Real-time quantitative Schlieren imaging by fast Fourier demodulation of a checkered backdrop

TL;DR

This paper introduces a fast Fourier demodulation method using a checkerboard pattern for real-time quantitative Schlieren imaging, achieving comparable accuracy to advanced techniques but with significantly improved speed.

Contribution

The paper presents the novel 'Fast Checkerboard Demodulation' method, enabling rapid and accurate quantitative Schlieren imaging using a periodic pattern instead of random dots.

Findings

FCD method is as accurate as multi-stage DIC and OF techniques.

FCD is significantly faster than traditional methods.

Open-source Matlab implementations are provided.

Abstract

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.