Page 170 - Chapter 3 - Laser/IPL Hair Removal

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Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
818nm – we utilised a standard ‘normalisation’ technique used by mathematicians to find a single, useful parameter) - Figure 96.
The depths used here are for the bulge, where the germ/stem cells reside on the follicle wall surfaces. Hence these thresholds only apply to the most superficial follicles – deeper follicles will require higher fluences.
These fluences assume a pulsewidth between 2 and 50 ms. Shorter, and longer, pulsewidths will not work so well – the cooking is less efficient – but this is only for fluences up to the threshold level! If your fluence is above the threshold, even by only 1 or 2 J/cm2, the importance of the pulsewidth is reduced. For even higher fluences, the importance of pulsewidth becomes, essentially, trivial.
Upper lip
Chin Arms Legs
Arm pits
Bikini
Threshold fluences - IPL
Figure 96 - Threshold fluences required to treat a range of body sites for IPL systems
We have also assumed a wavelength range between 600 and 1200 nm in these calculations. A smaller range will require higher fluences. Obviously, deeper follicles (bikini, armpits) require more fluence at the surface to ensure sufficient light gets deep enough to heat them. This means that more pre-cooling is also required, to protect the epidermis (since it will also get hotter!!).
The threshold at the skin surface for IPL devices is approximately 11.45 J/cm2, assuming a 10ms pulsewidth and a wavelength range from 600 to 1200nm. At this fluence the final value of Omega is 1 meaning that the cells are irreversibly denatured – these hairs cannot regrow.
Interestingly, a small increase in fluence (i.e. 1 J/cm2) above these threshold makes a large difference in terms of cell destruction! But, fluences below these levels will merely cook some of the stem cells, meaning that finer hairs will likely grow back in the future, due to the smaller quantity of surviving stem cells.
________________________________________________________________________ 170 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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FLuence (J/cm^2)