Product of the fluorescence quantum yield (Φ f) of a dye and the molar decadic absorption coefficient at the excitation wavelength, ɛ(λ), i.e., Φ f ɛ λ.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 310 (https://doi.org/10.1351/pac200779030293)