In the literature two different simplified expressions exist for the spontaneous emission factor β, being the fraction of spontaneous emission radiated into a specific optical mode. These expressions are valid in two mutually exclusive regimes. By treating the coupling between one discrete cavity mode and the outside optical continuum quasi one dimensionally in the spirit of Fano [Phys. Rev. 124, 1866 (1961)], we extend the validity range of both expressions and show that they lead to essentially the same result. The relevance and limitation of this result are discussed.© 1996 The American Physical Society.