Comparison of Visible and Infrared Video Plethysmography Captured from Different Regions of the Human Face

Recently, video plethysmography (VPG) - a heart rate estimation technique using a video camera - has gained significant attention. Most studies of VPG have used a visible RGB camera; only a limited number of studies investigating near-infrared light (wavelength 750-2500 nm), which can be used even in a dark environment, have been performed. The purpose of this study was to investigate the differences between VPG data collected using visible light (VPG_VIS) or near-infrared light (VPG_NIR) from four facial areas (forehead, right cheek, left cheek, and nose). An experiment was conducted to obtain both VPG_VIS and VPG_NIR simultaneously by alternately irradiating the face with NIR and VIS lights. Experimental results showed that the root mean squared error of heart rate estimated using VPG_NIR was 1 bpm higher than that of VPG_VIS. However, contrary to our expectations, the power of the heartbeat-related component included in VPG_NIR was not reduced despite the absorbance of hemoglobin in the NIR light range being 1/100 of that in the VIS light range. This result supports the hypothesis that a main factor in the generation of VPG waves was change in the optical properties caused by blood vessels compressing the subcutaneous tissue and the venous bed. Additionally, the accuracy of the heart rate estimation using VPG tended to be high when the nose was set as the ROI. This result was likely associated with the anatomical structure of the nose.