Page 174 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
As with the Alexandrite laser above, Mike applied a Nd:YAG laser to a skin tone 2 at a fluence of 100 J/cm2. Again, the trainee reported a low pain level of about 3 out of 10, and, again, she suffered no unwanted responses after the treatment.
In a different training session, Mike applied high fluences from another well-known Nd:YAG system to skin tones 5 and 6 (this was in a New York clinic which specialises in treating darker skin tones). These fluences were much higher than the laser operators were used to. The trainees also reported low pain sensations, after being pre-cooled with ice for two minutes. However, two of the trainees also reported a ‘stinging’ pain from the cryogen spurts. When this was switched off, their pain levels dropped significantly!!
Summary
The four types of devices commonly used for hair removal are not the same. They output different wavelengths, which all have different absorptions within melanin. This means that they cannot be viewed as being ‘equivalent’.
Device
Threshold fluence (J/cm2)
IPL (averaged to 818nm)
11.5
Diode
11.2
Nd:YAG
24.5
Alexandrite
9.4
Table 33 – Threshold fluences for four devices
The values in Table 33 indicate the absolute minimum fluences which should be applied to the skin surface to achieve good results, and this will only occur in the most superficial anagen follicles. These must be increased when targeting deeper follicles such as found in the pubic and underarm regions, otherwise finer, lighter-coloured hair may regrow at a later date.
The instantaneous temperature in the hair shaft after irradiation
The original computer model was designed to investigate the temperatures in a hair shaft as the heat flowed out into the surrounding dermis overtime. This is a one-dimensional model which solved the heat conduction equation numerically.
We ‘turned’ our mode through 90 degrees to look at a different question – how does the temperature profile appear along the length of a hair shaft immediately after it has absorbed light energy from various devices? This is still a one-dimensional analysis, so we can adapt our model accordingly.
________________________________________________________________________ 174 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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