Six types of aberrations in microscopes
Microscope imaging is affected by various aberrations. The main optical component of the microscope is the objective lens, and there are various types of objective lenses, such as achromatic objectives, plan objectives, etc. For example, the achromatic objective lens is used to eliminate chromatic aberration, and the flat field objective lens is used to eliminate field curvature. The following will introduce the general aberrations in microscopes
Chromatic aberration
Occurs when polychromatic light is the light source, and monochromatic light does not produce chromatic aberration.
White light is composed of seven kinds of red, orange, yellow, green, blue, blue, and purple. 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.
Elimination method:
Using monochromatic light (adding filters), optical design eliminates
Chromatic aberration
Spherical aberration
Spherical aberration is the monochromatic aberration of an on-axis point and is 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 the middle bright edge gradually blurred. This affects the image quality.
Elimination method:
Use a combination of convex and concave lenses
spherical aberration
Coma
Coma is a monochromatic aberration of off-axis points. When the off-axis object point is imaged with a large-aperture beam, the emitted beams do not intersect a point after passing through the lens, and the image of a light spot will get a strong comma, shaped like a comet, so it is called "coma".
Elimination method:
Use Axial Parallel Light
coma
Astigmatism
Astigmatism is also an off-axis point monochromatic aberration that affects sharpness. When the field of view is large, the object point on the edge is far away from the optical axis, and the beam is inclined greatly, which causes astigmatism after passing through the lens. Astigmatism makes the original object point become two separate and mutually perpendicular short lines after imaging, and after synthesis on the ideal image plane, an elliptical spot is formed.
Elimination method:
Eliminated by complex lens combinations.
Curvature of field
"Elephant Field Bending". When the lens has field curvature, the intersection of the entire 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 phase cannot be seen clearly during the microscopic examination, which makes observation and photography difficult.
The objectives of research microscopes are generally flat field objectives, which have corrected the field curvature.
Field curvature
Distortion
In addition to the field curvature, the various aberrations mentioned above all affect the clarity of the image. Distortion is another property of phase difference where the concentricity of the beam is not destroyed. Therefore, the sharpness of the image is not affected, but the image is distorted in shape compared to the original object.
