Page 85 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
 Figure 46: Fluence versus depth in skin. Click here for an animation showing the above effects.
The only way to get around this issue is to increase the fluence at the skin surface (see diagram below).
BUT.... If you do this, then you are firing more energy into the skin. This may cause more unwanted heating in the surrounding tissues such as epidermis. So, you must also increase the level of cooling to compensate for the increased fluence.
Alternatively, you may reduce the spot diameters to increase the fluence at the skin surface But, this has a negative effect in that it reduces the penetration depth too! In some situations, the only way forward is by increasing the surface fluence – but always balance this by increasing the cooling too!
  Back-scattering is the name given to the process when a photon is scattered back out of the skin.
● When you fire a laser/IPL beam into skin, the photons will scatter off molecules and atoms, and change direction. If the total change of direction tends towards 180° then it will be pointed back towards the skin surface.
● Some of those photons will eventually leave the skin entirely. This affects the potential results of all light-based treatments.
● Any light lost to back-scattering cannot be used in the treatment process!
● So, back-scattering is a very important consideration when treating the skin.
● PA’s Monte Carlo simulations (these are used to calculate where light energy ‘ends up’ where fired into some medium, like skin) showed that the amount of back-scattering depends heavily on the wavelength. In some cases, as much as 50% of the initial energy may be lost to back-scattering!
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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