Method for measuring the thickness d of a transparent layer, including: determining an approximate thickness d FFT based on a precisely known dispersion n ( λ) of the material of the layer by performing Fast Fourier Transformation, FFT, analysis on Fabry-Pérot oscillation, FPO, from the layer in a spectral reflectance measurement; performing a FPO full-spectral fitting procedure with the approximated thickness d FFT as starting value d 0,0 to determine an initial local best fitting thickness d FPO,0, characterised in that the FPO full-spectral fitting procedure includes determining the thickness difference Δ d fit between the initial local best fitting thickness d FPO,0 and the approximate thickness d FFT, or between neighboring local best FPO fitting results; wherein the method further includes: repetitively performing the FPO full-spectral fitting procedure (910-i) with the approximate thickness d FFT shifted by a limited set of integer multiples of the thickness difference Δ d fit as alternative starting values d 0,i to determine a global best fitting thickness d FPO; or repetitively performing the FPO full-spectral fitting procedure (910-i) with the initial local best fitting thickness d FPO,0 shifted by a limited set of integer multiples of the thickness difference Δ d fit as alternative starting values d 0,i to determine a global best fitting thickness d FPO. determining an approximate thickness d FFT based on a precisely known dispersion n ( λ) of the material of the layer by performing Fast Fourier Transformation, FFT, analysis on Fabry-Pérot oscillation, FPO, from the layer in a spectral reflectance measurement; performing a FPO full-spectral fitting procedure with the approximated thickness d FFT as starting value d 0,0 to determine an initial local best fitting thickness d FPO,0, characterised in that the FPO full-spectral fitting procedure includes determining the thickness difference Δ d fit between the initial local best fitting thickness d FPO,0 and the approximate thickness d FFT, or between neighboring local best FPO fitting results; wherein the method further includes: repetitively performing the FPO full-spectral fitting procedure (910-i) with the approximate thickness d FFT shifted by a limited set of integer multiples of the thickness difference Δ d fit as alternative starting values d 0,i to determine a global best fitting thickness d FPO; or repetitively performing the FPO full-spectral fitting procedure (910-i) with the initial local best fitting thickness d FPO,0 shifted by a limited set of integer multiples of the thickness difference Δ d fit as alternative starting values d 0,i to determine a global best fitting thickness d FPO. repetitively performing the FPO full-spectral fitting procedure (910-i) with the approximate thickness d FFT shifted by a limited set of integer multiples of the thickness difference Δ d fit as alternative starting values d 0,i to determine a global best fitting thickness d FPO; or repetitively performing the FPO full-spectral fitting procedure (910-i) with the initial local best fitting thickness d FPO,0 shifted by a limited set of integer multiples of the thickness difference Δ d fit as alternative starting values d 0,i to determine a global best fitting thickness d FPO.