Half Penta Prism

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A half penta prism is used to deviate the line of sight by 45° without inverting or reverting the image. It is widely used in telescopes and also has applications in projection and imaging system, in microscope tubes, and in geodesy. Since it deviates the light beam without changing the image it can be used for glasses that deflect the viewing angle either upward or downward—for instance, to allow relaxed reading while lying down, or for watching a climber while belaying.

The prisms have the geometrical structure of a penta prism cut in half. The beam is reflected twice inside the prism; first with total internal reflection and a second time on a coated reflecting surface.

Half penta prisms are also known as Bauernfeind prisms, after Karl Maximilian von Bauernfeind, a German geodesy expert.

Combing a half penta prism with a Schmidt prism produces a prism pair known as the Schmidt Pechan roof prism. These prisms are often sued in binoculars, since they provide a compact, conveniently shaped solution to the problem of inverting or reverting an image without deviating the line of sight.

Factory Standard 

  • Material: BK7 Grade A optical glass, Corning Fused Silica 7980,  JGS1, JGS2
  • Dimension Tolerance: +/-0.1mm
  • Angle Tolerance: <5 arc seconds
  • Flatness: l/4 l@ 632.8 nm
  • Reflectivity: R > 95% per face from 630 to 680 nm
  • Surface Quality: 60-40 scratch and dig
  • Coating: On request

Contact us for manufacturing limit or custom specifications.

Different Types of Half Penta Prisms

Shanghai Optics produces a variety of half penta prisms in various sizes, and we can customize size, substrate, coating, and surface quality as needed.

Our stock prisms are available in BK7 Grade A optical glass, Corning Fused Silica 7980, JG21 UV Grade Fused Silica, and JGS2 Fused Silica. Our quality control team uses state of the art meteorology equipment to ensure every optical piece we manufacture fully meets all standards, and we are able to manufacture half penta prisms with very low tolerances and high surface quality.

Our factory standard dictates reflectivity of greater than 95 percent per face, with light wavelengths from 630 to 680 nm.  Our angle tolerance is less than 5 arc seconds, and dimension tolerance standard is +/-0.1mm. We can provide custom specifications on request; please contact us to discuss manufacturing limits and receive a quote.

Surface quality of an optical prism describes the level of imperfections on the surfaces of the prism, and tends to be one of the most poorly understood specifications. We use the MIL-PRF-13830B performance specification. Under this standard, each optic is described with a two part scratch-dig number. The first number, the scratch number, describes the brightness of the scratches, and can be 10, 20, 40, 60, or 80, where the brightness of the scratches increases as the number increases. Inspection is done under specified darkfield illumination conditions.

The dig number represents the diameter of the largest component dig, given in 1/100th of a millimeter.  The sum of all scratch lengths with the specified scratch number may not exceed ¼ of the diameter of a circle with an area equal to that of the optical surface. The total number of allowable digs may not be greater than the diameter divided by twenty.

Our factory standard for penta prisms is 60-40 scratch dig. This number tells you that the prism may have scratches with no more brightness than 60, and the sum of the length of all scratches with brightness of 60 is no more than ¼ the diameter of a circle with an area equal to the prism’s surfaces. It also tells us that the maximum size of any dig on the prism is 0.4.

Please contact us to discuss manufacturing limits if your application requires tighter surface quality tolerances.


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