[Show abstract][Hide abstract] ABSTRACT: A diesel jet visualisation based on an ultra-short light pulse and a time-resolved detection is proposed. An optical gate, consisting of a beta barium borate crystal, allows to separate ballistic, refractive and scattered light. An imaging pulse passes through the studied medium, while a gating pulse is used to open the gate. With this configuration, a spatial resolution of 10 μm and a temporal resolution of 270 fs are obtained. The gate duration is compatible with the typical dimensions of a liquid jet generated by a commercial automotive fuel injector. Preliminary results show that spatio-temporal diagrams of transmitted light through a spray may be acquired, showing clearly the separation between ballistic, refracted and scattered light.
No preview · Article · Feb 2011 · Experiments in Fluids
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The extended geometrical optics approximation (EGOA) and the Monte Carlo ray tracing (MCRT) have been improved to include more scattering effects and used to explore the light scattering by a spheroidal droplet and bubble. The EGOA method takes into account not only deviation of each light ray and the divergence/convergence of ray bundle, but also the interferences between rays, the diffraction, the absorption and the incident beam profile. It has been extended to the oblique incidence case and used to study the light scattering of a plane wave by a spheroidal droplet or bubble of arbitrary ellipticity. The MCRT method can treat scattering by objects of any shape in 3D. The existed model has been improved by taking into account the interferences, absorption, polarization of the incident wave and its beam shape. The comparison of the two methods has been made for each aforementioned effect and the agreement is found always very good. It is also shown that the scattering patterns of a spheroidal bubble are quite different from those of a spheroidal droplet. High dependence of ellipticity on the scattering intensity is found for a spheroidal droplet.
[Show abstract][Hide abstract] ABSTRACT: 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).
No preview · Article · Oct 2009 · Journal of the Optical Society of America A
[Show abstract][Hide abstract] ABSTRACT: Forward light scattering of femtosecond pulses through strongly scattering media is investigated experimentally and numerically. Computations are based on a semi-Monte Carlo method including polarization effects when experiments depend on a Ti:sapphire regenerative amplifier (100 fs, 1 kHz, 1 mJ@ 800 nm). The temporal separation between ballistic light and scattered light is exhibited and used to perform optical depth measurements up to 22 (transmission factor of approximately 10(-10)). Quantitative comparisons between experiments and Monte Carlo simulations show a good agreement. Temporal forward scattered light evolutions with concentration and particle size are presented. Numerical results show that the early scattered light contains information on particle size, opening the way to particle sizing in strongly scattering media.
No preview · Article · Aug 2008 · Journal of the Optical Society of America A
[Show abstract][Hide abstract] ABSTRACT: The characterization of soot fractal aggregate, i.e., morphology, size, number and volume fraction, by optical diagnostics is very widespread since it is a sensitive and nonintrusive technique. In general, light absorption or scattering by fractal aggregates is quantitatively interpreted by the Rayleigh-Debye-Gans for Fractal Aggregates (RDG-FA) theory due to its simplicity. Nevertheless, important conditions have to be obeyed for the correct use of this theory. Among them, aggregates and primary particles have to be very small in comparison to the wavelength. In addition, internal scattering must be negligible. Moreover, the form factor functions that allow the fractal morphology to be taken into account have been established for a polydispersed population. In contrast to previous studies that evaluated the valid RDG-FA range by comparing the difference between rigorous calculations and the RDG-FA approach, the aim of the present study is to evaluate possible corrections that could be applied to the classical RDG-FA formula in order to take into account of aggregates and primary particles internal multiscattering. Therefore, new generalized form factors are proposed allowing the evaluation of the optical properties of one aggregate with a primary particle diameter of up to 90 nm, without any assumptions concerning aggregate complex internal interactions. The presented results are based on the assumption that DDSCAT (Discrete Dipole Approximation) provides accurate results, with a wavelength, λ = 632 nm, a soot fractal dimension of 1.8 and Dalzel and Sarofim optical index, m = 1.57 – i0.56.
No preview · Article · Apr 2008 · Particle and Particle Systems Characterization
[Show abstract][Hide abstract] ABSTRACT: An ultra-fast time-gated shadowgraph technique applied to high pressure spray visualization is presented. The spatial resolution limitation of the technique is discussed and a two color ballistic imaging set-up is proposed to improve it, with respect to a single color set-up. An unexpected elongation of the time gate in this configuration is identified, explained and solved.
