Two wavelengths, namely 660nm and 880nm, where the extinction coefficients (absorptivity) of the hemoglobin molecules are separated were used. Carboxyhaemoglobin (COHb) and methemoglobin (MetHb) are other hemoglobin molecules which normally do not have to be considered due to their low concentrations in the blood. Thus the total hemoglobin concentration in the blood sums up to ctotalHb and can, therefore, be used to determine the oxygen saturation SpO2 (see Equation 2) [34].