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Pulse Oximetry Arrives

In 1972 Takuo Aoyagi realized that the size of pulsing signals which had previously been a nuisance were related to oxygen saturation. He also realised that because these signals were pulsing at heart rate they must be originating from the arteries rather than being of venous origin. He measured the size of the pulsatile signals using two wavelengths, red and infra red. He normalised the red and infra red signals to allow for variations in led intensity and variations in photodiode sensitivity. He used the normalised value of the size of the red pulsatile signal divided by the normalised value of the size of the infra red pulsatile signal to calculate oxygen saturation of the blood. This technique is often referred to in older text books as the ratio of ratio technique.