Photon conversion layers are a possible way of improving the efficiency of existing solar cells, even above the Shockley-Queisser limit. The related concepts are often called downshifting, downconversion, and upconversion. Despite the variety of photon conversion systems proposed in the literature, understanding their real impact is often difficult due to joint effects. Here, a new methodology is provided to analyse the efficiency of such conversion layers and to be able to compare the different systems proposed. (C) 2014 Elsevier B.V. All rights reserved.