Page 115 - Mike Murphy's Blog Posts

If the black ink fades after five sessions, then, assuming everything else to be similar, the yellow ink will require about 45 sessions, if the same fluence is applied.
How do we get round this?
What this suggests is that we should be using higher fluences on those inks with lower absorption coefficients. Many laser operators automatically think that we should change wavelength.
What they are doing is precisely the same thing - choosing a wavelength with a higher absorption coefficient for that ink colour. Although, I suspect, most people don’t realise that’s what they’re doing!!
By changing to a different wavelength, you may be increasing the amount of absorption of the laser energy by the targeted ink colour. However, you might not!
There are, in fact, two choices here:
Route 1:
Stick with your original wavelength and use a higher fluence (see video). Most of us fall into the ‘trap’ of assuming that the fluence that works on black, or very dark, inks should also work on lighter ink colours too.
As we now know, that is simply not true. Lighter colours will inevitably have a lower absorption coefficient and so will require a higher fluence to achieve the same effect as with the black ink.
But the fact is that black ink is the easiest ink to excite with laser light. All the other colours are not so easy. They all require a higher fluence than black ink!
Route 2:
This is the route most people choose – change to another wavelength (if available!). As I pointed out above, this assumes that the chosen wavelength is more strongly absorbed by the target colour.
But, this may, or may not, be true. We can’t know until we try.
It may well be that some ink colour, say yellow, interacts strongly with the Nd:YAG 532nm wavelength at fluences above 5 J/cm2, whereas it only requires around 2.5 J/cm2 at the 694 nm wavelength (ruby)...
And, of course, not all yellow inks are the same!! Some may not react to the 532nm wavelength until the fluence reaches 15 J/cm2, or more!
Mike’s Blog Posts 115