Interaction entre une impulsion lumineuse ultra-brève et un nuage dense de particules
Author | : Cécile Calba |
Publisher | : |
Total Pages | : 139 |
Release | : 2008 |
ISBN-10 | : OCLC:834769551 |
ISBN-13 | : |
Rating | : 4/5 (51 Downloads) |
Book excerpt: Metrologies used to characterize two-phase flows are often based on non intrusive optical techniques (PDA, PIV, diffractometry, turbidimetry, ...), which use single scattering hypothesis. Their are ineffective in dense media where multiple scattering is predominant and scrambles the signal. This work is dedicated to study the potential of ultra-short light pulses to overpass this limitation, in association with a time-resolved detection. With this kind of illumination, scattered light is temporally divided versus different optical paths. Direct photons called ballistic photons are distinguished to scattered photons in forward detection. Time-resolved scattering phenomena have been quantified with help of numerical tools. An Monte Carlo code has been built. It uses single scattering characteristics describing interaction between one particle and an ultra-short pulse, obtained by Lorenz-Mie theory or Debye series. The originality of this code is to take into account the interaction time between incident pulse and particles, added to the propagation time between the scattered centers. This code is interesting for large particles (50-150 μm). In small sized particles case (small than 5 μm) a simplified semi Monte Carlo algorithm taking into account polarization can be uses, taking into account only propagation time between the scattered centers. Experiments have been made using a femtosecond system on dense calibrated solutions of particles (polystyrene or glass) immersed in water. Comparisons between computations and experiments show a good agreement. Concentration and particle size measurements have been demonstrated for optical depths until 20 and metrological perspectives are discussed.