an optically - excited atomic frequency standard that subjects alkali metal atoms to circularly - polarized optical radiation . the atomic frequency standard is improved by the use of a circular polarizer to control the intensity of the circularly - polarized optical radiation . the circular polarizer includes a linear polarizer and a quarter - wave retarder , with the light to be circularly polarized passing first through the linear polarizer and then through the quarter - wave retarder . in the atomic frequency standard , the optical radiation to which the circular polarizer is applied is itself linearly polarized , and the intensity of the circularly polarized light produced by the circular polarizer is controlled by rotating the circular polarizer . the degree of rotation determines how much of the linearly - polarized optical radiation passes through the linear polarizer , and thus how much circularly - polarized light is produced .