The behavior of achromatic 90 and 180° phase retarder devices has been computed. These devices are made of three right-angle prisms so that the transmitted beam is not deflected or deviated from its incoming direction although the beam has suffered four internal reflections. Conventional BK-7 prisms and quartz prisms can be used as achromatic 180 and 90° phase retarders if suitable dielectric coatings are applied to the reflecting surfaces. With other glass types extremely achromatic 180° retarders can be obtained with <0.2% deviation from 180° throughout the visible wavelength range. This is five times better than the so-called superachromatic retarders made of several birefringent plates of different materials.
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