In the past few decades various particle image based volumetric flow measurement techniques have been developed which showed their potential in accessing unsteady flow properties quantitatively in various experimental applications in fluid mechanics. In this article we would like to focus on physical properties and circumstances of 3D particle-based measurements and what knowledge can be used for gaining advancements in the reconstruction accuracy, spatial and temporal resolution and completeness. The natural candidate for our focus is 3D Lagrangian Particle Tracking (LPT), which allows determining position, velocity and acceleration along a large number of individual particle tracks in the investigated volume. With the advent of the dense 3D LPT technique Shake-The-Box in the past decade further possibilities for characterizing unsteady flows have been opened by delivering input data for powerful data assimilation techniques which use Navier-Stokes constraints. As a result, high-resolution Lagrangian and Eulerian data can be gained including long particle trajectories embedded in time-resolved 3D velocity- and pressure fields
