Page 96 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Figure 46 – Absorption and scattering in the skin – the red arrows represent individual photons
This is because the likelihood of being absorbed in relatively low in most parts of the dermis, until the photons encounter something which ‘wants’ to absorb them strongly – like blood or melanin or another chromophore. (Note: all absorption/scattering events are probabilistic – the chances of the photon being absorbed or scattered depends entirely on the optical ‘relation’ between the photon’s wavelength and the absorption/scattering coefficients of the atoms it encounters in its travels).
At some point a photon may be absorbed in the dermis (see Figure 46 – ‘Dermal absorption’).
If it not absorbed by an atom in the dermis, it may continue its ‘random walk’ through the dermis until it reaches the deeper, fatty layer – where it may be absorbed by an atom there. This process is known as ‘transmission’ since the photon is lost to the dermis (see Figure 46 – ‘Transmission’).
However, it may well be scattered back out of the fatty layer and be absorbed by something in the dermis.
Finally, a photon may be scattered numerous times until it finally leaves the skin altogether – this is known as back-scattering (see Figure 46 – ‘Back-scattering’) and can account for up to 60% of all the light energy entering the skin (depending on the wavelength).
Fluence – how does it change in the skin? Fluence is essentially the total number of photons fired into the skin, within a certain spot diameter. In Figure 47, we can see a bunch of photons entering the skin.
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