Page 74 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
Device Wavelength (nm) Absorption Coefficient (cm-1)
IPL 600-1200 122.2
Diode 810 128.5
Alex 755 160.4
Nd:YAG 1064 50.4
Table 11 - The wavelengths and corresponding melanin absorption coefficients for four typical hair removal systems. Many IPL systems typically use a range between 600 and 1200nm – this equates to an absorption coefficient of 122.2 cm-1, which is equivalent to a wavelength of 818nm.
Using a normalised analysis of the output spectrum of an IPL Xenon flashlamp, it is possible to calculate the “equivalent single wavelength” from the average absorption coefficient. This turns out to be 818nm – very close to the standard diode laser output used in these treatments (810 and 820nm). Interestingly, if the range 700-1200nm is used, then the average absorption coefficient drops to approximately 75 cm-1 with an equivalent wavelength of 950nm, while a 800-1200nm range yields a coefficient of 46.4 cm-1 with an equivalent wavelength of 1090nm (closer to the Nd:YAG wavelength). Using these wavebands would require higher incident fluences to account for the lower absorption by melanin.
In the hair shaft melanin:
How does the absorption of light differ from the absorption in the epidermal melanin?
Well, in both targets, the absorbing medium is melanin – it’s just at different concentrations. Below shows the absorption curves for skin tone 1 (very pale) and skin tone 6 (very dark) across the wavelength range from 600 to 1100nm.
The ST6 curve shows strong absorption at the lower wavelengths, as expected, while the pale, ST1 tone absorbs quite poorly across this range (Figure 38). Note that these are the absorption curves in the epidermal melanin.
If we now add the absorption curves for the hair shaft melanin, which is at much higher concentrations typically, we see the curves in the next figure. It is clear that the ‘black’ and ‘dark brown’ hairs have much greater absorptions than the surrounding skin. However, the ‘brown’ hair melanin concentration can vary widely, leading to a wide range of absorption curves too. Hence the ‘light brown’ absorption curve lies below the curve for black epidermis – meaning that light hitting a black skin with brown hair is more likely to be absorbed by the skin’s melanin than the hair’s melanin.
________________________________________________________________________ 74 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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