Page 206 - Chapter 3 - Laser/IPL Hair Removal

Pulsewidth
Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
How would the above unit deliver only 50 J/cm2, if that was what we actually required? Well, if we know that it can deliver 1000 J/cm2 in a 1 second time period, we just need to shorten the delivery time – or ‘pulsewidth’. In this scenario, if we shorten the pulsewidth to 0.05 seconds (which is 50 milliseconds), then only 0.05 * 1000 J/cm2 will be delivered to the skin – i.e. 50 J/cm2.
This is how most of today’s aesthetic diode lasers operate – by merely choosing an appropriate pulsewidth, they can deliver the required fluence.
A 20 J/cm2 fluence would require a pulsewidth of only 20 ms, and so on.
Laser Diode System Powers
So, what is happening when a laser is outputting, say, 3000 Watts, instead of 1000 W? And how can they do this???
Well, there are two ways to achieve this:
1. The simplest way is to merely add more arrays. Two 1000W arrays will give you a total power output of 2000W. But, the spot size has also doubled, since we now have two arrays, each of 1 cm2. So, the fluence is exactly the same as with one array. They are both sticking out 1000 Joules in each second into a 1cm2 area.
Why would anyone do this? Well, this will increase the area you can treat at any one time. So, if you wanted to treat a large area, like legs or a back, then a larger spot size will allow you to do this in a quicker time compared with a smaller spot size. Simples!
The only real advantage of adding more arrays to your laser is the increase in spot size area – there is no increase in fluence. But if you’re treating lots of legs and back, then this may be a good option.
2. The second method is to increase the energy which each array can deliver. This is trickier because it means that the device must also provide more cooling to keep those arrays cool.
We could change the power output of an array so that it now outputs 1500 W instead of 1000W in the same spot size. So now it can deliver 1500 Joules in 1 cm2 meaning that the fluence has increased by 50%.
In this case, a 50ms pulsewidth will now deliver a maximum of 75 J/cm2, (0.05 * 1500), compared with the 50 J/cm2 with the 1000W device above.
________________________________________________________________________ 206 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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