Glossary · Machine vision
Telecentric lens
Also known as: Telecentric objective
German: Telezentrisches Objektiv
In optics for machine vision, a telecentric lens is a lens whose chief rays are parallel to the optical axis on the object side, so magnification stays constant within a defined distance range and perspective distortion is eliminated. The front lens must be at least as large as the field of view.
- Machine vision
In one sentence
A telecentric lens keeps magnification constant over a depth range and removes perspective, making it ideal for precise measurement.
Example
A telecentric lens measures the length of screws accurately even though they lie at slightly different heights on the conveyor.
How it applies
- Engineering: Telecentric lenses are used for dimensional measurement, especially with telecentric Backlighting. They are large, heavy and expensive, and have a fixed working distance and field of view. Image-side telecentric designs additionally keep light incidence on the sensor uniform, which improves brightness evenness.
- Commissioning: The object must stay within the telecentric range; outside it, magnification errors return.
- Documentation: The documentation team should document lens type, fixed working distance and field of view, and state that replacing it with a standard lens invalidates the measurement.
Telecentric lens vs. entocentric lens
A standard (entocentric) lens shows closer objects larger, so height variations cause measurement errors. A telecentric lens shows objects at the same size regardless of distance within its range.