Mike Murphy's Blog

I recently wrote a post about tattoo ink absorptions and how they vary across the wavelengths we use today (click here for that post). After that, I thought more about the relative fluences required to induce the desired reaction in ink colours.

I used the graph which I nicked off the internet (see below) – I don’t know where it came from. Any pointers would be gratefully received.

The above curves show that relative absorptions for five ink colours across part of the spectrum. I added the four main laser wavelengths used today – 532, 694, 755 and 1064nm (Nd:YAG, ruby and alex).

To estimate the required minimum fluences, I took the relative absorption for each wavelength and each colour, and calculated the relative ratios.

Assuming the threshold fluence for black ink at 694nm to be 2 J/cm² (as per the Scheibner paper of 1990), I used this as the base point.

Using these data, I calculated the threshold (minimum) fluences that would be required to induce a reaction in those ink colours with those wavelengths (see above). Note that these fluences will be for the most superficial ink particles – higher fluences will be necessary for deeper ink.

The above curves show that black responds well to all the wavelengths at a low fluence (around 2 J/cm²).

Blue and green respond at a lower fluence with the ruby and alex wavelengths compared with the Nd:YAG wavelengths –  as observed in real life.

However, when we look at the red and yellow colours, the minimum fluences required for the ruby, alex and Nd:YAG fundamental (1064nm) jump up significantly. Only the YAG doubled line, 532nm, stays relatively low.

This all agrees with clinical experience – the 532nm wavelength is the natural choice for the ‘sunset’ colours (red, orange, yellow), while ruby and alex lasers are usually suggested for blues and greens.

However, it is not that simple!! I made a video showing how we can be ‘deceived’ by what we see. While we may not be able to discriminate between various ink colours in the skin, the laser wavelengths will react to the different colours according to their absorption profiles.

Never trust your eyes!!

Another issue is ‘anisotropy’ – this describes how scattering affects the laser light when it enters the skin (watch this video). It determines how deep the light can penetrate and how much is lost to back-scattering. Changing the wavelength is not trivial!!

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Conclusion

The fact is, all wavelengths we use today for tattoo removal can treat all ink colours – but with a huge variation. Therefore, to improve results we must ‘lower’ the threshold fluences by changing wavelengths for certain colours (which is why we choose different lasers for different colours!).

However, as this analysis shows, it is patently not true that you can “only” treat some colours with a particular wavelength – it’s just ‘easier’ if we do! Or, in other words, those lasers require lower fluences than others.

If higher fluences are applied to all colours, they will react – but those fluences might be damaging to the surrounding tissues. Some other constituents of the skin will inevitably absorb some of the energy from these wavelengths too – particularly melanin and blood with the 532nm line, and melanin with both the ruby and alex lasers. That is the problem here…

So we need to aim for a balance – a sufficiently high fluence to damage the ink particles without excessive damage to the surrounding tissues.

Or…..   a wholly new technique which does not involve light at all!!

Hope this helps,

Mike.

Come along to our MasterClass in Luton, London in September…