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!

DLR1 Original Version

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.

Most laser-induced processes in the skin depend on the energy density – not just the energy. If a large spot size is used, say 10 mm, then the energy density may be too low to induce the desired reaction (500 mJ in a 10 mm spot diameter is only 0.6 J/cm2!!).

The easiest way to find the spot diameter is to ‘burn’ some paper – use a magazine with a dark page. The photo below shows some burns I have done with various lasers. Please note that the spot diameter usually increases as you ‘turn up’ the laser – this is simply because the laser is more efficient at higher energy levels.



So, in summary, when an energy is quoted for any skin treatment, the corresponding spot diameter should also be quoted. If you don’t do that then you’re only giving half of the story. It’d be like saying you should cook a pie in the oven at gas mark 6, but without stating for how long!!

Incidentally, the energy density or fluence is never known as leprechauns or star bursts……


Bye for now,



Watch the following video for an explanation of energy density/fluence:

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