Modulated step-heating thermography for coating thickness measurement

In this paper, the modulated step-heating thermography is proposed for opaque coating thickness measurement. The surface is heated by a modulated heating flux at a frequency that the thermal wave is confined within the coating layer. The DC component of the thermal signal is normalized by the amplitude of the AC component to cancel out the influences from the variations in heating intensity, surface absorptivity and emissivity, resulting in a linear correlation to the coating thickness. Meanwhile, it eliminates the need for tedious frequency trail and the issue of the nonmonotonicity as suffered in conventional thermography. Both theoretical and experimental results confirmed the effectiveness. Specimens with thickness ranging from 100 to 400 mu m are measured with a relative error within +/- 7.9 %.