Smartwatches measure heart rate by using optical sensors (photoplethysmography or PPG) that shine light, typically green LEDs, onto your skin and then measure the amount of light reflected back. This method detects changes in blood flow to determine your heart rate.
The Science Behind It: Photoplethysmography (PPG)
PPG is a non-invasive optical technique used to detect volumetric changes in blood in peripheral circulation. Here's a breakdown of the process:
- Light Emission: The smartwatch emits light, commonly green, onto the skin. Green light is highly absorbed by hemoglobin in blood, making it effective for this purpose.
- Light Absorption and Reflection: When your heart beats, more blood flows through your wrist. This increased blood flow absorbs more of the green light. Between beats, less blood flows, and more light is reflected.
- Sensor Detection: A photodetector in the smartwatch measures the intensity of the reflected light.
- Signal Processing: The smartwatch's processor analyzes the changes in light intensity to calculate your heart rate (beats per minute or BPM). The periodic changes in light absorption correspond to the pulsating flow of blood caused by each heartbeat.
- Filtering Noise: Smartwatches use algorithms to filter out noise caused by movement, ensuring a more accurate reading.
Why Green Light?
While other colors can be used, green light is often preferred because hemoglobin, the protein in red blood cells that carries oxygen, absorbs green light more effectively than other colors, like red or infrared. This higher absorption rate allows the sensor to more accurately detect the changes in blood flow.
Factors Affecting Accuracy
Several factors can affect the accuracy of heart rate measurements on a smartwatch:
- Fit: A snug fit is essential. If the watch is too loose, light can leak in, and the sensor may not accurately detect blood flow changes.
- Skin Tone: Very dark skin tones may absorb more light, which can sometimes affect accuracy. However, modern smartwatches are designed to compensate for variations in skin tone.
- Movement: Excessive movement can introduce noise into the signal, making it harder to accurately measure heart rate. This is why smartwatches often require you to be relatively still during measurements, especially during activities.
- Dryness: Dry skin improves conductivity. If your skin is very dry, it could interfere with the sensor readings.
In Summary
Smartwatches utilize PPG technology to measure heart rate by detecting changes in blood flow through the use of light-emitting diodes (LEDs) and photodetectors. The reflected light is then processed by the device's algorithm to determine the number of heart beats per minute (BPM).
