Page 140 - Blog Posts v1

Fluence threshold vs pulsewidth
So, using the same calculation method as in Part 1, but with a ‘base point’ of now only 1 J/cm2, I have recalculated the minimum fluences required to induce a response, for different pulsewidths:
Wavelength
532nm
694 nm
755 nm
1064 nm
Ink colour
Black
1.2
1.0
1.0
1.0
Blue
2
1.2
1.2
1.6
Green
2
1.2
1.2
1.8
Red
1.2
2.8
3.1
2.6
Yellow
1.6
4.1
4.1
4.2
Minimum fluences required with pulsewidths less than around 10 ns
This clearly indicates that shorter pulses need less fluence. My original calculations were based on a study from 1990 when a ruby laser was used to determine the threshold fluence in black ink. But this system had a pulsewidth of around 40 nanoseconds – quite long by today’s standards.
Many of the Q-switched lasers we use today are in the range 5 to 10 ns, with the alexandrite and ruby lasers utilising longer pulses.
The tables show the difference between the pulsewidths – these are approximations since it is not possible to calculate these for all pulsewidths (insufficient data!!)
Wavelength
532nm
694 nm
755 nm
1064 nm
Ink colour
Black
2.2
2.0
2.0
1.9
Blue
3.8
2.2
2.3
3.1
Green
3.8
2.2
2.3
3.4
Red
2.2
5.3
5.9
4.9
Yellow
3.0
7.8
7.8
8.1
Minimum fluences required with pulsewidths more than around 10 ns
Picosecond lasers may use even lower fluences to generate the desired reaction (in superficial ink)!
The advantage of using lower fluences is that there is less chance of collateral damage to the surrounding tissues. Every laser tattoo removal procedure results in some micro- scarring, mostly due to the physical tears caused by high-speed fragments of ink tearing through the dermis and epidermis.
Mike’s Blog Posts 140