Abstract
Imaging through turbid environments is experimentally challenging due to multiple light scattering. Structured laser illumination has proven to be effective to minimize errors arising from this phenomenon, allowing the interior of optically dense media to be observed. However, in order to preserve the image spatial resolution while suppressing the intensity contribution from multiple light scattering, the method relies on multiple acquisitions and thus sequential illumination. These requirements significantly limit the usefulness of structured illumination when imaging highly transient events. Here we present a method for achieving snapshot visualizations using structured illumination, where the spatial frequency domain is increased by a factor of two compared to past structured illumination snapshots. Our approach uses two crossed intensity-modulated patterns, allowing us to expand the spatial frequency response of the extracted data. The snapshot capability of this imaging approach allows tracking single particles and opens up for the extraction of velocity vectors by combining it with standard particle tracking/image velocimetry (PTV or PIV) equipment. In this paper we demonstrate the capabilities of this new method and, for the first time, use structured illumination to extract velocity vectors in 2D in a transient turbid medium, in this case an optically dense atomizing spray.
Highlights
Atomizing sprays form the base for many combustion engine concepts commonly used in modern society, such as gasoline direct injection (GDI) engines, gas turbines and Diesel engines
The dual-frame imaging capability of the camera allowed for a minimum time separation between the two frames of around 200 ns
We have presented a novel Structured Illumination approach able to spatially resolve micrometric liquid structures moving at high velocities within an optically dense spray system
Summary
Atomizing sprays form the base for many combustion engine concepts commonly used in modern society, such as gasoline direct injection (GDI) engines, gas turbines and Diesel engines. Means to estimate the contribution from unperturbed/singly scattered light from a single modulated “subimage” (one-phase Structured Illumination, or 1p-SI) has been presented for snapshot imaging[30,31], holding promise to solve this particular issue (note that “phase” in one-phase Structured Illumination refers here to the use of a single sinusoidal intensity modulation and not to the different states of matter) The analysis of such a 1p-SI method is based on a spatial frequency-sensitive algorithm, where a band-pass filter is applied (in the Fourier domain) on the fundamental frequency component of the superimposed intensity modulation. The main drawback, with currently presented 1p-SI methods concerns the loss in spatial resolution, since it only extracts a small portion of the spatial frequencies offered by the imaging system
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