Page 75 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
Fresnel Reflections
● When light encounters a surface (skin, water, glass etc) it will usually undergo a change in ‘refractive index’. This induces a change in the light’s properties, particularly its wavelength and ‘polarization’.
● It’s not important to understand what polarization is (at this stage), but it is important to know what effect this has in treatments.
● Essentially, any change in the refractive index means that only a portion of the light will enter the medium. As a consequence of Fresnel Reflections, somewhere between 4 and 5% of any light falling on the sin surface will be reflected back out.
● This occurs when the light beam falls on the skin at right angles.
● If the beam is held at any other angle, the amount of light energy lost to this reflection increases.
  Always aim the light source at the skin at right angles to maximise the energy going into the skin/tissue!!
If it’s not a ‘right angle’, it’s a ‘wrong angle’!
● After undergoing countless scattering events in the skin, it may ‘escape’ from the dermis and enter a deeper layer.
● This is known as ‘transmission’.
● In most scenarios, the amount of transmitted light energy is fairly low – simply because it will have been absorbed by something in the dermis, or back-scattered out of the dermis altogether.
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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