What are Catadioptric Sensors ?

R. Andrew Hicks
Department of Mathematics
Drexel University

Catadioptric sensors are imaging sensors built with combinations of mirrors (catoptrics), and lenses (dioptrics). The main advantage of using mirrors with cameras is that by using a curved mirror a wide field of view can be obtained.

The word "catadioptric" alone used to be used often to describe telescope designs that used mirrors. Here is the definition that Google gives, from the Astrosoc Website Glossary :

Catadioptric (also spelled catadioptic): Reflecting telescope, so-called because the beam of light is 'folded', i.e. reflected, back through a hole in the main mirror, before reaching the eyepiece. The effect is to increase the telescope's focal length, thus producing a more portable but also costlier instrument. Catadioptrics use a lens-like correcting plate in the front for spherical aberration. The commonest types are the Schmidt-Cassegrain and Maksutov-Cassegrain.

Euclid wrote a book, the Catoprica, which consists 31 propositions on mirrors (convex, concave and flat). The word catadioptric appears in the dictionary of difficult to pronounce words.

So a catadioptric may be a telescope, microscope, projector or an optical device not used for imaging at all. In computer vision, the term catadioptric sensor is used for sensors consisting of cameras and mirrors. Catadioptric sensors are sometimes also known as omnidirectional sensors , although this terminology is misleading, since they rarely are omnidirectional in the sense that they capture an image in all directions.

Generally, but not always, the mirrors are rotationally symmetric and convex, as depicted above. This type of catadioptric sensor is much like a fisheye lens, although people don't generally think of fisheye lenses as being panoramic. Probably the reason that catadioptric sensors have become popular is that in the last 10 years it has become easy to digitize and process images. At the right is an image I took with a catadioptric sensor at the November 2003 EPADEL MAA meeting. In this case the mirror was a paraboloid. This image can be "unwarped" in software to give what we usually think of as a panorama:

Catadioptric Sensor Designs by Andy Hicks
The Page of Catadioptric Sensor Design

Last modified Wed Jan 14 12:54:28 EST 2004