Page 123 - An Introduction to Laser Tattoo Removal

Chapter 2 – Fundamentals of Laser Tattoo Removal v1.0
However, light from the far IR, e.g. the CO2 laser, is strongly absorbed by water and hence cannot penetrate very far into the skin since most of the energy is absorbed in the very superficial layers.
Figure 64: The penetration depth of light depends on the wavelength of the light.
Effect of Spot Diameter
The penetration depth of light into the skin can also be affected by the spot diameter of the beam (for lasers and IPLs).
At this point we should point out that ‘penetration depth’ is a physics concept – it is determined by finding the depth at which the value of the energy has dropped to a certain value (usually 50% or 36.8%).
Figure 65: Scattering causes the light beam to ‘spread out’ thereby reducing the fluence. The red lines show how the photons spread after entering the skin. The solid red areas above show where a ‘useful’ fluence exists within the skin. It is clear that a larger spot size at the skin surface results in a deeper ‘useful zone’ where the desired reactions may still occur. Smaller diameter spots result in shallower zones. (The above fluences are the same in both spot sizes). (Image courtesy of Caerwyn Ash – Effect of wavelength and beam width on penetration in light-tissue interaction using computational methods)
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Chapter 2 LEVEL A Laser Tattoo Removal
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