Page 86 - Mike Murphy's Blog Posts

Device electrical power
A percentage of the laser’s electrical power is used to ‘drive’ the diode lasers (perhaps between 50 and 80%). So, the unit might have a power rating of 5000 Watts, but only between 2500 and 4000W is actually used to ‘fire up’ the laser. (The rest is used to pump the water, run the screen etc...
A percentage of this electrical power is converted into laser (light) power. This is the “output” power – it is NOT the same as the “electrical” power of the device! Now, lasers are never 100% efficient so not all of this power will be converted into the laser power. It depends how good the diodes are, but this might be between 75 and 90% (or thereabouts).
So, a 5000W (electrical power) device might deliver an optical power (to the skin) of around 2500W – this is the power we should be interested in. This is what actually hits the skin surface.
As a consequence, higher powered lasers deliver more energy/fluence in same pulsewidth than lower power lasers
For a pulsewidth of 20 milliseconds in three different lasers:
Maximum fluence assuming a 1 cm^2 spot size
So, we can see that a higher powered laser will deliver more fluence to the skin in the same time (pulsewidth) than a lower powered laser. This means that the temperature rise in the target hair will be higher – simple because a higher fluence results in higher temperatures.
If the same fluence is delivered to the skin using a longer pulsewidth (say 40ms instead of 20ms), then the resulting temperature will be lower because more of the heat energy will have conducted away from the hair during the pulse!
This all makes it a bit tricky when comparing diode lasers of different powers. A 20J/cm2 fluence from a 1000W will generate a lower maximum temperature in the hair compared with a 20J/cm2 fluence from a 5000W laser – because the 5000W laser will deliver the energy much faster!
Repetition Rate
The repetition rate is the ‘number of shots per second’ that are fired to the skin. It is usually measure in Hertz (Hz).
Laser Output Power (optical) (Watts)
Maximum Fluences (J/cm2)
1000
20
2500
50
5000
100
Mike’s Blog Posts 86