spectral sensitivity, S λ

Referred to actinometers based on spectrophotometric measurements, S ac λ = Φ λ ɛ λ obs = sensitivity or actinometric factor. Φ λ is the quantum yield of the actinometer at the excitation wavelength and λ obs is the observation wavelength, which may be the same as or differ from the former.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 426