The Key Factor of Microscope Influencing Imaging - Aberration
Due to objective conditions, no optical system can generate theoretically ideal images, and the existence of various aberrations affects the imaging quality. Below is a brief introduction to various aberrations.
1. Color difference is a serious defect in lens imaging, which occurs when polychromatic light is used as the light source, and monochromatic light does not produce color difference. White light is composed of seven types: red, orange, yellow, green, blue, blue, and purple. Each type of light has different wavelengths, so its refractive index when passing through a lens is also different. This way, a point in the object may form a color spot in the image. The main function of an optical system is achromatic.
Color difference generally includes positional color difference and magnification color difference. The positional color difference causes the image to have color spots or halos when observed at any position, causing the image to become blurry. And the magnification chromatic aberration causes the image to have colored edges.
2. Spherical aberration is the monochromatic aberration of points on the axis, caused by 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 middle edges, which affects the imaging quality.
The correction of spherical aberration often uses lens combinations to eliminate it. As the spherical aberration of convex and concave lenses is opposite, different materials of convex and concave lenses can be selected to be glued together to eliminate it. The spherical aberration of the objective lens in the old model microscope was not completely corrected, and it should be matched with the corresponding compensating eyepiece to achieve the correction effect. The spherical aberration of general new microscopes is completely eliminated by the objective lens.
3. The aberration belongs to the monochromatic aberration of an off axis point. When an off-axis object is imaged with a large-aperture beam, the emitted beam passes through the lens and no longer intersects at a point. The image of a light point will be in the shape of a comma, resembling a comet, hence the term "coma".
4. Astigmatism Astigmatism is also an off axis monochromatic aberration that affects clarity. When the field of view is large, the object points on the edge are far 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 separate and perpendicular short lines after imaging, which are combined on the ideal image plane to form an elliptical spot. Astigmatism is eliminated through complex lens combinations.
5. Field bending, also known as "image field bending". When there is field curvature in the lens, the intersection point of the entire beam does not coincide with the ideal image point. Although clear image points can be obtained at each specific point, the entire image plane is a curved surface. This prevents the entire image surface from being clearly visible during microscopic examination, making it difficult to observe and take photos. Therefore, the objective lenses used for studying microscopes are generally flat field objectives, which have already corrected the field curvature.
6. The various aberrations mentioned earlier, except for the field curvature, all affect the clarity of the image. Distortion is another type of aberration, where the concentricity of the beam is not disrupted. Therefore, it does not affect the clarity of the image, but causes distortion in shape compared to the original object.
