Average Power vs Peak Power
Someone asked me recently “Should I buy a 5000 Watt diode laser or a 3500 Watt laser?” I was a bit puzzled…
I‘ve noticed a few laser companies promoting their systems om the basis of the “power”. With quotes like “the most powerful laser on the planet” and similar nonsense, they are trying to ‘hoodwink’ potential buyers into buying their devices.
They are, of course, playing on people’s ignorance. Most people don’t really understand what “power” is. Or how it relates to lasers and skin treatments.
So, here’s the lowdown.
A bit of physics…
Firstly, any cheap Q-switched laser is hugely more ‘powerful’ than any diode laser! Picosecond lasers are even more powerful. So, what does it actually mean?
Basic physics tells us that ‘power’ is simply how quickly or slowly energy is delivered. So, the shorter the time some energy is delivered, the higher the power. If you fire 10 Joules of energy at the skin in 1 millisecond, then the peak power of that pulse is simply 10/0.001 which is 10,000 Watts. (Note that 1 ms = 0.001 seconds).
If you deliver exactly the same energy over 2 ms, then the peak power becomes 10/0.002 = 5000 Watts.
Did you spot the word “peak”?
This is very important when discussing power. And this is where some laser suppliers are trying to pull the wool over your eyes…
The ‘peak’ power is the maximum possible power that might be generated in a laser pulse. It is a specific measurement – maximum energy divided by the pulsewidth. And this becomes a bit trickier when you need to define precisely how you measure the pulsewidth!!
In the image above, we can see three individual laser pulses (they could be IPL too – it doesn’t matter). The peak power could be found by dividing the maximum energy in each pulse by the pulsewidth (here it is measured at the ‘midpoint’ where the power is exactly half the maximum value – we call this the FWHM (Full Width Half Maximum) value.)
However, as you can see in the image, the ‘average power’ is significantly lower than the peak power. The ‘average power’ is defined as ‘total energy’ divided by the ‘total time’. Clearly, the total time, in the image, is much greater than the pulsewidth, so the average power is bound to be much lower. (Incidentally, the ‘rep rate’ is the repetition rate – the number of pulses per second, usually measured in Hertz (Hz)).
Now, in electronic systems, such as lasers and IPLs, the maximum available average power is essentially determined by the electrical power drawn from the wall socket. That supply can only deliver a certain amount of power, which is determined by the electrical supply to that building.
Inside these electronic devices are usually found various electronic components including capacitors. These can store electrical charge until required. This is how peak powers can exceed average powers in these devices. By storing a certain amount of electrical energy, and then using it in a very short time, it is possible to generate large peak powers, which greatly exceed the average power – but only for a very brief time.
But there are limits on this. In the image above, if the average power indicated, is the maximum power that can be taken from the electrical supply system, then the maximum energy per pulse, pulsewidth and rep rate will all be tied together, by this limit.
If you wanted to increase the rep rate (more shots per second) then you would need to lower the maximum energy in each pulse – to allow for more shots with the same average power. Likewise, if you wanted more energy in each individual pulse, then the rep rate would need to be slowed down so that fewer shots were fired per second.
This is why we often see, in diode lasers in particular, a change in one setting (fluence) when another one is changed (rep rate or pulsewidth). These three parameters are all usually tied together, to keep the average power at, or below, its maximum available limit.
In the skin
So, what does all this have to do with the skin?
When we are treating hair or blood vessels, we need to deliver a certain amount of energy in a certain pulsewidth – to induce the desired reaction.
If we deliver a 5000 Watt laser beam at the skin, then the targets will heat up more rapidly, compared with a 2500 Watt laser. A higher-powered laser will heat the hairs to a higher temperature than a lower-powered laser, simply because of intra-pulse conduction (but that’s another story).
But in hair, this is not so important! In hair, the real targets are the follicle germ cells, not the hair itself. My most recent research shows that pulsewidth merely changes when the germ cells get hotter. It makes very little difference to the final temperatures attained. The fluence is much more critical in terms of temperatures.
And this is true for all hair thicknesses too!
I’ll be presenting the results of this research at this year’s BMLA Conference in Cardiff in May.
Conclusion
How quickly or slowly we deliver energy to the hair follicle is really not important, as long as the fluence is correct for the targets. Consequently, the ‘power’ of the laser is not so important. Peak power is not same as average power – peak power determines the maximum temperature of the hair, but this is not that important either!
When I was young, before central heating was invented, we used to sit around a wee, three bar electrical fire. Each bar generated 1000 Watts. On a particularly cold day, my parents would switch on two bars. I don’t remember ever seeing three bars on at the same time…
Two bars (2000 Watts) heated the room up fairly quickly. I can’t imagine how such a power (average) being close to the skin would feel (bloody painful, I’m sure)!!
So, ask yourself, would you really want a 5000 Watt laser anywhere near your, or your client’s skin?
Next time a salesman says our laser is “blah blah power”, ask them if that’s the ‘peak’ or ‘average’ power. Then ask them the difference…
Thanks for reading,
Mike.
PS Our next MasterClass will be in Bury, Manchester on April 23/24th. Go to dermalasetraining.com for more details.
Mike Murphy's Blog 