For decades, pulse oximeters have been one of medicine’s most familiar instruments. Clip one onto a finger, and within seconds it reports how much oxygen is in your blood, a vital indication of health.

The technology is simple in concept: red and infrared light are passed through or reflected from tissue, oxygen-rich and oxygen-poor hemoglobin absorb those colors differently, and software turns the captured light into a measure of a patient’s health.

But light does not pass through or reflect from each person in the same way. Variations in skin pigmentation, blood flow, and age can alter the faint optical signal that pulse oximeters and other wearable monitors using light depend on. That matters because a small error in oxygen saturation can become a significant clinical problem, especially when doctors are deciding whether a patient is sick enough to need oxygen, hospitalization, or intensive care.