The Phase retardation in birefringent material is the shifting of the phase between two perpendicular polarization components of a light wave. Depending on the thickness of the crystal, light with polarization components along both axes with emerge in a different polarization state. The following formula is used: `Gamma=(2piDeltanL)/lambda_0`, where:
The formula and defintion are from Wikipedia (https://en.wikipedia.org/wiki/Waveplate).