Factors Affecting the Quality of Microscopic Imaging--Microscope Lens

Microscope lenses are of different types, but even for the same type of lens, the imaging quality is very different, mainly due to factors such as material, processing accuracy and lens structure, and also leads to different grades of lens prices. A huge difference from a few hundred yuan to several tens of thousands. More famous such as four three-piece Tiansei lens, six four-group double Gauss lens. For the lens design and manufacturer, the optical transfer function OTF (Optical Transfer Function) is generally used to comprehensively evaluate the lens imaging quality. The optical system transmits the information along the spatial distribution of the brightness. The optical system is transmitted when transmitting the information of the subject. The sinusoidal signal of each spatial frequency, the degree of modulation and the change of phase in the actual image, are all functions of spatial frequency. This function is called the optical transfer function. The OTF is generally composed of a modulation transfer function MTF (Modulation Transfer Function) and a phase transfer function PTF (Phase Transfer Function). Aberration is an important aspect that affects image quality. Common aberrations are as follows:

Ball difference:

A monochromatic conical beam emitted from an object point on the main axis to the optical system is refracted by the optical series. If the original beam has different angles of light, it cannot be placed 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 a blur circle) is formed, and the imaging error of the optical system is called a spherical aberration.

Hypothesis:

A monochromatic conical beam emitted from an off-axis object located outside the main axis to the optical system, after being refracted by the optical series, cannot form a clear point at the ideal image plane, but is formed by dragging a bright tail The illuminating star spot, the imaging error of this optical system is called coma.

Astigmatism:

An oblique monochromatic conical beam emitted from an off-axis object located outside the main axis to the optical system, after being refracted by the optical series, cannot form a clear image point, but can only form a diffuse spot. The imaging error of the optical system is called astigmatism.

Field music:

A clear image formed by a plane object perpendicular to the main axis through the optical system, if not in a plane perpendicular to the image plane of the main axis, and on a curved surface symmetrical on the main axis, that is, the best image plane is a curved surface, then The imaging error of the optical system is called field curvature. When the image is adjusted to the center of the screen, the image around the screen is blurred. When the image is blurred until the image around the screen is clear, the image at the center of the screen begins to blur.

Color difference:

A white object emits a white light to the optical system. After being refracted by the optical system, the light of each color cannot converge on one point, and a color image spot is formed, which is called chromatic aberration. The reason for the chromatic aberration is that the same optical glass has different refractive indices for light of different wavelengths, the short-wave light has a large refractive index, and the long-wave light has a small refractive index.

distortion:

The straight line outside the main axis in the object plane becomes a curve after being imaged by the optical system, and the imaging error of this optical system is called distortion. Distortion aberrations only affect the geometry of the image without affecting the sharpness of the image. This is the fundamental difference between distortion and spherical aberration, coma, astigmatism, and field music.

When evaluating lens quality, we usually judge several practical parameters such as resolution, sharpness and depth of field.

Resolution:

Also known as discrimination rate, resolution, refers to the ability of the lens to clearly distinguish the details of the fiber of the scene, the reason for restricting the resolution of the lens is the diffraction phenomenon of light, that is, the diffraction spot (Eryer spot). The unit of resolution is line pair / mm.

Acutance:

Contrast, also known as contrast, refers to the contrast of the brightest and darkest parts of the image.

Depth of Field (DOF):

In the scene space, the scene located within a certain distance from the plane of the focus object can also form a relatively clear image. The depth distance between the above-mentioned scenes that can form a relatively clear image before and after the focus 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.

Maximum relative aperture and aperture factor:

Relative aperture refers to the ratio of the incident aperture diameter (indicated by D) to the focal length (indicated by f) of the lens, ie relative aperture = D / f. The reciprocal of the relative aperture is called the aperture scale, also known as the f/aperture coefficient or the aperture number. The relative aperture of a typical lens is adjustable, and its maximum relative aperture or aperture factor is often indicated on the lens, such as 1:1.2 or f/1.2. If the scene is dark or the exposure time is short, you need to choose the lens with the largest relative aperture.

The interaction between the parameters of the lens

A good lens has a good reflection in terms of resolution, sharpness, depth of field, etc. It is also better for correcting various aberrations, but at the same time its price will increase several times or even hundreds of times. If we have some rules and experience, we can use the same grade lens to achieve better results.

1. The effect of the focal length

The smaller the focal length, the greater the depth of field;

The smaller the focal length, the larger the distortion;

The smaller the focal length, the more severe the vignetting phenomenon, and the illumination of the aberration edge is reduced;

2. Influence of aperture size The larger the aperture, the higher the brightness of the image;

The larger the aperture, the smaller the depth of field;

The larger the aperture, the higher the resolution;

3. Center and edge of the field

Generally, the image center has higher resolution than the edge.

Generally, the image center has higher illumination than the edge light field.

4. Effect of light length

Under the same camera and lens parameters, the shorter the wavelength of the light source of the illumination source, the higher the resolution of the resulting image. Therefore, in a vision system that requires precise size and position measurement, short-wavelength monochromatic light is used as an illumination source as much as possible, which has a great effect on improving system accuracy.