4.0.1. Mid Infrared Colours (PHT-P)
The mid-IR colour [P3-9] (Figure 2a) is measurable only if the exciting star is O4. For low or moderate optical depths (100 < 1), this colour can easily distinguish the density distributions r0 or r-1. The spectrum is redder for r0 than it is for r-1. The colour [P9-12] (Figures 2b, 3a) is measurable for O4 and O7 types. Once again the low and medium 100 cases can distinguish the density distribution (redder for r-1). On the other hand, the colour [P12-13] (Figures 2c, 3b, 4a) is measurable for all three types of exciting stars. At low optical depths (100 < 0.03), although this colour can not distinguish between the dust distributions, it is extremely sensitive to the type of the exciting star, hence can confirm the type as expected from total luminosity. If 100 is greater than 0.1, then the dust density distribution law can be determined with ease. The colour [P13-14] (Figures 2d, 3c, 4b) again can easily unravel the dust density distribution, if the stellar type estimate is already available.