\ Contributions from reconstruction, identification, and isolation to the total efficiency calculated using Eq. (7) of Ref. , as a function of the reconstructed photon \ptg for \pp (lower right frame) and \PbPb collisions for five centrality classes. 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 Efficiency obtained from PYTHIA 8 simulations of $\gamma$--jet processes, embedded into \PbPb minimum bias data in the \PbPb case.