What are the key factors affecting metallographic microscope imaging?
The metallographic microscope is suitable for observing metallographic structures and surface morphology, and is an ideal instrument for metallurgy, mineralogy, and precision engineering research. The metallographic microscope has the characteristics of good stability, clear imaging, high resolution, large and flat field of view. Because of its qualitative and quantitative analysis of the measured object, the metallographic microscope is a measuring instrument widely used in metallurgy, mechanical processing, technology and other industries.
Due to objective conditions, any metallographic microscope optical system cannot generate a theoretically ideal image, and the existence of various aberrations affects the imaging quality. The various aberrations are briefly introduced below.
1. Color difference
Chromatic aberration is a serious defect in the imaging of metallographic microscope lenses. It occurs when polychromatic light is used as the light source. Monochromatic light does not produce chromatic aberration. White light is composed of seven types: red, orange, yellow, green, cyan, indigo, and violet. The wavelengths of each light are different, so the refractive index when passing through the lens is also different. In this way, a point on the object side may form a color spot on the image side. The main function of the optical system is achromatic. Chromatic aberration generally includes positional chromatic aberration and magnification chromatic aberration. Positional chromatic aberration causes the image to have color spots or halos when viewed at any position, making the image blurry. And magnification chromatic aberration causes the image to have colored edges.
2. Spherical aberration of metallographic microscope
Spherical aberration is a monochromatic phase difference at an on-axis point due to the spherical surface of the lens. The result of spherical aberration is that after a point is imaged, it is no longer a bright spot, but a bright spot with gradually blurred bright edges in the middle, thus affecting the imaging quality of the metallographic microscope.
3. Astigmatism
Astigmatism is also an off-axis point monochromatic aberration that affects the clarity of a metallographic microscope. When the field of view is large, the object point on the edge is far away from the optical axis, and the beam tilts greatly, causing astigmatism after passing through the lens. Astigmatism causes the original object point to become two separated and mutually perpendicular short lines after imaging. After integration on the ideal image plane, an elliptical spot is formed. Astigmatism is eliminated through complex lens combinations.
5. Coma
Coma is a monochromatic aberration at an off-axis point. When an off-axis object point is imaged with a large-aperture beam, after the emitted beam passes through the lens of the metallographic microscope and no longer intersects, the image of a light point will be in the shape of a comma, shaped like a comet, so it is called "coma aberration" ".
6. Field song
Field curvature is also called "curvature of field". When there is field curvature in a metallographic microscope lens, the intersection point of the entire light beam does not coincide with the ideal image point. Although a clear image point can be obtained at each specific point, the entire image plane is a curved surface. In this way, the entire image plane cannot be seen clearly during microscopic examination, which makes observation and photography difficult. Therefore, the objective lenses of metallographic microscopes are generally flat-field objectives, which have corrected field curvature.
