A roof prism (also called a Dach prism or Dachkanten prism, from the German: "Dachkante", lit. roof edge) is a reflective optical prism containing a section where two faces meet at a 90° angle. These two 90° faces resemble the roof of building, giving this prism type its name. Reflection from the two 90° faces returns an image that is flipped laterally across the axis where the faces meet.
Characteristic for a roof prism is that the beam is split in half, with one half of the beam hitting first one face then the other face, while it is invert for the other half of the beam. Therefore a roof prism can be used only with some distance to focal planes, or the "edge" of the roof would introduce slight distortions. Furthermore the angle between the two faces has to be very close to 90°, or image quality would be degraded.
The simplest roof prism is the Amici roof prism, with other common roof prism designs being the Abbe–Koenig prism, the Schmidt–Pechan prism and probably the best known being the roof pentaprism pictured here. A porro prism is not a roof prism, contrary to popular notion, as the two 90° faces usually don't meet in a Porro prism and therefore don't form a roof edge.
Read more about Roof Prism: Phase Correction
Famous quotes containing the words roof and/or prism:
“The far-off clinching and mating of arches, the leap and thrust of the stone, carrying a great roof overhead, awed and silenced her.”
—D.H. (David Herbert)
“Neither can I do anything to please critics belonging to the good old school of “projected biography,” who examine an author’s work, which they do not understand, through the prism of his life, which they do not know.”
—Vladimir Nabokov (1899–1977)