Pulse oximetry is the technique of measuring arterial blood oxygenation through identification of pulsatile blood. The fundamental physiological principle that the technique uses is spectrophotometry, or the measurement of the absorption of light by blood and tissues. One simple way to understand the basic principle is to imagine a test tube filled with some substance and shining light of a certain wavelength upon it (incident light). Some of the light is transmitted (“transient” light) through the substance and some is absorbed. The amount of light transmitted depends upon the absorptive (and reflective) nature of the particular substance. The Beer-Lambert law relates this mathematically:

C = (1/d × α) log_e (I_i/I_t)

where C is the concentration of the particular substance in the test tube, d is the path length of the light through the test tube, α is an absorption coefficient for the substance in question at a particular wavelength of light, I_i is the intensity of the incident light, and I_t is the intensity of the transmitted light. When this principle is applied to blood oxygenation, then the question becomes one of determining the absorption of light by hemoglobin.

In adults, there are four forms of hemoglobin, oxyhemoglobin (HbO₂), methemoglobin (metHb), carboxyhemoglobin (HbCO), and “reduced” hemoglobin (Hb). Each form of hemoglobin absorbs a different amount of light at a given wavelength. Using four different wavelengths of light, the Beer-Lambert law can be solved for each of the four forms of hemoglobin to give a concentration in a sample of blood. With these concentrations, the fractional hemoglobin saturation (percentage of oxyhemoglobin in blood) can be calculated as follows: