How Your Watch Measures Blood Oxygen With TWO Colors of Light

Your watch can guess how much oxygen is in your blood using nothing but colored light.

How does shining light on your skin reveal something happening deep inside your bloodstream?

Oxygen rides through your blood clipped onto a protein called hemoglobin. Here is the key: blood full of oxygen lets more red light pass but absorbs more infrared. Blood low on oxygen does the opposite. So your watch fires two beams, one red near 660 nanometers, one infrared near 940, and a sensor measures how much of each returns as your blood pulses through. Compare the two ratios, and the math reveals what fraction of your hemoglobin is carrying oxygen. That number is your blood oxygen level, shown as a percentage.

The reading only works because your arteries pulse. The watch ignores the steady light blocked by skin and bone, and measures only the tiny flicker each heartbeat adds. That pulsing signal is pure arterial blood, exactly what it needs.

Follow The Explainer Vault — because everything has an explanation.