How a Smartwatch Measures Your Heart Rate With Light

A smartwatch finds your pulse by shining light into the skin of your wrist and measuring how much comes back. Each heartbeat briefly changes the amount of blood under the sensor, and blood absorbs light, so the returning light flickers in time with your pulse. The method is called photoplethysmography, or PPG. This article explains how it works and why it is less reliable in some situations.

The basic idea

Wikipedia describes a photoplethysmogram as an optical measurement of blood volume changes in tissue. It is usually made by lighting the skin with a light-emitting diode (LED) and using a photodiode, a light-sensitive component, to measure the light reflected from or transmitted through the tissue. Each heartbeat sends a pressure pulse that briefly widens the small arteries just under the skin, which changes how much light is absorbed. Each cardiac cycle shows up as a peak in the signal.

A watch sits on the back of the wrist, so it uses the reflected version: LEDs and photodiodes face the skin, and light goes in and part of it bounces back.

Two parts of the signal

Wikipedia explains that the signal has a steady component and a pulsing one. The steady (DC) level comes mostly from the bulk absorption of skin and tissue. The pulsing (AC) part is the change in blood volume with each beat, and its height is related to pulse pressure. The watch's software separates the small pulsing part from the large steady part, then counts the peaks per minute.

Why green light

Macworld, summarising Apple's own support documentation for the first Apple Watch, says blood absorbs green light, so measuring the shifts in green light absorption reveals heart rate. During workouts, it says, the watch flashes green light onto the wrist hundreds of times per second and uses photodiodes to measure how much is absorbed.

The same article says that when you are not exercising, that watch takes a reading about every 10 minutes using infrared LEDs, with green LEDs as a fallback if the infrared reading is not good enough. It adds that studies favour green LEDs for accuracy and that Apple did not explain why it does not use them all the time; the author suggests battery saving. This describes one maker's early design, and other brands and newer models may differ.

What can go wrong

Wikipedia says motion artefacts are often a limiting factor preventing accurate readings during exercise and everyday life. When the watch moves against the skin, the light path changes, and that change can look like a pulse. Macworld reports that Apple recommended a snug but comfortable strap so the sensors stay in place, and that cold weather and irregular movements such as tennis or boxing can make readings difficult.

• Poor blood flow in the skin: Wikipedia says low peripheral blood flow can give a signal with no clear pulse.
• Body differences: the waveform's shape differs between people and with where and how the sensor is attached. Some product guides also say that dark tattoos, body hair and skin tone can affect how much light is captured; check a maker's own documentation for its claims.
• Loose fit: a gap between the sensor and skin lets in stray light.

Raising the signal

Several sources note that sensors adjust to conditions. A watch may brighten its LEDs or sample more often when the signal is weak, which uses more battery. Software filtering and motion data from the watch's accelerometer can help separate arm movement from the pulse, although this is a maker-specific feature that this article does not describe in detail.

What the number means

A PPG heart-rate figure comes from light, not from the heart's electrical signal. It is not the same measurement as a hospital electrocardiogram. A watch's readings are general information and are not a medical diagnosis. Anyone worried about their heart should speak to a doctor rather than relying on a wearable.

Sources

• Photoplethysmogram, Wikipedia
• How the Apple Watch heart rate sensor works, according to Apple, Macworld