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

Digital inline holography (DIH), as a three-dimensional (3D) measurement technique, is widely used in characterizations of the particles, droplets or bubbles under different multi-phase flow circumstances. By analyzing the phase information carried by the interference pattern, the reconstruction of shape and the location of a test target is then achieved. However, such reconstruction mechanism produces different levels of uncertainty between the in-plane (the plane parallel to the hologram plane) direction and out-of-plane (the plane normal to the hologram plane) direction, and the uncertainty of the latter is larger than the former. Also, the reconstruction algorithm fails when the interference patterns of some sections of the target are overlapped on the hologram since the overlapped patterns are merged into a pure shadow which doesn’t carry any phase information. This paper tested a method, the Multi-view Digital Inline Holography (MvDIH), that combines the holograms recorded from multiple views to overcome the addressed defects of the single view DIH. This technique uses the similar setup as the DIH but applies a different post-process method to implement the reconstruction. As the DIH is applied to each view, one can not only acquire the cross-section of the target in the hologram plane but also the outline of such cross-section in the space. Then, two reconstruction methods with different ideologies are developed as, the one based on the outline and the one based on the cross-section. A post-process algorithm is developed to realize these two reconstruction methods with the holograms recorded from different views. To evaluate the performance of the Multi-view DIH, a test model which imitates the droplet and liquid ligament structure is 3D printed and measured by the proposed method. The results demonstrate that, with only three view, both method provides limited reconstruction result. When comparing to the true test model, for the outline based method, some parts of the reconstructed model are missing and some details are merged into one piece with simple geometry. Yet, for the cross-section based method, the reconstructed model contains redundant parts which also make such result unsatisfied. As the used holograms are increased to six views, the reconstructed result for cross-section based method is approaching to the true model, but still some sections are reconstructed with certain level of ambiguity.

Measurement of Cell Volume Using In-Line Digital Holography

The measurement of the volume of blood cells is important for clinical diagnosis and patient management. While digital holography microscopy (DHM) has been used to obtain such information, previous off-axis setups usually involve a separated reference beam and are thus not very easy to implement. Here, we use the simple in-line Gabor setup without separation of a reference beam to measure the shape and volume of cells mounted on glass slides. Inherent to the in-line holograms, the reconstructed phase of the object is affected by the virtual image noise, producing errors in the cell volume measurement. We optimized our approach to use a single hologram without phase retrieval, increasing distance between cell and hologram plane to reduce the measurement error of cell volume to less than 6% in some instances. Therefore, the in-line Gabor setup can be a useful and simple tool to obtain volumetric and morphologic cellular information.

A Comparison of Different Counting Methods for a Holographic Particle Counter: Designs, Validations and Results

Digital Inline Holography (DIH) is used in many fields of Three-Dimensional (3D) imaging to locate micro or nano-particles in a volume and determine their size, shape or trajectories. A variety of different wavefront reconstruction approaches have been developed for 3D profiling and tracking to study particles’ morphology or visualize flow fields. The novel application of Holographic Particle Counters (HPCs) requires observing particle densities in a given sampling volume which does not strictly necessitate the reconstruction of particles. Such typically spherical objects yield circular intereference patterns—also referred to as fringe patterns—at the hologram plane which can be detected by simpler Two-Dimensional (2D) image processing means. The determination of particle number concentrations (number of particles/unit volume [#/cm 3 ]) may therefore be based on the counting of fringe patterns at the hologram plane. In this work, we explain the nature of fringe patterns and extract the most relevant features provided at the hologram plane. The features aid the identification and selection of suitable pattern recognition techniques and its parameterization. We then present three different techniques which are customized for the detection and counting of fringe patterns and compare them in terms of detection performance and computational speed.

Weighted Constraint Iterative Algorithm for Phase Hologram Generation

A weighted constraint iterative algorithm is presented to calculate phase holograms with quality reconstruction. The image plane is partitioned into two regions where different constraint strategies are implemented during the iteration process. In the image plane, the signal region is constrained directly according to the amplitude distribution of the target image based on an adaptive strategy, whereas the non-signal region is constrained indirectly by total energy control of the hologram plane based on the energy conservation principle. The weighted constraint strategy can improve the reconstruction quality of the phase holograms by broadening the optimizing space of the iterative algorithm, leading to effective convergence of the iteration process. Finally, numerical and optical experiments have been performed to validate the feasibility of our method.

Reducing Computational Complexity and Memory Usage of Iterative Hologram Optimization Using Scaled Diffraction

A complex amplitude hologram can reconstruct perfect light waves. However, as there are no spatial light modulators that are able to display complex amplitudes, we need to use amplitude, binary, or phase-only holograms. The images reconstructed from such holograms will deteriorate; to address this problem, iterative hologram optimization algorithms have been proposed. One of the iterative algorithms utilizes a blank area to help converge the optimization; however, the calculation time and memory usage involved increases. In this study, we propose to reduce the computational complexity and memory usage of the iterative optimization using scaled diffraction, which can calculate light propagation with different sampling pitches on a hologram plane and object plane. Scaled diffraction can introduce a virtual blank area without using physical memory. We further propose a combination of scaled diffraction-based optimization and conventional methods. The combination algorithm improves the quality of a reconstructed complex amplitude while accelerating optimization.

Modelling a Holographic Particle Counter

In order to design an imaging unit of a novel holographic particle counter an aerosol particle model was developed to generate a virtual hologram plane of an aerosol volume of interest. The herein presented model combines the three essential components to help dimensioning a target detection unit: (i) an In-Line holography model with a reference light source and a basic transfer function of an imager to take into account imager size, pixel pitch and exposure time; (ii) an aerosol particle model with particles of variable count, size and spatial distribution; and (iii) the possibility to import fluid dynamics simulation data to simulate the particle flow in an arbitrary sampling volume.