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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
Summary of Light-Tissue Interactions
The important points to note about the way light interacts with tissues are:
The wavelength determines how deeply the light can penetrate. It also determines how much energy is absorbed by the targets (via absorption coefficients).
Energy is what does the “work” in raising temperatures in the absorbing targets.
          Pulse duration
Pulse duration is the time in which the light energy is delivered. This has a strong correlation with the resulting temperature rise in the absorbers. It can be anything from picoseconds (trillionths of a second) to many seconds or even minutes.
          Power / Power Density
Power is simply how quickly or slowly energy is delivered to the site. If the energy is delivered very quickly, then the power is ‘high’. If the same energy is delivered more slowly, then the power/power density is lower than before. We can calculate the power by dividing the energy by the pulse duration.
            Spot diameter
The spot diameter also determines how deeply the light can penetrate principally due to scattering.
          Energy Density (Fluence)
A laser will generate a beam with a certain amount of energy. We can focus this energy into a small spot size using lenses. Different lenses produce different spot diameters. The ‘concentration’ of energy in the spot diameter is known as the ‘fluence’ or ‘energy density’.
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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