Page 87 - An Introduction to Laser Tattoo Removal

Chapter 2 – Fundamentals of Laser Tattoo Removal v1.0
So, the correct choice of power is critical in achieving the desired response in tissue.
When we said that fluence was a critically important parameter we were not telling the whole truth. But, before you can understand the absolute importance of power density you needed to understand how fluence works (see Mike’s blog post).
Curiously, many experienced laser operators do not understand the paramount importance of power density. This is peculiar as we feel it is central to understanding how light-based treatments actually work.
As mentioned previously power is simply the rate of energy used per second. Now we know that fluence (or energy density) is energy per spot size area. It follows that power density is just the energy density per second, or fluence per second.
Energy Density Power density = _______________________
Pulse duration
and is measured in watts per cm2 (which is joules per second per cm2). In other words, power density is the total energy delivered in the time taken for the pulse in the spot area into which it has been fired.
Power density combines three essential elements of lasers –
● energy,
● time and
● spot size area
Since all light-based devices deliver light energy over time into a particular spot size, power density is a good way to compare different systems
However, the most important thing to understand about power density is that it absolutely determines what happens in tissue. Fluence determines whether anything happens at all, while power density determines what actually happens.
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Chapter 2 LEVEL A Laser Tattoo Removal
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