Abstract A widely published expression for dipole strengths of optical transitions is found to require correction. The proposed adjustment, which involves the refractive index of the solvent in which the strengths are measured, may lead to significant changes in predictions that have been based on the equation. We discuss a closely related issue, the vacuum dipole strength concept, in an empirical context. A simple mapping procedure for estimating index dependence of strengths is advocated as an alternative to effective field corrections. The technique is illustrated for chlorophyll a and bacteriochlorophyll a.