Page 113 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Back-scattering is more important in darker skin tones, where more of the light energy can be absorbed leading to higher temperatures, and potential damage. For this reason, darker skins must be pre-cooled for a longer period of time than lighter skin tones.
Understanding these processes will help to make proper choices about fluence settings, and, therefore, lead to better clinical results.
How do fluence and pulsewidth relate to each other?
This is not obvious. If a certain fluence is delivered to some hair in a certain pulsewidth, it will raise the temperature by a certain amount. If it is delivered in a shorter pulsewidth, then there is less time for the heat to conduct away from the hair shaft during the pulse. As a result, the temperature will be higher this time.
If a longer pulsewidth is used, with the same fluence, then heat can ‘escape’ from the hair during the pulse. The maximum temperature will not be as high as with a shorter pulse. But, this is not necessarily the important issue here!!
We must always remember that the end-goal is to denature the stem cells. They are not near the hair (relatively speaking!) The heat energy MUST conduct from the hair to the follicle wall when the stem cell is located. This takes some time – our calculations reveal that it can take between approximately 60 and 90 milliseconds for the stem cells to reach their maximum temperatures after the pulse has stopped.
Pulsewidth
Peak temperature achieved at time
Delay between end of pulse and peak temp
0.1
91.4
91.3
1
91.9
90.9
10
96.5
86.5
50
120.5
70.5
100
156.7
56.7
Table 23 – Times when the stem cells reach their maximum temperatures (all in milliseconds)
The table above shows that the maximum (peak) temperatures in the stem cells are attained a significant time after the pulses have stopped heating the hair shafts. In fact, there is little resemblance between the pulsewidths and the timings of the peak temperatures. This appears to show that the pulsewidths are only really important when determining the peak
________________________________________________________________________ 113 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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