Page 84 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Figure 35 – 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.
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