Exploring the usage of time-of-flight cameras for contact and remote photoplethysmography

The heart beat is one of the basic vital signs, but the pulse wave can communicate much more information than the beat frequency of the heart. Through the heart rate variability (HRV) among other things, stress level and drowsiness can be inferred. Reliable HRV measurements are commonly obtained by electrocardiography (ECG). In this paper we analyse the correlation between HRV and pulse rate variability (PRV) obtained from contact or remote photoplethysmography (PPG) on a Time-of-Flight (ToF) camera from PMD Technologies AG. The ToF camera is independent of passive illumination as it uses an infrared (IR) light source, which is invisible to the human eye. Therefore, potential use cases include driver monitoring or convenient heart rate measurement with smart phones. We will demonstrate methods for using a ToF camera for contact and remote PPG. For contact PPG the sensor is mounted directly on the subjects body. The pulse wave is calculated as the mean amplitude intensity of all pixels. For remote PPG the sensor is mounted to measure the subjects face. The pulse wave cannot be found in every part of exposed skin. Therefore, the part of the skin with the clearest signal is localised and the pulse wave is recovered from there, by means of passband filtering and blind source separation. Our results show that contact PPG on a ToF camera has high accuracy and highly correlates with a commercial IR pulse oximetry device, as well as with an ECG grade chest belt. For remote PPG our method shows correlation with the ToF contact PPG signal.