Limitations in measurement of subsurface temperatures using pulsed photothermal radiometry

A pulsed photothermal technique to calculate internal temperatures from noncontact surface temperature measurements is presented. The inversion process is based on approximating the integral equation describing the thermal interaction with a matrix equation. The matrix equation is then solved using singular-value decomposition. The method was evaluated using computer simulations and experiments with tissue phantoms and skin. The algorithm predicted internal temperatures within 10% for homogeneous samples down to a depth of about 500 mm. It did not predict internal temperatures accurately for inhomogeneous samples, but yielded fairly accurate estimates of the depths of subsurface absorbers and conserved energy. The uncertainty in the calculated depth of the absorber increased with depth. Currently this technique can probe depths to 500 mm.