Page 21 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
If we take an average value, then the IPL wavelengths is close to the absorption levels of the diode lasers. (See the section entitled ‘Absorption Coefficients’ later in this book).
Remember, the choice of wavelength is to maximise the amount of light energy absorbed by the melanin, which, in turn, generates more heat.
Fluence
Fluence is simply the ‘concentration’ of light energy. It is also known as the ‘energy density’ (although, technically this is incorrect!)
We usually measure this in Joules/cm2 (J/cm2).
If we shine a torch against a barn door, the light is spread out over a large area. The light hitting the door has a very low fluence since the concentration is very low.
If we then shine the same torch at our hand, so all of the light is striking our skin, then the area covered by the light is much smaller, compared to the barn door. This fluence is much greater than above, even though it is exactly the same amount of energy.
Precisely the same principle applies to lasers and IPLs. The fluence is simply the total amount of energy spread over the area striking the skin surface.
If we concentrate all the energy into a small area (say, 1 cm2), then all that energy will enter the skin through that square centimetre. That concentration may be enough to generate a lot of heat in the skin, if there is a strong absorber in there (like melanin).
If, however, we pull the laser/IPL tip away from the skin surface, then the light can spread out before hitting the skin (laser beams do spread – they are never truly parallel!!). If, by some amazing coincidence, the area that the lights hits the skin is now 2cm2, then the fluence is precisely half the value of above (the same energy spread over twice the area).
The beam concentration hitting the skin surface is now lower than before – the fluence has dropped by 50%. With a lower concentration of light energy, the temperatures generated in the skin will be lower than before. (The temperature rise does not drop by 50% unfortunately – it’s not that simple in the skin!!)
So, careful selection of the fluence is critical in achieving good results. If it is too low, then not enough temperature will be generated in the germ cells. If
________________________________________________________________________ 21 Chapter 3 Laser/IPL Hair Removal
© The Laser-IPL Guys, 2022