Page 74 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
it appears that the follicle diameters are typically between 3.5 and 5 times the diameters of their hairs.
This means that the heat must ‘travel’ from the hair shaft, where the absorbing melanin resides, out to the bulge in the follicle wall. For forehead hairs this is a relatively short trip – between 2 and 95 microns, whereas, for calf hairs the distance is between 95 and 175 microns (see table below).
Clearly, this requires some thought as to the optimum parameters to use. An easy way to think of this is the following:
the light energy is absorbed by the melanin in the hair shaft which heats up;
the hair shaft becomes the heat source – the heat energy conducts outwards in all directions;
as the heat transfers it raises the temperature of everything it ‘touches’;
the further it moves from the heat source (the hair shaft), the less effect it has (lower temperatures);
distant objects are cooler than nearby objects (relative to the hair shaft).
This means that we must consider which fluence/pulsewidth combination carefully for different hairs across the body! (See Section 3.6 – How to Choose the Parameters).
Hair shaft diameter
Follicle diameter
from
to
from
to
Forehead
8
25
10
120
Back
10
25
40
170
Thorax
10
23
55
140
Upper arm
10
23
60
120
Forearm
12
24
55
105
Thigh
17
41
80
175
Calf
30
55
125
220
Average
13.9
30.9
60.7
150
Table 9 – Hair shaft and follicle diameters across the body
Hair colour / thickness
Hair colour is determined principally by the melanin content within the medulla. The more melanin contained in the hair the darker it appears. Hence the reaction of the hair to light-based treatments depends very much on the hair colour. Quite simply, darker hair absorbs more light than lighter hair.
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