Why diffractive retarders are not in common use

Diffractive retarders fabricated from gratings in isotropic materials are analyzed by rigorous coupled wave analysis. Calculations show it is difficult to obtain substantial retardance with isotropic phase gratings. Even for an aspect ratio of two, diffractive retarders have a small retardance, < λ/12. Thus it is generally impractical to fabricate quarter wave retarders, much less half wave retarders in plastic or molded glass for example. The dispersion of these gratings is compared to the conventional materials used in the majority of retarders and is found to be very similar. Thus these gratings add little in terms of helping to achromatize retarders