Page 50 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
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!
Where does the concept of TRT in photothermal treatments come from? Anderson and Parrish wanted to selectively heat up the target vessels in port wine stains but without damaging the tissue surrounding those vessels. So, they decided to construct a set of rules to minimise the potential thermal conduction to adjacent structures. They chose the TRT of the target vessels to set the appropriate laser pulse duration.
This was a perfectly valid choice – it ties the duration of the laser pulse to the size of the target. In doing so, it reduces the time allowed for conduction of the heat generated by the laser energy, to flow into the surrounding tissues.
But it was a random choice! They could just as easily have chosen two times the TRT, or five times or ten times. It was arbitrary!
And it was wrong!!!
Why? Well, the premise of using the TRT as a guide for the pulse duration was flawed. The original idea was to prevent unwanted damage to other tissues by limiting the heating time so that adjacent tissues were not thermally damaged. But this idea did not consider the amount of
time required to damage the target cells.
If you put an egg into a pan of boiling water for 10 seconds, it will not cook through properly. That’s because the egg needs a certain amount of time to cook – typically between 3 and 5 minutes, depending on your preference.
As with all cooking, we know that we must cook our food at some temperature for a certain length of time. The same goes for cells – they must be maintained at the right temperature for the appropriate time, to ensure full denaturation (cell death).
________________________________________________________________________ 50 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
© The Laser-IPL Guys, 2025