An apparatus for spectroscopic ellipsometry, preferably for infrared spectroscopic ellipsometry, comprising a light source (11; 12; 20), a detector, a polarizer, an analyzer, and a measuring probe comprising an ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K) having at least one first surface having at least one measuring portion configured to be brought in optical contact with a measured object, and at least one second surface having at least one reflective portion, said ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K) being configured so that: at least a portion of polarized light entering the measuring probe undergoes an attenuated totally reflection at the least one measuring portion of the first surface, at least a portion of the totally reflected light is reflected back towards the first surface by the at least one reflective portion of the second surface, and at least a portion of the light reflected back by the at least one reflective portion of the second surface undergoes an attenuated total reflection at the at least one measuring portion of the first surface and is decoupled from the ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K), the difference between the magnitude of the angle (alpha_p) between the first surface and the second surface and the magnitude of the critical total reflection angle is less than 12°, the critical total reflection angle being equal to arcsin(n_m/n_p), wherein n_m is the reflective index of the measured object and n_p is the refractive index of the material forming the ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K); the first surface and the second surface intersect along a common line of intersection, thereby forming a cutting blade (53, 54) for cutting through the measured object or the ATR prism comprises a cutting blade (53, 54) for cutting through the measured object, wherein the cutting blade (53; 54) exhibits a cutting angle of less or equal to 60°. an ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K) having at least one first surface having at least one measuring portion configured to be brought in optical contact with a measured object, and at least one second surface having at least one reflective portion, at least one first surface having at least one measuring portion configured to be brought in optical contact with a measured object, and at least one second surface having at least one reflective portion, said ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K) being configured so that: at least a portion of polarized light entering the measuring probe undergoes an attenuated totally reflection at the least one measuring portion of the first surface, at least a portion of the totally reflected light is reflected back towards the first surface by the at least one reflective portion of the second surface, and at least a portion of the light reflected back by the at least one reflective portion of the second surface undergoes an attenuated total reflection at the at least one measuring portion of the first surface and is decoupled from the ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K), the difference between the magnitude of the angle (alpha_p) between the first surface and the second surface and the magnitude of the critical total reflection angle is less than 12°, the critical total reflection angle being equal to arcsin(n_m/n_p), wherein n_m is the reflective index of the measured object and n_p is the refractive index of the material forming the ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K); the first surface and the second surface intersect along a common line of intersection, thereby forming a cutting blade (53, 54) for cutting through the measured object or the ATR prism comprises a cutting blade (53, 54) for cutting through the measured object, wherein the cutting blade (53; 54) exhibits a cutting angle of less or equal to 60°. at least a portion of polarized light entering the measuring probe undergoes an attenuated totally reflection at the least one measuring portion of the first surface, at least a portion of the totally reflected light is reflected back towards the first surface by the at least one reflective portion of the second surface, and at least a portion of the light reflected back by the at least one reflective portion of the second surface undergoes an attenuated total reflection at the at least one measuring portion of the first surface and is decoupled from the ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K), the difference between the magnitude of the angle (alpha_p) between the first surface and the second surface and the magnitude of the critical total reflection angle is less than 12°, the critical total reflection angle being equal to arcsin(n_m/n_p), wherein n_m is the reflective index of the measured object and n_p is the refractive index of the material forming the ATR prism (50, 50A, 50B, 51A, 51B, 51C, 51D; 51E; 51F; 51G; 51H; 51K); the first surface and the second surface intersect along a common line of intersection, thereby forming a cutting blade (53, 54) for cutting through the measured object or the ATR prism comprises a cutting blade (53, 54) for cutting through the measured object, wherein the cutting blade (53; 54) exhibits a cutting angle of less or equal to 60°.