Page 120 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Figure 57 – The variation in Omega as the fluence is increased by 1 and 2 J/cm2
Interestingly, increasing the fluence by only 1 or 2J/cm2, has significantly different effects across the devices. The alexandrite laser induces a much greater change in Omega than an Nd:YAG laser, with diode lasers/IPLs coming in between these two (Figure 57).
IPL ‘Over-heating’ – the need for proper surface cooling
Using Steve Jacques value for “baseline dermal absorption coefficient” of 0.53 to 0.24 cm-1 (for the wavelength range 532 to 1064nm), the approximate temperature rise in the dermis during IPL treatments may be calculated (assuming no convection, scattering and an instantaneous temperature rise). Note that the melanin absorption coefficients for lasers/IPLs range between 50 and 164 cm-1 – significantly higher than the (blood-less, melanin-less) dermis!
The interesting result is that the maximum dermal temperature rise is typically around 1.3oC (at 1064nm) and 3oC (at 532nm) for an incident fluence of 10 J/cm2. This may appear surprising given that many patients report a painful experience when undergoing IPL treatments of hair and blood vessels.
The reason for this pain must be due mostly to a sharp temperature rise in the epidermal melanin during irradiation (see Figure 58). The thermal pain receptors are typically found near the epidermal-dermal junction. Hence, during and following irradiation the basal layer of the epidermis reaches a high temperature – this heat energy is then conducted towards the thermal pain receptors immediately beneath this layer.
________________________________________________________________________ 120 Chapter 3 Laser/IPL Hair Removal
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