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Back-scattering
What about back-scattering? What does this do to the fluence?
Well, this is a very important issue. Computer models over the last 40 years (from many locations across the planet) have consistently shown that back-scattering photons have an important effect on the fluence in the skin.
As photons scatter in the dermis, many of them are turned through 180o and start to make their way back towards the skin surface. Of course, this means that they may be absorbed on their way out. It doesn’t matter which direction the photons are coming from – absorptions will occur regardless (see figure 3).
Figure 3 – Fluence and back-scattering
In the figure above, the ‘blue’ photons are those heading back towards the surface – the back-scattered photons (I used blue here to show their change in direction; not the change in wavelength!!). It becomes very obvious that there are many more photons in the region just below the skin surface because of the presence of the back-scattered photons. These will encounter ‘new’ photons which are still entering the skin.
Many of these photons will be absorbed by the epidermal melanin, resulting in high temperatures there. Given that the thermal pain receptors are just below the basal layer, this explains why so much pain may be felt during photothermal treatments, unless proper surface cooling is applied during the treatment.
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