All hairs are part of a follicular unit which comprises the hair shaft, melanin (not always!!) and germ/stem cells. [There are other components such as the pilo-sebaceous units – but we’re not interested in those here.]
The germ cells are responsible for initiating and growing the hair in the anagen phase. They reside in a ‘bulge’ which is usually located about one third to one half of the way down the follicle.
In order to kill a follicle, ALL the germ cells in that unit must be destroyed (denatured). We can do this with heat generated by the absorption of light.
However, to ensure full, irreversible denaturation of all those germ cells, we MUST deliver sufficient energy – it is the heat energy which ‘cooks’ the germ cells.
And this is where the problem lies…
If there is insufficient energy delivered, then only some of the target germ cells will be properly (irreversibly) cooked. Some may survive and regenerate. Other cells may not be affected at all!
The difficulty is that we can never know how deep these cells are. If the hair is deep (such as underarm or pubic hair) then the germ cell bulge will be correspondingly deeper compared with more superficial hair. In these cases, we must deliver more energy to ensure a sufficient amount reaches those cells. Unfortunately, the concentration of this energy (fluence) drops exponentially as it penetrates deeper into the dermis (due to scattering).
So, what often happens, is that only a portion of the germ cells in a bulge are cooked properly, leaving a sufficient number to ‘re-group’ and take the follicle into the next anagen phase. But, since there are now fewer germ cells than before, the new hair is thinner (finer). Fine hairs do not generally contain as much melanin as thicker hairs since the hair shaft is also thinner.
As a consequence, fine hairs are harder to kill than thicker hair!!
When I hear someone saying “the hairs have grown back finer and lighter”, I immediately think “you didn’t use enough fluence!!” It’s obvious!
Lighter, finer hairs require even more fluence since they have less target melanin in them. The pulsewidth is not really that important (contrary to popular opinion!) It is the ENERGY which drives these processes – always.
Insufficient energy will always result in poor results. Using the correct fluence at the start of your treatments should always generate good results.
I hope this helps,
PS I will be writing a post soon about ‘pulsewidths’ – my recent research has shown that far too much importance has been placed on this parameter, and it’s incorrect!!
Don’t forget our upcoming MasterClass in Manchester on April 23rd/24th.