Fourier transform digital moiré interferometry

Abstract Processing interferometric images is a key task in optical interferometry for measurement purposes. It is well known that there are two main kinds of interferograms: those with open fringes and those with closed fringes. Often, processing a single interferogram with closed fringes can be challenging; therefore, applying phase-shifting is almost obligatory. However, it requires several images and a high precision mechanical device to achieve accurate results. On the other hand, interferograms with open fringes are more straightforward to process. However, precise knowledge of the carrier frequency of the fringes is essential for accurate processing. An alternative solution to the challenges posed above is the use of digital moiré interferometry, which consists of superimposing a reference interferogram and another obtained with the object to be measured. In this work we propose a digital moiré interferometry method that uses the Fourier transform and Butterworth-type filters. The feasibility and versatility of the proposed method is experimentally demonstrated using a Mach–Zehnder-type interferometer to analyze gas flow in a flame.