Mike Murphy's Blog

We all know that light energy can kill hair follicles. It has been around since about 1996. But, many people still don’t understand the relationship between fluence and depth in the skin.

Let’s begin with fluence – what is it? Simply put, fluence is just the concentration of light energy on the skin surface. If you increase the energy from your device, you increase the fluence. Likewise, if you decrease the spot size, you also increase the fluence.

My research shows that fluence is the single most important issue when thinking about light-based hair removal. So how do we apply it correctly?

Fluence drops as it penetrates the skin (because of absorption and scattering of the photons are they traverse the skin). By the time the light energy reaches the sub-cutis (fatty) layer, it has dropped to around 25% of the value at the skin surface. It doesn’t matter which laer/IPL device we use – they are all pretty much the same (Alexandrite, diode, IPL or Nd:YAG) in terms of penetration into the human skin.

To irreversibly kill a follicle, you need to raise its temperature for a certain amount of time. In that way you will ensure that all the target stem cells have been properly cooked (just like baking a cake – you need to put it in the over at the right temperature for the correct time).

BUT…

Any given fluence will kill all follicles, in the right phase, down to a certain depth in the skin. A higher fluence will kill even deeper follicles. The follicles are all in different phases across the body. That means we can only successfully kill a relatively small number of follicles at any one time – in early anagen. In this stage, the follicles are relatively close to the skin surface, making them easy targets.

But as they descend into the dermis, they become more difficult to kill since the target is further from the surface. If you stick a certain fluence, you will only kill follicles down to a certain depth. If you don’t increase your fluence, then you might easily miss some follicles, many times. This leads to some hairs never disappearing…

The trick is to apply a good, useful fluence to kill as many follicles as possible each time. Apply good skin cooling before and after the treatment, and you will avoid unwanted damage and reduce the pain sensation.

Incidentally, do NOT start at a low fluence and increase it each time your client returns – that’s a very silly idea!! You are merely creating problems for yourself down the line!

Hope this helps,

Mike.