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Wiktionary
folded optics

n. An optical system in which prisms and/or mirrors are used to reduce the overall physical length of the system.

Wikipedia
Folded optics

Folded optics is an optical system in which the beam is bent in a way to make the optical path much longer than the size of the system. Prismatic binoculars are a well-known example.

An early conventional film (35 mm) camera was designed by Tessina that used the concept of folded optics.

Usage examples of "folded optics".

It seemed to be a lensless system depending on folded optics and a paper-thin electronics sandwich of an image plane that transmitted the pictures on its face through a pea-size FM device.