Contributions from reconstruction, identification, and isolation to the total efficiency calculated using Eq. (7), as a function of the reconstructed photon $p_{\rm T}$ for pp (bottom left frame) collisions and Pb$-$Pb collisions for two centrality classes: 0$-$10% (upper left frame) and 70$-$90% (upper right frame). Green markers: reconstruction efficiency $\varepsilon^{\mathrm{rec}}$ Red or brown markers: efficiency additionally due to the photon identification by shower shape selection $\varepsilon^{\mathrm{rec}} \times \varepsilon^{\mathrm{id}}$ or the isolation criterion $\varepsilon^{\mathrm{rec}} \times \varepsilon^{\mathrm{iso}}$ Blue markers: efficiency due to the isolation criterion and shower shape selection $\varepsilon^{\mathrm{rec}} \times \varepsilon^{\mathrm{id}} \times \varepsilon^{\mathrm{iso}}$ Black markers: fraction $\kappa^{\mathrm{iso}}$ of generated photons which are isolated 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.