Page 145 - Mike Murphy's Blog Posts

B11 - Chasing the 'crack' in laser tattoo treatments! Is this right?
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.
It’s not!
There is absolutely no need for this cracking sound. In fact, it has nothing to do with the treatment at all.
https://youtu.be/3Oa1iHZ4EQg
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
Mike’s Blog Posts 145