Transfer functions from the SPIRE 250 μm simulations binned as a function of S/N, with 1.15 mm (top panel) and 2 mm (bottom panel) total noise (solid black lines; grey shaded areas show the associated uncertainties), i.e. the sum of high frequency (HF) and low frequency (LF) noise. The noise standard deviation σ_HF is from a simulation including only high-frequency noise (dashed line). The red curve and red shaded area show the cumulative fraction of the flux that is recovered and the associated uncertainties.