Schmidt–Pechan prism

The Schmidt- Pechan prism is an optical pentaprism which allows a complete image reversal ( image rotation by 180 °) without beam displacement.

It is used, among other things, compact binoculars, microscopes or periscopes.

Design and function

The Schmidt- Pechan prism comprises two prisms having a triangular base area, which are separated by an air gap. It can be conceived as a combination of a 45 ° Bauernfeind prism and a simple Schmidt prism. in the latter a triangular surface is folded into two mutually perpendicular roof prism surfaces.

The image reversal is achieved by a total of 6 reflections (2 of them in the roof prism surfaces ). Depending on the type of glass used, the second reflection surface and / or roof edge surfaces to be metallized, as there is the critical angle for total internal reflection will not be achieved.

The first deflecting prism is used as a " prism correction " for the second prism ( the prism Schmid ). It produces a 45 ° deflection. The light enters vertically and is totally reflected at the inner boundary surface with the air gap. By reflection on the lower surface of the prism, the light beam is then directed vertically to the boundary surface to the air gap and the incident surface of the second prism. The vertical incidence of a beam is used to prevent offset due to refraction and the minimization of reflection losses at the entrance and exit surfaces. The double reflection only of the beam path is deflected, the image orientation remains unchanged ( not mirrored ).

The triangular prism in the second angle are selected so that the beam path in the main section to 315 ° is more deflected. The total deflection is 360 degrees, the outgoing beam is aligned with the incoming. A transmitted upside-down at the top image is raised (rotation around a horizontal line through 180 ° ), and thus the usual purpose fulfilled. Thus, the image remains the right way, done with the roof prism faces an even number of reflections (4 in the second prism). In roof prism part of the image in the middle is cleaved, and the fields are each twice reflected separately before they recombine.

Pros and Cons

The Schmidt- Pechan prism offers compared to prisms with a similar function (eg, the Abbe-Koenig prism) the advantage of a very compact design.

The disadvantage, however, are the relatively long beam path in the prism and the like compared prisms (eg, the Abbe-Koenig prism or Räntsch ) slightly greater height. Moreover, it is somewhat more expensive than other systems, such as the Porro prism, due to its complexity. Requiring the accuracy of the 90 ° angle at the edge of the roof within a few seconds of arc is another cost factor.