Page 92 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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 Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
Figure 51: Light energy penetrating the skin, with no scattering
The ‘heated volume’ is simply the area of the spot times the depth, d, that the light reaches (which is 𝜋r2 * d). So, this volume of tissue will be heated by the light energy. However, the intensity of the energy falls with depth, so the upper (superficial) layers will absorb more energy (assuming volume homogeneous absorption) than the deeper layers, and hence, will become hotter.
Case 2 – With scattering
As we know, scattering has a very important effect on light as it moves through the skin. The beam ‘spreads’ out as it gets deeper, thereby reducing the fluence at each depth.
Consequently, this has an effect on the heated volume (Figure 52).
Figure 52: Light energy penetrating the skin, with scattering
Figure 52 shows that the spot, which measured a radius r1 at the skin surface, will have spread out to a radius of r2, at depth ‘d’. This means that the same energy (as shown in Figure 51 above) is now absorbed in a larger volume than above!
This means that the same amount of energy is now ‘occupying’ a larger tissue volume, meaning that the temperature rise will be lower than in the non-scattering case, particularly in the deeper regions.
Energy Absorption versus depth
As light penetrates into the skin it is absorbed by various tissue components (blood, melanin, water etc).
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
 Heated volume – with scattering
r1
Epidermis
Dermis
r2 Subcutis
      d
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