Pulse oximetry is a diagnostic method that measures the
proportion of oxygen-carrying molecules (hemoglobin) that
are actually carrying oxygen in the arterial blood, known as
oxygen saturation or SpO2. A conventional pulse oximeter
sensor shines two beams of light of different wavelengths
(red 660nm and infrared 910nm) through the blood that is
circulating in the small blood vessels of the finger or ear, and
then detects the amount of light that is able to pass through
the extremity. Hemoglobin carrying oxygen absorbs more
infrared light and allows more red light to pass than
hemoglobin without oxygen, which allows more infrared
light to pass. Oxygen saturation can be determined from the
absorption ratio.