Abstract
In the detection of particles using digital in-line holography, measurement accuracy is substantially influenced by the hologram processing method. In particular, a number of methods have been proposed to determine the out-of-plane particle depth (z location). However, due to the lack of consistent uncertainty characterization, it has been unclear which method is best suited to a given measurement problem. In this work, depth determination accuracies of seven particle detection methods, including a recently proposed hybrid method, are systematically investigated in terms of relative depth measurement errors and uncertainties. Both synthetic and experimental holograms of particle fields are considered at conditions relevant to particle sizing and tracking. While all methods display a range of particle conditions where they are most accurate, in general the hybrid method is shown to be the most robust with depth uncertainty less than twice the particle diameter over a wide range of particle field conditions.
Highlights
IntroductionDigital in-line holography (DIH) has been extensively applied to the detection and characterization of particle fields, where the particles can be tracer particles in flow measurements [1,2,3,4], droplets in spray diagnostics [5,6,7,8], micro-organisms in biological mobility studies [9,10], bubbles in multiphase flows [11, 12] and other particles of interest [13,14,15]
The results obtained from detection of a single particle represent the ideal performance of a particle detection method, because the hologram is free of experimental noises, e.g., diffraction patterns due to dusts on optics, aberrations in the planar wavefront, interference between light scattered by different particles, etc
A number of factors which are known to impact the accuracy of particle detection methods are investigated, including the Fresnel number, particle shape, particle number density, threedimensionality of the particle field and particle overlap in the x-y plane
Summary
Digital in-line holography (DIH) has been extensively applied to the detection and characterization of particle fields, where the particles can be tracer particles in flow measurements [1,2,3,4], droplets in spray diagnostics [5,6,7,8], micro-organisms in biological mobility studies [9,10], bubbles in multiphase flows [11, 12] and other particles of interest [13,14,15]. The first category includes methods that utilize the reconstructed intensity (amplitude) image of the particle. In the reconstruction of particle holograms without filtering of the DC term, the focused image of a particle appears as a dark region with sharp edges in contrast to the bright background. The measurement accuracy of the HYBRID method is quantified in terms of relative depth error and depth uncertainty using both synthetic and experimental holograms. This is followed by the numerical and experimental quantification of measurement accuracy and the conclusions
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