Page 93 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
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 53 (below) shows that at a depth, known as the ‘1/e’ depth, the value of the intensity at the surface will have dropped to 36.8% 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 53: 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!
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.
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
 Depth Intensity
Surface 100%
Epidermis
Dermis
Subcutis
Light energy distribution with depth
                    1//ee 36.8% 2
                 12//ee 13.5%
                 3
13//ee 5.0%
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