Imaging optics in the display engine are usually based on traditional free space optics but in compact form, including typically polarisation beam cubes (PBS) combined with catadioptric
birdbath optical architectures  to fold the optical path in various directions.
Image-based control of mobile Robot, with Central Catadioptric
Cameras, in: Proceedings of the IEEE International Conference on Robotics and Automation, ICRA'05, Barcelona, Spain, 3533-3538.
The approach, called a catadioptric
optical system, would enable Apple glasses to project very large virtual content from a very small projector.
* Don't try to guide a large reflector or catadioptric
telescope with a piggyback guidescope--use an off-axis (or on-axis) guider whenever possible.
Ren, "Distortion invariant joint-feature for visual tracking in catadioptric
omnidirectional vision," in Proc.
They can be composed of several cameras pointing towards different directions  or a unique camera and a reflective surface (catadioptric
vision systems) .
Compound, or "catadioptric
," telescopes use both lenses and mirrors to create a compact unit, such as the Schmidt-Cassegrain and Maksutov-Cassegrain designs.
Recent years have seen considerable progress in research on the use of catadioptric
Instead of using a reflective telescope, which is common in bathymetric lidar, they designed a catadioptric
telescope that uses mirrors and lenses to reflect and refract light.
So too are a two-tonne catadioptric
lens along with two tonnes of mercury.
 employed the master-slave camera in which the optical axis of catadioptric
panoramic camera coincides with the horizontal axis of PTZ camera.