Improvement of the size estimation of 3D tracked droplets using digital in-line holography with joint estimation reconstruction

International audienceDigital holography is a valuable tool for three-dimensional information extraction. Among existing configurations, the originally proposed setup (i.e. Gabor, or in-line holography), is reasonably immune to variations in the experimental environment making it a method of choice for studies of fluid dynamics. Nevertheless, standard hologram reconstruction techniques, based on numerical light back-propagation are prone to artifacts such as twin images or aliases that limit both the quality and quantity of information extracted from the acquired holograms. To get round this issue, the hologram reconstruction as a parametric inverse problem has been shown to accurately estimate 3D positions and the size of seeding particles directly from the hologram. To push the bounds of accuracy on size estimation still further, we propose to fully exploit the information redundancy of a hologram video sequence using joint estimation reconstruction. Applying this approach in a bench-top experiment, we show that it led to a relative accuracy of 0.13 % (for a 30 µm diameter droplet) for droplet size estimation, and a tracking accuracy of σ x × σ y × σ z = 0.15 × 0.15 × 1 pixels

Erratum: Generalized Lorenz-Mie theory for infinitely long cylinders with elliptical cross sections (J. Opt. Soc. Am. A)

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Lagrangian measurements of the fast evaporation of falling diethyl ether droplets using in-line digital holography and a high-speed camera

International audienceThe evaporation of falling diethyl ether droplets is measured by following droplets along their trajectories. Measurements are performed at ambient temperature and pressure by using in-line digital holography. The holograms of droplets are recorded with a single high-speed camera and reconstructed with an ''inverse problems'' approach algorithm previously tested (Chareyron et al. New J Phys 14:43039, 2012). Once evaporation starts, the interfaces of the droplets are surrounded by air/vapor mixtures with refractive index gradients that modify the holograms. The central part of the droplets holograms is unusually bright compared to what is expected and observed for non-evaporating droplets. The reconstruction process is accordingly adapted to measure the droplets diameter along their trajectory. The diethyl ether being volatile, the droplets are found to evaporate in a very short time: of the order of 70 ms for a 50-60 lm diameter at an ambient temperature of 25 C. After this time, the diethyl ether has fully evaporated and droplets diameter reaches a plateau. The remaining droplets are then only composed of water, originating from the cooling and condensation of the humid air at the droplet surface. This assertion is supported by two pieces of evidence: (i) by estimating the evolution of droplets refractive index from light scattering measurements at rainbow angle and (ii) by comparing the evaporation rate and droplets velocities obtained by digital holography with those calculated with a simple model of evaporation/condensation. The overall results show that the in-line digital holography with ''inverse problems''approach is an accurate technique for studying fast evaporation from a Lagrangian point of view

Transient internal and scattered fields from a multi-layered sphere illuminated by a pulsed laser

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Numerical predictions of microcavity internal fields created by femtosecond pulses, with emphasis on whispering gallery modes

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Introduction to the use of distributions for light scattering in elliptical coordinates

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Spatio-temporal analysis of a liquid jet using a sub-picosecond optical gate

International audienceA Diesel jet visualization based on an ultra-short light pulse and a time-resolved detection is proposed. An optical gate, consisting of a BBO (Beta Barium Borate) crystal, allows to separate ballistic, refractive and scattered light. An imaging pulse passes through the studied medium while a gating pulse is used to open the gate. With this configuration, a spatial resolution of 10 µm and a temporal resolution of 270 fs are obtained. The gate duration is compatible with the typical dimensions of a liquid jet generated by a commercial automotive fuel injector. Preliminary results show that spatio-temporal diagrams of transmitted light through a spray may be acquired, showing clearly the separation between ballistic, refracted and scattered light