Abstract
Submergible digital holographic camera can measure the in situ size and shape of suspended particles, such as complex flocs and biological organisms, without disturbance. As the number of particles in the water column increases, overlapping concentric rings (interference patterns) can contaminate the holographic images. Using light intensity (LI), this study proposes a practical method to assess the degree of contamination and screen out contaminated images. The outcomes from image processing support that LI normalized on a gray scale of 0 (black) to 255 (white) can be a reliable criterion for defining the contamination boundary. Results found that as LI increased, the shape of the particle size distribution shifted from a positively skewed to a normal distribution. When LI was lower than approximately 80, owing to the distortion of particle properties, the settling velocities derived from the contaminated holograms with mosaic patterns were underestimated compared to those from the uncontaminated holograms. The proposed method can contribute to a more accurate estimation of the transport and behavior of cohesive sediments in shallow estuarine environments.
Highlights
Digital holography is an imaging method that records holograms using a charge-coupled device (CCD) camera
In image processing for extracting particle properties, the evaluation of whether a hologram is contaminated is relatively subjective; further, the definition of the criterion for determining the contamination is not clear. This is because hologram contamination is primarily related to out-of-range suspended sediment concentration (SSC) (i.e., SSC lower than the lower limit or higher than the upper limit of the instrument) during LISST-Holo measurements (Zhao et al, 2018)
A critical SSC that generates mosaic patterns on a hologram depends on the particle size and beam attenuation with path length, as follows (Agrawal et al, 2008): c = − ln t/L
Summary
Digital holography is an imaging method that records holograms using a charge-coupled device (CCD) camera. The application of holographic techniques to cohesive sediments distributed in coastal environments has been somewhat limited It is because the high concentration of fine-grained suspended particles greatly reduces the optical transmittance within the water column (Sun et al, 2002). Flocculated cohesive sediments are readily settled and deposited on the bed during slack tides, and resuspended into the overlying layer during tidal acceleration periods Such cyclic behaviors form a high-concentration (greater than hundreds of mg l−1) near-bed layer, Practical Method to Screen Contaminated Holograms which makes it hard to meet the optical transmittance required to acquire the proper hologram and conduct post-processing for image analysis. The threshold for capturing proper hologram varies with the local bed properties because the SSC greatly depends on the size and shape of particles (DaviesColley et al, 2014; Merten et al, 2014), causing difficulties in the accurate detection of complex cohesive sediments
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