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