Page 85 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Figure 36 – Useful Penetration Depth
Fluence and penetration depth
Fluence is the ‘concentration’ of energy applied to the skin. However, as soon as the light enters the skin, it will scatter in all directions, including backwards, out of the skin (we call this ‘back- scattering’). As a direct consequence of this scattering the beam will diverge – the spot diameter increases in size.
So, if we deliver 0.6 Joules (600 millijoules) of light energy into a 5 mm diameter spot, then the resultant fluence is 3.1 J/cm2.
Figure 37 – An energy of 0.6 Joules concentrated into a 5 mm diameter spot will yield a fluence of 3.1 J/cm2
Light penetrates the skin and is absorbed by some constituents and scattered by others. Typically, for visible wavelengths, many scattering events for each absorption event.
As the light penetrates the skin layers its intensity (fluence) drops according exponentially according to the Beer-Lambert Law (please visit this site to learn what this actually means). Hence, in essence for every light-based treatment there are at least three zones of reaction:
________________________________________________________________________ 85 Chapter 3 Laser/IPL Hair Removal
© The Laser-IPL Guys, 2022
Hence, we have defined a more practical application – the ‘Useful Penetration Depth’. This description considers the wavelength (absorption coefficients), incident fluence, spot size and pulse duration of the incoming light energy (see video).