Abstract
A photothermal approach to the problem of characterizing the thermoplastic layer thickness sandwiched between two metal foils used in heat-sealed food containers is described. Real-time signal acquisition instrumentation for the photothermal radiometric signal based on impulse-response fast Fourier transform (FFT) processing via chirped laser-beam modulation and cross-correlation spectral analysis, instead of the conventional point-by-point discrete frequency scans with a lock-in amplifier has been introduced. A theoretical frequency-domain model for the signal generation due to laser heating, which contains both a thermal component and a mechanical component due to the thermal expansion of the thermoplastic layer, is presented. The time domain impulse response theoretical data have been obtained by a numerical FFT of the frequency-domain theoretical data. The total signal was measured via radiometric detection in both domains and was fitted to the theory to obtain thermal transport properties of the three-layered system. The thickness of the thermoplastic layer has been extracted with better than 5% precision.
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