EP2038622A1

EP2038622A1 - Method of characterizing the anisotropy of a scattering medium and device for implementing such a method

Info

Publication number: EP2038622A1
Application number: EP07788888A
Authority: EP
Inventors: Christophe Baravian; François CATON; Jérôme DILLET
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2006-06-22
Filing date: 2007-06-13
Publication date: 2009-03-25
Also published as: FR2902877A1; FR2902877B1; WO2007147959A1; JP2009541727A; US8199323B2; WO2007147959A8; US20100067006A1

Abstract

The invention relates to a method of characterizing a scattering medium. According to the invention, the processing on the electromagnetic radiation scattered by the scattering medium is carried out for an unpolarized signal. In this way, only the anisotropic incoherent transport of radiation induced by the scattering medium is obtained in the characterization according to the invention. According to the invention, the data representative of the angular variation of the first image representing the unpolarized scattered radiation is representative of the purely isotropic part of the scattering. Having obtained this purely isotropic part, it is then possible, according to the invention, to calculate a second image representative of the non-isotropic part of the scattering. This non-isotropic part represents the anisotropic transport of radiation induced by the medium at the moment of scattering.

Description

METHOD FOR CHARACTERIZING ANISOTROPY OF A DIFFUSING MEDIUM AND DEVICE FOR IMPLEMENTING THE SAME

OF SUCH A METHOD

The present invention belongs to the field of characterization of diffusing media.

An important property of diffusing media is their anisotropy. Indeed, when particles are deformable and / or anisotropic, two cases may occur. In the first case, the particles are oriented and randomly distributed, and the medium comprising these particles remains isotropic on a scale larger than the particle size. In this case, the propagation of light through the medium will be substantially identical in nature to that of isotropic particles randomly distributed. In the second case, the particles are oriented in a privileged direction of space. The oriented particles disadvantage the propagation of light in the direction of their orientation and favor it in all other directions. Thus, anisotropy of the propagation of light is observable on a large scale, for example when the diffusing medium is subjected to a shear or elongational flow.

In the prior art, methods for characterizing a diffusing medium comprising steps of:

- generation of at least one incident electromagnetic radiation

focusing said incident electromagnetic radiation on a surface of said scattering medium; collecting at least one diffusion electromagnetic radiation corresponding to said at least one electromagnetic radiation diffused by said diffusing medium; generating a first representative image of said at least one electromagnetic diffusion radiation;

treatment of said image;

characterizing said diffusing medium from said treatment.

Such a method and a device for implementing this method are for example known from US Pat. No. 6,021,626.

In this document, a characterization of an isotropic scattering medium is provided by analyzing an image representative of a radiation diffused by the scattering medium for several polarization states of a radiation incident on the scattering medium to be analyzed.

However, the device and method of this document have the disadvantage of not allowing rapid access to the incoherent anisotropic transport induced by the scattering medium, and therefore only applies to static or slowly changing media.

Indeed, since the incident radiation emitted on the scattering medium is itself polarized, the polarization effect of the incident radiation is combined with the anisotropic transport effect of the radiation induced by the anisotropies of the scattering medium and prevents access to transport. anisotropic radiation induced by the anisotropy of the medium.

In particular, the device of the aforementioned document involves calculations on all the coefficients of a Mueller matrix, as illustrated for example in Figure 15 of the aforementioned document. Thus, the device of this document does not in any way to access the anisotropic transport of radiation induced by the anisotropy of a scattering medium.

The present invention aims to overcome these disadvantages of the prior art.

An object of the present invention is therefore to objectively measure the anisotropic transport of radiation associated with anisotropy of a scattering medium.

Another object of the present invention is to quantitatively measure the anisotropic transport of radiation associated with anisotropy of a scattering medium.

Another object of the present invention is to provide a quantification of the degree of anisotropy of a scattering medium.

Another object of the present invention is to perform measurements of the anisotropic transport effect of radiation associated with anisotropy of a non-intrusive and in situ diffusing medium.

Another object of the invention is to perform anisotropic transport measurements associated with anisotropy of a scattering medium without requiring the use of a polarization analyzer.

Another object of the invention is to determine the axes of anisotropy of anisotropic scattering medium.

