Telecentric illuminators work by employing high-quality glass optical lenses to collimate light from a fiber optic light guide or LED spotlight. Divergent light from the source enters the glass assembly, becoming parallel and therefore highly concentrated as it exists. Nearly all light that enters the telecentric illuminator (neglecting back reflection and absorption through each optical lens) strikes the object under inspection. When combined with a LED pattern projector and a reticle, a standard telecentric imaging lens can be used as a telecentric illuminator.

As with a typical telecentric illuminator, light passing through the telecentric imaging lens is collimated, eliminating diffuse reflections when silhouetting an object. Unlike a telecentric illuminator, though, imperfections on the LED projector may be detected when using a telecentric imaging lens.