Page 116 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
It must be understood that a small increase in fluence results in a small increase in temperature in the germ cells, but that the rate of denaturation increases exponentially with temperature. So, even a 1oC rise in temperature in those cells will have a very significant effect on the outcome!
Diode Laser – Threshold fluences
The diode laser wavelengths (808, 810nm) are very similar to the averaged output from an IPL (with a Xenon lamp). As a direct consequence, the threshold fluences are also very similar.
The threshold at the skin surface for diode lasers (at 810nm) is approximately 11.1 J/cm2, assuming a 10ms pulsewidth. The 808nm laser will be very slightly different.
Body area
Threshold fluence (J/cm2)
Upper lip
11.1
Chin/arms
20.3
Arm pits/legs
23.2
Bikini
27.2
Table 27 - Threshold fluences for diode lasers
Diode systems come in a range of maximum output powers. Some units can deliver up to 2000 Watts, while others might go as high as 5000 Watts (claimed!). This means that they deliver their energy at different rates – so a 5000W laser will deliver its energy 2.5 ‘faster’ than a 2000W laser. But fluence doesn’t depend on this at all – when we consider the delivery rate we are discussing power density (or ‘intensity’/’irradiance’ in proper physics parlance).
So, if the fluence from a 2000W laser is 10 J/cm2, then a 10 J/cm2 from a 5000W laser is precisely the same – however, the 5000W laser will deliver it much more quickly into the skin and this changes the maximum temperature attained in the hair shaft. Such high temperatures can lead to rapid steam formation inside the follicle which can force the hair shaft to be ejected during treatments. Anecdotal evidence shows that such hairs may be simply ‘wiped’ away from the skin using a tissue or cloth. However, this does not guarantee a successful outcome since the follicle’s germ cells may survive this exposure.
Using a high-powered laser, such as a 5000W system, allows the user to apply shorter pulsewidths. This is equivalent to using a lower power with a longer pulsewidth – but this can have a different effect on the hair follicles.
________________________________________________________________________ 116 Chapter 3 Laser/IPL Hair Removal
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