The first pulse oximeter was developed by Millikan in the 1940s. It monitors the ratio of oxygen-carrying hemoglobin to oxygen-depleting hemoglobin in the arteries. A typical pulse oximeter has two light-emitting diodes (LEDs). These LEDs are pointed towards the area of the patient being tested-usually the fingertip or earlobe. One diode emits a beam of light at a wavelength of 660 nanometers, and the other emits at 905, 910, or 940 nanometers. Oxygen-carrying hemoglobin absorbs these two wavelengths very differently from oxygen-depleting hemoglobin. Using this property, the ratio of the two types of hemoglobin can be calculated. The test usually does not require drawing blood from the patient.
A typical pulse oximeter can also display the patient's pulse. According to Beer-Lambert's law, the ratio R/IR should have a linear relationship with arterial oxygen saturation (SaO2). However, because biological tissues are complex optical systems with strong scattering, weak absorption, and anisotropy, they do not fully conform to the classical Beer-Lambert law. This makes it difficult to establish a mathematical model expressing the relationship between the relative change in absorbance of red and infrared light (R/IR value) and arterial oxygen saturation (SaO2). The correspondence between R/IR and SaO2 can only be determined experimentally, i.e., through calibration curves. Most pulse oximeter manufacturers obtain empirical calibration curves through experimental methods to complete pre-calibration before product shipment.
