Page 132 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
A higher fluence will have more effect throughout the dermis since more energy can reach the deeper regions. The kill zone is now larger (deeper) than the previous low fluence. Any follicles down to the new depth should be properly killed. The stun and null zones are also now deeper, and smaller since the dermis is finite in size. In fact, our aim should be to fire a sufficiently high fluence so that all of the full-thickness dermis is a kill zone. In that way we ensure the highest chances of success. This is especially true for those regions of the body where the deepest hairs are found - the underarms and bikini areas.
Figure 78 - The 'zones' for a high fluence
We must use higher fluences there to be sure we are killing all the follicles, otherwise, thinner hairs will re-grow over time (Figure 78). Of course, higher fluences always require more pre- and post-cooling to protect the skin and minimise pain.
The above is true for all wavelengths (all devices) and skin tones. It's simple physics
Follicle sizes and TRTs
So, how does this apply to hair? Well, according to researchers, the diameter of hairs can range from 8 to around 100 microns. This is the size of the hair shafts, not the follicles. Remember, when we apply light treatments to hair, we are trying to heat up the melanin in the hair shaft since we know it can absorb light energy.
The melanin in the hair shaft becomes hotter than its surroundings, and this heat energy then conducts outwards to the germ/stem cells on the surface of the follicles. These are the real targets that must be destroyed to kill the follicles. But, unfortunately, they do not contain any chromophore which we can target directly, hence we need to use the melanin in the hair shaft itself.
Table 24 shows the difference between the TRTs and the denaturation times for two hair diameters. The denaturation times are the times that those hairs must be maintained at 70C and 80C to ensure complete destruction of those follicles (i.e. the germ/stem cells can be destroyed).
________________________________________________________________________ 132 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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