I’ve been doing a lot of work recently, looking at how lasers and IPLs differ when considering hair removal.
They’re all different!! In various ways…
Comparing diode lasers with Nd:YAG lasers with alexandrite lasers is like comparing chalk with cheeses. It is quite tricky trying to compare these devices properly.
Add in IPLs and it gets even more confusing!
So, here is my take on this topic…
When we fire light energy at the skin, a number of things happen. Some of it is immediately reflected from the surface. The remainder enters the skin and starts to scatter (bounces around off all those lovely molecules in there).
A fraction is absorbed in the epidermal melanin, raising its temperature, which can lead to pain. A significant amount of the light is back-scattered out of the skin altogether – lost forever!
A tiny portion (less than 5%) is finally absorbed by the melanin in the hair. This energy will raise the temperature of the hair shaft. If enough energy is absorbed then the heat will spread out to the germ cells, on the follicle wall, where, hopefully, it will ‘cook’ those cells into submission.
How ‘efficient’ are the different lasers/IPLs ?
This is a very interesting point. This whole process works by absorption of light energy – the melanin in the hair shaft will absorb some of the light energy. The amount it absorbs depends greatly on the wavelength of the light.
The above graph shows how melanin absorbs light in the visible and near infra-red parts of the spectrum. As we can see, there is much more absorption in the blue end of the spectrum (350 to 450nm). This is to be expected since melanin strongly absorbs ultra-violet and blue light to minimise cellular damage in the dermis.
However, as the wavelengths lengthen, the amount of absorption decreases, significantly.
This means that red light is not nearly as strongly absorbed as blue light!
It also means that if we fire different wavelengths at the skin, different amounts of the light energy will be absorbed, depending on which part of the spectrum they occupy.
Clearly, from the above graph, the alexandrite wavelength (755nm) is more strongly absorbed than the diodes (808 and 810nm), which are more strongly absorbed than the Nd:YAG (1064nm).
What does this mean?
Well, it means that these systems are NOT ‘equivalent’!!
If you fire 20 J/cm^2 of diode light energy at some hair, it will not have the same effect as firing 20 J/cm^2 of Nd:YAG, or alexandrite light energy. You will see different results.
So, we cannot compare these systems based purely on fluence. The same goes for IPL, which has a wide spread of wavelengths. Most devices use a 600nm filter (or thereabouts), which allows only the wavelengths between 600 and 1200nm into the skin.
Using a computer model, I calculated the ‘equivalent’ fluences. These are the fluences which should generate precisely the same outcomes, when fired at hair.
| Device | Fluence (J/cm^2) |
| IPL | 11.9 |
| Diode | 11.2 |
| Nd:YAG | 24.5 |
| Alexandrite | 9.4 |
This is very interesting. It shows that diode lasers and IPLs are quite similar! But the Nd:YAG laser needs a higher fluence, because the melanin absorption is so low at that wavelength. Likewise, the alexandrite laser requires less fluence, because the absorption at that wavelength is higher.
The IPL output is wide, compared with lasers. But, the absorption varies significantly across that range. However, it turns out that it has a similar effect to diode lasers!
There are a few more differences, which I will discuss later…
Hope this helps,
Mike.
PS Don’t forget to have a look at my upcoming Masterclass.
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