L'objectif de ce travail est la caracterisation des milieux semitransparents diffusants. Cette caracterisation s'appuie sur les mesures de transmission et de reflexion fournies par plusieurs dispositifs experimentaux, spectraux et bidirectionnels, operant dans le visible et dans l'infrarouge, ainsi que sur les integrations hemispheriques de ces mesures. La caracterisation s'appuie par ailleurs sur le modele radiatif le plus simple, compatible avec les conditions experimentales: utilisation d'un laser, d'un faisceau incident collimate ou d'une sphere integrante. Le modele a quatre flux suffit effectivement pour determiner, par ajustement parametrique, les coefficients d'absorption et de diffusion des milieux diffusants et pour approcher leur fonction de phase
L'objectif de ce travail est la caracterisation des milieux semitransparents diffusants. Cette caracterisation s'appuie sur les mesures de transmission et de reflexion fournies par plusieurs dispositifs experimentaux, spectraux et bidirectionnels, operant dans le visible et dans l'infrarouge, ainsi que sur les integrations hemispheriques de ces mesures. La caracterisation s'appuie par ailleurs sur le modele radiatif le plus simple, compatible avec les conditions experimentales: utilisation d'un laser, d'un faisceau incident collimate ou d'une sphere integrante. Le modele a quatre flux suffit effectivement pour determiner, par ajustement parametrique, les coefficients d'absorption et de diffusion des milieux diffusants et pour approcher leur fonction de phase
The aim of this work is the characterization of semi-transparent scattering media. Spectral and bidirectional devices, operating in the visible and infrared regions, provide transmission and reflection measurements. The hemispherical properties are computed by integration of BRDF and BTDF measurements. This characterization also requires a simple radiative model, according to the experimental conditions, such as the use of lasers, collimated beams or integrating spheres. Indeed, the four-flux model is enough to determine the absorption and scattering coefficients and to approximate the phase functions, using parametrical fittings. Results for spectralon, zircon ceramics and silicate fibrous samples are presented. These media appear to have definitely anisotropic phase functions.
We have developed a photothermal analysis device using randomly modulated local excitation. This method, which primarily makes it possible to obtain the pulse response, then the harmonic response from the sample, has a number of advantages over the other systems available up to now: lower thermal stress than in the pulsed regime, faster acquisition and lower frequency band than with sinusoidal excitation. These advantages can be made use of in studying different types of samples over a wide thickness range. The method and the device were first validated by comparing the experimental results with the theoretical results obtained using a 3D model for dielectric and metallic samples of known thermophysical properties. The method was then tried on sprayed ceramic coatings on metal substrates with intentional adhesion defects. The experimental results were compared with those obtained using a 1D theoretical model suitable for semi-transparent and scattering materials.
Photothermal radiometry is a non-destructive control method which is well suited to characterize thin coatings. We have undertaken the extension of this technique to ceramic materials on substrates. In order to take into account the scattering properties of ceramics, we developed a four-flux model including two directional fluxes issued from the laser beam and two isotropic fluxes due to scattering. We measured the diffuse transmission and reflection of ceramics by using a spectrometer equipped with an integrating sphere. Then we evaluated the absorption and scattering coefficients. We studied plasma-sprayed coatings by photothermal radiometry under modulated laser excitation for frequencies between 10 and 200 Hz. The experimental results we obtained, then compared to the model, allowed us to give values for the thermophysical parameters of the ceramic coatings. These properties depend strongly on the spraying process. We also detect bonding defects at the substrate-coating interface.
