Page 125 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
This treatment process is a bit like a battle between hot and cold. We are trying to ‘overheat’ the targets stem cells, by generating heat in the melanin in the hair shaft. But, at the same time, we are also trying to minimise the temperatures generated in the epidermis and upper dermis. These depend on the fluence, pulsewidth and skin tone. Results indicate that longer pulsewidths appear to be more ‘comfortable’ in darker skins – this makes sense since they will generate lower temperatures as the heat flows from the hair during the pulse.
Change in fluence with depth in the skin
When light energy is fired into the skin, it undergoes a number of processes which results in a change in the fluence.
Typically, the skin surface will reflect between 4 and 7% of all light falling on it. When it enters the epidermis, it encounters atoms and molecules, immediately. This causes the photons to scatter, in many directions. As a direct consequence, the beam diameter expands as the light penetrates deeper into the skin. Hence the fluence drops (since it’s just the energy divided by the spot diameter).
Total fluence in skin =
Reflection at skin surface
+
backscattered energy
+
bulk absorption in the dermis
+
chromophore(s) absorption
+
beam expansion (scattering)
+
transmitted energy to subcutis tissues
________________________________________________________________________ 125 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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