Luminescent down-shifting (LDS) is an elegant, purely optical method to improve the short-wavelength response of photovoltaic (PV) modules by red-shifting the incident solar spectrum. This paper in a first step develops a simple optical model to investigate the effect of LDS layers on top of PV modules on their external quantum efficiency. A meaningful figure of merit is the LDS efficiency eta(LDS) which quantifies the efficiency of the LDS layer to down-shift absorbed photons and send them towards the underlying solar cell. The simplified optical model developed here, extracts LDS efficiencies from experimental quantum efficiency spectra, and thereby enables assessing the current status and future potential of LDS for various solar cell technologies. Our simplified LDS model is fast, easy-to-use and universally applicable to analyze experimental LDS results. Moreover, it is especially useful for screening candidate LDS materials. (C) 2013 Elsevier B.V. All rights reserved.