Depth position detection of the particles in digital holographic particle image velocimetry (DHPIV)

Due to the fast improvement of digital sensors the digital holographic particle image velocimetry (DHPIV) becomes a promising measurement technique for the determination of the three dimensional velocity components for standard flow problems. Basically, this technique is based upon correlation and particle tracking methods for the extraction of the velocity information from the images. If a high-speed digital camera is used, the velocity information from the tracer particles is saved in discrete time steps in form of holograms. Each hologram implicitly contains the information about the 3D volume and accordingly the location of the tracer particles. Their 3D coordinates can be obtained after numerical reconstruction of the investigated volume from the corresponding hologram. Applying correlation and/or particle tracking methods to the holographic films yield the velocity components of the flow passing through the investigated volume. At present, one of the major problems is the accuracy of the particles depth coordinates determination, which is due to the still relative small resolution of the sensors. This paper aims to analyse the efficiency of existing particles depth coordinates extraction techniques, as well as their combination with known techniques of hologram pre-processing like the zero-order elimination in the in-line setup. Additionally, a novel technique for the extraction of the depth positions of the particles is presented, which improves the accuracy of the particles depth coordinate extraction.