Page 89 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Manufacturers who sell this type of system usually claim it is ‘superior’ to other diode lasers – this has not been confirmed by proper clinical studies, as yet.
The main issue, we perceive, with this approach is that the power output of each set of wavelengths needs to be carefully monitored. From our ‘Threshold Fluence’ calculations, it is clear that different wavelengths require different fluences to achieve the same result.
The threshold for the 1064nm wavelength is around 25 J/cm2 (see Error! Reference source not f ound.) whereas it is only 9.4 J/cm2 for the 755nm wavelength. This shows that the output from the 1064nm wavelength should be around 2.7 times (25/9.4) that of the 755nm wavelength, to ‘balance’ the light energies.
It must be understood that the different wavelengths are not equivalent – this is due entirely to their melanin absorption coefficients (Error! Reference source not found.). If a triple wavelength diode laser outputs the same power/energy across all three wavelengths, then the 755nm wavelength will have the ‘strongest’ effects because of its relatively high absorption in melanin, while the 1064nm wavelength will have very little effect (if set anywhere near the output fluence of the 755nm wavelength!)
Diode laser power – why does it matter?
Many of today’s diode lasers sold for laser hair removal are described in terms of their power outputs. This is not so common with IPLs, Nd:YAG or Alexandrite lasers. The reason for this is that the power output of a diode laser directly affects reactions in the hair and skin.
If a laser has a power output of 2000 Watts, then it means that it can deliver up to 2000 Joules per second. So, in a 25ms pulse (which is only 0.025 seconds) it can deliver a maximum of 50 Joules of energy in a single pulse (which is 0.025 times 2000).
A 4000 W diode laser will deliver twice as much energy, in the same time! In other words, in the same 25ms pulse, a 4000 W laser will deliver 100 Joules of energy. Assuming both lasers have the same spot size, this means that the 4000 W laser can output twice as much fluence as the 2000 W laser, in the same pulsewidth.
You will recall that the temperature rise in the absorbing target depends greatly on the absorbed energy. A 4000W laser pulse will generate a much greater temperature rise in the targets in the skin.
We have noticed that this confuses some people – imagine you set up your 2000W laser with a 25ms pulse in a 2 cm2 spot size. The total amount of energy it can deliver in that pulse is 50 Joules (as above). When this is delivered to the skin in a 2 cm2 spot, the fluence (which is energy /spot size area) is 25 J/cm2 (which is 50/2).
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