spectral sensitivity, S λ

https://dx.doi.org/10.1351/goldbook.ST07395
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.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 426