Page 212 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
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, 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. This is due to the fact that melanin absorbs the 755nm wavelength nearly 3 times more than the 1064nm wavelength.
It must be understood that the different wavelengths are not equivalent – this is due entirely to their melanin absorption coefficients (Table 11). 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!)
Triple Wavelength Diode Laser tests – what actually came out of it...
Triple wavelength diode lasers appeared on the market just a few years ago for hair removal. They claimed to enclose three sets of laser diodes emitting wavelengths at 755nm, 808 (or 810)nm and 1064nm.
The attached blurb (marketing) claimed that these three wavelengths generated a ‘better’ overall result when applied to unwanted hairs, simultaneously. As very sceptical physicists, we had our doubts....
Firstly, it seemed rather ‘convenient’ to me that the engineers had just happened to produce diode lasers which could emit the 755nm wavelength of the Alexandrite laser and the 1064nm wavelength of the Nd:YAG laser – both of which have been used in hair removal for a number of years.
Secondly, the fluences that these triple wavelength devices would need to output would have to correlate to the absorption coefficients of melanin (the target in hair shafts). This is not a straightforward calculation without a pretty good understanding of the principles involved.
However, these issues are difficult to determine without a spectrophotometer to properly analyse what is really coming out of such a device. Our engineer colleague did precisely this analysis (Figure 114). He took a commercially available unit and plugged it into his measuring device. What he found was very interesting...
Firstly, the peak powers output by this unit did not coincide with the stated wavelength of 755, 810 and 1064nm. Instead, they were closer to 796, 817 and 1061nm – this was not surprising to us. We had always questioned these numbers!
________________________________________________________________________ 212 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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