Total-internal-reflection deflectometry for measuring small deflections of a fluid surface

Abstract

We describe a method that uses total internal reflection at the water–air interface inside a large, transparent tank, to measure the interface’s deflections. Using this configuration, we obtain an optical set-up where the liquid surface acts as a deformable mirror. The set-up is shown to be extremely sensitive to very small disturbances of the reflecting water surface, which are detected by means of visualising the reflections of a reference pattern. When the water surface is deformed, it reflects a distorted image of the reference pattern, similar to a synthetic Schlieren set-up. The distortions of the pattern are analysed using a suitable image correlation method. The displacement fields thus obtained correlate to the local spatial gradients of the water surface. The gradient fields are integrated in a least-squares sense to obtain a full instantaneous reconstruction of the water surface. This method is particularly useful when a solid object is placed just above water surface, whose presence makes the liquid surface otherwise optically inaccessible. Graphical abstract

Surface Measurement of a Large Inflatable Reflector in Cryogenic Vacuum

“…Xu et al introduced a segmentation-based, data-fusion PMD approach to reconstruct the absolute surface of a monolithic, stepped multi-mirror array [14]. Taking advantage of total internal reflection at a water-air interface, PMD has also been used to measure disturbances of a fluid surface [15]. Willomitzer et al introduced stitched panoramic measurements of stained-glass windows with a mobile device [16].…”

Air entrapment at impact of a conus onto a liquid

“…2021; Cherdantsev et al. 2021; Jain, Gauthier & van der Meer 2021 a ) to reconstruct the interface deformation prior to contact. Let us note that the impact of a body onto the free surface of a liquid, with and without an air cushion, is an important problem in numerous engineering applications (Korobkin & Pukhnachov 1988; Ross & Hicks 2019).…”

“…The advantage of such studies, as compared to investigation of genuine droplet impacts with flat or structured surface, is the opportunity to observe the evolution of the air cushion at larger spatial and temporal scales. Therefore, it is possible to use relatively simple experimental techniques (Carrat et al 2021;Cherdantsev et al 2021;Jain, Gauthier & van der Meer 2021a) to reconstruct the interface deformation prior to contact. Let us note that the impact of a body onto the free surface of a liquid, with and without an air cushion, is an important problem in numerous engineering applications (Korobkin & Pukhnachov 1988;Ross & Hicks 2019).…”

A computer vision based optical method for measuring fluid level in cell culture plates

“…Several optical approaches for measuring fluids in culture plates exist. One method uses computer vision with well placed light sources and sensors placed at specific angles to measure ray deflection [7]. This method uses reflectance of the fluid surface, which makes it usable no matter the solid mass contents of the well.…”