The underlying principle of pulse oximeters is the Beer-Lambert law which stipulates that light intensity diminishes exponentially when traveling in an absorbing medium. Oxygenated (O2Hb) and de-oxygenated (Hb) hemoglobin have different extinction coefficients for different wavelengths and are the most important light absorbers in blood. In some wavelength ranges, Hb is more absorbent than O2Hb, and in others the absorption property is inverted. These regions are separated by isosbestic points. With each pulse, the volume of blood in the vessels and the optical path length increase, and therefore, the total absorbance also increases. As a result, the variation of volume can be registered as a variation of transmitted light intensity and recorded with a photodetector. This produces the PPG waveform. The difference in total absorbance at two distinct wavelengths allows the calculation of a modulation ratio R. The theoretical relationship between R and SaO2 can be described as: