4.1. Mid Infrared Colours (PHT-P)
The mid-IR colour [P3-9] (Figure 1a) is measurable only if the exciting star is of type O4. Except for a small range of 100 (0.06 - 0.04), the constant density case can be easily distinguished from other density distributions. The r-1 or r-2 cases can be separated for higher 100 (> 0.1) cases. The colour [P9-12] (Figures 1b, 2a) is measurable for O4 and O7 type stars. The dust density distribution of r-2 can easily be distinguished from the other distributions using this colour. The difference between r0 and r-1 can also be found for 100 < 0.02. To distinguish between the exciting stellar type, a very accurate measurement of this colour (< 0.005 dex) is required. The colour [P12-13] (Figures 1c, 2b, 3a) is measurable for all three types of exciting stars. Once again this colour can easily discriminate r-2 dust distribution law vis-a-vis r0 or r-1 for all three types of exciting stars. For the stellar types O4 and O7, the cases r0 and r-1 can be easily distinguished for 100 > 0.03, but requires better colour measurement for lower 100 values. For B0.5 type star, the same can be done for any value of 100 (except for a narrow region around 0.04) even with moderate accuracy of this colour. The colour [P13-14] (Figures 1d, 2c, 3b) with even moderate accuracy (0.005 - 0.1 dex), is extremely useful in distinguishing all the three density distributions for both O4 and O7 type of exciting stars. However, for B0.5 type star, only for 100 > 0.01, this discrimination is possible. The distinction between the spectral type of the exciting star is possible for most of the 100 range if the colour is available with good accuracy ( < 0.01 dex).