How do heart rate monitors work?

Just by light!

Published in

Shyam Cortex

3 min readOct 6, 2020

The heart rate sensor measures your heart rate in Beats per Minute(BPM) using an optical LED light source and an LED light sensor.

**Vital sign monitor** (Photo by Jair Lázaro on Unsplash)

The light shines through your skin, and the sensor measures the amount of light that reflects back. The light reflections will vary as blood pulses under your skin past the light. The variations in the light reflections are interpreted as heartbeats.

**Apple Watch** Photo by Fabian Albert on Unsplash

Optical heart-rate monitors are commonly found in smartwatches/wearables. Most of them obtain heart-rate data through “photoplethysmography” (PPG)**, or the process of using light to measure blood flow.

Wearables with optical heart-rate monitors have small LEDs on their undersides that shine green light onto the skin on your wrist.
The different wavelengths of light from these optical emitters interact differently with the blood flowing through your wrist.
When that light refracts (or reflects) off your flowing blood, another sensor in the wearable captures that information.
That data can then be processed, along with motion information detected by the device’s accelerometer, with algorithms to produce understandable pulse readings.

**Photoplethysmography (PPG) is a simple optical technique used to detect volumetric changes in blood. It is a low cost and non-invasive method that makes measurements at the surface of the skin.

So the accuracy of this depends on,

The algorithm gets better over time. So comparing to old and new smartwatches, the new ones have gotten better and more accurate.
Wristbands with optical monitors should be worn just above the wrist bone, or about two-fingers width from where your hand meets your wrist. If the monitor moves too low or too high on your arm, you can easily get an inaccurate reading.

We can also get an ECG report on a smartwatch!