Telecentric lens, the concept of telecentric lens, is mainly to correct the parallax of traditional industrial lens. It can keep the magnification of the obtained image unchanged within a certain object distance, which is a very important application for the situation that the measured object is not on the same object plane. Telecentric lens has always been favored by machine vision applications that need high lens distortion because of its unique parallel optical path design.
The principle advantage of telecentric lens The purpose of telecentric lens design is to eliminate the magnification difference caused by the distance between the measured object (or CCD chip) and the lens. According to the design principle of telecentric lens classification, they are as follows:
1) Design principle and function of telecentric optical path;
The object telecentric optical path is called the object telecentric optical path, in which the aperture stop is placed on the image focal plane of the optical system, and the convergence center of the object main light parallel to the optical axis is located at the object infinity. Its function is to eliminate the reading error caused by inaccurate focusing.
2) Design principle and function of image telecentric optical path:
The image telecentric optical path is called the image telecentric optical path, in which the aperture stop is placed on the object focal plane of the optical system, and the convergence center of the image main light parallel to the optical axis is located at the image infinity. Its function is to eliminate the measurement error caused by inaccurate image focusing.
3) Design principle and function of telecentric optical path on both sides:
It integrates the dual functions of object/image. Mainly used in the field of visual measurement and detection.
How to choose telecentric lens machine vision has developed rapidly in China in recent years. Everyone has a certain understanding of the choice of ordinary lens in system integration, but the choice of telecentric lens is often at a loss. Even with the help of technicians, we still don't know what problems we need to pay attention to in the process of use. In order to solve this problem, the matching principle of telecentric lens and camera is the same as that of ordinary industrial lens, as long as its target surface specification is greater than or equal to that of camera. Please note that the area below the objective lens of telecentric lens is telecentric imaging, while the area beyond this range is not strictly telecentric imaging, so attention must be paid in actual use, otherwise unnecessary deviation will occur.
When choosing a telecentric lens, customers should first know when to choose a telecentric lens. According to the principle, characteristics and unique advantages of telecentric lens, it is best to choose telecentric lens when the inspected object encounters the following six situations:
1) When it is necessary to detect an object with thickness (thickness >); 110 FOV diameter);
2) When it is necessary to detect objects that are not in the same plane;
3) When the distance between the object and the lens is unclear;
4) When it is necessary to detect three-dimensional objects with apertures;
5) When low distortion is required and the brightness of image effect is almost the same;
6) When defects can only be detected under parallel illumination in the same direction.
When choosing a telecentric lens, first of all, we should understand the use conditions corresponding to the relevant indicators of telecentric lens:
1) object size-shooting range.
2) Image size-the target size of the CCD used.
3) Working distance-the distance from the front surface of the objective lens to the object.
4) Resolution-CCD pixel size used.
5) Depth of field-the range where the lens can form a clear image. The greater the magnification of the image/object, the smaller the depth of field.
6) Interface-Camera interface, mostly C and T interfaces.
According to the use situation (object size and required resolution), select the objective lens and CCD or CMOS camera with appropriate object size, and get the image size at the same time, then calculate the magnification, and then select the appropriate image lens according to the product list. Attention should also be paid to the influence of depth of field index in the selection process, because the greater the magnification of image/object, the smaller the depth of field, and it may be necessary to re-select the lens to get the appropriate depth of field.
Comparison between telecentric lens and ordinary lens telecentric industrial lens is a lens specially designed to correct the parallax of traditional industrial lens. It can make the magnification of the obtained image not change with the change of the object distance within a certain object distance, which is a very important application when the measured object is not in the same object plane.
The closer the target object is to the lens (the shorter the working distance), the larger the image will be. When using ordinary lens for size measurement, the following problems will occur:
1) Because the measured object is not in the same measuring plane, the magnification is different;
2) large lens distortion
3) Parallax means that when the distance between objects increases, the magnification of objects also changes;
4) The resolution of the lens is not high;
5) the uncertainty of image edge position caused by the geometric characteristics of visual light source.
Telecentric lens can effectively solve the above problems of ordinary lens, and there is no judgment error of this nature, so it can be used for high-precision measurement and metrology. Telecentric lens is a kind of high-end industrial lens, which usually has superior imaging quality and is especially suitable for the application of size measurement.
No matter where you are, you will have the same magnification after refocusing at a specific working distance, because the maximum field of view of the telecentric lens is directly related to the proximity of the lens diaphragm. The larger the lens size, the greater the required field of view. Telecentric measuring lens can provide superior image quality with less distortion than traditional fixed-focus lens. This optical design makes the image plane more symmetrical and can be measured accurately by software.
Advantages of ordinary lenses: low cost, practicality and wide use.
Disadvantages of ordinary lens: the magnification will change and there will be parallax.
Ordinary lens application: imaging of large objects.
Advantages of telecentric lens: the magnification is constant, does not change with the change of depth of field, and ignores the difference.
Disadvantages of telecentric lens: high cost, large volume and heavy weight.