Referred to actinometers based on spectrophotometric measurements, S ac λ = Φ λ ɛ λ obs = sensitivi. Φ λ is the quantum yield of the actinometers at the excitation wavelength and λ obs is the observation wavelength, which may be the same as or differ from the former.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 426 (https://doi.org/10.1351/pac200779030293)