Page 178 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
IPL ‘Over-heating’ – the need for proper surface cooling
Using Steve Jacques’ values 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 99). 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.
Figure 99 – Thermal pain receptors are located just below the basal layer of the epidermis. These are triggered by the hot melanin following laser/IPL treatment. Pain is triggered at 45oC, so cooling the nerve endings reduces the pain sensation felt by the client/patient.
Cooling the nerves endings has the effect of ‘pushing’ them down from the skin surface!
The typical dermal temperature is around 37oC, while the pain receptors trigger at 45oC. This means that there is only an 8oC difference between the normal dermal temperature and the temperature at which the client/patient will feel thermal pain. By applying a little cooling at the
________________________________________________________________________ 178 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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