The ratio of observed mass and the real core mass for a series of models obtained by scaling the density of a one solar mass Bonnor-Ebert sphere. In frame a), the masses are determined with the 100 μm and 350 μm observations (thick lines) or the 250 μm and 500 μm observations (thin lines). The core is illuminated by the full ISRF (solid lines) or with a field that has been attenuated by A_V = 8^m. Frame b) shows results for background subtracted observations after including a uniform background that at 100 μm has an intensity of 0, 40, or 100 MJy sr^-1.