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

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 open source Matlab scripts.

Analog cosmology with two-fluid systems in a strong gradient magnetic field

“…A significant advantage of our proposed system is the ability to make non-destructive measurements of interface-waves. For example, using a "Fast Checkerboard Demodulation" (FCD) method [32] enables the time-resolved measurement of the full dynamics. In contrast to the observable universe (which was realised exactly once), in our system we can automise and repeat the experiment a large number of times in order to analyse the statistical properties of the state after inflation.…”

Space-Time Folding of the Wake Produced by a Supervelocity Rotating Point Source

“…1(c)), the observer thus only sees one image of the source. For Ma > 1, as the source rotates, an observer standing outside of the source trajectory observes the following sequence: i) a single image rotates in the same direction as the source, ii) a second image appears as the outer branch of the wakes reaches her iii) the new image splits into two images moving in opposite directions; see observer in We measure the wave fields using a quantitative real-time full-field measurement method [22]. A black and white square grid pattern is set beneath the plexiglass tank at a distance of 7 cm.…”

Slamming Liquid Impact and the Mediating Role of Air

“…This method is a spin on the well known Schlieren method, and is known as the free-surface synthetic Schlieren method. This was first proposed by Kurata et al [53], and since has been matured by the works of Moisy et al [40] and Wildeman [54] to result in a packaged method that is quick and inexpensive to arrange. In such methods, the optics of the problem are used to compute the spatial gradients of the FS.…”

“…Here we visualise the movements of the water surface by using it as a specularly reflecting surface in a total-internal-reflection (TIR) configuration. Taking inspiration from Moisy et al [40] and Wildeman [54], we use a fixed pattern, whose distortions by the moving FS are interpreted in a synthetic-Schlieren sense to obtain displacement fields. Note that contrary to Moisy et al [40] and Wildeman [54], we use the water surface as a mirror rather than as a lens.…”

“…Taking inspiration from Moisy et al [40] and Wildeman [54], we use a fixed pattern, whose distortions by the moving FS are interpreted in a synthetic-Schlieren sense to obtain displacement fields. Note that contrary to Moisy et al [40] and Wildeman [54], we use the water surface as a mirror rather than as a lens. From the point of view of a ray-optics problem, the presence of a mirror results in an additional complication as it is the reflecting 'mirror' that undergoes deformation, and not the apparent object that is behind the mirror.…”