An Amici roof prism, named for its inventor, the Italian astronomer Giovanni Amici, is a type of reflecting prism used to deviate a beam of light by 90° whilst simultaneously inverting the image. It is commonly used in the eyepieces of telescopes as an image erecting system.
The device is shaped like a standard right-angled prism with an additional "roof" section (consisting of two faces meeting at a 90° angle) on the longest side. Total internal reflection from the roof section flips the image laterally. The handedness of the image is unchanged.
The roof faces of the prism are sometimes coated to provide mirror surfaces. This allows the prism to be used with a wider range of beam entrance angles without being limited by total internal reflection.