Page 83 - An Introduction to Laser Tattoo Removal

Chapter 2 – Fundamentals of Laser Tattoo Removal v1.0
Wavelength Maximum back-scattered (nm) component %
532 29.1
755 59.4
1064 55.5
In treatment terms, this means that a significant amount of the fluence which is directed into the skin is lost. Clearly, the percentages lost are not always those shown in the table above – it depends on how deep the ink is, for one thing. But, the above table indicates the maximum possible loss due to back-scattering. As the number of treatments increases, and the depth of the target ink also increases, the above numbers become more accurate. Mike published an article on this phenomenon as it relates to laser tattoo removal.
This may explain why some of the deeper tattoos are so difficult to completely remove.
Pulsewidth
The pulsewidth is the length of time the pulse exists for. So, for most Q-switched lasers, this is typically between around 5 and 40 nanoseconds (1 nanosecond = 1 billionth of a second = 10-9 s).
When light energy is delivered in such short pulses, there is insufficient time for any heat generated to conduct away from the absorbing site (the ink particle). Consequently, virtually all of the energy is ‘retained’ by the particle which results in very high temperatures. It is believed that such high temperatures are necessary to induce the desired reaction.
It is the fact that the energy is delivered so quickly which allows the high temperatures to be generated.
This led to the development of the picosecond lasers for tattoo removal. It was reasoned that even shorter pulses would generate even higher peak temperatures, which should result in a more efficient reaction in the ink particles.
This appears to be true, but not for the reasons originally thought. It turns out that the process of laser tattoo removal is much more complicated than first thought...
A study from a South Korean research group found that picosecond laser energy was ‘better’ at altering ink colours than nanosecond lasers, in a small range of colours. Mike summarised this paper in a blog post.
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Chapter 2 LEVEL A Laser Tattoo Removal
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