BURIED THERMOPLASTIC LAYER DIAGNOSTICS BY THE USE OF COMBINED FREQUENCY-DOMAIN AND IMPULSE-RESPONSE PHOTO-THERMO-MECHANICAL RADIOMETRY

A photothermal approach to the problem of characterizing the thermopla stic layer thickness sandwiched between two metal foils used in heat-s ealed food containers is described. Real-time signal acquisition instr umentation for the photothermal radiometric signal based on impulse-re sponse fast Fourier transform (FFT) processing via chirped laser-beam modulation and cross-correlation spectral analysis, instead of the con ventional point-by-point discrete frequency scans with a lock-in ampli fier 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 respons e theoretical data have been obtained by a numerical FFT of the freque ncy-domain theoretical data. The total signal was measured via radiome tric detection in both domains and was fitted to the theory to obtain thermal transport properties of the three-layered system. The thicknes s of the thermoplastic layer has been extracted with better than 5% pr ecision. (C) 1998 American Institute of Physics.