At least one of these objects is achieved by the present invention which relates to a method for characterizing a diffusing medium comprising steps of: - generation of at least one incident electromagnetic radiation

focusing at least one incident electromagnetic radiation on a surface of said diffusing medium; collecting at least one diffusion electromagnetic radiation corresponding to said at least one electromagnetic radiation diffused by said diffusing medium;

generating a first representative image of said at least one electromagnetic diffusion radiation; treatment of said image;

characterizing said diffusing medium from said treatment,

characterized in that

said first image is representative of a non-polarized signal associated with said at least one electromagnetic diffusion radiation;

said processing step comprises substeps consisting of:

o determining a data representative of an angular variation of said first image; o generating a second image representative of a non-isotropic portion of said first image, said second image being calculated from said first image and said data representative of said angular variation; and in that - said characterizing step comprises sub-steps consisting of: o characterizing anisotropy of said scattering medium with the aid of said second image.

Thus, according to the invention, the treatments on the electromagnetic radiation diffused by the scattering medium are made for a non-polarized signal. In this way, only the anisotropic transport of radiation induced by the scattering medium is obtained in the characterization according to the invention. According to the invention, the data representative of the angular variation of the first image representing the non-polarized scattered radiation, is representative of the purely isotropic part of the diffusion. This angular variation is particularly dependent on the nature of the medium and its concentration but not on its anisotropy. Having obtained this purely isotropic part, it is then possible, according to the invention, to calculate a second representative image of the non-isotropic part of the diffusion. This nonisotropic part represents the anisotropic transport of radiation induced by the medium at the time of diffusion.

The data representative of the angular variation of the first image is, for example, an angular average of the first image, or an angular standard deviation.

It should be noted that in this way, unlike the process described in US Pat. No. 6,021,626, it is not necessary to use a polarization analyzer at the output of the scattering medium, since the treatments according to the invention are carried out directly on a representative image of the scattered radiation. In one embodiment for calculating the non-isotropic portion of the broadcast, said second image is calculated by the difference between said first image and said data.

In one embodiment for calculating the isotropic portion of the diffusion, said processing step comprises steps of:

- determining the center of gravity of said first image;

determining said datum representative of an angular variation of said first image from said center of gravity.

In order to obtain a first image which is representative of an unpolarized signal corresponding to said at least one electromagnetic scattering radiation, in the aforementioned method,

said step of generating at least one electromagnetic radiation comprises the steps of: generating a first incident electromagnetic radiation having a first polarization; generating a second incident electromagnetic radiation having a second polarization, said second polarization being antagonistic to said first polarization;

said step of collecting at least one diffusion electromagnetic radiation diffused by said diffusing medium comprises the steps of: collecting a first diffusion electromagnetic radiation corresponding to said first incident radiation diffused by said diffusing medium; o collecting a second diffusion electromagnetic radiation corresponding to said second incident radiation diffused by said scattering medium; and wherein said first image is representative of a non-polarized signal corresponding to said first diffusion electromagnetic radiation and said second diffusion electromagnetic radiation.

In order to simulate non-polarized scattering radiation from polarized incident radiation, the aforementioned method comprises steps of:

generating a third image representative of said first electromagnetic diffusion radiation;

generating a fourth image representative of said second diffusion electromagnetic radiation

wherein said first image is equal to half the sum of said third image and said fourth image.

The invention also relates to a device for characterizing the anisotropy of a diffusing medium comprising

at least one source of electromagnetic radiation capable of generating at least one incident electromagnetic radiation;

focusing means capable of transmitting said incident electromagnetic radiation onto a surface of said scattering medium; collection means capable of collecting at least one electromagnetic scattering radiation corresponding to said at least one electromagnetic radiation diffused by said diffusing medium;

generation means capable of generating a first representative image of said at least one electromagnetic scattering radiation; processing means able to process said first image;

characterization means able to characterize said diffusing medium,

in which

said processing means comprise processing subunits able to: determine a datum representative of an angular variation of said first image; o generating a second image representative of a non-isotropic portion of said first image, said second image being calculated from said first image and said data; and and wherein

the characterization means comprise subunits able to: characterize an anisotropy of said scattering medium by means of said second image.

