Mike Murphy's Blog

When light enters the skin, it immediatley begins to scatter. This is the process where photons are absorbed by atoms (making electrons ‘excited’), and new photons are emitted shortly after (when those electrons ‘de-excite’). Inevitably, they leave the atom in a different direction to their original direction. This is called ‘scattering’.

Consequently, the light spreads out within the skin (some of it back-scatters – it turns around and leaves the skin altogether).

But, the direction that the scattered photons follow is determined by something called the ‘anisotropy’. The anisotropy depends greatly on the wavelength of the incoming light.

So…. where does the light go after being scattered (I hear you ask!).

In a wee experiment, I fired three pointer lasers with blue, green and red light into my skin, to see how far the light spreads out (I had to do this in the dark so we could see properly…).

As we can clearly see, the blue light is hardly visible indicating very little lateral spread. The green light spreads further, but not too far. Meanwhile, the red light spreads significantly far from the beam entry point.

This is all very interesting but is it important?

Yes!! This indicates that we must be careful when treating the skin. It is clear that red light can travel quite far from the entry point at the skin surface, mostly within the dermis. This is entriely due to the scattering in there.

What this means, is that when treating issues such as hair or pigmentation, we must be aware that we could affect adjacent chromphores such as tattoo ink – paticularly with red or infrared wavelengths such as the light from diodes, alexandrites, Nd:YAGs and IPLs.

Putting white tape or white pencil on top of these things will not make much difference! We should not be targeting tattoos with any long-pulsed laser or IPL. But the real problem for them comes from the light scattering around inside the skin – not from above the skin surface!

White tape/pencil will make very little difference tbh. What we must do is keep a reasonable distance from potential problem sites. What is that distance? I don’t really know as yet, but I’d suggest keeping at least 8 cm (around 3 inches in old school money!) just to be sure of a minimal unwanted interaction with tattoos, or similar. This also applies to treating hairs on the forehead – stay clear of any PMU brows or you may cause damage.

And don’t rely on white tape or pencil – scattered light just ignores those…

Hope this helps,

Mike.