SLIPI Droplet Sizing – Next-Generation Spray Diagnostics

SLIPI Droplet Sizing is a cutting-edge development combining SLIPI imaging and polarization-ratio techniques for accurate droplet size analysis, without the use of complex UV laser setups or fluorescent tracers.

SLIPI Droplet Sizing builds on SLIPI by combining structured illumination with polarization ratio imaging for tracer-free droplet sizing in optically dense sprays. In contrast to conventional imaging with a uniform laser light sheet, SLIPI uses a spatially modulated laser light sheet to illuminate the measurement plane. Light scattered only once by the droplets retains this modulation, while multiply scattered light largely loses the modulation information. SLIPI processing extracts the modulated signal and suppresses contributions from multiple scattering, providing improved image contrast even in optically dense sprays.

For droplet sizing, the perpendicular and parallel polarized components of the Lorenz–Mie scattered light are recorded. Their polarization ratio is related to the D₂₁ surface mean diameter, enabling planar droplet size measurements without fluorescent additives. The system operates with continuous wave (cw) laser illumination at 532 nm and provides a compact and flexible setup for integration into existing systems.

LaVision offers multiple SLIPI configurations to meet different diagnostic needs– from quantitative size analysis to tracer-free, polarization-based measurements.

SLIPI droplet sizing in ammonia injection

The powerful capabilities of SLIPI Droplet Sizing are reflected in the award-winning study on ammonia injection– named best paper in Experimental Spray Diagnostics at ILASS Europe 2025.
With ammonia emerging as a leading carbon-free fuel, understanding its unique spray behavior, which is dominated by flash boiling, rapid evaporation, and strong radial dispersion, is essential. These effects improve mixing but also introduce challenges such as local cooling, making accurate droplet-scale measurements critical.

Droplet size distributions D₂₁ of ammonia sprays at different injection time

Conventional optical methods often fail under the dense, highly transient conditions of ammonia sprays. The awarded work shows how LaVision’s SLIPI Droplet Sizing solution overcomes these limitations by:

• Suppressing multiple scattering for clear, depth-resolved spray images
• Providing fast, reliable 2-D droplet size measurements (D21)
• Capturing detailed spatio-temporal evolution of flash-boiling phenomena
  
  

Using the highly relevant ECN Spray G 8-hole injector, the researchers revealed how ammonia droplet sizes shrink over time during flash boiling, a key information for optimizing injectors and advancing carbon-free combustion technology.

Laser Imaging in Dense Sprays: SLIPI

Structured Laser Illumination Planar Imaging (SLIPI) reduces multiple light scattering applying laser imaging in dense sprays.
SLIPI is based on spatially modulated laser light sheets for Mie or LIF imaging. While multiply scattered light loses the modulation information, it is maintained for singly scattered light. When the stripe-like phase shifted images are combined correctly, the resulting SLIPI image shows higher image contrast and reveals inner spray structures, which are hidden when using conventional planar laser imaging.

     

Laser light sheet imaging in a hollow cone spray with and w/o SLIPI

courtesy: Elias Kristensson, ILASS-Europe, 2013

References:
Y.N.Mishra, E. Kristensson, E. Berrocal, “Reliable LIF/Mie droplet sizing in
sprays using structured laser illumination planar imaging”, Optics Express,
Vol. 22, Issue 4, pp. 4480-4492 (2014),
http://dx.doi.org/10.1364/OE.22.004480

E. Berrocal, E. Kristensson, P. Hottenbach, M. Aldén , G. Gruenefeld,
“Quantitative imaging of a non-combusting diesel spray using structured
laser illumination planar imaging”, Appl. Phys. B (2012) 109:683–694