Mike Murphy's Blog

Answer – yes!! Why?

OK, so we need to look at where the light goes in the skin to answer that question…

Someone asked me, recently, the above question relating to the treatment of hair. She said that she had been trained to apply three shots of 10 J/cm² to the area, rather than one shot of 30 J/cm², and that it was the “same thing”.

Well, it’s not.

The fact is that light can only penetrate so far into the skin. As it does, it loses energy. So, the energy/fluence near the surface is much higher than it is deeper in the dermis. A lot of this light is lost due to ‘back-scattering’ when the photons turn around and leave the skin altogether (I’ll write a post about that soon).

So, at a depth of 3mm, the energy/fluence is only a fraction of what it was at the surface. It might easily be too low to induce the reaction we are after in the hair follicles.

For example, if the fluence at the surface was 10 J/cm2, then at a depth of 3mm it might only be 2 J/cm².

But, if the fluence at the surface was 30 J/cm², then at 3mm the fluence may be around 6 J/cm² – which may be sufficient to induce the desired response.

Penetration depth vs fluence

So, if you fire a fluence of 10 J/cm² three times at the same area, you will not increase the penetration of that energy to deeper regions – you will merely ‘cook’ the same tissue three times – which might be damaging!

But, if you fire 30 J/cm² just once, you may induce the correct amount of damage, as required for a good result.

This also explains why some treatments don’t work very well, particularly with hair removal. If you don’t fire a sufficient level of fluence at the hair, you may not damage the deeper follicles sufficiently. Those follicles will regenerate grow back finer hairs at a later time.

So, low fluences are never a good idea – they inevitably result in poor outcomes!

Hope this helps,

Mike.

BTW, out next MasterClass will be in Glasgow in February. We already have two people coming from America for this one (so don’t say “it’s too far…’)!! Contact lisa.dermalase@gmail.com for more details.