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CHAPTER 1 the other hand, have much smaller absorption coefficients compared to the haemoglobins. Their coefficients fluctuated within the 10-1 cm-1 range, and the two were almost inseparable. Figure 1.3 Absorption coefficient of major chromophores in the human body. Absorption coefficient in cm-1 of water, melanin, lipid, oxygenated haemoglobins (HbO2), deoxygenated haemoglobins (HbR), oxygenated myoglobins (MbO2), and deoxygenated myoglobins (MbR). Shaded area indicates the proposed optical window for photobiomodulation. Figure adapted from Beard (2011), Yao and Wang (2014), and Dean-Ben et al. (2017). By studying the pattern in the absorption coefficient of these chromophores, an ‘optical window’ has been proposed, within which the ‘maximum’ light penetration can be achieved. It starts around 650 nm when the absorption coefficient of HbO2 and HbR starts to drop below the 101 cm-1, and ends around 1200 nm, when the absorption coefficient of water starts to rise to 100 cm-1 in magnitude. Within this range, melanin still has the highest absorption coefficient amongst all the major chromophores. This phenomenon has led to various studies investigating the absorption coefficient and light penetration in different skin-toned participants. By combining absorption coefficient studies on melanin-free epidermis (Saidi, 1992), and melanosomes (Jacques and McAuliffe, 1991), Jacques (1998) proposed a function to calculate the absorption coefficient of different skin-toned people across a range of wavelengths. Using the average volume fraction of the epidermis occupied by melanosomes (fmel), one can construct an absorption coefficient graph in light (fmel = 3.5%), medium (fmel = 13.5%), and dark (fmel = 16%) skin-toned individuals (Figure 1.4). Over the range of the optical window, the absorption coefficient of different skin tones ranged between 102 cm-1 and 100 cm-1. The absorption coefficient of dark skin was ten times 19PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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