Reflective Photoplethysmography (PPG) vs. Transmittance Oximetry
Clinical fingertip pulse oximeters transmit light through translucent tissue (transmittance). In contrast, a Blood Oxygen SpO2 Fitness Tracker wears on the dorsal wrist, utilizing reflectance PPG. High-efficiency light-emitting diodes (LEDs) shine light into blood vessels beneath skin layers, and photodiode arrays measure the scattered light reflected back.
By leveraging differential optical absorption at specific light spectrums, wrist trackers calculate peripheral capillary oxygen saturation (SpO2) non-invasively during rest and exercise.
Optical reflection ratio: R = (AC660 / DC660) / (AC940 / DC940)