Pulse oximeters work by shining red (660 nm) and near-infrared (940 nm) LEDs through a finger and measuring how much light is absorbed by a photodiode on the other side. Oxyhemoglobin and deoxyhemoglobin absorb these wavelengths differently, allowing the device to calculate blood oxygen saturation (SpO₂). By isolating the pulsatile component of the signal — the small variation caused by arterial blood with each heartbeat — the device filters out static tissue, bone, and venous blood, leaving only the arterial signal. The ratio of red to infrared absorption is converted to an SpO₂ reading via a lookup table based on the Beer-Lambert law. The technology has limitations: it can't distinguish carboxyhemoglobin from oxyhemoglobin, is affected by motion, nail polish, and skin tone, but has nonetheless revolutionized patient monitoring in emergency care and surgery. The invention traces back to Takuo Aoyagi at Nihon Kohden in 1972.
