In the context of radiative energy transfer, the integral, J = (∞)∫(0) fD'(σ).ɛA(σ).dσ, which measures the overlap of the emission spectrum of the excited donor, D, and the absorption spectrum of the ground state acceptor, A; fD' is the measured normalized emission of D, fD' = fD(σ)/(∞)∫(0)fD(σ).⁢dσ, fD(σ) is the photon exitance of the donor at wavenumber σ, and ɛA(σ) is the decadic molar absorption coefficient of A at wavenumber σ. In the context of Förster excitation transfer, J is given by: \[J=\int _{0}^{\infty }\frac{f_{\text{D}}^{'}\left(\sigma \right)\ ɛ_{\text{A}}\left(\sigma \right)}{\sigma ^{4}} \ \mathrm{d}\sigma \] In the context of Dexter excitation transfer, J is given by: \[J=\int _{0}^{\infty }f_{\text{D}}\left(\sigma \right)\ ɛ_{\text{A}}\left(\sigma \right) \ \mathrm{d}\sigma \] In this case fD and ɛA, the emission spectrum of donor and absorption spectrum of acceptor, respectively, are both normalized to unity, so that the rate constant for energy transfer, kET, is independent of the oscillator strength of both transitions (contrast to Förster mechanism).