Page 103 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
We should look at fluence before going any further. Fluence is simply the concentration of energy - the number of photons passing through each square centimetre. So, a fluence of, say, 10 J/cm2, requires a certain number of, photons in each square centimetre. So, if we choose to use a bigger spot size, we must increase the number of photons to maintain the same concentration. That means more energy (since energy is merely the total number of photons!)
Now imagine the same scenario as before, but with a bigger spot diameter. To be the same fluence as before, this beam will need more energy (photons). As before, virtually every photon will be scattered on entering the skin. And, as before, the fluence will decrease the deeper it penetrates into the skin.
But, a significant proportion of the scattered photons will be directed back into the beam. If these two beams have the same wavelength, then the ‘spread’ of the beams will be approximately the same.
Figure 53 - Penetration depth as a function of spot diameter
You can imagine it as both beams losing photons, mainly from the edge of the beams. Smaller diameter spots will ‘lose’ as many as larger diameters. But larger diameters will ‘retain’ more photons inside them - meaning more fluence there.
Think of a small diameter spot as a three-lane motorway with exit roads on both sides. The cars on the outer edges of the lanes can easily drive off the motorway via the exit roads. If cars are leaving at every exit, then the number of cars left on the motorway after a few exits will be considerably less than the start of the motorway.
________________________________________________________________________ 103 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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This means that many of the photons inside the beam will be scattered to another point inside the beam. If we consider the ‘proportionality’, smaller spot sizes have fewer photons inside them, compared with larger spot sizes (of the same fluence). Consequently, larger beam diameters will have many more of the photons scattered ‘internally’ into the beam, whereas smaller beam diameters will essentially ‘lose’ more photons to the surrounding tissues (proportionally).
Same fluence in each beam
Both of these beams have the same fluence (concentration of energy). The beam on the left has a larger diameter than the beam on the right. Due to scattering in the skin, the larger beam diameter delivers more energy deeper into the skin.