I often hear people talking about listening for the ‘crack’ sound during laser tattoo removal treatments. Apparently some people have been trained to do this and they believe it’s an essential part of the process.
There is absolutely no need for this cracking sound. In fact, it has nothing to do with the treatment at all.
I took a video using a QS Nd:YAG laser at 1064nm on my own, non-tattooed, skin. As you can see in the video, I start with the handpiece tip on my skin – the correct position. The spot diameter is 5mm, the fluence is around 2.5 J/cm2. There is virtually no audible sound when I fire the laser – as it should be.
However, as I pull the laser tip from my skin, the spot size reduces, due to the focussing lens inside the handpiece. At around 10cm from my skin the spot diameter, on my skin surface, is around 1mm. This will generate a fluence around 25 times what it was above! For a Q-switched laser, this can generate a power density (irradiance) of around 1 GW/cm2 – that is 1,000,000,000 Watts/cm2!!! For picosecond lasers it could be ten times higher!!
The ‘cracking’ sound is now clearly heard – even though there is no ink to absorb the energy! At such fluences there is enough concentrated energy (and power density) to interact with the oxygen in the air. If enough energy is output, ozone can be generated by the laser energy.
This also generates a loud cracking sound due to the extreme and rapid pressure changes in the air.
However, this ‘crack’ has nothing to do with the tattoo treatment. In fact, it is more likely to damage the skin since excessive fluences are being used.
The second video shows how the spot size changes as I lift the handpiece tip from some photo paper. It’s clear that the spot begins at its proper diameter, but then reduces rapidly to around 1mm when the tip is about 10cm from the paper. As I increase the distance from the paper, the spot diameter starts to spread out again.
The point where the spot diameters is 1mm is known as the ‘focal point’ – the point where the spot diameter is a minimum. At any distance from this minimum, the spot size will be larger (and so the fluence will be lower).
This video shows why we should always use the handpiece tips properly according to the designed spacers. These are set by the design engineers to ensure the correct spot size, and fluence, is applied to the skin surface.
When I turn the photo paper over and expose the laser to the white side, the cracking sound still occurs when the handpiece tip is about 10cm from the surface.
So, even though the white paper is reflecting most of the laser energy, the crack sound is still generated.
But that’s because the crack has nothing to do with the surface!!!
So, don’t “chase the crack” – it will not help the treatment and could end up creating unwanted damage due to excessive fluencies!
ADDITIONAL – ADDED 24 JULY 2021
To ‘enhance’ the above argument I made a couple of videos using a QS Nd:YAG laser on my wife’s arm. She did agree, but only after much enticement and some bribes…
Ciao for now,
PS No spotty table cloths were injured in these videos…