So on this forum, the power settings are treated as a power per pulse. E.g. if you use 12J at 12mm, it is more power than 12J at 15mm.
However, I was looking at my technician’s machine this weekend (GentleLASE), I saw the power settings were set a power density (power/area), not a total output power. I don’t remember the units, but it was something like J/cm^2. So, why does everyone here say that the power output changes depending on the spot size?
The power settings are measured in Joules per centimetre squared or as you wrote J/CM^2, meaning how many joules of energy per square centimetre. The 15mm, 18mm etc is the spot size of the laser. A laser works in an upside down cone. Think of it as two pyramids, a bigger pyramid is taller than a smaller one right? Flip them upside down and the larger one therefore would go deeper since it’s taller. This is why a smaller spot size needs more energy because it doesn’t penetrate as deep. So we up the energy to compensate.
I am an electrical engineer, so please humor me while I try to understand what is going on here.
Oh, and I made a mistake earlier, Joule is a unit of energy not power. the “power setting” is given in an energy density, not a power density. nonetheless…
I was under the impression that the energy density setting refereed to the energy of the photos per unit area delivered to the skin per pulse. Thus regardless of the width of the laser spot size, the energy per unit area would be the same.
so is the energy density setting referring to something other than what I think it is?
Oh and just as a little side question, on the GentleLASE, what is the typical rage of power settings used at 18mm? I get the impression 12-18 J/cm^2 is the normal range.
It’s not the same energy per area, as I said a laser works in a cone, so larger spot size penetrates deeper, therefore you need higher energy on smaller spot sizes to compensate. 20J is the maximum on the 18mm spot size on GentleLASE, anything under 16J is a waste and won’t produce permanent results. The unit can certainly go to 12, even 8, but you won’t get permanent results under 16 it’s too weak.
Here’s a good paper for you to read to understand the cone-shape effect: http://www.candelalaser.com/products/gentleyag/gyagPA4.pdf
Also, check out this page for specifics on GentleLASE in particular (the above talks about GentleYAG): http://www.candelalaser.com/products/index.cfm?task=gentlelase#advantages
Spot size is the size of the laser tip. The larger ones are better for results because they enable deeper penetration. Joules/cm2 is the “energy” setting.
Most “powerful” settings include largest spot size and higher J/cm2 setting your skin can handle safely (as well as short pulse width…but on GentleLASE it’s fixed at 3ms, so no need to worry about it on this machine).