Page 213 - Mike Murphy's Blog Posts

E7 - How can there be more fluence inside the skin than applied at the surface?
Here’s a strange thing....
When we fire a laser or an IPL at the skin surface we usually know the fluence that is applied. So, we figure that a 10 J/cm2 will penetrate into the skin and diminish with depth.
Well, bizarrely, it doesn’t!!
This is entirely due to scattering. So, what is scattering?
The easiest way to think of scattering is to imagine each individual photon of light ‘bouncing’ off evert atom it encounters in the skin. It’s a bit like the Japanese Pachinko game which has steel balls dropping through a maze of metal bars (see video).
The animation shows a similar effect – imagine the blue balls are photons striking a single atom. You can see that they scatter all over the place!
Once they bounce off an atom they will quickly encounter another atom, and another and another. Clearly, they spread out in all directions, including back towards the surface. Many will leave the skin altogether.
However, this is the important bit – when those photons which are heading upwards towards the skin surface ‘pass’ the downward travelling photons, the total fluence is equal to both sets, arithmetically.
As a consequence, the fluence is a combination of the incoming photons and the outgoing photons, coming from all depths. For this reason, the total combined number of photons can easily exceed twice the initial number!!
The Model
My colleague, PA Torstensson, constructed an 8 layer Monte Carlo model to examine this. His model had a layer for the stratum corneum, the epidermis, the basal layer, 4 layers for the dermis (depending on the blood content in each layer) and the sub-cutis. This means that highly accurate calculations may be made to find out where the photons travel to.
He looked at three wavelengths - 532nm, 755nm and 1064nm. His results can be seen in the graph below:
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