A telecentric lens is a compound lens that has its entrance or exit pupil set to infinity. In the case of the entrance pupil, a rectangular (orthogonal or also parallel) image of the object is created. This means that the main rays (oblique rays that pass through the center of the aperture diaphragm) are parallel to the optical axis in front of or behind the system. These lenses always guarantee us the same size of the object, negligible distortion of the image in the direction from the center to the edges. And we also avoid the distortion given by the perspective. Therefore, they are suitable for very accurate measurements necessary in machine vision.
Only parallel rays pass through the lens (aperture located in its focus). The image therefore loses perspective, it is a so-called parallel projection. But this also has its disadvantages. As a side effect, the aperture providing telecentricity has a significant decrease in luminosity. The inspected object must therefore be adequately lit. It is also important that this inspected object must always be smaller than the lens diameter. For example, if we want to measure the dimensions of a square component with dimensions of 50×50 mm, try to have an optic diameter larger than this component. It is therefore necessary to use at least optics with a size of 72 mm. Or larger in order to really see the whole object – it has 71 mm diagonally.
Type of lenses
The lens can use telecentricity on the subject side, the camera, or even on both sides. The combined lens is called a bi-telecentric.
Telecentricity in the object area is used for accurate measurements. Telecentricity on the camera side is used in cases. And the camera sensor is sensitive to the angle of incidence of the beam. 3-CCD cameras are a typical case.
Telecentric lenses always guarantee us the same object size. In other words, if the subject is in the depth of field of the lens (we can see the subject sharply), it is always the same size.
A bi-telecentric lens can work in the same cases as a telecentric lens in the subject area, ie for accurate measurements. However, a bi-telecentric lens has several advantages. Due to the fact that the rays are parallel on both sides of the lens (a telecentric lens can only deflect the rays in the subject area), the bi-telecentric lens has a much more constant image (there are smaller variations in the size of the subject). Another great advantage is the higher contrast of the resulting image.
Example
To give an example from practice, a bi-telecentric lens was used. We put it in a device designed for inspection in the production of length gauges. Try to compensate for the fact that the meter was never set in the same direction towards the camera. It always depended on how well the operator managed to place him well in the inspection position. The constant size of the object, regardless of the distance from the camera, is then perfect here. The high accuracy of the bi-telecentric lens was also useful because this instrument served here as a verification device for the quality of accuracy class III meters. The high accuracy and reliability of the system is therefore a must, and the bi-telecentric lens has significantly helped.
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