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Reflections in a Bauernfeind prism for 60 deg beam deviation. Green side requires reflective coating.

A Bauernfeind prism is a type of reflecting prism used to deviate a beam of light by 45° to 60°, depending on its construction, while neither flipping nor rotating the image.[1] It is named for its inventor, the German expert of geodesy Karl Maximilian von Bauernfeind [de].[2]

The beam is reflected twice in the prism, with one reflection happening at less than the critical angle. Therefore, the prism requires a reflective coating for this surface to be usable in practice.

A Bauernfeind prism with 45° beam deviation is also known as a half-penta prism or semipentaprism.[3][4] A Bauernfeind prism is used together with a Schmidt roof prism to form a Schmidt–Pechan prism.


The Bauernfeind prism is commonly used in microscope tubes and in geodesy to deviate the path to the eyepiece in order make the device more ergonomic.[5]

More recent applications are glasses that deflect the viewing angle either downwards (for relaxed reading or watching TV[6]), or upwards (for watching a climber while belaying[7]).


  1. ^ Hanno Schmidt (26 May 2009). "6-PRISMA" (PDF). Retrieved 2015-03-22.
  2. ^ "Bauernfeind-Prisma - Techniklexikon". Retrieved 2015-03-22.
  3. ^ "". Retrieved 2015-03-22.
  4. ^ "VOB Lumex -- Prisms". Archived from the original on 2003-05-10. Retrieved 2015-03-22.
  5. ^ Optik: Physikalisch-technische Grundlagen und Anwendungen, p. 475, at Google Books (in German)
  6. ^ "Prismenbrillen, Liegebrillen, Teleskopspiegel - REHADAT-Hilfsmittel". Retrieved 2015-03-22.
  7. ^ Belay glasses