Absolute testing method for three optical flats using symmetric algorithms

Accurate surface flatness measurements are critical for the semiconductor and optical industries. The classical three-flat test enables absolute testing of optical flats, but it only yields a line profile, necessitating multiple rotational measurements for full-aperture evaluations. In this paper, we propose a full-field absolute testing method based on a symmetric algorithm, requiring three classical measurements and a single rotation measurement at various rotation angles. The algorithm, formulated using a linear combination of all the symmetrically transformed datasets of the measured phase maps, increases the cut-off frequency of the cross-talk noise in the Zernike modes by changing the rotation angle. Numerical simulations of the modulation transfer function and experimental results demonstrated improved accuracy compared with conventional methods.