Contributions to the systematic uncertainty of the isolated-photon purity and their quadratic sum as a function of $p_{\rm T}$ for $R=$ 0.2, in pp collisions (bottom left frame) and two Pb$-$Pb 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).