To calculate the molecule concentrations for computing the oxygen saturation, the Beer-Lambert law for light absorption in a medium can be used [35]. It puts the light intensity after passing a medium (I ) in relation to the incident light intensity (I0 ) as shown in equation IV. The attenuation then depends on the absorptivity α(λ) of the medium, its concentration c and the length of the optical path l . If the red and infrared light is being shone onto blood, the absorption can be determined with this principle and will mainly depend on the concentration of oxyhaemoglobin and deoxyhemoglobin. These two unknown concentrations cHb and cHbO2 can be directly determined by solving the system of two equations (one for each wavelength), assuming that the path length is known too.