Here are all the sensors on your Apple Watch — and what they do

apple watch rear sensor

Your Apple Watch comes jam packed with an impressive amount of technology for its size — including a number of health and utility sensors.

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Despite being intimately familiar with the health and general capabilities of our Apple Watch, few of us are likely as familiar with the sensors that make them possible. If you’ve ever wondered about the sensors on your Apple Watch, you’ve come to the right place.

All the sensors on an Apple Watch

apple watch diagram

From being able to read your heart rate to detecting the ambient amount of light in the room, the Apple Watch has a handful of key sensors. Here’s everything you need to know about the sensors on your Apple Watch.

Optical heart rate sensor

Apple Watch optical sensor

Your Apple Watch can continuously measure your heart rate using its optical heart rate sensors. Based on a technology known as photoplethysmography, this allows for day-to-day background heart monitoring.

This sensor works by flashing a green LED light into your wrist to measure the amount of blood flowing through it. When a typical heart beats, the amount of blood flow in the wrist increases. Between beats, it decreases. By flashing a green LED hundreds of times per second, your Apple Watch can measure your heart rate.

While useful for workout data and general monitoring, an optical heart rate sensor isn’t generally accurate enough for medical purposes. That bring us to the electrical heart sensor.

Electrical heart sensor

apple watch ecg

On the Apple Watch Series 4 and later models, you’ll find an ECG function that allows you to check your heart rate with a technology similar to the kind used by doctors. This sensor actually measures the electrical impulses across your chest.

When you open the ECG app and place your thumb on the Digital Crown, you’re actually creating a closed circuit between your heart and both arms. The Apple Watch sensor then measures these electric impulses.

The result is much more accurate heart rate monitoring, and the ability to detect irregular or unsteady heart rates. It’s accurate enough to have cleared the FDA regulations for a medical device.

Temperature sensor

wrist temperatureApple Watch Series 8 models and later, as well as all Apple Watch Ultra models, have a pair of temperature sensors. One of them is located underneath the display, while the other is mounted on the back crystal near your wrist’s skin.

The wrist temperature sensor, which takes readings at night, can help you get insight on your sleep and overall well-being. Currently, however, it’s not designed for use as a thermometer to check for fever.

Additionally, by measuring your body’s general temperature, it can more accurately predict menstrual cycles. This is because a woman’s body will generally rise in temperature during ovulation.

Always‑on altimeter

apple watch elevation trackingAn altimeter is a device that measures altitude. On the Apple Watch, there’s a barometric altimeter. This kind of sensor detects altitude based on barometric pressure, or the pressure within the atmosphere.

Since barometric pressure differs at different heights, the altimeter can accurately give you elevation data. This makes it useful for tracking hikes or walks across uneven terrain.

The altimeter is actually pretty accurate compared to industry-standard altimeters. It can also be used to track even relatively small gains or losses in elevation, such as during bouldering or rock climbing.

Accelerometer & gyroscope

gyroscope dataYour Apple Watch technically has two types of devices to measure motion: an accelerometer and gyroscope. Both of these sensors work in tandem to give you all types of data, from general activity to fall detection.

An accelerometer measures the acceleration of an object, or how it moves through space. The gyroscope, on the other hand, measures both orientation and rotation.

Together, these two sensors provide data about an Apple Watch’s position in space, as well as how it moves through it. The result is all types of data points about your workouts, workout intensity, and activity levels.

Compass

compass

The Apple Watch has a compass built-in, which may seem surprising for those that know how compasses work. (And, in fact, Apple even warns that strong magnets can interfere with the accuracy of the compass.)

Although you’d imagine that the Apple Watch uses GPS data for its compass feature, iFixit teardowns have shown that there is actually a physical compass in the device. Reportedly, this compass uses a tiny gyroscope to function. It also likely requires some type of mathematical offset to counteract the Apple Watch’s magnets.

The end result, of course, is a compass that can tell you what direction you’re facing. For those that hike frequently, this feature can be incredibly useful.

Ambient light sensor

time in daylightThe final sensor on current generation Apple Watch models is an ambient light sensor. As the name suggests, this sensor detects the light conditions in the area you’re currently in. In other words, it can tell how bright or dark your environment is.

This light sensor actually contributes to automatic screen brightness. You can try this out yourself by placing your Apple Watch screen next to a bright light, or out in sunlight. The display will appear brighter to compensate for the environmental light.

Additionally, the ambient light sensor can also be used for health purposes. On the Apple Watch Series 6 and later, you can actually check how much time you’ve spent in daylight — all thanks to the sensor.

What happened to the blood oxygen sensor?

blood oxygen levels

There’s one omission from the above list of Apple Watch sensors: the blood oxygen sensor. Available on the Apple Watch Series 6 and later, the system works to measure the percentage of oxygen that your red blood cells carry through your body.

Although newer Apple Watch models have a blood oxygen sensor, the system is now disabled on Apple Watch Series 9 and Apple Watch Ultra 2 models.

This is because Apple lost a patent lawsuit case against medical technology company Masimo. The lawsuit alleged that Apple infringed on Masimo’s patent for a blood oxygen sensor that can read a user’s pulse.

Back in October 2023, the U.S. International Trade Commission (USITC) found that Apple had violated Masimo’s patents. It then issued a ban on the import of watches that included a blood oxygen sensor.

On appeal, Apple has since won the ability to sell watches with a blood oxygen sensor — as long as changes were made in software to disable it, according to NPR.

Potential future Apple Watch sensors

Apple, of course, could also do a lot more in the realm of sensor technology. For example, it could add functionality to existing ones, like providing fever detection using the temperature sensor.

On the flip side, it could implement entirely new sensors, such as noninvasive blood glucose monitoring technology. Apple has long been rumored to be exploring a way to include a sensor that could accurately monitor blood sugar levels without piercing the skin.

The sensor could work by shining a light through a user’s wrist, which echoes the Apple Watch’s existing heart rate sensor. However, current rumors indicate that the system would not be able to continuously monitor a user’s blood sugar.

Current rumors also suggest that Apple may have a sensor that could also measure breathing patterns to help detect sleep apnea. Job postings first spotted by MyHealthyApple have also indicated that Apple may be working on sensors that can measure strain or variations in force.

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