Page 107 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Figure 42 – Penetration depths of four wavelengths
Pulsewidths
All of these wavelengths can penetrate deep into the dermis – the Nd:YAG penetrates about 3.5mm compared with the alexandrite which only goes as far as 2mm.
(These are the standard physics definitions of the “1/e” penetration depths which is where the incident energy drops to about 37% of the original value.)
The pulsewidths generated by different lasers and IPLs vary widely. Many lasers are limited by the technology or engineering to relatively short pulsewidths, whereas IPLs can generate longer pulses by using ‘trains’ of sub-pulses.
Some lasers, such as diodes, may be continuous which means that they can deliver very long pulses.
As a consequence, some lasers might deliver their energy in pulsewidths in the range 1 to 20 millisecond (ms) or so, while diodes and IPLs can output more than 100ms pulses.
The significance of the pulsewidth is that it essentially determines the maximum temperature achieved in the hair shafts. However, this is not necessarily the most important temperature since the real targets are the germ cells in the follicle bulges – not the hair! It takes a significant amount of time for the heat to conduct out to the bulges, which depends on the size of the follicle.
Fluences
As with pulsewidths, each device outputs a range of fluences. However, the fluence required to effectively destroy hair germ cells depends on the amount of energy which is absorbed by the melanin in the hair shaft. This is strongly dependent on the wavelength used - see next section.
This explains why the fluences used with a Nd:YAG laser (for example) is quite different to those used with diode lasers or IPLs. They are NOT equivalent!
The most important issue here is to ensure that the threshold fluence is exceeded when treating hair. This is the absolute minimum fluence that will denature the germ cells successfully. The minimum fluence depends on the depth of the bulges (see Table 14 – The maximum bulge
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