How an Optical Heart Rate Sensor Reads a Pulse

The green light visible on the back of most fitness watches and bands is part of an optical heart rate sensor, and the mechanism behind it has nothing to do with electricity moving through the body the way a chest-strap monitor works.

This covers how the sensor uses light to detect blood volume changes, what the light color choice is for, where this method differs from an electrical chest strap, and what actually limits its accuracy under certain conditions.

How Light Detects a Pulse

An optical sensor, using a method called photoplethysmography, shines light-emitting diodes into the skin and measures how much of that light is reflected back to a photodetector. Blood absorbs more light than the surrounding tissue, so as blood volume in the capillaries beneath the skin rises and falls with each heartbeat, the amount of reflected light rises and falls in a matching pattern. The device's processor detects that repeating pattern and converts it into a heart rate reading.

Most consumer devices use green LEDs specifically because blood absorbs green light more strongly than red or infrared light, which produces a clearer signal for this kind of measurement at the wrist. Some newer devices add additional wavelengths, including infrared, to improve signal reliability under difficult conditions like darker skin tones or looser device fit.

The photodetector itself is typically paired with multiple LEDs arranged around it, since combining signals from more than one light source and angle helps the processor filter out noise that a single LED and detector pair would be more susceptible to.

Sampling frequency for the optical sensor — how often it takes a light-reflection reading per second — also affects how quickly the device can detect a rapid change in heart rate, since a lower sampling frequency inherently smooths over faster fluctuations that a higher one would capture.

What Affects Signal Quality

Skin tone, hair, tattoos, motion, and how snugly the device sits against the skin can all affect how much light reaches the photodetector and how clean the resulting signal is. Devices compensate with algorithms that filter out irregular readings, but the underlying measurement is still dependent on consistent light contact with skin.

Ambient light leaking in around the edge of the sensor housing can also interfere with the signal, which is part of why most sensor housings are designed to seal tightly against the skin rather than sit loosely above it. Placement on the wrist relative to the wrist bone also affects signal quality, since the tissue and blood vessel density varies across that small area of the arm.

Where Optical Sensors Diverge From Chest Straps

A chest-strap monitor works on an entirely different principle — it detects the small electrical signal generated by the heart itself, similar in concept to a simplified electrocardiogram, rather than measuring blood volume optically. That electrical method is generally less affected by motion and skin tone than an optical wrist sensor, which is why chest straps are often used as the reference standard when testing a wrist-based device's accuracy.

Optical readings during high-motion activity are a known limitation, since rapid arm movement can introduce light interference that resembles a pulse signal but is not one. Manufacturers address this partly through motion-compensation algorithms that cross-reference the accelerometer's own motion data to filter out movement-related noise from the optical signal.

Cold conditions can also reduce blood flow near the skin's surface, which weakens the optical signal an LED-and-photodetector pair has to work with, independent of anything related to motion or skin tone.

What Accuracy Studies Actually Measure

Published accuracy comparisons typically measure a wrist-based optical sensor's readings against a chest-strap or clinical ECG reference under specific conditions — a figure that applies to that activity type and device generation, not a universal accuracy rating that holds across every use case. A study conducted during steady-state walking, for example, does not necessarily describe accuracy during interval training or cycling. Water-resistance ratings, listed separately from accuracy figures, describe housing sealing rather than anything about the optical sensor's own measurement performance.

Optical and electrical heart rate sensors solve the same measurement problem through entirely different physical mechanisms, which is part of why the two methods do not always agree in the same moment, and why understanding both is more useful than assuming either one is simply more accurate in every situation.

Note: This explains how fitness equipment and sensors work. It is not a workout program, it is not personal training, and it is not a substitute for a trainer or physician. Check the cited sources for current guidance.