The answer to this question is very simple – TEMPERATURE!!
Think about what we’re trying to do when treating hair, blood vessel or benign pigmentation. We are trying to ‘kill’ certain cells, using heat. That’s why we fire laser light energy at them.
We’re trying to force them to absorb that light energy, convert it into heat energy and raise the temperature of the target sufficiently to ‘cook’ those cells.
But how does this actually work?
Well, I know how much my readers just lurve equations…
….so here we go:
Above is a very simple equation which we (“super nerds”) use to calculate temperature rises after they’ve been hit by laser light energy.
The ‘H‘ is the fluence, or concentration of energy. We measure this in Joules/cm2 (often referred to as ‘Joules’, I hear…)
The energy is what generates heat. The light energy is absorbed by some cells in the target, and is converted into heat energy. We perceive this heat energy as temperature.
The funny Greek symbol to the right of ‘H’ is ‘mu‘. This is, essentially, the colour of the target (In more realistic terms, it is the absorption coefficient – but we can think of this as colour).
These are both divided by ‘C‘, which is just a boring, old constant. Let’s ignore this for the moment…
So, this equation is telling us that the temperature rise generated by laser energy absorbed in any target is simply dependent on the product of the fluence and the ‘colour’. This makes sense – darker hair are easy to remove compared with lighter hairs.
But what this equation also tells us is that the fluence is absolutely critical!
In fact, the success of ANY photo thermal treatment (by laser and/or IPL) depends strongly on the temperature rise attained in the target.
Clinical studies have shown, many times, that successful treatment outcomes occur when the correct fluence is applied. If it is too low, there is insufficient damage and some of the target cells will re-grow. If it is too high, you will damage too much of the adjacent tissues, which might lead to scarring!
Choosing the correct fluence is very important in all of these treatments. In fact, many treatments fail, of generate poor results, simply because the fluence is too low.
I hope this makes some sense to y’all.
You can learn more stuff like this at our DermaLase MasterClass, to be held in September in Birmingham.