Page 39 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
The good news is that this layer is only around 0.1mm below the skin surface, and so, is easy to cool effectively.
The trick is to cool the epidermis faster than it heats up! But, this becomes more difficult as the rate of temperature increase becomes faster – as with higher fluences. Consequently, high fluences applied to the skin are more ‘tricky’ to cool simply because the temperature rise is much quicker than with lower fluences – if the skin has not been sufficiently pre-cooled.
All of the above temperature calculations assume that there is no conduction during the delivery of the pulse – or, the pulse duration is extremely short. If a longer pulse is used to deliver the same amount of energy, then the peak (maximum) temperatures will be lower because some of the heat energy will conduct away from the basal layer during the pulse.
This can be significant! Longer pulses can induce much lower peak temperatures and hence reduce the pain sensation felt by the victim. It also allows the cooling to be more effective, since it doesn’t need to mitigate against the higher temperatures.
Given efficient skin surface cooling for each fluence applied, it may be possible to make laser/IPL hair removal ‘virtually’ pain-free whilst obtaining good results (i.e. at supra-threshold fluences). This will require sufficient pre-cooling before any light energy is applied, followed by plenty of cooling after the treatment has been completed (using ice packs).
It should be remembered that we are deliberately trying to over-heat the hair follicles to kill the stem cells. But, this will inevitably over-heat the basal melanin at the same time. We must mitigate against hits excess heating to reduce the pain felt and minimise any possibility of epidermal damage.
In summary, the only appropriate method to make an accurate judgement of epidermal light absorption is to use a high-quality melanin meter based on optical reflection measurement. The scarcity of such devices on the market is a typical sign of how immature this market segment still is after more than 30 years of common practice!
Epidermal temperature rise due to skin colour
The fluence must be chosen to kill the stem cells in the bulb and the bulge of the target follicles. Those are the real targets – not the melanin in the hair shaft. That melanin can be viewed as a
________________________________________________________________________ 39 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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