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