Page 112 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
The Importance of Back-scattering
When light enters the skin it is almost immediately scattered by the atoms that the photons encounter. The dermis is a highly scattering medium, so each photon may undergo many scattering events before being finally absorbed or ejected from the dermis.
The curves in Figure 62 show how the fluence behaves in the skin for three wavelengths, 532nm, 755nm and 1064nm; plus an ‘averaged’ data set representing the average of 755 and 1064nm data. The horizontal dotted line shows the fluence incident at the skin surface.
Back-scattered photons are those which, effectively, turn back on themselves, heading back towards the skin surface. In doing so, they ‘meet’ other, incoming photons. Back-scattered photons, combined with incoming photons increases the total fluence available in the region between the surface and a depth of around 1.4mm. As a consequence, any fluence applied at the skin surface will be increased in the top layer of the dermis – which may be beneficial in our treatments.
What does this mean in treatments?
Most laser/IPL treatments in the skin depend on the rise in temperature within the intended target (melanin, blood, tattoo ink). That temperature rise is dependent on the absorption coefficient of the target (which varies with wavelength) and the fluence which hits that target.
Clearly, the temperature rise drops as we go deeper into the dermis, since the fluence is lower there. From the above curves, it is obvious that the highest temperatures will be achieved near the skin surface, where the fluence is highest – for any wavelength.
The temperature rise also depends on the absorption coefficient, μa – this determines the probability of a photon being absorbed by that target. If μa is large, then the likelihood is that the photon will be absorbed, which will add to the temperature rise in that target. We should note that μa is very dependent on the wavelength.
So, if we need to attain some particular temperature in our target, then we need to be sure to apply sufficient fluence at the skin surface to ensure it is achievable. The results above indicate that this may be achieved by proper selection of the fluence at the skin surface. Increasing this will increase the amount of available energy at all depths. This means that we may need to use higher fluences to reach deeper targets.
Conclusion
The importance of back-scattering has been overlooked in laser/IPL skin treatments. Yet, it is clear from the above, that is it very important since it can have a significant effect on the fluences in the skin.
________________________________________________________________________ 112 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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