Temporal Moment Method in Pulsed Photothermal Radiometry: Application to Carbon Epoxy NDT

In pulsed back emission photothermal radiometry, a sample is heated for a short while by an IR or visible source. The surface temperature increase is observed by means of an IR camera [1]. The method of the zero-order temporal moment consists in integrating the temperature increase over the time. For a homogeneous medium, the temperature-time history being ΔTRco(t), this moment $${M^o} = \smallint _o^\infty \Delta {T_{R = 0}}\left( t \right)dt$$ tends to infinity. When an internal defect is present, characterized by its thermal resistance R and its depth z from the front surface, the perturbation of the temperature-time history, ΔT (t), induces a variation in the moment, and it may be analytically demonstrated by the use of the Laplace transform or the Fourier transform that the difference between the two moments is $$\Delta {M_o} = \smallint _o^\infty \left[ {\Delta {T_R}\left( t \right) - \Delta {T_{R = 0}}\left( t \right)} \right]dt = Q.R.{\left( {1 - z - L} \right)^2},$$ where Q is the energy density deposited in the sample and L its thickness. It is very important to point out that, if the sample is thick, M is equal to Q.R and is independent of the defect depth. Experimentally, it may be more interesting to use this parameter for identifying the thermal resistance of the defect than the correlation previously proposed [2]. As a matter of fact, obtaining ΔMo corresponds to the integration of temperature, an operation enhancing the signal-to-noise ratio. In this way, it becomes possible to obtain sensitivity as good as those of modulated flux methods using coherent detection. It is then possible to observe defects far in depth from the surface. We also mention that the temporal moment method may be used for correcting the pulsed thermogram for loss effects [3].