The spectral shapes are marred by the secondary inner-filter effect at the high-energy end of the spectrum which is caused by the overlap of UC emission and sample absorption. This is typically an issue in especially vis-to-UV UC, even though there are examples of sensitizers with limited UV absorption, thus somewhat mitigating this issue. The low-energy band peaking at around 440 nm is residual prompt fluorescence from 4CzBN, which is an inevitable loss-channel in all these systems. Interestingly, this feature can act as an approximate internal quantum yield reference since the prompt component of 4CzBN (with FPF = 0.11) should be virtually unaffected by the addition of annihilator species. Unfortunately, sensitizer degradation during measurements (vide infra) allows only approximate FUC values to be obtained using the prompt component. Coumarin 153 (FF = 0.53) 60 was employed as an external quantum yield reference instead, ensuring high reliability when evaluating FUC.