Three elementary spectra of dust (see Eq. (13)) in a log-linear scale. For illustration purposes, we take the case of 20 K dust with an emissivity of . The component of index 0 is the thermal brightness of dust. The component of index 1 comes from the first derivative of the Planck function (Eq. (6)) with , and the component 2 is obtained from the second derivative (Eq. (8)) using a logarithmic temperature dispersion ofσ = 0.15.