Infrared brightnesses for grains of different sizes (plotted as different curves in each panel) and various compositions: Si (left panels), Gra (middle panels) and PAH⁺ (right panels), heated by the diffuse radiation fields calculated for the best fit model of our prototype galaxy NGC 891. Infrared brightnesses for PAH⁰ are not plotted in this figure. The colour coding is as follows: red is for grains with radius a < 0.001   μm (0.00035, 0.00040, 0.00050, 0.00063 and 0.00100 μm), green is for grains with 0.001 < a ≤ 0.01 (0.00158, 0.00251, 0.00398, 0.00631, 0.01000 μm) and blue is for grains with a > 0.01 (0.0316, 0.10000, 0.31623, 0.7943 μm). The grain sizes considered in this plot are the same as those plotted in Fig. 6. Going from the top to the bottom panels the calculations are done for different positions in the model galaxy: R = 0 pc, z = 0 pc; R = 15   000 pc, z = 0 pc; R = 0 pc, z = 600 pc; and R = 15   000 pc, z = 600 pc.