Measuring film thickness using infrared imaging

Infrared imaging was used to measure the thickness of a Krylon® flat black spray paint coating applied to the surface of steel test coupons. Thermal signals were generated in the test samples by applying a cyclic tensile load at various frequencies, which generated a corresponding thermo-elastic temperature change. Since the substrate is a better thermo-elastic generator than the paint, the heat generated in the metal conducts through the paint during each cycle of loading and is easily distinguished from the paint signal. Test samples were prepared with a stepped progression of controlled paint thickness, and infrared images taken using a Delta Therm 1000 IR imaging system. The infrared signal intensity as well as the phase lag relative to the applied loading were directly related to the paint thickness. The experimental results are described well using existing heat conduction models. Though the proposed method was proven using thermoelastic heating, any heat source below the paint can be used to provide the necessary reference signal. Furthermore, the method can also be used for any coating type and is not limited to paint coatings.