Page 120 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Figure 70 - Temperature distributions for a 180 μm hair diameter, irradiated by an Alexandrite laser at 10 J/cm2 with three pulsewidths – 1, 10 and 100ms
The above graphs (Figure 70) show the temperature distributions in, and around, a 180 micron thick hair, after being hit by a 10 J/cm2 fluence from an Alexandrite laser. The upper blue, red and green lines are the temperature distributions from 1, 10 and 100 millisecond pulsewidths. Clearly, the 1ms (upper blue line) shows very high temperatures near the hair surfaces, as we might expect. This distribution is very ‘lumpy’ in that the temperatures near the hair centre are relatively low compared with the walls. (This is due to the very brief pulsewidth and the way light interacts with the melanin).
The 10ms pulse (upper red line) shows a more evenly spread range of temperatures inside the hair, but all lower than the 1ms distribution.
Then the 100ms pulse (upper green line) shows a more expected ‘Gaussian’ temperature distribution, again at lower temperatures than above. This is because there is much more time for the heat to dissipate out into the surrounding dermis during the 100ms pulse.
But, the thing to notice is that the lower blue, reg and green lines are almost identical! These are the temperature distributions around 282, 287 and 338ms after the start of the pulse, when the stem cells have reached their maximum temperatures. In other words, after the light pulse has stopped, the temperature distributions change as the heat conducts out of the hair into the surroundings. The hair “cools”.
________________________________________________________________________ 120 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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