Phase retardance of periodic multilayer mirrors

A periodic multilayer reflector composed of two nonabsorbing materials is found to have a phase retardance that is a function of the refractive indices of the materials, the periodic design, and the incidence angle. The addition of a reflection enhancing periodic multilayer to a mirror causes the phase retardance to change to a unique value that depends on the properties of the added layers and the incidence angle. After reaching this limiting value the retardance is not changed by the further addition of reflection enhancing layers with the same periodic structure. Equations for the limiting phase retardance of a periodic multilayer reflector are provided.