According to one embodiment of the above-mentioned device, this device comprises

a radiation source capable of generating initial electromagnetic radiation; a first polarizer capable of polarizing said initial electromagnetic radiation so as to generate a first incident electromagnetic radiation having a first polarization;

a second polarizer capable of biasing said initial electromagnetic radiation so as to generate a second incident electromagnetic radiation having a second polarization, said second polarization being antagonistic to said first polarization;

in which,

said collection means comprise a collection unit able to: collect a first diffusion electromagnetic radiation corresponding to said first incident radiation diffused by said diffusing medium; o collecting a second diffusion electromagnetic radiation corresponding to said second incident radiation diffused by said scattering medium; and wherein said first image is representative of a non-polarized signal corresponding to said first diffusion electromagnetic radiation and said second diffusion electromagnetic radiation.

Finally, in order to simulate a non-polarized diffusion radiation, the aforementioned device comprises a calculation unit able to:

and said first image is equal to the half-sum of said third image and said fourth image. Other objects and advantages of the invention will become apparent in the light of the detailed description below with reference to the appended figures in which: FIG. 1 represents a device for characterizing the anisotropy of a diffusing medium according to FIG. invention;

FIG. 2 represents an output field calculation as a function of an input field in a scattering medium using Mueller matrices; FIG. 3 represents a calculation of the output field following the passage through a scattering medium for an input field having a first polarization and a second antagonistic polarization;

FIG. 4 is an exemplary image obtained at the level of a CCD camera according to the invention before processing;

FIG. 5 is a representation of the characteristic elements of the image of FIG. 4;

FIG. 6 is a graph representing the angular average of the intensity of an image according to FIG. 4 as a function of the radius at the center of the image;

FIG. 7 is an exemplary characterization image of the anisotropy according to the invention after treatment;

FIG. 8 is a representation of the characteristic elements of the image of FIG. 7; FIG. 9 is a graph representing the intensity of the anisotropic image according to the invention as a function of the measurement angle, for a radius fixed at a diffusion distance;

FIG. 10 is a graph illustrating the evolution of the degree of polarization of a diffusing medium corresponding to a suspension of red blood cells as a function of the shear applied to the scattering medium of red blood cells. Illustrated in FIG. 1, a device 1 for characterizing the anisotropy of a scattering medium according to the invention comprises a laser diode 2 capable of emitting monochromatic electromagnetic radiation at 635 nm, and polarization means 3 comprising either a circular polarizer right 3A, a left circular polarizer 3B. The device 1 also comprises a mirror 4, a beam splitter 5, capable of transmitting the electromagnetic radiation to an assembly comprising a scattering medium 8 to be characterized, positioned between a rheometer 9 and a glass plate 7. The electromagnetic radiation is backscattered after its passage through the scattering medium 8 to the separator 5, which then transmits this backscattered beam to a CCD camera 6. The CCD camera 6 is connected to calculation means 10, for example in the form of a calculator on which are loaded programs for analyzing the properties of the diffusing medium 8 via one or more analysis images generated by the CCD camera 6. The rheometer 9 is able to apply a stress to the scattering medium 8. This constraint can be a tangential or radial constraint variable speed.

Illustrated Figure 2, we recall the relationship between the output field and the field of entry to the crossing of a scattering particle. The rriij matrix is called the Mueller matrix. This matrix is known per se in the theory of electromagnetic radiation. In FIG. 2, the parallel and perpendicular component input field E ^s i and E ^s _{2 respectively} is given as a function of the parallel and perpendicular component input field respectively E 'i and E ^! 2. This also provides the intensity S ^s of the output field as a function of the intensity S ¹ of the input field, as a function of the Mueller mij matrix. A comprehensive description of this theory of electromagnetic interaction and an interpretation of the terms of the Mueller matrix can be found in publication CF Bohren and DR Huffman "Absorption and scattering of light by small particles", Wiley science (USA) ISBN 0-471-29340-7 (1983).

Illustrated in FIG. 3, according to the invention, a first image h is acquired by means of the CCD camera 6 of the device 1 described in FIG. 1 when the polarization means are arranged so that the incident light radiation Ei on the medium 8 has a first parallel polarization, and a second image I ₂ when the polarization means are arranged so that the incident light radiation E2 on the scattering medium 8 has a second perpendicular polarization.

