Introduction

Telecentricity is a unique property of certain multi-element lens designs in which the chief rays are collimated and parallel to the optical axis. A telecentric lens is a special optical lens that has its entrance or exit pupil, or both, at infinity. The size of images produced by a telecentric lens is insensitive to either the distance between an object being imaged and the lens, or the distance between the image plane and the lens, or both. This optical property is called telecentricity.

Telecentric Lenses

Telecentric lenses are used for precision optical two-dimensional measurements, reproduction (e.g., photolithography), and other applications that are sensitive to the image magnification or the angle of incidence of light. The simplest way to make a lens telecentric is to put the aperture stop at one of the lens’s focal points. Commercially available telecentric lenses are often compound lenses that include multiple lens elements, for improved optical performance. Telecentricity is not a property of the lenses inside the compound lens but is established by the location of the aperture stop in the lens.

Types of Telecentric Lenses

A telecentric lens can be object-space telecentric, image-space telecentric, or bi-telecentric (also double-telecentric). In an object-space telecentric lens the image size does not change with the object distance, and in an image-space telecentric lens the image size does not change with the image-side distance from the lens.

Benefits of Telecentric Lenses

The primary advantage of a telecentric lens is that its magnification does not change with respect to depth. Telecentric lenses eliminate the parallax error characteristic of conventional lenses by having a constant, non-angular field of view. At any distance from the lens, a telecentric lens will always have the same field of view. This results in high-accuracy measurements because their lack of parallax error makes objects appear the same size independent of their location.

Telecentric lenses eliminate the concern about measurement errors that would otherwise occur due to factors such as a vibrating conveyor or inexact part locations. Other benefits include reduced distortion, increased sharpness, improved color reproduction, and reduced vignetting (dark corners in an image).

Telecentric lenses, with their unique properties and benefits, play a crucial role in applications requiring high precision, such as machine vision systems, metrology, gauging, and microlithography inspection. They provide superior image performance when doing high precision measurement and inspection, making them an invaluable tool in these fields.