The photothermal deflection technique has been used to determine the thermophysical properties of artificially aged paper samples. The deflection amplitude ratio and the phase difference of the deflection angle between front and rear surface illumination have been measured as a function of the pump beam modulation frequency, in order to avoid the dispersion effects mainly introduced by the probe beam vertical offset with respect to the sample surface. A theoretical model for the photothermal signal generation, taking into account the unavoidable light scattering occurring at the paper fibers–air interfaces, has been developed. A photothermal deflection method has been also proposed to measure the transmittance and the reflectance values of a given paper sheet, thus offering a proper constraint to the whole fitting procedure. In this way it has been possible to estimate the thermal diffusivity, the optical absorption and the scattering coefficient values of all the samples examined. A correlation between these values and the aging state of the paper has been found.
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Bertolotti et al. (1999) studied this question.
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