Page 117 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Now this does depend on the particular technology, but generally, with continuous lasers, longer pulsewidths will result in higher temperature increases in the skin (Figure 67).
Figure 68 - If we increase the pulsewidth, the local temperatures will decrease – with pulsed lasers
However, for pulsed systems, it is usually the other way round!! Shorter pulsewidths will generate higher temperatures, simply because there is less time for the heat to escape from the absorbing target during the pulse (Figure 68).
This is a bit more confusing.
So, we can see how changing the fluence, absorption coefficient and pulsewidth can change the rise in temperature. Note that these rises are all “linear”. In other words, the longer you apply the energy/fluence, the higher the temperature will rise in a corresponding, straightforward manner.
But, this is where it gets a bit weird. The way cells react to increased temperatures is most definitely NOT linear – it is exponential (as can be seen in the Arrhenius Equation) (Figure 69).
A relatively small increase in fluence (1 or 2 J/cm2) can lead to a very large increase in cell damage. So, we must always be careful when increasing the fluence – it’s not just a straightforward process.
Plus, an increase from 20 to 21 J/cm2 will be much more aggressive than a change from 15 to 16 J/cm2, because that 1oC increase is starting from a higher temperature, which means it has a greater effect on the cells (due to the higher temperature)!
________________________________________________________________________ 117 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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