On our blog we often talk about things around light, or things light can be and can do. Here is another example of what a light can do: measure your blood pressure. Two LED flicker while you wear it on your wrist and an optic sensor “reads” how the blood pulsates.
This technology is known as photoplethysmography. Blood absorbs light and fluctuations of light correlate to heart rate. LED emits light in AC and DC current (alternating current flickers, and direct gives steady light). The waveform of the latter corresponds to the detected transmitted or reflected optical signal from the tissue. DC component also changes slowly with respiration. The AC component shows changes in the blood volume during the phases phases of the cardiac cycle; the fundamental frequency of the AC component depends on the heart rate and is superimposed onto the DC component.
If you ever used such device you might have mused about the choice of green color LEDs. The answer lies in the spectrum or wavelengths (to be even more scientific, between 500 and 600 nm). Green or green-yellow light is much better reflected and detected by a sensor when blood pulsates through skin, and gives a more accurate result compared to infra-red.
From purely medical use, these sensors found their way to popular wearables, and many a runner already tear off their chest-strapped monitors. Devices like my Fitbit combine BPM with distance and speed, sleep monitor and all at a convenience of a wrist-band.
Reference: Paper on Wearable Photoplethysmographic Sensors by Toshiyo Tamura, Yuka Maeda, Masaki Sekine and Masaki Yoshida