Page 127 - An Introduction to Laser Tattoo Removal

Chapter 2 – Fundamentals of Laser Tattoo Removal v1.0
At double that depth (1/e2), the intensity will have fallen to only 13.5% of the original value. So, this means that the total amount of light energy absorbed between the 1/e and 1/e2 depths is only around 23.3%. (36.8%-13.5%).
Hence, the vast majority of energy is absorbed in the upper layer (63.2%) while only around 23.3% is absorbed in the deeper layer (of the same thickness).
Below this depth (1/e2) and only around 13.5% is available for deeper regions. In most clinical situations, this will not be sufficient to induce the desired reaction.
This explains why some deep tattoo ink is just impossible to reach, safely. We could increase the fluence at the surface, but this would likely damage the upper layers of skin!
Short pulses versus long pulses – effect of heating on tattoo particles
What is the difference between short and long pulsewidths on tattoo particles? The main difference is that the maximum, or peak, temperatures are different. Longer pulsewidths result in lower peak temperatures, simply because there is more time for the heat energy to conduct, or dissipate, away from the particle into the surrounding dermis.
Figure 66 shows how the shorter pulsewidths ‘force’ more energy into a volume of ink particle, thereby pushing its temperature higher than with a longer pulse.
Figure 67: Shorter laser pulses result in less thermal conduction during the pulse. This results in higher temperatures at the absorbing sites. Longer pulsewidths allow for heat energy to conduct away from the absorbing site during the pulse resulting in lower peak temperatures.
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Chapter 2 LEVEL A Laser Tattoo Removal
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