Page 52 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Site
Number of hairs
% of 1 cm2
Thin
Thick
Chin
500
0.11
12.9
Upper lip
500
0.11
12.9
Arm pits
65
0.01
1.7
Pubic
70
0.02
1.8
Legs
60
0.01
1.5
Table 9 - Percentage of hair coverage in various body sites
Table 9 shows the range of area hair can cover in a number of body sites, depending on whether the hair is thin or thick. So, for example, chin hair can range anywhere from 0.11% to nearly 13% coverage, depending on the hair diameters.
This is quite remarkable, and counter-intuitive. I’m sure we all imagine that the hair coverage is significantly greater than these figures, yet, we are wrong! Unless the data in Table 8 is completely wrong, these are the facts!
So, the maximum hair area densities are found in the chin and upper lip at between 0.11% to 12.8%. For all other sites, these range from 0.02% to 1.8%, depending on the hair diameters.
This correlates to the amount of light which may be absorbed. As the light penetrates into the skin, this ratio decreases since the beam diameter spreads!
No. % light NOT density Total area of hair (cm2) absorbed
Thin to thick % of 1cm2 area
Chin
500 1.13E-07 1.29E-05 0.11 12.9
87 - 99.9%
Upper lip 500 1.13E-07 1.29E-05 0.11 12.9 87 - 99.9%
Arm pits
65 1.47E-08 1.67E-06 0.01 1.7
98.3 - 99.99%
Pubes 70 1.59E-08 1.80E-06 0.02 1.8
98.2 - 99.98%
Legs
60 1.36E-08 1.54E-06 0.01 1.5 98.5 - 99.99%
Table 10 - % of light NOT absorbed by hair
What does this mean for light-based hair removal treatments?
This is an interesting question. We set up our lasers and IPLs to deliver a certain amount of energy per square centimetre (the ‘fluence’ - see this video on this topic). We choose certain wavelengths (yet another video) that we know will be absorbed preferentially by the chromophores in the targets (melanin in hair, haemoglobin in blood etc).
________________________________________________________________________ 52 Chapter 3 Laser/IPL Hair Removal
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