Waveplates (retarders) are made from birefringent materials which introduce a phase difference between the fast and slow principal axis of the waveplate. The optical axis of waveplate is parallel to the face. Light incident normal to the surface will be split to components polarized parallel and perpendicular to the optical axis with different refractive index and velocity in this device. The difference in velocities gives rise to a phase shift which is called retardance. At any specific wavelength the phase retardance is governed by the thickness of Waveplates. 

OPMOUNT provided include half waveplates and quarter waveplates, other custom waveplates can also be provided upon request