where r is the separation of the donor from the acceptor, L is the sum of the Van der Waals radii of the donor and the acceptor, and J ′ is the normalized spectral overlap integral, where normalized means that both emission intensity and extinction coefficient have been adjusted to unit area. It is important noticed that J ′ , because it is normalized, does not depend on the actual magnitude of extinction coefficient nor on the donor fluorescence quantum yield. This difference is an important distinction from the situation for dipole-dipole energy transfer. The overlap integral J ′ simply derives from the density of degenerate states that couple donor and acceptor (from Fermi's golden rule), i.e. is a measure of the number of states that are capable of satisfying the resonance condition. Because K is a constant that is not related to any spectroscopic data, it is difficult to characterize the exchange mechanism experimentally.