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Structured Laser Illumination Planar Imaging: New horizons for the study of spray dynamics, thermometry and droplet sizing
Edouard Berrocal

Division of Combustion Physics
Lund University
Sweden


The first reported applications of laser sheet imaging for the study of atomizing sprays date from the mid 1980th. Since those early tries, blurring effects from the light being scattered multiple times by the surrounding droplets were already observed and reported. While strategies in suppressing part of this multiple light scattering contribution, were known for transmission imaging (e.g. spatial Fourier filtering, polarization filtering, time-gating) no robust solution was found for laser sheet imaging until the development of Structured Laser Illumination Planar Imaging (SLIPI) in 2008.
The originality of SLIPI resides in using a laser sheet with a spatially modulated light intensity. This light structure encodes the incident illumination beam which can be then decoded after image recording. As multiply scattered photons have “short memory” they do lose the modulation information while singly scattered photons fully preserve it. Therefore the two components, single and multiple scattering, can be separated using a demodulation post-processing algorithm on the recorded images.
This article is a review of the SLIPI technique from its earliest to latest developments. It describes the traditional way of applying structured illumination using three modulated sub-images and its use for averaged imaging of droplet sizes, extinction coefficients and spray thermometry. Recently the technique has been adapted for single-shot applications in order to study spray dynamics and liquid breakups. This is performed using two modulated sub-image instead of three, known as 2p-SLIPI, opening new horizons for the study of spray dynamics, single-shot droplet sizing and thermometry, even through optically dense situations. The limitations and advantages of SLIPI, in comparison with conventional laser sheet imaging, will also be discussed.