Telecentric lens
Definition: A telecentric lens is a type of optical system that places its entrance pupil or exit pupil at infinity. It is designed to correct the parallax of traditional lenses. It can keep the magnification of the obtained image within a certain range of object distance .
Telecentric means that the entrance (or exit) pupil is at infinity, corresponding to it, and near center means that the entrance (or exit) pupil is inside the lens. The core point of telecentric lens imaging principle is the combination of traditional lens and pinhole imaging principle. Telecentric lenses are divided into object-space telecentric lenses, image-space telecentric lenses, and bi-telecentric lenses . The picture below is the physical picture of the telecentric lens.
The following figure
Left : Internal splines of a cylindrical object captured with a standard lens (top) and a telecentric lens (bottom).
Right : Two identical machine screws (100 mm apart) taken with a standard lens (top) and a telecentric lens (bottom), respectively.
When the object remains within a certain range, the size of the image obtained by the telecentric lens will not change with the displacement of the object. This range is usually called "depth of field" or "telecentric range". From the above analysis of the telecentric optical path, it can be seen that only the light emitted from the object side and incident parallel to the optical axis of the lens can be detected by the telecentric lens. Therefore, the diameter of the front facet of the telecentric lens must be at least as large as the diagonal of the object field of view , and therefore, compared to traditional optical lenses, the telecentric lens is larger, heavier and more expensive.
In addition, since the telecentric lens only receives parallel light and filters out almost all diffuse reflection light sources, the image is dark in the natural environment, so the selection of parallel light sources (that is, coaxial lighting) can maximize the advantages of the bi-telecentric lens. Make the edge of the measured object clear and stable, and effectively remove the noise in the imaging process.
main points
1: Coaxial fill light is required
2: How big is the object, how big is the lens