  A quantitative synthetic Schlieren imaging (SSI) method based on fast Fourier
demodulation is presented. Instead of a random dot pattern (as usually employed
in SSI), a 2D periodic pattern (such as a checkerboard) is used as a backdrop
to the refractive object of interest. The range of validity and accuracy of
this "Fast Checkerboard Demodulation" (FCD) method are assessed using both
synthetic data and experimental recordings of patterns optically distorted by
small waves on a water surface. It is found that the FCD method is at least as
accurate as sophisticated, multi-stage, digital image correlation (DIC) or
optical flow (OF) techniques used with random dot patterns, and it is
significantly faster. Efficient, fully vectorized, implementations of both the
FCD and DIC/OF schemes developed for this study are made available as Matlab
scripts.