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Jan 2008 · Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: The purpose of the present study is to estimate the weldability of a polymeric material couple according to their thermal and optical properties. A first model based on Mie theory and Monte Carlo method describes the laser beam behavior in semi-transparent media and makes it possible to approximate the laser power distribution at the interface of the two materials. A second model based on finite element method permits the temperature field estimation into both parts to be welded. The results are validated by infrared thermography.
[Show abstract][Hide abstract] ABSTRACT: A semi-analytical Monte Carlo (SMC) simulation was developed to simulate the propagation of laser-induced fluorescence (LIF)
in an optically participating spray, which simultaneously exhibits spectrally dependent emission, anisotropic scattering,
absorption, and re-emission. The SMC simulation is described and then applied to an experimental configuration of a cloud
of polydisperse droplets composed of water and sulforhodamine B dye. In the SMC simulation, the collected LIF flux on the
remote receptor element is calculated as the global contribution from the emissive source, single, twice, … and nth collision events in any sequence. The effects on the fluorescence photons propagation of spray parameters like the dye
concentration, droplets concentration, and droplets size are examined. Three spectral bands representing different optical
properties are chosen to analyze the interference of absorption, scattering and re-emission on the detected LIF flux. The
obtained spectral LIF flux distribution on the receptor demonstrates a “red shift” phenomenon.
Full-text · Article · Jun 2007 · Journal of Zhejiang University SCIENCE
[Show abstract][Hide abstract] ABSTRACT: In this paper a numerical model for laser beam scattering in the semi-transparent polymers is presented, using a Monte Carlo algorithm and the Mie theory. The algorithm correctly accounts for the independent multiply-scattered light. We describe the algorithm, present a number of important parameters that account in the welding process, and explicitly show how the algorithm can be used to estimate the laser beam intensity both inside the semi-transparent component and at the welding interface and the beam widening. For the model validation an experimental bench test has been realized and some results from two test cases are presented.
No preview · Article · Mar 2007 · Optics and Lasers in Engineering
[Show abstract][Hide abstract] ABSTRACT: Two-flux method can be used, as a simplification for the radiative heat transfer, to predict heat flux in a slab consisting
of gas and particles. In the original two-flux method (Schuster, 1905 and Schwarzschild, 1906), the radiation field was assumed
to be isotropic. But for gas-particles mixture in combustion environments, the scatterings of particles are usually anisotropic,
and the original two-flux method gives critical errors when ignoring this anisotropy. In the present paper, a multilayer four-flux
model developed by Rozé et al. (2001) is extended to calculate the radiation heat flux in a slab containing participating
particles and gas mixture. The analytic resolution of the radiative transfer equation in the framework of a two-flux approach
is presented. The average crossing parameter ε and the forward scattering ratio ζ are defined to describe the anisotropy of
the radiative field. To validate the model, the radiation transfer in a slab has been computed. Comparisons with the exact
analytical result of Modest (1993) and the original two-flux model show the exactness and the improvement. The emissivity
of a slab containing flyash/CO2/H2O mixture is obtained using the new model. The result is identical with that of Goodwin (1989).
No preview · Article · Dec 2004 · Science in China Series E Technological Sciences
[Show abstract][Hide abstract] ABSTRACT: During long term missions, white paints, used as thermal control
coatings on satellites, are severely damaged by the effect of space
environment. Reflectance spectra, showing broad absorption bands, are
characteristic of the coatings optical degradation. In this paper, a
numerical model simulating optical degradation of white paints is
presented. This model uses Mie's theory, coupled with a random walk
Monte Carlo procedure. With materials like white paints, we are faced to
several major difficulties: high pigment charging rate, binder
absorption, etc.. The problem is even worse in the case of irradiated
paints. In parallel with the description of the basis of the model, we
will make an overview of the encountered problems. Simulation results
are presented and discussed, in the case of zinc oxide/PDMS type white
paints, irradiated by 45 keV protons, in accordance with geostationary
orbit environment conditions. The effects of the optical properties of
the pigment, the pigment volume concentration, the absorption by the
binder on hemispherical reflectance are examined. Comparisons are made
with experimental results, and the interest of such a numerical code for
the study of high charged materials degradation is discussed.
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · May 2003 · Optics Communications
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Oct 2001 · Atmospheric Environment
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: 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.