How does a telecentric lens work?

Answer 1

Telecentric lenses accept ray cones where the "principal ray" is parallel to its mechanical axis because the entrance pupil is placed at infinity. For this reason an orthonormal view of the object (i.e. where no image of the object sides is present) is frequently possible.

Standard lenses generate images of different size if a same object changes its distance from the lens. On the other hand objects of different size can be viewed as if they had the same dimension, if they subtend the same viewing angle. In a telecentric system rays get into the optics only with an almost parallel-to-the-axis path. This effect is due to the specific path of the rays: in the case of common optics the geometric information "parallel" to the main optical axis shows a component on the detector plane direction, while in a telecentric lens this perpendicular component is not present at all.

You can think as if common lenses would build a correspondence between the 3-dimensional object space and the 2-dimensional detector (image) space: in the case of a telecentric lens the third dimension in object space is not displayed. Some interesting information can be found reading this tutorial online: http://www.opto-engineering.com/telecentric.php

Answer 2

A telecentric lens is designed to ensure that light rays are parallel when they reach the image plane, producing images with consistent magnification regardless of the object distance. This design minimizes perspective distortion and simplifies measurements by providing accurate and distortion-free images. By maintaining parallel light rays, telecentric lenses are commonly used in machine vision systems for accurate measurement and inspection tasks.