Referred to actinometer based on spectrophotometric measurements, \(S_{\rm{ac}}(\lambda) = \varPhi (\lambda)\ \varepsilon(\lambda _{\rm{obs}}) = \rm{sensitivity\ or\ actinometric\ factor}\). \(\varPhi(\lambda)\) is the quantum yield of the actinometer at the excitation wavelength and \(\lambda_{\rm{obs}}\) is the observation wavelength, which may be the same as or differ from the former.