Unity in Diversity: Sampling Strategies in Wearable Photoplethysmography

& PHOTOPLETHYSMOGRAPHY OPTICALLY MEASURES the pulsating blood volume flow in the human skin so that primary vital signs such as the heart rate can be determined. Most known is this sensing principle from the mysterious illumination that can sometimes be seen at the back of fitness trackers and smartwatches. Since powering these high-intensity light puts a large dent into a wearable’s energy budget, this paper delves into the sampling schemes and strategies used by current off-the-shelf wearables to save energy and yet obtain good readings. As it turns out, the devices are following very different approaches. Plenty of commercial wearables have been introduced to the general public that benefit their users in terms of personal health care and fitness. Besides the detection of physical activity, these devices can also monitor the user’s heartbeat throughout daily life. This initially triggered interest from the quantified-self movement, but recently has been adopted by organizations that use the devices to support and motivate people to practice more physical exercise. The wearables that apply photoplethysmography (PPG) are loosely attached to the body and tend to be more comfortable and less obtrusive to wear long-term than electrocardiogram (ECG) stick-on sensors. While ECG sensing requires perfectly attached electrodes, wearables at the wrist can be as comfortable as traditional watches. Although most PPG research strives to match ECG in terms of accuracy, most devices are still used for fitness instead of medical care. PPG needs a strong illumination of the skin to capture the pulsating blood just below the human skin. The light-emitting diodes (LEDs) are utilized to generate short flashes to sample the blood volume of the moment, and are turned off Digital Object Identifier 10.1109/MPRV.2019.2926613