Page 83 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
● This change in direction is known as “scattering”. Click here for a video of this process.
Figure 44: Scattering, backscattering and absorption in the skin
● All visible light will undergo many scattering events in skin. As you sit on your imaginary photon you will scatter around inside the dermis (most of the time) changing direction again and again. Watch an animation here.
● Eventually you will meet a chromophore particle (remember chromophores? They are the things in tissue which absorb photons).
  Blue light entering the skin
Fresnel reflections
                                    Epidermis Basal layer Dermis
Blue photons
Fresnel reflected photons Back-scattered photons
Absorbing melanocytes in basal layer Scattered photons
Transmitted photons
Main Absorbing Chromophores:
Haemoglobin (oxy & deoxy) Water
                                                  Blood vessels Subcutis fat
Not to scale
The curious thing is that a photon may be scattered many millions of times before it is absorbed, transmitted or back-scattered out of the skin. Or it may undergo only one scattering event before it is absorbed! It is a completely random event whether it is absorbed or scattered again.
A photon generated at the heart of the Sun will undergo many, many scattering events before finally reaching the surface and escaping. This process could take anywhere up to 20,000 years!!!
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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