# Bauernfeind prism

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.[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.

## Applications

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 (as in belay glasses for watching a climber while belaying).

## References

1. Hanno Schmidt (26 May 2009). "6-PRISMA". Retrieved 2015-03-22.
2. Techniklexikon.net. "Bauernfeind-Prisma - Techniklexikon". techniklexikon.net. Retrieved 2015-03-22.
3. "www.edmundoptics.com/optics/prisms/image-rotation-prisms/half-penta-prisms/3249". edmundoptics.com. Retrieved 2015-03-22.
4. "VOB Lumex -- Prisms". Retrieved 2015-03-22.
5. Optik: Physikalisch-technische Grundlagen und Anwendungen, p. 475, at Google Books (in German)