A method of determining a phase shift caused by reflection at, or transmission through, a dielectric coating as a function of wavenumber includes obtaining a nominal phase shift for the dielectric coating as a function of wavenumber, 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 a function of wavenumber, and determining the phase shift as a function of wavenumber based on the wavenumber shift and the nominal phase. Further described is a method of determining a layer design for a dielectric coating, wherein the dielectric coating comprises a plurality of stacked layers.