Surface Brightness Spectra of medium size cold cores as a function of the mass: 0.4, 5 and 20 solar Masses. The typical cirrus galactic brightness emission (red dashed-dot line) is modelled as a grey body at 17K with a spectral index β = 2. The averaged Galactic plane emission, over-plotted in red dash-dot-dot-dot line, gives an estimate of the background Galactic confusion level. The more massive the cold cores are, the more warmer they appear, and the less detectable they are.