Thierry Girasole

Vectorial complex ray model and application to two-dimensional scattering of plane wave by a spheroidal particle

A vectorial complex ray model is introduced to describe the scattering of a smooth surface object of arbitrary shape. In this model, all waves are considered as vectorial complex rays of four parameters: amplitude, phase, direction of propagation, and polarization. The ray direction and the wave divergence/convergence after each interaction of the wave with a dioptric surface as well as the phase shifts of each ray are determined by the vector Snell law and the wavefront equation according to the curvatures of the surfaces. The total scattered field is the superposition of the complex amplitude of all orders of the rays emergent from the object. Thanks to the simple representation of the wave, this model is very suitable for the description of the interaction of an arbitrary wave with an object of smooth surface and complex shape. The application of the model to two-dimensional scattering of a plane wave by a spheroid particle is presented as a demonstration.

Comparison of iterative and monte carlo methods for calculation of the Aureole about a point source in the earth's atmosphere.

Point sources in the atmosphere are surrounded by aureoles because of atmospheric scattering. The properties of an aureole were calculated by use of a Monte Carlo approach and an iterative method for an isotropic source and an axially symmetric emission source inside an infinite homogeneous atmosphere. The influence of single-scattering albedo, optical depth between source and observer, and source intensity anisotropy were studied from both approaches. For each situation, the limits and advantages of the Monte Carlo technique and the iterative method are described.

Time gate, optical layout, and wavelength effects on ballistic imaging

A method to distinguish a hidden object from a perturbing environment is to use an ultrashort femtosecond pulse of light and a time-resolved detection. To separate ballistic light containing information on a hidden object from multiscattered light coming from the surrounding environment that scrambles the signal, an optical Kerr gate can be used. It consists of a carbon disulfide (CS(2)) cell in which birefringence is optically induced. An imaging beam passes through the studied medium while a pump pulse is used to open the gate. The time-delayed scattered light is excluded from measurements by the gate, and the multiple-scattering scrambling effect is reduced. In previous works, the two beams had the same wavelength. We propose a new two-color experimental setup for ballistic imaging in which a second harmonic is generated and used for the image, while the fundamental is used for gate switching. This setup allows one to obtain better resolution by using a spectral filtering to eliminate noise from the pump pulse, instead of a spatial filtering. This new setup is suitable for use in ballistic imaging of dense sprays, multidiffusive, and large enough to show scattered light time delays greater than the gate duration (tau=1.3 ps).

Measurement of bubbles by Phase Doppler technique and trajectory ambiguity

That paper is devoted to the analysis of the importance of the finite size of the laser beam on the quality of bubble measurements by a Phase Doppler Anemometer (PDA) (trajectory ambiguity or Gaussian beam effect). Detection at 30 ° and 70° are studied for a standard PDA, a non-standard PDA and dual burst configurations. The possibility to sort bubbles and perfectly reflecting particles is discussed.

Interaction between ultra short pulses and a dense scattering medium by Monte Carlo simulation: consideration of particle size effect

Abstract With an enough short-pulse incident to an individual particle, elementary scattering modes can be observed: internal or external reflection, refraction and diffraction. Simulation of pulse propagation in dense scattering medium is usually computed for large observation time, so that time delays of pulse interaction with the particles are negligible compared to propagation times between particle. A Monte Carlo method is proposed to compute the propagation of an incident 100 fs laser pulse in dense medium taking into account time-dependent scattering characteristics of particle: observation time of scattered light is less than 5000 fs. Two extreme cases are exemplified: predominance of direct and single-scattered photons appears in a thin time window for small particles (1 μm). On the contrary multiple scattering is always predominant and scrambles the transmitted signal for large particles (100 μm).

Multilayer four-flux model of scattering, emitting and absorbing media

Four-flux model allows to compute diffuse and collimated flux through a slab containing absorbing and scattering particles in an absorbing medium. An extension of this model is proposed so that the slab can be composed of an arbitrary number of layers. Moreover, emission of the particles and of the surrounding matrix is taken into account.

Average crossing parameter and forward scattering ratio values in four-flux model for multiple scattering media

Abstract The four-flux model for multiple scattering media allows one to evaluate scattering properties (reflectances, transmittances) by using simple formulae which may even be readily handled by pocket calculators. Unfortunately, it involves parameters ϵ , called the average crossing parameter, and ζ , called forward scattering ratio, which cannot be a priori evaluated in the framework of the four-flux model, excepted in special cases (collimated radiation or semi-isotropic diffuse radiation). It however happens that the four-flux model can be extended to larger fields of applications if the average crossing parameter and the forward scattering ratio, originally defined as physical quantities, are viewed adjustable parameters. This paper is devoted to the evaluation of these free parameters, allowing one to carry out more accurate four-flux computations.

Particle Imaging Sizing: GLMT Simulations

Defocused off-axis images of particles are computed in the framework of the generalized Lorenz-Mie theory. Two exemplifying cases are studied: interferometric sizing at large off-axis angles and imaging in near forward directions (shadow Doppler velocimetry)

Digital image computation of scenes during daytime fog

A model for digital image computation of a driving scene during daytime fog has been implemented in order to evaluate targets visibility. Using a semi-Monte Carlo method to solve the radiative transfer equation inside fog, we establish a physically based rendering with an accurate lighting simulation. The fog is modeled as a layer of water droplets described by a Deirmendjian size distribution, and the scattering properties of each droplet are computed using Mie theory. The boundary conditions are sky luminance and sun illuminance on top of the fog layer, and a bidirectional reflectance distribution function (BRDF) model derived from measurements on road samples. The veiling luminance inside the layer in all directions and for several altitudes is computed and allows us to evaluate the attenuation of every surface existing in the scene. As an example, the visibility of a Lambertian reflective silhouette is given.

Theoretical evaluation of a shadow Doppler velocimeter

The shadow Doppler velocimeter (SDV) allows one to measure the velocity and size of non-spherical and optically non-homogeneous particles with high temporal and spatial resolution. The technique is based on coherent, near forward off-axis imaging of the particles. This paper is devoted to the rigorous evaluation of the image formation of a spherical particle illuminated by two continuous laser beams and to define the theoretical limits and the sizing accuracy of the SDV approach.