Broad-band superachromatic retarders

A simple achromatic retarder made of two plates of different birefringent materials can only be used over a limited range of wavelengths. One way to overcome this limitation is by using a combination of three such achromatic retarders. However, such a combination has the drawback that the orientation of its principal axis varies with the wavelength. An alternative method that is free from this drawback is to use a combination of three plates of different birefringent materials. This paper shows how this method of achromatization can be extended and presents a systematic design procedure for a superachromatic retarder using four plates of different birefringent materials giving an almost constant retardation over a very wide wavelength range.