Accumulation of carotenoid (Car) triplet states was investigated by singlet-triplet annihilation, measured as chlorophyll (Chl) fluorescence quenching in sunflower and lettuce leaves. The leaves were illuminated by Xe flashes of 4 μs length at half-height and 525-565 or 410-490 nm spectral band, maximum intensity 2 mol quanta m -2 s -1 , flash photon dose up to 10 μmol m -2 or 4-10 PSII excitations. Superimposed upon the non-photochemically unquenched F md state, fluorescence was strongly quenched near the flash maximum (minimum yield F e ), but returned to the F md level after 30-50 μs. The fraction of PSII containing a 3 Car in equilibrium with singlet excitation was calculated as T e = (F md -F e )/F md . Light dependence of T e was a rectangular hyperbola, whose initial slope and plateau were determined by the quantum yields of triplet formation and annihilation and by the triplet lifetime. The intrinsic lifetime was 9 μs, but it was strongly shortened by the presence of O 2 . The triplet yield was 0.66 without nonphotochemical quenching (NPQ) but approached zero when NP-Quenched fluorescence approached 0.2 F md . The results show that in the F md state a light-adapted charge-separated PSII L state is formed (Sipka et al., The Plant Cell 33:1286-1302, 2021) in which Pheo - P680 + radical pair formation is hindered, and excitation is terminated in the antenna by 3 Car formation. The results confirm that there is no excitonic connectivity between PSII units. In the PSII L state each PSII is individually turned into the NPQ state, where excess excitation is quenched in the antenna without 3 Car formation.