Page 97 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Figure 47 – Fluence drop in 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.
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!
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!!
What about back-scattering? What does this do to the fluence?
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