Abbe-König prism
An Abbe-König prism , sometimes just called König or Abbe prism (in English often also Abbe prism Type A ), is an erecting prism , i.e. a special reflection prism that is used to invert an image (rotation by 180 °). It is commonly used in binoculars and telescopes. The prism, named after Ernst Abbe and Albert König , is very similar in form and function to the Brashear-Hastings prism (in English often also Abbe prism Type B ), which however does not consist of two, but three firmly joined partial prisms.
Structure and functionality
The Abbe-König prism consists of two glass prisms of different shapes, which are firmly joined (cemented) in a symmetrical, flat, V-shaped arrangement. The first partial prism has the shape of a Dove prism ( trapezoidal base, 60 ° inclined entry / exit surfaces), but is used in such a way that no refraction occurs in it. The second partial prism is slightly larger (interior angles 90 °, 60 ° and 30 °) and has a "roof" on the side opposite the 60 ° angle, consisting of two surfaces that meet at a 90 ° angle (see roof prism ). It is usually cut off at the "90 ° corner", as this area is not involved in the optical function (less space requirement and weight).
The light falls perpendicular to the entrance surface of the first partial prism and is totally internally reflected on the longitudinal surface inclined by 30 ° . Then it passes into the second partial prism, where it is again totally reflected on the roof. The light beam passes the roof in two parts. Each one is totally reflected twice. They "meet" between the two roof surfaces and meet again as a whole on the next longitudinal surface inclined by 30 ° before they leave the prism combination vertically again through the exit surface. In total, with these four reflections, the image is not reversed (or a mirror-reversed image is not reversed). The picture is rotated 180 °, for example a picture that was previously upside down is erected. The prism is straight: no deflection and no parallel offset of the optical axis .
With the more frequently used Porro prism , the optical axis experiences both a vertical and a horizontal offset (the latter not with the Porro prism of the 2nd type). The Abbe-König prism has an advantage over it and is used in some instruments. It is also less bulky than a Porro prism. Its disadvantage, however, is the higher production costs: the 90 ° angle between the roof surfaces must have a high degree of accuracy, because otherwise two non-colliding fields will result.
In a simplified variant of the Abbe-König prism, the “roof” is replaced by a (mirrored) surface. The number of reflections is three and odd. The odd number of reflections basically leads to a mirror image. For example, an upside-down image is erected, but at the same time made mirror-inverted, similar to the Dove prism.
Notes and individual references
- ↑ This short term but mainly for dispersing Abbe prism used
- ↑ Optical design . In: US Department of Defense (Ed.): Military Standardization Handbook . MIL-HDBK-141, 1962, pp. 13–28 ( PDF: Go to page 264 ).
- ↑ Optical design . In: US Department of Defense (Ed.): Military Standardization Handbook . MIL-HDBK-141, 1962, pp. 13–29 ( PDF: Go to page 265 ).
- ^ William L. Wolfe: Chapter 4. Nondispersive Prisms. In: Michael Bass (Ed.): Handbook of optics, Vol. 2: Devices, Measurements, and Properties. 2nd ed. McGraw-Hill, New York 1995, ISBN 0-07-047740-X , pp. 4-8 ( online ).
- ^ Warren J. Smith: Optical engineering. The design of optical systems . 4th edition. McGraw-Hill, New York 2008, ISBN 978-0-07-147687-4 , pp. 143 .