Page 142 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
In essence, fluence, balanced with the appropriate amount of pre-cooling, leads directly to successful outcomes.
In many situations, laser users reduce the fluence when they see a ‘darker skinned’ patient –
THIS IS A BIG MISTAKE!!
Darker skins contain more melanin, per unit volume, than lighter skins. Hence, they will become ‘hotter’ for any given fluence. It is quite obvious – if someone has more melanin in their skin, then it will “steal” more light energy as the light passes through the epidermis. Consequently, the epidermis will become hotter compared to a pale skinned person.
BUT.....
The task in hand is to destroy the stem cells in the follicle. This requires a certain amount of fluence to reach those stem cells, regardless of the skin colour.
So, we choose the fluence according to the depth of the hair follicle, and the amount of pre- cooling according to the skin colour.
The curious fact which emerges from this line of thinking is that darker skins allow through less light energy into the dermis. This suggests we need to use higher fluences on darker skins, to achieve the same results as found in paler skins.
This is very counter-intuitive!!! Yet, there we have it - darker skins ‘steal’ more light and so this should be compensated for by applying more light energy at the surface. Which means, yet more cooling.
This explains why many people use long-puled Nd:YAG lasers on very dark skin tones. The 1064nm wavelength is poorly absorbed in melanin, so it does not induce much of a temperature rise in the epidermis. As long as enough fluence reaches the deeper follicle melanin, the stem cells can be cooked properly.
Interestingly, the colour of the skin really does not matter with the Nd:YAG laser – it can be used on all skin tones successfully, but you need to apply relatively high fluences to ensure sufficient absorption in the melanin.
________________________________________________________________________ 142 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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