Page 78 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
If we multiply those numbers with the cross-sectional areas of the hairs, we can find how much hair (by area) there is in each square centimetre of skin:
Site
Number of hairs
%of1cm2
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 15 - Percentage of hair coverage in various body sites
Table 15 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 14 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 directly 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. density
Chin 500
Total area of hair (cm2)
% light NOT absorbed
87 - 99.9%
87 - 99.9% 98.3 - 99.99% 98.2 - 99.98% 98.5 - 99.99%
Upper lip Arm pits Pubes Legs
500 65 70 60
Thin to
1.13E-07 1.13E-07 1.47E-08 1.59E-08 1.36E-08
thick
1.29E-05 1.29E-05 1.67E-06 1.80E-06 1.54E-06
% of 1cm2 area
0.11 12.9 0.11 12.9 0.01 1.7 0.02 1.8 0.01 1.5
Table 16: % 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).
________________________________________________________________________ 78 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
© The Laser-IPL Guys, 2025