Page 137 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Why is skin cooling so critical in photothermal treatments?
All photo-thermal treatment use light to deliver energy into the target, where it is converted into heat energy after being absorbed by something. This heat then raises the local temperatures until the targets are, hopefully, cooked.
But, as described previously, some of this light energy will be ‘stolen’ by epidermal melanin. It is inevitable. In pale-skinned people this may as little as 5% of the incident energy, but it could be nearer 50% in very dark-skinned people. The temperature of the epidermis is raised, as a result, which conducts down to the thermal pain nerves, causing pain.
In addition, this heat energy can also induce blistering and pigmentary changes. None of these are wanted, so we must use pre-cooling to reduce their likelihoods. Cooling the skin surface will lower the temperature of the epidermis, the upper dermis and the pain nerves. The longer these are cooled, the colder they all will become.
Figure 79 – When three skin colours are lasered by a diode laser at 20J/cm2, the basal layer of the epidermis becomes hot. By applying sufficient surface cooling for the appropriate times, we can minimise the effects of this excess heat in that layer. However, in the very darkest skin types, SC6, surface cooling may not be sufficient to counteract the very high temperatures generated in the epidermis. In such cases, an Nd:YAG is a much safer choice since the peak temperatures generated by this laser are much lower.
________________________________________________________________________ 137 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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