Porro prism

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The Porro prism , named after its inventor Ignazio Porro , who patented it in 1854, is the classic erecting prism in traditional binoculars with Kepler's telescopes . A Porro prism usually consists of two simple right-angled reflection prisms - so-called half - cube prisms - through which the light beam passes one after the other. Since the two half-cube prisms are rotated by 90 ° in relation to each other, the fourfold reflection of the light beam when passing a Porro prism produces a total of 180 ° of the image. The 180 ° rotation of the original image caused by a Kepler telescope is thus canceled by the Porro prism.

The beam path is reflected twice, i.e. in an even number, in a half-cube prism, so that the image remains correct. It is deflected by 180 °, which is used in the first half-cube prism to erect the upside-down image, it is turned upside down. The unwanted 180 ° deflection - one would have to look backwards with the telescope - is compensated with the help of the second prism. This turns the image around a vertical axis and the telescope remains straightforward. A process analogous to erecting the upside-down image is omitted because the observer participates in the second turn of the beam path. It does not follow the first turn because it does not turn itself upside down. The physical-optical formulation of the condition to be met is as follows: The image is completely reversed when the reflections take place in two different main sections of a prism system.

There is also the Porro prism of the 2nd type , which also deflects the beam path four times 90 °. A double-reflecting half-cube prism is placed between two smaller half-cube prisms that only reflect once. The main sections of the outer prisms are arranged at right angles to the central prism. Its advantage is that there is no vertical offset of the beam path. Another variant with the same function is the Porro-Abbe prism composed of two partial prisms

The two-part version of the Porro prism of the 2nd type has been further developed into the Perger prism , which combines the properties of requiring only a small offset of the beam path by changing the angle of the reflective surfaces and the putty surface and also making it possible to reflect a measuring beam or an illuminated display . Perger prisms are z. B. used in compact binoculars with integrated rangefinder.

  • twice reflected beam path in a half-cube prism

  • Beam path in a pair of half-cube prisms (Porro prism)

  • Schematic cross section through porro prism binoculars

  • Cross section through porro prism binoculars

  • Beam path in a Porro prism, type 2

  • Beam path in the Perger prism (blue cemented surface)

Side effects

  1. By folding the beam path, the overall length of the telescope is reduced by up to 60 percent (e.g. for 7 × 50 binoculars from around 40 cm ( focal length of the lens ) to around 15 cm).
  2. The mutual distance between the partial binoculars increases so that spatial vision is noticeably improved, but close-up focus is made more difficult.
  3. Due to the greater distance between the binoculars, their diameter and thus the light intensity can be greater.

As a component in binoculars, the Porro prism is now in competition with the straight roof-prism pentaprism . The latter is geometrically more complex, but allows a less wide construction of the binoculars (to the detriment of spatial vision, see 2.). The smaller offset of the beam path, which is desired for compact external dimensions of binoculars, can also be achieved with the Perger prism patented in 2012, which avoids the basic disadvantages of the roof prism.

See also

Web links

Commons : Porro prisms  - collection of images, videos and audio files

Individual evidence

  1. ↑ Patent GB195402377 : Certain applications of total or partial reflection of light on transparent surfaces either alone or combined with the reflection. Published on November 9, 1854 (currently not available on Espace). ‌
  2. ↑ Fritz Hodam: Technical Optics . VEB Verlag Technik Berlin, 1967, p. 254.
  3. ↑ Heinz Haferkorn: Optics - physical-technical basics and applications . Barth, Leipzig 1994, ISBN 3-335-00363-2 , p. 485.
  4. ↑ M. Bass (Ed.): Handbook of Optics. Volume I - Geometrical and Physical Optics, Polarized Light, Components and Instruments . 3. Edition. McGraw-Hill Professional Publishing, 2009, ISBN 978-0-07-162925-6 , pp. 19.6 .