Aspheric Surface Measurement Using Moire Deflectometry based on Digital Fourier Transform

Moire deflectometry is a wave front technique combining Talbot effect with Moire technique for measuring and testing phase objects or reflective surfaces. Moire deflectogram provided by light deflection from tested phase object or reflective surface can be used to calculate the deflection angle which represents the reflective index of the phase object or the height of reflective surface. A moire deflectometric measurement of aspheric surface using Digital Fourier Transform method is described in this paper. The experimental set-up of a moire deflectometer is of the form of a Kepler telescope for obtaining magnified images. For the first time the Digital Fourier Transform (DFT) is employed to process moire deflectograms. Phase-shifting technique is also used in data acquisition. The theory for combing Fourier Transform with phase-shifting is presented in the paper, and the equations for phase calculation for both infinite and finite moire deflectograms are deducted. Moire deflectograms in both x and y directions are acquired and processed by means of DFT method and integrated to reconstruct the profile of the measured surface. Moire deflectometry with phase-shifting and DFT has the advantages in simple set-up and high measuring accuracy. Since only the spatial coherence is required in moire deflectometry, the set-up has a strong capability against mechanical interference and a good stability.