How an apple watch uses light to measure heart rate.

By Anne Kuckertz

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If you look closely at an apple watch while it takes a person’s heart rate, you’ll notice bright green light shining from the bottom of it right on to their wrist. Why does a watch need light to measure heartrate? It’s actually pretty ingenious.

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The watch takes advantage of the physics of blood. When the heart beats it sends blood containing lots of oxygen out into the body. When it contracts, the amount of blood in places like the hands, feet, and ears decrease. This causes the amount of oxygen in the blood to decrease as well.

Packets of light energy called photons are sent from a watch through the skin to try and find these oxygen atoms. Each oxygen atom has rings of electrons orbiting its center, like planets orbiting the sun. The closer a ring is to the center of the oxygen, the less energy the ring has. Electrons like to be in as a low an energy state as possible, and therefore as close to the center as possible. However, when a photon from a watch hits an atom, the atom might absorb that energy, giving the light energy to its electrons. This extra energy forces the electrons to stray farther from where they like to be. In order to return to home base, they have to release the extra energy from the photon back out into their surroundings.

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Once the energy is back in the blood, it makes its way to where the apple watch sits against the surface of the skin. The bottom of the watch contains a photodiode, in addition to the light, that detects photons and tells the apple watch how much energy it received back from the blood. The watch uses this information to decide how much of the green light it originally emitted actually hit oxygen. Once it knows how much oxygen the light found it stores this information. The watch repeats this process of light shining and detecting until it can figure out how fast the heart is pumping blood in and out of the hand. And boom, heart monitored.

 

But light doesn’t have to just measure oxygen. It is used to detect other things too such as sepsis and cancer. In fact, the military used it to find bullets in injured soldiers. Unlike the apple watch, where the photodiode and the light source are on the same side of the skin. The light in these bullet finding devices was located on the front of the body and the photodiode was on the back. The light shined through the body and, if a bullet was present, was stopped by the bullet, preventing the light from traveling to the photodiode. If the photodiode did not see the light it was supposed to, it knew that a bullet was present.

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Unfortunately, a major hurdle that medical light detecting devices must overcome is all the ambient light that is present in a user’s surroundings. If a person is outside on a sunny day or inside with the lights on, the photodiode might accidentally detect that light energy in addition to or instead of the energy emitted from the oxygen atoms or the energy blocked by the bullet. A big focus in the field recently is on developing algorithms that know what this extra energy noise looks like so that they can ignore it. A lot of these commercially available light-based sensors included in personal fitness trackers are still not as accurate as medical grade sensors though. So, when somebody consults a medical professional with concerns regarding their apple watch readings, their doctor will be confirming the patient’s symptoms using medical grade equipment.

But light is still an immensely powerful thing. It provides energy, it brightens the day, and it can be a powerful medical tool. With more development, light has the potential to detect a whole host of diseases in the body, providing affordable, portable, and easy alternatives to current diagnostic techniques.