Page 102 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Penetration Depth and Spot Size
Why does spot size affect penetration depth?
This is a little tricky because it’s not intuitive. We know, from both clinical tests and computer modelling, that larger spot sizes result in more energy/fluence reaching deeper into the dermis. But how does that happen?
Well, it all comes down to scattering. The dermis is mostly composed of collagen fibres. These fibres are all scattering centres for the wavelengths we typically use in many laser-skin treatments (400 to 1200nm). As a consequence, when light enters the skin, the photons are usually scattered many, many times before being absorbed by something, or emitted back out of the skin altogether (known as ‘back-scattering’).
When a photon is scattered, it is essentially absorbed by an atom, and then re-emitted as a new photon, usually in a new direction. That direction is determined by something called the anisotropy. Longer, red/infrared, wavelengths tend to be ‘forward scattered’ - roughly in the same direction as the original photon direction. Shorter wavelength, blue/green, photons may be directed into a much wider angle.
It is clear that the anisotropy has an effect on the penetration depth of the photons - blue/green photons generally don’t go as deeply as red/infrared photons.
So that’s how wavelength affects penetration depth. But what has that to do with the spot size?
Imagine a ‘column’ of photons fired into the skin, in a small diameter beam. As soon as this beam hits the skin, the photons begin to scatter. Many of the photons will move outside of the original beam. This means that the beam diameter will increase in size as it penetrates further into the dermis. The fluence is the total energy divided by the beam diameter, so, the fluence will decrease as the diameter increases. In other words, the fluence decreases in value as the beam penetrates further into the dermis.
________________________________________________________________________ 102 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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Hence, we have defined a more practical application – the ‘Useful Penetration Depth’. This description considers the wavelength (absorption coefficients), incident fluence, spot size and pulse duration of the incoming light energy (see video).
Figure 52 – Useful Penetration Depth