Page 126 - An Introduction to Laser Tattoo Removal

Chapter 2 – Fundamentals of Laser Tattoo Removal v1.0
Heated volume following light energy absorption - tattoos
Laser-tissue interactions is a very tricky subject! There are so many things to consider when trying to figure out where the light energy goes. Once it reaches its target, is there enough left to induce the desired reaction? Here is a very brief introduction to this subject.
Energy Absorption versus depth
As light penetrates into the skin it is absorbed by various tissue components (blood, melanin, water etc).
Hence, the light which reaches the deeper parts of the skin is not nearly as intense as the light near the surface. In fact, we can calculate how this intensity changes with depth.
Figure 65 (below) shows that at a depth, known as the ‘1/e’ depth, the value of the intensity at the surface will have dropped to 63.2% of that initial value. (Don’t worry about the ‘1/e’ stuff – this is a mathematical definition which depends on the absorption coefficient of the target tissues!!!)
Figure 66: The fall in light energy intensity depends on the depth and the absorption coefficient of the tissue(s).
(We are assuming no scattering in this example, just for simplicity).
These calculations show that the tissue volume above the 1/e depth will have absorbed around 63.2% of all the light energy (100% – 36.8%) – that’s close to two thirds of all of the incident light energy in the ‘top’ layer!
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Chapter 2 LEVEL A Laser Tattoo Removal
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