Page 114 - Mike Murphy's Blog Posts

B1 - Laser tattoo treatments - how do we approach different ink colours??? Part 1.
What is happening with laser tattoo removal treatments?
As we all know, the laser energy is absorbed by the tattoo inks, and they ‘break up’. Well, something like that...
But what does ‘absorbed’ actually mean? And how much of the light energy is absorbed by each ink colour?
To answer these questions, we need to understand the concept of ‘absorption coefficient’. This, essentially, represents how strongly or poorly an ink colour absorbs the incoming light energy.
It’s all to do with quantum physics, so I won’t go into detail here. But, in essence, the absorption coefficient is a probability. It is the likelihood that an atom will absorb (or ‘keep’) the energy carried by an individual photon of light, when they interact.
That probability depends greatly on the wavelength (more accurately, the frequency!) of that photon. So, an atom in some tattoo ink, might have a very high probability of absorbing the energy from a 1064nm photon (Nd:YAG); but the same atom might not absorb a 532nm photon (frequency-doubled Nd:YAG) very well, due to a very low probability.
In other words, the absorption coefficient of that photon at 1064nm is high, while it is low at the 532nm wavelength. In reality, the absorption coefficient of any atom changes with wavelength from virtually zero to almost 100%, depending on the atom’s quantum states etc, etc, etc...
So, how does this translate to laser tattoo removal?
Well, the reality is that all tattoo ink colours will absorb all the laser wavelengths used
today (Nd:YAG 1064 and 532nm, ruby 694nm and alexandrite 755nm) – to some extent.
The important point is that the absorption coefficients range enormously across the ink colours and wavelengths.
So, let’s look at some examples...
Black inks tend to absorb most wavelengths very strongly. Let’s say that a particular black ink absorbs the 1064nm wavelength with a probability of around 90%. This means that laser energy, at 1064nm, will be very strongly absorbed by that ink. Say, for this discussion, that this ink can be fully removed, in a tattoo, after five sessions.
Now, say that same tattoo has some yellow ink in it, and this ink has an absorption probability of around only 10% (at 1064nm). This means, that using the same incident fluence, the yellow ink will absorb around 0.111 (1/9) times the energy compared with the black ink.
Mike’s Blog Posts 114