Rotating-compensator ellipsometer, comprising the following elements in the light path in this order according to the propagation direction of light: a light source; a polarizer; a compensator with an optical axis, arranged in a supporting and rotating assembly; an analyzer; a detector, wherein the rotating-compensator ellipsometer further comprises a control unit that is operably connected to at least one of the above elements, characterized in that the compensator is a concave prism, having at least five planar surfaces that are perpendicular to a median plane of the prism, said median plane including the optical axis, wherein a first planar surface of the prism is perpendicular to the optical axis; a second planar surface of the prism forms an angle of 90°-β with the optical axis; a third planar surface of the prism is parallel to the optical axis, is perpendicular to the first planar surface and is provided with a reflective coating; a fourth planar surface of the prism forms an angle of 90°-β with the optical axis; a fifth planar surface of the prism is parallel to the first planar surface and is perpendicular to the third planar surface, and wherein 45° < β < 65°, the compensator being arranged in the light path to generate an effective 90° phase shift between s and p polarization components of light emitted by said light source by means of two total internal reflections and a further specular reflection. a light source; a polarizer; a compensator with an optical axis, arranged in a supporting and rotating assembly; an analyzer; a detector, wherein the rotating-compensator ellipsometer further comprises a control unit that is operably connected to at least one of the above elements, characterized in that the compensator is a concave prism, having at least five planar surfaces that are perpendicular to a median plane of the prism, said median plane including the optical axis, wherein a first planar surface of the prism is perpendicular to the optical axis; a second planar surface of the prism forms an angle of 90°-β with the optical axis; a third planar surface of the prism is parallel to the optical axis, is perpendicular to the first planar surface and is provided with a reflective coating; a fourth planar surface of the prism forms an angle of 90°-β with the optical axis; a fifth planar surface of the prism is parallel to the first planar surface and is perpendicular to the third planar surface, and wherein 45° < β < 65°, the compensator being arranged in the light path to generate an effective 90° phase shift between s and p polarization components of light emitted by said light source by means of two total internal reflections and a further specular reflection. wherein the rotating-compensator ellipsometer further comprises a control unit that is operably connected to at least one of the above elements, characterized in that the compensator is a concave prism, having at least five planar surfaces that are perpendicular to a median plane of the prism, said median plane including the optical axis, wherein a first planar surface of the prism is perpendicular to the optical axis; a second planar surface of the prism forms an angle of 90°-β with the optical axis; a third planar surface of the prism is parallel to the optical axis, is perpendicular to the first planar surface and is provided with a reflective coating; a fourth planar surface of the prism forms an angle of 90°-β with the optical axis; a fifth planar surface of the prism is parallel to the first planar surface and is perpendicular to the third planar surface, and wherein 45° < β < 65°, the compensator being arranged in the light path to generate an effective 90° phase shift between s and p polarization components of light emitted by said light source by means of two total internal reflections and a further specular reflection. a first planar surface of the prism is perpendicular to the optical axis; a second planar surface of the prism forms an angle of 90°-β with the optical axis; a third planar surface of the prism is parallel to the optical axis, is perpendicular to the first planar surface and is provided with a reflective coating; a fourth planar surface of the prism forms an angle of 90°-β with the optical axis; a fifth planar surface of the prism is parallel to the first planar surface and is perpendicular to the third planar surface, and wherein 45° < β < 65°, the compensator being arranged in the light path to generate an effective 90° phase shift between s and p polarization components of light emitted by said light source by means of two total internal reflections and a further specular reflection.