Rotatable retarders have been developed for applications in spectroscopic, full Mueller Matrix ellipsometry in the far-IR spectral range. Several materials, such as silicon. KRS-5. and a commercial polymer plastic (TOPAS) have been utilized to achieve a fully adjustable retardation between 0 degrees and 90 degrees. Experimental characteristics of the rotatable retarders that utilize three- and four-bounce designs are compared with calculations. We discuss the effect of light focusing on the performance of these rotatable retarders. (C) 2010 Elsevier B.V. All rights reserved.