Schematic summarizing different procedures to determine refractive indices from spectroscopic and ellipsometric data spanning UV to far-IR. The data include transmission (T), reflection (R), and the ellipsometric angles (Δ and Ψ). The values f and s refer to film and substrate, respectively; θ_i is the angle of incidence on the sample during the measurement; and d refers to the thickness of the film (d_f) and substrate (d_s). SSKK refers to the singly subtractive Kramers–Kronig equation (Eq. (11)) describing the correlation between n and k values.