Left: Isolated-photon differential cross section measured in pp and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=$ 5.02 TeV for five Pb$-$Pb centrality classes for $R=$ 0.2. Error bars and boxes are the statistical and systematic uncertainties, respectively. The bands correspond to NLO pQCD calculations with JETPHOX, for Pb$-$Pb collisions calculated for the 0$-$100% centrality class and scaled by $\langle N_{\rm coll} \rangle$. Right: Ratio of data over JETPHOX NLO pQCD calculations. The bands centred at unity correspond to the JETPHOX pQCD calculations, their width represents the scale (blue) uncertainty and PDF (orange) uncertainty. The normalisation uncertainties are not included in the left panel but they are shown in the right panel as a grey box on the right of each of the frames around unity.