Page 87 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

If we then take the same 0.6 Joules of light energy and focus it into a 2 mm spot diameter, then it becomes much more ‘concentrated’ than before. In this scenario, the fluence has jumped to 19.1 J/cm2 – more than 6 times the previous value!
Figure 39 – The same 0.6 J concentrated into a 2 mm spot diameter will yield a fluence of 19.1 J/cm2 – more than 6 times the fluence in a 5 mm spot
Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Since fluence is defined as energy / spot size area, and the area is increasing with depth, then the fluence will drop, as the light penetrates deeper into the skin.
In Figure 41, a fluence of 5 J/cm2 is applied to the skin surface on the left side of the image. At some depth, due to the beam spreading out and increasing the beam diameter, the fluence will drop to 4 J/cm2. At a deeper depth, the beam diameter will be larger and the fluence will drop to 3 J/cm2. Then, it will be 2 J/cm2 at a deeper level.
6 J/cm2 4 J/cm2
Figure 40 - Fluence vs Penetration into the skin
Figure 41 – Increasing the fluence at the skin surface will increase the fluence within the skin throughout its thickness
________________________________________________________________________ 87 Chapter 3 Laser/IPL Hair Removal
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