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CHAPTER 1 greater than that of the light skin, while that of the medium skin is somewhere in between. A more recent study (Saager et al., 2015) used spatial frequency domain spectroscopy to study the absorption coefficient of different skin-toned individuals in vivo. They recruited participants that fell into all six categories of Fitzpatrick skin types, which is a numerical scale used to classify skin colours (Fitzpatrick, 1988). They found that the volume fraction of the epidermis occupied by melanosomes (fmel) ranged from 5% to 20%, which was similar to Jacques’ study. However, they found a much smaller difference between the different Fitzpatrick skin types, suggesting that the effect of melanin concentration could almost be ignored in the red to NIR range. Figure 1.4 Absorption coefficient of human skin. Absorption coefficient (cm-1) of different skin tones showing both theoretical values and measured values. Dashed lines are theoretical values calculated using the equation suggested by Jacques (1998), assuming 3.5%, 13.5%, and 16% of epidermidis volume occupied by melanosomes in light, medium, and dark skins respectively. Solid lines are measured absorption coefficient in vivo in Fitzpatrick skin types II, IV, and VI participants (Saager et al., 2015). The reported volume fraction of melanosomes in these participants were around 6%, 11%, and 16% in Fitzpatrick skin types II, IV, and VI respectively. Shaded area indicates the optical window. 1.2.2 Molecular mechanisms of PBM Photobiomodulation is essentially delivering photons of a particular wavelength, or electromagnetic energy, to tissues. The absorbed photons interact with the tissues in different ways to conserve the total energy of the tissue. They can excite some electrons to a higher energy state which later return to equilibrium by giving off the extra energy as vibrations or heat. In addition to this process, electrons that are excited to a higher energy state can give out photon emissions leading to fluorescence. A third possible fate for the absorbed energy is to react with 20PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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