4.0.2. Far Infrared Colours (PHT-C)
The far-IR colour [C4-8] (Figures 2e, 3d, 4c), at higher optical depths (100 > 0.03), can surprisingly confirm the type of the exciting star, though the colour measurement should be rather accurate (better than 0.05 dex). However, it is insensitive to the dust density distribution law except at the largest optical depths. At low 100, the density distribution can be inferred provided this colour is known accurately. The colour [C8-11] (Figures 2f, 3e, 4d) is rather insensitive and demands even higher accuracy in measurement for extracting information. However, with modest accuracy, it can distinguish between a O4 and B0.5 star clearly. Inferring density distribution law needs about 0.02 dex accuracy.