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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 . It consists of a block of material shaped like 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. When the light enters and therefore exits the glass 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 entry point. 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 an 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. Porro prisms can reflect light rays that are not parallel to the optical axis in such a manner that they are internally reflected off the hypotenuse of the prism. Such an abaxial ray then emerges from the prism having been reflected a third time, thus introducing non image-forming stray light and reducing contrast. Abaxial reflections can be eliminated by putting a groove or notch across the width of the hypotenuse face center of the prism, which blocks these detrimental reflections. Porro prisms are most often used in pairs, forming a double Porro prism.

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