PaulBriard 2012 DH Miami .pdf

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3D relative locations and diameters measurements of
spherical particles by Fourier Interferometry Imaging

Paul BRIARD, 1Sawitree SAENGKAEW, 1Siegfried MEUNIER-GUTTIN-CLUZEL, 2Xue Cheng WU,
Ling Hong CHEN and 1Gérard GREHAN

CORIA UMR 6614 CNRS/Université et INSA de Rouen, Département ‘Optique & Laser’, BP 8, 76800 Saint Etienne du Rouvray, France
State key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, China
corresponding author:


Abstract: This paper presents the possibility to measure the diameters and relative locations of
a set of spherical particles. The method is based on the analysis of the interference patterns in
in the 2D associated Fourier space.
OCIS codes: (070.0070) Fourier optics and signal processing; (080.0080) Geometric optics

The study of dispersed two-phase flows, for example the evaporation of a spray of fuel or CO2 capture by means
of a spray, requires the understanding of the interactions between particles. To quantify these interactions it is
necessary to characterize these particles by measurable parameters such as distances between particles, their size
and/or their refractive index. Fourier Interferometry Imaging is an optical method used to measure 3D relative
locations, diameters and refractive indices of a set of spherical particles. 3D relative locations measurement for
monodisperse set of particles has already been shown [1]. In this paper, diameters and 3D relative locations
measurements are shown for a polydisperse set of particles.
The first section explains principle of the method. The second and the third section focus on diameter and
3D relative location measurements, respectively. The fourth section is a conclusion.
1. Principle of Fourier Interferometry Imaging (FII)
Consider the case of several particles illuminated by a pulsed laser (Fig. 1). These particles scatter the light in all
directions. The interferences between the waves generated by each particle can be recorded by a camera. The
challenge is to be able to extract the information from such a complex interference pattern.

Figure 1. Principle of Fourier Interferometry Imaging. 3 particles illuminated by an incident wave scatter the light toward a detector 2D
(CCD camera). For this paper only the toward scattering is discussed (20°< 0 <90°).

To evaluate such complex interference patterns, a code which explicitly includes interference effects has
been written to rigorously simulate the field scattered by a cloud of spherical particles [2]. This code, based on
the near field Lorenz-Mie theory, allows one to simulate the scattering by an arbitrary cloud of spherical and

homogeneous particles. To identify the interferences between each pair of particles, the 2D Fourier transform of
the interference field is computed.

Figure 2. Picture recorded by CCD camera (numerical simulation) and Fast Fourier Transform 2D (or FFT 2D). Here, three droplets of water
are illuminated (3D locations and diameters randomly defined)

There are several spots in Figure 2.b with an axis of symmetry parallel to the axis. The spot located in the
center of the 2D Fast Fourier transform (FFT) corresponds to interference fringes between the reflected (p=0
order) and refracted (p=1 order) light, scattered from each particle. The others spots correspond to interference
fringes arising from light scattered by the pair of particles. For each pair of particles, there is a corresponding
pair of spot in the FFT which is symmetric through the center of the transformed field.
The number of illuminated particles can be determined by counting the number of spots in the FFT. The
locations of the spots in the FFT allow us to determine the relative 3D positions of the particles. Using the
topography of the spots we can also extract the diameter of each particle, as explained in section 2.
2. Diameters measurement by FII
The spot generated by a pair of particles is composed of many peaks. The three peaks with the highest gray level
correspond to interferences between the light reflected (p=0) or refracted (p=1) by the pair. Figure 3 shows an
example of a profile of a spot.

Figure 3. Example spot profile. The peaks shown here are the result of interference between light scattered by 2 water droplets (diameters :
80 and 100 m, 0 = 20±5°).

The two mains peaks on the left and right sides of the spot (peaks number 1 and 3 in the figure 3)
correspond to the interferences of the light refracted by a particle (p=1) and the light reflected by another particle
(p=0). The main peak near the center of the spot (peak number 2 in the figure 3) corresponds to interferences
between either the light refracted by two particles, or the light reflected by two particles, according to the angular
location of the CCD camera. The differences between the locations of these peaks (1 and 2 ) are
proportional to the particle diameters (equation 1, which be found from paper [3]).

Δi  f ( Ni ,0 )CRi


The factor C depends on experimental conditions unrelated to the particles (wavelength of laser light, CCD
camera position and size …). The function f depend only Ri and Ni which are the radius and the refractive index
of each particle. The size of the particles is extracted by measuring i with accuracy close of 1%. The
measurement of 3D relative locations is shown in section 3.
3. 3D relative locations measurement by FII
The position (,) of a spot in the FFT is defined by equation 2. It is measured from the position of the main
peak of the spot and the size of the particles. The particle-pairs associated with the spot appear at positions
(x1,y1,z1) and (x2,y2,z2).

   x2  x1  cos 0   z2  z1  sin 0 
 

   C 
y2  y1
 


The relative coordinate y2-y1 is extracted from  measurement. The extraction of the relative coordinates x2-x1
and z2-z1 require two CCD cameras with two different angular positions 0 [1]. If more than three particles are
illuminated, an algorithm for rebuilding the field of particles must be used. Figure 4 is an example of an initial
field of a set of six illuminated particles (in red) and the field rebuilt (in grey) using the FII method. With a
resolution of CCD cameras equal to 512x512, the accuracy on diameters and 3D relative locations measurement
is better than one micrometer.

Figure 4. Example of random initial particles field (red) and rebuilt particle field (in grey). Particles are water droplets in a cube of 6003 m3
and the diameters of particles are between 10 and 100 m.

4. Conclusion
In this paper, the 3D relative locations and diameter measurements of spherical particles using the FII method
has been introduced. The measurement accuracy is micrometric. The focus of our current research is the study of
interferences fringes for a CCD camera near the rainbow angle of particles. The aim is to measure the refractive
index of the illuminated particles, with an accuracy better than 10-3.
[1] Briard P., Saengkaew S., Wu X.C., Meunier-Guttin-Cluzel S., Chen L.H., Cen K.F. et Gréhan G., “Measurements of 3D relative
locations of particles by Fourier Interferometry Imaging (FII)” in Optics Express, Vol 19, n° 13 , 12700-12718(2011),.
[2] Wu X.C., Meunier-Guttin-Cluzel S., Saengkaew S., Lebrun D., Brunel M., Coetmellec S., Cen K.F. and Gréhan G. “Particle field digital
Holography : a numerical standard” in Submitted to optics communication (2011).
[3] König G., Anders K., and Frohn A., “A new light-scattering technique to measure the diameter of periodically generated moving
droplets” in Journal of Aerosol Science, 17(2), 157-167. (1986)

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