Page 200 - Mike Murphy's Blog Posts

This process is known as ‘transmission’ since the photon is lost to the dermis (see figure 1 – ‘Transmission’).
However, it may well be scattered back out of the fatty layer and be absorbed by something in the dermis.
Finally, a photon may be scattered numerous times until it finally leaves the skin altogether – this is known as back-scattering (see figure 1 – ‘Back-scattering’) and can account for up to 60% of all the light energy entering the skin (depending on the wavelength).
Fluence – how does it change in the skin?
Fluence is essentially the total number of photons fired into the skin, within a certain spot diameter. In figure 2, we can see a bunch of photons entering the skin.
Figure 2 – Fluence drop in the skin
We can see that they spread out as they penetrate further into the skin. This reduces the fluence, simply because the spot diameter is increasing with depth. But, not all the photons will reach the deeper parts of the dermis – many will be absorbed in the upper regions. This, therefore, leaves less photons (energy) available for the deeper parts.
For these two reasons the fluence drops rapidly with depth. In fact, it drops exponentially! This one fact explains why it is ‘difficult’ to effectively treat very deep targets in the skin – there are simply not enough photons reaching there!!
Mike’s Blog Posts 200