Total isolated-photon efficiency as a function of $p_{\rm T}$ 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.