A retarder system and method of its use in ellipsometers and polarimeters are disclosed. The retarder system is a parallelogram shaped element which, as viewed in side elevation, has top and bottom sides which are parallel to one another. The retarder system also has right and left sides which are parallel to one another, with both the right and left sides being oriented at an angle to the top and bottom sides. The retarder system is made of a material with an index of refraction greater than that of a surrounding ambient. In use an input beam of electromagnetic radiation is caused to enter the right or left side of the retarder system, is diffracted inside the retarder system, and follows a locus which causes it to essentially totally internally reflect from internal interfaces of both the top and bottom sides, then emerge from the retarder system at the opposite, left or right respectively, side in a direction which is essentially undeviated and undisplaced from the locus of the input beam of electromagnetic radiation, even when the retarder system is caused to rotate about an axis coincident with the locus of the electromagentic beam.