Modelled hemispherical emissivity ε_h of a regolith of grains following a power-law size distribution with p = 2.8, a_max = 5 cm, and a_min = 1 µm. Water ice is crystalline (Ih90) and contaminants are AmC or Th, intra-mixed with fraction f. Two estimates of the emissivity ε_F of the unlit B ring with the CIRS instrument on April 5, 2017, are displayed (blue dots, see text for details). Both esa (solid line) and δ-Edd (dash-dotted line) diffraction corrections are shown, assuming ξ = 0.