Page 101 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Figure 51 – Scattering in the skin - anisotropy
The penetration depth is how far light can penetrate into the skin. However, the physics definition is given by ‘1/absorption coefficient’. So, given that the absorption coefficient of the dermis is 0.244 cm-1 with the 1064nm (Nd:YAG laser) wavelength, then this light will penetrate to a depth of 4.1 cm.
This is the depth at which the energy/fluence drops to around 36.8% of the incident value at the skin surface. For a wavelength of 532nm (frequency-doubled Nd:YAG laser), the absorption coefficient in the dermis is 0.527 cm-1, yielding a penetration depth of 1.9 cm – less than half the depth of the fundamental wavelength of 1064nm! (Absorption coefficient data from Steve Jacques paper, “Skin Optics”). But, this is merely the depth at which the energy/fluence has diminished to an arbitrary value!! It has no real bearing on clinical treatments. When considering results, we need to know the absolute value of the fluence – not a relative value.
Where it does become important though, is when we are trying to target deeper tissues. If we choose the ‘wrong’ wavelength for some particular tissue at some particular depth, then it must ‘match’ that depth – otherwise insufficient fluence will reach the target cells!
________________________________________________________________________ 101 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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