Here, UC and r denote upconversion and reference sample, respectively. F is the integrated emission intensity, A the absorption at 405 nm, and h the refractive index of the solvent. Except for the fitting procedure no additional corrections for reabsorption were invoked, even though the UC sample absorption at the fitting wavelength was non-negligible. The presented values of FUC,g are, thus, potentially somewhat underestimated. The procedure for calculating FUC was identical, but the high-energy end of the spectrum (of which parts were typically reabsorbed) was not accounted for (Figure , red spectra). Note that the employed definition for FUC,g gives a theoretical maximum of 50%.