Page 104 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Now imagine a ten-lane motorway. Only those cars near the edges can easily leave - the central cars are much less likely to leave the motorway. They may move between lanes (analogous to scattering), but leaving the motorway completely is much more difficult for most of the cars.
The upshot of this is that larger spot diameter beams, with the same fluence as a smaller diameter beam, will deliver more photons deeper into the skin, than the small spots. Or, in other words, the fluence penetrates further!
Summary
The way light travels in the skin is mostly determined by the scattering. The wavelength and anisotropy determine the way light scatters. Larger spot diameters have more photons in them than smaller diameter beams, with the same fluence (concentration).
In large diameter beams, much of this scattering will merely put the photons into other locations within the beam. Proportionally, smaller beams will lose more photons from the original beam, compared with larger diameter beams. So larger diameter beams will inevitably deliver more photons deeper into the skin than smaller beams.
This section has looked at the effects of scattering on fluence as the light travels deeper in to the skin. It did not discuss what occurs if that fluence encounters any absorbers, such as melanin or blood or anything else that might ‘steal’ some energy.
If the light energy encounters an absorber, then it might be absorbed (it might not – it’s a probabilistic effect – the higher the absorption coefficient, the greater the probability of the light being absorbed). Once that light is absorbed, it cannot travel any further – obviously!!
Consequently, the fluence drops simply because it has been absorbed and scattered into a large area.
So, the progression of the fluence through the skin is determined by both the scattering and absorption properties of the skin, for that wavelength(s). This includes potential “barriers’ such as the basal layer melanin in the epidermis. For the wavelengths used in laser/IPL hair removal, the basal layer melanin presents a real challenge since it will absorb some of the fluence, consequently, reducing the fluence available for the hair follicles beneath.
Penetration Depth and Fluence
Light penetrates the skin and is absorbed by some constituents and scattered by others. Typically, for visible wavelengths, many scattering events for each absorption event.
________________________________________________________________________ 104 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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