The ultrasonic waves are emitted from the transducer into the coated — painted surface — substrate through the used coupling material (hydrogel/water). The ultrasound propagates from the transducer to the painted substrate and consecutively to the various layers of the paint. Echoes are generated by the various interfaces between the different kinds of materials with different mechanical properties. The echoes are received by the transducer at different time instances, which are proportional to the distance between the interfaces and the transducer. The signal that contains the echoes is called a-scan signal. From the Time of Flight (TOF) of two successive echoes, generated by the surface of the coating (A) and the interface between the coating and the substrate material the thickness can be calculated. This is a real-time measurement, lasting a few seconds in the case that a set of measurements is acquired in a region of interest of the sample.
Based on the measurement of the acoustic wave propagation speed in the paint layer, and the measurements of the TOF of the wave propagation in the samples, the thickness of the paint layer is measured – calculated.