Propagated light can be absorbed or scattered by the tissue. In the near-infrared range (NIR), 600 to 1000 nm optical window, the major absorbing components—chromophores—in the soft tissues are water, oxyhemoglobin () and deoxyhemoglobin (Hb). There are also minor contributions from other tissue chromophores, such as melanin, lipids, and so on. Each one has a different level of absorption at each wavelength. Components such as water, lipids, CFS, and melanin can be assumed to keep a constant concentration during the test period (static absorbers) and have a little contribution to the overall attenuation in the specific window. On the other hand, concentration of dynamic absorbers—oxygenated and deoxygenated hemoglobin—changes during the experiment according to the function and metabolism of the tissue.