3. The fraction $$a$$ of photons emitted by D and absorbed by A is given by $a = \frac{1}{\mathit{\Phi}_{\text{D}}^{0}}\int _{_{\lambda }}I_{\lambda}^{\text{D}}(\lambda)\left [ 1 - 10^{-\varepsilon_{\text{A}}(\lambda)c_{\text{A}}\, l} \right ]\text{d}\lambda$ where $$c_{\text{A}}$$ is the molar concentration of acceptor, $$\mathit{\Phi} _{\text{D}}^{0}$$ is the @F02453@ @Q04991@ in the absence of acceptor, $$l$$ is the thickness of the sample, $$I_{\lambda}^{\text{D}}(\lambda)$$ and $$\varepsilon_{\text{A}}(\lambda )$$ are the @S05813@ of the @S05827@ of the donor @F02453@ and the @M03972@ of the acceptor, respectively, with the @NT07086@ condition $$\mathit{\Phi} _{\text{D}}^{0} = \int_{\lambda}I_{\lambda}^{\text{D}}(\lambda)\, \text{d}\lambda$$.
For relatively low @A00028@, $$a$$ can be approximated by $a = \frac{2.3}{\mathit{\Phi}_{\text{D}}^{0}}c_{\text{A}}\, l\int _{\lambda}I_{\lambda}^{\text{D}}(\lambda)\varepsilon_{\text{A}}(\lambda)\text{d}\lambda$ where the integral represents the overlap between the donor @F02453@ spectrum and the acceptor @A00043@.