Page 131 - Chapter 3 - Laser/IPL Hair Removal

Figure 77 - Where are the 'zones' for a low fluence?
Research by Emerson and Town has shown that follicles in the ‘stun’ zone are forced into a “catagen-like” state. They have not been sufficiently damages to kill the follicles outright, so they will survive. After a relatively short period, they go into a “telogen-like” phase of quiescence. After some time, they will be triggered into a new anagen phase, but due to a smaller quantity of stem cells, their new hairs will be thinner and lighter in colour.
Below that zone is the null zone, where the fluence is now so low that it merely 'warms' the follicle stem cells but has no real effect on them. Those hairs will survive this treatment easily.
Clearly, this 'low' fluence will not kill all the follicles since the energy reaching the deeper hairs is just too low. So, we need to increase the fluence to push down the depth of the kill zone (see image below).
________________________________________________________________________ 131 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
If we fire a relatively 'low' fluence into the skin, then the effectiveness of this fluence will permeate to some depth. The targets down to that depth should be properly cooked. But, just below that effective kill zone, the fluence will be too low to have the desired effect on the follicles. Some of the stem cells will be cooked, while some will remain alive. These surviving cells will re- generate those follicles in the next anagen phase and grow finer, lighter-coloured hairs.