telecentric
Relating to a compound lens which has its entrance or exit pupil at infinity; in the prior case, this produces an orthographic view of the subject
telecentric: parallel light rays that ignore angle
A telecentric optical system produces an image in which all rays passing through the object space are parallel to the optical axis. This happens when the entrance pupil or exit pupil (or both) is positioned at infinity. The result is an orthographic projection, not a perspective one: an object's image size does not change as you move it closer to or farther from the lens, provided it stays in focus.
Telecentricity comes in two flavors. Object-space telecentric systems have the entrance pupil at infinity, making incoming light parallel; this erases perspective distortion on the object being viewed. Image-space telecentric systems have the exit pupil at infinity, producing parallel rays in the image space. Bi-telecentric systems achieve both simultaneously, though they demand more complex optical designs and are expensive.
Telecentricity is essential in machine vision for precision measurement and quality control. When a part's size must be measured or its edges detected consistently regardless of slight depth variations, perspective error becomes unacceptable. A telecentric camera does not enlarge an object if it moves 5 mm closer to the lens; a standard lens would. This property makes telecentricity standard in automotive inspection, PCB defect detection, and pharmaceutical tablet metrology.
The trade-off is aperture and light gathering. Telecentric lenses are slower (lower f-number) than conventional optics of similar magnification, require longer working distances for equivalent fields of view, and cost significantly more. The optical design must use additional corrective elements to achieve the pupil-at-infinity condition, adding weight and bulk.
In microscopy and machine vision, telecentricity is specified by numerical aperture and working distance, not focal length alone. A telecentric system designed for 1:1 magnification at 100 mm working distance cannot be repurposed for a different magnification without redesign. The term itself derives from the Greek tele (far) and centrum (center), referring to the pupil position rather than the optical behavior.