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This article is transferred from: New Machine Vision
1. Introduction and selection of telecentric lens principle:
1) Principle of telecentric lens :
Telecentric lens is designed to correct the parallax of traditional lens. It can make the magnification of the obtained image not change within a certain object distance range. far-flung relationship.
Telecentric lenses are further divided into object-side telecentric lenses, image-side telecentric lenses and bilateral telecentric lenses. Below are actual photos of the two telecentric lenses.
Regarding the principle of telecentric lens, there are some introductions on the Internet, but the writing is very obscure. In fact, it can be made clear in a few words. The core point is that the telecentric lens is a combination of ordinary lenses and the principle of pinhole imaging.
This article tries to use three diagrams to illustrate the principle of telecentric lenses.
1. Telecentric lens
Optical path of traditional lens:
If we put a small hole at the focal point of the square, the light path becomes like this:
The function of this small hole is to allow only parallel incident light rays from the object side to achieve image plane imaging. From the geometric relationship, it can be seen that there is no relationship between near and far. The principle of the object-side telecentric lens is as simple as that. The reason why it is called object-side telecentricity is that it receives parallel light imaging, which is equivalent to the object being at infinity.
The disadvantage of object-side telecentric lenses is that magnification is directly related to image distance. In actual use, the distance the camera is installed will affect the magnification. Therefore, each lens system must be individually calibrated for magnification.
2. Image-space telecentric lens
We know that the optical path is reversible, so the optical path of the object-side telecentric lens is reversed to become the optical path of the image-side telecentric lens.
The characteristic of this lens is that the magnification has nothing to do with the image distance, and the magnification is not affected by whether the camera is far or near.
3. Double telecentric lens (double telecentric, bi-telecentric)
Combining the optical path of the object-side telecentricity and the image-square-circle center becomes a bilateral telecentric lens. Here is the light path:
The characteristic of this lens is that the distance of the object or the distance of the camera does not affect the magnification. Therefore, it is widely used in the field of machine vision measurement and inspection.
Of course, the aperture diaphragm in the actual center lens cannot be infinitely small, so the incoming light will be too small. Therefore, the actual center lens will still have a certain relationship between near and far (this index is called telecentricity, and the telecentricity of a telecentric lens is usually less than 0.1°). Object distance isn't arbitrary either, but it's much larger than a normal lens' depth of field.
2) Selection method of telecentric lens :
The selection method of the telecentric lens is actually similar to the lens in the ordinary optical system. The points that need to be paid attention to are as follows:
①.Compatible CCD target surface size
This is similar to the selection of ordinary lenses. The CCD target surface compatible with the telecentric lens is required to be larger than or equal to the matching camera target surface, otherwise the resolution will be wasted.
②. Interface type
At present, the type of interface provided by the telecentric lens is similar to that of the ordinary lens, including C-mount, F-mount, etc., which can be used as long as it is matched with the camera.
③. Magnification, or imaging range
When the magnification and CCD target surface are determined, the imaging range is determined, and vice versa.
④. Working distance
Generally, when the above three points are selected, the working distance has been determined within a range, which is determined by its imaging optical path. What needs to be paid attention to is whether this working distance meets the actual use requirements. When choosing a telecentric system for inspection, we recommend selecting a lens first and designing other mechanical structures according to its working distance.
⑤. Depth of field range
Under the premise of meeting the previous conditions of use, the larger the depth of field range, the better the optical characteristics of the telecentric system, which can be used as a reference when selecting models.
2. Principle and selection of bi-telecentric lens
In recent years, companies that often do machine vision precision measurement will hear some relatively new terms, such as bilateral telecentricity, unilateral telecentricity, object-side telecentricity, and square telecentricity, etc. These are not often mentioned before. The optical concept of the lens is confusing, and I don’t know how to understand it. The collected data are often highly specialized and difficult to understand. Today, I will briefly describe the related principles of bi-telecentric industrial lenses from the perspective of practical application.
1) The principle of double telecentric lens and the problems that can be solved :
1. The principle of convex lens imaging
Feature 1: All light passing through the optical center does not change its propagation direction
Feature 2: The convex lens has a converging effect on parallel light, and the imaging of the lens takes advantage of this
2. Imaging principle of double telecentric lens
Principle: By placing a diaphragm in the middle of the lens, the light entering and leaving the lens is parallel light, and other light is blocked by the diaphragm and cannot reach the imaging chip. The object side solves the problem of depth of field, and the image side solves the problem of magnification change.
3. Problems solved by bi-telecentric lens
①. Resolution problem:
The resolution of ordinary industrial lenses cannot keep up with the improvement of chip resolution. It is limited by the principle of its optical imaging. The best can only be about 10um, and it can be used with cameras with 1000W pixels at most, which cannot meet the current high resolution. Cameras and high-precision measurement inspection requirements.
②. Depth of field problem:
The depth of field of ordinary lenses is relatively small. When the object to be measured exceeds its range in the depth direction of the lens, detection or measurement cannot be performed.
③.Magnification problem:
The magnification varies with the working distance. This characteristic can lead to intolerable errors when our vision systems are used to perform precision measurement tasks.
2) Selection method of double telecentric lens :
A: Mainly pay attention to the following points: field of view, compatible CCD target surface, interface type, etc. meet the requirements, other such as working distance, depth of field range, external dimensions, etc., as long as it does not affect the use.
3) Frequently asked questions about bi-telecentric lenses:
Q: Why is the volume of the bi-telecentric lens usually relatively large?
A: Because the bi-telecentric lens is parallel light in and out, the larger the shooting area, the larger the area of the parallel light is required to enter, so the larger area of the lens barrel is required, so the bi-telecentric lens is usually larger in size and has a larger field of view. The bigger the volume, the bigger the volume.
Q: What kind of light source is better with the bi-telecentric lens?
A: Since the telecentric lens only accepts parallel light and filters out almost all diffuse reflection light sources, the image is relatively dark in the natural environment. Therefore, the use of parallel light sources can maximize the advantages of the double telecentric lens and make the edge of the object to be measured. Clear, stable, and effectively remove noise during detection.
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