What is the relationship between fluence at the skin surface and depth into the skin?
This is very important.
When we fire laser/IPL light energy into the skin, it encounters many, many, many atoms and molecules, which determines how that light will progress and where it will end up. As soon as it hits the skin, the light will begin to scatter – this has two major implications:
- The light beam will essentially ‘spread out’ as it penetrates deeper into the skin. This means that the spot size will increase with depth, meaning that the fluence will inevitably drop as it gets deeper into the dermis.
- Much of the light energy will ‘back-scatter’. It will turn around and heads back towards the skin surface. Now this is very interesting as it means that this back-scattered light energy will ‘meet’ the incoming energy. Imagine a sports stadium emptying of people, all looking for their cars. Half of them suddenly realise that their cars are on the other side of the stadium and so they turn back to walk around the stadium. Now we have people leaving the stadium ‘meeting’ those people trying to get past them. There is now a huge ‘throng’ of people just outside the stadium doors – much more than left the stadium originally.
This is what happens to photons in the skin. There is a region in the upper dermis, down to a depth of around 1.5mm, where there are more photons than originally entered the skin – due to all those photons which have turned around.
Fluence in the skin for three wavelengths – 532, 755 and 1064nm.
This situation can easily double the fluence in the top part of the skin. So, if you fired 10 J/cm2 at the surface, there can easily be 20 J/cm2 just under the surface. This value will decrease until about 1.5mm (for ‘red’ light) where it will be almost exactly the same as the incident fluence at the surface. For the green 532nm wavelength, this depth is only around 0.35mm.
As the light penetrates deeper, the fluence will drop below the original value.
A wee side note – Monta Carlo calculations of these processes show that we can lose up to 60% of all the red light energy through back-scattering out of the skin!
What does this all mean in hair, blood vessel or tattoo removal?
Well, it means that are actually hitting the targets with more fluence than we think! But that’s not a bad thing – we’ve been doing it for decades.
But the most important issue here is that we must understand that the reaction we are trying to induce in the hair, blood or tattoo ink, is very dependent on the fluence which hits it.
Let’s look at hair specifically.
Computer model calculations show that we must hit the hair follicles with a particular fluence to ensure total destruction of the follicle. I built a model which looks at this process for all the wavelengths we use today. The results are very interesting.
Quite simply, if we hit the follicles with the correct fluence, they cannot re-grow. If the fluence is a little too low, they will usually survive and grow back – normally finer and lighter in colour.
If the fluence is too low, they will not be affected at all. I wrote about this recently in my ‘zones’ post (read here).
Depth
Knowing that the fluence changes in the skin, significantly, we need to figure out the optimum fluence at the skin surface to ensure that the ‘correct’ fluence reaches the targets, at all depths.
So, I calculated the fluences for a range of depths for the diode, Alexandrite and Nd:YAG lasers and for IPLs.
To ensure total follicular destruction (100% irreversible denaturation) at a depth of 2mm we must apply the following fluences at the skin surface:
| Device | Fluence (J/cm2) |
| Alexandrite | 31 |
| Diode | 39 |
| Nd:YAG | 89 |
| IPL | 41 |
The wide range of values here are due to the large differences in the absorption coefficients at these wavelengths. The 755nm wavelength of the Alexandrite laser is much more strongly absorbed than the 1064nm of the Nd:YAG, for example (approximately three times stronger, which is why we need three times the fluence with the 1064nm compared with the 755nm wavelength).
Now these fluences will only be 100% effective to a depth of 2mm. Below that and the follicles will not be irreversibly cooked. This explains why so many treatments have regrowing, finer hairs after a couple of sessions. Those follicles were too deep for the applied fluence to kill them effectively.
More recent research indicates that we probably need to kill both the germ cells in the bulge and the bulb to generate good results. The bulb descends from around 1.5mm at the beginning of the anagen phase, to anywhere between 3 and 5mm deep in the later anagen stage. If we don’t catch the bulb when it is near the skin surface, we will likely not achieve good results!
Conclusion
What this all means is that we need higher fluences to effectively target deeper follicles. But there comes a point where our fluence is just insufficient to reach the deeper follicles – the technologies just cannot generate sufficiently high fluences in a decent spot size. We will not be able to target those until they go into the next anagen phase, and the bulb is near the surface again.
Of course, higher fluences always means more pre- and post-skin cooling – with ice!! Especially on darker skin tones – they must have more pre-cooling to protect the epidermis and upper dermis.
So, if you are firing your diode laser at the skin surface with only 10 or 12 or 14 J/cm2, you will never obtain good results. The fluence is just too low. Likewise with the other devices.
Or, if you are using an Alexandrite laser at 18 J/cm2, you will only be able to effectively target the more superficial hair follicle down to a depth of around 1.2mm. Anything deeper than that will not be properly cooked. Those hairs will grow back finer and lighter in colour.
So, it should be clear now that fluence and depth are inextricably linked! It appears that many laser/IPL operators are using low fluences routinely, and seeing poor results routinely too. This is not surprising given the above!
Finally, we must choose the fluence according to the depth of the target follicles (or blood vessels or tattoos);
and choose the pre-cooling times according to the skin tone on the day of treatment.
Incidentally, the light does NOT ‘travel down the hair shaft’ – that’s just an urban myth! That would ‘break the laws of physics, captain…’
Hope this helps,
Mike.
PS Our next MasterClass will be in Liverpool on June 2nd and 3rd. If you wish to attend please contact us on DermaLaserMasterClass@gmail.com.
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