Porro prism
In optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

, a Porro prism, named for its inventor 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....

, 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 optical instruments to alter the orientation of an image
An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...


It consists of a block of glass shaped as a right geometric prism
Prism (geometry)
In geometry, a prism is a polyhedron with an n-sided polygonal base, a translated copy , and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a...

 with right-angled triangular end faces. In operation, light enters the large rectangular face of the prism, undergoes total internal reflection
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...

 twice from the sloped faces, and exits again through the large rectangular face. Because the light exits and enters the glass only at normal incidence, 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...


An image travelling through a Porro prism is rotated by 180° and exits in the opposite direction offset from its entrance point. Since the image is reflected twice, the 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...

 of the image is unchanged.

Porro prisms are most often used in pairs, forming a double Porro prism. A second prism, rotated 90° with respect to the first, is placed such that the beam will traverse both prisms. The net effect of the prism system is a beam parallel to but displaced from its original direction, with the image rotated 180°. As before, the handedness of the image is unchanged.

Double Porro prism systems are used in small optical telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....

s to re-orient an inverted image (an arrangement is known as a image erection system), and especially in many 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...

 where they both erect the image and provide a longer, folded distance between the objective lenses and the eyepieces.

Commonly, the two components of the double Porro system are cemented together, and the prisms may be truncated to save weight and size.

A single Porro prism is a type of roof prism
Roof prism
A roof prism is in general any kind of reflective optical prism containing a section where two faces meet at a 90° angle...

, though it is not used in this way in binoculars.

A variation on the double Porro prism is the Porro-Abbe prism
Porro-Abbe prism
A Porro–Abbe prism , named for Ignazio Porro and Ernst Abbe, is a type of reflection prism used in some optical instruments to alter the orientation of an image...

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