Pulse Oximetry: The use of pulse oximetry to measure arterial oxygen saturation in anesthesiology is standard of care. Pulse oximetry relies on the Beer-Lambert law, which relates the concentration of a chromophore to light absorption, optical path length, and extinction coeffi cient. In pulse oximetry, the chromophores of interest are oxy and deoxyhemoglobin. Because there are two chromophores, two wavelengths in which the extinction coeffi cient of oxy and deoxyhemoglobin differ are required to determine