Page 90 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Just as with most ‘cooking’ devices, the longer the pulsewidth, the more cooking will occur. So, with many laser/IPL devices, a longer pulsewidth will deliver more light energy into the target. Consequently, longer pulses usually means higher temperatures.
But, it’s not quite as simple as that...
It really depends on whether the device being applied to the skin is ‘continuous’ or ‘pulsed’. In the simplest scenarios, the following tends to be true:
Longer pulsewidths usually mean higher fluences resulting Continuous/Pulsed in higher temperatures when applied to the skin.
Like IPLs, the pulsed output from a long-pulsed Nd:YAG Pulsed laser will generate lower temperatures when longer pulses
are used.
So, pulsed laser/IPL systems typically generate higher maximum temperature in the hair when shorter pulses are used, whereas continuous lasers (including diodes) are the opposite. This is discussed on more detail in Section 3.4.
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 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!!
________________________________________________________________________ 90 Chapter 3 Laser/IPL Hair Removal
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Device Delivery Result
Diode laser
IPL
Nd:YAG Alexandrite
Longer pulsewidths usually deliver the same amount of
energy as shorter pulses, for a given fluence. This will Pulsed result in lower maximum temperature sin the target since
a longer pulse allows more time for the heat energy to conduct from the absorbing target.
Pulsed As with the IPL and Nd:YAG technologies.