Digital holographic near-field reconstruction applied to particles characterization in astigmatic optical systems and refractive index measurement

Digital in-line holography (DH) is widely used to characterize two-phase flows as it allows determining, simultaneously and in one shot, the shape, 3D center position and size of particles. In this communication, two extensions of DH are presented. The first one concerns particles characterization in astigmatic optical systems (e.g. optical lenses, cylindrical cells ). Prob-lems induced by astigmatism are solved, in the paraxial approximation, with propagators de-rived from the generalized diffraction integral that is expressed with a classical ray transfer matrix formalism. These generalized propagators associated to suitable sampling conditions allow to simulate or back propagate holograms in an accurate, flexible and numerically effec-tive way.The second extension relies on the measurement of the refractive index of spherical and trans-parent particles, for material recognition purposes and extensive multiphase flows characteriza-tion. The method is based on the analysis of the photonic jet generated by these kind of parti-cles. Direct calculations with the Lorenz-Mie theory show that the structure of this optical near-field caustic is strongly dependent upon the particle refractive index and diameter. Even if the particle near-field appears to be roughly reconstructed with DH, we demonstrated in this communication that the particle refractive index can be straightforward retrieved using the lo-calization of the photonic jet maxima and geometrical considerations. In this presentation, all the mathematical backgrounds of these solutions are detailed and illustrated by numerical and experimental results.