Multiple Scattering Suppression In Planar Laser Imaging Of Dense Sprays By Means Of Structured Illumination

A novel method to reduce the multiply scattered light contribution to images recorded with planar laser imaging is demonstrated. The technique, structured laser illumination planar imaging (SLIPI), is based on spatially modulated excitation light and is tested here within the dense region of a hollow-cone spray. The main idea is to use a laser sheet that is spatially modulated along the vertical direction. By both shifting the spatial phase of the modulation and using adequate image post processing of the successive recorded images, it is possible to remove a significant amount of the multiply scattered light detected. In this paper, SLIPI is applied for imaging within a typical hollow-cone water spray generated in ambient air at 50 bars injection pressure from a pressure-swirl nozzle. Because this type of spray has a known inner structure, the method can be evaluated, demonstrating that 47% of the detected light arising from multiple scattering can be suppressed, resulting in an increase from 61% to 89 % in image contrast. Such an improvement allows more accurate interpretation and analysis of the near-field region of atomizing sprays. The possibility of extracting instantaneous flow motion is also demonstrated for the case of a dilute nebulizer. All these results indicate promising applications of the technique in denser turbid media, such as air-blast atomizer or diesel sprays