Porro prism

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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 image.

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 rectagular 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. Since the image is reflected twice, the handedness of the image is unchanged.

A single Porro prism is a type of roof prism, though it is not used in this way in binoculars.