The most achromatic quarterwave plates appear to be rhomb-type devices. General relations are given for calculating the phase retardation of these devices and are used to determine the variation of the phase retardation with wavelength and with acceptance angle for different rhomb designs. Length-to-aperture ratio, beam deviation, surface coatings, effect of strain birefringence, and other parameters are also considered. The classical version of the Fresnel rhomb can be optimized to improve its performance, and several of the more recent rhomb designs have excellent characteristics. The choice of which type of rhomb to use depends on the requirements of the particular optical system.
J. M. Bennett, "A Critical Evaluation of Rhomb-Type Quarterwave Retarders," Appl. Opt. 9, 2123-2129 (1970)