How can there be more fluence inside the skin than is 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 every 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 below 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!!

And… much of this fluence may be absorbed by a chromophore, at that depth. This means that superifical chromophores are much more likley to be heated than deeper ones.

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:

His model found that the back-scattering element (the photons heading back towards the skin surface) is hugely significant.

The graph above shows a red dotted line at the ‘100’ level. This represents the initial fluence fired into the skin. The three curves show, very clearly, that the fluence increases significantly just below the surface, down to the bottom of the basal layer. The amount of increase depends on the wavelength (which is not surprising).

What does this mean for treatments?

The maximum fluences are around 2.3, 2.0 and 1.5 times the initial fluences for the three wavelengths 755, 1064 and 532nm, respectively.

The red wavelengths (755 and 1064nm) remain above the initial fluence until a depth of around 1.5mm, whereas the 532nm drops below this level at a depth of around 0.4mm.

This has important ramifications of treatments, since most photothermal treatments are heavily dependent on the fluence.

What it actually means is, that if an initial fluence of, say, 10 J/cm2, is applied to the skin surface, then it will reach a maximum value of 23 and 20 J/cm2 for 755nm and 1064nm, respectively, at the top of the dermis.

These fluences will drop back to the original value at the depth of 1.5mm.


This model data is quite counter-intuitive. But that’s simply because we don’t tend to think of photons ‘turning around’ inside the skin and moving back towards the surface!! Once you understand that, then this data becomes much more sensible.

I am using this to calculate the optimum fluences needed for the removal of hair follicles and blood vessels.

I’ll keep you posted…

Ciao for now,


Dont forget about our MasterClass in Birmingham in September

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