A prism with one 90° and two 45° apexes, used in binoculars. Light entering via the long face is totally internally reflected and emerges through the same face. A pair of such prisms are used in prismatic binoculars to both invert the image and fold the light path, making the binoculars more compact. This system was devised in 1851 by the Italian geodesist and optician Ignazio Porro (1801–75).
Subjects: Astronomy and Astrophysics.