Page 298 - Blog Posts v1

E40 - Laser energy, energy density, fluence, leprechauns and star bursts......
I constantly hear laser users talking about the 'energy' they are using to treat tattoos, hair, blood vessels etc.
However, it is clear that many don't really understand what they mean. I often read about treatments using "500" or "250" or "never above 600" joules, millijoules or whatever!! I think these are the numbers displayed on the laser screen. However, there are a couple of issues I want to raise about this.....
Firstly, these numbers are, in the main, pretty meaningless! They merely indicate an approximate energy output from the laser head, but often they are not accurate. This may be due to either poor initial calibration - if any was carried out in the first place! Or, it may be due to the fact that laser flashlamps drop in efficiency with use. As the shots mount up the real energy delivered by a laser falls. So, if the laser was outputting an energy of, say, 750 mJ, when it was new, it might only be delivering half of that after 200,000 shots (or so).
So, the numbers that appear on the laser's screen may, or may not, be accurate!
Secondly, and this is the important bit, the thing that really counts when treating the skin is the "energy density (sometime called 'fluence')". This is the amount of energy delivered into the spot size which 'hits' the skin.
If a certain amount of energy, say 500 millijoules, is fired into a 5 mm spot diameter, the resultant energy density is 2.5 joules/square centimetre (J/cm2). If, however, the spot diameter is only 3 mm, then the ED is now 7.1 J/cm2 - nearly 3 times the density in the 5 mm spot.
That's because the energy density depends on the square of the radius (the area of the spot). So, as the spot size decreases, the energy density increases rapidly.
Mike’s Blog Posts 298