Presentation by Walter Schäfer on “The Time-Shift Technique for the Measurement of Particle Size and Velocity” at the LIP 2012 – Lasers and Interactions with Particles Conference, held in Rouen, France, on March 28, 2012. The work presented was carried out at Technische Universität Darmstadt by Walter Schäfer and Cameron Tropea.
Abstract
In this study, we present a novel optical measurement approach based on the Time-Shift Time-of-Flight (TSTOF) technique for the simultaneous determination of droplet size and velocity in sprays. The technique leverages time-resolved light scattering signals generated as individual droplets traverse a shaped laser beam. These time-shift signals encode critical information about the spatial displacement of scattering modes, which can be directly related to the physical properties of the droplet. Experimental investigations were conducted on two contrasting types of spray systems: paint droplets containing pigments and dispersions, and milk droplets, which represent a natural emulsion system. Both present challenges in optical diagnostics due to internal light scattering, absorption, and signal distortion caused by their complex compositions. Using a near-backscatter configuration and dual-detector setup, we demonstrate that the TSTOF technique remains robust even in the presence of secondary scattering and poor signal-to-noise conditions. For milk sprays, clear time-shift signatures corresponding to first-order refraction and surface reflections were resolved, enabling accurate estimation of droplet diameter and velocity. In the case of pigmented paint sprays, the system was able to detect and process distorted signals by employing a relaxed validation criterion, offering meaningful ensemble-based statistics. The results highlight the potential of TSTOF as a compact, alignment-tolerant, and material-independent method for analyzing both transparent and complex, multi-phase droplet systems.