Interferometrical profilometer for high precision 3D measurements of free-form optics topography with large local slopes

Abstract

SIOS Meβtechnik GmbH developed a universal interferometrical profilometer for 3D measurements of freeform optics topography. Due to the measurement principle using a scanning differential interferometer, no expensive and individually shaped reference optics are required. All optic shapes such as plane-,spherical-, and freeform-optics with local slopes up to 7 mrad and sizes up to 100 × 100 mm2 can be measured with sub-nanometer resolution. The capability of the setup has been proven by measurements of highly precise machined silicon mirrors (plane and spherical). A maximum of ± 3 nm peak-valley deviation between two subsequent measurements of a 30 mm × 100 mm plane mirror topography has been achieved, which proves a very good repeatability. Furthermore, measurement results show very good accordance with those from Fizeau interferometer measurements of this precision plane mirror. The maximum deviation was ± 10 nm, which is a hint to a very good accuracy of our measurements. Furthermore, form parameters such as the radii of spherical mirrors can be determined precisely due to the interferometer-based synchronous measurements of the x- and y- positions of the z- topography. A reproducibility of 1.4 × 10-4 of the radius measurements of a 29 m radius mirror was achieved, whereat the mirror was measured on different supports and in different orientations.