Total isolated-photon efficiency as a function of $p_{\rm T}^{\gamma}$ calculated using Eq. (7) with the systematic uncertainty discussed in Sect. 4 for $R=0.2$ and $0.4$, for pp collisions and Pb$-$Pb collisions for five centrality classes. The efficiency is obtained from PYTHIA 8 simulations of pp collisions $\gamma-$jet processes, embedded into data in the considered centrality class for the Pb$-$Pb collision case.