Smartwatches monitor heart rate primarily by using optical sensors to detect changes in blood flow near the wrist. These sensors illuminate the skin with LEDs, often green, and measure the light reflected back to the sensor.
The Process Explained:
- Illumination: The smartwatch shines a light, typically green LED light, onto the skin of your wrist.
- Light Absorption: Green light is highly absorbed by hemoglobin in red blood cells. This absorption varies with changes in blood flow.
- Detection: The watch contains a photodiode, a light-sensitive sensor, that measures the amount of light reflected back.
- Analysis: As your heart beats, more blood flows through your wrist. This increased blood flow absorbs more green light, resulting in less reflected light reaching the sensor. The sensor detects these changes in light reflection.
- Calculation: The smartwatch's algorithm analyzes these fluctuations in light reflection to determine the heart rate in beats per minute (BPM). It identifies the peaks in blood flow associated with each heartbeat.
Why Green Light?
Green light is often chosen because it is strongly absorbed by hemoglobin, the protein in red blood cells that carries oxygen. This strong absorption makes green light particularly effective at detecting changes in blood flow. Other colors are also used in some devices, sometimes in combination with green.
Factors Affecting Accuracy:
Several factors can impact the accuracy of heart rate monitoring:
- Fit: A snug fit ensures consistent contact between the sensor and the skin. A loose fit can allow ambient light to interfere or cause the sensor to lose contact with the skin.
- Skin Tone: Darker skin tones may absorb more light, potentially affecting the accuracy of the readings. Manufacturers often use complex algorithms to compensate for these variations.
- Movement: Excessive movement can introduce noise into the signal, making it difficult to accurately detect heart rate.
- Sensor Type: While most smartwatches use optical sensors (PPG), some also incorporate electrocardiogram (ECG or EKG) sensors for medical-grade heart rate monitoring. ECG/EKG sensors measure the electrical activity of the heart.
ECG (EKG) vs. Optical Sensors:
| Feature | Optical (PPG) Sensors | ECG/EKG Sensors |
|---|---|---|
| Measurement | Blood flow | Electrical activity |
| Accuracy | Generally less accurate | More accurate |
| Complexity | Simpler | More complex |
| Use Cases | General fitness tracking | Medical monitoring |
| Availability | Widely used | Less common; may require specific placement on the body |
