What is an optical HR sensor?
Optical heart-rate sensors are the most common pulse sensors in wearables. Most of them glean heart-rate data through "photoplethysmography" (PPG) or the process of using light to measure blood flow.
Wearables with optical heart-rate monitors have small LEDs underneath that shine light onto the skin on your wrist. The light interacts differently with the blood flowing through your wrist depending on the wavelength. When the light is reflected off your blood back to the watch, another sensor in the wearable picks up the information. The data is then processed to produce accurate pulse readings.
PPG sensors use two primary components to measure your heart rate:
- Optical emitters - most wearables have at least 2 LED's that send light waves into the skin. These light wavelengths will interact differently with different levels of skin, tissue, and ultimately, your blood
- Photodetector(s) - the photodetector captures the light refracted from the user of the device and translates it into information that can be processed by the device
When it comes to data accuracy, OHRM wearables face a few challenges:
- Optical noise - Motion can produce a lot of noise that can disrupt the sensors. When a light is shined on skin, only a small fraction of the light returns to the sensor, and of the total light collected, only approx. 1/100th of it may indicate blood flow. The rest are scattered signals due to other material light skin, muscle, tendons, etc
- Skin tone / colouring - PPG optical sensors use green light because our bodies and blood are good at absorbing it, and it's less affected by movement. However, it can have problems with dark tattoos, sweat, and even arm hair
- Sensor location - The wrist is not the best place for accurate PPG monitoring of heart rate because of the much higher optical noise created in that region (muscle, tendon, bone, etc) and because of high variability in vascular structure and blood perfusion across people
- Low perfusion - Perfusion in the process of a body delivering blood to capillary beds. Low perfusion that occurs closer to the body's extremities can cause a lower signal-to-noise ratio making blood flow signals more difficult to interpret as accurate recordings of heart rate
Why is it important?
Alongside the accelerometer, the PPG sensor is arguably one of the most important aspects of wearables technology given the broad range of biometric data that can be derived from the readings. Here is a list of some of the data that can be taken:
- Breathing rate
- VO2 max
- SPO2 / oxygen saturation
- R-R interval
- Blood pressure
- Cardiac efficiency
Through the Terra API, we provide connections to a range of different devices that contain optical HR sensors.
Below is a list of the wearables that have optical HR sensors today:
- Garmin Fenix series, Venu series, Lily, Approach series, Vivoactive, Forerunner series, Instinct series, Enduro, Descent series, Mk2 series, Epic, Tactix series, D2 series, Vivomove series, Vivofit, Marq series, Quatix series
- Fitbit Versa series, Charge series, Inspire 2, Sense, Luxe, Ace 3
- Apple Watch all series
- Polar Grit X pro, Grit X, Ignite series, M430, Unite, Vantage series
- Withings ScanWatch series, Steel HR, Move ECG
- Oura ring Gen 3, Gen 2
- Samsung Galaxy Watch all series, Galaxy Fit2
- Suunto 9 range, 7 range, 5 range, 5 peak, 3 range, Traverse
- Wahoo Elemnt Rival Multisport GPS watch
- Xiaomi Mi Watch
