Perger prism

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Beam path in the Perger prism; the cement surface is drawn in blue.

A Perger prism is an erecting prism , i.e. a special reflective prism that is used to invert an image (rotation by 180 °). The specialty of this prism is that, like a Porro prism , it manages with only four beam deflections and has neither a roof edge , a mirrored surface nor an air gap. In contrast to the Porro prism, however, it leads to a significantly reduced axis offset, which allows the construction of comparatively compact binoculars . The inversion system was patented by Andreas Perger in 2012 and was initially used in the Geovid HD binoculars (third generation) from the manufacturer Leica .

Structure and functionality

The Perger prism consists of two glass prisms of different shapes that can be cemented together. The two-part variant of the Porro prism of the 2nd type was initially used as a template for its design . In the next development steps, the cemented surface was first inclined in relation to the beam path, and the first and last reflective surfaces were tilted in such a way that the beam each had an angle of incidence which exceeded the 45 ° usual in the Porro prism. In this way, the axial offset could be reduced to about 70% of that in the Porro prism 2nd type, i.e. H. can be reduced to about half of the value present in the Porro prism.

Since the partial prisms are cemented and the beam deflection is based entirely on the principle of total reflection , there are no reflection losses within this reversing system. The roof prisms , which are widespread in binocular construction, lead to a loss of image quality due to diffraction effects on the roof edge, and also require a phase correction coating - complications that are avoided in the Perger prism.

Another advantage of the Perger prism is that the inclination of the cemented surface to the optical axis can be used to couple a measuring beam or a display, which enables this type of prism to be used in binoculars with integrated rangefinders. For this purpose, the putty surface is provided with a dichroic coating that is matched to the wavelengths of the measuring beam and the display.

Individual evidence

  1. ↑ a b Patent EP2463692A1 : Prisma. Published on June 13, 2012 , inventor: Andreas Perger. ‌
  2. ↑ H. Merlitz: Hand binoculars: function, performance, selection. Verlag Europa-Lehrmittel, 2013, ISBN 978-3-8085-5774-7 , p. 42.
  3. ↑ A. Weyrauch, B. Dörband: P- Belag: Improved imaging in binoculars through phase-corrected roof prisms. In: Deutsche Optikerzeitung. No. 4, 1988.