Page 47 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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                                       Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
● Likewise, lasers and IPL systems generate light energy which we can utilise to cause changes in the skin. The components within the skin absorb some of this energy which is converted to heat (thermal energy) which results in a change in the tissue structure.
● ‘Energy Density’ and ‘Fluence’ are interchangeable expressions. The understanding of fluence is absolutely critical. You simply must understand what fluence is and how it relates to the real world. Click here for an animation on fluence.
● Simply put, fluence is the total delivered energy divided by the effective spot size. It is often called the energy density although this is not strictly correct (a ‘density’ refers to a volume, not an area!)
● Given that energy is measure in joules and spot size area is measured in cm2 it follows that fluence is measured in joules per square cm (J/cm2).
  'Energy density’ or ‘fluence’ is essentially how concentrated the laser energy is into a spot
If you fire a certain amount of laser energy into a spot size of, say, 6 mm diameter, then you will generate a certain fluence. This fluence may induce a certain amount of thermal damage in cells and tissue
If you fire exactly the same energy into a smaller spot diameter, then the fluence is greater because the energy is more concentrated into a smaller area. This, higher, fluence will induce more damage than with a bigger spot.
If you reduce the spot size even more but still use the same energy, then the fluence is even greater. Such a high fluence could cause a lot of unwanted tissue damage!
                  Figure 24: The top image shows a fluence of 5 J/cm2 of laser/IPL light on the skin surface in a spot diameter of 5 mm. If that spot diameter is reduced to only 2 mm, without changing the energy, then the fluence will increase to 32 J/cm2. This is due to the fluence being dependent on the square of the spot diameter – so, a relatively small decrease in diameter results in a large increase in fluence!
We can think of fluence as a ‘concentration’ of energy in a spot area.
● The importance of fluence is quite simply put – it is the fluence which determines whether anything happens at all! If the fluence is too low then nothing will happen in the tissues. If it is too high then unwanted damage is certain.
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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