Ratio of isolated-photon cross section measured with $R=$ 0.4 over $R=$ 0.2 for Pb$-$Pb and pp collisions at $\sqrt{s_{\rm NN}}=$ 5.02 TeV. Each panel for each Pb$-$Pb collisions centrality class, bottom right panel for pp collisions. Error bars and boxes are the statistical and systematic uncertainties, respectively. The violet band corresponds to pQCD calculations with JETPHOX, the width represents the PDF uncertainty, and the blue vertical bars indicate the statistical uncertainty from the Monte Carlo approach.