Page 122 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Figure 71 - Temperature distributions for a 180 μm hair diameter, irradiated by an Alexandrite laser at 30 J/cm2 with a pulsewidth of 1ms
So, this highlights an interesting situation. When discussing pulsewidth, most people, correctly state that ‘shorter pulsewidths result in higher temperatures’. This is correct – but only in the hair shafts. It is not necessarily true in the target stem cells in the bulge! As we can see from the curve in Figure 72, the stem cell temperature in the bulge hardly changes at all, as we change the pulsewidth, even over such a large range.
However, there is an important point to note here – when the pulsewidths exceed 100ms, the total amount of denatured cells drops (the grey curve – Omega). So, in a real-world setting, these pulsewidths would result in a poorer clinical outcome. In fact, these curves indicate that the applied fluence of 10 J/cm2 is not sufficient to achieve the desired endpoint.
As discussed in section “Diode Laser – Threshold Fluence”, the minimum (threshold) fluence we must apply with a diode laser is 11.1 J/cm2 (on the upper lip where the follicles tend to be nearer to the skin surface). Using this fluence pushes the value of Omega to just under 100%, for all the pulsewidths.
________________________________________________________________________ 122 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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