Method for analysing an object comprising several superposed layers, by optical reflectance measurements

1 . A method for determining a variation of absorption properties of an object, between a first instant and a second instant later than the first instant, the object being delimited by a surface, the object comprising a surface layer and a deep layer, the surface layer extending between the surface and the deep layer, the method comprising:

a) illuminating the object by a light source, the light source emitting an illumination beam forming an illumination zone on the surface of the object; and

b) detecting photons backscattered by the object, after being propagated in the object, by a photodetector, the detected backscattered photons emanating from a detection zone on the surface of the object, the detection zone being situated at a detection distance from the illumination zone, the detection distance being chosen from among, in ascending order:

a first detection distance, forming a first detection zone;

a second detection distance, forming a second detection zone;

a third detection distance, forming a third detection zone;

wherein detecting the photons, at the first detection zone, at the second detection zone and at the third detection zone, generate a detection signal at the first detection zone, at the second detection zone and at the third detection zone respectively, wherein the method comprises, chronologically:

(i): at the first instant, implementing steps a) and b) by detecting the photons in the second and third detection zones;

(ii): from the detection signals at the second and third detection zones resulting from (i), estimating an absorption coefficient of the object, the surface layer and the deep layer being considered as having a same absorption coefficient;

(iii): from the absorption coefficient resulting from (ii), estimating an average distance travelled by the photons, within the surface layer, between the illumination zone and the first detection zone;

(iv): at the second instant, implementing steps a) and b) by detecting the photons backscattered in the first detection zone;

(v): from the detection signal at the first detection zone resulting from (iv), and the average distance resulting from (iii), estimating a variation of the absorption coefficient in the surface layer between the first instant and the second instant;

(vi): at the second instant, implementing steps a) and b) by detecting, in the step b), the photons in the second and third detection zones; and

(vii): from the detection signals at the second and third detection zones resulting from (vi), and the variation of the absorption coefficient in the surface layer, resulting from (v), estimating the absorption coefficient in the deep layer at the second instant.

2 . The method of claim 1 , wherein the step (i) further comprises:

from the detection signals measured at the second detection zone and the third detection zone, at the first instant, determining a spatial variation of the absorbance of the object at the first instant;

from the spatial variation of the absorbance of the object, at the first instant, estimating a first estimated absorption coefficient in the surface layer and in the deep layer at the first instant; and

applying a first absorption calibration function to the first estimated absorption coefficient, so as to determine the absorption coefficient, in the surface layer and in the deep layer, at the first instant.

3 . Method according to claim 2 , wherein the step (vii) further comprises:

from the detection signals measured at the second detection zone and the third detection zone, at the second instant, determining a spatial variation of the absorbance of the object, at the second instant;

from the spatial variation of the absorbance of the object, at the second instant, estimating a first estimated absorption coefficient in the deep layer at the second instant; and

applying a second absorption calibration function to the first estimated absorption coefficient in the deep layer at the second instant, so as to determine the absorption coefficient, in the deep layer, at the second instant, the second absorption calibration function taking into account the variation of the absorption coefficient in the surface layer between the first instant and the second instant.

4 . The method of claim 3 , wherein step (vii) further comprises:

from the variation of the absorption coefficient in the surface layer resulting from (v), estimating an average distance travelled by the photons, in the surface layer, between the illumination zone and the first detection zone, at the second instant;

calculating a ratio between the average distances travelled by the photons resulting respectively from the preceding substep and the step (ii); and

using the ratio to form the second absorption calibration function.

5 . The method according to claim 3 , wherein

the method comprises estimating the absorption coefficient, in the surface layer, at the second instant; and

the second absorption calibration function is established using modellings or experimental measurements performed on phantoms, each phantom comprising:

a surface layer, the absorption coefficient of which corresponds to the absorption coefficient estimated, in the surface layer, at the second instant; and

a deep layer, the absorption coefficient of which is variable between the different phantoms.

6 . The method of claim 1 , wherein

the first detection distance is less than 2 cm; and

the second and third detection distances are greater than 2 cm.

7 . A device configured to be applied facing a surface of an object during at least one first instant and a second instant, the device comprising:

a light source configured to emit an illumination beam, forming an illumination zone, on the surface of the object;

a photodetector, configured to form a detection signal from a detection of photons backscattered by the object, in;

a first detection zone, extending to a first detection distance from the illumination zone;

a second detection zone, extending to a second detection distance from the illumination zone, the second detection distance being greater than the first detection distance; and

a third detection zone, extending to a third detection distance from the illumination zone, the third detection distance being greater than the second detection distance; and

at least one processor, programmed to implement a process from detection signals formed by the photodetector, the process including

(i): at the first instant, implementing steps a) and b) by detecting the photons in the second and third detection zones, steps (a) and (b) including

a) illuminating the object by a light source, the light source emitting an illumination beam forming an illumination zone on the surface of the object;

b) detecting photons backscattered by the object, after being propagated in the object, by a photodetector, the detected backscattered photons emanating from a detection zone on the surface of the object, the detection zone being situated at a detection distance from the illumination zone, the detection distance being chosen from among, in ascending order:

a first detection distance, forming a first detection zone;

a second detection distance, forming a second detection zone; and

a third detection distance, forming a third detection zone;

(ii): from the detection signals at the second and third detection zones resulting from (i), estimating an absorption coefficient of the object, the surface layer and the deep layer being considered as having a same absorption coefficient;

(iii): from the absorption coefficient resulting from (ii), estimating an average distance travelled by the photons, within the surface laver, between the illumination zone and the first detection zone;

(iv): at the second instant, implementing steps a) and b) detecting the photons backscattered in the first detection zone;

(v): from the detection signal at the first detection zone resulting from (iv), and the average distance resulting from (iii), estimating a variation of the absorption coefficient in the surface layer between the first instant and the second instant;

(vi): at the second instant, implementing steps a) and b) by detecting, in the step b), detecting the photons in the second and third detection zones; and

(vii): from the detection signals at the second and third detection zones resulting from (vi), and the variation of the absorption coefficient in the surface layer, resulting from (v), estimating the absorption coefficient in the deep layer at the second instant,

wherein detection signals include

in the step (ii), detection signals at the second and third detection zones;

in the step (v), detection signal at the first detection zone; and

in the step (vii), detection signals at the second and third detection zones.

8 . The device of claim 7 , wherein

the first detection distance is less than 2 cm; and

the second and third detection distances are greater than 2 cm.