A method of determining a phase shift caused by reflection at, or transmission through, a dielectric coating as function of wavenumber, the method comprising: obtaining a nominal phase shift for the dielectric coating as function of wavenumber, wherein the nominal phase shift is a known phase shift that is applied to a wave front in reflection or transmission by the dielectric coating as designed; determining a first wavenumber and a second wavenumber for performing measurements of phase shift at these wavenumbers, based on the nominal phase shift; determining a wavenumber shift based on a first measurement of phase shift at the first wavenumber, a second measurement of phase shift at the second wavenumber, and the nominal phase shift as function of wavenumber; and determining the phase shift as function of wavenumber based on the wavenumber shift and the nominal phase shift, wherein the determining the phase shift as function of wavenumber based on the wavenumber shift and the nominal phase shift comprises: determining a first contribution to the phase shift as function of wavenumber, the first contribution depending on thickness variations of stacked layers of the dielectric coating from their respective nominal thickness; and determining a second contribution to the phase shift as function of wavenumber, the second contribution depending on local deformation of the dielectric coating due to substrate deformation, and wherein the phase shift as function of wavenumber is based on a sum of the first contribution and the second contribution. obtaining a nominal phase shift for the dielectric coating as function of wavenumber, wherein the nominal phase shift is a known phase shift that is applied to a wave front in reflection or transmission by the dielectric coating as designed; determining a first wavenumber and a second wavenumber for performing measurements of phase shift at these wavenumbers, based on the nominal phase shift; determining a wavenumber shift based on a first measurement of phase shift at the first wavenumber, a second measurement of phase shift at the second wavenumber, and the nominal phase shift as function of wavenumber; and determining the phase shift as function of wavenumber based on the wavenumber shift and the nominal phase shift, wherein the determining the phase shift as function of wavenumber based on the wavenumber shift and the nominal phase shift comprises: determining a first contribution to the phase shift as function of wavenumber, the first contribution depending on thickness variations of stacked layers of the dielectric coating from their respective nominal thickness; and determining a second contribution to the phase shift as function of wavenumber, the second contribution depending on local deformation of the dielectric coating due to substrate deformation, and wherein the phase shift as function of wavenumber is based on a sum of the first contribution and the second contribution. determining a first contribution to the phase shift as function of wavenumber, the first contribution depending on thickness variations of stacked layers of the dielectric coating from their respective nominal thickness; and determining a second contribution to the phase shift as function of wavenumber, the second contribution depending on local deformation of the dielectric coating due to substrate deformation, and wherein the phase shift as function of wavenumber is based on a sum of the first contribution and the second contribution.