Processing method for near-field in-line holograms (Fresnel number ≥1)

• An application of an enhanced Digital Holography in near field is described.
• An optimization algorithm allows achieving Royer criteria in emulsions.
• An original processing method based on hologram segmentation.
• An efficient droplet׳s path tracking highlight effects of the local flow structure.

Liquid–liquid extraction is used in many industrial processes. Such processes involve liquid–liquid flows, called emulsions, which can be successfully characterized by especially suited holographic techniques. Because the droplets are millimeter size, a near-field diffraction regime is observed. This regime departs from conventional holography and requires adapted numerical solutions. Due to droplets transparency, positioning could not be achieved by usual focusing methods based on research of singularities. The only suitable method is based on calculating an indicator. To be effective a segmentation of the image into individual diffraction patterns is needed. As initial reconstruction can lead to errors, to further improve robustness, the hologram restitution is checked, and then the four parameters, radius and position, are optimized to fit the acquired hologram. If some missing droplets are found, they are identified and included. As a result about slightly less than twice as many drops are included, leading to a shadow density of 20%. Outlooks are regarding the adaptation to cylindrical geometry of industrial columns and a two channel system to improve the maximum hold-up to be measured.