Page 59 - Mike Murphy's Blog Posts

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)!!
Mike’s Blog Posts 59