Microscope Lens Parameters and Their Influence on Microscopic Imaging Quality
There are different types of optical microscope lenses, but even for the same type of lens, the imaging quality is very different, which is mainly due to factors such as material, processing accuracy, and lens structure, and also leads to different grades of lenses. The price varies greatly from a few hundred yuan to tens of thousands of yuan. The most popular ones are four-piece three-group Tiansai lens and six-piece four-group double Gauss lens. For lens design and manufacturers, the optical transfer function OTF (Optical Transfer Function) is generally used to comprehensively evaluate the imaging quality of the lens. The optical system transmits information about the distribution of brightness along the space. The sine wave signal of each spatial frequency, its modulation degree and phase change when forming an actual image, are all functions of the spatial frequency, and this function is called the optical transfer function. OTF generally consists of two parts, the modulation transfer function MTF (Modulation Transfer Function) and the phase transfer function PTF (Phase Transfer Function ). Aberration is an important aspect that affects image quality. There are six common aberrations as follows:
Ball difference:
The monochromatic conical beam emitted from a certain object point on the main axis to the optical system, after being refracted by the optical series, if the rays of different aperture angles of the original beam cannot intersect at the same position on the main axis, so that the ideal image on the main axis At the plane, a diffuse spot (commonly known as the circle of confusion) is formed, and the imaging error of this optical system is called spherical aberration.
coma:
The monochromatic conical light beam emitted to the optical system from an off-axis object point located outside the main axis, after being refracted by the optical series, if it cannot form a clear point at the ideal image plane, it will form a bright tail Comet-shaped spot, the imaging error of this optical system is called coma.
Astigmatism:
The oblique monochromatic conical light beam sent to the optical system by an off-axis object point located outside the main axis cannot form a clear image point after being refracted by the optical series, but can only form a diffuse spot. The imaging error of an optical system is called astigmatism.
Scene:
If the clear image formed by the optical system of a plane object perpendicular to the main axis is not in an image plane perpendicular to the main axis, but on a curved surface symmetrical to the main axis, that is, the best image plane is a curved surface, then this The imaging error of an optical system is called field curvature. When the image at the center of the screen is clear, the images around the screen are blurred; and when the image at the center of the screen is clear, the image at the center of the screen begins to blur again.
Color difference:
A beam of white light is sent from a white object to the optical system. After being refracted by the optical system, the various colors of light cannot converge at one point, and a colored image spot is formed, which is called chromatic aberration. The reason for chromatic aberration is that the same optical glass has different refractive indices for different wavelengths of light, the refractive index of short-wave light is large, and the refractive index of long-wave light is small.
distortion:
The straight line outside the main axis in the plane of the subject becomes a curve after being imaged by the optical system, and the imaging error of the optical system is called distortion. Distortion aberrations only affect the geometry of the image, not its sharpness. This is the fundamental difference between distortion and spherical aberration, coma, astigmatism, and curvature of field.
When we evaluate the quality of the lens, we generally judge from several practical parameters such as resolution, sharpness and depth of field.
Resolution:
Also known as discrimination rate and resolution, it refers to the ability of the lens to clearly distinguish the fiber details of the scene being photographed. The reason that restricts the resolution of the lens is the diffraction phenomenon of light, that is, the diffraction spot (Airy spot). The unit of resolution is line pair/mm.
Sharpness (Acutance):
Also known as contrast, it refers to the contrast between the brightest and darkest parts of an image.
Depth of Field (DOF):
In the scene space, the scenes located within a certain distance before and after the focusing object plane can also form a relatively clear image. The above-mentioned depth distance between the scenes that can form a relatively clear image before and after the focusing object plane, that is, the depth range of the scene space that can obtain a relatively clear image on the actual image plane, is called the depth of field.
