An Abbe–Koenig prism is a type of reflecting

prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...

, used to invert an image (rotate it by 180°). They are commonly used in

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

and some

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

s for this purpose. The prism is named after

Ernst Abbe Ernst Karl Abbe HonFRMS (23 January 1840 – 14 January 1905) was a German physicist, optical scientist, entrepreneur, and social reformer. Together with Otto Schott and Carl Zeiss, he developed numerous optical instruments. He was also a co-ow ...

and Albert Koenig. The prism is made from two glass prisms, which are optically cemented together to form a symmetric, shallow Vee-shaped assembly. Light enters one face at normal incidence, is internally reflected from a 30° sloped face, and then is reflected from a "roof" (German ''Dach'') section (consisting of two faces meeting at a 90° angle) at the bottom of the prism. The light is then reflected from the opposite 30° face and exits, again at normal incidence. The net effect of the

four 4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest semiprime and composite number, and is considered unlucky in many East Asian cultures. In mathematics Four is the smallest c ...

internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...

s (two reflections are on roof plains) is to flip the image both vertically and horizontally. Since the light is reflected an even number of times, this produces a 180° image rotation (without changing the image's

handedness In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to it being stronger, faster or more Fine motor skill, dextrous. The other hand, comparatively often the weaker, less dextrous or sim ...

and allows use of the prism as an ''image erecting system''. Unlike the more common double

Porro prism In optics, a Porro prism, named for its inventor Ignazio Porro, is a type of ''reflection prism'' used in optical instruments to alter the orientation of an image. Description It consists of a block of material shaped like a right geometric ...

configuration, the Abbe–Koenig prism does not displace the output beam from the input beam, making it advantageous in some instruments. The prism is also less bulky than the double Porro design. The prism is sometimes simply called a "

roof prism A roof prism, also called a Dachkanten prism or Dach prism (from German: ''Dachkante'', lit. "roof edge"), is a reflective prism containing a section where two faces meet at a 90° angle, resembling the roof of a building and thus the name. R ...

", although this is ambiguous, because other roof prisms exist, such as the Amici and Schmidt–Pechan designs. A variant of the Abbe–Koenig prism replaces the "roof" section of the prism with a single mirror-coated reflecting surface. This type of prism flips the image vertically, but not laterally, changing the handedness of the image to the opposite sense.

Phase correction problem

The Abbe–Koenig prism shares the phase correction problems with other roof prisms. Abbe–Koenig prism and other roof prism binoculars benefit from phase-correction coatings to minimize these problems and substantially improve resolution and contrast.

Commercial market share in binoculars

Abbe-Koenig prisms are significantly longer than

Schmidt–Pechan prism A Schmidt–Pechan prism is a type of optical prism used to rotate an image by 180°. These prisms are commonly used in binoculars as an ''image erecting system''. The Schmidt–Pechan prism makes use of a roof prism section (from the German: ...

s, and were and are rarely used except in large roof prism type binoculars, where light weight, compact size and cost are less important than ultimate light-gathering power. In the early 2020s the commercial market share of Abbe–Koenig prism type binoculars is relatively small compared to other prism designs.

References

{{DEFAULTSORT:Abbe-Koenig Prism Prisms (optics)