A Porro–Abbe prism (sometimes called a Abbe–Porro prism), named for

Ignazio Porro Ignazio Porro (25 November 1801 – 8 October 1875) was an Italian inventor of optical instruments. Porro's name is most closely associated with the prism system which he invented around 1850 and which is used in the construction of Porro prism ...

and

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 ...

, is a type of reflection

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 in some

optical instruments An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common examples include periscopes, microscopes ...

to alter the orientation of an image. It is a

variant Variant may refer to: In arts and entertainment * ''Variant'' (magazine), a former British cultural magazine * Variant cover, an issue of comic books with varying cover art * ''Variant'' (novel), a novel by Robison Wells * " The Variant", 2021 e ...

of the more common

double Porro prism A double is a look-alike or doppelgänger; one person or being that resembles another. Double, The Double or Dubble may also refer to: Film and television * Double (filmmaking), someone who substitutes for the credited actor of a character * ' ...

configuration. It is made from a piece of glass shaped like four right-angled reflecting prisms joined face-to-face in a twisted fashion. Light enters one flat face, is internally reflected four times from the sloping faces of the prism, and exits the second flat face offset from, but in the same direction as the entrance beam. The image is rotated 180° in the process, and for this reason the prism is used as an ''image erection system'' in some

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

camera A camera is an Optics, optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), ...

viewfinder In photography, a viewfinder is what the photographer looks through to compose, and, in many cases, to focus the picture. Most viewfinders are separate, and suffer parallax, while the single-lens reflex camera lets the viewfinder use the main ...

s. The Porro–Abbe system reduces the lateral beam axis offset by 23% compared to a traditional double Porro prism system in binoculars.Binocular Optics and Mechanics Chapter from Binocular Astronomy by Stephen Tonkin, page 14 – 16
/ref> The prism is not dispersive since light enters and exits the prism only at normal incidence. Since the light is four times reflected, an even number of times, 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 ...

is not changed. For ease of manufacture, the prism is often made as a pair of double-right-angled prisms and the two halves cemented together. A single half of the assembly is sometimes also called a Porro–Abbe prism.

References

Images of a Porro–Abbe prism
Prisms (optics) {{Optics-stub es:Prisma Porro