From the Einstein population rate equation, spectral absorption coefficient σ ph,ω in ERT is, σ σ p h , ω ω = ℏ ℏ ω ω γ γ ˙ ˙ p h , a n e u p h , where γ γ ˙ ˙ p h , a is the interaction probability (absorption) rate or the Einstein coefficient B 12 (J m s), which gives the probability per unit time per unit spectral energy density of the radiation field (1: ground state, 2: excited state), and n e is electron density (in ground state). This can be obtained using the transition dipole moment μ e with the FGR and relationship between Einstein coefficients. Averaging σ ph,ω over ω gives the average photon absorption coefficient σ ph.