The half-sum of these two images h and I2 makes it possible to obtain the mn coefficient of the Mueller matrix.

The image I obtained using the calculation means 10 by the half-sum of the images I ₁ and I ₂ corresponds to a non-polarized incident light. In the example described, this unpolarized incident light is obtained using two beams of polarization. According to a variant not shown, the mn coefficient can also be obtained directly using a non-polarized incident light such as white light.

The image I obtained is as shown in FIG. 4 and FIG. 5. FIG. 4 is an image obtained on a real sample, and FIG. 5 is a representation of the composition of the real image of FIG. 4.

Illustrated on these two images, there is an angular anisotropy in the form of a central zone oriented 1 1. This central zone oriented 1 1 is surrounded by a substantially circular zone 12. This image makes it possible to obtain a characterization of the induced anisotropy of the diffusing medium under the constraint of the rheometer 9 of FIG. 1. The Applicant has in fact discovered that when the visualized image is isotropically angular, it is because the objects are either spherical or randomly oriented. When an angular anisotropy exists, as is the case in FIGS. 4 and 5, an overall orientation of anisotropic particles or scattering objects takes place.

Using the image I, the anisotropy is measured by difference between the image I corresponding to an oriented state, and an image of an isotropic or random reference state.

This reference state can be calculated by the angular average of the image I starting from the center of this image.

FIG. 6 illustrates the intensity of the image I 'corresponding to an angular average of the image I, as a function of the radius starting from the centroid of the image I. In a manner known per se, the isotropic part of the diffusion does not depends only on the diffusion distance I *. This diffusion distance I ^* is the characteristic distance between two diffusion events in a scattering medium.

In this figure, the points correspond to a real measurement, and the curve in solid line corresponds to a modeling of the real points for example as in the publication C. Baravian, F. Caton, J. Dillet and J. Mougel. This modeling can be used to determine the diffusion distance I *. FIGS. 7 and 8 illustrate the image I "obtained by subtracting the angular average I 'of the image I from the same image I. FIG. 7 is an image obtained for a real sample, and FIG. 8 is a representation schematic of the structure of the image of Figure 7.

The image I "of FIGS. 7 and 8 comprises two sets of positive and negative lobes defining preferred axes corresponding to the global orientation of the anisotropic particles of the scattering medium under the rheometer constraint.This orientation is defined by the corresponding Y axis. negative lobes.

FIG. 9 is a graph showing the intensity of the image I "as a function of the measurement angle, for a radius fixed at the diffusion distance I ^* . It is noted that the angular average of the intensity I" is null by definition.

The degree of polarization is then defined as the standard deviation of the intensity of the image I ", that is to say the standard deviation of the images of FIGS. 7 and 8.

The degree of anisotropy is thus d = _

This degree of anisotropy is a representative dimensional value of the anisotropic incoherent transport induced by the orientation under the effect of the rheometer. This parameter allows an objective and quantitative measurement of the degree of anisotropy of the medium broadcasting. It is noted that this degree of anisotropy is zero for an isotropic medium on average.

FIG. 10 is a graph illustrating the evolution of the degree of anisotropy of a diffusing medium corresponding to a suspension of red blood cells as a function of the shear applied to the medium by the rheometer 9.

In this figure, it is observed that spherical red blood cells, represented by hatched circles and for example prepared by osmotic stress, deform less than disk red globules represented by solid circles.

Represented by empty circles, both types of globules have been hardened and show no particular anisotropy.

This type of measurement therefore makes it possible to measure the deformability of red blood cells and thus to compare different types of blood with each other.

This measurement can notably be used in the detection of clinical or blood pathologies, since it has been demonstrated that a large number of these pathologies are related to the deformability of red blood cells, for example in the publication Langenfeld JE, Machiedo GW, Lyons M, Rush BF Jr, Dikdan G, Lysz TW. "Correlation between red blood cell deformability and changes in hemodynamic function", Surgery 1 16 (5): 859-67 (1994).

