We report an interferometric method for smooth freeform optics from the amount of sub-aperture surface profiles. Theoverall experimental system is composed of a 5-axis precision stage and a sub-aperture measuring interferometer, whichis carefully calibrated to achieve 2 nmRMS precision. The sub-aperture interferometer adopts a broadband source inorder to maximize the reliability of profile measurement, and a preliminary assumption of the overall surface is derivedfrom the measured local 2nd derivatives which is robust to tip/tilt alignment errors during the sub-aperture acquisition.The optical surface of a 200 mm diameter 2D polynomial freeform mirror is measured based on this system andtraditional contact surface profiler for the cross-validation. The experiment shows the effectiveness of the system that themismatch against a commercial interferometer is less than 20 nmRMS.
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