Methods of dry objective lens and liquid immersion objective lens for metallographic microscope
According to the difference in the object space medium when the objective lens is used, special objectives are divided into two categories. Any objective lens whose medium between the front lens and the sample is air is called a dry objective lens; any objective lens whose medium between the front lens and the sample is liquid The object is called a liquid immersion objective.
(1) Numerical aperture of dry objective lens
The theoretical maximum value of the aperture angle of a dry objective lens is s=90°. Because A, the maximum theoretical numerical aperture value of the objective lens in air is 1si n90°; 1, the minimum resolvable distance is 8=in/2. But in fact, when the objective lens is used in air, the maximum aperture angle can only reach u=72°. Therefore, the approximate aperture of the dry-huan objective lens can only reach 1 sin72°, o. 95l is longer than the theoretical value, and the minimum resolvable distance is greater than /2.
(2) Liquid immersion environment depends on numerical aperture
Liquid immersion objectives have larger numerical apertures than dry-system objectives. Commonly used infusion solutions include distilled water (n=1.333), cedar oil (n=1.515), glycerol (n=1.455) and diiodomethane (M=1.N2). The maximum aperture angle of the water immersion objective lens can only reach u=65. , so the maximum numerical aperture of the water immersion objective lens is 1.333sinn65°=1.20, the maximum aperture angle of the oil immersion objective lens can reach u=67°, so the maximum numerical aperture of the Poison immersion objective lens can reach 1.515sin67°= 1.40.
(3) Advantages of liquid immersion objective lens
Oil immersion objectives have many advantages: 1) For objective lenses with the same focal length and numerical aperture, the liquid immersion objective lens has better aberration correction than the dry objective lens, so the image quality is better. 2) Since the refractive index of the immersion lens is greater than that of air, so , the liquid immersion objective lens has a higher resolution than the dry objective lens; 3) Since the refractive index of the immersion liquid is the same as the refraction of the glass material of the front lens of the objective lens, the imaging light entering the objective lens increases, and the harmful reflected light is greatly shortened. Improves the brightness, contrast and clarity of objects in the field of view.
All liquid immersion objectives are specially made, and each has its own corresponding immersion liquid. The objective lens housing is engraved with the symbol of the medium that should be used, and the immersion liquid is provided with the instrument (except distilled water) and cannot be selected at will.
(4) Use of liquid immersion objective lens
The method of using the immersion objective lens is: first use a low-magnification objective lens, and then use a higher-magnification dry objective lens to observe, move the selected target to the center of the field of view, and mark the lens arm and micro-motion seat. Then raise the cutting table, turn the immersion objective lens to the working position, and use a glass rod or dropper to drop the immersion liquid on the front lens of the objective lens. When dripping the liquid, do not let the glass rod or dropper touch the surface of the front lens to prevent Scratches and liquid dripping on the front telescope are not allowed to have bubbles. Then move the load-carrying member to the original marking line, so that the liquid is in contact with the surface of the sample, and use the fine-tuning handwheel to adjust the object image. Detailed observation and analysis of the target can be carried out. After the study is completed, raise the cutting table, remove the objective lens, and use absorbent cotton, aviation gasoline or xylene to clean the immersion liquid on the specimen surface and the lens. When using a liquid immersion objective lens, special attention should be paid to cleaning. If residual liquid remains on the objective lens, it will be difficult to remove and the imaging quality of the objective lens will be reduced.
When taking photos of metallographic structure details, you can choose an oil immersion objective lens or a drying system objective lens with a large N numerical aperture; for photos of tissue outlines or for evaluating particle size, use a lamp with a smaller numerical aperture (less than 0.65). Dry objective lens. When taking pictures, you must use photography eyepieces specially designed for photography to match the objective lens, rather than using eyepieces for general observation.
