Imaging photoplethysmography: What are the best locations on the face to estimate heart rate?

The potential of imaging photoplethysmography for cardiovascular monitoring applications has been demonstrated recently. Various processing schemes have been proposed to extract heart rate (HR) from a defined region of interest (ROI) on the face. However, the reasons that motivate the choice of the ROI are often unclear. This study aimed at investigating the spatial distribution of the HR-related information on the subject face, based on a partition into 260 small ROIs. A power spectral density (PSD) analysis was performed to determine the amount of HR-related information in each ROI. Normalized color maps were used to visualize the spatial distribution of the HR-information and clearly showed that color fluctuations due to blood volume changes are always more pronounced in the forehead region. After face segmentation, for the R/G/B/NIR channels, average percentages of power of 27%b/43%b/27%b/36%b for the forehead region, 17%/28%/18%/21%for the cheek region and 16%/24%/16%/20% for the whole face were obtained.