Disclosed is a triangular shaped retarder system, for entering retardation between orthogonal components of an electromagnetic beam of radiation. The triangular shaped element, as viewed in side elevation, has first and second sides which project to the left and right and downward from an upper point. A third side is oriented essentially horizontally and is continuous with, and present below, the first and second sides. During use in a spectroscopic ellipsometer/polarimeter system, an entered beam of electromagnetic radiation exits in a propagation direction which is essentially undeviated and undisplaced from the direction of its incidence, even when the retarder system is caused to rotate about the locus of the beam of electromagnetic radiation.