Mike Murphy's Blog

When treating skin with lasers/IPLs we are often told that darker skin tones must be treated with lower fluences.

This is wrong!

Our skin tones come from the concentration of melanin in the epidermis – mostly in the basal layer in lighter tones. In darker skins, the melanosomes may be found throughout the epidermis, not just in the basal layer.

All visible light, and most of the near-infrared light, is absorbed by melanin (especially down at the blue end of the spectrum). Consequently, any light, in this wavelength range, that we fire at the skin will encounter epidermal melanin. This presents a ‘barrier’ to the incoming light.

In the image below, we can see that the skin tone 1 (ST1) barrier is significantly lower and thinner than the skin tone 6 (ST6) barrier. That’s because both the concentration and distribution of the melanosomes is greater in darker skin tone.

If we fire a fluence, F₀, into these two skin tones, the melanosomes in ST6 will absorb much more of the light energy than the ST1. So, the fluence reaching the dermis will be significantly reduced by the ST6 layer.

The light energy emerging from the epidermis in the ST1 scenario is ‘F₁’, while the fluence emerging from ST6 is ‘F₂’. We can clearly see that F₁ is always larger than F₂, because more fluence is ‘stolen’ by the melanosomes in the ST2 epidermis. This is true for all darker skin tones, 2 to 6.

This has two significant consequences:

1. The epidermal temperature in the ST6 scenario will be much greater than in the ST1 scenario. This could easily lead to more unwanted tissue damage in the epidermis and upper dermis in that skin tone;
   
2. Less fluence reaches the dermis after traversing ST6 than with the ST1 scenario.

We can easily compensate for the over-heating of the ST6 epidermis with more pre-cooling before laser/IPL energy is applied.

However, the second consequence is more troubling….

If less fluence is reaching the dermis, where our intended targets are found (hair, blood vessels, tattoos), then our chances of a successful treatment outcome are reduced.

What this means is that we must apply MORE fluence to darker skin tones than we do with the lighter skin tones, to achieve the same goal. However, in doing this, we will increase the epidermal temperature even more!!

So, we must apply even more pre-cooling to darker skin tones, if we want to use higher fluences and more successful outcomes.

But there is a limit – there is a point where the epidermis has just too much melanin, which cannot be sufficiently cooled to prevent damage or pain.

Conclusion

This is a bit disturbing. We’ve been told for years that we should reduce the fluence when treating darker skins – but that will also reduce our success rates. The science shows that this approach is incorrect!! We should be increasing the fluence to counter the increased loss due to the greater melanin concentration in dark skins. Plus, this requires greater cooling too.

Unless, of course, you’re using the 1064nm wavelength from an Nd:YAG laser…

Hope this makes sense,

Mike.