Page 26 - Blog Posts v1

A8 - Why we should NOT reduce the fluence when treating darker skins!!!
The fluence must be chosen to kill the germ cells in the target follicles. Those are the real targets – not the melanin in the hair shaft. The melanin can be viewed as a surrogate which we can heat using light energy. But, the germ cells do not have a chromophore which we can exploit using light.
So, to kill the follicles we must heat the hair shaft melanin sufficiently to denature the germ cells irreversibly.
The skin colour is completely irrelevant in this consideration! It doesn’t matter what the skin colour is – only the hair colour. The skin colour (in the basal layer of the epidermis is simply a barrier we must surmount).
Let’s consider the skin colour. There are two important issues we should think about:
1. The melanin in the basal layer will absorb light energy, inducing a temperature rise in that layer. This heat will diffuse towards the thermal pain receptors (just below the basal layer) and trigger them. If the temperature is high (above 45oC), then pain will be felt.
2. Any fluence absorbed in the basal layer will not be available to the deeper melanin in the hair shaft. So, darker skins will result in lower fluences reaching the hair melanin.
So we have a conundrum...
To successfully kill the hair follicles we must apply a certain minimum fluence at the skin surface.
But the skin’s natural colour will absorb some of this fluence, leaving less to reach the hair melanin.
This suggests that darker skins will require higher fluences than lighter-coloured skins...
Most laser/IPL devices have a “Skin Type” selection option - usually when a higher skin type is chosen, the fluence automatically decreases.
This is entirely WRONG!!!! For two reasons:
Firstly, the darker skin will ‘lose’ more fluence because of the higher absorption in the basal melanin;
and secondly, applying a lower fluence at the surface will reduce the likelihood of a successful treatment!
Mike’s Blog Posts 26