Page 227 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Some lasers, such as diodes, may generate many sub-pulses, which means that they can deliver very long pulses overall.
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, in a train of sub-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 stem cells in the follicle bulges and bulb – 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 stem 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 stem cells successfully. The minimum fluence depends on the depth of the bulges (around 1.0 to 1.5 mm deep).
It becomes obvious that using the same fluence on a patient’s upper lip will not have the same effect if used on their arms or legs, simply because those follicles are deeper! So, what are these minimum fluences that we need to use?
We built a computer model looking at the temperatures generated by different wavelengths at different depths in the skin.
Figure 97 shows the required minimum fluences to successfully denature the stem cells in a number of body areas. These are all based on the depths of those bulges. Clearly, as the depth increases, the required fluences also increases. We can also see that the fluences increase as we go from Alexandrite to diode/IPLs to Nd:YAG lasers, as a direct consequence of their absorption coefficients in melanin.
Now, we’re not saying that this model is ‘perfect’ by any means. It is only a 1-dimensional numerical analysis, but it is useful in indicating the ‘ballpark’ numbers. A more detailed model would require much more time to build – and time is precious!!
________________________________________________________________________ 227 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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