Page 99 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Selective Photothermolysis and hair removal – a quick review
The theory of Selective Photothermolysis was devised in 1980 by two American scientists, John Parrish and Rox Anderson. They wanted to figure out a better method of treating blood vessels in the skin, without damaging the surrounding tissues – as was ‘normal’ back in those days.
So, they came up with the idea that by carefully selecting the wavelength, the fluence and the pulsewidth of the laser, we could minimise collateral damage whilst effectively damaging the target tissue.
Their experiments on blood vessels worked very well. It appeared that their selection process was correct. We have been using their ideas in skin treatments ever since.
While this theory worked well, it did not describe the processes fully (we discussed this in more detail in Chapter 1, Section 1.3). One of the cornerstones of this theory was the idea of ‘thermal relaxation time’ which is the time taken for a hot object to drop in temperature by 50%. The idea was that by limiting the pulsewidth of the light energy to this value, we would minimise collateral damage to other skin tissues, and prevent scarring etc.
However, there is a flaw in this idea.
Thermal relaxation time – how important is it?
There is a fundamental premise behind the theory of selective photothermolysis, which underpins many of today’s laser/IPL treatments. The idea is that we should select a pulse duration (the time duration of the pulse of light) which is ‘shorter’ than the ‘thermal relaxation time’ (TRT) of the target – that might be a blood vessel, hair or pigmentary target (usually, melanosomes).
The TRT of an object is simply how quickly it cools from its maximum temperature to some other temperature – this is usually chosen to be 50% of the maximum temperature. When an object becomes hot, it loses heat energy to its surroundings due to conduction (and convection, if there is a flowing liquid nearby, like blood).
The rate at which it loses this heat is dependent on the size of the object and its specific heat capacity. So, larger objects take longer to cool, compared to smaller objects made from the same material.
Consequently, large hairs and blood vessels take more time to cool down than smaller hairs and vessels. However, the original idea did NOT consider intra-pulse conduction of heat during the pulse – this allows for cooling to occur while the light energy is still being delivered. This clearly happens during longer pulses!
________________________________________________________________________ 99 Chapter 3 Laser/IPL Hair Removal
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We discussed the basic principles of selective Photothermolysis on Chapter 1, but we will review them here.
4 J/cm2