Tracer particles within the measurement volume are illuminated by a high power pulsed light source and the scattered light pattern is recorded simultaneously from typically 4 viewing directions using CCD cameras. The 3D particle distribution is reconstructed by a tomographic reconstruction algorithm (MART) as a 3D light intensity distribution for each voxel. The particle displacement within a chosen interrogation volume is then obtained by the 3D cross-correlation of the reconstructed particle distribution at the two exposures using advanced iterative multi-grid algorithms with deformed interrogation volumes.