Abbe–Koenig prism

An Abbe- Koenig prism, sometimes called only king or Abbe's prism ( in English often Abbe prism Type A), a reversing prism, that is, a specific reflection prism is used to invert an image ( rotation about 180 °). It is commonly used in binoculars and telescopes. Named after Ernst Abbe and Albert King prism is similar in form and function very much the Brashear -Hastings prism ( prism in English often Abbe type B), which, however, does not consist of two, but three tight together part prisms.

Design and operation

The Abbe - Koenig prism consists of two glass prisms of different shapes, laid together in a symmetrical, flat, V -shaped arrangement ( cemented ) are. The first sub-prism has the shape of a Dove prism ( trapezoidal base, 60 ° inclined Eintritts-/Austrittsflächen ), but is used so that no refraction takes place in it. The second sub-prism is slightly larger ( interior angles 90 °, 60 ° and 30 ° ) and has, at the side opposite the 60 ° angle side of a "roof ", consisting of two colliding at a 90 ° angle surfaces ( see roof prism ). At the " 90 ° corner ", it is usually cut off, as this area is not involved in the optical function ( smaller footprint and weight).

The light is perpendicularly incident to the entrance surface of the first sub-prism and is totally internally reflected at the inclined 30 ° to the longitudinal surface. Then, in the second sub-prism occurs about where it is totally reflected on the roof again. The light beam passes the roof is in two parts. Each is totally reflected twice. Between the two roof surfaces " meet " them and meet again as a whole to the next by 30 ° inclined longitudinal surface before they leave the prism combination perpendicularly through the exit surface again. In sum, the picture in these four reflections do not mirror (or a reversed image is not reversed ) is made. The image is rotated 180 °, a pre- upside-down image is erected for example. The prism is geradsichtig: No deflection and no parallel displacement of the optical axis.

When frequently used Porro prism optical axis undergoes both a vertical and a horizontal offset (the latter not at Porro prism type 2 ). The Abbe - Koenig prism is opposite him in the advantage of the in some instruments are exercised. Furthermore, it is less bulky than a Porro prism. But its disadvantage is the higher production costs: the 90 ° angle between the roof surfaces must have high accuracy, because otherwise two non- colliding half images.

In a simplified version of the Abbe - Koenig prism the "roof" by a ( mirrored ) surface is replaced. The number of reflections is three and odd. The odd number of reflections basically leads to a mirror- image. An example upside-down image is upright but made ​​the same mirror, similar to the Dove prism.

Notes and References

  • Reflection prism
  • Ernst Abbe
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