Page 128 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Fluence – how does it change in the skin? Fluence is essentially the total number of photons fired into the skin, within a certain spot diameter. In Figure 75, we can see a bunch of photons entering the skin.
We can see that they spread out as they penetrate further into the skin. This reduces the fluence, simply because the spot diameter is increasing with depth. But, not all the photons will reach the deeper parts of the dermis – many will be absorbed in the upper regions. This, therefore, leaves less photons (energy) available for the deeper parts.
Figure 75 – Fluence drop in the skin
For these two reasons the fluence drops rapidly with depth. In fact, it drops exponentially! This one fact explains why it is ‘difficult’ to effectively treat very deep targets in the skin – there are simply not enough photons reaching there!!
Have you ever wondered how many photons are fired in a pulse of light energy? Well, using Planck’s relation we can calculate this quite easily. It depends on the wavelength (actually, the frequency!!). One Joule of 1064nm light will contain around 5,356,221,986,807,990,000 photons while the same energy in a 532nm pulse with contains only around 2,678,110,993,403,990,000 photons!
________________________________________________________________________ 128 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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