Page 98 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
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 48).
In the figure below, the ‘blue’ photons are those heading back towards the surface – the back- scattered photons (We 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.
Figure 48 – Fluence and back-scattering
Monte-Carlo simulations reveal that this increase in fluence can extend as far as 1.4mm into the skin. This means that the fluence applied at the skin surface will increase down to a depth of 1.4mm, then will drop rapidly due to absorption and scattering.
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