Page 49 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Side effects after IPL treatments due to improper device settings. In this case, it appears that there was insufficient skin surface cooling during, and after, the treatment, leading to excessive thermal damage in the dermis.
It is very easy to damage the skin using modern lasers and IPLs. This can be significantly reduced with proper understanding and training.
Figure 16 - A badly done IPL treatment due to incorrect choice of settings and lack of cooling
Selective Photothermolysis and hair removal – a quick review
4 J/cm2
We discussed the basic principles of Selective Photothermolysis on Chapter 1, but we will review them here.
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
________________________________________________________________________ 49 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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