Measurement of air gap thickness underneath an opaque film by pulsed photothermal radiometry

We describe an experimental method to detect and measure a thin air gap between an opaque film and a substrate. The method is pulsed photothermal radiometry with signal shape analysis at suitable delayed times. This relies on the use of a short light pulse to heat up the surface of the opaque film by∼10°C, and detecting the infrared thermal emission from the surface as a function of time for a sufficiently long time. A numerical computation as well as an analytical approximation is developed to explain the dependence of the photothermal radiometry signal shape on the air gap thickness in the range of ten to hundreds of microns. Our work has applications not only for detecting subsurface air gaps and delaminations, but also for measuring the thermal resistance between layers for nondestructive characterization of adhesion bond strengths.