A dual rotating compensators ellipsometer has been developed. It is based on the optical PC1SC2A configuration, where C1 and C2 are the synchronized continuously rotating compensators [1]. The systematic errors if the compensators are quarter-wave plates have been already studied [2]. Smith [3] has demonstrated that the optimum retardance of a dual-rotating-retarder (DRR) instrument must be equal to 127° compared to the quarter-wave (90°) retarders generally used.