A method for determining the thickness of a plurality of coating layers. The method comprises the steps of performing a calibration analysis on calibration data to determine initial values and search limits of optical parameters of the plurality of coating layers, irradiating the plurality of layers with a pulse of THz radiation in the range from 0.01 THz to 10 THz, detecting the reflected radiation to produce a sample response derived from the reflected radiation, producing a synthesized waveform using the optical parameters and predetermined initial thicknesses of the layers, varying the thicknesses and the optical parameters within the search limits to minimize the error measured between the sample response and the synthesized waveform, and outputting the thicknesses of the layers.