Reconstruction of the EBL normalization with Monte Carlo simulated air showers passing through the analysis chain. Three samples of Monte Carlo events are represented: the first one (orange squares) corresponds to the observation conditions of PKS 2155 - 304, the second and third (triangles) correspond to a poor background estimation. These two last sets were used to estimate the systematic uncertainty represented with the grey shaded area. Top panel: reconstructed EBL normalization as a function of the simulated normalization. Bottom panel: residuals, defined as the difference between the reconstructed and simulated optical depth normalizations.