"Pulse oximetry is a noninvasive method for monitoring blood oxygen saturation.
Peripheral oxygen saturation readings are typically within 2% accuracy of the more accurate reading of arterial oxygen saturation from arterial blood gas analysis. A standard pulse oximeter passes two wavelengths of light through tissue to a photodetector.
Taking advantage of the pulsate flow of arterial blood, it measures the change in absorbance over the course of a cardiac cycle, allowing it to determine the absorbance due to arterial blood alone, excluding unchanging absorbance due to venous blood, skin, bone, muscle, fat, and, in many cases, nail polish.
The two wavelengths measure the quantities of bound and unbound hemoglobin, and from their ratio, the percentage of bound hemoglobin is computed. The most common approach is transmissive pulse oximetry".