Snapshot 3D reconstruction of liquid surfaces

Abstract

In contrast to static objects, liquid structures such as drops, blobs, as well as waves and ripples on water surfaces are challenging to image in 3D due to two main reasons: first, the transient nature of those phenomena requires snapshot imaging that is fast enough to freeze the motion of the liquid. Second, the transparency of liquids and the specular reflections from their surfaces induce complex image artefacts. In this article we present a novel imaging approach to reconstruct in 3D the surface of irregular liquid structures that only requires a single snapshot. The technique is named Fringe Projection-Laser Induced Fluorescence (FP-LIF) and uses a high concentration of fluorescent dye in the probed liquid. By exciting this dye with a fringe projection structured laser beam, fluorescence is generated primarily at the liquid surface and imaged at a backward angle. By analysing the deformation of the initial projected fringes using phase-demodulation image post-processing, the 3D coordinates of the liquid surface are deduced. In this article, the approach is first numerically tested by considering a simulated pending drop, in order to analyse its performance. Then, FP-LIF is applied for two experimental cases: a quasi-static pending drop as well as a transient liquid sheet. We demonstrate reconstruction RMS errors of 1.4% and 6.1% for the simulated and experimental cases respectively. The technique presented here demonstrates, for the first time, a fringe projection approach based on LIF detection to reconstruct liquid surfaces in 3D. FP-LIF is promising for the study of more complex liquid structures and is paving the way for high-speed 3D videography of liquid surfaces.

High-speed 3D imaging of liquid sheet surfaces using the FP-LIF technique

“…Here, A is the background light intensity in the image, B is the amplitude of the fringes, epsilon is noise and finally φ is the phase of the fringes. In this imaged fringe pattern, all interesting information of the third dimension is found in the phase φ and to extract it a phase demodulation of the image performed, see [12] for more information on this. With the estimated phase, the third dimension, depth d(x, y), can be estimated for the surface with the following equation,…”

“…The scale s of how many pixels there are per meter and finally the principal image point [x 0 , y 0 ] of the camera. This is the general approach for the 3D reconstruction using FP-LIF, for more in depth information see [12]. The FP-LIF requires the use of fluorescing liquid.…”

“…High-speed 3D imaging of liquid breakups in sprays, showing detailed temporal surface deformation with a fairly good spatial resolution and by means of a single camera remains a challenge. With this goal in mind, we have recently developed a technique called Fringe Projection -Laser Induced Fluorescence (FP-LIF) [12]. FP-LIF is based on the Fringe Projection concept where a structured laser beam is used to illuminate a liquid structure.…”

Three-dimensional encoding of a gas–liquid interface by means of color-coded glare points

“…Ihrke et al (2005) exploited the emission from fluorescent dye dissolved in the fluid imaged from multiple viewpoints for the volumetric reconstruction of a stream of water. More recently, Roth et al (2020) combined laser-induced fluorescence (LIF) (Kinsey 1977) and the projection of a fringe pattern to reconstruct the 3D-surface of a pendent droplet with a single camera setup. Horbach and Dang (2010) used the reflection of a structured light pattern on a specular surface for a 3D reconstruction through a region-growing approach that considers local curvature features.…”