Where do photons go in the skin?

When photons (light) enter the skin they can undergo only two processes – absorption or scattering.

Once a photon is absorbed, its energy is ‘gained’ by the absorbing site. This may be a melanosome in the epidermal basal layer (see figure 1 – ‘Epidermal absorption’). This absorption will transfer the photon’s energy into the melanin and raise its temperature (by a very small amount!)

Figure 1 – Absorption and scattering in the skin

More likely, the photons will scatter throughout the dermis as they ‘bounce’ off many atoms (this can be anywhere between 1 and several million atoms!!). This is because the likelihood of being absorbed in relatively low in most parts of the dermis, until the photons encounter something which ‘wants’ to absorb them strongly – like blood or melanin or another chromophore.

At some point a photon may be absorbed in the dermis (see figure 1 – ‘Dermal absorption’).

If it not absorbed by an atom in the dermis, it may continue its ‘random walk’ through the dermis until it reaches the deeper, fatty layer – where it may be absorbed by an atom there. 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!!


What about back-scattering? What does this do to the fluence?

Well, this is a very important issue. Computer models over the last 40 years (from many locations across the planet) have consistently shown that back-scattering photons have an important effect on the fluence in the skin.

As photons scatter in the dermis, many of them are turned through 180º and start to make their way back towards the skin surface. Of course, this means that they may be absorbed on their way out. It doesn’t matter which direction the photons are coming from – absorptions will occur regardless (see figure 3).

Figure 3 – Fluence and back-scattering (the fluence increases down to a depth of around 1.4 mm, not ‘1.25mm’ as shown in this figure!)

In the figure above, the ‘blue’ photons are those heading back towards the surface – the back-scattered photons (I used blue here to show their change in direction; not the change in wavelength!!). It becomes very obvious that there are many more photons in the region just below the skin surface because of the presence of the back-scattered photons. These will encounter ‘new’ photons which are still entering the skin.

Many of these photons will be absorbed by the epidermal melanin, resulting in high temperatures there. Given that the thermal pain receptors are just below the basal layer, this explains why so much pain may be felt during photothermal treatments, unless proper surface cooling is applied during the treatment.

Monte-Carlo simulations reveal that this increase in fluence can extend as far as 1.4mm into the skin. This means that the fluence applied at the skin surface will increase down to a depth of 1.4mm, then will drop rapidly due to absorption and scattering.

Figure 4 – Change in fluence with depth for three wavelengths


The importance of back-scattering has been overlooked in laser/IPL skin treatments. Yet, it is clear from the above, that is it very important since it can have a significant effect on the fluences in the skin.

Understanding these processes will help to make proper choices about fluence settings, and, therefore, lead to better clinical results.

All of this, and more, will be discussed in our MasterClass in Birmingham in September 2022.

Ciao for now,


For more on our upcoming MasterClass please visit https://www.dermalasetraining.com/news-and-tips

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