Typically, the relative angle between the reflecting surfaces (as in a roof) needs to have a critical tolerance in order to maintain a maximum allowable angular deviation. However, depending on placement in a system, the other angle(s) could be toleranced to limit aberration effects. Angle tolerances for prisms are inspected using an autocollimator with the prism oriented as a retro-reflector. This is only suitable for testing 90° and 45° angles; i.e. as in a right angle prism. Note that although this specification relates to the physical edge of two reflecting surfaces, it is typically tested as beam deviation.