Page 109 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Absorption
Once the light energy reaches the required depth, how strongly will it be absorbed? This depends very much on the wavelength and the associated absorption coefficient. These coefficients can be measured in a laboratory and melanin has been studied intensively for many years due to its importance in our general skin health.
Figure 43 shows the melanin absorption curve over the range 600 to 1200nm - normally used for hair removal. As we can clearly see, the shorter wavelengths (at the 600nm end) are much more strongly absorbed than the longer wavelengths. This means that these wavelengths do not need as much energy (fluence) as the longer wavelengths to generate high temperatures in the hairs. This explains why Nd:YAG laser need to deliver higher fluences than diodes or alexandrite lasers to achieve the same results.
But, these wavelengths cannot penetrate as deeply as the longer wavelengths! So, this tells us that longer wavelength will require more fluence to achieve the same temperatures as shorter wavelengths. See Table 5 for the absorption coefficient values used in this analysis.
Figure 43 – The absorption of light energy by melanin depends on the wavelength. Shorter wavelengths are more strongly absorbed than longer wavelengths.
Skin colours
As discussed before, skin colour is merely the concentration of melanin in the epidermis. The more melanin there, the more light it will absorb and hence, the higher the temperatures generated in that layer, and the lower the fluence available for the deeper hair follicle melanin.
So, people with darker skins will definitely feel more ‘heat’ than lighter coloured people. Consequently, they must receive more skin surface cooling to minimise the pain sensation and potential epidermal damage. ________________________________________________________________________ 109
Chapter 3 Laser/IPL Hair Removal
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