Most laser systems have a lens to focus the laser energy down to a sale spot diameter (see figure 1 below). However, in many systems, the focal point (where the spot diameter is the smallest) is outside the handpiece assembly.
This has a potentially hazardous effect if not properly understood! The problem with this design is that the spot diameter depends on the distance from the lens!!
Most laser systems have some kind of ‘spacer’ on the handpiece tip (see figure 2).
This tip is designed by the laser engineers to ensure you apply the ‘correct’ spot diameter to the skin surface.
If you pull the handpiece away from the skin, such that the tip is no longer in contact with the skin then you cannot know what the spot diameter is!!
 |  |
In figure 2, the left handpiece shows a scale of spot diameters. This scale assumes that the circular black ring at the base is in touch with the skin surface – otherwise it is meaningless.
The following video shows how much the spot diameter can vary:
You can see that the spot become slightly smaller as the tip is moved away from the page, then increases near the top of the page.
The main effect of this is that the fluence (energy density) changes enormously. The fluence changes with spot diameter – a smaller diameter results in a higher fluence. But, the value of fluence changes with the square of the diameter!!! This means that even a relatively small change in diameter can result in a large change in fluence.
This can result in either damage to the skin or zero effect on your target – neither is good.
So, always keep your handpiece tip close to, or touching, the skin surface to ensure the correct applied fluence.
(Incidentally, no models were harmed in the production of this video….)
Ciao for now,
Mike.
Mike Murphy's Blog 