Optical Heart Rate Sensors: LEDs, Photodiodes, Wavelengths

How optical heart rate sensors work

Optical heart rate sensors, sometimes called photoplethysmography (PPG) sensors, use light to estimate blood flow. The sensor includes at least one light-emitting diode (LED) and a photodiode. The LED shines light through the skin, and the photodiode measures how much light reflects back. Each heartbeat changes the volume of blood in your capillaries, which changes the amount of absorbed light. The sensor converts those variations into a pulse reading. This is the same approach used for blood oxygen measurements, which we cover in What Is Blood Oxygen Monitoring on a Smartwatch.

Different wavelengths serve different purposes. Green light is commonly used for heart rate because blood absorbs it strongly, creating a distinct signal. Red and infrared light penetrate deeper into the tissue and are often used for blood oxygen and other measurements. Many modern sensors combine multiple LEDs with different colors to improve versatility and adapt to various skin depths. For example, Apple Watch uses a sensor with green and infrared LEDs, and it can also use the electrical heart sensor for ECG, as described in the Apple Watch User Guide. The choice of wavelengths helps the sensor cope with different blood volumes and motion conditions.

Why more LEDs does not guarantee accuracy

Marketing materials often highlight the number of LEDs in a sensor assembly, but that number is only one factor. A sensor with more LEDs can in theory illuminate a larger area and capture more light, yet that does not automatically improve accuracy. The quality of the photodiode, the lens geometry, the wavelength of the light, and the algorithms that interpret the signal all play important roles. A poorly optimized multi-LED module may actually introduce more reflections and motion artifacts.

Motion artifacts are a major challenge for wrist-based sensors. When you move your arm, the sensor shifts slightly, and the background light changes. Sophisticated signal processing helps separate the pulse signal from these distractions. Newer generations of sensors often include additional photodiodes or accelerometer data to cancel out motion, but this is not simply a function of LED count. For instance, Apple Watch has a sensor that includes four photodiodes and multiple LEDs, but the real magic happens in the watchOS algorithms that analyze the raw optical data. Fitness bands and budget watches often use a single LED and one photodiode, which can be less accurate during high-intensity interval training or when your wrist moves a lot.

What sensor array differences mean

The table below summarizes the key elements of an optical heart rate sensor array and how each affects real-world performance. When you see a block diagram of a sensor, these are the components that determine how well it can track your pulse.

Optical sensor limitations and how to work around them

Optical heart rate sensors are not perfect. They can be affected by ambient light, skin temperature, poor contact with the wrist, and especially by irregular movements. When you are running, your arm swing creates a rhythmic motion that has a similar frequency to your pulse, and without good noise cancellation the watch might read your stride instead of your heartbeat. To improve accuracy, some watches use a multi-chamber design that isolates the LEDs from external light, or they rely on tighter strap tension, which you can adjust yourself.

Another limitation is that optical sensors measure the pulse at the surface, not the electrical signal of the heart. That is why some smartwatches include an electrical ECG sensor, which requires you to touch a button that completes a circuit. The ECG app on Apple Watch uses the electrical heart sensor to record the timing and strength of the electrical signals that make the heart beat, as described by Apple. For everyday fitness tracking, an optical sensor is usually enough, but if you have a history of heart issues, you might want a watch with an ECG feature, which you can learn more about in our guide on ECG vs Optical Heart Rate on a Smartwatch.

What to compare when choosing a smartwatch

When you evaluate a smartwatch for heart rate tracking, focus on these aspects instead of just the LED count:

• Look for a sensor with multiple wavelengths if you care about blood oxygen measurements, as covered in What Is Blood Oxygen Monitoring on a Smartwatch.
• Check if the watch uses a wrist-based optical sensor or an electrical sensor for ECG, which is a different approach, explained in ECG vs Optical Heart Rate on a Smartwatch.
• Consider the watch's health platform and how frequently it updates algorithms, since software updates can improve accuracy over time. See How Accurate Are Smartwatch Health Metrics? What the Research Says.
• Think about your skin tone and tattoos, because some sensors struggle with darker skin or ink. Our article about Does Skin Tone Affect Smartwatch Heart Rate Readings covers this.
• Check the exercise modes and whether the sensor is optimized for activities like running, cycling, or swimming. Why Smartwatch Heart Rate Lags During Interval Workouts gives more context.
• Read reviews from other users about how the watch performs during real workouts, but keep in mind that individual experiences vary. The best way to know is to try the watch on your own wrist.

Sensor generations in practice

Smartwatch makers often talk about their latest sensor generation. Newer generations typically add more LEDs, different wavelengths, or improved signal processing. However, not every watch with a recent release date has the newest sensor. Some budget models reuse older components. The operating system also plays a role: Apple Watch uses a proprietary sensor that works with watchOS, while Wear OS watches often rely on third-party sensors from suppliers like InvenSense or Broadcom. Garmin has its own Elevate sensor line, which has evolved over multiple generations. If you want the most advanced heart rate tracking, look at high-end models from brands like Apple, Garmin, and Samsung, which we round up in our Best Apple Smartwatches in 2026 and Best Garmin Smartwatches in 2026 guides.

How to evaluate heart rate accuracy claims

You cannot easily test a watch's heart rate accuracy in a store, but you can look for clues. Check the manufacturer's specifications for the sensor type and any mention of the hardware. See if the watch lists specific features like a multi-LED or multi-wavelength sensor. Most importantly, read the documentation and see what algorithms or modes are mentioned. If you have a medical condition, consult your doctor rather than relying on a smartwatch reading. The FDA also provides guidance on the distinction between wellness and medical software, which you can review in the device software functions guidance. The FDA notes that software functions intended for a healthy lifestyle that are unrelated to the diagnosis or treatment of a disease are not considered devices. That means most heart rate apps are not held to the same standards as clinical equipment.

What to pick for your use

Questions

Do more LEDs always mean a better heart rate sensor?

No. LED count is only one part of the optical design. Photodiode quality, wavelength selection, and signal processing often have a larger impact on accuracy. A well designed single LED sensor can outperform a poorly optimized multi LED module.

What is a photodiode in a heart rate sensor?

A photodiode is a light sensitive semiconductor that converts reflected light into an electrical signal. It works together with the LED to measure changes in blood volume under the skin. Some sensors have multiple photodiodes to capture light from different angles and reduce motion artifacts.

Do I need multiple wavelengths in a smartwatch sensor?

Multiple wavelengths can improve blood oxygen measurements and help the sensor adapt to different skin depths. However, for basic heart rate tracking, a single green light sensor is often sufficient. The advantage depends on the use case. For example, if you want to measure SpO2 at altitude, you need red and infrared LEDs.

Can I trust heart rate readings from any smartwatch?

Most smartwatches are wellness devices, not medical instruments. The FDA treats them as general wellness products, so they do not undergo the same validation as clinical equipment. Use readings for fitness and general awareness, and consult a doctor for medical decisions.

How does skin tone affect optical heart rate accuracy?

Darker skin contains more melanin, which absorbs more light and can make it harder for the sensor to distinguish blood flow changes. Some manufacturers use additional wavelengths or adjusted algorithms to improve performance. See our guide on Does Skin Tone Affect Smartwatch Heart Rate Readings for more details.

What is the difference between an optical heart rate sensor and an ECG sensor?

An optical sensor uses light to estimate blood flow, while an ECG sensor measures the electrical activity of the heart. ECG is generally more accurate for detecting rhythm issues, but it requires a different sensor design, often found on higher-end watches. See ECG vs Optical Heart Rate on a Smartwatch for a full comparison.