Principle and Selection of Telecentric Lens

来源: 发布时间:2020-04-06 00:00:00 浏览量:3443

Telecentric lenses are designed to correct the parallax of traditional lenses. In a certain range of object distance, it can make the obtained image magnification not change. In short, the images taken by this lens have nothing to do with the distance. As high quality optical lens elements manufacturer for telecentric lenses, JUKA Optical bing you to understand the telecentric lenses and how to select it.


1. The principle of telecentric lenses


Telecentric lenses are divided into object-side telecentric lenses, image-side telecentric lenses, and double telecentric lenses. Below are real photos of two telecentric lenses. 


Regarding the principle of a telecentric lens, the core point is that a telecentric lens is a combination of a common lens and a small hole imaging principle. Here I try to illustrate the principle of a telecentric lens with three pictures.


1.1 Object-side telecentric lens


Light path of traditional lens:



If we place a small hole at the focal point of the image side, the light path becomes like this:



The purpose of this small hole is to allow only parallel incident object rays to reach the image plane. From the geometric relationship, it can be seen that the image has 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 an object-side telecentric lens is that it receives parallel light imaging, which is equivalent to an object at infinity.


The disadvantage of object-side telecentric lenses is that the magnification is directly related to the image distance. The distance of the camera in actual use will affect the magnification. Therefore, each lens system must be individually calibrated for the magnification.


1.2 Image-side telecentric lens

We know that the light path is reversible. Then if the light path of the object-side telecentric lens is reversed, it becomes the light path of the side telecentric lens.



 The characteristic of this lens is that the magnification is independent of the image distance. Whether the camera is far or near does not affect the magnification.


1.3 Double Telecentric Lenses

Combining the optical path of the center of the object-side telecentric lens and the image-side lens, it becomes a double telecentric lens. Below 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 pinhole aperture in an actual circle lens cannot be infinitely small, so the light coming in is too small. Therefore, the actual center of the lens will still have a certain near-large, far-small phenomenon (this index is called telecentricity, and the telecentricity of telecentric lenses is usually less than 0.1 °). The object distance is not arbitrary, but it has a much larger depth of field than a normal lens.



2. How to select telecentric lenses


2.1 Applcation range of telecentric lens

Under what circumstances should I choose a telecentric lens? According to the author's years of experience in machine vision product selection, I will give the reader some reference again. In the following cases, it is recommended to use a telecentric lens.


1) When the thickness of the detected object is large and more than one plane needs to be detected, typical applications such as food boxes, beverage bottles, etc.


2) When the placement of the measured object is uncertain, it may be at a certain angle with the lens.


3) When the measured object bounces up and down during the detection process, such as when the working distance changes due to vibrations on the production line.


4) When the measured object has an aperture or a three-dimensional object.


5) When a low distortion rate is required and the brightness of the image effect is almost completely consistent.


6) When the defects to be detected can only be detected under the same direction of parallel illumination.


7) When it is necessary to exceed the detection accuracy, if the allowable error is 1um.



2.2 Selection method of telecentric lens

The selection method of the telecentric lens is actually similar to the lens in the ordinary optical system.The following points need attention:


1) Compatible CCD target surface size. This point is similar to the selection of ordinary lenses, and requires that the CCD target surface compatible with the telecentric lens is greater than or equal to the supporting camera target surface, otherwise it will waste resolution.


2) Interface Type. At present, the types of interfaces provided by telecentric lenses are similar to ordinary lenses, with C and F ports, etc., as long as they are used with the camera.


3) Magnification, or imaging range. When the magnification and the 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 to be within a range, which is determined by its imaging optical path. What needs to be noticed is whether this working distance meets the requirements of actual use. When using a telecentric system for testing, we recommend that you firstly select the lens and design other mechanical structures based on its working distance.


5) Depth of field range. On the premise that the previous conditions of use are met, 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 a model.



3. FAQ of Telecentric Lenses


Q1: Why is the volume of bi-telecentric lenses usually larger?

A1: Because a bi-telecentric lens enters and exits in parallel light, it requires a large shooting area and a large area of parallel light to enter. Therefore, a large area of the lens barrel is required. So the size of a bi-telecentric lens is usually large and the field of view The larger, the larger the volume.


Q2: What kind of light source works better with a bi-telecentric lens?

A2: Since the telecentric lens only accepts parallel light, and it filters out almost all diffuse reflection light sources, the imaging is dark in the natural environment. Therefore, the use of a parallel light source can maximize the advantages of the bi-telecentric lens and make the edge of the measured object. It can make the edge of the measured object clear and stable, and effectively remove the noise during the detection process.

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