Page 53 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
But how much of this energy is actually absorbed by the hair? Well, going from the calculations in Table 10, it appears very little is absorbed, since the targets areas are so small!! (To be more precise, we’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 we suspect the actual numbers would be very similar to the above, much simpler, calculation.
So, let’s look at the maximum amount of light energy that may be absorbed in the hairs. From Table 9, 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 PA, reveal that much of this light energy is either reflected or back-scattered out of the skin altogether. This can be more than 60% of the incident energy, depending on the wavelength.
Of the remaining 40%, a portion is absorbed in the hair melanin (depending on the Skin Colour). Most of the rest of it will be absorbed in the dermis where it will be converted into heat energy. This is not good! This will cause the temperatures in the epidermis and dermis to rise, potentially damaging those tissues, and may lead to scarring.
Skin Cooling
When we target hair follicles, we are deliberately trying to overheat them with light energy. However, less than 10% of the applied light energy actually generates heat in the follicles – the remainder is sloshing around the dermis, looking to “cook” something!
So, we MUST mitigate against this excess heat, otherwise the likelihood of unwanted tissue damage is high.
To do this effectively, we much cool the skin. In the section ‘Optimal Skin Cooling in Laser Hair Removal’, we discussed the various techniques employed to do this. In this section we’ll discuss the basics behind these ideas.
Air is a very poor conductor of heat. That’s why we wear layers of clothing on cold days – the layers essentially traps air, which reduces the flow of heat from our bodies. If those clothes are wet, then we will lose heat much faster.
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