Fig. 5. (A) Emission spectra arising from the Yb^III : ²F_5/2 → ²F_7/2 transition recorded under 375 nm excitation at different temperatures (from 80 to 320 K). In the inset, a zoom-in of the 0′–0 Stark component is shown. (B) Sensitivities (S_r) of the optothermometric approache relying on the three different thermometric parameters (Δ). In (C–E) the thermometric parameters for the three optothermometric approaches are reported (C(i) – ratio between integrated regions I₁, I₂, and I₃; D(i) – ratio between 0′–0 and 0′–1 Stark components; E(i) – 0′–0 peak width) along with the respective fits (C(i) and E(i) – cubic function; D(i) – Boltzmann function; as implemented in OriginPro® software), and their associated uncertainties (ii). In A, red shaded areas indicate the integration ranges (I₁, I₂, I₃) used for the first luminescence thermometry approach, blue arrows evidence the intensity drop of the Stark components used for the second approach, and the green double-headed arrow in the inset highlights the broadening of the 0′–0 Stark component upon increasing the temperature. In B, the region for S_r > 1 % K^–1 is highlighted with a grey-shaded area.