Page 49 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
1. Some light is always reflected from the skin surface (the top surface of the skin which is the stratum corneum). This can vary between 4 and 5%, typically, for ‘normal’ skin. Wet skin may reflect slightly more. This leaves between 95 and 96% of the original light energy to enter the skin.
2. As the light penetrates into the skin, it is absorbed by various constituents such as melanin, blood, keratin, bilirubin, water etc. In particular, all visible wavelengths are absorbed in the basal layer by melanin, to some extent. So, the amount of light energy entering the dermis is always less than the energy fired at the skin surface. By this point there may only be around 50 to 80% of the light energy remaining, to penetrate into the dermis. Darker skin types will absorb a significant amount of energy in the basal layer due to the higher concentration of melanocytes there. In the dermis, the light will be absorbed by blood, water and melanin (particularly around hair follicles) and various other potential chromophores. In the visible part of the spectrum, very little energy will reach the bottom of the dermal layer.
3. Once the photons enter the skin, they encounter atoms and molecules. This will inevitably result in scattering events. This causes the beam to ‘spread’ out, resulting in an increase in the spot diameter. The fluence drops as a direct consequence (since fluence is energy divided by the spot area). Some of this scattering will result in photons ‘turning round’ and leaving the skin altogether – this is known as ‘back-scattering’. PA found, through Monte Carlo simulations, that the amount of light energy lost to back- scattering is very significant – up to 50% in some cases!
Where does the light ‘go’ in the skin? Click here for an animation....
This means that the amount of light energy finally reaching the targets can be much less than
the amount at the skin surface.
The importance of this is that each treatment requires a certain amount of energy to induce the desired response. If the fluence falls below the threshold required, then the reaction cannot occur. This shows that all treatments will stop at some depth, where the fluence has fallen below the threshold (watch this video on what happens here – pdf here).
This depends on the wavelength, energy and spot size. Hence, careful selection of these parameters is essential to ensure good clinical results.
Quick calculator for fluence
There is a quick way to calculate fluence if you know the applied energy and spot diameter. We can use the ‘multiplying factors’ in the table below:
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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