A method of determining physical and optical properties of samples using a terahertz frequency electromagnetic radiation, comprising the steps of: providing a terahertz ellipsometer or polarimeter system sequentially comprising: a source of Terahertz frequencies (BWO); a first rotatable polarizer that operates at THZ frequencies; a first rotating element (RE1); a stage (STG) for supporting a sample (S); a second rotating element (RE2) a second rotatable polarizer that operates at THZ frequencies; a detector (DET) of THZ frequencies; wherein one of the first (RE1) and second (RE2) rotating elements is a rotating polarizer and the other is a rotating retarder (RRET) comprising, in any functional order, a first reflective element (RP) which is a prism, and a second (RM1), third (RM2) and fourth (RM3) reflective elements, placing a sample (S) on said stage (STG); causing said source of Terahertz frequencies to direct a beam (BI) of terahertz frequency electromagnetic radiation of a fundamental frequency to pass through said first rotatable polarizer (WGP1), then through said first rotating element (RE1), then reflect from a sample (S) placed on said stage (STG), then pass through said second rotating element (RE2), then pass through said second rotatable polarizer (WGP2), and enter said detector (DET) of THZ frequencies as output beam (BO); obtaining sample describing data from said detector (DET). providing a terahertz ellipsometer or polarimeter system sequentially comprising: a source of Terahertz frequencies (BWO); a first rotatable polarizer that operates at THZ frequencies; a first rotating element (RE1); a stage (STG) for supporting a sample (S); a second rotating element (RE2) a second rotatable polarizer that operates at THZ frequencies; a detector (DET) of THZ frequencies; wherein one of the first (RE1) and second (RE2) rotating elements is a rotating polarizer and the other is a rotating retarder (RRET) comprising, in any functional order, a first reflective element (RP) which is a prism, and a second (RM1), third (RM2) and fourth (RM3) reflective elements, a source of Terahertz frequencies (BWO); a first rotatable polarizer that operates at THZ frequencies; a first rotating element (RE1); a stage (STG) for supporting a sample (S); a second rotating element (RE2) a second rotatable polarizer that operates at THZ frequencies; a detector (DET) of THZ frequencies; wherein one of the first (RE1) and second (RE2) rotating elements is a rotating polarizer and the other is a rotating retarder (RRET) comprising, in any functional order, a first reflective element (RP) which is a prism, and a second (RM1), third (RM2) and fourth (RM3) reflective elements, placing a sample (S) on said stage (STG); causing said source of Terahertz frequencies to direct a beam (BI) of terahertz frequency electromagnetic radiation of a fundamental frequency to pass through said first rotatable polarizer (WGP1), then through said first rotating element (RE1), then reflect from a sample (S) placed on said stage (STG), then pass through said second rotating element (RE2), then pass through said second rotatable polarizer (WGP2), and enter said detector (DET) of THZ frequencies as output beam (BO); obtaining sample describing data from said detector (DET). <EOS>