A method of determining a three-dimensional velocity field in a volume having particles, the particles within the volume being excited to radiate by illuminating the volume, including two or more cameras simultaneously capturing images of the observation volume at two different instants of time, the observation volume being divided into small volume elements (voxels), each voxel being projected onto image points of the cameras, the intensity of all the voxels being reconstructed from the measured intensity of the respective associated image points, a plurality of voxels being combined to form an interrogation volume, and a displacement vector being determined by a three-dimensional cross correlation of the two interrogation volumes.