The roof (or Amici) prism deviates or deflects the image through an angle of 90 degrees. It is a Right Angle prism whose hypotenuse has been replaced by a 90-degree roof. Glass that does not contribute to the clear aperture has been trimmed away to reduce size and weight.

Roof prisms are suitable for applications that demand both Right Angle deflection and image erection (a combination left-to-right angle deflection and a top-to-bottom inversion, equivalent to a 180-degree rotation about the optical axis).

Roof prisms normally are uncoated and are used in a TIR (total internal reflection) mode; however, protected aluminum or internal silver coatings may be specified for roof surfaces when used in wide-field applications beyond TIR limits.

Antireflection coated entrance and exit faces are also available.