Apple explains Heart Rate on Apple Watch technology

Apple has posted the following information regarding Heart Rate and Apple Watch on Apple.com:

The average heart rate is 72 beats per minute (bpm). As you exercise, your muscles need more oxygen, and your heart beats faster to deliver it. During workouts, the heart rate sensor in Apple Watch measures your heart rate and displays it right on your wrist. You can also check your heart rate at any time using the Heart Rate Glance. And throughout the day, Apple Watch measures your heart rate every 10 minutes and stores it in the Health app. All this information, as well as other data it collects, helps Apple Watch estimate how many calories you’ve burned. And by checking your heart rate during workouts, you can see how both your intensity level and your heart rate change over time.

The heart rate sensor in Apple Watch uses what is known as photoplethysmography. This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate.

The heart rate sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate every 10 minutes. However, if the infrared system isn’t providing an adequate reading, Apple Watch switches to the green LEDs. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.

More info via Apple here.

9 Comments

  1. Not sure why the green LED needs to “flash hundreds of times per second”, or flash at all. Seems like it would be more power efficient to be always-on. The photodiode measures the change in reflectivity and the processor can calculate this rate of change to provide heart rate. What function does flashing provide?

    1. Spark, I think they don’t check heart rate all the time:

      “During workouts, the heart rate sensor in Apple Watch measures your heart rate and displays it right on your wrist. You can also check your heart rate at any time using the Heart Rate Glance.”

      So it only measures more often or continuously during workouts and if you want a reading when not working out you can get one with a Glance. The flashing might actually use less power since if it’s off half the time when flashing, it will use 1/2 the power for each check. Or there might be some technical reason for the flashing. That’s my understanding of what Apple stated.

      1. I’m going to guess that each green LED flash and subsequent pickup by the photosensor represents a data point of green absorption. This should help determine the period between heart beats.

        Totally wasted on Vulcans.

    2. LEDs are NEVER always on. They’re always pulse modulated to save power and to dissipate heat. Depending on the usage the modulation rate can be slower or faster.

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