What are the factors that affect microscope resolution?
1. Color difference
Chromatic aberration is a serious defect in lens imaging. 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.
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
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 a bright center and gradually blurred edges. Thereby affecting the image quality.
The correction of spherical aberration is often eliminated by lens combination. Since the spherical aberration of convex and concave lenses is opposite, convex and concave lenses of different materials can be selected and glued together to eliminate it. In old model microscopes, the spherical aberration of the objective lens is not completely corrected, and it should be matched with the corresponding compensating eyepiece to achieve the correction effect. Generally, the spherical aberration of new microscopes is completely eliminated by the objective lens.
3. Coma
Coma is a monochromatic phase difference at off-axis points. When an off-axis object point is imaged by a large-aperture beam, after the emitted beams pass through the lens and no longer intersect at one point, the image of a light point will be in the shape of a comma, shaped like a comet, so it is called "coma".
4. Astigmatism
Astigmatism is also an off-axis point monochromatic phase difference that affects clarity. 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 being integrated on the ideal image plane, an elliptical spot is formed. Astigmatism is eliminated through complex lens combinations.
5. Field music
Field curvature is also called "curvature of field". When the lens has field curvature, 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 face cannot be seen clearly during microscopic examination, which makes observation and photography difficult. Therefore, the objectives of research microscopes are generally flat-field objectives, which have been corrected for field curvature.
6. Distortion
The various phase differences mentioned above, except field curvature, all affect the clarity of the image. Distortion is another type of phase difference in which the concentricity of the beam is not destroyed. Therefore, the clarity of the image is not affected, but the shape of the image is distorted compared with the original object.
(1) When the object is located beyond twice the focal length on the object side of the lens, a reduced inverted real image is formed within twice the focal length on the image side and outside the focus;
(2) When the object is located at twice the focal length of the object side of the lens, an inverted real image of the same size is formed at twice the focal length of the image side;
(3) When the object is located within two times the focal length of the object side of the lens but outside the focus, an enlarged inverted real image will be formed beyond two times the focal length of the image side;
(4) When the object is located at the object-side focus of the lens, the image side cannot be imaged;
(5) When the object is within the focus of the object side of the lens, no image is formed on the image side, and an enlarged upright virtual image is formed on the same side of the object side of the lens at a position farther than the object.
Resolution The resolution of a microscope refers to the minimum distance between two object points that can be clearly distinguished by the microscope, also known as the "discrimination rate". The calculation formula is σ=λ/NA, where σ is the minimum resolution distance; λ is the wavelength of light; NA is the numerical aperture of the objective lens. It can be seen that the resolution of the objective lens is determined by two factors: the NA value of the objective lens and the wavelength of the illumination source. The larger the NA value, the shorter the wavelength of the illumination light, the smaller the σ value, and the higher the resolution. To improve the resolution, that is, reduce the σ value, the following measures can be taken:
(1) Reduce the wavelength λ value and use short-wavelength light sources.
(2) Increase the medium n value to increase the NA value (NA=nsinu/2).
(3) Increase the aperture angle u value to increase the NA value.
(4) Increase the contrast between light and dark.
