What does the wavelength designation mean for quartz retarders and how does this relate to the film specifications?

To achieve the phase shift designated for any given retarder, the optical thickness of the material is selected to give the desired shift at a specific wavelength. Retarders are very wavelength dependent. Wavelengths close to the design or slight thickness differences will result in a slightly inaccurate phase shift of the transmitted beam. Since white light is composed of a range of wavelengths, no single material thickness can correspond to the proper shift across the entire region and, as a result the design must be generalized for the region of interest. The film material achieves the desired retardation for the visible due to its design at the center of the visible spectrum (560nm). Our quartz and film retarders are available in phase shifts of λ/2 and λ/4 in several wavelength options