Page 52 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Mike presented this research at the BMLA Conference in 2022 in front of a number of puzzled- looking delegates...
How do pulsewidth, thermal relaxation time and denaturation time relate?
So, what is the relation between pulsewidth, TRT and the cooking (denaturation) time of tissues and cells?
Well, it turns out that there is no direct relation, although they are interlinked. Let us define each of these times in turn:
1. The pulsewidth
This is easy – it is simply the time it takes for the device to deliver its energy to the skin or target. In many photothermal lasers it is typically in the millisecond range, while for tattoo laser treatments it is usually nano- or picoseconds. It is merely a ‘delivery’ time – that’s all. It does not depend on the physical attributes of the target in any way. However, the maximum (peak) temperature attained in that target does depend on the pulsewidth (and the fluence, obviously!).
2. The thermal relaxation time (TRT)
As described above, this is the time taken for the maximum (or peak) temperature in an object to cool to 50% of that peak value. It is a ‘cooling’ time. We can see in figure 13.2 that this is the time, after the end of the pulse, where the temperature has fallen to a point in the cooling part of the graph, where the temperature is half of the maximum temperature (which is usually reached at the end of the pulse).
The TRT depends quite strongly on the physical size of the hot object – this determines how quickly or slowly the cooling part of the temperature-time curve drops (figure 13.2).
________________________________________________________________________ 52 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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