longitudinal chromatic aberration and lateral chromatic aberration for complex light. Early microscopes mainly focused on the correction of chromatic aberration and partial spherical aberration, and there were achromatic and apochromatic objectives according to the degree of correction. With the continuous development, aberrations such as field curvature and distortion of metallographic microscope objects have also been given enough attention. After the objective lens and eyepiece are corrected for these aberrations, not only the image is clear, but also its flatness can be maintained in a large range, which is particularly important for metallographic microphotography. Therefore, plan achromatic objectives, plan apochromatic objectives and wide-field eyepieces have been widely used. The degree of aberration correction mentioned above is marked on the objective lens and eyepiece respectively in the form of lens type.
Light source The earliest metallographic microscopes used general incandescent bulbs for lighting. In order to improve the brightness and lighting effect, low-voltage tungsten filament lamps, carbon arc lamps, xenon lamps, halogen lamps, mercury lamps, etc. appeared. Some special microscopes require a monochromatic light source, and sodium lamps and thallium lamps can emit monochromatic light.
Illumination mode Metallographic microscope is different from biological microscope, it does not use transmitted light, but reflected light imaging, so there must be a special additional illumination system, that is, vertical illumination device. In 1872, V.von Lang created this device and made the first metallographic microscope. The original metallographic microscope only had brightfield illumination, and later developed oblique illumination to improve the contrast of certain tissues
