An optical heart rate sensor counts your pulse by shining light into your skin and watching how much comes back. It sits on the back of most smartwatches and fitness bands. It needs no chest strap and no wires.
This guide explains the method in plain words. It also covers what can throw a reading off, and how the sensor differs from an ECG.
How an optical heart rate sensor works
The method has a long name: photoplethysmography, or PPG. It is a simple, low-cost way to detect changes in blood volume in the tiny vessels just under the skin. Doctors have used it for decades. Fingertip pulse clips in clinics work the same way.
A light source shines on the skin. A light-sensitive part, called a photodiode, measures how much light returns. Blood absorbs light. When your heart beats, more blood fills the vessels and less light comes back. Between beats, there is less blood and more light returns.
The signal rises and falls with each heartbeat. The device counts those peaks over time and reports beats per minute.
Sources: Allen J, Photoplethysmography and its application in clinical physiological measurement, Physiological Measurement, 2007 (PubMed 17322588).
Why many wearables use green light
Apple describes its sensor this way: it pairs green LED lights with photodiodes to detect how much blood flows through the wrist at any moment. The LEDs flash hundreds of times per second. The company says blood absorbs green light, so the green signal changes clearly with each beat.
The same Apple page says the sensor can also use infrared light. That mode is used for background readings and for heart rate notifications. Apple lists a measuring range of 30 to 210 beats per minute for its sensor.
Other brands use similar parts. The idea is the same: light in, light out, count the pulses.
Sources: Apple Support, Monitor your heart rate with Apple Watch (published September 21, 2023).
What can disturb the reading
Optical sensors are handy, but they are not perfect. Apple's own support pages name several factors that can affect the sensor.
Skin perfusion is one. This means how much blood flows through your skin. It varies a lot from person to person. Cold weather can lower it. Apple notes that exercising in the cold may leave too little blood flow at the wrist for a reading.
Skin changes matter too. Apple says the ink, pattern and saturation of some tattoos can block light from the sensor. That makes reliable readings harder.
Motion is a third factor. Apple says rhythmic movement, such as running or cycling, gives better results than irregular movement, such as tennis or boxing.
Fit also counts. For workouts, Apple advises a snug fit on top of the wrist, with the sensor close to the skin. If a wrist reading will not settle, Apple suggests pairing a Bluetooth chest strap.
Sources: Apple Support, Get the most accurate measurements using your Apple Watch; Apple Support, Monitor your heart rate with Apple Watch.
How PPG differs from ECG
An ECG, or electrocardiogram, measures something else. It records the tiny electrical signals that make the heart beat. A PPG sensor does not see those signals. It only sees the blood moving in the skin as a result.
That is why the two are not interchangeable. Apple Watch Series 4 and later add electrodes in the Digital Crown and on the back case for ECG. The optical sensor and the electrical sensor are separate parts that do different jobs.
The US Food and Drug Administration reviewed the Apple ECG app through its De Novo pathway. The decision is dated August 14, 2018. The FDA document says the app creates a single-channel ECG similar to a Lead I ECG and checks it for signs of atrial fibrillation or normal sinus rhythm. It says the output is for informational use. It is not meant to replace usual methods of diagnosis or treatment. The document also says the app is not meant for people under 22.
The review covers the ECG app. It does not cover the optical sensor's pulse counting.
Sources: FDA, De Novo Classification Request for ECG App, DEN180044 (decision dated August 14, 2018); Apple Support, Monitor your heart rate with Apple Watch.
How to get a better reading
A few habits help. Apple describes the right fit as not too tight and not too loose, and says to wear the watch on top of the wrist.
For hard workouts, a chest strap is still the steadier choice. Apple's own page points to one when the wrist sensor struggles.
Treat the number as an estimate, not a verdict. This guide is general information. It is not medical advice. If a reading worries you, ask a licensed clinician.
Sources: Apple Support, Get the most accurate measurements using your Apple Watch (fit and chest strap guidance); Apple Support, Monitor your heart rate with Apple Watch.
Sources
- Allen J. Photoplethysmography and its application in clinical physiological measurement. Physiological Measurement, 2007. PubMed 17322588.
- Apple Support. Monitor your heart rate with Apple Watch. Published September 21, 2023.
- Apple Support. Get the most accurate measurements using your Apple Watch.
- US Food and Drug Administration. De Novo Classification Request for ECG App, DEN180044. Decision dated August 14, 2018.