Page 66 - Mike Murphy's Blog Posts

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 2: Percentage of hair coverage in various body sites
Table 2 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 1 is completely wrong, these are the facts!
What does this mean for light-based 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 my wee video on this topic). We choose certain wavelengths (yet another wee video) that we know will be absorbed preferentially by the chromophores in the targets (melanin in hair, haemoglobin in blood etc).
But how much of this energy is actually absorbed by the hair? Well, going from the calculations in Table 2, it appears very little is absorbed, since the targets areas are so small!! (To be more precise, I’d have to do a volume calculation. This would involve calculating the energy distribution in the skin, which depends on the absorption and scattering coefficients of each constituent. This approach would give a more accurate set of answers, but I suspect the actual numbers would be very similar to the above, much simpler, calculation. Plus, I can’t be bothered doing that particular calculation at the moment...)
So, let’s look at the maximum amount of light energy that may be absorbed in the hairs. From Table 2, it appears that the hair in the chin and upper lip will absorb around 13% of the incident light energy. But in the armpits, pubic region and legs this drops to less than 2%.
This is very surprising!!
What this tells us is that the vast majority of the light energy is not doing the job we are attempting. In fact, Monte Carlo calculations by my Swedish colleague, PA Torstensson, reveal that much of this light energy is either reflected or back-scattered out of the skin altogether. This can be more than 50% of the incident energy, depending on the wavelength.
Mike’s Blog Posts 66