spectral sensitivity, \(S(\lambda)\)

https://doi.org/10.1351/goldbook.ST07395
Referred to actinometers based on spectrophotometric measurements, \(S_{\text{ac}}\left(\lambda \right)=\varPhi \left(\lambda \right)\ \varepsilon\left(\lambda _{\text{obs}}\right)=\text{sensitivity or actinometric factor}\). \(\varPhi \left(\lambda \right)\) is the quantum yield of the actinometers at the excitation wavelength and \(\lambda _{\text{obs}}\) 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)