Effect of the photometric uncertainties in the flux density distribution at 500 μm. Black dashed line: distribution of the flux density measured at the position of the 24 μm sources. Red solid line: the same distribution after adding a 5.1 mJy random Gaussian noise to each measured flux density. Due to photometric noise, some sources have a negative flux density (put to zero in our iterative algorithm) and are not represented here. Black dotted line: flux density cut used in our analysis (20 mJy).