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With the continuous development of machine vision, the technology of lens selection is becoming more and more important. Many people will encounter various difficulties in the selection of lens and camera. This article introduces the basic content of lens selection.
1. The principle of lens imaging
The principle of convex lens imaging is based on the lens combination, and the reflected light from the object is imaged on the plane (coinciding with the chip surface).
2. Introduction of technical indicators
1. Focus/focal length
Focal length is a measure of the concentration or divergence of light in an optical system, and refers to the distance from the optical center of the lens to the focal point of light concentration when parallel light is incident [1]. An optical system with a short focal length has a better ability to gather light than an optical system with a long focal length. Simply put, the focal length is the distance from the focal point to the center point of the mirror. In the camera, the focal length f<image distance<2f can form an image.
2. Aperture
Inside the lens, there is a polygonal or circular hole-like grating device with a variable area. This device is called the aperture. The aperture controls the light flux of the lens and the aperture coefficient describes its size. The aperture factor refers to the ratio of the focal length f' of the lens to the diameter D of the entrance pupil of the entire lens, usually represented by f/#. The formula f/#=f'/D. The smaller the f/#, the larger the aperture. Generally, the value of f/# increases by root sign 2, and the area is doubled.
3. Working distance
When the lens is in focus, the distance between the measured object and the front end of the lens.
4. Field of view / field of view
WD=, f'<<, field of view >>, FOV>>; f'=, field of view ==, WD>>, FOV>>; (
under the same working distance, the shorter the focal length, the shorter the field of view The larger the lens, the larger the field of view; the same focal length lens, the field of view is fixed, the farther the working distance is, the larger the field of view)
5. Magnification
The ratio of the size of the image to the size of the object, the calculation formula β=sensor size/field of view size.
Once the camera resolution and pixel size are determined, the sensor size can be calculated; the
sensor size divided by the field of view (FOV) is equal to the optical magnification.
β = CCD / FOV (The ratio of the size of the image to the size of the object, the calculation formula β = sensor size / field of view size.)
6. Resolution
Resolution is a key indicator of the ability of an imaging system to reproduce the details of an object. Units are line pairs per millimeter (lp/mm).
Lens matching pixel size (um) = {(1000um/mm)} / {2*lens resolution (lp/mm)} When selecting a model, it is recommended that the
lens matching pixel size <= camera pixel
size Resolution (um) (the characteristic size of the smallest object that the system can distinguish) (resolution accuracy) = sensor pixel size (um) / magnification
7. Depth of field (small depth of field, clearer)
The depth of the object space that can obtain a clear image on the image plane
The main factors affecting the depth of field (1. Lens aperture 2. Lens focal length 3. Shooting distance)
1. The larger the aperture, the smaller the depth of field; and vice versa; (aperture ( F)>>, depth of field <<; aperture (F)<<, depth of field>>)
2. The longer the focal length of the lens, the smaller the depth of field; and vice versa; ( f'>>, depth of field<<; f'<<, Depth of field>>)
3. The farther the shooting distance, the greater the depth of field (WD>>, depth of field>>; WD<<, depth of field<<）
8. Distortion
Distortion: Due to the different magnifications of the lens in different fields of view, the image loses its similarity with respect to the object. This aberration of image deformation is called distortion.
Pincushion distortion (concave), barrel distortion (convex)
9. TV distortion
Visual Distortion Metrics of Images
10. Back intercept
The back focus Fb generally refers to the distance from the center of the last surface of the lens group to the focal point, and the distance from the last surface of the lens or optical system to the image plane is the optical back focus (BFL). Flange surface:
method If the flange refers to the flange, it is the connecting fitting of the metal pipe. It is welded vertically at the end of the pipe. After the two flange faces are aligned, they are connected together with bolts and nuts, and the pipe is connected together. The corresponding lens is mechanical, and the circular surface is a little larger and one size above the optical back focus
Flange distance: the distance from the lens flange surface to the image plane (chip)
Mechanical back intercept: the distance from the last mechanical surface of the lens to the image plane Optical
back focus: the distance from the vertex of the mirror surface at the end of the lens to the image plane
3. Lens classification
Lens classification
Function: fixed focus lens (normal), zoom lens (normal), fixed aperture lens
Use: telecentric lens FA lens line scan lens macro lens (micro lens)
angle of view: ordinary lens wide-angle lens telephoto lens focal
length: short focal length, medium focal length, long focal length
Common industrial lenses, fixed focus lenses, telecentric lenses. Fixed-focus lens, as the name suggests, has a fixed focal length; telecentric lens (to overcome parallax, generally high precision)
4. Lens selection
1. Selection ideas
In the machine vision system, the main function of the lens is to image the workpiece onto the camera chip, so the selection of the lens will directly affect the overall performance of the machine vision system. Generally, you can choose the model according to the following ideas.
2. Points to note
a. Focus ring and optical interface
The structural part that adjusts the relative position of the lens group or the back focus of the optical system to make the image clear is called the focus ring (focus ring). The optical
interface refers to the interface between the lens and the camera. The optical interfaces commonly used in the industry have formed general specifications, such as c-port, cs-port, F-port, K-port
b. Maximum compatible camera chip size
The maximum compatible camera chip size refers to the maximum clear imaging range that the lens can support. When actually selecting a camera and lens, it should be noted that the maximum compatible chip size of the selected lens must be greater than or equal to the size of the selected camera chip.
3. Selection process
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