Schmidt-Pechan prism
The Schmidt-Pechan prism is an optical inverting prism that enables complete image inversion (image rotation by 180 °) without any beam offset.
It is used in compact binoculars , microscopes and periscopes , among other things .
Layout and function
The Schmidt-Pechan prism consists of two prisms with a triangular base, which are separated from one another by an air gap. It can be seen as a combination of a 45 ° Bauerfeind prism and a simple Schmidt prism . In the case of the latter, a triangular surface is folded into two rectangular roof edges .
The image reversal is achieved by a total of six reflections (two of them in the roof edges). Depending on the type of glass used, the second reflection surface and / or the roof edge surfaces must be mirrored, since the critical angle for total reflection is not reached there.
The first prism serves as a deflecting “correction prism” for the second prism (the Schmidt prism). It creates a 45 ° deflection. The light enters vertically and is totally reflected at the inner interface to the air gap. As a result of a reflection on the underside of the prism, the light beam is then directed perpendicularly to the interface with the air gap or the incidence surface of the second prism. The perpendicular incidence serves to prevent a beam offset due to refraction and to minimize reflection losses at the entry and exit surfaces. Due to the double reflection, only the beam path is deflected, the image orientation remains unchanged (not mirror-inverted).
The triangular angles in the second prism are chosen so that the beam path in the main section is deflected further by 315 °. The entire deflection is 360 °, the exiting beam is aligned with the entering one. A transmitted picture that is upside down at the beginning is erected (rotation around a horizontal straight line by a total of 180 °) and thus the usual purpose is fulfilled. To ensure that the image remains correct, an even number of reflections (4 in the second prism) takes place with the help of the roof edge surfaces. In the roof prism part, the image is split in the middle, and the fields are reflected twice separately before they reunite.
Advantages and disadvantages
Compared to prisms with a similar function (e.g. the Abbe-König prism ), the Schmidt-Pechan prism offers the advantage of a very compact design.
On the other hand, the relatively long beam path in the prism and the somewhat larger overall height compared to similar prisms (e.g. the Abbe-König or Räntsch prism ) are disadvantageous . In addition, due to its complexity, it is somewhat more expensive than other systems, for example the Porro prism . The requirement for the accuracy of the 90 ° angle at the roof edge within a few arc seconds is another cost factor.
Individual evidence
- ↑ a b Daniel Malacara: Geometrical and Instrumental Optics . Academic Press, 1988, ISBN 978-0-12-475970-1 , pp. 65 .
- ^ Paul R. Yoder: Design and Mounting of Prisms and Small Mirrors in Optical Instruments . SPIE Press, 1998, ISBN 978-0-8194-2940-7 , pp. 43-44 .
- ↑ Matthias Haag: System-technical optimization of the beam quality of high-power diode lasers . Herbert Utz Verlag, 2000, ISBN 978-3-89675-840-8 , p. 104 .