Exploration of the optimal skin-camera distance for facial photoplethysmographic imaging measurement using cameras of different types

Recent studies have provided strong evidence that photoplethysmographic imaging (PPGi) techniques can serve for measuring the heart rate from video recordings. However, in the works on PPGi technology so far, motion artifacts, which limit the accuracy, were unavoidable. In this work, we present an exploration of our assumption that for a particular model of camera, there is an optimal measurement distance, which ensures the minimal influence of artifacts in PPGi measurement.We conducted experiments using two cameras of different types that are commonly integrated with modern consumer electronic devices. First, we demonstrated that the both types of cameras are applicable for PPGi-based non-contact measurement. We used the both cameras simultaneously to record the face regions of 10 subjects and then extracted the information about their heart rates from each of the recordings. To verify the results obtained by PPGi method, we compared them with those measured using a “gold standard” technique. We then explored the relation between the camera-face distance and the measurement error. For each kind of camera, we determined the optimal face-camera distance that ensured reducing the error from motion artifacts to the possible minimum.