Waveplate (retardation plates or phase shifters) is made from birefringence material. The velocities of the extraordinary and ordinary rays through the birefringent materials vary inversely with their refractive indices. The difference in velocities gives rise to a phase difference when the two beams recombine. At any specific wavelength the phase difference is governed by the thickness of the retarder. Please specify the wavelength, phase retardation, dimension and type when you purchase it. Custom designed waveplate is available upon request.