Non-contact on-line layer thickness measurement in industrial coating processes

A non-contact and non-destructive method for the measurement of the layer thickness in an industrial coating process is described. The method is based on the time-resolved measurement of the temperature of the coated surface during and after the irradiation with a laser pulse. The measurement time of this photothermal technique is only several 10 ms for coating thicknesses in the order of 100 mu m, which is significantly shorter than the measurement with thermal waves. This enables the use of the technique even on moved objects without moving the measurement system. The measurement is done on the wet paint and calibrated on the thickness of the dry coating after stoving of the paint. The resolution related to the layer thickness after stoving is about +/-1 mu m for a thickness range from 10 to 100 mu m. The new measurement technique is able to measure the distribution of the layer thickness on the coated object just after paint application in order to optimize the paint application process.