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This work investigates the imaging performance, in terms of contrast and resolution, of two different time-gated ballistic imaging setups commonly used in spray research. It is shown that the two setups generate similar spatial resolution in the presence of scattering media. The simpler (2f) setup, however, is less sensitive to component misalignments and time-gate induced aberrations than the commonly used (4f) system. Measurements comparing both arrangements indicated slightly higher contrast for the 2f system under the densest conditions for small scatterers. Subsequent computational modeling confirmed the observed tolerance of the 2f system to misalignment and gate effects. The best performing setup was also compared experimentally to its non-time-gated shadow-imaging equivalent, to establish when the time-gate enhances imaging performance. It is shown that the time-gated setup generates higher contrast under almost all of the scattering conditions tested, while the non-time-gated setup generates higher spatial resolution only in the lower scatterer size range at the lowest scatterer concentrations.

We investigate transmission through falling snow using a 3D imaging lidar at 1.56-μm wavelength. The lidar is based on the time-correlated single-photon counting technique. Experimental transmission data are compared with Mie theory transmission calculations based on snow particle size distribution simultaneously measured with a laser disdrometer. The calculations were performed in two ways, using the Beer-Lambert approach where all radiation interacting with a hydrometeor is considered extinct and an approach that includes effects of the forward scattering. Comparison of these two methods shows that inclusion of the contribution from forward scattering gives better agreement between experiment and calculations than using extinction only. When comparing the results using scattering calculations with a curve fit approach based on precipitation rate, it is evident that both the Beer-Lambert approach and the forward scattering approach give a much better correlation between experiment and calculations than relying on precipitation rate, as measured with the disdrometer, only. © The Authors.Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Probing the dynamics of structures in turbid media is important for understanding the internal forces which drive the time-evolution of many fluid systems; the breakup of fuel injection sprays is a prime example. We demonstrate a three-pulse configuration for timegated ballistic imaging, applied to a turbulent, steady spray allowing the acquisition of time-correlated image data. Coupled with targeted region-matching analysis, the detected image-triplets are used to generate timeresolved velocity and acceleration vectors representing motion and forces involved in spray development.