Contributions to the systematic uncertainty of the isolated-photon purity and their quadratic sum as a function of \ptg for $R=0.2$, in \pp collisions (bottom left frame) and two \PbPb collisions centrality classes: 0--10\% (top left frame) and 30--50\% (top right frame). The statistical uncertainty is also shown and may appear larger than the total uncertainty as a result of the smoothing done by the fit over the purity (see text).