The method as just described thus makes it possible to characterize the anisotropy of an opaque system in visible light. It can be applied to any dispersion of anisotropic particles and / or deformable presenting collective orientations at the microscopic scale.

A variant of the invention is now described.

The invention has been described above by calculating the angular standard deviation of the image I. According to the variant, the anisotropic transport can be determined by the angular standard deviation calculated at the diffusion distance of the center of gravity. Figure I of Figure 9. The diffusion distance I ^* can be obtained from the image I as described above with reference to Figure 6.

More generally, any analysis of the angular anisotropy of the scattering of radiation diffused by the medium obtained by an analysis of the angular variation in the image I can be used in the context of the present invention.

Claims

1. A method of characterizing a diffusing medium comprising steps of: - generating at least one incident electromagnetic radiation

focusing said incident electromagnetic radiation on a surface of said scattering medium;

collecting at least one diffusion electromagnetic radiation corresponding to said at least one electromagnetic radiation diffused by said diffusing medium;

generating a first representative image of said at least one electromagnetic diffusion radiation;

treatment of said image; characterizing said diffusing medium from said treatment,

characterized in that

said processing step comprises substeps consisting of:

o determining a data representative of an angular variation of said first image; o generating a second image representative of a non-isotropic portion of said first image, said second image being calculated from said first image and said data representative of said angular variation; and in that

said characterization step comprises substeps consisting in: characterizing anisotropy of said scattering medium with the aid of said second image.

2. Method according to claim 1, wherein said data representative of an angular variation is an angular average of said image or an angular standard deviation associated with said image.

The method of claim 1 or 2, wherein said second image is computed by difference between said first image and said data.

The method according to one of the preceding claims, wherein said processing step comprises steps of:

- determining the center of gravity of said first image; determining said datum representative of an angular variation from said centroid.

5. Method according to one of the preceding claims, wherein

said step of generating at least one electromagnetic radiation comprises the steps of: generating a first incident electromagnetic radiation having a first polarization; o generating a second incident electromagnetic radiation having a second polarization, said second polarization being antagonistic to said first polarization;

said step of collecting at least one diffusion electromagnetic radiation diffused by said diffusing medium comprises the steps of:

o collecting a first diffusion electromagnetic radiation corresponding to said first incident radiation diffused by said scattering medium; o collecting a second diffusion electromagnetic radiation corresponding to said second incident radiation diffused by said scattering medium;

wherein said first image is representative of a non-polarized signal associated with said first diffusion electromagnetic radiation and said second diffusion electromagnetic radiation.

The method of claim 5, said method comprising steps of:

7. Device (1) for characterizing the anisotropy of a diffusing medium (8) comprising at least one source of electromagnetic radiation (2) capable of generating at least one incident electromagnetic radiation;

focusing means (4, 5, 7) capable of transmitting said incident electromagnetic radiation onto a surface of said scattering medium;

- Collecting means (5, 6) capable of collecting at least one electromagnetic scattering radiation corresponding to said at least one electromagnetic radiation diffused by said scattering medium;

generation means (6) capable of generating a first representative image of said at least one electromagnetic diffusion radiation;

processing means (10) able to process said first image;

characterization means (10) able to characterize said diffusing medium,

characterized in that

said processing means comprise processing subunits able to: determine a datum representative of an angular variation of said first image; o generating a second image representative of a non-isotropic portion of said first image, said second image being calculated from said first image and said data representative of an angular variation; and in that

8. Device according to claim 7, comprising: - a radiation source capable of generating initial electromagnetic radiation;

a first polarizer capable of biasing said initial electromagnetic radiation so as to generate a first incident electromagnetic radiation having a first polarization;

in which

said collection means comprise a collection unit able to: collect a first diffusion electromagnetic radiation corresponding to said first incident radiation diffused by said diffusing medium; o collecting a second diffusion electromagnetic radiation corresponding to said second incident radiation diffused by said scattering medium; and wherein said first image is representative of an unpolarized signal associated with said first scattering electromagnetic radiation and said second scattering electromagnetic radiation.

9. Device according to claim 8, comprising a computing unit capable of:

generating a third image representative of said first electromagnetic diffusion radiation; generating a fourth image representative of said second diffusion electromagnetic radiation