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It consists of a block of glass shaped as a right geometric prism with right-angled triangular end faces. In operation, light enters the large rectangular face of the prism, undergoes total internal reflection 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.
An image travelling through a Porro prism is rotated by 180° and exits in the opposite direction offset from its entrance point.
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 light 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°.
Double Porro prism systems are used in small optical telescopes to re-orient an inverted image (an arrangement is known as an image erection system), and especially in many binoculars 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.
While a single porro prism can be constructed to work as well as a roof prism, it is seldom used as such. Therefore, to reduce the cost of production for a porro prism, the edge of the roof is usually left out. Sometimes only one small window as entrance surface and one window as exit surface are polished. The distinction between a roof prism and a porro prism is that for the roof prism the roof edge lies in the same plane as entrance and exit beam, while for a porro prism the (left out) roof edge is orthogonal to the plane formed by the beams. Furthermore, the roof prism has no displacement and a deviation typically between 45° and 90°, while in a single porro prism the beam is typically deviated by 180° and displaced by a distance of at least one beam diameter.
A variation on the double Porro prism is the Porro-Abbe prism.
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