What are Catadioptric Sensors ?
R. Andrew Hicks
Department of Mathematics
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).
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