Curvature measurement using phase shifting in-line interferometry, single shot off-axis geometry and Zernike's polynomial fitting

This paper describes the difference between phase shifting in-line interferometry, single shot off-axis geometry and Zernike's polynomial fitting methods for measuring the curvature of a spherical smooth surface by using the Michelson interferometer. In phase shifting in-line interferometry, four interferograms shifted by a piezoelectric actuator (PZT) were captured by a digital detector and corrected by using the flat fielding method. In off-axis geometry, single shot off-axis interferogram was obtained by tilting the reference and the object wave of the off-axis interferogram was reconstructed in the central region of the observation plane by using the digital reference wave concept. The demodulated phase map was obtained and unwrapped to remove the 2 pi ambiguity. The unwrapped phase map was converted to height and the sagittal length that used for curvature measurement was calculated accurately. The results extracted from phase shifting in-line interferometry and single shot off-axis geometry methods were compared with the results extracted from single shot Zernike's polynomial fitting method and the results were in excellent agreement. A new trial was done to overcome the fringes produced from the object interfaces. Some factors of uncertainty which affected on the measurement were estimated in the order of 6.0 x 10(-5) mm or 0.003 dioptre (del). (C) 2011 Published by Elsevier GmbH.