A Porro–Abbe prism named for Ignazio Porro

Ignazio Porro

Ignazio Porro 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 binoculars....

 and Ernst Abbe, is a type of reflection prism

Prism (optics)

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...

 used in some optical instruments to alter the orientation of an image. It is a variant of the more common double Porro prism

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....

 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

Total internal reflection

Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is...

 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

Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

, and camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

 viewfinder

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 optical system. Viewfinders are used in many cameras of...

s.

The prism is not dispersive

Dispersion (optics)

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...

 since light enters and exits the prism only at normal incidence. Since the light is reflected an even number of times, the image's handedness

Handedness

Handedness is a human attribute defined by unequal distribution of fine motor skills between the left and right hands. An individual who is more dexterous with the right hand is called right-handed and one who is more skilled with the left is said to be left-handed...

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.