Digital in-line holography
Since phonation produces aerosols, speech has proven to be a pernicious, invisible, yet potent viral transmission route among asymptomatic individuals in the COVID-19 pandemic. To develop comprehensive mitigation strategies and mechanisms to control transmission, it is crucial to measure both the amount of aerosols produced by a speaking individual and the associated atomization processes. However, these biological emissions are hard to characterize since they are intermittent, faint, range drastically in size between 1 and 1000 microns, and form and travel rapidly, with speeds of a few to tens of meters per second. We implemented digital in-line holography (DIH) as a lens-less and powerful tool to overcome many of these experimental challenges.
We combine high-speed imaging with DIH, and a schematic of the setup is shown in schematic on the left. Briefly, the head of the subject is positioned to the side of a collimated blue (488 nm) laser beam (of diameter ∼3 cm), facing perpendicular to the direction of laser propagation, such that the lips are at the height of the beam. As the subject speaks, the lips are inside the laser beam.
This allows for the full visualization of the formation of saliva filaments between the lips, followed by their extension and eventual fragmentation into droplets.