Photothermal radiometry using normalized DC component for coating thickness evaluation

An improved photothermal technique for evaluating opaque coating thicknesses using a normalized DC component is proposed. The pump beam is modulated at a frequency that the generated thermal wave only exists in the coating layer and becomes invariant to the thickness changes. The DC component is normalized by the amplitude of the AC signal in order to eliminate the dependency on the pump intensity, surface absorptivity, and emissivity. Both theoretical analysis and experimental results demonstrate that the normalized DC component is linearly correlated to the coating thickness over a broad range. This method offers a significant advantage over existing photothermal methods by avoiding the tedious procedure of frequency trails for coatings of an unknown thickness. Also, the non-monotonic issue is successfully resolved.