G - Physics – 01 – N
Patent
G - Physics
01
N
G01N 21/84 (2006.01) G01N 21/21 (2006.01) G01N 21/41 (2006.01)
Patent
CA 2051977
18 ABSTRACT A method and a device for the non destructive measurement of the fibre orientation anisotropy in a fibrous structure. The method relies on the difference of refractive indices for light beams polarized in two perpendicular directions to measure the value of the anisotropy on the surface of a fibrous structure. Because the use of an infrared light, with a wavelength in the 9 to 11 µm spectral range, the penetration depth of the light is of the order of a few tenths of microns and the fibrous structure shows a large birefringence directly associated with the fibre orientation anisotropy. This birefringence can be measured as a difference in the refractive indices of the fibrous structure in two different directions. A light beam produced with an infrared light source, such as a CO2 laser, is modulated by a chopper, linearly polarized and sent to a photoelastic modulator which varies the state of polarization of the light at a given frequency. After directing the modulated light towards the surface of the fibrous structure, the light is reflected and polarized again. The intensity of the second harmonic and the total intensity of the reflected polarized beam is measured according to a plurality of angles of incidence. These angles are obtained by the rotation of the sample platform parallel to the plane of incidence. The angles of incidence at the minimum ratio between the intensity of the second harmonic and the total intensity (Brewster's angle) are calculated for two different orientations and the fibre orientation anisotropy is determined by subtracting the two Brewster's angles: the higher is the difference between the Brewster's angles, the higher is the fibre orientation anisotropy. The average refractive index is obtained from the mean value between the Brewster's angles.
Bernard Pierre
Charlebois Alain
Institut National D'optique
Robic
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