The traditional video camera with its limited field of view makes it difficult for use in some of the applications in the field of computational vision. Catadioptric sensors are proving to be an apt solution for this issue.
Catadioptric sensors are imaging sensors that are designed to capture a wider field of view using a combination of curved mirrors and traditional lenses. They are also referred to as panoramic or omni-directional sensors.
The term ‘catadioptric’ or ‘catadioptic’ was derived from curved mirrors. Any device that uses these two combinations is termed 'catadioptrics'. For example, catadioptric telescopes are fitted with a lens-like correcting plate at the front surface to provide spherical aberration. The Schmidt-Cassegrain and Maksutov-Cassegrain telescopes are prominent catadioptric telescope types. Thus, a catadioptric device can be non-imaging devices such as telescopes, projectors, microscopes, or optical devices.
Over the last decade, catadioptric sensors have become popular as they are simple and enable easy digitization and processing of images.
Image credit: Photos.com.
Types and Operation of Catadioptric Sensors
The lenses and mirrors in the catadioptric sensors have to be meticulously configured so as to capture a much wider field of view. The shape of the mirrors is crucial in designing a catadioptric sensor that provides an effective single viewpoint. All of the captured rays have to pass through the single viewpoint, thereby ensuring distortion-free reconstruction of an image.
Catadioptric devices are grouped according to single viewpoint constraints.
The first group consists of catadioptric devices using spherical-shaped mirrors that do not conform to the single viewpoint constraint, but are preferred because they are less complex compared to manufacture and have advantages such as compact size and equal characteristics over azimuth. It is, however, difficult to reconstruct an image with this type of system.
The second group consists of catadioptric devices containing mirrors that conform to single viewpoint constraints. The shapes of these mirrors are parabola, hyperbola, and ellipse. For example, a parabola mirror should be used in conjunction with orthographic lens so as to achieve capturing an image using the single viewpoint property.
Apart from their application in the field of computational vision and robotics, catadioptric sensors are also used in the following:
- Military – For mobile robots
- In surveillance and video security systems
- Egomotion estimation and object tracking,
- Teleconferencing and advertising
- Model acquisition for virtual reality
- Unknown environment exploration and tourism.
Catadioptric optics-based combination systems are also customizable for application in headlamps, microscopes, early lighthouse focusing systems, search lights, optical telescopes, and telephoto lenses.