To determine FUC,g a fitting procedure that accounts for reabsorption was employed, and it is explained in detail in Section 2.3 of the Supporting Information. To our delight, all systems investigated yielded relatively high FUC,g, with the system consisting of 4CzBN/TIPS-Naph in particular yielding a high value of 16.8% (out of a 50% maximum, see Figure for the UC spectrum). This value is to the best of our knowledge the highest vis-to-UV FUC,g reported to date, and an almost 2-fold improvement on the previous record. The remaining systems yielded FUC,g values ranging from 4 to 14%, and full results are presented in Figure and Table . It should be noted that the values achieved for TP (12.6%) and PPD (5.8%), which both emit from singlet states just shy of 4 eV, are multifold improvements on that previously reported, and likely results from more efficient TET and, subsequently, more efficient TTA between triplets. The external FUC measured for our specific setup yielded Fout between 0.7-0.85, resulting from significant reabsorption of the samples. In TIPS-Naph and PPF this results from very small Stokes shifts (Figure ), causing ground state annihilators to reabsorb the UC light to a larger extent than in systems with larger Stokes shifts. In PPD and TP relatively low Fout instead results from the pronounced absorption feature of 4CzBN between 300-350 nm (Figure ), which is part of the spectral region where these annihilators emit. Measurements on all annihilators were also performed in THF, typically yielding lower FUC,g and much more pronounced sample degradation (Table and Figure ).