Contour plots of the observable fraction of the dust as functions of redshift and the observed wavelength, for a dust temperature of 20 K (left) and 40 K (right). Contour levels are indicated in the plots. The contours are calculated using Equation (18) from da Cunha et al. (2013; see also their Figure 3), and indicate the difference between the intrinsic dust brightness temperature and that of the CMB, normalized by that intrinsic dust brightness temperature.