In the recent years an increasing number of smart accessories became available on the market. These are able to monitor the user’s physiological signals during exercise or also 24 hours a day. Among these the most popular types are the smart bracelets and smart watches, which utilize different kinds of sensors to measure body temperature, number of steps, galvanic skin response, heart rate, oxygen saturation and ECG. A frequently used methodology in these devices is the so called photoplethysmography (acronym: PPG).
Photoplethysmography is a procedure to measure the pulsation of blood vessels using light, based on the results various physiological values can be calculated, including heart rate and oxygen saturation. Earlier it was not possible to perform this measurement outside of clinical environment, but due to the technological advancements today photoplethysmography can easily be integrated into a smartwatch, since the necessary electrical components are cheap and tiny, with low power consumption. The measurement however is very sensitive to external effects including motion and temperature change, which makes it difficult to perform reliably in free-living conditions.
In the first part of my thesis I describe the theory of PPG measurements and the options of implementations, and also the distorting effects present in free-living condition measurements. In the second part of my thesis I present the steps of designing my own PPG measurement system. Next, I analyze the mentioned distorting effects based on publications and my own measurement results, and also, I describe my recommendation for robust PPG measurement. Finally, I summarize the acquired knowledge and experience and the possibilities of further developments in this topic.