Measure for integrated intensity of electronic transitions and related to the Einstein transition probability coefficient \(A_{ij}\) by: \[f_{ij} = 1.4992\times 10^{-14}\ \frac{A_{ij}}{s^{-1}}\ (\frac{\lambda }{\rm{nm}})^{2}\] where \(\lambda \) is the transition wavelength. There are, however, differing uses of \(f\).