Pulse Oximeters Measure Blood Oxygen With Light

Why this is here: The device relies on two specific wavelengths of light—660 nm (red) and 940 nm (near-infrared)—because these easily penetrate tissue and highlight the differing light absorption properties of oxygenated and deoxygenated hemoglobin.

Takuo Aoyagi, an electrical engineer at Nihon Kohden in Tokyo, developed pulse oximetry in 1972 while researching non-invasive cardiac output measurements. The device uses LEDs to shine red and infrared light through a finger—or ear—and a photodiode to measure how much light passes through. By comparing the absorption of these wavelengths, the device determines the percentage of oxygen in arterial blood, also displaying pulse rate.

Aoyagi initially saw the patient’s pulse as interference, but realized the signal could be used to isolate arterial blood absorption from other tissues. Oxyhemoglobin and deoxyhemoglobin absorb light differently, creating a measurable distinction. The device alternates the LEDs, subtracts ambient light, and uses a lookup table based on the Beer-Lambert law to display oxygen saturation as “SpO₂.”

While pulse oximetry is broadly useful, it isn’t foolproof. Movement, nail polish, skin tone, and limited bloodflow can affect accuracy.

Critically, the device cannot differentiate between oxygenated hemoglobin and carboxyhemoglobin—present in carbon monoxide poisoning—potentially giving a falsely high reading. Despite these limitations, the technology remains essential for monitoring patients and is now even available in consumer smartwatches.