A smartwatch can estimate heart rate by shining green light into your wrist and measuring the light that comes back. This method is called photoplethysmography, or PPG. It matters because it lets a small wearable device track pulse during rest, exercise, sleep, and recovery without using a chest strap or medical wires.
The same idea connects biology, optics, electronics, and data processing in one tiny sensor system.
Inside the watch, green LEDs send light into the skin while a photodiode measures reflected light. Blood absorbs more green light than the surrounding tissue, so the reflected light changes slightly as blood volume rises and falls with each heartbeat. The watch turns these repeating changes into a waveform, then counts the peaks to estimate beats per minute.
Good contact, proper wrist position, and reduced motion help the sensor collect a cleaner signal.
Key Facts
- PPG stands for photoplethysmography, a method that measures changes in blood volume using light.
- Green light is often used because hemoglobin in blood absorbs green wavelengths strongly.
- Heart rate in beats per minute can be found with BPM = 60 / T, where T is the time in seconds between pulse peaks.
- If N beats are counted in t seconds, BPM = 60N / t.
- A photodiode converts reflected light intensity into an electrical signal.
- Motion, loose fit, cold skin, tattoos, and poor sensor contact can make the heart rate reading less accurate.
Vocabulary
- Photoplethysmography
- A light-based method for measuring changes in blood volume under the skin.
- LED
- A light-emitting diode that produces light when electric current flows through it.
- Photodiode
- An electronic sensor that converts incoming light into an electrical signal.
- Capillary
- A tiny blood vessel where oxygen, nutrients, and waste are exchanged with body tissues.
- Pulse waveform
- A repeating graph pattern caused by changes in blood volume during each heartbeat.
Common Mistakes to Avoid
- Thinking the watch sees the heart directly, which is wrong because it measures changing blood volume in the wrist using reflected light.
- Assuming brighter light always means a better reading, which is wrong because too much light, poor contact, or signal saturation can reduce accuracy.
- Counting every small wiggle as a heartbeat, which is wrong because motion and noise can create false peaks that do not come from pulse changes.
- Wearing the watch loosely on the wrist, which is wrong because gaps let outside light in and allow the sensor to move during measurement.
Practice Questions
- 1 A smartwatch detects 12 pulse peaks in 10 seconds. Use BPM = 60N / t to calculate the heart rate in beats per minute.
- 2 The time between two neighboring pulse peaks is 0.80 seconds. Use BPM = 60 / T to calculate the heart rate.
- 3 A student gets an unstable heart rate reading while running with the watch loose on the wrist. Explain two engineering or body-related reasons the signal may be noisy and one way to improve the measurement.