Half Penta Prisms

Half penta prisms deviate light by a 45 degree angle. They can be used to deviate the line of sight without inverting or reverting the image; a property which makes them very popular for telescopes, imaging systems, microscopy and geodesy.

A half penta prism is sometimes called a semi penta prism or Bauernfeind prism (after German geodesy expert Karl Maxmilian von Baurenfeind). At Avantier we produce custom half penta prisms for a wide variety of applications.

Half Penta Prisms
Half Penta Prisms

Structure of Half Penta Prisms

A half penta prism is shaped like a penta prism (a five sided reflecting prism) cut in half. It will have four lateral sides, two of which are transparent. A third reflecting surface must be treated with a mirror coating, and the fourth (short) surface has no optical application. Sometimes black paint and Inconel are used to protect the mirror surface of the third reflecting side and increase the efficiency of the system. 

 Light enters the prism through one lateral side, then is reflected with total internal reflection, and then reflected again on a surface that has been treated with a reflective coating.  It then leaves the prism on the same side as it initially underwent TIR. 

Preferred substrates for half penta prisms include BK7 Grade A optical glass, Corning Fused Silica 7980, JG21 UV Grade Fused Silica, and JGS2 Fused Silica. While a reflective coating on the reflecting face is standard, anti-reflective or protective coatings may also be used on the  entrance and exit face.

Applications of Half Penta Prims

One key application of half penta prisms is in telescopes, but it is also used in many other microscopes and imaging systems. One use of half penta prims is within the eyepiece of glasses, to deflect the using angle. Practical uses of these glasses might be to read a book at an ergonomic angle while lying down comfortably, or to watch a climber without craning ones head.

A half-penta prism can also be combined with a Schmidt prism to create a Pechan prism, which does not deviate the line of sight but does invert (or revert) the image. Pechan prisms are often used for system size reduction.

Details of Half Penta Prisms

  • Materials: These prisms are available in a variety of materials including BK7 Grade A Optical Glass, Corning Fused Silica 7980, JG21 UV Grade Fused Silica, and JGS2 Fused Silica. Material selection is based on the specific optical requirements of the application.
  • Reflectivity: Avantier manufactures half-penta prisms with high reflectivity, typically greater than 95% per face for light wavelengths from 630 to 680 nm. This high reflectivity is critical to maintaining optical signal quality.
  • Angle Tolerance: The angular tolerance of these prisms is less than 5 arc seconds, ensuring precise light divergence.
  • Dimensional Tolerance: The standard dimensional tolerance is +/-0.1mm, indicating that the prisms are manufactured with high dimensional precision.
  • Surface Quality: Surface finish is a critical specification for optical prisms and describes the level of imperfections on the prism surfaces. We follow the MIL-PRF-13830B performance specification, which includes a two-part scratch dig number. The first part, the scratch number, describes the brightness of the scratches (e.g., 10, 20, 40, 60, or 80). The dig number represents the diameter of the largest component dig in 1/100th of a millimeter. The company’s standard for penta prisms is a 60-40 scratch dig, which ensures that surface imperfections meet specific brightness and size criteria.
  • Customization: We can provide custom specifications based on customer requirements, including materials, coatings, and surface finishes. If your application requires tighter surface quality tolerances, you can discuss your needs with the company to determine if they can be accommodated.
Half Penta Prisms
Half Penta Prisms

Factory Standard


BK7 Grade A optical glass, Corning Fused Silica 7980,  JGS1, JGS2

Dimension Tolerance


Angle Tolerance

<5 arc seconds


l/4 l@ 632.8 nm


R > 95% per face from 630 to 680 nm

Surface Quality



On request

Custom Half Penta Prisms at Avantier

At Avantier, we pride ourselves in the way we produce custom optics made to order for every. When we manufacture a half penta prism, it is not just any half penta prism; it is a half penta prism carefully designed and crafted to meet the exact specifications of a valued customer. Whether you’ve got a blueprint you’d like produced or you simply have a vague of idea of what your prism should do, our engineers and designers are ready to work with you to make your idea a reality. Call us today to schedule your first consult or place a custom order.


Contact us NOW for sales & expert advice.