Melanin
Definition: Melanin, a poly-indolequinone biopolymer, is a high refractive index brown-black pigment produced and stored by melanosomes, organelles of the melanocytes at the base of the epidermis. Melanin is composed of nanoparticles that are 30–50 nm in size, in which melanin monomers are cross-linked together and form stacking structures. Melanin absorbs light in the UV, visible, and near-infrared regions and protects tissues by scattering or dissipating light. Darker skin tones have significantly larger and more concentrated melanosomes that increase the absorption cross section of melanin resulting in enhanced light absorption. Extracranial contamination (including skin melanin concentration) do not affect NIRS measurements, since the concentration of melanin remains constant over the measuring period. It thus affects noise level, but not the changes in optical properties due to the hemodynamic response.
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References: https://doi.org/10.1152/physrev.00059.2017https://doi.org/10.3389/fnins.2023.1086208
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