Illustration of the spectral energy density response of each component in the microwave emission. The shaded bands indicate the nine different observation frequencies of the Planck space observatory. Our goal is to create a map of each component in the sky; “CMB” emission, “Thermal dust” emission, and so on, after making assumptions about the spectral behaviour of each component such as we do in this figure. We model the spectral behaviour as slightly different in each pixel; hence it is indicated using bands rather than lines. Figure 2 adds further detail. This figure is reproduced directly from Planck Collaboration X (2015).