T and β values for pixels with 350 μm surface brightness ~10 MJy sr^-1 (red band) and ~100 MJy sr^-1 (blue band). The values are plotted as a function of multiplicative (left frames) and additive (right frames) modifications of the measured 250 μm surface brightness. The solid curves are the median values (calculated over all fields) and the shaded region between the 25% and 75% percentiles indicates the field-to-field dispersion. The grey vertical bands correspond to the probable uncertainties of ~2% in the relative gain calibration and ±0.2 MJy sr^-1 in the surface brightness zero point.