Measured reflectance spectra of Si + 800 nm SiO₂ stacks with and without an additional absorbing layer, showing the impact of the thinnest and thickest layers of Ag or Au studied in this work: (a) 1 nm Ag; (b) 50 nm Ag; (c) 1 nm Au; and (d) 41 nm Au. For the thinnest layers (a and c), the effect on the reflectance spectrum is significant but small; a thinner layer will produce an even smaller effect, which may harm the accuracy of the fitting. For the thickest layers (b and d), the prominence of the oscillatory fringes is reduced; for thicker layers than this, the fringes are reduced further or even eliminated, which likewise reduces the accuracy of the fitting.