Page 32 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
● In figure 13a we see a parade of soldiers marching is step, at the same speed in the same direction. These soldiers are highly coherent. In figure 13b we can see a crowd of people who appear to be moving in different directions, probably at different speeds with a plethora of different goals. Those people are highly incoherent.
● So, you can clearly see that laser light is ‘special’ in that it has (at least) three unique properties which differentiates it from ordinary light. As mentioned above, you cannot find a natural source of laser light in the universe – so far.
 What happens to laser light when it enters the skin?
As soon as any light, laser included, enters the skin the photons are subjected to many, many scattering events (click here for a video on scattering). This has significant effects on the laser properties:
• Scattering causes the photons to fly out in all directions – hence, the beam diverges in many directions. The laser’s initial, very low divergence is lost;
• In addition, some of the photons lose some coherence. The once ‘regimented’ order of the laser’s photons disappears after a relatively low number of scattering events.
Another important effect is the change in refractive index between the air and the skin. This change forces the wavelength of the laser beam to change, to a lower value.
Hence, the incident laser changes wavelength and loses both its original coherence and low divergence properties.
Such a beam cannot be considered a laser!
Here’s a blog post on what it might be like to ‘sit’ on a photon while it is entering the skin.
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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