Porro Prisms operate on the principle of total internal reflection (TIR). A beam entering perpendicular to the entrance / exit surface is reflected by the two roof surfaces and emerges parallel to itself. For applications in which either the acceptance angle for TIR is exceeded, or the reflecting surfaces cannot be kept sufficiently clean for TIR, a metal or dielectric coating can be applied to the reflecting surfaces.

Antireflection coated entrance and exit faces are also available.