TWIL #056 - Your Smartwatch Measures Your Heart Rate Using Light
The green LEDs on the back of a smartwatch are not decorative. They are a miniature optical sensor that detects blood volume changes in your wrist 100+ times per second.
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How the heart rate sensor works:
The technique is called photoplethysmography (PPG). The green LEDs on the underside of your smartwatch shine light into your wrist. Blood absorbs green light strongly. As your heart beats, a pulse of blood moves through the capillaries under the skin - a small but measurable change in blood volume. More blood means more green light absorbed; less blood means more reflected. A photodetector on the watch measures the intensity of reflected light many times per second, and the resulting waveform directly corresponds to your pulse.
Green LEDs are chosen over other colours because haemoglobin absorbs green light particularly well. The main downside: green is also absorbed by skin pigment (melanin), which is one reason accuracy can vary across different skin tones - an issue that has been extensively criticised and studied in consumer wearables research.
ECG (electrocardiogram): Some smartwatches (Apple Watch Series 4+, certain Samsung and Withings models) can record a single-lead ECG. This works differently from PPG - it measures electrical signals generated by the heart. Two electrodes are required: one on the watch back touching the wrist, one on the Digital Crown (or a dedicated button). When the user places a finger on the crown, the electrical circuit closes across the body, creating a Lead I configuration equivalent to a clinical ECG lead. This can detect atrial fibrillation (AFib) with meaningful clinical sensitivity.
Blood oxygen (SpO2): Uses red and infrared LEDs (not green). Oxygenated haemoglobin absorbs infrared light but not red; deoxygenated haemoglobin absorbs red light but not infrared. By comparing the ratio of reflected red to infrared light, the sensor estimates oxygen saturation in the blood.
Step counting: A three-axis accelerometer measures acceleration in all directions. Algorithms detect the characteristic rhythmic acceleration pattern of walking or running and count steps. Gyroscopes assist with orientation context.
The accuracy gap: Consumer wearable PPG sensors are accurate within a few beats per minute under normal conditions but degrade with movement (motion artefact), low perfusion (cold wrists), tattoos, or very dark skin tones. Clinical-grade pulse oximetry uses medical sensors on fingertips with controlled light wavelengths - a different class of device.