where (TT) is an electronically coupled and spin-coherent triplet pair, and 1 (T...T) is a triplet pair that maintains spin coherence, but has lost electronic coherence, and thus behaves chemically and spectroscopically like free triplets. The generation of two triplet excitons from one singlet exciton, and hence a single photon, could increase the efficiency of solar cells beyond the detailed-balance limit, and hence has attracted significant recent attention. However, sizeable improvements to solar-cell efficiency using SF have not been achieved to date, so there are continuing efforts to increase the efficiency of SF as well as the "harvestability" of the triplet excitons. But for SF to be optimized, it is important to be able to accurately identify and characterize the process, including distinguishing SF from other photophysical processes that result in triplet excitons.