<p>Digital inline holography (DIH), as a three-dimensional (3D) measurement technique, is widely used to characterizations of particles, droplets, or bubbles in different multi-phase flow circumstances. The setup of DIH contains a laser source, a spatial filter, a collimating lens, and a digital camera. The laser beam from the source first passes the spatial filter and the collimating lens to form a collimated beam. By having the beam pass the target object, the hologram is generated and recorded by a digital camera. The post-processing algorithm uses the phase information carried by the interference pattern to reconstruct the shape and the location of the target object. However, such a reconstruction mechanism produces a different level of uncertainty between the in-plane (the plane parallel to the image plane) direction and out-of-plane (the plane normal to the image plane) direction, and the uncertainty of the latter is larger than the former. Also, the reconstruction algorithm fails when the interference patterns of multiple objects are overlapped on the hologram since the overlapped patterns are merged into a pure shadow without any phase information carried. This paper proposes a method, the Multi-view Digital Inline Holography (Multi-view DIH), that combines the holograms recorded from multiple views to overcome the addressed defects of the single view DIH. This technique uses a similar setup as the DIH but records the hologram from multiple views simultaneously. A post-process algorithm is developed to combine the reconstruction results from different views. For each view, such an algorithm is able to correct the out-of-plane direction result by making use of the information from the other views. However, an individual calibration process for the measurement is needed to set up a relation between the coordinates between different views. To evaluate the performance of the Multi-view DIH, a test model that imitates the droplet and liquid ligament structure is 3D printed and measured by Multi-view DIH. The results demonstrate that, when compared with the single view DIH, the Multi-view DIH has higher accuracy and shows more details of the model. The proposed method also is compared with another well-developed 3D reconstruction technique, the traditional tomography. The results show that, as more and more views added, the difference between the Multi-view DIH and the tomography is vanishing, while the Multi-view DIH has better reconstruction results when these are less view to use. Other than the number of views, the effects of viewing angles are also analyzed. Similar to tomography, the smaller the angles between the views are, the larger the uncertainty is. In sum, this paper proposes the Multi-view DIH as an improved technique of the DIH and demonstrates its potential in the 3D reconstruction of the fluid structures. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;</p>

Shape Reconstruction of Liquid Ligaments and Droplets Model via Multi-View Digital Inline Holography

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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