Apple Watch PPG Sensor: Technical Deep-Dive
Apple Watch uses reflective optical sensing with green LEDs and photodiodes for heart rate. Models that support Blood Oxygen also use red and infrared light for wellness SpO2 estimates; Apple does not publish a universal photodiode count or exact LED wavelengths for every generation.
Apple describes the optical heart sensor as green LEDs paired with light-sensitive photodiodes. It can also use infrared light for background heart-rate readings. Models that support Blood Oxygen add red and infrared illumination, but layouts vary by generation and Apple does not publish the exact wavelength and detector specifications asserted in many third-party summaries.
Apple Watch measures heart rate continuously during workouts and for three minutes afterward. In the background it measures throughout the day when the wearer is still and periodically while walking; Apple says the interval varies with activity rather than promising a fixed five-minute schedule. Irregular Rhythm Notifications periodically evaluate pulse timing for patterns suggestive of AF and advise users to seek clinical evaluation; they are not continuous monitoring or a diagnosis.
Blood Oxygen measurements use red and infrared light and require the wearer to remain still during a 15-second reading. Apple markets this feature for general fitness and wellness, not medical use. It has not been cleared as a medical pulse oximeter, so claims that it meets an FDA pulse-oximeter standard are inappropriate; ISO 81060-2 is a blood-pressure standard, not an SpO2 standard.
Frequently Asked Questions
What PPG wavelengths does Apple Watch use?
Apple documents green light for optical heart-rate measurements and red plus infrared light for Blood Oxygen measurements on supported models. It does not publish exact nominal wavelengths for every generation.
How accurate is Apple Watch heart rate?
Apple Watch heart rate achieves ±5% or 5 bpm (whichever is greater) during exercise per Apple's specifications. Independent studies show mean absolute error of 2–4 bpm at rest and 3–8 bpm during moderate exercise, comparable to clinical-grade chest strap monitors.
Is Apple Watch SpO2 FDA cleared?
The Blood Oxygen app is available as a wellness feature but is NOT FDA cleared as a medical pulse oximeter. The Heart Rate sensor functions as an FDA-cleared medical device for irregular rhythm notification, but SpO2 did not receive independent clearance as of 2024.