How can a fingertip clip estimate blood oxygen?

A fingertip clip can estimate how much of the hemoglobin in arterial blood carries oxygen by measuring light. Hemoglobin carrying oxygen and hemoglobin without it absorb red and infrared light differently. A pulse oximeter compares the two wavelengths rather than counting oxygen molecules or taking a blood sample.

Much of the light is affected by tissue and blood that contribute a relatively steady background. With each heartbeat, the arterial blood volume at the measurement site changes slightly. This produces a small pulse-related change in the detected light. Separating that changing component from the background helps the instrument focus on arterial blood.

The electronics compare the pulse amplitude with the background level separately for each wavelength, then compare those normalized values. The red and infrared LEDs operate in separate time slots so the detector can distinguish their contributions. An unlit interval can help measure ambient light that did not come from either LED.

In the invented example, the red pulse amplitude is 1 against a background of 100, giving 0.01. The infrared amplitude is 2 against 100, giving 0.02. Their ratio is 0.01 ÷ 0.02 = 0.5. This dimensionless ratio is not an oxygen-saturation percentage. A real instrument uses an empirically calibrated relationship to turn the optical ratio into an estimate.

The result depends on both the light measurement and that calibration. Motion, poor perfusion, skin pigmentation and other factors can affect accuracy. The mechanism illustrates how a changing biological signal can be separated from a larger background, while also showing why a calculated optical quantity and a reliable physiological estimate are different things.