Examples for the basis functions for W and V. Upper panel: first three basis functions φ_i(u), i = 1, 2, 3 for the observational spectra for BP and RP. The corresponding wavelength axes are indicated. Lower panel: first three basis functions ϕ_j(λ), j = 1, 2, 3, for the SPDs for BP and RP, and the combined basis functions covering the entire wavelength range. The corresponding sample axes are indicated.